Mechanical Transfer of Large-Scale Two-Dimensional Materials Onto Arbitrary Substrates Via Water-Penetration-Assisted Method
Authors: Lai Shen , Lee Sungjoo 
Published: February 2020

Recently, two-dimensional (2D) materials have emerged as an important research topic. Single layer molybdenum disulfide (MoS2) is one of the most widely investigated 2D materials, and possesses outstanding properties with applications in band gap engineering, optical and electrical devices, and spin-orbit research. Large-area and uniform single layer MoS2 can be synthesized by performing chemical vapor deposition (CVD) on various substrates. The full exploitation of the potential of CVD MoS2 requires a method for the clean transfer of the as-grown MoS2 from the growth substrate to the target substrate without degeneration of its properties. The current approach to the transfer of CVD MoS2 usually involves coating a polymer carrier film on top of the CVD MoS2 as a supporting layer and the chemical etching of the growth substrate to separate MoS2 from its surface. However, this etching process causes serious damage to the 2D material due to the harsh corrosivity of the etchant, usually hydrogen fluoride (HF) or a strong base (NaOH or KOH), and the removal of polymer carrier film leaves a residue. Thus the development of a gentle transfer approach that is highly efficient, repeatable, and environmentally friendly is required.
In this work, we report a method that with the assistance of water penetration enables the mechanical transfer of MoS2 from the growth substrate to a target substrate without any etching of the growth substrate or leaving any polymer residue on the MoS2 layer. The whole transfer process is as below:
After CVD growth of MoS2 on SiO2, a thin Cu carrier film (50–100 nm) is prepared with e-beam deposition on top of the MoS2 layer. Then a layer of polymethyl methacrylate (PMMA) is spin-coated onto Cu/MoS2/SiO2, then baking process is performed, and a layer of polydimethylsiloxane (PDMS) is attached onto PMMA/Cu/MoS2/SiO2. Next, PDMS/PMMA/Cu/MoS2/SiO2 is immersed in deionized (DI) water (We also carried out peeling off out of water under ambient conditions as in other mechanical transfer processes as a contrast experiment). Once the peeling off process starts, water will immediately flood into the space generated by the separation of PDMS/PMMA/Cu/MoS2 because of this natural tendency and the hydraulic pressure. The penetrated water mechanically supports the upper PDMS/PMMA/Cu/MoS2 with buoyancy force, which prevents physical damage. After separation of PDMS/PMMA/Cu/MoS2, it is dried under ambient conditions, and then PDMS/PMMA/Cu/MoS2 is placed and pressured onto a target substrate. The whole sample is heated at 130°C on a hotplate. Once PDMS has lost adhesion, it can easily be peeled off leaving PMMA/Cu/MoS2 on top of the target substrate. In the next step, the PMMA/Cu/MoS2/target substrate is dipped in acetone to remove PMMA and then in Cu etchant to remove Cu. In contrast to the residue left on the MoS2 layer after the removal of the polymer contact material in previous studies, the Cu layer is completely removed by the Cu etchant due to the de-wetting of metals on 2D materials. We also experimentally demonstrated that Cu, which has a strong adhesion force with MoS2, is an ideal contact material for the clean peeling off of the carrier film and MoS2. Finally, perfectly uniform, large-area CVD MoS2 with a clean surface is obtained on the target substrate with our transfer process.
The morphology of MoS2 before and after transfer under or out of water were observed with optical microscopy (OM), scanning electron microscopy (SEM) and atomic force microscopy (AFM). The MoS2 before and after transfer under water both appear to be uniform, continuous, and clean without any wrinkles, cracks or polymer residue. However, many holes were observed in the sample transferred out of water which were generated by the abrupt forces generated by the peeling off process; the absence of support by water means that the MoS2 layer, which is just one atomic layer thick, can easily be damaged. Samples were also studied with Raman and photoluminescence (PL) spectroscopy. Raman spectra and PL peak positions are same for all three samples. But the PL peak intensity of the MoS2 sample transferred out of water is much weaker than the other two samples due to the physical damage generated by the transfer process without water support. All above results confirmed the important role of water penetration in our mechanical transfer process.
Our transfer method protects the original quality and morphology of large area MoS2 without leaving any polymer residue, and enables the reuse of the growth substrate. This clean transfer approach is expected to facilitate the realization of industrial applications of MoS2 and other 2D materials.