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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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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
4.2.2.1Fabrication of sensors ........................................................................................................ 74
4.2.2.2Measurements .................................................................................................................... 74
4.2.3 Results and discussion ....................................................................................................... 75
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
4.3.2.1Fabrication of sensors ........................................................................................................ 85
4.3.2.2Measurements .................................................................................................................... 85
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4.3.3 Results and discussion ....................................................................................................... 86
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
5.1.2.1Fabrication of sensors ........................................................................................................ 98
5.1.2.2Measurements .................................................................................................................... 99
5.1.3 Results and discussion ....................................................................................................... 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
5.3.2.1Fabrication of sensors ...................................................................................................... 110
5.3.2.2Measurements .................................................................................................................. 111
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5.3.3 Results and discussion ..................................................................................................... 111
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
5.4.3 Results and discussion ..................................................................................................... 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
6.1.2.1Fabrication of sensors ...................................................................................................... 136
6.1.2.2Measurements .................................................................................................................. 137
6.1.3 Results and discussion ..................................................................................................... 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
6.2.2.1Fabrication of sensors ...................................................................................................... 145
6.2.2.2Measurements .................................................................................................................. 146
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6.2.3 Results and discussion ..................................................................................................... 146
6.2.4 Conclusions ...................................................................................................................... 149
6.3 Carbon nanotubes based strain sensors ............................................................................ 151
6.3.1 Introduction ...................................................................................................................... 151
6.3.2 Experimental .................................................................................................................... 152
6.3.2.1Fabrication of sensors ...................................................................................................... 152
6.3.2.2Measurements .................................................................................................................. 152
6.3.3 Results and discussion ..................................................................................................... 153
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
6.4.2.1Fabrication of sensors ...................................................................................................... 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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XXIV
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