My interest in materials dated from high school, when I found love in Chemistry. I self-studied
several books on College Chemistry and even audited Organic Chemistry course in a
university. My interest and hardworking finally result in my success winning the first prize in the
National Chemistry Contest.
When I stepped into college, I chose Material Science and Engineering as my major, which I
thought could found more application. I studied very hard. Even when preparing for GRE test, I
never skipped a class, and usually worked out very late to revise the knowledge taught in
daytime. The hardship during this period helped me build a solid foundation on my major
courses, which was quite useful for my further research. For instance, the knowledge of lattice
structure learned in the major course, Solid State Physics, was frequently used in TEM and
XRD analysis in my research.
But neither the high GPA in my major courses (1st in my class) nor GRE score could satiate me.
I wanted to know more than the theoretical knowledge written in the books. Amazed by the
miracles happened in the Physics Lab, I managed to attend several research projects in
various fields, including steels, light alloys, polymers and nanomaterials, most of which gave
me a deeper insight and research experience in my major. After spending much time in
laboratories working in different groups, I finally found nanomaterials interested me most.
Because of the quantum confinement and the increase of surface to volume ratio; unique
property would show up, such as high catalysis of Au nano particles. And that was the very
driving force that motivated me continuing my further study as a Ph.D. in this field.
My research on nanomaterials started with the synthesis of Ga-doped ZnO (GZO)
nanocrystals (NCs). Among Group III elements, I chose Ga as the dopant to enhance the
electrical conductance of pure ZnO due to its low reactivity to oxygen and the little deformation
induced with the replacement of Zn2+ by Ga3+. Besides, hot injection method was used in the
experiment because of good colloidal stability, small size and narrow size distribution of the
product. The GZO nanocrystals were then dispersed into a stable nanocrystal ink, which was
spun coated into a thin film and annealed at relatively low temperature. I also utilize TEM,
SAED, SEM, EDS, UV spectrometer and four-point probe to characterize the structure,
composition, optical and electrical property of GZO, demonstrating its potential as a
Transparent Conducting Oxides (TCO), which could be applied in LEDs and thin film solar
cells.
In a similar approach as described before, I conducted another research on Cu2ZnSnS4xSe4(1-x)
(CZTSSe) NCs, which could be used as light absorber in heterojunctional solar cells. A tunable
band gap was found in CZTSSe NCs with the change the Se/(S+Se) ratio, which could be
used to maximize the energy transformation efficiency. In addition, S rich samples and Se rich
samples were observed different in shape with different Zn content. By using the knowledge of
crystal nucleation and growth learned in my major course, Fundamentals of Material Science, I
explained these phenomena successfully.
Hot injection, the method used in my research, could be a successful pathway for the
synthesis of high-quality nanocrystals and of a large number of different materials, but still
suffered from several defects, mainly the low yield of the final product and incontrollable
chemical stoichiometry in mutli-element compounds. Some papers reported that di-n-pentyl
phthalate (DPP) could be added in the solution to enhance the NC yield. Nonetheless, the
removal of the surfactant around the NCs, which would lower down the yield, might still be a
problem. In addition, to achieve controlled stoichiometry, different precursors as well as
reaction condition needed to be tested, as the mechanism of the reaction was still not fully
understood. The above mentioned problems were what I was trying to solve in my current
research on rare earth element nanophosphors.
Except nanocrystals and thin films, nanowires, which can exhibit amazing properties because
of their low dimension structure, also attracted my attention. Intensive study on nanowires is
needed to find a cost-effective way to synthesize them and an efficient way to utilize them.
Currently, MOCVD process based on V-L-S mechanism has been developed to synthesize
single crystalline nanowires with a controlled shape or pattern. Nevertheless, the catalysts will
unavoidably influence the purity of the nanowires. The need of high temperature, sample
uniformity, substrate choice, and low product yield are also the major problems in this field. In
my opintion, solution-based synthesis, such as hot injection, might serve as the key to solve
this problem. Shape control means, such as induced magnetic field, or the addition of NC
seeds in the solution, might be used to facilitate this process. The study on self-assembly of
the nanowires into optical or electronic devices, such as FETs and photovoltaic cells, might be
the next step in the research. In sum, nanowire is a very promising yet not fully-developed field.
Therefore, I wanted to attend a Ph.D. program to carry on my research interest relating this
field and made even break-through discoveries.
When it came to daily life, I often discussed with my friends on the hot topics in my interested
areas, such as the synthesis of novel nanocrystals and nanowires. Sometimes I found even a
short discussion with them could broaden my horizon, informing me new ideas that I had never
heard of. By working with my colleagues, I did not only learn how to cooperate with other
people, but also acquired the knowledge on the operation of several apparatuses, including
SEM, EDS and UV spectrometer. Besides, when I felt free, I usually went to the library to read
some books to help me better understand the fundamentals of nanomaterials. All of these
would contribute significantly to my further Ph.D. study.
To sum up, I truly hope to receive favorable consideration from the Admission Commitee in
your esteemed university. It would be a great pleasure for me to join in your doctoral program
to continue my further research.