[en] Molecular dynamics simulations and free energy calculations are employed to investigate the evolution, formation probability, detailed balance, and isomerization rate of small C cluster isomer at 2500 K. For C_1_0, the isomer formation probability predicted by free energy is in good agreement with molecular dynamics simulation. However, for C_2_0, C_3_0, and C_3_6, the formation probabilities predicted by free energy are not in agreement with molecular dynamics simulations. Although the cluster systems are in equilibrium, detailed balance is not reached. Such results may be attributed to high transformation barriers between cage, bowl, and sheet isomers. In summary, for mesoscopic nanosystems the free energy criterion, which commonly holds for macroscopic systems in dynamic equilibrium, may not provide a good prediction for isomer formation probability. New theoretical criterion should be further investigated for predicting the isomer formation probability of a mesoscopic nanosystem. (paper)

[en] Metal-air batteries have an outstanding advantage of combining high-energy-density metal anodes with active air cathodes. The advanced properties of layered materials can be utilized to improve the internal reaction rate and charge storage of metal-air batteries. Layered MoSi${}_2$N${}_4$ is a newly synthesized MAX phase (where M is transition metal, A is Al or Si, and X is C, N, or both). This work investigates the possibility of MoSi${}_2$N${}_4$, a layered MAX phase, as both the cathode and anode of Zn-air batteries. The mechanism of Zn storage is revealed. As a Zn storage material, phase transition from state I to state III occurs with increasing Zn loading in MoSi${}_2$N${}_4$. The maximum theoretical capacity of Zn-loaded MoSi${}_2$N${}_4$ is 257 mAh g${}^{-<!--\; ???\; -->1}$. On MoSi${}_2$N${}_4$ surface as the cathode, the two-electron mechanism of O${}_2$ reduction to ZnO${}_2$ is more efficient than general sluggish four-electron aqueous O${}_2$ redox reactions. The work reveals the possibility of MAX phases as the electrodes of Zn-air batteries and the mechanism of Zn storage in 2D MAX layered materials. (© 2022 Wiley‐VCH GmbH)

[en] Highlights: • The existence of 2D ionic crystals is explored. • Our model predicts the existence of 2D ferromagnetic semiconductors. • The mechanism of magnetic anisotropy in maintaining the magnetic order is revealed. Modern advanced technologies have been employed to synthesize new two-dimensional (2D) materials of various compositions. Since ionic bonds have no orientation, the ability of 2D ionic crystals to resist thermal fluctuation perpendicular to the 2D surface is worth studying. In this paper, we propose a theoretical analysis on the existence of 2D ionic EuS crystals. Although the Mermin-Wagner theorem denies the possibility of long-range magnetic order in 2D systems, our theoretical model predicts that 2D EuS crystals are ferromagnetic semiconductors. The mechanism of magnetic anisotropy in maintaining the magnetic order is uncovered. The model indicates the effect of the low-energy gap in the spin-wave spectrum on preventing thermal disturbance from destroying the ferromagnetism. A new type of magnetic interaction in 2D EuS crystals is revealed, which leads to the enhancement of Curie temperature by applying a gate voltage. Our theoretical research can benefit future research of other magnetic 2D crystals.

[en] In recent years, two-dimensional confined catalysis, i.e., the enhanced catalytic reactions in confined space between metal surface and two-dimensional overlayer, makes a hit and opens up a new way to enhance the performance of catalysts. In this work, graphdiyne overlayer was proposed as a more excellent material than graphene or hexagonal boron nitride for two-dimensional confined catalysis on Pt(111) surface. Density functional theory calculations revealed the superiority of graphdiyne overlayer originates from the steric hindrance effect which increases the catalytic ability and lowers the reaction barriers. Moreover, with the big triangle holes as natural gas tunnels, graphdiyne possesses higher efficiency for the transit of gaseous reactants and products than graphene or hexagonal boron nitride. The results in this work would benefit future development of two-dimensional confined catalysis.

[en] Recently, two-dimensional materials and nanoparticles with robust ferromagnetism are even of great interest to explore basic physics in nanoscale spintronics. More importantly, room-temperature magnetic semiconducting materials with high Curie temperature is essential for developing next-generation spintronic and quantum computing devices. Here, we develop a theoretical model on the basis of density functional theory calculations and the Ruderman-Kittel-Kasuya-Yoshida theory to predict the thermal stability of two-dimensional magnetic materials. Compared with other rare-earth (dysprosium (Dy) and erbium (Er)) and 3d (copper (Cu)) impurities, holmium-doped (Ho-doped) single-layer 1H-MoS₂ is proposed as promising semiconductor with robust magnetism. The calculations at the level of hybrid HSE06 functional predict a Curie temperature much higher than room temperature. Ho-doped MoS₂ sheet possesses fully spin-polarized valence and conduction bands, which is a prerequisite for flexible spintronic applications.

[en] Magnetic order in two-dimensional systems was not supposed to exist at finite temperature. In recent years, the successful preparation of two-dimensional ferromagnetic materials such as CrI₃, Cr₂Ge₂Te₆, and Fe₃GeTe₂ opens up a new chapter in the remarkable field of two-dimensional materials. Here, we report on a theoretical analysis of the stability of ferromagnetism in Fe₃GeTe₂. We uncover the mechanism of holding long-range magnetic order and propose a model to estimate the Curie temperature of Fe₃GeTe₂. Our results reveal the essential role of magnetic anisotropy in maintaining the magnetic order of two-dimensional systems. The theoretical method used here can be generalized to future research of other magnetic two-dimensional systems. (rapid communication)

[en] The radiation-induced fragmentation of the C₆₀ fullerene was investigated by the tight-binding electron–ion dynamics simulations. In intense laser field, the breathing vibrational mode is much more strongly excited than the pentagonal-pinch mode. The fragmentation effect was found more remarkable at long wavelength λ⩾800 nm rather than the resonant wavelengths due to the internal laser-induced dipole force, and the production ratio of C and C₂ rapidly grows with increasing wavelength. By such fragmentation law, C atoms, C₂ dimers or large C_n fragments could be selectively obtained by changing the laser wavelength. And the fragmentation of C₆₀ by two laser pulses like the multi-step atomic photoionization was investigated. -- Highlights: ► The radiation-induced fragmentation of the C₆₀ fullerene was investigated by the tight-binding electron–ion dynamics simulations. ► The production ratio of C and C₂ is remarkable at wavelengths above 800 nm. ► C, C₂ or large C_n fragments could be selectively obtained by changing the laser wavelength

[en] Van der Waals (vdW) heterojunctions with type-II band alignment, in which electrons and holes are localized in distinct layers, play a central role in optoelectronic devices and solar cells. The present study analyzes a type-I→II band alignment transition in InSe-MoS₂ vdW heterostructure, proposed to be controlled via changing interlayer distance or applying perpendicular external electric field. The band position shift of InSe relative to that of MoS₂ attributes to a surface polarization mechanism. Changing band offset into type II facilitates possible use and allows greater flexibility for band engineering of InSe-MoS₂ heterostructure in optoelectronic and solar energy applications. The present findings provide theoretical guidance to a new approach to improve the optoelectronic properties of vdW heterostructures. (copyright 2018 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)

[en] A simple model based on the statistics of single atoms is developed to predict the diffusion rate of thermal atoms in (or on) bulk materials without empirical parameters. Compared with vast classical molecular-dynamics simulations for predicting the self-diffusion rate of Pt, Cu, and Ar adatoms on crystal surfaces, the model is proved to be much more accurate than the Arrhenius law and the transition state theory. Applying this model, the theoretical predictions agree well with the experimental values in the presented paper about the self-diffusion of Pt (Cu) adatoms on the surfaces

[en] Highlights: • Fe and Ru adatoms on germanene were proposed as catalysts for removing the CO contamination in H₂. • Under the catalysis of Fe and Ru adatoms, CO in H₂ could be transformed into HCHO. • Germanene substrate could prevent the aggregation of Fe and Ru adatoms. Single-layer germanene was proposed as substrate for single-atom transition metal catalysts with much larger adsorption energies and higher thermal migration barriers than graphene. By density functional theory calculations, the electronic properties, thermal stabilities and catalytic abilities of Au, Fe, Ni, Pd, Pt and Ru adatoms on single-layer germanene were theoretically investigated. The results indicate that the CO contamination in H₂ could be transformed into HCHO under the catalysis of Fe and Ru adatoms. For Fe and Ru adatoms, the three-step catalytic reactions were found both thermodynamically and kinetically favorable, with stable reaction intermediates and low potential barriers. Furthermore, Fe and Ru adatoms could be stably adsorbed on germanene with large adsorption energies and high thermal migration barriers, preventing the aggregation of Fe and Ru on germanene. However, for Au, Ni, Pd and Pt adatoms, the reaction intermediates are thermodynamically unfavorable so that the catalytic reactions should be difficult to proceed. By contrast, Fe and Ru adatoms on single-layer germanene should be excellent catalysts for removing the CO contamination from H₂ feed gas of fuel cells.