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Fabrication and Investigation of Organic and Nano

Materials Based Sensors

By

Muhammad Tariq Saeed

Supervisor

Prof. Dr. Fazal Ahmad Khalid SI

Co-Supervisor

Prof. Dr. Khasan S. Karimov

THIS DISSERTATION IS SUBMITTED TO GIK INSTITUTE, IN PARTIAL FULFILLMENT OF THE

REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY IN MATERIALS ENGINEERING

Faculty of Materials Science and Engineering

GIK Institute of Engineering Sciences and Technology, Topi, Pakistan

Spring 2012

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DEDICATED TO

MY BELOVED GRAND MOTHER (AYESHA MAI) AND AUNTIES

(SAFIA BIBI AND FAZALAN MAI) WHO LEFT ME FOREVER

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IV

Acknowledgements

All praises and glories are for Almighty Allah the most Merciful and The Most Beneficent Who

blessed me with ability and courage to complete this dissertation. I supplicate to Almighty Allahto shower his countless blessings on his Last Prophet Muhammad (PBUH) who is the persistent

source of knowledge and torch of guidance to all mankind; and messenger of peace for all

creatures.

I am highly indebted to my supervisor Prof. Dr. Fazal Ahmad Khalid and co-supervisor Prof. Dr.

Khasan Sanginovich Karimov for their kind supervision and sincere guidance. I can not convey

my entire feelings in words but I can pray to Almighty Allah (SWTA) to reward them for their

kindness. Really it is the great honor for me that I am student of well renowned scholars who areamong the leaders of their respective fields.

I am thankful to Higher Education Commission of Pakistan for providing me MS leading to PhD

Scholarship through its Indigenous PhD program. I am also highly indebted of Pakistani nation

and the Pakistani soil for financial support provided by HEC during this period.

I am grateful to all the faculty members Faculty of Materials Science and Engineering especially

Dr. Fida Muhammad, Dr. Zain ul Abdein, Dr. Fahd Nawaz Khan, Dr. Aqeel A. Taimoor, Mr.

Sheraz M. Khakwani and Syed Zameer Abbas. I am also thankful to my colleagues particularly

Mr. Mustasim Billah Bhatty, Muhammad Umer Farooq, Dr. Hamid Zaighum, Dr. Rana Abdul

Shakoor, Mr. Nabi Buksh, Hafiz Tareq Manzoor, Ghazanfar Saeed, Irfan Haider Abidi, Adnan

Maqbool and Asif Hussain.

I would like to appreciate the assistance of FMSE lab staff particularly Shahzad Raza, Shoaib

Rasool, Afsar Khan, Maroof, Atiq Qurashi and Safeer Ahmad. Moreover, I will never forget the

cooperation of Mr. Khalid Khan, Mr. M. Shafiq, Mr. Nizakat Khan and Jameel Shah.

I highly acknowledge my family for their love, kindness and support for my whole life. They

have always been a source of inspiration for me. By virtue of their prayers, I succeeded in my

life. I am also thankful to my uncle Malik Muhammad Hussain Chani and his family for their

moral support and encouragement.

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Declaration

This is to certify that the results presented in this dissertation are my original work performed

entirely by myself during the course of my PhD studies at Ghulam Ishaq Khan Institute of

Engineering Sciences and Technology, Topi, Pakistan. This dissertation has not been submitted

to any other university in whole or in part. Prior approval must be accorded to use the material

contained in or derived from this dissertation.

Muhammad Tariq Saeed

Reg. No. MM0618

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Journal Publications

1. Carbon nanotubes-cuprous oxide composite based pressure sensors, Khasan SanginovichKarimov, Muhammad Tariq Saeed Chani, Fazal Ahmad Khalid, Adam Khan, RahimKhan, Chinese Physics B. Vol. 21, No. 1 (2012) 016102-1-5.

2. Carbon nanotubes-cuprous oxide composite based strain sensors, Khasan SanginovichKarimov, Muhammad Tariq Saeed Chani, Fazal Ahmad Khalid, Adam Khan Physica E,

Vol. 44, No. 4 (2012) 778-781.

3. Effect of displacement on resistance and capacitance of the polyaniline film, KhasanSanginovich Karimov, Muhammad Tariq Saeed, Fazal Ahmad Khalid, Syed Abdul Moiz,

Chinese Physics B, Vol. 20, No. 4 (2011) 040601-5.

4. Carbon nanotubes film based temperature sensor Khasan Sanginovich Karimov,Muhammad Tariq Saeed Chani, Fazal Ahmad Khalid, Physica E, Vol. 43, No. 9 (2011)

1701-1703.

5. Organic Cu/Cellulose/ PEPC/Cu Humidity Sensor, Muhammad Tariq Saeed, F. AhmadKhalid, Kh. S. Karimov, M. Shah, OAM-RC, Vol. 4, No. 6 (2010) 88-892.

6. Carbon nanotubes based strain sensors, Khasan Sanginovich Karimov, Fazal Ahmad Khalid ,Muhammad Tariq Saeed Chani,Measurements, 10.1016/j.measurement.2012.02.003.

7. Carbon Nanotubes Based Flexible Temperature Sensor, Kh.S. Karimov, F.A. Khalid, M.Tariq Saeed Chani, A. Mateen, M. Asif Hussain, A. Maqbool, OAM-RCVol. 6, No. 1-2

(2012) 212-214.

8. Orange dye-polyaniline composite based impedance humidity sensors, Muhammad TariqSaeed Chani, Kh.S. Karimov, F. Ahmad Khalid, S. Zameer Abbas, Chinese Physics B

(Accepted).9. Polyaniline based impedance humidity sensors, Muhammad Tariq Saeed Chani,

Kh.S.Karimov, F. Ahmad Khalid, S.A.Moiz, Solid State Sciences (under review).

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Conference Publications

1. A study of V2O4-PEPC composite based resistance temperature sensors MuhammadTariq Saeed, Kh.S. Karimov, F. Ahmad Khalid, M. Farooq, M. Saleem, , 1

st

SaudiInternational Electronics, Communications and Photonics Conference (SIECPC-2011),

April 23-26, 2011, Riyadh Saudi Arabia.

2. Temperature Sensing Properties of Organic-Inorganic Ag/p-CuPc/n-GaAs/Ag Cell, Kh. S.Karimov, F. Ahmad Khalid, Muhammad Tariq Saeed, T.A. Qasuria, Z.M. Karieva and

M. Farooq, International Symposium on Vacuum Science and Technology (ISVST),

November 2-6, 2010, Islamabad, Pakistan.

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Abstract

This work presents the fabrication and investigation of organic and nano materials based sensors

for humidity, temperature and electromechanical applications. Polyaniline (PANI), orange dye(OD)-PANI composite and of cellulose- poly-N-epoxypropylcarbazole (PEPC) have been used

for the fabrication of surface type humidity sensors. The sensors are fabricated by depositing

films of various thicknesses on glass substrates between pre-deposited metallic electrodes. The

sensing mechanism is based on the impedance and capacitance variations due to the absorption

or desorption of water vapors. The consequences of annealing, measuring frequency and

absorption-desorption behavior of the sensor have been discussed in detail. For all sensors

impedance-humidity relationship shows more uniform change as compared to capacitance-

humidity relationship in the given range humidity.

The temperature sensors have been fabricated by using multiwalled carbon nanotubes

(MWCNTs), V2O4-PEPC composite and CuPc on n-GaAs. The CNTs based sensors are

fabricated by the deposition CNT nanopowder on a paper substrate and on adhesive elastic

polymer tape. The nominal thickness of the CNT films on paper substrates is 3040 m while

that of elastic substrate is ~ 300-430 m. The DC resistance of the sensors decreases with

increase in temperature. For both types sensors, the resistance-temperature relationship shows

wide range sensitivity.

The V2O4-PEPC composite based temperature sensors are fabricated by drop-casting the blend of

composite into the gap between preliminary deposited silver electrodes on glass substrates. The

thickness of the V2O4-PEPC films is in the range of 20-40 m. It is found that with increase in

temperature the AC resistance of the samples decreases by 10-12 times. The response recovery

time is also measure.

The Ag/p-CuPc/n-GaAs/Ag cells are fabricated by the deposition of p-type copperphthalocyanine on n-type GaAs single-crystal semiconductor substrate. The temperature sensing

and photoconductive behavior of the cells are investigated. The results reveal that with increase

in temperature from 33-75 C the resistance temperature coefficients (RTC) for the reverse and

forward bias resistances are equal to -2.0 %/C and -1.5 %/C, respectively.

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Electromechanical sensors based on PANI, CNTs and CNTs-Cu2O composites have been

fabricated and investigated. The 20-80 m thick PANI films are deposited by drop-casting on Ag

electrodes, which are preliminary deposited on glass substrates. The effect of displacement on

the resistance and capacitance of film is investigated. It is observed that with increases in

displacement the resistance decreases and the capacitance increases.

For the fabrication of CNTCu2O composite based pressure sensors tablets of composite are

made at a pressure of 353 MPa. The average diameter and the average thickness of the tablets are

10 mm and 4 mm, respectively, and both sides of the tablet are covered by silver paste. By

varying pressure from 0-37 kN/m2, the change in DC resistance of the sensor is measured.

The CNTs and CNTsCu2O composite based strain sensors have been fabricated by pressed

tablets and elastic polymer beam. The 1 mm thick tablets of CNTs and CNTsCu2O composite

are fabricated at a pressure of 200-300 MPa and 353MPa, respectively. The samples are installed

on the polymer elastic beam by glue. The electric contacts to the samples are made by silver

paste. The inter-electrodes distance (length) and diameter of the surface-type samples are in the

range of 68 mm and 10 mm, respectively. It is found that DC resistance of the strain sensors

increases under tension and decreases under compression, while the average strain sensitivities

are in the range of 50-80 and 4446 for CNTs and CNTsCu2O composite based sensors,

respectively.

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Table of contents

Acknowledgements..... IV

Declaration....V

Journal Publications ...VI

Refereed Conference Publications ....VII

Abstract.....VIII

Table of Contents..........X

List of Figures.........XVII

List of Tables.......XXII

List of Abbreviations ........XXIII

Chapter-1 Introduction......1

1.1 Preamble .............................................................................................................................. 1

1.2 Aims and objectives ............................................................................................................. 2

1.3 Outline of the dissertation .................................................................................................... 2

1.3.1 Chapter-2.............................................................................................................................. 2

1.3.2 Chapter-3.............................................................................................................................. 2

1.3.3 Chapter-4.............................................................................................................................. 3

1.3.4 Chapter-5.............................................................................................................................. 3

1.3.5 Chapter-6.............................................................................................................................. 3

1.3.6 Chapter-7.............................................................................................................................. 3

References ....................................................................................................................................... 4

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Chapter-2 Literature Survey......6

2.1 Sensors and thei classifications ............................................................................................ 6

2.2 Sensing materials ................................................................................................................. 8

2.2.1 Organic materials ............................................................................................................... 11

2.2.2 History of organic semiconducting materials in electronic devices .................................. 12

2.2.3 Importance of organic semiconducting materials .............................................................. 12

2.2.4 Chemical nature of organic semiconducting materials ...................................................... 14

2.2.5 Charge transport properties/conduction mechanism of organic semiconductors .............. 15

2.3 Properties of various organic sensing materials................................................................. 18

2.3.1 Polyaniline (PANI) ............................................................................................................ 18

2.3.2 Carbon nanotubes (CNTs) ................................................................................................. 20

2.3.3 Copper phthalocyanine (CuPc) .......................................................................................... 21

2.4 Sensing mechanism ............................................................................................................ 22

2.4.1 Percolation theory .............................................................................................................. 23

2.4.1.1Introduction ........................................................................................................................ 23

2.4.1.2Percolative transport .......................................................................................................... 23

2.5 Applications ....................................................................................................................... 24

References ..................................................................................................................................... 26

Chapter-3 Materials and Experimental.......35

3.1 Materials ............................................................................................................................ 35

3.1.1 Polyaniline (PANI) ............................................................................................................ 35

3.1.2 Cellulose ............................................................................................................................ 35

3.1.3 Poly-N-epoxypropylcarbazole (PEPC) .............................................................................. 36

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3.1.4 Copper phthalocyanine (CuPc) .......................................................................................... 36

3.1.5 Orange dye (OD)................................................................................................................ 36

3.1.6 Carbon nanotubes (CNTs) ................................................................................................. 41

3.1.7 Cuprous oxide (Cu2O)........................................................................................................ 41

3.1.8 Vanadium oxide (V2O4) ..................................................................................................... 41

3.1.9 Gallium arsenide (GaAs) ................................................................................................... 41

3.2 Fabrication techniques ....................................................................................................... 43

3.2.1 Substrate preparation ......................................................................................................... 43

3.2.2 Metallization ...................................................................................................................... 44

3.2.3 Vacuum thermal evaporation ............................................................................................. 44

3.2.4 Drop casting ....................................................................................................................... 44

3.2.5 Adhesive tape ..................................................................................................................... 45

3.2.6 Glued film .......................................................................................................................... 45

3.2.7 Cold compaction ................................................................................................................ 45

3.3 Film characterization ......................................................................................................... 463.3.1 Crystal thickness monitor .................................................................................................. 46

3.3.2 Optical microscope (OM) .................................................................................................. 46

3.3.3 Scanning electron microscope (SEM) ............................................................................... 47

3.4 Experimental setups ........................................................................................................... 49

3.4.1 Apparatus for thin film deposition ..................................................................................... 49

3.4.2 Apparatus for testing of humidity sensors ......................................................................... 51

3.4.3 Apparatus for testing of temperature sensors..................................................................... 51

3.4.4 Apparatus for testing of displacement sensors................................................................... 52

3.4.5 Apparatus for testing of pressure sensors .......................................................................... 52

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3.4.6 Apparatus for testing of strain sensors ............................................................................... 52

References ..................................................................................................................................... 57

Chapter-4 Humidity Sensors...........60

4.1 Polyaniline based humidity sensors ................................................................................... 60

4.1.1 Introduction ........................................................................................................................ 60

4.1.2 Experimental ...................................................................................................................... 61

4.1.2.1Fabrication of sensors ........................................................................................................ 61

4.1.2.2Measurements .................................................................................................................... 62

4.1.3 Results and discussion ....................................................................................................... 63

4.1.4 Conclusions ........................................................................................................................ 69

4.2 Fabrication and investigation of orange-dye-polyaniline composite film based humidity

sensors ........................................................................................................................................... 73

4.2.1 Introduction ........................................................................................................................ 73

4.2.2 Experimental ...................................................................................................................... 74


4.2.2.2Measurements .................................................................................................................... 74


4.2.4 Conclusions ........................................................................................................................ 82

4.3 Organic Cu/Cellulose/ PEPC/Cu humidity sensors ........................................................... 84

4.3.1 Introduction ........................................................................................................................ 84

4.3.2 Experimental ...................................................................................................................... 85


4.3.2.2Measurements .................................................................................................................... 85

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4.3.4 Conclusions ........................................................................................................................ 90

References ..................................................................................................................................... 92

Chapter-5 Temperature Sensors..........97

5.1 Carbon nanotubes film based temperature sensors ............................................................ 97

5.1.1 Introduction ........................................................................................................................ 97

5.1.2 Experimental ...................................................................................................................... 98


5.1.2.2Measurements .................................................................................................................... 99


5.1.4 Conclusions ...................................................................................................................... 103

5.2 Carbon nanotubes based flexible temperature sensors .................................................... 104

5.2.1 Introduction ...................................................................................................................... 104

5.2.2 Experimental .................................................................................................................... 104

5.2.2.1Fabrication of sensors ...................................................................................................... 104

5.2.2.2Measurements .................................................................................................................. 105

5.2.3 Results and discussion ..................................................................................................... 105

5.2.4 Conclusions ...................................................................................................................... 107

5.3 A study of V2O4-PEPC composite based resistance temperature sensors ....................... 110

5.3.1 Introduction ...................................................................................................................... 110

5.3.2 Experimental .................................................................................................................... 110


5.3.2.2Measurements .................................................................................................................. 111

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5.3.4 Conclusions ...................................................................................................................... 117

5.4 Temperature sensing properties of organic-inorganic Ag/p-CuPc/n-GaAs/Ag cell ........ 121

5.4.1 Introduction ...................................................................................................................... 121

5.4.2 Experimental .................................................................................................................... 123

5.4.2.1Fabrication of cells ........................................................................................................... 123

5.4.2.2Measurements .................................................................................................................. 124


5.4.4 Conclusions ...................................................................................................................... 126

References ................................................................................................................................... 130

Chapter-6 Electromechanical Sensors.......135

6.1 Effect of displacement on resistance and capacitance of polyaniline film ...................... 135

6.1.1 Introduction ...................................................................................................................... 135

6.1.2 Experimental .................................................................................................................... 136


6.1.2.2Measurements .................................................................................................................. 137


6.1.4 Conclusions ...................................................................................................................... 141

6.2 Carbon nanotubes-cuprous oxide composite based pressure sensors .............................. 144

6.2.1 Introduction ...................................................................................................................... 144

6.2.2 Experimental .................................................................................................................... 145


6.2.2.2Measurements .................................................................................................................. 146

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6.2.4 Conclusions ...................................................................................................................... 149

6.3 Carbon nanotubes based strain sensors ............................................................................ 151

6.3.1 Introduction ...................................................................................................................... 151

6.3.2 Experimental .................................................................................................................... 152


6.3.2.2Measurements .................................................................................................................. 152


6.3.4 Conclusions ...................................................................................................................... 155

6.4 Strain sensors based on carbon nanotubes-cuprous oxide composite .............................. 157

6.4.1 Introduction ...................................................................................................................... 157

6.4.2 Experimental .................................................................................................................... 158


6.4.2.2Measurements .................................................................................................................. 158

6.4.3 Results and discussions .................................................................................................... 1586.4.4 Conclusions ...................................................................................................................... 160

References ................................................................................................................................... 163

Chapter-7 summary and Future Work.......169

7.1 Summary .......................................................................................................................... 169

7.2 Future Work ..................................................................................................................... 172

Appendix...173

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List of Figures

Figure 2.1 Classification of chemical sensors [10] ......................................................................... 9

Figure 2.2 Classification of physical sensors [10] ........................................................................ 10

Figure 2.3 Conjugated molecules a: copper phthalocyanine, b: polythiophene, c: fullerene, d:

polyacetylene and e: polyaniline ................................................................................................... 16

Figure 3.1 Molecular structure of Polyaniline (PANI) (n=m=0.5) [8] ......................................... 37

Figure 3.2 Molecular structure of Cellulose ................................................................................. 37

Figure 3.3 Molecular structure of Poly-N-epoxypropylcarbazole (PEPC) (where n=4-6) ........... 38

Figure 3.4 Molecular structure of Copper Phthalocyanine (CuPc)............................................... 38

Figure 3.5 General azo-dye structure ............................................................................................ 39

Figure 3.6 Molecular structure of Orange Dye 25 (OD) [7] ......................................................... 39

Figure 3.7 Schematic diagram of thermal evaporator ................................................................... 50

Figure 3.8 Apparatus for testing humidity sensors ....................................................................... 53

Figure 3.9 Schematic of testing setup for the characterization of temperature sensors ................ 54

Figure 3.10 Schematic diagram of the experimental setup for characterization of displacement

sensors [28] ................................................................................................................................... 55

Figure 3.11 Schematic of apparatus for the investigation of pressure sensors [29] ..................... 56

Figure 3.12 Simplified schematic diagram of the elastic beam of constant resistance to bending

without load; (a) side view, (b) top view, and under load (c) side view [26]. .............................. 56

Figure 4.1 Schematic diagram of the Au/PANI/Ag humidity sensor. .......................................... 65

Figure 4.2 Surface morphology of PANI film at low (a) and high (b) magnification. ................. 65

Figure 4.3 Capacitance and impedance-relative humidity (RH) relationships for the Au/PANI/Ag

sensor at 120 Hz. ........................................................................................................................... 66

Figure 4.4 Impedance-relative humidity (RH) relationships for the Au/PANI/Ag sensor at 120

Hz and 1 kHz. ............................................................................................................................... 67

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Figure 4.5 Impedance-relative humidity (RH) relationships for the Au/PANI/Ag sensor at 120

Hz during increasing and decreasing of humidity. ....................................................................... 67

Figure 4.6 Normalized experimental and simulated impedance-relative humidity (RH)

relationships for the Au/PANI/Ag sensor at 120 Hz. ................................................................... 71

Figure 4.7 Circuit diagram of Op-amp logarithmic amplifier. ..................................................... 71

Figure 4.8 The response (res) and recovery (rec) times of the Au/PANI/Ag sensor. ................... 72

Figure 4.9 Schematic diagram of the Ag/OD-PANI/Ag Humidity Sensor. ................................. 77

Figure 4.10 Optical Micrograph of the OD-PANI films surface................................................. 78

Figure 4.11 SEM image of the OD-PANI films surface. ............................................................ 78

Figure 4.12 Capacitance and impedance-humidity relationships for the Ag/OD-PANI/Ag Sensor

at 120 Hz. ...................................................................................................................................... 79

Figure 4.13 Impedance-humidity relationship for the thick film (170 m) Ag/OD-PANI/Ag

sensor at 120 Hz during increasing and decreasing of humidity; Inset shows the Impedance-

humidity relationship for the thin film (50 m) Ag/OD-PANI/Ag sensor. .................................. 79

Figure 4.14 Impedance-humidity relationships for the Ag/OD-PANI/Ag sensor at 120 Hz and 1

kHz. ............................................................................................................................................... 80

Figure 4.15 Normalized experimental and simulated impedance-humidity relationships for the

Ag/OD-PANI/Ag sensor at 120 Hz. ............................................................................................. 83

Figure 4.16 The response (res) and recovery (rec) times of the Ag/OD-PANI/Ag sensor. .......... 83

Figure 4.17 Cross-sectional view of the Cu/ Cellulose /PEPC /Cu sensor. .................................. 88

Figure 4.18Relationship among capacitance, resistance, impedance and relative humidity ....... 88

Figure 4.19 Comparison of experimental and simulated capacitance with respect to relative

humidity ........................................................................................................................................ 91

Figure 4.20 Comparison of experimental and simulated resistance with respect to relative

humidity ........................................................................................................................................ 91

Figure 5.1 Schematic diagram of the CNTs film based resistance temperature sensor. ............. 100

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Figure 5.2 Resistance-temperature relationships of the one of CNTs film based sensors at

heating-cooling processes. .......................................................................................................... 100

Figure 5.3 Experimental and simulated relative resistance-temperature relationships of the CNTs

sensors (Ro andR are initial resistance and resistance at elevated temperatures respectively). . 102

Figure 5.4 Al/CNT/Al resistance temperature sensor ................................................................. 106

Figure 5.5 Resistance-temperature relationship of the one of Al/CNT/Al sensors .................... 106

Figure 5.6 Experimental (solid line) and simulated (dashed line) relative resistance-temperature

relationships of the Al/CNT/Al sensors (Ro and R are initial resistance and resistance at elevated

temperatures respectively) .......................................................................................................... 109

Figure 5.7 Schematic diagram of V2O4-PEPC composite based resistance temperature sensor 112

Figure 5.8 SEM image of V2O4-PEPC composite film .............................................................. 112

Figure 5.9(a and b) Resistance-temperature relationships for two of the V2O4-PEPC sensors

during heating process ................................................................................................................ 113

Figure 5.10(a and b). Derivatives of the resistance (dR/dT)-temperature relationships for the two

of the V2O4-PEPC sensors during heating process. .................................................................... 114

Figure 5.11 Resistance-temperature relationships at heating-cooling processes for the one of the

V2O4-PEPC sensors at 100 Hz. Experimental and simulated relative resistance-temperature

relationships of the V2O4-PEPC based temperature sensor ........................................................ 118

Figure 5.12 The log relative resistance-temperature relationship (a) and relative resistance-log

temperature relationship (b) for the V2O4-PEPC sensor ............................................................. 119

Figure 5.13 Relative resistance-time relationship for the V2O4-PEPC composite temperature

sensor. ......................................................................................................................................... 120

Figure 5.14 Cross sectional view of organicinorganic heterojunction sensor fabricated by usingn-type GaAs substrate and a thin p-CuPc film: 1-GaAs, 2-CuPc, 3-Ag, 4-Ag (semi-transparent),

5 and 6- terminals; (a) Side view (b) Top view. ......................................................................... 125

Figure 5.15 SEM image of the deposited CuPc film .................................................................. 125

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Figure 5.16 Dependence of reverse (RR) and forward (RF) bias resistances of the Ag/p-CuPc/n-

GaAs/Ag cell on temperature. ..................................................................................................... 128

Figure 5.17 Dependence of the simulated and experimental relative reverse (RR/RoR) bias

resistances of the Ag/p-CuPc/n-GaAs/Ag cell on temperature................................................... 128

Figure 5.18 Dependence of the simulated and experimental forward (RF/RoF) bias resistances of

the Ag/p-CuPc/n-GaAs/Ag cell on temperature ......................................................................... 129

Figure 6.1 SEM image of porous polyaniline film. .................................................................... 140

Figure 6.2 Relative resistance-displacement and capacitance-displacement relationships for the

Ag/PANI/Al sensor: polyaniline films thickness is 80 m (solid line ) and 20 m (dashed line).

..................................................................................................................................................... 140

Figure 6.3 Comparison of experimental and simulated relative resistance-displacement

relationships for the Ag/PANI/Al displacement sensor. ............................................................. 143

Figure 6.4 Comparison of experimental and simulated relative capacitance-displacement

relationships for the Ag/PANI/Al sensor. ................................................................................... 143

Figure 6.5 Photographs of sample from various angles. ............................................................. 148

Figure 6.6 Resistance-pressure relationship for the CNT-Cu2O pressure sensor. ...................... 148

Figure 6.7 Experimental (solid lines for the sensors shown in Figure 6.6) and simulated (dashed

line) relative resistance-pressure relationships for the CNT-Cu2O pressure sensor (Ro and R are

resistances at atmospheric pressure and under uniaxial pressure, respectively). ........................ 150

Figure 6.8 Schematic diagram of the pressure sensors arrangement in the Wheatstone bridge (Vo

= output voltage). ........................................................................................................................ 150

Figure 6.9 CNT based resistive strain sensor installed on the elastic beam. .............................. 154

Figure 6.10 Resistance-strain relationships under tension for the sample 1 and 2 fabricated at apressure of 300 and 200 MPa, respectively. ............................................................................... 154

Figure 6.11 CNT-Cu2O composite resistive strain sensor installed on the elastic beam. ........... 161

Figure 6.12 Resistance-strain relationships under tension and compression for the sample

fabricated at 353 MPa. ................................................................................................................ 161

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Figure 6.13 Comparison of Experimental and Simulated results ............................................... 162

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List of Tables

Table 2-1 History of organic semiconductor materials and devices ............................................. 13

Table 3-1. Physical properties of organic materials ...................................................................... 40

Table 3-2. Physical properties of inorganic materials .................................................................. 42

Table 3-3 FTM5 Crystal thickness monitors technical data [24] ................................................ 48

Table 4-1 Relative standard deviation (R.S.D) of the measured data ........................................... 66

Table 4-2 Humidity sensing properties of Sensor ......................................................................... 72

Table 4-3 Humidity sensing properties of Sensor ......................................................................... 80

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List of Abbreviations

AC Alternating current

AlPc Aluminium phthalocyanine

BP Bucky-paper

CNTs Carbon nanotubes

CPs Conductive polymers

CTC Charge transfer complexes

CuPc Copper phthalocyanine

DC Direct current

DNA Deoxyribonucleic acid

DWCNTs Double-walled carbon nanotubes

EA Electron affinity

EB Emeraldine base

Eg Energy gap

EM Emeraldine

ES Emeraldine salt

FM Frequency modulated

GaAs Gallium arsenide

GTS Generator-type sensors

HOMO Highest occupied molecular orbital

IE Ionization energy

LBL Layer by layer

LCDs Liquid crystal displays

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LM Leuco-emeraldine

LUMO Lowest unoccupied molecular orbital

LVDT Linear variable differential transformer

MPcs Metal-phthalocyanine

MTS Modulated-type sensors

MWCNTs Multi-walled carbon nanotubes

NA Nigr-aniline

NIR Near infrared

OD Orange dye

OFETs Organic field effect transistors

OLEDs Organic light emitting diodes

OM Optical microscopy

OMCs Organic molecular crystals

Op-amp Operational amplifier

OS Organic semiconductorOSCs Organic solar cells

OTFT Organic thin film transistor

PANI Polyaniline

Pc Phthalocyanine

PEPC Poly-N-epoxypropylcarbazole

PMMA Poly (methyl methacrylate)

PNA Pernigr-aniline

RH Relative humidity

RTDs Resistance temperature detectors

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SEM Scanning electron microscopy

SGS Small-gap semiconducting

SPANI Anilinesulfonic acid

SWCNTs Single-walled carbon nano tubes

TCNQ Tetracyanoquinodimethane

UV Ultraviolet

VCO Voltage-controlled oscillator

VO Vanadium oxide

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Chapter-1Introduction1

1

1.1 PreambleFrom last few decades extensive research is being done on the organic semiconducting materials

for their application in electronic devices [1-4]. As the conventionally used inorganic thin film

transistors have high cost because of high processing temperature and complicated fabrication

technology. Moreover inorganic semiconductors are not suitable for flexible and large area

electronics. Comparative to inorganic semiconductors, organic semiconductors have advantages

of low cost, simple technology, excellent flexibility and large area deposition [1, 5-8]. For thefabrication of cheaper, flexible and large area electronic devices, organic thin films are deposited

by drop-casting, spin coating, spraying, printing, and physical vapor deposition technique [9-13].

Although the thin films of organic semiconductors have very low carrier mobility as compared to

their inorganic counter parts [14, 15], but in low cast applications they are competitive to

inorganic devices. Although recently few types of organic sensors has been commercialized [16]

but the organic light emitting diode is the first commercial device based on organic

semiconductor[17, 18], while the organic field effect transistors and solar cells are expected to

be commercialized in near future. Some statistics predict that in year 2020 the organic

electronics will have $ 96 billion business which will be further flourish upto $ 250 billion in

year 2025 [19]. The sensitivity of several organic semiconductors towards various types of gases,

chemicals, bio species, radiations, humidity, temperature and pressure is the highly attractive

field of current research. Keeping in view the above mentioned advantages and the potential of

organic semiconductor based devices in the future market, the fabrication of various types of

sensors based on organic and nano materials by using simple technology is very promising. This

dissertation aspires at the fabrication and investigation of sensors for humidity, light,

temperature, pressure and strain sensing applications.

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1.2 Aims and objectivesThe main objective of this dissertation is fabrication and investigation of organic and nano

materials based sensors by using low cost materials and simplest technology to introduce low

cost devices for various industrial, environmental and medical applications. By considering thedemand of industry for sensors with adequate sensitivity, low cost and simplicity of structure, the

objectives of the current research work were the

I. Use of commercially available organic and nano materials to explore their newapplications or enhance their capabilities in already existing applications.

II. Fabrication of organic-inorganic composites films and organic-on-inorganic layeredstructure to improve stability and performance of sensors

III.

Use of simple film fabrication techniques like drop casting, gluing, adhesive tape andcold compaction to make the technology easy.

IV. Simple processing of materials for time efficient and cheaper fabrication.V. Electrical characterization of sensors to find their suitability for specific applications.

1.3 Outline of the dissertationThis dissertation consists of four major sections, which include literature survey (chapter-2),

materials and experimental (chapter-3), results and discussion (chapter-4 to chapter-6) and

summary and future work (chapter-7). The chapter wise detail of the dissertation is the following

1.3.1 Chapter-2This chapter gives the overview of sensor, sensing materials, organic semiconductors and their

history and conduction mechanism. This chapter is comprised of five sections. The section 2.1 is

about sensors and their classification, while section 2.2 provides a little description of sensing

materials, particularly, the nature of organic semiconductors with their history. The discussion

about conduction mechanism is given in section 2.3. Brief description about some of the

materials is given in section 2.4. Last section is about the application of sensors.

1.3.2 Chapter-3Detailed information about the materials used for the fabrication of sensors is given in this

chapter. More over the fabrication techniques and the experimental setups used for fabrication

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and characterization of various types of sensors have been discussed in detail. There are four

sections, which present the materials, fabrication techniques, characterization and the setup for

the testing of sensors in a sequence.

1.3.3

Chapter-4

The results and discussion about various types of humidity sensors along with their introduction

and experimental procedure have been presented in this chapter. This chapter is subdivided into

three sections and these sections are comprised on the basis of sensors materials. These

materials are polyaniline, orange dye-polyaniline composite and cellulose-PEPC composite.

1.3.4 Chapter-5This chapter is comprised of four sections that present four different types of temperature

sensors. These are the resistance-temperature sensors, which include multiwall carbon nanotubes

(CNTs) film, CNTs flexible, CNTs-Cu2O composite, V2O4-PEPC composite and organic on

inorganic (CuPc on GaAs) sensors. The experimental results have been discussed in detail.

Moreover detailed introduction with complete fabrication processes of each type of sensor is

given in this chapter.

1.3.5 Chapter-6The design, fabrication and investigation of three types of electromechanical sensors have been

presented in four sections of this chapter. First section presents the investigation of effect of

displacement on polyaniline film, while the second section is about Carbon nanotubes-cuprous

oxide composite based pressure sensors. The third and fourth sections describe the CNTs and

CNTs-cuprous oxide composite based strain sensors, respectively.

1.3.6 Chapter-7In this chapter the results have been summarized and the future work has been presented.

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References

1. N.A.M. Barakat, A. Hamza, S.S. Al-Deyab, A. Qurashi and H.Y. Kim: 'Titanium-based

polymeric electrospun nanofiber mats as a novel organic semiconductor', Materials

Science and Engineering: B, 2012, 177, 34-42.

2. Y. Gao: 'Surface analytical studies of interfaces in organic semiconductor devices',

Materials Science and Engineering: R: Reports, 2010, 68(3), 39-87.

3. S. Palaniappan and A. John: 'Polyaniline materials by emulsion polymerization pathway',

Prog. Polym. Sci., 2008, 33(7), 732-758.

4. M.X. Zhang, S. Chai and G.J. Zhao: 'BODIPY derivatives as n-type organic

semiconductors: Isomer effect on carrier mobility', Org. Electron., 2012, 13, 215-221.

5. C.J. Brabec: 'Organic photovoltaics: technology and market', Sol. Energy Mater. Sol.

Cells, 2004, 83(2), 273-292.

6. K. Itaka, M. Yamashiro, J. Yamaguchi, S. Yaginuma, M. Haemori and H. Koinuma:

'Combinatorial approach to the fabrication of organic thin films', Appl. Surf. Sci., 2006,

252(7), 2562-2567.

7. A. Manor and E.A. Katz: 'Open-circuit voltage of organic photovoltaics: Implications of

the generalized Einstein relation for disordered semiconductors', Sol. Energy Mater. Sol.

Cells, 2012, 97, 132-138.

8. Y. Wang, J. Ren, X. Yuan, Z. Dou and G. Hu: 'Effects of electric field and magneticinduction on spin injection into organic semiconductors', Physica B: Condensed Matter,

2011, 406(4), 926-929.

9. N.A. Azarova, J.W. Owen, C.A. McLellan, M.A. Grimminger, E.K. Chapman, J.E.

Anthony and O.D. Jurchescu: 'Fabrication of organic thin-film transistors by spray-

deposition for low-cost, large-area electronics', Org. Electron., 2010, 11(12), 1960-1965.

10. A.A.M. Farag and I.S. Yahia: 'Structural, absorption and optical dispersion characteristics

of rhodamine B thin films prepared by drop casting technique', Optics Communications,

2010, 283(21), 4310-4317.

11. F.C. Krebs: 'Fabrication and processing of polymer solar cells: A review of printing and

coating techniques', Sol. Energy Mater. Sol. Cells, 2009, 93(4), 394-412.

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12. V. Nadazdy, R. Durny, J. Puigdollers, C. Voz, S. Cheylan and M. Weis: 'Defect states in

pentacene thin films prepared by thermal evaporation and LangmuirBlodgett technique',

J. Non-Cryst. Solids, 2008, 354(19-25), 2888-2891.

13. S.A. Wilson, R.P.J. Jourdain, Q. Zhang, R.A. Dorey, C.R. Bowen, M. Willander, Q.U.

Wahab, S.M. Al-hilli, O. Nur and E. Quandt: 'New materials for micro-scale sensors and

actuators: An engineering review',Materials Science and Engineering: R: Reports, 2007,

56(1), 1-129.

14. B. Lucas, A. El Amrani, A. Moliton, A. Skaiky, A. El Hajj and M. Aldissi: 'Charge

transport properties in pentacene films: Evaluation of carrier mobility by different

techniques', Solid-State Electronics, 2012, 69, 99-103.

15. Z. Wang, M. Helander, M. Greiner, J. Qiu and Z. Lu: 'Carrier mobility of organic

semiconductors based on current-voltage characteristics', J. Appl. Phys., 2010, 107(3),034506-034506-034504.

16. R. Zhang: 'Conductive TPU/CNT composites for strain sensing', PhD thesis, Queen

Mary, University of London, London, 2009.

17. W. Brutting: 'Physics of organic semiconductors'; 2005, KGaA, Weinheim, Wiley-VCH

Verlag GmbH & Co.

18. I. Yahia, M. Abd El-sadek and F. Yakuphanoglu: 'Methyl orange (CI Acid Orange 52) as

a new Organic Semiconductor: Conduction mechanism and Dielectrical relaxation', Dyes

and Pigments, 2012, 93(1434-1440).

19. IDTechEx.com. '

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Chapter-2Literature Survey2

1

Sensors are based on active sensing materials, which include inorganic and organic materials.

The organic materials have some advantages over the inorganic counterparts such as low density,

easy processing, flexibility and low cost [1-5]. The carbon is the element on which organic

materials are based. Examples of organic materials are the polymers, low molecular weight

organic materials, fullerene, carbon nanotubes etc. In the beginning, organic materials were

considered as insulators. In the last few decades of the 20 th century [6, 7], the discovery of

conductivity in organic materials attracted the scientists and researchers to explore their potential

to replace traditional inorganic electronic materials. History, evolution in conducting properties,

mechanisms involving in conduction and suitability of materials for various applications are the

interesting points which have been discussed in this chapter.

This chapter gives an overview of sensors and their classification, sensing materials, organic

semiconductors and their history and conduction mechanism. This chapter is also comprised of

four sections. The section 2.1 is about sensors and their classification, while section 2.2 provides

a little description of sensing materials, particularly, the nature of organic semiconductors with

their history. The discussion about conduction mechanism is given in section 2.3. In the last

section (section 2.4) few of the materials are described briefly with their applications.

2.1 Sensors and their classificationsSensors are the devices that usually convert real-world data into electrical signals. Sensor can

provide the information on the physical, chemical and biological environment [8]. Use of sensors

in each and every field of life is common and ever increasing. In the modern era, sensors are

considered one of the prime requirements for better and easy life because sensors playing vitalrole in the field of health and medicine, food and agriculture, business and industry, environment

and security, defense and aerospace and also communication. Research is in continuation for the

development of efficient and low cost sensors for wide variety measurands. To obtain the

comprehensive overview for the facilitation of comparison, classification of sensors is important

[9]. In literature various classifications are devised, which include the simplest as well as

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complex schemes. These schemes have their own advantages and disadvantages. But because of

diversity in the growth of sensing technology, standard classification could not come into

existence so far[10].

White [9] in 1987 presented a classification system, which is derived from literature of Hitachiresearch laboratory. He claims that this system is flexible with intermediate complexity and it is

apposite for personnel involve in computer based storage and retrieval system. This classification

categorizes the sensors on the basis of mesurands, technological aspects of sensors, detection

means used in sensors, sensors conversion phenomena, sensors materials and field of

applications. In year 2000 G. Harsanyi [11] classified the sensors on the basis of quantity to be

measured, basic sensor structure and sensing effect. On the basis of device structure which can

be used to measure the sensing properties of materials can be categorized as

i. Impedance type sensorsii. Sensing semiconducting devicesiii. Sensors based on acoustic wave propagationiv. Electrochemical cells as sensorsv. Image sensingvi. Charged particle dependent sensing

Another classification is devised on the base of source of excitation, which divided the sensors

into Generator-type sensors (GTS) and Modulated-type sensors (MTS). The sensors which

operate without external excitation are called Generator-type sensors, while that need excitations

are called modulated sensors. Examples of GTS and MTS are thermocouple and thermistors

respectively.

Hulanicki et al. [12] categorized the sensors on the basis of operating principles of transducers.

These transducers contain number of sensors. Major types of devices are presented below

a.

Optical devicesb. Electrochemical devicesc. Electrical devicesd. Mass sensitive devicese. Magnetic devicesf. Thermometric devices

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g. Devices used to determine chemical compositionWang et al. [10] classify the biomedical sensors in to physical, chemical and biological sensors

on the basis of working principle. Figure 2.1 and Figure 2.2 show the classifications of chemical

and physical sensors respectively. Moreover on the basis of measuring quantity, biomedicalsensors are classified as flow sensors, temperature sensors, displacement sensors, speed sensors,

pressure sensors etc. Each type of these sensors can be further classified on the basis of materials

or the working principles of sensors. The hearing sensors, vision sensors, capacity sensors and

olfaction sensors are the sensors which are classified according to human sensing organs. These

sensors are used to replace the human organs for the similar activities.

2.2 Sensing materialsThe materials which have semiconducting or conducting behavior can be used for sensor

fabrication. These materials include silicon, ferrous and non-ferrous metals and alloys, ceramics,

glasses, organic and nanomaterials [13]. There are few factors on the basis of which materials

are categorized as the most suitable, suitable and not suitable which include the ease in

fabrication, processing technology, availability of materials etc [14, 15]. According to their

nature materials are classified into inorganic, organic and composite materials. Generally

inorganic sensing materials are high density and difficult to process. The organic materials due to

low density, simple processing technology, high surface area and flexibility have potential toreplace conventional inorganic materials [15-18]. To improve the electrical and mechanical

properties of electronic devices, organic-organic or organic-inorganic composites and organic-

inorganic layered structures are also developed [13].

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Figure 2.1 Classification of chemical sensors [10]

Ion selective electrodes

Ion selective field effect transistors

Light addressable potentiometric sensors

Microelectrode array

Electrochemical gas sensors

Semiconductor gas sensors

Solid electrolyte gas sensors

Surface acoustic wave sensors

Capacitive humidity sensors

Resistive humidity sensors

Thermal conductivity humidity sensors

Chemical Sensors

Humidity Sensors

Gas Sensors

Ion Sensors

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Figure 2.2 Classification of physical sensors [10]

Photo resistor

Photodiode and transistor

Photovoltaic sensors

Fiber optic sensors

Photoelectric

Sensors

Space-variant capacitance sensors

Area-variant capacitance sensors

Medium-variant capacitance sensors

Capacitive pressure sensors

Electret microphone

2-D capacitive sensors

Capacitive

Sensors

Physical Sensors

Resistance strain sensors

Piezoresistive sensors

Self induced sensors

Differential transformer sensor

Sensors based on piezoelectric effect

Sensors based on piezoelectric resonator

Magnetoelectric induction sensor

Hall magnetic sensor

Hall viscor sensors

Hall respiration flow sensor

Magnetoelectric

Sensors

PiezoelectricSensors

Resistance

Sensors

Inductive Sensors

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2.2.1 Organic materialsThe carbon compounds are the base for organic materials, while the conjugation is the basic

feature of organic semiconductors. The chain of carbon atoms consisting of alternating single

and double bond is called conjugation, which in the range of semiconductors results in openingof band gap and splitting of the energy level, and charge delocalization in these levels. The Sp2

hybridization based bonding distinguishes the organic semiconductors from inorganic one. The

ring like structure of benzene is the archetype of the organic molecule. Difference among various

organic semiconducting molecules is based on the number of benzene rings, the polymerization,

the carbon atoms substitution by sulfur or nitrogen or the hydrogen substitution by side groups

[13, 19, 20].

On the basis of charge transport mechanism organic materials are classified as p-type (hole-

transport) and n-type (electron- movement). The majority of organic semiconductors have p-type

characteristic, while n-type organic materials also exist which have few environmental stability

problem [21]. The organic semiconducting materials are also categorized on the basis of

molecular weight as low molecular weight materials (oligomers) and polymers for which basic

unit is the monomer. The carbon atoms of the main chains of the molecules which forms the

backbone of molecule are strongly bonded by -bond which do not takes part in electronic

conduction. In the molecules of both types of organic materials the p z orbitals of sp2 hybridized

carbon atoms form weakly bonded conjugated -electron system. These -electrons undergo -

* transitions which is considered lowest electronic transition with energy band gap of 1.5-3.0

eV [19, 22]. Normally polymers are comprised of carbon atoms in combination with one or more

of the elements like nitrogen, oxygen, hydrogen, silicon, sulfur, chlorine and fluorine. For the

fabrication of electronic devices, the thin films of oligomers and polymers are deposited which

are amorphous in nature. The organic molecules can be crystallized by van-der Waals interaction

to form organic molecular crystals. A variety of methods is used for the growth of single organic

molecular crystals (OMCs), which include vapor phase growth or growth by solution,Czochralski and Bridgman-type methods and sublimation. Although OMCs show better

performance but the thin films are most practical for the fabrication of electronic devices [13, 19,

20].

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2.2.2 History of organic semiconducting materials in electronic devicesAs the invention of transistor in 1950s brought a revolution in electronic industry, the inorganic

semiconducting materials started to replace the already being used metallic materials. In the

beginning of new century it has become possible that inorganic semiconductors may replaced byorganic semiconductors. As a first commercial product organic light emitting diode (OLED)

display is available in market [19]. The discovery of semiconducting behavior in organic

materials and their commercialization have a long history, which is spread over a century. From

the beginning it was general concept that organic materials are insulators. This concept proved

wrong when in the start of last century photoconductivity was observed in the anthracene, and it

was the first known photoconductive organic material [23-25]. The research on organic

molecular crystal during 1960s resulted in exploration of basic phenomena of optical excitation

and charge carrier transport [26, 27]. In 1970s the discovery of conducting polymer was the

outcome of a laboratory accident which was granted a Nobel Prize in chemistry in the year 2000

[28-30]. This was the revolution which opened a new horizon for synthesis of conjugated

polymers and tailoring of their properties by controlled doping [31]. Brief history of organic

semiconductor materials and devices is shown in Table 2-1.

With the revolution in electronic materials, the electronic devices also revolutionized. In the

second half of 20th century inorganic materials based solid state electronics has replaced the

vacuum tube based electronics. Currently the wide spread microelectronic devices for our daily

life use are the gift of development of solid state electronics. After the discovery of

semiconducting and conducting organic materials the expectation for realization of low cost,

light weight, large area, printed integrated circuits, plastics solar cells and flexible light sources

and displays brought revolutionary developments in these materials [19].

2.2.3 Importance of organic semiconducting materialsAlthough the inorganic semiconductors have capability to perform as a sensor but the organic

semiconductors because of their salient feature related to electrical and physical parameters have

a potential for better performance as a sensing materials. Regarding electrical parameter, the

change in electrical properties of organic materials in response to external agents like humidity,

temperature, pressure, displacement, light, electromagnetic radiations, gases and chemicals etc

make the organic materials competitor of inorganic materials. The ease in tailoring of electronic

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Table 2-1 History of organic semiconductor materials and devices

Sr.

NoYear Development Researcher Ref.

1Before

1941

Organic materials are insulator, the metal-like appearance of few of them

intrigued chemists

2 1941-46 Initial interest in OS; -electron transfer in biological system Gyorgi[32,

33]

3 1948 Observation of weak electric conductance in bulk phthalocyanine Eley [34]

4 1950

Assumption; -electron contribute in electrical Conduction

Introduction; term of organic semiconductor Akamatu et al. [35]

5 1955 Electroluminescence response reported in organic molecular solids Bernanose [36]

6 1970Development of organic semiconductor thin film by vacuum evaporation

and Longmuir-Blodgett methods.William et al. [37]

7 1970 Field effect phenomena reported in organic semiconductor Brabe [38]

8 1976 Discovery of metallic conductivity in polyacetylene Heeger [6]

9 1983 Development of first polyacetylene based OFET

10 1983 Photovoltaic effect observed in organic semiconductors Chamberlain [39]

11 1987 Development of first polythiophene based thin film transistor (OTFT) Koezuka et al. [40]

12 1987 Fabrication of first OLED Tang [41]

13 1987-97 Research focused on improvement of charge carrier mobility ------- [42]

14 2000 Successful synthesis and development of conductive polymers ------- [43]

15 2000-09 Commercialization of some organic materials based sensors ------- [44]

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properties of organic materials along with physical properties such as light weight, greater

flexibility, high absorption coefficient and larger surface area make organic materials superior

than their inorganic counterparts [13]. The higher biocompatibility of organic materials makes

them ideal for use in bioelectronics which includes bio-detectors or signal processing in bio-

molecular electronics, while the DNA and its constituents can be used as active materials in thin

film transistors [45-49]. In addition to above mentioned features simplicity of fabrication

techniques and lower cost attract the researchers to explore the potential of organic materials for

efficient and reliable electronic devices.

2.2.4 Chemical nature of organic semiconducting materialsAs the carbon atom is the basic element of organic materials; these atoms are interconnected

with alternating single and double bonds to form a conjugated molecule, which is an organic

semiconductor[20, 50, 51]. Few examples of conjugated molecules are shown in Figure 2.3.

These semiconducting molecules are formed by the binding of positively charged nuclei and

negatively charged electrons in the atomic shells of constituent atoms. In the molecular chain

each carbon atom contains four electrons out of which three electrons exist in the Sp 2 hybridized

orbitals, while fourth one exists in pz orbital. The electrons in Sp2 orbitals involve in the

formation of covalent bonds between the adjacent carbon atoms on any side in molecular chain

through molecular orbitals. These electrons also bond the carbon atom with hydrogen atoms or

side groups. While the electron in pz orbital forms a covalent bond with adjacent carbon atom in

a chain on only one side through bond. This type of bonding results in the formation of chains

with alternative single () and double ( & ) bonds. The only two electrons from each atom

take part in bonding; two electrons out of four bonding electrons remain in and two in

molecular orbitals. The remaining two electrons on each carbon atom in non bonding orbitals are

available for bonding of remaining chains or ligands [51].

It is the basic feature of organic molecules that due to the interaction between pz atomic orbitals

the splitting between and * molecular orbitals takes place. Additional smaller splitting of the

levels also takes place because of these interactions further away from the chain. The and *

molecular orbitals are also known as highest occupied molecular orbit (HOMO) and lowest

unoccupied molecular orbital (LUMO) respectively. Between HOMO and LUMO there is an

energy gap and these states are also recognized as ionization energy (IE) and electron affinity

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(EA) respectively. The and * states in polymers are broadened into the valance (filled) and

conduction (empty) states due to orbitals coupling along the chain [20, 30].

In organic materials, the splitting of HOMO and LUMO energy levels in the semiconductors

range is due to the formation of alternative single and double bonds in the chains. The materialshaving molecular chains with only single bonds () or double bonds () are difficult to include in

the category of semiconductors because of wider energy band gap. So, for semiconducting

properties, the organic materials should have alternative single and double bonds (conjugation)

[51].

2.2.5 Charge transport properties/conduction mechanism of organic semiconductorsIn organic molecular solids the ionic molecular states are involved in the transport of electrons or

holes. To create a hole in a neutral moleculeM, it is necessary to remove electron to transform itinto radical cation M+. This hole (deficiency of electron) move from molecule to molecule.

Similarly electron transport is the result of negatively charged radical ions M-. In the case of

conjugated polymers, instead of charged states term of negative and positive polarons is used

[19]. As in organic semiconductors, the bonding orbitals and quantum mechanical wave -

function overlap are responsible for the charge transport. But due to inadequacy of -bonding

overlapping between the molecules of disordered organic semiconductors, the concept of

quantum mechanical tunneling is considered appropriate to explain the charge transport in these

materials [52]. The transport properties of organic semiconductors are categorized by the

following [20]

i. Polaronic Effectii. Hopping Conductioniii. Low Mobility, Low Saturation Velocity

The formation of polaron is the result of interaction of charges with lattice deformation and

polarization [20,51]. The extension of this deformation in organic materials is on atomic scaleand the mobility of charges reduces due to self trapping [51]. If the same deformation is shared

by two charges is called bi-polaron while the attraction between oppositely charged polaron is

similar to exciton. Polarons are responsible for conduction in single chain molecule [20]

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Figure 2.3 Conjugated molecules a: copper phthalocyanine, b: polythiophene, c: fullerene,

d: polyacetylene and e: polyaniline

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As the molecule to molecule movement (conduction between different molecules) of charge

carriers is the basis for charge transport, which is generally called as hoping transport. This name

is given to charge transport process because of its quantum mechanical tunneling nature and

reliance on the probability function [53]. This thermally activated process is dependant on the

energy gap between HOMO and LUMO Levels. The thermal activation can be expressed as [20]

2.1

where Ea is an energy of the order of 1 eV.

In organic semiconductors the mechanism of charge carrier transport can lay between band and

hopping transport, which are two extremes. Normally, the band transport is observed in highly

purified molecular crystals at temperatures which are not too high. But as compared to inorganic

semiconductors the weak delocalization with small band width is found in organic

semiconductors, which result in low mobility (in the range of 1-10 cm2/Vs) at room temperature.

Equation 2.2 shows that temperature dependence of the band transport follows the power law

[19].

2.2

Hopping conduction is somewhat different from band conduction. It occurs in amorphous

organic solids and leads to lower mobility values (around 10-3 cm2/Vs). Only the localized states

exist or play a role in hoping conduction rather than delocalized bands, which are too far or the

required temperature for the charge carrier is too high. As the charges expend their time on

localized states and conduction of charges takes place by hopes between these states. This type

of carrier conduction (mobility) may be influenced by temperature and electric field of the

system as given in Eq.2.3 [19].

2.3

At room temperature the maximum low-field mobility of the most of organic semiconductors is

about 1 cm2/Vs and it is less reliant on temperature. This mobility is comparable with amorphous

silicon but much small than that of crystalline silicon. The low temperature causes to increase the

mobility as for example mobility of naphthalene increases below 100K. This increase in mobility

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is credited to the freeze-out of phonons and hoping to band transport transition. Moreover, for

the description of charge transport in organic solids trapping effect, space charge and charge

carrier injection mechanism should be considered [19, 51]

The drift velocity and charge carrier density are the parameters which define the current throughmaterial on a microscopic level. As given in Eq.2.4, the drift velocity is articulated by mobility

and electric field [19].

2.4

At high field, the drift velocity shows saturation and its values are much smaller even at low

temperature. The charge carrier density n is also the most important parameter. In a

semiconductor having energy band gap Eg and No effective density of states, the intrinsic carrier

density is given as

2.5

A hypothetical density of at room temperature can be lead by

and , which is never reachable until much densities of impurities can be achieved

in real materials. In organic semiconductors the carrier density can be improved by the following

ways:

1. Electro-chemical doping2. Carrier injection from contacts3. Photo generation of carriers4. Field effect doping

2.3 Properties of various organic sensing materials2.3.1 Polyaniline (PANI)Polyaniline as a potential conductive polymer has developed great research interest because of its

significant properties like excellent environmental stability, reversible acid-base and redox

chemistry, optical and electrochemical properties [54-56]. Moreover its cheapness, easy

availability of raw materials and simple synthesis technique make polyaniline relatively popular

among the conductive polymers [17, 57]. PANI undergoes solution or counter ion-induced

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process-ability and it may also be crystallized. To enhance its potential applications, the

secondary doping of PANI is carried out [56].

The general formula for the polyaniline is [(-B-NH-B-NH-)y, (-B-N=Q=N-)1-y]x , where B and Q

are the benzenoid and quinonoid forms of C6H4 rings. Depending on the neutral intrinsic redoxstates PANI is classified as [56].

1. Pernigraniline (PNA, y = 0), which is the fully oxidized state with blue/violet color.2. Nigraniline (NA, y = 0.75), which is the 75% intrinsically oxidized with blue/violet color.3. Leucoemeraldine (LM, y = 1), which is the fully reduced state having white/clear &

colorless appearance.

4. Eemeraldine (EM, y = 0.5), which is the 50% intrinsically oxidized. The emeraldine salthas green while emeraldine base has blue color

Emeraldine exist in two forms which are emeraldine base (EB) and emeraldine salt (ES). Among

these the emeraldine base (EB) is the basic form of PANI which consists of four-ring tetramer

structure having two segments of amine and two of imine, and is non-conductive. The dopants

can be doped (included) into PANI or de-doped (released) from it reversibly due to their non-

redox and physical interaction. The conductive emeraldine salt (ES) is produced by the

electrostatic attraction between the anions of incorporated dopants and the nitrogen on the

backbone of polyaniline [54].

The polyaniline has capability to sense acidic, basic and some neutral vapors or liquids. The

sensing mechanism is based on change in electrical conductivity and color upon exposure to such

environments. These features make polyaniline suitable for sensors, indicators and detectors [16,

57-66]. Fuke et al. [60,61] also studied the Co-polyaniline and Ag-polyaniline nanocomposite

based humidity sensors. They evaluated Co-polyaniline based sensors with various film

thicknesses for humidity sensing and measured the hysteresis and response & recovery times. In

the study of sensors based on Ag-polyaniline nanocomposite clad on optical fibers; the clad

length and nao-particle size were optimized for humidity sensing. A rapid response humidity

sensor based on ultra thin film layer by layer (LBL) assembly of poly (anilinesulfonic acid)

(SPANI) were fabricated by Nohria et al. [64] and compared with SPANI based sensors

fabricated by spin coating. They compared the sensitivity and response and recovery times of

humidity sensors. Pandey et al. [67] fabricated pellets of polyaniline and studied the change in

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resistance with change in relative humidity. Parvatikar et al. [68] synthesized

polyaniline/tungsten oxide composite by in situ deposition technique and the fabricated pellets

and then characterize them for temperature and humidity sensing.

2.3.2

Carbon nanotubes (CNTs)

The discovery of carbon composed spherical molecule (fullerene) lead the Harry Kroto, Robert

Curl, and Richard Smalley for the award of Nobel Prize in Chemistry in 1996. Research on

theory and synthesis of fullerene was in its full bloom during 1980s and early 1990s [69] when

Iijima [70] discovered multiwall carbon nano tube, which he observed with the help of

transmission electron microscope. At that time this structure was considered as filamentous

carbon. The single wall carbon nanotube was also discovered in 1993 by Iijima [71]. The

discovery of CNT made it the most popular material of global scientific arena and it became one

of the most investigated materials of the 20th century.

Carbon nano tubes are rolled sheets of graphene in a cylindrical fashion, while the graphene is a

sheet of hexagonally arranged carbon atoms which are bonded by SP 2 hybridization, while in

CNTs there is very little mixing of and orbitals. These graphene sheets are classified as

single layer or few layer graphene. Depending upon the nature of graphene sheet CNTs are

categorized as single wall, double wall triple wall and multiwall carbon nanotubes [20].

Properties of the CNTs are dependant on chiral angle (angle at which sheets are rolled) and the

radius. Electronically, carbon nanotube (CNT) can be metallic, semiconducting, or small-gap

semiconducting (SGS) materials, it depends on the orientation of the graphene lattice with

respect to the axis of the tube [72]. The numbers of unit vectors along two direction of graphene

crystal lattice defines the electronic behavior of CNTs and these vectors are represented by

indices n and m which are commonly written as (n, m). In case of n=m the nanotube is metallic

and if n-m is the multiple of 3, the tube is small band-gap semiconducting, while in all other

cases normal semiconducting.

The excellent electrical, electronic and mechanical properties along with chemical stability make

the CNTs a popular material for engineering applications. These properties are regarded to the

extremely small size with hollow cylindrical shape and high aspect ratio of CNTs [73, 74]. Due

to their small particle size and large surface area, their ability to promote electron transfer and

bio-sensing properties, the CNTs have proven to be the best material for various kinds of sensors

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[73]. An extensive research is being carried out to explore the potential of CNTs for

electrocatalytic and sensing applications [75] which include temperature, pressure, strain,

humidity, chemical, gas and bio sensing etc. In various sensing applications, the CNT-based

sensors have advantages of exceptionally higher sensitivity and a faster response at room

temperature over conventional solid state sensors, which are operated at high temperatures (300-

500 C) for higher chemical reactivity [76]. Various types of sensors have been fabricated and

investigated on the basis of single-walled carbon nanotubes (SWCNTs), double-walled carbon

nanotubes (DWCNTs), and multi-walled carbon nanotubes (MWCNTs) [77-86].

It has been confirmed theoretically and experimentally that CNTs are compatible with strain

sensors. In response to torsion or axial strain, the CNTs undergo band-gap changes, which are

chirality dependent [87]. The thin films based on randomly oriented CNTs and CNTs reinforced

composite have proved sensitive to mechanical loading, which demonstrate the macro scale

fabrication of strain sensors. The direct change in resistance results in response to mechanical

loading [88].

2.3.3 Copper phthalocyanine (CuPc)Phthalocyanine (Pc) is planar aromatic macro-cyclic compound with dark green-blue color

which forms coordination complexes with more than 70 metals including Cu, Al, Ni, Co, Fe and

V etc. These compounds are constituted by alternatively arranged carbon and nitrogen atoms

which forms four isoindole units with a cloud of 18 delocalized electrons [89]. Initially, since

their discovery in 1928 Pcs were used as inks, paints, colors and dyestuff for textiles. Later on,

because of their peculiar physical properties, the phthalocyanines and their derivatives became

fascinating subject of research for materials scientists to explore their potential applications in

various fields including nano-technological devices and molecular materials [89, 90]. Currently

Pcs along with other materials are being used as active materials in LCDs, information storage

systems, photovoltaics, laser dyes, semiconductor and electro-chromic devices and sensors [89,

91, 92].

The copper phthalocyanine is a complex of copper (Cu) with phthalocyanine having blue color,

which is because of to * transition at wavelength of ~610 nm. It has exceptional properties

like high stability (i.e. not deg

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