Date of Award

Spring 2011

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Electrical & Computer Engineering

Committee Director

Hani E. Elsayed-Ali

Committee Member

Helmut Baumbart

Committee Member

Gon Namkoong

Committee Member

Abdelmageed Elmustafa

Abstract

The advancement in ultrafast electron crystallography (UEC) over the past few decades facilitated the study of structural dynamics in all phases of matter induced by femtosecond laser pulses. This technique became very powerful when the spatial resolution was combined with the temporal resolution, and succeeded in studying chemical reactions by ultrafast electron diffraction, bulk crystal phonons and melting by X-ray diffraction.

In this dissertation, I demonstrate the uniqueness of UEC and its potential in monitoring in real time the structural dynamics of bismuth (Bi) nanoclusters and islands induced by femtosecond laser pulses. Our approach to accomplish this task includes building a time-resolved high energy electron diffraction setup that is capable of delivering high energy and short electron pulses, less than 3 ps, which will facilitate the real time measurement of the Bragg diffraction ring intensity, shift in the peak position and the diffraction ring full width at half maximum (FWHM) at different delay times with respect to the femtosecond excitation. Additionally, the temperature evolution of the same parameters, intensity, position and FWHM of the diffraction peaks, was monitored by using conventional direct current heating stage.

Another task was accomplished in which I utilized the pump-probe ultrafast electron diffraction setup that I built and tested with picoseconds laser pulses in PERI lab - Old Dominion University, transferred later to the Applied Research Center where a femtosecond laser system was used to characterize the transient effects induced in Bi nanoclusters due to femtosecond laser excitation. The sample under consideration is excited by a femtosecond laser pulses with moderate fluence just to induce an observable change in the diffraction pattern and far from sample damage. The femtosecond laser pulses induce changes in the charge carrier distribution function of Bi nanoclusters, which leads to a disturbance in the lattice potential and drives the solid-liquid phase transformation. The melting is detected as decrease in the integrated intensity of the Bragg peaks with time delay.

Another interesting behavior is observed in these experiments in which a lattice contraction following femtosecond laser excitation and proceeding over a time period of ∼ 6ps precedes the lattice expansion in Bi (012) planes. Again, the electronic excitation, here, plays an important role in inducing a sudden change in the interatomic forces which leads to A1g phonon excitation. Due to the limited resolution of our system (2–3 ps) we were not able to detect the A1g oscillation frequency/wavelength, but its effects which appear as lattice contraction upon its decay can be seen from the temporal evolution of the Bragg peak position over the time period, 0 < t < 6 ps.

The incident laser fluence was not high enough to induce full melting, but was enough to induce partial lattice melting. This was observed as a gradual increase in the FWHM of the Bragg peaks as a function of delay time, i.e., formation of thin liquid layer which increases in size with time when the lattice temperature increases through electron-phonon and/or phonon-phonon relaxation.

Also, the time evolution of the relative Bragg peak intensity, Δd/d and FWHM were monitored for Bi islands. Bi islands were prepared by annealing the as-deposited Bi thin film (5 nm, average coverage) solely by either raising its temperature slowly up to ∼ 525 K or with ultrafast laser pulses of fluence 0.8 mJ/cm² - 2.4 mJ/cm².

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DOI

10.25777/afr9-1j61

ISBN

9781124664019

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