Mixed multilayered vertical heterostructures utilizing strained monolayer WS₂

Abstract

Creating alternating layers of 2D materials forms vertical heterostructures with diverse electronic and opto-electronic properties. Monolayer WS2 grown by chemical vapour deposition can have inherent strain due to interactions with the substrate. The strain modifies the band structure and properties of monolayer WS2 and can be exploited in a wide range of applications. We demonstrate a non-aqueous transfer method for creating vertical stacks of mixed 2D layers containing a strained monolayer of WS2, with Boron Nitride and Graphene. The 2D materials are all grown by CVD, enabling large area vertical heterostructures to be formed. WS2 monolayers grown by CVD directly on Si substrates with SiO2 surface are easily washed off by water and this makes aqueous based transfer methods challenging for creating vertical stacks on the growth substrate. 2D hexagonal Boron Nitride films are used to provide an insulating layer that limits interactions with a top graphene layer and preserve the strong photoluminescence from the WS2. This transfer method is suitable for layer by layer control of 2D material vertical stacks and is shown to be possible for all CVD grown samples, which opens up pathways for the rapid large scale fabrication of vertical heterostructure systems with atomic thickness depth control and large area coverage.

Determining the Optimized Interlayer Separation Distance in Vertical Stacked 2D WS2:hBN:MoS2 Heterostructures for Exciton Energy Transfer

“…To fabricate the vertical stacks, we used our recently reported sliding transfer method in isopropanol (IPA) to preserve the underlying TMD materials. The use of highly volatile solvent ensures no residues to be remained so that cleaner interfaces between the 2D layers can be generated compared to the conventional methods based on water . The cleanliness of interface is evidenced by the strong interlayer coupling of WS 2 :MoS 2 (Figure S3 and Table S1, Supporting Information) and MoS 2 :WS 2 (Figure S4 and Table S2, Supporting Information) heterostacks, as well as the annealing effects shown in Figure S13 (Supporting Information).…”

“…After that, a piece of clean cover glass was submerged into the DI water to fish the film. Then a few drops of IPA were applied onto the cover glass to expel the residual water before the film slid slowly onto the designated substrate . After being air dried overnight, the sample was baked at 150 °C for 30 min to enhance the interface adhesion.…”

Blister-based-laser-induced-forward-transfer: a non-contact, dry laser-based transfer method for nanomaterials

“…Monolayer crystals of MoS2 were grown by CVD 19 and deposited on a BB-LIFT substrate (250 nm thick Ti) using a dry transfer method 3 . This provided a sample with a higher density of more homogeneous crystals, Fig.…”

Deterministic and Etching‐Free Transfer of Large‐Scale 2D Layered Materials for Constructing Interlayer Coupled van der Waals Heterostructures

“…As is reported previously, water droplet will lift the hydrophobic 2DLMs from the substrate and break them into small pieces, while organic solvents (e.g., acetone) will not . To demonstrate the deterioration of 2DLMs in water and the necessity of introducing PMMA, we carried out a control experiment of transferring MoS 2 directly by using PVA.…”

“…However, these approaches require exquisite manipulations and are not universally achievable since the material‐substrate adhesion differs much with various growth conditions. Soaking CVD‐grown 2DLMs in water can also bring cracks to the materials resulting from the hydrophobic nature of 2DLMs . Another common problem of these methods is that the exfoliation is not on‐target, i.e., the entire 2D material on the substrate is transferred at one time, causing inevitable waste of materials.…”