Diamond formation mechanism in chemical vapor deposition.

It is a key challenge to prepare large-area diamonds by using the methods of high-pressure high-temperature and normal chemical vapor deposition (CVD). The formation mechanism of thermodynamically metastable diamond compared to graphite in low-pressure CVD possibly implies a distinctive way to synthesize large-area diamonds, while it is an intriguing problem due to the limitation of in situ characterization in this complex growth environment. Here, we design a series of short-term growth on the margins of cauliflower-like nanocrystalline diamond particles, allowing us to clearly observe the diamond formation process. The results show that vertical graphene sheets and nanocrystalline diamonds alternatively appear, in which vertical graphene sheets evolve into long ribbons and graphite needles, and they finally transform into diamonds. A transition process from graphite (200) to diamond (110) verifies the transformation, and Ta atoms from hot filaments are found to atomically disperse in the films. First principle calculations confirm that Ta-added H- or O-terminated bilayer graphene spontaneously transforms into diamond. This reveals that in the H, O, and Ta complex atmosphere of the CVD environment, diamond is formed by phase transformation from graphite. This subverts the general knowledge that graphite is etched by hydrogen and sp3 carbon species pile up to form diamond and supplies a way to prepare large-area diamonds based on large-sized graphite under normal pressure. This also provides an angle to understand the growth mechanism of materials with sp2 and sp3 electronic configurations.

Two-dimensional tetragonal transition-metal carbide anodes for non-lithium-ion batteries.

Searching for high-performance anode materials with high energy-density, fast kinetics, and good stability is a key challenge for non-lithium-ion batteries (NLIBs), such as Na+, K+, Mg2+, Ca2+, Zn2+ and Al3+ ion batteries. Here, we systematically investigated the performance of a new class of two-dimensional tetragonal transition-metal carbides (tetr-MCs) using first-principles calculations, as anodes for NLIBs. The results show that tetr-MCs are ideal anode materials with good stabilities, favorable mechanical properties, intrinsic metallic properties, high theoretical capacities, and fast ion diffusion rate for NLIBs. Among all tetr-MCs, we found that the energy barrier of Mg atoms on tetr-TiC is only 54 meV and that of Al atoms on tetr-VC is 101 meV, which are lower than the energy barriers of 230-500 meV of the well-studied MXenes, indicating that tetr-VC and tetr-TiC monolayers are promising anodes for NLIBs. Therefore, compared to MXenes, tetr-MCs show many advantages for NLIB applications, such as a lower diffusion barrier (minimum 54 meV), a high theoretical capacity (up to 1450 mA h g-1), and a lower average open circuit voltage (0.05-0.77 V). The results are of great significance for the experimental preparation of excellent anode materials for NLIBs.

A novel hydrogenated boron-carbon monolayer with high stability and promising carrier mobility.

Although immense research on the extension of the two-dimensional (2D) material family has been carried out, 2D materials with a satisfactory band gap, high carrier mobility, and outstanding thermodynamic stability under ambient conditions are still limited. In this work, using first principles calculations, we proposed new 2D ternary materials consisting of C, B, and H atoms, namely hexagonal-BCH (h-BCH) and tetragonal-BCH (t-BCH). Both phonon calculations and ab initio molecular dynamics simulations show that these proposed sheets are thermodynamically stable phases. The electronic structure calculations indicate that h-BCH and t-BCH sheets are semiconductors with a band gap of 2.66 and 2.22 eV, respectively. Remarkably, the h-BCH (t-BCH) sheet exhibits electron mobility as high as 7.41 × 103 (1.09 × 103) cm2 V-1 s-1, which is higher than that of the MoS2 monolayer, though the hole mobility is about one (two) order of magnitude lower. Equally important is the fact that the position of both the conduction and valence band edges of the h-BCH sheet matches well with the chemical reaction potential of H2/H+ and O2/H2O, giving a 2D photocatalyst as a potential candidate for overall visible-light-driven water splitting. Therefore, the designed h-BCH and t-BCH monolayers have promising applications in future electronics and photocatalysts.

Application of Nano-Crystalline Diamond in Tribology.

Nano-crystalline diamond has been extensively researched and applied in the fields of tribology, optics, quantum information and biomedicine. In virtue of its hardness, the highest in natural materials, diamond outperforms the other materials in terms of wear resistance. Compared to traditional single-crystalline and poly-crystalline diamonds, nano-crystalline diamond consists of disordered grains and thus possesses good toughness and self-sharpening. These merits render nano-crystalline diamonds to have great potential in tribology. Moreover, the re-nucleation of nano-crystalline diamond during preparation is beneficial to decreasing surface roughness due to its ultrafine grain size. Nano-crystalline diamond coatings can have a friction coefficient as low as single-crystal diamonds. This article briefly introduces the approaches to preparing nano-crystalline diamond materials and summarizes their applications in the field of tribology. Firstly, nano-crystalline diamond powders can be used as additives in both oil- and water-based lubricants to significantly enhance their anti-wear property. Nano-crystalline diamond coatings can also act as self-lubricating films when they are deposited on different substrates, exhibiting excellent performance in friction reduction and wear resistance. In addition, the research works related to the tribological applications of nano-crystalline diamond composites have also been reviewed in this paper.

Monodispersed Transition Metals Induced Ordinary-Pressure Phase Transformation from Graphite to Diamond: A First-Principles Calculation.

High pressure and high temperature are normally required for the transformation of graphite to diamond; thus, finding a method that allows the transformation to occur under ordinary pressure will be extremely promising for diamond synthesis. Here, it is found that graphite spontaneously transforms into diamond without any pressure by adding monodispersed transition metals, and the universal rules that will help predict the role of certain elements in the phase transition were studied. The results show that the favorable transition metals possess an atomic radius of 0.136-0.160 nm and an unfilled d-orbital of d2s2-d7s2, which allow more charge transfer and accumulation at the proper position between the metal and dangling C atoms, leading to stronger metal-C bonds and a lower energy barrier for the transition. This provides a universal method to prepare diamond from graphite under ordinary pressure and also provides a way for the synthesis from sp2 to sp3 bonded materials.

Highly Stable Two-Dimensional Iron Monocarbide with Planar Hypercoordinate Moiety and Superior Li-Ion Storage Performance.

Stable planar hypercoordinate motifs have been recently demonstrated in two-dimensional (2D) confinement systems, while perfectly planar hypercoordinate motifs in 2D carbon-transition metal systems are rarely reported. Here, by using comprehensive ab initio computations, we discover two new iron monocarbide (FeC) binary sheets stabilized at 2D confined space, labeled as tetragonal-FeC (t-FeC) and orthorhombic-FeC (o-FeC), which are energetically more favorable compared with the previously reported square and honeycomb lattices. The proposed t-FeC is the global minimum configuration in the 2D space, and each carbon atom is four-coordinated with four ambient iron atoms, considered as the quasi-planar tetragonal lattice. Strikingly, the o-FeC monolayer is an orthorhombic phase with a perfectly planar pentacoordinate carbon moiety and a planar seven-coordinate iron moiety. These monolayers are the first example of a simultaneously pentacoordinate carbon and planar seven-coordinate Fe-containing material. State-of-the-art theoretical calculations confirm that all these monolayers have significantly dynamic, mechanical, and thermal stabilities. Among these two monolayers, the t-FeC monolayer shows a higher theoretical capacity (395 mAh g-1) and can stably adsorb Li up to t-FeCLi4 (1579 mAh g-1). The low migration energy barrier is predicted as small as 0.26 eV for Li, which results in the fast diffusion of Li atoms on this monolayer, making it a promising candidate for lithium-ion battery material.

Enhanced SiV photoluminescence by oxidation-induced nano-structures on diamond particle surfaces.

We successfully constructed nanostructures on the surface of diamond particles by oxidation, which drastically enhanced their silicon-vacancy (SiV) photoluminescence (PL) intensity. The {100} plane of the diamond crystal initially had a smooth surface and strong anti-oxidation ability, which converted to a nano-pyramid structure with the sides resembling a {111} crystal plane orientation after oxidation. The {111} plane originally presented vertically layered or scale-like structures, but exhibited irregular nanoporous structures with some ridges perpendicular to the {111} plane in the edge area after oxidation. Since the crystal orientation of these nano-structures matches the <111> aligned split-vacancy structure of the SiV center, the collection efficiency of SiV luminescence increased, such that the SiV emission intensity increased by 27-fold and 4-fold for the nano-pyramid and the irregular nano-porous structure, respectively. Oxidation also significantly improved the crystal quality of diamond, such that the lattice stress around the SiV color center reduced, narrowing the linewidth of the SiV PL peak to nearly 3 nm. This study provides a feasible way to optimize SiV photoluminescent properties by building nanostructures on the surface of diamond particles.

Evolution of Granular Structure and the Enhancement of Electron Field Emission Properties of Nanocrystalline and Ultrananocrystalline Diamond Films Due to Plasma Treatment Process.

The present work reports the plasma post treatment (ppt) process that instigates the evolution of granular structure of nanocrystalline diamond (NCD), consequently conducing the enhancement of the electron field emission (EFE) properties. The NCD films contain uniform and nanosized diamond grains (∼20 nm) with negligible thickness for grain boundaries that is distinctly different from the microstructure of ultrananocrystalline (UNCD) films with uniformly sized ultrananodiamond grains (∼5 nm) having relatively thick grain boundaries (∼0.1 nm). The turn-on of the electron field emission (EFE) process occurs at ( E0)NCD = 24.1 V/µm and ( E0)UNCD = 18.6 V/µm for the pristine NCD and UNCD materials, respectively. The granular structure of the starting diamond films largely influenced the microstructure evolution behavior and EFE properties of the materials subject to plasma annealing. The CH4/(Ar-H2) ppt-process leads to formation of a hybrid granular structured diamond (HiDNCD and HiDUNCD) via isotropic conjoining of nanosized diamond grains, whereas the CH4/N2 ppt-process leads to the formation of acicular granular structured diamond films (NNCD and NUNCD) via inducing aeolotropic growth of nanodiamond grains. While both of the HiDNCD and HiDUNCD films contain hybrid granular structure, the HiDUNCD films contain a larger proportion of nanographite phase and result in improved EFE properties, viz. ( E0)HiD-UNCD = 7.7 V/µm and ( E0)HiD-NCD = 12.3 V/µm. In contrast, when the films were CH4/N2 ppt-processed, the acicular diamond grains were formed for NUNCD and NNCD films; however, carbon nanoclusters attached to the diamond grains of NNCD films and the nanographitic layers encasing diamond cores are not crystallized very well, as compared with NUNCD films. Therefore, the NNCD films exhibit slightly inferior EFE properties than the NUNCD films, viz. ( E0)N-UNCD = 5.3 V/µm and ( E0)N-NCD = 11.8 V/µm. The difference in EFE properties for ppt-processed NCD and UNCD films corresponds to the dissimilar granular structure evolution behavior in these films that is, in turn, due to the distinct different microstructure of the pristine NCD and UNCD films.