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(shí)間：2016年9月23日下午3:00

地點(diǎn)：逸夫樓一階

報(bào)告人簡(jiǎn)介：

張繩百教授于1982年獲吉林大學(xué)學(xué)士學(xué)位，后獲李政道獎(jiǎng)學(xué)金到美國(guó)加州大學(xué)伯克利分校師從著名固體物理學(xué)家Malvin L. Cohen教授攻讀博士學(xué)位，博士畢業(yè)先在美國(guó)國(guó)家可再生能源實(shí)驗(yàn)室工作，2008年被美國(guó)倫斯勒理工學(xué)院聘為 Kodosky 教授。他致力于半導(dǎo)體表面物理、納米物理、再生能源及儲(chǔ)氫材料等領(lǐng)域的研究，在Phys. Rev. Lett., PNAS, JACS, Nano Lett.等頂級(jí)國(guó)際期刊上發(fā)表論文400余篇，SCI 引用22000多次。曾獲美國(guó)能源部Chunky Bullet 獎(jiǎng)，美國(guó)再生能源國(guó)家實(shí)驗(yàn)室杰出服務(wù)獎(jiǎng)。2001年當(dāng)選為美國(guó)物理學(xué)會(huì)會(huì)士。

報(bào)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(wù)處

                                                                                          物理與光電工程學(xué)院