Here, it is seen that the islands tend to align with one side against the HOPG edge, and one corner pointing at a near-perpendicular angle to it. 1(b) shows MoS 2 islands arranged on a pair of HOPG step edges in another sample location, with the profile taken along the white line indicating a single layer height of around 0.6 nm. 1(a), the majority of islands are aligned roughly with one corner pointing along the step edge, and one side perpendicular to it. Also of interest is the ordered orientation of the islands. Most of these triangles form across an HOPG step, or in contact with the step at one of their edges. The HOPG substrate's step-terrace morphology is overlaid by triangular MoS 2 thin film islands. The micrometer-scale AFM morphology of sub-monolayer MoS 2 on HOPG is shown in Fig. These findings offer a microscopic explanation for the apparently ordered orientations of micron sized MoS 2 islands as observed in AFM images. As ab initio calculations indicate that the difference in surface energy for different orientations is insufficient to explain this tendency, we instead construct a simple model attributing it to the type of graphite edge (zig-zag or armchair) at which islands nucleate. However, we find that the MoS 2 islands show a preference for forming with a small relative angle of rotation with respect to the substrate, with angles above a few degrees found to be rare. An investigation of the superstructure dependent surface energy using ab initio calculations indicates that inter-layer interactions are not sufficient to impose such a constraint on the MoS 2 islands' lattice orientation. We find that there exist at least five possible orientations of MoS 2 islands with respect to the HOPG substrate lattice. 21 By analyzing the atomic lattice and moiré pattern, the stacking orientation between the MoS 2 adlayer and the HOPG substrate can be obtained. We find that the triangular islands are atomically clean and defect-free, and that clear moiré patterns can be observed, which in general arise due to a lattice mismatch or rotational mis-alignment between a weakly interacting adlayer and substrate. 19 MoS 2 grown using the CVD method has previously been investigated using STM and photoluminescence techniques, 20 but the detailed atomic scale structure at the CVD MoS 2/HOPG interface has not yet been elucidated. 17,18 The CVD process, as opposed to solution transfer used in previous reports, can prevent contaminations such as trapped water at the interface. In this work, we perform atomic force microscopy (AFM) and scanning tunneling microscopy (STM) measurements on sub-monolayer chemical vapor deposition (CVD) grown MoS 2 on a substrate of highly oriented pyrolytic graphite (HOPG). The results encourage the fundamental exploration of the interaction between MoS 2 and hexagonal graphene or graphite. 16 Although there is a large lattice mismatch between the MoS 2 and the graphene structure, graphene can serve as an epitaxial substrate for MoS 2. have recently reported the formation of MoS 2 flakes on the graphene surface via thermal decomposition of ammonium thiomolybdate. 14 Hence, the study of the interfaces between MoS 2 and graphene (or graphite) is critically important and may provide useful hints for various applications. 12 Graphene/MoS 2 heterostructures have also been adopted to demonstrate an extremely high photosensitivity and gain 13 as well as the ultrasensitive detection of DNA hybridization. 5–9 Such heterostructures offer the possibility to create devices with new functionalities or better performance in electronic logic and memory devices, 9–11 and also offer great potential in the hydrogen evolution reaction. Recently, many proposed novel devices are based on heterostructures of MoS 2 and graphene. Molybdenum disulfide (MoS 2), a layered semiconductor whose layers are weakly bound by the van der Waals force, 1–4 has a great potential for application in electronic devices.
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