Design, construction, and operation of a compact, ultrafast 100kV electron diffraction instrument

Abstract

Elucidating the structure and properties of nanomaterials at greater resolutions necessitates the continuing development of novel imaging techniques. Electron imaging methods (such as electron microscopy/diffraction) are well-suited for probing matter at the nanoscale; for a given energy, the electron scattering cross-section is ~10⁵⁻⁶ higher than X-rays and ~10³ times less damaging [1]. Ultrafast electron imaging techniques are capable of spatial and temporal resolutions down to ~0.1 nm and 100 fs (femtosecond), respectively. This enables the observation of fundamental dynamic processes including photoinduced phase transitions, electron-phonon energy transfer, and the evolution of coherent phonons [2]. At present, open user access to the incredible power of these ultrafast techniques is generally limited to one ultrafast electron diffraction (UED) facility. Existing, well-established methods used to study nanomaterials such as X-ray diffraction and conventional electron microscopy have a plethora of commercially available, laboratory scale instruments which can be used to carry out experiments. In contrast, there are no similar turn-key devices that enable the study of ultrafast dynamic processes. The construction of an in-house ultrafast electron diffraction apparatus is one solution to the problem of instrument accessibility and the realization of time-demanding experiments with proper controls. In this thesis, I document the design, assembly, and use of a compact laboratory scale UED instrument. The instrument is capable of stable operation at 100 kV, with subsequent development and testing suggesting that it can reach voltages in excess of ~130 kV. The instrument is able to produce electron pulses with a temporal length of ~200 fs while containing a sufficient number of electrons for adequate signal-to-noise level. Two experiments were then carried out using the UED apparatus in order to showcase its time and spatial resolutions: electron deflection by photoinduced plasma, and the investigation of the charge density wave (CDW) material NbTe2. Analysis of the time-resolved diffraction data collected from the NbTe2 measurements suggests at 60 kV demonstrate sub-picosecond resolution in agreement with the predicted instrument response obtained from N-particle tracer simulations.