Department of Materials Science and Nanoengineering, Rice University , Houston, Texas 77005, United States.
Sandia National Laboratories , Albuquerque, New Mexico 87185, United States.
ACS Nano. 2017 Aug 22;11(8):8223-8230. doi: 10.1021/acsnano.7b03242. Epub 2017 Jul 19.
The values of the ionization energies of transition metal dichalcogenides (TMDs) are needed to assess their potential usefulness in semiconductor heterojunctions for high-performance optoelectronics. Here, we report on the systematic determination of ionization energies for three prototypical TMD monolayers (MoSe, WS, and MoS) on SiO using photoemission electron microscopy with deep ultraviolet illumination. The ionization energy displays a progressive decrease from MoS, to WS, to MoSe, in agreement with predictions of density functional theory calculations. Combined with the measured energy positions of the valence band edge at the Brillouin zone center, we deduce that, in the absence of interlayer coupling, a vertical heterojunction comprising any of the three TMD monolayers would form a staggered (type-II) band alignment. This band alignment could give rise to long-lived interlayer excitons that are potentially useful for valleytronics or efficient electron-hole separation in photovoltaics.
过渡金属二硫属化物（TMD）的电离能对于评估其在高性能光电半导体异质结中的潜在用途是必需的。在这里，我们报告了使用深紫外光照明的光电子显微镜系统地确定三种典型 TMD 单层（MoSe、WS 和 MoS）在 SiO 上的电离能。电离能从 MoS 到 WS 再到 MoSe 逐渐降低，与密度泛函理论计算的预测一致。结合布里渊区中心价带边缘的测量能量位置，我们推断，在没有层间耦合的情况下，由三种 TMD 单层中的任何一种组成的垂直异质结将形成交错（type-II）能带排列。这种能带排列可能会产生长寿命的层间激子，这对于谷电子学或光伏中的高效电子-空穴分离可能是有用的。
