2, University of Dayton, Dayton, Ohio, United States
3, University of Illinois at Urbana-Champaign, Champaign, Illinois, United States
Flexible and stretchable photodetector devices based on two dimensional (2D) materials have been demonstrated to exhibit a rare combination of excellent optoelectronic performance with the ability to accommodate large amounts of strain during device operation. This unique coupling is enabled by the broad optical absorption in graphene and other 2D systems, quantum confinement of energy carriers in the 2D plane resulting in ultrafast transport dynamics, the van der Waals bonding between the layers, and the enhanced electromechanical properties that arise due to the extreme thinness of the material. Practical realization of flexible 2D materials in these applications is limited by the lack of large area, transfer-free processing schemes that enable the layered materials to be incorporated on soft, organic substrates and allow for bottom-up device fabrication. Wafer-scale photonic crystallization of amorphous precursors to layered 2D materials directly on soft substrates, as demonstrated herein, can facilitate the future 2D flexible electronic and optoelectronics that are easily processed at low temperatures for devices including ultra-thin photodetectors.
High quality growth of 2D materials typically require high temperature growth in the range of 500~1000°C coupled with an epitaxial template to facilitate the thermodynamic reactions in forming non-defective and stoichiometric materials. To overcome the processing limitations for flexible electronics where the substrate typically cannot accommodate for either of these, the 2D layers must either be grown polycrystalline at a much lower substrate temperature, transferred from a rigid substrate to the flexible substrate of interest, or processed by non-thermal annealing techniques in order to achieve similar device performance. Recently, techniques including additive manufacturing have emerged that allow for the transfer of liquid-based 2D inks onto flexible substrates for device manufacturing. While 3D printing provides for patternability and low temperature solution processing, issues with repeatability and film discontinuity require advances before practical devices can be achieved.
Herein, we present a technique that utilizes large area pulsed light source in order to initiate controllable heating and transformation of amorphous ultrathin MoS2 on flexible PDMS. The phase transformation in this case occurs through absorption of the incoming light, resulting in local heating just in the absorbate material and allowing for crystallization of materials to occur at temperatures as high as 390°C. Photonic crystallization has been utilized to process materials including graphene inks, metallic nanoparticles, and other nanomaterials, but has yet to be demonstrated in the phase transformation of 2D materials directly on soft substrates. If optimized, the use of large area photonic annealing technique can unlock new device constructs not achievable through conventional deposition processes.