Real-Space Imaging of Carrier Dynamics of Materials Surfaces by Second-Generation Four-Dimensional Scanning Ultrafast Electron Microscopy

Abstract

In the fields of photocatalysis and photovoltaics, ultrafast dynamical processes, including carrier trapping and recombination on material surfaces, are among the key factors that determine the overall energy conversion efficiency. A precise knowledge of these dynamical events on the nanometer (nm) and femtosecond (fs) scales was not accessible until recently. The only way to access such fundamental processes fully is to map the surface dynamics selectively in real space and time. In this study, we establish a second generation of four-dimensional scanning ultrafast electron microscopy (4D S-UEM) and demonstrate the ability to record time-resolved images (snapshots) of material surfaces with 650 fs and ∼5 nm temporal and spatial resolutions, respectively. In this method, the surface of a specimen is excited by a clocking optical pulse and imaged using a pulsed primary electron beam as a probe pulse, generating secondary electrons (SEs), which are emitted from the surface of the specimen in a manner that is sensitive to the local electron/hole density. This method provides direct and controllable information regarding surface dynamics. We clearly demonstrate how the surface morphology, grains, defects, and nanostructured features can significantly impact the overall dynamical processes on the surface of photoactive-materials. In addition, the ability to access two regimes of dynamical probing in a single experiment and the energy loss of SEs in semiconductor-nanoscale materials will also be discussed.

Charge dynamics in aluminum oxide thin film studied by ultrafast scanning electron microscopy

“…The SE dynamics measured on a slab of B-doped Si (1 0 0) allows to define the zero of the delay and to evaluate the effect of primary electron current on the time resolution of the system [25]. The dynamics obtained on silicon using different values of the electronic current is shown in Fig.…”

“…The rise time (see Table 1) appears to be strongly dependent on the number of electrons per pulse reaching the sample; this is attributed to a broadening of the electron probe pulse along the longitudinal direction of the microscope column due to coulombian repulsion forces acting among electrons during propagation from the tip to the sample [25], which limits the time resolution of the system.…”

Enhanced Optoelectronic Performance of a Passivated Nanowire‐Based Device: Key Information from Real‐Space Imaging Using 4D Electron Microscopy

“…This technique is capable of an unprecedented spatial resolution that cannot be achieved by any purely optical techniques . Very recently, as a new direction in 4D electron microscopy, a second‐generation scanning ultrafast electron microscope (S‐UEM) with temporal resolution of 650 fs was developed, providing the unique opportunity to selectively visualize the dynamics of charge carriers at the material surface, which is inaccessible by both transmission electron microscopy and ultrafast laser spectroscopy . In S‐UEM, the optical pulse generated from the fs‐laser is used to generate photoelectron wave packets instead of continuous electron beams, as in conventional electron microscopes, and it is synchronized with another optical pulse that initiates carrier dynamics in the sample.…”

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

“…[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.…”