To study the effect of the substrate support on the nanoscale contact, three different regimes, i.e., graphene on rigid (ultra-crystalline diamond) and on elastic (Polydimethylsiloxane) supports and free-standing graphene, were considered. The contribution of the graphene support to the mechanical and electrical characteristics of the graphene/metal contact was studied using the conductive atomic force microscopy (AFM) technique.The results revealed that the electrical conductivity of the graphene/metal contact highly depends on the nature of the graphene support. The conductivity increased when transitioning from suspended to elastic and then to rigid substrates, which is attributed to the changes in the contact area being higher for the suspended graphene and lower for the rigid substrate. The experimental observations showed good agreement with theoretical results obtained from modeling of the studied material systems. Further, the results indicated that in addition to the substrate support, the nature of the contact, static or dynamic, results in large variations of the electrical conductivity of the graphene/metal contacts. In case of the static mode, the contact made with supported graphene was very stable for a wide range of applied normal loads. Transitioning to the dynamic mode led to instability of the graphene/metal contact as demonstrated by lowering in …

In this dissertation, tribo-corrosion behavior of several single-phase and multi-phase high entropy alloys were investigated. Tribo-corrosion of body centered cubic MoNbTaTiZr high entropy alloy in simulated physiological environment showed very low friction coefficient (~ 0.04), low wear rate (~ 10-8 mm3/Nm), body-temperature assisted passivation, and excellent biocompatibility with respect to stem cells and bone forming osteoblast cells. Tribo-corrosion resistance was evaluated for additively manufactured face centered cubic CoCrFeMnNi high entropy alloy in simulated marine environment. The additively manufactured alloy was found to be significantly better than its as-cast counterpart which was attributed to the refined microstructure and homogeneous elemental distribution. Additively manufactured CoCrFeMnNi showed lower wear rate, regenerative passivation, less wear volume loss, and nobler corrosion potential during tribo-corrosion test compared to its as-cast equivalent. Furthermore, in the elevated temperature (100 °C) tribo-corrosion environment, AlCoCrFeNi2.1 eutectic high entropy alloy showed excellent microstructural stability and pitting resistance with an order of magnitude lower wear volume loss compared to duplex stainless steel. The knowledge gained from tribo-corrosion response and stress-corrosion susceptibility of high entropy alloys was used in the development of bio-electrochemical sensors to sense implant degradation. The results obtained herewith support the promise of high entropy alloys in outperforming currently used …

NASCION-type lithium-aluminum-germanium-phosphate (LAGP) glass-ceramic is one of the most promising solid electrolyte (SEs) material for the next generation Li-ion battery. Based on the crystallization of glass-ceramic material, the two-step heat treatment was designed to control the crystallization of Li-ion conducting crystal in the glass matrix. The results show that the LAGP crystal is preferred to internally crystalize, Tg + 60%∆T is the nucleation temperature that provides the highest ion conductivity. The compositional investigation also found that, pure LAGP crystal phase can be synthesized by lowering the amount of GeO2. To fill gap of atomic structure in LAGP glass-ceramic, molecular dynamic (MD) simulation was used to build the crystal, glass, and interfacial structure LAGP. The aliovalent ion substitution induced an simultaneously redistribution of Li to the 36f interstitial site, and the rapid cooperative motion between the Li-ions at 36f can drop the activation energy of LAGP crystal by decreasing the relaxation energy; furthermore, an energy model was built based on the time-based analysis of Li-ion diffusion to articulate the behavior. The glass and interfacial structure show and accumulation of AlO4, GeO4 and Li at the interface, which explains the Li-trapping on the intergranular glass phase. An in-situ synchrotron X-ray study found …

This dissertation tested the hypothesis that energy transferred from a plasma or plume can be used to optimize the structure, chemistry, topography, optical and electrical properties of pulsed laser deposited and sputtered thin-films of ZnO, a-BOxNy, and few layer 2H-WS2 for transparent electronics devices fabricated without substrate heating or with low substrate heating. Thus, the approach would be compatible with low-temperature, flexible/bendable substrates. Proof of this concept was demonstrated by first optimizing the processing-structure-properties correlations then showing switching from accumulation to inversion in ITO/a-BOxNy/ZnO and ITO/a-BOxNy/2H-WS2 transparent MIS capacitors fabricated using the stated processes. The growth processes involved the optimization of the individual materials followed by growing the multilayer stacks to form MIS structures. ZnO was selected because of its wide bandgap that is transparent over the visible range, WS2 was selected because in few-layer form it is transparent, and a-BOxNy was used as the gate insulator because of its reported atomic smoothness and low dangling bond concentration. The measured semiconductor-insulator interfacial trap properties fall in the range reported in the literature for SiO2/Si MOS structures. X-ray photoelectron spectroscopy (XPS), Hall, photoluminescence, UV-Vis absorption, and X-ray diffraction (XRD) measurements investigated the low-temperature synthesis of ZnO. All films are nanocrystalline with …

In the first study, superconducting 2D NbSe₂ was examined towards its prototypical demonstration as a transition-edge sensor, where photoexcitation caused a thermodynamic phase transition in NbSe₂ from the superconducting state to the normal state. The efficacy of the optical absorption was found to depend on the wavelength of the incoming radiation used, which ranged from the ultra-violet (405 nm), visible (660 nm), to the infrared (1060 nm). In the second case involving WSe₂, the UV-ozone treatment revealed the presence of localized excitonic emission in 1L WSe₂ that was robust and long-lived. Our third material platform dealt with hybrid 0D-2D ensembles based on graphene and WSe₂, specifically graphene–endohedral, WSe₂–fullerene (C₆₀), and WSe₂–Au nanoparticles, and exhibited exceptional performance gains achieved with both types of hybrid structures. Next, we investigated WSe₂ based mixed dimensional hybrids. Temperature T-dependent and wavelength λ-dependent optoelectronic transport measurements showed a shift in the spectral response of 1L WSe₂ towards the SPR peak locations of Au-Sp and Au-BP, fostered through the plexciton interactions. Models for the plexcitonic interactions are proposed that provide a framework for explaining the photoexcited hot charge carrier injection from AuNPs to WSe₂ and its influence on the carrier dynamics in these hybrid systems. Last, we …

In this study, we have investigated the relationship between the process-microstructure to predict and modify the material's properties. Understanding these relationships allows the identification and correction of processing deficiencies when the desired properties are not achieved, depending on the microstructure. Hence, the co-relation between process-microstructure-properties helped reduce the number of experiments, materials & tool costs and saved much time. In the case of high entropy alloys, friction stir welding (FSW) causes improved strength due to the formation of fine grain structure and phase transformation from f.c.c to h.c.p. The phase transformation is temperature sensitive and is studied with the help of differential scanning calorimetry (DSC) to calculate the enthalpy experimentally to obtain ΔGγ→ε. The second process discussed is heat treatment causing precipitation evolution. Fundamental investigations aided in understanding the influence of strengthening precipitates on mechanical properties due to the aging kinetics – solid solution and variable artificial aging temperature and time. Finally, in the third case, the effect of FSW parameters causes the thermal profile to be generated, which significantly influences the final microstructure and weld properties. Therefore, a computational model using COMSOL Multiphysics and TC-Prisma is developed to generate the thermal profile for different weld parameters to understand its …

In the present study both fusion based - laser powder bed fusion (LPBF), and solid state - additive friction stir deposition (AFSD) additive manufacturing processes were employed for the manufacturing of a metastable high entropy alloy (HEA), Fe40Mn20Co20Cr15Si5 (CS-HEA). A processing window was developed for the LPBF and AFSD processings of CS-HEA. In case of LPBF, formation of solidification related defects such as lack of fusion pores (for energy density ≤ 31.24 J/mm3) and keyhole pores (for energy density ≥ 75 J/mm3) were observed. Variation in processing conditions affected the microstructural evolution of the metastable CS-HEA; correlation between processing conditions and microstructure of the alloy is developed in the current study. The tendency to transform and twin near stress concentration sites provided excellent tensile and fatigue properties of the material despite the presence of defects in the material. Moreover, solid state nature of AFSD process avoids formation of solidification related defects. Defect free builds of CS-HEA using AFSD resulted in higher work hardening in the material. In summary, the multi-processing techniques used for CS-HEA in the present study showcase the capability of the AM process in tailoring the microstructure, i.e., grain size and phase fractions, both of which are extremely …