jim mara msc nanobio science thesis afm bias induced electrochemistry redox processes at the solid...

AFM Bias Induced Electrochemistry:

Redox Processes at the Solid Liquid Interface

by

Jim Mara B.Eng, CA, CTA, PDA

The thesis is submitted to University College Dublin

in part fulfilment of the requirements for the degree of

Master’s (MSc) NanoBio Science

School of Physics

UCD Conway Institute of

Biomolecular and Biomedical Research

Head of School: Prof. Padraig Dunne

Principal Supervisor: Dr. Brian Rodriguez

ii

Table of Contents 1 Abstract ......................................................................... 3

2 Abbreviations ................................................................ 4

3 Acknowledgment ........................................................... 5

4 Introduction .................................................................. 6

4.1 Atomic force microscope ............................................................................ 8

5 Materials...................................................................... 15

5.1 Highly Oriented Pyrolytic Graphite (HOPG) ............................................17

5.2 Silver Nitrate ............................................................................................. 23

6 Method ........................................................................ 26

7 Results ......................................................................... 28

8 Discussion ................................................................... 45

9 Conclusions ................................................................. 54

10 Future Work ................................................................ 57

11 References ................................................................... 59

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1 Abstract

Our experiments are aimed at investigating the nano-redox processes that occur in an

electrolyte between a negatively biased AFM tip and underlying substrate. We explore

the possibility of modifying substrates in a 0.01M AgNO3 electrolyte solution using an

Atomic Force Microscope (AFM) in contact mode. The voltage potential difference

between the AFM tip and substrate was pulsed in order to initiate redox processes at

the tip. All our work was geared towards gaining a better understanding and control

of nanofabrication methods in order to construct e.g. nano scale interconnects. The

motivation for conducting this work lies is in the field of nanowire based biosensors-

detection and quantification of biological and chemical species are critical to many

areas of health care and the life sciences

The approach we used to fabricate the nanostructures was contact based AFM voltage

induced electrochemical reduction of aqueous silver cations (‘Ag+’), accompanied by

sometime oxidative pit formation on primarily Highly Ordered Pyrolytic Graphite

(HOPG) substrates. We did this in a meniscus of aqueous silver nitrate. All substrates

were imaged using amplitude modulation-AFM (AM-AFM) pre and post bias in order

to confirm the presence of any nano-redox reaction. All biases were applied in AFM

contact mode.

We varied the duration and magnitude of the bias in order to quantify their effect on

the nano-redox process. We suggest reaction mechanisms for the nano-redox

processes studied, including the pit formation, and nano/micro deposits formed.

We conduct the majority of our experiments on HOPG, and study the effect of the

following experimental parameters: voltages between -5.0V to -3.75V; voltage pulse

durations between 2.5 to 30 seconds; AFM metal coated tips versus pure Si AFM tips,

and AFM bias contact forces between 4.5nN and 9.1nN. We see resulting Ag

deposition volumes of between 2,833.3 nm3 to 1,475,104.0 nm3. Our experiments also

produce sometime pit formation on the substrates of between 803.9 nm3 and

283,636.6 nm3 volume. Our experiments on gold substrates show similar trends but

at lower voltages and pulse durations.

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2 Abbreviations

Atomic Force Microscope (AFM)

Highly Ordered Pyrolytic Graphite (HOPG)

Nanowire (NW)

Scanning probe microscopy (SPM)

Amplitude Modulation-AFM (AM-AFM)

Noncontact Atomic Force Microscopy (NC-AFM)

Scanning Tunnelling Microscopy (STM)

Nano Electrochemical Lithography (EL)

Dip-pen nanolithography (DPN)

Full Width at Half Maximum (FWHM)

Overpotential Deposition (OPD)

Scanning probe microscopy (SPM)

Local anodic oxidation (LAO)

Electron beam lithography (EBL)

Nanoimprint lithography (NIL).

Electrochemical force microscopy (EcFM)

Electrochemical “dip-pen” nanolithography (E-DPN).

Transmission electron microscopy (TEM)

Scanning Electron Microscopy (SEM)

Chemical Vapour Deposition (CVD)

Silver Nitrate (AgNO3),

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3 Acknowledgment

I would like to express my gratitude to Dr. Brian Rodriguez for his patience and

insightful help over the course of this work.

I would like, in general, to thank all members of the UCD Nanoscale Function Group

who were always very generous with their time and knowledge, with special mention

for Craig Carville, Liam Collins and Bart Lukasz.

Also, my family and friends, especially Mam and Da, who always supported me even

when I least deserved it.

Finally, a word of mention for the unknown passer-by who smiled at me and looked

away at just the right moment.

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4 Introduction

The objective of our experiment was to explore the possibility of modifying surfaces

in an electrolyte solution using an Atomic Force Microscope (AFM) in contact mode.

The voltage potential difference between the AFM tip and substrate was pulsed in

order to initiate redox processes at the tip. All our work was geared towards gaining a

better understanding and control of, nanofabrication methods in order to construct

e.g. nano scale interconnects. Furthermore our aim was to fabricate the

nanostructures on a number of substrates namely: Highly Ordered Pyrolytic Graphite

(HOPG) and Gold.

The motivation for conducting this work lies is in the field of nanowire based

biosensors- detection and quantification of biological and chemical species are

critical to many areas of health care and the life sciences, from diagnosing disease to

the discovery and screening of new drug molecules. Central to detection is the

transduction of a signal associated with the selective recognition of a species of

interest.1 Nanostructures, such as nanowires and nanocrystals offer new and

sometimes unique opportunities to develop novel sensors. The diameters of these

nanostructures are comparable to those of the biological and chemical species being

sensed. Therefore, they represent excellent primary transducers for producing signals

that ultimately interface to macroscopic instruments. In particular, inorganic

nanowires (NWs) and nanocrystals exhibit highly reproducible electrical and optical

properties. The size-tuneable colours of semiconductor nanocrystals together with

their highly robust emission properties offer advantages over conventional organic

molecular dyes for labelling and optical-based detection of biological species. The

combination of tuneable conducting properties of semiconducting NWs and the

ability to bind analytes on their surface yields direct, label-free electrical readout,

which is exceptionally attractive for many applications.1

Figure 4.a (below) shows schematically the parts comprising a typical biosensor:

a) bio-receptors that specifically bind to the analyte; b) an interface architecture

where a specific biological event takes place and gives rise to a signal picked up by c)

the transducer element.

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The transducer signal (which could be anything from the in-coupling angle of a laser

beam to the current produced at an electrode) is converted to an electronic signal and

amplified by a detector circuit using the appropriate reference and sent for processing

by, e.g., d) computer software to be converted to a meaningful physical parameter

describing the process being investigated; finally, the resulting quantity has to be

presented through e) an interface to the human operator. Biosensors can be applied

to a large variety of samples including body fluids, food samples, cell cultures and can

be used to analyze environmental samples.2

Figure 4.a

Elements and selected components of a typical biosensor2

The approach we used to fabricate the nanostructures was AFM based voltage

induced electrochemical reduction of aqueous silver cations (‘Ag+’). We did this in a

meniscus of aqueous silver nitrate (AgNO3), on various substrates (see above) to form

solid silver nano structures on said substrate.

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4.1 Atomic force microscope

An AFM was originally a tool for imaging/studying samples: An AFM senses inter-

atomic forces occurring between a sharp probe tip and a sample surface to produce

images of sample surfaces such as ceramic materials, biological membranes, metals,

polymers, and semiconductors with subnanometer resolution. The images produced

are 3-D with resolution on the order of 0.1 to 1 nm. The AFM uses a microcantilever,

with a sharp probe tip on its lower surface, which is scanned over a sample surface.

Deflection of the cantilever, due to interatomic forces between the probe tip and the

sample, at each scan point is representative of the sample height. By plotting the

sample height versus the horizontal position of the probe, a 3-D image of the surface

can be obtained. The high image resolution of the AFM is due to the size of the probe

tip, which may be only a few atoms wide. This gives the AFM an advantage over

optical microscopes, which are limited by the wavelength of visible light, which is

approximately 400–700 nm.

Most commonly, the probe tip is dragged across the sample at a constant force, which

is referred to as contact mode imaging. Continuous lateral force on the sample from

the probe tip may cause damage to softer fragile samples. Tapping mode (amplitude

modulation) was developed to reduce lateral forces on such samples. A schematic

showing the typical instrumentation of an AFM is shown in Figure 4.b (below)3

For softer less stable samples, it is therefore very challenging to study their nano-

mechanical properties by applying tip forces directly to them.

For that reason, we used amplitude modulation-AFM (AM-AFM) for imaging the

substrates, which minimizes the lateral forces between the tip and the sample. Jim V.

Zoval et al17, note that for optimum imaging quality ‘noncontact atomic force

microscopy (NC-AFM) was employed in an attempt to overcome the noise problems

inherent to the Scanning Tunnelling Microscopy (STM) and repulsive mode AFM

imaging modes.’17

Previous work by other researchers has established that NC-AFM is superior to STM

or repulsive mode AFM for the investigation of many compliant surfaces and for

observing weakly adsorbed or laterally mobile molecular species.4

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Figure 4.b

Schematic of the instrumentation of an AFM.3

In AM-AFM the cantilever is oscillated at a drive frequency which is slightly lower

than the resonant frequency. As the lever approaches the sample, tip interactions

(either contact or non-contact) cause a change in the amplitude of oscillation. These

oscillation changes yield information about the sample topography, and the phase of

the oscillation gives information on the material. The attractive force between the

cantilever and the sample will cause higher amplitude, and the repulsive force will

cause lower amplitude. This mode is suitable for studying any silver nano-structure

we form as it results in a much lower lateral force on the sample than contact mode,

therefore less damage is done to the nanostructure.5

NC-AFM was employed in an attempt to overcome the noise problems inherent to the

STM and repulsive mode AFM imaging modes. Like the conventional repulsive mode

AFM, the operational principle of the NC-AFM (also called the “dynamic”, “attractive

mode”, and “ac” atomic force microscope) involves the detection and maintenance of

a small force which is exerted locally by the probe tip on the sample surface. In

contrast to the repulsive mode AFM experiment, however, the NC-AFM probe tip is

located in the attractive region of the tip-sample interaction potential and at greater

distances from the surface of 10-15 Å.

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The attractive tip-sample forces present in this separation regime are smaller than

those involved in repulsive mode AFM imaging (10-11-10-15 N vs 10-9 N), with the result

that the NC-AFM tip is a less perturbative probe of the surface topography. The

attractive force imparted by the tip to the sample is detected optically as a deflection

of the AFM cantilever toward the sample surface.17 Prior work has established that

NCAFM is superior to STM or repulsive mode AFM for the investigation of many

compliant surfaces6 and for observing weakly adsorbed or laterally mobile molecular

species.7

It was further recognized that scanning probe microscopy (SPM) allows not only

imaging, but also manipulation of matter. The tip-surface force interactions enabled

approaches such as molecular and atomic manipulation and nano-scratching, while

voltage control enabled polarization switching in ferroelectric materials and

electrochemical reaction based nano-oxidation. In recent years, much attention is

focused toward probing the local mechanism(s) of tip-induced processes. In these

processes, tip force- and bias-induced physical and chemical changes occur in

materials, and the simultaneously measured functional response provides

information on the thermodynamics and kinetics of the associated processes.8

Nano electrochemical lithography (EL) methods are based on the building of an

electrochemical cell, which, in the simplest configuration, consist of two conductive

surfaces (electrodes) separated by either a liquid or solid conductive phase

(electrolyte). By applying an appropriate bias, an electrochemical reaction, namely a

charge transfer process localized at the electrode/electrolyte interface, occurs. In

electrochemical lithography, the electrodes are, respectively, the substrate and a

conductive structure that could be the tip of a scanning probe microscope or the

protrusion of a stamp. For our work we used the conductive tip of the AFM. In

principle, electrochemical lithography offers a broad spectrum of possible

applications such as the local control of the reactions (electrochemical oxidation or

electrochemical reduction), to change the chemical nature of materials in confined

and specific places, local deposition of materials, and the fabrication of chemical

patterns. Moreover, unlike all of the other techniques for nanofabrication, in EL the

substrate can be directly exploited as the reactive layer.

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This capability allows us to overcome the concepts of bottom-up and top-down

nanofabrication and represents an example of the so-called “third way” for

nanofabrication.9

Yan Li et al reported in 2001 on a method focusing on electrochemical AFM “Dip-

Pen” Nanolithography. The “dip-pen” nanolithography (DPN) method uses an atomic

force microscope tip as a “nib” to directly deliver organic molecules onto suitable

substrate surfaces, such as Au.10

When an AFM is used in air to image a surface, the narrow gap between the tip and

surface behaves as a tiny capillary that condenses water from the air. “Dip-pen” AFM

lithography uses the water meniscus to transport organic molecules from tip to

surface. However, unlike in the previous AFM “dip-pen” methods where water is only

used as a solvent for the molecules, the researchers used this tiny water meniscus as a

nanometer-sized electrochemical cell in which metal salts could be dissolved, reduced

into metals electrochemically, and deposited on the surface, see Figure 4.c (below).

The DC voltage needed for metal deposition depends on the type of precursor salt and

the resistivity between the AFM tip and the surface.11

Figure 4.c

Schematic sketch of the E-DPN experimental setup.11

The above researchers, amongst others, endeavours and experimental techniques

informed our approach to fabricating nanostructures using an AFM on various

substrates. However there are a couple of peculiarities to our approach; unlike Yan

Li’s we are not using the AFM tip to deliver the silver cations into the electrochemical

cell: we are forming a meniscus of silver cations on the substrate, and in this way

delivering the reagents to the substrate and AFM tip site.

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Furthermore the general approach amongst researchers to date is to apply positive

voltages to the tip with respect to the substrate. Our approach is to apply a negative

bias to the tip and thus deliver the electrons via the tip. No matter the set up we are

still expecting nano redox reactions in the vicinity of the tip: between the biased tip

and the electrolyte and between the biased tip and substrate.

Nanometer-scale metal particles possess chemical and physical properties which

differ significantly from macroscopic metal phases. In recent years, the list of particle

size dependent properties has grown to include bond distances, the van der Waals

attractive force operating between particles, the surface plasmon resonance, the

melting point, the standard electrode potential, and the photoelectric yield. One or

more of these properties becomes size-dependent for metal particles having

dimensions below a critical threshold which is in the range from 2 to 10 nm,

depending on the particular property and metal considered.17

Janousek et al researched AFM local anodic oxidation on graphene12. Graphene is an

extensively studied material due to its remarkable electronic and mechanical

properties. Local anodic oxidation (LAO) by means of AFM with a conductive tip is

the method used by the researchers for device patterning in the nanometer scale at

laboratory conditions. It represents a clean alternative to up-to-date nano-

lithographic techniques such as electron beam lithography (EBL) and nanoimprint

lithography (NIL). Oxidation reaction between the tip and a conductive substrate

creates an oxide line, which acts as a potential barrier. This enabled the researchers

to make narrow constrictions of 20 nm in width.

They did this by applying LAO on single-layer graphene chemical vapour deposition

(CVD) grown on copper foil and graphene grown on SiC prepared by high

temperature annealing. Under optimum conditions for LAO the graphene oxide is

created. The thickness of the oxide line depends on various parameters e.g. a tip bias,

speed of the tip and ambient relative humidity. By applying negative bias to the

conductive AFM tip with respect to the conductive substrate, dissociated OH− ions

oxidize the substrate and thus create a pattern. The quality of oxide lines depends on

parameters as bias voltage applied to the tip, velocity of the tip, relative humidity and

set-point.

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The main difference between this work and the work presented in this thesis is that

we are utilizing a reduction reaction at the tip substrate interface as follows:

Ag+ + e- = Ag reduction of Ag+ and nucleation at the tip/substrate interface.

Janousek et al local anodic oxidation reaction on graphene12:

H2O + e- = OH- +H

OH- + Cx = CxO + H- oxidation of the Graphene substrate

The researchers analyzed the properties of LAO oxide lines for different negative

voltages applied on the tip from -3.5 V to -7 V with respect to the sample (Figure 4.d

below). The visible oxidation started at the bias -4 V. Note the authors did not

provide details of the AFM tip they were using which is critical to predicting the

electric field between the tip and substrate.

Figure 4.d

Influence of tip bias from -3.5 to -7.0 V. For bias voltage -3.5 V, the oxidation process

is not observable. Height profiles in the two sections are at the bottom. Tip velocity was

100 nm/s. (B) Oxide lines for different tip velocity from 25 to 2000 nm/s. Height

profiles in the two sections are at the bottom. Tip bias was -6 V12

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Further research in this area was conducted by Sergei Kalinin et al and in their 2014

paper13, they report on bias dependent mechanisms of irreversible cathodic and

anodic processes on a pure CeO2 film studied using a modified AFM. For a moderate

positive bias applied to the AFM tip an irreversible electrochemical reduction

reaction is found, associated with significant local volume expansion. Simultaneous

detection of tip height and current allows the onset of conductivity and the

electrochemical charge transfer process to be separated, further elucidating the

reaction mechanism. The researchers’ observations suggest that the desired

electrochemical reaction proceeds only after the surface activation energy has been

surpassed. For a tip positive bias the summarized observation are:

(a) At sufficiently low biases, the surface is reduced

(b) The onset of the electronic conduction reverses the electrochemical process

direction

(c) The process is strongly mediated by the water layer on the surface.

Further the researchers state that the detected current represents a sum of two

contributions, namely Faradaic current (electrical current mediated by ions as

opposed to electrical current purely mediated by electron movement) of

electrochemical reaction (changes in material below the tip) and, conductive

electronic and ionic currents. Here, the conductive electronic current is classical

electronic transport from the bottom electrode to the tip through the material. The

ionic current is associated with the electrochemical process on the bottom electrode,

ionic transport through the material, and the electrochemical process on the top

electrode without affecting the material.13

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5 Materials

Cantilevers:

HQ:DPE-XSC11 tip B14

Conductive Pt-coated tip (HQ:DPE-XSC11 tip B), manufacturer Nanoandmore.

The coating thickness is increased on this cantilever, which gives more freedom for

using it in contact electrical modes. The conducting Pt coating covers the entire

Silicon chip, cantilevers and tips. It provides high conductivity and enhances the laser

reflectivity.

Manufacturers’ Specifications:

Tip Shape Tip Height Tip Radius Full Cone Angle

Rotated (12-18 µm) <40 nm 40°

Cantilever

Shape Length Width Thickness Force Const. Res. Freq.

Beam 210 µm 30 µm 2.7 µm 1.1 - 5.6 N/m 60 - 100 kHz

Calibrated Values: 447.60 pN/nm

The tip used (tip B) was suitable for imaging the deposition because it is a relatively

soft cantilever and very responsive to forces encountered- as a result the agitation of

any Ag nucleation is minimized under the cantilever pressure. Also, the tips used had

a high resonant frequency which is needed for taping mode AFM.

Figure 5.a

SEM image of the DPE silicon tip14

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Si tip NanoWorld SSS-NCH AUD15

We also conducted testing with a used Si doped tip with no reflex coating

(NanoWorld SSS-NCH AUD), Veeco model.

Manufacturers’ Specifications:

Shape:

Polygon based pyramid

Tip Height Tip Radius Full Cone Angle

10 - 15 µm 2 nm 20°

Length Width Thickness Force Const. Res. Freq.

120 – 130µm 25 – 35µm 3.5 - 4.5µm 21 – 78N/m 250 – 390kHz

NanoWorld Pointprobe® NCH probes are designed for non-contact or tapping mode

imaging. This probe type combines high operation stability with outstanding

sensitivity and fast scanning ability.

The probe is made from monolithic silicon which is highly doped to dissipate static

charge. They are chemically inert and offer a high mechanical Q-factor for high

sensitivity.

The purpose of using Si probes was so that we could juxtapose results obtained from

conductive metal coated tips with those results obtained from conductive non-metal

coated tips. This was in order to determine if the tips metal coating dissolved under

the applied bias and was thus the source of our ‘deposition’, see Discussion 8 (below)

for further discussion on this point. An indication of tip deterioration during bias

application is a reduction in the intensity of the reflected laser into the photodiode (a

reduced SUM, or changes in the resonance frequency or tip deflection figure).

AFM used: MFP3D (Asylum Research)

Bias pulses were generated using a custom code developed in Igor Pro

(Wavetrics) and was used to control the bias output of the AFM controller.

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Substrates:

HOPG:

Windsor Scientific HOPG/ZYH/DS/2-1, 10mm x 10mm

Agar Scientific, HOPG 3.5 +/- 1.5 Mosaic Spread 10mm x 10mm x 2mm

Gold Substrate, Arrandee 11x11mm metal coated substrates

BioForce NanoSciences UV/Ozone ProCleaner used for tip and Gold

irradiation.

Silver nitrate solution volumetric, 0.01M AgNO3 (0.01N), Sigma-Aldrich

manufacturers.

Oscilloscope- Hamey Instruments, 50mHz, Analogue oscilloscope, HM 504-2

Lens Paper- Fischerbrand lens cleaning tissue 80 x 100mm, Fischer Scientific

Wicking Paper- Filter paper ‘Whatman’ Circular (90mm)

5.1 Highly Oriented Pyrolytic Graphite (HOPG)

We conducted the vast majority (81%) of our AFM electrochemical deposition

experiments on HOPG. HOPG is an allotrope of carbon and part of the graphite

family.

"Usual" graphite, especially natural one, exhibits quite imperfect structure due to an

abundance of defects and inclusions. A number of technologies have developed for

the preparation of perfect graphite samples to take advantage of its unique structure.

Of these, pyrolysis of organic compounds is the most common and effective. Pyrolytic

graphite is a graphite material with a high degree of preferred crystallographic

orientation of the c-axes perpendicular to the surface of the substrate.

It is manufactured by graphitization heat treatment of pyrolytic carbon or by

chemical vapour deposition at temperatures above 2500K.16

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Hot working of pyrolytic graphite by annealing under compressive stress at

approximately 3300K results in HOPG. Thus HOPG is a highly-ordered form of high-

purity pyrolytic graphite (impurity level is of the order of 10 ppm ash or better).16

HOPG is characterized by the highest degree of three-dimensional ordering. The

density, parameters of the crystal lattice, preferable orientation in a plane (0001) and

anisotropy of the physical properties of the HOPG are close to those for natural

graphite mineral. In particular, like mica, HOPG belongs to lamellar materials

because its crystal structure is characterized by an arrangement of carbon atoms in

stacked parallel layers – this two-dimensional and single-atom thick form of carbon

is called graphene. Graphite structure can be described as an alternate succession of

these identical staked planes. Carbon atoms within a single plane interact far more

strongly than with those from adjacent planes- this explains characteristic cleaving

behaviour of graphite.16

Furthermore, graphene is a planar, hexagonal arrangement of carbon atoms. The

lattice of graphene consists of two equivalent interpenetrating triangular carbon sub-

lattices A and B, see Figure 5.b (below). Each one contains one half of the carbon

atoms. Each atom within a single plane has three nearest neighbours: the sites of one

sub-lattice (A – marked by red) are at the centres of triangles defined by three nearest

neighbours of the other one (B – marked by blue).16

The lattice of graphene thus has two carbon atoms, designated A and B, per unit cell,

and is invariant under 120° rotation around any lattice site. The network of carbon

atoms is connected by the shortest bonds in a honeycomb like shape. However in

bulk HOPG, even in bi-layer graphene, A- and B-sites carbon atoms become in-

equivalent (including those on the surface): two coupled hexagonal lattices on the

neighbour graphene sheets are arranged according to Bernal ABAB stacking, where

every A-type atom in the upper (surface) layer is located directly above an A-type

atom in the adjacent lower layer, whereas B-type atoms do not lie directly below or

above an atom in the other layer, but sit over a void – a centre of a hexagon. Figure

5.b illustrates the assumed non-equivalent types of carbon atoms.16

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Thus in each layer the atoms form a grid of correct hexagons with distances between

atoms equal 0.1415 nm. The distance between layers is equal 0.3354 nm which results

in a theoretical calculated value of density ρ = 2.265 g/cm3.16

HOPG terminated with graphene layer is an excellent tool for using in scanning probe

microscopy as a substrate- this is an easily renewable material with an extremely

smooth surface. This is vital for any SPM measurements that require uniform, flat,

and clean substrates, for samples where elemental analysis is to be done.

Figure 5.b

Schematic representation of the structure of the bulk hexagonal graphite crystal. The

dashed lines show the axes of bulk unit cell. Side insets: top view of the basal plane

of graphite and schematic representation of the surface structure (carbon atoms) of

graphite most viewed by SPM, where every other atom is enhanced (right-side inset)

and viewed under ideal conditions, where every single atom is seen (left-side inset).16

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Similar to mica, HOPG specimens are layered polycrystals. Each bulk polycrystal

looks like mosaic of microscopic mono-crystal grains of different sizes. The structure

is columnar, the columns run vertically within the flat slab of the material, and the

grain boundaries can be seen on the lateral surfaces. The grains are slightly

disoriented with respect to each other. An angular spread of the c-axes of the

crystallites is of the order of 1 degree. The surface of specimen consists of many

randomly placed steps – result of the cleaving process: single atomic steps and steps

of several or dozens of atomic layers.16

Although the heights of multilayer hills and valleys are not calibrated, single steps

have the well defined height of 0.34 nm. To characterize the angle of deviation of the

grain's boundaries from the perpendicular axis of the columnar structure, a measure

of the parallelism of grains – perfectness of HOPG samples, a "mosaic spread" term is

used. The lower the mosaic spread, the more highly ordered the HOPG. The term

originates from X-ray crystallography.16

The disordering results in broadening of the (002) diffraction peak: the more

disordering, the wider the peak. Therefore, ‘perfectness’ of HOPG can be easily

related to a Full Width at Half Maximum (FWHM) of the Cu-Ka rocking curve

(radiation peak) measured in degrees – "mosaic spread angle". Thus, the smaller this

angle, the higher the quality of HOPG. The size of grains also varies with the mosaic

spread. The lower mosaic spread results in a freshly cleaved surface that exhibits the

smaller number of the steps due to the bigger size of grains. The higher the quality is-

the less the roughness of the surface. The lower level grade material is also more

"cleavable" allowing the bigger number of cleavings per sample.

All the other physical characteristics of graphite, including atom-to-atom distance,

that is an atomic property of carbon, are independent of its grade and remain the

same for all types of HOPG. Due to the anisotropic nature of HOPG such

characteristics as thermal conductivity and electrical resistivity are different in

different directions: along the basal plane and along the principal axis c

(perpendicular to the basal plane).16

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HOPG is a highly stable material. It remains stable at the temperatures up to 500°C

in air and up to two-three thousand degrees Celsius in a vacuum or inert

environment. It exhibits high chemical inertness to just about everything.16

Zoval, Jim V., et al also report that the graphite basal plane surface is

electrochemically ‘very inert’17. Furthermore the same group go on to outline how

silver nanocrystallites interact weakly with the graphite surface and are removed by

the sweeping action of the AFM probe tip from the imaging area.17 This effect has

been previously documented for gold particles on graphite by Schaefer et al.18

Silver micro and nanocrystallites which nucleate at defect sites are observed by STM

and AFM, and, it has been concluded that silver overpotential Deposition (OPD) on

graphite is initiated by nucleation exclusively at defects, such as step edges, on the

graphite surface. Further they report that the NC-AFM data presented for low-defect

density surfaces such as the graphite basal plane, STM and repulsive-mode AFM data

can provide a misleading view of nucleation by “ignoring” the presence of weakly

adsorbed metal nanocrystallites which are not associated with defects.17

It is worth bearing in mind that these researchers experiments were conducted within

the following parameters: voltage pulses having amplitudes of 100, 250, and 500 mV

vs. Ag0 and durations of 10 or 50 ms were applied to graphite surfaces immersed in

dilute (≈1.0 mM) aqueous silver nitrate. Also, the potentiostatic deposition of silver

was accomplished by using a silver wire reference electrode immersed directly in the

silver plating solution. In contrast our experiments were conducted in a meniscus of

AgNO3 using conductive AFM tips at higher voltage and longer durations (see Results

7 below for details).17

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Figure 5.c (below) demonstrates Zoval, Jim V., et al’s experimental set up.

Figure 5.c

Schematic diagram of the instrument employed by Zoval, Jim V., et al for pulsed

potentiostatic deposition of silver nanocrystallites.17

Zoval, Jim V., et al go on to note that the capacitance of a graphite basal plane surface

was 1.70 µF cm-2, which is in the normal range. Further in successive silver deposition

trials in which the graphite surface was cleaved prior to each experiment, the

apparent capacitance of the surface fluctuated by 10-20%, presumably due to

fluctuations in the defectiveness of the graphite surface which is exposed during

cleavage.17 This observation can have important implications for the consistency of

results in our experiments.

Further they highlight ‘isolated silver nuclei were never observed on atomically

smooth regions of the graphite surface in any of these experiments’.

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5.2 Silver Nitrate

It is worth remembering that humans have been experimenting with depositing silver

ions for generations and it is thus an ideal system to be studied on the nanoscale

using AFM electrochemical techniques. Indeed any macroscopic deposition must, at

least transiently, by obtained by nanoscopic deposition en route to the macro scale.17

Digital photography now replaces most of the chemical and physical applications that

chemical photography employed over its relatively short history of about 170 years.

The colour and form of a digital image after capture with a camera can be

manipulated through Photoshop, allowing a photographer an exacting control over

the finished image, with little if nothing left to chance and with the ability to

reproduce numerous identical copies.

In the early development of chemical photography, individual images were more

unique in their nature, as a silver image produces something that is far more difficult

to control and exhibits a more random nature from chemical reaction. This is

particularly the case when the print maker is controlling the chemistry and exposure.

Early photographic chemistry can exhibit enormous variations in reaction to colour,

sensitivity and stability, making the visual outcome unique and often unrepeatable.

By experimenting with some of these more antiquated processes, it is possible to

produce these on paper without the need for a suspension medium such as gelatine.

Thus, early efforts revolved around the use of chemistry as a printmaking medium

and together with others, such as ink and graphite. The development of digital

photography has therefore helped establish this chemical art form in its rightful place

within the context of printmaking; see Figure 5.d (below) for an early example of

early silver ion chemical photography.

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Figure 5.d

Early print making using silver ions.

To explore the fascinating possibilities, it is necessary to look back at the dawn of

photography and some of the experiments made by practitioners working at that

time. Robert Hunt who experimented with light sensitive substances, in his

“Researches on Light in its chemical reactions” published in 1844, points out many

interesting reactions of substances with light, including charcoal:

“If a stick of charcoal is placed in a bottle in which is some solution of nitrate of

silver, so that one half of the charcoal is in the solution, and the other half above it,

there will in a short time appear little spangles of silver upon the upper portion of

the charcoal, if it is exposed to diffused light. In full sunshine the effect is greatly

retarded. If the bottle is placed in a dimly illuminated place, there will in the course

of a few weeks, form in the solution around the charcoal, a series of the most

delicate thread-like crystallisations of the silver. After these have formed, if the

bottle is exposed to sunshine they are gradually re-dissolved into the fluid.”

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There is still much to be learnt by looking in depth at the work of these experimenters

and reactions they dismissed at that time, which did not follow the objective of

achieving a practical working photographic process. As these may now prove useful

when employed in combination with 21st century technology, such as scanning and

flash photography, to record precise moments of chemical reaction and

decomposition. Let us bear in mind that technological development and in this case

chemical discoveries, are not always in step with chronological progression.

Fox Talbot who developed the first real practical form of photography, employing a

negative and positive, used the word Calotype to describe his process, which is

derived from the Greek word Kalos, meaning beautiful. Looking at some of the early

examples, beautiful is surely a very apt word.

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6 Method

1. We fixed the HOPG substrate to a copper supporting plate using conductive silver

paint, and further soldered a conductive wire to the copper plate to facilitate

attaching of a crocodile clip. The crocodile clip was then routed through the AFM

BNC to ground the circuit.

2. We cleaved the HOPG with adhesive tape, and then pipette 100μL AgNO3 onto the

HOPG.

3. We then wicked away the AgNO3 leaving only a meniscus of AgNO3 on the

substrate, taking care not to touch the substrate surface with the wicking paper if

at all possible.

4. We absolutely minimized the time between pipetteing the AgNO3 onto the

substrate and eventual bias application. This is because the AgNO3 is

photosensitive (see materials 5.2 above). Similarly the AFM light was switched off

during bias and image acquisition. This is to reduce any static noise interference

from the AFM light.

5. All AFM tips were UV irradiated (using a UV/Ozone ProCleaner) for 10 minutes

prior to loading into the AFM cantilever holder- this was to clean the tips.

6. We first AC mode imaged the substrate surface (for subsequent juxtaposition

with post bias images).

7. In order to determine if there was any deposition, we AC mode imaged the

substrate (per and post bias).We applied small tip-substrate forces when imaging

and applying bias in order to avoid unnecessary agitation of any nucleated

particles. We did this by using small amplitudes, and small set points. It is

important to only use topographic images of the substrate for confirmation

purposes, as other images such as ‘phase’ are not a true reflection of any

deposition. See Results 7 (below) for details of set points used.

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8. All biases were applied using custom code for the MFP3D. The biases were

applied in a grid fashion (see Figure 7.dd below) at predetermined sites, for set

durations at specified voltages. The voltage was applied so that the AFM tip was

at a negative bias with respect to the substrate which was grounded. In order to

apply a negative bias to the AFM tip we attached a conductive wire underneath

the cantilever holder and this wire was then routed through a BNC output

channel, thus enabling us to apply a specified voltage to the tip. We applied all

biases in contact mode with the surface. The experiment was carried out in

ambient air with temperature of ≈ 298 +/- 2K.

9. For any experiments that we conducted using a gold substrate (Arrandee gold

11 x 11 mm), we prepared the surface as follows- Ozone irradiate for 10 minutes,

rinse in isopropanol, ethanol and then polish with lens paper (Fisherbrand lens

cleaning tissue range 0960F00021, not with Kim wipes- as these scratch the

substrate) , Milli-Q water and then desiccate using a dry nitrogen gun.

10. We also calibrated the cantilever using the Thermal method (in order to ascertain

the spring constant) and by performing a force distance curve on glass to get the

Invols (inverse optical lever sensitivity) of the cantilever.

11. All experiments were performed in air at room temperature. See Results 7

(below) for details of voltages applied, bias duration and set points used in the

different experiments.

12. AFM topography images were processed using Gwyddion (64 bit) freeware,

Nanotec Electrónica WSxM freeware, and Igor Pro software.

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7 Results

The nano-deposits generated during the contact bias application are shown below.

Figure 7.a (below) shows Ag deposition (bright silver feature in upper left) on the

HOPG surface following an applied bias of -3.75V for 5.5 seconds using a set point of

0.6V which corresponds to a tip-surface force of 6.8nN. The volume of this Ag

deposition is 1,475,104.0 nm3.

Figure 7.b (below) shows the cross section profile of line 1 from Figure 7.a.

Figure 7.c (below) shows the same surface prior to any bias in a meniscus of AgNO3

for comparison purposes. The step edges and basal planes of the freshly cleaved

HOPG are clearly visible.

There is no evidence of any pit associated with this deposition.

Figure 7.a

Ag deposition on HOPG after an applied bias of -3.75V for 5.5 seconds in contact

mode.

The deposition is non symmetric, and is not associated with a step edge, and is

generally conical in shape being wider at the base.

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Figure 7.b

Cross-sectional profile of deposition for an applied bias of -3.75V for

5.5 seconds.

Figure 7.c

Surface of HOPG prior to any bias being applied. Image acquired in AM-AFM mode

Figure 7.d below shows Ag deposition on HOPG in a meniscus of AgNO3. The applied

voltage was -3.85V for 3.5seconds in contact mode with a set point of 0.8V (9.1nN

tip-HOPG contact force). The larger deposition (bottom middle) volume is

199,556.3nm3., the smaller deposition volume is 20,194.0nm3, total deposition of

219,750.3nm3;.

Figure 7.f shows the same surface of HOPG prior to any bias being applied. Image

acquired in AM-AFM mode, set point 0.8V

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Figure 7.d

Ag deposition on HOPG, bias applied -3.85V for 3.5 seconds in contact mode. Image

acquired in AM-AFM

Figure 7.e

Cross-sectional profile of deposition for an applied bias of -3.85V for 3.5 seconds.

Again the deposition is conical in shape and not closely associated with a step edge.

The deposition volume is markedly reduced from that obtained in Figure 7.a above.

The deposition in Figure 7.a was done at a higher voltage and for a longer duration.

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Figure 7.f

Surface of HOPG prior to any bias being applied. Image acquired in AM-AFM mode

Figure 7.g (below) shows Ag deposition on HOPG in a meniscus of AgNO3. The

applied voltage was -3.90V for 2.5 seconds in contact mode with a set point of 0.8V

(9.1nN tip-HOPG contact force).The deposition volume is 687,901.2 nm3; the smaller

deposition volume is 1.2 nm3.

Figure 7.g


acquired in AM-AFM mode.

The bias was applied in contact mode at a single location (just below the beginning of

the finger of deposition). This finger of deposition was in contrast to the localised

deposition of Figure 7.a & Figure 7.b (above). It is possible the finger of deposition

was originally localised deposition that was spread out by the action of the AM-AFM

tip motion.

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Figure 7.h

Cross-sectional profile of deposition from Figure 7.g for an applied bias of -3.90V for

2.5 seconds.

Figure 7.i (below) shows Ag deposition on HOPG in a meniscus of AgNO3. The


(4.5nN tip-HOPG contact force).The deposition volume is 113,187.1 nm3.

Figure 7.i


acquired in AM-AFM mode, set point 0.4V, free air amplitude 0.50V.

Again a finger like deposition is observed in contrast to localised deposition, even

though the bias was applied at a single spot.

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Figure 7.j

Cross-sectional profile of deposition for an applied bias of -3.85V for 3.0

seconds.

Figure 7.k

Surface of HOPG shown in Figure 7.i prior to any bias being applied. Image


Figure 7.l (below) shows Ag deposition on HOPG in a meniscus of AgNO3. The


(4.5nN tip-HOPG contact force).The deposition volume is 72,617.3 nm3; the volume

of the pit is 200,864.4nm3.

Figure 7.l

Ag deposition on HOPG Bias applied -3.80V for 3.0 seconds in contact mode. Image

acquired in AM-AFM mode, set point 0.4V.

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It is clear from Figure 7.l above and Figure 7.m below that any deposition is now

accompanied by the formation of a pit. The deposition volume is 36.2% of the pit

volume.

Figure 7.m

Cross-sectional profile of deposition shown in Figure 7.l for an applied bias of -

3.80V for 3.0 seconds.

Figure 7.n

Surface of HOPG shown in Figure 7.l prior to any bias being applied. Image


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Figure 7.o (below) shows another site of Ag deposition on HOPG in a meniscus of

AgNO3. The applied voltage was -3.80V for 3.0 seconds in contact mode with a set

point of 0.4V (4.5nN tip-HOPG contact force).The deposition volume is 69,235.7nm3;

the volume of the pit is 283,636.6 nm3. The deposition volume is 24.4% of the pit

volume.

Figure 7.o



Figure 7.p

Cross-sectional profile of deposition and pit shown in Figure 7.o for an applied bias

of -3.80V for 3.0 seconds.

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Figure 7.q (below) shows another site of Ag deposition on HOPG in a meniscus of

AgNO3. The applied voltage was -3.80V for 3.0 seconds in contact mode with a set

point of 0.4V (4.5nN tip-HOPG contact force). The deposition volume is

44,2983.4nm3; there is no associated pit, and the deposition is not along a step edge.

Figure 7.q



Figure 7.r

Cross-sectional profile of deposition shown in Figure 7.q for an applied bias of -

3.80V for 3.0 seconds.

Figure 7.s (below) shows Ag deposition on HOPG in a meniscus of AgNO3 using a Si

tip. The applied voltage was -4.5V for 4.5 seconds in contact mode with a set point of

0.39V. The deposition volume is 6,865.2 nm3; with the associated pit volume being

30,868.9nm3. The deposition volume is 22.2% of the pit volume.

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Figure 7.s


acquired in AM-AFM mode, set point 0.39V, free air amplitude 0.49337V, using a Si

tip.

Figure 7.t

Cross-sectional profile of deposition shown in Figure 7.s for an applied bias of -

4.50V for 4.5 seconds using a Si tip.

Figure 7.u

Surface of HOPG shown in Figure 7.s prior to any bias being applied. Image


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Figure 7.v (below) below shows Ag deposition on HOPG in a meniscus of AgNO3

using a Si tip. The applied voltage was -5.0V for 3.0 seconds in contact mode with a

set point of 0.39V. The deposition volume is 671,390.0 nm3; with the associated pit

volume being 8,257.2 nm3. The deposition volume is 8,131% per the pit volume.

Figure 7.v



Figure 7.w

Cross-sectional profile of deposition shown in Figure 7.v for an applied bias of -5.0V

for 5.0 seconds using a Si tip.

Figure 7.x (below) shows Ag deposition on HOPG in a meniscus of AgNO3 using a Si



803.9 nm3. The deposition volume is 455% per the pit volume.

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In addition to the bias site deposition decorating the atomically-smooth regions of

the graphite surface, it is clear from the AM-AFM image shown in Figure 7.x (below)

that nanoscopic silver particles/ribbons are also appearing along step edges on the

graphite surfaces. This deposition could be triggered by the photosensitive nature of

the AgNO3, see 5.2 (above) and or/coupled with the higher conductivity of the step

edges compared to basal planes (see above).

Figure 7.x


acquired in AC mode, set point 0.4V.

Figure 7.y

Cross-sectional profile of deposition shown in Figure 7.x for an applied bias of -


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Figure 7.z (below) shows Ag deposition on HOPG in a meniscus of AgNO3 using a Si



26,416.8nm3; the deposition volume being 974% per the pit volume.

Figure 7.z



Figure 7.aa

Cross sectional profile of deposition shown in Figure 7.z for an applied bias of -5.0V

for 4.0 seconds using a Si tip

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Figure 7.bb (below) shows further Ag deposition on HOPG in a meniscus of AgNO3


set point of 0.4V. The deposition volume is 41,283.7 nm3; with the associated pit

volume being 37,044.1 nm3; the deposition volume being 111% per the pit volume.

Figure 7.bb



Figure 7.cc

Cross-sectional profile of deposition shown in Figure 7.bb for an applied bias of -


Figure 7.dd (below) shows the typical grid scenario that we employed for bias site

selection for testing the bias/duration/nucleation parameters.

Figure 7.dd

Typical grid scenario used for testing the bias/duration/nucleation

parameters. The Nucleation shown corresponds to Figure 7.o (above)

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Figure 7.ee (below) shows Ag deposition on a gold substrate in a meniscus of AgNO3


set point of 0.39V. The deposition volume is 9.25 x 10-2 m3; with no associated pit.

Figure 7.ee

Ag deposition on Gold, Bias applied -2.5V for 3.0 seconds in contact mode. Image


Figure 7.ff

Cross-sectional profile of deposition shown in Figure 7.ee for an applied bias of -


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HOPG Substrate Results

Experiment

No.

Tip Voltage, V Duration, s Set Point, V Tip Force,

nN

Deposit

Volume, nm3

Pitt Volume, nm3

1 HQ:DPE -5.00 30.00 0.8 9.1 88,539.1 119,192.8

2 HQ:DPE -3.75 5.50 0.6 6.8 1,475,104.0 n/a

3 HQ:DPE -3.80 5.50 0.65 7.4 2,833.3 n/a

4 HQ:DPE -3.85 5.5 0.64 7.2 205,089.6 n/a

5 HQ:DPE -3.85 3.5 0.80 9.1 219,750.3 n/a

6 HQ:DPE -3.85 3.5 0.80 9.1 135,985.9 n/a

7 HQ:DPE -3.90 2.5 0.80 9.1 687,902.4 n/a

8 HQ:DPE -3.82 3.4 0.40 4.5 27,038.2 n/a

9 HQ:DPE -3.85 3.0 0.40 4.5 113,187.1 n/a

10 HQ:DPE -3.80 3.0 0.40 4.5 72,617.3 200,864.4

11 HQ:DPE -3.80 3.0 0.40 4.5 69,235.7 283,636.6

12 HQ:DPE -3.80 3.0 0.40 4.5 44,298.4 n/a

13 Si, SSS -4.5 4.5 0.39 * 6,865.2 30,868.9

14 Si, SSS -5.0 3.0 0.39 * 671,390.0 8,257.2

15 Si, SSS -5.0 3.5 0.40 * 3,658.6 803.9

16 Si, SSS -5.0 4.0 0.40 * 257,457.2 26,416.8

17 Si, SSS -5.0 4.5 0.40 * 41,283.7 37,044.1

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Gold Substrate Results

Experiment

No.

Tip Voltage, V Duration, s Set Point, V Tip Force,

nN

Deposit

Volume, nm3

Pitt Volume, nm3

18 Si, SSS -2.5 3.00 0.39 * 92,529,623.7 n/a

19 HQ:DPE -1.0 1.50 0.47 5.3 7,208,942.9 23,294.6

20 HQ:DPE -1.25 0.75 0.36 4.1 3,219,836.0 n/a

21 HQ:DPE -1.0 1.75 0.066 0.7 3,704,274.9 n/a

* Tip Force not calculated, this is because the Si tips Invols data was not collected at the time of experiment. The Si tips are far stiffer than the HQ:DPE tips, (see

Materials 5 (above)) so the tip-substrate force will be far higher for experiments using Si tips than HQ:DPE tips.

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8 Discussion

Jiang, Yan et al in their 2008 paper19 investigate convex and concave nanodots they

created on HOPG in ambient air by applying a voltage pulse between a metal-coated

AFM tip and the sample surface. Using a linear scan with a positive substrate bias,

nanoscale lines were also etched on the HOPG surface. Depending on the amplitude

and duration of the voltage pulse, the nanostructures were either convex or concave.

The depth of the concave structure sharply increased with the amplitude and

duration of the voltage pulse, while the height of the convexity stayed at a low level

and varied in a small range with the voltage lower than a threshold value. Under

negative substrate bias or in a vacuum, no change occurred on the HOPG surface in

the experimental range.

The formation of the nanostructures was ascribed by the authors to the primary

dissociative adsorption of water and oxygen in air induced by the intensive hole

concentration and the subsequent defect-assisted oxidation of graphite (with the

proviso that in our experiments we also have additional dissociated NO3- ions

available to play a role).24 The external electric field can induce a reaction of the

carbon surface with absorbed gases, which has been used in the fabrication of carbon-

based nanostructures20. Pits with minimum diameter down to 2 nm were produced by

applying positive voltage pulses of 3–8 V to the HOPG for 10–100 μs (note the

substrate is positively biased with respect to the tip, so electrons are flowing from the

tip to substrate which mirrors our experimental set up).21 When the HOPG and the tip

are immersed in liquid, convex structures on the HOPG surface can be created by

using electrical methods22

Jiang, Yan et al19 report that both concave and convex topography nanostructures

were produced in ambient air. The convex structures were created on the surface at

low electric voltage or short pulse duration. On increasing the amplitude or duration

of the voltage, ‘the convex profile will convert into concave morphology’.

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The authors propose that the convex profiles are attributed to concentrated holes

inducing water and oxygen dissociative adsorption onto the graphite layers while the

concave ones are formed by defect-assisted oxidation of the carbon layers under

enhanced electric field.24 We see evidence of pit formation in our experiments

(experiment no. 1, 10, 11, 13 through 17, & 19), which probably follow the mechanism

as outlined by Jiang, Yan et al. When pulses with short duration (<=1,000 ms) were

applied, nanodots with convex profiles were formed, However, where the voltage

pulses with longer durations were applied, concave dots were produced. The depth of

nanodots formed increased apparently with the duration. It is shown that the pulse

duration has a strong influence on the formation of the nanostructures.

When lower voltage pulses were applied for the same duration of 10s, the

nanostructures were not observed at the corresponding sites. This indicates that the

formation of nanodots occurs only at voltages larger than a threshold value.24 In

Jiang, Yan et al’s case, the threshold voltage is estimated to be in the range of 5–6 V,

and is dependent on the etching speed and air humidity.20Convex etching lines were

formed at the voltage lower than a specific threshold. On increasing the voltage over

the threshold, the etching lines became concave. It is dependent on the etching speed

and air humidity.20

The authors go on to suggest that the reaction between the water or oxygen and the

carbon layers most likely causes the formation of the nanostructures. The authors

further go on to outline the mechanism for nanodot and nanopit formation: under

the applied voltage one of the oxygen-free23 electron pairs of a water molecule close to

the HOPG surface can interact with the hole and a C–O bond is formed. The

formation of C–O bonds may cause the carbon lattice to produce a strain of the

topmost layer of HOPG and form a protrusion. This can be described as the first

stage. At the following stage, on increasing the amplitude or duration of the applied

voltage, more C–O bonds are formed and the carbon lattice strain increases. When

the strain reaches a limitation, some C–C bonds on the top area of the protrusions

begin to fracture and a small pit is produced. The whole process is shown in Figure

8.a (below).24

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Figure 8.a

Schematic diagrams of nanostructures on the HOPG surface produced in ambient

air by the AFM tip under an external electric field.24

Whilst our experiments were not conducted under the same conditions as Jiang, Yan

et al.: our experiments were conducted in a meniscus of AgNO3 and not solely in

ambient air (i.e. H2O meniscus/electrochemical cell) which would affect the reactions

occurring at the substrate surface, also our applied electric field was stationary- no

scan-speed. We did observe pit formation (experiments 1, 10, 11, 13, 14, 15, 16, 17 &

19). Pits were preferentially formed in our experiments that were conducted with Si

tips, but not exclusively (experiments 1, 10, 11 & 19, see results tables above). It is

possible that our pit formation follows a similar path as outlined above ‘defect

induced oxidation of graphite facilitates the formation of concave structures’24.

Furthermore the stiffer Si tips would make a better contact with the HOPG surface

thus lowering the impedance and facilitating the oxidation of the HOPG. This could

be further investigated by repeating our experiments whilst at the same time

measuring voltage and current.

Park, Jin Gyu, et al.24 report in their 2007 paper that sub-100 nm holes were made on

HOPG surfaces using a metal-coated AFM tip and carbon nanotubes. The hole-

formation mechanism is related to the chemical reaction of graphite with adsorbed

water and tunnelling electrons from the tip to substrate. The authors suggest that

chemical reactions between HOPG and tunnelling electrons are a more important

mechanism than field-emission electrons. The substrate (HOPG) was always

maintained at a higher voltage than the AFM tip (this is the same configuration as our

experiments). However the authors applied −10 V pulses to the metal-coated tip with

a 50 ms pulse width (50% duty ratio). With 1000 repetition times, a hole was

fabricated on the HOPG surface.24

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The authors go on to contend that ‘hole diameter depended on the applied voltage,

contact force, number of pulses, and humidity; there was a threshold voltage to make

a hole and any amount over this voltage created a larger hole proportional to the

voltage amplitude.’

Further the authors submit that ‘the surface experienced too much damage with high

current (several μA) over the threshold voltage. Therefore higher contact force in this

voltage range gives higher current; hence the size cannot be controlled.’ The authors

suggest several mechanisms for hole fabrication such as mechanical contact between

tip and sample, heating, electro-migration, electronic contact, field-induced

electrochemical etching, and field-induced evaporation. However, the mechanical

effect can be ruled out based upon previous tests and other reports25. Our contact

force is below the HOPG yield strength (see ensuing discussion below on this point).

The local heating effect was controversial due to the high thermal conductivity of

graphite and metals.26 If the tip apex has low thermal conductivity, then the local heat

can build up to 1000K which is enough to dissociate graphite (C) to gas (CO2).26

Again this experimental set-up was similar to our configuration in that the substrate

(HOPG) was always maintained at a higher potential than the AFM tip, however the

voltages were larger (10V) and applied for shorter durations 50 ms pulse width (50%

duty ratio) with 1000 repetitions. However there are enough similarities between the

studies for the authors suggestion that the possible machining mechanism can be

ascribed to the chemical reaction of graphite with tunnelling electrons to be

applicable in our work, but as the authors note ‘further research is needed in a more

controlled atmosphere’ before we can make firm conclusions in this regard.

Xu (2003)27 reported on the nano-indentation of HOPG surface and pure elastic

deformation up to a maximum load of ≈610 μN.28 Fraxedas et al.29 (2002) concluded

that the plastic yield threshold was not reached when forces as large as 16 μN are

applied,29 with the penetration at plastic yield larger than 25 nm. Compared with the

above references, the constant contact force ≈4 nN in our work (with the HQ-DPE

tips) shall be negligible in playing a part in patterning or modification of the HOPG

sample surface.

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Zoval, Jim V. et al report on voltage pulses employed to electrochemically deposit

silver nanocrystallites on atomically smooth graphite basal plane surfaces.17 Voltage

pulses with amplitudes of 100, 250, and 500 mV vs Ag0 and durations of 10 or 50 ms

were applied to graphite surfaces immersed in dilute (≈1.0 mM) aqueous silver

nitrate. Whilst the experimental set up is not identical to ours the studies are similar

enough for us to gain insight from their findings. Zoval’s experimental set up is

shown in Figure 5.c (above).

Further the authors state that any nucleated silver nanocrystals interact weakly with

the graphite surface and are removed by the sweeping action of the probe tip from the

imaging area. This effect has been previously documented for gold particles on

graphite by Schaefer et al18 (see Introduction 1 above).

Further the authors state that silver micro and nanocrystallites which nucleate at

defect sites are observed by STM and AFM, and, consequently, it had been

concluded30 that silver over potential deposition (OPD) on graphite is initiated by

nucleation exclusively at defects, such as step edges, on the graphite surface. The NC-

AFM data presented by the authors suggest that on low-defect density surfaces such

as the graphite basal plane, STM and repulsive-mode AFM data can provide a

misleading view of nucleation by “ignoring” the presence of weakly adsorbed metal

nanocrystallites which are not associated with defects.17 The researchers also note

that in successive silver deposition trials in which the graphite surface was cleaved

prior to each experiment, the apparent capacitance of the surface fluctuated by 10-

20%, presumably due to fluctuations in the areal density of defects on the graphite

surface which is exposed during fresh cleavage.

The researchers note that the diameters of the silver particles associated with defects

such as step edges were smaller by 20-50% compared with silver particles which were

present on nearby basal plane regions of the same graphite surface.17 Further they

elucidate that the size disparity probably derives from the fact that the diffusional

transport of Ag+ to growing nuclei arrayed along step edges had a cylindrical

symmetry, whereas nuclei on the basal plane, which were farther removed on average

from nearest neighbours, experienced a more efficient hemispherical diffusional flux

leading to faster growth and a larger terminal radius, see Figure 7.z above for graphic

nucleation/deposition similarities in our work.

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However we are not seeing these results replicated entirely in our experiments: it is

worth bearing in mind our experimental parameters were different in that we

generally applied higher voltages for longer durations in a meniscus (as opposed to

bath) which may lead to a masking of the Zoval trends observed at smaller time

frames and lower voltages.

Significantly the researchers note that at values of coulometric loading (quantity of

matter transformed during an electrolysis reaction by measuring the amount of

electricity in coulombs consumed or produced) 31, QAg greater than ≈15 μC cm-2, a

branching occurs in which an ever increasing fraction of the deposition charge is

consumed with the deposition of silver onto micron-scale crystallites instead of onto

nanocrystallites on the surface. The results of our experiments are generally micron

sized deposits with some smaller associated nano-deposits in the general vicinity

(and sometime nano-pits (see above)). Probably our results are predominantly

micron sized because the voltages and durations we employed are higher than in

Zoval’s experiments, and our coulometric loading was therefore higher and favoured

eventual micron sized deposits. We would need to measure and monitor the induced

current during deposition to confirm this.

Zoval et al conclude that the silver electrodeposition mechanism is the following:

within ≈5 ms of the application of a large potentiostatic pulse to the graphite surface,

critical silver nuclei are established both at defect sites on the surface and at high

areal density on the defect-free graphite basal plane. The number density of these

silver nuclei does not increase appreciably with time (up to ≈50 ms). Following their

formation and for the ensuing 15 ms, critical nuclei grow at a rate limited by the

hemispherical (for deposition not located on step edges) diffusive flux of Ag+ to each

nucleation site. At approximately 15ms, the rate of growth of most silver

nanocrystallites slows dramatically, and further silver deposition is concentrated at a

small fraction of crystallites which increase very rapidly in size-attaining micron-scale

dimensions within ≈20-40ms. Again Zoval conducted his experiments in a bath of

AgNO3, but the tendency toward micron scale nucleation at a higher bias and

duration concur with our findings.

http://en.wikipedia.org/wiki/Coulomb

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The recent development of electrochemical force microscopy (EcFM)32 and its ability

to probe local bias- and time-resolved ion dynamics and electrochemical processes at

the solid–liquid interface would be invaluable for delving further into this

phenomenon.

Li, Yan et al11 report on the deposition of several metals and semiconductors on Si

surfaces at room temperature using the electrochemical “dip-pen” nanolithography

(E-DPN).technique, see Figure 4.c (above) for experimental set-up.

In a typical experiment, an ultra-sharp silicon cantilever coated with H2PtCl6 was

scanned on a cleaned P-type Si (100) surface with a positive DC bias applied on the

tip. During this lithographic process, H2PtCl6 dissolved in the water meniscus is

electrochemically reduced from Pt(IV) to Pt(0) metal at the cathodic silicon surface

and deposits as Pt nano-features according to the following equation:

PtCl62- + 4e → Pt + 6Cl-

The authors state that the height and width of the fabricated features, using the E-

DPN technique, depend on several factors, including the humidity, scan speed, and

applied voltage. They show that by varying these factors, they can change the height

and width of the created features. Using a similar process they authors succeeded in

creating features made of Au, Ge, Ag, Cu, Pd, etc. Further they state that in principle,

any metal or semiconductor that can be electrochemically deposited from an aqueous

solution of salts could be delivered to a surface with precise control of position to

form features with nanometer dimensions using E-DPN.11

See Figure 8.b (below) for an example of a silver line drawn by E-DPN method, with

AgNO3 solution as the ink, on a Si substrate.

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Figure 8.b

A silver line drawn by E-DPN method with AgNO3 solution as the ink. Experimental

conditions: relative humidity: 42%, voltage: 4V, scan speed: 20nm/s.

Although Li, Yan et al’s experimental set up is different to ours in that they work with

an AFM tip at a positive potential with respect to the substrate, they use a capillary

instead of meniscus to deliver the ionic reactants, and the working substrate was Si;

their work on biased deposition of Ag+ from AgNO3 bears relevance to our

investigations and offers corroborative evidence that the nucleation in our

experiments is biased induced electrochemical deposition of Ag.

The cantilever deflection is related to the force applied to the sample as follows:33

F = k. Invols. ∆V

F = 447.60 pN/nm . Defl InvOLS 25.28 nm/V . Set point 400.0mV

F = 4.5 nN (force applied during imaging)

Using Hook’s law (F= -kx), the amplitude of the cantilever during imaging is given by:

Amplitude = 4.5 nN / 447.60 pN/nm = 10.05nm

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In AC mode imaging it is important to manipulate the Set Point and Integral Gain

(‘IG’) in order to improve the image quality; the IG determines how fast the AFM

responds to perturbations. The set point determines the force applied to the tip (and

hence sample), it is this force/value which the feedback loop maintains. In order to

optimize the IG and set point a systematic approach should be adopted - first adjust

the set point to an appropriate level for your objective (a lower set-point ≡ bigger

force applied to sample). Initially engage the surface with a high set point and then

adjust downward; then set the IG to a level which induces noise in the image quality,

now set the IG to a level just below the noise producing level. In this way the IG

amplification is maximised thus improving image quality.

During amplitude mode operation, the cantilever is oscillated near its resonance

frequency, ωo, resulting in a ‘free air’ amplitude, Ao. When the cantilever is placed in

close proximity to the surface, the tip – surface/deposition interactions result in

decreased cantilever oscillation amplitude from the ‘free air’ amplitude. The

cantilever oscillates at amplitude, A, after interacting with the surface. The AFM

image is acquired during raster scanning over the sample by measuring the necessary

adjustments to the vertical displacement of the scanner, via a feedback loop, to

maintain the constant value of set point ratio s = A/Ao. The feedback loop provides

precise control over the cantilever amplitude, and thus, the force between the tip and

surface.34 The force which the tip exerts on the sample is described by Hooks Law,

F = -k.x ; by using a cantilever with smaller length, the cantilever displacement is

reduced and so the force exerted on the sample is also reduced. Note shorter

cantilevers oscillate at higher frequencies and this affects the feedback loop and scan

speed.

Similar to conventional repulsive mode AFM, the operational principle of the NC-

AFM (also called the “dynamic”, “attractive mode”, and “ac” atomic force microscope)

involves the detection and maintenance of a small force which is exerted locally by

the probe tip on the sample surface. In contrast to the repulsive mode AFM

experiment, however, the NC-AFM probe tip is located in the attractive region of the

tip-sample interaction potential and at greater distances from the surface of 10-15 Å.

The attractive tip-sample forces present in this separation regime are smaller than

those involved in repulsive mode AFM imaging (10-11-10-15 N vs 10-9 N), with the

result that the NC-AFM tip is a less perturbative probe of the surface topography.

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The attractive force imparted by the tip to the sample is detected optically as a

deflection of the AFM cantilever toward the sample surface.17

It is also worth remembering that any Ag deposition dimensions obtained from cross

sectional analysis shown in Results 7 (above) are affected by tip convolution. The

apparent diameters obtained from NC-AFM image data are exaggerated by

convolution with the geometry of the probe tip. Since several different probe tips

were employed for our measurements, and the exact dimensions of each were not

measured, it is therefore impossible for us to deconvolve the tip contribution from the

NC-AFM image data. Other studies suggest that the apparent particle diameter is

larger than the true particle diameter by about twice the nominal tip radius. 17

9 Conclusions

It is clear from viewing our experimental results that we are inducing nano and micro

scale modifications on top of and into the HOPG surface (deposition and pits).

Our experimental set-up is not identical to any of the prior works that we studied, but

we can draw insight from the findings of Jiang, Yan et al. It is probable our stationary

electric field induces anion adsorption on the HOPG surface, weakening the HOPG

lattice structure. Thereafter this weakened lattice structure suffers further anion

adsorption and is simultaneously oxidized (forming C-O bonds). This process

increases the strain on the HOPG lattice eventually causing C-C bonds to fracture,

and the onset of pit formation.

Whilst our results do not consistently show pit formation, this may be due to a

varying AFM-HOPG contact, and also varying basal plane conductivity at each freshly

cleaved HOPG layer (change in areal density of defects per fresh layer).

Further it is possible that we are consistently producing pits but that depending on

the variable geometry of the pit and surrounding electrolyte; we may in fact

subsequently ‘fill’ in these pits with reduced/nucleated Ag. It is also important to bear

in mind that the AFM tip-HOPG contact is not an entirely static environment. The

AFM constantly tries to maintain the contact force we set, but as a pit forms under

the AFM tip the feedback loop must respond to this changed tip-surface dynamic. All

of these factors are an influence on the eventual pit-deposition structure under the

applied bias.

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Park, Jin Gyu, et al’s observations are also relevant; they note that hole diameter

depended on the applied voltage, contact force, number of pulses, and humidity.

There was a threshold voltage to make a hole and any amount over this voltage

created a larger hole proportional to the voltage amplitude. We see similar trends in

that longer duration voltage pulses created pits of larger volume (contrast pits formed

in experiments (1 vs. 10 vs. 11) and (13 vs. 16 vs. 17). Also the general trend follows the

magnitude of the voltage (higher voltage ≈ bigger volume pit).

We are presuming our deposits are Ag, and proceed according to the following

reaction:

Ag+ + e- = Ag

The electrons being supplied by the AFM tip and Ag+ from the silver nitrate solution.

The composition of our deposits could be confirmed by spectroscopic analysis (see

Section 10, Future Work, below).

We believe our deposits follow the mechanism outlined by Zoval et al. Zoval’s main

contention is that micron deposits succeed and are initiated by primary nano deposits

(nano deposits in the time frame ≈5 ms to ≈50 ms). Our experiments were run over

durations 3.8 to 5.0seconds and generally showed only micron scale deposits. Whilst

our experiments did not always follow the trend of longer durations resulting in

larger deposition volumes, this may be because the initial biased induced nucleation

absorbed/used up the majority of the available Ag+ ions in the surrounding meniscus;

Zoval conducted his experiments in a bath of AgNO3: we conducted our work in a

meniscus.

Li, Yan et al’s findings also corroborate Zoval and our findings, in that humidity,

voltage and scan speed (or lack thereof in our experiments) determines the quantum

of any deposition.

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We can conclude that the deposition is not metal dissolving from the AFM tip

because we get nucleation and pits when we use a pure Si tip (no metal coating). The

vastly increased quantity of deposition for the gold substrate versus the HOPG

substrate is a reflection of the inertness of HOPG versus Gold (high valence).

Furthermore, the HOPG has a lower areal density of defect sites which are conducive

for initial nucleation.

Also, the graphite basal plane surface is electrochemically very inert, and strongly

coordinating stabilizers are not involved in the silver electro-synthesis procedure.

Therefore the graphite-supported silver particles obtained by the voltage pulse

method have immediate applications for investigations of the intrinsic

electrochemical reactivity of silver particles over a wide range of particle diameters

The fabrication results show the AFM bias induced electrochemistry is a low-cost,

flexible and promising nanofabrication method which can modify a surface above and

below the surface layer.

These nano and microscopic deposits possess chemical and physical properties which

differ significantly from macroscopic metal phases. The list of particle size dependent

properties includes amongst others bond distances, the van der Waals attractive force

operating between particles, the surface plasmon resonance, the melting point, the

standard electrode potential, and the photoelectric yield. The variety of applications

which these properties can be put to work in is only limited by the imagination. In

our case, we are specifically aiming the results at biosensor applications, but this is a

small window into the endless possibilities.

Whilst the future is in the small, it is about to get a lot bigger.

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10 Future Work

Zoval et al17 report that it was sometimes possible to dislodge silver nanocrystallites

from the graphite surface by ultrasonication. The resulting suspension of particles

was then drop-coated onto a carbon-coated gold TEM grid for analysis. The TEM

data allowed formal identification of the silver nanocrystallites. We should use this

technique in future work to confirm the chemical identity of our deposits.

Furthermore, future work should include a spectroscopic analysis of any material

nucleated on the surface in order to confirm its elemental composition.

We should measure and document the current during the experiment. It is generally

accepted the deposition current is indicative of an instantaneous nucleation and

three-dimensional growth mode of deposition.35

Gunawardena et al.36 have shown that experimental data for several metals (including

silver) are consistent with the expression

Where:

D is the diffusion coefficient for the soluble form of the metal,

C* is the concentration of the metal in solution in units of mol cm-3,

M is the atomic weight of the metal (or the formula weight of the soluble metal

complex),

N is the total number of metal nuclei present on the electrode surface,

F is the density of the metal.17

Using this equation we can become more predictive and judgmental of our

experimental results.

We should SEM image the surface and tip pre and post experiment. Any nucleation

or deformation of the tip would result in a change to the applied electric field between

the tip and substrate. SEM analysis of the surface pre and post nucleation would

reveal the contribution of surface defects to the nucleation process.

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The z component of the tip deflection should also be recorded during bias dwell time.

Any change to the tip deflection is indicative of nucleation growth under/around the

tip. This information can be used in conjunction with the onset of tip-surface current

flow to elucidate the initiation condition of any nucleation.

The photo-sensitivity of the AgNO3 must be wavelength dependent. Therefore, if we

can isolate this wavelength, and then send a focused laser at this wavelength into the

AgNO3 solution on the HOPG, we may be able to achieve laser guided nucleation of

Ag on a HOPG or any substrate for that matter. The actuators of an AFM should allow

for the control of the laser beam.

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