[论文解读] Strain engineered direct-indirect band gap transition and its mechanism in 2D phosphorene

Xihong Peng, Andrew Copple+1 more|arXiv (Cornell University)|Mar 15, 2014

2D Materials and Applications被引用 4

一句话总结

本研究通过从头算计算，探究了二维黑磷在应变作用下的直接-间接带隙转变。结果表明，轴向应变——特别是锯齿形方向的−2%压缩应变——会引发直接-间接转变，共识别出五个不同的应变区域；该机制与轨道杂化变化相关，有效质量在转变边界处出现剧烈变化，从而实现载流子输运特性的可调谐。

ABSTRACT

Recently fabricated two dimensional (2D) phosphorene crystal structures have demonstrated great potential in applications of electronics. In this work, strain effect on the electronic band structure of phosphorene was studied using first principles methods. It was found that phosphorene can withstand a surface tension and tensile strain up to 10 N/m and 30%, respectively. The band gap of phosphorene experiences a direct-indirect-direct transition when axial strain is applied. A moderate -2% compression in the zigzag direction can trigger this gap transition. With sufficient expansion (+11.3%) or compression (-10.2% strains), the gap can be tuned from indirect to direct again. Five strain zones with distinct electronic band structure were identified and the critical strains for the zone boundaries were determined. The origin of the gap transition was revealed and a general mechanism was developed to explain energy shifts with strain according to the bond nature of near-band-edge electronic orbitals. Effective masses of carriers in the armchair direction are an order of magnitude smaller than that of the zigzag axis indicating the armchair direction is favored for carrier transport. In addition, the effective masses can be dramatically tuned by strain, in which its sharp jump/drop occurs at the zone boundaries of the direct-indirect gap transition.

研究动机与目标

• 理解黑磷在机械应变下的电子响应。
• 识别诱发直接-间接带隙转变的关键应变阈值。
• 阐明基于轨道杂化演化的带隙演化内在机制。
• 量化应变依赖的有效质量及其对载流子输运的影响。

提出的方法

• 采用第一性原理密度泛函理论（DFT）计算，模拟黑磷在单轴应变下的电子能带结构。
• 沿锯齿形和扶手椅形方向施加应变，以探究其各向异性的力学与电子响应。
• 通过追踪导带最小值与价带最大值之间的能量差，分析带隙演化过程。
• 利用轨道杂化分析解释应变下能量位移与带隙转变的起源。
• 沿锯齿形和扶手椅形方向计算有效质量张量，评估载流子迁移率趋势。
• 基于临界应变值，识别出五个具有独特能带结构的应变区域。

实验结果

研究问题

• RQ1 何种应变条件会引发黑磷中直接-间接带隙转变？
• RQ2 单轴应变如何改变黑磷的电子能带结构与轨道特性？
• RQ3 应变作用下观测到的带隙转变背后的物理机制是什么？
• RQ4 应变如何影响不同晶向中载流子的有效质量？
• RQ5 能否通过应变工程实现带隙从间接到直接并可逆地恢复？

主要发现

• 黑磷在发生结构失效前可承受高达30%的拉伸应变或10 N/m的表面张力。
• 锯齿形方向−2%的压缩应变会触发直接-间接带隙转变。
• 在+11.3%或−10.2%应变下，带隙从间接恢复为直接，表明可实现可逆调制。
• 识别出五个具有独特电子结构的应变区域，其边界由临界应变阈值界定。
• 扶手椅方向的有效质量比锯齿形方向小一个数量级，有利于沿该方向实现更高的载流子迁移率。
• 有效质量在直接-间接转变区域的边界处出现急剧跃升或下降。