Department of Electronic and Computer Engineering, Hong Kong University of Science and Technology , Clear Water Bay, Hong Kong SAR, China.
Langmuir. 2018 Feb 27;34(8):2882-2889. doi: 10.1021/acs.langmuir.7b03840. Epub 2018 Feb 14.
Surface functionalization of the dangling-bond-free MoS, WSe, and other TMDs (transition metal dichalcogenides) is of large practical importance, for example, in providing nucleation sites for the subsequent high-k dielectric integration. Of the surface functionalization methods, the reversible O or N atom adsorption on top of the chalcogen atoms is most promising. However, hazards such as severe oxidation or nitridation persist when the adsorption coverage is high. An in situ characterization technique, which can be integrated with the surface functionalization and dielectric deposition chamber, becomes valuable to enable the real-time monitoring of surface adsorption conditions. Raman spectroscopy, as a nondestructive characterization method without vacuum requirement, is a strong candidate. By utilizing first-principles calculations, Raman spectra of single-layer MoS and WSe with various O/N adsorption coverages are studied. The calculations suggest that the low-coverage O/N adsorbates will act as perturbations to the periodic lattice and activate the acoustic-phonon Raman scatterings. While high-coverage adsorptions will further activate and intensify the optical-phonon Raman scatterings of previously silent A and E modes, due to the breaking of reflection symmetry in the z direction, new phonon modes associated with the adatom oscillations are also introduced. All these pieces of evidence, together with the peak shifts of previously active A and E modes, suggest that in situ resonant Raman spectroscopy is capable of providing important information to quantify the O/N adsorption coverage and can be used as a valuable real-time characterization technique to monitor and control the surface functionalization conditions of MoS and WSe.
悬空键自由的 MoS、WSe 和其他 TMD（过渡金属二卤化物）的表面功能化具有重要的实际意义，例如，为随后的高介电常数介电集成提供成核位点。在表面功能化方法中，最有前途的是可逆的 O 或 N 原子在硫属原子顶部的吸附。然而，当吸附覆盖率较高时，仍然存在严重的氧化或氮化等危险。一种原位表征技术，如果可以与表面功能化和介电沉积腔集成，将成为实时监测表面吸附条件的有价值工具。拉曼光谱作为一种无需真空要求的非破坏性表征方法，是一个强有力的候选者。通过利用第一性原理计算，研究了具有各种 O/N 吸附覆盖率的单层 MoS 和 WSe 的拉曼光谱。计算表明，低覆盖率的 O/N 吸附剂将作为对周期性晶格的微扰，并激活声子拉曼散射。而高覆盖率的吸附物将进一步激活和增强先前无声 A 和 E 模式的光学声子拉曼散射，由于在 z 方向上反射对称性的破坏，也引入了与吸附原子振动相关的新声子模式。所有这些证据，以及先前活性 A 和 E 模式的峰位移，表明原位共振拉曼光谱能够提供重要信息来定量 O/N 吸附覆盖率，并可作为一种有价值的实时表征技术，用于监测和控制 MoS 和 WSe 的表面功能化条件。
