4D Scanning Ultrafast Electron Microscopy: Visualization of Materials Surface Dynamics

Abstract

The continuous electron beam of conventional scanning electron microscopes (SEM) limits the temporal resolution required for the study of ultrafast dynamics of materials surfaces. Here, we report the development of scanning ultrafast electron microscopy (S-UEM) as a time-resolved method with resolutions in both space and time. The approach is demonstrated in the investigation of the dynamics of semiconducting and metallic materials visualized using secondary-electron images and backscattering electron diffraction patterns. For probing, the electron packet was photogenerated from the sharp field-emitter tip of the microscope with a very low number of electrons in order to suppress space-charge repulsion between electrons and reach the ultrashort temporal resolution, an improvement of orders of magnitude when compared to the traditional beam-blanking method. Moreover, the spatial resolution of SEM is maintained, thus enabling spatiotemporal visualization of surface dynamics following the initiation of change by femtosecond heating or excitation. We discuss capabilities and potential applications of S-UEM in materials and biological science.

Real-Space Mapping of Surface Trap States in CIGSe Nanocrystals Using 4D Electron Microscopy

“…56 However, the dark contrast observed here suggests that SEs in the conduction band lose energy during transit to the surface and they do not reach the detector because of an energy deficit. 58,[61][62] In this case, scattering processes with photo-generated electron-hole pairs are most likely responsible for the energy loss.…”

“…[43][44][45][46][47][48][49][50][51][52][53][54][55] In particular, the invention of scanning ultrafast electron microscopy (S-UEM) provided the unique opportunity to selectively map charge carrier dynamics on the surface of a material with nm spatial and subpicosecond temporal resolutions. [56][57][58][59][60][61][62] In S-UEM, the optical pulse generated from a femtosecond (fs) laser system is used to generate electron packets from the tip of the scanning electron microscope, instead of the continuous electron beam used in the conventional setup. This pulse is synchronized with another optical excitation pulse that initiates carrier dynamics in the sample (Scheme 1).…”

“…A schematic diagram of the 4D S-UEM setup is provided in Figure S4 of the Supporting Information; detailed information on the principle of its operation has been published elsewhere. 56,59 Briefly, the output of a fs Clark- 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 8 MXR fiber laser system is integrated with a modified Quanta FEI-650 SEM. The IR pulses delivered by the laser are centered at 1030 nm with a pulse width of ~270 fs and the repetition rate ranging from 200 kHz to 25.4 MHz.…”

Visualization of carrier dynamics in p(n)-type GaAs by scanning ultrafast electron microscopy

“…Unlike UEM, which operates in the transmission mode, scanning UEM techniques exploit the time evolution of secondary electrons (SEs) produced in the specimen, and provide additional marked advantages over the transmission mode. These include a relatively facile sample preparation requirement, an efficient heat dissipation, a lower radiation damage, and an accessibility to low-voltage environmental study (2,3). Since its development this technique has been used to study carrier excitation dynamics in several prototypical semiconducting materials surfaces.…”

Imaging carrier dynamics on the surface of the N-type silicon

“…In this work, we introduce the scanning ultrafast electron microscopy (SUEM) technique, which allows the "direct image" of the spatiotemporal dynamics of carriers [2,3]. Specifically, we report the SUEM imaging of the surface photovoltage (SPV) dynamics in silicon, which results from the transient repopulation of the surface states by minority carriers [4].…”