Structural Dynamics and Atomic Motion in Thin Films Studied by Ultrafast Electron Diffraction and Transient Optical Spectroscopy

Abstract

Because of their unique structure, thin films provide an unprecedented view into the fundamental physics of a two-dimensional world. There is also an enormous demand for such materials in applied fields, and many thin films find use as platforms for device applications.

To further our understanding of such materials, this thesis investigates the properties of thin films on the time- and length-scales associated with atomic motions. To do this, two techniques with access to these extreme scales were used: transient spectroscopy, and ultrafast electron diffraction. Transient spectroscopy is equipped with the temporal resolution required to witness chemical dynamics; this fact is demonstrated in this thesis by an experiment which probes the ultrafast formation of graphene from an oxidized precursor. However, only ultrafast electron diffraction has the spatial resolution required to watch atoms move in real time.

Thin films add another layer of complexity to such already challenging experiments due to the fact that they typically have a minimal response to optical and electron probes as a result of their low-dimensionality. To address this issue, this thesis introduces several novel design principles in order to optimize ultrafast electron diffraction for studying thin films and monolayers. This culminates in the construction of a low-energy electron diffractometer, the first of its kind in the world. The successful demonstration of this machine to study the transient electric fields produced near a laser-irradiated graphene surface confirms that the electron diffraction can interrogate the two-dimensional world.

However, this device, as with all conventional ultrafast electron diffractometers, is poorly-suited to investigate irreversible reactions, a restriction that has recently been lifted with the advent of ultrafast streak cameras. Because streaking is a fairly new technique, there are many open questions as to how to interpret the streaked diffraction data. In this thesis, ultrafast streaking is put on firm theoretical grounds through the development of a new analysis approach that allows the entire time-dependent diffraction pattern to be recovered from a single streaked diffraction image. This development enables access to the entire range of possible thin film dynamics, both reversible and irreversible.