RESUMO
Phase stability and the phase transition of Janus transition metal chalcogenides (TMDs) have become interesting issues that have not been fully resolved since their successful synthesis. By fitting the results from first principles calculations, a tight-binding dynamics matrix of the 1T' phase is constructed and the eigenvectors are also obtained. We propose a method to project the atomic motion causing the phase transition from 2H to 1T' onto these eigenvectors, and identify four key phonon modes which are the major factors to trigger phase transition. Temperature excitation is used to excite the key modes and the free energy criterion is used to determine the phase stability. The relatively large enthalpy difference between the 2H and 1T' phases favours the 2H one as the stable phase at low temperature. While the 1T' phase has a quick increase in vibrational free energy with rising temperature, especially for 1T' Janus TMDs which have a quicker increase in the total free energy than that of 1T' non-Janus TMDs, making them show a lower phase transition temperature. Our work will deepen our understanding of the phase transition behavior of 2D Janus TMDs, and the tight-binding dynamics matrix and the method to obtain the key modes will be a useful tool for further study of the phase transitions of 2D Janus TMDs and other related materials.
RESUMO
Searching for two-dimensional (2D) ferromagnetic materials is one of the key steps in 2D spintronics. 2D metal carbide/nitride materials (MXene) are widely regarded as promising candidates for this kind of material. However, when the surfaces are saturated with some functional groups during the preparation, the ground states of most of the MXenes transit from ferromagnetic (FM) to antiferromagnetic (AFM) or non-magnetic (NM) states. In this article, we propose a new method to avoid this problem by adopting asymmetric decoration of the MXene surface, which can make MXenes ferromagnetic ground states. Based on hybrid density functional theory calculations, our results show asymmetrical adsorption of negative ions or metal atoms makes the Ti atoms have different valence states, such as one sublayer Ti4+ and another Ti+, which prefer FM ground states. This research will deepen our understanding of the magnetic properties of 2D materials and contribute to the design of new 2D ferromagnetic materials.
RESUMO
Optical thermometry based on up-conversion (UC) fluorescent intensity ratio (FIR) with 808 nm excitation is preferable in water-rich environments, and investigation of the ambiguous intrinsic influencing factors on host-dependent sensitivity is a prerequisite for the development of highly sensitive thermometry. Herein, MIn2O4:Nd3+/Yb3+/Er3+ (M = Ca, Sr, and Ba) microcrystals with low phonon energy are synthesized via a sol-gel method. Intense UC luminescence with tunable emission color from green to red is obtained by controlling the Yb3+ content, and the UC mechanisms and successive energy transfer of Nd3+ â Yb3+ â Er3+ are elaborated using lifetime measurements. The thermal sensing properties of the samples based on the thermally coupled levels (4S3/2/2H11/2) of Er3+ are assessed, and their sensitivities increase gradually with an increase in temperature and reach the maximum of about 0.0048, 0.0033 and 0.0058 K-1 at 490 K for M = Ca, Sr, and Ba, respectively. By analysing the host structure, site symmetry of M2+ ions and characteristics of the M-O bonds, it is proposed that the higher Ca-O bond covalency in CaIn2O4 leads to better sensitivity than SrIn2O4 with the same structure, and the optimal sensitivity in BaIn2O4 is mainly attributed to the specific local crystal field of the Ba2+ site with higher ligancy and longer chemical bonds. These results provide insight for the selection of appropriate matrix materials to achieve higher temperature detection sensitivity.
