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Solution-free Ion Exchange on Lead Halide Perovskite Thin-film and Infrared Characterization
No full textLead halide perovskite is an emerging material for photovoltaic applications with the standard
formula ABX3. CsPbBr3 has outstanding stability compared to other lead halide perovskite
materials. Lead halide perovskite band gap energy is tunable by ion exchange in the B (lead) and
X(halogen) sites. Close-Spaced Sublimation (CSS) is a scalable Physical Vapor Deposition
(PVD) method that can be used for CsPbBr3 thin-film manufacturing and ion exchanging on
CsPbBr3 thin-film. This thesis investigates the CsPbBr3 thin-film ion exchange on the B and X
sites in the CSS system and applies the infrared spectrum as a novel tool to characterize the
treated CsPbBr3 thin-films
Reliability and Condition Monitoring of SiC Power MOSFETs
No full textSilicon Carbide (SiC) power devices have witnessed increasing mainstream adoption in
transportation over the past decade. This is mainly because of the SiC’s high breakdown
strength, and thermal conductivity which translates to higher blocking voltage capability,
efficiency, and switching frequency compared to their silicon (Si) counterparts. Being a
relatively new technology, the long-term reliability of SiC MOSFETs in the field is not
well-known, and some reliability concerns still warrant attention. In particular, the gate-oxide
in SiC is inherently more susceptible to degradation when compared to Si devices. To prevent
potentially catastrophic failures especially in safety critical applications, it is essential to
address existing reliability concerns and improve the overall system reliability.
This dissertation aims to study the reliability and condition monitoring of SiC power MOS-
FETs. The dissertation first proposes two novel precursors, namely the Miller capacitance
and gate-source capacitance changes, which are independent of temperature, for monitoring
gate oxide degradation in SiC MOSFETs. These precursors allow for gate oxide degradation
monitoring without the need for decoupling the effects of package degradation. Using these
findings, the study presents a straightforward in-situ circuit for early warning monitoring of
gate oxide aging. Next, this dissertation introduces an effective solution for monitoring gate-
oxide degradation in SiC devices using transfer characteristics. In particular, this dissertation
proposes a plug-in circuit for the gate driver that can extract transconductance (gm) and
threshold voltage (Vth) values from the transfer characteristic with high precision. Lastly,
this dissertation proposes a 120 KVA AC power cycling test setup designed for high-power
Silicon Carbide (SiC) modules. Currently, there is a lack of early warning signals for drivers
to replace critical components in traction applications. To address this issue, this dissertation
proposes suitable precursors for all dominant failure mechanisms and corresponding condition
monitoring tools to monitor device aging on power converters. These monitoring tools are
integrated into the built-in desaturation protection circuit of the gate driver for low-cost and
practical implementation. The study monitors the on-resistance of all twelve switches online
as a temperature-sensitive electrical parameter (TSEP) to measure the devices’ junction
temperature. To reduce the processing load on the microcontroller, the study also intro-
duces an out-of-order equivalent time sampling technique for data sampling, which yields a
measurement error of less than 1.5%
Essays on Empirical Corporate Finance
No full textThis dissertation consists of two essays on empirical corporate finance.
The first essay, included in Chapter 1, is “Product Market Decisions and Subprime Lending
by Captive Finance Companies”. I study whether companies strategically utilize captive
financing, a form of providing funding to consumers, to manage product demand. Using
detailed data on auto loans, I show that captive lenders alter the financing terms and lending standards throughout the product life cycle. They reduce interest rates, allow longer maturity, charge lower down payments, and relax loan standards (1) when the underlying car models become outdated; (2) when competitors release new models; and (3) when
they experience exogenous shocks such as recalls. While the lower interest rates offered by
captive lenders reduce the likelihood of consumer default in the short term, the average
default rate eventually increases in the long horizon because captive lenders’ willingness to
dispense higher-risk loans allows more subprime borrowers to access credit. For consumers
who cannot find a loan from non-captive lenders, borrowing from captive lenders help them
in purchasing a car, but they could potentially be approved for a loan they cannot afford.
These findings collectively suggest that captive financing is a tool manufacturers use to boost
car sales throughout the product life cycle, while this tool could induce overleveraging by
consumers.
The second essay, included in Chapter 2, is “Franchising Dreams: Corporate Expansion
and Local Growth”. It is a joint work with Umit Gurun and Steven Xiao. In this study,
we investigate franchising strategies by constructing a comprehensive dataset of franchise
establishments. We find that companies tend to franchise their stores in new, remote, and
rural markets. Moreover, while local franchisee investments predict future local house price
growth, franchisor investments do not, suggesting that franchisees have information about
the economic prospects of local markets. The proportion of establishments directly invested by franchisors increases over time if existing establishments perform well, and the
follower establishments tend to perform better than the pioneer establishments. These findings collectively suggest that companies use franchising as an information-production tool for geographic expansion
Geometric Algorithms for Trajectory Planning and Facility Location Optimization Problems
No full textGeometry has long functioned as a bridge between abstract and real-world problems. In the
field of computational geometry, we design algorithms and data structures to solve computational problems efficiently by exploiting their intrinsic geometric properties. This dissertation
showcases our algorithmic contributions in utilizing computational geometry to solve a set
of problems in two distinct subject areas – i) robot motion planning and ii) facility location
theory.
First, we describe geometric algorithms for computing feasible trajectories for an articulated
two-segment robotic probe subject to specific motion constraints. We examine generalizations of the trajectory planning problem, in both two and three dimensions, with a fixed or
variable end segment. Our algorithmic solutions are non-trivial and exact, as opposed to approximations and heuristics that are often employed for complex motion planning problems
involving robots with restrictions and high degrees of freedom. The development of these
algorithms is primarily driven by the need for precise planning in minimally invasive robotic
surgeries in the medical domain.
Secondly, we analyze several variations of facility location optimization problems from a
geometric perspective. These problems involve finding the optimal location for a facility,
either a line segment or a point, based on its distances from a set of demand points in
fixed dimensions. We study variations such as discrete and center median line segments,
continuous median line segment, and medoid (i.e., discrete point). To solve these problems,
we create new geometric algorithms that are efficient, either exact or approximate with a
relative performance guarantee. These optimization problems are considered fundamental in
location science and an integral part of many industrial as well as data-science applications
Thin-film Schottky Diodes on Softening Polymer Substrates for Radio-frequency Bioelectronics
No full textThe next generation of implantable electronics for biomedical medicines must include features
which minimize the impact of the chronic inflammatory response to improve operational lifetime
and minimize discomfort in the body. For this reason, eliminating the mechanical mismatch and
effects that wires have on biological tissue could improve the state of present-day implantable
electronics. Wireless devices could serve longer lifetimes and reduce the likeliness of
complications that often arise in tethered electronics post-implantation. Moreover, by employing
flexible substrates for wireless bioelectronics, the inflammatory response could be further
mitigated and conformability to biological tissue enhanced. Yet, the design and fabrication of
wireless electronic components on flexible substrates is limited, and flexibility of the devices is
often sacrificed in exchange for the performance of rigid, silicon-based devices. Schottky diodes
are rectifying electronic components crucial in the development of implantable wireless
technology for biomedical medicines. In this work, we developed Schottky diodes on novel
stimuli-responsive flexible substrates. By incorporating Schottky diode technology on novel
flexible substrates that soften in response to temperature and moisture, the tradeoffs of
flexibility, conformability, and performance are explored on electronic components for wireless
technology. This work could pave the way for the next generation of soft, flexible electronics for
biomedical applications
Essays on Novelty in Online Reviews
No full textThe popularity of electronic word-of-mouth (e.g., online reviews and ratings) has increased
dramatically over the years. Consequently, it has become important to understand the
various ways in which online reviews impact different stakeholders, such as consumers, businesses, platforms, and reviewers. Despite the efforts to understand the relationship between
online reviews and consumers’ purchase decisions, a surprising omission in the existing research is the impact of new information provided in online reviews. Within this context,
my dissertation consists of three essays that focus on the impact and the use of novel information in online reviews. In the first essay, I examine how novel information in reviews
influences restaurant check-ins and review helpfulness and how this relationship changes as
review volume changes. I show that novel information positively impacts the helpfulness of
reviews and restaurant check-ins—however, the impact becomes negative when the review
volume becomes high. To provide a better understanding of the relationship between novel
information in reviews and restaurant check-ins, I do a mediation analysis and show that
review helpfulness partially mediates the effect of novel information on restaurant check-ins.
The indirect effect (through helpfulness) is moderated by review volume. I also show that
the impact of novel information on check-ins changes with restaurant type, with the impact
higher for high-priced restaurants. In the second essay, I focus on a common problem that
review platforms face when a large number of reviews are posted for a product or service.
Because readers are typically unable or unwilling to read all posted reviews, platforms aim
to identify a small subset of reviews that can provide enough information to help readers
make a decision. To address this problem, I present a formulation to determine the novelty of a review subset and propose an efficient review selection algorithm to maximize the
amount of novel information in a review subset given the subset size. The proposed approach greatly improves the amount of novel information relative to benchmark approaches.
I design heuristics for real-time environments and show the proposed approaches can be extended to scenarios preserving the average opinion and aspect coverage in the corpus. The
third essay examines the moderating role that credibility can play on the effect of novelty on
review helpfulness. This is motivated by prior studies from the psychology literature that
have shown the impact of credibility on the perception of information that people receive in
traditional environments. Research on review platforms has identified two types of credibility characteristics: source credibility (e.g., a platform-designated status of a reviewer) and
rating credibility (closeness of a review’s star rating to the product’s average star rating). I
examine the interaction between the two credibility characteristics (reviewer’s source credibility and review’s rating credibility) and a review’s novelty on a review’s helpfulness. I
show that while source credibility has a substitution effect on the impact of the novelty of
a review on its helpfulness, rating credibility has a complementary effect on the impact of
novelty
Olivier and Beyond: Film Adaptations of Shakespeare’s Hamlet
No full textAdaptations of Shakespeare’s plays have a long and wide-ranging history. For over four hundred
years, there have been many theatrical versions that have, more or less, followed the written
“text” of the play using various venues, settings, and casts. Beyond the stage, there are
novelizations and children’s stories, paintings and photographic tableaux, radio plays, and
symphonies and operas all inspired by Shakespeare’s works. More to the point of this
dissertation, filmmakers are especially fascinated with the works of Shakespeare. As long as
there have been movies, there have been Shakespearean—loose, traditional, or far from
traditional—film adaptations all over the world. Even television has been no stranger to
Shakespeare with its filmed stage productions, adapted films versions, themed episodes, or entire
seasons based on the plays. Scholarship treats just about every example mentioned above;
however, I am interested in how filmed adaptations of Shakespeare’s plays, beginning with
Laurence Olivier’s 1948 version of Hamlet, exhibit a unique tension in the ways they mix
innovation with preservation that can exert influence over subsequent versions and affect our
understanding and enjoyment of the play. In other words, this dissertation investigates how
adaptations of Shakespeare’s plays both embrace and resist alteration of their “original” source,
in this case Olivier foundational Hamlet film—seeking sometimes to change the material yet
wanting to return to or preserve some authentic founding “text” just as often.
This dissertation argues that filmed adaptations of Shakespeare’s plays demonstrate the tension
between tradition and innovation specifically because film as a medium asks different questions
of the plays. For example, film emphasizes a type of “realism” that is different from theatrical
illusion while also often elevating technological spectacle over language. Furthermore, film’s
conventional running times also affect the plays’ structure, sometimes causing substantial cuts to
the text. I contend that we can find similar traditional versus modern tension in most adaptations
of Shakespeare’s plays; however, his most filmed play, Hamlet, Prince of Denmark will be the
exclusive focus of my dissertation
Terahertz Up-conversion Mixers Using Varactors in CMOS and Their Applications
No full textWireless communication at frequencies above 100 GHz is drawing attention due to its high data
rate capability resulting from the wide available bandwidth. The recent advances of the high
frequency performance of complementary metal oxide semiconductor (CMOS) technology have
made it an affordable way for implementing the wireless systems. In order to support high-order
modulations to increase the data rate, and an increased range, the transmitter must have a high
output 1-dB compression point (OP1dB) and a wide bandwidth. Since the transistor fmax in CMOS
has peaked at ~350 GHz, it is challenging to implement 300-GHz transmitters in CMOS.
Consequently, the performance of the last up-conversion mixer in a transmitter is a key factor
determining its performance.
A 300-GHz sub-harmonic up-conversion mixer using symmetric varactors (SVAR’s) is
demonstrated. This mixer takes an IF signal centered at 150 GHz and up-converts to RF at 290
GHz with an LO of 70 GHz. Implemented in 65-nm CMOS, the mixer achieves the maximum
conversion gain (CG) of -16 dB and OP1dB of -11.4 dB. The OP1dB when reported was more than
10 dB higher compared to that of the other CMOS sub-harmonic up-conversion mixers in the
literature.
Fundamental mixing has superior conversion efficiency and output power. To increase CG and
OP1dB, a fundamental up-conversion mixer with a similar structure using asymmetric varactors
(ASVAR’s) is demonstrated. Using a similar transformer-based hybrid structure, this mixer
achieves measured CG of -12.5 dB. The OP1dB is greater than -2 dBm with LO power of 15 dBm
at 140 GHz. Due to the imbalance, a -21-dBm leakage at 2fLO is presented at the output.
To reduce the generation of unwanted harmonic terms, a double-balanced up-conversion mixer
using ASVAR is demonstrated in 65-nm CMOS. It utilizes a power-splitting-transformer hybrid
for differential signal isolation. The up-converter achieves measured OP1dB of -6.2 dBm and
maximum CG of -11.2 dB including input and output baluns, and a 3-dB bandwidth of ~25 GHz.
The CG and OP1dB are the highest among all up-converters in CMOS with RF at ~300 GHz. These
results are particularly critical for mixer-last transmitters operating near 300 GHz for high datarate communication.
A 280-GHz transmitter using the proposed double-balanced mixer is experimentally demonstrated
in 65-nm CMOS. The transmitter has a maximum output power of -8 dBm. The spectrum
measurement shows the capability of transmitting 30-Gbps QPSK signals. This transmitter is the
first ever demonstration of transmitters using varactor-based mixer above 100 GHz and supporting
such a data rate
Effect of Ionic Liquid Coatings on Early Healing and Osseointegration of Titanium Implants
No full textAlthough titanium (Ti) dental implants are known to achieve high success rates and
osseointegration in vivo, a higher incidence of implant failures have been recently reported.
Implant failures are caused by several factors; however, the nature and intensity of the
inflammatory response at the titanium-tissue interface determines the healing outcome of an
implant. Surface modifications performed on titanium implants have attempted to directly
address both patient and external factors that interfere with constructive inflammation, but often
do not address multiple complications that impact osseointegration while maintaining
regenerative healing. Dicationic imidazolium-based ionic liquids (IonL) have demonstrated low
toxicity, antimicrobial, lubricant, and anticorrosive activities in vitro making them a potential
candidate as a multifunctional dental implant coatings. However, the biological response to these
coatings in vivo is unknown. The goal of this dissertation was to evaluate the effect IonLs have
on inflammation, healing, and osseointegration of titanium dental implants. This research is
divided into three aims (i) to investigate the biocompatibility of IonL in a subcutaneous model,
(ii) to define and validate success criteria for an oral implantation model and (iii) to investigate
the impact of IonL on early healing and osseointegration in an oral implantation model. In each
aim a combination of clinical evaluation, histopathology, immunohistochemistry, molecular
analysis, and MicroCT was used to track inflammation and healing from 2-30 days (d) in the
Lewis rat.
In aim 1, an initial evaluation of both IonL-Phe and IonL-Met indicated that IonL appeared in
peri-implant tissues and increased acute inflammation at 2d compared to uncoated Ti. At 14d,
inflammation receded with more developed peri-implant tissue in coated and uncoated samples
with no foreign body giant cells. IonL was no longer observed at 14d, suggesting elution or
resorption by macrophages. This aim demonstrated that medium dose IonL-Phe does not
significantly interfere with Ti foreign body response in an aseptic environment.
In aim 2, a new pre-clinical oral implantation model defined an appropriate baseline for
successful Ti osseointegration. Healing was similar to other rodent models: hematoma and acute
inflammation at 2d, initial bone formation at 7d, advanced bone formation and remodeling at
14d, and bone maturation at 30d. Overall, this model resulted in a 78.5% osseointegration
success rate (>60% bone-to-implant contact (BIC)), similar to human osseointegration.
Therefore, This model combines the advantages of a rodent model while maximizing BIC,
making it an excellent candidate for evaluation of IonLs. Following aim 2, a pilot in vivo
assessment determined medium dose IonL-Phe demonstrated the best histogical response and
BIC for the remaining evaluation. In aim 3, IonL-Phe-coated and uncoated cpTi screws were
implanted into several demographic groups of rats to represent biological variations that could
affect healing. Molecular and histological analysis indicated IonL heightened acute inflammation
compared to uncoated Ti. However, the coating was released/resorbed by 7 days and did not
negatively affect subsequent bone remodeling in all demographics. Overall, IonL-Phe coating
did not disturb oral Ti osseointegration and may provide additional control over the healing
environment in scenarios known to be challenged by bacteria, such as peri-implantiti
Control of a Soft 3D-printed Artificial Finger Actuated by Coiled Shape Memory Alloy Muscles With Thermoelectric Cooling
No full textArtificial hands with many unique designs and capabilities have been presented in the literature;
however, most of these hands only have binary finger states, meaning only open or closed states.
Position control of fingers for these hands enables more precise manipulation of objects for a
wide variety of applications, expanding their usage beyond simple grasping tasks. An additional
area of focus, particularly with thermally actuated artificial muscles, is increasing actuation
frequency, which is currently limited by heat transfer rates. Improving frequency will allow for
faster response of the hands to input stimuli and accelerate their potential usage in robotic and
prosthetic applications.
The objective of this research is to improve upon existing robotic hand designs that utilize
thermally actuated artificial muscles. Such muscles have a high strength-to-weight ratio, low
profile and silent actuation, but suffer from low actuation frequency and energy inefficiency. In
this thesis, coiled shape memory alloy (SMA) muscles were studied and utilized for the design of
a robotic hand using soft 3D-printed thermoplastic polyurethane (TPU). First, discussion on the
design of the hand, manufacturing and characterization are presented. Next, finger position
control to the hand using embedded flex sensors to create a closed-loop system is presented. This
strategy maintains the ease of manufacturing of the TPU hand via 3D-printing, while also
introducing new sensing capability. An investigation into improving the actuation frequency of
the coiled SMAs using thermoelectric cooling via Peltier plates is also conducted. Holistically,
these additions to the 3D-printed hand are projected to increase its overall actuation speed and
precision, allowing for greater manipulation capabilities, but also adding additional weight and
complexity to the overall system