Exfoliation , Synthesis , and Characterization of Nanoscale

Since the experimental discovery of graphene, two dimensional materials have enjoyed more attention and emphasis in academic research than nanowires, but the latter are an important area of study for creating 1D materials, or single atom chains, the next generation materials for advancing electronic devices. Atomically thin layers can be generated from 2D materials with weak bonds in one direction, and by applying this concept to one dimensional weakly bonded materials, we hypothesize that single atom chains with atomic-scale diameters may be produced. Tellurium (Te) and selenium (Se) have lattices consisting of spiral chains oriented along the caxis, and each chain weakly bonds with others to form a bulk crystal. Since Te exhibits higher mobility than Se, nanoscale Te was the primary focus of this work. Trigonal Te single crystals were exfoliated, without tape, on oxidized silicon substrates. Atomic force spectroscopy revealed that Te nanowires with heights of only 1–2 nm and widths less than 100 nm were successfully fabricated. Anisotropic Te flakes with a thickness of 15 nm showed ridges running along the length of the flake surfaces. A1 and E Raman modes of a 30 nm thick flake were consistent with those of bulk Te, but they showed a slight blueshift (4 cm). Polarized Raman spectroscopy was used to determine the crystal orientation of the flakes. Te and Se crystals were also grown on SiO2 substrates by physical vapor deposition. Te flakes and wires with heights of 60 nm and 30 nm were obtained; Se flakes with a height of 30– 40 nm were also obtained. The synthesized Te wires were utilized in electron transport measurements. A four-terminal device showed a mobility of 51 cmVs at 2.8 K, a mobility lower than predicted due to the low quality of the Te wire. The contact resistance and resistivity of Te were estimated to be 9.4 kΩ and 9.4×10 Ω∙m by applying a correction factor of 0.6. Another successfully fabricated two-terminal device showed a mobility of 1032 cmVs at 10 K, which is a promising enough result to suggest that such a device could be used for quantum transport measurements. Acknowledgment I would like to express my deep gratitude and sincere thanks to Dr. Hugh O. H. Churchill for his continuous support and advice throughout my Master’s study and research. Furthermore, I would like to thank Dr. Shui-Qing Yu and Dr. Gregory J. Salamo for their support over the past two years and the rest of my thesis committee, Dr. Hameed A. Naseem and Dr. Samir M. ElGhazaly, for helping to review and critique this research. I would also like to acknowledge and thank Dr. Ishiang Shih, who provided single crystal tellurium, Dr. Jin Hu, who helped me with using furnaces, Rabindra Basnet, Krishna Pandey, who performed physical vapor deposition of Te and Se onto substrates, Mohammad Zamani Alavijeh, who aided with my AFM measurements, and Xian Hu, who helped me with my Raman spectroscopy measurements. I am grateful also to Dr. Satish Shetty for our many discussions about research, to Arash Fereidouni for all of his help, especially with the optical cryostat, and to my other lab members Hasan Doha, Josh Thompson, Jeb Stacy, and Shiva Davari. Thank you Thach Ngoc Pham, Yiyin Zhou, Huong Tran, and Joshua Grant for helping me so much when I started working in Dr. Yu’s lab group. Finally, I’d like to sincerely thank my friends and family for their love and support throughout my Master’s degree. My research and accomplishments over the past two years would not have been possible without each and every one of you, and I am so grateful to have spent this chapter of my life with you all. Parts of chapter 2 were originally published as (1) H. O. H. Churchill et al., "Toward Single Atom Chains with Exfoliated Tellurium," Nanoscale Research Letters, vol. 12, Aug 1