Yuan Pengyu, Tan Hong, Wang Ridong, Wang Tianyu, Wang Xinwei
Department of Mechanical Engineering, Iowa State University Ames IA 50011 USA
School of Energy and Power Engineering, Nanjing University of Science and Technology Nanjing Jiangsu 210094 China.
RSC Adv. 2018 Apr 3;8(23):12767-12778. doi: 10.1039/c8ra01106k.
The currently reported optical-phonon-scattering-limited carrier mobility of MoS is up to 417 cm V s with two-side dielectric screening: one normal- side and one high- side. Herein, using picosecond energy transport state-resolved Raman (ET-Raman), we demonstrated very fast hot carrier diffusion in μm-scale (lateral) unconstrained MoS (1.8-18 nm thick) on a glass substrate; this method enables only one-side normal- dielectric screening. The ET-Raman method directly probes the diffusion of the hot carrier and its contribution to phonon transfer without contact and additional sample preparation and provides unprecedented insight into the intrinsic of MoS. The measured values span from 0.76 to 9.7 cm s. A nonmonotonic thickness-dependent trend is discovered, and it peaks at 3.0 nm thickness. This is explained by the competition between two physical phenomena: with an increase in sample thickness, the increased screening of the substrate results in higher mobility; moreover, thicker samples are subject to more surface contamination, loose substrate contact and weaker substrate dielectric screening. The corresponding carrier mobility varies from 31.0 to 388.5 cm V s. This mobility is surprisingly high considering the normal- and single side dielectric screening by the glass substrate. This is a direct result of the less-damaged structure of MoS that is superior to those of MoS samples reported in literature studies that are subjected to various post-processing techniques to facilitate measurement. The very high hot carrier mobility reduces the local carrier concentration and enhances the Raman signal, which is further confirmed by our Raman signal studies and comparison with theoretical studies.
目前报道的在双面介质屏蔽（一侧为常规屏蔽，另一侧为高屏蔽）情况下，二硫化钼（MoS）的光学声子散射限制载流子迁移率高达417 cm² V⁻¹ s⁻¹。在此，我们使用皮秒能量传输状态分辨拉曼光谱（ET - Raman），证明了在玻璃衬底上微米级（横向）无约束的二硫化钼（厚度为1.8 - 18 nm）中热载流子的快速扩散；该方法仅能实现单侧常规介质屏蔽。ET - Raman方法无需接触和额外的样品制备即可直接探测热载流子的扩散及其对声子转移的贡献，并为二硫化钼的本征特性提供了前所未有的深入见解。测量得到的扩散系数值范围为0.76至9.7 cm² s⁻¹。我们发现了一种非单调的厚度依赖性扩散系数趋势，其在3.0 nm厚度时达到峰值。这可以通过两种物理现象之间的竞争来解释：随着样品厚度增加，衬底屏蔽增强导致迁移率提高；此外，较厚的样品更容易受到表面污染、与衬底接触松散以及衬底介质屏蔽较弱的影响。相应的载流子迁移率在31.0至388.5 cm² V⁻¹ s⁻¹之间变化。考虑到玻璃衬底的常规和单侧介质屏蔽，这种迁移率出奇地高。这是二硫化钼结构损伤较小的直接结果，优于文献研究中报道的经过各种后处理技术以利于测量的二硫化钼样品。极高的热载流子迁移率降低了局部载流子浓度并增强了拉曼信号，这通过我们的拉曼信号研究以及与理论研究的比较得到了进一步证实。
