題目：Transition Metal Dichalcogenides: from Edge Passivation, Impurity Doping, to Fast Charge Transfer at Heterojunctions

主講人：張繩百  教授

主持人：陳元平  教授

時間：2016年9月23日下午3:00

地點：逸夫樓一階

報告人簡介：

張繩百教授于1982年獲吉林大學學士學位，后獲李政道獎學金到美國加州大學伯克利分校師從著名固體物理學家Malvin L. Cohen教授攻讀博士學位，博士畢業先在美國國家可再生能源實驗室工作，2008年被美國倫斯勒理工學院聘為 Kodosky 教授。他致力于半導體表面物理、納米物理、再生能源及儲氫材料等領域的研究，在Phys. Rev. Lett., PNAS, JACS, Nano Lett.等頂級國際期刊上發表論文400余篇，SCI 引用22000多次。曾獲美國能源部Chunky Bullet 獎，美國再生能源國家實驗室杰出服務獎。2001年當選為美國物理學會會士。

報告摘要：

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, 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. More intriguingly, we show  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 . 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  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, to the study of ultrafast charge transfer between MoS₂ and WS₂ upon optical excitation. We show that the collective motion of the excited carriers could be important for the explanation of the experimental findings.

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                                                                                                          教務處

                                                                                          物理與光電工程學院