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

主讲人：张绳百  教授

主持人：陈元平  教授

时间：2016年9月23日下午3:00

地点：逸夫楼一阶

报告人简介：

张绳百教授于1982年获吉林大学学士学位，后获李政道奖学金到美国加州大学伯克利分校师从著名固体物理学家Malvin L. Cohen教授攻读博士学位，博士毕业先在美国国家可再生能源实验室伟德体育下载，2008年被美国伦斯勒理工学院聘为 Kodosky 教授。他致力于半导体表面物理、纳米物理、再生能源及储氢材料等领域的研究，在Phys. Rev. Lett., PNAS, JACS, Nano Lett.等顶级国际期刊上发表论文400余篇，SCI 引用22000多次。曾获美国能源部Chunky Bullet 奖，美国再生能源国家实验室杰出服务奖。2001年当选为美国物理学天津体育直播天津体育直播士。

报告摘要：

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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                                                                                                          教务处

                                                                                          物理与光电工程学院