Department of Mechanical Engineering , University of Connecticut , Storrs , Connecticut 06269 , United States.
Institute of Materials Science , University of Connecticut , Storrs , Connecticut 06269 , United States.
Nano Lett. 2019 Mar 13;19(3):1527-1533. doi: 10.1021/acs.nanolett.8b04269. Epub 2019 Feb 26.
Isotopes of an element have the same electron number but differ in neutron number and atomic mass. However, due to the thickness-dependent properties in MX (M = Mo, W; X = S, Se, Te) transition metal dichalcogenides (TMDs), the isotopic effect in atomically thin TMDs still remains unclear especially for phonon-assisted indirect excitonic transitions. Here, we report the first observation of the isotope effect on the electronic and vibrational properties of a TMD material, using naturally abundant WSe and isotopically pure WSe bilayer single crystals over a temperature range of 4.4-300 K. We demonstrate a higher optical band gap energy in WSe than in WSe (3.9 ± 0.7 meV from 4.41 to 300 K), which is surprising as isotopes are neutral impurities. Phonon energies decrease in the isotopically pure crystal due to the atomic mass dependence of harmonic oscillations, with correspondingly longer E and A phonon lifetimes than in the naturally abundant sample. The change in electronic band gap renormalization energy is postulated as being the dominant mechanism responsible for the change in optical emission spectra.
一种元素的同位素具有相同的电子数，但中子数和原子质量不同。然而，由于 MX（M = Mo，W；X = S，Se，Te）过渡金属二卤代物（TMD）中存在厚度相关的特性，原子层厚度的 TMD 中的同位素效应仍然不清楚，特别是对于声子辅助间接激子跃迁。在这里，我们报告了首例对 TMD 材料电子和振动性质的同位素效应的观察，使用在 4.4-300 K 温度范围内天然丰度的 WSe 和同位素纯 WSe 双层单晶。我们证明了在 WSe 中比在 WSe 中具有更高的光学带隙能量（4.41 到 300 K 时为 3.9 ± 0.7 meV），这令人惊讶，因为同位素是中性杂质。由于简谐振动的原子质量依赖性，同位素纯晶体中的声子能量降低，相应地比在天然丰度样品中的 E 和 A 声子寿命更长。电子能带隙重整化能量的变化被假定为导致光发射光谱变化的主要机制。
