Abstract Since its discovery, graphene has been the object of study for several and manifold applications, ranging from photocatalysis to mechanics and electronics of materials. However, the use of pure graphene in certain opto- and microelectronic applications is sometimes limited because of its zero band gap. Among the different methods to widen the band gap, in the present work the attention is focused on the so-called van der Waals composites (or heterostructures, or heterojunctions), namely the stacking of two monolayers of different materials that are held together by weak van der Waals interactions. An interesting composite is the graphene/molybdenite (MoS2) heterostructure, where the second material is a bidimensional semiconductor characterized by strong in-plane covalent bonds and weak out-of-plane interactions, which means that it is possible to exfoliate MoS2 into monolayers of atomic thickness. Also, from the crystallographic point of view, both graphene and the monolayer of molybdenite (MoS2-1H) have a 2D hexagonal lattice and their stacking could be structurally favourable. In this work, the electronic band structure and density of states, complex dielectric function and optical properties of the stacked van der Waals bilayer heterojunction graphene/MoS2-1H were calculated and compared to both the single monolayers of graphene and molybdenite, to understand how the interaction between the two materials may alter the above cited properties. The analysis of the band structure in this van der Waals composite clearly showed a small direct band gap related to the transition π → π ∗ (ca. 2.5 meV) in correspondence of the high symmetry point K. The heterojunction showed also, as expected, some important variations in the complex dielectric function and related properties in the visible-light spectral region. The results obtained in the present work could be of use for future development and applications of this kind and similar 2D composite materials with tailored electronic and optical properties.

中文翻译：

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石墨烯/辉钼矿双层复合材料的电子和光学性能

摘要 自发现以来，石墨烯已成为多种应用的研究对象，从光催化到材料的力学和电子学。然而，由于其零带隙，纯石墨烯在某些光电和微电子应用中的使用有时会受到限制。在加宽带隙的不同方法中，在目前的工作中，注意力集中在所谓的范德华复合材料（或异质结构或异质结）上，即将两个不同材料的单层堆叠在一起，由弱的范德瓦尔斯相互作用。一个有趣的复合材料是石墨烯/辉钼矿 (MoS2) 异质结构，其中第二种材料是二维半导体，其特征在于强的面内共价键和弱的面外相互作用，这意味着可以将 MoS2 剥离成原子厚度的单层。此外，从晶体学的角度来看，石墨烯和单层辉钼矿 (MoS2-1H) 都具有二维六方晶格，它们的堆叠在结构上可能是有利的。在这项工作中，计算了堆叠范德华双层异质结石墨烯/MoS2-1H 的电子能带结构和态密度、复杂的介电函数和光学性质，并将其与单层石墨烯和辉钼矿进行比较，以了解两种材料之间的相互作用可能会改变上述特性。对这种范德华复合材料中的能带结构的分析清楚地表明，与高对称点 K 对应的跃迁 π → π ∗ (ca. 2.5 meV) 相关的小直接带隙。正如预期的那样，异质结还显示出复杂介电函数和可见光光谱区域相关特性的一些重要变化。目前工作中获得的结果可用于此类以及具有定制电子和光学特性的类似二维复合材料的未来开发和应用。