Achieving Ultraclean Graphene: Characterization and Applications

Maintaining the cleanliness of graphene remains one of the most critical challenges for realizing its full potential for advanced electronic and quantum applications. Contaminants introduced during synthesis, transfer, and handling can obscure intrinsic properties and hinder atomic-scale characterization. We approach the goal of making graphene ultra-clean through the development of polymer-free transfer and in-situ radical-cleaning techniques. Two complementary transfer methods were established. A polymer-free exfoliation technique using Ni/In/Cu stacks was developed to transfer epitaxial graphene grown on SiC substrates without introducing organic residues. In contrast, a metal-assisted transfer approach employing a thin Pd interlayer was demonstrated for CVD-grown graphene, enabling polymer-free transfer onto SiO₂ and other robust substrates while preserving surface integrity. Both methods eliminated polymer contamination and, in the case of epitaxial graphene, simultaneously removed surface adlayers during transfer. To further enhance cleanliness, an in-situ electron-beam-induced radiolysis process was developed, in which water radicals generated within the transmission electron microscope react with hydrocarbon residues to form volatile products. This process effectively removed organic contaminants without damaging the graphene lattice, yielding atomically clean regions hundreds of nanometers wide. Characterization using scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy (EELS) confirmed the exceptional cleanliness and structural integrity of the resulting graphene. Finally, ultra-clean graphene was explored as an interfacial layer for remote epitaxy on GaSb substrates, demonstrating the feasibility of transferring clean graphene to chemically sensitive materials. The combination of polymer-free transfer, metal-assisted exfoliation, and in-situ radical cleaning provides a scalable, reproducible route to fabricate ultra-clean graphene suitable for next-generation electronic and quantum device applications.