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过渡金属二硫族化合物(TMDs)是二维材料家族中的重要成员, 具有丰富多样的晶体结构和物理特性, 是近年来在科学研究和器件应用领域关注度较高的材料之一. 本文通过第一性原理计算研究了单层RuSe2的结构和相变, 在确定其基态为二聚相(
$T^\prime$ 相)的同时, 发现该材料存在能量相近的三聚相($T^{\prime\prime\prime}$ 相). 我们分别从动力学和热力学角度预测了该晶相结构的稳定性. 结合相变势垒的计算和分子动力学模拟, 我们预测在室温下对$T^\prime$ 相结构施加较小的应力就可以实现晶格从$T^\prime$ 相到$T^{\prime\prime\prime}$ 相的转变. 相变后的能带结构以及载流子迁移率都发生了明显的改变, 其带隙由1.11 eV的间接带隙转变为0.71 eV的直接带隙, 载流子迁移率有了大幅的提升, 空穴迁移率达到了$3.22 \times 10^3 \, {\rm cm}^{2}\cdot{\rm V}^{-1}\cdot{\rm s}^{-1}$ . 我们的工作对比研究了RuSe2单层中可能共存的两种畸变相, 分析了不同晶相的电子结构和迁移率, 为实验上研究二维RuSe2材料及其在未来器件中的应用提供了理论依据.Transition metal dichalcogenides (TMDs) represent an important family of two-dimensional materials with diverse crystal structures and physical properties, offering a broad platform for scientific research and device applications. The diversity of TMDs' properties arises not only from their relatively large family but also from the variety of their crystal structure phases. The most common structures of TMDs are the trigonal prismatic phase (H phase) and the octahedral phase (T phase). Studies have shown that, in addition to these two high-symmetry phases, TMDs also have other distorted phases. Distorted phases often exhibit different physical properties from symmetric phases and can perform better in certain systems. Given the structural differences between different distorted phases can sometimes be very small, it is experimentally challenging to observe the coexistence of multiple distorted phases. Therefore, it is meaningful to theoretically explore the structural stability and physical properties of different distorted phases. In this study, we investigate the structure and phase transition of monolayer RuSe2 through first-principles calculation. While confirming its ground state as the dimerized phase ($T^\prime$ phase), we identify the existence of another energetically competitive trimerized phase ($T^{\prime\prime\prime}$ phase). By comparing the energies of four different structures and combining the results of phonon spectra and molecular dynamics simulations, we predicted the stability of the$T^{\prime\prime\prime}$ phase at room temperature. Given that the H and T phases of two-dimensional RuSe2 have already been observed experimentally, and considering that the energy of the$T^{\prime\prime\prime}$ phase is much lower than that of the H and T phases, it is highly likely that the$T^{\prime\prime\prime}$ phase exists in experiment. Combined with calculations of the phase transition barrier and molecular dynamics simulations, we anticipate that applying slight stress to the$T^\prime$ phase structure at room temperature can induce a lattice transition from$T^\prime$ to$T^{\prime\prime\prime}$ phase. This phase transition results 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$ 0.82 \times 10^3 \, {\rm cm}^{2}\cdot{\rm V}^{-1}\cdot{\rm s}^{-1}$ to$3.22 \times 10^3 \, {\rm cm}^{2}\cdot{\rm V}^{-1}\cdot{\rm s}^{-1}$ , an approximately threefold enhancement. Our work contrasts and investigates two possible coexisting distorted phases in monolayer RuSe2, analyzing their electronic structures and carrier mobilities, thereby facilitating experimental investigations of two-dimensional RuSe2 materials and their applications in future electronic devices.
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Keywords:
- RuSe2 /
- Phase transition /
- Carrier mobility /
- First-principles calculation
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图 1 (a) 单层RuSe2四种结构的相对能量, 以图中矩形超胞的长度作为横坐标进行排列. 绿球代表Ru原子, 红球代表Se原子; (b) RuSe2的$ T^\prime $相晶体结构; (c) RuSe2的$ T^{\prime\prime\prime} $相晶体结构.
Fig. 1. (a) The relative energies of four structures of monolayer RuSe2, arranged along the lengths of rectangular supercells. Green spheres represent Ru atoms, while red spheres represent Se atoms; (b) Crystal structure of RuSe2 in the $ T^\prime $ phase; (c) Crystal structure of RuSe2 in the $ T^{\prime\prime\prime} $ phase.
图 2 (a) AIMD模拟300 K时$ T^{\prime\prime\prime} $相单层RuSe2总能量随时间的变化, 插图为始末状态的晶体结构图; (b) $ T^{\prime\prime\prime} $相单层RuSe2的声子谱.
Fig. 2. (a) Variation of the total energy of monolayer RuSe2 in the $ T^{\prime\prime\prime} $ phase during AIMD simulation at 300 K. The crystal structures of the initial and final states shown in the inset; (b) Phonon spectrum of monolayer RuSe2 in the $ T^{\prime\prime\prime} $ phase.
图 3 (a) 单层RuSe2在$ T^\prime $相的能带结构; (b) 单层RuSe2 在$ T^{\prime\prime\prime} $相的能带结构; (c) $ T^\prime $相中的Ru-Se八面体结构; (d) $ T^{\prime\prime\prime} $相中的Ru-Se八面体结构; (e) Ru-Se 八面体的Ru-d轨道在不同晶相结构中的分裂情况示意图.
Fig. 3. (a) Band structure of monolayer RuSe2 in the $ T^\prime $ phase; (b) Band structure of monolayer RuSe2 in the $ T^{\prime\prime\prime} $ phase; (c) Octahedral structure of Ru-Se in the $ T^\prime $ phase; (d) Octahedral structure of Ru-Se in the $ T^{\prime\prime\prime} $ phase; (e) Schematic illustration of the splitting of Ru-d orbitals in the Ru-Se octahedron in different crystal phases.
图 4 (a) 单层RuSe2在$ T^\prime $与$ T^{\prime\prime\prime} $相下能量随zigzag方向晶格长度变化的关系曲线, 插图为部分位置处发生相变的NEB势垒图; (b) 300 K, 下$ T^\prime $ 相在Zigzag方向施加6.5%压缩应变时的分子动力学模拟, 其中的插图分别是0 ps 和15 ps的结构图
Fig. 4. (a) The energy of monolayer RuSe2 in the $ T^\prime $ and $ T^{\prime\prime\prime} $ phases variation with the lattice along the zigzag direction. The insets are the NEB barriers showing phase transitions at certain positions; (b) AIMD of the $ T^\prime $ phase under 6.5% compressive strain along the zigzag direction at 300 K, with insets showing crystal structures at 0 ps and 15 ps.
表 1 计算得到的单层RuSe2在armchair和zigzag方向上的载流子有效质量$ m^*\left(\text{m}_{0}\right) $, $ m_0 $为单个电子的质量; 弹性模量$C\;\left( {\rm N}\cdot{\rm m}^{-1}\right) $; 形变势$ E_1 \;\text{eV} $和迁移率$ \mathit{µ}\;\left({\rm cm}^{2}\cdot{\rm V}^{-1}\cdot{\rm s}^{-1}\right) $.
Table 1. Calculated effective mass of carriers $ m^*\left(\text{m}_{0}\right) $, here the $ {m}_{0} $ represents the mass of a single electron; elastic modulus $ C\;\left({\rm N}\cdot{\rm m}^{-1}\right) $; deformation potential $ E_1 \; \left(\text{eV}\right)$; and mobility $\mathit{µ}\; \left({\rm cm}^{2}\cdot{\rm V}^{-1}\cdot{\rm s}^{-1}\right)$ of monolayer RuSe2 in the armchair and zigzag directions.
Phase/Carrier $ m^*_{arm} $ $ m^*_{zig} $ $ C_{11} $ $ C_{22} $ $ E_{1 arm} $ $ E_{1 zig} $ $ \mu_{\rm arm} $ $ \mu_{\rm zig} $ $ {m}_{0} $ ($ {\rm N}\cdot{\rm m}^{-1} $) (eV) $ \left({\rm cm}^{2}\cdot{\rm V}^{-1}\cdot{\rm s}^{-1}\right) $ $ T^\prime \text{/e}$ 37.59 9.64 95.73 92.01 0.74 –2.87 2.44 3.53 $ T^\prime \text{/h}$ 3.21 1.22 1.46 –0.71 224.42 823.25 $ T^{\prime\prime\prime}\text{/e} $ 3.48 1.16 97.80 99.31 –1.46 –2.98 70.53 150.57 $ T^{\prime\prime\prime} \text{/h}$ 1.10 0.60 1.78 0.50 1182.65 3219.20 
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