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Register a new userFirst principles study of stability of MXenes under electron beam
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Interactions of two-dimensional MXene sheets and electron beam of (scanning) transmission electron microscope are studied via first-principles calculations. We simulated the knock-on displacement threshold for Ti$_3$C$_2$ MXene sheet via ab initio molecular dynamics simulations and for five other MXenes (Ti$_2$C, Ti$_2$N, Nb$_2$C, Mo$_2$TiC$_2$, and Ti$_3$CN) approximately from defect formation energies. We evaluated sputtering cross section and sputtering rates, and based on those the evolution of the surface composition. We find that at the exit surface and for low TEM energies H and F sputter at equal rates, but at high TEM energies the F is sputtered most strongly. In the enter surface, H sputtering dominates. The results were found to be largely similar for all studied MXenes, and although the displacement thresholds varied between the different metal atoms the thresholds were always too high to lead to significant sputtering of the metal atoms. We simulated electron microscope images at the successive stages of sputtering, and found that while it is likely difficult to identify surface groups based on the spot intensities, the local contraction of lattice around O groups should be observable. We also studied MXenes encapsulated with graphene and found them to provide efficient protection from the knock-on damage for all surface group atoms except H.
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A theoretical study proposes the atomic configuration of electron-beam irradiated C$_{60}$ thin films. We examined the electronic structure and electron-transport properties of the C$_{60}$ clusters using density-functional calculations and found that a rhombohedral C$_{60}$ polymer with $sp^3$-bonded dumbbell-shaped connections at the molecule junction is a semiconductor with a narrow band gap while the polymer changes to exhibit metallic behavior by forming $sp^2$-bonded peanut-shaped connections. Conductance below the Fermi level increases and the peak of the conductance spectrum arising from the $t_{u1}$ states of a C$_{60}$ molecule becomes obscure after the connections are rearranged. The present rohmbohedral film, including the [2+2] four-membered rings and peanut-shaped connections, is a candidate to represent the structure of the metallic C$_{60}$ polymer at the initial stage of electron-beam irradiation.
NMR is the technique of election to probe the local properties of materials. Herein we present the results of density functional theory (DFT) textit{ab initio} calculations of the NMR parameters for fluorapatite (FAp), a calcium orthophosphate mineral belonging to the apatite family, by using the GIPAW method [Pickard and Mauri, 2001]. Understanding the local effects of pressure on apatites is particularly relevant because of their important role in many solid state and biomedical applications. Apatites are open structures, which can undergo complex anisotropic deformations, and the response of NMR can elucidate the microscopic changes induced by an applied pressure. The computed NMR parameters proved to be in good agreement with the available experimental data. The structural evaluation of the material behavior under hydrostatic pressure (from --5 to +100 kbar) indicated a shrinkage of the diameter of the apatitic channel, and a strong correlation between NMR shielding and pressure, proving the sensitivity of this technique to even small changes in the chemical environment around the nuclei. This theoretical approach allows the exploration of all the different nuclei composing the material, thus providing a very useful guidance in the interpretation of experimental results, particularly valuable for the more challenging nuclei such as $^{43}$Ca and $^{17}$O.
This article reports the study of SnO by using the first-principles pseudopotential plane-wave method within the generalized gradient approximation (GGA). We have calculated the structural, elastic, electronic and optical of SnO under high pressure. The elastic properties such as the elastic constants Cij bulk modulus, shear modulus, Young modulus, anisotropic factor, Pugh ratio, Poisson ratio are calculated and analyzed. Mechanical stability of SnO at all pressure are confirmed by using Born stability criteria in terms of elastic constants and are associated with ductile behaviour based on G/B ratios. It is also found that SnO exhibits very high anisotropy. The energy band structure and density of states are also calculated and analyzed. The results show the semiconducting and metallic properties at 0 (zero) and high pressure, respectively. Furthermore, the optical properties such as dielectric function, refractive index, photoconductivity, absorption coefficients, loss function and reflectivity are also calculated. All the results are compared with those of the SnO where available but most of the results at high pressure are not compared due to unavailability of the results.
The transition metal carbides (namely MXenes) and their functionalized derivatives exhibit various physical and chemical characteristics and offer many potential applications in electronic devices and sensors. Using density functional theory (DFT), it is revealed that the nearly free electron (NFE) states are near the Fermi levels in hydroxyl (OH) functionalized MXenes. Most of the OH-terminated MXene are metallic, but some of them, e.g. Sc2C(OH)2, are semiconductors and the NFE states are conduction bands. In this paper, to investigate the NFE states in MXenes, an attractive image-potential well model is adopted. Compared the solutions of this model with the DFT calculations, it is found that due to the overlap of spatially extensive wave functions of NFE states and their hybridization between the artificial neighboring layers imposed by the periodical boundary conditions (PBCs), the DFT results represent the properties of multiple layers, intrinsically. Based on the DFT calculations, it is found that the energy gap widths are affected by the interlayer distances. We address that the energetics of the NFE states can be modulated by the external electric fields and it is possible to convert semiconducting MXenes into metals. This band-gap manipulation makes the OH-terminated semiconducting MXenes an excellent candidate for electronic switch applications. Finally, using a set of electron transport calculations, I-V characteristics of Sc2C(OH)2 devices are investigated with the gate voltages.
We perform a systematic first-principles study of phosphorene in the presence of typical monovalent (hydrogen, fluorine) and divalent (oxygen) impurities. The results of our modeling suggest a decomposition of phosphorene into weakly bonded one-dimensional (1D) chains upon single- and double-side hydrogenation and fluorination. In spite of a sizable quasiparticle band gap (2.29 eV), fully hydrogenated phosphorene found to be dynamically unstable. In contrast, full fluorination of phosphorene gives rise to a stable structure, being an indirect gap semiconductor with the band gap of 2.27 eV. We also show that fluorination of phosphorene from the gas phase is significantly more likely than hydrogenation due to the relatively low energy barrier for the dissociative adsorption of F2 (0.19 eV) compared to H2 (2.54 eV). At low concentrations, monovalent impurities tend to form regular atomic rows phosphorene, though such patterns do not seem to be easily achievable due to high migration barriers (1.09 and 2.81 eV for H2 and F2, respectively). Oxidation of phosphorene is shown to be a qualitatively different process. Particularly, we observe instability of phosphorene upon oxidation, leading to the formation of disordered amorphous-like structures at high concentrations of impurities.
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