Zhao Jianfeng, Bakr Osman M, Mohammed Omar F
Division of Physical Science and Engineering, King Abdullah University of Science and Technology, Thuwal 23955-6900, Saudi Arabia.
Struct Dyn. 2020 Mar 30;7(2):021001. doi: 10.1063/4.0000007. eCollection 2020 Mar.
The performance of optoelectronic devices strongly depends on charge carrier dynamics on top of surfaces of the absorber layers. Unfortunately, this information cannot be selectively probed using conventional ultrafast laser spectroscopic methods, due to the large penetration depth (tens of nm to m) of the photon pulses in the pump-probe configurations. Therefore, ultrafast time-resolved approaches that can directly and selectively visualize the behavior of the surface carrier dynamics are urgently needed. Here, we introduce a novel methodology of low-voltage scanning ultrafast electron microscopy that can take ultrafast time-resolved images (snapshots) of the surface of materials at the sub-nanometer level. By this approach, the surface of the photoactive materials is optically excited and imaged, using a pulsed low-voltage electron beam (1 keV) that interacts with the surface to generate secondary electrons with an energy of a few eV, and that are emitted only from the top surface of materials, providing direct information about the carrier dynamics and the localization of electron/holes in real space and time. An outlook on the potential applications of this low voltage approach in different disciplines will also be discussed.
光电器件的性能在很大程度上取决于吸收层表面的电荷载流子动力学。不幸的是，由于泵浦-探测配置中光子脉冲的穿透深度较大（几十纳米到数米），使用传统的超快激光光谱方法无法选择性地探测到这些信息。因此，迫切需要能够直接且选择性地可视化表面载流子动力学行为的超快时间分辨方法。在此，我们介绍一种新型的低电压扫描超快电子显微镜方法，该方法能够在亚纳米尺度上获取材料表面的超快时间分辨图像（快照）。通过这种方法，利用脉冲低电压电子束（1 keV）对光活性材料的表面进行光学激发和成像，该电子束与表面相互作用产生能量为几电子伏特的二次电子，且这些二次电子仅从材料的顶面发射，从而提供有关载流子动力学以及电子/空穴在实空间和时间中的局域化的直接信息。还将讨论这种低电压方法在不同学科中的潜在应用前景。
