PhD, University of Cincinnati, 2006, Engineering : Materials Science

High-temperature electronics and MEMS (Micro-Electro-Mechanical Systems) based on polycrystalline diamond (PCD) are promising because of its wide band gap, high thermal conductivity, and large carrier mobility. To take advantage of this opportunity, research was undertaken to develop techniques for the synthesis of both undoped and doped high quality PCD films with good surface flatness suitable for the fabrication of high temperature electronics and MEMS devices. One way to fabricate smooth films is to decrease the grain size because diamond films with large grain size bring forth problems in contact formation and device fabrication due to the large surface roughness. Consequently, there is a need to fabricate nanocrystalline films with small grain size and good smoothness. In addition, the electrical properties and conduction mechanisms in nanocrystalline diamond (NCD) films have not been sufficiently analyzed. This study also aims at achieving high resistivity nanocrystalline diamond films and to study the electrical conduction mechanism. Several approaches have been used in our research to achieve these goals. Initially microcrystalline diamond (MCD) films were grown on silicon (100) substrates by the microwave plasma enhanced chemical vapor deposition (MPCVD) method using methane in a hydrogen plasma environment. Introduction of small amounts of argon into the Argon / Hydrogen plasma was used to deposit diamond films with a range of microstructures from microcrystalline to nanocrystalline grains. A detailed quantitative study of the sp3, sp2 content in the films grown with varying amounts of argon in the plasma was done using Raman spectroscopy. The optimum gas composition that gave the best quality diamond film consisting of 1.6 µm grains was 60% Ar/ 39% H2/ 1% CH4. The optimum gas composition that gave nanocrystalline grains of size in the order of <50 nm was 95% Ar/ 4% H2/ 1% CH4. A change in the cross-sectional microstructure from the columnar to grain-like (open full item for complete abstract)

Committee: Dr. Raj Singh (Advisor) Subjects: Engineering, Materials Science

MS, University of Cincinnati, 2002, Engineering : Materials Science

Molybdenum nitride thin film was deposited on silicon wafer by the reactive sputter deposition. Single phase γ–Mo2N thin film was obtained with N2/(Ar+N2) flow ratios in sputtering gas varying from 10% to 30% whereas an amorphous structure was obtained at N2/(Ar+N2) flow ratios of 50%. The deposition rate of the molybdenum nitride thin film varies significantly as nitrogen partial pressure in sputtering gas increases. A decrease in peak intensity along with peak shift and broadening was observed in X-ray diffraction spectra as the nitrogen partial pressure sputtering gas increased. The XPS analysis of the as-deposited thin films shows that the Mo 3d3/2, Mo 3d5/2 and Mo 2p3/2 peak gradually shift to the higher binding energy direction as nitrogen partial pressure is increasing. The intensity of N 1s peak also increase with increasing nitrogen partial pressure. Although the XRD examination shows no evidence of long range order of the phase structure for the amorphous thin film sputtered at 50% N2/(Ar+N2) flow ratio, the existence of Mo–N bond in the film was confirmed by XPS examination. The nitrogen partial pressure in the sputtering gas was found to have significant influence on the surface morphologies and cross section structures of the thin film. Thermal annealing of the amorphous thin film in a nitrogen atmosphere revealed that the film could survive 700°C/5min thermal annealing without obvious crystallization but failed after 800°C/5min thermal annealing, in which the crystalline γ-Mo 2N and h–MoSi2 phases were observed simultaneously.

Committee: Dr. Ray Y. Lin (Advisor) Subjects: Engineering, Materials Science

Doctor of Philosophy, The Ohio State University, 2007, Evolution, Ecology, and Organismal Biology

Nitrogen (N) is the nutrient most limiting to plant growth (NPP) in temperate forests. In N-limited temperate forests, most of the N required for NPP is recycled between soil and plant N pools by the microbial process of N-mineralization (Nmin). However, human activities have increased atmospheric N deposition (Ndep) to forests in the last 50-100 years, and this surplus N may increase NPP. But, forest responses to Ndep are not satisfactorily understood, and depend on how atmospheric N inputs are partitioned between soils and plants. From my field data collection at a mature forest site, I estimated that NPP required 51 kg N ha-1 yr-1, most of which was used for fine root and leaf production (62% and 31%, respectively). Each year, Nmin supplied 87% of Nreq, and Ndep contributed an additional 13%, 4% of which was due to canopy retention of Ndep (Ncr). Data from my mesocosm 15N-labelling experiment suggested that very little (<10%) of Ncr observed in the field was actually taken up by trees, and the majority of Ndep (>85%) was assimilated into soil pools. These results suggest that Ndep could not have significantly increased forest NPP at UMBS over the time scale of my studies. My greenhouse experiment corroborated this conclusion, with tree seedlings showing no significant increase in photosynthesis or growth in response to Ndep at ambient rates. However, Ndep to forest ecosystems has been occurring for decades in industrialized regions, and most of the N inputs have been incorporated into soil organic matter (SOM). Research across temperate forests has suggested that forests exposed to large N inputs over time exhibit decreased soil C/N ratios, which are associated with faster Nmin rates. Using meta-analysis, I verified this pattern in the literature, and discovered novel relationships between forest soil properties and their responses to N inputs. My results demonstrated a long-term, quantitative relationship between Ndep and Nmin, and suggest that NPP may increase (open full item for complete abstract)

Committee: Peter Curtis (Advisor) Subjects: Biology, Ecology

Master of Science (MS), Ohio University, 1991, Chemical Engineering (Engineering)

Remote microwave-enhanced chemical vapor deposition of silicon-nitrogen (Si xN y) thin films

Committee: Daniel Gulino (Advisor) Subjects: Engineering, Chemical

Master of Science, Miami University, 2012, Zoology

Human-induced changes such as nitrogen deposition and forest harvest can alter biogeochemical cycling in temperate forests. However, it is still unclear what impacts increased N availability and successional stage have on productivity. Nitrogen (N) and phosphorus (P) availability were examined along an age gradient in northern hardwoods in the Bartlett Experimental Forest, New Hampshire, USA. Net N mineralization decreased with age, but no patterns were discovered for available P. However, there was strong evidence for N and P coupling in these sites with results suggesting that N availability influenced P availability through the production of phosphatase enzymes. The apparent interaction of N and P, via phosphatase, is good evidence that resource optimization processes balance nutrient availability across a wide gradient in fertility. This could be viewed as contributing to colimitation, but it could also be that N is the primary limiting nutrient because of its underlying effects on P availability.

Committee: Melany C. Fisk PhD (Advisor); Michael J. Vanni PhD (Committee Member); Thomas O. Crist PhD (Committee Member) Subjects: Biogeochemistry; Ecology; Forestry; Soil Sciences