UC Riverside UC Riverside Electronic Theses and Dissertations Title Two - Dimensional van der Waals Materials: Characterization and Electronic Device Applications Permalink https://escholarship.org/uc/item/4453831z Author Samnakay, Rameez Rauf Publication Date 2016 Peer reviewed|Thesis/dissertation eScholarship.org Powered by the California Digital Library University of California UNIVERSITY OF CALIFORNIA RIVERSIDE Two - Dimensional van der Waals Materials: Characterization and Electronic Device Applications A Dissertation submitted in partial satisfaction of the requirements for the degree of Doctor of Philosophy in Materials Science and Engineering by Rameez Rauf Samnakay August 2016 Dissertation Committee: Dr. Alexander A. Balandin, Chairperson Dr. Roger K. Lake Dr. Alexander Khitun Copyright by Rameez Rauf Samnakay 2016 The Dissertation of Rameez Rauf Samnakay is approved: Committee Chairperson University of California, Riverside Acknowledgements I would like to express my thanks and gratitude to a multitude of people who I have met and interacted with over the course of my life and who helped mould me and make what I am today. First, I would like to thank my advisor, Dr. Alexander Balandin, for giving me guidance and direction as well as giving me an opportunity to conduct research at the highest level. What I learned from his expertise and professionalism to approaching problems and research will remain me with me throughout my life. I would like to thank Dr. Roger Lake and Dr. Alexander Khitun for serving in my dissertation committee as well as providing invaluable advice during the course of my studies. In addition, I was lucky to interact and collaborate with some great researchers and scientists especially Dr. Sergey L. Rumyantsev and Dr. Michael Shur at Rensselaer Polytechnic Institute. I learnt an immense amount by working with them and benefited from their profound knowledge and understanding of device physics. I would like to thank my fellow group members of the Nano-Device Laboratory, both past and present, for their discussions and support along the way and in particular Dr. Jacqueline Renteria for teaching me so much and Dr. Pradyumna Goli for his guidance as I was learning the ropes. The other group members are Dr. Zhong Yan, Dr. Guanxiong Liu, Dr. Richard Gulotty, Dr. Sylvester Ramirez, Dr. Denis Nica, Dr. Monica Lacerda, Hoda Malekpour, Alfred Jiang, Maxim Stolyarov, Fariborz Kargar, Mohammed iv Saadah, Ruben Salgado, Stas Legedza, Aditya Ravidhar, Ece Aytan, Edward Hernandez, Dineth Gamalath and Robert Hernandez. During my time at UCR, I was able to interact and collaborate with some brilliant colleagues namely Dr. Darshana Wickramaratne, Dr. Mahesh Neupane and Walid Amamou whose advice and collaboration I found invaluable. I would like to thank Dr. Purushothaman Srinivasan (GlobalFoundries) and Peter Paliwoda (GlobalFoundries) for guiding me during my internship at GlobalFoundries. I greatly benefited from our experiments on 1/f electronic noise. Finally, I also wish to thank my family, which has supported me through all my endeavors with their love and encouragement. Of these special thanks go to my grandmother Fatima Samnakay, Nishat and Sohail Khan, Dr. Saeed and Mina Samnakay, Rafia and Hanif Khan, Shahid and Nasreen Samnakay and Azra, Atif, Akif, Shumaila, Humaa and Nidaa Darr. On top of everything, all praise goes to the Almighty Allah; the most Gracious, the most merciful for blessing me with all good things I have, and for showing me the enlightened path. Part of this dissertation is based on or is a reprint of the published material as it appears in the following journals: v Reproduced/Reprinted from: R. Samnakay, C. Jiang, S. L. Rumyantsev, M. S. Shur, and A. A. Balandin, Appl. Phys. Lett. 106, 023115 (2015). http://dx.doi.org/10.1063/1.4905694 with permission from AIP Publishing. R. Samnakay, D. Wickramaratne, T. R. Pope, R. K. Lake, T. T. Salguero and A. A. Balandin, Nano Lett. 15, 2965 (2015). DOI: 10.1021/nl504811s with permission from Copyright (2015) American Chemical Society. vi Dedicated To My Mother and Grandparents vii ABSTRACT OF THE DISSERTATION Two - Dimensional van der Waals Materials: Characterization and Electronic Device Applications by Rameez Rauf Samnakay Doctor of Philosophy, Graduate Program in Materials Science and Engineering University of California, Riverside, August 2016 Dr. Alexander A. Balandin, Chairperson The successful exfoliation of graphene and studies into its unique electrical and thermal properties has motivated searches for other quasi two-dimensional (2D) materials with interesting properties that could be used for practical applications. In this dissertation, I describe my research of the properties of inorganic van der Waals materials – layered transition metal dichalcogenides and devices based on these materials. The first part of the dissertation deals with the selective gas sensing using MoS2 thin-film transistors. The sensing is enabled by the change in the channel conductance, characteristic transient time, and spectral density of the low-frequency current fluctuations. The back-gated MoS2 thin- film field-effect transistors were fabricated on Si/SiO2 substrates. The exposure to ethanol, acetonitrile, toluene, chloroform, and methanol vapors resulted in drastic changes in the source-drain current. It was established that the transient time of the current change and the normalized spectral density of the low-frequency current fluctuations can be used as additional sensing parameters for selective gas detection with thin-film MoS2 transistors. The second part of this dissertation involves the Raman study viii of 1T-TaSe2 thin-films. Bulk 1T-TaSe2 exhibits unusually high charge density wave (CDW) transition temperatures of 600 and 473 K below which the material exists in the incommensurate (I-CDW) and the commensurate (C-CDW) charge-density-wave phases, respectively. The C-CDW reconstruction of the lattice coincides with new Raman peaks resulting from zone-folding of phonon modes from middle regions of the original Brillouin zone back to Γ. The C-CDW transition temperatures as a function of film thickness were determined from the evolution of these new Raman peaks, and they are found to decrease from 473 to 413 K as the film thicknesses decrease from 150 to 35 nm. The results of the dissertation contribute to better understanding of properties of 2D materials and may lead to their practical applications in electronics. ix Table of Contents Acknowledgements…………………………………………….………….……………...iv Dedication……………………………………………………………………….…..…...vii Abstract……………………………………………………...…………………………..viii CHAPTER 1 ....................................................................................................................... 1 INTRODUCTION............................................................................................................. 1 1.1 Motivation ............................................................................................................................................. 1 1.2 The van der Waals Materials ................................................................................................................. 4 1.3 Transition Metal Dichacolgenides ......................................................................................................... 6 1.4 Charge Density Waves ........................................................................................................................... 8 1.5 Low-Frequency Noise .......................................................................................................................... 10 References ..................................................................................................................................................... 12 CHAPTER 2 ..................................................................................................................... 14 EXPERIMENTAL METHODS .................................................................................... 14 2.1 Raman Spectroscopy ........................................................................................................................... 14 2.1.1 Applications of Raman Spectroscopy ......................................................................................... 16 2.1.2 Experimental Raman Setup ........................................................................................................ 17 2.1.3 Raman Spectroscopy Measurements ......................................................................................... 20 References ..................................................................................................................................................... 22 x CHAPTER 3 ..................................................................................................................... 23 2D CHANNEL VAN DER WAAL DEVICES ............................................................. 23 3.1 Fabrication of 2D Channel Van der Waal Devices ................................................................................ 23 3.1.1 Substrate and Wafer Preparation .............................................................................................. 23 3.1.2 Transfer of 2D VDW Flakes on Substrate ................................................................................... 25 3.1.3 Two-Run Electron Beam Lithography Process ............................................................................ 28 3.2 Electrical Characterization