inkjet printing of carbon nanotubes for electronic...

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ISBN 978-951-42-9308-5 (Paperback)ISBN 978-951-42-9309-2 (PDF)ISSN 0355-3213 (Print)ISSN 1796-2226 (Online)

U N I V E R S I TAT I S O U L U E N S I SACTAC

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TECHNICA

OULU 2009

C 346

Tero Mustonen

INKJET PRINTING OF CARBON NANOTUBES FOR ELECTRONIC APPLICATIONS

FACULTY OF TECHNOLOGY,DEPARTMENT OF ELECTRICAL AND INFORMATION ENGINEERING,MICROELECTRONICS AND MATERIAL PHYSICS LABORATORIES,UNIVERSITY OF OULU;INFOTECH OULU,UNIVERSITY OF OULU

C 346

ACTA

Tero Mustonen

C346etukansi.kesken.fm Tuesday, November 3, 2009 2:47 PM

A C T A U N I V E R S I T A T I S O U L U E N S I SC Te c h n i c a 3 4 6

TERO MUSTONEN


Academic dissertation to be presented with the assent ofthe Faculty of Technology of the University of Oulu forpublic defence in Auditorium TS101, Linnanmaa, on 4December 2009, at 12 noon

OULUN YLIOPISTO, OULU 2009

Copyright © 2009Acta Univ. Oul. C 346, 2009

Supervised byDoctor Krisztián Kordás

Reviewed byProfessor Thomas F. GeorgeDoctor Asta Kärkkäinen

ISBN 978-951-42-9308-5 (Paperback)ISBN 978-951-42-9309-2 (PDF)http://herkules.oulu.fi/isbn9789514293092/ISSN 0355-3213 (Printed)ISSN 1796-2226 (Online)http://herkules.oulu.fi/issn03553213/

Cover designRaimo Ahonen

OULU UNIVERSITY PRESSOULU 2009

Mustonen, Tero, Inkjet printing of carbon nanotubes for electronic applications. Faculty of Technology, Department of Electrical and Information Engineering, Microelectronicsand Material Physics Laboratories, University of Oulu, P.O.Box 4500, FI-90014 University ofOulu; Infotech Oulu, University of Oulu, P.O.Box 4500, FI-90014 University of OuluActa Univ. Oul. C 346, 2009Oulu, Finland

Abstract

In this thesis, preparation of carbon nanotube (CNT) inks and inkjet printing of aqueousdispersions of CNTs for certain electrical applications are studied. The nanotube inks prepared inthis work are based on chemically oxidized CNTs whose polar side groups enable dispersion inpolar solvents. Subsequent centrifugation and decanting processes are used to obtain stabledispersions suitable for inkjet printing. The inks are based on either carboxyl functionalized multi-walled carbon nanotubes (MWCNTs), carboxyl functionalized single wall carbon nanotubes(SWCNTs) or SWCNT-polymer composites.

The applicability of MWCNT inks is firstly demonstrated as printed patterns of tanglednanotube networks with print resolution up to ~260 dpi and surface resistivity of ~40 kΩ/. whichcould be obtained using an ordinary inkjet office printer. In addition, MWCNT inks are found toexhibit spatial ordering in external magnetic fields due to entrapped iron catalyst nanoparticles inthe inner-tubular cavity of the nanotubes. Ordering of nanotubes in the inks and in drying dropletsplaced in relatively weak magnetic fields (B ≤ 1 T) is demonstrated and studied.

The high electrical conductivity and optical transparency properties of SWCNTs are utilizedfor enhancing the conductivity of transparent poly(3,4-ethylenedioxythiophene):poly(styrenesulphonate) (PEDOT:PSS) films. Polymer-nanotube composite materials are inkjetprinted on flexible substrates. It is demonstrated that incorporation of SWCNTs in the thinpolymer films significantly increases the electrical conductivity of the film without losing the hightransparency (> 90%). The structure of composite films is studied using atomic force microscopy(AFM).

The electronic properties of deposited random SWCNT networks are studied. The amount ofdeposited SWCNT is controlled by the inkjet printing technique. In dense networks the current-voltage behaviour is linear whereas for sparse films the behaviour is nonlinear. It is shown that theconduction path in dense films is through the metallic nanotubes, but in sparse films thepercolation occurs through random networks of metallic and semiconducting SWCNTs havingSchottky-type contacts. The existence of Schottky-junctions in the films is demonstrated withfield-effect transistors (FET) on Si-chips and on polymer substrates. The latter is demonstrated asfully printed transistors using a single ink as a material source. FETs are further utilized aschemical-FET sensor applications. The performance of resistive CNT sensors and theircomparisons with chem-FETs in terms of selectivity are studied for H2S gas.

Keywords: alignment, carbon nanotube, electrical transport, inkjet printing, printedelectronics, sensor, transistor, transparent conductive coating

“Discovery consists of seeing what everybody has seen and thinking what nobody has thought”

Albert Szent-Györgyi (1893–1986)

7

Acknowledgements

This work was done at the Microelectronics and Materials Physics Laboratories

of the University of Oulu and the EMPART research group of Infotech Oulu

during the years 2005–2008. The work was mainly carried out in the framework

of the PrinTag project (Nanomaterials in printable electronics RFID solutions)

which was funded by the Finnish Funding Agency of Technology and Innovations

(TEKES), UPM-Kymmene Oyj and Perlos Oyj. Parts of the work were also done

in the framework of the SANES project (self-adjusted nanoelectronic sensors)

which was funded by the European Union (EU-STREP) and LaserProbe Oy.

I wish to thank my supervisor and friend Dr. Krisztián Kordás who gave me

the opportunity to work with the subject of this thesis and also for many fruitful

discussions and off-topics in front of pizza and foaming beer. I also gratefully

thank Prof. Heli Jantunen for this opportunity to work in this field and for helpful

guidance in the post graduate studies. I especially thank Dr. Peter Heszler (1958–

2009) for his contribution to this work and for his unforgettable lectures that I

have attended. Unfortunately, he didn’t have enough time to see the finish of this

work.

I am grateful to Dr. Asta Kärkkäinen and Prof. Thomas F. George for serving

as official reviewers of the thesis. I also would like to acknowledge the help of

Prof. Arthur E. Hill for the language revision of the thesis.

In addition, the following persons deserve thanks for their support, hospitality,

discussions, arguments, knowledge and ideas which guided me in this work (in

alphabetical order): M.Sc. Niina Halonen, M.Sc. Santtu Heinilehto, Dr. Panu

Helistö, Dr. Swastik Kar, Dr. Ákos Kukovecz, Dr. Jyrki Lappalainen, M.Sc.

Hannu Moilanen, Jani Mäklin, Dr. Jari Penttilä, Prof. K. Venkat Rao, Dr. Siegmar

Roth, M.Sc. Sami Saukko, Prof. Heikki Seppä, Dr. Géza Tóth, Dr. Antti Uusimäki,

Dr. Robert Vajtai, Prof. Jouko Vähäkangas and the staff in Microelectronics and

Materials Physics Laboratories and the Institute for Electron Microscopy. Finally,

I would like to thank Aino for patience and understanding for all these years when

I have spend too many hours in the lab and bringing work home.

The work was financially supported by the Research Foundation of Seppo

Säynäjäkangas, Nokia Foundation, Finnish Chemical Society, Tauno Tönning

Foundation and Tekniikan Edistämissäätiö (TES). All foundations are gratefully

acknowledged.

9

List of symbols and abbreviations

The list of units and symbols are in order as they appear in the text and the list of

abbreviations is in alphabetical order.

W Watt

m metre

K Kelvin

Pa Pascal

h hour

min minute

dpi dots per inch

g gram

l litre

rpm revolutions per minute

°C degree Celsius

Ω Ohm

square

T Tesla

emu electromagnetic unit

Oe Oersted

at% atomic per cent

wt% weight per cent

A Ampere

V volt

eV electron volt

ppm parts per million

mmHg millimetre of mercury

a1, a2 unit vectors of graphene

ch lattice vector of graphene

I-V current-voltage

M magnetization

Mf magnetic flux

H magnetic field strength

φ polarization angle

B magnetic flux density

10

P power

T0 zero-field transmission

T transmission parallel

T transmission perpendicular

ΔA optical dichroism

I0 zero-field intensity

I intensity parallel

I intensity perpendicular

E electric field

S nematic ordering parameter

θ angle

ΔU potential energy difference

Ekin kinetic rotational energy

λ wavelength

T transmittance

R resistance

J current density

V, V* potential

T temperature

Фb Schottky barrier height

A* Richardson’s constant

e, q elementary charge

m* effective mass of carriers

me mass of electron

k Boltzmann’s constant

h Planck’s constant

A average area of cross section

m number of Schottky junctions

Δr change in resistance

r0 initial resistance

U applied bias

S-D source-drain

VSD, US-D source-drain voltage

VG, UG gate voltage

IS-D source-drain current

11

AFM atomic force microscopy

Al2O3 aluminium(III) oxide or alumina

CCVD catalytic chemical vapour deposition

chem-FET chemical field-effect transistor

CIJ continuous inkjet

CNT carbon nanotube

COOH carboxylic acid (functional group)

DC direct current

DMF dimethyl formamide

DoD drop-on-demand

EDS energy dispersive (X-Ray) spectroscopy

EFTEM energy-filtered transmission electron microscopy

FC field-cooled

FES fluctuation enhanced sensing

FESEM field-emission scanning electron microscopy

FET field-effect transistor

FT-IR Fourier-transform infrared

H2S hydrogen sulphide

LED light-emitting diode

m-CNT metallic carbon nanotube

MWCNT multi-walled carbon nanotube

OLED organic light-emitting diode

PECVD plasma-enhanced chemical vapour deposition

PEDOT:PSS poly(3,4-ethylenedioxythiophene):poly(styrenesulphonate)

PET poly(ethylene terephthalate)

R2R roll-to-roll or reel-to-reel

RFID radio frequency identification

RBM radial breathing mode

RIE reactive ion etching

RND random

s-CNT semiconducting carbon nanotube

SEM scanning electron microscopy

SQUID superconducting quantum interference device

STM scanning tunnelling microscopy

SWCNT single-wall carbon nanotube

TMS temperature modulated sensing

ZFC zero-field-cooled

13

List of original papers

This thesis consists of an overview and the following five publications:

I Kordás K, Mustonen T, Tóth G, Jantunen H, Lajunen M, Soldano C, Talapatra S, Kar S, Vajtai R & Ajayan PM (2006) Inkjet printing of electrically conductive patterns of carbon nanotubes. Small 2(8–9): 1021–1025.

II Kordás K, Mustonen T, Tóth G, Vähäkangas J, Uusimäki A, Jantunen H, Gupta A, Rao KV, Vajtai R & Ajayan PM (2007) Magnetic-field induced efficient alignment of carbon nanotubes in aqueous solutions. Chemistry of Materials 19(4): 787–791.

III Mustonen T, Kordás K, Saukko S, Tóth G, Penttilä JS, Helistö P, Seppä H & Jantunen H (2007) Inkjet printing of transparent and conductive patterns of single-walled carbon nanotubes and PEDOT-PSS composites. Physica Status Solidi B 244(11): 4336–4340.

IV Mustonen T, Mäklin J, Kordás K, Halonen N, Tóth G, Saukko S, Vähäkangas J, Jantunen H, Kar S, Ajayan PM, Vajtai R, Helistö P, Seppä H & Moilanen H (2008) Controlled Ohmic and nonlinear electrical transport in inkjet-printed single-wall carbon nanotube films. Physical Review B 77(12): 125430 1–7.

V Mäklin J, Mustonen T, Halonen N, Tóth G, Kordás K, Vähäkangas J, Moilanen H, Kukovecz Á, Kónya Z, Haspel H, Gingl Z, Heszler P, Vajtai R & Ajayan PM (2008) Inkjet printed resistive and chemical-FET carbon nanotube gas sensors. Physica Status Solidi B 245(10): 2335–2338.

Paper I describes the preparation of stable dispersions from catalytic chemical

vapour deposition (CCVD) grown carbon nanotube (CNT) forests and inkjet

printing of the inks produced. The surfaces of CNTs were partially oxidized to

create polar functional groups, thus aiding dispersion of nanotubes in aqueous

media. FT-IR and Raman spectroscopy were utilized to study the surface species

of functionalized nanotubes. Electronic properties of printouts were studied by

two-probe DC measurements. The obtained results showed inkjet printing to be an

efficient method for depositing carbon nanotubes on paper and plastic substrates

with reasonably good accuracy. The demonstrated method brings us a step closer

to high throughput and cost-effective large area manufacturing for printable

sensors, transistors and other electronic devices.

The topic of Paper II was to utilize the ferromagnetic catalyst particles

trapped inside multi-walled carbon nanotubes in aligning CNTs in external

magnetic fields. The magnetic field-induced alignment was studied by measuring

transmittance of a polarized laser beam through aqueous dispersions of carbon

nanotubes. Furthermore, films of aligned carbon nanotubes were demonstrated on

14

silicon substrates by deposition and subsequent drying of droplets of inks in the

presence of a magnetic field.

Paper III introduces the use of single-wall carbon nanotubes (SWCNTs) to

increase the conductivity of PEDOT:PSS polymer. Transparent and conductive

films of nanotube-polymer composites with various thickness were inkjet printed

on photo-paper and plastic substrates. Using composites, the electrical

conductivities of films could be improved without decreasing the optical

transmittance of the film in the visible spectrum. Regions of the two-component

polymer and improved percolation of the conductive phase via interconnects with

the added carbon nanotubes were identified by AFM/STM techniques.

In Paper IV electrical transport of inkjet printed SWCNT films was studied.

SWCNT films of varying surface coverage were deposited on alumina (Al2O3)

and on silicon chips. Dense nanotube networks showed ohmic behaviour, while

nonlinear current-voltage characteristics were measured for sparse networks of

SWCNTs printed on alumina substrates. The two conduction processes were

explained on the bases of percolated metallic nanotubes (ohmic films) as well as

semiconducting and metallic nanotubes forming a network of Schottky junctions

(nonlinear films). Silicon chips with integrated gate electrodes were used to

demonstrate field-effect transistor operation using sparse inkjet-printed SWCNT

films as channels. The results suggested the possibility of patterning metallic and

semiconducting regions on a substrate using the same material. Accordingly, a

fully inkjet printed carbon nanotube transistor with a sparse nanotube layer acting

as channel and dense layers as source drain and gate electrodes on a plastic foil is

shown in the thesis. Paper IV was selected for the April 2008 issue of the Virtual

Journal of Nanoscale Science & Technology, which is published by the American

Physical Society in cooperation with numerous other societies and publishers, and

constitutes an edited compilation of links to articles from participating publishers,

covering a focussed area of frontier research.

The field-effect characteristics of an inkjet printed sparse SWCNT film and a

dense metallic SWCNT film were demonstrated as a chemical-FET sensor and as

a conventional resistive sensor in Paper V, respectively. The response and

selectivity of sensors towards H2S gas analyte were tested and compared. It was

shown that selectivity was obtained by applying a positive gate voltage which

increased the response of the chemical-FET sensor towards the analyte gas.

The planning of experiments, ink preparation, inkjet printing, optical

characterization, analyses (excluding electrical measurements) and data

processing were mostly the contribution of the author. In Paper III STM/AFM

15

measurements were done in association with M.Sc. Sami Saukko. Substrates and

Si-chip dies used in the experiments were provided by M.Sc. Niina Halonen, and

the suspended chip package was prepared by M.Sc. Hannu Moilanen (Paper IV

and Paper V). The manuscripts were written by the author together with the kind

help of co-authors.

17

Contents

Abstract Acknowledgements 7 List of symbols and abbreviations 9 List of original papers 13 Contents 17 1 Introduction 19

1.1 Printing technologies ............................................................................... 19 1.2 Carbon nanotubes .................................................................................... 20 1.3 Objective and outline of thesis ................................................................ 24

2 Inkjet printing of carbon nanotubes 27 2.1 Background ............................................................................................. 27 2.2 Methods for preparing carbon nanotube inks .......................................... 29 2.3 Inkjet printing performance and characteristics of line pattern ............... 33

3 Magnetic-field induced alignment of multi-walled carbon nanotubes 37 3.1 Background ............................................................................................. 37 3.2 Magnetic properties of multi-walled carbon nanotubes .......................... 38 3.3 Optical characterization of aligned carbon nanotubes in

dispersions ............................................................................................... 39 4 Inkjet printing of single-wall carbon nanotubes and polymer

composites 45 4.1 Background ............................................................................................. 45 4.2 Preparation and printing of composite ink .............................................. 45 4.3 Structural and optical characterization of nanotube-polymer

composite films ....................................................................................... 46 5 Electrical characterization of single-wall carbon nanotube films 51

5.1 Background ............................................................................................. 51 5.2 Film deposition and substrate characteristics .......................................... 51 5.3 Electrical characterization ....................................................................... 53 5.4 Inkjet printed carbon nanotube transistors .............................................. 56

6 Carbon nanotube gas sensors 61 6.1 Background ............................................................................................. 61 6.2 Resistive and chem-FET gas sensors ...................................................... 62

7 Conclusions 67 References 69 Original publications 75

19

1 Introduction

1.1 Printing technologies

In the graphical industry, printing technologies are divided into conventional and

digital printing technologies. Conventional printing consists of offset, flexography,

gravure and screen printing technologies, in which the image/pattern is

transferred to substrates from prefabricated plates, cylinders or screens. High

throughput products such as newspapers, books, and magazines are made using

conventional printing methods. In contrast to conventional technology, printing

plates are not required for patterning in digital printing technology, thus every

print can be different. Digital printing enables applications such as personalized

printing, variable data printing and print-on-demand for small print runs.

Commonly used printing technologies are presented in Figure 1.

Fig. 1. Printing methods and technologies (Heilmann 2007).

A young rapidly growing industry – printed electronics – is a versatile field in

which the components and applications used in electronics are manufactured by

using the aforementioned technologies. The field of printed electronics is more

demanding compared to graphical industry with respect to resolution, print

quality and reproducibility. Small deviations in printing may cause large

differences in application performance, e.g., printing of transistors requires low-

dimensional features in the components. Printed electronic components are

multilayer devices, so that during the past several years more input has been put

20

on roll-to-roll (R2R) technology, where different printing units are in series and

the device is printed in one operation. In R2R technology, several different

printing technologies can be combined. However, the technology as a cost-

effective method to produce large volume printed electronics is not seen as a

replacement for traditional silicon technology due to a number of limitations in

quality and resolution of printed solution-based materials. Printing is seen rather

as a complementary way to produce novel applications and devices with key

drivers from large area and flexible electronic devices, e.g., displays, light sources,

antennas (RFIDs), batteries and solar cells (Hutton 2006, Hecker 2008).

1.2 Carbon nanotubes

Early findings of nanosized tubular carbon structures were reported by Russian

scientists Radushkevich and Lukyanovich in 1952 and published in the Journal of

Physical Chemistry of Russia. Due to the cold war, access to the journal was

difficult for Western scientists, and many of the papers written in Russian were

not mentioned in literature databases at that time. In addition, studies on carbon

filaments were applied mainly in the steel industry (e.g., coolant channels of

nuclear reactors), so that materials scientists (as a whole community) were not

aware of these findings for forty years (Monthioux & Kuznetsov 2006). After the

re-discovery of carbon nanotubes by Iijima (1991), research was truly launched,

and since then the number of publications has increased dramatically to date

(Figure 2).

21

Fig. 2. Amount of publications and patents found using keyword “carbon nanotube” in

SciFinder Scholar 2007 program. Search was performed on 5.4.2009.

The unique properties of carbon nanotubes promise a vast number of potential

applications in multidisciplinary fields of science. Their high thermal conductivity

(~6600 W/mK) (Berber et al. 2000), mechanical strength (tensile strength of 60–

100 GPa, and Young’s modulus of 0.2–0.7 TPa) (Troiani et al. 2003,

Sammalkorpi et al. 2004) and optional metallic/semiconducting properties

(Yorikawa & Muramatsu 1995, Kleiner & Eggert 2001) give rise to applications

ranging from chip-cooling elements (Kordás et al. 2007) and composite

reinforcements (Coleman et al. 2006) to electronic applications such as transistors

(Tans et al. 1998, Zhou et al. 2004), chemical sensors (Kong et al. 2000) and

field-emission displays (Chung et al. 2001, Choi et al. 2008), just to name a few.

It is worth noticing that commonly used carbon fibres (or nanofibres) should

be distinguished from carbon nanotube structures. In fibres, the graphitic sheets are

stacked on each other parallel to the fibre axis or in amorphous hollow form (Baird

et al. 1971, Muchopadhyay et al. 2002) whereas carbon nanotubes consist of

rolled sp2 hybridized graphene sheets in a tubular form of single- or multi-walled

structures. The electronic properties of carbon nanotubes depend mostly on the

type of nanotube and the chirality (helicity) of the rolled graphene sheet (Figures

3 and 4). In single-wall carbon nanotubes (SWCNTs) the chirality plays a

22

significant role in their optical and electronic properties. Metallic conduction

occurs in the armchair nanotubes as the valence and conduction bands always

cross each other at the Fermi energy. Semiconducting properties of chiral

nanotubes arise from the opening of the gap at the Fermi energy in the one-

dimensional band structure (Ajayan 1999).

Statistically, 1/3 of the SWCNTs are metallic and 2/3 are semiconducting

with varying band gaps. The chirality and diameter of SWCNT is specified by the

vector

),( 212211 nnananch ≡+=

, (1)

where n1,n2 are integers, a1,a2 unit vectors of graphite, and ch connects two

crystallographically equivalent sites A and A’ of graphene. The SWCNT cylinder

is formed by connecting A and A’. All armchair nanotubes (n1 = n2) are metallic

and zigzag nanotubes (n2 = 0) are metallic when n1 is a multiple of three. Chiral

nanotubes are metallic when

qnn 32 21 =+ , (2)

where q is an integer (Saito et al. 1992, Figure 3).

23

Fig. 3. (a) Carbon nanotubes are made by rolling a graphene sheet into a cylinder. (b)

Possible vectors of chiral nanotubes (Saito et al. 1992, published by permission of

American Institute of Physics).

In multi-walled carbon nanotubes (MWCNTs), the walls inside are superimposed

by outer walls, and this causes overlapping of the density of states. In addition,

due to the large diameter of MWCNTs, the band gap of semiconducting

MWNCTs becomes small, resulting in a practically metallic nature at room

temperature (Ebbesen et al. 1996, Carroll et al. 2007).

24

Fig. 4. Armchair- (left), zigzag- (middle) and chiral (right) carbon nanotube structures.

As the diameter of nanotubes decreases, they tend to form bundles of tens of

nanotubes due to van der Waals forces (π-π interaction) (Kiang et al. 1995). The

typical diameter of a single-wall nanotube is 1–2 nm, while multi-walled carbon

nanotubes can have diameters between 10–100 nm. The length of CNTs can be

suitably varied from a few tens of nanometres up to the centimetre regime. Both

the diameter and length of nanotubes depend on the type of synthesis process and

on the applied parameters.

1.3 Objective and outline of thesis

The overall objective of the thesis was to demonstrate feasible applications made

with carbon nanotube inks dispensed by inkjet printing. The milestone of the

work was to develop a method for (nearly) invisible electronic applications using

inkjet-printed carbon nanotubes as transparent conductive material which was

further combined with printed transparent organic conductors. To reach this goal,

methods for producing carbon nanotube inks were first investigated together with

related printing techniques. Other objectives were to demonstrate the diversity of

carbon nanotube inks in inkjet printed electrical applications, to use the CNT

films for fully inkjet printable carbon nanotube transistors, and to produce carbon

nanotube transistor chemical sensors.

Chapter 2 describes the preparation of multi-walled carbon nanotube inks and

their subsequent inkjet printing to obtain electrically conductive patterns of

MWCNTs on paper and plastic substrates. In Chapter 3, the magnetic properties

25

of nanotubes used in the inks are investigated. Orientation and manipulation of

multi-walled carbon nanotubes in dispersion using an external magnetic fields is

described. The manipulation enables the deposition of aligned carbon nanotube

films that could be utilized, for example, in optical coatings. In Chapters 4, 5 and

6, the type of nanotube is changed from multi-walled to single-wall because of

their superior optical and semiconducting properties. Chapter 4 describes the

inkjet printing of transparent conductive polymer composite films that show

enhanced conductivity compared to polymer films with similar optical properties.

These films could be applied as transparent conductive coatings. In Chapter 5, the

electrical properties of single-wall carbon nanotube films are studied. It was

found that the film properties could be controlled from semiconducting to

metallic by varying the density (average layer thickness) of the printed SWCNT

networks. The preceding findings were utilized in printing the channels of field

effect transistors on Si chips and also in the production of completely inkjet-

printed transistors on a plastic substrate. In Chapter 6, the field-effect behaviour

of SWCNTs is further exploited in printable sensor devices. It was demonstrated

that the sensor selectivity towards a gas can be increased by using chem-FET

sensors instead of traditional resistive sensors.

In this thesis, a conventional mass-producible method is presented as a novel

tool for carbon nanotube deposition, and its applicability towards printed

transparent conductors and printed transistors and sensors is explored. Also, the

electrical characterization of printed single-wall carbon nanotube films revealed

another unique characteristic of carbon nanotubes. This is utilized as completely

inkjet-printed carbon nanotube transistors on flexible polymer film using only one

material for all of the transistor components. These results contribute to potential

future mass production of printable electronics applications using carbon

nanotubes or other nanomaterials.

27

2 Inkjet printing of carbon nanotubes

Background information and inkjet printer technologies are described to give

specifications for the printers used in the thesis. The procedure for ink preparation

and printing of carbon nanotube inks using thermal inkjet printers are

demonstrated for further studies toward transparent conductive films and

transistor applications.

2.1 Background

Since the first appearance of inkjet-type technology in the late 19th century

(Thomson 1867), inkjet printers have become a precision fluid microdispensing

technology for use in electronics, optics and biomedical instrumentation, most

notably in the last twenty years. The advantages of inkjet printing technology

compared to other conventional pattern generating methods are the non-contact

direct dispensing of materials, the ability to use non-planar substrates, and the

avoidance of unnecessary masking. In addition, inkjet printing is an additive

process and is thus environmentally friendly, with only the necessary amount of

material being deposited. In contrast, subtractive methods such as

photolithography waste over 90% of the applied material. In the case of expensive

materials, the additive nature of inkjet printing can result in significant cost

savings (Wallace & Hayes 2005).

Inkjet printing technology can be described as a technique in which an

extremely repeatable formation of small fluid droplets is directed to a specific

location with high accuracy. Inkjet printing technology can be divided into two

categories: continuous mode and drop-on-demand (DoD) mode. In the continuous

mode, ink is continuously jetted and the charged ink droplets are guided by

deflection plates onto the substrate (Figure 5).

28

Fig. 5. Schematic of a continuous inkjet (CIJ) printer (Wallace & Hayes 2005, published

by permission of Woodhead Publishing Ltd.).

In the drop-on-demand (DoD) mode (or demand mode), which is widely used in

office printers, two sub-categories predominate: piezoelectric and thermal inkjet

printers. The latter method is also called the bubble-jet technique. In the demand

mode, a small transducer is used to displace ink by creating a pressure wave

which travels to an orifice or nozzle and ejects a droplet. In piezoelectric inkjet

printers the pressure is generated by piezoelectric actuators, while in thermal

inkjet printers a heating element evaporates a small amount of solvent resulting in

vapour generation and volume expansion, and hence displacement and ejection of

the ink (Figure 6). In this work both types of drop-on-demand mode inkjet

printers have been used (Wallace & Hayes 2005, Ballarin et al. 2004).

29

Fig. 6. Schematics of (a) piezoelectric and (b) thermal printers (Ballarin et al. 2004

published by permission Elsevier).

Recent developments in large area printing and speed of inkjet printing machinery

have taken inkjet printing a step closer towards traditional high throughput

techniques such as gravure and flexography printing (Table 1).

Table 1. Recent developments in inkjet printing machinery (Heilmann 2009).

Name Printhead

manufacturer

Printing method Speed Resolution

Fujifilm Jet Press

720

Dimatix DoD, piezo 2700 sheet/h,

180 pages/min

1200 dpi

HP Inkjet Press Hewlett-Packard DoD, thermal 122 m/min 1200 dpi

Kodak Strem

Concept Press

Kodak CIJ 304 m/min 600 dpi

Océ JetStream Kyocera Mita DoD, piezo 150 m/min,

675-2700 pages/min

600 x 600 dpi

Screen Truepress

Jet SX

Epson DoD, piezo 1600 sheets/h,

106 pages/min

1440 x 720 dpi

2.2 Methods for preparing carbon nanotube inks

To be able to disperse CNTs efficiently in water, the hydrophobic surface of the

nanotubes needs to be changed to hydrophilic by attaching polar chemical groups

that interact and form hydrogen bonds with water molecules. It should be noted

30

that CNT ink is not a true solution in a molecular sense. CNT ink can be

described as a colloidal suspension which is either stable (colloid) or sediments

with time (suspension) depending on the concentration. A common term

describing both types of inks is dispersion which is used throughout this thesis

(Rosca et al. 2005).

MWCNTs used in the experiments were grown on Si/SiO2-wafers by

catalytic chemical vapour deposition (CCVD) in a continuous tube reactor from

ferrocene dissolved in xylene (10 g/l) acting as a precursor for the metal catalyst

and carbon source (Wei et al. 2002, Halonen et al. 2008). The nanotubes were

detached from the substrate by dissolving the SiO2 surface underneath the

nanotube forest with hydrofluoric acid in ethanol. They were subsequently

oxidized using nitric acid and potassium permanganate solutions to obtain

carboxyl functionalities on the surface (Figure 7) (Paper I). Single-wall carbon

nanotubes were obtained from a commercial source (Sigma-Aldrich) and were

already in carboxyl functionalized form (SWCNT-COOH), so that SWCNT inks

for printing were prepared without any further chemical functionalization steps

(Papers III and IV). Oxidized carbon nanotubes having polar functional groups on

the surface can be dispersed in polar solvents, such as water, alcohols and

dimethyl formamide (DMF) as relatively stable dispersions. Due to van der Waals

forces between functionalized CNTs and π-π –interactions of delocalized

electrons in an aromatic carbon structure, carbon nanotubes tend to form bundles

(Kiang et al. 1995). Severe bundling and agglomeration causes precipitation of

nanotubes in the dispersion. Bundling and precipitation with time also takes place

with oxidized CNTs; however, the inks prepared by the following method are

stable over several weeks of storage. A typical method for making stable carbon

nanotube dispersions is the use of ultrasound agitation in solvents for 1 hour to

debundle and disperse nanotubes (Papers I, III and IV). The dispersions are then

centrifuged at 3500 rpm for 15 min to sediment large bundles and precipitates to

the bottom of the container. Subsequently, the supernatant suspension is collected

and centrifuged again. This was continued until a stable dispersion was achieved,

which was typically repeated 4 times.

31

Fig. 7. Schematics of carbon nanotube fuctionalization.

Chemically oxidized carbon nanotubes were analyzed using FT-IR and Raman

spectroscopy (Figures 8 and 9). After nitric acid treatment, vibrations assigned for

C-O stretching and out-of-plane deformations at 1158 cm-1 and C=O stretching

(1723–1735 cm-1) of the carboxyl groups were seen in the infrared spectrum. The

nitric acid-treated carbon nanotubes were already dispersible in aqueous media. In

order to increase the number of carboxyl groups on the surface of the CNTs,

subsequent oxidation of hydroxyl and carbonyl functionalities to carboxyl acid

functional groups with potassium permanganate (KMnO4) and perchloric acid

(HClO4) was carried out. In the second oxidation step, increased absorbencies for

O-H bending (1401cm-1) and C=O stretching (1723–1735 cm-1) were found in

accordance with the oxidation of hydroxyl and carbonyl functionalities on the

surface. C=C stretching of MWCNTs was found in all spectra (Kim et al. 2005,

Paper I).

32

Fig. 8. FT-IR spectra of functionalized carbon nanotubes. Top: as-produced MWCNTs;

middle: nitric acid treated MWCNTs; and bottom: perchloric acid and potassium

permanganate treated MWCNTs (Paper I, published by permission of Wiley-VHC).

Covalent functionalization of the graphitic surface of carbon nanotubes was

analyzed by evaluating lattice vibrations in the D band (1350 cm-1) and G band

(1582 cm-1) of the Raman spectrum. The D (diamond) and G (graphite) bands

denote sp3 and sp2 hybridized carbon atoms, respectively. The functionalized

carbon nanotubes have increased relative intensity of the D band, which can be

attributed to sp3 hybridization of carbon due to covalent oxygen functionalities on

the surface. Also, the D’ mode (1618 cm-1) appears as a shoulder to the G band

because of strained C=C vibration of neighbouring sp2 hybridized carbons due to

attached functional groups. (Eklund et al. 1995, Paper I)

33

Fig. 9. Raman spectra from as-produced (left) and functionalized multi-walled carbon

nanotubes (right) (Paper I, published by permission of Wiley-VHC).

Recent studies show that as a result of the functionalization, not only polar

functional groups give the ability to disperse nanotubes in polar solvents, but also

functionalized carbon fragments which are formed during oxidation and are

adhered on the surface of CNTs (Salzmann et al. 2007). The effect of

functionalized carbonaceous fragments in the preceding results can not be

neglected; however, the obtained FT-IR and Raman data are relevant regardless of

the fact that they may be dependent on the functionalities on the nanotube surface,

on carbonaceous fragments or on both.

Carbon nanotube inks were printed using a desktop bubble-jet (thermal inkjet)

printer Canon BJC 4550 (Papers I and III) and a piezoelectric drop-on-demand

inkjet printer Dimatix DMP-2831 (Papers IV and V). With the desktop printer, the

cartridges were opened and purified of any remaining dye ink and then refilled

with carbon nanotube inks.

2.3 Inkjet printing performance and characteristics of line pattern

Multi-walled carbon nanotubes were firstly printed on paper and transparency

foils. The lines printed were rectangular (0.3 × 4.2 cm) with multiple repetitions

up to 90 prints. Four platinum electrodes were sputtered with 1 cm spacing for 2

and 4 point I-V measurements. Sheet resistivities of printed films on paper and

plastic substrates are presented in Figure 10. Due to the rough surface of the paper

and to the low amount of MWCNTs deposited in a single printing step, multiple

prints were needed to achieve reasonable electrical conduction (Paper I). It is also

34

worth noticing that possible clogging of printhead nozzles may cause an

uncontrolled decrease in the amount of ink deposited, so that the number of prints

do not always correspond to the quantity of dispensed nanotubes (Paper I and III).

The sheet resistivities of inkjet-printed MWCNTs could be well controlled

between ~1 MΩ/ to ~1 kΩ/.

Fig. 10. Sheet resistivities as a function of print number of inkjet-printed carbon

nanotube films on transparency foil and on paper (Paper I, published by permission of

Wiley-VHC).

The printing resolution using the desktop printer was 300 dpi in a single print.

When using the desktop printer (Canon BJC 4550), the paper/transparency sheets

were fed several times through the printer. Since the registering accuracy of the

feeding mechanism of such office printers is not optimized for printing

subsequent layers, a limited lateral paper positioning of only ±200 µm could be

obtained (Paper I). However, in the case of the drop-on-demand materials

deposition printer (Dimatix DMP-2831), the accuracy for drop deposition was

±2.5 µm with a resolution of 5080 dpi. Also, the nanotube coverage was well

controlled using the single nozzle of the printer. The drop formation was

monitored by a stroboscopic camera to ensure single drop formation by varying

the piezo waveform and applied voltage of the printer as seen in Figure 11

(Papers IV and V).

35

Fig. 11. Stroboscopic image series of drop formation from a printing nozzle. The

image recorded after 42 μs delay shows that the injected nanotube dispersion forms a

single droplet. The orifice of nozzle is the dark central hole having diameter size of

21.5 µm. Due to the stroboscopic camera the photographs also show a mirrored image

of the ejected droplets (Paper V, published by permission of Wiley-VHC).

Multiple prints of multi-walled carbon nanotubes on transparency foil or paper

formed a random network with good percolation. On paper substrates, the

cellulose fibres were 1–10 µm in diameter, and the voids between the fibres

limited the formation of a continuous network of carbon nanotubes (Figure 12a-d).

The patterning of various structures and images by using the inkjet printing

technique is within the limits of printer resolution (Figure 12e) (Paper I).

Fig. 12. Field-emission scanning electron microscopy (FESEM) images of MWCNTs

printed on transparency foil (a,b) and on paper (c,d). (e) Photograph printed with

carbon nanotubes on paper in order to demonstrate patterning and print performance

with CNT inks using a desktop printer (Paper I, published by permission of Wiley-VHC).

37

3 Magnetic-field induced alignment of multi-walled carbon nanotubes

An increased electrical conductivity is expected along the aligned direction of

carbon nanotubes; hence a few carbon nanotubes can reach relatively good

conductivity, leading to higher transmittance. The possibilities of alignment of

aqueous nanotube dispersions in relatively weak magnetic fields were studied.

3.1 Background

Pure carbon nanotubes are predicted to have weak-field magnetic susceptibility.

This could be either diamagnetic or paramagnetic, depending on the chirality,

field direction and Fermi energy (Lu 1995). Functionalization and defects on a

carbon nanotube surface result in changes in the density of states near the Fermi

level. Such defects act as localized magnetic moments with Curie behaviour, i.e.,

the atoms have incomplete electron subshells and therefore non-zero dipole

moment (Ellis & Ingham 2006, Paper II). Carbon nanotubes could be aligned by

this weak paramagnetic behaviour. However, the magnetic fields used for this

purpose are typically higher than 10 T, requiring special facilities and hence

disabling practical applications (Gonnet et al. 2006).

The ferromagnetic properties of CNTs arise from catalyst particle (Fe, Ni, Co)

impurities located on the surface and also entrapped inside the carbon nanotubes.

The origin of such catalyst particles is related to the growth process. In the course

of the acid functionalization process, the catalyst particles are removed from the

surface; however, the rod-shaped metal particles remain inside the hollow cavity

of the nanotubes (Figure 13). Post-deposition of ferromagnetic particles (Correa-

Duarte et al. 2005) or proton irradiation to introduce ferromagnetic H-vacancies

and H-adatom complexes (Ma et al. 2005) are options to increase the magnetic

susceptibility; however, these are not necessary in order to obtain alignment in

relatively weak magnetic fields (Paper II).

38

Fig. 13. Energy-filtered transmission electron microscopy (EFTEM) images of carboxyl

functionalized carbon nanotubes. The black arrows show catalyst particles entrapped

in the inner tubular cavity of nanotubes (Paper II, supporting information, published

by permission of American Chemical Society).

3.2 Magnetic properties of multi-walled carbon nanotubes

The nanotubes which were used in the inks (Paper I) had ferromagnetic impurities

deriving from the catalyst particles in the CCVD growth. The magnetic properties

of as-produced and carboxyl functionalized carbon nanotubes were measured by

using a superconducting quantum interference device (SQUID). In the

measurements of the field dependence of magnetization, the as-grown nanotubes

show ferromagnetic polarization saturating at 0.65 emu g-1 at 300 K and at 0.77

emu g-1 at 5 K, which decreases after carboxyl functionalization to 0.12 emu g-1

and 0.17 emu g-1, respectively (Figure 14). The decrease of magnetization with

functionalized carbon nanotubes is due to purification and dissolution of Fe

nanoparticles from the CNT surface. Some ferromagnetic polarization remains

due to entrapped Fe nanoparticles inside the functionalized carbon nanotubes

(Figure 13). Hysteresis in the field dependence measurements confirms

ferromagnetic ordering of spins in the Fe nanoparticles. The temperature

dependence of magnetization shows an increase in the magnetization at 5 K. This

confirms the increased paramagnetic properties of functionalized carbon

nanotubes that stem from the defects acting as localized magnetic moments.

39

Fig. 14. SQUID measurements from as-produced (a,b) and carboxyl functionalized (c,d)

carbon nanotubes. Temperature dependence (a,c) of magnetization measured for both

zero-field cooling, ZFC and field cooling, FC and field dependence (b,d) of

magnetization were measured at 5 and 300 K (Paper II).

3.3 Optical characterization of aligned carbon nanotubes in dispersions

Inks (dispersions) made from carboxyl functionalized carbon nanotubes having

iron impurities are expected to show spatial ordering of the fluid components in

external magnetic fields. A stable multi-walled carbon nanotube dispersion with

concentration of 0.1 g/l was prepared as described previously (Paper I). A part of

the dispersion was diluted with deionized water to 0.025 g/l for measurements at

lower concentration. The dispersions were placed in 10 mm quartz cuvettes which

were inserted in the bore of an electromagnet, and the optical transmission of a

40

polarized laser beam was measured through the dispersion with reference to the

magnetic field direction (0° ≤ φ ≤ 180°) as shown in Figure 15.

Fig. 15. Measurement setup for optical characterization of alignment in carbon

nanotube dispersions (Paper II, published by permission of American Chemical

Society).

The transmitted laser beam intensity through the nanotube dispersion could be

modulated by applying relatively low magnetic fields. The original zero-field

transmission of a 0.025 g/l dispersion (T0) 38.6% is reduced to 20.8% with

perpendicular transmission (T) and increased to 54.8% with parallel transmission

(T). With the dispersion of 0.1 g/l concentration, the absolute values of the

induced transmission changes were much lower than those for the diluted

dispersion due to increased absorption and scattering of the laser beam (Figure

16).

The optical dichroism of ordering was calculated according to Equation 3

(Shobaki et al. 1996, Paper II)

0 0

ln ln ,II

A A AI I

⊥⊥

Δ = − = − − −

(3)

where ΔA represent the optical dichroism, and A and A are the absorption

perpendicular and parallel to the laser polarization, respectively. I0, I and I

denote the transmitted intensities in zero-field, field perpendicular and field

parallel to the laser polarization, respectively. For the diluted 0.025 g/l dispersion,

the optical dichroism becomes 0.97, and with the original 0.1 g/l concentration its

3.02. The obtained optical anisotropies suggest very good field-induced ordering.

41

Fig. 16. Magnetic field and polarization vs. optical transmission plots for CNT

dispersions of 0.025 g/l and 0.1 g/l concentrations (Paper II, published by permission

of American Chemical Society).

The quantitative account of the ordering was calculated by the single-parameter

orientation model with an ordering parameter S (Islam et al. 2004, Paper II),

( ) ( )1 2

1

1 3coscos ,

2S f d

θθ θ−

− +=

(4)

where ( )

2

2

cos

1cos cos

1

( )d

ef

e

α θ

α θ θθ

−

−

−

=

and where

kin

U

Eα Δ≡ . (5)

Here, θ is the polar angle the nanotube axis makes with magnetic alignment field,

ΔU is the difference in the magnetic potential energy of randomly oriented and

aligned nanotubes, and Ekin is kinetic rotational energy. Values for ΔU and Ekin are

determined in Paper II. Finally, the nematic ordering parameter S becomes 0.99,

meaning excellent alignment at room temperature with a relatively low magnetic

field applied. It is worth noting that the values obtained for calculations were

statistical averages, since in reality the ordering parameter depends on the amount

of iron entrapped in a particular nanotube, and in the dispersions the amount of

iron particles varied from nanotube to nanotube. However, these calculations

were supported by considering pure nanotube parameters, and from that the

42

nematic ordering parameter S becomes close to zero, implying random orientation

in the dispersion (Paper II. Note: due to miscalculation, the value obtained in

Paper II was –0.48, which denotes perpendicular alignment. The correct zero

value was obtained in recalculations, see Paper II Additions and Corrections).

The heterogeneous nature of the magnetization was demonstrated by drop-

casting the carbon nanotube dispersion on a polished silicon wafer in the presence

of a magnetic field. FESEM imaging of a dried droplet shows that some of the

nanotubes are aligned in the field, while others are randomly oriented (Figure 17).

Nanotubes that are on the perimeter are aligned parallel to the magnetic field,

whereas the rest of the nanotubes form a randomly oriented network. The ring-

shaped pattern is formed due to the “coffee-drop” effect that is caused by

capillary flow in the drying droplet, in which pinning of the contact line of the

drying drop ensures that liquid evaporating from the edge is reloaded by liquid

from the interior (Deegan et al. 1997). Nanotubes which have iron nanoparticles

entrapped inside are aligned and stick together by mutual magnetic attraction.

Stacked nanotubes form aggregates that are deposited near the drop edge and are

aligned. The non-magnetic nanotubes remain mobile and are found in the middle

part of the droplet. This was supported by EDS analysis proving the absence of

iron in the randomly oriented carbon nanotube areas (data not shown).

43

Fig. 17. FESEM images of dropcast carbon nanotubes on polished silicon wafer.

Aligned carbon nanotubes at the edge (a,b,f). Transition zone (c) and zone of randomly

oriented carbon nanotubes (d). In the middle (e), both types of nanotubes were

present. The main panel size is 2.5 × 2.5 mm2, and the size of panels (a-f) are 14 × 7

µm2 (Paper II, published by permission of American Chemical Society).

45

4 Inkjet printing of single-wall carbon nanotubes and polymer composites

Coexistent conductivity and high transparency are made possible when using

single-wall carbon nanotubes instead of multi-walled ones. Also, the addition of a

small amount of single-wall carbon nanotubes in polymers increases the

conductivity of the host material without remarkably changing the optical

properties in the visible spectrum. Accordingly, the electrical conductivity of

highly transparent and conductive polymer was improved with SWCNTs when

depositing thin films of SWCNT-polymer composites using an inkjet printer.

4.1 Background

The optical properties of CNT films depend on their type and structure, which in

turn are influenced by how the nanotubes are produced. Due to their size and also

to the lack of band gap in their electronic structure, MWCNTs are more visible

than SWCNTs. Furthermore, the degree of nanotube dispersion upon the surface,

nanotube purity, diameter, defects and also the CNT-substrate interaction play an

important role in the optoelectric performance of deposited films. (Li et al. 2008).

Transparent and conductive CNT films are applied as microwave shielding

(Xu et al. 2007) and as transparent heating elements on glass substrates (Yoon et

al. 2007). In composites with conductive polymers such as polyaniline and

poly(3,4-ethylenedioxythiophene):poly(styrenesulphonate) (PEDOT:PSS), CNTs

are used as transparent electrodes in electrochromic devices (Hu et al. 2007),

organic solar cells (Rowell et al. 2006) and in organic light-emitting diodes

(OLEDs) (Li et al. 2006).

4.2 Preparation and printing of composite ink

Carboxyl functionalized single-wall carbon nanotubes (SWCNTs) were obtained

from a commercial source (Sigma-Aldrich), and stable dispersions were prepared

as explained previously (Paper I) with the exception of having ethanol as solvent

instead of deionized water. According to the datasheet, the nanotube content of

commercial SWCNTs was 90% having a carboxylic acid composition of 3–6 at%

and bundle dimensions of 4–5 nm × 0.5–1.5 µm.

To reduce the ring formation of a drying droplet (coffee-drop effect), usually

mixtures of higher boiling point additives or lower boiling point solvents such as

46

ethylene glycol or isopropanol, are used with aqueous solvents (de Gans &

Schubert 2004). Accordingly, ethanol – as a low boiling point solvent – was used

in the ink to accelerate the drying time for deposited ink droplets, thus preventing

ring formation (Paper III). Alternatively, by increasing the substrate temperature,

the rate of evaporation can be increased to prevent or at least minimize the ring

formation effect (Soltman & Subramanian 2008).

The polymer (PEDOT:PSS) solution (1.3 wt% in water) was obtained from

the same source as the nanotubes. The solution was diluted with deionized water

with a volumetric ratio of 6:4 to make stock solution for polymer and composite

inks. The composite ink was prepared by mixing 8 parts of SWCNT ink in

ethanol and 2 parts of polymer stock solution with vigorous stirring. As reference,

polymer ink was prepared similarly using pure ethanol instead of SWCNT ink.

Multiple prints (up to 30) of SWCNTs, polymer and composite inks were

deposited using a thermal desktop printer (Canon BJC-4550) on high gloss photo

paper (Canon PP-101, 270 g/m2) and on 75 µm thick poly(ethylene terephtalate)

(PET) sheets. Platinum was sputtered for contact electrodes, and the sheet

resistances of the deposited films were measured for both types of substrates. Also,

the optical spectra of PET films were measured for optical characterization. In the

case of multiple prints, the substrate feeding mechanism caused inaccuracy of

pattern overlapping. In addition, peeling of SWCNTs from the PET surface was

observed; therefore, optical characterization of nanotubes without polymer was

practically impossible. However, nanotube adhesion on photo paper was found to

be good, and comparison of sheet resistance between the polymer and composite

films could be made.

4.3 Structural and optical characterization of nanotube-polymer composite films

The addition of carboxylated SWCNTs in PEDOT:PSS showed improved

conductivity, i.e., decreased resistivity, in inkjet-printed films on both substrates

as shown in Figure 18. The main goal besides decreasing resistivity was to keep

the films’ transparency as high as possible. When printing multiple layers, the

amount of nanotubes was increased, and transmission in the visible spectrum

became higher for polymer than for the composite. However, with the addition of

SWCNTs for thin films of PEDOT:PSS (which is a requisite for nearly invisible

conductive films), the sheet resistivity is decreased an order of magnitude.

Another remarkable point for multiple prints of SWCNT-polymer composite films

47

in the infrared region (2500 nm) of transmitted light is that the addition of

SWCNTs does not decrease the resistivity of the film. This result contradicts the

other data presented on the left side of Figure 18, where the composites have

decreased sheet resistivity compared to polymers with increased number of prints.

The explanation for this contradiction is the nozzle clogging that could not be

controlled in the desktop printer, so that the number of prints is not an exact

parameter for evaluation of the amount of deposited material. However, the

optically determined amount of polymer is quantitative, and therefore it can be

used as a proper method for film characterization. For some reason, the polymer

printed with multiple times clogged the printhead more than the composite ink,

explaining the lower conductivity in multiple prints. The increase of conduction

in the thin films occurs from the improved percolation of the conductive polymer

phase by the SWCNTs interconnecting the regions. In the case of thick films, the

percolation in the polymer is contiguous, and the effect of SWCNTs is only

optical (Paper III).

Fig. 18. Upper: Sheet resistivities of polymer and composite patterns printed on PET

film and photopaper substrates as a function of print number (left) and transmittance

(right). Lower: Photographs of film where the printed film is connected in series with a

green light-emitting diode (LED) and placed between the LED and camera. The

corresponding data of the photograph are shown by arrows (Paper III, published by

permission of Wiley-VHC).

48

Structural characterization of PEDOT:PSS gives more insight into the conduction

mechanisms of the film. PEDOT:PSS consists of water insoluble PEDOT

polymer as the conductive part, and by adding excess of PSS polymer, a water

soluble compound is formed (Sirringhaus et al. 2000) (Figure 19). Water soluble

grades of PEDOT:PSS have a conductive PEDOT:PSS phase and an insulating

PSS phase in excess that could be distinguished from each other using atomic

force microscopy, AFM (Crispin et al. 2006). The resistivity is high with a few

prints of PEDOT:PSS due to the lack of a continuous network of conductive

PEDOT:PSS islands in the partly separated insulating PSS. The addition of

SWCNTs increases the percolation through bundled nanotube networks. For

thicker films the percolation of conductive PEDOT:PSS is continuous and the

addition of SWCNTs does not play any role in the conduction due to the large

contact resistance between polymer and nanotube that limits the current to flow

only in the polymer (Paper III).

Fig. 19. Chemical structure of poly(3,4-ethylenedioxythiophene):poly(styrenesulphonate)

(PEDOT:PSS) polymer (Sirringhaus et al. 2000).

Composite films were studied using energy filtered transmission electron

microscopy (EFTEM), field-emission scanning electron microscopy (FESEM)

and AFM using a non-contact tapping mode (Figure 20). The EFTEM image

shows a good dispersion of SWCNTs in the polymer. The FESEM image, which

is taken from a crack induced by scratching the surface, shows clearly how the

polymer coats the nanotubes. The surface analysis made with the tapping-mode

AFM shows the smoothness of the printed films with <40 nm roughness and

without signs of the typical ring formation of inkjet-printed droplets. The phase

images reveal more features from the film, and different soft and hard phases can

be distinguished from the tip movement. These phases turn out to be conductive

PEDOT:PSS networks (light) and insulating PSS islands (dark) (Crispin et al.

2006, Paper III). From the AFM phase images, it is clearly seen that nanotubes

49

form bridges between polymer islands and therefore increase the conductivity in

sparse polymer networks.

Fig. 20. (a) EFTEM image of well-dispersed SWCNTs in PEDOT:PSS. (b,c) FESEM

images of nanotubes embedded in the polymer, where the image is taken from crack

induced by scratching surface nearby. (d,e) Tapping-mode AFM topography images of

printed composite and corresponding phase images (f,g), where the various material-

phases can be distinguished (Paper III, published by permission of Wiley-VHC).

51

5 Electrical characterization of single-wall carbon nanotube films

Deposition in a controlled manner, and an understanding of the electronic

properties of inkjet-printed single wall carbon nanotube films, are essential in

applications aimed at practical devices. Once individual droplet deposition was

made possible, controlled printing and film generation of various films were

achieved. From the results obtained, carbon nanotube transistors on silicon chip

and on plastic substrates were demonstrated.

5.1 Background

The electronic properties of individual single-wall carbon nanotubes are diverse

and fascinating. A number of factors have an influence on their electrical transport

mechanism, such as nanotube chirality (Yorikawa & Muramatsu 1995, Kleiner &

Eggert 2001), temperature (Ebbesen et al. 1996), electrical contact (Avouris

2002), chemical functionalization (Strano et al. 2003) and surrounding chemistry

(Collins et al. 2000). The transport mechanism of metallic single-wall nanotubes

is ballistic, i.e., electrons travel through a conjugated electron cloud of carbon

nanotubes without electrical resistivity caused by scattering (Mann et al. 2003).

Semiconducting nanotubes have Schottky-type barriers when contacted with

metallic counterparts, as is the case with other semiconductor materials (Avouris

2002).

With networks of carbon nanotubes, the situation is different and more

complex. The adjacent nanotubes or bundles of nanotubes may lead to carrier

hopping from one nanotube to another (Skákalová et al. 2006), or can result in a

series of Schottky junctions when metallic and semiconducting nanotubes cross

each other (Yao et al. 1999, Paper IV).

5.2 Film deposition and substrate characteristics

An aqueous dispersion of carboxyl functionalized single-wall carbon nanotubes

(SWCNTs) was prepared as described previously in section 2.2 (Paper IV) and

loaded into cartridges of a Dimatix DMP-2831 printer. SWCNTs were deposited

on alumina (Al2O3) substrates between thick-film gold electrodes of 200 µm gap

with different surface coverage. Gold electrodes were used to minimize the

rectifying SWCNT-to-electrode behaviour of semiconducting nanotubes (s-CNTs)

52

(Avouris 2002, Paper IV). Various SWCNT coverings were achieved by printing a

different number of layers over the same pattern (3–20 layers). SWCNTs were

deposited as bundles and individual nanotubes in random orientation. Typical

bundle lengths were 0.5–2.0 µm with diameters of 10–20 nm as measured by

EFTEM and SEM as well as AFM. The bundles were in intimate contact with the

substrate; however, suspended nanotubes were bridging at the boundaries of

Al2O3 crystallites (Figure 21).

Fig. 21. Optical and scanning electron microscopy (SEM) images of the samples

printed on alumina substrates. (a) Optical microscopy image of a sparse SWCNT film

(3 prints) on alumina substrate between gold electrodes. (b) SEM image showing a

partially connected sparse SWCNT network on the alumina substrate. (c) Low

magnification image of a dense SWCNT network (20 prints). (d) SEM image of dense

SWCNT network. (e) Close-up image of SWCNTs bridging two alumina crystallites.

Because of good percolation, the dense network is well grounded through the contact,

and resolution is good. Sparse networks’ lack of good percolation and local charging

of SWCNTs caused by electron beam are seen in image (b) (Paper IV, published by

permission of American Physical Society).

53

5.3 Electrical characterization

The printed SWCNT films of variable surface coverage were measured using a

dual sourcemeter (Keithley 2612) in ambient air and in nitrogen atmosphere at 20

to 150 °C. Current-voltage (I-V) measurements show coverage-dependent

electronic transport as seen in Figure 22.

Fig. 22. Current-voltage characteristics of inkjet-printed SWCNT films on alumina

substrate with different surface coverage (number of print repetitions) measured in

ambient air at temperature range up to 80 °C (Paper IV, published by permission of

American Physical Society).

Nanotube films of high density behave as ohmic conductors characterized by

linear I-V curves. The curves are expected to be linear due to the higher surface

coverage, which enables direct contact between the metallic carbon nanotubes

(m-CNTs) in the random network, thus providing continuous and highly

conductive paths for the current to pass through the film. In contrast to dense

films, sparse networks obtained with a low number of print repetitions show

nonlinear features. In these films, the probability for a contiguous network of m-

CNTs becomes low, and other conduction mechanisms contribute to the transport.

It is worth considering that the effect of self-heating from the applied voltage

would be a very unlikely explanation for nonlinear characteristics, because such

an effect would be more pronounced for denser films where significantly higher

power is generated compared to sparse networks (Paper IV).

The nonlinear characteristics arise from the film properties consisting of

random networks of metallic (m-CNTs) and semiconducting carbon nanotubes (s-

CNTs). Statistically, the amount of s-CNTs is twice that of m-CNTs. In sparse

networks, only a few contiguous paths exist, leading to a random sequence of

Schottky junctions between the m-CNTs and s-CNTs. The Schottky barrier

between two types of nanotubes is formed because the valence bands of s-CNTs

are below the Fermi energy of m-CNTs. Consequently, carriers that have energy

54

larger than the barrier height will be injected via thermionic emission into the

nanotube. The potential and temperature-dependent current density J(V,T) for a

metal semiconductor interface is described as (Sze 1981):

( )*

* 2, exp exp 1b qVJ V T A T

kT kT

Φ = − −

(6)

Here, Фb is the Schottky barrier height, A*=4πqm*k2/h3 is Richardson’s constant, q

is the elementary charge, m*=0.037me is the effective mass of carriers (Jarillo-

Herrero et al. 2004), and k and h are Boltzmann’s and Planck’s constants,

respectively. The potential drop across the series intrinsic resistance R of the CNTs

is taken into account by considering V*=V-RI, where the R is the reciprocal of the

slope of the I-V curve at large bias voltages. When applying Equation 6 for a thin

film with average cross section, A, which is estimated from the amount of CNTs

deposited in the gap between the electrodes, and m number of Schottky junctions

along a percolated path, the current I becomes (Paper IV)

( ) ( )* 2 /, exp exp 1b

q V RI mI V T AA T

kT kT

−Φ = − −

(7)

By fitting Equation 7 to the measured I-V plots, the parameters Фb and m are

resolved, as presented in Table 2.

Table 2. Apparent intrinsic resistance of the nanotube network R, effective Schottky

barrier height Фb, and number of Schottky junctions m in series in a percolated path

as a function of SWCNT coverage (Paper IV).

Number of prints R (kΩ) Фb (meV) m

3 4875.0 ± 24.1 253.8 ± 3.0 66

5 1348.2 ± 17.1 220.0 ± 2.6 53

10 70.33 ± 0.29 148.6 ± 1.3 35

15 26.40 ± 0.03 117.9 ± 1.3 26

20 16.27 ± 0.01 107.0 ± 1.2 17

The values obtained for Schottky barrier heights in the measured films are in

good agreement with measurements performed on single junctions of m-CNTs

and s-CNTs (Yao et al. 1999). The barrier height gradually decreases due to

increased percolation of m-CNTs in the network. Also, the number of Schottky

junctions in series in a percolated path gives a reasonable estimate since the

55

distance between electrodes (200 µm) is connected with bundles having length of

1 µm. It also decreases due to parallel short circuits within a percolated path

caused by the increased number of m-CNTs (Paper IV).

The conductance of each sample increases with temperature as seen in

Figures 22 and 23. In order to analyze the temperature dependence, I-V

characteristics and resistances at constant 5 V bias were measured in nitrogen

atmosphere, and the relative change of resistance values Δr/r0 were plotted for

dense (20 print numbers) and sparse (3 print numbers) networks as a function of

temperature.

Fig. 23. Left: Current-voltage characteristics of inkjet deposited dense (20 prints) and

sparse (3 prints) SWCNT films measured in nitrogen atmosphere. Right: Relative

change of film resistances ∆r/r0 upon heating (h1), and subsequent cooling (c1) and

reheating (h2) (Paper IV, published by permission of American Physican Society).

The resistance drop for sparse networks is higher than in dense networks. The

reason for this behaviour is ambiguous, and only a qualitative analysis could be

made from the results (Paper IV). In sparse networks, a negative temperature

coefficient is expected due to enhanced Schottky emission at the interfaces and

56

also to the increased carrier concentration in the s-CNTs as the temperature is

increased (Kaiser et al. 1998, Skákalová et al. 2006). In the case of m-CNTs, the

electron-electron interactions can cause a slow negative temperature coefficient of

resistance in ballistic transport which could occur in short m-CNTs (Bockrath et

al. 1999). For long m-CNTs which are diffusive due to defects in the structure, a

positive temperature coefficient of resistance is caused by scattering on the radial

breathing mode (RBM) phonons (Rinzler et al. 1998, Yin et al. 2007, Paper IV).

The large negative temperature coefficient seen in sparse networks is

compensated for by the combination of these processes and results in a less

negative temperature coefficient in dense films (Paper IV).

Carbon nanotube applications that are meant to be used in ambient

atmosphere need consideration of adsorbed moieties and how they change the

electronic properties of the films. Electron donors or proton acceptor molecules

on nanotubes increase the electrical resistance due to induced downshift of the

valence band of p-type nanotubes away from the Fermi level (Kong et al. 2000).

After the first heating cycle, by cooling samples back to room temperature and

reheating them in nitrogen atmosphere, Δr/r0 vs. temperature curves split at

100 °C, and the resistance value follows the previous value which indicates the

presence of adsorbed moisture on the nanotubes that is desorbed during the first

heating cycle.

5.4 Inkjet printed carbon nanotube transistors

To demonstrate the applicability of results obtained previously, sparse and dense

networks were applied as channels in field-effect transistor (FET) devices. Field-

effect transistor measurements were carried out on silicon chips having an inkjet

printed nanotube channel (Paper IV) and also on polymer foils by printing all the

transistor electrodes and the channel using either dense or sparse networks

(Mustonen et al. 2008). On silicon chips, individual droplets were deposited

between electrodes of 15 µm gap. Si chips were fabricated by standard thin film

process sequences including sputter deposition, plasma-enhanced chemical vapour

deposition (PECVD), lithography, reactive ion etching (RIE) and wet etching

(Figure 24).

57

Fig. 24. (a) Layout of the Si chip used in the SWCNT FET measurements. The optical

micrograph shows a set of Pt electrodes (source-drain) upon which the SWCNTs were

inkjet printed. (b) Atomic force microscopy images of the electrode area. The higher

magnification shows networks of SWCNTs printed between the electrodes (Paper IV,

published by permission of American Physical Society).

Sparse films having nonlinear I-V characteristics could be modulated with the

application of a gate voltage with an on-off ratio up to 10. The low on-off ratio is

attributed to the thick gate oxide (3.15 µm) compared to that conventionally used

(few nanometres) and is expected to increase with thinner gate oxides. The large

hysteresis is expected to have arisen from charge injection to the surrounding

58

dielectric (Vijayaraghavan et al. 2006). In contrast, a chip a with dense network of

SWCNTs having linear I-V characteristics showed negligible gate effect (Figure

25)

Fig. 25. I-VSD and I-VG sweeps on sparse (a) and dense (b) SWCNT networks on Si-chip

(Paper IV, published by permission of American Physical Society).

Because the films could be made either metallic or semiconducting simply by

depositing either dense or sparse films from the same SWCNT ink, all the

transistor electrodes and the channel could be printed using the same material. In

addition, when using an insulating polymer as a gate dielectric layer, all the

transistor components could be inkjet printed onto any thin dielectric substrate.

This was demonstrated by inkjet printing a multiple layer SWCNT source and

59

drain electrodes of 1 × 1 mm size with a channel length of 700 µm and width of

200 µm on polyimide foil of 25.4 µm thickness. Subsequently, sparse SWCNT

networks were deposited between the electrodes acting as the channel in the

transistor. The dense SWCNT gate electrode was printed on the opposite side of

the polyimide film. A 20 layer SWCNT printed channel showed nonlinear I-V

behaviour which could be modulated with a gate bias. A 50 layer channel had a

linear I-V and showed a negligible gate effect (Mustonen et al. 2008). Figure 26

shows the schematics and transistor data of fully printed SWCNT transistors

using a single ink.

Fig. 26. Top: Schematic cartoon and photograph of a completely inkjet printed SWCNT

transistor. In the photograph, the polyimide foil is bent in order to apply the gate bias

through the lowest probe needle. Two other probe needles were placed on the source

and drain electrodes. Bottom: I-VSD and I-VG sweeps on completely printed SWCNT

transistors (Mustonen et al. 2008).

The resolution of tens of micrometres and heterogeneous nature of the printout in

the inkjet printing technique limits the transistor performance drastically.

However, recent advances in the printing resolution down to 1 µm scale have been

demonstrated by using electrohydrodynamic jet printing (Park et al. 2007) and

sub-femtolitre inkjet printing (Sekitani et al. 2008).

61

6 Carbon nanotube gas sensors

Gaseous molecules tend to adsorb on a carbon nanotube’s surface. Depending

upon the oxidative or reductive nature of the gaseous moieties, the doping can

cause an increase or a decrease in the electrical conductivity of semiconducting

SWCNTs. These characteristics are utilized for gas sensing applications, such as

resistive sensors and more mature and selective transistor-type chem-FET sensors.

6.1 Background

The adsorption of an oxidative gas, e.g. NO2 , shifts the Fermi level in the

semiconducting nanotube closer to the valence band which causes an enrichment

of hole carriers in the tubes and thus enhances the conductance. In contrast, a

reductive gas, e.g. NH3 , shifts the valence band away from the Fermi level which

results in hole depletion and the conductivity is reduced (Kong et al. 2000). The

doping effects of reductive or oxidative species are similar with metallic

nanotubes (Varghese et al. 2001). In bulk SWCNT samples, the effect of doping

is averaged over metallic and semiconducting nanotubes (Kong et al. 2000). Also,

a semiconductor-metal interface, for example as seen in contacts between

semiconductor and metal electrodes, is affected by the doping of the adsorbed

moiety. The Schottky contacts between metallic and semiconducting SWCNTs act

in a similar manner. The major drawback for most types of electrical gas sensors is

their selectivity. Because they tend to sense any kind of moiety, in the case of

mixtures of species no distinction can be made by measuring only the changes in

the conductivity. Several methods to solve this selectivity problem have been

introduced lately. In temperature modulated sensing (TMS), periodical signals are

applied to the sensor’s heating element, which alter the adsorption and desorption

kinetics of gas moieties and results in a signal that is characteristic for the gas

under investigation. The periodic signal data is extracted and converted using

various computing algorithms (Ionescu et al. 2005).

Another method to discriminate gas species is to use fluctuation enhanced

sensing (FES), where the fluctuations in the resistance are measured, i.e., the

noise of the system is under investigation. Nanostructured materials, e.g., carbon

nanotubes (Haspel et al. 2008), proved to be excellent for FES due to their grainy

and porous structure, which causes significant electronic fluctuations compared to

single crystalline materials. The selectivity is obtained by analyzing the

fluctuation data with mathematical tools. (Kish et al. 2000).

62

6.2 Resistive and chem-FET gas sensors

CNT networks having an ohmic behaviour can be used only as resistive gas

sensors. MWCNTs – which are all metallic – printed on paper showed resistance

values which varied in the presence of gas moieties according to Table 3 (Paper I).

Table 3. Relative resistance change ∆r/r0 of MWCNT patterns printed on paper. The

pressure value corresponds to the saturated vapour pressure of each substance at

295 K (Paper V).

Substance Vapor pressure (mmHg) ∆r/r0

water 17 ~1.0

ammonia 580 ~1.5

methanol 128 ~0.5

ethanol 50 < 0.1

2-propanol 44 < 0.1

Dense SWCNT networks also have a metallic nature and can be used similarly.

However, sparse networks of SWCNTs have nonlinear I-V characteristics (Paper

IV), and the field-effect of the gate can tune semiconducting nanotubes in the film

and hence increase selectivity of sensors towards certain gases; thus the effect can

be utilized in chem-FET sensors.

The active sensing films of SWCNTs were inkjet printed onto silicon chip

dies (Paper IV) using a Dimatix DMP-2831. For rapid measurements, the rate of

heating and cooling of the sensors was improved from conventional ceramic

package chips by using a custom made package design with suspended chip dies,

where the chips were hung on the bonding wires above the substrate carrier

(Figure 27). The heating and cooling curves of both types of chip packages are

shown in Figure 28.

63

Fig. 27. (a) Conventional ceramic and (c,d) custom-made suspended chip die

packages. Panel (b) shows a micrograph of the die area where the nanotubes were

inkjet printed (Paper V, published by permission of Wiley-VHC).

Fig. 28. Heating-cooling curves of sensor chips in the two setups: Temperature vs.

time for a suspended die (solid line, heating with 2.5 V) and for an adhesive mounted

die (dashed line, heating with 5.0 V) (Paper V, published by permission of Wiley-VHC).

Dense and sparse networks of SWCNTs were printed on chips in order to produce

both ohmic and nonlinear I-V characteristics, respectively. Measurements were

carried out in a gas chamber of ~500 ml volume. Prior to each measurement, the

sensors were annealed at 130 °C for 10 minutes by using the integrated Pt micro

heater of the chip. The dry sensors were exposed to H2S gas (1 to 100 ppm in

64

synthetic air buffer) at constant gas flow of 1 l/min, and both I-V measurements

and gate voltage sweeps were carried out using a Keithley 2612 dual sourcemeter.

I-V measurements were made using gate biases of 0 V, 50 V and -50 V and gate

voltage sweeps using constant source-drain bias voltages of 1 V and 5 V. For gate

voltage sweep measurements, the sampling of analyte gas was started after the

channel current was stabilized (Paper V).

The relative change in the resistance (Δr/r0) for applied 100 ppm H2S was

less than 8% for the dense SWCNT network resistive sensor and, due to the ohmic

behaviour of the film, the gate control was negligible. In the case of sparse

SWCNT networks, gate control was observed which increased the possibilities to

extract more information from the chem-FET sensor device than from the

resistive sensor (Figures 29 and 30).

Fig. 29. Top, source-drain I-V data of resistive gas sensors (20 prints, dense network)

under various gate voltages (0 V, +50 V and -50 V) for synthetic air as reference (left

panel) and 100 ppm H2S gas in synthetic air buffer as analyte (right panel) measured at

room temperature. Bottom, similar data obtained from sparse networks (3 prints)

(Paper V, published by permission of Wiley-VHC).

65

By plotting the relative difference of H2S analyte and synthetic air (reference) in

the channel conductance (IS-Danalyte – IS-D

ref)/IS-D as a function of source-drain

voltage (US-D) and gate bias (UG), the differences between resistive and chem-FET

sensors are revealed (Figure 30).

Fig. 30. Relative difference of channel conductance for 100 ppm H2S gas in synthetic

air (analyte) and synthetic air (reference) for the resistive (left panel) and chem-FET

sensors (right panel) (Paper V, published by permission of Wiley-VHC).

As seen in Figure 30, the addition of 100 ppm H2S showed an increased

conductivity compared to the reference gas without H2S analyte in both cases. For

a sparse SWCNT network the analyte response is slightly higher. However, a

significant difference is observed for low source-drain voltages at positive gate

bias. When using 50 V positive gate voltage (UG) and 0.2 V source-drain voltage

(US-D), the channel conductivity is ~60% higher than with the reference gas.

The reason for selectivity for the sparse network chem-FETs occurs because

of simultaneous gate effect modulation of the Schottky barriers between m-CNTs

and s-CNTs (Paper IV) and the doping with gas analyte of the semiconductor-

metal interface. At low applied voltage, the carriers have limited energy to get

through the barriers and doping of adsorbed gas contributes to the event by

changing the Fermi level. In the dense films, there is only a small Schottky barrier

or no barrier at all, so that the modulation is restricted only to changed carrier

scattering events caused by the action of the H2S on the nanotubes (Tans et al.

1998, Martel et al. 1998, Paper V). At higher voltages, the difference between

chem-FET and resistive sensors is insignificant due to the fact that the Schottky

junctions behave as diodes with a large forward bias and the transport behaviour

is close to the ohmic regime (Kong et al. 2000).

67

7 Conclusions

The main objective for this thesis was to develop electronic applications

incorporating carbon nanotube (CNT) inks dispensed using inkjet printing

technology. Inks of multi-walled carbon nanotubes (MWCNTs) were prepared

from chemically functionalized nanotubes. The functionalization process is based

on a partial oxidation of the surface of the carbon nanotubes with oxidizing acids

introducing polar carboxyl groups, thus enabling dispersion in polar solvents.

Subsequent preparation steps, ultrasonic agitation and centrifugation were applied

to give stable dispersions of the produced inks. Carboxyl functionalized single-

wall carbon nanotubes (SWCNTs) were commercially available and were used as

received from the vendor. The inks produced from both types of carbon nanotubes

were stable and could be dispensed using drop-on-demand type inkjet printers.

The inkjet printing of MWCNT inks using an office desktop printer and

characterization of ink and line pattern have been demonstrated in the thesis.

Subsequently, a magnetic field was applied for aqueous MWCNT dispersions to

control the ordering of the deposited nanotubes. It was realized by using a

polarized laser beam that nanotubes having ferromagnetic iron particles could be

aligned according to a magnetic field. The orientation of carbon nanotubes in the

magnetic field was quantified by calculating the ordering parameter using a

single-parameter orientation model.

Transparent conductive coatings were demonstrated by inkjet printing of the

conductive polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulphonate)

(PEDOT:PSS). It was shown that by incorporating SWCNTs in the polymer, the

conductivity was further improved without loss of transparency in the visible

region. It was also noticed that the improved conductive properties apply only for

thin films with a small number of prints. The reason for this behaviour was

studied from the deposited films’ structure using atomic force microscopy. From

this study it was found that the impregnation of SWCNTs builds bridges between

conductive parts and increases percolation in the film.

The printing performance was improved by controlled deposition using a

single nozzle and adjustable printing parameters when an advanced piezoelectric

printer was acquired. As the film deposition was controlled and the amount of

deposited nanotubes was able to be determined quantitatively, the electronic

properties of inkjet-printed SWCNT films of various surface coverage were

studied using current-voltage (I-V) measurements. It was noticed that the I-V

characteristics of SWCNT films varied from linear to nonlinear as a function of

68

surface coverage. In dense films, the percolation of metallic nanotubes enables

ohmic (linear) conduction. In the case of sparse films, the carriers need to pass

through Schottky junctions between metallic and semiconducting nanotubes,

resulting in a nonlinear behaviour. Sparse film performance was demonstrated with

inkjet printed field-effect transistors (FETs) on a prefabricated silicon chip

structure. Sparse films showed gate voltage response with modulation on-off

ratios up to 10, which is a reasonably good result because the gate oxide was very

thick compared to that of conventional transistors. Dense SWCNT films showed

no gate response in FETs.

Moreover, the controlled ohmic and nonlinear SWCNT films’ properties were

utilized in fully printed transistors by using the ink as a single source for printing

metallic and semiconducting parts of the FET structure. In this thesis, the first

demonstration of printing all the transistor components on a flexible polymer film

using a single ink for source, drain and gate electrodes (dense network) and

channel (sparse network) is described. The polymer substrate in the fully printed

FETs acted as a dielectric material.

In the last part of the thesis, inkjet printed SWCNTs were further utilized as

conventional resistive sensors (dense network) and chemical-FET (chem-FET)

sensors (sparse network) for H2S gas analysis. The films of the chem-FET sensors

showed an increased selectivity towards H2S when compared to similar films with

ohmic behaviour, which could be used as traditional resistive sensors.

The significance of the results presented in this thesis prove inkjet printing as

a valuable and precise tool for large-area processing of nanomaterials. The

versatility of carbon nanotubes’ properties is utilized in various applications, e.g.,

transparent conductive coatings, field-effect transistors and sensors that were

realized by using inkjet printing process. Most of the applications presented in the

thesis are proof-of-concept demonstrations that pave the way for the research

community to continue these experiments and to enhance the results for achieving

even better devices and concepts for commercial products in the future of printed

electronics.

69

References

Ajayan PM (1999) Nanotubes from carbon. Chemical Reviews 99(7): 1787–1799. Avouris P (2002) Carbon nanotube electronics. Chemical Physics 281(2-3): 429–445. Baird T, Fryer JR & Grant B (1971) Structure of fibrous carbon. Nature 233(5318): 329–

330. Ballarin B, Fraleoni-Morgera A, Frascaro D, Marazzita S, Piana C & Setti L (2004)

Thermal inkjet microdeposition of PEDOT:PSS on ITO-coated glass and characterization of the obtained film. Synthetic Metals 146(2): 201–205.

Berber S, Kwon Y-K & Tománek D (2000) Unusually high thermal conductivity of carbon nanotubes. Physical Review Letters 84(20): 4613–4616.

Bockrath M, Cobden DH, Lu J, Rinzler AG, Smalley RE, Balents L & McEuen PL (1999) Luttinger-liquid behavior in carbon nanotubes. Nature 397(6720): 545–632.

Carroll DL, Redlich P, Ajayan PM, Charlier JC, Blase X, De Vita A & R Car (1997) Electronic structure and localized states at carbon nanotube tips. Physical Review Letters 78(14): 2811–2814.

Choi YC, Lee JW, Lee SK, Kang MS, Lee CS, Jung KW, Lim JH, Moon JW, Hwang MI, Kim IH, Kim YH, Lee BG, Seon HR, Lee SJ, Park JH, Kim YC & Kim HS (2008) The high contrast ratio and fast response time of a liquid crystal display lit by a carbon nanotube field emission backlight unit. Nanotechnology 19(23): 235306.

Chung DS, Park SH, Jin YW, Jung JE, Park YJ, Lee HW, Ko TY, Hwang SY, Kim JW, Kwon NH, Yoon MH, Lee CG, You JH, Lee NS & Kim JM (2001) Field emission display using self-aligned carbon nanotube field emitters. Proceedings of the 14th International Vacuum Microelectronics Conference, 2001, IVMC 2001, Davis, California, USA: 179–180.

Coleman JN, Khan U & Gun’ko YK (2006) Mechanical reinforcement of polymers using carbon nanotubes. Advanced Materials 18(6): 689–706.

Collins PG, Bradley K, Ishigami M & Zeitl A (2000) Extreme oxygen sensitivity of electronic properties of carbon nanotubes. Science 287(5459): 1801–1804.

Correa-Duarte MA, Grzelczak M, Salgueiriño-Maceira V, Giersig M, Liz-Marzán LM, Farle, M, Sierazdki K & Diaz R (2005) Alignment of carbon nanotubes under low magnetic fields through attachment of magnetic nanoparticles. Journal of Physical Chemistry B 109(41): 19060–19063.

Crispin X, Jakobsson FLE, Crispin A, Grim PCM, Andersson P, Volodin A, van Haesendonck C, Van der Auweraer M, Salaneck WR & Berggren M (2006) The origin of the high conductivity of poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT-PSS) plastic electrodes. Chemistry of Materials 18(18): 4354–4360.

de Gans B-J & Schubert US (2004) Inkjet printing of well-defined polymer dots and arrays. Langmuir 20(18): 7789–7793.

Deegan RD. Bakajin O, Dupont TF, Huber G, Nagel SR & Witten TA (1997) Capillary flow as the cause of ring stains from dried liquid drops. Nature 389(6653): 827–829.

70

Ebbesen TW, Lezec HJ, Hiura H, Bennett JW, Ghaemi HF & Thio T (1996) Electrical conductivity of individual carbon nanotubes. Nature 382(6586): 54–56.

Eklund PC, Holden JM & Jishi RA (1995) Vibrational modes of carbon nanotubes; spectroscopy and theory. Carbon 33(7): 959–972.

Ellis AV & Ingham B (2006) Magnetic properties of multi-walled carbon nanotubes as a function of acid treatment. Journal of Magnetism and Magnetic Materials 302(2): 378–381.

Gonnet P, Liang Z, Choi ES, Kadambala RS, Zhang C, Brooks JS, Wang B & Kramer L (2006) Thermal conductivity of magnetically aligned carbon nanotube buckypapers and nanocomposites. Current Applied Physics 6(1): 119–122.

Halonen N, Kordás K, Tóth G, Mustonen T, Mäklin J, Vähäkangas J, Ajayan PM & Vajtai R (2008) Controlled CCVD synthesis of robust multi-walled carbon nanotube films. Journal of Physical Chemistry C 112(17): 6723–6728.

Haspel H, Ionescu R, Heszler P, Kukovecz A, Kónya Z, Mäklin J, Mustonen T, Kordás K, Vajtai R & Ajayan PM (2008) Fluctuation enhanced gas sensing on functionalized carbon nanotube thin films. Physica Status Solidi 245(10): 2339–2342.

Hecker K (2008) Organic Electronics Association Roadmap for Organic and Printed Electronics. VDMA Whitepapers 3-6.

Heilmann J (2007) Inkjet as a tool in publication, packaging and manufacturing applications. Ink Jet as a Manufacturing Process Symposium, 2007, Information Management Institute, Barcelona, Spain: presentation slides.

Heilmann J (2009) Mustesuihkutekniikka syrjäyttää offsetin, mutta milloin? Mitä mustesuihkutekniikan kehityksessä on tapahtunut ja miten se vaikuttaa painamiseen ja julkaisemiseen? GT-seminar, 2009, VTT, Espoo, Finland: presentation slides

Hu, L, Grüner G, Li D, Kaner RB & Cech J (2007) Patternable transparent carbon nanotube films for electrochromic devices. Journal of Applied Physics 101(1): 016102.

Hutton M (2006) The Future of Printed Electronics to 2010. Pira International Ltd., Surrey 20.

Iijima S (1991) Helical microtubules of graphitic carbon. Nature 354(6348): 56–58. Ionescu R, Hoel A, Granqvist CG, Llobet E & Heszler P (2005) Low-level detection of

ethanol and H2S with temperature-modulated WO3 nanoparticle gas sensors. Sensors and Actuators B: Chemical 104(1): 132–139.

Islam MF, Milkie DE, Kane, CL, Yodh AG & Kikkawa JM (2004) Direct measurement of the polarized optical absorption cross section of single-wall carbon nanotubes. Physical Review Letters 93(3): 037404.

Jarillo-Herrero P, Sapmas S, Dekker C, Kouwenhoven LP & van der Zant HSJ (2004) Electron-hole symmetry in a semiconducting carbon nanotube quantum dot. Nature 429(6990): 389–392.

Kaiser AB, Düsberg G & Roth S (1998) Heterogeneous model for conduction in carbon nanotubes. Physical Review B 57(3): 1418–1421.

Kiang C-H, Goddard III WA, Beyers R & Bethune DS (1995) Carbon nanotubes with single-layer walls. Carbon 33(7): 903–914.

71

Kim UJ, Furtado CA, Liu X, Chen G & Eklund PC (2005) Raman and IR-spectroscopy of chemically processed single-walled carbon nanotubes. Journal of the American Chemical Society 127(44) 15437–15445.

Kish LB, Vajtai R & Granqvist CG (2000) Extracting information from noise spectra of chemical sensors: single sensor electronic noses and tongue. Sensors and Actuators B: Chemical 71(1-2): 55–59.

Kleiner A & Eggert S (2001) Band gaps of primary metallic carbon nanotubes. Physical Review B 63(7): 073408.

Kong J, Franklin NR, Zhou C, Chapline MG, Peng S, Cho K & Dai H (2000) Nanotube molecular wires as chemical sensors. Science 287(5453): 622–625.

Kordás K, Tóth G, Moilanen P, Kumpumäki M, Vähäkangas J, Uusimäki A, Vajtai R & Ajayan PM (2007) Chip-cooling with integrated carbon nanotube microfin architectures. Applied Physics Letters 90(12): 123105.

Li J, Hu L, Wang L, Zhou Y, Grüner G & Marks TJ (2006) Organic light-emitting diodes having carbon nanotube anodes. Nano Letters 6(11): 2472–2477.

Li Z, Kandel HR, Dervishi E, Saini V, Xu Y, Biris AE, Lupu D, Salamo GJ & Biris AS (2008) Comparative study on different carbon nanotube materials in terms of transparent conductive coatings. Langmuir 24(6): 2655–2662.

Lu JP (1995) Novel magnetic properties of carbon nanotubes. Physical Review Letters 74(7): 1123–1126.

Ma Y, Lehtinen PO, Foster AS & Nieminen RS (2005) Hydrogen-induced magnetism in carbon nanotubes. Physical Review B 72(8): 084451.

Mann D, Javey A, Kong J, Wang Q & Dai H (2003) Ballistic transport in metallic nanotubes with reliable Pd Ohmic contacts. Nano Letters 3(11): 1541–1544.

Martel R, Schmidt T, Shea HR, Hertel T & Avouris P (1998) Single- and multi-wall carbon nanotube field-effect transistors. Applied Physics Letters 73(17): 2447–2449.

Monthioux M & Kuznetsov VL (2006) Who should be given the credit for the discovery of carbon nanotubes? Carbon 44(9): 1621–1623.

Mukhopadhyay K, Dwivedi CD & Mathur GN (2002) Conversion of carbon nanotubes to carbon nanofibres by sonication. Carbon 40(8): 1373–1376.

Mustonen T, Mäklin J, Tóth G, Kordás K, Vajtai R & Ajayan PM (2008) Inkjet printing of carbon nanotube transistors on rigid and flexible substrates. 22nd International Winterschool of Electronic Properties of Novel Materials, 2008, IWEPNM 2008, Kirchberg, Austria: 138–139.

Park J-U, Hardy M, Kang SJ, Barton K, Adair K, Mukhopadhyay DK, Lee CY, Strano MS, Alleyne AG, Georgiadis JG, Ferreira PM & Rogers JA (2007) High-resolution electrohydrodynamic jet printing. Nature Materials 6(10): 782–789.

Rinzler AG, Liu J, Dai H, Nikolaev P, Huffman CB, Rodríguez-Macías FJ, Boul PJ, Lu AH, Heymann D, Colbert DT, Lee RS, Fischer JE, Rao AM, Eklund PC & Smalley RE (1998) Large-scale purification of single-wall carbon nanotubes: process, product, and characterization. Applied Physics A: Materials Science & Processing 67(1): 29–37.

72

Rosca ID, Watari F, Uo M & Akasaka T (2005) Oxidation of multi-walled carbon nanotubes by nitric acid. Carbon 43(15): 3124–3131.

Rowell MW, Topinka AM, McGehee MD, Prall H-J, Dennler G, Sariciftci NS, Hu L & Grüner G (2006) Organic solar cells with carbon nanotube network electrodes. Applied Physics Letter 88(23): 233506.

Saito R, Fujita M, Dresselhaus G & Dresselhaus MS (1992) electronic structure of chiral graphene tubules. Applied Physics Letters 60(18): 2204–2206.

Salzmann CG, Llewellyn SA, Tobias G, Ward MAH, Huh Y & Green MLH (2007) The role of carboxylated carbonaceous fragments in the functionalization and spectroscopy of a single-walled carbon-nanotube material. Advanced Materials 19(6): 883–887.

Sammalkorpi M, Krasheninnikov A, Kuronen A, Nordlund K & Kaski K (2004) Mechanical properties of carbon nanotubes with vacancies and related defects. Physical Review B 70(24): 245416.

Sekitani T, Noguchi Y, Zschieschlang U, Klauk H & Someya T (2008) Organic transistors manufactured using inkjet technology with subfemtoliter accuracy. Proceedings of the National Academy of Sciences of the United States of America. 105(13): 4976–4980.

Shobaki J, Manasreh D, Yusuf NA & Abu-Aljarayesh I (1996) A scaling model for the optical anisotropy in magnetic fluids: dichroism and birefringence. IEEE Transactions on Magnetics 32(6): 5245–5250.

Sirringhaus H, Kawase T, Friend RH, Shimoda T, Inbasekaran M, Wu W & Woo EP (2000) High-resolution inkjet printing of all-polymer transistor circuits. Science 290(5499): 2123–2126.

Skákalová V, Kaiser AB, Woo Y-S & Roth S (2006) Electronic transport in carbon nanotubes: From individual nanotubes to thin and thick networks. Physical Review B 74(8) 085403.

Soltman D & Subramanian V (2008) Inkjet-printed line morphologies and temperature control of the coffee ring effect. Langmuir 24(5): 2224–2231.

Strano MS, Dyke CA, Usrey ML, Barone PW, Allen MJ, Shan H, Kittrell C, Hauge RH, Tour JM & Smalley RE (2003) Electronic structure control of single-walled carbon nanotube functionalization. Science 301(5639): 1519–1522.

Sze SM (1985) Semiconductor devices, physics and technology. 2nd ed. John Wiley & Sons, New York, 72.

Tans SJ, Verschueren RM & Dekker C (1998) Room-temperature transistor based on single carbon nanotube. Nature 393(6680): 49–52.

Thomson W (Lord Kelvin) (1867) Improvements in telegraphic receiving and recording instruments. GB Patent 2.147.

Troiani HE, Miki-Yoshida M; Camacho-Bragado GA, Marques MAL, Rubio A, Ascencio JA & Jose-Yacaman M (2003) Direct observation of the mechanical properties of single-walled carbon nanotubes and their junction as at the atomic level. Nano Letters 3(6): 751–755.

Varghese OK, Kichambre PD, Gong D, Ong KG, Dickey EC & Grimes CA (2001) Gas sensing characteristics of multi-wall carbon nanotubes. Sensors and Actuators B: Chemical 81(1): 32–41.

73

Vijayaraghavan A, Kar S, Soldano C, Talapatra S, Nalamasu O & Ajayan PM (2006) Charge-injection-induced dynamic screening and origin of hysteresis in field-modulated transport in single-wall carbon nanotubes. Applied Physics Letters 89(16): 162108.

Wallace D & Hayes D (2005) Ink-jet printing as a tool in manufacturing and instrumentation In: Bucknall DG (ed) Nanolithography and Patterning Techniques in Microelectronics. Woodhead Publishing Ltd., Cambridge, 267–298.

Wei BQ, Vajtai R, Jung Y, Ward, J, Zhang R, Ramanath G & Ajayan PM (2002) Organized assembly of carbon nanotubes. Nature 416(6880): 495.

Xu H, Anlange SM, Hu L & Grüner G (2007) Microwave shielding of transparent and conducting single-walled carbon nanotube film. Applied Physics Letters 90(18): 183119.

Yao Z, Postma HWC, Balents L & Dekker C (1999) Carbon nanotube intramolecular junctions. Nature 402(6759): 273–276.

Yin Y, Vamivakas AN, Walsh AG, Cronin SB, Ünlü MS, Goldberg BB & Swan AK (2007) Optical determination of electron-phonon coupling in carbon nanotubes. Physical Review Letters 98(3): 037404.

Yoon Y-H, Song J-W, Kim D, Kim J, Park J-K, Oh S-K & Han C-S (2007) Transparent film heater using single-walled carbon nanotubes. Advanced Materials 19(23): 4284–4287.

Yorikawa H & Muramatsu S (1995) Energy gaps of semiconducting nanotubules. Physical Review B 52(4): 2723–2727.

Zhou Y, Gaur A, Hur S-H, Kocabas C, Meitl MA, Shim M & Rogers JA (2004) p-channel, n-channel thin film transistors and p-n diodes based on single wall carbon nanotube networks. Nano Letters 4(10): 2031–2035.

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Original publications

I Kordás K, Mustonen T, Tóth G, Jantunen H, Lajunen M, Soldano C, Talapatra S, Kar S, Vajtai R & Ajayan PM (2006) Inkjet printing of electrically conductive patterns of carbon nanotubes. Small 2(8–9): 1021–1025.

II Kordás K, Mustonen T, Tóth G, Vähäkangas J, Uusimäki A, Jantunen H, Gupta A, Rao KV, Vajtai R & Ajayan PM (2007) Magnetic-field induced efficient alignment of carbon nanotubes in aqueous solutions. Chemistry of Materials 19(4): 787–791.

III Mustonen T, Kordás K, Saukko S, Tóth G, Penttilä JS, Helistö P, Seppä H & Jantunen H (2007) Inkjet printing of transparent and conductive patterns of single-walled carbon nanotubes and PEDOT-PSS composites. Physica Status Solidi B 244(11): 4336–4340.

IV Mustonen T, Mäklin J, Kordás K, Halonen N, Tóth G, Saukko S, Vähäkangas J, Jantunen H, Kar S, Ajayan PM, Vajtai R, Helistö P, Seppä H & Moilanen H (2008) Controlled Ohmic and nonlinear electrical transport in inkjet-printed single-wall carbon nanotube films. Physical Review B 77(12): 125430 1–7.

V Mäklin J, Mustonen T, Halonen N, Tóth G, Kordás K, Vähäkangas J, Moilanen H, Kukovecz Á, Kónya Z, Haspel H, Gingl Z, Heszler P, Vajtai R & Ajayan PM (2008) Inkjet printed resistive and chemical-FET carbon nanotube gas sensors. Physica Status Solidi B 245(10): 2335–2338.

Reprinted with permission from Wiley-VHC (I,III,V), American Chemical

Society (II) and American Physical Society (IV).

Original publications are not included in the electronic version of the dissertation.


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