Surface potentials can be measured with Kelvin probe force microscopy, 15 but the scanning probe may be difficult to combine with electron beam imaging of the sample. In addition, sensitivity to induced surface photovoltages is lacking. All-optical transient absorption techniques are traditionally used to measure charge carrier dynamics 13,14 but are limited by light optical diffraction, and the large penetration depth of photons may lead to reduced contrast from thin samples or surface layers. USEM is unique in its combination of direct electron beam microscopy, ultrafast carrier dynamics imaging, surface sensitivity, and surface photovoltage measurement.
#MORTAL KOMBAT PROJECT 4.1 SEASON 2.9 RESOLUTION FREE#
12 In addition, the change in the surface potential induced by the presence of photo-excited free charge carriers can be simultaneously measured with USEM through the local influence of this surface potential on the secondary electron (SE) trajectories and thereby SE collection efficiency. 10 The impressive surface sensitivity of USEM enabled by detection of low-energy secondary electrons also allows for resolving the influence of surface termination on carrier dynamics, as demonstrated on CdTe 11 and GaAs.
Materials and specimens whose charge carrier dynamics have been studied with USEM to date include bulk samples of silicon of various doping, 5–7 GaAs, 8 CdSe, 2,9 and alumina. 3,4 The majority of reports to date have targeted the imaging of semiconductor carrier dynamics, creating carriers through fs laser excitation and tracking them with electron probe pulses through a carrier induced change in the secondary electron signal. 1,2 In USEM, a pulsed laser beam excites (or pumps) the sample while a pulsed electron beam, scanning the sample at a fixed delay with respect to the laser pulse, probes the material response with high resolution. We illustrate our approach using molybdenum disulfide, a two-dimensional transition metal dichalcogenide, where we measure ultrafast carrier relaxation rates and induced negative surface potentials between different flakes selected with the scanning electron microscope as well as on defined positions within a single flake.įour-dimensional or ultrafast scanning electron microscopy (USEM) has in recent years been pioneered as a promising technique to study temporal dynamics in nanostructured materials with electron beam resolution. We further show that temporal laser pump resolution can be maintained inside the scanning electron microscope by pre-compensating dispersion induced by the components required to bring the beam into the vacuum chamber and to a tight focus. We demonstrate an order of magnitude improvement in optical pump resolution, bringing this to sub-micrometer length scales. Here, we present a system capable of focusing the laser using an inverted optical microscope built into an ultrafast scanning electron microscopy setup to enable high numerical aperture pulsed optical excitation in conjunction with ultrafast electron beam probing. Current implementations have left a four order of magnitude gap between optical pump and electron probe resolution, which particularly hampers spatial resolution in the investigation of carrier induced local surface photovoltages.
Ultrafast scanning electron microscopy images carrier dynamics and carrier induced surface voltages using a laser pump electron probe scheme, potentially surpassing all-optical techniques in probe resolution and surface sensitivity.
0 kommentar(er)
