Xiaojun Hu

n-type conductivity and phase transition in ultrananocrystalline diamond films by oxygen ion implantation and annealing

Ultrananocrystalline diamond (UNCD) films were implanted by oxygen ion and annealed at different temperatures. The electrical and structrual properties of O+-implanted UNCD films were investigated by Hall effects, high-resolution transmission electron microscopy (HRTEM) and uv Raman spectroscopy measurements. The results show that O+-implanted nano-sized diamond grains annealed at 800 °C and above give n-type conductivity to the sample and the UNCD film exhibits n-type resistivity with the carrier mobility of 1∼11 cm2 V−1s−1. With O+ dose increasing from 1015 to 1016 cm−2, diamond phase transits to the amorphous carbon phase accompanied by n-type semiconduction transforming to metallic conduction. In the 1014 cm−2 O+-implanted UNCD film, some amorphous carbon at grain boundaries transits to diamond phase with annealing temperature (Ta) increasing from 500 °C to 800–900 °C, and some of diamond grains are found to be converted to amorphous carbon phase again after 1000 °C annealing. This phase transition is...

Novel bonding patterns and optoelectronic properties of the two-dimensional SixCy monolayers

The search for new two-dimensional (2D) materials with novel optical and electronic properties is always desirable for material development. Herein, we report a comprehensive theoretical prediction of 2D SiC compounds with different stoichiometries from C-rich to Si-rich. In addition to the previously known hexagonal SiC sheet, we identified two types of hitherto-unknown structural motifs with distinctive bonding features. The first type of 2D SiC monolayer, including t-SiC and t-Si2C sheet, can be described by tetragonal lattice. Among them, t-SiC monolayer sheet is featured by each carbon atom binding with four neighboring silicon atoms in almost the same plane, constituting a quasi-planar four-coordinated rectangular moiety. More interestingly, our calculations demonstrate that this structure exhibits a strain-dependent insulator–semimetal transition, suggesting promising applications in strain-dependent optoelectronic sensors. The second type of 2D SiC sheet is featured by silagraphyne with acetylenic linkages (–CC–). Silagraphyne shows both high pore sizes and Poissons ratio. These properties make it a potentially important material for applications in separation membranes and catalysis. Moreover, one of the proposed structures, γ-silagraphyne, is a direct-band-gap semiconductor with a bandgap of 0.89 eV, which has a strong absorption peak in the visible-light region, giving a promising application in ultra-thin transistors, optical sensor devices and solar cell devices.

Two-dimensional stoichiometric boron carbides with unexpected chemical bonding and promising electronic properties

Exploring new two-dimensional materials with novel properties is becoming a particularly important task due to their potential applications in future nano-mechanics, electronics, and optoelectronics. In the present study, the hitherto unknown stable two-dimensional boron carbides with various stoichiometries are revealed via the structure swarm optimization method combined with first-principles calculations. The predicted new compounds are energetically more favorable compared with the previously proposed counterparts. Counterintuitively, we identify two B–C bonding patterns: pyramidal-geometry tetra-coordinated and hexa-coordinated sp2 carbon moiety. The intriguing covalent bonding modes create distinct and fascinating physical and chemical properties. For instance, we discover that the predicted B4C3 has an ultrahigh Youngs modulus that can even outperform graphene; the B2C sheet is metallic with a relatively high superconducting transition temperature (Tc ≈ 21.20 K). On the other hand, the well-located band edge makes β-B3C2 a potentially promising metal-free optoelectronic material for visible-light water splitting.

D-carbon: Ab initio study of a novel carbon allotrope

We have investigated the structural, mechanical and electronic properties of D-carbon by ab initio calculations, a new phase of crystalline sp carbon (pace group D2h , Pmma-orthorhombic). Totalenergy calculations demonstrate that D-carbon is energetically more favorable than previously proposed T6 structure. State-of-the-art theoretical calculations show that this new phase is dynamic, mechanical, and thermal stable at zero pressure, and more stable than graphite beyond 63.7 GPa. Importantly, the calculations reveal that D-carbon possesses high Vickers hardness (86.58 GPa) and bulk modulus (369 GPa), which are comparable to diamond. D-carbon is a semiconductor with a band gap of 4.33 eV, lower than diamond’s gap (5.47 eV). The simulated X-ray diffraction pattern is in satisfactory agreement with the previously experimental data in chimney or detonation soot, suggesting its possible presence in the specimen. Equally important, the possible transition path from diamond to D-carbon has been investigated, indicating a possible approach to synthesize this new phase.By means of ab initio computations and the global minimum structure search method, we have investigated structural, mechanical, and electronic properties of D-carbon, a crystalline orthorhombic sp3 carbon allotrope (space group Pmma [ D2h5 ] with 6 atoms per cell). Total-energy calculations demonstrate that D-carbon is energetically more favorable than the previously proposed T6 structure (with 6 atoms per cell) as well as many others. This novel phase is dynamically, mechanically, and thermally stable at zero pressure and more stable than graphite beyond 63.7 GPa. D-carbon is a semiconductor with a bandgap of 4.33 eV, less than diamonds gap (5.47 eV). The simulated X-ray diffraction pattern is in satisfactory agreement with previous experimental data in chimney or detonation soot, suggesting its possible presence in the specimen.

Synthesis of strong SiV photoluminescent diamond particles on silica optical fiber by chemical vapor deposition

The separated silicon-vacancy (SiV) photoluminescent diamond particles were synthesized on a silica optical fiber by hot filament chemical vapor deposition (HFCVD). The effects of the pre-treated method and chamber pressure on the microstructure and photoluminescence of the diamond particles were investigated. The results show that the diamond particles are homogeneously distributed on the surface of the optical fiber. With the chamber pressure increasing from 1.6 kPa to 3.5 kPa, the shape of the particles transforms from flake to circle, while the diamond particles cannot be deposited on the fiber with the pressure further increased to 4.5 kPa. The samples synthesized under 2.5 kPa chamber pressure are composed of diamond particles with size around 200–400 nm, exhibiting stronger SiV photoluminescence with the width of around 6 nm.