The Impact of Expanding Technical Knowledge on the Beginning Industrial Arts Teacher

This study measures technical preparation and job demands among North Texas State University industrial arts graduates teaching in high schools. In addition to data from professional literature and the NTSU Bulletin, questionnaire mailings reveal that most graduates consider themselves qualified although recommending more semester hours of industrial arts for certification. They also affirm the practical value to the teacher of experience in industry. The study recommends narrowing the number of areas in industrial arts preparation and providing a more specialized teacher-training program with greater uniformity of semester hours

Measuring the Electric Field Dependence of Photocurrent Quantum Yield in Suspended Carbon Nanotube pn Junctions

In nanoscale materials, the Coulombic interaction between electrons are stronger than in bulk materials. These stronger interactions, caused by confinement and reduced dielectric screening, have interesting consequences for light-matter interactions. In carbon nanotubes (CNTs), strong interactions can enhance the impact ionization process, and thus assist photocurrent generation in CNTs. Conversely, the strength of the attraction between photo-excited electrons and holes can hinder photocurrent generation by binding electrons and holes to each other. An understanding of both impact ionization and electron-hole binding is needed to determine which will dominate the system and under what circumstances. The central question of this research is whether high efficiency photocurrent can be generated in a material that has strong Coulombic interactions between charge carriers. In this thesis I explore the possibility of using electric fields to ionize excitons and thereby access a regime where highly efficient photocurrent generation can be achieved. I begin by introducing simple theoretical models that explain the general features of CNT optical properties. Increasing the complexity of the models reveals finer details of the system and allows us to estimate the effect of electric field on the exciton energy state and ionization rate. To maximize Coulmbic interactions between charge carriers, I made fully suspended CNTs by growing the nanotubes over a trench using a fast-heat chemical vapor deposition process. Split-gate electrodes at the bottom of the trench are used create a pn junction in the nanotube by applying opposite voltages to each gate, populating each half of the nanotube opposite-sign charge. The devices form near ideal diodes, and the samples are remarkably clean. The same design of CNT devices that I used are also used by our collaborators at University of Utah to study strongly interacting transport phenomena at low temperature. A number of computational models were used to quantify the CNT device parameters. Working with Dr. Andrea Bertoni, we created and experimentally verified an electrostatics model that calculates the electric field in the CNT pn junction and determines the intrinsic region length. Experimental verification of the simulations was achieved using scanning photocurrent microscopy. Simulations of the optical interference patterns near the nanotube were performed using finite-difference-time-domain software to determine the number of photons absorbed by the CNT. Dr. Vasili Perebeinos computed the effect of electric field on the excited states in carbon nanotubes by solving the Bethe-Salpeter equation for a CNT exciton in a static electric field. Photocurrent spectra were measured around the S22 resonance on eight different CNTs over a range of electric fields up to ~10 V/μm. The spectra are processed to extract the photocurrent quantum yield (PCQY) from the photocurrent peak. The PCQY increases with field in all cases, and increases by a factor of 35 to a value of 1.85 electrons per photon in the largest diameter CNT (with D = 2.8 nm). The same procedure was performed for the S33 resonance which shows a weaker field dependence, but larger PCQY ~ 0.3 at low field. The results show that the photocurrent quantum yield can exceed 100% at the S22 exciton resonance sufficiently large axial electric field and CNT diameter. This suggests that impact ionization can coexist with efficient exciton dissociation when the electric field is ~10 V/"μm" and the CNT diameter is ~ 2.8 nm. We observed a different PCQY field dependence for S33 excitons. A large fraction of S33 excitons autoionize at low fields, resulting in a high PQCY at low field. We observe a gradual increase in PCQY with respect to field. The gradual increase is attributed to reduced recombination of the free carriers by sweeping them out of the device more quickly. To interpret our data we compared our experimental results with theoretical calculations of the decay products of S22 and S33 excitons. This research serves as a framework that can be extended to other systems with strong interactions, and motivates future work tuning the carrier interaction strength to achieve high-efficiency photocurrent generation

Ecosystem carbon & nitrogen cycling across a precipitation gradient of the central Great Plains

The SGS-LTER research site was established in 1980 by researchers at Colorado State University as part of a network of long-term research sites within the US LTER Network, supported by the National Science Foundation. Scientists within the Natural Resource Ecology Lab, Department of Forest and Rangeland Stewardship, Department of Soil and Crop Sciences, and Biology Department at CSU, California State Fullerton, USDA Agricultural Research Service, University of Northern Colorado, and the University of Wyoming, among others, have contributed to our understanding of the structure and functions of the shortgrass steppe and other diverse ecosystems across the network while maintaining a common mission and sharing expertise, data and infrastructure.Regional analyses have shown that ecosystem pools of carbon (C) and nitrogen (N) increase as precipitation increases from the semi-arid shortgrass steppe to the tallgrass prairie of the Central Great Plains. Models based on our functional understanding of biogeochemical processes predict that ecosystem C and N fluxes also increase across this community gradient; however, few field flux data exist to evaluate these predictions. We measured decomposition rates, soil respiration, and in situ net nitrogen mineralization at five sites across a precipitation gradient in the Great Plains region. Soil respiration (SResp) and the decomposition constant, k, for common substrate litter bags were significantly higher in the sub-humid mixed and tallgrass prairie (growing season average mid-day SResp = 7.20 μmol CO2 m-2 sec-1, k = 0.66 yr-1) than the semi-arid shortgrass steppe (SResp = 4.55 μmol CO2 m-2 sec-1, k = 0.32 yr-1). In contrast, in situ net nitrogen mineralization was not significantly different across sites. The C flux data concur with predictions from current biogeochemical models; however, the in situ net nitrogen mineralization results do not. We hypothesize that this discrepancy results from the difficulties associated with measuring in situ net nitrogen mineralization in soils with vastly different immobilization potentials

Alkaloid quantities in endophyte-infected tall fescue are affected by the plant-fungus combination and environment

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Photocurrent Quantum Yield in Suspended Carbon Nanotube p–n Junctions

We study photocurrent generation in individual suspended carbon nanotube pn junctions using spectrally-resolved scanning photocurrent microscopy. Spatial maps of the photocurrent allow us to determine the length of the pn junction intrinsic region, as well as the role of the n-type Schottky barrier. We show that reverse-bias operation eliminates complications caused by the n-type Schottky barrier and increases the length of the intrinsic region. The absorption cross-section of the CNT is calculated using an empirically verified model, and the effect of substrate reflection is determined using FDTD simulations. We find the room temperature photocurrent quantum yield is approximately 30% when exciting the carbon nanotube at the S₄₄ and S₅₅ excitonic transitions. The quantum yield value is an order of magnitude larger than previous estimates

A nanoscale pn junction in series with tunable Schottky barriers

PN junctions in nanoscale materials are of interest for a range of technologies including photodetectors, solar cells, and light-emitting diodes. However, Schottky barriers at the interface between metal contacts and the nanomaterial are often unavoidable. The effect of metal-semiconductor interfaces on the behavior of nanoscale diodes must be understood, both to extract the characteristics of the pn junction, and to understand the overall characteristics of the final device. Here, we study the current-voltage characteristics of diodes that are formed in fully suspended carbon nanotubes (CNTs). We utilize tunable Schottky barrier heights at the CNT-metal interface to elucidate the role of the Schottky barriers on the device characteristics. We develop a quantitative model to show how a variety of device characteristics can arise from apparently similar devices. Using our model we extract key parameters of the Schottky barriers and the pn junction, and predict the overall I-V characteristics of the device. Our equivalent circuit model is relevant to a variety of nanomaterial-based diode devices that are currently under investigation

The effect of polar lipids on tear film dynamics

In this paper we present a mathematical model describing the effect of polar lipids on the evolution of a precorneal tear film, with the aim of explaining the interesting experimentally observed phenomenon that the tear film continues to move upwards even after the upper eyelid has become stationary. The polar lipid is an insoluble surface species that locally alters the surface tension of the tear film. In the lubrication limit, the model reduces to two coupled nonlinear partial differential equations for the film thickness and the concentration of lipid. We solve the system numerically and observe that the presence of the lipid causes an increase in flow of liquid up the eye. We further exploit the size of the parameters in the problem to explain the initial evolution of the system