Enhancing Device Performance of Field-Effect Transistors Based on 2D Materials Using Nanostructured Substrates via Block Copolymers Self-Assembly

Two-dimensional (2D) materials are attractive for use in next-generation electronic devices due to their atomically thin structures, high flexibility/transparency, and unique electrical/optical properties. However, despite their vast potential, the practical application of 2D materials devices presents numerous challenges, particularly the substrate effect issue that directly deteriorates device performance. The substrate effect refers to the interactions with the interfacial area between the device substrate and the atomic layer structure of 2D materials. This substrate effect leads to significant performance degradation and poses a major hurdle for applications of 2D materials-based devices. Previous research has proposed to decrease the substrate effect, such as fabricating 2D material devices without substrates, but this severely compromises the mechanical stability of the devices, making them difficult to utilize effectively.<br/><br/>In this study, we employed self-assembly of block copolymers to implement nanostructures on the substrate surface at scales of nanometers, thereby achieving partially supported 2D materials. We fabricated a polystyrene-<i>block</i>-poly(methylmethacrylate) diblock copolymers thin film and induced self-assembly into cylindrical nanostructures through thermal treatment. Using this film as a mask in micro-electromechanical systems (MEMS) processes, we fabricated the substrates with nanostructures on their surfaces, demonstrating partially supported graphene field-effect transistors (FETs). Our strategy enabled us to suspend the channels of the devices in a regular nano-scale manner, preventing channel collapse and ensuring excellent stability in terms of mechanical properties. Minimizing extrinsic scattering of surface phonons using nanostructured substrates, we successfully demonstrated high performance FETs with up to three times higher field-effect mobility than transistors fabricated on conventional flat substrates.<br/><br/>Furthermore, we controlled the size of the nanostructures (diameter = 30~40 nm) on the substrate surface and analyzed their effect on the 2D materials in terms of contact area. Graphene transferred to flat substrates exhibited p-doped properties due to contact doping with the substrate, while graphene transferred to nanopatterned substrates retained its intrinsic properties due to partial suspension. Considering that graphene on nanostructured substrates also showed relatively n-doped characteristics compared to those on flat substrates, it is evident that the geometry of the substrates significantly influences the doping level of graphene.<br/><br/>In conclusion, we addressed the problem of performance degradation caused by the adverse interactions between substrates and 2D materials using the block copolymers nanopatterning process. The implementation of partially supported 2D materials-based FETs using BCP lithography that decreases substrate effect ensures high mechanical stability and device performance. Moreover, FETs on nanostructured substrates exhibited higher mobility than devices on flat substrates, attributed to reduced charge impurities and extrinsic scattering. Therefore, this strategy can contribute to enhance the device performance of next-generation electronic applications based on 2D materials.