Title: Salient physics of transition metaldichalcogenides 

Speaker: Prof. Shengbai Zhang (Department of Physics, Applied Physics, & Astronomy, Rensselaer Polytechnic Institute, Troy, New York 12180, USA）  

Time: Nov. 10, 2016 10:00AM 

Venue: No. 101 meeting room of the library, IOS, CAS 

Abstract: Materials physics of two-dimensional (2D) semiconductors can be noticeably different from that of traditional three-dimensional semiconductors, both in standard and in 2D-specific properties. Taking transition metal dichalcogenides (TMDs) as an example: while a semiconductor surface tends to reconstruct according to the electron counting model (ECM) [1], the same model does not apply to TMD. More intriguingly, first-principles calculations reveal that the multi-valency of transition metal element can be critically important to the edge reconstruction [2]. The latter is vital for the understanding of strong photoluminescence observed in MoS2 flakes [3]. The MoS2 can also be stacked withWS2 to form an atomically-thin heterostructure from which experiment has observed a ultrafast femtosecond hole transfer upon optical excitation [4].Using the time-dependent DFT approach coupled with molecular dynamics, we propose that the collective motion of excited carriers could be responsible for the ultrafast dynamics [5]. For such a van der Waals interface, charge transfer is also a matter of criticality where the timescale of the transfer varies discontinuously (from very fast to very slow) with respect to the interfacial dipole coupling.

Reference：

[1] M. D. Pashley, Phys. Rev. B 40, 10481 (1989).

[2] M. C. Lucking, J. Bang, H. Terrones, Y.-Y. Sun, and S. Zhang, Chem. Mater.27, 3326 (2015).

[3] H. R. Gutiérrez, et al., Nano Lett. 13, 3447 (2012).

[4]X. Hong, et al., Nat. Nanotech. 9, 682 (2014); Y. Yu, et al., Nano Lett. 15, 486 (2015).

[5] H. Wang, J. Bang, Y. Sun, L. Liang, D. West, V. Meunier, and S. Zhang, Nat. Comm. 7, 11504 (2016).

Biography：Prof. Zhang’s research involves first-principles and multiscale calculations of structural and electronic properties of materials, which range from inorganic crystalline, amorphous semiconductors, metals, and their nanostructures to organic materials, bio functional groups, and solutions. While his research interests are primarily in the fundamental physics of materials, they have always been inspired by the needs for sustainable energy and environment, technology leadership, and national security. Prof. Zhang has published about 280 peer reviewed papers with high impact to the fields (citations >11,000, Hirsch Index = 56).