Atomically thin 2D materials have gained the interest of the scientific community in the past decade due to their exotic electronic and optoelectronic properties, thus emerging as potential candidates for the next generation of nano-devices. Quantum confinement in one of the dimensions is the primary reason for these exotic properties. However, it has been seen that these properties are widely inconsistent, and they are controlled by variety of factors such as material synthesis, device fabrication, testing environment, etc. Due to low dimensional nature of these materials, defects are inevitable. These defects typically originate from either the presence of bulk impurities or interface between sample and substrate. These defects manifest as mid-gap states in semiconductor channel and act as trapping centers for charge carriers, thus often referred to as trap states. The presence of trap states is not necessarily a detrimental thing. In this dissertation, I will focus on the role these trap states play in the emergence of a few electronic and optoelectronic properties.High responsivity (R) in photodetectors based on 2D materials is mainly associated with a presence of photogating effect in which trap states dynamics plays a crucial role. Photogating also results in fractional power (γ) dependence of the photocurrent (Iph) on an effective illumination intensity (Peff). Chapter 2 presents photoconductivity studies of few layers of rhenium diselenide (ReSe2) based field-effect transistors (FETs) over a wide range of applied gate voltages (-48 V ≤ Vg ≤ 60 V) and temperature (20 K ≤ T ≤ 300 K). A very high responsivities ≈ 16500 A/W and external quantum efficiency (EQE) ~ 106 % (at 140 K, Vg = 60 V and Peff = 0.2 nW) was obtained. Investigating R and γ at various gate voltages and over a wide range of temperatures leads to a strong correlation between R and γ. Such correlations indicate the importance of trap states and photogating in governing high responsivities in these materials. It is expected that thicker samples will aid in photoconduction by effectively increasing photon absorption. In chapter 3, a layer dependent study of optoelectronic properties of indium selenide (InSe) based FETs shows that responsivity decreases for thicker InSe devices. In these devices, photogating remains constant (similar γ) and responsivity depends predominately upon field-effect mobility (μFE). Interlayer resistance regulates the mobility and (consequentially) responsivity. Thus, mobility dominates the responsivity and trap states play second fiddle. The presence of metal−insulator transition (MIT) in two-dimensional (2D) systems leads to tunable material properties by regulating parameters such as charge carrier density. Chapter 4 shows our observation on MIT in the 2D copper indium selenide (CuIn7Se11) flakes by electrostatic doping via the SiO2 back gate. A temperature and gate voltage dependence of conductivity (σ) of CuIn7Se11 FET shows clear evidence of the metallic and insulating phase. Evidence of 2D variable-range hopping (VRH) and percolation critical conductivity confirms the presence of charge density inhomogeneity originating from trap states. The low effective mass and high dielectric of copper indium selenide systems result in a lower critical charge carrier density required for percolation-driven MIT, attended by conventional SiO2 dielectric gate. Even though findings reported in this dissertation are performed on specific materials, fundamental understandings can be easily extrapolated to other 2D systems. Understanding the role of trap states will provide valuable insights for the design and development of high-performance devices using 2D materials.

京都大学 / 新制・課程博士 / 博士(農学) / 甲第24243号 / 農博第2522号 / 新制||農||1094(附属図書館) / 学位論文||R4||N5414(農学部図書室) / 京都大学大学院農学研究科応用生物科学専攻 / (主査)教授 松浦 健二, 教授 大門 高明, 教授 日本 典秀 / 学位規則第4条第1項該当 / Doctor of Agricultural Science / Kyoto University / DFAM

Melatonin is a natural antioxidant that has been investigated for properties as a potential spin trap to identify short-lived free radicals. Computational quantum chemistry studies have been performed for the oxidation of melatonin to N1-acetyl-N2-formyl-5-methoxykynuramine. This research focused on modification of melatonin into derivatives and analyzing the change in total molecular energy from melatonin to its oxidation product, as well as the corresponding derivatives. Each of the molecular geometries were optimized at the DFT/B3LYP/6-31G(d), DFT/B3LYP/cc-pVXZ (X = D, T), HF/6-31G(d), HF/cc-PVXZ (X = D, T), MP2/6-31G(d), and MP2/cc-PVXZ (X = D, T) levels of theory. Single point energies were extrapolated to the complete basis set. The results demonstrated that some electron-withdrawing groups increased the total energy of the system. The electron-withdrawing functional group which lowered the total energy of the system was a peroxyl functional group, and this is believed to be due to overlapping constructive interference between molecular orbitals.

World poverty is a persistent phenomenon despite international efforts and the improvements achieved in the last few decades. For many people it can be a chronic condition. This thesis aims at testing that the main reason some people are poor is due to a poverty trap, i.e., to some contextual mechanisms which limit their ability to escape poverty, reproducing it over time. To investigate this hypothesis, this thesis is guided by three different questions. First, does a poverty trap emerge in the aftermath of an extreme weather shock? Second, do communities in a humanitarian context have the same wealth dynamics? Third, what is the role of income diversification for agricultural households for asset accumulation, and does it depend on their wealth? The analysis focuses (a) on the case of Nigeria and a devastating flood, (b) on refugees and host communities in Uganda and (c) on agricultural households in Tanzania, respectively. Results show that Nigerian flooded households have poverty traps dynamics, condemning the poorest in a destitute state over time. Refugees and host communities in Uganda have similar wealth dynamics but both converge to a low-wealth equilibrium, suggesting a structural poverty trap that worse for refugees. Income diversification in Tanzania shows important nonlinearities according to households’ wealth: it fosters the accumulation of durable assets for better-off households only, while helping the poorest to accumulate livestock. These findings shed light on the interaction of low-income conditions and contextual challenges and opportunities, suggesting policy actions able to lift poor people above a wealth threshold, improve their living conditions and favouring their profitability.

Individual organisms often use cues from their natural environments to determine many behavioral and life-history "decisions." These "decisions" are usually adaptive, i.e. a response to selection, because the environmental cues on which they are based reliably correlate with increased fitness over time. When the selected behavioral response to a natural cue no longer provides a fitness benefit, then selection for a new response may occur but individuals maintaining the previously selected response may suffer reduced survival and reproduction. Especially in human-modified landscapes individuals making a maladaptive behavioral or life-history choice based on those formerly reliable environmental cues may be faced with an "evolutionary trap". In urban, or suburban, environments many factors have been altered in ways that could lead to evolutionary traps. Inappropriate behavioral responses by many individuals could lead to reduced demographic performance of urban populations relative to their wildland counterparts and to the decline of entire urban populations. In birds, maladaptive patterns of nest provisioning or vigilance may occur (a) when human-provided adult foods are easier to feed young because they are more abundant and predictable than foods appropriate for nestlings, or (b) when birds' perception of predation risk, which can be influenced by human disturbance, is greater than the real risk. By provisioning or attending their nests more or less than what is appropriate given the environmental level of resources and risks, the behavior of suburban parents may be contributing to high levels of nest failure during the nesting stage. To determine whether maladaptive parental care influences nest survival during the nestling stage, I conducted an experiment using Florida Scrub-Jays (Aphelocoma coerluscens). Suburban scrub-jays have lower nest survival during the nestling stage but higher survival during the incubation stage relative to wildland jays. Both predators and food abundance vary greatly between suburban and wildland scrub. The suburbs have a greater abundance of predators that may prey on both adult scrub-jays and their nests and more foods appropriate for adults but less nestling-appropriate food. This variation in risks and resources should affect the parental care behavior of suburban scrub-jays, which in turn may affect patterns of nest survival. In pre-treatment observations, I found that suburban females spent more time brooding than wildland birds but suburban males did not provision any more than wildland males. Experimentally increasing the perception of adult predation risk reduced parental care in both suburban and wildland females. Increasing the availability of nestling food reduced parental care in suburban females but had no effect in wildland females. Increasing food availability, but not predation risk, decreased call rates but increased call frequency in nestling scrub-jays from both habitats. However, neither parental care nor food availability had much influence on nest survival during the nestling stage. Instead, side nest concealment and the presence of helpers were the most important variables in nest survival analyses prompting other explanations besides maladaptive parental behavior or lack of nestling food resources for the habitat-specific difference in nest survival during the nestling stage.

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Over the past several decades, moose (Alces alces) populations in New England have been in decline due to winter tick (Dermacentor albipictus) parasitism. Winter ticks have been known to infest moose, with over 90,000 ticks being recorded on a single moose. These severe infestations, known as epizootics, are associated with high annual calf mortality rates (> 50%) and reductions in annual productivity (adult calving and twinning rates < 60% and 5%, respectively). Given this, it is increasingly important to effectively monitor moose and winter ticks to address consistent population decline of moose due to winter tick epizootics. The objectives of this work were to measure off-host winter tick abundance and associated environmental variables and compare off-host tick abundance in relation to moose abundance. To assess moose and winter tick abundance, I used a combination of camera traps and winter tick sampling. I set an array of 60 cameras across central-western Massachusetts, which contribute data to the Northeast Wildlife Monitoring Network. I used a plot-based sampling strategy that is effective for heterogenous habitat types to sample winter ticks at sites in western and central Massachusetts as well as the White Mountain National Forest and Umbagog National Wildlife Refuge in New Hampshire. The Penobscot Nation developed the tick sampling approach and contributed additional winter tick data from their sovereign trust lands in Maine. I used N-mixture models to generate estimates of tick abundance for each of the three study areas. Results from this study can inform monitoring strategies for ticks, predict epizootic severity, and develop tools to mitigate threats to moose. This is important for managers looking to prevent further declines in moose populations due to winter ticks and tribes seeking to maintain populations for sustenance.

To develop effective conservation planning and mitigate biodiversity loss, standardized metrics for monitoring and assessing biodiversity are needed. This information is particularly vital in Botswana, where knowledge of many of the diverse wildlife populations is lacking. To address this knowledge gap, my dissertation research evaluated the distributions, densities, and ecology of the wildlife community in northern Botswana, with a focus on terrestrial carnivore species given their importance both ecologically and economically. My objectives were threefold: 1) estimate the distributions of the mammal community (n = 44 species) and evaluate community, group, and species-specific responses to anthropogenic and environmental variables, 2) test whether the presence of intraguild species or resource availability had a larger influence on the seasonal distributions of carnivore species, and 3) simultaneously estimate the population densities of 7 carnivore species. To accomplish these objectives, I completed a multi-year camera trap survey in a 1,154-km2 study area and analyzed the data using occupancy models (single and multi- species) and spatially explicit capture-recapture models. Estimates of species richness ranged from 8 to 27 unique species, species had a mean occurrence probability of 0.32 (95% credible interval = 0.21–0.45), and estimated densities ranged from 1.8 aardwolves (Proteles cristata) to 12.7 spotted hyenas (Crocuta crocuta) per 100 km2. The occupancy and richness of terrestrial mammals was negatively associated with human disturbance and in general, positively associated with open grasslands/floodplains. Carnivore species, specifically, tended to have greater species richness and larger population densities in open habitats than in closed. I also found carnivore distributions were positively associated with the detection rate of intraguild competitors and predators, suggesting competitor/predator avoidance did not play a large role in shaping carnivore community structure. My research highlights the pivotal role protected areas and grasslands play in conserving wildlife populations in northern Botswana. Additionally, my research helps progress camera trap analyses from single to multi-species assessments. Broader application of this multi-species approach would likely result in a better understanding of wildlife and carnivore communities which in turn, may help inform management actions aimed at addressing the loss of wildlife populations globally. / Ph. D.

My research addressed pheromone lure design and the activity of the grape berry moth, Paralobesia viteana, flight and infestation across three years of study. In my lure evaluations, I found all commercial lures contained impurities and inconsistencies that have implications for management. First, sex pheromone concentration in lures affected both target and non-target attraction to traps, while the blend of sex pheromones impacted attraction to P. viteana. Second, over the duration of study, 54 vineyard blocks were sampled for the pest in and around cultivated wine grape in Virginia. The trapping studies indicated earliest and sustained emergence of the spring generation in sex pheromone traps placed in a wooded periphery. Later, moths were detected most often in the vineyard, which indicated that P. viteana emerged and aggregated in woods prior to flying and egg-laying in vineyards. My research supports use of woods and vineyard trap monitoring at both the height of 2 meters and in the periphery of respective environments. These conditions should improve grower efficiency when using trapping as a tool. Cluster infestation was assessed relative to vineyard growth stage and location in the vineyard. My results are in agreement with historical studies that suggest infestation is highest in the periphery of vineyards. However, I found that infestation peaked at veraison before falling somewhat around pre-harvest. This indicated that growers should manage this pest prior to veraison, perhaps as early as budbreak, to prevent the insect from reaching damaging levels when the crop is more susceptible to loss. Lastly, by combining field-based data collection with remote climate and landscape monitoring, growers can achieve a greater level of surveillance of P. viteana. A mash up of remote and vineyard-based data can provide substantive pertinent information for management at both local and regional scales. If one grower identifies moth emergence, then growers nearby potentially can use that same information to initiate models to monitor development of the moth population in their own vineyard. This research complements the notion that the more a community shares information, the broader its applicability becomes to neighbors. / Ph. D.