High Resolution S/TEM Study of Vertical and In-Plane Heterostructures Formed by Two-Dimensional MoS₂ and ReS₂

Session

PSA.1 - 1D & 2D Materials

Authors

Mr. Saiphaneendra Bachu (1), Ms. Lauren Stanton (1), Mr. Chenhao Qian (1), Dr. Danielle Reifsnyder Hickey (1), Dr. Nasim Alem (1)

Affiliations

1. Pennsylvania State University

Keywords

Anisotropy, Epitaxy, HAADF-STEM, Heterostructures, Two-dimensional (2D) materials

Abstract text

Low dimensional (zero, one and two dimensional) materials attracted great attention from researchers owing to their novel properties arising from quantum confinement and surface/interface effects [1]. Two-dimensional (2D) transition metal dichalcogenides (TMD), such as MoS₂ and WSe₂, are particularly interesting because of their semiconductor nature and transition from indirect to direct band gap when thinned down to a single layer [2]. This opened up a lot of opportunities in band structure engineering towards applications in optoelectronics. One such example of band structure engineering is integrating multiple 2D TMD into heterostructures in vertical and in-plane geometries. For example, C. Zhang et al. observed that the lattice mismatch strain at the interface of MoS₂-WSe₂ in-plane heterostructures is relieved by forming dislocations at the interface and that changes the band alignment from type II to type I [3]. This means the performance of the heterostructures towards targeted applications is highly dependent on careful selection of constituent TMD and the interfaces formed between them.

This study explores vertical and in-plane heterostructures formed by MoS₂ and ReS₂. MoS₂-ReS₂ heterostructures are a fascinating material system to study because MoS₂ is isotropic in nature whereas ReS₂ has an anisotropic structure thus forming isotropic-anisotropic interface [4]. Such anisotropy in the system could introduce modulations in the atomic and interfacial structure and lead to polarization dependent optical properties [5]. Therefore, it is important to understand the atomic structure of the MoS₂-ReS₂ interface before this material system could be incorporated into any applications.

This work covers synthesis and atomic scale structural characterization of vertical and in-plane MoS₂-ReS₂ heterostructures. Synthesis of the heterostructures was carried out using a two-step powder-source chemical vapor deposition (CVD) process on sapphire substrates [6]. The as-grown heterostructures were characterized using aberration-corrected high-angle annual dark-field (HAADF) scanning transmission electron microscopy (STEM) to understand interface atomic structure and, epitaxy and strain between the MoS₂ and ReS₂ layers. An FEI Titan³ G2 operated at 80 kV was employed to perform HAADF-STEM imaging. Atomic resolution HAADF-STEM image of a vertical heterostructure showing the underlying monolayer MoS₂ with monolayer ReS₂ crystal grown on top is presented in Figure 1a. A closer look at the image reveals the Re chains in ReS₂ align with the zigzag direction of MoS₂ which means ReS₂ grows epitaxially on top of MoS₂. This is also confirmed by the matching orientation of FFT spots of MoS₂ and ReS₂ in Figure 1b. Dashed red circles in Figure 1a indicate vacancy clusters present in the ReS₂ lattice highlighting the defective nature of ReS₂ layer grown during synthesis. Figure 1c is a low magnification TEM image showing in-plane heterostructures of MoS₂ and ReS₂, wherein ReS₂ grows in the gap between MoS₂ triangles during the second step of the synthesis procedure. An atomic resolution HAADF-STEM image taken from the edge of a MoS₂ triangle, as outlined by a red box in Figure 1c, is shown in Figure 1d. The presentation will further dwell into quantitative analysis of various features seen in the heterostructures [7].

Figure 1 MoS₂-ReS₂ vertical heterostructures: (a) atomic resolution HAADF-STEM image of vertical heterostructure showing monolayer ReS₂ grown on top of monolayer MoS₂ and (b) binned FFT pattern obtained from (a) highlighting the epitaxy between MoS₂ an ReS₂; MoS₂-ReS₂ in-plane heterostructures: (c) Low-magnification TEM image showing MoS₂ triangles with ReS₂ grown between the triangles and (d) atomic-resolution HAADF-STEM image of in-plane heterostructure formed at the edge of a MoS₂ triangle, obtained from the region marked by the red box in (c).

References

[1] Pakdel, Amir, et al. Materials Today 15.6 (2012): 256-265.

[2] Jariwala, Deep, et al. ACS nano 8.2 (2014): 1102-1120.

[3] Zhang, Chendong, et al. Nature Nanotechnology 13 (2018): 152–158.

[4] Lin, Yung-Chang, et al. ACS Nano 9.11 (2015): 11249-11257.

[5] Neupane, Guru Prakash, et al. Small 15 (2019): 1804733.

[6] Chen, Bin, et al. Advanced Materials 29.34 (2017): 1701201.

[7] This work was supported by the National Science Foundation (NSF), in part under the CAREER program (DMR-1654107), in part by the program EFRI 2-DARE: 2D Crystals by Activated Atomic Layer Deposition (EFRI-1433378), and in part by the Penn State 2D Crystal Consortium-Materials Innovation Platform (2DCC-MIP) under NSF cooperative agreement DMR-1539916.