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POLITECNICO DI MILANO
SCHOOL OF INDUSTRIAL AND INFORMATION
LAUREA MAGISTRALE IN MATERIALS ENGINEERING
AND
NANOTECHNOLOGY
Assessment of semi-conducting Carbon Nanotube networks for
fully printed field effect transistors
Supervisor: Prof. Chiara CASTIGLIONI
Co-supervisor: Dr. Mario CAIRONI
Prabir MAHATO
836126
i
ABSTRACT
Printed electronics is a branch of electronics where functional materials, such as
conductors, semiconductors and dielectrics, are deposited one by one in the forms of
inks with high-throughput printing machines inherited by the graphical arts. Printed
electronics allows to add electronic functionalities with a cost-effective process on a
wide variety of substrates, including plastic and paper, and it allows to target large-
area electronics applications. Rather than competing with established silicon
electronics, it offers alternative applications where lightweight, flexibility and ease of
integration are critical figure of merits. Organic semiconductors, and in particular
conjugated polymers, are typically considered among the best options for printed
electronics, because of their chemical tunability, which simplifies inks formulations,
their advantageous mechanical properties and electronic and optoelectronic properties
which have improved over the last few decades.
However, semiconducting polymers still show limitations, both in terms of maximum
field-effect charge carrier mobility, which is below 5 cm2V
-1s
-1 for most of largely
available materials, and in terms of contact resistance, owing to high energetic barriers
for injection of carriers from most of printable electrodes, thus complicating
downscaling of devices. Both limitations represent a strong bottleneck for
improvement of switching performances of printed electronics devices, such as printed
transistors, therefore strongly limiting the overall expected speed of a printed polymer
circuit. With the aim to find alternatives to printed polymers to overcome their current
limitations, this work has the goal of assessing the use of semiconducting single-
walled carbon nanotubes (swCNTs) for the fabrication of fully printed field-effect
transistors on plastic substrates. While CNTs offer in principle much better electronic
performances than polymers, especially in terms of charge mobility, economically
viable methodologies to produce them lead to mix of conducting and semi-conducting
nanotubes. Moreover their dispersion in suitable solvents to formulate stable, printable
inks is still a challenge. One possible solution to both criticalities is to adopt polymer
wrapping, where a semiconducting polymer capable of selectively wrapping specific
chiralities can be used both as a sorting agent and a dispersant. The formulations of
polymer wrapped swCNTs studied here have been provided by the group of Prof.
ii
M.A. Loi at the University of Groningen. An optimization of inkjet printing of
swCNTs formulations has been performed in order to control the formation of suitable
nanotubes networks. These printed networks were integrated in field-effect transistors
printed on plastic by combining only inkjet printing and bar-coating in ambient air,
demonstrating working devices with holes field-effect mobility reaching 1.90 cm2V
-1s
-
1. The obtained result is a first promising step towards high performance, fully printed
CNTs based flexible circuits, which can be achieved by further optimization of the
quality of the printed CNTs networks, especially in terms of coverage.
iii
ACKNOWLEDGEMENT
It was a great opportunity to work on my thesis at Center for Nano Science and
Technology(CNST), under Italian Institute of Technology, Milano, Italy. I give my
heartfelt thanks to Dr. Mario Caironi, Track Tenure Researcher at CNST, Milano and
Prof. Chiara Castiglioni, professor at Politecnico Di Milano to provide me such an
enriching environment to learn. Discussions with them on regular basis on the progress
of my work were highly fruitful and provided me insight to my work.
I am thankful to Francesca Scuratti, PhD student at CNST, for her guidance, time and
patience. As a master’s student, mostly having theoretical knowledge, she played an
important role to translate it into the practical space in these nine months. I thank
Matteo Cesarini, Diego Nava, Michele Garbugli and others from the CNST lab for
their extended support whenever required during different stages of my work.
Lastly, I thank my beloved parents, Smt. Ahalya , Shri Harendra Kr. Mahato and my
dear sister Jyotsna being great support system. Their commitment, patience and
unconditional love has been extraordinary and words are less to describe it. I am
thankful to my amazing lot of friends Abhinov, Praveen, Kishan, Santosh,
Dharanidharan, Sunny, Amberker, Idham and Junaidi, Punit and others for being great
friends especially through these two years. I take this opportunity to thank my mentor
Andrea Bonfanti who aspires to strive for excellence all the time. My sports coach
Pino Guarnaccia along with my sports mates Mauro Oliva, Giuseppe Neutro, and
others to keep me in the right space on and off the field to complete this work.
Prabir Mahato
iv
TABLE OF CONTENTS
Abstract i
Acknowledgement iii
List of Figures vi
Introduction 1
Chapter 1 4
Background Knowledge
1.1. Introduction to printed electronics 4
1.1.1 Organic semi conductors 5
1.1.2 Carbon nanotubes as semi-conductors 8
1.1.3 Inkjet printing 8
1.1.3.1 Classification of Inkjet printers 9
1.1.4 Bar coating 10
1.2. Carbon nanotubes 13
1.2.1 Structure of carbon nanotubes 14
1.2.2 Electronic properties of SWCNTs 16
1.2.3 Charge Transport of CNTs 18
1.2.4 Transport in SWCNT Networks 21
1.2.5 Contacts to nanotubes: Schottky barriers 22
1.2.6 Synthesis and sorting of CNTs 24
1.2.7 Polymer wrapping of CNTs 27
1.3 Transistors and Field effect transistors 30
1.3.1 History of FETs 32
1.3.2 Geometry of FETs 33
1.3.3 Working principle of FETs 34
1.3.4 Electrical characterization 39
1.3.5 Single wall carbon nanotube FETs 42
1.3.6 Carbon nanotube networks FETs 44
Chapter 2
Materials, equipments and fabrication method 46
2.1 Materials 46
2.1.1 Substrate- PEN 46
2.1.2 PEDOT: PSS 47
2.1.3 Dielectric Material- PMMA 48
v
2.1.4 P3DDT Polymer 49
2.1.5 Semi conductor- Carbon nanotube networks 50
2.2 Equipments 51
2.2.1 Dimatix Printer 51
2.2.2 TQC Bar Coater 54
2.2.3 Profilometer 56
2.2.4 Glove box Prober 59
2.2.5 Hot Plate- Annealing 61
2.2.6 Scanning Electron Microscope 62
2.3 Fabrication Method 63
Chapter 3
Results and Analysis 66
3.1 Reference devices based on printed semi conductor
tovalidate the architecture 67
3.2 Bar coating of CNTs and sonication effect 68
3.3 Spin coating and bar coating of the delectric layer 70
3.4 Bar coating of the dielectric, completely printed
Device 72
3.5 Optimization of the carbon nanotube networks 75
3.5.1 Bar coater wire pitch 76
3.5.2 Bar coating with different formulation 76
3.5.3 Effect of dielectric thickness 78
Conclusions 80
Bibliography 82
vi
vii
List of Figures
Figure No. Page No.
Figure 1.1: A brief summary between printed and conventional electronics..........5
Figure 1.2: Sketch of the double bond between two sp2 hybridized carbon.....6
Atom
Figure 1.3: Band structure containing a positive polaron and bi polaron...............7
Figure 1.4: Charge transport; 1-2; a backbone charg..............................................7
Figure 1.5: Shows different types of bar...............................................................11
Figure 1.6: Types of Carbon nanotubes.................................................................14
Figure 1.7: Building of CNTs from graphene sheets, the primitive vectors
and an example of chiral vector and chiral angle...............................................15
Figure 1.8: Bravais Lattice of graphene.................................................................15
Figure 1.9: Different types of CNT: depending on the rolling.............................16
Figure 1.10: shows the band structure in the extended first
Brillouin zone of graphene.................................................................................17
Figure 1.11: Energy diagram and Density of States(DOS) a)of m-SWNTs
and b)s-SWNTs …………………………………………………………………..17
Figure 1.12 (a) A schematic of the non aligned metal and nanotube
energy levels and other related figures............................................................23
Figure 1.13: (a) Density gradient ultracentrifugation (DGU)
(b) multicolumn gel chromatography, and (c) Agarose gel technique...................26
Figure 1.14: Showing the Carbon Nanotube wrapping with polymers.................27
Figure 1.15: wrapping orientation with different polymers...............................28
Figure 1.16: Different polymers used for separation of s CNTs
from metallic ones..............................................................................................29
Figure 1.17: John Bardeen(with spectacles), William Shockley and
WalterBrattain at Bell Labs, 1948.........................................................................30
Figure 1.18: History of transistors.........................................................................31
Figure 1.19: Moore’s law.......................................................................................32
Figure 1.20: Shows different configuration of FETs.......................................33
Figure 1.21: a) showing the positions of HOMOs and LUMOs
with respect to the Fermi levels of the source and drain and their change......35
Figure 1.22: a) Carrier concentration profile of TFT in different regimes et.........37
Figure 1.23: showing ID – VD curves for TFTs at different values of VG.........39
Figure 1.24: showing ID –VG curves at a constant value of VD .........................40
Figure 1.25: Showing first SWNT- FET device.............................................42
Figure 1.26: Device architecture of the first Si based FETs...........................42
Figure 1.27: the output characteristics of the first CNT FETs developed
by Dekker’s group...............................................................................................43
Figure 1.28: Device architecture for inkjet printed transistors work by
Sadiral e.................................................................................................................45
Figure 1.29: Mobility plots extracted from devices made by increasing the
number of printed passes work by Sadir et al.........................................................45
Figure 2.1: Chemical structure of Polyethylene Naphthalate............................46
Figure 2.2: Chemical structure of Poly ethylene Terephthalate........................47
Figure 2.3: Chemical structure of PEDOT: PSS...............................................47
Figure 2.4: Chemical structure of PMMA........................................................48
Figure 2.5: Structure of P3DDT monomer.......................................................49
viii
Figure 2.6: Absorption spectrums of HiPCO: P3DDTin toluene
and oDCB after the 1st centrifugation..........................................................51
Figure 2.7: View of Dimatix Printer................................................................52
Figure 2.8: A Dimatix Matrix Materials Cartridge............................................53
Figure 2.9: Photo of Dimatix Printer..................................................................54
Figure 2.10: TQC bar coater...............................................................................54
Figure 2.11: The coating rods with different wire spacing diameters..................55
Figure 2.12: Photo view of TQC coater...............................................................56
Figure 2.13: Schematic representation of Stylus profilometer............................56
Figure 2.14: Photo view of the place where the sample is place profilometer.......58
Figure 2.15: Photo view of the profilometer setup..............................................58
Figure 2.16: Photo view of the Stylus prober...................................................59
Figure 2.17: Prober in the Glove Box................................................................60
Figure 2.18: Annealing of samples.....................................................................61
Figure 2.19: Working of Scanning Electron Microscope....................................62
Figure 2.20: Architecture of the device..............................................................65
Figure 3.1: Mobility curves of spin coated N2200 and CNTs device................68
Figure 3.2 Mobility curve showing sonication effect.........................................69
Figure 3.3: Plot showing the difference in the performances of the devices
fabricated with spin and bar coating of the dielectrics.........................................71
Figure 3.4: Shows transfer curves of the best fully printed device......................72
Figure 3.5: Shows output curves of the best fully printed device.........................73
Figure 3.6: Mobility curve of the best working printed device............................74
Figure 3.7: shows the SEM image of CNT solution with o-Xylene after bar
coating.....................................................................................................................75
Figure 3.8: The transfer characteristic curve of device plotted with 10 µm pitch
bar and 20 µm coater. ........................................................................76
Figure 3.9: The mobility curves of the device with the semi-conductor
formulation with CNTs solution and o-Xylene......................................................77
Figure 3.10: Shows the transfer characteristics of a device fabricated with
dielectric PMMA prepared at concentration 90 mg/ml and 80mg/ml................78
1
INTRODUCTION
Printing has very ancient origins, and since the invention of the printing press by
Gutenberg in the 15th
century, it has evolved to replicate information in mass volumes
at low cost, up to the contemporary roll-to-roll industrial graphical arts technology.
The recent development of solution-processable functional materials with electronic
properties, including semiconductors, conductors and dielectrics that can be
formulated in the form of inks has allowed to develop what is called “Printed
Electronics”. The latter is a branch of electronics where graphical arts printing
technology has been and is continuously being adapted to the needs of functional inks
and electronic applications, to deploy electronic and sensing functionalities, at low
temperature and high-throughput, typically on plastic or paper substrates, targeting
large-area and flexible applications. It is not a competitor of established silicon
technology, but it rather aims at turning passive surfaces of even low cost objects into
active components, integrating micro- and opto-electronic functionalities. Examples
are disposable point-of-care applications, low cost smart tags, smart packaging,
flexible and/or rollable displays, wearables, local body area networks.
Among printable materials, both polymer conductors, such as PEDOT:PSS
(conductivity up to 1000 S/cm) and highly conductive metallic inks, such as silver
nanoparticles or metal complexes based inks (conductivity up to 104 S/cm) are
largely available. Printable dielectrics are as well very well covered, comprising a lot
of commodity polymers. Different possibilities have been developed for what concerns
semiconducting inks, among which organic conjugated materials have clearly
represented one of the most promising options for large-area printed electronics,
because of their chemical tunability, which simplifies inks formulations, their low
temperature processing. their robust mechanical properties and electronic and
optoelectronic properties which have steadily improved over the last few decades.
However semiconducting polymers still show strong limitations, which are reflected in
relatively poor electronic performances of fully printed electronic devices. As an
example, despite the steady improvement of recent years, polymer field-effect
transistors (FETs), which can be considered the fundamental building blocks of
2
polymer electronic circuits, show a carrier mobility which for the best semiconductors
is in the range of 1 to 10 cm2/Vs, and typically lower than 1 cm
2/Vs in case of fully
printed FETs. Another strong limitation is represented by the Schottky barriers
developing at polymer semiconductor and electrodes interface, leading to strong
contact resistance effects, posing a severe constraint on the device downscaling. The
previous limitations, combined with the typical coarse resolution of state-of-the-art
printing technologies, typically lead to poor switching performances of fully printed
FETs, therefore strongly limiting the overall expected speed of a printed polymer
circuits, and as a consequence, limiting also their possible applications. It is therefore
desirable to find alternative solution-processable semiconductors, which ideally
maintain the ease of formulation and printability of polymers, but at the same time
offer improved electronic performances.
This thesis focused on the assessment of semiconducting single-walled carbon
nanotubes (swCNTs) formulations as alternative to polymer semiconductors for the
fabrication of fully printed field-effect transistors on plastic substrates. While CNTs
offer in principle much better electronic performances with respect to polymers,
especially in terms of charge mobility, economically viable methodologies to produce
them lead to strongly entangled mats of conducting and semi-conducting nanotubes.
Moreover, their dispersion in suitable solvents to formulate stable, printable inks is
still a challenge. In this work we focused on the use of swCNTs well-sorted and
dispersed thanks to a polymer wrapping approach, where a semiconducting polymer
capable of selectively wrapping specific chiralities, and therefore swCNTs with
specific bandgaps, can be used both as a sorting agent and a dispersant. The
formulations adopted have been provided by the group of Prof. M.A. Loi at the
University of Groningen, an international leader in this approach which has first
proven the possibility of achieving FETs with solution-processed swCNTs networks
with high mobility, in excess of 10 cm2/Vs. The specific goal of the present work was
to study the possibility of depositing by means of bar-coating, a very simple and
scalable technology compatible with roll-to-roll printing, carbon nanotubes networks
from formulations of swCNTs wrapped by Poly(3-dodecylthiophene-2,5-diyl)
P3DDT , which is a conductive polymer. More specifically, the study has first aimed
at fabricating and validating a reference fully printed FET architecture on PEN
3
substrate based on a well-known model semiconducting polymer, namely Naphthalene
diimide(N2200) . Such polymer FET, that was realized by inkjet printing PEDOT:PSS
source/drain and gate contacts, and comprised a bar-coated (PMMA) Poly Methyl
methacralate,dielectric and a bar coated N2200 semiconductor achieved a field-effect
mobility of 0.62 cm2V
-1s
-1 and served as a benchmark for swCNTs based devices.
Subsequently, deposition of swCNTs networks both by spin-coating and bar-coating
was optimized especially in terms of coverage of the substrate and integrated into FET
devices to assess their performances. Thanks to the achievement of controlled
deposition of swCNTs networks by bar-coating on PEN, functional fully printed FETs
with hole mobility reaching 1.90 cm2/Vs were finally achieved, a result which
represents a first promising step towards high performance, fully printed CNTs based
flexible circuits.
This work was performed in collaboration with Istituto Italiano di Tecnologia (IIT),
and the experiments were conducted in the laboratories of the “Printed and Molecular
Electronics” group at the Center for Nano Science and Technology of IIT in Milan. All
swCNTs formulations were provided by the group of Prof. M.A. Loi at the University
of Groningen, Netherlands.
The thesis is organized as follows:
Chapter 1 provides a general introduction to printed electronics- polymers, inkjet
printing, carbon nanotubes and filed effect transistors.
Chapter 2 relates to experimental part that is the materials, equipments and the
fabrication methods\
Chapter 3 provides theresults and analysis of the experiments conducted during the
this thesis work.
4
Chapter 1
Background Knowledge
1.1 Introduction to printed electronics
Printed electronics is a subject of electronics which deals with solution printing of thin
films on various substrates. Electrically functional electronic or optical inks are
deposited on the substrate creating active and passive components such as thin film
transistors, resistors, capacitors and coils. Here the inks can be composed of different
carbon based compounds and these can be deposited by solution based techniques.
The main feature of printed electronics is that, it gives us a possibility to print many
micro-structured layers one above another in a much simpler and cost effective
manner. This gives a wide opportunity to implement new and improved functionalities
to the device. The resolution of printing is very high (in µm or nm) with precision.
Printing technologies divide between sheet-based and roll-to-roll-based approaches.
Sheet-based inkjet and screen printing are best for low-volume, high-precision
work. Gravure, offset and flexographic printing are more common for high-volume
production, such as solar cells, reaching 10,000 square meters per hour
(m²/h).[1][2]
While offset and flexographic printing are mainly used for
inorganic[3][4]
and organic[5][6]
conductors (the latter also for
dielectrics),[7]
gravure printing is especially suitable for quality-sensitive layers like
organic semiconductors and semiconductor/dielectric-interfaces in transistors, due to
high layer quality.[7]
If high resolution is needed, gravure is also suitable for inorganic
[8] and organic
[9] conductors. Organic field-effect transistors and integrated
circuits can be prepared completely by means of mass-printing methods.[7]
Both organic and inorganic ink materials can be used for the printing methods
provided with proper control in their viscosity, surface tension and solid content.
Solubility, wetting, adhesion as well as post treatment conditions after deposition play
a very vital role, since they have the ability to change the morphology of the layers.
With respect to the conventional electronic methodologies, printed electronics
provides an opportunity to modify the mechanical flexibility and functionality by
chemical adjustment.
5
Obviously, the performance is not expected to be as great as that of the conventional
electronics, since they are solution processed and the charge carrier mobility is lower
than conventional method.
Figure 1.1 A brief summary between printed and conventional electronics
Conductive polymers are the materials which can be used for solution processed
applications. Other alternative materials with high performance are also researched.
1.1.1 Organic Semi Conductors
Organic semiconductors are the carbon based conjugated systems having delocalized
π bonds. Carbon have an electron configuration of 1s2 2s
2 2p
2, in total of six electrons,
with four valence electrons available for pairing. Through the linear combination of
the 2s and 2p atomic orbitals, the atom can form hybridized sp, sp2 or sp
3
orbitals, with its four valence electrons that can participate to the formation of up
to four bonds. While the electrons in the hybridized orbitals can form strong
covalent σ-bonds with other atoms, defining the organic compound backbone,
those of the 2p orbitals (not involved in the hybridization) form the weaker π-
6
bonds (fig.1.8). This type of bond allows the delocalization of the free electrons
over the carbon atomic plane, attributing semiconducting properties to the system.
Two or more atoms connected by σ-bond and π-bond constitute a conjugated
molecule.[10]
Figure 1.2: Sketch of the double bond between two sp2 hybridized carbon atom
The repetition of these conjugated sub-units (monomers) results into the polymers.
With the increase in the monomers the conjugation increases, facilitating the
electronic delocalization. Delocalization is eased by reduction in the energy gap
between the HOMO (highest occupied molecular orbitals) and LUMO (least
occupied molecular orbitals) levels.
To briefly, understand the charge transport in the organic semiconductors, it is
important to know the concept of polarons. Since, contrary to the metals, the
electrons are localized in the semi-conductor results into local deformation of the
molecule. This charge structure interaction can be described as a strong electron-
phonon coupling and therefore for charge transport this phonon and charge must
move coordinately. This quasiparticle couple is called polaron. By applying an
external field, the polaron can travel across the system carrying the local deformation
of the lattice. Thus, the polaron can be positive or negative with spin of (+,-) ½ and
can be considered as a defect due to the vacancy of an electron. Furthermore, in the
case of a second doping stage, if we remove a second electron, the formation of a
bipolaron occurs. This new spineless now particle, can be again positive or negative but
stable bipolarons are known to exist in a few materials, including polyacetylene and a
number of oxides. [11]
7
Figure 1.3: Band structure containing a positive polaron and a positive bi polaron
The charge transport within a molecular solid has two main components; an intra-
molecular, or rather the motion of the polaron along the chain and the inter-molecular
that regards the jump of the charges between the molecules composing the molecular
solid.
The first is a coherent event which takes place within extended molecular states. On
Figure 1.4: Charge transport; 1-2; a backbone charge transfer -intra-molecular. 2-3, inter-
chain transfer though hopping
8
the other hand, the inter-chain transfer is the most spread charge transport
mechanism,plays a crucial role in the charge transport [12]
and is governed by the
presence of weak inter-chain reaction and by the degree of disorder in the organic
system. Some of the semi-conducting polymeric materials are N2200, PEDOT: PSS,
P3HT, P3DDT.
1.1.2 Carbon Nanotubes as semi conductors
Carbon nanotubes as discussed earlier in the section 1.1, provides an alternative to the
use conventional polymers as semiconductors. They having outstanding electrical and
mechanical properties, along with processibility in solution deposition methods makes
them suitable candidate as semi-conductors. Although it has to be mentioned that
various techniques such as gel chromatography, polymer wrapping etc. needs to be
employed in order to separate tthe semi-conducting carbon nanotubes from the
metallic ones with very high purity for better performances in the devices.
Carbon nanotube based polymer solution have been successfully used in coating
methods like spin coating[13,14],
spray coating[15]
, self assembly coating[16]
, dip
coating[17]
, rod coating[18]
to printing techniques such as screen printing[19]
, inkjet
printing [20]
aerosol printing [21–23]
, transfer printing [24],
and contact printing [25]
techniques have eliminated the need of using a high-vacuum environment and
multistage patterning process, thus paving the way for scalable manufacturing of
large-area, low-cost, and flexible electronics.
1.1.3 Inkjet Printing
Inkjet printing offers unique advantages over other methods of printing. It requires
absolutely no prefabrication of templates, allowing for a rapid printing process at low
cost. Additionally, due to its precise method of patterning, post-printing steps are not
necessary. Furthermore, multiple materials can be deposited simultaneously with the
use of multiple ink cartridges, and the amount of deposited material can be controlled
with great precision. Finally, due to the nature of inkjet printing technology, multiple
layers can be printed on top of one another with great ease.
9
1.1.3.1 Classification of Inkjet Printers
Inkjet printers can be classified based on its principle of printing technology into two
categories- Continuous and Drop on Demand.
Continuous
Continuous inkjet printing supplies a continuous stream of ink droplets. The principle
is that these droplets are charged upon leaving the nozzle and are then deflected by
voltage plates, where the applied voltage determines whether the droplet will be
deposited onto the substrate or recycled through the gutter. Therefore, even though
when the printer is not actually printing anything onto a substrate, a stream of droplets
is still being ejected from the nozzle and recycled through the gutter.
Drop on Demand
As the name suggests, a drop-on-demand inkjet printer ejects a droplet of ink only
when it is told to do so. Therefore, when the printer is not actually printing anything
onto a substrate, there are no droplets being ejected from the nozzle. Drop-on-demand
inkjet printers can be further split into two categories, namely thermal and
piezoelectric.
Thermal inkjet printers
Thermal inkjet printers, at times referred to bubble jet printers, contain a thin film
resistor in the nozzle. In order to eject a droplet, this thin film resistor is heated by
passing current through it. This causes the ink in the nozzle to vaporize, creating a
bubble and a large increase in pressure, which forces ink droplets out of the nozzle.
Hewlett-Packard, Canon, and Lexmark employ this type of drop-on demand inkjet
printer.
Piezoelectric inkjet printers
They contain a piezoelectric transducer in the nozzle. When voltage is applied to the
piezoelectric transducer, it deforms and causes an increase in pressure, which forces
10
ink droplets out of the nozzle. In terms of consumer printers, Epson employs this type
of drop-on-demand inkjet printer. The Fujifilm Dimatix printer, which is specialized
commercial inkjet employ the piezoelectric drop-on-demand technology as well.
Dimatix printer 2830 is used in this work, because of control over drop volume and
spacing and therefore better resolution can be obtained.
Based on the usage, the printers be classified into consumer and commercial printers.
Consumer based printers
Consumer inkjet printers are quite cheap and offer familiarity, so there is no need to
learn new software or hardware. Nevertheless, these printers are made to print a
specific type of ink, so developing useable ink can be a bit more difficult. The new ink
must match the original ink in all aspects. Furthermore, in the instance where a new
ink clogs the nozzle, some consumer inkjet printers are easier to clean than others. In
general, each Hewlett-Packard printer cartridge has its own nozzle, allowing the user
to easily remove the cartridge and clean it. On the other hand, the nozzle for Epson
printer cartridges is built into the printer itself and cannot be easily removed for
cleaning. The most prominent disadvantage for consumer inkjet printers is their
overall lack of control. In particular, the drop volume and spacing cannot be adjusted,
and the resolution is relatively low.
Commercial based printers
Commercial printers are used since they have a great deal of control over drop volume
and spacing, and they provide better resolution. Though expensive, these printers, like
the Fujifilm Dimatix are specifically made for printing various types of materials.
1.1.4 Bar coating
Bar coating is used in order to print the carbon nanotube semi conducting active layer
and the dielectric. Bar coating set up consists of the substrate placing plate. A bar is
moved at a certain set velocity over this, after placing the substrate. Depending on the
type of bar configurations and the velocity the thickness of the layer is determined.
11
Different types of bar
Figure 1.5: Shows different types of bar
12
Wire wound rods
The original “Mayer” rod. Popularized by Charles Mayer in the 1920's and still the
most popular. Wire wound rods can be stripped and rewound to save cost and reduce
waste. There are no minimums and no setup charges for wire wound rods.
Formed rods
A formed (threaded, grooved, etc.) rod is roll formed from a solid steel bar and closely
resembles a wire wound rod. The size of the groove determines the coating thickness
the same way the cavity between wire winding does. However, the radius in the
groove of a formed rod is less likely to clog and cleans up easier than its wire wound
equivalent.
Smooth rods
Commonly used to smooth previously metered coatings to paper, paperboard and
plastic films. The coating thickness is governed by pressure (against a backer roll) or
web tension. RDS smooth rods can be ground to meet any surface finish requirement
and hard chrome plating can extend rod life significantly.
Gapped rods
For higher viscosity coatings, gapped rods reduce clogging and eliminate lines. By
winding a smaller wire with spacing (see image) the cavity created is wide and
shallow allowing heavy coatings to flow better. A smaller wire creates a smaller
interruption in the coating flow enabling the coating to wet out more completely.
While a gapped rod is less durable than a double wound rod, it is also less expensive.
Double wound rods
For higher viscosity coatings, double wound rods reduce clogging and eliminate lines.
By winding a smaller wire on top of a larger one (see image) the cavity created is
wide and shallow allowing heavy coatings to flow better. A smaller top wire creates a
smaller interruption in the coating flow enabling the coating to wet out more
completely. While a double wound rod is more expensive than a gapped rod, it is also
more durable.
13
1.2 Carbon Nanotubes
The history of carbon nanotubes goes back farther than most people think.
In fact, a US patent was given to two British men in 1889, on the production of carbon
nanotubes utilizing marsh gas, otherwise known as methane. The method employed is
essentially the same as used today. The patent describes the production of “hair-like
carbon filaments” for electrical lighting. This patent also described about some of the
unique electrical and mechanical properties, “Carbon filament may be bent and
twisted into various shapes and will spring back to their original form on being
released.”[26]
In 1960’s a group at National Carbon Company in Parma, Ohio and later
in the 1970’s at and the University of Canterbury in Christchurch, New Zealand,
helped to characterize the carbon nanotubes.
Later in 1983, a company called Hyperion Catalysis perfected ways to produce
nanotubes and began incorporating them into fuel line for cars. The nanotube’s high
conductivity would dissipate any electrical charge that could potential build up and
spark. Today, 60 percent of the cars on the road use Hyperion’s nanotube incorporated
fuel lines. Hyperion’s nanotubes are also incorporated into plastic wing panels. This
allows the plastic on the wing to be grounded and then sprayed with paint droplets that
are charged up to 20,000 volts. The drops are attracted to the wing and stick, thus
reducing the wasted paint that would normally mist away in the air.
The real research and work with carbon nanotubes began in 1991, when Sumino
Limijima and his colleagues created some nanotubes at the research laboratory of the
electronics multinational NEC in Tsukulba, Japan[27]
. Here they used, high-resolution
transmission electron microscopy to observe carbon nanotubes, that the field really
started to take off.
Researchers at the Institute of Chemical Physics in Moscow independently discovered
carbon nanotubes and nanotube bundles at about the same time, but these generally
had a much smaller length-to-diameter ratio. The shape of these nanotubes led the
14
Russian researchers to call them "barrelenes“.
1.2.1 Structure of Carbon Nanotubes
Carbon nanotubes are rolled up graphene layers, which are generally 0.2- 5 µm long
having diameters up to 1-2 nm. The honey comb sheets of carbon are rolled up into
cylinder. Multiwall carbon nanotubes and single wall nanotubes are the two different
types of nanotubes depending upon the rolling.
Figure 1.6: Types of Carbon nanotubes
The possible ways of rolling of the graphene layer gives different CNTs -Zig Zag,
chiral and arm chair. The characteristics of these CNTs are that they are metallic or
semi-conducting in nature depending upon their rolling up.
is a chiral vector which is related to the diameter of the tube.
A chiral vector is defined as in terms of and , and is a linear combination.
and are unit vectors which define the graphene lattice and θ is the angle
between one of the two unit vectors, either or and the chiral vector .
15
Figure 1.7: Building of CNTs from graphene sheets, the primitive vectors and an
example of chiral vector and chiral angle.
Figure 1.8: Bravais Lattice of graphene
Based on interpreted with the θ angle, which is the angle between the chiral vector,
carbon nanotubes can be classified into some specific structures.
Chiral : 0⁰<θ<30⁰
Zig-Zag : θ = 0⁰ or 60⁰
Arm chair : θ = 30⁰
16
Figure 1.9: Different types of CNT: depending on the rolling.
From the study of band structure, we get to the conclusion that the zig-zag and the
chiral have 1/3rd
metallic and 2/3rd
semi conducting CNTs and for arm chair
confurigation all the CNTs are metallic in nature.
Hence in the device we can use only zig zag and chiral ones, as they can be used as
semi-conductors. Very high purity of semi-conducting carbon nanotubes must be used
for the active channel. This is important since, it affects the performance of carbon
nanotube transistors affecting the current ON/OFF ratio.
Sorting of carbon nanotubes is required. Many techniques are used in order to sort the
semi-conductor and the metallic tubes.
1.2.2 Electronic properties of SWCNTs
Since, the SWNTs are rolled up graphene layers; hence their electronic properties of
SWNTs derive from the electronic configuration of graphene, in which the bonding π
orbitals form valence states and the anti-bonding π* orbitals form conduction states.
17
Figure 1.10: shows the band structure in the extended first Brillouin zone of graphene
Figure 1.11: Energy diagram and Density of States(DOS) a)of m-SWNTs and b)s-SWNTs .
The shapes peaks are the van Hove singularities of SWNTs
The unique electronic properties of SWNTs originate from the quantum confinement
of the electrons normal to the nanotube rolling axis.
The periodic boundary conditions around the nanotube circumference require that the
component of the momentum ( ) along the circumference is quantized ( ,
where j is a non-zero integer).[28]
This quantization leads to the formation of a set of
discrete sub-bands for each nanotube as described by the red parallel lines in Figure
1.11. The crossing of these lines with the band structure of graphene determines the
electronic structure of the nanotube. If the lines pass through the Fermi point (K or
K’), the nanotube is a metal (m-SWNT); otherwise the nanotube is a semiconductor
(s-SWNT).
18
The nature of the tube in terms of metallic or semiconducting behaviour is also
related to the values of the indices (n, m). When |n-m| = 3q (where q is an integer), the
nanotubes are metallic or semi-metallic. In any other case, they are semiconducting.
Theoretically, in an as-synthesized crude mixture of SWNTs there are approximately
one-third metallic/semi-metallic SWNTs and two-third semiconducting SWNTs
available.[29]
The transition energy (band-gap) of semiconducting nanotubes Eg can be described
by using a simple tight-binding model, given by the equation:
Where,
is electron Fermi velocity
γ is the nearest neighbour interaction energy
is the nearest neighbour C-C distance and
is the nanotube diameter.[30]
Their electronic states are organized in discrete bands, with a one-dimensional density
of electronic (DOS) states known as van Hove singularities. Two examples of energy
diagram and DOS of m-SWNT and s-SWNT are presented in Figure 1.11a and b,
respectively. No real band gap is observed in the m-SWNT system.
1.2.3 Charge Transport in CNTs
Due to the confinement along the circumferential direction, SWCNTs are in the
class of the quantum mechanical systems. Thus the transport properties can be
described considering the model for 1D nanowires, with the lateral dimensions W,
H much smaller than the length L (W, H « L).[31] [32]
To understand the different
transport regimes, we define the important length scales that are involved in the
electron motion:
λF = 2π/KF , the Fermi wavelength, which is the wavelength of the electrons at
the Fermi level. It is related to the Fermi energy of the free electron gas
19
This coincides with the length for which the quantum effects are relevant, i.e.
the quantization of the bands in SWCNTs due to the lateral confinement.
λcoh, the coherence length, which indicates the mean distance that the
electron can travel without losing its phase due to phonon-electron,
electron-electron or defects scattering.
λel, the elastic mean free path which represents the travelling length of the
electron before it undergoes an elastic scattering event. The elastic
scattering leaves the electron phase unchanged, differently from the
inelastic scattering events, but changes the electron momentum.
λloc, the localization length which indicates the average spatial extent of
the states in the system under study. In a infinite 3D pure crystal λloc =
∞ and the electron is completely delocalized. In the presence of a certain
degree of disorder, λloc and the electron can be weakly or strongly
localized.
In nanowires with lateral dimensions W, H <= λF and length L » λ
F , the quantum
lateral confinement produces the discrete one dimensional sub-bands.
Consequently the conduction is quantized and occurs though the 1D sub-bands
that are named conduction channels. In this situation the transport along the wire
axis depends on the characteristics length scales and could be diffusive, in which
the L » λel and the resistance grows linearly (incoherent transport) or exponentially
(coherent transport), or ballistic, in which no scattering events are present and the
resistance is determined by the number of conduction channel and it is
independent of the length. In this last regime, to estimate the conductance in a
single channel, we can use the Landauer theory considering the conductor
connected with two electrodes. The electron waves that approach the conductor
from the electrodes are partially transmitted and reflected through the electrodes.
For a given energy, several modes propagate inside the electrodes. Thus we can
20
define the Landauer conductance as:
G= GοT(E) with
where T(E) is the sum of the transmission probability over all the modes, that at zero
temperature is limited to the mode at EF
Go is called conductance quantum
In a situation of transparent electrical contacts, in which no reflection occurs, and
highly order in the system (T = 1), the conductance formula becomes:
G = GoN with
where N is the number of transmitted modes and Go represents the conductance of the
single mode. m-SWCNTs, i.e. armchair[10, 10], possesses two sub-bands which cross
the Fermi level, resulting in two conducting channels.
If the scattering length within the nanotube is much longer than the length of probed
nanotube and the contacts are transparent, the conductivity of the nanotube has the
ideal value G = 2Go.[33]
However, in the real experiment, the environment (substrate),
the presence of centers of scattering such as defects, impurities and distortion on the
structure and the choice of the contacts material negatively affects the theoretical
conductance.
21
1.2.4 Transport in CNT Networks
To understand the transport in carbon nanotube networks and also the various factors
involved in the carbon nanotube charge transport various research works have been
done and theories established on various factors which may be affecting the transport
phenomena in the carbon nanotube networks. It is quite obvious to understand that the
carbon nanotubes’ transport processes would be different in the case charge transport
in single carbon nanotubes and when in networks. The proximity of the CNTs even
modifies the intra-nanotube transfer. This is affected through bending and distortion
of the lattice as well as Wan der Waal’s inter-nantube interaction. These two effects
results into energetic barrier that hinders the charge transport and consequently
decreases the conductivity.
The observed the side walls of nanotube are same as the graphene sheets in the
graphite is around 3.5⁰ A. When the simulations in the model with length higher than
3⁰A done, it was demonstrated that the interaction between the orbitals of the two
nanotubes decreases and the quantum conductance presents discrete resonant peaks.[34]
This makes the hopping phenomena responsible for the charge transport. Thus, charge
can hop from one tube to another tube through resonant tunneling and the conductivity
of the network depends on the intertube tunneling efficiency. With this principle, the
phenomena why the ordered networks have higher conductivity can be demonstrated.
This is because, in the ordered networks, the contact surface between the nanotubes, is
higher than the randomly distributed networks, hence increasing the probability of the
tunneling event and the conductivity overall.
In the device, practically, the other aspects like different deposition methods,
synthesis, presence of different chiralities and introduction of impurities or other
chemical species as in the case of functionalized nanotubes play a very important
role.[35]
The different methodologies used for synthesis of Carbon nanotubes lead to metallic
and semiconducting species, with wide range of chirality that are randomly interfaced
in a network.
22
According to the perturbation theory, the resonant tunneling events between two states
happen at the Fermi-point of two nanotubes only if the conservation of the momentum
and energy is generated. Different chiralities posses’ different Fermi momenta, hence,
poor tunneling, thus are lowering the values of conductivity.
Another important factor which affects the transport phenomena of the carbon
nanotubes is the density of the networks or the number of the carbon nanotubes
involved in the bulk film. Modeling the nanotubes, as conducting sticks and implying
the percolation theory, it predicts that
with
Where
1.2.5 Contacts to Nanotubes: Schottky Barriers
Two types of barriers can form at the metal/tube interface and increase the contact
resistance beyond the ideal h/4e2 value. The first is a barrier created by an imperfect
interface between the contact metal and the nanotube. Its resistance is a function of the
cleanliness of the interface and the overlap of the metal–nanotube electronic states. Au
and Pd have proven to make the best contacts to nanotubes, with near-perfect
transmission frequently obtained [36].
More fundamental is the Schottky barrier that can form at the interface of a metal and
a semiconducting nanotube. The properties of the Schottky barrier will depend on the
band alignment at the interface (Fig. 1.12a). In the absence of interface states believed
to be a good approximation for the metal/tube interface [37]
, the heights of the
Schottky barriers for hole and electron injection are given by the work function of the
metal contacts, φM, the work function of the nanotube, φNT, and its energy gap, Eg.
23
When the Fermi level of the metal lies at the midgap of the nanotube (Fig. 1.12b)
there will be a Schottky barrier for both n and p carrier injection, whereas when it
aligns with the hole band (Fig. 1.12c) there would be no barrier to hole injection but a
large barrier to electron injection.
a)
b)
c)
Figure 1.12 a) A schematic of the non aligned metal and nanotube energy levels depicting the
relation between the Schottky-barrier heights for hole (/electron) injection, φpSB (φn
SB) and the workfunction of the metal, φM, the work function of the tube, φNT, and its energy
gap Eg.When both are put in contact, band bending at the interface occurs to compensate
for the work-function difference. (b) The case for contacts with the work function at
the nanotube midgap. (c) p-type contacts.
From different experiments, it has been made clear that the contact properties very
sensitive to the ambient environment and device history. It was also noted [36]
that a
high work function by itself is not sufficient to assure ohmic contacts. For example, Pt
24
has a larger work function than Pd but forms more resistive contacts, probably due to
additional tunneling barriers formed as a result of poor wetting of the tube by the
contact metal.
Since the energy gap grows with decreasing diameter ( it is clear from the
above equation that for every metal there exists a critical nanotube diameter below
which a Schottky barrier with finite height will form and the height of this barrier will
increase with decreasing diameter.
The assessments in the past have provided a consistent picture of contact properties of
semiconducting nanotubes, although a major challenge remains to make good ohmic
contacts to tubes with diameters 1.5 nm and smaller. Surprisingly, even small-
bandgap/metallic tubes show significant barriers in this range [38]
. The origin of this
behaviour is still not completely understood.
1.2.6 Synthesis and sorting of Carbon Nanotubes
By the different synthesis methods, the diameter of the SWNTs can be modulated;
consequently the band gaps and their conducting and metallic nature. HiPCo method,
High pressure carbon monoxide disproportionation process which uses of flowing CO
gas with Fe(CO)5 as catalyst produces SWNTs with diameter 0.7- 1.2 nm.
Another method the CoMoCAT method uses molybdenum and cobalt as catalysts
results into SWNTs with 0.8 nm, where as with the arc discharge method tubes of
diameter around 1.5 nm. This method uses high current to carbon rods. Another
technique called the laser ablation method too produces CNTs of diameter around 1.5
nm, using vaporization of carbon atoms from graphite target in an inert atmosphere by
a high power laser. It must be noted that just after their synthesis, SWCNTs have a
natural tendency to form bundles because of the Van der Waals’s forces between their
walls.
25
The methods that are used to separate metallic and semi conducting SWNTs are based
on the functionalization of the SWNTs walls. These processes can be divided into two
categories: covalent and non covalent functionalization.
Covalent functionalization is a technique which is used from 1998 and in order to
increase the solubility of SWNTs. This discrimination is based on the electronic
properties. This method affects the band gap converting the m-carbon nanotubes into
semi conducting ones. The disadvantage of this method is that it affects the charge
transport by introducing scattering sites for charge carriers.
The non covalent functionalization method which is widely used is using Sodium
dodecyl sulphate (SDS), sodium cholate, sodium dodecylbenzene sulphonate (SDBS),
and many other bile salts to disperse SWNTs in aqueous solution. The principle is
based on the hydrophobic group (tail) in contact with the nanotube and hydrophilic
group (head) in contact with the water. There is no efficient separation by these
surfactant-dispersed SWNTs, due to the interactive nature of the CNTs.
Density gradient ultracentrifugation (DGU) (see Figure 1.13a) is the first method
which showed the separation between semiconducting nanotubes and the metallic
ones. [39]
This method used a mixture of two surfactants in different ratios. Next step
was ultracentrifugation in a density gradient medium, hence, the carbon nanotubes
(CNTs) were sorted based on the density difference and therefore by diameter and
band gap . It was quite interesting that this resulted in a multi-layer coloured solution,
where different colours corresponded to tubes of different band gaps. Repetition
centrifugation processes resulted in to SWNT fractions having a narrow distribution of
nanotube diameters. This is a very effective technique applied for commercial
purposes, separating both SWNTs produced Arc Discharge method with a high 99%
semiconducting and 98% metallic purity.
Gel chromatography is another method which is (Figure 1.13b) based on the specific
adsorption of SWNTs in an allyl dextran-based gel. The interaction of this gel media
is differently with s-SWNT and m-SWNT, such as van der Waals forces.[40]
Different
interaction of the nanotubes with different curvature will have been given by different
surfactant coverage. Hence, by performing multicolumn gel chromatography, and
26
In different columns, by successively injecting SDS surfactant to the vertically stacked
column, SWNTs with different diameter and chiralities can be obtained . Using this
agarose gel technique, it is possible to obtain large-scale separation of SWNT, up to
the scale of liters. This technique allows to obtain separation of SWNTs in large scale
up to litres.
Figure 1.13: (a) Density gradient ultracentrifugation (DGU) (b) multicolumn gel
chromatography, and (c) Agarose gel technique
Another technique obtained by Krupke et al. to discriminate metallic and
semiconducting SWNTs from a raw mixture using dielectrophoresis.[41]
In this
method, an electric field gradient generated by a microelectrode array and placed on a
SDS-based SWNT dispersion. Hence, in this way the semiconducting SWNTs remain
in a stationary position, while the metallic tubes migrate towards the electrodes due to
their greater dipole moment and is collected [42]
. For large scale separation of s-
SWNTs, agarose gel electrophoresis is used.
27
1.2.7 Polymer wrapping of Carbon Nanotubes
Polymer wrapping method uses molecules to wrap and select s-SWNTs. This method
was adopted from the interesting feature of DNAs is that their nucleobases have π
stacking interaction with the surface of the s-SWNTs, resulting in helical wrapping at
the surface of the tubes.
Figure 1.14: Showing the Carbon Nanotube wrapping with polymers
The principle includes, that the conjugated back bone wraps up around the nanotube
wall, while the polymer chains which are hairy wraps the SWNTs wall with the Wan
der Waal’s interaction. Several simulations performed by Gao et al. give an indication
that the alkyl tails of neighbouring polymer chains zip and align around the nanotubes
due to the Wan der Waal’s interaction. Polyfluorene derivatives have an affinity
towards different chiralities and diameters of SWNTs. Literature shows that these
polymers in particular have selectivity towards CNTs produced by the HiPCo method
with larger diameter. Poly(9,9 didodecylfluorene-2,7-diyl-altanthracene-1,5-diyl)
(PF12-A) shows selectivity to nanotubes with larger diameter, and it is claimed that
28
this large diameter selectivity is due to the anthracene unit. In contrast to other
fluorene copolymers, poly(9,9-dioctylfluorenyl-2,7-diyl and bipyridine) (PFO-BPy)
can extract almost single chirality small diameter nanotubes
(97% of (6,5)-SWNTs) using p-xylene as solvent. This result opened the opportunity
for extracting single chirality s-SWNTs using polymer wrapping.
Figure 1.15: wrapping orientation with different polymers
It has been reported by Bao et al demonstrating that poly thiopenes with long alkyl
chains have a good efficiency to disperse and separate carbon nanotubes. The best
polythiophene derivative they reported is regioregular poly(3-dodecylthiophene) (rr-
P3DDT).
29
Figure 1.16: Different polymers used for separation of semi conductive CNTs from metallic
ones
30
1.3 Transistors and Field Effect Transistors
The history of transistors goes back to the year 1947, when John Bardeen (1908-
1991), William B. Shockley (1910-1989) and Walter H. Brattain (1902-1987)
invented the first transistor made up semi conductor material which was able to do
both- amplify an electrical signal and secondly, allow flow of current. This was
developed in the Bell labs when they were trying to find a replacement of the old and
bulky vacuum tubes. Before this, the vacuum tubes were used in the telephone
technology, amplifying current. Since they are not so reliable and excessive power
was consumed.
Figure 1.17: John Bardeen(with spectacles), William Shockley (using the microscope) and
Walter Brattain (standing behind) at Bell Labs, 1948 [43]
.
After the invention of germanium transistor the three terminal device known as the
“point contact” device (in the middle of last century), inorganic semiconductors have
been started to utilize as a dominant materials in electronics.
31
Figure 1.18: History of transistors
In 1960, Kahng and Atalla [44]
fabricated the first silicon-based metal-oxide-
semiconductor transistor. This was ground breaking and since then technology and
processing of transistors based on silicon has seen an upsurge.
In these recent years, increasing interest has been focused on Organic Electronics,
since there an ease of fabrication of these transistors, along with the cost effectiveness.
A research field focused on this has been working continuously to improve the
performance of the transistors. The current ON/OFF ratio, mobility, working at low
voltages hence lower power consumption are the main parameters which are mainly
optimized on. This would lead to a low cost, large area plastic electronics employing
transistors and diodes based on organic semiconductors. For the advances in the field
of electronics, realization of Moore’s law is necessary so miniaturization of transistors
play a very vital role.
The Moore’s law as observed in 1965, states that the number of transistors in a
dense integrated circuit doubles approximately every two years. The observation is
named after Gordon Moore, the co-founder of Fairchild Semiconductor and Intel,
whose 1965 paper described a doubling every year in the number of components per
integrated circuit,[45]
and projected this rate of growth would continue for at least
another decade.[46]
32
Figure 1.19: Moore’s law
1.3.1 Field Effect Transistors
The first demonstration of field effect in a carbon based material was observed in an
organic molecule, phtalocyanine, in 1964[47],
but only later, in the 1987, the first
organic polythiophene-based FET was realized by Koezuka[48].
This development
consequently buoyed up intense study of carbon based compounds, leading to the
synthesis of organic semiconductors with increasingly performance. However, over
the last twenty years, the discovery of new carbon allotropes, such as graphene and
carbon nanotubes, and their outstanding electrical properties gave a huge impetus to
the fabrication of FETs with the highest performance ever.
1.3.2 Geometry of Field Effect Transistors
Field effect transistors, also known as thin film transistors, when their fabrication is
done by deposition of thin films, consists of three terminals- the source, drain and the
gate. In a top gate bottom contact, the contacts are printed first on the substrate, on top
of that the semiconductor layer and the gate is separated by a thin dielectric film. W is
the width of the channel, L the length or the channel while t is the thickness of the
dielectric.
33
The thickness of the semi conducting film and the dielectric plays a crucial role in the
performance of the transistor, which would be later discussed in the sections.
Figure 1.20: Shows different configuration of FETs
[80].
Each of the four FETs or realized as TFT geometries shown in Fig. 22 has certain
advantages and disadvantages. For example, the presence of an energy barrier at the
interfaces between the source and drain contacts and the organic semiconductor layer
and is expected to impede the exchange of charge carriers between the contacts and
the semiconductor. Various experiments and simulations on such geometries have
shown that for the same energy barrier height, TFTs with a staggered structure (a, d)
have the advantage of being less affected by this energy barrier than TFTs with a
coplanar structure (b, c) [49–53]
. It has been inferred however, that in case of the
bottom-gate coplanar structure (b), the effect of the energy barrier on the carrier
exchange efficiency can be substantially reduced by modifying the surface of the
source and drain contacts with a thin organic monolayer carrying an appropriate
dipole moment [54, 55,56,57,58,59]
or with a thin metal oxide[60–63]
.
In the bottom-gate coplanar structure (b) the gate dielectric layer and the source and
drain contacts are deposited before the organic semiconductor layer film. This is
advantageous for especially very high mobility organic semiconductors, especially
those that are vacuum-deposited small-molecule materials, but also many high-
mobility polymers, adopt a thin-film microstructure that is very sensitive to external
34
perturbations. Hence, with the bottom-gate coplanar structure (b), methods involving
thermal treatments and/or solvents can be safely employed to prepare the gate,
dielectric and the contacts first without harming the semiconductor layer.
Hence to conclude, depending upon the materials, the deposition techniques and
fabrication methods adopted the best possible suitable architecture for the film thin
transistor is opted keeping in mind the desired results from the devices. The source
drain channel lengths, L typically is in the range of 10-100µm and W, channel widths
between 100µm- 1000µm.
1.3.3 Working Principle of the FETs
The field effect transistor works as a voltage controlled current source. A voltage VG
is applied across the gate dielectric in order to modulate the current between the
source and the drain, when a given voltage VD is applied between the source and the
drain. The semiconductor film and the gate electrode are capacitively coupled, since
there is a dielectric layer between the two, such that application of a bias on the gate
induces charge in the semiconductor film. Much of this charge is mobile and moves in
response to the applied source drain voltage VD. Ideally, when no gate voltage is
applied, the conductance of the semiconductor film is extremely low because there are
no mobile charge carriers; i.e., the device is off. It is only when the gate voltage is
applied, mobile charges are induced, and the transistor is on, hence the name field
effect transistors.
35
Figure 1.21: a) showing the positions of HOMOs and LUMOs with respect to the Fermi
levels of the source and drain contacts and how they change on application
of the b) c)bias gate voltage and d) e) drain voltage.
The electronic energy level diagrams in Figure 1.21a shows the positions of the
highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular
orbitals (LUMOs) of the organic semiconductor with respect to the Fermi levels of the
source and drain contacts, when no gate voltage is applied. Even now, if a small gate
bias were applied, there would be no conduction because there are no mobile charges
in the semiconductor, as this is shown in the fig 1.21b . Although there is a shift in the
position of the HOMO and the LUMO levels on application of positive and negative
gate bias. Application of positive bias shifts the HOMO and LUMO levels down,
hence lowering the energy. Only sufficient amount of gate bias brings the LUMO
level in resonance with the Fermi level of the contacts, therefore allowing a possibility
to have mobility of the charges. At this point, mobility of charges take place only if
the VD that is the drain voltage is applied and hence there can be a current flow
between the source and the drain as in fig 1.21d. Similarly, whereas the application of
negative bias across the source and the drain shift the HOMO and LUMO levels up
36
such that the HOMO level is in resonance with the contact Fermi levels and electrons
move out of the semiconductor and into the contacts, leaving positively charged holes.
These holes are now the mobile charge carriers on application of the drain voltage as
shown in fig. 1.21e. It must be noted in the fig. 1.21d and 1.21e that the source is
always the charge injecting contact regardless of the sign of the gate voltage.
The diagrams in the above figure are important as they make us visualize the
mechanism by which the organic thin film transistors are modulated by the gate bias.
The given description on the above paragraph is simplistic and for a real and
quantitative description of the process an account of the presence of the charge traps
and residual dopants must be taken. In additionally, the diagrams in Figure 1.21 might
lead one to believe that any organic semiconductor can be made to conduct holes or
electrons, depending on the sign of the gate voltage. This is not true; in general, since a
given organic semiconductor can be made more conductive with either a positive or a
negative gate voltage, but not both, with only a few recent exceptions like that of
carbon nanotubes which are ambipolar in nature . This leads to the conclusion that
organic semiconductors are classified according to whether they are hole (p-channel)
conductors or electron (n-channel) conductors.
Observing again at Figure 1.21b for the case of n-type conduction and VD = 0, and
assuming ohmic contacts and zero-threshold operation (these assumptions will be
qualified shortly), application of a positive VG induces positive charges at the
gate/insulator interface and an
equal number of negative charges near the organic/insulator interface (supplied by the
source and drain contacts). With no source-drain bias applied, this negative charge
density will be uniform across the channel. For a positive VD the areal density of
charge (C/cm2) induced at a given position x alongthe channel is proportional to the
voltage difference VG - V(x):
qind(x) = n(x) et = Cox(VG - V(x))
where Cox is the capacitance of the insulator per unit area, typically reported in
nF/cm2,
n(x) is the number density of charges in the channel (no./cm3)
e is the fundamental unit of charge, and
37
t is the thickness of the charged layer in the channel.
Figure 1.22: a) Carrier concentration profile of TFT in the linear regime b) pinch-off occurs
when VDVG- VT c) Carrier concentration profile of TFT in the saturation
regime.
It is observed that in all most occasions, it is never at VG= 0 that there is introduction
of charges into the active channel. This can be due to various reasons- like a mismatch
in the Fermi level of the metal contact and the LUMO, resulting into a charge transfer
between the metal and inorganic semiconductor, consequently causing a dipole and
band bending in the organic semiconductor, hence a VG has to be applied in order to
achieve the flat zone condition. Also in another situation of traps present in the film,
they need to be filled in order to make the channel conduct. On the contrary, if there is
a doping, this results into the channel to be conductive even at VG= 0. In order to
handle all these situations a threshold voltage VT is introduced. For example for an n
type channel, doping shifts VT negatively while traps shift it, VT positively, while
38
depending of the alignment of the mismatch of the Fermi level of the metal and the
LUMO, the VT may be either positive or negative.
Hence the equation changes to qind(x) = n(x) et = Cox(VG - VT - V(x))
Figure 1.22a shows that, when VD = 0, the channel charge density is uniform for a
given VG (since ideally VT is not a function of x, and when VD = 0, V(x) =0).
However, when VD is nonzero but less than VG, there is a linear gradient in the charge
concentration, Figure 1.22b For a given small value of VD, the average value of qind is
Cox(VG - VT - VD/2), which is the induced areal charge density in the center of the
channel. To the left of center, the charge density will be higher, and to the right, the
charge density will be lower.
Now using Ohm’s law and the definition of conductivity
where σ is the conductivity, μ is the carrier mobility (i.e.,velocity per unit electric
field), and nind, av is the average carrier concentration in the channel. Solving the
equations we obtain
which is commonly rewritten as the linear regime.
In this Linear regime for |VG – VT | >> VD, the charge density is uniformly distributed
along the channel, and the device operates as a resistor. The drain current is linear
with applied drain voltage.
But as the VD is increased, as shown in the fig the carrier concentration becomes non-
uniform, decreasing from the source to the drain such that when VG-VT VD , the
39
channel becomes pinched as a result of no potential difference between the gate and
the part of the channel nearest to the drain. This is shown in fig 1.22b
Now on further increasing the VD, it would push the voltage always toward the source,
without any increase in the current due to the fact that the integrated resistance of the
channel from the source to the pinch point remains the same, and carriers are swept
across the narrow space charge region from the pinch point to the drain by the
(comparatively) high electric field in the depletion region. Hence on substituting VD=
VG-VT , we get the saturation regime, and the current in the saturation regime is given
by
1.3.4 Electrical characterization of FETs
The TFTs are typically characterized in one of two ways-
Output curves - By holding VG constant and sweeping VD (commonly referred
to as ID-VD or output curves; see Figure 1.23
Figure 1.23: showing ID – VD curves for TFTs at different values of VG.
Transfer curves- by holding VD constant and sweeping VG (commonly referred
to as ID-VD or transfer curves; see Figure 1.24)
40
Figure 1.24: showing ID –VG curves at a constant value of VD . Here VT is
the threshold or pinch off voltage.
Note: If contact effects and trapping are not too problematic, these traces can be
modelled quantitatively using I-V relationships derived from Ohm’s law.
Field Effect Mobility- The mobilities in the different regimes give us a direct
indication of the performance of the field effect transistors.
Linear Mobility calculated in the linear regime for | VG- VT| > |VD|
and saturation mobility calculated in the saturation regime for |VD|>| VG-
VT|>0
ION/OFF- Current ON OFF ratio- It is relates to maximizing mobility. A higher
ION/OFF ratio is an indication of better performance. It is generally expressed as
10x.
41
Sub threshold swing- S It is a measure of how rapidly the device switches
from the off state to the on state in the region of exponential current increase
and is typically reported in V decade-1
or mV decade-1
. Sn , sub threshold
swing = CoxS (VnF cm-2
decade-1
), which yields the required number of gate-
induced carriers to effect a one-decade increase in the current near Vo (the
exponential increase in current shown on the adjacent axes ) and thus allows
meaningful comparison of films on different dielectrics. A large subthreshold
swing generally implies a large concentration of shallow traps, i.e., a diffuse
turn-on region. Both ION/IOFF and Sn are dependent on the device geometry,
operating conditions, and measurement apparatus and thus are only truly
useful when cited along with this information or when used to compare
devices tested under identical circumstances [64]
.
Since these are the features define the performance of the field effect transistors, it is
necessary to have a good semiconducting material, which is to an extent fulfilled by
carbon nanotubes.
42
1.3.5 Single Wall Carbon nanotube-FETs
Figure 1.25: Showing first SWNT- FET device
The CNT FETs were first developed by Dekker’s group at Delft university and by
IBM for a convenient back-gate geometry [65,66]
. While the device (Fig. 1.25) is
similar to the Si-based FET as shown in Fig. 1.26 [66]
, the detailed device physics are
very different [67,68]
. In Si or other conventional semiconductor based FETs, carriers
are generated by thermal excitation from dopant levels, and the polarity of the FET is
determined by the type of dopants, that is either donor (n-type) or acceptor (p-type)
[69].
Figure 1.26: Device architecture of the first Si based FETs
[64]
In a CNT FET, the situation is very different. This is because CNT is naturally
intrinsic onto which substitutional or stable doping is unlikely [70–72]
. As a result,
43
controlled doping is very difficult for CNT, which prevented the CNT technology
from being developed into a high performance CMOS technology for a long time
[73,74]. Indeed, doping in CNT is neither easy nor desirable. Since the high carrier
mobility in a CNT mainly results from the stable and perfect sp2 structure of the CNT,
and the introduction of dopants in CNT would distort the otherwise perfect structure
and compromise its electric prop-erties, leading to stronger scattering, lower carrier
mobility and thus lower performance [73]
.
In a high performance CNT FET fabricated via the doping-free process [75]
, the
conduction CNT channel is intrinsic with perfect sp2 lattice in which carriers are
provided directly from the metal electrodes rather than by thermal excitation [76,77]
,
making the CNT FETs suitable for low temperature electronics applications [78]
. In
addition, the properties of the contact also determine the polarity of the CNT FETs.
When a metal electrode is brought into contact with a semi-conductor, the Fermi
levels of the metal and the semiconductor are usually not identical. This misalignment
in the Fermi levels would lead to certain degree of charge transfer between the metal
and the semiconductor, and as a result a Schottky barrier (SB) forms between the
metal and semiconductor [79]
. For high performance devices, this is highly undesirable
since the presence of a SB severely limits the injection of carriers from the metal
electrode to the semiconductor, or from the semiconductor to the metal.
Figure 1.27: the output characteristics of the first CNT FETs developed by Dekker’s group in
44
1998 [65]
.
The fabrication of the back-gated CNT FET is fairly straightforward. Pt electrodes
were used as the source and drain, which was pre-patterned on a SiO2 surface, and
CNT was transferred onto these metal electrodes to form channels. The doped Si
substrate beneath the SiO2 was used as the back-gate, which may effectively modulate
the carrier concentration in the channel and in the region near the metal/
semiconductor CNT interface. In particular, the SB may be significantly thinned at
large gate bias, allowing the carrier injection into the CNT to change from thermionic
emission to thermally assisted tunneling mode, leading to a large on and off state
current ratio. The transfer characteristic of the device (Fig. 1.27, inset) reveals clearly
that the device is a p-type FET, whose conductance can be changed over six orders of
magnitude from on-state at negative gate voltage to off-state at large positive gate
voltage. It should be noted, however, that the device current is relatively small, of the
order of tens nA. This is because there exist a SB between Pt and CNT, limiting the
hole injection efficiency from Pt into the CNT channel. The low current level not only
leads to slow speed of the device, but also limits the driving ability of the device and
makes it unsuitable for high performance applications. Perfect ohmic contacts can be
made to the SWNTs either to the valence band using Pd [76]
or to the conductance band
of the CNT using Sc [77]
, rendering both n-type and p-type high performance CNT
FETs operating close to the ballistic limit available.
1.3.6 Carbon nanotube network FETs
Carbon nanotube being easily processible in solution state, provides an opportunity to
be printed. Inkjet printing method has been used in cases making the process being
cost effective and allowing quick fabrication.
These works on glass substrate and top gate bottom contact approach architecture
have employed a high-yield sorting method based on non-covalent functionalization
through P3DDT chains to select only semiconducting chirality. Through an
enrichment process, a stable formulation in oDCB with high loading (up to 0.2 mg
mL−1) has been obtained resulting in a dense network of nanotubes already after a
45
single printing pass. By varying the printing passes, balanced ambipolar FETs, with
saturation mobilities of 10 and 7 cm2 V
−1 s
−1, for holes and electrons respectively, as
well as unipolar p-type FETs, with a maximum saturation mobility of 15 cm2 V
−1 s
−1,
have been realized, showing hysteretic free behavior and on–off ratio in linear regime
up to 107 [20]
.
Figure 1.28: Device architecture for inkjet printed transistors work by Sadir et al
Figure 1.29 a) Mobility plot extracted from devices made by increasing the number of printed
passes (channel width W = 200 μm and channel length
L = 40 μm). The holes mobility (blue lines) and electrons mobility (red lines) in linear regime
(empty symbols) and saturation regime (filled symbols) are reported. b) Threshold voltages of
the same devices.
Other works on single wall carbon nanotube networks on flexible substrates have
shown up that the mobilities can be reached up to average 4.3 cm2V
-1s
-1 and on off
ratios 105- 10
6 [80]. This gives me motivation to carry on my work.
46
CHAPTER 2
MATERIALS, EQUIPMENTS
AND FABRICATION TECHNIQUE
In this section, the materials which are required for the fabrication of the devices are
listed, with their properties. Like for carbon nanotube active layer, the complete
procedure for its preparation is given. The equipments used in the experiments are
listed included here. A brief description of their features and working principle
explained. Lastly, in this part of thesis, the complete fabrication procedure is
comprehensively described, outlining the device architecture which is used for the
fabrication.
MATERIALS
2.1.1 PEN SUBSTRATE
Figure 2.1: Chemical structure of Polyethylene Naphthalate
Polyethylene Naphthalate (poly (ethylene 2,6-naphthalate or PEN) is
a polyester and a polymer of naphthalene-2,6-dicarboxylic acid and ethylene glycol.
It provides a very good oxygen barrier.
The structure of PEN consists of two aromatic rings, which results in enhancing of
strength and modulus, gaseous barrier, chemical and hydrolytic resistance, also
47
thermo and thermo-oxidative resistance. It is also resistive to Ultra violet light as
compared to Polyethylene Terephthalate (PET).
Figure 2.2: Chemical structure of Poly ethylene Terephthalate
Cleaning of the substrate PEN
The substrate is cleaned with Iso-Propyl Alcohol (IPA), and then with N2 gas to dry it.
Then we use the Dimatix Printer in order to print the PEDOT: PSS transistors and on
top of that the silver source and drain contacts.
2.1.2 PEDOT: PSS
Figure 2.3: Chemical structure of PEDOT: PSS
PEDOT: PSS consists of a polymer mixture of two ionmers-
poly(3,4 ethyenedioxythiophene ) and poly sodium polystyrene sulfonate.
48
The component poly(3,4-ethylenedioxythiophene) or PEDOT which is a conjugated
polymer and carries positive charges and is based on polythiophene. Together the
charged macromolecules form a macromolecular salt.
The other component in this mixture which is made up of sodium polystyrene
sulfonate is a sulfonated polystyrene. Here the sulfonyl groups are de-protonated and
carry a negative charge.
It is a transparent, conductive polymer with high ductility and used in many thin
flexible films for flexible, solar cells, organic light emitting diodes and field effect
transistors.
Their conductivity can be increased many times by adding up high boiling point
solvents like dimethyl sulfoxide, sorbitol, other ionic liquids and solvents.
2.1.3 Dielectric Material- PMMA
Figure 2.4: Chemical structure of PMMA
Poly Methyl Methcrylate (PMMA), chemically known poly( methyl 2-methyl
propenoate) as also known as acrylic glass, is the material used as a dielectric since
i) It can withstand temperatures up to 160⁰C, hence, can be annealed
to high temperatures
ii) High environmental stability
iii) Ease in solution processibility
The dielectric PMMA solution which is prepared in n-butyl acetate and has been
produced by Sigma-Aldrich, Mw = 120 kg mol−1 with a concentration of 80 g/l.
49
This used for spin coating and bar coating. The spin coating conditions are supposed
to be 1000 acceleration, velocity 1200 rpm for 70 seconds and then annealing. While
the bar coating settings for the dielectric are velocity 30 mm/s and with a wire coater
of 8 µm.
2.1.4 P3DDT Polymer
Figure 2.5: Chemical structure of P3DDT monomer.
Poly(3-dodecylthiophene-2,5-diyl) known as P3DDT is a conducting polymer
It is used in rechargeable battery electrodes, electrochromic devices, chemical and
optical sensors, light-emitting diodes, microelectrical amplifiers, field-effect
transistors and non-linear optical materials. P3DDT can be used in organic field effect
transistors with enhanced carrier mobility prepared by a drawing method.
Being a p-type polymer semiconductor. Here, P3DDT ((Mn = 26,800 g/mol, Mw =
29,000 g/mol) is used to wrap the carbon nanotubes.
50
2.1.5 Semiconductor -Carbon Nanotube solution
The sorting and formulation was done by the group of Prof. Maria Antonietta Loi
in Groningen, Netherlands. The synthesis of the wrapping polymer Poly(3-
dodecylthiophene-2,5-diyl) (P3DDT) (shown in figure 2.5) was done via GRIM
method. Gel permeation chromatogra- phy (GPC) was used to determine the
molecular weight found to be Mn = 26,800 g/mol, Mw = 29,000 g/mol. High-
pressure carbon monoxide conversion (HiPCO) SWCNTs were purchased from
Unidym, Inc. and were used as received. The P3DDT was solubilized in toluene
using a high power ultrasonicator (Misonix 3000) with cup horn bath (output
power 69 W). Afterwards, SWCNTs were added to form the HiPCO: polymer
dispersions with weight ratio 1:2. The solution was then sonicated for 2 hours at
69 W and 16° C. After ultrasonication, the dispersion was centrifuged at 40,000
rpm (196 000 g) for 1 h in an ultra- centrifuge (Beckman Coulter Optima XE-90;
rotor: SW55Ti) to remove all the remaining bundles and heavy-weight
impurities. After the centrifugation, the highest density components precipitate at
the bottom of the centrifugation tube, while the low density components,
including individualized s-SWCNTs wrapped by the polymer and free polymer
chains, stay in the upper part as supernatant. An extra step of ultracentrifugation
was implemented to decrease the amount of free polymer in solution
(enrichment). For this purpose, the supernatant obtained after the first
ultracentrifugation was centrifuged for 5 hour, 55,000 rpm (3,67,000 g), the
individualized s-SWCNTs were now precipitated to form a pellet and the free
polymer was kept in the supernatant. Finally, the pellet of CNTs 0.2g/l was re-
dispersed by mild sonication in ortho-dichlorobenzene (oDCB). The optical
characterization of the solutions was performed using a UV–Vis–NIR
spectrophotometer to check the concentration of the carbon nanotubes selected
by the polymers, as well as the amount of the polymer in the solution. In figure
2.6 are reported the spectra of the solution in (a) toluene and (b) o-DCB and the
CNTs chiralities present in the ink.
51
Figure 2.6: a) Absorption spectrum of HiPCO: P3DDT in toluene after the 1st
centrifugation. b) Absorption spectrum of HiPCO: P3DDT in oDCB after 2nd
centrifugation. The chiralities of the SWNTs present in the sample are determined
using an empirical equation.
2.2 EQUIPMENTS
2.2.1 Dimatix Printer
Fujifilm Dimatix, a commercial inkjet printer. is specifically made for printing various
types of materials. As a result, they have a great deal of control over drop volume and
spacing, and they provide better resolution. Although these specialized inkjet printers
can be expensive, they seem to be a good choice for printing due to their superior
functionality
Model-DMP-2830
The Dimatix Materials Printer (DMP) is a bench top materials deposition system
designed jetting with micro-precision. A large range of functional fluids like
polymers, solutions, can be deposited onto virtually any surface, including plastic,
glass, ceramics, and silicon, as well as flexible substrates from membranes, gels, and
thin films to paper products.
DMP is a complete ready to use system; it also facilitates developing and testing
manufacturing processes and product prototypes. The prototyping of products from
wearable electronics, flexible circuits, and displays to DNA arrays and RFID tags
52
makes it interesting. With inexpensive and exchangeable cartridges, allowing the users
to fill them with their own desired fluid materials makes the DMP system more user
friendly. By employing inexpensive exchangeable cartridges that researchers can fill
with their own fluid materials, the DMP system minimizes waste of expensive fluid
materials, This facilitates not only minimal complexity which is associated with
traditional way of product development and it’s prototyping, but also eliminates cost
which is un called for.
Figure 2.7: Dimatix Printer View
System Features
1. Flat substrate with xyz stage, inkjet deposition technique
2. User-fillable cartridges with low cost
3. Piezo-based inkjet print
4. Fiducial camera for substrate alignment and measurement
5. Drop jetting observation system which is built in
6. The jetting resolution and the pattern can be controlled from the PC
7. Graphical User Interface (GUI) application software
8. Capable of jetting a wide range of fluids
9. Heated vacuum platen
53
10. Cartridge cleaning station
11. Includes PC, monitor, and software
Dimatix Materials Cartridge
Figure 2.8: A Dimatix Matrix Materials Cartridge
Whereas the Dimatix Materials Cartridge is a cartridge-based inkjet printhead used
with the DMP and available in 1 pl and10 pl drop volumes. Based on FUJIFILM
Dimatix’s proprietary Silicon MEMS technology, the 16-jet Dimatix Materials
Cartridge is designed for high-resolution, non-contact jetting of functional fluids in a
broad range of applications. The industry-first 1 pl cartridge can deposit features as
small as 20 μm (20 millionths of a meter) to fabricate products such as organic thin-
film transistors (TFTs) and printed circuits. In biotechnology, the Dimatix Materials
Cartridge allows researchers to closely pack large numbers of elements in DNA
arrays, to permit more accurate and efficient analyses. Cartridges can easily be
replaced to facilitate printing of a series of fluids. Each single-use cartridge has 16
nozzles linearly spaced at 254 microns with typical drop sizes of 1 and 10 pico litres.
.
54
Figure 2.9: Photo of Dimatix Printer
2.2.2 TQC Bar Coater
Figure 2.10: TQC bar coater
The TQC Bar coater consists of the spiral or wire bar draw down rod applicator is
ideal for applying a thin film even on materials which are very flexible like plastics
55
and thin sheets. It’s feature to work on flexible substrates and with motorized film
applicators make the coater unique. The bar made up of a very high grade stainless
steel is not affected by acid or base elements.
Figure 2.11: The coating rods with different wire spacing diameters
For the coating of CNTs in oDCB with o-Xylene, a wire bar coater of 320 mm long
and 20 um. Later, to compare between the experiments, a wire bar coater of 320 mm
long and 20 um is used.
For the coating of PMMA dielectrics, a wire bar coater of 320 mm long and 8um.
The thickness of the PMMA dielectrics is estimated to be around 568 nm with the help
of profilometer.
56
Figure 2.12: Photo view of TQC coater
2.2.3 Profilometer
A profilometer is used in order to measure the thickness of the film but comparing the
two flat regions of the substrate and the film laid region.
Figure 2.13: Schematic representation of Stylus profilometer
A profilometer consists at least of two stages- the detector and the sample stage. The
detector is what determines where the points on the sample are and the sample stage is
57
what holds a sample. In some systems, the sample stage moves to allow for
measurement, in others the detector moves and in some both move.
There are two types of profilometers: stylus and optical. Here Stylus profilometer is
used. These type of profilometers use a probe to detect the surface, physically moving
a probe along the surface in order to acquire the surface height. This is done
mechanically with a feedback loop that monitors the force from the sample pushing up
against the probe as it scans along the surface. A feedback system is used to keep the
arm with a specific amount of torque on it, known as the ‘setpoint’. The changes in
the Z position of the arm holder can then be used to reconstruct the surface.
Stylus profilometry requires force feedback and physically touching the surface, so
while it is extremely sensitive and provides high Z resolution, it is sensitive to soft
surfaces and the probe can become contaminated by the surface. This technique can
also be destructive to some surfaces.
Because a stylus profilometer involves physical movements in X, Y and Z while
maintaining contact with the surface, it is slower than non-contact techniques. The
stylus tip size and shape can influence the measurements and limit the lateral
resolution.
Working with the profilometer
First, we place the substrate deposited with the thin film. Scratch it with a pick and put
it under the stylus. It is important to make sure that the experimental set up is
performed under less 50 decibels and almost no vibrations. This is because the
measurement is sensitive to noise and vibrations.
The stylus is placed on the edge of the scratch in order the scanning to be done on
both the plane substrate as well as the scratch. Then we adjust to Valley Bias. Double
click on down in order to make the stylus touch the surface of the substrate. A click on
Up. Adjustment is then changed to Centre Bias and the scan speed set in the range in
between 50 µm/s - 20 µm/s. If the surrounding is noisy- generally the lower range that
is 20 µm/s is selected. Then the scanning is started. On obtaining two flat zones –
58
select levelling two zones, and verify the thickness. The height difference gives the
thickness of the film.
Figure 2.14: Photo view of the place where the sample is placed in the
profilometer
Figure 2.15: Photo view of the profilometer setup
59
2.2.4 Glove Box Prober
Glove box prober is used to for the electrical characterization of the devices. It is done
under controlled environment- N2 filled and oxygen and water less than 20 PPM. The
completely fabricated that is a set of working devices are placed on the plate under the
optical microscope. The microscope is then adjusted to get the correct focus on the
devices. There are four probes which are placed on the supports fixed on the working
plane with magnets. The supports are fitted out with mico-manipulators that enable to
contact the sample in a very precise way, without risking any damage to the devices.
The probes are connected through coaxial cables to an Agilent B1500A
Semiconductor Device Analyzer, a system able to perform I-V and C-V
measurements. The instrument is a single cabinet provided with an LCD screen
for the display of the graphic interface of the Easy EXPERT software, which
controls the whole system. The resolution of the system is 0.1 fA in current and 0.5
µV in voltage.
Figure 2.16: Photo view of the Stylus prober
60
Figure 2.17: Prober in the Glove Box
61
2.2.5 Annealing
After printing, annealing helps in reducing the grain boundaries between the dielectric
and the semiconductor, by having an amorphous morphology between them,
enhancing charge transport and better mobilities. It also helps to escape the oxygen
trapped in the sample.
Annealing is done for 40 minutes at 120⁰ C in the fume hood.
Figure 2.18: Annealing of samples
62
2.2.6 Scanning Electron Microscopy
Scanning Tunneling Microscope uses beam of electrons which are finely focused in
order to produce high resolution of the sample which is to be examined.
Working of an SEM
Figure 2.19: Working of Scanning Electron Microscope
An electron gun is located at the top of the device and which shoots a beam of
electrons. This beam of electrons is highly concentrated. There are two types of guns –
thermionic gun and field emission guns.
The thermionic gun uses heating of the filament until it produces electrons, where as
field emission guns use electric field in order to rip away the electrons from their
atoms. This is a more popular method. A series of lenses make up the microscope
which directs the electrons towards the specimen in order to maximize their
efficiency. Magnification depends upon the intensity of the electrons; more the
intensity, higher is the magnification.
63
A vacuum chamber is required where the electron beams do not get obstructed as it
passes through the body of the microscope. In case of small particles, they could
deflect the electrons onto the sample itself obstructing the results. As the beam of
electrons, known as the incident beam, hits the specimen, there are three kinds of
electrons: primary backscattered, secondary and auger electrons are emitted with X
rays. Only the primary backscatter and secondary electrons are used by SEM. An
electron recorder is placed which picks up the rebounding electrons and draws up
textual information in a very coherent and consistent manner.
The samples are prepared, mostly if they are non conductive in nature. A thin layer of
metal, usually is provided by the sputter coaters. The gold is deposited through the use
of an electric field and an argon gas as the electric field dislodges an electron from the
argon, resulting in positively charged ions. Then these positive ions are then attracted
to the negatively charged gold foil. The argon ions settle on the gold, expelling gold
atoms, making them to fall on the specimen, hence covering it with a thin conductive
coating.
2.3 Fabrication Method
The substrate PEN is first cleaned with IPA Iso Propyl Alcohol and then dried with N2
gas. This is done under the fume hood. The transistor pattern is printed on it with
Dimatix printer. The substrate is taken and kept on the platen. The PJ700 PEDOT:
PSS cartridge is taken, it is then cleaned in the fume hood dipped in water and dried
with N2 gas. A syringe of 1 ml taken, fitted on the cartridge and filled with 300 µl of
PEDOT: PSS. The settings of the printer are configured.
Firstly, select the correct settings of PEDOT: PSS cartridge. The transistor pattern is
selected and the substrate height is put at 300 µm. The jetting of the nozzles is
checked, if not jetting properly, it is adjusted. Now, checking with the fuducial
camera, the printer nozzle is put on the substrate and the printing process is started.
After printing of the transistor patterns, annealing is done at 120⁰ C for 40 minutes
under the fume hood, by keeping the samples on the hot plate. Silver contacts are
printed with Dimatix printer. The silver cartridge is taken cleaned, the settings of the
Dimatix correctly set with the pattern of the contacts selected and the thickness of the
64
substrate set to 300 µm. The patterned substrate kept on the hot plate, and looked
with the fuducial camera, the patterns printed on it. Annealing is done at 120⁰ C for 40
minutes. With the optical microscope or in the Dimatix printer screen, the channel
length and the channel width is calculated.
Next the Carbon Nanotube ODCB wrapped in P3DDT semi conductor channel is bar
coated. The bar coating conditions are as follows-
The specific bar coating rod for CNTs is taken as 10 µm or 20 µm. Cleaned with IPA
(Iso Propyl Alcohol) and put on its slot. The patterned sample is placed on the bar
coater plastic plate and a hot plate maintained around 110⁰ C is placed underneath it.
Velocity set at 30 mm/s. The bar coated substrate is kept for annealing at 120⁰ C for
40 minutes.
The dielectric PMMA is bar coated. A communal bar coating rod of 8 µm is taken and
cleaned with Acetone and IPA and subsequently dried, bar coating velocity kept at 30
mm/s. The bar coating done, again the cleaning of the bar is performed. The devices
are now kept for annealing at the same temperature of 120⁰ C for 40 minutes.
Printing of Gate is done by the Dimatix printer with PEDOT: PSS. The process is
similar to that of printing of transistor pattern, but different Gate pattern “pad” used.
To explain briefly, the PEDOT: PSS filled up in the cartridge taken after cleaning, and
then put into the Dimatix cartridge slot. The Dimatix printer is configured to get the
best jetting of the nozzles. With the help of the fuducial camera, the nozzle placed on
the devices and the gate pattern is printed properly. A subsequent annealing is done at
temperature of 120⁰ C and for 40 minutes.
The completely fabricated devices are then electrically characterized with the probers
placed in the Glove box. The glove box conditions are controlled keeping in the
nitrogen gas environment and oxygen kept at less than 20 PPM and moisture
minimum.
The transfer curves and output curves are plotted on the system with the easy Expert
software attached to the probes. Agilent technologies make this software. The
measurement data saved and transferred to the personal computer. The mobility plots
are done with the OriginPro 8.5.
65
Figure 2.20: Architecture of the device
With the top gate staggered architecture, advantage of being less affected by this
energy barrier compared to the TFTs with a coplanar structure.
66
CHAPTER 3
RESULTS AND ANALYSIS
In this section the experimental work which is done during these nine months of thesis
is reported. The aim of the work is to obtain fully printed carbon nanotube transistors
on plastics. Bar coating is the deposition technique which is implemented in order to
print the semiconducting layer. This method of deposition is opted because it is easier
as well as the thickness of the semi conducting CNT layer can be maintained uniform.
In the literature[82] [83]
, it shows that blade coating and rod coating partially aligns the
carbon nanotube networks in the solution due to shear. This method of deposition
reduces sheet resistance. In order to research any substantial results on the alignment
of the carbon nanotube active layer, bar coating has been employed. Subsequent
attempts have been made to improve the performance of the transistors. Optimization
of the transistors is done by modulating different parameters during the fabrication of
the transistors. The best and most indicative results obtained during this alteration of
the fabrication process are reported here. The process modifications stated with why
such modifications are being carried out and the possible outcomes being commented.
Electrical characterization is done with transfer curves and linear and saturated
mobilites being computed which gives the performance of the carbon nanotube
transistors. Scanning electron Microscopy is also done for analysis.
67
3.1 Reference devices based on printed semi conductor to validate the
architecture
In order to verify the feasibility of fabrication of printed CNT transistors on plastics,
N2200 a naphthalenediimide based polymer was used as the semi-conducting
material. N2200 is a n type conductive polymer which is used in printed electronics.
The reason for using N2200 is that it can be prepared easily in the lab and also has
good charge transport properties with respect to other polymers. The devices are
fabricated by spin coating of N2200. Firstly, the substrate is taken and cleaned with
Iso Propyl Alcohol (IPA), and dried with Nitrogen gas. The transistor pattern is
printed with the dimatix printer. It is then put to annealing on the hot plates at 120⁰ C
for 40 minutes. The source and drain silver contacts printed with the dimatix printer
and subsequently annealed. Here, the spin coating conditions of N2200 are velocity
1000 rpm, time 60 seconds and acceleration 1000. Annealing is done at 120⁰ C for 40
minutes. Then the dielectric PMMA (80 mg/ml) layer is spin coated at conditions of
1000 rpm velocity, 1000 acceleration and time for 60 seconds. Subsequent annealing
is followed up at 120⁰ C for 40 minutes. The gate of PEDOT: PSS is printed using
dimatix printer and afterwards kept for annealing for 40 minutes at 120⁰ C. The
performance of these transistors is recorded with mobility curves. Under similar
conditions of processing, carbon nanotube transistors are also fabricated by spin
coating. The results obtained are given below.
68
Figure 3.1: Mobility curves of spin coated N2200 and CNTs device
From the figure 3.1 it is clear that in the case of N2200, the linear 0.20 cm2V
-1s
-1 and
the saturation mobilities 0.61 cm2V
-1s
-1 are not close to each other whereas when
observed in the case of CNT transistors, the linear and the saturation mobilities of
0.77 cm2V
-1s
-1 and 0.83 cm
2V
-1s
-1 are close to each other (as in figure 3.1 blue). This
is because in CNT transistors the lateral contact resistances are less as compared. An
indication of higher mobilities than N2200 is also observed. Hence, this motivates me
to carry on further optimization on carbon nanotube transistors and also showing a
more likely possibility to be down scaled.
3.2 Bar coating of CNTs and the sonication effect
In order to optimize the performance of carbon nano tube transistors, different
parameters are employed to find the best results. It is mentioned in the synthesis of
carbon nanotubes that they have a natural tendency to form bundles due to the Van der
Waal’s forces. A batch of experiments is carried out at same conditions to demonstrate
69
this effect. The fabrication procedure is as followed. Firstly, we take up a sonicated
(for 5 minutes) carbon nanotube solution (in oDCB) wrapped with P3DDT polymer.
Then on the printed transistor pattern and source drain contacts, the active layer is bar
coated with 20µm pitch bar coater at 30 mm/s, and subsequently annealed at every
alternate step after deposition. The spin coating of the dielectric PMMA done at 1000
rpm, at acceleration of 1000 and for 60 seconds. The subsequent annealing step at
120⁰ C for 40 minutes; in order to remove the trapped air in the thin films and to
enhance amorphous morphology with less grain boundaries. Printing of gate and then
annealing done, the devices are tested.
Now, the same solution is taken for the next batch of experiment, but with ignorance
to sonication. A completely similar fabrication process carried out.
Figure 3.2 Mobility curve showing sonication effect
The above curves show, the mobilities in both the situations. A mark decrease in the
mobilities is observed in the figure 3.2 as when compared to the sonicated carbon
nanotube devices in with saturation mobilities of 1.2 cm2
V-1
s-1
and 0.20 cm2
V-1
s-1
respectively. This is because of entanglements or sticking together of the carbon
70
nanotubes. The carbon nanotubes are in bundles, hence there is a constraint in the
charge transport in a particular direction.
3.3 Spin coating and bar coating of the Dielectric layer
In order to have a control on the thickness of the dielectric layer, bar coating of the
dielectric PMMA was done. The thickness of the dielectric plays a crucial role
influencing the capacitance of the device and hence the overall mobility.
Spin coating of the Dielectric
After deposition of the semi conducting layer that is CNTs in oDCB wrapped with
P3DDT and o-Xylene in the formulation 3:1, on the printed substrate with bar coating,
the spin coating of the dielectric is done for 70 seconds, at 1000 rpm and 1000
acceleration. After this the consequent steps of complete fabrication of CNTs
transistors followed.
Bar coating of the dielectric
Bar coating of the dielectric PMMA is done with a wire of 8µm velocity of 30 mm/s.
Consequent annealing of the dielectric is done at 120⁰ C for 40 minutes , the gate
electrode is printed with the dimatix printer, annealed and the transfer characteristics
71
recorded.
Figure 3.3: Plot showing the difference in the performances of the devices fabricated with
spin and bar coating of the dielectrics
The results show that there is a better control on the thickness of the dielectric. This
can be analysed from the fact that the figure.3.3 shows a higher value of saturated
drain current up to 8.08 x 10-5
A at -60 V gate voltage, while from the spin coated
dielectric devices shows the transfer curves, the saturated drain current up to 1.96 x 10-
5A at -60 V gate voltage. This can be related to the fact that as the thickness is
controlled with the bar coating of the dielectric. It is measured that with the
profilometer that the bar coating thickness of the dielectric PMMA is 328 µm, while in
spin coating the thickness is measured 568 µm. The current and the dielectric
thickness relation is given in the source drain current equations in the two regimes as
in linear regime and
in the saturated regime,
where is the capacitance of the dielectric per unit area. Hence with the decrease in
thickness the capacitance increases and the increase in current can be justified.
72
3. 4 Bar coating of the dielectric, completely printed device
Since bar coating of the dielectric showed much better performance compared to that
of the spin coated one, we fabricate a complete device with bar coating of CNTs in o-
Xylene and bar coating with dielectric. The procedure of fabrication is given in the
module 2.3.
Figure 3.4: Shows transfer curves of the best fully printed device
The figure 3.4 shows the saturation current value of 1.55 x 10-4
A at voltage bias of -
50V. Through the work, this is the maximum value of current which is achieved by
the device using PMMA as the dielectric. The current ON/OFF ratio is 104 .
73
Figure 3.5: shows output curves of the best fully printed device.
Output curves are swapped from 10V to -40V at constant gate voltages. It was found
that at lower voltages around -5V voltage there was charge transport and the drain
current with modulation with drain voltage is shown in the above graph.
74
Figure 3.6: Mobility curve of the best working printed device.
The mobility plot (figure 3.6) which is obtained for this device shows that the contact
resistance is negligible or vey less, since the two saturation and linear mobility values
are very close to each other. This can be attributed even to high leakage current.
Leakage current is high. The values maximum values obtained are 1.90 cm2V
-1s
-1
saturation mobility and the linear mobility 1.89 cm2V
-1s
-1. Scanning Electron
Microscopy of the bar coat deposited carbon nanotube film is done to analyse the
carbon nanotube network in the active layer.
75
Figure 3.7: shows the SEM image of CNT solution with o-Xylene after bar coating.
The SEM image, it can be seen that there are many voids in the network. Hence, it can
be said that the coverage of the carbon nanotubes is limited, losing part of the active
area. This can be a major reason for constraint values of mobility. It also leads to the
fact that the junction between the contacts and the semi conductor are not efficiently
in contact. The image further does not show any orientation of the carbon nanotube
networks by shear. In literature [83, 84]
it showed that with rod coating and blade coating
there is a partial alignment of the carbon nanotube networks.
3.5 Optimization of Carbon nanotube networks
In order to further optimize the performance of the transistors and out of curiosity, the
bar coating conditions are being changed.
3.5.1 Bar coater wire pitch
All above bar coating of the carbon nanotubes semi conducting layer are done with a
20 µm pitch bar coater wire at a bar coating velocity of 30 mm/s. Now 10µm pitch bar
coater wire is used. The fabrication process is as follows. Firstly, the transistor pattern
and the silver source drain contacts is printed with dimatix printer. With 10 µm pitch
76
bar coater and at a velocity of 30 mm/s the formulation of semi conducting layer of
CNT in oDCB wrapped with P3DDT and o-Xylene in the ratio 3:1 is bar coated,
annealed and rest of the standard fabrication procedure followed.
Figure 3.8: The transfer characteristic curve of device plotted with 10 µm pitch
bar and 20 µm coater.
From the above transfer curves it can be commented that the hysteresis is large in this
case. Due to the lower pitch of the bar coater wire, the semi conducting layer is a thin
layer. Therefore makes the charge carriers being vulnerable trapping in the interfaces
of the different layers. Hence, with low coverage and thin layer of the nanotube semi
conducting layer the hysteresis is more prominent.
3.5.2 Bar coating with different formulation
In the view of having more of coverage of CNTs in the solution, the formulation of
the solution is changed. Now, for bar coating solely CNTs in oDCB wrapped with
P3DDT polymer is used. The formulation of CNTs in oDCB with o-Xylene was used
so as to decrease the boiling point of the solution. oDCB has a boiling point of 180.5⁰
77
C while o-Xylene has a boiling point of 144⁰ C. With the similar fabrication process,
complete fabrication of the devices is done. The percentage yield of the working
transistors in this case was 65%. Most of the devices were not functional due to short
circuit of the transistor pattern during printing.
Figure 3.9: The mobility curves of the device with the semi-conductor formulation with
CNTs solution (solvent oDCB and P3DDT wrapped)
Figure 3.9 shows a saturation mobility value of the device of 0.1 cm2
V-1
s-1
with
oDCB and saturatiom mobility value of 1.03 cm2
V-1
s-1
when the device is fabricated
with CNTs with oXylene in the ratio 3:1. The values are one order in magnitude
lower than obtained and expected. Also another observation which is evident in these
devices is that, their performances varied as we moved to the different regions of the
substrate. This is due to de wetting of the substrate.
Now finally, on knowing the effect of different parameters on the behaviour of the
carbon nanotube transistor and optimizing the conditions, a batch of transistors is
fabricated. The following fabrication specifications are implemented- Formulation of
carbon nanotube in oDCB and oXylene in the ratio 3:1. Sonication of the
semiconductor solution is done for five minutes. Bar coating of this layer with
78
specifications of bar coater pitch of 20 µm at velocity of 30 mm/s and placing 110⁰ C
hot plate beneath. Bar coating of the dielectric with PMMA (80g/l) with 8µm pitch bar
coater wire obtaining a thickness of 568 µm. And on complete fabrication, the result is
obtained.
3.5.3 Effect of the Dielectric thickness
With the aim to have lower leakage current, PMMA dielectric of higher concentration
is prepared. On complete fabrication of the devices, as intuitive and demonstrated in
figure 3.9 that a high thickness value decreases the leakage current.
Figure 3.10: shows the transfer characteristics of a device fabricated with dielectric PMMA
prepared at concentrations 90 mg/ml and 80mg/ml.
A fresh PMMA solution is prepared with a higher concentration. Here PMMA
((Sigma-Aldrich, Mw = 120 kgmol−1
) in n-butyl acetate at a concentration of 90 g/l is
taken. As intuitive, due to the high thickness of the dielectric, the gate current is less.
79
But this inference cannot be generalized. There is a high leakage current also because
of improper fabrication – which leaves behind non uniform film deposition.
Note: Also it must be taken into account that the current values in n transfer curves
was in pico Amperes and hence not used for analysis.
80
Conclusions
The main purpose of this thesis work has been the assessment of sorted, single-walled
carbon nanotube (swCNTs) formulations for the fabrication of fully printed field-
effect transistors (FET) on plastic foils, as a possible alternative to printed organic
semiconductors for future, high performance printed electronic circuits. The work has
comprised a study of different techniques for the uniform and controlled deposition of
functional inks, including spin-coating, adopted as a reference technique, and inkjet
printing and bar-coating, scalable processes compatible with low-cost mass printing
manufacturing. At first, a benchmark, fully printed polymer FET on PEN was
fabricated, adopting as semiconductor the well-known N2200 as polymer based on
Naphthalene Diimide. The device was obtained by printing PEDOT: PSS electrodes
on plastic, on top of which a thin layer of the semiconductor was deposited by spin
coating. As a dielectric, a 560 nm thick poly methyl methacralate layer was deposited
by bar-coating, on top of which a gate contact was patterned thanks again to inkjet
printing. Such device showed typical n-type FET characteristics with a typical
electron mobility of around 0.62 cm2V
-1s
-1
Formulations of swCNTs wrapped by the polymer P3DDT in dichlorobenzene were
studied for the development of printed CNTs FETs, since they were previously
demonstrated to yield good performances in FET devices fabricated on silicon
substrates. P3DDT wrapped swCNTs networks, formed either by spin-coating or bar-
coating, were integrated in the benchmark device architecture. The optimization of the
FETs performances comprised: the control of the thickness and uniformity of the
semiconducting network, the formulation of the CNTs solutions by adding different
solvents, and the control of the dielectric thickness. The control of the thickness of the
printed network was tackled by bar-coating the inks with different bars, characterized
by different gaps in between the wires. The best result was obtained with a bar with 20
μm wire, while a thinner wet layer achieved with a 10 μm showed higher hysteresis
and reduced currents owing to lower coverage. As far as the formulation is concerned,
it was found that the addition of xylene in the oDCB formulation, in 1:3 volume ratio,
leads to improved performances 1.5 cm2V
-1s
-1, thanks to a better wetting of the plastic
substrate leading to a more uniform coverage. The optimization of the printed PMMA
dielectric thickness was instead crucial in keeping under control the leakage currents.
81
By combining all these optimizations, fully printed swCNTs FET were successfully
demonstrated, achieving a hole mobility of 1.90 cm2V
-1s
-1 and a current ON/OFF ratio
of 104. Electron mobility was typically quite low, likely as an effect of ambient
contamination, since all processing was performed in air.
SEM images revealed a limited surface coverage even in the best devices, indicating
that future work should focus on obtaining denser networks to boost device currents.
As a matter of fact, the reported mobility has to be considered an effective mobility,
and there is plenty of room of improvement to achieve much more performing printed
CNTs FETs in future. The first step demonstrated here is therefore very promising and
it allows to foresee a realistic path towards high performance printed CNTs circuits on
plastic which have the true potential to rival printed polymer counterparts.
82
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