Speaker: Shengbai Zhang
Department of Physics, Applied Physics, & Astronomy, Rensselaer Polytechnic Institute
Time: 9:30a.m., Aug.8
Avenue: Physics Building 216 Meeting Room
Abstract: The physics of two-dimensional (2D) semiconductors can be fundamentally different from that of traditional bulk semiconductors, as fully reflected in the properties of transition metal dichalcogenides (TMDs). By a density functional theory (DFT) calculation of the TMD edges, we show [1] that a general electron counting model emerges for edge and surface reconstructions, which no longer depends on the crystal structures of the semiconductors as it does in the past [2]. More intriguingly, we show [1] that the multi-valency nature of the transition metal elements can be critically important for the stability of and for the opening up of the band gap at the edges. Our results for MoS2 may explain the strong luminescence recently observed near the edges of TMD flakes [3]. For doping studies, band structure analysis is often not enough. Rather, calculation of total-energy-based defect transition level is necessary, which requires the calculation of charged defects in a jellium background. It has, however, not been fully recognized that the jellium approach could be fundamentally flawed for 2D, as the results will diverge with the size of the vacuum region used in the calculation. We have devised [4] for the first time an appropriate procedure to calculate the converged dopant ionization energies for 2D within the jellium approximation. Application to standard dopants in TMDs and other 2D materials suggests that none of them can be easily ionized. Finally, we apply the time-dependent DFT approach, coupled with molecular dynamics, [5] to the study of ultrafast charge transfer between MoS2 and WS2 upon optical excitation [6]. We show that the collective motion of the excited carriers could be important for the explanation of the experimental findings [6]. The ability for the charge transfer appears to be a matter of criticality, with the timescale of charge transfer depending discontinuously on the magnitude of the dipole coupling strength at the interface.
[1] M. C. Lucking, J. Bang, H. Terrones, Y.-Y. Sun, and S. Zhang, Chem. Mater. 27, 3326 (2015).
[2] M. D. Pashley, Phys. Rev. B 40, 10481 (1989).
[3] H. R. Gutiérrez, et al., Nano Lett. 13, 3447 (2012).
[4] D. Wang, et al., Phys. Rev. Lett. 114, 196801 (2015).
[5] S. Meng and E. J. Kaxiras, J. Chem. Phys. 129, 054110 (2008).
[6] X. Hong, et al., Nat. Nanotech. 9, 682 (2014); Y. Yu, et al., Nano Lett. 15, 486 (2015).
