Structural phase transition induced enhancement of carrier mobility of monolayer RuSe<sub>2</sub>
Authors: Lu Kang-Jun , Wang Yi-Fan , Xia Qian , Zhang Gui-Tao , Chen Qian ,   Key Laboratory of Quantum Materials and Devices of Ministry of Education, School of Physics, Southeast University, Nanjing 211189, China
Published: June 2024

Transition metal dichalcogenides (TMDs) is an important member of two-dimensional material family, which has various crystal structures and physical properties, thus providing a broad platform for scientific research and device applications. The diversity of TMD's properties arises not only from their relatively large family but also from the variety of their crystal structure phases. The most common structure of TMD is the trigonal prismatic phase (<i>H</i> phase) and the octahedral phase (<i>T</i> phase). Studies have shown that, in addition to these two high-symmetry phases, TMD has other distorted phases. Distorted phase often exhibits different physical properties from symmetric phases and can perform better in certain systems. Because the structural differences between different distorted phases are sometimes very small, it is experimentally challenging to observe multiple distorted phases coexisting. Therefore, it is meaningful to theoretically investigate the structural stability and physical properties of different distorted phases. In this study, we investigate the structure and phase transition of monolayer RuSe<sub>2</sub> through first-principles calculation. While confirming that its ground state is a the dimerized phase (<inline-formula><tex-math id="M7">\begin{document}$T^\prime$\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20240557_M7.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20240557_M7.png"/></alternatives></inline-formula> phase), we find the presence of another energetically competitive trimerized phase (<inline-formula><tex-math id="M8">\begin{document}$T^{\prime\prime\prime}$\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20240557_M8.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20240557_M8.png"/></alternatives></inline-formula> phase). By comparing the energy values of four different structures and combining the results of phonon spectra and molecular dynamics simulations, we predict the stability of the <inline-formula><tex-math id="M9">\begin{document}$T^{\prime\prime\prime}$\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20240557_M9.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20240557_M9.png"/></alternatives></inline-formula> phase at room temperature. Because the <i>H</i> phase and <i>T</i> phase of two-dimensional RuSe<sub>2</sub> have already been observed experimentally, and considering the fact that <inline-formula><tex-math id="M10">\begin{document}$T^{\prime\prime\prime}$\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20240557_M10.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20240557_M10.png"/></alternatives></inline-formula> phase has much lower energy than the <i>H</i> and <i>T</i> phases, it is highly likely that the <inline-formula><tex-math id="M11">\begin{document}$T^{\prime\prime\prime}$\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20240557_M11.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20240557_M11.png"/></alternatives></inline-formula> phase exists in experiment. Combining the calculations of the phase transition barrier and the molecular dynamics simulations, we anticipate that applying a slight stress to the <inline-formula><tex-math id="M12">\begin{document}$T^\prime$\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20240557_M12.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20240557_M12.png"/></alternatives></inline-formula> phase structure at room temperature can induce a lattice transition from <inline-formula><tex-math id="M13">\begin{document}$T^\prime$\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20240557_M13.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20240557_M13.png"/></alternatives></inline-formula>phase to <inline-formula><tex-math id="M14">\begin{document}$T^{\prime\prime\prime}$\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20240557_M14.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20240557_M14.png"/></alternatives></inline-formula> phase, resulting in significant changes in the band structure and carrier mobility, with the bandgap changing from an indirect bandgap of 1.11 eV to a direct bandgap of 0.71 eV, and the carrier mobility in the armchair direction increasing from <inline-formula><tex-math id="M15">\begin{document}$ 0.82 \times $\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20240557_M15.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20240557_M15.png"/></alternatives></inline-formula><inline-formula><tex-math id="M15-1">\begin{document}$ 10^3 \, {\rm cm}^{2}{\cdot}{\rm V}^{-1}{\cdot}{\rm s}^{-1}$\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20240557_M15-1.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20240557_M15-1.png"/></alternatives></inline-formula> to <inline-formula><tex-math id="M16">\begin{document}$3.22 \times 10^3 \, {\rm cm}^{2}{\cdot}{\rm V}^{-1}{\cdot}{\rm s}^{-1}$\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20240557_M16.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20240557_M16.png"/></alternatives></inline-formula>, an approximately threefold enhancement. In this work, two possible coexisting distorted phases in monolayer RuSe<sub>2</sub> are compared with each other and studied, and their electronic structures and carrier mobilities are analyzed, thereby facilitating experimental research on two-dimensional RuSe<sub>2</sub> materials and their applications in future electronic devices.