interfacial characterization of chemical vapor deposition
TRANSCRIPT
APPROVED: Oliver M. R. Chyan, Major Professor Guido Verbeck, Committee Member Justin Youngblood, Committee Member Rob Petros, Committee Member William E. Acree, Chair of the Department of
Chemistry Mark Wardell, Dean of the Toulouse Graduate
School
INTERFACIAL CHARACTERIZATION OF CHEMICAL VAPOR DEPOSITION (CVD) GROWN
GRAPHENE AND ELECTRODEPOSITED BISMUTH ON RUTHENIUM SURFACE
Jafar Abdelghani, B.S., M.Sc.
Dissertation Prepared for the Degree of
DOCTOR OF PHILOSOPHY
UNIVERSITY OF NORTH TEXAS
May 2014
Abdelghani, Jafar. Interfacial characterization of chemical vapor deposition (CVD) grown
graphene and electrodeposited bismuth on ruthenium surface. Doctor of Philosophy
(Chemistry-Analytical Chemistry), May 2014, 130 pp., 1 table, 71 figures, 96 numbered
references.
Graphene receives enormous attention owing to its distinctive physical and chemical
prosperities. Growing and transferring graphene to different substrates have been investigated.
The graphene growing on the copper substrate has an advantage of low solubility of carbon on
the copper which allow us to grow mostly monolayer graphene. Graphene sheet of few
centimeters can be transferred to 300nm silicon oxide and quartz crystal pre-deposited with
metal like Cu and Ru. Characterization of the graphene has been done with Raman and contact
angle measurement and recently quartz crystal microbalance (QCM) has been employed.
The underpotential deposition (UPD) process of Bi on Ru metal surface is studied using
electrochemical quartz crystal microbalance (EQCM) and XPS techniques. Both Bi UPD and Bi
bulk deposition are clearly observed on Ru in 1mM Bi (NO3)3/0.5M H2SO4. Bi monolayer
coverage calculated from mass (MLMass) and from charge (MLCharge) were compared with respect
to the potential scanning rates, anions and ambient controls. EQCM results indicate that Bi UPD
on Ru is mostly scan rate independent but exhibits interesting difference at the slower scan. Bi
UPD monolayer coverage calculated from cathodic frequency change (ΔfCathodic) is significantly
smaller than the monolayer coverage derived from integrated charge under the cathodic Bi UPD
peak when scan rate is at least 5 mV/s. XPS is utilized to explore the detailed chemical
composition of the observed interfacial process of Bi UPD on Ru.
Copyright 2014
by
Jafar Abdelghani
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ACKNOWLEDGEMENTS
First and foremost I would like to thank God. You have given me the power to believe in
myself and pursue my dreams. I could never have done this without the faith
I have in you, the Almighty. I take immense pleasure to express my sincere and deep sense of
gratitude to my advisor Dr. Oliver M. R. Chyan for his guidance, help, encouragement, and
support. I learned a lot from his knowledge and his critique of this work which helped me to
overcome the obstacles and succeed in the tough challenges.
Many thanks to my committee members Dr. Guido Verbeck, Dr. Justin Youngblood, Dr.
Rob Petros, and Dr. Ben Jang (external member Texas A&M) who have importantly helped to
make my research more interesting and improve the content of this dissertation. Also, JD Fox
from Dr. Verbeck group for helping analyze some samples for me. For all my friends back home
who supported and helped me when I needed them. Also, to my first friend in USA Fuad Noor
who opens his home for me with a great hospitality.
Last but not least I would like to thank my family, my parenst, my sisters, and my
brothers for their encouragement and support. They have supported me finically all these years,
so they are the ones who have this success. For my lovely wife Mozinah Khreisat who joined me
in this journey to fulfill this dream and endure my tough times and took care of our daughters.
Special thanks for my aunt Aisha Al-Faqeeh and their family for her kindness and support in the
USA.
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TABLE OF CONTENTS
LIST OF TABLES ............................................................................................................................... vii
LIST OF FIGURES ............................................................................................................................ viii
CHAPTER 1: INTRODUCTION OF INSTRUMENTS .......................................................................... 1
1.1 Introduction .................................................................................................................. 1
1.2 Thin Film Deposition ..................................................................................................... 1
1.2.1 Chemical Vapor Deposition (CVD) Reactor for Graphene Growth ................ 1
1.2.2 Physical Vapor Deposition (PVD), Sputtering Instrument .............................. 3
1.3 Electrochemistry ........................................................................................................... 5
1.3.1 Fundamentals of Electrochemistry ................................................................ 5
1.3.2 The Nature of Electrochemical System .......................................................... 6
1.3.3 Three Electrode System ................................................................................. 7
1.3.4 Open Circuit Potential (OCP) .......................................................................... 8
1.3.5 Cyclic Voltammetry (CV)................................................................................. 9
1.3.6 Electrochemical Quartz Crystal Microbalance (EQCM)................................ 10
1.3.7 Rotating Disk Electrode System (RED) and Rotating Ring Disk Electrode
System (RRDE) ....................................................................................................... 12
1.4 Surface Analysis Techniques ....................................................................................... 13
1.4.1 X-ray Photoelectron Spectroscopy (XPS) ..................................................... 14
1.4.2 Fourier Transform-Infrared (FTIR) Spectroscopy.......................................... 15
1.4.2.1 Attenuated Total Reflectance (ATR) .......................................................... 18
1.4.3 Raman Spectroscopy .................................................................................... 19
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1.5 Surface Microscope ..................................................................................................... 24
1.5.1 Atomic Force Microscopy (AFM) .................................................................. 24
1.5.2 Scanning Electron Microscope (SEM) .......................................................... 26
1.6 References ................................................................................................................... 28
CHAPTER 2: GRAPHENE ................................................................................................................ 31
2.1 Introduction ................................................................................................................ 31
2.2 Chemical Vapor Deposition (CVD) Procedure to Grow Graphene .............................. 32
2.2.1 Growth of Graphene on Copper Substrate .................................................. 36
2.3 Transfer of CVD Graphene........................................................................................... 37
2.3.1 The Procedure of Transferring Graphene Grown on Copper ....................... 39
2.3.2 The Procedure of Transferring Graphene Grown on Nickel ......................... 40
2.4 Characterization of Graphene on Different Substrates, Raman Spectroscopy, Water
Contact Angle Measurements, and QCM Frequency ....................................................... 41
2.4.1 Detection of Graphene Monolayers Using QCM ......................................... 44
2.4.2 Detection Graphene Using AFM .................................................................. 46
2.5 Graphene Plasma Hydrogenation (Graphane) and Characterization by FTIR ............. 47
2.6 Adsorption of Carbon Monoxide (CO) on the Graphene Surface ............................... 55
2.7 Electrochemical Properties of Graphene .................................................................... 58
2.7.1 Electrodepositing of Cu on Graphene .......................................................... 61
2.8 References ................................................................................................................... 63
CHAPTER 3: BISMUTH (Bi) UNDERPOTENTIAL DEPOSITION (UPD) ON RUTHENIUM (Ru) ........... 66
3.1 Introduction ................................................................................................................ 66
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3.2 The Effect of the Ambient on Bismuth under Potential Deposition (UPD) on
Ruthenium ........................................................................................................................ 68
3.3 Anion Effect on Bismuth under Potential Deposition (UPD) on Ruthenium............... 72
3.4 XPS Analysis of Bi UPD on Ru under Different Conditions .......................................... 77
3.5 The Ring Rotating Disc Electrode (RRDE) Result of Bi Deposition on Ru .................... 82
3.6 Bi Bulk on Ru Result .................................................................................................... 90
3.7 Proposed Mechanism of Bi Deposition on Ru ............................................................ 92
3.8 Conclusion ................................................................................................................... 96
3.9 References ................................................................................................................... 97
CHAPTER 4: CHARACTERIZATION OF PLASMA TREATED INTER-LAYER DIELECTRIC (ILD) ULTRA-
LOW-K USING XPS AND FTIR ......................................................................................................... 99
4.1 Introduction ................................................................................................................ 99
4.2 Characterizations of Dielectric Damages on Blanket Wafer Using FT-IR .................. 101
4.3 XPS Characterizations of Dielectric Damages on Blanket Wafer .............................. 104
4.4 Characterization of ILD Plasma Treatment Using AFM and Optical Microscope ...... 111
4.5 Conclusion and Future Work ..................................................................................... 115
4.6 References ................................................................................................................. 117
CHAPTER 5: CONCLUSION AND FUTURE WORK………………………………………………………………………119
5.1 Conclusion ................................................................................................................. 119
5.2 Future Work .............................................................................................................. 121
COMPREHENSIVE LIST OF REFERENCES ...................................................................................... 123
vi
LIST OF TABLES
Table 4.1: The measurement of AFM roughness of TEL blanket wafer before and after treatment
with O2 plasma…………………………………………………………………………………………………………………….115
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LIST OF FIGURES
Figure 1.1: Chemical vapor deposition (CVD) reactor for graphene growth system with the gas
flow controller and vacuum pumo……………………………………………………………………………………………3
Figure 1.2: Dual magnetron gun sputtering system of the Denton vacuum desktop pro, used for
physical vapor deposition (PVD).………………………………………………………………………………………………..5
Figure 1.3: Schematic of layers of electrolyte in electrochemical system …...…………………………….7
Figure 1.4: A three electrode system illustration …...………………………………………………………………….8
Figure 1.5: Example of the CV of the Bi UPD on Ru with the background.…………………………………10
Figure 1.6: Electrochemical quartz crystal microbalance (EQCM) cell setup.…………………………….11
Figure 1.7: (a) The arrangement of RRDE showing the disc electrode in the center and the ring
electrode separated by isolated material next to it. (b) The flow pattern on the RDEE of the
electrolyte while rotating ……..…………………………………………………………………………………………………13
Figure 1.8: X-ray photoelectron spectrometer (XPS) PHI 5000 VersaprobeTM, CART XPS
instruments.…………………………………………………………………………………………………………………………….15
Figure 1.9: FTIR spectrometer layout ……..……………………………………………………………………………….17
Figure 1.10: Attenuated total reflectance ATR representing the mechanism of the reflected light
and the sample characterization.……………………………………………………………………………………………..18
Figure 1. 11: Anti-Stokes and Stokes lines in Raman spectroscopy. …………………………………………20
Figure 1.12: Almega XR Raman spectrometer from Thermo Scientific Nicolet, CART Raman
instrument.………………………………………………………………………………………………………………………………23
Figure 1.13: Atomic force microscope AFM picture in our lab.………………………………………………….25
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Figure 1.14: Scanning electron microscope (SEM) FEI Nova NanoSEM 230, CART SEM
instrument. ……………………………………………………………………………………………………………………………27
Figure 2.1: Diagram of CVD growth on copper…………………………………………………………………………33
Figure 2.2: Chemical vapor deposition (CVD) graphene system in our lab.……………………………….35
Figure 2.3: Graphene growth process diagram.………………………………………………………………………..36
Figure 2.4: a) Photo of copper foil before graphene growth b) Photo of copper foil after
graphene growth.…………………………………………………………………………………………………………………….37
Figure 2.5: Process of transfer CVD graphene to other substrate.…………………………………………….38
Figure 2.6: Water contact angle measurement.………………………………………………………………………..41
Figure 2.7: Raman spectra of graphene on Cu and graphene on 300 nm SiO2 developed in our
lab.…………………………………………………………………………………………………………………………………………..42
Figure 2.8: Raman spectra difference between graphene and graphite ……..……………………………43
Figure 2.9: a) Shows the frequency of the QCM before transfer graphene. b) Shows the
frequency after the transfer of graphene.………………………………………………………………………………..45
Figure 2.10: a) Reported AFM image of a boundary between a graphene monolayer and a SiO2
substrate. b) Reported AFM image of a boundary between a graphene monolayer and a mica
substrate……...………………………………………………………………………………………………………………………….46
Figure 2.11: FT-IR spectra of hydrogenation of graphene and loss after 45min.……………………….48
Figure 2.12: Anticipated graphene hydrogenation structure in certain spots show vacant
between the carbons of graphene and not covered all the carbon …….…………………………………..49
Figure 2.13: Differential FT-IR spectra of hydrogenated graphene using H2 plasma.…………………50
Figure 2.14: Graphene hydrogenation, in this case, there is one carbon atom separating
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them ………….……………………………………………………………………………………………………………………………51
Figure 2.15: Graphene hydrogenation, in this case, the 2 H atoms are separated by a multiple
carbon atoms, which classifies it as the “Far” structure……...……………………………………………………51
Figure 2.16: MIR-IR spectra of graphene with few atoms Cu sputtered plasma hydrogenation
detected using MR-IR for 45degree polarization.……………………………………………………………………..53
Figure 2.17: Raman spectra of graphene before and after plasma hydrogenation.…………………..55
Figure 2.18: The adsorption of CO on pristine graphene transferred to Ru QCM.……………………..56
Figure 2.19: Schematic view of a single gas molecule (NH3) adsorption on pristine graphene
(a)TM–graphene (b). T, top site; B, bridge site; H, hollow site; M, transition metal atom (Au).
Carbon atom in gray, H in white, N in blue and Au atom in yellow ……..……………………………………57
Figure 2.20: The adsorption of CO on graphene transferred to Ru QCM embedded with 5nm Cu.
…………………………………………………………………………………………………………………………………………………58
Figure 2.21: Comparison of the CVs between Ru electrode and graphene on Ru wafer electrode.
…………………………………………………………………………………………………………………………………………………59
Figure 2.22: Comparison of the CVs between GC electrode and graphene on Ru wafer electrode.
…………………………………………………………………………………………………………………………………………………60
Figure 2.23: CV’s for Cu foil, graphene, and GC electrodes in hexacyanoferrate(III).…………………61
Figure 2.24: a) Graphene on Ru wafer before electroplating with 2mMCu. b) Graphene on Ru
wafer after electroplating with 2mMCu.…………………………………………………………………………………..62
Figure 3.1: Ambient effect of Bi deposition on Ru. (a.) CV of Bi deposition on Ru. Scan rate of
5mV/s. (b.) Zoom-in view of CV in Bi UPD region. (c.) Mass response. (d.) Zoom-in view of mass
response in Bi UPD region………………………………………………………………………………………………………..70
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Figure 3.2: (a.) Bi UPD MLMass (Cathodic) calculated in different scan rate, N2 purge (black line),
lab ambient (red line), and O2 purge (Green line). b.) Bi UPD MLCharge (Cathodic) calculated in
different scan rate, N2 purge (black line), lab ambient (red line), and O2 purge (Green line).
…………………………………………………………………………………………………………………………………………………70
Figure 3.3: Anion effect of Bi deposition on Ru. (a.) CV of Bi deposition on Ru. Scan rate of
5mV/s. (b.) Zoom-in view of CV in Bi UPD region. (c.) Mass response. (d.) Zoom-in view of mass
response in Bi UPD region.……………………………………………………………………………………………………….73
Figure 3.4: Bi UPD (MLCharge – MLMass) vs scan rate of Bi3+ containing solution (a) in H2SO4 and (b.)
in HClO4.…………………………………………………………………………………………………………………………………74
Figure 3.5: Scan rate effect and ambient effect of Bi UPD ML coverage on Ru in 0.5M H2SO4
+1mM Bi3+ (a.) MLMass (cathodic) (b.) MLMass (anodic) (c.) MLCharge (cathodic) (d.) MLCharge
(anodic). …………………………………………………………………………………………………………………………………75
Figure 3.6: Scan rate effect and ambient effect of Bi UPD ML coverage on Ru in 0.5M HClO4 +
1mM Bi3+ (a.) MLMass (cathodic) (b.) MLMass (anodic) (c.) MLCharge (cathodic) (d.) MLCharge (anodic).
…………………………………………………………………………………………………………………………………………………76
Figure 3.7: XPS of Bi UPD N2 purge deposition on Ru cathodic region in 0.5M H2SO4 with high
scan rate (50mV/s) of depositing Bi on Ru electrode.……………………………………………………………….77
Figure 3.8: XPS of Bi UPD non- purge deposition on Ru cathodic region in 0.5M H2SO4 with high
scan rate (50mV/s) of depositing Bi on Ru electrode.……………………………………………………………….78
Figure 3.9: XPS Bi UPD on Ru (no N2 purge) Ru 3d Region at 280 eV which represent the Ru
metal or native oxide……………………………………………………………………………………………………………….79
Figure 3.10: XPS of Bi UPD deposition on Ru cathodic region in 0.5M H2SO4 with low scan rate
xi
(5mV/s) of depositing Bi on Ru electrode.………………………………………………………………………………..80
Figure 3.11: XPS of Bi UPD deposition on Ru cathodic region in 0.5M HClO4.……………………………81
Figure 3.12: XPS of Bi UPD deposition on Ru cathodic region in 0.5M H2SO4 under N2 with rinse
with water after deposition.…………………………………………………………………………………………………….82
Figure 3.13: RRDE system sketch.……………………………………………………………………………………………..84
Figure 3.14: The result of Bi UPD on Ru RRDE system (O2- saturated) a.) The ring current b.) The
disc current c.) Koutecky- Levich plot at varying potential.………………………………………………………86
Figure 3.15: The result of Ru background RRDE system (O2- saturated) a.) The ring current b.)
The disc current c.) Koutecky- Levich plot at varying potential.………………………………………………..86
Figure 3.16: The result of Ru background RRDE system in 0.5M HClO4 (O2- saturated) a.) The ring
current b.) The disc current c.) Koutecky- Levich plot at varying potential.……………………………….89
Figure 3.17: The result of of Bi UPD on Ru RRDE system in 0.5M HClO4 (O2- saturated) a.) The
ring current b.) The disc current c.) Koutecky- Levich plot at varying potential.………………………..89
Figure 3.18: XPS of Bi bulk on Ru. N2 purge. 5 mV/s and hold at ~-0.6V for 7 mins. Sputter rate:
0.5 nm/min and/or 2.3 nm/min (calibrated to SiO2). (a.) Bi 4f (H2SO4) (b.) Bi 4f (HClO4) (c.) O 1s
(H2SO4) (d.) O 1s (HClO4)…………………………………………………………………………………………………………..90
Figure 3.19: XPS of Bi bulk on Ru. N2 purge. 5 mV/s, hold at ~-0.6V for 7 mins, and then expose
to air for 1 hour. Sputter rate: 0.5 nm/min and/or 2.3 nm/min (calibrated to SiO2). (a.) Bi 4f
(HClO4) (b.) O 1s (HClO4) (c.) Ru 3d (HClO4) (d.) Cl 2p (HClO4).………………………………………………….91
Figure 3.20: (a.) CV and mass response of Bi deposition on Ru. Scan rate of 5 mV/s. (b.) Zoom-in
view of CV and mass response in Bi UPD region.………………………………………………………………………92
Figure 4. 1: Organsilicate glass (OSG) proposed structure [13]……………………………………………….102
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Figure 4.2: Schematic representation of Multiple internal reflections infrared (MIR-IR) with Si
attenuated total reflection (ATR).…………………………………………………………………………………………..102
Figure 4.3: FTIR spectra time dependent O2 plasma ashing on blanket wafer.………………………..103
Figure 4.4: ILD/low-k blanket wafer O2 treatment plots of IR absorption peak heights of CH3 and
OH vs. etching time for 1540sec.……………………………………………………………………………………………103
Figure 4.5: C 1s core-level spectra for pristine and O2 plasma treatment etching for 70sec un-
patterned low-k material. The C1s peaks show at 283.4 eV corresponds to Si-C, C-H [21] and the
peaks at 286.4 eV corresponds to C-O [21, 22].………………………………………………………………………105
Figure 4.6: C 1s core-level spectra for pristine and O2 plasma treatment etching from 100 to
1540sec un-patterned low-k material. The C1s peaks show at 284.2 eV corresponds to Si-C, C-H
[21] and the peaks at 286.4 eV corresponds to C-O [22].……………………………………………………….106
Figure 4.7: TIR spectra of a 300 nm blanket low-k films measured and normalized curve XPS C
(1s) of integrated area 280-290 eV after O2 plasma etching vs. etching time for 1540sec.……..107
Figure 4.8: C 1s core-level spectra for pristine and O2 plasma treatment etching for 70sec un-
patterned low-k material. The C1s peaks shows at 284.0eV correspond to Si-C, C-H [23] and the
peaks at 286.4 eV corresponds to C-O [23].……………………………………………………………………………109
Figure 4.9: C 1s core-level spectra for pristine and H2 plasma treatment etching for 70sec un-
patterned low-k material. The C1s peaks shows at 283.6 eV correspond to Si-C, C-H [23], and the
peaks at 285.6 eV corresponds to C=C and C-C, and the peaks at 286.4 eV C-O.…………………….109
Figure 4.10: Normalize curve Hydrogen plasma treatment of blanket wafer ILD/ low-k, analysis
of XPS C1s integrated area from 280-290eV.…………………………………………………………………………..110
Figure 4.11: Wafer before and after treatment of O2 plasma magnify for 100x. a) TEL blanket
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wafers as is wafer. b) TEL blanket wafers 100sec O2 plasma. c) TEL blanket wafers 640sec O2
plasma. d) TEL blanket wafers 1540sec O2 plasma.…………………………………………………………………112
Figure 4.12: a) AFM image roughness of as is blanket wafer. b) AFM image roughness of 100sec
O2 plasma treatment blanket wafer.……………………………………………………………………………………….113
Figure 4.13: a) AFM image roughness of 640sec O2 plasma treatment blanket wafer. b) AFM
image roughness of 1540sec O2 plasma treatment blanket wafer.………………………………………….114
Figure 5.1: CV and Mass response on the deposition of the Bi UPD on Ru…………………………….122
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CHAPTER 1
INTRODUCTION OF INSTRUMENTS
1.1 Introduction
In this dissertation work, several surface characterization instruments alongside with the
electrochemical characterization tools were used in our lab. On graphene project which is one
of the major part of my dissertation, a chemical vapor deposition (CVD) reactor was built in
house for graphene growth. Raman, FT-IR and some electrochemical techniques was used for
graphene characterization. Moreover, rotating ring disk electrode (RRDE), X-ray photoelectron
spectroscopy (XPS), and electrochemical quartz crystal microbalance (EQCM) were the
instruments utilized to accomplish the work on bismuth (Bi) under potential deposition (UPD)
on ruthenium (Ru). Beside all the instruments mentioned earlier some work was needed to use
other instruments or techniques such as atomic force spectroscopy (AFM), physical vapor
deposition (PVD), and scanning electron microscope (SEM) for other purposes or to check same
experiment with different techniques.
1.2 Thin Film Deposition
1.2.1 Chemical Vapor Deposition (CVD) Reactor for Graphene Growth
Chemical vapor deposition (CVD) is a process used to deposit a solid thin film material
on a substrate [1]. It is a very popular method in the semiconductor industry to produce thin
film layers. Usually the source of the thin film deposited on the substrate is gases which
introduce under vacuum with heat or high frequency voltage. There are different types of CVD
processed such as, atmospheric pressure chemical vapor deposition (APCVD), low pressure
1
chemical vapor deposition (LPCVD), and plasma enhanced chemical vapor deposition (PECVD).
The mechanism of all types of CVD process is rather similar. The precursor gases are introduced
to the reaction chamber which elevated to certain temperature of interest. When the gases
pass through the chamber, it will contact the hot substrate surface which will be good
environment for a reaction or decompose forming a thin film layer at the cooled down system.
There are several factors that can affect the quality of the thin film produced on the
substrate like, the temperature, the purity and concentration of the gases, and the vacuum
pressure. The CVD process mostly produces well control and uniform film with high quality, fine
grained, and impervious. As figure 1.1 represents the basic component of the CVD machine
includes gas delivery system, reactor chamber, and vacuum system. CVD technique has very
important applications in the industry and research. For instance, CVD has been used to
produce optical fibers in the telecommunications industry, coating films in the semiconductors
industry, and graphene which is new material with promising applications in the computer
technology [2].
2
Not only can the CVD process deposit a thin layer on the substrate, but it can also
control the thickness and quality. The thin layer deposit using this system can reach up to 0.1nm
(10pm) for a monolayer [3, 4]. Figure 1.1 shows the actual CVD machine which was created in
Dr. Chyan’s lab in order to grow a large area of graphene.
1.2.2 Physical Vapor Deposition (PVD), Sputtering Instrument
Physical vapor deposition (PVD) is another technique used mainly to deposit thin film on
other substrates which is wildly used on the solar cells and semiconductors industries. The
process on the PVD is similar to the chemical vapor deposition (CVD) except the precursor of
the PVD is in solid form instead of gas like in the CVD. The PVD process is carried out under high
Figure 1.1: Chemical vapor deposition (CVD) reactor for graphene growth system with the gas flow controller and vacuum pump.
3
vacuum conditions which could reach up to 10-6 torr [5]. PVD go through several steps to
achieve thin film deposition. The first step of the process is evaporation which will be related to
the target (the material that the researchers are interested to be deposited on the substrate). To
change the target from the solid phase to vapor, the target was bombarded with high energy
source of ions or electrons beams, like argon (Ar). The second step transports or transfers the
vaporized atoms to deposit on the substrate by line of sight deposition. Then next is the
reaction which is extra step which could be carried out, depend on the study and concern. For
example, if adding oxidation to the coated sample was involved, oxygen gas during PVD process
can be included or any other chemical of interest specific to the intent of application. Finally, the
deposition of the thin film layers on to the substrate surface. Depending on the characteristics
of both of the target and the substrate, the resulting coating can be very strong, mild, or weak
in term of mechanical properties. There are many applications and objectives. Many coatings
employed to prevent corrosion and resist the frictions. Also, it can increase the hardness of
some material, and improve smoothness of the surface [6].
4
Figure 1.1: Dual magnetron gun sputtering system of the Denton vacuum desktop pro, used for physical vapor deposition (PVD).
As it can be seen from figure 1.2, the PVD machine consists of different component. Our
Denton vacuum desktop pro sputtering machine has two different guns technique to attach the
target for sputtering, DC and RF. This sputtering system prepared a direct current (DC) power
and a radio frequency (RF) power magnetron guns. The high radio frequency links to the
alternating currents which permit to coat insulating material. Using the RF gun isolated material
such as Si or SiO2 can be sputtered. Also, our instrument has an advantage to deposit two
materials simultaneously.
1.3 Electrochemistry
1.3.1 Fundamentals of Electrochemistry
Electrochemistry is one of the branches of chemistry which is concerned with
5
monitoring and determining the charge transfer between the materials. Most of the
electrochemistry is studied in solution that requires good conductivity. Therefore, an electrolyte
must be added to the solution to complete the circuit. During the charge transfer between
materials (electrodes), it involves electron movement which is produce electricity that can be
useful for so many applications such as fuel cell and solar system. Moreover, electrochemistry
can be employed to detect trace amount of some chemicals which has unique characteristics for
redox and electron structure, such as tracing the transition state of some reactions.
1.3.2 The Nature of Electrochemical System
Electrochemistry system is not homogenous, it is heterogeneous. That is because of the
electrochemical interaction taking place across the interface of the solid electrode and the
electrolyte solution. There are three distinctive layers on the electrochemical reaction. The first
is the double layer which is the interaction between the electrode and the electrolyte. The
double layer mostly has about 100Ao thickness. Then the diffusion layer which comes between
the double layer and the bulk layer. This layer is resulted from the difference in the
concentration of the electroreactant and product. The last layer is the bulk layer which is the
rest of the component that is far away from the electrode.
6
Figure 1.2: Schematic of layers of electrolyte in electrochemical system [7].
Some of the electrochemical reaction is simple and easy to monitor, and some is very
complicated. There is a lot of thing happened on the surface of the electrode all of sudden
which need special attraction and preparation. In this dissertation many electrochemical
reactions were observed and investigated using different technique as it will be shown later.
1.3.3 Three Electrode System
Three electrode cells is one of the most used electrochemical techniques to study the
properties of materials and electrodes. As it’s well known the regular two electrode system
consists of the reference and working electrode. The three electrode system added another one
which is called as counter electrode or auxiliary electrode. The function of the counter electrode
is to keep the charge balance of the system which will provide electrons when there is a loss
7
and gain electrons when there is excess. Most of the counter electrodes are made of noble or
inert materials such as, noble metals or graphite. The other electrode is the reference electrode
which has known half-cell potentials that the analyst can refer the measurement to it. The last
electrode is the working electrode which is the electrode that the characteristic and reaction on
its surface is explored. Using the electrochemical technique the potential, current and charge
can be controlled to investigate different kind of properties and find out new catalysts and much
more. Three electrode system was used frequently on the bismuth UPD on ruthenium study.
Figure 1.3: A three electrode system illustration [8].
1.3.4 Open Circuit Potential (OCP)
Open circuit potentials (OCP) is one of the electrochemical techniques which can
measure the potential of the working electrode surface versus the reference electrode when
there is no external potential or current applied on the system. OCP used to measure the
equilibrium between the electrode and the solution which reflects to the cleanliness and
8
oxidation state of the electrode.
1.3.5 Cyclic Voltammetry (CV)
Cyclic voltammetry (CV) is one of the most often used electrochemical techniques which
can reveal rich information about the oxidation reduction state of the chemical of interest. The
principle of the CV is to measure the current developed on the working electrode surface when
excess of potential is applied. Moreover, by sweeping the potential linearly back and forth, the
CV data provide efficient monitoring of the reaction status developed and study the reaction at
any point. This process can enlighten us with the characteristics and the properties of the
electrode as well as the analyte.
As it can be seen on figure 1.5 the difference of the cyclic voltammetry between the
background and the target deposit material on the surface. The feature of the deposition
characteristics and the changes happening during the sweeping can clearly be identified and
studied.
9
1.3.6 Electrochemical Quartz Crystal Microbalance (EQCM)
Electrochemical quartz crystal microbalance (EQCM) is a powerful tool for the
investigation of the dynamics system changes on the materials of the electrodes. EQCM
connected the change of the mass with the change of the electrical charges using the Sauerbrey
equation together with Faraday's law [9]. The theory of electrochemical quartz crystal
microbalance (EQCM) is to combine the electrochemistry technique with quartz crystal
microbalance (QCM) to study the property of an electrode.
Figure 1.4: Example of the CV of the Bi UPD on Ru with the background.
10
Figure 1.5: Electrochemical quartz crystal microbalance (EQCM) cell setup.
EQCM technique gives us the advantages to measure the mass change during the
deposition and stripping as well as the electrochemical charge change. By comparing these
variables in the same time rich information can be found on the side reactions or deposition.
Moreover, it is very sensitive to any change on the surface of the conditions of the surroundings
which in our interest to explore; the sensitivity of the EQCM in our set up can reach up to sub
nano-gram which can translate to how many monolayers. Calculating the mass change using
Sauerbrey equation [10] which accounts for the relationship between the mass change and the
frequency change, reveal much information about mechanism and kinetic of the reaction.
Sauerbrey equation shows the linear relationship between the mass change and its
corresponding change in vibration frequency. Also, variation of the deposition time on the QCM
can give us the good information on the growth mechanism. By calculate the number of the
11
monolayers in different times, the growth mechanisms of layer by layer or 3D mechanism can
be determined. The electron transfer can be monitored simultaneously with the mass change as
different conditions to determine if there is a co-deposition of the reaction products on the
surface of the electrode or not.
1.3.7 Rotating Disk Electrode System (RED) and Rotating Ring Disk Electrode System (RRDE)
Rotating disk electrode system (RED) is one of the three electrode system where the
working electrode is rotating during the experiment [11]. This rotation creates a flux which
provides the electrode with electrolyte constantly. The continuous flow keeps the reaction going
and give better chance to make it happened. The extra rotation to the working electrode, give
good information to study the reaction mechanism and investigate the rate of the chemical
reaction. Rotating ring disk electrode system (RRDE) is similar to the RDE with double working
electrode. The function of the second working electrode is to reverse what is happening on the
first one. For example if there is and oxidation reaction happened on the first electrode, some
potential can be applied to reduce it and vice versa. In this way the quantity of the reaction can
be determined and measured the rate. RRDE is one of the best techniques used to determine
the oxygen reduction reaction (ORR) which is very important aspect in the proton exchange
membrane fuel cells (PEMFC). There three different movement that control the reaction and
charge transfer on the RDE and RRDE which are diffusion, convection, and migration.
.As it can be seen from the figure 1.6 The RDE and RRDE are similar in the all of the component
expect for second working electrode for the RRDE. The RDE and RRDE were used to determine
the ORR of the Bi UPD on Ru.
12
a) b)
Figure 1.6: (a) The arrangement of RRDE showing the disc electrode in the center and the ring electrode separated by isolated material next to it [12]. (b) The flow pattern on the RDEE of the electrolyte while rotating [13].
1.4 Surface Analysis Techniques
Several instrument can be identify as a surface analysis techniques. At University of North
Texas (UNT), there is a big facility providing a lot of surface instruments called the Center for
Advanced Research and Technology (CART) which can be used by student and faculty. In this
dissertation, some of these instruments such as X-ray photoelectron spectroscopy (XPS),
scanning electron microscopy (SEM), and Raman as well as the instrument in our lab, Fourier
transform-infrared spectroscopy and atomic force spectroscopy were utilized. The surface
analysis instruments are very important in the industrial work because it will provide more
details on the surface chemical characterization, the functional bonds and types, and thin film
characterization. The surface sensitivity of some of these instruments can reach few
nanometers like the XPS [14].
13
1.4.1 X-ray Photoelectron Spectroscopy (XPS)
X-ray photoelectron spectroscopy, or ESCA (electron spectroscopy for chemical analysis),
is a powerful instrument. XPS can determine the surface element and chemical bonds
composition which can lead to the chemical structure. The samples exposed with high energy
monochromatic x-ray source such as kα-Al of 1486.6eV or kα-Mg of 1253.6eV [15] which
stimulate the electron on the core level from the atoms within the surface. The kinetic energy
(KE) of these released electrons can be measured from the instrument, and the binding energy
(BE) can be calculated using the following equation:
BE= hv –KE
where:
KE: is the measured kinetic of the photoelectron ejected from the atom.
BE: is the binding energy.
hv: is the photon energy of the x-ray.
Then the detected energy of photoelectron is recorded by an analyzer. This detection showed in
a monitor and a signal or peak [16, 17]. Depending on the structure and environment of the
material, the BE has special characteristics which translate to an intensity and location. Almost
every element in the periodic table has a distinctive binding energy and chemical shift which
can guide the analyst to figure out the structure and composition except for hydrogen (H) and
helium (He).
14
Even though XPS is very powerful technique, it has some limitation. For example, the depth
of the surface analysis of the sample depending on the taking-off angle is between 1-10nm.
Also, XPS is line of sight technique which mean it can analyze what on the surface not what
behind or in between like tranches if they bigger than 10 nm. Using the depth profiling to do
after the 10nm, make it less accurate and a destructive technique. Regardless these limitations
still XPS is one of most important instrument used in the researches and industry. It use in the
polymer industry, semiconductors, and corrosion research.
1.4.2 Fourier Transform-Infrared (FTIR) Spectroscopy
Infrared spectroscopy has been one of the best techniques to identify the chemical
structure and the functional group. Infrared spectra correspond to a special fingerprint of each
Figure 1.7: X-ray photoelectron spectrometer (XPS) PHI 5000 VersaprobeTM, CART XPS instruments.
15
structure and bond connection with response of absorption peak. This absorption is related to
the vibrations and frequencies of each bond in the sample. Almost every different structure has
unique frequencies that can identify that difference. Nowadays, the IR techniques have been
extensively improved to be not just an identification tool, but also a quantitative one. Using the
new modern software with the algorithms, make IR more comprehensive instrument. There are
many types and modifications of the infrared spectroscopy that can enhance the instrument
work. For example, developing of the Fourier transforms infrared (FT-IR), enhances the
sensitivity and the resolution [18]. Moreover, the detect time using FT-IR is greatly reduced
which make it more productive and save a lot of many. In FT-IR the sample preparation and type
play a big role in getting good result. For instance, the KBr disk is used for the powder samples;
the liquid cell for the liquid samples, attenuated total reflectance (ATR) is a sampling technique
for solid and liquid, and others. In general the mechanism of the FT-IR is very simple. The
infrared light is emitted from a light source. The light beam passes through an aperture which
determines the size spot of the light. The beam transmitted or reflected on the sample
depending on the nature of the material. This is, also, where the absorption corresponds to the
frequencies of the bonds on the sample. Finally the light goes to detector to measure the
differences between the initial light and the final one and translate the differences to a signal on
the computer (IR-scheme R-drive).
%T = I/Io
where %T is transmittance; I is the intensity measured with a sample in the beam (from the
sample single beam spectrum); Io is the intensity measured from the back ground spectrum
16
The absorbance spectrum can be calculated from the transmittance spectrum using the
following equation.
A = -log10 T
where A is the absorbance.
Due to the fact that FT-IR is a subtraction technique, background spectrum should be collected
to get a relative scale for the absorption which shows more details and better resolution.
Figure 1.8: FTIR spectrometer layout [19].
17
1.4.2.1 Attenuated Total Reflectance (ATR)
Attenuated total reflectance (ATR) is a sampling technique which increase the sensitivity
up to 100times the normal one. This sensitivity huge jump allows FT-IR to detect change to the
sub-nanometer. the Multiple internal reflection MIR-IR which accompany with the ATR make the
light to reflect inside the sample many times and collect more information which enhance the
sensitivity and resolution As it can be seen in figurer 1.9.
Figure 1.9: Attenuated total reflectance ATR representing the mechanism of the reflected light and the sample characterization.
The bounce of the light inside the crystal is due to the reflective index between the
crystal and the air. If another material is added on the surface of the crystal the bounce of the
light still going, but there is some light escape which called surface evanescent wave [18]. This
evanescent wave is very important because it can interact with the material on Si ATR surface
that the chemical bonding information can be identified with the subtraction from the
background.
To detector Infrared light ATR Crystal
Sample
18
1.4.3 Raman Spectroscopy
Raman and IR are the most common vibrational spectroscopies for assessing molecular
motion and fingerprinting species. Even though the Raman effect was revealed more than
eighty years ago, Raman spectroscopy was not used until the discovery of the laser in the late
1950s. Raman spectroscopy gives information about molecular vibrations to identify and
quantize [20]. Raman involves shining a laser monochromatic light source on a sample and
detecting the scattered light. Most of the scattered light will be in the same frequency as the
excitation source which is called Rayleigh or elastic scattering. The rest of the scattered light is
shifted in energy because of the interactions between the confrontation of the vibrational
energy levels and electromagnetic waves of the molecules in the sample which will be showed
as a spectrum [21, 22]. Some examples of sources used in Raman spectrometers are:
helium/neon laser, argon-ion laser, and neodymium yttrium aluminum garnet (Nd: YAG) laser.
The most common laser used in Raman spectroscopy is operating at 532 nm and 785 nm
especially for biological and medical samples. The scattered radiation is measured by a detector
at some angle, usually at 90 degrees. Germanium (Ge) detector is an example of a detector used
in Raman spectrometer [20].
Raman basic principle is based on inelastic scattering of a monochromatic excitation
source. When the radiation goes through a transparent medium, the chemical sample
ineleastically scatters radiation in all directions and this small fraction of radiation is scattered
differently from the incident beam. This difference in the incident beam and the shifts in
wavelength depend upon the chemical structure of the sample [20, 21]. The routine energy
range for most of Raman spectroscopy is between wavenumber 200 to 4000 cm–1.
19
In molecular systems, these frequencies are principally in the ranges associated with
rotational, vibrational and electronic level transitions. The scattered radiation happens in every
direction and can have a detectable change in its polarization as well as its wavelength. Rayleigh
scattering is when there is a scattering process with no change in frequency. Raman scattering
is the difference in wavelength or frequency of light. Raman can shift photons higher or lower
energy, depending on the vibrational state of the molecule. Stokes scattering is stronger than
Anti-Stokes. In Stokes scattering a photon is scattered at lower energy and shifts the
wavelength toward the end of the spectrum. Raman scattering effect is when the molecule
starts at ground vibrational state due to the Raman spectra taken at room temperature. Anti-
Stokes Raman scattering is when a small fraction of molecules will have a higher vibrational
level, thus the scattered photon can be scattered at a higher energy.
Figure 1. 10: Anti-Stokes and Stokes lines in Raman spectroscopy.
20
Raman is closely related to infrared absorption in that they use the same type of
quantized vibrational changes and rotational energy levels. However IR is a direct absorption
process where as Raman is a scattering process. Raman requires a changing polarizability while
IR requires a dipole moment. Also, the light source is different in the IR they used white light
source like Deuterium, while in Raman they used laser to illuminate the sample. Some of the
matrix effect like water is eliminated in Raman because Raman doesn’t respond to water while
in IR water is all over the spectra. Not being able to see water in the spectra makes Raman ideal
for biological research. Although they affect the samples in different ways they complement
each other very well. For example, with a sample that does not have a dipole moment, IR
would not be able to detect the chemical bonds; however Raman spectroscopy can detect it.
Likewise, for a sample which has photoresist properties and can’t be excited by Raman
irradiation, it can be detected on the IR.
Resonance Raman happens when the wavelength of the laser is close to an excited
electronic state of a bond in molecule, i.e. where it is strongly absorbed or fluoresces, the signal
enhancement can be increased by a factor between 100 and 106. In this case the concentration
of the analyte can be very low up to 10-8 molar and Raman detector still can detect it. Since the
Raman resonance is restricted to the Raman bands associated with the chromophore, the
Raman resonance spectra usually only consists of a few lines. As the line intensities in a Raman
resonance experiment enhance quickly as the excitation wavelength gets near the wavelength
of the electronic absorption peak [20]. To receive the best signal sensitivity, a laser with a
mechanical attunement is required. When the laser radiation increased, the sample
decomposition became a big problem. For this reason, the electronic absorption peaks are
21
usually found in the ultraviolet region. The solution to this problem is to flow the sample past
the focused beam of the laser. This way only a small piece of the sample is irradiated at any
moment during the process [16]. Although Raman resonance has this big advantage which you
can select a wavelength to enhance the sensitivity to a particular type of bond or vibrations, it
has a potential disadvantages which are increased fluorescence and increase
absorption/heating [22].
Raman is a very powerful instrument and specific toward chemical structures. It can
analyze samples in gas, liquid and solid forms. Raman spectroscopy can provide information
about crystallite dimension, clustering of the sp2 phase, the existence of sp2-sp3 hybridization,
the chemical impurities, the optical energy gap, the amount of the mass density, and strain [21].
Furthermore, it can be used to detect graphene layers, nanotube diameter, defects and other
crystal disorder, doping, elastic constants, edge structure chirality, curvature, and lastly the
metallic and semiconducting behavior. Raman is the best technique to be used to detect the
single carbon nanotube and graphene and distinguish between the numbers of layers of
graphene in the sample. Also, it can tell if the graphene is pure or it has impurities [22].
22
Figure 1.11: Almega XR Raman spectrometer from Thermo Scientific Nicolet, CART Raman instrument.
Raman spectroscopy became in popularity among analytical chemists and in
pharmaceutical fields. It is powerful and easy technique with so many applications which make
it very useful. No sample preparation is needed specifically to use this technique which saves
time and effort. Raman spectroscopy has become a standard method for materials
characterization and it is finding greater acceptance in industrial process monitoring. In
addition, these days Raman is employed in the pharmaceutical applications to determine
content in various quantity forms, and its necessity increase and look promising.
23
1.5 Surface Microscope
Recent advancements of surface imaging microscope allow effective nanometer scale
imaging of sample surface. The atomic force microscope (AFM) is one of surface imaging
microscopes which has the ability to magnify 1000 times more than the regular microscope.
Also, the scanning electron microscope is another one which used x-ray to and the secondary
electron to capture the image of the sample and go down to ~1pm size of the image sample.
Many others microscopes can do surface analysis under different condition such as
environmental scanning electron microscope (ESEM), transmission electron microscope (TEM),
and others. In this dissertation the AFM and SEM is our emphasis since they used directly in my
work.
1.5.1 Atomic Force Microscopy (AFM)
Atomic force microscope (AFM) is one of best scanning probe techniques to sense and
scan the feature of the sample to draw a 3D image. AFM is using a silicon nitride tip to contact
with the sample. The oscillation of the tip is the feedback to draw and build an image on the
screen. AFM can help nano-technology researchers to do nano-scan on the sample surface of
their interest. Also, it can be employed to locate the defect and measure the roughness of the
sample. The principle of the AFM is based on the force between the cantilever angle on the tip
and the sample topographies which is moves up and down to adjust accordingly is adjusted
accordingly. AFM uses a laser light that reflect on the back of the cantilever tip to detect the
change on the sample surface which is oscillated. The change of the laser position give the
feedback on the image adjusted on the screen. The topography of the sample surface is a
24
translation of the force between the sample feature and the cantilever with the laser reflection.
If the tip is scanning through the specimen, the tip is adjusted by the feedback to keep the
cantilever oscillation between the sample and the tip constant. Several kind of contact modes
can be used to scan the surface of the sample depending on the nature and sensitivity of the
sample. AFM have contact mode for insensitive samples, and non-contact and tapping modes
for sensitive samples. The contact mode is employed with a static mode of the tip. The static tip
deflect is the way the feedback can be acquired depending on the repulsive force between the
tip and the electrons of the atoms. Non-contact mode, the tip doesn’t touch the sample surface,
but uses the oscillation within few nanometers [23] to draw the picture based on the
differences of the van der Waals forces occurs during the approach. The last kind is the tapping
mode which is used to image the soft samples that could be damaged with contact mode of the
AFM. As the name refers the tip oscillate as the sample scans. The difference of the height of
the oscillation is used as the feedback signal to draw the picture. AFM can scan conductive and
nonconductive samples using these modes [24, 25].
Figure 1.12: Atomic force microscope AFM picture in our lab.
25
1.5.2 Scanning Electron Microscope (SEM)
Scanning electron microscope (SEM) is a microscope that uses electron instead of light
to image the surface of the sample. In the SEM there is an electron gun firing electrons, coils,
and lenses. Most of the specimen of this type of electron should be conductive. For non-
conductive samples the image will not be clear, it will show a glow. There are several solutions
to picture non-conductive samples such as sputter some gold particles and then take the
picture. The principle of SEM of images the sample surface is depending on the secondary
electrons detected after the primary electron (gun electron) hit the surface and eject some of
the surface electrons. The high energy beam electrons sometimes can penetrate to further that
the surface atoms located which ejected the secondary electrons. These electrons called backed
scattered electrons which can be detected with different detector. If the gun electron went
deeper in the sample it can cause x-ray which need special detector to catch it [26- 28].
26
Figure 1.13: Scanning electron microscope (SEM) FEI Nova NanoSEM 230, CART SEM instrument.
As it can be seen on figure 1.13 the scanning electron microscope is consist of a column
generate the primary electrons. Specimen chamber contains the stage where the samples can
place. The secondary detector is located above the sample chamber. Then, the Software which
is control the SEM and transfer the signal to monitor to visualize the image of the sample.
27
1.6 References
1. Stoffel, A.; Kovacs, A.; Kronast, W.; Muller, R. J. Micromech. Microeng. 1996, 6, 1-13.
2. Chen, I.; Jang, C.; Xiao, S.; Ishigami, M.; Fuhrer, M. Nature Nanotechnology 2008, 3, 206 –
209.
3. Suntola, T. Handbook of Crystal Growth, D. T. J. Hurle, Editor, Chap. 14, Elsevier Science B. V.,
Amsterdam 1994, 3.
4. Leskela, M.; Ritala, M. Thin Solid Films 2002, 409, 138.
5. Willey, R. R. Practical Design of Optical Thin Films, (Willey Optical, Consultants, Charlevoix,
Mich., 2007), pp.45-46.
6. http://www.ionfusion.com/technology
7. Xiliang, L.; Jason, J. D. Chem Soc Rev. 2013, 42, 5944.
8. http://physicalchemistryresources.com/book_1_sections_graphicfiles_06152009/MPC_ec_
Voltammetry_06152009/PC_ec_Voltammetry_05262009_web_05302009.htm
9. Flatgen, G.; Wasle, S.; Lubke, M.; Eickes, C.; Radhakrishnan, G.; Doblhofer, K.; Ert, G.
Electrochim. Acta. 1999, 44, 4499.
10. Sauerbrey, Günter (April 1959) Zeitschrift für Physik 155.
11. http://www.neoscience.co.kr/products/prod_view.php?cls=030110&prod_code=0000255
12. http://electroanalisis.blogspot.com/2012/05/hydrodynamic-voltammetry.html
13. Bard, A.J.; Faulkner, L.R. Electrochemical Methods: Fundamentals and Applications. New
York: John Wiley & Sons, 2nd Edition, 2000.
14. Briggs, D.; Seah, M. Practical Surface Analysis. Volume 1 – Auger and X-ray Photoelectron
Spectroscopy. Second Edition, John Wiley and Sons, Chichester, 1990.
28
15. Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. F. Handbook of X-ray Photoelectron
Spectroscopy, 1st ed. (G. E. Muilenberg, editor), Perkin-Elmer Corporation (Physical
Electronics), 1979.
16. Ebel, M.F., Absolute calibration of an X-Ray photoelectron Spectrometer, Journal of Electron
Spectroscopy and Related Phenomena 1976, 8, 213.
17. Vickerman, J. C., Ed. Surface Analysis - The Principal Techniques; John Wiley & Sons: New
York, 1997.
18. Stuart, H. Infrared Spectroscopy. Fundamentals and Applications 2004, John Wiley & Sons
Ltd.
19. http://chemwiki.ucdavis.edu/Physical_Chemistry/Spectroscopy/Fundamentals/The_Power_
of_the_Fourier_Transform_for_Spectroscopists
20. Skoog, D.; Leary, J. Principles of Instrumental Analysis, fourth edition. Fort Worth: Saunders
College Publishing. 1992.
21. Prajapati, P.; Prajapati, A. Raman Spectroscopy: A Versatile Tool in Pharmaceutical Analysis,
International Journal of Pharmaceutical Sciences Review and Research 2011, 9, 1, 57-64.
22. Robert, B. Resonance Raman spectroscopy, Photosynth Res 2009, 101,147–155.
23. Gross, L.; Mohn, F.; Moll, N.; Liljeroth, P.; Meyer, G. Science 2009, 325.
24. http://www.olympusconfocal.com/theory/confocalintro.html
25. Wickramashinghe, H. K. Sci. Am. 1989, 98.
26. Pool, R. Science, 1990, 247, 634.
29
27. Jeffree, C. E.; Read, N. D. Ambient- and Low-temperature scanning electron microscopy. In
Hall, J. L.; Hawes, C. R. Electron Microscopy of Plant Cells 1991. London: Academic Press.
28. Goldstein, J.; Newbury, D E. ; Joy, D C. ; Lyman, C E. ; Echlin, P.; LIfshin, E.; Sawyer, L.;
Michael, J R. Scanning Electron Microscopy and X-ray Microanalysis (3 ed). Springer 2003.
30
CHAPTER 2
GRAPHENE
2.1 Introduction
Graphene is a single layer of carbon wherein the carbon atoms are arranged in a pattern
like that of a honeycomb. Its unique mechanical and chemical properties make it very
interesting for many researchers. The sp2 graphene sheet is of great interest, especially in the
electronics industry, because of its inherent conductivity and transparency, which make it
effective for many applications in the industry, as well as because of its large surface area. The
sp2 sheet provides a large surface area and excellent electrical conductivity. These properties
make graphene an excellent candidate for applications in touch screens and display devices [1].
There are several ways to produce graphene, such as chemical vapor deposition (CVD),
physical vapor deposition (PVD), exfoliation directly from graphite, etc. One of the most
effective and practical ways to produce grapheme is to use a CVD machine because this results
in a large, uniform area of high quality; this is our focus in this dissertation. Furthermore, using
the CVD technique is effective for the direct growing of graphene on certain specific substrates
like copper. The graphene also can be transferred to different substrates, as will be discussed in
the section on this technique. These properties of CVD graphene make it highly compatible with
uses for various applications and studies. For example, graphene can be used as an
electrocatalyst for a fuel cell, and in applications taking advantage of its conductivity and
transparency, it can be used for conductive touchable and flexible screens and for high speed
transistors. Moreover, graphene can be reformed to produce different types of carbon
allotropes. If it is rolled, graphene forms a nanotube, graphene can be wrapped to form a bulky
31
ball (florescence), and if layers of graphene are stacked on top of each other, the result is
graphite [2, 3].
2.2 Chemical Vapor Deposition (CVD) Procedure to Grow Graphene
The procedure of chemical vapor deposition depends on choosing a metal substrate; in
our case copper was chosen as the substrate because it is self-limited in dissolving carbon, so no
more than few layers (1-10 layers) at maximum will be produced [4, 5]. Then, using a furnace,
the metal substrate is heated to almost the melting point. The next step is to introduce
methane and hydrogen gas. Methane gas is a source of carbon which starts to dissolve on the
melting surface of copper. Hydrogen gas works as the catalyst to induce the dissolving of carbon
in the copper surface [5]. Also, it is very important to keep the surface free of oxygen and in the
reduced form; that is why hydrogen is used throughout the CVD process until the growing of
graphene is completed. Figure 2.1 illustrates the mechanism and the process of carbon
deposition on the copper surface.
Figure 2.1: Diagram Illustrating the Process of CVD growth on copper [6].
The furnace is cooled down quickly to keep the deposition of carbon on the metal
32
surface and prevent the formation of bulky layers of graphene, which is more likely in the
deposition of graphite layers [7].
Growing graphene using this method has some limitations related to material properties
such as impurities, continuity, and uniformity. These limitations can be minimized, for graphene
CVD reaches almost the same purity of exfoliation graphene. One of the important parameters
in growing CVD graphene is the use of high purity hydrogen and methane gases, which will
minimize the impurities and enhance the uniformity and continuity of the graphene formation.
Another challenge in producing graphene using the CVD process is the wrinkling of the
graphene which happens due to the difference in the expansion between the copper atoms and
the graphene atoms during the processes of annealing and cooling. This can be avoided using
proper annealing and a small sized copper surface. Several factors can affect the quality and
number of layers of graphene grown using CVD. For instance, more methane gas results in more
carbon atoms, leading to a greater number of layers of graphene. Also, if the time for the
annealing process is lengthened, the percentage of two and three monolayers is increased
dramatically, but the continuity and uniformity of the graphene are also enhanced significantly.
Moreover, the temperature and purity of the copper substrate affect the quality of graphene
grown using the CVD process. The higher the temperature used to grow graphene, the faster
the growth of graphene and the greater the number of layers produced. The purity of the
copper plays a significant role in growing continuous, un-oxidized, and uniform graphene layers.
Finally, the vacuum and the input of the precursor gases are very important to control in the
process of the growth of graphene. Under a higher vacuum, the flowing of the gases is more
uniform and controllable than under low vacuum; a higher vacuum thus results in higher quality
33
of monolayer graphene. Leaks in the system lead to flow of ambient air, which oxidizes the
copper surface, and that will result in the growth of graphene oxide or in preventing the growth
of graphene.
Growth of graphene by CVD can be accomplished on many different substrate metals,
such as nickel, ruthenium, and iridium, and some researchers have used cobalt. However, the
most commonly used substrates for the growth of graphene are copper and nickel due to the
cost effectiveness and ease of handling of these metals. The most significant difference
between growing graphene on copper and on other substrates is the number of layers of
graphene that can be grown based on the amount of carbon the substrates can dissolve. Nickel,
for example, dissolves a greater amount of carbon than copper, which is self-limited. Using
nickel, between five to hundreds of graphene layers can be grown while the use of copper will
produce between one and ten layers maximum [6-9]. It is important to mention here that 90-
95% of graphene grown on copper will form 1-3 monolayers and the rest will form 3-10
monolayers. The differentiation between these numbers of layers will be related to the use of
different techniques in next section. Also, different substrates will have different defects and
wrinkle problems.
34
In order to grow a large area of graphene, a chemical vapor deposition (CVD) system was
recently built in Dr. Oliver Chyan’s lab at the University of North Texas (Figure2.2). Using the CVD
machine makes it possible to grow graphene on any substrate with very high quality. Copper
(Cu) is one of the best substrates for the growth of graphene because of its self-limited carbon
solubility, which is limited to the formation of one to three monolayers of graphene in 95% of
the area. In our experiments, 25 micron thick copper foil was used with a purity of 99.98% as
the metal substrate for the production of graphene. Moreover, Cu is easy to remove using the
wet enchant to obtain a large pure graphene sheet ready to use in any test or application.
In general, ultra-high purity gases such as hydrogen, argon, and methane are used in the
synthesis of graphene. The process begins with the removal of the air using a rough pump the
insertion of argon gas, and then hydrogen is permitted to enter to clean the surface and initiate
Figure 2.2: The chemical vapor deposition (CVD) graphene system in our lab.
35
the synthesis of graphene. Finally, a source of carbon, methane in our case, is allowed into the
chamber to dissolve on the copper and form graphene, while the hydrogen gas is kept running
to avoid any oxidation on the surface. The process of growing graphene is shown in figure 2.3.
Figure 2.3: Diagram of the graphene growth process.
2.2.1 Growth of Graphene on Copper Substrate
After the copper foil is inserted in the furnace chamber, the chamber is closed and
vacuumed to about 20mTorr. Argon is then inserted at a pressure of 450mTorr, and the furnace
is heated to 850°C to allow the copper to anneal. Then the flow of argon is stopped and
hydrogen is inserted to the pressure of 220mTorr for 50 min while the temperature is raised to
950oC. The hydrogen used in this step plays a critical role in reducing the surface and preparing
it for the growth of graphene. Then, methane was flowed simultaneously with hydrogen at the
same temperature for more 30min. Here it is important to mention that the total pressure
during the process of growing graphene should be less than 500mTorr. That will give graphene
of higher quality and better continuity.
36
The number of monolayers that are obtained depends on how long methane gas is used
and the speed of the cooling of the system. After 30 min have passed, the flow of methane gas
is stopped without the turning off of the hydrogen gas, the furnace heating system is then
turned off, and the door is opened to allow graphene to form on the copper surface. A fan can
be used to accelerate the process of cooling to limit the percentage of multilayers that will form.
When the temperature reaches 40°C, the pumping stops and argon is inserted to raise the
pressure to around to the atmospheric pressure. After the flow of argon is stopped, the copper
with graphene on top is removed and kept under vacuum until it is used for the experiments
[10].
a b
Figure 2.4: a) Photo of copper foil before graphene growth b) Photo of copper foil after graphene growth.
2.3 Transfer of CVD Graphene
After the synthesis of the graphene on a metal substrate, transference to a different
substrate is necessary for the operation of some experiments. The process of transferring
graphene depends on the metal or non-metal on which the graphene has been grown. For
copper and nickel, the procedure of the transference is as demonstrated in figure 2.5, with
37
slight differences as will be described later.
First, a surface is cast with a polymer such as poly methyl methacrylate (PMMA) to hold
up the graphene sheet during the copper corrosion. Then the copper substrate is corroded with
a solution of iron (III) nitrate Fe (NO3)3 or ferric chloride FeCl3. After the metal is corroded, the
resulting PMMA/ graphene is fished and cleaned using deionized water. Then the PMMA/
graphene is transferred to any other substrate which is desirable, a step which is followed by the
adding of drops of PMMA onto the PMMA/graphene/substrate to re-dissolve the first PMMA
and make graphene relax and lie flat.
Finally, the PMMA/graphene/substrate is dipped in acetone to dissolve the PMMA,
which results in graphene having been transferred to the new substrate successfully. It has been
Cu foil
Graphene
Step 8
Step 7
Step 6
Step 5
Step 4
Step 3
Step 2
Step 1
Figure 2.5: The process of transfer of CVD graphene to another substrate.
38
noticed that the use of acetone alone will not remove all PMMA; therefore, some researchers
have discovered that treating the transferred graphene with hydrogen under vacuum and low
heat can remove the residual PMMA [7].
2.3.1 The Procedure of Transferring Graphene Grown on Copper
The desired size of Graphene/Cu is cut, and, if it is not flat, it is placed between two pre-
cleaned and fresh cover slides to make it flat. It is important to avoid any dust on the cover glass
that might create holes in the graphene sheet after transfer. Then 2% of PMMA polymer
dissolved in Anisole solvent is used to drop-cast the graphene on the copper substrate. Next,
the PMMA/Graphene/Cu is flipped over on a clean Kim wipe tissue and left there for 20sec. This
allows the PMMA to spread only on the front/graphene side; simultaneously, the excess PMMA
will be absorbed by the underlying Kim wipe tissue paper. The PMMA/Graphene/Cu is air dried
for 20 minutes or under pumping using a water aspirator to speed up the PMMA drying. Then it
is placed in 1M Fe (NO3)3 to etch the Cu. This step will take about 45-60 min; however, it is
better to leave it for at least 2 hr to make sure all the copper is etched. The color of the Fe
(NO3)3 shouldn’t turn to green; if it does, then it should be disposed of, and use fresh solution
should be used. After that, the PMMA/graphene is transferred with a wet cover glass by
inserting the glass slide 30-45 degrees into the etching solution to catch the PMMA/graphene
slowly (in order to avoid big wrinkles), then the PMMA/graphene is transferred into a clean DI
water-petri dish. This step is repeated using another clean DI water petri dish to make sure the
graphene is clean of the etching solution. One of the best adhesion substrates to which
graphene can be transferred is 300 nm SiO2/Si. Pre-cleaning of the substrate, which includes
sonication in acetone, blow drying, and ensuring that there are no dust particles, should be
39
performed. Then the graphene is transferred to the SiO2 with a compressed air gun; this should
be directed straight down on the center of the PMMA/graphene, and the water should be
blown away. Some wrinkles may be observed on the PMMA/Graphene surface, but the
wrinkles may disappear gradually when it is dry. It is important to mention that extended
immersion in water could lead to the loss of graphene as water seeps in between the graphene
and the substrate. After the drying, a few drops of PMMA are added and left for 5 minutes; then
the graphene and substrate are spun to accomplish the removal of excess PMMA. This will help
to relax the graphene sheet and remove the wrinkles. Then the graphene and substrate are
dipped in acetone and shaken for 30 minutes. The graphene and substrate are spray cleaned
with acetone, then air dried; they are then placed back into the acetone and dried again.
Samples are kept in a desecrator for Raman analysis and test experiments. For additional
cleaning and to ensure that all PMMA was removed, the graphene transferred to substrate is
put in a vacuum with flowing hydrogen gas at 400oC for 20 min [11].
2.3.2 The Procedure of Transferring Graphene Grown on Nickel
Most of the steps for transferring graphene grown on nickel are the same as with copper
if 1-10 monolayers have been grown. However, if more than one hundred layers have been
grown, there is no need to use PMMA to hold the graphene layer while the nickel is being
etched; it can be transferred without the polymer. Also, another difference between copper and
nickel is that copper will etch and dissolve completely while nickel will separate from the
graphene and precipitate on the bottom of the beaker.
Transferring of graphene has been successfully done on different kinds of substrates
40
using this method. Graphene was transferred from Cu to ruthenium (Ru) wafer, Ru quartz crystal
microbalance (QCM), attenuated total reflectance (ATR) Si crystal, and SiO2 wafer. After these
transferals are complete, several experiments can be accomplished such as hydrogenation,
electrodepositing, and detection of gases.
2.4 Characterization of Graphene on Different Substrates, Raman Spectroscopy, Water Contact
Angle Measurements, and QCM Frequency
Various techniques were used for graphene characterization before and after the
transferring process, such as Raman spectroscopy, the difference in the frequency of QCM, and
water contact angle measurement. It has been found that the water contact angle of the
graphene sheet is about 95o, which is consistent with what has been reported previously in the
literature [12].
Figure 2.6: Water contact angle measurement.
The quality and presence of a graphene layer on SiO2 substrate can be checked by
optical contrast and verified by Raman spectroscopy. Figure 2.7 shows Raman spectra done
before transfer of graphene to SiO2 and after transfer. The results from Raman spectroscopy
41
were also consistent with reports in the literature [13] (Figure 2.8). The new way to check the
transferring process employed in this study is to use the difference of the frequency in the QCM
before and after transferring. After that, the Sauerbrey equation, which accounts for the
relationship between the mass change and the frequency change of a thickness-shear mode
quartz crystal, can be employed [14].
Figure 2.7: Raman spectra of graphene on Cu and graphene on 300 nm SiO2, developed in our lab.
42
Figure 2.8: Raman spectra differences between graphene and graphite [13].
Raman analysis can give important information about the quality and number of
graphene layers grown or transferred. The two distinguishing bands for graphene are the 2D
band ~2700cm-1 and the G band ~ 1600cm-1. Other bands, such as the D band ~ 1350cm-1 and
(D+D’) band ~ 2850cm-1, can be useful to detect defects in graphene [4]. To determine the
number of monolayers of graphene and the defects in the Raman analysis, the G band and 2d
band ratio is calculated. If the intensity ratio is ca. 0.5, one monolayer of graphene was formed.
A ~1 G/2D intensity indicates about 2 to 3 monolayers of graphene. The higher the G band the
greater the number of layers of graphene formed; also, it is an indication of defects in the
graphene growth [4].
43
2.4.1 Detection of Graphene Monolayers Using QCM
The QCM technique has advantages in the measurement of the mass change during the
deposition and stripping as well as of the charge change. Through comparison of these variables
it the same time, rich information was obtained regarding the side reactions or deposition.
Moreover, the QCM technique is very sensitive to any change on the surface of the surrounding
conditions which is of interest for exploration. Calculation of the mass change using the
Sauerbray equation [14], which accounts for the relationship between the mass change and the
frequency change, reveals much information about what is happening on the surface, and that
can be translated into the number of monolayers as well as the mechanism and kinetics of the
reaction if this is required. The sensitivity of the QCM technique is very high. It can detect the
change of sub-monolayer or mass change on the surface. The sensitivity of the QCM technique
used for this study is ca. 0.708ng of the change of 1Hz.
The Sauerbrey equation shows the linear relationship between the mass change and the
corresponding change in vibration frequency. The frequency of the Ru QCM is taken within the
stabilized 1Hz frequency, which can be identified by means of changing of the frequency within
100 sec, before transfer of graphene. After that, the graphene is transferred to the Ru QCM and
is dried for a couple of hours. The frequency of the Ru QCM which has graphene on it is
measured again. Then the number of monolayers of graphene is calculated using the Sauerbrey
equation. It has been found based on the area of the QCM that ~100Hz difference before and
44
after the transfer of graphene to QCM will indicate 1 monolayer of graphene (see Figure 2.9).
Furthermore, Raman analysis was used to verify that graphene was transferred to QCM. Ru and
Au QCM were used to transfer graphene and measure the number of layers. The number of
monolayers was calculated to be ca. 1.2 ML for the graphene transferred to Ru QCM. This
indicates that high quality graphene was grown, and to confirm that finding, Raman analysis
was carried out and indeed gave the same result. From these experiments of transferring
graphene to metal QCM, it was discovered that the adhesion of graphene is metal sensitive.
Other metal QCM was tried, such as that of Cu, but graphene did not adhere to the surface.
Figure 2.9: a) Shows the frequency of the QCM before the transfer of graphene. b) Shows the frequency after the transfer of graphene.
Through the use of this technique, a new method was employed to determine the
b
a
45
number of graphene monolayers and the quality of transference in addition to the other
methods which were mentioned earlier. In addition, the process of cleaning can be monitored,
and its efficiency can be detected. The next step is to use the same technique to perform the
test and detect any changes in the electrodepositing and charge transferring of metal
deposition or other reactions occurring on the surface of the graphene.
2.4.2 Detection Graphene Using AFM
Atomic force microscopy can also be performed on graphene that has been transferred
to a flat rigid substrate in order to directly measure the thickness and thus the number of layers
of the graphene. The flatness of the graphene surface area can be tested using the atomic force
microscope (AFM) [15]. AFM images can reveal the quality of the graphene sheet transferred to
substrate and the flatness of the graphene surface area. The ultra-flatness of graphene on mica
substrate is reported with height variations ca. 0.25Ao [15]. The flatness of graphene on SiO2 is
about 1.3nm as reported by Shen et al. [16] Atomic force microscope (AFM) can be used to
measure the uniformity of the deposition and surface roughness.
a b
Figure 2.10: a) Reported AFM image of a boundary between a graphene monolayer and a SiO2 substrate. b) Reported AFM image of a boundary between a graphene monolayer and a mica substrate [15].
46
2.5 Graphene Plasma Hydrogenation (Graphane) and Characterization by FTIR
According to Diankov opening the band gap energy of graphene will have more
significant applications especially in the semiconductor industry [17]. There are many
approaches to achieving the gap opening such as depositing graphene on lower dielectric or
functionalizing graphene with a different functional group. Hydrogenation of graphene is one of
these ways to open the band gap to a level which will be useful for many applications related to
semiconductors. Different ways can be used to hydrogenate graphene; one of them is to use
hydrogen plasma. There are two types of hydrogen plasma which can be used, DC and RF, and
there are several differences between them in the way they hydrogenate graphene, the rate,
and the preferential number of monolayers [18]. For example, DC plasma hydrogenation has a
low rate of hydrogenation with preference for monolayers of graphene, while RF plasma has a
high rate of hydrogenation with preference for multilayers [19, 20].
Raman analysis and AFM were used to characterize and demonstrate the hydrogenation
of graphene; however, someone might argue that the change on the Raman spectra or even on
the AFM image might come from the hydrogenation of graphene or from a defect of the
graphene surface. By means of using the FTIR in our lab, the olefinic C-H stretch peak which
corresponds to the hydrogenation of graphene, can be detected as can be seen in figure 2.11.
Moreover, in the case of MIR-IR, the orientation of the hydrogenation on the graphene can lead
to understanding and predicting the structure of the graphane.
The observed result in the hydrogenation of our CVD graphene which is transferred to an
ATR crystal is single side hydrogenation, until it reaches the maximum of the hydrogenation.
Then it starts to hydrogenate the other side, which changes the conformation structure to a
47
chair structure, and the graphane pops up and breaks from the surface. This is the reason why it
reaches maximum on hydrogenation and then the intensity starts to go down again.
Figure 2.11: FT-IR spectra of hydrogenation of graphene and loss after 45 min.
The time required for this process to happen depends on the sample size, the
hydrogenation rate, and the parameters which are used to achieve the hydrogenation. To avoid
this problem, a free standing graphene sheet can be tested to see if it is observed to have the
same problem. In this experiment, two mode of FTIR were used, the transmission (TR-IT) and
the multiple internal reflections (MIR-IR).
48
Figure 2.12: Anticipated graphene hydrogenation structure in which certain spots show vacancies between the carbons of graphene and not all the carbon is covered [21].
Surprisingly, at the beginning the C-H stretch peak can be seen using transmission infra-
red (TR-IR) more clearly than the multiple internal reflection infra-red (MIR-IR). That can be
explained by the fact that the technique using MIR-IR is an angle-dependent technique, so
depending on the angle at which the surface is hit, it will collect the information of that angle
on the surface, and because the hydrogenation of the graphene must be oriented to a certain
angle every time, it is necessary to choose the right angle in order to detect the hydrogenation.
However, the transmission detects the changes in all the angles; that is why the C-H stretching
was seen at the beginning using this technique with a height of 0.18 milli absorbance(mAU)
after 45 min plasma hydrogenation (see figure 2.13). According to Lord, the C-H stretch is
sensitive to the ring size and the position of 3017cm-1 refers to a six-member ring. Also, the
49
position of this vibration is effective for unsaturated compounds [22].
Figure 2.13: Differential FT-IR spectra of hydrogenated graphene using H2 plasma.
It is important to mention that MIR-IR is more sensitive and specific to this region. The
next step was to use the MIR-IR with the polarizer so that the angle at which the light hit the
surface could be varied from 0-180o. It has been found that the best angle polarization is
between 40-60o, which results in higher intensity; again, the peak shows at the same position,
3017cm-1, which confirms that it is the right position of the C-H olefinic vibration. Therefore, it
can be concluded from these experiments that the hydrogenation of graphene is oriented; it is
embedded with an angle to the graphene surface. Some researchers have mentioned that the
hydrogenation of graphene may be one of the best ways to control the band gap opening of
graphene, which will make it compatible with semiconductors applications. Moreover, different
hydrogenation sites and close structures will give different band gaps, which is of interest for
controlling the band gap opening so it can be directed to different applications (see Figures 2.14
Transfer graphene 0.07mAU
25min 0.09mAU
45min 0.18mAU
C-H Olefinic
3017cm-1
50
and 2.15) [23].
In the process of graphene hydrogenation, the RF plasma chamber was used with low
pressure up to 0.1 bar with 30 watts. The hydrogenation with time was studied from 5 sec to ca.
H
H
H
H
Figure 2.14: Graphene hydrogenation. In this case, there is one carbon atom separating the 2 H atoms [23].
Figure 2.15: Graphene hydrogenation. In this case, the 2 H atoms are separated by multiple carbon atoms, which classifies it as the “Far” structure [23].
51
2hr. The use of these conditions revealed the maximum hydrogenation to be at about 40-60
min. After the 60 min, if it continued, the intensity of the C-H vibration signal would start to
decrease, which means that some of the graphane somehow changed. Future work is required
to explain why this phenomena occurs.
It is interesting to mention that not only can H2 gas hydrogenate graphene, but also the
residue of H2O in the chamber or adsorb on the surface can cause the hydrogenation. According
to Jones, the H2O molecules in the plasma chamber split to H- and H radical, which can do the
work of the hydrogenation. Also, he mention that the vacuum is not sufficient to remove all the
water molecules [24, 25].
Another experiment which was employed to explore the qualities of the graphene
produced was to perform hydrogenation of the graphene transferred to ATR crystal and then to
sputter a few spot with Cu using mask dots ~2nm. It is well known that the use of metal with
graphene enhances the adsorption of the gases, which increases the probability of
hydrogenation of the graphene [26]. As can be seen in figure 2.16, the peak of the C-H olefinic
shows up clearly after 10 min of plasma hydrogenation.
52
This experiment shows better hydrogenation and a sharper peak using the MIR-IR
polarizer at 45o because the edge of the graphene might be increased around the Cu atoms; it
could also be that the metal itself works as a catalyst to increase the hydrogenation process.
Moreover, the constancy of occurrence at the same position of C-H olefinic, at 3017cm-1, proves
that that is the right location for graphene hydrogenation. This is a special signature for the
graphene properties, confirmed in the literature for C-H olefinic in a six member ring; this
proves that the graphene was partially hydrogenated [27].
The intensity of the C-H vibration increases from 0.2 milli absorbance (mAU) to 0.8 mAU.
The initial 0.2 mAU comes from the edge graphene termination hydrogenation after the process
of transfer, and the rest of the increase is due to the plasma hydrogenation. There is not much
increase between 10 min and 45 min, but there is a significant difference in that it is easier
Figure 2.16: MIR-IR spectra of graphene with a few atoms of Cu sputtered; plasma hydrogenation was detected using MR-IR for 45 degree polarization.
53
initially to hydrogenate the surface when all the sites are open, but it becomes more difficult
after that. This could provide an explanation for the loss of intensity after continuation of the
plasma hydrogenation because there could be competition with the graphane which has
already hydrogenated; some of this might be removed as hydrogenation occurs again, which
might cause damage to the surface.
Raman analysis also was used to detect the hydrogenation of graphene and to
determine what had happened on the surface. The result of the Raman analysis confirmed that
hydrogenation of the graphene was present based on the change of the intensity and the
appearance of the D at (~1350cm-1), D’ at (~1620cm-1) and D+D’ at (~2890cm-1) peaks, as is
shown in figure 2.17. The fact that D’ and D+D’ show in the Raman analysis proves that the
graphene is hydrogenated, a result which is consistent with the literature [28]. These
experiments, also, have been repeated and confirmed by the collaboration of Dr. Wonbong
Choi’s group from the Materials Science and Engineering Department at UNT.
54
Figure 2.17: Raman spectra of graphene before and after plasma hydrogenation.
2.6 Adsorption of Carbon Monoxide (CO) on the Graphene Surface
Graphene gas sensors have received a lot of attention for the fact that they can detect as
little as one molecule or atom. Several theoretical studies have explored the effectiveness and
performance of pristine and embedded metals graphene in detecting gas molecules such as
NH3, NO2, CO, CO2, and O2. In general, the embedded graphene is more sensitive and stable
than non-embedded graphene. High performance gas sensor graphene based catalysts and the
adsorption/ desorption mechanism have been investigated by many researchers [26, 29]. The
adsorption/ desorption properties of graphene were studied using our QCM system to explore
the efficiency of graphene in detecting and adsorbing gases; in our case, carbon monoxide (CO)
55
was used. A graphene layer was transferred to Ru QCM and hooked inside the dissector, which
was pumped and filled with N2 first and then with the CO the second time. The pressure inside
the dissector was left at atmospheric pressure, and the dissector was closed. Use of the QCM
technique to measure the frequency after a period of time will determine how much CO
adsorbs on the surface of the graphene. Our experiment shows that the adsorption of CO on
pristine graphene surface is easily shaken off, which indicates that the interaction is a very weak
van der Waals interaction. That was concluded because after the QCM had vibrated for few
minutes, the frequency reading returned to the same reading from before the CO adsorption
and stabilized. In other words, the vibration of the QCM shakes off the adsorption layer of CO as
can be seen in Figure 2.18. From the differences in the frequency, the number of monolayers
can be calculated. The coverage of the CO on the pristine graphene was found to be ca. 0.5
monolayer.
Figure 2.18: The adsorption of CO on pristine graphene transferred to Ru QCM.
56
It is reported in the literature that embedded metal graphene works better for
adsorption of gases. There are several theoretical studies which address adsorption of NH3 and
CO on pristine and embedded graphene, and they explain the locations and the strength of the
bond. According to Zhou, the stable adsorption site for CO on pristine graphene is at the hollow
(H) site [26].
Figure 2.19: Schematic view of a single gas molecule (NH3) adsorption on pristine graphene (a) TM–graphene (b). T, top site; B, bridge site; H, hollow site; M, transition metal atom (Au). Carbon atom in gray, H in white, N in blue and Au atom in yellow [26].
The next experiment on detection of CO gas was to investigate the adsorption of CO gas
on the embedded graphene with Cu metals. A few atoms of Cu ca. 5nm, which were not
sufficient to cover all the graphene spots, were sputtered on the surface, and the same
experiment of the adsorption of CO was repeated.
57
Figure 2.20: The adsorption of CO on graphene transferred to Ru QCM embedded with 5nm Cu.
The results show that the adsorption of CO on graphene embedded with Cu is stronger.
Some of the CO shakes off after vibration, corresponding to the sites of pristine graphene, and
some are held, as is shown in Figure 2.20. The frequency of the QCM did not return to the same
reading from before the adsorption of CO this time, which indicates that some of the CO was
retained on the surface. The percentage of the retained CO was ~68% and 32% was shaken off.
Future experiments should be conducted to investigate the adsorption under different
conditions, i.e., with different temperature, pressure, and gases. Two QCMs were tested, one
with graphene and one without, to see if they would show the same CO adsorption as in the
conditions of the open lab, and the result did not show the same trend.
2.7 Electrochemical Properties of Graphene
The electrochemical properties of graphene were also explored. The differences
58
between the Ru, glassy carbon (GC), and graphene were investigated. As shown in Figure 2.21,
there is a difference between the Ru cyclic voltammetry (CV) and the CV of the graphene
electrode. Moreover, the CV of graphene is more similar to that of the glassy carbon electrode
as appears in the figure 2.22.
Figure 2.21: Comparison of the CVs between the Ru electrode and Graphene on the Ru wafer electrode.
59
Also, the interactions between graphene and the simple redox molecule
hexacyanoferrate (III) were explored; the results are shown in figure 2.23. Cyclic voltammetric
measurements were used to characterize and find the differences between graphene and
different substrates. Moreover, the difference between the Cu substrate on which graphene was
grown and glassy carbon (GC), which is expected to behave similarly to graphene, was tested.
Observation of these CVs shows that there is a significant difference between them, and that, as
expected, graphene transferred to SiO2 behaves in a way more similar to GC. That also proves
that our transferred graphene is clean, continuous, and uniform.
Figure 2.22: Comparison of the CVs between the GC electrode and graphene on the Ru wafer electrode.
60
2.7.1 Electrodepositing of Cu on Graphene
Currently, the combination of nanomaterials on electrodes as a catalyst to improve
electrochemical sensors is very common due to the resulting excellent conductivity, high surface
area, and better catalytic activity. For example, Ag nanoparticles (Ag NPs) electrocatalytic
materials are employed for a reduction of H2O2, and Pt nanoparticles (Pt NPs) electrocatalytic
materials are used to reduce the H2O2 [30]. These nanomaterials can be used as electrode
materials or as catalytic labels for improved electrochemical detection [31]. Also, they can be
combined with thermo-sensitive polymers to develop a thermo electrocatalytic process [32].
Using graphene as a platform, a smooth large area of deposition is provided, with small NPs of
metals which will enhance the electrochemical and electroactivity properties [33]. Other
platforms require a preparation process before they are used, such as polishing or
electrochemical cleaning, but the use of graphene requires no polishing or electrochemical
GC elect.
Graphene on the SiO2 elect.
Copper foil elect.
Graphene foil elect.
Figure 2.23: CVs for Cu foil, graphene, and GC electrodes in hexacyanoferrate (III).
61
cleaning, which makes graphene one of the best candidates for this kind of application.
Figure 2.24: a) Graphene on Ru wafer before electroplating with 2mMCu. b) Graphene on Ru wafer after electroplating with 2mMCu.
After the initial determination of the characteristics of the electrochemistry of the
graphene, the deposition of Cu on the graphene surface was explored. As can be seen in Figure
2.24, the deposition of Cu on graphene occurs when the potential on the -0.2V vs. Ag/AgCl is
held for about 30 min [34]. This was the first step needed to prove that Cu can indeed be
deposited on a graphene sheet. The next step was to observe whether the deposition and
striping can occur without damage to the surface of graphene. Observation by means of a
microscope reveals little difference before and after; however, Raman analysis is needed to
confirm that damage has not occurred to the surface. Moreover, measuring of the open circuit
potential (OCP) shows that the reading is the same after Cu is stripped from the graphene
sheet. This suggests that decorating graphene with Cu particles may be advantageous for use in
many applications; however, additional investigation employing other metals needs to be done.
a b a
62
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65
CHAPTER 3
BISMUTH (Bi) UNDERPOTENTIAL DEPOSITION (UPD) ON RUTHENIUM (Ru)
3.1 Introduction
Underpotential deposition is a term that refers to electrochemical deposition of a single
layer or monolayer of atoms, usually metals, on a substrate with positive potential for bulk
deposition. The underpotential deposition (UPD) of this monolayer on the substrate can bring
about significant changes in the catalytic properties of the substrate, which is usually another
metal. The UPD phenomenon has been the subject of a large number of studies over the last
two decades. The UPD process, in general, requires a bi-metallic catalytic surface design for a
new catalyst. These new surfaces will have different characteristics from either the substrates or
the deposited metals, and this will result in the possibility of new applications for these
catalysts. Even though the UPD is supposed to result in one monolayer deposited on the surface
of the substrate, this is not the case for all metals. Some of them could have more than one and
others could have less than one monolayer depending on different conditions and parameters
such as the size or the morphology of the metals deposited.
There are different kinds of UPD, reversible, quasi-reversible, and irreversible,
depending on the characteristic of the metals. Most of the new UPD has unique properties that
are not predictable on the basis of the metal, substrate, and other UPD metals; therefore, it is
necessary to study and investigate the structure and the characteristics of these new UPDs and
explore the catalytic behavior under different conditions. This will reveal important information
regarding how the deposition occurs. Electrochemical quartz crystal microbalance (EQCM) is a
very sensitive technique in which the difference in the mass, simultaneous with the change of
66
the charge, can be measured. Monitoring the differences in both mass and charge transfer on
the surface leads to a profound understanding of the result [1].
In previous work, one of our members (Lin, Po-Fu) had investigated the features of Bi
underpotential deposition on Ru and analyzed the differences of the deposition under different
substrate conditions like Ru and RuOx [2]. The interesting part of this work was that when the
EQCM was used to do the deposition and calculate the number of monolayers (ML) of the UPD
of Bi on Ru, a discrepancy was found between the number of monolayers calculated on the
mass basis and the number of monolayers calculated on the charge basis [2]. This situation
indicates that the extra monolayers based on the calculation of charge as compared to that of
mass could imply the involvement of other parameters or reactions. To provide a clear picture
regarding this discrepancy, the influences of anions and the ambient effect were investigated.
The exploration of these influences was carried out using a variety of techniques such as EQCM,
XPS, and RRDE, as will be explained in detail later.
E-chem experiments were completed under the same conditions with variability of
certain parameters. The three-electrode system was used to study Bi UPD on Ru. Ru QCM and
wafers were employed as working electrodes (WE), platinum (Pt) wire was used as counter
electrodes (CE), and mercury mercurous sulfate Hg/Hg2SO4 was used as the reference electrode
(RE). XPS analysis procedures were accomplished using the PHI 5000 VersaprobeTM instrument.
The high energy monochromatic x-ray source kα-Al at 1486.6eV was employed as the source of
the x-ray. 123eV was selected for the survey energy pass, and the high resolution energy pass
was at 23.5eV. The Ar sputtering rate was 5Å/min based on the SiO2 removal calibration.
67
3.2 The Effect of the Ambient on Bismuth under Potential Deposition (UPD) on Ruthenium
Different ambient were investigated in this chapter to see the effect on the UPD as well
as the effect on the discrepancy. The lab ambient (air) was undertaken first, then nitrogen (N2)
was tested, and finally the effect of excess oxygen (O2) was explored. It is expected that the
ambient will have significant impacts in changing the number of ML due to the fact that there is
a discrepancy on the mass and charge calculation of the number of monolayers (ML).
As it can be seen in figure 3.1 there is different in the reduction current and oxidation
current in the UPD region and bulk region under N2 and O2. In the Bi UPD anodic scan the
reduction current increases within the potential region of [-0.47, 0.03], with the oxygen amount
increasing in the ambient above the test solution (0.5M H2SO4+ 1mMBi3+).
Figure 3.1: Ambient effect of Bi deposition on Ru. (a.) CV of Bi deposition on Ru. Scan rate of 5mV/s. (b.) Zoom-in view of CV in Bi UPD region. (c.) Mass response. (d.) Zoom-in view of mass
d
c
a
b
68
response in Bi UPD region.
The Bi UPD oxidation current decreases to a point that the surface starts undergo
reduction reaction, which is evident by negative current in Bi UPD region. The mass response of
Bi UPD with N2 purge shows similar Bi MLMass coverage as the mass response without N2 purge.
This can be explained that the oxygen reduction reaction (ORR) taking places in Bi UPD region
even in anodic scan. However, the mass response with O2 purge shows significantly higher Bi
MLMass coverage than the mass response with or without N2 purge. This drastically increases in
reduction current and mass response with O2 purge can be explained that the surface of Bi UPD
had undergone oxidation, probably Bi2O3, while large amount for ORR reduction current taking
places, making the anodic scan current become negative. Another explanation can be that
higher concentration of O2 facilitating the ORR reaction and making the surface of Bi UPD
become negatively charged, which attracts BiOH2+ or BiO+ from solution to Bi UPD surface,
making this large mass response.
Calculating the number of ML based in the charge and mass gave us different amount
from the scan rate range of 5-50 mV/s. ML coverage based in the mass calculation with different
scan rate did not show significant difference. Based on the reduction charge at scan rate
5mV/s, it goes from ca. 3 ML for nitrogen, to ca. 5 ML for lab ambient, and ca. 18 ML for oxygen
as shown in figure 3.2 which is indicating that the amount of O2 play an essential role in these
experiments. Figure 3.2 also shows that the difference between the numbers of ML based in
charge and mass in the scan rate 100mV/s and higher is not that significant.
69
In the early stage of cathodic scan, ORR taking places on the fresh Ru surface before Bi
UPD take places. This explains the onset potential of reduction current is earlier than the onset
potential of mass response. As the cathodic scan reaches to Bi UPD region, mass started to
increase while ORR still take place. Soon after the completion of Bi UPD monolayer on Ru,
oxygen reduction reaction decreases. As the cathodic scan passed the Bi UPD region, Bi bulk
deposition take places. The mass response and current of Bi bulk deposition shows similar
amount of increase in Bi ML coverage indicates that electrons from the working electrode are
solely use to reduce Bi3+ to Bi on the surface. Notice that a slight difference in the Bi bulk
stripping coverage between the Bi MLMass and Bi MLCharge can be contributed to unpreventable
oxidation of Bi in solution. Once the CV stops at the Bi bulk region, the Bi is soon oxidized to
Bi2O3, which is then dissolved in acidic solution, releasing Bi3+ back to solution. During the
0 50 100 150 200 2500.85
0.90
0.95
1.00
1.05
1.10
1.15
1.20
1.25
1.30
1.35
Bi U
PD M
L (fr
om M
Man
ual)
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Bi UPD ML (from MCathodic)(H2SO4)(N2 purge) Bi UPD ML (from MCathodic)(H2SO4)(no N2 purge) Bi UPD ML (from MCathodic)(H2SO4)(O2 purge)
0 50 100 150 200 250-10123456789
10111213141516171819
Bi U
PD M
L (fr
om Q
Man
ual B
asel
ine
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cted
)
Scan Rate (mV/s)
Bi UPD ML (from QCathodic)(H2SO4)(N2 purge) Bi UPD ML (from QCathodic)(H2SO4)(no N2 purge) Bi UPD ML (from QCathodic)(H2SO4)(O2 purge)
b
a
Figure 3.2: (a.) Bi UPD MLMass (Cathodic) calculated in different scan rate, N2 purge (black line), lab ambient (red line), and O2 purge (Green line). b.) Bi UPD MLCharge (Cathodic) calculated in different scan rate, N2 purge (black line), lab ambient (red line), and O2 purge (Green line).
70
anodic scan, the Bi bulk oxidized and dissolved rapidly until the Bi coverage reaches to
monolayer. When stopping CV near the end of the Bi bulk oxidizing region, the remaining Bi is
soon oxidized to Bi2O3, and then dissolved in acidic solution in the form of Bi3+[3]. When CV
scans to anodic region, a significant reduction current occurs while Bi UPD monolayer
undergoes oxidation. This reduction current is contributed to oxygen reduction reaction which
reduces the partial-exposed Ru surface while Bi UPD adatoms are oxidizing. This can be
explained that O2 purge makes Bi UPD monolayer harder to strip away from surface. Notice that
in Bi bulk region, there is no significant difference in Bi MLMass coverage between O2 purge and
N2 purge. This similarity in Bi MLMass coverage signifies the Bi bulk should be mainly Bi metal
instead of Bi2O3 or other Bi oxide species. In addition, Bi UPD ML in O2 purged solution shows
higher resistant to oxidation indicating that the ORR does take place. From the observation of
the very small change on the number of monolayers under the O2 ambient, it can be concluded
that ORR (which occur on the surface) prevents the oxidation of Bi layer and increases the mass.
This explanation also infers that ORR can take place at Bi UPD-modified Ru electrode surface
even with more than one Bi monolayer coverage such as 2.5 Bi MLcharge coverage or less.
A comparison of the ambient differences in the above sample evinces that with an
increase in the concentration of oxygen the Bi UPD MLCharge increases without effecting any
change in the MLMass (see figure 3.2). This is another evidence of additional electron transport
processes other than Bi UPD which occurs on Ru electrode. This significant increase of
reduction current in both cathodic and anodic scan during Bi UPD function indicates that it does
change the catalytic property of the Ru electrode. One possible explanation for this charge-
mass discrepancy is an evidence of possible ORR or formation of peroxide catalyzed by Bi UPD-
71
modified Ru electrode. Notice that O2 purge does not change the Bi UPD MLMass drastically as
with air-saturated Bi UPD MLMass. This indicates that the Bi UPD in O2 purge and in air-saturated
ambient are likely to have the same chemical composition. In other words, Bi UPD in oxygen-
saturated and in air-saturated conditions are more likely to be Bi oxide or co-deposit of Bi and Bi
oxide instead of Bi adatoms. In contrast, if any oxides are forming during the processing of Bi
UPD in N2 purging conditions, it is more likely to form Bi adatoms and less Bi oxide. The ratio of
Bi oxide-to-Bi adatoms of Bi UPD on Ru is expect to be higher in O2-saturated and air-saturated
conditions than that in N2 purge condition.
3.3 Anion Effect on Bismuth under Potential Deposition (UPD) on Ruthenium
Wieckowski found that anion adsorption (e.g. HSO4- or SO4
2-) on the surface of Pt
electrode decreases the catalytic activity of Pt on ORR [4]. This experiment explores the effect
of anion adsorption on the catalytic property of Bi UPD-modified Ru electrode. To explore this
anion effect on Bi UPD, this study has undertaken a comparison between SO42- and ClO4
- [5].
While performing the experiment, the anodic bulk and UPD region is shifted slightly to the
positive region. This results in a weakening of the stronger absorbing SO42 and the prevailing of
the weaker absorbing ClO4- in the solution, as showing in figure 3.3
72
Figure 3.3: Anion effect of Bi deposition on Ru. (a.) CV of Bi deposition on Ru. Scan rate of 5mV/s. (b.) Zoom-in view of CV in Bi UPD region. (c.) Mass response. (d.) Zoom-in view of mass response in Bi UPD region.
Furthermore, multiple LSV oxidation stripping shows that Bi UPD on Ru is more stable in
HClO4 than in H2SO4; in other words, Bi UPD on Ru in HClO4 is harder to strip away than in H2SO4
by electrochemical oxidation. One explanation of this shift can be the strong adsorption of Bi
with the OH- anion. Because prechlorate anion has lower tendency to adsorb on Bi as reported
while sulfate anion is adsorbed more strongly with Bi and compete with the hydroxium anion
[5]. To overcome the adsorbing anion more energy (ΔE) is needed in the OH- than SO42- which
means, OH- is stronger. Another justification is due the lack of adsorption of the anion on the
HClO4 anion, the Bi deposition is adhering stronger on the Ru. The ΔE difference between the
HClO4 is greater than H2SO4 by 14.5 kJ/mole. In one hand, under the same scan rate (5 mV/s),
73
the Bi UPD MLMass coverage in H2SO4 (1.03 ML) is higher than Bi UPD MLMass coverage in HClO4
(0.37 ML). In the other hand, the Bi UPD MLCharge gives similar ML coverage in both H2SO4 (3.06
ML) and HClO4 (2.71 ML), as figure 3.4 present. One of the interpretations of these results is
because of the anion effects. For example, co- adsorption of SO42- with Bi UPD on Ru in H2SO4
gives higher Bi UPD MLMass coverage while ClO- is weakly adsorb on the Ru surface. In figure 3.3
the anion changing from SO42- to ClO4
- effect took place clearly in the striping of the Bi bulk and
Bi UPD under N2 purge. Nevertheless under other ambient the difference is not significant. This
finding can be justified by the fact that the lack of oxygen lower the oxygen reduction reaction
(ORR) and allow anion effect to take place. It was reported that sulfate anion adsorbed strongly
on the Pt surface to interfere with ORR. The same behavior of the sulfate anion on Ru is
expected and our experiment data support this assertion [6].
Figure 3.4: Bi UPD (MLCharge – MLMass) vs. scan rate of Bi3+ containing solution (a) in H2SO4 and (b.) in HClO4.
When comparing the number of monolayer based on mass under O2- saturated
condition in H2SO4 with under nitrogen ambient, it is noticed that under O2 condition the
a
b
74
number of monolayers is higher than the other ambient (see Figure 3.5(b) with Figure 3.6 (b)).
Whereas the number of MLMass in HClO4 under O2-saturated condition has similar coverage as it
in air-saturated condition, but slightly higher than it in N2-purge condition. This may indicates
that the surface of Bi UPD MLMass in H2SO4 during anodic scan may not be just Bi oxide or Bi
adatoms. Anion adsorption of sulfate/bisulfate ions are possible to adsorb on the surface of Bi
oxide. These adsorbed anions interact with the dissolved oxygen and cause delay in Bi UPD
stripping under O2-saturated condition during anodic scan. XPS data supports the finding of
sulfate/bisulfate ions adsorption on Bi oxide surface as it will be shown. Under all the scan rates
(up to 250 mV/s), Bi UPD MLMass in H2SO4 are higher than it in HClO4 under three ambient
conditions.
Figure 3.5: Scan rate effect and ambient effect of Bi UPD ML coverage on Ru in 0.5M H2SO4 +1mM Bi3+ (a.) MLMass (cathodic) (b.) MLMass (anodic) (c.) MLCharge (cathodic) (d.) MLCharge
75
(anodic).
Figure 3.6: Scan rate effect and ambient effect of Bi UPD ML coverage on Ru in 0.5M HClO4 + 1mM Bi3+ (a.) MLMass (cathodic) (b.) MLMass (anodic) (c.) MLCharge (cathodic) (d.) MLCharge (anodic).
The calculation of MLMass coverage in H2SO4 is higher than it in HClO4 which can be
explained by sulfate/bisulfate anions has higher tendency to adsorb on metal surfaces in acid
solutions than ClO4-. XPS data in later section support this anion adsorption. In addition, Bi
UPD MLCharge gives higher coverage in HClO4 under O2-saturated condition than it in H2SO4
(Figure 3.5 (c) (d) vs. Figure 3.6 (c) (d)). This indicates that the Bi UPD-modified Ru electrode has
higher catalytic property in HClO4 than in H2SO4 under O2-saturate condition.
76
3.4 XPS Analysis of Bi UPD on Ru under Different Conditions
In addition to the aforementioned anion effect and the ORR reaction, oxidation of Bi
adatoms on Ru could also affect the Bi UPD process. Using XPS, the Bi oxidation was explored,
and the results indicate that the dominated oxidation product is Bi2O3 observed at 158.6(Bi 4f),
see Figure 3.7. Furthermore, more than one complex formed on the surface; however, the
dominant one was Bi2O3 at 158.6eV in both SO42- and ClO4
- solutions. It has been noticed that
the SO42- anion peak shows up at 168.5 eV under the nitrogen purge condition, as shown in
figure 3.7, on the surface, whereas under the non-purge condition there was no SO42-
adsorption or only a very little amount (see figure 3.8). That indicates that the purge of nitrogen
strengthens the sulfate anion adsorption, or it reduces the ORR reactions on the surface,
allowing more SO42- to stick on the surface.
Figure 3.7: XPS of Bi UPD N2 purge deposition on Ru cathodic region in 0.5M H2SO4 with high
77
scan rate (50mV/s) of depositing Bi on Ru electrode.
Figure 3.8: XPS of Bi UPD non- purge deposition on Ru cathodic region in 0.5M H2SO4 with high scan rate (50mV/s) of depositing Bi on Ru electrode.
For all of the anion conditions, the deposition of Bi on the Ru substrate prevents further
oxidation of Ru. This points to the conclusion that the Bi layer protects the Ru substrate, as
presented in figure 3.9. As can be seen from the XPS data, the Ru surface remains at Ru(0) state
after sputtering, which means it is kept oxide free by the protection of the Bi layer.
78
The second investigation was to explore the scan rate effect by changing the scan rate of
the deposition (UPD) of the Bi on the surface under N2 from 50 mV/s to 5 mV/s. Our XPS data
show that changing the scan rate will change the surface composition of Bi (0) at 156.5 eV metal
and Bi2O3 at 158.6 eV (figure 3.10). For instance, the higher scan rate was found to have more Bi
(0) metal coverage than the lower scan rate. The lower scan rate gave more time for the Bi
surface to form the complex or to become oxidized, while on the contrary the higher scan rate
left more Bi (0) metal with less Bi2O3. It is observed here that with the low scan rate, shown in
Figure 3.8, the Bi2O3 form shifted slightly after sputtering for 3min with argon (Ar) at the rate
5Ao (based on the SiO2 rate). Moreover, the sputtering didn’t change the oxidation state of the
Bi (0) metal, which means that it was adsorbed strongly to the surface. This means forming
more of Bi2O3 on the surface, making the adhesion stronger to the substrate. Therefore, there is
Figure 3.9: XPS Bi UPD on Ru (no N2 purge) Ru 3d Region at 280 eV which represent the Ru metal or native oxide
79
no shifting or decreasing in the intensity after sputtering, in the case of lower scanning rate. On
the other hand, the higher scan rate, in figure 3.5, shows the mixture of Bi (0) and Bi2O3 is less
stable because after sputtering for 0.5 min, some of the oxide was removed, and the Bi2O3
shifted to Bi (0). Moreover, with further sputtering, the Bi layer began to disappear. The Bi UPD
continued to protect the Ru from becoming oxidized.
Figure 3.10: XPS of Bi UPD on Ru cathodic region in 0.5M H2SO4 with low scan rate (5mV/s) of depositing Bi on Ru electrode.
Next, the effect of another anion (ClO4-) was tested in the XPS for comparison with SO4
2-.
The Bi2O3 complex remained the dominant form with little Bi (0) (see Figure 3.11). It was noted
that with the use of the ClO4- anion, the adsorption of the Bi UPD and its complex was not as
strong as that of H2SO4. Therefore, after 1 min sputtering almost all the complex had shifted to
Bi (0) metal. Also, it was noted that after 2 min of sputtering, the Bi (0) began to disappear.
80
Figure 3.11: XPS of Bi UPD on Ru cathodic region in 0.5M HClO4.
Finally, the effects of dissolving the anion by rinsing the final sample with water before
analyzing it in the XPS machine were explored. It was observed that the rinse doesn’t remove
the anion completely, but it dissolved some of it. This process led to splitting the peak of the
Bi2O3 to two peaks, which represent two different characteristic of that complex. The big
difference which was detected between the UPD with the rinse and the UPD without rinse was
that the one without the rinse showed more resistance to sputtering of Ar at the rate 5Ao
(based on the SiO2 rate), while the sample with the rinse after sputtering with 1min (5Ao)
shifted to Bi (0) metal and after 2 min it started to disappear as demonstrated in figure 3.12.
81
Figure 3.12: XPS of Bi UPD deposition on Ru cathodic region in 0.5M H2SO4 under N2 with rinse with water after deposition.
3.5 The Ring Rotating Disc Electrode (RRDE) Result of Bi Deposition on Ru
The ring rotating disc electrode (RRDE) is a powerful instrument for detection of the ORR
process on the surface of the electrode. By means of the RRDE, the number of electrons
transferred can be determined in the ORR process. Then the effectiveness of that electrode as
an ORR catalyst can be evaluated. The oxygen reduction reaction (ORR) can go through different
pathways as shown in schema 3-1[7].
82
Schema 3-1: The pathways of ORR.
Oxygen can go through a four electron reduction reaction to form water (k1) and
generate electricity. Alternatively it can go through an intermediate pathway (k2) of two electron
reduction to form peroxide. Then subsequently another two electron reduction can take place,
forming water. Formation of hydrogen peroxide in the proton membrane exchange fuel cell
(PEMFC) can cause corrosion in the fuel cell, decrease the electroactivity, lower the efficiency,
and lead to complete failure. As a result, many studies are exploring new electrocatalysts. The
new catalyst should have higher ORR catalytic efficiency and lower generation of undesirable
hydrogen peroxide as a byproduct. The Pt electrode is considered as one of the best
electrocatalyst for the PEMFC. However, the conversion energy drops from the theoretical value
of 1.229V to 0.78V [8] because of the slow kinetic rate of ORR due to its high overpotential [9,
10]. Therefore, exploration of new catalysts that are economical, durable, resulting in lower
generation of peroxides, and efficient [11, 12] is crucial.
Different techniques can be used to test the rate of ORR and H2O2 formation of the
electrode surface. A high-quality catalyst should have a high, fast ORR, and at the same time the
rate of H2O2 generation should be very low. These two property requirements are the most
important for screening a new electrocatalyst for use in the PEMFC system. To measure the ORR
83
performance and the formation of H2O2, the rotating ring-disc electrode (RRDE) and the
scanning electrochemical microscope (SECM) are the best techniques currently being used to
evaluate the performance of a new catalyst. In the RRDE system, the rate of ORR and H2O2 can
be detected. A constant positive voltage is applied on the ring electrode, and this will oxidize
and detect what has been produced by the reduction process at the disc electrode. Using the
Koutecky-Levich equation, the number of electrons transferred and the percentages of
peroxides forming on the surface can be calculated, and the ORR efficiency can be evaluated.
Linear sweep voltammetry (LSV) is an electrochemical technique that can pinpoint the
onset potential of the ORR catalyst. However, use of the LSV technique alone can’t determine
how much peroxide is forming. That is why the RRDE combined with the LSV technique is used
for this purpose [13]. To determine the onset potential of the new catalyst, the LSV results with
N2 and O2 are overlapped. The two sets of LSV data will clearly show the onset potential point. If
that overlap doesn’t show the onset potential clearly, the point at which the LSV curve of the
Ring disk
Ring Disk
Flo
w
Figure 3.13: RRDE system sketch.
84
reduction in current begins to curve will be taken as the point of onset potential. Prior to the
ORR measurement, the Ru electrode surface is polished with a 0.5micron cloth pad. Then it is
electrochemically cleaned and reduced at -0.6V vs. Hg/Hg2SO4. Comparing the nitrogen
background with the oxygen ambient, the LSV can pinpoint the onset potential of ORR at 0.25V
(vs. Hg/Hg2SO4) on the Ru catalyst.
Figure 3.14 (a) and (b) shows the result of Bi UPD on Ru RRDE in 0.5 M H2SO4 at varying
rotating speeds under the condition of saturated O2 compared with the background Ru as
shown in figure 3.15. What’s more, figure 3.14 (c) demonstrates the limiting current at different
applied potentials. The limiting current depends on the applied potential and it increases when
the corresponding overpotential is increased. In figure 3.15 (c), which shows the inverse square
root of rotating speeds versus the current density of disk electrode, the non-linear curve (such
as -0.25V) indicates a complex electron transfer situation. In other words, the reaction rate does
change even when the supply of reactant is increased by an increase in the rotating rate.
85
Figure 3.14: The result of Bi UPD on Ru RRDE system (O2- saturated) a.) The ring current b.) The disc current c.) Koutecky- Levich plot at varying potential.
Figure 3.15: The result of Ru background RRDE system (O2- saturated) a.) The ring current b.) The disc current c.) Koutecky- Levich plot at varying potential.
86
The electron transfer number n can be calculated by following equations
where Neff is the collection efficiency, Id is the disk current, and Ir is the ring current [14, 15].
From the RRDE result, it was observed that Bi UPD enhances the effectiveness of ORR. It
was noted that there was increase in the current in the RRDE result for the disc and ring as well,
see Figures 3.14 and 3.15. A different ambient was studied with a different rotation speed to
discover the change and the effectiveness of Bi UPD on Ru. Under N2, comparing the Bi UPD and
Ru as backgrounds, the result shows increase in the disc and ring current, which indicates that
greater reaction occurred on both surfaces. The onset potential for ORR on the Ru electrode is
slightly lower than the onset potential of Bi UPD toward ORR, as can be observed in figure 3.10.
The percentage of H2O2 forming on the Ru and Bi UPD under these conditions was calculated to
be ca. 1%, which is a good indication of efficient catalyst activity in the fuel cell. The number of
electrons transferred at a potential -0.3V was found to be ca. 3.78 and 3.71 for Bi UPD and Ru
respectively at 900rpm. This means that the O2 was reduced on the surface to form H2O
efficiently, primarily through the 4e- process. These characteristics, also, were explored under
air and under O2 ambient. Both of these conditions show very similar behavior, very close
87
percentages of H2O2 formation, and close numbers of electrons transferred. The significant
difference was that the disc and ring current was increased dramatically because of the increase
of the O2 content.
Owing to the fact that Bi deposition is unstable in the acidic media [2], the RDE and
RRDE experiments were executed in the solution containing Bi3+. The Bi UPD was deposited, and
then the RRDE LSV was run in the same solution. The advantages of this method are that it
leads to the saving of time during the depositing of Bi UPD and to a guarantee that the Bi layer
still exists. However, the disadvantage is when the LSV is run, the region of the ORR is mixed
with the deposition of Bi on the Ru region even though the rotation is proceeding. That can
explain the incline of the curve at -0.5V in the RRDE figure 3.14; also, it can explain the non-
linear curve on the Koutecky- Levich plot.
Another electrolyte, HClO4 was tested for the purpose of comparing the difference in the
catalytic effectiveness under N2, air, and O2 ambient with different rotation speeds. It has been
noticed that HClO4 has higher catalytic activity for ORR, which can be seen from the significant
increase of the current on the disc and ring. This is consistent with the result that was obtained
from the EQCM. Figure 3.16 and 3.17 evince that the percentage of H2O2 forming on the surface
was ~ 1%, and the number of electrons transferred depends on the rotation speed; it is about
3.75 at 900rpm, which is very efficient.
88
Figure 3.16: The result of Ru background RRDE system in 0.5M HClO4 (O2- saturated) a.) The ring current b.) The disc current c.) Koutecky- Levich plot at varying potential.
Figure 3.17: The result of Bi UPD on Ru RRDE system in 0.5M HClO4 (O2- saturated) a.) The ring current b.) The disc current c.) Koutecky- Levich plot at varying potential.
a
b
c
a
c
b
89
3.6 Bi Bulk on Ru Result
Bi bulk deposition in all three ambient conditions gives almost the same MLmass based on
the mass calculation. XPS results show Bi2O3 and Bi adatoms appear in all three ambient
conditions, with a majority of Bi2O3. Also, Bi2O3 is the dominant composition in both
electrolytes, H2SO4 and HClO4, as can be seen in figure 3.18(a) and (b); the location of Bi2O3 is at
158.6eV, and there is a small amount of Bi (0) at 156.5eV. The anodic stripping of the Bi bulk is
more difficult in HClO4 than in H2SO4, as can be noticed from the CV previously shown in figure
3.2. In addition, from the XPS it is noticed that more sputtering is necessary to remove the Bi
bulk in the case in which the HClO4 electrolyte is more than H2SO4, see figure 3.18(a) (b). These
observations lead to the conclusion that the co-deposition of Bi and Bi oxide could occur during
the Bi bulk deposition in both H2SO4 and HClO4.
Figure 3.18: XPS of Bi bulk on Ru. N2 purge. 5 mV/s and hold at ~-0.6V for 7 mints. Sputter rate: 0.5 nm/min and/or 2.3 nm/min (calibrated to SiO2). (a.) Bi 4f (H2SO4) (b.) Bi 4f (HClO4) (c.) O 1s (H2SO4) (d.) O 1s (HClO4)
90
For further investigation of the bulk surface characterization, another sample was
prepared of Bi bulk deposition under N2 in HClO4 and left in the air for one hour. The purpose of
that sample was to distinguish between instant analysis and analysis of an aged sample. It was
found that the longer the exposure to air, the thicker the oxide formation of Bi2O3, as shown in
figure 3.19(a) (b). That means the oxide layer of Bi is not close packed, with the result that it
allows O2 to penetrate and form more oxide; this is unlike the case of Al2O3, which prevents
further oxidation. This means that the Bi UPD may just be composed of Bi adatoms, but it is
oxidized to Bi2O3 during time in which CV is stopped in the solution, or during the rinsing and
cleaning of the sample, its transportation, the XPS sample preparation, etc., before XPS analysis
is run. In all of the ambient and anion cases, the layer/s of Bi UPD and Bi bulk protect the Ru
substrate form becoming oxidized, see figure 3.15(c).
Figure 3.19: XPS of Bi bulk on Ru. N2 purge. 5 mV/s, holding at ~-0.6V for 7 mins, and then
91
exposure to air for 1 hour. Sputter rate: 0.5 nm/min and/or 2.3 nm/min (calibrated to SiO2). (a.) Bi 4f (HClO4) (b.) O 1s (HClO4) (c.) Ru 3d (HClO4) (d.) Cl 2p (HClO4).
3.7 Proposed Mechanism of Bi Deposition on Ru
Based on all of these experiments, the mechanism of the Bi UPD deposition and Bi bulk
deposition was explored. It occurs in different regions but can be divided into three major
categories as can be seen in Figure 3.20(a) and (b).
Figure 3.20: (a.) CV and mass response of Bi deposition on Ru. Scan rate of 5 mV/s. (b.) Zoom-in view of CV and mass response in the Bi UPD region.
The above experiments show evidence of ORR reaction on a Bi UPD-modified Ru surface.
Therefore, the mechanism of Bi UPD deposition on Ru in Figure 3.16 is proposed as follows.
Region I—Bi UPD Region
Ru
Bi Bi Bi e- e- e-
Bi3
Ru e- e-
O2 H2O or H O
e-
Ru
O2 H2O or H O
e- Bi Bi e- e-
Stop CV Ru
Bi Bi2O3 Bi2O3
Bi3
Dissolution
+ H2O
H+
e-
e-
H2O H+
92
In the early stage of cathodic scan, ORR takes place on the fresh Ru surface before Bi
UPD takes place. This explains the earlier onset potential of reduction in current as compared to
the onset potential of mass response. As the cathodic scan reached the Bi UPD region, the mass
of deposition of Bi UPD started to increase while ORR was still taking place. Soon after the
completion of the Bi UPD monolayer on Ru, the oxygen reduction reaction stopped.
Region II—Bi Bulk Deposition Region
As the cathodic scan passed the Bi UPD region, it showed that BI bulk deposition had
taken place. The mass response and current of Bi bulk deposition show a similar amount of
increase in the Bi ML coverage. This indicates that electrons from the working electrode are
exclusively occupied in reducing Bi3+ to Bi on the surface. Notice that the slight difference in the
Bi bulk stripping coverage between the Bi MLMass and Bi MLCharge, see figure 3.1, can be
attributed to unpreventable oxidation of Bi in solution. Depending on the calculation of the
number of monolayers, it can be concluded that no or very little ORR takes place in the Bi bulk
region. Once the CV ceases at the Bi bulk region, the Bi is soon oxidized to Bi2O3, which is then
dissolved in the acidic solution, releasing Bi3+ back to the solution.
Ru
Bi Bi Bi
Bi Bi Bi
Bi Bi Bi e- e- e-
Stop CV Ru
Bi Bi Bi
Bi3+
e- e- e-
Bi3+
Dissolution
+ H2O
H+
Ru
Bi Bi Bi
Bi Bi Bi
Bi Bi2O3 Bi2O3
e-
e-
H2O H+
93
Region III—Bi Bulk Stripping Region
During the anodic scan, the Bi bulk oxidized and dissolved rapidly until the Bi coverage extended
to the completion of the monolayer. Upon the ceasing of the CV near the end of the Bi bulk
oxidizing region, the remaining Bi is soon oxidized to Bi2O3, and then it is dissolved in acidic
solution in the form of Bi3+.
Region IV (1.2 to 0.6 Bi MLMass) & V (< 0.6 Bi MLMass)—Bi UPD Stripping Region
When the anodic CV scans to region IV, a significant reduction in current occurs while
the Bi UPD monolayer undergoes oxidation. This reduction in current is contributed to an
oxygen reduction reaction which reduces the partially-exposed Ru surface while Bi UPD
adatoms are oxidizing. This ORR mechanism is strongly supported by an increase in the
concentration of oxygen in the solution. Notice that in figure 3.2b, the Bi UPD MLcharge is
Ru
Bi Bi Bi e-
Bi3+
Ru
O2 H2O or H2O2
e- Bi Bi e-
Bi3+
Ru
O2 H2O or H2O2
e-
Ru
Bi Bi Bi e-
Bi3+
Stop CV Ru
Bi Bi Bi
Bi Bi Bi
Bi Bi
Bi3+
e- Ru
e- Bi Bi2O3 Bi2O3
Bi3+
Dissolution
+ H2O
H+ e-
H2O H+
94
significantly larger for O2 purge than for N2 purge or for no gas purge. This can be explained by
the conclusion that O2 purge makes Bi UPD monolayer harder to strip away from the surface.
Notice that in the Bi bulk region, there is no significant difference in Bi MLMass coverage between
O2 purge and N2 purge. This similarity in Bi MLMass coverage signifies that the Bi bulk must be
mainly Bi metal instead of Bi2O3 or another Bi oxide species. In addition, Bi UPD ML in O2
purged solution shows higher resistance to oxidation indicating that the ORR does take place.
Because electrons are provided to the electrode’s surface, Bi UPD adatoms undergo the
reduction reaction instead of oxidation during the anodic scan.
Based on Le Chatelier’s Principle, electrons from ORR shift the reaction Bi Bi3+ + 3e- to
the left, which reduces Bi on the electrode surface and results in higher Bi MLMass coverage in
the Bi UPD region. This explanation implies that ORR can take place at the Bi UPD-modified Ru
electrode surface, despite the fact that there is more than one Bi monolayer coverage, such as
2.5 Bi MLMass coverage or less. Notice that in figure 3.16b region V, the oxidation current starts
appear when the potential of anodic scan reaches to -0.2 V. This can be explained by the
conclusion that electrons from ORR cannot sustain the rate of Bi oxidation. This phenomenon
can also be seen in figures 3.1b and 3.1d, especially in the reaction with O2 purge, where a slight
increase in anodic current and a sudden drop in Bi MLMass near -0.2V are observed. This could
signify that two reactions occurred during the anodic scan: ORR and Bi UPD oxidation. That
means that they were competing for the existing O2. Thus, when there is O2 starvation, one of
these two reactions will be sacrificed to the other or reduced to a minimum.
95
3.8 Conclusion
The average Bi UPD MLMass (~1.5 Bi ML) coverage is lower than the average Bi UPD
MLCharge (~ 5.0 Bi ML) in H2SO4 under lab ambient. There is little or no variation on the Bi ML
based on mass for all three of N2, air, and O2 ambient, whereas, based on the charge calculation
there is a significant difference in Bi UPD ML coverage between N2, air, and O2. Bi UPD-modified
Ru enhances ORR reaction, both in H2SO4 and in HClO4. The increase in the current of the disc
and ring in the RRDE supports the conclusion that Bi UPD enhances the ORR. The XPS data show
that Bi forms oxide on the surface in all cases. Also, it is found that a low scan rate forms more
uniform Bi2O3 and forms a stronger layer than that which is formed at a higher scan rate; this
layer can’t be removed easily. XPS data strongly support the presence of Bi and Bi oxide,
possibly Bi2O3, on the Bi UPD monolayer. The presence of Bi oxide is possibly due to the
inevitable oxidation of Bi during the time of the stopping of the CV scan, transporting of
samples, and recording of XPS data. The stability of Bi UPD on Ru also supports the evidence of
the presence of Bi oxide (Bi2O3, BiO+, or BiOH2+) when the Bi UPD monolayer or sub-monolayer
is exposed to air or to an oxygen-containing solution for a short period of time. In addition,
multiple LSV oxidation stripping of Bi UPD on Ru shows relatively more stable Bi UPD in HClO4
than in H2SO4. Furthermore, XPS data support the finding of sulfate ion (SO42-) adsorption on Bi
UPD monolayer in H2SO4. However, the stability problem of Bi UPD adatoms on Ru needs to be
solved if it is to be fully utilized as an alternative fuel cell catalyst. Finally, a detailed mechanism
of Bi deposition on Ru is proposed.
96
3.9 References
1. Cheng, T.J., Li, K.F., Chang, H.C. The Chinese Chem. Soc. 2001, 59, 2, 219-233.
2. Lin, P. M.Sc. Thesis, University of North Texas, 2011.
3. Kathryn, E.; Toghill, G.; Wildgoose, A.; Moshar, B. Chris, B. Richard, G.; Electroanalysis
20, 2008, 16, 1731 – 1737.
4. Kolics, A.; Wieckowski, A. J. Phys. Chem. B 2001, 105, 2588-2595.
5. Brian, K.; Niece, A.; Gewirth, A. Langmuir 1996, 12, 4909-4913.
6. Uhm, S.; Yun, Y.; Tak, Y.; Lee, J. EQCM analysis of Bi oxidation mechanism on a Pt
electrode. Electrochemistry Communications 2005, 7, 1375–1379.
7. Wroblowa, H.S.; Pan, Y.D.; Razumney, G. J. Electroanal. Chem 1976, 60, 195.
8. U.S. Department of Energy, (2009). Technology Installation Review [online]. Available:
http://www.eere.energy.gov/femp/ [accessed 10 November 2009].
9. Chen, Y.; Li, M.; Liao, L.; Jie, X.; Shen, Y. Electrochemistry Communications 2009, 11,
1434.
10. Venkatewara Rao and B. Viswanathan, J. Phys. Chem. C 2009, 113, 18907.
11. Bashyam, R.; Zelenay, P. NATURE 2006, 443, 7.
12. Thomas E. Wood, Zhongshu Tan, Alison K. Schmoeckel, David O’Neill, Radoslav
Atanasoski, Journal of Power Source 2008, 178, 510.
13. Bard. A.J.; Faulkner, L.R. Electrochemical Methods: Fundamentals and Applications, 2nd
ed., 2001, John Wiley & Sons, Inc.
14. Sethuraman, V.; Weidner, J.; Haug, A.; Motupally, S.; Protsailob, L. Journal of The
Electrochemical Society 2008,155, B50-B57.
97
15. Yu, K. Ph.D. Dissertation, University of North Texas, 2011.
98
CHAPTER 4
CHARACTERIZATION OF PLASMA TREATED INTER-LAYER DIELECTRIC (ILD)
ULTRA-LOW-K USING XPS AND FTIR
4.1 Introduction
Moore’s Law (1965), which is predicted by the Intel co-founder Gordon E. Moore
(Chemistry Ph.D. CalTeck), states that the number of transistors that can be placed inexpensively
on an integrated circuit will be doubled approximately every two years [1]. The trend has
continued for more than half a century and is not expected to stop until 2015 or later. To keep
up with Moore’s law, new low-k and ultra-low-k materials have to be invented. The definition of
low- k dielectric is an insulating material that shows weak polarization when exposed to an
external-applied electric field. The dielectric constant, k, is a physical measure of the electric
polarizability of a material, and electric polarizability is the tendency of a material to permit an
externally applied electric field to make electric dipoles [2, 3]. Moreover, the properties of these
materials should have criteria such as stability and isolatation. Silicate and organosilicate are the
most promising material candidates so far [2].
Two different ways to deposit these materials are spin coating and chemical vapor
deposition (CVD). Most researchers use the CVD process to deposit the thin film or low-k
because it gives better control and better distribution of organo-materials like carbon and
fluorine. With the CVD process, silicon dioxide (SiO2) can be doped with fluorine (fluorine-doped
oxide) (F-SiO2), or carbon (carbon-doped oxide) (C-SiO2) [4-8]. These new doped SiO2 materials
are expected to lessen the moisture content in the dielectric. Alternatively, a dielectric with
minimum hydrophilic properties needs to be designed. Since k constant of water is ~ 80, a low-k
99
dielectric only needs to absorb very small traces of water before losing its permittivity
advantage [9-12].
Organsilicate glass (OSG) composed of Si, C, O, and H, is one of the materials that is proposed as
a promising low-k classification. In these kinds of materials, the carbon bond (Si-C) replaces one
of the oxygen (Si-O) bonds in the SiO2 network, which gives the material porosity and softness
[13-15]. The replacements of carbon in the SiO2 reduce the dielectric value and increase the
isolation properties. The replacement of O with the C in the Organsilicate glass (OSG) structure
can be seen in figure 4.1.
Figure 4. 1: Organsilicate glass (OSG) proposed structure [13].
The first marketable example of a low-k interlayer dielectric arose around 2000. The process
substituted SiO2 with fluorosilicate glass for the 180 nm technology node [16]. The direction of
current research is to create a less than 16nm integrated circuit which, requires a very low-k
100
dielectric to isolate and function in these circuits. Nevertheless, these materials will need to go
through some modern semiconductor manufacturing chemical and physical processes, which is
an extremely challenging task. The plasma processes of low-k materials are necessary for the
semiconductor chips to sculpt and prepare the pattern, remove the photoresist, and clean the
surface from residues. Even though the plasma treatment can be useful and helpful to do all
these tasks, it can be very harmful in the performance of the chip if not used correctly and in
the right conditions. The changing of the SiO2 to C-SiO2 will add additional porosity to the
material. At the same time, it will lower the dielectric constant, so this new porosity is the
reason for the ultra-low-k (ULK) performance.
To simulate the effect of plasma treatment, two gases were chosen (O2 and H2) to
investigate the chemical modifications on un-patterned ULK without the TEOS layer. ULK film
was 300nm thickness on Si substrate, which is obtained from one of our collaboration
companies. The samples were cut and exposed to plasma for different time periods to study the
time-dependent effect. A Harrick plasma etcher chamber pressure of 100mTorr and a power of
30W was used for O2 and H2 etching. One of the processes that this low-k material should go
through is plasma cleaning, which could induce damage to the surface and cause a failure in the
circuit. In this chapter, some studies will be examined that show the changes on the low-k
surfaces and the composition employed on just the blanket wafer without pattern using
different technique such as the XPS, FT-IR, and AFM [17].
4.2 Characterizations of Dielectric Damages on Blanket Wafer Using FT-IR
Multiple internal reflections infrared (MIR-IR) is when an infrared beam passes through a
101
surface that has a higher reflective index and bounces inside that object multiple times, which
can increase the sensitivity and resolution of the sample. MIR-IR metrology has been utilized in
our lab to study the effect of plasma treatment on blanket ILD wafers. This metrology can be
successfully characterized as the chemical bonds and the change of these bonds. Furthermore,
the effect of the plasma damage can be quantized. Using our lab tools a fabrication technique
was developed to study the induced damage of plasma on the low-k material. As the picture
shows in figure 4.2, a polished 6X1 cm crystal of Si with the low-K material was fabricated based
on previous procedure [18]. This 45o bevel angle face allows the IR light to travel smoothly and
reflect inside the crystal ca. 90 times, collecting information to the level of nano change [19, 20].
To explore the effect of plasma chemistry on porous low-k material, oxygen (O2) and
hydrogen (H2) plasma etching were investigated on the blanket wafer that was received from
TEL Company. As can be seen on figure 4.3, the oxygen plasma causes significant changes to the
low-k material and changes the bonding structure, which indicates the damage occurred on
these materials due to the plasma treatment.
Figure 4.2: Schematic representation of Multiple internal reflections infrared (MIR-IR) with Si attenuated total reflection (ATR).
102
Figure 4.3: FTIR spectra time dependent O2 plasma ashing on blanket wafer.
-200 0 200 400 600 800 1000 1200 1400 1600
0.000
0.005
0.010
0.015
0.020
0.025
0.030
0.035
0.040
IR A
bsor
banc
e (m
abs)
etching time (sec)
OH peak TIR CH3 peak TIR
Figure 4.4: ILD/low-k blanket wafer O2 treatment plots of IR absorption peak heights of CH3 and OH vs. etching time for 1540sec.
103
The study of the time variable from figure 4.3 and 4.4 show clearly that there is a
gradual increase of the content of the OH group at 3200-3700cm-1. Also, a decrease of the
content of the CH3 group at 2970cm-1 was observed. It can be conclude from these result that
the low-k material was damaged with a time increase in the plasma treatment. Moreover, there
is a forming of double bond carbon and an increase of additional C=O peak at ca. 1720cm-1 to a
certain level. Then it decreases, which means that C=O is formed in the transition state to
change the Si-CH3 to Si-O-H.
4.3 XPS Characterizations of Dielectric Damages on Blanket Wafer
To confirm the result obtained from the FT-IR, the changes of the bond conformation
were investigated using the x-ray photon spectroscopy. First, the effect of O2 plasma was tested
on the blanket wafer for different plasma etching times. As it is shown in figure 4.5, the general
trend of the low-k material shows degradation of the carbon content and changing of the
bonding form.
104
Figure 4.5: C1s core-level spectra for pristine and O2 plasma treatment etching for 70sec un-patterned low-k material. The C1s peaks show at 283.4 eV corresponds to Si-C, C-H [21] and the peaks at 286.4 eV corresponds to C-O [21, 22].
For the original surface sample, the C1s spectrum contains an unresolved, intense peak located
at 283.4 eV coming from the carbon atoms in Si-C bonds of the Si-CH3 group [21]. After the
plasma treatment, significant modifications of the surface composition and structure appear.
105
From the XPS result, figure 4.6 shows that the carbon content nearly disappeared from
the low-k film when treated with O2 plasma for about twenty five minutes. However, the MIR-IR
spectra display (not shown) indicates some carbon content still exists. This discrepancy can be
explained by the escaped depth of the XPS which is approximately 10nm and is limited to
surface change. While FT-IR penetration goes through the whole crystal and explores the
changes in the whole unit. From the results of the XPS and FT-IR, it can be concluded that the O2
plasma treatment damages the low-k material. Plasma treating of low-k film changes bond
conformation on the surface by breaking the Si-C-H and forming Si-O/H, which is alters the
Figure 4.6: C1s core-level spectra for pristine and O2 plasma treatment etching from 100 to 1540sec un-patterned low-k material. The C1s peaks show at 284.2 eV corresponds to Si-C, C-H [21] and the peaks at 286.4 eV corresponds to C-O [22].
106
dielectric constant. These changes can cause the chip to absorb a lot of water, negatively
affecting chips efficiency and even leading to total device failure.
-200 0 200 400 600 800 1000 1200 1400 1600
0.0
0.2
0.4
0.6
0.8
1.0
C1s XPS peakarea CH3 peak TIR
etching time (sec)
Nor
mal
ize c
urve
XPS
C(1
s)In
tegr
ated
are
a
-2
0
2
4
6
8
10
12
14
IR A
bsor
banc
e (m
abs)
Figure 4.7: TIR spectra of a 300 nm blanket low-k films measured and normalized curve XPS C (1s) of integrated area 280-290 eV after O2 plasma etching vs. etching time for 1540sec.
As shown in figure 4.7, the C1s XPS analysis showed 50% reduction in carbon (C) counts
within 20-40 sec of the etching time and a negligible reduction of C after 220 sec. This result is
inconsistent with the MIR-IR results, indicating that XPS was only able to probe a few nm deep
and thus unable to see damage in the bulk. However, the transmission infrared TIR shows the
same behavior of the XPS, which means that TIR is also more sensitive to surface change than
107
the bulk damage. Moreover, TIR is more reliable in the lower range of the IR spectrum, from 400
to 1600cm-1, than the upper range from 1600 to 4000cm-1.
Based on these results and the previous analysis, the best technique to use in analyzing
the ILD damages is the MIR-IR, which was able to detect the changes of the film on the surface
and the bulk. Also, the MIR-IR is extremely sensitive, as it can detect minute film changes
smaller than nanometers. The XPS the technique, however, only registers what is happening on
the surface within a few nm. Furthermore, the technique requires researches to break down its
spectra before they can study what happens beneath the film surface.
Figure 4.7 displays the differences in the sample and after progressive O2 plasma
treatments. As it can be seen from the figure, the total integrated area of the carbon (C1s) XPS
peak was used. Also, the biggest change in bond and largest decrease in carbon occurred within
the first 200sec; for the rest of the time, the oxygen plasma continued removing a slight amount
of the carbon. Again, the limit of the C1s XPS signal is a result of this sample analysis and within
the limit of escaped depth of an XPS photoelectron, which is about 10nm.
The other plasma treatment gas tested on the sample was hydrogen (H2) which yields
different results than the O2 plasma. From figure 4.8 and 4.9, it can be seen that in the
treatment of the low-k sample with hydrogen plasma, there is little influence on the change in
carbon content.
108
Figure 4.8: C1s core-level spectra for pristine and O2 plasma treatment etching for 70sec un-patterned low-k material. The C1s peaks shows at 284.0eV correspond to Si-C, C-H [23] and the peaks at 286.4 eV corresponds to C-O [23].
300 298 296 294 292 290 288 286 284 282 280 278 276
400
600
800
1000
1200
1400
1600
1800
Inte
sity a
.u.
Binding energy (eV)
100secH treatment 220secH treatment 340secH treatment 640secH treatment 940secH treatment 1540secH treatment
C1s 283.6. eV Si-C, C-H
General trend
C1s 286.2 eV, C-O
C1s
C1s 285.6 eV, C-C, C=C
Figure 4.9: C1s core-level spectra for pristine and H2 plasma treatment etching for 70sec un-patterned low-k material. The C1s peaks shows at 283.6 eV correspond to Si-C, C-H [23], and the peaks at 285.6 eV corresponds to C=C and C-C, and the peaks at 286.4 eV C-O.
109
Also, with H2 plasma, the bond configuration remains relatively consistent; thus, when
compared to the O2 plasma, the damage caused by the H2 plasma is insignificant, meaning that
the latter is a better choice for cleaning the surface during the post cleaning.
-200 0 200 400 600 800 1000 1200 1400 16000.00.10.20.30.40.50.60.70.80.91.01.11.21.3
C(1s) H treatement integrated area
Norm
aliz
e In
tegr
ated
are
a
etching time (sec)
Figure 4.10: Normalized curve Hydrogen plasma treatment of blanket wafer ILD/ low-k, analysis of XPS C1s integrated area from 280-290eV.
By displaying the normalized curve of H2 plasma treatment, figure 4.6 shows less
removed of carbon over time. Also, after 40sec, the carbon content increase, which can be
explained this way: during first 40sec, the H2 plasma cleans the top surface without any
changing the actual content of the ILD/low-k material. After 70sec of H2 plasma, the low-k
material starts to change and the carbon bonds break; that is why the carbon intensity begins to
110
decline. However, not much carbon was removed. As it can be seen from the figure, with H2
plasma half was lost compared to the original, while with O2 plasma all surface carbon was
removed. In the hydrogen plasma treatment, the configuration of the ILD/low-k film changed
after 100sec, while with the oxygen plasma treatment, changes became observable in the first
10sec.
4.4 Characterization of ILD Plasma Treatment Using AFM and Optical Microscope
Because carbon increase the porosity of the ILD/low-k material, it is expected that the
O2 and H2 treatment will reduce the porosity and increase the roughness. For that purpose, the
optical microscope and atomic force microscope (AFM) were used to explore the differences in
surface features and reflection color. From the optical picture, it can be seen that with each
treatment, the color changed dramatically, changes due to alteration in the structure’s bond
confirmation and porosity. It has been observed that the changes in color correspond to
changes of the penetration and reflection of the light from and in the surface before and after
treatment. As it can be seen figure 4.11, after each treatment, the color of each sample is differs
dramatically.
111
Figure 4.11: Wafer before and after treatment of O2 plasma magnify for 100x. a) TEL blanket wafers as is wafer. b) TEL blanket wafers 100sec O2 plasma. c) TEL blanket wafers 640sec O2 plasma. d) TEL blanket wafers 1540sec O2 plasma.
The next experiment is to investigate the change of ILD/low-k material using the AFM
instrument, which can detect the roughness of the surface and smoothness. Figure 4.12 it
shows that the roughness of the surface increases significantly after treatment with O2 plasma.
a b
c d
112
a)
b)
Figure 4.12: a) AFM image roughness of as is blanket wafer. b) AFM image roughness of 100sec O2 plasma treatment blanket wafer.
113
Figure 4.13: a) AFM image roughness of 640sec O2 plasma treatment blanket wafer. b) AFM image roughness of 1540sec O2 plasma treatment blanket wafer.
114
As one can notice from figure 4.11 and 4.13, the roughness of the ILD/low-k material
begins to increase after the treatment of O2 plasma which means that the porosity of the
sample starts to collapse, and the carbon bonds begin to break down. Interestingly, at the
beginning, the roughness decreases, which is related to a more transitional state where the
chemical bond is dangling and not totally broken. However, at the end of the treatment, the
surface is extremely rough due to the removal of most of the carbon from the surface. The
roughness measurement is shown in table 4.1.
As is (nm) 100 sec (nm) 640sec (nm) 1540sec (nm)
4.61 0.88 8.23 91.3
1.4 1.156 25.2 39.6
21.12 3.671 3.47 56.4
17.71 1.22 14.6 164.3
7.566 12.4 11.2 99.2
Average 10.4812 3.8654 12.54 90.16
Table 4.1: The measurement of AFM roughness of TEL blanket wafer before and after treatment
with O2 plasma.
4.5 Conclusion and Future Work
Different types of plasma gases can cause variable damages on the low-k materials. This
115
will be interesting to investigate, as researches and companies should target the right one for
cleaning and ensuring device preservation. Choosing the right technique is also crucial when
evaluating the plasma damage for these materials. As it was shown from the result of the
analysis of the FT-IR compared to the outcome from the XPS, there is variation, which could be
significant for the evaluation of plasma damage. Since XPS is limited to surface analysis for the
damage, it could provide less accurate results while the FT-IR processes through the whole
sample, making for a more comprehensive assessment. Future work will engage the effects of
other plasma gases such as CH4, N2 and CO2. Additionally, the same processes on the trench
pattern wafer will be investigated and evaluated with regard to side wall damage. More
chemical interactions before and after these treatments will be explored and observed the
effect of these chemicals on cleaning such as water, acids, or bases will be observed. Using our
powerful FT-IR technique alongside XPS will give a clearer picture for the semiconductor
industries and help researchers see the plasma effect on ILD pattern structure, leading to better
and more reliable devices.
116
4.6 References
1. Shul, R.J.; Pearton, S.J. Handbook of Advanced Plasma Processing Techniques. Springer
2000.
2. Baklanov, M.; Ho, P.; Zschech, E. Advanced interconnects for ULSI Technology. John Wiley
& sons, Chichester, UK, 2012.
3. Baklanov, M.; Green, M.; Maex, K. Dielectric Films for Advanced Microelectronics. John
Wiley & sons, 2007.
4. Tagami, M.; Ogino, A.; Miyajima, H.; Shobo, H.; Baumann, F.; Ito, F.; Sponner, T. ECS
Transaction. 2011, 40, 405-413.
5. Chaudhari, M.; Du, J.; Behera, S; Manandhar, S.; Gaddam, S.; Kelber. J. Appl. Phys. Lett.
2009, 94, 204102.
6. Behera, S.P.; Wang, Q.; Kelber, J. J. Appl. Phys. Lett. 2011, 44, 155204.
7. Bao, P.; Shi, H.; Liu, J.; huang, H.; Ho, S.; Goodner, D.; Moinpour, M.; Kloster, G. J. Vac. Sci.
Technol. B. 2008, 26, 219.
8. Jnnai, B.; Nozawa, T.; Samukawa, S. J. Vac. Sci. Technol. 2008, 26, 1926.
9. Baklanov, R.; Travaly, Y.; Le, T.; Shamiryan, D.; Vanhaelemeersch, S. Silicon Nitride, Silicon
Dioxide, Thin Insulating Films and Other Emerging Dielectrics YIII. ECS. 2005, 179-198.
10. Le, T.; Baklanov, R.; Kesters, E.; Azioune, A.; Struyf, H.; Boullart, W.; Pireaux, J.;
Vanhaelemeersch, S. Electrochem. Solid State Lett. 2005, 8, F21- F24.
11. Ijima, T.; Lin, Q.; Chen, S.; Labelle, C.; Fuller, N.; Ponoth, S.; Lloyd, J.; Dunn, D.; Muzzy, C.;
Gill, J.; Nitta, S.; Spooner, T.; Nye, H. Proceedings of IEEE IITC 2006, 21.
117
12. Bao, J.; Shi, H.; Liu, J.; Huang, H.; Ho, S.; Goodner, D.; Moinpour, M.; Kloster, G.;
McSwiney, M. J. Vac. Sci. Technol. A. 2010, 28,2.
13. Morgen, M.; Ryan, E.; Zhao, J.; Hu, C.; Cho, T.; Ho, P. Annual Review of
Materials Science 2000, 30, 645.
14. Kittel, C. Introduction to solid state physics, 7th edition. 1996, Wiley, New York
15. Maex, K.; Baklanov, M.; Shamiryan, D.; Iacopi, F.; Brongersma, S.; Yanovitskaya, Z. J.
Appl. Phys. 2003, 93, 8793.
16. Grill, A.; Neumayer, A. J. Appl. Phys. 2003, 94, 6697.
17. Volksen, W.; Miller, R. D.; Dubois, G. Chem. Rev. 2010, 110, 56.
18. Chyan, O.; Wu, J.; Chen, J. J. Applied Spectroscopy 1997, 51, 1905.
19. Chyan, O.; Chen, J.; Xu, F.; Wu, J. Anal. Chem. 1997, 69, 2434.
20. Abell, T.; Lee, J.; Moinpour, M. Mater. Res. Soc. Symp. Proc. 2006, 914.
21. Le, Q. T.; Baklanov, M. R.; Kesters, E.; Azioune, A.; Struyf, H.; Boullart, W.; Pireaux, J.;
Vanhaelemeersch, S. Electrochemical and Solid-State Letters 2005, 8, 7, F21-F24.
22. Bao, J.; Shi, H.; Huang, H.; Ho, H. Oxygen plasma damage to blanket and patterned
ultralow- surfaces. J. Vac. Sci. Technol. A 2010. 28, 2, 207-215
23. Bao, J.; Shi, H.; Liu, J.; Huang, H.; Ho, P. International Interconnect Technology
Conference, IEEE 2007.
118
CHAPTER 5
CONCLUSION AND FUTURE WORK
5.1 Conclusion
Graphene grown using the chemical vapor deposition (CVD) has special requirements for
formation in the desired way. It is necessary to have the right set-up in the CVD machine to
grow continuous uniform graphene. The pressure plays a significant role in terms of whether
graphene will form or not. Most of the graphene grown should be uniform between 3-10
monolayers with less than 5% defects for Cu substrate. If more than 5% defects or breaking is
observed that is means the system has to be checked and the parameters have to be reset. A Ni
substrate it will grow more graphene layers, up to approximately 50 monolayers. There are
many applications for CVD graphene. Some of the applications have been explored. For
example, as a gas detector graphene can be sensitive enough to detect one molecule on the
surface. Carbon monoxide and dioxide can be detected using the quartz crystal microbalance
(QCM) up to a sub-nona monolayer.
In the Bi work the catalytic effect of the Bi UPD on Ru shows promising features
regarding the ORR. The discrepancy in the reduction region between the charge and the mass
calculation of the monolayer is most likely a result of enhancing of the ORR on the surface. The
ambient plays important roles in increasing the current on the reduction region. The anion
effect also shows an effect on the adsorption of the Bi UPD combined under the N2 ambient.
119
Figure 5.1: CV and Mass response on the deposition of Bi UPD on Ru.
The fact that this discrepancy was observed on the UPD and not on the Bi bulk proves
that the Bi UPD on the Ru is the reason for the charge differences. Bulk Bi has some
characteristics leading to ORR. However, this could be because of the open structure deposition
or because of the Bi behavior, a question that should be investigated further in future work.
The Inter-layer dielectric (ILD) ultra-low-k work demonstrates that the XPS shows that a
plasma type does affect how much damage can be caused to the ILD. Moreover, there is a
change on the bond conformation which forms from the reaction of the plasma radical and the
material of the ILD. Some of this material attaches to the surface, and some is washed off. The
time dependent study showed that there are many complicated changes to the ILD, depending
-0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2
-80
-60
-40
-20
0
20
Current Density vs. Potential (5 mV/s) Delta Mass vs. Potential (5 mV/s)
Potential (V) (vs. Hg/Hg2SO4)
Curre
nt D
ensi
ty (u
A/cm
2 )
-0.20-0.15-0.10-0.050.000.050.100.150.200.250.300.350.400.450.500.550.600.650.700.750.80
Bi Monolayer (by M
ass)
120
on the time of the treatment; it also demonstrated the vulnerability of the surface to catch
moisture.
5.2 Future Work
Future work on graphene in our lab will be begin with growing graphene using Ni foils
and comparing the characteristics of the graphene grown on Cu and Ni for the same
experiment. For example, the hydrogenation of the graphene can be tested on both graphene
transferred from Cu and graphene transferred from Ni. Moreover, some electrochemistry
experiments can be carried out to demonstrate the effect of more monolayers on conductivity
and the catalytic effect. Some metal deposition can be tested to demonstrate the
reproducibility of the surface and the decoration system. Graphene shows good resistance to
metal corrosion, which can be explored widely with and without the inhibitors. Different types
of toxic gases, such as NOx, SOx, and carbon base gases, can be tested on the surface of the
graphene. Graphene can be transferred or grown on the substrate; it will be interesting to
distinguish between these two types with regard to the applications which were mentioned
previously. Furthermore, several materials can be tested for the growth of graphene on the
substrate directly, including metal. If graphene can be attached directly to the substrate without
any process of etching or dissolving, that will make it more reliable and effective. Graphene can
be used as a base in electric devices, which can be decorated with imbedded metals or
semiconductor materials.
More work can be carried out on questions regarding Bi deposition such as the effect of
the cations, and more testing can be done to verify the ORR behavior. Investigation of the
121
structure of Bi UPD and Bulk on Ru is another interesting area that may be important for
understanding this is. As has been mentioned before, Bi bulk shows an interesting behavior in
relation to ORR, and it may be useful to study Bi bulk on Ru using the RRDE system to clarify
that behavior. Because of the instability of the Bi deposited on Ru in the acid media to test the
ORR directly and accurately, a nafion cast can be used to hold the layer during the test.
The Inter-layer dielectric (ILD) ultra-low-k work is still in the beginning stages, and still
has long way to go. Changing the plasma gases such as CH4, N2 and CO2 treatment will be an
interesting area of exploration related to the blanket wafers. More investigation can be done on
the pattern wafers with the plasma gases, and the results can be compared to those with the
blanket. Wet reaction, such as HF, can be investigated with both kinds of wafers. XPS can be a
powerful tool to explore what is on the surface, but for the ILD change composition, FT-IR can
be more comprehensive. The direction of the beam in x-ray analysis of the sample on the XPS
can be critical to determining the intensity and even the type of bonds shown.
122
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