Deterministic assembly of arrays of lithographicallydefined WS2 and MoS2 monolayer features directly frommultilayer sources into van der Waals heterostructures
Vu Nguyen⸸, Hannah Gramling⸸, Clarissa Towle§‡, Wan Li※, Der-Hsien Lien||‡, Hyungjin Kim||‡,Daryl Chrzan§, Ali Javey||‡, Ke Xu※‡, Joel Ager§‡, Hayden Taylor⸸ ⃰Department of Mechanical Engineering, §Department of Materials Science and Engineering,
Department of Electrical Engineering and Computer Sciences, ※Department of Chemistry, Universityof California, Berkeley, Berkeley, California 94720, United States⸸||‡Materials Sciences Division, Lawrence Berkeley National Laboratories, Berkeley, California 94720,
United States⃰ email contact: hkt@berkeley.edu

One of the major challenges of van der Waals (vdW) integration of 2D materials, the process ofassembling dissimilar 2D materials together using the universal vdW force, is achieving high-yieldand high-throughput assembly of pre-defined sequences of monolayers into heterostructurearrays. Mechanical exfoliation has recently been studied as a promising technique to transfer
monolayers from a multilayer source synthesized by other techniques [1], allowing thedeposition of a wide variety of 2D materials without the constraints imposed by the harsh
synthesis conditions. Although a variety of techniques have been developed to exfoliate the 2D
materials mechanically from the source and place them deterministically onto a target substrate,they typically can transfer only either a wafer-scale blanket [2, 3] or one small flake at a timewith uncontrolled size and shape [4]. Here we present a method to exfoliate arrays oflithographically defined monolayer WS2 and MoS2 features from multilayer sources anddirectly transfer them in a deterministic manner onto target substrates. This exfoliate–align–release process, without the need of an intermediate carrier substrate, was enabled by a newtransfer medium fabricated by spin-coating a partially-crosslinked and transparent adhesive
onto a transparent, electrostatically active backing material with low surface energy. WS2/MoS2
vdW heterostructure arrays produced by this method were characterized, showing the expected
coupled exciton between the monolayers. Light-emitting devices using WS2 monolayers werealso demonstrated, proving the functionality of the fabricated materials. Our work demonstrates
a significant step towards developing mechanical exfoliation as a scalable transfer technique
for the manufacturing of functional, atomically thin materials.

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Gramling, H.M., et al., Spatially precise transfer of patterned monolayer WS2 and MoS2 withfeatures larger than 104 µm2 directly from multilayer sources. ACS Applied ElectronicMaterials, 2019.
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heterostructures. Nature, 2017. 550(7675): p. 229.
Shim, J., et al., Controlled crack propagation for atomic precision handling of wafer-scale twodimensional materials. Science, 2018: p. eaat8126.
Masubuchi, S., et al., Autonomous robotic searching and assembly of two-dimensional crystals
to build van der Waals superlattices. Nature communications, 2018. 9(1): p. 1413.

Figure 1: Fabrication of monolayer and heterostructure arrays by the Covalent-Bond ExfoliateAlign-Release (CoBEARs) process . (a) Optical microscopy images of the array of WS2 micro-features
after critical steps. The target substrate is 50 nm SiO2/Si. The monolayer array is obtained after onesingle exfoliation. Scale bars are 200 µm. (b) WS2/MoS2 heterostructure array obtained by repeating thesteps in (a), with the first cycle depositing MoS2 and the second cycle depositing WS2. From left to right,
scale bars are 200 µm, 200 µm and 100 µm. (c) Raman spectra of the samples in (b). The MoS2-onlyspectrum was obtained before WS2 deposition. The WS2-only and WS2/MoS2 spectra correspond to thetwo red spots numbered 1 and 2 respectively in the high-magnification image of (b).