Spatially Precise Transfer of Patterned Monolayer WS₂ and MoS₂ with Features Larger than 10⁴ μm² Directly from Multilayer Sources

Abstract

A current challenge in the processing of 2D materials, or “van der Waals (vdW) solids”, is the transfer of 2D layers from source crystals and growth substrates onto target substrates. Transferas opposed to direct growth and patterning on the targetenables low-temperature processing of the target as well as the use of diverse target materials. These two attributes will allow the assembly of vdW heterostructures to realize devices exploiting the unique properties of vdW materials. Until now, however, there has been no effective method for transferring regions of monolayer material of controlled shape from a multilayer source. We introduce such a method and demonstrate its use in the spatially controlled transfer of arrays of single-layer MoS2 and WS2 sheets from multilayer crystals onto SiO2 substrates. These sheets have lateral sizes exceeding 100 μm and are electronically continuous. The method offers a scalable route to parallel manufacturing of complex circuits and devices from vdW materials.

Deterministic Assembly of Arrays of Lithographically Defined WS2 and MoS2 Monolayer Features Directly From Multilayer Sources Into Van Der Waals Heterostructures

“…S6). Rough surfaces resembling nanoparticles scattered on the material have also been observed previously on monolayer features that had undergone energetic and extended O2 plasma cleaning before the Au etch [30]. In that case, X-ray photoelectron spectroscopy suggested that some Au remained on the surface after cleaning [30].…”

“…Rough surfaces resembling nanoparticles scattered on the material have also been observed previously on monolayer features that had undergone energetic and extended O2 plasma cleaning before the Au etch [30]. In that case, X-ray photoelectron spectroscopy suggested that some Au remained on the surface after cleaning [30]. As an alternative to the solution of using a mild and short O2 plasma recipe, a thicker metal protection layer or a non-plasma-based recipe, such as atomic hydrogen cleaning [40], could be considered to remove polymer residue with minimal effect on the monolayer.…”

“…The WS2 multilayer sources were obtained as crystals synthesized through chemical vapor transport (CVT) (HQ Graphene, Groningen, Netherlands), while the MoS2 sources were naturally occurring crystals (Crystal Age, Bristol, UK). The as-received source crystals were prepared for exfoliation by applying patterned Au and photoresist handles using the procedure described in our previous work [30], which is summarized as follows. A 100 nm Au film was thermally evaporated on top of the TMDC crystal to provide monolayer exfoliation selectivity and also to protect the monolayer in subsequent processing steps.…”

“…In an effort to meet the need for a scalable technique to transfer pre-patterned monolayer arrays, we have recently developed a thin-film-mediated process to transfer arrays of TMDC monolayer features with pre-defined geometry directly from multilayer sources [30,31]. A key advantage of this process is that it incorporates the use of an evaporated gold film which bonds to and strains the topmost monolayer of the TMDC source crystal, providing monolayer selectivity [32][33][34] and thus easing the layer-controlled requirement of the source.…”

“…2b indicates that excess polymer was effectively cleaned. The ability to release micro-features reliably and cleanly with this encapsulation approach promises to address the weaknesses of the widely used commercial thermal release tape, which employs heat-expandable microspheres that limit the scaling down of transferred feature size [35], and leaves behind residue that requires an extended and high-power O2 plasma-cleaning recipe [30].…”