Zhang Yingchao, Voronine Dmitri V, Qiu Shangran, Sinyukov Alexander M, Hamilton Mary, Liege Zachary, Sokolov Alexei V, Zhang Zhenrong, Scully Marlan O
Texas A&M University, College Station, TX 77843, USA.
Xi'an Jiaotong University, Xi'an, Shaanxi 710049, China.
Sci Rep. 2016 May 25;6:25788. doi: 10.1038/srep25788.
Two-dimensional (2D) materials beyond graphene such as transition metal dichalcogenides (TMDs) have unique mechanical, optical and electronic properties with promising applications in flexible devices, catalysis and sensing. Optical imaging of TMDs using photoluminescence and Raman spectroscopy can reveal the effects of structure, strain, doping, edge states, and surface functionalization from materials to bioscience. However, Raman signals are inherently weak and so far have been limited in spatial resolution in TMDs to a few hundred nanometres which is much larger than the intrinsic scale of these effects. Here we overcome the diffraction limit by using resonant tip-enhanced Raman scattering (TERS) of few-layer MoS2, and obtain nanoscale optical images with ~20 nm spatial resolution. This becomes possible due to electric field enhancement in an optimized subnanometre-gap resonant tip-substrate configuration. We investigate the limits of signal enhancement by varying the tip-sample gap with sub-Angstrom precision and observe a quantum quenching behavior, as well as a Schottky-Ohmic transition, for subnanometre gaps, which enable surface mapping based on this new contrast mechanism. This quantum regime of plasmonic gap-mode enhancement with a few nanometre thick MoS2 junction may be used for designing new quantum optoelectronic devices and sensors with a wide range of applications.
除石墨烯之外的二维（2D）材料，如过渡金属二硫属化物（TMDs），具有独特的机械、光学和电子特性，在柔性器件、催化和传感等领域有着广阔的应用前景。利用光致发光和拉曼光谱对TMDs进行光学成像，可以揭示从材料到生物科学领域中结构、应变、掺杂、边缘态和表面功能化等方面的影响。然而，拉曼信号本身较弱，到目前为止，TMDs中拉曼信号的空间分辨率仅限于几百纳米，这比这些效应的本征尺度大得多。在这里，我们通过使用少层MoS2的共振针尖增强拉曼散射（TERS）克服了衍射极限，并获得了空间分辨率约为20 nm的纳米级光学图像。这是由于在优化的亚纳米间隙共振针尖 - 基底配置中电场增强才得以实现。我们通过以亚埃精度改变针尖 - 样品间隙来研究信号增强的极限，并观察到对于亚纳米间隙存在量子猝灭行为以及肖特基 - 欧姆转变，这使得基于这种新的对比度机制进行表面映射成为可能。这种具有几纳米厚MoS2结的等离子体间隙模式增强的量子机制可用于设计具有广泛应用的新型量子光电器件和传感器。
