Yuhong Zhou

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.

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.