Fabrication and characterization of ZnO nanostructures for ...

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Linköping Studies in Science and Technology Dissertation No. 1412

Fabrication and characterization of ZnO nanostructures for sensing and photonic device applications

Syed Muhammad Usman Ali

Physical Electronics and Nanotechnology Division

Department of Science and Technology (ITN) Campus Norrköping, Linköping University

SE-60174 Norrköping Sweden

Linköping 2011

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Copyright © 2011 by Syed Muhammad Usman Ali

E mail; [email protected]

[email protected]

ISBN: 978-91-7393-015-4

ISSN 0345-7524

Printed by LiU-Tryck, Linköping University, Linköping, Sweden

November, 2011

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Motivation from ALLAH:

Surah Al-'Ankabut [29:20]

ALLAH (GOD) Say: “Travel through the earth and see how ALLAH did originate creation; so will ALLAH produce a later creation: for ALLAH has power over all things”.

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Dedication:

When I was a kid, my father (deceased) always encouraged and motivated me for the higher

education (especially for PhD studies) and I promised him that inshah ALLAH I will try my

best to fulfil your desire. Today by the grace of almighty ALLAH, I have fulfilled his desire

but I have tears in my eyes that unfortunately, he is not alive to see and hug me that I have full

filled his desire as I promised (May ALLAH grant an eternal peace to his soul in heaven,

Ameen). I dedicated this thesis to my father, mother, all brothers and sisters and in laws, my

beloved wife Soofia Usman, her sacrifices & supports are countless for achieving this goal

and at the end my lovely and beloved children, Syeda Hiba Fatima, Syeda Aiman Fatima,

Syeda Kinza Fatima and my little lovely son Syed Hussam Muhammad Ali for all your

sacrifices, patience and support to me.

Finally I am quoting a short quotation, which my father wrote on my note book when I got

the admission in electronic engineering department; and he always said to me that ups and

down are the part of life but a person should keep his moral and hopes high, he wrote as:

“People become really quite remarkable when they start thinking that, they can do

things. When they believe in themselves they have the first secret of success......”

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Fabrication and characterization of ZnO nanostructures for sensing and photonic device

applications

Syed Muhammad Usman Ali

Department of Science and Technology

Linköping University, 2011

Abstract:

Nanotechnology is an emerging inter-disciplinary paradigm which encompasses diverse fields

of science and engineering converge at the nanoscale. Nanotechnology is not just to

grow/fabricate nanostructures by just mixing nanoscale materials together but it requires the

ability to understand and to precisely manipulate and control of the developed nanomaterials

in a useful way. Nanotechnology is aiding to substantially improve, even revolutionize, many

technology and industry sectors like information technology, energy, environmental science,

medicine/medical instrumentation, homeland security, food safety, and transportation, among

many others. Such applications of nanotechnology are delivering in both expected and

unexpected ways on nanotechnology’s promise to benefit the society.

The semiconductor ZnO with wide band gap (~ 3.37 eV) is a distinguish and unique material

and its nanostructures have attracted great attention among the researchers due to its peculiar

properties such as large exciton binding energy (60 meV) at room temperature, the high

electron mobility, high thermal conductivity, good transparency and easiness of fabricating it

in the different type of nanostructures. Based on all these fascinating properties, ZnO have

been chosen as a suitable material for the fabrication of photonic, transducers/sensors,

piezoelectric, transparent and spin electronics devices etc. The objective of the current study

is to highlight the recent developments in materials and techniques for electrochemical

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sensing and hetrostructure light emitting diodes (LEDs) luminescence properties based on the

different ZnO nanostructures. The sensor devices fabricated and characterized in the work

were applied to determine and monitor the real changes of the chemical or biochemical

species. We have successfully demonstrated the application of our fabricated devices as

primary transducers/sensors for the determination of extracellular glucose and the glucose

inside the human fat cells and frog cells using the potentiometric technique. Moreover, the

fabricated ZnO based nanosensors have also been applied for the selective determination of

uric acid, urea and metal ions successfully. This thesis relates specifically to zinc oxide

nanostructure based electrochemical sensors and photonic device (LED) applications.

The first part of the thesis includes paper I to V. In this part, we have demonstrated the

electrochemical sensing characterization and wireless remote monitoring system for glucose

based on the well aligned vertically fabricated ZnO nanowires based sensors.

In paper I, we have presented a potentiometric electrochemical glucose sensor based on zinc

oxide nanowires. Glucose oxidase (GOD) was electrostatically immobilized on the surface of

the well aligned zinc oxide nanowires resulting in sensitive, selective, stable and reproducible

glucose biosensors. The potentiometric response vs. Ag/AgCl reference electrode was found

to be linear over a relatively wide logarithmic concentration range (0.5 µ to 10 mM) suitable

for extra/intracellular glucose detection.

In paper II, we have demonstrated the another technique for the determination of the glucose

using immobilized ZnO nanowires interfaced/coupled as an extended gate to the metal oxide

semiconductor field effect transistor (MOSFET). The potentiometric response of presented

sensor was directly connected to the gate of a commercial MOSFET to study the I-V response

variation with respect to the change in the concentration of the test electrolyte glucose

solutions. Here we have successfully showed that the ZnO nanowires grown on any thin wire/

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substrate can be interfaced with conventional electronic component to produce a sensitive and

selective biosensor.

In paper III, we have successfully demonstrated the measurements of an intracellular glucose

using the functionalised ZnO-nanorod-based glucose selective electrochemical sensor in

human adipocytes and frog Xenopus laevis. The electrochemical response of the sensor was

linear for a wide concentration range (0.5 µ to 1000 µM). The measured values of the glucose

concentration inside the human fat cells (adipocytes) or frog oocytes using our proposed

sensor were close to the values reported in the literature. We have also investigated the impact

of insulin, we added insulin to the cell medium to stimulate glucose uptake and as a result an

increase in an intracellular glucose was observed.

In paper IV, this paper presents a prototype wireless remote glucose monitoring system

interfaced with ZnO nanowire arrays based glucose sensor, which can be effectively apply for

the monitoring of glucose levels in diabetes. A communication protocol that facilitates remote

data collection using SMS has been utilized for monitoring patient’s sugar level. In this study,

we demonstrate the remote monitoring of the glucose levels with existing GPRS/GSM

network infra-structures using our proposed functionalized ZnO nanowire arrays sensors

integrating with standard available mobile phones. The data can be used for centralized

monitoring and other purposes. Such applications can reduce the health care costs and provide

caregivers to monitor and support to their patients especially in the rural area.

In paper V, we have presented a potentiometric uric acid selective sensor using the zinc

oxide (ZnO) nanowires fabricated on the surface of a gold coated flexible substrate. Uricase

was electrostatically immobilized on the surface of well aligned ZnO nanowires for the

selective determination of a uric acid. The sensor showed a linear response covering a wide

logarithmic concentration range from 1 µ to 650 µM suitable for human blood serum. By

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incorporating the Nafion® coating on the surface of the sensor, the linear range could be

extended to 1 µ to 1000 µM at the expense of an increased response time from 6.25 s to less

than 9 s.

The second part of this thesis, different ZnO nanostructures were fabricated on p-GaN to form

a p-n heterojunction light emitting diodes (LEDs). The luminescence properties of these p-n

heterojunctions based LEDs were also comparatively investigated.

In paper VI, we have fabricated the different ZnO nanostructures like nanorods, nanotubes,

nanoflowers, and nanowalls on the p-type GaN substrates and the luminescence properties of

these heterojunction LEDs were comparatively investigated by EL and PL measurements. The

highest emission in the visible region was observed from nanowalls structures while highest

emission for UV region was observed from the nanorods structures due to their good crystal

qualities. It has also been observed that nanowalls structures demonstrated a strong white light

emission with high colour rendering index (CRI) of 95 along with correlated colour

temperature (CCT) of 6518 K.

Keywords: Nanotechnology, zinc oxide, nanowires/ nanorods, nanotubes, nanoporous/nanoflakes, electrochemical sensor and photonic devices.

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List of publications included in the thesis

1. A fast and sensitive potentiometric glucose microsensor based on glucose oxidase

coated ZnO nanowires grown on a thin silver wire

Syed M. Usman Ali, O. Nur, M. Willander, B. Danielsson

Sensors and Actuators B 145 (2010) 869-874.

2. Glucose detection with a commercial MOSFET using a ZnO nanowires extended

gate

Syed M. Usman Ali, Omer Nur, Magnus Willander, and Bengt Danielsson

Nanotechnology, IEEE Transaction on 8 (2009) 678-683.

3. Functionalized ZnO nanorod based intracellular glucose sensor

M. H. Asif, Syed M. Usman Ali, O. Nur, M. Willander, Cecilia Brännmark, Peter

Strålfors , Ulrika Englund , Fredrik Elinder and B. Danielsson

Biosensors and Bioelectronics 25 (2010) 2205-2211.

4. Wireless remote monitoring of glucose using functionalized ZnO nanowire arrays

based sensor

Syed M. Usman Ali, Tasuif Aijazi, Kent Axelsson, Omer Nur, Magnus Willander

Sensors 2011, 11, 8485-8496; doi:10.3390/s110908485.

5. Selective potentiometric determination of uric acid with uricase immobilized on

ZnO nanowires

Syed M. Usman Ali, N.H. Alvi, Zafar Ibupoto, Omer Nur, Magnus Willander, Bengt

Danielsson

Sensors & Actuators: Chem. B 2 (2011) 241-247.

6. Fabrication and comparative optical characterization of n-ZnO nanostructures

(nanowalls, nanorods, nanoflowers and nanotubes)/p-GaN white light emitting

diodes

N. H. Alvi, Syed M. Usman Ali, S. Hussain, O. Nur, and M. Willander

Scripta Materialia 64 (2011) 697-700.

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List of publications not included in the thesis

Journal papers

1. Alimujiang Fulati, Syed M. Usman Ali, Muhammad Riaz, Gul Amin, Omer Nur and

Magnus Willander. Miniaturized pH sensors based on zinc oxide nanotubes/nanorods.

Sensors 2009, 9(11), 8911-892.

2. M. Willander, L. L. Yang, A. Wadeasa, S. U. Ali, M. H. Asif, Q. X. Zhao and O. Nur,

Zinc oxide nanowires: controlled low temperature growth and some

electrochemical and optical nano-devices, J. Mater. Chem., 2009, 19, 1006-1018.

3. Alimujiang Fulati, Syed M. Usman Ali, Muhammad H. Asif, Naveed ul Hassan Alvi ,

Magnus Willander, Cecilia Brännmark, Peter Strålfors , Sara I. Börjesson, Fredrik Elinder,

Bengt Danielsson, An intracellular glucose biosensor based on nanoflake ZnO,

Sensors and Actuators, Chem. B 150 (2010) 673-680.

4. Muhammad H. Asif , Syed M. Usman Ali , Omer Nur , Magnus Willander, Ulrika H.

Englund, Fredrik Elinder, Functionalized ZnO nanorod-based selective magnesium ion

sensor for intracellular measurements, Biosensors and Bioelectronics 26 (2010) 1118-

1123.

5. Syed M. Usman Ali, Muhammad H. Asif , Alimujiang Fulati , Omer Nur, Magnus

Willander, Cecilia Brännmark, Peter Strålfors, Ulrika H. Englund, Fredrik Elinder and

Bengt Danielsson, Intracellular K+ determination with a potentiometric

microelectrode based on ZnO nanowires, IEEE Transaction on Nanotechnology,

volume 10, Issue 4, pp. 913-919.

6. M. Willander, O. Nur, M. H. Asif, S. M. Usman Ali, and K. Sultana, Zinc oxide

nanorods for intracellular sensing of biological analytes, metallic ions and localized

photodynamic therapy, (Manuscript).

7. Magnus Willander, O. Nur , M. Fakhar-e-Alam, J. R. Sadaf, M. Q. Israr , K. Sultana, Syed

M. Usman Ali , M. H. Asif, Applications of zinc oxide nanowires for bio-photonics

and bio-electronics, Proc. of SPIE 7940, 79400F (2011); doi:10.1117/12.879497.

8. Th. S. Dhahi, U. Hashim, T. Nazwa, M. Kashif, Syed M. Usman Ali, Magnus Willander,

pH measurement using micro gap structure, International journal of mechanical and

materials engineering, Malaysia, accepted).

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9. Faraz Mahmood, Imran Mohsin, Syed M. Usman Ali , Abid Karim, Design of an ultra-

wideband monopole antenna for handheld devices, Asian journal of engineering,

sciences and technology Vol. 1 issue 1(2011).

10. M. Kashif, Syed M. Usman Ali, M. E. Ali, H. I. Abdul gafour, U. Hashim M. Willander

and Z. Hassan, Morphological, optical and raman characterization of ZnO

nanoflakes prepared via sol-gel method, Phys. Status Solidi A, 1-5 (2011) / DOI

10.1002/pssa.201127357.

11. Syed M. Usman Ali, Zafar H. Ibupoto, Salah Salman, Omer Nur, Magnus Willander,

Bengt. Danielsson, Selective determination of urea using urease immobilized on ZnO

nanowires, Sensors & Actuators: B. Chem. 160 (2011) pp. 637-643.

12. Syed M. Usman Ali , M. Kashif , Zafar Hussain Ibupoto, M. Fakhar-e-Alam, U.

Hashim, Magnus Willander, Functionalized ZnO nanotubes arrays as electrochemical

sensor for the selective determination of glucose, Micro & Nano Letters, 2011, Vol. 6,

issue. 8, pp. 609-613.

13. Syed M. Usman Ali, Zafar Hussain Ibupoto, C. O. Chey, Omer Nur, Magnus Willander,

Bengt Danielsson, Functionalized ZnO nanotube arrays for the selective

determination of uric acid with immobilized uricase, Chemical Sensors 2011, 1: 19.

14. N. H. Alvi, Syed M. Usman Ali, K. ul Hasan, O. Nur, and M. Willander, Optical and

electro-optical properties of n-ZnO nanoflakes/p-GaN heterojunction light emitting

diodes, (Manuscript).

15. K. ul Hasan, N. H. Alvi, Syed M. Usman Ali, Jun Lu, O. Nur, and M. Willander Single

ZnO nanowire biosensor for detection of glucose interactions, (Manuscript).

16. C. O. Chey, Syed M. Usman Ali, Z. Ibupoto, K. Khun, O. Nur, M. Willander,

Potentiometric creatinine biosensor based on immobilization of creatinine deiminase

(CD) on ZnO nanowires, J. Nanosci. Lett. 2012, 2: 24.

17. Z. H. Ibupoto, Syed M. Usman Ali, C.O. Chey, K. Kimleang, O. Nur, Magnus

Willander, Functionalized ZnO nanorods coated with selective ionophore for the

potentiometric determination of Zn+2 ions, (accepted in Journal of Applied Physics).

18. Z. H. Ibupoto, Syed M. Usman Ali, K. Kimleang, C.O. Chey, O. Nur, Magnus

Willander, ZnO nanorods based enzymatic biosensor for the selective determination

of Penicillin, Biosensors 2011, 1(4), 153-163.

19. K. Khun, Z. H. Ibupoto, Syed M. Usman Ali, C. O. Chey, O. Nur, M. Willander, The

selective iron (Fe3+) ion sensor based on functionalized ZnO nanorods with selective

ionophore (accepted in Electroanalysis).

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20. Syed M. Usman Ali, Zafar H. Ibupoto, O. Nur, M. Willander, Synthesis of ZnO

nanowalls for enzymatic determination of urea using immobilized urease,

21. Z. H. Ibupoto, Syed M. Usman Ali, K. Kimleang, M. Willander, L-Ascorbic acid

biosensor based on immobilized enzyme on ZnO nanorods, (accepted, Journal of

Biosensors and Bioelectronics).

22. Z. H. Ibupoto, Syed M. Usman Ali, K. Kimleang, M. Willander, Synthesis of ZnO

nanorods in PBS and their morphological and optical characterization, (Manuscript).

23. Z. H. Ibupoto, Syed M. Usman Ali, K, Khun and M. Willander, Thallium (I) ion sensor

based on functionalized ZnO nanorods, (submitted in Talanta journal).

24. Magnus Willander, Omer Nur and Syed M. Usman Ali, Zinc oxide nanostructures

based bio and chemical extra and intracellular sensors, submitted in African physical

review journal).

25. Z. H. Ibupoto, K. Khun, Syed M. Usman Ali, M. Willander, Potentiometric l-lactic acid

biosensor based on immobilized ZnO nanorods by lactate oxidase, (submitted).

26. Syed M. Usman Ali, Z. H. Ibupoto, M, Kashif, U. Hashim, Magnus Willander,

Construction of potentiometric uric acid sensor based on ZnO nanoflakes with

immobilized uricase (manuscript).

Conference papers

27. Kashif, Syed M. Usman Ali, K. L. Foo, U. Hashim, Magnus Willander, ZnO

nanoporous structure growth, optical and structural characterization by aqueous

solution route, enabling science and nanotechnology: 2010 International conference on

enabling science and nanotechnology Escinano 2010. AIP Conference proceedings,

volume 1341, pp. 92-95 (2011).

28. Muhammad H. Asif , Syed M. Usman Ali , Omer Nur, Magnus Willander , Ulrika H.

Englund, Fredrik Elinder, Functionalized ZnO nanorod-based selective magnesium ion

sensor for intracellular measurements, Biosensor world congress 2010, Glasgow UK,

26-28 May.

29. Syed M. Usman Ali , U. Hashim, Zafar Ibupoto, M, Kashif, M. Fakhar-e-Alam, Magnus

Willander, ZnO nanoporous arrays based biosensor for highly sensitive and selective

determination of uric acid using immobilized uricase, INSC 2011 4th to 5th July, 2011

Seri Kembangan Selangor, Malaysia.

(manuscript).

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30. M, Kashif, U. Hashim, Syed M. Usman Ali , Magnus Willander, Effect of Sn doping on

crystal structure and optical properties of ZnO thin films, INMIC 2011, Karachi

Pakistan Accepted.

31. M. Kashif, Syed M. Usman Ali, U. Hashim, Magnus Willander, Fabrication of n-ZnO-

NPs/p Si heterojunction and its electro-optical characterization, INSC 2011 4th to 5th

July, Seri Kembangan Selangor, Malaysia.

32. M. Kashif, Syed M. Usman Ali, U. Hashim, Magnus Willander, Structural and

electrical study of ZnO: Al nanorods, IPEC 2011, international Conference in Malaysia.

33. Syed M. Usman Ali, M. Kashif, Faraz Mahmood, Aamir H. Khan, Uda Hashim, Magnus

Willander, SMS based remote monitoring of glucose using ZnO nanotubes based

nanosensor, IPEC 2011, 22-23 October, international Conference in Malaysia.

34. Faraz Mahmood, Syed M Usman Ali, M. Kashif, U. Hashim, Magnus Karlsson and

Magnus Willander, Design of a Broadband Monopole Antenna for Handheld

Applications, IPEC 2011, 22-23 October, international Conference in Malaysia.

35. Syed M. Usman Ali, C. O .Chey, Z. H. Ibupoto, M. Kashif, U. Hashim, Magnus

Willander, Selective determination of cholesterol using functionalized ZnO nanotubes

based sensor, CLV-02, Vinh city, 11-15 October 2011, Cambodia,

36. K.L. Foo, M. Kashif, U. Hashim, Syed M. Usman Ali, M. Willander, Growth of ZnO

thin film on silicon substrate for optical application by using sol–gel spin coating

method, Accepted in ICOBE 2012, international Conference, Malaysia.

37. Faraz Mahmood, Syed M Usman Ali, C. O. Chey, H. Ing, Magnus Willander, Design of

a broadband monopole antenna for mobile handsets, CLV-02, Vinh city, 11-15

October 2011, Cambodia.

38. Faraz Mahmood, Syed M Usman Ali, Mahmood Alam and Magnus Willander, Design

of WLAN patch and UWB monopole antenna, IMTIC ’12, submitted to international

multi-topic conference, 28-30 March 2012, Jamshoro, Sindh, Pakistan

39. Syed M. Usman Ali, C. O. Chey, Z. H. Ibupoto, O. Nur, M. Willander, Fabrication and

characterization of hetro-junction light emitting diode based on n-ZnO nanoporous

structure grown on p-GaN, CLV-02, Vinh city, 11-15 October 2011, Cambodia.

40. C. O. Chey, Syed M. Usman Ali, Z. H. Ibupoto, C. Sann, Kimleang Khun, K. Meak, O.

Nur, M. Willander, Fabrication and characterization of light emitting diodes based on

n-ZnO nanotubes grown by a low temperature aqueous chemical method on p-GaN,

CLV-02, Vinh City, 11-15 October 2011, Cambodia.

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41. Syed M. Usman Ali, M. Kashif, Z. H Ibupoto, C. O. Chey, U. Hashim, Magnus

Willander, Sensing and optical characteristics of ZnO nanotubes fabricated through

two step aqueous chemical route, IPEC 2011, 22-23 October, international conference in

Malaysia.

42. M. Kashif, Syed M. Usman Ali, Z .H Ibupoto, Mojtaba Nasr-Esfahani, U. Hashim,

Magnus Willander, Growth of ZnO nanorods and effect of seed layer on

interdigitated electrode (IDE) impedance, submitted to Nanotech 2012, International

conference in Iran.

43. Syed M. Usman Ali, Z. H. Ibupoto, M. Kashif, Mojtaba Nasr-Esfahani, U. Hashim, M.

Willander, Synthesis and electro-optical characterization of n-ZnO nanoflakes/p-GaN

heterojunction light emitting diode, submitted to Nanotech 2012, International

conference in Iran.

44. Syed M. Usman Ali, M. Kashif, Z. H Ibupoto, Mojtaba Nasr-Esfahani, U. Hashim.

Magnus Willander, Optical and electrochemical sensing characterization of ZnO

nanoflakes, submitted to Nanotech 2012, International conference in Iran.

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Acknowledgments

All praise goes to ALLAH, who created the whole universe and selected human as the best among all creation. This is a memorable occasion in my life to finish the writing of my PhD thesis. I begin my acknowledgement while expressing my thanks to Almighty ALLAH who always blessed and granted me the capabilities to comprehend and learn the new inter-disciplinary field named “Nanotechnology” in the execution of this research work. In the course of completion of this PhD thesis, many people have directly or indirectly supported. That includes my family members, teachers, colleagues and all friends. At this moment, I am deeply indebted to all of them and my gratitude is beyond the words.

Firstly, I would like to express my heartiest gratitude to my supervisor Prof. Magnus Willander for his useful and inspiring guidance, and consistent encouragement without which this thesis would have not been materialized. I greatly appreciate his supervision during entire PhD studies.

I would like to thank my co-supervisor associate Prof. Omer Nour for his contribution, patience and guidance during my study and research work.

I would like to pay my sincere thanks to Prof. Bengt Danielsson for his magnanimous guidance and support to work successfully on ZnO based nanosensors and collaboration at Lund University, Sweden.

I would like to thank the ex-research administrator Lise-Lotte Lönndahl Ragnar and our present research administrator Ann-Christin Norén for their administrative help during my studies and research work.

I am also thankful to Prof. Igor Zozoulenko, Prof. Shaofang Gong, Dr. Qingxiang Zhao, Dr. Adriana Serban, Dr. Magnus Karlsson, Dr. Alim Fulati, Dr. Lili Yang, Dr. Ari, Dr Amir Baranzahi, Dr. Daniel Simon, Prof. Uda Hashim (Malaysia) and M. Kashif (Malaysia), Annelie Eveborn for their endless cooperation in my research works and studies.

Besides, Zafar Hussain Ibupoto, Chey Chen, Kimleang Khun, Naveed, Kamran, Gul, Faraz Mahmood, Mazhar, Dr. LiLi, Amal, Olga, Kristin Persson, Azam, Mushtaque, Yousaf, Zaka Ullah Sheikh, Owais Khan, Saima Zaman, Ahmed, Asif, Kishwer, Zia Ullah and all the other group members; thank you very much for the insightful collaboration, friendship, and help. My sincerest wishes and warmest thanks to all my group members and I will never forget sharing the difficult and happy moments during my stay here in Norrköping.

Words are lacking to express my heartiest gratitude to the authorities of the NED University of Engineering & Technology, Karachi Pakistan for nominating me for the PhD studies at Linköping University, Sweden. I would also like to thank for providing me the partial financial help for completing my PhD studies over here.

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For my family, Mom, Mom in law, all brothers and sisters and all in laws; my words cannot describe my immense feeling of appreciation for them. Mom, even though, I haven’t been there with you for all these years but I missed you a lot and always prayed for your good health. I know you always prayed for me and my success. Thanks for your prayers, encouragement and unforgettable sacrifices with patience throughout my life and PhD studies abroad.

Last but not least, my beloved wife, Soofia who did a great care of me and my sweet children Syeda Hiba Fatima, Syeda Aiman Fatima, Syeda Kinza Fatima and Syed Hussam Muhammad Ali. Words are hardly enough to express my gratitude to all of them and their endurance for my PhD studies. May Allah bless on all of us; Ameen. “I especially acknowledge the sacrifices of my wife Soofia who missed and did not attend the marriage ceremony of her beloved brother Meraj ul Haq, held in December 2009 and the funeral ceremony of her most beloved brother Ibtehaj ul Haq who died suddenly in heart failure in October 2010, due to my limited scholarship and stiff financial status. May ALLAH grant her Saber-e-Jameel (Patience) and bless the soul of her deceased brother with eternal peace (Ameen)”.

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CONTENTS

CHAPTER 1 ......................................................................................................................... 1

Introduction ................................................................................................................ 1

CHAPTER 2 ......................................................................................................................... 7

Material properties of ZnO ......................................................................................... 7

2.1 Semiconductor ZnO basic properties .......................................................... 7

2.2 Physical properties of ZnO .......................................................................... 9

2.3 Defects and emission properties of ZnO ................................................... 11

2.4 Electrical properties of ZnO ...................................................................... 15

2.5 ZnO nanostructures based electrochemical sensors .................................. 17

CHAPTER 3 ....................................................................................................................... 25

Fabrication of ZnO nanostructures and device processing ....................................... 25

3.1 Substrate preparation ................................................................................. 25 3.1.1 Substrate cleaning ........................................................................ 26 3.1.2 Fabrication of ZnO nanostructures .............................................. 27

3.2 Bottom contacts deposition ....................................................................... 33

3.3 Photoresist and plasma etching ................................................................. 34

3.4 Top contacts deposition ............................................................................. 35

CHAPTER 4 ....................................................................................................................... 37

Experimental and characterization techniques ......................................................... 37

4.1 Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) .................................................................................. 37

4.2 Atomic force microscope .......................................................................... 38

4.3 X-ray diffraction ........................................................................................ 39

4.4 Electrochemical measurements using ZnO nanostructure based sensors . 41

4.5 Photoluminescence .................................................................................... 43

4.6 Electroluminescence .................................................................................. 45

4.7 Electrical (current voltage I-V) characterizations ..................................... 45

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CHAPTER 5 ....................................................................................................................... 48

Results and discussions ............................................................................................ 48

5.1 Electrochemical nano-sensors ................................................................... 48 5.1.1 Potentiometric electrochemical glucose sensor (Paper I) ............ 48 5.1.2 Zinc oxide nanowires as extended gate MOSFET for

glucosedetection (Paper II) .......................................................... 51 5.1.3 An intracellular glucose sensor using the functionalised ZnO nanorods (Paper III) ..................................................................... 54 5.1.4 Wireless remote glucose monitoring system (paper IV) .............. 58 5.1.5 Selective determination of uric acid (Paper V) ............................ 62

5.2 Emission properties of nanostructures based photonic devices (Paper VI) .................................................................................................. 66

CHATPER 6 ....................................................................................................................... 73

Conclusion and outlook ..................................................................................................... 73

CHAPTER 7 ....................................................................................................................... 75

My contributions to included papers ................................................................................ 75

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List of figures

Figure 1: Scanning electron microscope (SEM) images of some ZnO nanostructures fabricated on different substrate using the aqueous chemical growth technique ................... 3

Figure 2.1: The hexagonal wurtzite structure of ZnO unit cell. The blue circle represents the zinc ions and brown circle represents the oxygen ions coordinated tetrahedrally ............................................................................................................................ 8

Figure 2.2: Showing the PL spectra of ZnO nanoflowers and EL spectra of ZnO nanorods based light emitting diodes (LED) at room temperature [1] ................................ 11

Figure 2.3: The current voltage (I-V) characterization of different ZnO (nanostructures)/p-GaN LEDs [1] ........................................................................................ 16

Fig. 3.1: Schematic diagram showing the different steps of the device (LED) fabrication ............................................................................................................................. 26

Figure 3.2: SEM image of ZnO nanorods fabricated on p-type GaN substrate using low temperature aqueous chemical growth technique ......................................................... 27

Figure 3.3 (a-d): SEM images for ZnO nanorods/nanowires fabricated under different growth parameters ................................................................................................................ 29

Figure 3.4: SEM image of ZnO nanotubes fabricated on the p-type GaN substrate .......... 30

Figure 3.5: SEM image of ZnO nanowalls on p-type GaN substrate ................................. 31

Figure 3.6: SEM image of ZnO nanoflowers fabricated on p-type GaN substrate ............. 32

Figure 3.7: SEM image of ZnO nanowires fabricated through sol gel method on p-type GaN substrate ............................................................................................................... 33

Figure 4.1: EDX spectrum of ZnO nanowires on a gold coated plastic substrate .............. 38

Figure 4.2: AFM (10µm x10µm) image of ZnO nanowalls fabricated on p-type GaN substrate ................................................................................................................................ 39

Figure 4.3: A schematic diagram of Bragg reflection from crystalline lattice planes having interplan distance “d” between two lattice plane ..................................................... 40

Figure 4.4: Display the Ө-2Ө XRD spectra of ZnO (a) nanowalls, (b) nanorods, (c) nanoflowers, and (d) nanotubes grown on p-GaN substrates, respectively ......................... 41

Figure 4.5: Schematic diagram of potentiometric measuring setup .................................... 42

Figure 4.6: A schematic diagram illustrating the selective intracellular glucose sensor .... 43

Figure 4.7: Schematic diagram of photoluminescence (PL) setup ..................................... 44

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Fig. 5.1 (a): Calibration curve showing the time response of the sensor electrode in 50 µM glucose solution (b) Calibration curve showing electrochemical response (EMF) vs. logarithmic glucose concentrations using ZnO sensor electrode and Ag/AgCl reference electrode [1] .......................................................................................................... 50 Fig. 5.2: Schematic diagram illustrating the configuration used for glucose detection with MOSFET using extended-gate functionalized ZnO nanowires as working electrode and Ag/AgCl as a reference electrode [6] ............................................................ 52

Figure 5.3 (a): Typical drain current (ID ) versus gate voltage (VG) for the extended-gate MOSFET, the upper curve (line) is for 50 μM glucose solution while the lower curve (dotted line) is for the case of 100 μM of glucose concentration. (b) Relation between the drain current and glucose concentration for a range of 1–100 μM glucose concentration [6] .................................................................................................................. 54

Figure 5.4: Scanning electron microscopy (SEM) images of the ZnO nanorods fabricated on Ag-coated glass capillaries using ACG method :( a and b) before enzyme immobilisation and (c) after enzyme immobilisation [7] ..................................................... 55

Figure 5.5: A calibration curve showing the electrochemical potential difference versus the glucose concentration (0.5–1mM) using functionalised ZnO-nanorod-coated probe as a working electrode and an Ag/AgCl microelectrode reference microelectrode [7] ......................................................................................................................................... 56

Figure 5.6: (a) Intracellular mechanism for insulin-induced activation of glucose uptake. (b) Output response (EMF) with respect to time for intracellularly positioned electrodes when insulin is applied to the extracellular solution [7] .................................... 58

Figure 5.7: The proposed system block diagram of wireless remote monitoring system for the functionalized ZnO nanowire arrays based glucose sensor [12] .................. 59

Figure 5.8: (a) Calibration curve of the sensor electrode showing the stable and smooth signal in 50 µM glucose solution (b) inset curve showing the time response of the sensor [12] ...................................................................................................................... 61

Figure 5.9: The proposed system circuit diagram of the designed prototype circuit board [12] ............................................................................................................................. 61

Figure 5.10: (a) Calibration curves for the uric acids sensor with membrane [14] ............ 62

Figure 5.11: (a) Time response of the sensor in 100 µM test solution of uric acid without membrane coating [14] ........................................................................................... 63

Figure 5.11: (b) Time response of the sensor in 100 µM test solution of uric acid with membrane coating [14] ......................................................................................................... 64

xxi

Figure 5.12: (a) Calibration curves from three different experiments using the same sensor electrode and Ag/AgCl reference electrode [14] ..................................................... 65 Figure 5.12: (b) The sensor to sensor reproducibility of five (n = 5) ZnO nanowires/uricase/ Nafion® electrodes in 100 µM test solution of uric acid [14] .............. 65 Figure 5.13: (a-e) Showing the room temperature PL spectrum for ZnO nanostructures (a) nanowalls (b) nanorods (c) nanoflowers and (d) nanotubes on p-GaN and (e) showed the combined PL spectra of all the four nanostructures [15] ............. 67 Figure 5.14: (a-e) showing the EL spectra for n-ZnO (nanostructures)/p-GaN LEDs, in (a) nanowalls, (b) nanoflowers (c) nanorods, and (d) nanotubes, (e) showing the combined EL spectra of all the nanostructures, and (f) shows the CIE 1931 x, y chromaticity space of ZnO nanostructures based LEDs [15] ............................................... 69 Figure 5.15: Showing the combined current-voltage (I-V) characterizations for the different ZnO (nanostructures)/p-GaN based LEDs [15] ..................................................... 70

xxii

List of tables

Table 2.1: Basic physical parameters of ZnO at room temperature [17-20] ....................... 10

Table 4.1: Different alloys combination (metallization) for the ohmic contacts for p-type GaN .............................................................................................................................. 34

Table 4.2: Different ohmic contacts combination enlisted for n-type ZnO ........................ 35

1

CHAPTER 1

Introduction

Today, semiconductor devices have become an integral and indispensible part of our daily life

and we could not think to live without them. The current technological advances in the

semiconductor devices based on different semiconducting materials is the backbone of the

modern electronics industry including high tech. laptops, TV, cellular phones (iPhones) and

many other devices. Currently, the semiconductor silicon (Si) keeps the dominant position in

the modern electronic industry, which is used to fabricate the discrete and very large scale

integrated circuits (VLSI) for different application such as computing, data storage and

telecommunications etc. Moreover, the modern industrial trend is to miniaturize the electronic

devices and increase their efficiency. The process of miniaturization was well defined by

Gordon E. Moore in his famous ‘‘Moore’s law’’ which describe that the number of transistors

on a chip doubles every second year [1]. However, as the size of the devices continues to

reduce but the process of miniaturization will eventually have reached to the point where

existing Si devices could not follow the ‘‘Moore’s law’’ anymore and where quantum

mechanical effect dominates and becomes a reality that is indispensable in device design. In

addition, Si is not a promising candidate for optoelectronic devices due to its indirect band

gap such as white light emitting diodes (LEDs) and laser diodes. To overcome this problem

GaAs with direct band gap was chosen but due to the rapid development of information

technologies, the requirement of ultraviolet (UV) / blue light emitter applications has become

vastly increased which is beyond the limits of GaAs. Therefore, scientists have attracted

towards the other wide bandgap semiconductors such as SiC, GaN and ZnO, i.e. the third

generation semiconductors, due to their especial features in the field of semiconductor.

2

Nanotechnology has an inter-disciplinary nature which emerged from the efforts made

between sciences and engineering by applying the bottom-up or top-down methodologies. In

the nanotechnology, Low-dimensional structures possess novel physical and chemical

properties, and hence they are of basic building blocks with today’s technology.

Nanostructures such as one dimensional, two-dimensional or even zero-dimensional can be

reproducibly fabricated on different substrates and explored for different applications to

fabricate the “nanodevices”. Among these low-dimensional structures, nanowires, nanotubes,

nanoflakes and etc., have become the promising candidates for the researchers in science and

engineering due to their unique and interesting properties for the device application at

nanoscale. In past decade, nanorods based on different materials have been successfully

synthesised such as Si, GaN, SnO and ZnO and reported in literature [2-5]. Among the

diverse materials, ZnO is one of the most exciting contenders for the fabrication of different

nanostructures and probably has the richest variety of different nanostructures and few are

shown in figure 1. Due to the various advances in the fabrication of nanoscale materials and

their characterization tools have triggered the research activities in this area. As a result,

theses nanoscale materials may find a wide range of applications in optoelectronic devices,

sensors/transducers, nano-sensors (chemical/biosensing), nano-laser, nano-electromechanical

systems (NEMS), nano-electronics, and nano-cantilevers etc. Moreover, there is a potential in

employing such nanostructures as “wireless” devices with self-powering capability, in some

applications, such as an electrochemical potentiometric nanosensors, and devices based on

piezoelectric effect etc. However, the challenge is the conversion of the property in focus to

electrical signal. When this is achieved, different nano-integrated systems can be made

available very easily.

3

Figure 1: Scanning electron microscope (SEM) images of some ZnO nanostructures fabricated on different substrate using the aqueous chemical growth technique.

Zinc oxide (ZnO) is II-VI compound semiconductor material in periodic table and it has been

under intensive focused among the researchers because of its special properties such as high

electron mobility with undoped state, high thermal conductivity, good transparency, wide

band gap (~3.37 eV), large exciton binding energy (60 meV) which is much larger than that

of GaN (21 meV) and even room temperature thermal excited energy (25 meV). Moreover, a

simple process for fabricating its different nanostructure by adopting the various techniques to

make ZnO nanostructures suitable for optoelectronics and in light emitting diodes [6],

chemical sensors [7], hydrogen storage [8] etc. The ZnO possess unique physical properties

4

and can be fabricated into different morphologies including one dimensional (1D)

nanorods/nanowires, nanotubes, nano-belt, and nano-needles [9-12] and two dimensional

(2D) ZnO nanostructures, such as nanosheets, nanoplates, nanowalls, and nanoporous [13-16]

etc., have high surface to volume ratios and making them useful for a variety of applications

such as catalysts, nano-sieve filters, gas sensors [17] and etc. The use of nanomaterials has

allowed the introduction of many new signal transduction technologies in sensors/transducers

resulting in improved sensitivity and performance. Moreover, due to the unique properties of

nanostructures/nanomaterials in the electrochemical sensing area, nanosensors offer some

significant advantages owing to their small size and high surface area to volume ratios

allowing larger signals, better catalysis and the more rapid movement of analytes through

sensors. In general, nanostructures such as ZnO nanowires, nanotubes and nonporous are

attractive for their versatile roles in bioelectronics and nanoelectronics applications and they

are increasingly being used as main building blocks for electrochemical sensing designs. In

addition, it has been reported that ZnO possess the conducive properties like excellent

biological compatibility, non-toxicity, bio-safety, high-electron transfer rates, enhanced

analytical performance, increased sensitivity, easy fabrication and low cost [18-19].

Moreover, ZnO has a high isoelectric point (IEP) of about 9.5, which should provide a

positively charged substrate for immobilization of low IEP proteins or enzyme such as

glucose oxidase (IEP ≈ 4.5) and etc. In addition, ZnO has high ionic bonding (60%), and it is

dissolve very slowly at biological pH values. The proposed p-n heterojunction LEDs

possessing a promising future as a white light source for the future low power consumption

lightening applications because they emits light covering the whole visible spectrum without

applying any conversion methodologies. The through studies for the optical properties of p-n

heterojunction like (n-ZnO/p-GaN) LEDs are still under investigations. The ZnO nanorods

and nanotubes based p-n heterojunction (n-ZnO/p-GaN) LEDs are highly attractive due to

5

their potential to enhance the light extraction [20] as compared to its counterpart ZnO

nanostructures/p-GaN based thin films LEDs. The first objective of the present thesis is to

describe the electrochemical sensing application of ZnO nanostructures and make them

suitable and convenient for wireless sensing/remote monitoring systems applications. Second,

different n-ZnO nanostructures were fabricated by using low cost aqueous chemical growth

(ACG) technique on p-type GaN substrates to form a white light emitting LEDs. The colour

qualities of emitted spectra and luminescence properties of the fabricated LEDs were also

studied.

The present thesis has been devised in the following sequence; Chapter 1 Introduction,

Chapter 2 describes some of the basic properties of ZnO related to this thesis. Chapter 3

describes the fabrication of ZnO nanostructures and device processing used in current studies,

Chapter 4 presents the characterization tools applied for the experiments in the present

investigations, Chapter 5 presents the results and discussion and finally, the thesis is

concluded in Chapter 6.

6

References:

[1] G. E. Moore, Electronics, 1965, 38, 33.

[2] P. Kim, C.M. Lieber, Science 1999, 286, 2148.

[3] Z.R. Dai, J.L. Gole, J.D. Stout, Z.L. Wang, J. Phys. Chem. B. 2002, 106,1274.

[4] S. Gradecak, F. Qian, Y. Li, H. Park, C.M. Lieber, Appl. Phys. Lett. 2005, 87, 173111.

[5] Z. L. Wang, J. Song, Science 2005, 312, 242.

[6] N. H. Alvi, S. M. Usman Ali, S. Hussain, O. Nur, and M. Willander, Scripta Materialia.

2011, 64, 697.

[7] A. Umar, M. M. Rahman, S. H. Kim, and Y.-B. Hahn, Chem.Commun. 2008, 166.

[8] Q. Wan, C.L. Lin, X.B. Yu, and T.H. Wang, Apply. Phys. Lett. 2004, 84, 124.

[9] A. Manekkathodi, M. Y. Lu, C. W. Wang, and L. J. Chen, Adv. Mater. 2010, 22, 4059.

[10] Y. Xi, J. Song, S. Xu, R. Yang, Z. Gao, C. Hu, and Z. L. Wang, J. Mater. Chem. 2009,

19, 9260.

[11] B. Q. Cao, Z. M. Liu, H. Y. Xu, H. B. Gong, D. Nakamura, K. Sakai, M. Higashihata,

and T. Okada, Cryst. Eng. Commun. 13. 2011, 4282.

[12] S. Cho and K. H. Lee, Cryst. Growth Des. 2009, 10, 1289.

[13] N. Wang, L. Jiang, H. Peng, and G. Li, Cryst. Res. And Technol. 2009, 44, 34.

[14] J. P. Cheng, Z. M. Liao, D. Shi, F. Liu, and X. B. Zhang, J. Alloys Compd.2009, 480,

741.

[15] M. Mäder, J. W. Gerlach, T. Höche, C. Czekalla, M. Lorenz, M. Grundmann, and B.

Rauschenbach, phys. status solidi RRL. 2008, 2, 200.

[16] M. Kashif, S. M. U. Ali, K. L. Foo, U. Hashim, and M. Willander, AIP Conference

Proceedings. 2010, 1341, 92.

[17] J.F. Chang, H.H. Kuo, I.C. Leu, and M.H. Hon, Sens. Actuators B. 2002, 84, 258.

[18] P. D. Batista, and M. Mulato, Appl. Phys. Lett. 2005, 87, 143508.

[19] B. S. Kang, F. Ren, Y. W. Heo, L. C. Tien, D. P. Norton, and S. J. Pearton, Appl. Phys.

Lett. 2005, 86, 112105.

[20] A. M. C. Ng, Y. Y. Xi, Y. F. Hsu, A. B. Djurisic, W. K. Chan, S. G. wo, H. L. Tam, K.

W. Cheah, P. W. K. Fong, H. F. Lui, and C. Surya, Nanotechnology. 2009, 20, 445201.

7

CHAPTER 2

Material properties of ZnO

During the last decade, new nanomaterials/nanostructures based device structures have

attracted a great attention because of their fascinating properties and potential as building

blocks for electronics, optoelectronics, and sensor applications. These properties make the

ZnO a promising material for the fabrication of the nanodevices such as light emitting diodes

[1-2], electrochemical sensors [3-4], ultra-violet (UV) detectors [5-6], nanogenerators [7] and

etc. Currently, zinc oxide is the most studied material among metal oxides due to its broad

application list related to its semiconducting, optical and piezoelectric properties and etc.,

respectively. For instance, ZnO-based devices can be used in optoelectronics,

sensors/transducers and lasers etc. Here some of the properties of ZnO are highlighted:

2.1 Basic properties of ZnO

2.2 Physical properties of ZnO

2.3 Optical properties of ZnO

2.4 Electrical properties of ZnO

2.5 Electrochemical sensing aspect of ZnO.

2.1 Semiconductor ZnO basic properties

ZnO normally forms in the hexagonal (wurtzite) crystal structure as illustrated in figure 2.1, it

has the lattice parameter a = 3.25 Å and c = 5.12 Å. The large difference in the values of

electronegativity (Oxygen = 3.44 and Zinc = 1.65) responsible for the strong ionic bonding

between them. In the wurtzite structures, the zinc (Zn) atoms are tetrahedrally co-ordinated to

four oxygen (O) atoms stacked alternately along the c-axis. Generally, ZnO unit cell is neutral

in which an oxygen anion is encircled by four zinc cations at the corner of a tetrahedron, and

8

vice versa. The distribution of the cations and anions could take specific configuration as

determined by crystallography technique, so that some surfaces can be terminated entirely

with cations or anions, resulting in positively or negatively charged surfaces, called polar

surfaces. These polar surfaces of the ZnO have untransferable and unchangeable ionic charges

and their interaction at the surface depends on their distribution. Thus, in results the structures

have been shaped with a minimal electrostatic energy which is responsible for the fabrication

of polar surface dominated nanostructures. This phenomenal effect results for the fabrication

of different ZnO one-dimensional (1D) nanostructure such as nanowires, nanorods,

nanotubes, nanospring, nanocages, nanobelts and etc., [8-9].

Figure 2.1: The hexagonal wurtzite structure of ZnO unit cell. The blue circle represents the zinc ions and brown circle represents the oxygen ions coordinated tetrahedrally.

Generally, wurtzite structure of ZnO comprises on four common surfaces, two of them are

polar i.e., Zn (0001) and O (000 1) which have terminated faces along the c axis and two are

non-polar (11 20) and (10 10) faces and these nonpolar surfaces possess equal number of zinc

(Zn) and oxygen (O) atoms. In contrast, the polar surfaces are responsible for the different

9

chemical and physical properties of ZnO. The most common polar surface is the basal plane.

The presence of polarized charged ions, different surfaces like positively charged Zn-(0001)

and negatively charged O-(000 1) polar surfaces are produced, resulting in a normal dipole

moment and spontaneous polarization along the c-axis as well as a divergence in surface

energy. To maintain a stable structure, the polar surfaces generally have facets or exhibit

massive surface reconstructions, but ZnO ± (0001) are exception, which are atomically flat,

stable and without reconstruction [10-11]. Understanding the superior stability of the ZnO ±

(0001) polar surfaces is a forefront research in today’s surface physics [12-14]. In addition to

the wurtzite structure, ZnO can be transformed to the rocksalt (NaCl) structures at relatively

modest external hydrostatic pressures. In ZnO, the pressure-induced phase transition from the

wurtzite (B4) to the rock salt (B1) phase occurs at approximately 10 GPa [15]. Thus, the

several properties of ZnO nanostructured materials depend on its polarity, growth, etching,

defect generation and plasticity, spontaneous polarization and piezoelectricity. ZnO is a

versatile wideband semiconductor as compared to its contenders like GaN in properties and

applications. In fact, ZnO have several advantages as compared to the existing devices

fabricated from other wideband semiconductors in which the most important property of ZnO

is its high exciton binding energy of ZnO i.e. 60 meV at room temperature compared to its

counterpart GaN (25 meV). This high exciton binging energy is responsible to enhance the

efficiency of light emission. Several reviews on ZnO bulk, thin film, and one-dimensional

materials have been reported in the literature. A comprehensive review on various aspects of

ZnO bulk material, thin films, and nanostructures is reported [16].

2.2 Physical properties of ZnO

There are few basic physical parameters for the ZnO at the room temperature which is listed

in table 2.1 [17-20]. There is still some uncertainty in the values of the thermal conductivity

10

due to the presence of some crystal defects in the material [21]. In addition, a stable and

reproducible p-type doping in ZnO is still a challenge and cannot be achieved. The findings

regarding the values related to the mobility of hole and its effective mass are still arguable.

The values of the carrier mobility can surely be enhanced after achieving good control on the

defects in the material [22].

Table 2.1: Basic physical parameters of ZnO at room temperature [17-20].

S.No Parameters Values

1 Lattice constants at 300 K a = 0.32495 nm, c = 0.52069 nm

2 Density 5.67526 g/cm3

3 Molecular mass 81.389 g/mol

4 Melting point 2250 K

5 Electron effective mass 0.28 m0

6 Hole effective mass 0.59 m0

7 Static dielectric constant 8.656

8 Refractive index 2.008, 2.029

9 Bandgap energy at 300 K 3.37 eV

10 Exciton binding energy 60 meV

11 Thermal conductivity 0.6 – 1.16 W/Km

12 Specific heat 0.125 cal/g°C

13 Thermal constant at 573 1200 mV/K

14 Electron mobility ∼210 cm2/Vs

11

2.3 Defects and emission properties of ZnO

The semiconductor materials electro-optical properties are mainly dependent on the intrinsic

and the extrinsic defects which are present in the crystal structures. Recently, the optical

properties of ZnO, particularly ZnO nanostructures, have been a main focused among the

researchers due to its wide band-gap (~3.37 eV at room temperature), which makes ZnO a

promising material for photonic applications in the UV or blue spectral range, while the high

exciton-binding energy (60 meV), which is much larger than that of GaN (25 meV), allows

efficient excitonic emission even at room temperature. The efficient radiative recombinations

have made ZnO very attractive in optoelectronics applications. There are various techniques

through which the optical/ luminescence properties of ZnO (both nanostructures and bulk)

have been thoroughly investigated at low and room temperatures. The spectra obtained from

photoluminescence (PL) measurements of ZnO nanoflowers and spectra from

electroluminescence (EL) of ZnO nanorods based heterojunction LED at room temperature

are shown in figure 2.3 (a-b).

Figure 2.2(a-b): Showing the PL spectra of ZnO nanoflowers and EL spectra of ZnO nanorods based light emitting diodes (LED) at room temperature [1].

12

In the PL spectra, the ultra-violet (UV) emission band and a broad visible emission band were

observed. The UV peak generally observed due to the phenomena of transition

recombinations of free excitons (F.E) in the near band-edge of ZnO. The excitons may have

activities like they can be free and able to move through the crystal or they can be bound to

donors and accepters with neutral or charged states [1]. The broad visible region (420 nm -

750 nm) as shown in the above figure 2.3 (a) is attributed due to the presence of deep level

defects in ZnO. The optical and electrical properties of ZnO can be altered due to the changes

of these deep level defects in the crystal structure of ZnO. These defects can be introduced

during the fabrication process or by applying other techniques like the post annealing or ion

implantation. The optical properties of the ZnO associated with the extrinsic and intrinsic

defects and are still under moot since 1960. Especially, the origin of intrinsic emission from

ZnO is still arguable due to the presence of native point defects in the structure. The ZnO

structure possess the donor and accepter energy levels and these are present at below and

above the conduction band (CB) and valance band (VB) respectively and responsible for the

near-band edge emissions. Moreover, the emission of whole visible region (400-750 nm) is

due to the presence of different deep energy levels within the bandgap and the origin of these

defects are still under moot and several research groups have reported different origins for

these deep level defects as described in references [8, 18, 23-39]. The defects can be

categorized into three types, like the line defects, point defects and complex defects which are

present in the crystal structure. The line defects occurred due to the disruptions into the rows

of atoms, whereas the point defects are generated due to the isolated atoms in localized

regions and complex defects were formed when more than one point defects have merged.

The extrinsic point defects are generated if impurities/foreign atoms were incorporated in the

structure, while for intrinsic defects comprises only on the host atoms. The intrinsic optical

recombinations occurred between the electrons and holes present in the CB and VB

13

respectively [18]. In addition, the deep level emission (DLE) band or whole visible

range(400-750 nm) in ZnO has been previously attributed due to the presence of various

intrinsic defects in the structure like the oxygen vacancies (VO) [40-44], oxygen interstitial

(Oi) [29-32], zinc vacancies (VZn) [33-36], zinc interstitial (Zni) [37-38] and oxygen anti-site

(OZn) and zinc anti-site (ZnO) [39]. However the extrinsic defects such as permutation of Cu

and Li [31, 45] are also suggested to be involved in deep level emissions. ZnO crystal

structures also possess two types of intrinsic vacancy defects recognized as oxygen vacancy

(Vo) and zinc vacancy (Vzn). The green emission from ZnO is due to the presence of single

ionized oxygen vacancies. However, in case of zinc rich growth, the oxygen vacancy has

lower formation energy than the zinc interstitial and dominates, whereas doubly ionized

oxygen vacancies are responsible for the red emission from ZnO [46]. The origin of the green

emission in ZnO is still arguable and several hypotheses have been reported for this emission

[23, 47-54]. Zinc vacancies were soundly studied and reported in the literature by some

groups to be the source of the green emission positioned at 2.4-2.6 eV below the CB in ZnO

[55-56]. Some researchers have also reported that oxygen vacancies are responsible for the

green emission as well in ZnO as described in [57-58, 53]. In addition, it has also been

reported that the oxygen interstitials and extrinsic deep levels defects such as Cu are sources

of the green emission in ZnO [59]. Recently, it has been reported that many deep level defects

are responsible for the green emission in ZnO along the VO and Vzn both contribute to the

green emission [46, 59-60]. The zinc vacancies are also considered to be the main source of

blue emission in ZnO. The recombination process between the zinc interstitial (Zni) energy

level to Vzn energy level is also responsible for the blue emission and this corresponds to ~

2.84 eV(436 nm). These phenomena can be described by utilizing the full potential linear

muffin-tin orbital method, which define the position of the Vzn level that is placed at ~ 3.06

eV below the CB, while the position of the Zni level is theoretically calculated at a position at

14

~ 0.22 eV below the CB [61]. In the structure of ZnO, oxygen interstitial (Oi) and zinc

interstitial (Zni) are the two common defects exists and intrinsic in nature. Typically zinc

interstitial defects are positioned at ~ 0.22 eV below the CB and play important part for the

visible emissions in ZnO due to the recombination process among Zni and different defects

that exists in the deep levels like oxygen and zinc vacancies, oxygen interstitials which are the

main source for the different colour emission such as blue, red and green emissions in ZnO

[61]. Oxygen interstitials defects are normally positioned at 2.28 eV below the CB and

generate the orange-red emissions in ZnO [86-98]. The oxygen interstitials defects are also

responsible for yellow emission as reported in literature [35, 64]. Some research groups have

also reported recently that by adding Oi and Li impurities in the growth material using ACG

method were also responsible for yellow emission [31]. The presence of Zn (OH)2 that is

attached to the surface of the nanorods during the chemically growth process is also

responsible for yellow emission. There are some defects known as the anti-site defects which

are generated in the ZnO structure due to the occupying of wrong lattice position, for example

it happened when the zinc fills the oxygen position or oxygen fills zinc position in the lattice.

Such types of defects can be merged into ZnO by applying the ion implantation or irradiation

processing. Few other types of defects like the cluster defects also exist in ZnO that are

occurred by merging of more than one point defect. Such cluster defects can also be generated

when the point and extrinsic defects such as VO Zni cluster, and this cluster is formed due to

the merging of oxygen vacancy and zinc interstitial and it has been reported that it has a

positioned at 2.16 eV below the CB [39].

Finally some brief discussion about the extrinsic defects, because they also play an important

role for the emission properties of ZnO. It has been reported that the UV emissions in ZnO is

positioned at 3.35 eV due to the excitons bound to the extrinsic defects like Li and Na

accepters present in ZnO structures [18]. The emission due to incorporation of copper

15

impurities in ZnO is placed at 2.85 eV [65]. Similarly, after the doping of Li in ZnO thin film

a yellow emission was observed at positioned 2.2 eV below the CB [66-67] with the Li related

defects. There are some more extrinsic defects associated with Cu, Li, Fe, Mn, and OH which

are also responsible for luminescence from ZnO. It has been also reported that the defects

having different energies can also produce the same colour emission such as the combinations

of ZnO: Cu and ZnO: Co have different energies but they emit the same (green) colour [68].

At the end, defects related to hydrogen, because it has an interesting role in the emission from

ZnO. Although, defects related to hydrogen are not deep level and located at 0.03 to 0.05 eV

below the CB [69]. The emission spectra obtained from ZnO indicated that it has a great

potential to emit luminescence covering the whole visible region and it has a promising

future to be used as low power consumption white light emitting source.

2.4 Electrical properties of ZnO

To comprehend completely the electrical behaviour of ZnO nanostructures prior to utilize

them in fabrication of nanodevices/nanoelectronics is very important. Inherently, the undoped

ZnO nanostructures is n-type in nature and it has been reported in the literature that it is due to

the presence of native defects in its crystal structure like oxygen vacancies and zinc

interstitials [70]. The numerical values of the electron mobility in a ZnO nanostructures in an

undoped state are estimated 120 to 440 cm2 V/s at room temperature and arguable which also

depend on the fabrication methods [18]. It has also been reported that after doping of ZnO, the

highest carrier concentration for holes and electrons i.e. 1019 cm-3 and 1020 cm-3 respectively

were obtained [71]. However, these levels of p-conductivity were uncontrollable and not

reproducible. It has also been observed that after the doping, the carrier mobility has reduced

as compared to the undoped ZnO due to the carrier scattering mechanism which includes

ionized impurity, non-ionized impurity, polar optical-phonon and acoustic phonon scatterings

16

[18]. The mobility of electrons and holes were estimated 200 cm2 V/s and 5 to 50 cm2 V/s at

room temperature. Similarly, the effective mass of electron and holes were estimated to 0.24

m0 and 0.59 m0 respectively. Thus, holes have very less mobility as compared to the electrons

mobility due the large differences in their effective mass [72].We have demonstrated the

fabrication of ZnO nanostructures based p-n heterojunction LEDs [1]. The electrical

parameter such as current voltage (I-V) curves of these fabricated LEDs are shown in figure

2.4 All the LEDs exhibited good rectifying behaviour as expected. Reasonable p-n hetero-

junctions are achieved and the threshold voltages of these LEDs were around 4 V. It has been

observed that the ZnO nanotubes based LED exhibits higher current when comparing other

nanostructures based LEDs with same sets of operating conditions and it may be due to the

more oxygen sub-vacancies and large surface area of the nanotubes as compared to other

nanostructures.

Figure 2.3: A typical I-V characteristic for different ZnO (nanostructures)/p-GaN LEDs [1].

17

2.5 ZnO nanostructures based electrochemical sensors

The rapid advancements in development of a miniaturized nanodevices based on

semiconductor nanomaterials have attracted a significant interest among the researchers due

to the special physical properties of these materials at low dimensions [73-77]. Determination

of biological or biochemical/chemical processes is of utmost importance for medical,

environmental and biotechnological applications. However, converting the biological signal to

an easily processed electronic signal is challenging due to the complexity of connecting an

electronic device directly to a biological environment. Electrochemical biosensors provide an

attractive means to analyze the content of a biological sample due to the direct conversion of a

biological event to an electric signal. Over the past decades, several sensing concepts and

related devices have been developed. The area of biosensors started to be active with the

introduction of the first generation of glucose oxidase (GOD) biosensors in 1962 [78]. This

GOD sensor concept is still the most widely used, although many improvements (generations)

have been added since 1960’s [79].

Recently, electrochemical sensing based on various nanomaterials with a wide variety of low

dimensional nanostructures has attracted considerable attentions due to their special physical

properties. Among these materials, ZnO has attracted great interests in the applications of

sensors/transducers because it has a wide variety of nanostructures such as

nanowires/nanorods, nanotubes, nanoporous/nanoflakes and etc., and their remarkable

properties such as large surface-to-volume ratio, biosafety, bio-compatibility, nontoxicity,

high-electron transfer rates, enhanced analytical performance, increased sensitivity, easy

fabrication, and low cost. In addition, a high isoelectric point of ZnO (IEP 9.5) provides

convenient micro-environment to form a good matrix with low isoelectric point acidic

proteins or DNA for immobilization by electrostatic interactions with high binding stability

[80-86]. Moreover, ZnO possess high ionic bonding (60%), and its dissolution is slow at

18

biological pH values. Moreover, Z. Li et al. [87] reported that ZnO nanorods are bio-

compatible and bio-safe when they are used in biological environment at normal

concentration range. In addition, ZnO is relatively stable around biological pH-values which

make ZnO compatible with biological fluids and species [88] which also makes it attractive in

vivo environment. Currently, we have successfully demonstrated that ZnO

nanorods/nanowires can be used to measure the intracellular glucose and K+ concentrations

using micro injection technique in human adipocytes and frog oocytes [89, 4]. The main effort

has been focused to fabricate the ZnO nanorods/nanowires selectively on the borosilicate

glass capillary tips (0.7 µm outer diameters), suitable and capable to gently penetrating the

cell membrane and immobilized with glucose oxidase (GOD) and coating of ionophore

(Valinomycin) for the selective determination of glucose and K+ ions concentrations

respectively. Thus, the ZnO nanostructures are suitable for extra and intracellular sensing

applications.

19

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25

CHAPTER 3

Fabrication of ZnO nanostructures and device processing Currently, the on-going miniaturization of electronic devices/circuits led to an emerging

interest in nano-scaled materials. In addition, inorganic structures confined in several

dimensions within the nanometre range, exhibit peculiar and unique properties superior to

their bulk counterparts. In recent years, various semiconducting nanomaterials like quantum

dots (0-dimensional) and one-dimensional (1D) structures, like nanowires, nanobelts,

nanotubes and nanowalls etc., have attained enormous attention. Among the huge variety of

1D nanostructure, semiconducting nanowires have gained particular interest due to their

potential applications in optoelectronic, sensing and electronic devices. Among these

materials, ZnO has a richest family of nanostructures including the carbon nanotubes [1].

There are several techniques to synthesize and fabricate the ZnO nanostructures on different

substrates including aqueous chemical growth (ACG) method [2], vapor-liquid-solid (VLS)

method [3-10], metal organic chemical vapor deposition (MOCVD) [11] and

electrodeposition (ED) [12,13] etc. In the present research work, ZnO nanostructures were

synthesized using low cost and low temperature aqueous chemical growth method on silver

wires (250 µm in diameter), gold coated plastic substrate and p-type GaN. The fabrication of

ZnO nanowires/nanorods, nanotubes and nanowalls based devices on different substrates

followed several steps such as described below;

3.1 Substrate preparation

ZnO nanowires/ nanorods, nanotubes, and nanowalls used in the present work were grown on

silver, gold, and p-type GaN substrates. Before the fabrication of the ZnO nanostructures, the

substrates were cleaned properly for the purpose of eliminating unwanted dirty particles and

chemicals on the surface of substrates.

26

3.1.1 Substrate cleaning

Pre-cleaning treatment of substrates is very crucial for fabricating the high quality and

vertically aligned nanowires/nanorods, nanotubes and nanowalls because the unwanted

chemicals, unwanted particles on the surface of the substrates and oxide layers from the

surface can generate the instability and unpredictability in the development processing.

Generally, the cleaning process begins with the steps like; first the substrates were immersed

in the solution prepared in de-ionized water (DI-H2O) with hydrofluoric acid (HF) in a

proportion of nine: one (9:1) for three minutes in order to remove the native oxide layers from

the surface of the substrates(especially in case of Si substrate). Then in second step, we

immersed the substrates in the acetone for sonication bath for 5 minutes at 40 oC and this

sonication process was repeated in isopropanol and ethanol as well. During the sonication

process, the substrates were cleaned with de-ionized. After the sonication, the substrates were

cleaned properly and dried with nitrogen.

In the present work we followed the ACG method, first we make sure to remove the possible

organic contaminants gathered in oxide layer by using etching method in diluted hydrofluoric

acid (HF) if we choose to grown ZnO nanostructures on Si substrate. Then the organic

contaminations on the substrates are removed by sonication of the substrates in the solution of

acetone at 40 °C for 5 minutes and then in isopropanol at 40 °C for 5 minutes. Between the

sonication processes the substrates are cleaned with DI water. Thereafter, the substrates are

ready for the ACG growth.

Figure 3.1: Schematic diagram showing the different steps of the device (LED) fabrication.

27

3.1.2 Fabrication of ZnO nanostructures

ZnO nanowires and nanorods

The aqueous chemical growth (ACG) method is the low temperature (<100 oC), simplest and

cost effective for the fabrication of ZnO nanowires and nanorods as described by Vayssieres

et al [2]. In this technique, an equimolar concentration (0.025-0.01 M) of hexa-methylene-

tetramine (HMT, C6H12N4) with zinc nitrate (Zn (NO3)2.6H2O) was mixed in a deionized

water for fabrication of ZnO nanowires and nanorods. Before the substrates were placed into

the solution, a small part of the gold coated plastic was covered in order to use it as a contact

pad for electrochemical measurements and then a nucleation layer was generated on the plane

substrates by spin coating technique for the purpose of improving the alignment and

orientation of ZnO nanowires and nanorods (for p-type GaN and gold coated plastic). This

nucleation layer is prepared by the procedure describe in Ref. [14]. This seed layer results in

excellent alignment and orientation of ZnO nanorods as shown in Figure 3.2.

Figure 3.2: SEM image of ZnO nanorods fabricated on p-type GaN substrate using low temperature aqueous chemical growth technique.

The substrates were annealed at 250 °C (for GaN) and 100 oC (for gold coated plastic)

respectively to solidify the seed layer. Where as in case of silver wires probes, we dipped the

28

wires in the seed solution for two minutes and dried for one minutes at room temperature and

this process was repeated twice. After seeding, the substrates were put inside solution and was

heated up to 90 °C for 3 to 7 h. After the completion of fabrication process, the fabricated

devices were cleaned with deionized-water and dried with nitrogen. The fabrication of the

ZnO nanowires/nanorods proceeds through the following chemical reactions described in

[15]. At first the HMT reacts with water and produces ammonia as described by chemical

equation:

(CH2)6 N4 + 6H2O 6HCHO + 4NH3

As a result of above reaction, the ammonium and hydroxide ions were produced as describes

by the reaction given below:

NH3 + H2O NH4+ + OH-

The produced hydroxide ions react with zinc ions to grow solid ZnO nanowires/nanorods on

the substrate as described by the reaction:

2OH- + Zn+2 ZnO (s) + H2O

The ZnO nanowires/nanorods fabricated by the above technique were highly dense and

vertically align with respect to surface of a substrate. In order to obtain the different

diameters, length and density of the fabricated nanowires/nanorods, we need to control and

optimize the growth parameter such as molar concentration of the HMT and the zinc nitrate,

temperature, and time. Scanning electron microscopy (SEM) images of the obtained ZnO

nanowires/nanorods with different density, diameters and length by changing some growth

parameters are shown in figure 3.3 (a-d).

29

Figure 3.3 (a-d): SEM images for ZnO nanorods/nanowires fabricated under different growth parameters.

Fabrication of ZnO nanotubes

ZnO nanotubes were fabricated by etching the ZnO nanorods along the c-axis direction. After

the fabrication of ZnO nanorods, the sample was dipped into the solution of KCl with a

concentration in the range from 2 M to 7 M for time periods ranging from 3 to 18 h. During

the process, the temperature of the solution was kept constant at 90 °C. The etching

mechanism is that Cl- ions in the solution preferentially adsorbed onto the top of the ZnO

nanorods to decrease the positive charge density of the (0001) ZnO surface, therefore, makes

the (0001) ZnO surface less stable to easily etch through c-axis while chloride adsorption onto

lateral walls seems to be less probable because the surface (101 0) faces appear to be the most

stable ZnO surface [16]. Typical SEM images of ZnO nanotubes are shown in Figure 3.4.

30

Figure 3.4: SEM image of ZnO nanotubes fabricated on the p-type GaN substrate.

Fabrication of ZnO nanowalls

To fabricate the ZnO nanowalls structure onto the surface of p-type GaN substrate. First we

cleaned the substrates then the substrates were affixed inside the vacuum chamber of

evaporator (Satis CR 725). Then a thin film of aluminium with a thickness of 10 nm was

uniformly evaporated. Second, the seed solution was spun coated three times with a speed of

2000 rpm and the spinning time was kept 45 s for each turn and after completing the step

second, the substrate was annealed at ~ 220 °C for 15 minutes. Finally, the substrate was

inserted in the nutrient solution which contained equimolar concentration (0.03 M) of zinc

nitride hexa-hydrate [(Zn (NO3)2.6H2O)] and hexa methylene tetramine (HMT) [(C6H12N4)].

The substrates were placed in the growth solution using Teflon sample holder upside down to

the bottom at 90 °C for 2-5 hours. During the fabrication process, hydroxide zinc acetate

(LHZA) layered has formed i.e., Zn5 (OH)8(CH3COO)2.2H2O, as a self-template and then

31

after that the LHZA films were transformed into nano-crystalline ZnO nanowalls without

morphological deformation by heating at 150 °C in air [17-18]. The SEM image of the

fabricated ZnO nanowalls is shown in figure 3.5. The thickness of the walls was estimated

from the SEM images and it was found to be around 50 nm.

Figure 3.5: SEM image of ZnO nanowalls on p-type GaN substrate.

Growth of ZnO nanoflowers

In order to fabricate ZnO nanoflowers on p-type GaN substrates, ammonium solution

(NH4OH) was used to control the pH of the growth solution. The pH of the solution to

fabricate the nanoflowers was kept at around 10.5. The substrate is placed in the solution with

the sample surface upside down to the bottom of the solution container, and heated at 90 ºC

32

for 3-6 hours [19]. Figure 3.6 shows the SEM image of the fabricated ZnO nanoflowers. The

length of the leaves of the flowers is ~ 2.2 μm and the diameter is about 200 nm.

Figure 3.6: SEM image of ZnO nanoflowers fabricated on p-type GaN substrate.

Fabrication of ZnO nanorods by sol-gel method

ZnO nanorods/nanowires can also be fabricated by sol-gel method at low temperature (<100

oC) [20]. In this method, first a sol-gel seed layers solution was made by mixing zinc acetate

(Zn (CH3COO)2. 2H2O; 0.7 M) solution in the mixture solution of 2-methoxyethanol and

mono-ethanolamine (MEA) with a molar ratio of (1:1). Then the resultant solution was stirred

at 60 oC for 30 minutes to yield a clear and homogeneous solution. Sol-gel solution was spun

coated on the surface of the substrate with a spin speed of 3000 rpm for one minute. This

process was repeated three times. The samples were then annealed at ≤ 500 oC in air for 1 h

[21]. After the annealing, the substrate was put in zinc nitrate (Zn (NO3)2.6H2O) and hexa-

methylene-tetramine (HMT, C6H12N4) solution prepared with equimolar concentration for 4-8

33

hrs. The SEM image of resulting ZnO nanowires grown through sol gel route is shown in

figure 3.7.

Figure 3.7: SEM image of ZnO nanowires fabricated through sol gel method on p-type GaN substrate.

3.2 Bottom contacts deposition

The efficiency and durability of the light emitting diodes based on ZnO nanostructures rely on

the evolution of metallization schemes with low specific contact resistance. The electro-

optical, electrical characteristics and life time of the prepared devices depend on the contact

resistance between the semiconductor and metal. If this contact resistance is high then it will

affect the efficiency of the devices [22]. Basically, the main objectives for these ohmic

contacts are to assist to inject and exit the current into the devices and should ideally have

symmetric and linear I-V relation. In the present work, we have chosen the combination of

Pt/Ni/Au alloy with thickness of Pt, Ni, and Au layers of 20 nm, 30 nm, and 80 nm,

34

respectively to make the bottom contacts. This chosen alloy possesses a specific contact

resistance of ~ 5.1×10-4 Ω-cm-2 [23]. The sample was annealed at 350 ºC for 1 min in a

flowing nitrogen (N2/Ar) gas atmosphere. Several combinations of different alloys for ohmic

contacts for the p-GaN are listed in table 4.1.

Table 4.1: Different alloys combination (metallization) for the ohmic contacts for p-type

GaN.

Combination of

alloys

(metallization)

Annealing

temperature

Lowest ρc (Ω-cm-2)

Ref

Pd/Au 500 oC 4.3×10-4 20

Pt/Au 750 oC 1.5×10-3 21

Ni/Pd/Au 550 oC 4.5×10-6 22

Ni/Au 750 oC 3.3×10-2 21

Pd/Pt/Au 600 oC 5.5×10-4 23

Pt/Ni/Au 350 oC 5.1×10-4 24

Pd/Ni/Au 450 oC 5.1×10-4 25

Ni/Au 500 oC (annealed under partial O2 ambient to

form NiO)

1.0×10-4 26

3.3 Photoresist and plasma etching (RIE)

A photoresist was spun coated in order to isolate and fill the gap between the fabricated ZnO

nanorods on the p-type substrates. This process also eliminates the carrier cross talk between

the fabricated nanostructures like nanorods or nanotubes etc. After the spun coting of

photoresist, the tips of the nanorods were etched using oxygen plasma ion etching or reactive

ion etching (RIE).

35

3.4 Top contacts deposition

In order to deposit the top contacts on etched ZnO nanorods tips, we have chosen the

combination of non-alloyed Pt/Al metals to make the top ohmic contacts to the ZnO

nanostructures. The thickness of the chosen metals i.e. Pt and the Al layers were evaporated

as 50 nm and 60 nm, respectively. This contact resistance of the chosen contact was estimated

~ 1.2×10-5 Ω-cm-2 [24]. There are different ohmic contacts combinations were reported in the

literature for n-ZnO as enlisted in the table 4.2.

Table 4.2: Different ohmic contacts combination enlisted for n-type ZnO.

Combinations of

metal alloys

(metallization)

Annealing temperature

Lowest ρc (Ω- cm-2) Ref

Ti/Al/Pt/Au 200 oC 9×10-7 27, 28

Ti/Au 300 oC 2.0×10-4 29, 30

Ti/Au 600 oC 3.0×10-3 31

Ti/Au 500 oC 1.0×10-3 29, 30

Ti/Au None 4.3×10-5 32

Al/Pt None 2.0×10-6 33,34

Ti/Al/Pt/Au None 8.0×10-7 27, 28

Al None 4.0×10-4 33

Zn/Au 500 oC 2.36 ×10-5 34

In None 7 ×10-1 35

Re/Ti/Au 700 oC 1.7 ×10-7 36

Ru 700 oC 3.2 ×10-5 37

Pt-Ga None 3.1 ×10-4 38

36

References:

[1] Z. L. Wang, Materials Today 2004, 7, 26.

[2] L. Vayssieres, K. Keis, S. E. Lindquist, A. Hagfeldt, J. Phys. Chem. B 2001, 105, 3350.

[3] R. S. Wagner, and W. C. Ellis, Appl. Phys. Lett 1964, 4, 89.

[4] P. D. Yang, H. Yan, S. Mao, R. Russo, J. Johnson, R. Saykally, N. Morris, J. Pham, R. He,

and H. J. Choi, Adv. Funct. Mater 2002, 12, 323.

[5] X. D. Wang, C. J. Summers, and Z. L. Wang, Nano. Lett. 2004, 4, 423.

[6] X. D. Wang, J. H. Song, P. Li. J. H. Ryon, R. D. Dupuis, C. J. Summers, and Z. L. Wang,

J. Am. Chem. Soc 2005, 127, 7920.

[7] N. S. Ramgir, D. J. Late, A. B. Bhite, M. A. More, I. S. Mulla, D. S. Joag, and K.

Vijayamohanan, J. Phys. Chem. B 2006, 110, 18236.

[8] J. H. Song, X. D. Wang, E. Riedo, and Z. L. Wang, J. Phys. Chem. B 2005, 109, 9869.

[9] J. H. Park, and J. G. Park. Curr. Appl. Phys. 2006, 6, 1020.

[10] Q. X. Zhao, P. Klason, and M. Willander, Appl. Phys. A 2007, 88, 27.

[11] W. Lee, M. C. Jeong, and J. M. Myoung, Nanotechnology 2004, 15, 254.

[12] J. Cembrero et al., Thin Solid Films 2004, 198, 451.

[13] B. Mari, M. Mollar, A. Mechkour, B. Hartiti, M. Perales, J. Cembrero, Microelectron. J

2004, 35, 79.

[14] H. Womelsdorf, W. Hoheisel, G. Passing, DE-A 199 07 704 A 1, 2000.

[15] M. Wang, C. H. Ye, Y. Zhang, G. M. Hua, H. X. Wang, and M. G. Kong, J. Cryst.

Growth 2006, 291, 334.

[16] B. Meyer, and D. Marx, Phys. Rev. B 2003, 67, 035403.

[17] E. Hosono, S. Fujihara, T. Kimura, and H. Imai, J. Coll. Inter. Science, 2004, 272, 39.

[18] E. Hosono, S. Fujihara, and T. Kimura, J. Mater. Chem.2004, 14, 88.

[19] Y. J. Kim, J. Yoo, B. H. Kwon, Y. J. Hong, C. H. Lee, and G. C. Yi1, Nanotechnology,

2008, 19, 315202.

[20] K. Prabakar, and, H. Kim, Thin Solid Films. 2010, 518, 136.

[21] Y. S. Bae, D. C. Kim, C. H. Ahn, J. H. Kim, H. K. Choa, Surf. Interface Anal. 2010, 42,

978.

[22] M. E. Lin et al., Appl. Phys. Lett. 1994, 64, 1003.

[23] J. S. Jang, I. S. Chang, H. K. Kim, T. Y. Seong, S. Lee, and S. Park, J. Appl. Phys. Lett.

1999, 74, 70.

[24] H. K. Kim, K. K. Kim, S. J. Park, T. Y. Seong, I. Adesida, J. Appl. Phys.2003, 94, 4225.

37

CHAPTER 4

Experimental and characterization techniques

After the successful fabrication of desired ZnO nanostructures based devices on silver wires,

gold coated plastic and p-type GaN substrates, different experimental and characterization

techniques were applied to investigate the electrochemical sensing response (EMF),

morphological, structural, optical, electrical and electro-optical properties of different

nanodevices presented as an electrochemical sensors and light emitting diodes (LEDs). We

have used the following characterization and experimental techniques:

(1) Scanning electron miscropy (SEM)

(2) Atomic force microscopy (AFM)

(3) X-ray diffraction (XRD)

(4) Photoluminescence (PL)

(5) Electroluminescence (EL)

(6) Electrical (current voltage I-V) characterization

4.1 Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy

(EDX)

Scanning electron microscopy (SEM) is a highly important tool for the investigation of the

surface morphology of nanostructures. By employing it we can investigate the diameter,

length, thickness, density, shape and orientation of the nanostructures. The equipment named

JEOL JSM-6301F and LEO 1550 scanning electron microscopes respectively were utilized in

this research work. The maximum resolution of the SEM was up to 10 nm. Figure 3.2 and

figure 3.4 showed the SEM images of ZnO nanostructures fabricated on p-GaN substrate by

ACG methods. This can be clearly seen from these SEM images that fabricated ZnO nanorods

and nanotubes were well aligned, hexagonal and vertical with high density. The fabricated

nanorods and nanotubes uniformly spread all over the substrate. This can be seen from the

SEM images that the diameters of our as fabricated ZnO nanorods and nanotubes were around

150-250 nm and the length of the both were around 1.2 μm. The SEM (LEO 1550) also has

38

energy-dispersive X-ray spectroscopy (EDX) that analyzes the chemical composition of

object nanostructures. Figure 4.1 shows a typical EDX spectrum of ZnO nanowires on gold

coated plastic substrate, which indicates that the as-grown samples are indeed ZnO.

Figure 4.1: EDX spectrum of ZnO nanowires on a gold coated plastic substrate.

4.2 Atomic force microscope

Atomic force microscope (AFM) is a high resolution scanning probe microscopy. This tool is

very important to investigate the surface profiles of materials in three dimensions (3D) at

nanoscale and can also be utilize for manipulating the atoms. Moreover, AFM can also be

used to measure the force at nano-newton scale [1-2]. In comparison to SEM, AFM is

superior in many aspects, like it provides 3D visualization of the material images, in AFM the

substrate/samples do not need any specific preparation to improve the conductivity of the

surface as it is required for the SEM. Thus, it is helpful to study the biological surfaces.

Figure 4.2 is showing the AFM image 10 μm × 10 μm of ZnO nanowalls fabricated on p-

GaN.

39

Figure 4.2: AFM (10 µm x10 µm) image of ZnO nanowalls fabricated on p-type GaN

substrate.

4.3 X-ray diffraction

The distance between the atoms in a solids has been estimated around one angstrom (Å) and

the energy corresponds to this distance can be estimated by the formula given below;

E= hc / λ for λ= 1 Å E ≈ 12 × 103 eV

Generally, we applied the X-ray diffraction (XRD) characterization technique for

semiconductor materials in order to determine the material quality, composition, lattice

parameters, orientation, defects, stress and strain. X-rays have approximately same energy (K

eV) as the interatomic distance energies of solids. In 1913 W.H and W.L. Bragg demonstrated

that the X-rays have the characteristics patterns after reflection from crystalline materials and

these patterns were different from those that were obtained from liquids. After this successful

demonstration, X-ray diffraction patterns became an important tool for the material science

research. Figure 4.3 showing a schematic diagram of Bragg reflection from crystalline planes

40

having inter-plane distance d. The incident and reflected X-rays from the two planes are also

shown. Bragg concluded that the path difference between the two X-rays diffracted from two

consecutive lattice planes is 2dsinθ and it leads to Bragg’s law, which states that the condition

for diffraction of X-rays for a crystalline material is:

nλ = 2d sinθ

Here θ is the angle of incidence and λ is the wavelength of the X-rays, n is an integer and it is

the order of reflection, and d is the distance between the lattice planes.

Figure 4.3: Shows a schematic diagram of Bragg reflection from crystalline lattice planes having interplan distance “d” between two lattice planes.

Figure 4.4 (a-d) showing the XRD patterns obtained from different ZnO nanostructures. This

XRD pattern have a strong (002) peaks and weak (004) peaks. The (002) peak has a strong

and sharp intensity and narrower spectral width as compared to the other peaks obtained in the

XRD pattern. This indicates that the grown ZnO nanostructures have a high quality wurtzite

hexagonal ZnO phase [3-4].

41

Figure 4.4: Display the Ө-2Ө XRD spectra of ZnO (a) nanowalls, (b) nanorods, (c) nanoflowers, and (d) nanotubes grown on p-GaN substrates, respectively.

4.4 Electrochemical measurements using ZnO nanostructure based sensors

Electrochemical measurements were carried out using a two-electrode configuration

consisting of ZnO nanowires/nanorods as the working electrode and an Ag/AgCl as a

reference electrode. The electrochemical responses were measured using a Metrohm pH meter

model 826 (Metrohm Ltd, Switzerland) at room temperature (23 2 °C). The electrochemical

response was observed until the equilibrium potential has reached and stabilized, then the

42

electrochemical potential was measured. Figure 4.5 shows the schematic diagram of extra

cellular measuring set up.

Figure 4.5: Schematic diagram of potentiometric measuring setup.

For the intracellular glucose measurements, we have applied the microinjection technique to

insert the prepared sensor inside the cells. To perform these experiments, we have fabricated

the ZnO nanowires based sensor on the tip borosilicate glass capillaries (sterile Femtotip II)

possess the inner diameter 0.5 μm, tip outer diameter of 0.7 μm suitable for the intracellular

measurements as a working electrode and another femtotip prepared as a reference

microelectrode. ZnO nanorods were selectively fabricated on the tip of capillaries using low

43

temperature ACG method. Figure 4.6 shows the schematic diagram of measuring set up for

the intracellular glucose sensing.

Figure 4.6: A schematic diagram illustrating the selective intracellular glucose sensor.

4.5 Photoluminescence

Photoluminescence (PL) is a non-destructive technique used for the determination of extrinsic

and intrinsic properties of the materials (semiconductors). In this technique the semiconductor

under investigation is excited using laser irradiation and then the PL spectrum of the

spontaneous emission from radiative recombinations in the material band gap was acquired.

During the PL process both radiative and non-radiative phenomena occurred but in the present

work we have focussed our studies for the radiative recombinations only. The PL

measurement technique has been widely utilized for the determination of the bandgap,

44

impurity levels, defects and recombination process in the semiconductor materials. Further

details are described in the reference [5]. In the present work, a diode laser irradiation with a

wavelength of 266 nm was pumped using resonant doubly unit (MBD 266) as an excitation

source. A schematic diagram illustrating the PL setup is shown in figure 4.7 given below.

Figure 4.7: Schematic diagram of photoluminescence (PL) setup.

The operation of PL measurement can be described in three steps.

(1) First, the materials whose optical properties are under investigation were optically

excited by laser to generate the electron-hole pairs. Different types of laser sources

such as He-Cd laser and Ar+ laser with wave lengths of 325 nm and 316 nm are

commonly utilized for the excitations in ZnO. The laser irradiation was directed

towards the specific target (sample) with the help of a setup as shown in the schematic

diagram.

(2) Second, after the excitation, the generated electron-hole pairs recombine radiatively

and emit light.

45

(3) Third, the emitted light is observed and disseminated by a double grating mono-

chromator and photomultiplier detectors. The final spectrum is collected and analyzed

in a computer.

4.6 Electroluminescence (EL)

The electroluminescence (EL) is a process in which optical devices were characterized with

electrical input and optical output. During this characterization, an electrical signal (current or

voltage) were applied to properly bias the pn junction who emits the light. The acquired

emitted spectrum is the result of electron-hole radiative recombinations process in the device.

The electroluminescence measurement is very crucial and has considered being a figure of

merit for the light emitting diodes (LEDs).

In the present work, we have performed the electroluminescence characterization at room

temperature of our prepared p-n heterojunction based LEDs for a possible future application

of ZnO nanostructures for white LEDs based lighting. There is a special detector known as

photomultiplier detector present in EL equipment was used to observe the emitted light from

the fabricated LEDs during the EL measurements. The acquired spectra were measured from

the top contacts of the LEDs by detecting the light escaping from the edge of the top contact

electrode.

4.7 Electrical (current voltage I-V) characterizations

Semiconductor parameter analyzer (SPA) is very important equipment utilized for the

measurement of electrical parameters such as current voltage (I-V) characterization of

electronic devices like transistors, light emitting diodes, schottky diode, logic gates, and etc.

In the present work, it has been utilized for the I-V measurements of the fabricated ZnO

nanostructures based LEDs. The I-V characterizations provide a useful data about the

fabricated devices (LEDs) like turn on voltages or the threshold voltage, forward bias and

reverse bias currents, different current regions, break-down voltage, ideality factor, and

46

parallel and series resistance of the fabricated LEDs. The I-V curves obtained during the

investigation are shown in Figure 2.4 (chapter 2) [3]. The curves exhibited a good rectifying

behaviour as we were expected from these LEDs. This can be seen from the figures that an

appropriate p-n heterojunctions were achieved. The turn on voltage of these fabricated

heterojunctions LEDs is around 4 V.

47

References [1] http://en.wikipedia.org/wiki/Atomic_microscopy.

[2] http://www.chembio.uoguelph.ca/educmat/chm729/afm/introdn.htm.

[3] N. H. Alvi, S. M. Usman Ali, S. Hussain, O. Nur, and M. Willander, Scripta Materiala

2011, 64,697.

[4] L. B. Freund, and S. Suresh (Eds.), Thin Film Materials, Cambridge University Press,

Cambridge (2003).

[5] G. D. Gilliland, Mater. Sci. Eng. 1997, R18, 99.

[6] Peter Klason, Zinc oxide bulk and nanorods, a study of optical and mechanical properties,

PhD thesis, University of Gothenburg., (2008).

[7] N. H. Alvi, K. ul Hasan, M. Willander, and O. Nur, (Accepted in Lighting Research and

Technology) DOI: 10.1177/1477153511398025.

48

CHAPTER 5 Results and discussions In this chapter I am presenting some results obtained during my research studies. These

results are divided into two parts.

In part one, results from papers I to V are included. We have successfully demonstrated the

fabrication and utilization of ZnO nanowires/nanorods based electrochemical nano-sensors for

the selectively determination of glucose (extra and intra cellular) and uric acid etc. In addition

to that we have successfully employed these developed sensors in extended gate MOSFET

configuration for the electrochemical detection of glucose and wireless monitoring of glucose

using existing GSM infrastructures.

In part second, we demonstrate the photonic applications of different ZnO nanostructures such

as nanorods, nanotubes, nanoflowers, and nanowalls fabricated using the ACG approach on p-

GaN substrate to form hetrostructure p-n junctions and the emission properties of these

photonic devices were comparatively studied.

5.1 Electrochemical nano-sensors After the successful and controlled synthesis of ZnO nanostructures such as

nanorods/nanowires on different substrates, they have been the target of numerous

investigations due to their unique properties. The diameters of these one dimensional

nanostructures are comparable to the size of the biological and chemical species being sensed,

which intuitively makes them excellent primary transducers for producing electrical signals.

5.1.1 Potentiometric electrochemical glucose sensor (Paper I) An electrochemical glucose sensor was fabricated using ZnO nanowires grown on thin silver

wire of 250 µm diameter. Enzyme glucose oxidase (GOD) was electrostatically immobilized

on the surface of the well aligned zinc oxide nanowires resulting in sensitive, selective, stable

49

and reproducible glucose biosensors. The experimental setup for the two electrodes

potentiometric measurements is shown in Fig. 4.5 (chapter 4). The changes in the

electrochemical response i.e., an electromotive force (EMF) can be observed when the

concentration of analyte in the test solution was varied by applying a calibration procedure.

The potentiometric response vs. Ag/AgCl reference electrode was found to be linear over a

relatively wide logarithmic concentration range 0.5 µ to 10 mM [1].

The sensing mechanism of most electrochemical glucose sensors is based on an enzymatic

reaction catalyzed by glucose oxidase (GOD) according to the following:

----------- (1)

As a result of this reaction, δ-gluconolactone and hydrogen peroxide are produced. These two

products and the oxygen consumption can be used for the glucose determination. With H2O

availability in the reaction, gluconolactone is spontaneously converted to gluconic acid, which

at neutral pH, form the charged products of gluconate - and proton (H+), according to the

equation below:

-----------(2)

This proteolytic reaction of the δ-gluconolactone to gluconic acid shown in Eq. (2), which

results in a decrease of the medium pH, can be used for the determination of the glucose

concentration [2]. Concentration changes of ions surrounding the ZnO nanowires will change

the electrode potential [3]. The potentiometric response of the sensor electrodes were studied

in glucose solutions made in buffer (PBS pH 7.4) with concentration ranging from 0.5 µM to

10 mM. Avery fast response time was noted over the whole concentration range with 95% of

the steady state voltage achieved within 4 s for a sensor electrode in conjunction with a

Nafion® membrane Fig. 5.1(a). The tested sensor configuration showed large dynamic ranges

with an output response (EMF) that was linear vs. the logarithmic concentration of glucose

50

going from -10 mV for 0.5 µM and -154 mV for 10 mM glucose as shown in Fig. 5.1 (b).

(a)

(b)

Figure 5.1 (a): Calibration curve showing the time response of the sensor electrode in 50 µM glucose solution (b) Calibration curve showing electrochemical response (EMF) vs. logarithmic glucose concentrations using ZnO sensor electrode and Ag/AgCl reference electrode [1].

This corresponds to slope of around 35 mV/ decade. These obtained signals were reproducible

and strong enough to open a gate channel in a commercial MOSFET having low threshold

gate voltages. Moreover, these signals can be integrated in an electronic circuitry for remote

monitoring the patient’s diabetes level from remote locations and wireless sensor

configurations. The excellent performance in sensitivity, stability, selectivity, reproducibility

and freedom from interference was achieved when the sensor was exposed to glucose

51

solution. All these advantageous features can make the proposed biosensor applicable in

medical, food or other areas. Moreover, the fabrication method is simple and can be extended

to immobilize other enzymes and other bioactive molecules with small isoelectric points for a

variety of biosensor designs.

5.1.2 Zinc oxide nanowires as extended gate MOSFET for glucose detection (Paper II) In this study, we have demonstrated another technique for the selective determination of

glucose using the immobilized ZnO nanowires as an extended gate of a commercial MOSFET

with low threshold voltages. We report how instead of growing ZnO nanowires on the gate

area inside the transistor (e.g., on the AlGaN/GaN HEMT device), ZnO nanowires can be

integrated on the surface of an Ag wire (with a diameter of around 250 μm) as an extended

gate [4]. In this way, the chemically sensitive gate is then separated from the rest of the

transistor construction, and the sensing area increases significantly as compared to gate areas

of some published sensors based on transistors, e.g., HEMT [5]. Thereby, the biosensor

construction is much facilitated as the enzyme can be readily immobilized on the wire, and

applied in a variety of different probes or flow systems designs without problems arising

from, e.g., encapsulation of the electronics, etc. In addition, we report on the effect of the

uniformity and vertical orientation of ZnO nanowires on response time of the sensor.

We have coupled/interfaced the fabricated glucose sensor with a commercial MOSFET

having very low/zero threshold voltages in an extended gate configuration [6]. The extended-

gate MOSFET sensor approach demonstrates the possibility and potential of the use of

nanostructures coupled to standard electronic components for sensing applications. In order to

interface the output signal from the electrochemical measurement with a commercial

MOSFET, we selected the highly sensitive n-channel zero threshold (Vth = 0 V) ALD/110900

commercial n-MOSFET (Advanced Linear Devices, Sunnyvale, CA), which can operate in

52

precision zero threshold mode. This transistor was integrated with the extended-gate sensor

together with an Ag/AgCl electrode and connected to a Keithley 2602 unit, as schematically

shown in Fig. 5.2.

Figure 5.2: Schematic diagram illustrating the configuration used for glucose detection with MOSFET using extended-gate functionalized ZnO nanowires as working electrode and Ag/AgCl as a reference electrode [6].

In addition, a pH meter (Model 744, Metrohm) was used to measure the potentiometric

output voltage (EMF) of the different ZnO nanowires sensors presented here. Moreover, time

response measurements were also performed to study the stability. For the time response

measurements, a model 363 A potentiostat/galvanostat (EG and G, Inc., Idaho Falls, ID) was

used. The working electrode (ZnO nanowires) is negatively charged due to oxidation [see eqn.

(1) and (2)]. The gate voltage must be positive in order to invert carriers at the n-MOSFET

channel and observe the drain current modulations. The output voltage can be made positive

53

by interfacing instrumentation amplifier in an inverting mode with unity gain between the

sensor output and gate terminal of the MOSFET. If a p-channel MOSFET is used, then there

is no need for an instrumentation amplifier interfacing.

When the extended gate was immersed in 50 µM glucose solution, an induced voltage of

around 60 mV was added to the gate. As a result, a strong modulation of the drain current was

observed. In this case, when the extended-gate transistor was stable, 0.540 μA was added to

the drain current, as shown in Fig. 5.3 (a). This increment is due to the reaction between the

glucose and GOD/ZnO/Ag electrode leading to an electron transfer to the ZnO nanowires. It

is important to mention that on contrary to the non-stable behaviour of both the GOD/Ag and

bare Ag wire, the modulation observed here prevail for a long time with no observable signal

decay. The dependence of drain current (ID) on the glucose concentration is shown in figure

5.3 (b). As evident form figure 5.3 (b), the ID increased as the glucose concentration is

increased, as expected. It is also clear that the dependence of the ID on the glucose

concentration is showing a linear relationship.

In summary, the extended-gate MOSFET sensor concept presented here is robust, and opens

the possibility of externally integrating nano-sensing element to commercial transistors giving

the advantages of simplicity and low cost. In addition, the extended gate makes it easier and

more practical to sense elements when the available sample volume is relatively small. This

simple method of fabricating ZnO/GOD sensor can also be extended to immobilize other

enzymes and other bioactive molecules on various nanostructures, and form versatile

electrodes for sensing applications/studies.

54

Figure 5.3 (a): Typical drain current (ID ) versus gate voltage (VG) for the extended-gate MOSFET, the upper curve (line) is for 50 μM glucose solution while the lower curve (dotted line) is for the case of 100 μM of glucose concentration. (b) Relation between the drain current and glucose concentration for a range of 1–100 μM glucose concentration [6].

5.1.3 An intracellular glucose sensor using the functionalised ZnO nanorods (Paper III)

Based on all the advantageous features observed during the extracellular investigations, we

implied this proposed sensor for intracellular configuration for the determination of glucose

using micro-injection technique.

55

To prepare the sensor suitable for an intracellular glucose measurement, we have evaporated a

silver thin film onto the tip of borosilicate glass capillary, then a ZnO nanorods were

fabricated on the capillary tip using ACG method as shown in figure 5.4 (a-c), whose outer

diameter was 0.7 µm and immobilized them with the enzyme glucose oxidase by employing

simple physical adsorption method.

Figure 5.4: Scanning electron microscopy (SEM) images of the ZnO nanorods fabricated on Ag-coated glass capillaries using ACG method :( a and b) before enzyme immobilisation and (c) after enzyme immobilisation [7].

The functionalized glucose oxidase retained its enzymatic activity due to excellent

electrostatic interaction between ZnO and glucose oxidase. This prepared nano-electrode

device was successfully applied using micro-injection techniques to selectively determine the

glucose concentrations in human adipocytes and frog oocytes [7]. In the human body, the

hormone insulin only stimulates glucose transport into the muscle and the fat cells. However,

56

it has been observed that the insulin affect the glucose uptake in human adipocyte and oocytes

from frog Xenopus laevis [8, 9]. We have demonstrated a glucose transport system that is

markedly activated by insulin in both cells [10]. The extra cellular response of the

electrochemical potential difference of the ZnO nanorods to the changes in buffer electrolyte

glucose was measured for the range of 500 nM to 1 mM and shows that this glucose

dependence is linear and has sensitivity equal to 40.27 mV/ decade at around 23 oC as shown

in figure 5.5.

Figure 5.5: A calibration curve showing the electrochemical potential difference versus the glucose concentration (0.5–1 mM) using functionalised ZnO-nanorod-coated probe as a working electrode and an Ag/AgCl microelectrode reference microelectrode [7].

To start intracellular measurements, first we used the nanosensor to measure the free

concentration of intracellular glucose in a single human adipocyte and frog oocytes by the

procedure described in [7]. The intracellular glucose concentration was estimated to be 50±15

µM for (n = 5). The obtained results were in good agreement as compared with the 70 µM

intracellular concentration determined by nuclear magnetic resonance spectroscopy in rat

muscle tissue in the presence of a high, 10 mM extracellular glucose concentration as reported

57

[19]. In order to investigate the impact of insulin, we added insulin to the cell medium to

stimulate glucose uptake. Insulin stimulates glucose uptake by binding to its receptor at the

cell surface, which initiates intracellular signal transduction, causing translocation of insulin

sensitive glucose transporters (GLUT4, glucose transporter-4). After integration in the plasma

membrane, GLUT4 allows glucose to enter the cell along a concentration gradient, as shown

in Fig. 5.6 (a). Thus, when we achieved a stable potential for intracellular measurement, 10

nM insulin was added to the cell medium. After several minutes insulin increased the glucose

concentration in the cell from 50±15 µM to 125±15 µM as shown in Fig. 5.6 (b). In another

set of experiments, we have applied the nanosensor to measure intracellular glucose

concentration in single frog oocytes. The intracellular glucose concentration was 125±23 µM

for (n = 5). This is slightly higher than what has been reported earlier i.e. < 50 µM [11]. We

do not know the reason for this difference, but one possibility is that the electrodes behave

slightly differently inside the oocyte than outside, where they were calibrated. However, to

test whether the electrode is measuring the glucose concentration inside the oocytes, we added

10 nM of insulin to the cell medium to stimulate glucose uptake. Indeed, the glucose

concentration in the frog oocytes increased from 125±23 to 250±19 µM. The proposed

intracellular biosensor showed a fast response time less than 1 s and has quite a wide linear

range from 0.5 to 1 mM suitable for the intracellular measurements. The performance

regarding sensitivity, selectivity, and freedom from interference when the sensor was exposed

to intra- and extracellular glucose measurements were quite acceptable. These results

demonstrate the capability to perform biologically relevant measurements of glucose within

living cells. The ZnO-nanorod glucose electrode thus holds promise for minimally invasive

dynamic analyses of single cells.

58

(a)

(b)

Figure 5.6: (a) Intracellular mechanism for insulin-induced activation of glucose uptake. (b) Output response (EMF) with respect to time for intracellularly positioned electrodes when insulin is applied to the extracellular solution [7].

5.1.4 Wireless remote glucose monitoring system (paper IV) In this paper, we have presented another application of ZnO nanowires based glucose sensor.

We have applied our fabricated glucose sensor for remotely monitoring the glucose levels

from remote locations using existing GSM/GPRS mobile communication infra structures

[12]. As the GSM/GPRS infrastructures have proven to be reliable and cost effective, the

59

services provided by these systems are inevitably used for data acquisition and monitoring

applications. Thus, we have chosen this system in our present work due to its wide area

coverage and can reach to doctor/caregiver at any time.

Figure 5.7: The proposed system block diagram of wireless remote monitoring system for the functionalized ZnO nanowire arrays based glucose sensor [12].

Figure 5.7 showing the proposed system block diagram of wireless remote monitoring system

for the ZnO nanowires based glucose sensor. The electrical signals generated by our glucose

sensor are stable and strong enough with ranging from 10 mV to hundreds of mV with

varying glucose concentrations as shown in figure 5.8 (a-b). These signals were firstly

collected by electrodes, then pass through amplifier and filters to get rid of noises using

flexible shielded cables. After that the signals are connected to the input port of built-in ADC

60

of the PIC18F452 microcontroller. The input signal from the sensor is first interfaced to an

instrumentation amplifier using flexible coaxial cables whose gain was adjusted to an

appropriate level accordingly and then this signal amplified by instrumentation amplifier (IA).

The output from the amplifier is then fed to the input of the ADC built in with microcontroller

which converts this signal to corresponding digital signals readable by the microcontroller.

Software in the controller, which is written in C language, then reads the signal and compares

it to a lookup table. This algorithm has the key responsibility to read the sugar level in terms

of input electrical signal and from the lookup table convert it into corresponding molar

concentrations. After conversion it generates instruction set for the GSM mobile device

connected with RS 323 serial data cable to the circuit board shown in figure 5.9. This

instruction set when passed to the mobile using the serial port sends sugar levels as SMS

messages to the physician mobile and medical data storage system for immediate consultation

and medications. SMS message typically take 10 to 30 seconds to deliver but depending upon

the network load, it may take longer than 30 seconds. In this study the developed

communication protocol [13] is used for the monitoring of sugar levels. The system detected

the sugar levels in modelled glucose solutions and sent an SMS messages as designed. During

the test, SMS delay was found to be varying between 8 to 30 seconds. The system is also

tested in manual simulation mode and similar results were obtained.

61

Figure 5.8: (a) Calibration curve of the sensor electrode showing the stable and smooth signal in 50 µM glucose solution (b) inset curve showing the time response of the sensor [12].

Figure 5.9: The proposed system circuit diagram of the designed prototype circuit board [12]

62

5.1.5 Selective determination of uric acid (Paper V) In this paper, we have also successfully demonstrated the selective determination of uric acid

using the zinc oxide (ZnO) nanowires fabricated on a gold coated flexible plastic substrate

[14]. For the selective determination of uric acid, enzyme uricase was chosen and

immobilized on the surface of ZnO nanowires, drawing on the fact that there is a large

difference between the isoelectric points of ZnO and uricase. The isoelectric point of ZnO is

9.5, making it a suitable to immobilize a low isoelectric proteins or enzymes such as uricase

(~ 4.6) by electrostatic adsorption in proper buffer solutions.

The electrochemical measurements has been carried out by utilizing two electrodes system

and the resultant slope is drawn from calibrated values of electrochemical response (EMF) vs.

the logarithmic concentrations of uric acid solution ranging from 1 µ to 1000 µM as shown in

figure 5.10.

Figure 5.10: Calibration curves for the uric acids sensor with membrane [14].

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The sensing mechanism of most electrochemical uric acid sensors is based on an enzymatic

reaction catalyzed by Uricase according to the following:

Uric acid has oxidized in the presence of Uricase into Allantoin along with carbon dioxide

and hydrogen peroxide. Due to the presence of H2O, the Allantoin is accepting proton from

H2O and converted to Allantoinium ion, which in results interact with ZnO nanowires and

producing a potential at the electrode. The potentiometric response of the sensor electrodes

were studied in uric acid solutions made in buffer (PBS pH 7.4) with concentration ranging

from 1 µM to 1000 µM. A very fast response time was noted over the whole concentration

range with 95 % of the steady state voltage achieved within 6.25 s as shown in figure 5.11 (a)

for a sensor electrode without membrane and within 9 s for a sensor electrode with membrane

figure 5.11 (b).

Figure 5.11: (a) Time response of the sensor in 100 µM test solution of uric acid without membrane coating [14].

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Figure 5.11: (b) Time response of the sensor in 100 µM test solution of uric acid with membrane coating [14].

The linearity, stability, and reproducibility of the uric acid sensor have been evaluated by

performing three repeated experiments by utilizing a same sensor electrode. The results of

these experiments reveal good consistency in the calibration traces as shown in Fig 5.12 (a).

The reproducibility and long term stability was evaluated by using 5 different uric acid sensor

electrodes constructed independently under same conditions; the relative standard deviation of

the fabricated sensor electrodes in standard uric acid solutions was less than 7 %. The sensor

to sensor reproducibility in 100 µM uric acid solution has shown in Fig.5.12 (b).

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(a)

Figure 5.12: (a) Calibration curves from three different experiments using the same sensor electrode and Ag/AgCl reference electrode [14].

(b)

Figure 5.12: (b) The sensor to sensor reproducibility of five (n = 5) ZnO nanowires/uricase/ Nafion® electrodes in100 µM test solution of uric acid [14].

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5.2 Emission properties of nanostructures based photonic devices (Paper VI) The second part of this thesis, we have fabricated the photonic devices based on different

ZnO nanostructures on p-type GaN substrates to form the hetrostructure pn junctions and the

emission properties of these hetrostructure based photonic devices (LEDs) were

comparatively studied.

In Paper VI, the emission properties of different ZnO nanostructures fabricated on the p-type

GaN substrates were studied [15]. The core objective of the current study was to investigate

the difference in emission properties of the different ZnO nanostructures fabricated on p-type

GaN substrates and point out which ZnO nanostructure possess the excellent emission

properties and to be utilize an integral part of future ZnO based white light emitting diodes

(LEDs). To fabricate the all four types of ZnO nanostructures on to the p-type GaN substrates,

we followed the aqueous chemical growth (ACG) technique. The electrical, optical and

emission characteristics of these different nanostructures were extensively studied and

compared.

The PL spectra from the ZnO nanowalls fabricated on p-type GaN substrate has shown in

5.13 (a). This can be seen from the figure that the PL emission peaks are located at places

around at 361 nm (3.43 eV), 378 nm (3.29 eV), and 490 nm (2.53 eV) respectively. The peak

at 378 nm (3.29 eV) related to the band-edge emission (BEE) which indicated that the as

fabricated ZnO nanowalls have good crystalline quality [2]. The peak located at 490 nm (2.53

eV) related to the deep level emission (DLE) and this can be attributed to oxygen vacancies

(Vo) related defects [5]. Similarly, the PL spectra from other three different ZnO

nanostructures have shown in figure 5.13 (b-d). The PL intensity peaks approximately located

at 376 nm (3.29 eV) and a broad peak (covering the visible region from 400 nm to 750 nm)

were observed in all three types of nanostructures. During the investigation, it has been also

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observed that the emission peak located at 361 nm (3.43 eV) in the nanowalls structures can

be attributed to the band edge emissions in p-GaN substrate and this emission peak indicated

that the commercially purchased p-GaN substrates possess a high optical quality.

Figure 5.13: (a-e) Showing the room temperature PL spectrum for ZnO nanostructures (a) nanowalls, (b) nanorods, (c) nanoflowers, and (d) nanotubes on p-GaN and (e) showed the combined PL spectra of all the four nanostructures [15].

In order to see the highest intensity from all these different ZnO nanostructures, we have

merged all obtained PL spectra from these fabricated nanostructures as shown in figure 5.13

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(e). This can be noticed that between all fabricated ZnO nanostructures, ZnO nanorods have

the highest excitonic band edge PL emissions at 376 nm (3.29 eV), which indicates that ZnO

nanorods have the best crystal quality as compared to all other nanostructures. While in the

visible region, the ZnO nanowalls structures possess the higher PL intensity as compared to

other three ZnO nanostructures. This shows that the nanowalls structure possess more deep

level defects density as compared to the other three nanostructures.

The EL spectra for the fabricated LEDs based on the four different types of nanostructures

were acquired at room temperature as shown in figure 5.14 (a-d). During the EL

measurements, the LEDs were under forward biasing of 25 V with injection current of 4 mA.

The EL spectra of the n-ZnO nanowalls/p-type GaN based LEDs possess the violet, a violet-

blue and a broad EL peaks covering the whole visible region from 480 nm to 700 nm as

shown in figure 5.14 (a). The EL peaks were approximately located at 420 nm (2.95 eV), 450

nm (2.75 eV) and a broad peak covering EL emissions from 480 nm to 700 nm. The broad

peak includes the green, yellow, orange and red emissions. The EL spectrum for n-ZnO

nanoflowers/p-GaN LED was also acquired which possess the violet, violet-blue and the

broad peaks as shown in figure 5.14 (b). These observed peaks were located approximately at

400 nm (3.09 eV), 450 nm (2.75 eV) and a broad peak covering the EL emission from 480 nm

to 700 nm. Similarly, the EL spectra for the LEDs based on n-ZnO nanorods and nanotubes

were acquired respectively as shown in figure 5.14 (c-d) [4]. Both LEDs have the same

spectra because the ZnO nanotubes were achieved from ZnO nanorods by chemical etching.

There is only difference of the charge injection surface area and the ZnO nanotubes have

more surface area as compared to the ZnO nanorods. The electrons can also be injected from

the inner side of the hallow nanotube and this increases the emission intensity. A violet, a

violet-blue and a green EL peaks were observed in both LEDs and these peaks were located

approximately at 400 nm (3.09 eV), 450 nm (2.75 eV) and 540 nm (2.29 eV), respectively.

69

This is important to notice that after comparing the obtained PL spectra from all four types of

fabricated LEDs, it has been found that the nanorods have the highest PL intensity peak in the

UV region as compared to the PL intensity of the all other nanostructures. It means that ZnO

nanorods have good crystal quality and more suitable for UV diodes. ZnO nanowalls

structures have the highest PL intensity emission in visible region when compared to other

nanostructures. As we know that the deep levels defects are responsible for emissions in the

visible range in ZnO. This means that ZnO nanowalls structure possess high deep level

defects density as compared to all other nanostructures.

Figure 5.14: (a-e) showing the EL spectra for n-ZnO (nanostructures)/p-GaN LEDs, in (a) nanowalls, (b) nanoflowers (c) nanorods, and (d) nanotubes, (e) showing the combined EL spectra of all the nanostructures, and (f) shows the CIE 1931 x, y chromaticity space of ZnO nanostructures based LEDs [15].

70

This is interesting to note that when comparing EL spectra of the fabricated LEDs based on

four types of nanostructures, it has been observed that nanorods and the nanotubes possess the

similar EL spectra. The only difference is that the nanotubes possess the higher EL intensity

in the visible region as expected because nanotubes have more oxygen sub-vacancies and

inject more charges as compared to other nanostructures. In order to compare the all four

acquired EL spectra, we have merged them into one graph as shown in figure 5.4 (e). It has

been observed that the nanowalls showed higher defects related emission but possess small

charge injected surface under the contact as compared to other three nanostructures. The I-V

characterization was performed using semiconductor parameter analyzer and all I-V curves

have been combined in one graph as shown in figure 5.15. This can be seen from figure that

the LEDs based on ZnO nanotubes showed the highest current as compared to all other LEDs

under the same set of operating conditions.

Figure 5.15: Showing the combined current-voltage (I-V) characterizations for the different ZnO (nanostructures)/p-GaN based LEDs [15].

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Finally, in order to evaluate the colour quality of the emitted emission, we have performed the

colour rendering indices (CRIs) and correlated colour temperatures (CCTs) measurements for

all four types of fabricated LEDs. It has been observed that the ZnO nanowalls based LEDs

have the highest colour rendering index (CRI) with a value of 95 with a correlated colour

temperature of 6518 K. Whereas the LED based on the nanorods possess the lowest CRI with

a value of 87 and correlated colour temperature of 4807 K.

We have also plotted the CIE 1931 colour space chromaticity diagram in the (x, y)

coordinates system as shown in figure 5.14 (f). The observed values of the plotted

chromaticity coordinates for the fabricated LEDs based on ZnO nanowalls, nanorods,

nanoflowers, and nanotubes were (0.3131, 0.3245), (0.3332, 3470), (0.3558, 0.3970), and

(0.3555, 0.3935), respectively. As per US standard ANSI_ANSLG C78, 377-2008 for solid

state lighting which described that the light sources with chromaticity coordinates less than

three MacAdam ellipses from the Planckian locus can be considered as white light. Thus,

after analyzing the obtained data from the chromaticity coordinates for the fabricated LEDs, It

has been observed that the values of the chromaticity coordinates of ZnO nanowalls and

nanoflowers are very close to the Planckian locus, i.e., around less than one Macadam ellipses

away and considered to be white light emitting diodes. Similarly, the values of the

chromaticity coordinates for the ZnO nanorods and nanotubes are about 3 Mac-Adam ellipses

away and these are also very close to white light region. Thus, it is concluded that the

fabricated LEDs based on different nanostructures are white light emitting diodes [15].

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REFERENCES [1] Syed M. Usman Ali, O. Nur, M. Willander, B. Danielsson, Sens. Actuators B Chem. 2010,

145, 869.

[2] T. V. Anh Dam, D. Pijanowska, W. Olthuis and P. Bergveld, Analyst. 2003, 128, 1062.

[3] S.M. Al-Hilli, R.T. Al-Mofarji, P. Klason, M. Willander, J. Appl. Phys.2008, 103,

014302.

[4] A. Offenh¨auser and W. Knoll, Trends Biotechnol. 2001, 19, 62.

[5] B. S. Kang, H. T. Wang, F. Ren, S. J. Pearton, T. E. Morey, D. M. Dennis, J. W. Johnsons,

P. Rajagopal, J. C. Roberts, E. L. Piner, and K. J. Linthicum, Appl. Phys. Lett., 2007, 91,

252103-1.

[6] Syed M. Usman Ali, Omer Nur, Magnus Willander, and Bengt Danielsson

Nanotechnology, IEEE Transaction on, 2009, 8, 678.

[7] M. H. Asif, Syed M. Usman Ali, O. Nur, M. Willander, Cecilia Brännmark, Peter Strålfors

, Ulrika Englund , Fredrik Elinder and B. Danielsson, Biosensors and Bioelectronics. 2010,

25, 2205.

[8]M. Janicot, and M. D. Lane, Proc. Natl. Acad. Sci. 1989, 86, 2642.

[9]I. A. Simpson, and S. W. Cushman, Annu. Rev. Biochem. 1986,55, 1059.

[10] Cline, G.W., Jucker, B.M., Trajanoski, Z., Rennings, A.J.M., Shulman, G.I., Am. J.

Physiol. 1998, 274 (2 (Endo-crinol. Metab.)), E381–E389.

[11] Umbach, J.A., Coady, M.J., Wright, E.M., Biophys. J. 1990, 57, 1217.

[12] Syed M. Usman Ali, Tasuif Aijazi, Kent Axelsson, Omer Nur, Magnus Willander

Sensors 2011, 11, 8485-8496; doi:10.3390/s110908485.

[13] T. Ozkul, A. Al-Homoud, Comput. Stand. Interfaces 2003, 25, 553.

[14] Syed M. Usman Ali, N.H. Alvi, Zafar Ibupoto, Omer Nur, Magnus Willander, Bengt

Danielsson, Sensors & Actuators: B 2 (2011) 241-247.

[15] N. H. Alvi, Syed M. Usman Ali, S. Hussain, O. Nur, and M. Willander Scripta Materialia

64 (2011) 697-700.

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CHAPTER 6 Summary and the future prospects The core objectives of the current research studies were to investigate the sensing /

transducing phenomena and photonic properties of ZnO nanostructured based nanodevices.

The design, low cost and simple fabrication process for the nanostructures is very crucial for

the effective and efficient applications of the future nanodevices based on these

nanostructures. Recently, scientists and engineers are working to overcome the problems

associated with the trend of the device miniaturization at nanoscale. We believed that till year

2020, a new paradigm of nanotechnology (nanoelectronics) will take place the current era of

existing micro-technology.

In the first part of the thesis, we have fabricated ZnO nanowires/nanorods on different

substrates and effectively characterized them for the sensing (electrochemical transduction)

applications. The diameters of these one dimensional (1D) nanostructures are comparable to

the size of the biological and chemical species being sensed, which intuitively makes them

excellent primary transducers/sensors for producing electrical signals. Among a variety of

nanosensor systems, our proposed nanowire / nanorods based electrochemical sensor is one

that can offer high sensitivity, high selectivity, and real-time detection, suitable for diagnostic

applications. We have successfully demonstrated the utilization of ZnO nanowires based

electrochemical sensors for the selectively determination of glucose (extra and intra cellular),

metallic ions (K+) and uric acid and urea etc. In addition to that we have successfully

employed these developed sensors in extended gate MOSFET configuration and wireless

remotely monitoring of glucose using existing GSM infrastructures.

For the future prospects, as the electrical signal produced from these electrochemical

nanosensors are strong enough (≥ 4 to 20 mV, required for data acquisition system) to

interface them with an appropriate electronic control systems to develop a lab on chip, a

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multichannel nanosensor chip/system capable to determine the multiple analytes or species at

the same time and transmit them through wireless medium to the concerned specialist/Doctors

for the immediate analysis and considerations. I would like to continue my research work to

develop an efficient wireless nanosensor device in the future as well.

In second part of the thesis, we focussed on the photonic properties of ZnO nanostructures

and colour quality of the light emitted from ZnO nanostructures based LEDs. There is indeed

a high demand to develop a cost effective low power consuming white light source to

overcome the high power consuming of the conventional light sources and their overheads.

We believed that the ZnO nanostructures based white LEDs can be an alternate choice to

replace conventional light sources in the near future.

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CHAPTER 7

MY CONTRIBUTIONS TO INCLUDED PAPERS IN THE THESIS

The papers which are included in the present PhD thesis mainly describe two aspects of ZnO

nanostructures i.e., sensing/electrochemical transduction phenomena and photonic

applications (nanostructures based white light emitting diodes). Characterization and

measurements were done by using SEM, XRD, PL, EL, AFM, EDX, SPA, AUTO-LAB,

POTENTIOSTAT / GALVANOSTAT and pH meter respectively.

1. Contribution to paper I

I fabricated the nanosensor devices, designed the experiments, performed the morphological

characterizations, data analysis and wrote the manuscript.

2. Contribution to paper II

I fabricated the nanosensor devices, generate an idea to work on, designed the experiments,

performed the morphological characterizations, data analysis and wrote the manuscript.

3. Contribution to paper III

I fabricated the nanosensor devices, designed the experiments, performed the morphological

characterizations, data analysis and wrote my part of the manuscript.

4. Contribution to paper IV



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5. Contribution to paper V



6. Contribution to paper IV

I fabricated the ZnO nanotubes and nanowalls on p-type GaN substrate and designed the

experiments, performed the morphological characterizations, data analysis and wrote my part

of the manuscript.

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