二硫化鉬奈米結構的邊緣態

Abstract

本篇論文是以現有的多能帶緊密束縛法模型來探討晶體結構改變時，在奈米緞帶以及奈米碎片有/無缺陷結構的能帶結構在色散關係、態密度、狀態數以及電子機率密度的變化，並與第一原理計算結果作比較。 在過度金屬硫屬化物的奈米緞帶中相對於平行方向的平移對秤性會被破壞，因此將重新定義以一維奈米緞帶寬的鏈狀結構為超晶胞，並計算奈米緞帶的能帶結構。當緊密束縛法與第一原理計算比較時，奈米緞帶的能帶結構能夠涵蓋主要的能帶特徵，但是仍然會遺失一些屬於邊緣態的能帶，而遺失的能帶分別屬於扶手狀結構中能帶成分為鉬原子的 的原子軌道以及鋸齒狀結構中能帶成分為硫原子的 的原子軌道。 在具有缺陷的過度金屬硫屬化物的奈米碎片中，已不具有平移對秤性，因此Bloch sum已不合適作為基底來展開波函數，取而代之的是以每顆原子的原子軌道的波函數來展開局域的波函數。在三角形奈米碎片會形成兩種結構分別為硫或鉬為邊緣的結構，並且以11能帶計算在三角形奈米碎片伴隨單顆鉬或單顆硫缺陷位於碎片中心的態密度，結果顯示在硫邊緣下缺陷態會分布於塊材能態與邊緣態的能隙間，但是在鉬邊緣則缺陷態與邊緣態混合出現。

This thesis theoretically investigates the energy dispersion relations, density of states(DOS), number of states(NOS) and charge densities of nanoribbons, nanoflakes with/without defects by using multi-orbital tight binding(TB) models in comparison with the first principle(FP) results. For MoS2-nanoribbons, the translational symmetry is reduced to be with respect to only the direction in parallel with the ribbon. One thus can redefine a super cell contains a chain of atoms crossing over the width of the1D nanoribbon, and calculate the sub-band structure of the ribbon. The calculated result by the tight binding theory does recover the major feature of the band structure of nanoribbon, but misses some subbands of edge states as compared with the FP results. The missing of edge states correspond with the subband composed of the orbital of Mo atoms for armchair-edge nanoribbins and the subband composed of the orbital of S atoms for zigzag-edge nanoribbons, respectively. For defective MoS2-flakes where the translational symmetry is absent, the extended Bloch sum is not any more an appropriate basis set to expand the wave function of electron. Instead, one should take the atomic orbital wave function of individual atoms as basis to expand the localized wave functions. The triangle MoS2 flakes have two kinds of structures whose edges are terminated by S and Mo, respectively. A preliminary calculated the density of states of a triangle MoS2 with the point defect with removal of a S atom or Mo atom at the center of the flake using the eleven orbital tight binding model show that the defect states are lie in the gap between the bulk states and edge states in the case of S-edge flake, but the defect states are mixed with the edge states in the case of Mo-edge flake.