Fabrication of nanostructures using atomic force microscope assisted nanolithography

3.2 Experimental Section 94 3.3 Results and Discussion 95 3.3.1 Nanopatterning of electroactive polymer film 95 3.3.2 Electropolymerization of precursor polymer film 98 3.3.3 Nanowriti

FABRICATION OF NANOSTRUCTURES USING

ATOMIC FORCE MICROSCOPE ASSISTED

NANOLITHOGRAPHY

SUBBIAH JEGADESAN

M Sc.,(Madurai Kamaraj University, India)

M Phil., (Cochin University of Science & Technology, India)

A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY

DEPARTMENT OF CHEMISTRY

NATIONAL UNIVERSITY OF SINGAPORE

2007

i

Dedicated to my parents

ACKNOWLEDGEMENTS

It is my very great pleasure to express my heartfelt gratitude and sincere thanks to

Associate Professor Suresh Valiyaveettil for his guidance, support and encouragement

during the course of this work

I am very thankful to Associate Prof Regoberto C Advincula, University of

Houstan, for his useful suggestion and valuable advice during this work

My sincere thanks to all the current and former members of the group for their

cordiality and friendship I thank Dr C Basheer, Dr R Lakshminarayanan, Dr P K

Ajikumar, Ms R Renu, Ms J Akhila, Ms S Gayathri Dr G A Rajkumar, Dr M

Vetrichelvan, Dr Sivamurugan, Li Hairong, Michelle Low, Sheeja Bhahulayan,

Nurmawati, Ankur and Satyanand for all the good times in the lab Also, i am very much

grateful to Dr Sindhu for her constant support and valuable suggestion during my work

I owe my gratitude for the technical assistance provided by the staff of the XRD,

UV, IR, Mass spectroscopy, Elemental Analyses and Thermal Analysis Laboratories at

department of chemistry Also, my sincere thanks to the staff of department general

office and chemical store

I would like to thank Dr Xie Xianning, Mr Chung Hong jing, Miss Li Hui, Mrs

Ghee lee and all staff from NUS- Nanoscience Nanotechnology Initiative for their help

and assistance during my work

I wish to express my deep gratitude to my family for their constant support and

motivation with full of kindness I wholeheartedly thank my parents, sisters, brothers,

brother-in-law and parents-in-laws for their encouragement and support My thanks are

also to all my friends and well-wishers

I thank the NUS - Nanoscience and Nanotechnology Initiative for granting the

research scholarship for my research work

Table of Contents s

Chapter 1 Introduction

1.1 Evolution of Nanotechnology 4

1.2 Development of Micro and Nanoscale Fabrication 5

1.2.1 Necessity of nanoscale fabrication 7

1.4 Scanning Probe Microscopy 24

1.4.1 Scanning tunneling microscope 25

1.4.2.1 Basic components of an AFM 27

1.4.2.3 Lateral force microscopy 33 1.4.2.4 Force curve measurements 34 1.4.2.5 Tip–sample interaction 35

1.5 Applications and challenges of SPM 37

1.6 Nanolithography of Polymer Films 39

Chapter 2 Fabrication of conducting nanopattern

on PVK film using electrochemical Nanolithography

2.3.2 Formation of an electrochemical bridge for electropolymerization 70

2.3.3 Conductivity of PVK film on Au (111) substrate 73

2.3.4 Nanopatterning of Carbazole monomer on Si (100) 74

2.3.5 Conductive and thermal properties of patterned carbazole film 76

2.3.6 Nanopatterning of PVK film on Si (100) 78

2.3.7 Conductive and thermal properties of patterned PVK film 80

2.3.8 Comparison of PVK and carbazole monomer film patterning 83

Chapter 3 Nano/micro scale surface modification

of conjugated precursor polymer film

3.2 Experimental Section 94

3.3 Results and Discussion 95

3.3.1 Nanopatterning of electroactive polymer film 95

3.3.2 Electropolymerization of precursor polymer film 98

3.3.3 Nanowriting on polymer film 100

3.3.4 Electrical conductivity of corona pattern 101

3.3.5 Effect of electron scavenger in pattern formation 102

Chapter 4 Fabrication of polymer nanostructures

via electrostatic nanolithography

4.2 Experimental Section 113

4.3 Results and discussion 114

4.3.1 Nanopatterning of PMAA film 114

4.3.2 Kinetics and pattern formation of PMAA polymer film 115

4.3.3 Conduction during pattern formation 119

Chapter 5 Effect of hydrophobicity on meniscus

formation in nanopatterning of polymer film

5.3 Results and discussion 128

5.3.1 Patterning of PMA polymer film 129

5.3.2 Patterning of PAA polymer film 132

5.3.3 Hydrophobic effect on patterning 134

5.3.4 Ablation of polymer during patterning 138

Chapter 6 Influence of surface properties in

corona pattern formation on polymer films

6.2 Experimental Section 145

6.3 Results and Discussion 147

6.3.1.1 Patterning of polyvinylalcohol Vs polyvinylphenol 149 6.3.1.2 Patterning of polymethylmethacrylate Vs polybenzylmethacrylate151 6.3.1.3 Patterning of polyvinylchloride Vs polybenzylchloride 152 6.3.1.4 Patterning of polyvinylacetate Vs polystyrene 154

6.3.1.5 Patterning of polymethacrylate Vs polymethylstyrene 155

Chapter 7 Synthesis and nanofabrication of

oligomers using AFM lithography

7.2.1 Synthesis of triphenylene oligomers 164

7.3 Results and Discussion 172

7.3.1 Nanofabrication of oligomer 173

7.3.1.1 Patterning of hydrophilic oligomer 1 173

7.3.1.2 Patterning of amphiphilic oligomer 2 175

7.3.1.3 Patterning of amphiphilic oligomer 3 176 7.3.2 Optical properties of the oligomers 178

7.3.3 Transport properties of the oligomers 180

Summary

Fabrication of nanostructure using polymeric materials is a key technique in the

application of organic materials to nanodevices, molecular electronics, and nanosensors

AFM lithography has been used to manipulate soft materials using biased nano-probe and

it led to the development of surface patterning methodologies at nanoscale The focus of

this thesis is aimed towards the patterning of various organic surfaces including polymers

and oligomers to develop functional nanostructures using electrochemical and electrostatic

nanolithography Due to the increasing demand and necessity for the nanoscale fabrication

using organic/polymer materials for organic electronics, we explored here the surface

effects of pattern formation, importance of the water meniscus formation to facilitate

patterning, the choice of method as well as parameter to develop nanostructures and

analyzed their importance We have explored many polymers with different functional

groups on the polymer backbone, co-polymer, electro-active polymer and oligomers for

the pattern formation and then the physical and chemical properties of the patterns are

investigated A brief summary of the concepts of nanofabrication, various lithography

technique, atomic force microscope technique and nanolithography of polymer film have

been explained in Chapter One

In chapter 2, fabrication of nanopatterns with PVK polymer on Au (111) substrate

through elecropolymerization of precursor polymer film was reported The second part of

this chapter describes the patterning of both carbazole monomer and PVK polymer on Si

(100) substrate and exhibit how the conductive nanopattern can be formed from insulating

polymer through electrochemical nanolithography Using a voltage-biased atomic force

microscope (AFM) tip, electric-field-induced polymerization through cross-linking of

carbazole moieties were demonstrated with the formation of nanopattern which is

controlled by AFM probe writing speed and bias voltages Also, the conducting property

(current-voltage (I-V) curves) of these nanopatterns was also investigated using a

conducting-AFM (C-AFM) and the thermal stability of the patterns was evaluated by

annealing the polymer/monomer film above the glass transition (Tg) temperature of the

precursor polymer

In chapter 3, we explored the formation of conductive nanopattern from binary

electro active polymer film through electro-oxidation process In addition to the pattern

formation, corona pattern formation, electric field distribution during pattern formation

and the flow of electron from the tip to the polymer film were analyzed in detail

In the subsequent chapters, we discuss the patterning of various polymer films and

discussed how the patterning is differentiated with various polymers using electrostatic

nanolithography In chapter 4, we describe the fabrication of nanostructure from

polymethacrylic acid (PMAA) on Si(100) substrate using electrostatic nanolithography

The kinetics, growth, and optimization of the conditions such as writing speed and bias

voltages, were investigated for nanopattern formation The nanostructure of size 28 nm

was created using the biased AFM tip on the PMAA film coated on Si (100) substrate and

found that this method is a direct and reliable method to produce uniform nanostructures

on a polymer film

The role of water meniscus on the polymer film during the dynamic writing

process is reported in chapter 5 The effect of hydrophobicity on water meniscus

formation between the AFM tip and substrate during the patterning of polymer film were

demonstrated and discuss how such a meniscus formation facilitate the continuous pattern

formation on the polymer films The patterning process was done on a hydrophobic (PMA)

and hydrophilic (PAA) polymer film at various tip speeds and applied biases and the

results were compared to elucidate the surface effect of polymer film for pattern formation

In chapter 6, we explored the surface effect of corona type patterning formation by

comparing the pattern formation on different polymers such as non-aromatic and aromatic

polymers Here, we found that polymers with aromatic ring structures facilitate corona

pattern formation as against non-aromatic containing polymers doesn’t show any such

corona pattern other than the dot pattern The formation of corona pattern attributed to the

combined effect of discharge of electrons between AFM tip and substrate and the electron

rich aromatic/electro active surface groups on the polymer backbone

Finally, the synthesis and patterning ability of few oligomer molecules with

different functional groups are described in chapter 7 Here we show that functional

groups such as hydrophilic and amphiphillic groups, on the oligomer affects nanopattern

formation In addition to the patterning, optical and transport properties of ultra thin

organic molecules are explored and the results were discussed

ABBREVIATIONSAAND SYMBOLS

13 C-NMR Carbon nuclear magnetic resonance

1 H-NMR Proton nuclear magnetic resonance

ECNL Electrochemical nanolithography

EPN Electro pen nanolithography

ESI-MS Electron spray ionization mass spectrum

i.e That is (Latin id est)

IRRAS IR reflection absorption spectroscopy

Maple Matrix-assisted pulsed-laser evaporation

MEK Methyl ethyl ketone

MeOH Methanol

mg Milligram(s)

ml Milliliter(s)

MOS Metal oxide semiconductor

NMR Nuclear magnetic resonance

PSMe Poly (4-methylstyrene)

PVAc Polyvinyl acetate

PVC Polyvinylchloride

PVK Polyvinylcarbazole

PVPh Polyvinyl phenol

RE Reference electrode

RPM Redox probe microscopy

sec Second

SPL Scanning probe lithography

SPM Scanning probe microscopy

STM Scanning tunnelling microscopy

LIST OF TABLES

No.

Chapte 6

Table 6.1 List of polymers, company, molecular weight and their

corresponding solvents to make solution for spin coating of polymer for patterning

146

Table 6.2 Chemical structure of the polymers used for patterning 147

Chapte 7

Table 7.1 Kinetics of pattern formation at various voltage with tip speed of

0.1 µm/s (X Æ denotes no pattern formation was observed)

177 Table 7.2 Optical properties of Oligomer 1,2 & 3 179

Figure 1.1 Schematic diagram shows the outline of the evolution of electronic

devices from lithographic techniques

6

Figure 1.2 Schematic representation of evolution of top-down and bottom

approach for nanofabrication

10 Figure 1.3 Schematic representation of the nano-oxidation process on Si

substrate

16

Figure 1.4 Schematic representation of DPN 18

Figure 1.5 Components of a scanning probe instrument 28

Figure 1.6 Beam-deflection set-up for the detection of interacting force in an

AFM

31 Figure 1.7 Distance dependence of Van Der Waals and electrostatic forces

compared to the typical tip-surface separations in the contact mode (CM), non-contact mode (NCM), and intermittent contact mode

35

Figure 1.8 Chemical lithography of self assembled monolayer of organic

molecules

41

Figure 1.9 schematic representation of the nanopatterning of polymer film by

electrochemical oxidation method

42 Figure 1.10 Flow chart showing the outline of the work done in this thesis 45

Chapte 2

Figure 2.1 (a) Schematic diagram of electrochemical nanolithography (b)

chemical structure and possible polymerization sites of PVK (c) mechanism for electropolymerization (cationic) and cross-linking

of PVK

68

Figure 2.2 (a) Three-dimensional nanostructure on the polymer, patterned

using a tip voltage of -7V at a speed of 0.5 µm/s and (b) nanopattern of lines drawn at varying voltage of -3V to -10V at constant tip speed of 1 µm/s and the feature size ranging from 35

nm to 150 nm was observed For figure a,b), the height profile below corresponds to the part of the white line marked in the respective figure above

69

Figure 2.3 (a) Cyclic voltammogram of PVK in 0.1 M LiClO4 / THF solution

(WE: Pt plate, CE: Pt plate, RE: Ag / AgCl) at 1, 5, 10, 15, and 20 cycles at scan rate 50 mV s-1 (b) FT-IR Spectra on ITO substrate

PVK-spin coated and PVK electrodeposited potentiostatically (cross-linked)

70

Figure 2.4 (a) Square patterning of PVK surface of 1µm by2 electrochemical 74

oxidation (b) I-V curve measurements of the patterned (i) and unpatterned area (ii)

Figure 2.5 (a) Nanopatterns drawn on carbazole film at constant bias of -7V

at various tip speed 1 µm/s, 2 µm/s, 4 µm/s, 6 µm/s, 8 µm/s and 10 µm/s corresponds to the line width of 187 nm, 162 nm, 140 nm,

128 nm, 87 nm and 78 nm respectively and imaged by contact mode AFM (height) (b) 3D image of carbazole monomer patterned at -7V with a pattern width of 86 nm Height is at ~ 2

nm

75

Figure 2.6 (a) Square patterning of size 1 µm2

on carbazole film at a scanning speed 1Hz with a tip bias of -5V imaged by contact mode AFM, and (b) the corresponding current mapping image (C-AFM) of conductive square pattern with a conducting current of 10.0 pA at

an applied bias of +5V for imaging

77

Figure 2.7 The AFM height image of the patterned character “NUS” before

(a) and after (b) heating at 270° C for 3 hrs of the carbazole film

77 Figure 2.8 (a) Nanolines written on PVK film at constant tip speed of 1 µm/s

with the different biases of -5V, -7V, -9V and -11V with line widths of 83 nm, 128 nm, 162 nm, and 231 nm, respectively, (b) AFM image with height profile of the pattern “PVK” drawn at -7V

at a tip speed 1 µm/s

79

Figure 2.9 Variation of pattern width (a) and height (b) with applied bias for

polymer and monomer, (c) Plot of line width Vs AFM tip speed during PVK and carbazole patterning

80

Figure 2.10 (a) AFM height images of the electrochemically patterned

polygons at different tip voltages and speeds (b) Corresponding C-AFM image of polygon patterning with a conductive current of

10 pA (c) Square(1 µm2

) patterning of PVK film at tip scanning speed of 1Hz with a tip bias of -5V and imaged by contact mode AFM (d) corresponding I-V curve hysteresis measurements on the patterned square of the PVK film

82

Figure 2.11 3D height AFM images of the patterns (a) before and (b) after

heating at a temperature of 270 °C for 3 hrs on PVK polymer film

83

Chapte 3

Figure 3.1 Electropolymerization of neutral precursor polymer A (PMTC) to

cross-linked conducting polymer B (CPMTC)

93

Figure 3.2 AFM images show (a) the surface morphology of the polymer film

(polymer A) spin coated on the substrate and (b) the thickness measurement and corresponding height profile Film thickness was found to be around 46 nm

95

Figure 3.3 (a) Dot pattern of polymer (A) film at various bias of -7V -9V and

-11V with tip contact time of 2s and corresponding diameter of

658 nm, 2778 nm and 5320 nm respectively (b) Line pattern of polymer A at constant tip speed of 1 µm/s with different applied

97