Scanning ultrafast electron microscopy: A novel technique to probe photocarrier dynamics with high spatial and temporal resolutions

Abstract

The dynamics of photo-excited charge carriers, particularly their transport and interactions with defects and interfaces, play an essential role in determining the performance of a wide range of solar and optoelectronic devices. A thorough understanding of these processes requires tracking the motion of photocarriers in both space and time simultaneously with extremely high resolutions, which poses a significant challenge for previously developed techniques, mostly based on ultrafast optical spectroscopy. Scanning ultrafast electron microscopy (SUEM) is a recently developed photon-pump-electron-probe technique that combines the spatial resolution of scanning electron microscopes (SEM) and the temporal resolution of femtosecond ultrafast lasers. Despite many recent excellent reviews for the ultrafast electron microscopy, we dedicate this article specifically to SUEM, where we review the working principle and contrast mechanisms of SUEM in the secondary-electron-detection mode from a users' perspective and discuss the applications of SUEM to directly image photocarrier dynamics in various materials. Furthermore, we propose future theoretical and experimental directions for better understanding and fully utilizing the SUEM measurements to obtain detailed information about the dynamics of photocarriers. To conclude, we envision the potential of expanding SUEM into a versatile platform for probing photophysical processes beyond photocarrier dynamics.

Imaging photoinduced surface potentials on hybrid perovskites by real-time Scanning Electron Microscopy

“…Light-assisted SEM analyses can be provided with Time-Resolved detection schemes (TR-SEM), conceived and designed to address the different dynamical ranges. Ultrafast dynamics in photoinduced SE contrast have actually been retrieved stroboscopically, by exploiting pulsed PE beams (May et al, 1987) and in pumpprobe configurations (Cho et al, 2014;Liao and Najafi, 2017;Mohammed et al, 2011). Specifically, ultrafast SEM monitors light-induced charge dynamics, deriving out-of-equilibrium and hot-charge transport parameters at semiconductor surfaces and pn-junctions.…”

Lock-in Ultrafast Electron Microscopy Simultaneously Visualizes Carrier Recombination and Interface-Mediated Trapping

“… 14 − 17 In ultrafast scanning electron microscopy (USEM), a focused electron beam is used to probe the dynamics of a charge carrier distribution after laser excitation, thus bringing electron beam resolution into the traditional pump–probe schemes. 18 − 21 With USEM, carrier dynamics has been studied in bulk materials such as Si and GaAs, in crystals including CIGSe and CdSe, and in layered materials like black phosphorus and across a silicon p–n junction. 22 − 31 In all schemes, low-energy, 0–10 eV, secondary electrons (SEs) are used as the probe signal.…”

Micro‐ and Nanoscopic Imaging of Enzymatic Electrodes: A Review

“…One or several detectors collect these signals to form images . Resolutions achieved with modern SEMs can be as low as 1–20 nm …”

“…100 nm STM [38] 10-1000 ms atomic scale atomic scale Ultrafast STM [38] ns atomic scale atomic scale SEM [13] ca. 10 nm Ultrafast SEM ( [13] ) a few ps ca. 10 nm Wide-field FM ( [39] ) ca.…”