title: 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.
authors.forename: Amin
authors.initials: A
authors.lastname: Azizi
authors.forename: Sarah
authors.initials: S
authors.lastname: Eichfeld
authors.affiliation: §Department of Physics and Astronomy, Stony Brook University, Stony Brook, New York 11794-3800, United States.
authors.forename: Gayle
authors.initials: G
authors.lastname: Geschwind
authors.forename: Kehao
authors.initials: K
authors.lastname: Zhang
authors.affiliation: ∥FEI Company, 5350 NE Dawson Creek Drive, Hillsboro, Oregon 97124, United States.
authors.forename: Bin
authors.initials: B
authors.lastname: Jiang
authors.forename: Debangshu
authors.initials: D
authors.lastname: Mukherjee
authors.forename: Lorraine
authors.initials: L
authors.lastname: Hossain
authors.forename: Aleksander F
authors.initials: AF
authors.lastname: Piasecki
authors.forename: Bernd
authors.initials: B
authors.lastname: Kabius
authors.forename: Joshua A
authors.initials: JA
authors.lastname: Robinson
authors.forename: Nasim
authors.initials: N
authors.lastname: Alem
keywords: MoS2; WSe2; atomic and chemical structure; freestanding heterostructures; graphene; transition metal dichalcogenides; transmission electron microscopy