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During this reporting time period (2010-2011), our work focused on (i) the development of a direct, self-catalyzed, near-ambient pressure plasma-assisted method to synthesize ZnO nanocrystalline thin films, (ii) studying the formation of nanostructures, especially nanowires, resulting from the thermal annealing of nanocrystalline thin films.

1.1. Self-catalyzed, rapid thermal plasma CVD synthesis of nanocrystalline ZnO films

With an improved close-looped temperature control system, we have successfully developed and demonstrated a combined method—inductive heating plus rapid thermal plasma chemical vapor deposition (CVD)—to synthesize ZnO nanocrystalline thin films with good control of growth rate, grain size, uniformity, porosity and preferred grain orientation. Synthesis of ZnO is performed in a quartz process tube at a base pressure of 130 torr, using argon and oxygen as carrier and reactant gases at a ratio of 99.67% to 0.33% at a total flow rate of 301 sccm. The films are characterized using a combination of scanning electron microscopy (SEM), x-ray diffraction, and optical spectroscopy. The as-deposited films are conformal, nanocrystalline, conductive, and highly transparent (with UV-vis spectroscopy showing a transition wavelength corresponding to the optical bandgap of ZnO at 375nm). 20nm and 200nm thick ZnO films are showing transparency at 100% and 80%, respectively. The 200nm film also has a sheet resistance of 80 Ω/square.

In addition, we found the following:

(i) Film thickness of the as-synthesized ZnO film is linearly proportional to synthesis time at or above the melting point of Zn (420°C), which disproves the hypothesis that boiling of Zn was necessary to produce Zn vapor and hence ZnO. Instead, a possible mechanism of a thermal plasma-based, rapid, near-ambient pressure growth seems to hinge on first the melting of Zn and subsequently the ion bombardment and ejection of low-energy Zn from the molten Zn.

(ii) The type of substrates has no observable influence on the grain morphology and texture. We have deposited on substrates including ceramics (silicon(100), c-sapphire, a-sapphire, quartz, borosilicate glass, and muscovite) and metals (gold, titanium) at 330°C and polymer (polyimide) at 160°C—all substrates except polyimide appears to yield similarly textured ZnO grain morphology. This shows that substrate temperature, rather than substrate type, exert a larger influence on grain morphology.

1.2. Effect of thermal annealing on the morphology of ZnO nanocrystalline thin films.

Thermal annealing in pure argon environment at temperatures ranging from 750°C to 950°C is performed to elucidate the effect of heat treatment in the absence of oxygen on the grain morphologies and dimensionalities of ZnO grains. Annealing at 750°C (0.46 Tm) results in the growth of ZnO grains into larger grains with greater definition at the grain boundaries. At 800°C (0.48 Tm), restructuring of the grain texture produces conspicuous (002) facets along with increased grain sizes and lower grain density. As annealing temperature increases from 800°C to 900°C (0.52 Tm), the increasing thermal energy input causes further surface texture restructuring due to grain boundary diffusion and bulk diffusion. This in turn accelerates grain growth in the direction perpendicular to the (002) plane—the ZnO plane with the lowest surface energy—and produces columnar ZnO grains that continue to elongate along the c-axis. Accompanying this change is noticeable growth in the direction parallel to the surface of the substrate, i.e. in direction normal to {100} planes. During grain growth, the larger grains are formed by consuming smaller adjacent grains, which lowers the grain density. The columnar ZnO crystals would act as seeds for the seeded growth of nanorods as temperature is further increased to 950°C (0.54 Tm). SEMs show that at 950°C, nascent nanorods form on the aligned ZnO nanocrsytals. At the same time, it is also observed that the grain density continues to decrease from 900 to 950°C. This is likely due to increased bulk diffusion that provides for the growth of the nanorods.

2. Impact on the PI’s career

This grant has contributed significantly to the PI’s research. Results obtained from my research have been instrumental in the following:

- The awarding of an NSF MRI grant to SFSU in 2010 for the procurement of a temperature-controlled semiconductor probe station.

- Submission of one journal paper to Springer’s Nanoscale Research Letters (impact factor: 2.6).

- Publishing of 3 conference proceedings manuscripts in 2011 MRS Spring Meeting, 2011 IEEE NANO, and 2011 ASME IMECE Conference (with two undergraduate co-authors).

- The establishing of collaborative research between this PI and a faculty in the Department of Chemistry to investigate the growth mechanics of doped metal oxide including ZnO, SnO₂ and TiO₂.

- The winning of a first place in the annual SFSU Science Competition by the three undergraduate researchers in my lab supported by this grant.

3. Impact on Students Involved in Research

During the second year of this grant, the PI was able to (1) set up collaborative research with the chemistry department and leverage advanced characterization tool for the experimental work involved in this work, (2) use the funding to procure hardware and apparatus essential to the success of the project, and (3) hire 1 engineering undergraduate researcher during regular  Fall/Spring semesters and 1 engineering undergraduate student during the summer months (June – August of 2011); indirectly support 1 additional undergraduate student for research during Fall/Spring semesters. One of the 3 undergraduates is a female student and all three students are from an economically disadvantaged background. Funding from this grant also allowed these undergraduate researchers to present their work at the following conferences: (i) 2011 MRS Spring Meeting, (ii) 2011 IEEE NANO, and (iii) 2011 ASME IMECE.  In addition, one of the students was awarded the highly competitive 2011-2012 California State University Sally Casanova Pre-Doctoral Fellowship and the other an NNIN REU international internship in 2011.

Four additional undergraduate researchers who worked in my lab in summer 2009 and who were indirectly supported (cost of consumables) by this grant graduated in May 2011. One of them was offered employment at Applied Materials Inc. as a full-time thin film engineer in spite of the tough job market for fresh undergraduates due to his work in my lab.