Carrier transport engineering in wide bandgap semiconductors for photonic and memory device applications

Wide bandgap (WBG) semiconductors play a crucial role in the current solid-state lighting technology. The AlGaN compound semiconductor is widely used for ultraviolet (UV) light-emitting diodes (LEDs), however, the efficiency of these LEDs is largely in a single-digit percentage range due to several factors. Until recently, AlInN alloy has been relatively unexplored, though it holds potential for light-emitters operating in the visible and UV regions. In this dissertation, the first axial AlInN core-shell nanowire UV LEDs operating in the UV-A and UV-B regions with an internal quantum efficiency (IQE) of 52% are demonstrated. Moreover, the light extraction efficiency of this UV LED can be further improved by 63% by utilizing appropriate hexagonal photonic crystal structures. The carrier transport characteristics of the LEDs have been carefully engineered to enhance the carrier distributions and reduce the current leakage, leading to a significantly improved IQE of the LEDs. In this regard, the p-type AlGaN electron blocking layer (EBL) has been utilized to suppress electron leakage. Although the EBL can suppress the electron leakage to an extent, it also affects the hole injection due to the generation of positive polarization sheet charges at the hetero interface of EBL and the last quantum barrier (QB). Moreover, the Mg acceptor activation energy of the Al-rich AlGaN EBL layer is elevated, affecting the Mg doping efficiency. To mitigate this problem, in this dissertation, EBL-free UV LED designs are proposed where the epilayers are carefully band-engineered to notably improve the device performance by lowering the electron overflows. The proposed EBL-free strip-in-a-barrier UV LED records the maximum IQE of -61.5% which is -72% higher, and IQE droop is -12.4%, which is -333% less compared to the conventional AlGaN EBL LED structure at 284.5 nm wavelength. Moreover, it is shown that the EBL-free AlGaN deep UV LED structure with linearly graded polarization-controlled QBs instead of conventional QBs in the active region could drastically reduce the electrostatic field in the quantum well (QW) region due to the decreased lattice mismatch between the QW and the QB. The carrier transport in the EBL-free deep UV LEDs is significantly improved, attributed to the increased radiative recombination, quantum efficiency, and output power compared to the conventional EBL LEDs. Overall, the study of EBL-free UV LEDs offers important insights into designing novel, high-performance deep UV LEDs for practical applications. Further, it is demonstrated that novel WBG materials could be perfectly employed for emerging non-volatile memory (resistive random access memory, RRAM) applications. The resistive switching (RS) capability has been observed in Ga2O3 at low power operation. Importantly, for the first time, the multi-bit storage capability of this types of RRAM devices with a reasonably high Roff/Ron ratio is experimentally demonstrated. In addition, integrating a thin SiNx layer in the conventional SiO2 RRAM device could effectively facilitate the formation of a conducting filament. It is reported that the proposed RRAM device exhibits excellent RS characteristics, such as highly uniform current-voltage characteristics with concentrated SET and RESET voltages, excellent stability, and high Roff/Ron (\u3e 103) even at ultra-low current (10 nA) operation. The multi-bit RS behavior has been observed in these RRAM devices, which pave the way for low-power and high-density data storage applications

A role of radioprotective agents in cancer therapeutics: a review

In the field of medicine or dentistry, cancer is one of the most common causes for death of the individual worldwide, in which oral cancer accounts for about 10% of it. One of the most important treatment modalities for cancer includes radiotherapy. During radiotherapy, exposure of the normal tissue to these ionizing radiations, results in mutagenesis and cell death. Several modalities and clinical approaches have been made to reduce these early and late complications of the radiotherapies and one among them is, by the means of pharmacological agents. Many experimental and clinical studies have given rise to new concepts of chemical and molecular pharmacological agents that could be effective in protection and treatment of radiation damage to surrounding normal tissues. Clinical Significance: To reduce the significant complications in irradiated patients, the clinical implication of these radioprotective agents have emerged as potential drugs and with anti-tumour effect in the radiotherapy of various cancers including oral carcinomas

UBQLN4 Represses Homologous Recombination and Is Overexpressed in Aggressive Tumors

Genomic instability can be a hallmark of both human genetic disease and cancer. We identify a deleterious UBQLN4 mutation in families with an autosomal recessive syndrome reminiscent of genome instability disorders. UBQLN4 deficiency leads to increased sensitivity to genotoxic stress and delayed DNA double-strand break (DSB) repair. The proteasomal shuttle factor UBQLN4 is phosphorylated by ATM and interacts with ubiquitylated MRE11 to mediate early steps of homologous recombination-mediated DSB repair (HRR). Loss of UBQLN4 leads to chromatin retention of MRE11, promoting non-physiological HRR activity in vitro and in vivo. Conversely, UBQLN4 overexpression represses HRR and favors non-homologous end joining. Moreover, we find UBQLN4 overexpressed in aggressive tumors. In line with an HRR defect in these tumors, UBQLN4 overexpression is associated with PARP1 inhibitor sensitivity. UBQLN4 therefore curtails HRR activity through removal of MRE11 from damaged chromatin and thus offers a therapeutic window for PARP1 inhibitor treatment in UBQLN4-overexpressing tumors

Nuclear poly(A)-binding protein 1 is an ATM target and essential for DNA double-strand break repair

The DNA damage response (DDR) is an extensive signaling network that is robustly mobilized by DNA double-strand breaks (DSBs). The primary transducer of the DSB response is the protein kinase, ataxia-telangiectasia, mutated (ATM). Here, we establish nuclear poly(A)-binding protein 1 (PABPN1) as a novel target of ATM and a crucial player in the DSB response. PABPN1 usually functions in regulation of RNA processing and stability. We establish that PABPN1 is recruited to the DDR as a critical regulator of DSB repair. A portion of PABPN1 relocalizes to DSB sites and is phosphorylated on Ser95 in an ATM-dependent manner. PABPN1 depletion sensitizes cells to DSB-inducing agents and prolongs the DSB-induced G2/M cell-cycle arrest, and DSB repair is hampered by PABPN1 depletion or elimination of its phosphorylation site. PABPN1 is required for optimal DSB repair via both nonhomologous end-joining (NHEJ) and homologous recombination repair (HRR), and specifically is essential for efficient DNA-end resection, an initial, key step in HRR. Using mass spectrometry analysis, we capture DNA damage-induced interactions of phospho-PABPN1, including well-established DDR players as well as other RNA metabolizing proteins. Our results uncover a novel ATM-dependent axis in the rapidly growing interface between RNA metabolism and the DDR

CONTROLLED ELECTRON LEAKAGE IN ELECTRON BLOCKING LAYER FREE InGaN/GaN NANOWIRE LIGHT-EMITTING DIODES

In this study, we have proposed and investigated the effect of coupled quantum wells to reduce electron overflow in InGaN/GaN nanowire white color light-emitting diodes. The coupled quantum well before the active region could decrease the thermal velocity, which leads to a reduced electron mean free path. This improves the electron confinement in the active region and mitigates electron overflow in the devices. In addition, coupled quantum well after the active region utilizes the leaked electrons from the active region and contributes to the white light emission. Therefore, the output power and external quantum efficiency of the proposed nanowire LEDs are improved. Moreover, the efficiency droop was negligible up to 900 mA injection current

Facile Synthesis of Ordered Mesoporous Orthorhombic Niobium Oxide (T-Nb2_2O5_5) for High-Rate Li-Ion Storage with Long Cycling Stability

Herein, we describe the synthesis and evaluation of hierarchical mesoporous orthorhombic niobium oxide (T-Nb$_2$O$_5$) as an anode material for rechargeable lithium-ion batteries (LIB). The as-synthesized material addresses key challenges such as beneficial porous structure, poor rate capability, and cycling performance of the anode for Li-ion devices. The physicochemical characterization results reveal hierarchical porous nanostructure morphology with agglomerated particles and a 20 to 25 nm dimension range. Moreover, the sample has a high specific surface area (~65 m$^2$ g$^{−1}$) and pore volume (0.135 cm3 g$^{−1}$). As for the application in Li-ion devices, the T-Nb$_2$O$_5$ delivered an initial discharging capacity as high as 225 mAh g$^{−1}$ at 0.1 A g$^{−1}$ and higher rate capability as well as remarkable cycling features (~70% capacity retention after 300 cycles at 250 mA g$^{−1}$) with 98% average Coulombic efficiency (CE). Furthermore, the scan rate-dependent charge storage mechanism of the T-Nb$_2$O$_5$ electrode material was described, and the findings demonstrate that the electrode shows an evident and highly effective pseudocapacitive Li intercalation behaviour, which is crucial for understanding the electrode process kinetics. The origin of the improved performance of T-Nb$_2$O$_5$ results from the high surface area and mesoporous structure of the nanoparticles

Power assisted exercise for people with complex neurological impairment : a feasibility study

Background/Aims: Participation in physical activity and exercise presents a challenge for adults with complex neurological impairment. Power assisted exercise facilitates combined limb and trunk movement and may be a feasible exercise option for people with movement impairment. The aim of this study was to determine whether power assisted exercise is a feasible activity option for people with complex neurological impairment. Methods: Seven adults with complex neurological impairment were recruited to take part in a four week programme of twice weekly power assisted exercise. Programme attendance and completion was recorded and adverse events or effects documented. Mobility was monitored using the Timed Up and Go test (TUG). Upon completion of the programme, participants were interviewed regarding their experience of using the equipment. Findings: All seven participants completed the programme and the overall attendance was 96%. No adverse events occurred; two participants reported minor adverse effects which were temporary. The TUG scores improved and participants enjoyed the programme, reporting perceived benefits in physical function and wellbeing. Conclusions: The findings of this study demonstrate that people with complex neurological impairment can participate in a programme of power assisted exercise. Reported improvements in physical mobility suggest that further research in this area is indicated. Key words: Power Assisted Exercise, Neurology, Feasibility, Mobilit

Cord Blood Stem Cells Inhibit Epidermal Growth Factor Receptor Translocation to Mitochondria in Glioblastoma

Overexpression of EGFR is one of the most frequently diagnosed genetic aberrations of glioblastoma multiforme (GBM). EGFR signaling is involved in diverse cellular functions and is dependent on the type of preferred receptor complexes. EGFR translocation to mitochondria has been reported recently in different cancer types. However, mechanistic aspects of EGFR translocation to mitochondria in GBM have not been evaluated to date.In the present study, we analyzed the expression of EGFR in GBM-patient derived specimens using immunohistochemistry, reverse-transcription based PCR and Western blotting techniques. In clinical samples, EGFR co-localizes with FAK in mitochondria. We evaluated this previous observation in standard glioma cell lines and in vivo mice xenografts. We further analyzed the effect of human umbilical cord blood stem cells (hUCBSC) on the inhibition of EGFR expression and EGFR signaling in glioma cells and xenografts. Treatment with hUCBSC inhibited the expression of EGFR and its co-localization with FAK in glioma cells. Also, hUCBSC inhibited the co-localization of activated forms of EGFR, FAK and c-Src in mitochondria of glioma cells and xenografts. In addition, hUCBSC also inhibited EGFR signaling proteins in glioma cells both in vitro and in vivo.We have shown that hUCBSC treatments inhibit phosphorylation of EGFR, FAK and c-Src forms. Our findings associate EGFR expression and its localization to mitochondria with specific biological functions in GBM cells and provide relevant preclinical information that can be used for the development of effective hUCBSC-based therapies