题目：Controlled growth, growth mechanism, and device applications of two-dimensional semiconductors

报告人：Bilu Liu (刘碧录)

Department of Electrical Engineering, University of Southern California, Los Angeles, CA 90089, United States

E-mail: biluliu@usc.edu

时间：2015年9月19日上午10:00

地点：西安交通大学曲江校区西一楼3层A306会议室

 

报告人简介:

Dr. Bilu Liu received a B.S. in materials chemistry from University of Science and Technology of China (USTC) in 2006, and a Ph.D in materials science from Institute of Metal Research (IMR), Chinese Academy of Sciences (CAS) in 2012. He is now a Research Associate at Department of Electrical Engineering, University of Southern California (USC). His research interest covers controlled growth and growth mechanism of carbon nanotubes (CNTs) and two-dimensional (2D) materials, and the use of these materials for nano-electronics, thin-film transistors, and related electronic and optoelectronic applications. He has published more than 50 papers in prestigious journals in these research areas and holds four Chinese and US patents. His overall citation is more than 3000, and h-index = 25.

 

摘要:

Atomically thin two-dimensional (2D) transition metal dichalcogenides (TMDCs) are a large family of layered materials possesses very diverse properties, covering metals, semiconductors, superconductors, charge density waves, etc. These materials, especially semiconducting TMDCs like MoS₂ and WSe₂, have attracted lots of attention recently due to their high surface-to-bulk atomic ratios, mechanical flexibility, and semiconducting properties, making them strong candidates for many applications including nanoelectronics, optoelectronics, flexible devices, sensors, etc.^[1] In this talk, I will first present our recent progress on the controlled growth and growth mechanism studies of 2D TMDCs. I will talk about vapor phase methods for the controlled growth of large single crystalline domains of WSe₂ with lateral sizes up to tens of micrometers.^[2] Effects of growth temperatures and durations on the sizes, shapes, and layer numbers of WSe₂ will be discussed. Substrate atomic-step-guided nucleation and growth of aligned WSe₂ on single crystalline sapphire substrate will also be presented.^[3] In addition, by reducing the supply of source materials, we observed a novel phenomenon of screw-dislocation-driven (SDD) growth of 2D few layer and pyramid-like WSe₂ flakes.^[4] Later, I will discuss device applications of CVD-grown WSe₂. We show that the device characteristics of CVD WSe₂ can be tuned into either p-type or ambipolar behavior, by changing the types of contact metals.^[2] We further developed an efficient method to convert as-grown semiconducting 2H-phase WSe₂ into metallic 1T-phase WSe₂, by controlled reacting with n-butyl lithium (n-BuLi). By using metallic WSe₂ as contact regimes and intact semiconducting WSe₂ as channel regimes, i.e., WSe₂ transistors with phase-engineered contacts, we successfully made ohmic contacted WSe₂ transistors and achieved a hole mobility of 66 cm²/V·s and on/off ratio of 10⁷ for monolayer CVD WSe₂.^[5] This phase transition between 2H and 1T phase WSe₂ is demonstrated to be reversible, thus offering a facile means to modify the chemical, electronic, and optical properties of WSe₂ and potentially many other 2D TMDCs. Our research shows great potential of 2D WSe₂ for electronic applications.

 

In the second part, I will introduce new layered semiconductors, black arsenic-phosphorus (b-AsP), with highly tunable chemical compositions and electronic and optical properties.^[6] Transport and infrared absorption studies demonstrate the semiconducting nature of b-AsP with tunable bandgaps, ranging from 0.3 to 0.15 eV. These bandgaps fall into long-wavelength infrared (LWIR) regime and cannot be readily reached by other layered materials. Moreover, polarization-resolved infrared absorption and Raman studies reveal in-plane anisotropic properties of b-AsP. This family of layered b-AsP materials extend the electromagnetic spectra covered by 2D layered materials to the LWIR regime, and may find unique applications for future all 2D layered material based devices.

 

[2]   B. L. Liu, M. Fathi, L. Chen, A. Abbas, Y. Q. Ma, C. W. Zhou, ACS Nano 2015, 9, 6119.