Van der Waals heterostructures of MoSe2 and graphene studied by transmission electron microscopy

Abstract

Two dimensional layered transition metal dichalcogenides (TMDs) have attracted much attention for future electronics and optoelectronics due to their unique semiconducting features [1]. Nonetheless their properties are strongly influenced by the structural and chemical atomic arrangement in these atomically thin layers. The control and understanding of the atomic structure of synthesized TMD monolayers are thus crucial to exploit the potential properties predicted and/or to be newly discovered. In addition, compared to graphene which is a mono‐atomic planar structure, the structural and chemical configuration of the TMD materials can have a lot of variations and it can be a way to tune their semi‐conducting features. For instance, a ternary mixture such as Mo x W 1‐x S 2 and vertical/horizontal heterostructures between TMD structures with different chemical components or with other layered structures such as graphene and boron nitride can open the possibility of unique architectures [2]. In particular, vertical heterostructures are promising building blocks for novel semiconducting future materials because these layers have no surface dangling bond and vertically stacked layers are connected with van der Waals (vdW) forces. This allows to create atomically sharp interface with a desired structure design down to single atomic layered scale. Today a lot of efforts have been made to fabricate vdW heterostructures [3]. In this work, vdW vertical heterostructures of MoSe 2 and graphene are studied using a transmission electron microscopy (TEM). The vdW stacks are fabricated by two step growth process. First graphene is grown by conventional CVD technique on Pt substrate, then followed by MoSe 2 growth via vdW epitaxy by Molecular beam epitaxy (MBE) technique in another reactor. The direct growth approach presents various interests compared to the manual stacking, such as clean interface and large surface production. In addition, using as grown CVD graphene, the obtained stack layers can be easily transferred on appropriate substrates. The synthesized MoSe 2 /graphene layers are studied from micron down to atomic scale by several TEM techniques mainly using Low Voltage Aberration Corrected (LVAC) TEM in order to understand the growth mechanism of the vdW epitaxy by MBE and the correlation between grown MoSe 2 layer and graphene substrate. Using (S)TEM techniques, abundant information on synthesized structures can be provided. Local number of layers can be determined by several STEM techniques such as STEM HAADF imaging (Figure 1) and PACBED ( Partially averaged convergent beam electron diffraction ) [4]. Domains in graphene and MoSe 2 layers were independently recognized together with their local orientation using diffraction information, which allowed to study the local structural relationship between MoSe 2 and graphene substrate. Figure 2 shows a TEM image of stack layer and the orientation of MoSe 2 and graphene is determined by Fourier transform shown in the inset. MoSe 2 layers are often grown oriented to graphene with small range of misorientation 0 to 5°. The edge of MoSe 2 monolayer are observed along the zig‐zag line of graphene in the case of non‐continuous MoSe 2 monolayers. In addition, typical line defects are observed in a continuous domain (Figure 3a). This line defect consists of a symmetrical mirror structures (Figure 3b and 3c) [5], considered to be related to the stoichiometry control during the growth. Local chemical quantitative analysis by energy dispersive X‐ray spectroscopy (EDX) was also applied on MoSe x in order to exploit the sensitivity of the measurements, which will be a powerful method applicable at multi scale to predict various defect structures influencing their stoichiometry. Finally the MoSe 2 layers grown on CVD graphene with different experimental conditions were characterized using TEM and STEM based techniques. The influence of process parameters on the atomic configuration such as line defects are studied and the crystal mosaicity in MoSe 2 monolayer related to graphene substrate will be discussed by local structural analysis with a theoretical support.