Cost-Effective Synthesis of Diamond Nano-/Microstructures from Amorphous and Graphitic Carbon Materials: Implications for Nanoelectronics

The synthesis of diamonds with different microstructures is important for various applications including nanoelectronic devices where diamonds can be implemented as heat spreaders. Here we report the synthesis of functional diamond microstructures and coatings, including diamond microfibers, microspheres, tubes, and large-area thin film, using amorphous and graphitic carbon precursors by hot filament chemical vapor deposition. The characteristics of microstructures depend upon initial carbon precursors and their laser annealing pretreatments. Low-cost and abundant carbon precursors act as diamond nucleation sites and accelerate diamond growth, while laser annealing can further promote the nucleation and growth of diamond. As a result, carbon microfibers are converted to diamond microfibers, while large diamond microspheres are formed from multipulse laser-annealed carbon microfibers. Both of the diamond structures consist of 5-fold twinned microcrystallites. Highly dense and phase-pure diamond films are observed using porous carbon seed, and individual diamond tubes with porous walls are obtained by using carbon nanotube hollow fibers. The electron backscatter diffraction analysis confirms the diamond cubic lattice structure, while sharp diamond peaks (1331–1333 cm–1) in Raman spectra demonstrate the excellent diamond quality of prepared diamond microstructures

Spin Engineering of VO₂ Phase Transitions and Removal of Structural Transition

Vanadium dioxide undergoes a metal-to-insulator transition, where the energy of electron–electron, electron–lattice, spin–spin, and spin–lattice interactions are of the same order of magnitude. This leads to the coexistence of electronic and structural transitions in VO2 that limit the lifetime and speed of VO2-based devices. However, the closeness of interaction energy of lattice-electron-spin can be turned into an opportunity to induce some transitions while pinning others via external stimuli. That is, the contribution of spin, charge, orbital, and lattice degrees of freedom can be manipulated. In this study, spin engineering has been exploited to affect the spin-related interactions in VO2 by introducing a ferromagnetic Ni layer. The coercivity in the Ni layer is engineered by controlling the shape anisotropy via kinetics of growth. Using spin engineering, the structural pinning of the monoclinic M2 phase of VO2 is successfully achieved, while the electronic and magnetic transitions take place

Electrochemical Performance of Carbon-Nanotube-Supported Tubular Diamond

Tubular diamond structures with high surface areas are very desirable for various potential electrochemical applications. Here, we report a simple and cost-effective two-step method for the synthesis of a diamond tube with a porous tube wall from carbon nanotube (CNT) hollow fibers via pulsed laser annealing (PLA) and hot filament chemical vapor deposition (HFCVD). These diamond tubes exhibit high double-layer capacitances of 11.65–18.07 mF cm–2, three orders of magnitudes higher than the equivalent flat diamond films. Scanning electron microscopy (SEM) shows the presence of diamond microspheres composed of both micro- and nanocrystallites on the entire tube after 3–6 h HFCVD. The number density of the diamond, the average size of diamond microspheres, and the nanocrystallite content on the microspheres can be controlled by HFCVD time and laser annealing parameters of CNT hollow fibers. The electron back-scattered diffraction analysis shows the crystallographic orientation of the prepared diamond along the ⟨101⟩ plane. Raman spectra show a sharp characteristic/signature diamond peak at ∼1332 cm–1, corresponding to an unstrained high-quality diamond. The magnificent electrochemical performances of these CNT-supported diamond tubes are explained by their significantly enhanced electroactive surface area and the presence of a very small fraction (0.73–1.03%) of sp2 carbon in diamond tubes for electron conduction. The density of states, band gaps, and outmost quantum capacitance (∼200 μF/cm2 at −2.2 V electrode potential) of the tubular diamond are calculated by the density functional theory calculations, which support our experimental findings and suggest its future potentiality as an efficient supercapacitor electrode material