In particular, the presence of a 2D layer in the semiconductor/2D-material heterostructure can lead to integrating such heterostructure devices on foreign 2D materials through van der Waals interactions. Various semiconductor nanostructures and microstructures have been successfully fabricated on 2D layered materials exhibiting their flexibility and wearability, similar to 2D devices 9, 10. Semiconductor heteroepitaxy ensures the monolithic fabrication of sophisticated device structures to achieve high device performance 8. However, most of these issues can be resolved by fabricating semiconductor/2D-material heterostructures. To fabricate a sophisticated device, such as light-emitting diodes (LEDs) comprising multiple stacks of quantum wells, barriers, and n-type and p-type doped layers, numerous and time-consuming transfer steps would be required to build the device structure. In addition, the lack of chemical reactivity on the 2D film surface makes it difficult to grow sequential structures of different compositions of 2D layers. Since it is difficult to satisfy all the demanded devices only by 2D materials, the integration process can be restricted for specific device applications. Accordingly, the van der Waals integration facilitates the combination of heterogeneous devices with minor compatibility issues.Äespite the promising features of van der Waals integration, however, there are still some limitations in the integration approach that need to be overcome. Furthermore, the fabrication process involves simple wet or dry transfer techniques 6, 7. The van der Waals interaction among the 2D films enables the stacking of different kinds of 2D layers on top of each other, while various 2D layered materials have become available for utilization in electronic and optoelectronic device applications, such as light-emitting devices, field-effect transistors, photodetectors, photovoltaics, energy storage, and biosensors 3, 4, 5. Two-dimensional (2D) van der Waals heterostructures have gained significant importance in fabricating multifunctional devices because of their ability to integrate heterogeneous 2D materials with a compact form factor 1, 2. Furthermore, the reliable adhesiveness allowed us to achieve device wearability, while the LEDs exhibited homogeneous light emissions under various bending conditions because of negligible external stress in the discrete micro-LEDs. The transferred micro-LEDs showed well-aligned crystallographic orientations as well as reliable device performances, including strong light emissions, good rectifying behaviors of the current density–voltage curve, and good simultaneity between the electroluminescence intensity and the applied currents, ensuring reliable electrical connections and mechanical adhesions of the light-emitting layer to the foreign graphene films.
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After device fabrication, the LED/graphene heterostructure device was piled on the foreign graphene layers using a typical wet transfer technique of 2D crystals where the bottom graphene layer of the heterostructure was adhered to the foreign graphene only by van der Waals interactions. GaN micro-LEDs were selectively grown on a graphene substrate using a patterned SiO 2 mask, and then the whole device structure was laterally fixed by a polyimide insulator to form a united layer. We report the van der Waals integration of micropatterned GaN light-emitting diodes (LEDs) onto foreign graphene films.