The typical two-dimensional semiconductors, group IIIA chalcogenides, have garnered tremendous interest for their outstanding electronic, mechanical, and chemical properties. However, so far, there have been rare reports on boron monosulfides (BS) binary material. Here, four two-dimensional BS sheets, namely, the α-, β-, γ-, and δ-BS sheets, are proposed and discussed from first principles calculations. State-of-the-art calculations reveal all these structures are thermally and dynamically stable, indicating the potential for experimental synthesis. Specifically, for α-BS, it has a calculated exfoliation energy of 0.96 J m−2, suggesting that the preparation of α-BS is feasible by the exfoliation of bulk rhombohedral-BS. Our results show that α-, β-, and γ-BS are semiconductors, whereas δ-BS is a metallic system. Remarkably, our calculations indicate that δ-BS is a superconductor with a large electron-phonon coupling (λ ≈ 1.51), leading to a high superconducting critical temperature (Tc ≈ 21.56 K), which is the interesting property with intrinsic superconducting among all two-dimensional group IIIA chalcogenides. The potential of semiconducting BS monolayers as the gas-sensor or thermoelectric materials is also demonstrated.
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 ( E) = 24.1 V/μm and ( E) = 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 CH/(Ar-H) ppt-process leads to formation of a hybrid granular structured diamond (HiD and HiD) via isotropic conjoining of nanosized diamond grains, whereas the CH/N ppt-process leads to the formation of acicular granular structured diamond films (N and N) via inducing aeolotropic growth of nanodiamond grains. While both of the HiD and HiD films contain hybrid granular structure, the HiD films contain a larger proportion of nanographite phase and result in improved EFE properties, viz. ( E) = 7.7 V/μm and ( E) = 12.3 V/μm. In contrast, when the films were CH/N ppt-processed, the acicular diamond grains were formed for N and N films; however, carbon nanoclusters attached to the diamond grains of N films and the nanographitic layers encasing diamond cores are not crystallized very well, as compared with N films. Therefore, the N films exhibit slightly inferior EFE properties than the N films, viz. ( E) = 5.3 V/μm and ( E) = 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.
We report on the theoretical discovery of Iron monocarbide binary sheets stabilized at two-dimensional confined space, which we call tetragonal-FeC (t-FeC) and orthorhombic-FeC (o-FeC), respectively. From the energy viewpoint, the proposed t-FeC is the global minimum configuration in the 2D space, and each carbon atom is four-coordinated with ambient four Iron atoms. Strikingly, the o-FeC monolayer is an orthorhombic phase with planar pentacoordinate carbon moiety and planar seven-coordinate Fe moiety. To our knowledge, this monolayer is 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, t-FeC monolayer shows a higher theoretical capacity (395 mAh g -1 ), and can stably adsorb Li up to t-FeCLi 3 . Low migration energy barrier is predicted as small as 0.26 eV for Li, which result in the fast diffusion of Li atom on this monolayer. Moreover, electron-phonon calculations coupled with Bardeen-Cooper-Schrieffer arguments suggest t-FeC can be potential two-dimensional superconductors with 6.77 K superconducting transition temperature.
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