This dissertation presents the recent developments and experiments performed using the third generation femtosecond electron
diffractometer in Professor Jianming Cao's group as well as experiments performed using the previous second generation diffractometer now
located at Shanghai Jiao Tong University. Two techniques of ultrafast electron diffraction (UED), time-resolved reflection high energy electron
diffraction (Tr-RHEED) and time-resolved transmission electron diffraction (Tr-TED) were developed and applied to study the ultrafast lattice
dynamics in semiconductor nanostructures. Tr-RHEED provides the ability to directly monitor the thermal transport across an interface inside a
semiconductor quantum well (QW) by measuring the temperature evolution of the first few atomic layers. Tr-TED allows for a measurement of the
laser-induced ultrafast structural dynamics of 5 nm PbSe quantum dots (QDs) in real time by diffracting through the entire sample thickness. In
the first project, the setup of the first Tr-RHEED experiments and the first successful collection of Tr-RHEED data in our laboratory's history
is discussed. The ultrafast temperature evolution of the GaAs nanofilm was measured and numerically modeled using the well known heat conduction
equation and also a three-temperature model. These models were fit to the experimental data, allowing for the extraction of the thermal boundary
conductance (TBC) and providing a method of measuring TBC in epitaxially grown semiconductor heterostructures. Surprisingly, the TBC was found
to increase with increasing temperature even for temperatures above the Debye temperature, opening up questions about the exact mechanisms
governing heat transfer at interfaces between very similar semiconductor nanoscale materials. In the second project, we directly monitored the
lattice dynamics in PbSe quantum dots induced by laser excitation using Tr-TED. The energy relaxation between the carriers and the lattice took
place within 10 ps, showing no evidence of any significant phonon bottleneck effect. Meanwhile, the lattice dilation exhibited some unusual
features that could not be explained by the available mechanisms of photon-induced acoustic vibrations in semiconductors alone. The heat
transport between the QDs and the substrate deviates significantly from Fourier's Law, which furthers studies about the heat transfer under
nonequilibrium conditions in nanoscale materials. In addition to the UED projects, femtosecond transient spectroscopy (FTS) experiments were set
up and tested on 20 nm gold nanofilms for various optical excitation laser fluences. The experimental data obtained agrees well with many
previous published results. The well known two-temperature model (TTM) was used to describe the temperature evolution and the energy
redistribution from the electronic to lattice systems. Using similar experimental and data analysis techniques to the ones developed in this
dissertation will pave the way for future FTS experiments performed in conjunction to UED experiments to gain a more complete picture of the
ultrafast dynamics in carriers and phonons in complex materials. / A Dissertation submitted to the Department of Physics in partial fulfillment of the requirements for the degree
of Doctor of Philosophy. / Fall Semester 2018. / October 19, 2018. / GaAs/AlGaAs Quantum Wells, Lattice Dynamics, Nanoscale Thermal Transport, Phonon Bottleneck, Thermal Boundary
Conductance, Ultrafast Electron Diffraction / Includes bibliographical references. / Jianming Cao, Professor Directing Dissertation; Wei Yang, University Representative; Peng Xiong, Committee
Member; Mark Riley, Committee Member; Nicholas Bonesteel, Committee Member.
| Identifer | oai:union.ndltd.org:fsu.edu/oai:fsu.digital.flvc.org:fsu_661140 |
| Contributors | Gorfien, Matthew Charles (author), Cao, Jianming (professor directing dissertation), Yang, Wei (university representative), Xiong, Peng (committee member), Riley, Mark A. (committee member), Bonesteel, N. E. (committee member), Florida State University (degree granting institution), College of Arts and Sciences (degree granting college), Department of Physics (degree granting departmentdgg) |
| Publisher | Florida State University |
| Source Sets | Florida State University |
| Language | English, English |
| Detected Language | English |
| Type | Text, text, doctoral thesis |
| Format | 1 online resource (160 pages), computer, application/pdf |
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