MEES Seminar Series, Dr. Kai Wang, Oak Ridge National Laboratory, "Bandgap Engineering in Two-Dimensional Transition Metal Dichalcogenides: Doping, Interlayer Interaction, and Conformal Growth"

November 29, 2016 - 12:30 PM to 1:30 PM
Duke 345
Contact: 
Department of Mechanical Engineering and Engineering Science, 704-687-8253

As direct bandgap semiconductors, monolayers (MLs) of transition-metal dichalcogenides (TMDs) exhibit exotic properties that arise from quantum confinement and crystal symmetry effects in the two-dimensional (2D) limit. Incorporating dopants in ML TMDs and stacking two different MLs to build heterobilayers can enable one to manipulate their electrical and optical properties. In this talk, I will first show the incorporation of molybdenum (Mo) atoms in monolayer WS2 during chemical vapor deposition (CVD), and correlate the distribution of Mo atoms with photoluminescence properties, ultrafast transient absorption dynamics, and electrical properties derived from field-effect transistor fabrication. The key finding was a gradual in-plane distribution of Mo in WS2, effectively providing an approach to engineer the band gap of WS2. I will then discuss attempts to tailor the electronic structure in heterobilayer by manipulating the twist angles between the two constituent TMDs. Here, monolayers of WS2 and WSe2 were grown by CVD, and artificially stacked. We found that although interlayer excitons were indicated by femtosecond pump-probe spectroscopy, complimentary measurements using photoluminescence and absorption spectroscopies did not show any evidence of them, suggesting that the interlayer exciton transition in this heterobilayer is very weak. More specifically, the interlayer coupling indicated by substantial PL quenching, enhanced absorption, and rapid charge transfer were found to be insensitive to the relative twist angle, indicating that stamping provides a robust approach to realize reliable optoelectronics. Lastly, I will discuss the conformal growth of highly crystalline TMD MLs on well-designed structures, an approach that may hold great promise for the development of novel optoelectronics with high performance.

About the Speaker:

Dr. Kai Wang is now working as a postdoctoral research associate at Center of Nanophase Material Science in Oak Ridge National Laboratory. He received his Ph.D. in Engineering and Applied Science program/Physics department from the University of New Orleans in 2012. Before that, he received his bachelor and master degrees in Materials Science & Engineering from Southwest Jiaotong University in China. His research is focused on the synthesis and characterization of low-dimensional materials and thin films for gas detection and photovoltaic applications. In particular, he is interested in the relationships between material structure and morphology, their growth mechanisms, and the resulting optoelectronic properties.