In situ Laser Synthesis, Processing and Characterization in the TEM

Rapid materials synthesis, processing and characterization enables a wide variety of materials systems with tuned properties. The objective of this dissertation is to demonstrate how a prototype setup allows laser illumination to be coupled into a (scanning) transmission electron microscope (TEM) for real-time observations of synthesis, processing, and characterization.

The laser synthesis of two-dimensional (2D) crystals and van der Waals (vdW) heterostructures is investigated through stepwise laser crystallization within a TEM. Amorphous tungsten selenide that was deposited by pulsed laser deposition (PLD) evolves through a series of metastable nanophases as crystallizing and coalescing into continuous 2D WSe₂ domains on monolayer graphene or MoSe₂ substrates. The lattice-matched MoSe₂ substrate is shown to play a guiding role in the formation of heteroepitaxial vdW WSe₂/MoSe₂ bilayers both during the crystallization process and afterwards, when crystalline nanosized domains are observed to coalesce by rotation, and grain boundary migration processes. In addition, the controllable implantation of hyperthermal species from PLD plasmas is introduced as a top-down method to compositionally engineer 2D monolayers and form Janus monolayers using in situ diagnostics. The chalcogen atoms on both sides of transition metal dichalcogenide (TMD) were resolved by grid tilting and the Janus structure of TMD was confirmed in atomic resolution for the first time. These in situ studies of pulsed laser-driven crystallization and implantation represent a transformational tool for the rapid exploration of synthesis pathways and lend insight to the growth of 2D crystals by PLD and laser processing methods.

Laser characterization within the TEM is demonstrated via experimentally accessing photon-stimulated electron energy-loss (sEEL) and electron energy-gain (EEG) responses of individual plasmonic nanoparticles via photon-plasmon-electron interactions induced by simultaneous irradiation of a continuous wave laser and continuous current electron probe. EEG and sEEL probabilities are equivalent and increase linearly in the low irradiance range; importantly the photon energy must be tuned in resonance with the plasmon energy for the sEEG and sEEL peaks to emerge. This study opens a fundamentally new approach to explore the quantum physics of excited-state plasmon resonances that does not rely on high intensity laser pulses or any modification to the EELS detector.