Kan Luo

Role of the surface effect on the structural, electronic and mechanical properties of the carbide MXenes

The two-dimensional material MXene has recently attracted interest for its excellent performance in diverse perspectives. Etched from the parental MAX phase with hydrofluoric acid, the synthesized MXene surface is normally functionalized by oxygen (-O), fluorine (-F) or hydroxyl (-OH) groups. Herein, using first-principles density functional calculations, we investigate the structural, mechanical and electronic properties of the carbide MXene M2CT2 (M=Sc, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, W; T=-O, -F, -OH). Both the M atom and the surface group T have a significant effect on the MXenes properties. Generally, oxygen functionalized MXenes present smaller lattice parameters and stronger mechanical strength compared to those functionalized by fluorine and hydroxyl groups. Sc2CO2 exhibits the smallest interlayer thickness and W2CO2 shows the strongest mechanical strength. In regard to electronic properties, five oxygen functionalized members M2CO2 (M=Sc, Ti, Zr, Hf, W), two fluorine functionalized members M2CF2 (M=Sc, Mo), and hydroxyl functionalized Sc2C(OH)(2) present semiconducting characteristics, but only Sc2C(OH)(2) exhibits a direct band gap. Copyright (C) EPLA, 2015

Electronic structures and mechanical properties of Al(111)/ZrB2(0001) heterojunctions from first-principles calculation

Employing first-principles density functional theory (DFT), the structures and electronic and mechanical properties of Al(111)/ZrB2(0001) heterojunctions are investigated. It is found that both B-terminated ZrB2(0001) and Zr-terminated ZrB2(0001) can form heterojunction interfaces with Al(111) surface. The heterojunction with B-terminated ZrB2(0001) is demonstrated to be most stable by comparing the surface adhesion energies of six different heterojunction models. In the stable configurations, the Al atom is found projecting to the hexagonal hollow site of neighbouring boron layer for the B-terminated ZrB2(001), and locating at the top site of the boron atoms for Zr-terminated ZrB2(001) interface. The mechanisms of interface interaction are investigated by density of states, charge density difference and band structure calculations. It is found that covalent bonds between surface Al atoms and B atoms are formed in the B-terminated heterojunction, whereas the Al atoms and Zr atoms are stabilised by interface metallic bonds for the Zr-terminated case. Mechanical properties of Al/ZrB2 heterojunctions are also predicted in the current work. The values of moduli of Al/ZrB2 heterojunctions are determined to be between those of single crystal Al and ZrB2, which exhibit the transition of mechanical strength between two bulk phases. DFT calculations with the current models provide the mechanical properties for each heterojunction and the corresponding contributions by each type of interface in the composite materials. This work paves the way for industrial applications of Al(111)/ZrB2(0001) heterojunctions.

New insight into the helium-induced damage in MAX phase Ti3AlC2 by first-principles studies

In the present work, the behavior of He in the MAX phase Ti3AlC2 material is investigated using first-principle methods. It is found that, according to the predicted formation energies, a single He atom favors residing near the Al plane in Ti3AlC2. The results also show that Al vacancies are better able to trap He atoms than either Ti or C vacancies. The formation energies for the secondary vacancy defects near an Al vacancy or a C vacancy are strongly influenced by He impurity content. According to the present results, the existence of trapped He atoms in primary Al vacancy can promote secondary vacancy formation and the He bubble trapped by Al vacancies has a higher tendency to grow in the Al plane of Ti3AlC2. The diffusion of He in Ti3AlC2 is also investigated. The energy barriers are approximately 2.980 eV and 0.294 eV along the c-axis and in the ab plane, respectively, which means that He atoms exhibit faster migration parallel to the Al plane. Hence, the formation of platelet-like bubbles nucleated from the Al vacancies is favored both energetically and kinetically. Our calculations also show that the conventional spherical bubbles may be originated from He atoms trapped by C vacancies. Taken together, these results are able to explain the observed formation of bubbles in various shapes in recent experiments. This study is expected to provide new insight into the behaviors of MAX phases under irradiation from electronic structure level in order to improve the design of MAX phase based materials.

Controllable magnitude and anisotropy of the electrical conductivity of Hf3C2O2 MXene

Hf3C2O2, a new MXene member synthesized recently, was predicted to be a semi-metal with high mechanical strength. Based on the unique electronic structure, the energy bands and electrical conductivities of the MXene under various strains are comprehensively investigated in this paper. Biaxial and two orthogonal uniaxial strains in both compressive and tensile manners are studied. Results from this study suggest that Hf3C2O2 shows a transition between semi-metal and semi-conductor under both biaxial and uniaxial strains. A compressive strain generally induces a larger energy overlap between the conduction band minimum and the valance band maximum, while a tensile strain reduces the energy band overlap and even opens a band gap. As a consequence, the magnitude of electrical conductivity decreases drastically from compressive to tensile strains applied. Moreover, the uniaxial strains are determined to be efficient in manipulating the anisotropy of the electrical conductivity. These data imply that the Hf3C2O2 MXene is a promising candidate material for devices such as strain sensors.

Ab Initio Studies on the Clathrate Hydrates of Some Nitrogen- and Sulfur-Containing Gases

Ab initio calculations are performed to investigate the host-guest interactions and multiple occupancies of some sulfur- (H2S, CS2) and nitrogen-containing (N2, NO, and NH3) molecules in dodecahedral, tetrakaidecahedral, and hexakaidecahedral water cages in this work. Five functionals in the framework of density functional theory are compared, and the M06-2X method appears to be the best to predict the binding energies as well as the geometries. Results show that N2 and NO molecules are more stable in the 51264 cage, while NH3 and H2S prefer to stabilize in the 51262 cage. This suggests that the sI hydrates of NH3 and H2S exhibit higher stability than the sII structures and that sII NO hydrate is more stable than sI NO hydrate. N2 is found to be more stable in type II structure with single occupancy and to form type I hydrate with multiple occupancy, which is consistent with the experimental observations. As to the guest molecule CS2, it may undergo severe structural deformation in the 512 and 51262 cage. For multiple occupancies, the 512, 51262, and 51264 water cages can trap up to two N2 molecules, and the 51264 water cage can accommodate two H2S molecules. This work is expected to provide new insight into the formation mechanism of clathrate hydrates for atmospherically important molecules.

Hybridization of inorganic CoB noncrystal with graphene and its Kubas-enhanced hydrogen adsorption at room temperature

Hybridization of inorganic compounds with graphene-based materials can give rise to various enhanced properties, in which constructing a chemical bond stabilized hybrid structure is of primary importance. In this work, an archetypical hybrid material has been prepared by the reaction of an inorganic CoB noncrystal with graphene at room temperature with a high-energy ball-milling process. Experimental characterization results prove that the inorganic CoB noncrystal is stably pinned to graphene through B–C chemical bonds. The hybrid material shows a high electrochemical hydrogen storage capacity at room temperature. Based on the detailed experimental measurements and theory calculations, the enhanced electrochemical hydrogen storage ability is identified as a Kubas-enhanced hydrogen adsorption mechanism induced by the Co–B–C structure.

First-principles study on the electrical and thermal properties of the semiconducting Sc3(CN)F2 MXene

The two-dimensional materials MXenes have recently attracted interest for their excellent performance from diverse perspectives indicated by experiments and theoretical calculations. For the application of MXenes in electronic devices, the exploration of semiconducting MXenes arouses particular interest. In this work, despite the metallic properties of Sc3C2F2 and Sc3N2F2, we find that Sc3(CN)F2 is a semiconductor with an indirect band gap of 1.18 eV, which is an expansion of the semiconducting family members of MXene. Using first-principles calculations, the electrical and thermal properties of the semiconducting Sc3(CN)F2 MXene are studied. The electron mobilities are determined to possess strong anisotropy, while the hole mobilities show isotropy, i.e. 1.348 × 103 cm2 V−1 s−1 along x, 0.319 × 103 cm2 V−1 s−1 along the y directions for electron mobilities, and 0.517 × 103 cm2 V−1 s−1 along x, 0.540 × 103 cm2 V−1 s−1 along the y directions for hole mobilities. The room-temperature thermal conductivity along the Γ → M direction is determined to be 123–283 W m−1 K−1 with a flake length of 1–100 μm. Besides, Sc3(CN)F2 presents a relatively high specific heat of 547 J kg−1 K−1 and a low thermal expansion coefficient of 8.703 × 10−6 K−1. Our findings suggest that the Sc3(CN)F2 MXene might be a candidate material in the design and application of 2D nanoelectronic devices.