Multimodal Ultrafast Scanning Electron Microscopy

Ultrafast Transmission Electron Microscopy (UTEM) with field-emitter sources [1] is one of the most advanced techniques to study materials dynamics on the nanoscale. Spatially coherent femtosecond electron pulses serve as nearly universal probes in ultrafast imaging, diffraction and spectroscopy. Despite the reduced complexity of electron-beam instrumentation at lower energies, Ultrafast Scanning Electron Microscopy (USEM) [2], has experienced somewhat fewer advancements in recent years, in part due to limitations in available contrast modes. In this contribution, we present the development and application of a newly designed multimodal USEM allowing for the simultaneous measurement of different electron probe signals (Fig. 1a, b). The instrument is based on pulsed photoemission from a Schottky field emitter driven by femtosecond laser pulses, enabling adjustable pulse duration and repetition rates. Electron transmission or reflection, as well as secondary electron signals can be recorded over a wide range of electron energies from 100- eV to 30 keV. First proof-of-concept studies with the multimodal USEM address the structure and dynamics of the charge-density wave compound 1T-TaSe2. Thin flakes of 20-50 nm thicknesses are deposited onto a gold aperture with a diameter of 2 µm, allowing for transmission of the electron beam through the sample. Direct real-space imaging is performed by recording secondary electrons with a scintillator-based in-lens detector at each scan position (inset of Fig. 1a), providing navigation on the sample and rapid survey images. Electrons transmitted through the sample are recorded using a phosphor-screen microchannel plate detector and a CMOS camera. This configuration enables the acquisition of electron diffraction patterns with high sensitivity and speed (bottom inset of Fig. 1a). Raster-scanning the focused electron probe across the sample and recording a diffraction pattern at each scan position, we realize a four-dimensional scanning transmission electron microscopy (4D-STEM) modality, in which two real-space dimensions are complemented by two reciprocal-space dimensions. The bottom inset of Fig. 1a shows an exemplary diffraction pattern acquired at the marked location in the corresponding real-space SE image. Diffraction patterns exhibit well-defined diffraction reflexes associated with the crystalline lattice of 1T-TaSe2, as well as an array of additional superstructure reflections stemming from the commensurate room-temperature charge density wave (CDW) (zoom-in). From this four-dimensional raster-scanned dataset, real-space images are constructed by integrating the intensity of selected regions in reciprocal space for each scan position. Specifically, three complementary image types are displayed in Fig. 1: (I) a bright-field (BF) image obtained from the intensity of the fundamental beam ((0,0) spot), (II) image constructed by integrating the intensity of fundamental lattice reflections (main lattice spots), and (III) an image based on the intensity of the CDW-related superstructure peaks (see top row in Fig. 1c). These images map local structural inhomogeneities in thickness, laminar structure and strain within the sample. Optical excitation of the sample by femtosecond pump pulses at 1030 nm wavelength induces a suppression of the superstructure reflexes on a time scale of few picoseconds (Fig. 1d) [3,4] for prior ultrafast electron diffraction studies on this material without spatial resolution. In the data shown, the time scale of the suppression of the CDW peaks is limited by the ∼2 ps space-charge-limited electron pulse duration used. Optimizing electron-gun and photoemission conditions, we anticipate electron pulse durations comparable to the 200 fs obtained for Ultrafast TEMs [1]. Moreover, optical excitation induces a time-independent spatially heterogeneous change in contrast across the membrane (Fig. 1c), indicative of thermal strain and bending, which is analyzed quantitatively using dynamical diffraction simulations. Finally, we will use the instrument to explore complementary secondary electron contrast changes resulting from changes in electronic structure upon the phase transition. In conclusion, the multimodal USEM allows for versatile ultrafast imaging of nanoscale structural and electronic dynamics in materials, combining high spatial and reciprocal-space resolutions, secondary electron detection, as well as enabling future extensions using energy filtering and cathodoluminescence emission [5]. Spatially and momentum resolved, laser-induced phase transition in 1T-TaSe2 thin-films (20-50 nm). a) Schematic of the ultrafast scanning electron microscope and experimental setup, including secondary-electron images (SE), bright- and dark-field (BF and DF) specimen images at 20 keV electron energies. Exemplary diffracted primary electron pattern in transmission at the point marked in the SE image, showing main-lattice (Main) and charge-density wave (CDW) spots. b) Photograph of the prototype electron microscope. c) Series of nano-beam diffraction images, taken with a photoemitted (continuous-wave laser) 20 keV electron beam. The images display BF-, Main-lattice-peak- and CDW-peak-filtered intensity maps. Top and bottom rows show the 1T-TaSe2 thin film without and with laser excitation corresponding to the commensurate (C) and incommensurate (IC) phases, respectively. The diffraction images on the right show the suppression of the CDW peaks above a critical temperature TC. The scans in the middle row were taken with the laser beam two laser-focus diameters misaligned from the aperture, which results in an indirect excitation and a reduced mean temperature at TaSe2 flake. d) Pump-probe delay measurement on the sample shown in c). Inset shows a difference image of the diffraction after and before time zero.