Clean Transfer of 2D Transition Metal Dichalcogenides Using Cellulose Acetate for Atomic Resolution Characterizations

Abstract

Atomically thin two-dimensional (2D) transition metal dichalcogenides (TMDs) can be easily synthesized on SiO2/Si substrates by chemical vapor deposition (CVD). However, for practical applications, those 2D crystals usually need to be retrieved and placed onto target substrates. Hence, a robust and effective transfer process is required. Currently, the most widely used approach for transferring CVD-grown TMDs involves the spin-coating of a poly­(methyl methacrylate) (PMMA) support layer, followed by the wet etching of the SiO2 layer in hot NaOH. This transfer process often causes substantial accumulation of polymer residues as well as severe structural damage of TMDs induced during the etching of substrates at elevated temperatures. In this work, we present an alternative approach for the transfer of CVD-grown TMDs that can address the issues mentioned above. In this process, we replaced PMMA with cellulose acetate (CA) as a support layer and used buffered oxide etch (BOE) as an effective room-temperature etchant for SiO2. The CA-transferred TMDs exhibit well-preserved structural integrity and unaltered optical properties as well as largely improved microscale and nanoscale cleanliness with reduced wrinkles and cracks. Furthermore, we integrated our CA-transfer method with a deterministic positioning system that allowed microprecision transfer of the TMD layers. For example, a WS2–MoS2 vertical heterojunction with an electronically coupled and uniform interface was successfully created. The CA-transfer technique developed in this work represents a cleaner alternative to the PMMA-transfer method, thus permitting atomic resolution characterizations and the implementation of novel applications of CVD-grown TMDs and their heterostructures.

Transfer of large-scale two-dimensional semiconductors: challenges and developments

“…Since the early stage use of polymer supporting layers such as PMMA to transfer CVDgrown graphene [130], the method was successfully adopted to CVD-grown MoS 2 [132] and thereafter extended. There now exist a range of supporting layers beyond PMMA that can be used to transfer large area films of 2D TMDs, from flexible low surface energy polymers such as PDMS [19,57,85], to more environmentally friendly natural polymers such as CA [66] and even to rigid non-polymer supports such as thin Cu films [44,158]. Moreover, there now exist methods which forgo the use of any support, relying on surface assisted delamination in an aqueous solution [64].…”

“…The CA/TMD assembly is then placed on the surface of BOE to etch the SiO2 layer and detach the TMD, and is then transferred to a target substrate and rinsed in acetone and IPA. Reproduced with permission [66]. Copyright (2019) American Chemical Society.…”

“…For this reason, they deserve to be highlighted in this review. Citing the well-known problems of using PMMA-based transfer methodologies, specifically polymer residues which degrade performance (see for example [22,51,52,92,153]), Zhang et al used cellulose acetate (CA) as a support layer to transfer CVD-grown TMDs onto a SiO 2 /Si substrate [66]. To avoid the problems associated with using the conventional hot NaOH etching method to detach the film (such as cracks or wrinkles from bubbles), the authors used a combination of NH 4 F and HF (known as buffered oxide etch) which works at room temperature.…”

“…Given the potential for using 2D TMDs for novel technological applications, it is vital that the transferred films are of a high quality and fidelity, in order to preserve the material properties. To this end, the structural, chemical and electronic properties of transferred TMD films have been investigated [29,44,64,66,84,85,104,171,172]. This is important as many transfer methodologies suffer from process related weaknesses, such as trapped bubbles, polymer residues, cracks or wrinkles.…”

Facile Morphological Qualification of Transferred Graphene by Phase‐Shifting Interferometry

“…It has been reported that wet transfer using the cellulose acetate supporting layer produces a clean graphene surface because it has a higher solubility than PMMA by acetone. [16,50] In the case of S DT , as known in the literatures, huge amount of polymer residues are left on the transferred film. Quantitative analysis on the residues can be extracted by analyzing the PSI images.…”