| 报告题目 | Nano-optical spectroscopic imaging of monolayer MoS2 |
| 报告人 | Dr. Nicholas J. Borys |
| 报告人单位 | Molecular Foundry, Lawrence Berkeley National Lab, Berkeley CA, USA |
| 报告时间 | 2015-03-31 |
| 报告地点 | 合肥微尺度物质科学国家实验室一楼科技展厅 |
| 主办单位 | 合肥微尺度物质科学国家实验室 |
| 报告介绍 | Abstract： Monolayer MoS2 (ML-MoS2) has emerged as a prototypical semiconductor for high performance, flexible 2D atomically thin optoelectronics devices including field effect transistors, ultrafast photodetectors, sensors, light emitting devices and even photovoltaics. At only 0.7 nm thick, dielectric screening is reduced, enhancing the coulombic interaction between electrons and holes and yielding tightly bound exciton states with binding energies that exceed room temperature. This complex manifold of excitons in conjunction with local structure and electronic properties govern the performance and capabilities of ML-MoS2 and other transition metal dichalcogenides, and because these materials form direct bandgap semiconductors, these exciton states show remarkably strong light-matter interactions which are readily probed with optical measurement techniques such as photoluminescence and absorption spectroscopy. In particular, spatially mapping the photoluminescence intensity and energy that arises from radiative relaxation of the exciton states provides direct access to variations in crucial optoelectronic properties such as exciton diffusion length, local carrier concentration, defect density, strain and exciton energy. However, the spatial resolution of traditional optical techniques is constrained by the diffraction limit to hundreds of nanometers, which is well above the characteristic nanoscale dimensions of these optoelectronic properties in ML-MoS2 and related 2D materials. Although scanning nearfield optical microscopy (SNOM) is able to image with sub-diffraction resolution, applying the technique to inelastic light-matter interactions such as photoluminescence in 2D systems constitutes a formidable challenge. Using the recently developed Campanile probe, nearfield optical microscopy is performed for the first time to map the photoluminescence of synthetically grown ML-MoS2 with sub-diffraction spatial resolution that approaches critical optoelectronic length scales. The unique structure of the Campanile probe enables background-free, full spectroscopic imaging of the photoluminescence and resolves nanoscale variations in emission intensity and energy. The enhanced resolution combined with detailed spectral analysis reveals a unique disordered peripheral edge state that surrounds a pristine, crystalline interior. Furthermore, the spatial extent of excited state quenching by grain boundaries in polycrystalline ML-MoS2 is quantified, providing an upper bound on quantities such as the exciton diffusion length. In addition to providing fundamental insight into the optoelectronic properties of ML-MoS2 and other transition metal dichalcogenides, the results elucidate crucial considerations for the development of reliable, large-scale synthesis procedures for these 2D semiconductors. |
