Freestanding van der Waals Heterostructures of Graphene and Transition Metal Dichalcogenides

Abstract

Vertical stacking of two-dimensional (2D) crystals has recently attracted substantial interest due to unique properties and potential applications they can introduce. However, little is known about their microstructure because fabrication of the 2D heterostructures on a rigid substrate limits one's ability to directly study their atomic and chemical structures using electron microscopy. This study demonstrates a unique approach to create atomically thin freestanding van der Waals heterostructures-WSe2/graphene and MoS2/graphene-as ideal model systems to investigate the nucleation and growth mechanisms in heterostructures. In this study, we use transmission electron microscopy (TEM) imaging and diffraction to show epitaxial growth of the freestanding WSe2/graphene heterostructure, while no epitaxy is maintained in the MoS2/graphene heterostructure. Ultra-high-resolution aberration-corrected scanning transmission electron microscopy (STEM) shows growth of monolayer WSe2 and MoS2 triangles on graphene membranes and reveals their edge morphology and crystallinity. Photoluminescence measurements indicate a significant quenching of the photoluminescence response for the transition metal dichalcogenides on freestanding graphene, compared to those on a rigid substrate, such as sapphire and epitaxial graphene. Using a combination of (S)TEM imaging and electron diffraction analysis, this study also reveals the significant role of defects on the heterostructure growth. The direct growth technique applied here enables us to investigate the heterostructure nucleation and growth mechanisms at the atomic level without sample handling and transfer. Importantly, this approach can be utilized to study a wide spectrum of van der Waals heterostructures.

Mapping unoccupied electronic states of freestanding graphene by angle-resolved low-energy electron transmission

“…Possible candidates for further studies are few layer graphene sheets, single layer hexagonal boron nitride/graphene compounds, exfoliated single layer molybdenum disulphide [31], or heterostructures of graphene and transitional metal dichalcogenides [32].…”

Trivial and inverted Dirac bands and the emergence of quantum spin Hall states in graphene on transition-metal dichalcogenides

“…The next best candidates are two-dimensional transition-metal dichalcogenides (TMDCs) which are direct band-gap semiconductors [16,17]. Graphene on TMDCs has already been grown [18][19][20] and investigated for transport [21,22] as well as considered for technological applications [23][24][25][26]. It was recently predicted that monolayer MoS 2 will induce a giant spin-orbit coupling in graphene, of about 1 meV (compared to 10 μeV in pristine graphene [27]).…”

Recent Advances in Controlling Syntheses and Energy Related Applications of MX2 and MX2/Graphene Heterostructures

“…[60] Subsequently, Alem et al, reported the syntheses of nearly freestanding MX 2 /Gr heterostructures on Quantifoil TEM grids with ≈2 μm holes by the APCVD growth of monolayer MX 2 on transferred monolayer Gr (from Cu foils) (Figure 8c). [127] Notably, this work enabled the on-site investigation of the growth dynamics of MX 2 on Gr surfaces at an atomic resolution. [51] Copyright 2015, American Chemical Society.…”