Development and analysis of ZnO-based semiconductor photodetectors for UV detection

The entire thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file; a non-technical public abstract appears in the public.pdf file.Title from PDF of title page (University of Missouri--Columbia, viewed on October 27, 2011).Thesis advisor: Dr. Ping Yu.Vita.Ph. D. University of Missouri--Columbia 2010.The objective of this thesis is to develop ZnO-based semiconductor photodetectors for UV detection with low dark current, high responsivity, and fast response time. To achieve this objective, an understanding of carrier recombination and transport mechanisms of the devices is necessary by investigating their electrical properties and optical properties. The photoresponse under continuous wave excitation and pulse excitation along with the frequency photoresponse provide characterizations and useful mechanism information of the devices. These measurements are also helpful to the optimization of the structures in the UV detectors. In this thesis, various ZnO UV detectors are investigated, including ZnO photoconductors, metal-semiconductor-metal (MSM) ZnO photodetectors with Ohmic contacts, post-processed MSM ZnO photodetectors with Schottky contacts, and p-i-n ZnO photodetectors. Experimental data and analysis give four unique results: (1) Very high photoelectric gain was confirmed in ZnO photoconductors. Persistent photoconductivity was observed in these detectors, which is due to carriers trapped in surface states. (2) A high gain and high speed photo-detection was validated in ZnO MSM photodetectors with Ohmic contacts. (3) A extremely low dark current and very high UV-Visible rejection was observed in ZnO MSM photodetectors with Schottky contacts. (4) Two photocarrier processes were found for the first time in ZnO p-i-n photodetectors. Our results show that semiconductor photodetectors based on ZnO are a promising candidate for UV detection.Includes bibliographical reference

Global nonexistence of solutions for the viscoelastic wave equation of Kirchhoff type with high energy

In this paper we consider the viscoelastic wave equation of Kirchhoff type: $$u_{tt}-M(\|\nabla u\|_{2}^{2})\Delta u+\int_{0}^{t}g(t-s)\Delta u(s){\rm d}s+u_{t}=|u|^{p-1}u$$ with Dirichlet boundary conditions. Under some suitable assumptions on $g$ and the initial data, we established a global nonexistence result for certain solutions with arbitrarily high energy.Comment: 12 page

Surface Modification of CNTs with N-Doped Carbon: An Effective Way of Enhancing Their Performance in Supercapacitors

This document is the Accepted Manuscript version of a Published Work that appeared in final form in ACS Sustainable Chemistry and Engineering, copyright © American Chemical Society after peer review and technical editing by the publisher.To access the final edited and published work see http://dx.doi.org/10.1021/sc500069hCarbon nanotubes have been successfully coated with a N-doped carbon layer via hydrothermal carbonization in the presence of a nitrogen-containing carbohydrate, i.e., glucosamine hydrochloride. By controlling the amount of glucosamine added, it was possible to tune the N content of the composites N-doped carbon/CNT between 1.8–2.5 wt %. The prepared composites exhibited superior supercapacitor performance in comparison to bare CNTs even though they possess lower textural properties. Thus, a 2- to 4-fold increase in specific capacitance per surface area was registered at low current densities and sweep rates and a 2-fold increase in energy density, while keeping the power density. Besides, the composites possess superb long-term stability, losing only 4–6% of specific capacitance after 10,000 cycles at 10 A g–1.This research work was supported by Spanish MINECO (MAT2012-31651 and CTM2011- 23378). M.S. acknowledges the award of the Ramón y Cajal contract.Peer reviewe

Efficacy of non-invasive brain stimulation for disorders of consciousness: a systematic review and meta-analysis

ObjectiveThe aim of this study is to evaluate the efficacy of non-invasive brain stimulation (NIBS) in patients with disorders of consciousness (DoC) and compare differences in efficacy between different stimulation modalities.MethodsWe searched the PubMed, Cochrane Library, Web of Science, and EMBASE databases for all studies published in English from inception to April 2023. Literature screening and quality assessment were performed independently by two investigators. Weighted mean differences (WMDs) and 95% confidence intervals (CIs) were used to evaluate the therapeutic effects of NIBS. The Cochrane Q test and I2 statistic were used to evaluate heterogeneity between studies. Subgroup analysis was performed to identify the source of heterogeneity, and differences in efficacy between different stimulation modalities were compared by Bayesian analysis.ResultsA total of 17 studies with 377 DoC patients were included. NIBS significantly improved the state of consciousness in DoC patients when compared to sham stimulation (WMD: 0.81; 95% CI: 0.46, 1.17; I2 = 78.2%, p = 0.000). When divided into subgroups according to stimulation modalities, the heterogeneity of each subgroup was significantly lower than before (I2: 0.00–30.4%, p >0.05); different stimulation modalities may be the main source of such heterogeneity. Bayesian analysis, based on different stimulation modalities, indicated that a patient’s state of consciousness improved most significantly after repetitive transcranial magnetic stimulation (rTMS) of the left dorsolateral prefrontal cortex (DLPFC). Diagnosis-based subgroup analysis showed that NIBS significantly improved the state of consciousness in patients with a minimal consciousness state (WMD: 1.11; 95% CI: 0.37, 1.86) but not in patients with unresponsive wakefulness syndrome or a vegetative state (WMD: 0.31; 95% CI: −0.09, 0.71). Subgroup analysis based on observation time showed that single treatment did not improve the state of consciousness in DoC patients (WMD: 0.28; 95% CI: −0.27, 0.82) while multiple treatments could (WMD: 1.05; 95% CI: 0.49, 1.61). Furthermore, NIBS had long-term effects on DoC patients (WMD: 0.79; 95% CI: 0.08–1.49).ConclusionAvailable evidence suggests that the use of NIBS on patients with DoC is more effective than sham stimulation, and that rTMS of the left DLPFC may be the most prominent stimulation modality

Being a morning man has causal effects on the cerebral cortex: a Mendelian randomization study

IntroductionNumerous studies have suggested a connection between circadian rhythm and neurological disorders with cognitive and consciousness impairments in humans, yet little evidence stands for a causal relationship between circadian rhythm and the brain cortex.MethodsThe top 10,000 morningness-related single-nucleotide polymorphisms of the Genome-wide association study (GWAS) summary statistics were used to filter the instrumental variables. GWAS summary statistics from the ENIGMA Consortium were used to assess the causal relationship between morningness and variates like cortical thickness (TH) or surficial area (SA) on the brain cortex. The inverse-variance weighted (IVW) and weighted median (WM) were used as the major estimates whereas MR-Egger, MR Pleiotropy RESidual Sum and Outlier, leave-one-out analysis, and funnel-plot were used for heterogeneity and pleiotropy detecting.ResultsRegionally, morningness decreased SA of the rostral middle frontal gyrus with genomic control (IVW: β = −24.916 mm, 95% CI: −47.342 mm to −2.490 mm, p = 0.029. WM: β = −33.208 mm, 95% CI: −61.933 mm to −4.483 mm, p = 0.023. MR Egger: β < 0) and without genomic control (IVW: β = −24.581 mm, 95% CI: −47.552 mm to −1.609 mm, p = 0.036. WM: β = −32.310 mm, 95% CI: −60.717 mm to −3.902 mm, p = 0.026. MR Egger: β < 0) on a nominal significance, with no heterogeneity or no outliers.Conclusions and implicationsCircadian rhythm causally affects the rostral middle frontal gyrus; this sheds new light on the potential use of MRI in disease diagnosis, revealing the significance of circadian rhythm on the progression of disease, and might also suggest a fresh therapeutic approach for disorders related to the rostral middle frontal gyrus-related

Extended Kerr effect of polymer-stabilized blue-phase liquid crystals

Electric-field-induced birefringence of a polymer-stabilized blue-phase liquid crystal (BPLC) is investigated. In the low field region, conventional Kerr effect holds. As the electric field increases, the induced birefringence gradually saturates and deviates from Kerr effect. An exponential convergence model, called extended Kerr effect, is proposed to fit the experimental data. Good agreement between experiment and model is obtained. This extended Kerr effect will make a significant impact to the optimization of emerging BPLC display devices

Exponential energy decay of solutions for a system of viscoelastic wave equations of Kirchhoff type with strong damping

The initial boundary value problem for a system of viscoelastic wave equations of Kirchhoff type with strong damping is considered. We prove that, under suitable assumptions on relaxation functions and certain initial data, the decay rate of the solutions energy is exponential