Browsing by Author "Li, Chenxi"

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    Super-elasticity of three-dimensionally cross-linked graphene materials all the way to deep cryogenic temperatures
    (AAAS, 2019) Zhao, Kai; Zhang, Tengfei; Chang, Huicong; Yang, Yang; Xiao, Peishuang; Zhang, Hongtao; Li, Chenxi; Tiwary, Chandra Sekhar; Ajayan, Pulickel M.; Chen, Yongsheng
    Until now, materials with high elasticity at deep cryogenic temperatures have not been observed. Previous reports indicated that graphene and carbon nanotube–based porous materials can exhibit reversible mechano-elastic behavior from liquid nitrogen temperature up to nearly a thousand degrees Celsius. Here, we report wide temperature–invariant large-strain super-elastic behavior in three-dimensionally cross-linked graphene materials that persists even to a liquid helium temperature of 4 K, a property not previously observed for any other material. To understand the mechanical properties of these graphene materials, we show by in situ experiments and modeling results that these remarkable properties are the synergetic results of the unique architecture and intrinsic elastic/flexibility properties of individual graphene sheets and the covalent junctions between the sheets that persist even at harsh temperatures. These results suggest possible applications for such materials at extremely low temperature environments such as those in outer space.
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    Surface Modification of Polycrystalline Diamond
    (2024-04-12) Li, Chenxi; Ajayan, Pulickel; Vajtai, Robert; Mohite, Aditya
    The dissertation investigates surface modification techniques on polycrystalline diamond surfaces, exploring their impacts on properties relevant to electronic, surface- cleaning, and nanofabricating applications. Firstly, a comparative study elucidates the oxidation of microcrystalline diamond powder (DP) and polycrystalline diamond film (PCD) via wet chemical treatments and dry processes. The investigation reveals that sulfuric/nitric acids (H2SO4/HNO3) treatment at 360°C demonstrates superior oxidation performance, while oxygen (O2) plasma treatment enhances oxygen content on PCD surfaces. This study provides insights into oxidation mechanisms and guides the optimization of diamond surface cleaning conditions. Secondly, a novel strategy for aminating boron-doped diamond (BDD) via UV irradiation in ammonia (NH3) is presented. By employing hydrobromic acid (HBr) treatment, primary amine dominance is achieved, enhancing amination efficiency. The study also demonstrates the influence of preoxidation states on amine group coverage, offering insights into surface cleaning effects and mechanisms through theoretical simulations. Thirdly, the dissertation explores the functionalization of hydrogen (H)-terminated diamond surfaces with nitrogen (N) and I sulfur (S) heteroatoms, revealing improved electrical conductivity compared to H- terminated diamonds. Pre-functionalization with S promotes sequential amination efficiency on diamond surface, facilitating reduced UV-exposure times. Density functional theory (DFT) simulations indicate downshifts in bandgap upon functionalization, suggesting enhanced surface conductivity for various electronic applications. Finally, a top-down approach for fabricating diamond nanostructures using metal masks and reactive-ion etching (RIE) process is presented. Silver (Ag) mask exhibits distinct etching profiles, where diamond nanorods (DNRs) cluster is preferably formed after etching and preserves single crystallinities with features resembling diamond nanotubes. Preliminary electrical measurements show Schottky-like conductivity features, indicating potential applications in nanodiamond-based electronics. Collectively, these investigations contribute to a deeper understanding of surface modification techniques on polycrystalline diamond surfaces, offering insights into their utilities across diverse technological domains.