Alfa Chemistry offers a variety of layer transfer methods, including polymer-assisted transfer methods and polymer-free transfer methods, to provide expert 2D material layer transfer services to our customers. Please continue reading to learn more about our various techniques. Additional technological methods for layer transfer investigations are also being developed. For further information, please contact us.
Polymer-Assisted Transfer Methods
The polymer-assisted transfer method is our common approach. We generate 2D films covered with a polymer layer on a growth substrate and execute a transfer of the 2D material using chemicals that etch the growth substrate or capillary forces or bubble formation, followed by removal of the polymer with a chemical solvent. Polymethyl methacrylate is the most often used polymer (PMMA).
Chemical Etchant-Assisted Wet Transfer Method
This approach is used to make devices out of CVD-grown large-area films. As a mechanical support layer, we rotate the PMMA film onto the graphene/copper foil stack's surface. By suspending the PMMA/graphene/metal stack on the surface of the etchant solution, the metal foil is etched away. The PMMA/graphene stack is subsequently cleaned and transferred to the target substrate in deionized water (DI) bath, and the support layer is then dissolved in an acetone bath. Ferric chloride (iii), hydrochloric acid, nitric acid, ferric nitrate (iii), and copper chloride are some of the most frequent etchant solutions.
Fig 1. Polymer-assisted wet transfer methods. (a) A polymer mechanical support layer is applied to the surface of the grown 2D film. (b) Chemical etching transfer. (c) Capillary-driven transfer. (d) Bubble transfer. (e) Chemical removal of the polymer support layer using a solvent bath after transfer to the target substrate. (Schranghamer T. F, et al. 2021)
Etchless Wet Transfer Method
Alfa Chemistry utilizes capillary forces or interfacial bubble formation in an aqueous-based method for etchless transfer of 2D materials.
We first rotate the PMMA onto the 2D film/substrate and leave it overnight. After this, the whole is immersed in hot DI water at an angle of about 45° with respect to the water surface. Due to capillary forces, the water begins to penetrate the film/substrate interface, causing the PMMA/2D film to delaminate from the substrate. The delaminated film floats to the surface and is subsequently transferred to another desired substrate. Finally, acetone is used to remove the support layer.
This transfer method has been used for several different TMDC/substrate combinations. The substrate remains intact during the transfer process due to the lack of chemicals and can be reused multiple times.
Polymer-Free Transfer Methods
Despite the fact that the polymer support layer is dissolved using a solvent at the end of the transfer process, the film still has a large quantity of polymer residue. As a result, we apply various polymer-free transfer methods that avoid the support layer and are etchant-free for damage-free and uniform transfer of 2D materials.
Water-Soluble Layer Transfer Method
Before growing the 2D film, we place a water-soluble sacrificial layer on the growth substrate. After growth, DI water is used to treat the complete film/soluble layer/substrate stack. When the water-soluble layer dissolves, the 2D film is separated from the substrate. After that, the floating film is transferred to the target substrate in the same way as wet transfer is done. This mode of transfer could be a good choice for making TMDC-based energy harvesting devices.
Fig 2. Polymer-free transfer methods. (a) Water-soluble layer transfer. Prior to 2D film growth. (b) Metal-assisted transfer. (Schranghamer T. F, et al. 2021)
Metal Assisted Transfer Methods
We also looked at resist-free metal-assisted transfer methods for the clean transfer of 2D materials because of the stronger adhesion between the metal and TMD and the greater mechanical rigidity of the metal film to give greater stability. The objective is to eliminate polymer contamination by employing metal sheets as a support layer instead of PMMA or other polymers.
We used a copper-assisted transfer method for MoS2 transfer. Thermal evaporation is used to form a thin Cu coating on the grown MoS2. The MoS2 film delaminates from the growing substrate below after a piece of thermal stripping tape (TRT) is lightly pushed into the Cu-coated substrate and peeled off. The TRT/Cu/MoS2 stack is then pushed onto the target substrate and heated to 120°C to separate the TRT from the Cu film. After that, the Cu film is etched in a solution of ammonium persulfate and DI water, and the MoS2 film is cleaned using a modified RCA method.
During the transfer process, the Cu sacrificial layer offers stronger mechanical support for the MoS2 film and decreases wrinkles and strains. In metal-assisted transfers, strong mechanical support layers such as gold and nickel are also utilized.
Reference
- Schranghamer T. F, et al. (2021). "Review and Comparison of Layer Transfer Methods for Two-Dimensional Materials for Emerging Applications." Chem. Soc. Rev. 50: 11032-11054.
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