Layered transition metal dichalcogenides (TMDs) are a diverse group of materials whose properties vary from semiconducting to metallic with a variety of many body phenomena, ranging from charge density wave (CDW), superconductivity, to Mott-insulators. Recent interest in topologically protected states revealed also that some TMDs host bulk Dirac- or Wyle-semimetallic states and their corresponding surface states. In this review, we focus on the synthesis of TMDs by vacuum processes, such as molecular beam epitaxy (MBE). After an introduction of these preparation methods and categorize the basic electronic properties of TMDs, we address the characterization of vacuum synthesized materials in their ultrathin limit-mainly as a single monolayer material. Scanning tunneling microscopy and angle resolved photoemission spectroscopy has revealed detailed information on how monolayers differ in their properties from multi-layer and bulk materials. The status of monolayer properties is given for the TMDs, where data are available. Distinct modifications of monolayer properties compared to their bulk counterparts are highlighted. This includes the well-known transition from indirect to direct band gap in semiconducting group VI-B TMDs as the material-thickness is reduced to a single molecular layer. In addition, we discuss the new or modified CDW states in monolayer VSe₂ and TiTe₂, a Mott-insulating state in monolayer 1T-TaSe₂, and the monolayer specific 2D topological insulator 1T′-WTe₂, which gives rise to a quantum spin Hall insulator. New structural phases, that do not exist in the bulk, may be synthesized in the monolayer by MBE. These phases have special properties, including the Mott insulator 1T-NbSe₂, the 2D topological insulators of 1T′-MoTe₂, and the CDW material 1T-VTe₂. After discussing the pure TMDs, we report the properties of nanostructured or modified TMDs. Edges and mirror twin grain boundaries (MTBs) in 2D materials are 1D structures. In group VI-B semiconductors, these 1D structures may be metallic and their properties obey Tomonaga Luttinger quantum liquid behavior. Formation of Mo-rich MTBs in Mo-dichalcogenides and self-intercalation in between TMD-layers are discussed as potential compositional variants that may occur during MBE synthesis of TMDs or may be induced intentionally during post-growth modifications. In addition to compositional modifications, phase switching and control, in particular between the 1H and 1T (or 1T′) phases, is a recurring theme in TMDs. Methods of phase control by tuning growth conditions or by post-growth modifications, e.g. by electron doping, are discussed. The properties of heterostructures of TMD monolayers are also introduced, with a focus on lateral electronic modifications in the moiré-structures of group VI-B TMDs. The lateral potential induced in the moiré structures forms the basis of the currently debated moiré-excitons. Finally, we review a few cases of molecular adsorption on nanostructured monolayer TMDs. This review is intended to present a comprehensive overview of vacuum studies of fundamental materials' properties of TMDs and should complement the investigations on TMDs prepared by exfoliation or chemical vapor deposition and their applications.

层状过渡金属二硫化碳（TMD）是多种材料，其性质从半导体到金属，从电荷密度波（CDW），超导电性到Mott绝缘体，都具有多种人体现象。最近对拓扑保护状态的兴趣还表明，某些TMD拥有体Dirac-或Wyle-半金属态及其相应的表面态。在这篇综述中，我们专注于通过真空工艺（例如分子束外延（MBE））合成TMD。在介绍了这些制备方法并分类了TMD的基本电子性能后，我们将超薄极限条件下的真空合成材料表征为主要是单层材料。扫描隧道显微镜和角度分辨光发射光谱学已揭示出有关单层与多层和块状材料的性能如何不同的详细信息。对于有数据可用的TMD，给出了单层属性的状态。突出显示了与它们的整体对应物相比单层性质的明显改变。这包括众所周知的从VI-B型TMD半导体中间接带隙到直接带隙的跃迁，因为材料厚度减小到一个分子层。另外，我们讨论单层VSe中的新的或修改的CDW状态 突出显示了与它们的整体对应物相比单层性质的明显改变。这包括众所周知的从VI-B型TMD半导体中间接带隙到直接带隙的跃迁，因为材料厚度减小到一个分子层。另外，我们讨论单层VSe中的新的或修改的CDW状态 突出显示了与它们的整体对应物相比单层性质的明显改变。这包括众所周知的从VI-B型TMD半导体中间接带隙到直接带隙的跃迁，因为材料厚度减小到一个分子层。另外，我们讨论单层VSe中的新的或修改的CDW状态_如图2和TiTe _2所示，单层1T-TaSe _2中的Mott绝缘状态，以及单层特定的2D拓扑绝缘体1T'-WTe ₂，产生量子自旋霍尔绝缘体。本体中不存在的新结构相可以通过MBE在单层中合成。这些相具有特殊的性质，包括莫特绝缘体1T-NbSe ₂，1T'-莫特的2D拓扑绝缘体₂，和CDW材料1T-VTE ₂。在讨论了纯TMD之后，我们报告了纳米结构或改性TMD的特性。2D材料中的边缘和镜面孪晶晶界（MTB）是1D结构。在VI-B组半导体中，这些一维结构可能是金属的，其特性服从Tomonaga Luttinger量子液体行为。讨论了在二硫代双氰化物中形成富Mo的MTB以及在TMD层之间的自嵌入，这是可能在TMD的MBE合成过程中发生或在生长后修饰过程中有意诱导的潜在组成变异。除了组成修改之外，尤其是在1H和1T（或1T'）阶段之间进行相位切换和控制是TMD中反复出现的主题。讨论了通过调节生长条件或通过生长后修饰（例如通过电子掺杂）进行相控制的方法。还介绍了TMD单层异质结构的特性，重点是VI-B组TMD的莫尔结构中的侧向电子修饰。莫尔结构中产生的侧向电势构成了目前争论的莫尔激子的基础。最后，我们回顾了分子吸附在纳米结构单层TMD上的几种情况。这篇综述旨在对TMD的基本材料性能进行真空研究的全面概述，并应补充对通过剥落或化学气相沉积制备的TMD的研究及其应用的补充。莫尔结构中产生的侧向电势构成了目前争论的莫尔激子的基础。最后，我们回顾了分子吸附在纳米结构单层TMD上的几种情况。这篇综述旨在对TMD的基本材料性能进行真空研究的全面概述，并应补充对通过剥落或化学气相沉积制备的TMD的研究及其应用的补充。莫尔结构中产生的侧向电势构成了目前争论的莫尔激子的基础。最后，我们回顾了分子在纳米结构单层TMD上吸附的几种情况。这篇综述旨在对TMD的基本材料性能进行真空研究的全面概述，并应补充对通过剥落或化学气相沉积制备的TMD的研究及其应用的补充。