Revisiting Intercalation‐Induced Phase Transitions in 2D Group VI Transition Metal Dichalcogenides
Revisiting Intercalation‐Induced Phase Transitions in 2D Group VI Transition Metal Dichalcogenides
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DOI：
10.1002/aesr.202100027
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发表时间：
2021-05
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影响因子：
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通讯作者：
Mengjing Wang;Shiyu Xu;J. Cha
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作者：
Mengjing Wang;Shiyu Xu;J. Cha
二维材料表现出许多奇异的结构和电子相。特别是，二维层状过渡金属二硫化物（TMDC） MX2，其每层都包含一个过渡金属层(M)，与顶部和底部的硫层（X1 / 4 S, Se, Te）共价结合，具有多种具有不同物理和化学性质的结构多晶，如图1A所示。例如，三角形棱柱2H相具有半导体性质，具有与厚度相关的带隙，适合作为场效应晶体管中的导电通道；八面体1T相为半金属相，具有优异的电化学催化性能；扭曲的八面体1t0 /Td相受到拓扑保护，表现出相关的电子行为，如量子自旋霍尔效应、自旋分辨费米弧，甚至潜在的拓扑超导性。所有的多晶（2H, 1T， 1t0和Td）都可以在VI组TMDCs （M1 / 4Mo， W）中被接触和改变，过渡金属保持d电子构型，如图1B所示。值得注意的是，相的热力学稳定性高度依赖于硫元素。例如，在环境条件下，2H相在WS2中是热力学稳定的，而Td相在WTe2中是首选的。这种热力学稳定性的区别可以用1T/ 1t0和2H相之间的能量差来解释，其中硫化物的能量差最大，碲化物的能量差最小。VI族TMDCs中典型的相工程策略包括碱金属插入、电荷掺杂、应变、高压、热处理（加热/冷却）和激光照射，其中大多数是通过交替相变来实现相的相变。已经写了全面的评论，详细介绍了各种阶段工程方法。在各种相工程方法中，碱金属嵌入二维TMDCs得到了广泛的研究。从70年代开始，对二维材料的嵌入进行了广泛的研究。Dresselhaus， Yoffe等人探索了插层对二维材料物理性质的影响，如结构变化，二维超导性和电荷密度波。尽管在插层方面进行了广泛的研究和悠久的历史，但由于最近多模态原位实验探针的发展，在原子水平上对插层诱导的相变的微观和机理的理解才刚刚出现。由于二维VI族TMDC包含了几乎完整的TMDC结构多型集，并且它们是过去十年中研究最多的化合物，因此我们将在这里缩小讨论范围至VI族TMDC。在这个观点中，我们将首先回顾我们目前对ⅵ族TMDCs中碱金属的相变和电化学插层的理解。然后，我们将解决我们在理解上的差距，并提出关键的主题，以帮助建立对插层诱导相变的更完整的理解，在第六组王m .，徐s .，查J. J.教授耶鲁大学机械工程与材料科学系，纽黑文，CT 06511，美国E-mail: judy.cha@yale.edu
2D materials exhibit many exotic structural and electronic phases. Particularly, a 2D-layered transition metal dichalcogenide (TMDC) MX2, with each layer comprising a transition metal layer (M) covalently bonded with top and bottom chalcogen layers (X1⁄4 S, Se, Te), possesses multiple structural polymorphs with distinct physical and chemical properties, as shown in Figure 1A. For example, the trigonal prismatic 2H phase is semiconducting with a thickness-dependent bandgap suitable as a conducting channel in field-effect transistors; the octahedral 1T phase is semimetallic with superior electrochemical catalytic properties; and the distorted octahedral 1T 0/Td phase is topologically protected, exhibiting correlated electronic behaviors, such as the quantum spin Hall effect, spin-resolved Fermi arcs, and even potential topological superconductivity. All the polymorphs (2H, 1T, 1T 0, and Td) can be accessed and transmuted in group VI TMDCs (M1⁄4Mo, W) with transition metals maintaining a d electron configuration, as shown in Figure 1B. It is noted that the thermodynamic stabilities of the phases highly depend on the chalcogen element. For instance, the 2H phase is thermodynamically stable in WS2 at ambient conditions, whereas the Td phase is preferred in WTe2. Such thermodynamic stability distinction can be explained by the energy difference between the 1T/1T 0 and 2H phase, which has a maximum in sulfides and a minimum in tellurides. Typical phase engineering strategies in group VI TMDCs include intercalation of alkali metals, charge doping, strain, high pressure, thermal treatment (heating/cooling), and laser irradiation, most of which accomplish phase conversions by alternating the energetics of the phases. Comprehensive reviews have been written to detail various phase engineering methods. Among the various phase engineering approaches, intercalation of alkali metals into 2D TMDCs has been studied extensively. Intercalation into 2D materials was extensively researched, starting in the 70s. Dresselhaus, Yoffe, and others explored the effects of intercalation on physical properties of 2D materials, such as structural changes, 2D superconductivity, and charge density waves. Despite the extensive investigations and long history in intercalation, microscopic and mechanistic understanding of the intercalation-induced phase transition at the atomic level is just emerging, enabled by the recent developments of multimodal, in situ experimental probes. Because 2D group VI TMDC encompasses an almost complete set of structural polytypes in TMDCs and they are the mostly investigated compounds in the past decade, we will narrow the scope of discussion to group VI TMDCs here. In this Perspective, we will begin by reviewing our current understanding of the phase transition and electrochemical intercalation of alkali metals in group VI TMDCs. Then, we will address gaps in our understanding and suggest critical topics to help build a more complete understanding of the intercalation-induced phase transition in group VI Dr. M. Wang, S. Xu, Prof. J. J. Cha Department of Mechanical Engineering and Materials Science Yale University 15 Prospect St, New Haven, CT 06511, USA E-mail: judy.cha@yale.edu