Zichao Ye
Personal Statement 8.0
Every natural phenomenon has its hidden mechanism. It is attractive for me that insignificant
motions of particles could cause various macroscopic effects. For example, what micro process is
responsible for the photoelectric conversion? What micro mechanism could be the most efficient
way of information transfer? I believe it is quantum mechanism and solid state physics that could
pave the way for me to understand them. Therefore, after being recommended to the research team
of nanoscale electronic materials and devices, I found semiconductor material is my dream field
and today I lay my dream on Stanford University / UCB.
As my dream field, electronic information material is a realm waiting to be profoundly
explored. Hence creative and critical thinking is necessary. With my great passion in the field, I
successfully produced Ga-doped ZnO (GZO) nanocrystals with sizes of 5-7 nm at low temperature
via hot-injection method and fabricated GZO thin film using spin casting its nano ink. As a novel
representative of wide band gap materials which might replace traditional ZnO, GZO nanocrystals
were first synthesized under relatively low temperature and vacuum, which partially overcomes
the high-cost problem facing the development of the wide band gap materials and proves the
method to be a promising candidate for the large scale fabrication of high efficiency GZO thin
film in solar cells.
However, low productivity is the main disadvantage of hot-injection which would harm the
quality and morphology of the following GZO thin films. Knowing that the type of solvent greatly
related to the reaction process, I compared oleylamine, toluene and xylene. Consequently, no
quantum dots obtained in oleylamine because of its high boiling point, making the reaction mainly
processed in a relatively low-efficiency liquid phase. On the contrary, though toluene was much
better in forming high productivity quantum dots, its boiling point is too low which resulted in a
high nucleation rate, making crystals gather due to the size effect. But compared to 300℃, the
reaction temperature, xylene owns an appropriate boiling point which not only prevented the
above obstacles but improve the productivity since the fierce gas phase reaction accelerates the
nucleation as well as restrains the growth of crystals.
Additionally, once I have critically commented a paper named “Cu2ZnSnS4 (CZTS) thin films
deposited by DC reactive magnetron sputtering”. My uppermost point is the paper was not
original for its synthesis method was almost the same as the one described in a former paper
because it only replaced sulfur with hydrogen sulfide as a precursor. The editor of Materials
Letters confirmed my job and returned the paper. From then on, I began to think about
Cu2-II–IV–VI4 quaternary compounds, which are potential substitute for well-known
CuInxGa1-xS2ySe2(1-y) (CIGSSe) absorber material, as their constituents are nontoxic and
earth-abundant. Among them, Cu2ZnSnS4xSe4(1-x) (CZTSSe), whose efficiency far exceeds that of
CZTS, is regarded as most likely to be commercialized. Hence, through hot-injection, I
synthesized CZTSSe nanocrystals with diverse Se/(S+Se) ratios.
The following optical measurements presented CZTSSe nanocrystals have a tunable band
gap from about 1.28 to 1.50 eV, indicating a nature of parabola with the increase of Se/(S+Se)
ratio. The band gap of CZTSe could be inferred to be 1.5 eV from such a parabola figure, which in
my view proves SeJin Ahn’s paper need to be reconsidered. He said the band gap of CZTSe is 1.0
eV because of the existence of secondary phase ZnSe and CuxSe in samples whose Eg=1.5 eV. He
gave the absence of Sn in CZTSe films as evidence. However, it is not the case in my Se rich
samples, as the ratios of all five elements are measured ideal. If ZnSe formed in my experiment,
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Zichao Ye
the ratio of Zn and Se would be abnormal. Besides, there are no extra peaks of CuxSe in XRD
patterns, as shown in SeJin Ahn’s paper. Thereby, even though my results suggest the band gap of
CZTSe is 1.5 eV, no such secondary phases exist, which though might not be rigorous enough to
assert Eg=1.0 eV is wrong, it at least reflects SeJin Ahn’s work needs revision.
Besides research experiences, I was one of a few students who finished every course deign
independently for individual experiments and as a leader for group tasks. In the physics
experiments, I completed all the reports myself with lots of related extra problems concerning
general theories discussed, such as the mechanism of LED and common characteristics of high
temperature superconductors. While during the course of material experiments, I was always the
team leader, arranging all the complexities in order and cooperating with teammates to prepare
samples, measure their mechanical properties and analyze metallurgical structures. Therefore,
thanks to my responsibility and hard-working, I accomplished double 90+ in two consecutive
semesters, which only less than ten students could achieve in the history of this course.
Since Stanford University located in the famous Silicon Valley, it has powerful groups
working on almost every aspects of electronic information materials, including organic electronics,
patterning materials, nano structured inorganic materials for applications in electronics, unique
compound semiconductor materials and so on. As you have seen, with all the characters above, I
am competent for exploring semi-conductor materials, especially materials with novel
optoelectronic properties. Besides, it could be relatively easier for me to start my career in IT
firms there. Hence I am determined to reveal the profound mechanics behind this valuable field.
And Stanford is the right place to realize all these dreams.
If I joined into the group of XXX, I would get a bright future there because it is a correct way
leading to my world of dream.
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