Ultra-clean Dry Transfer And Nanolithography Of Two-dimensional Materials And Layered Heterostructures

Research of two dimensional materials has been one of the most important branches of condensed matter physics since the discovery of graphene in 2004.Recently,layered heterostructures composited of different 2D materials fabricated by mechanical assembly are growing as a novel system to explore exotic physical phenomena.Mechanical transfer of ultra-clean 2D heterostructures and nano-fabrication of high quality nanostructure of 2D materials are crucial steps for such studies.In this thesis:1.We developed an all-dry mechanical transfer method for transferring ultraclean and non-encapsulated graphene samples by employing the superlubricity in graphene/hexagonal boron nitride interface.Transferring high-quality exfoliated graphene flakes onto different substrates while keeping the graphene free of polymer residues is of great importance,but at the same time very challenging.Currently,the only feasible way is the so-called all-dry “pickand-lift” method,in which a hexagonal boron nitride(hBN)flake is employed to serve as a stamp to pick up graphene from one substrate and to lift it down onto another substrate.The transferred graphene samples,however,are always covered or encapsulated by hBN flakes,which leads to difficulties in further characterizations.We improved “pick-and-lift” method,which allows ultra-clean graphene flakes to be transferred onto a variety of substrates without hBN coverage.Basically,by exploiting the superlubricity at the graphene/hBN stack interface,we are able to remove the toplayer hBN stamp by applying a tangential force and expose the underneath graphene.2.We developed an electrode-free local anodic oxidation(LAO)technique,which enables in-situ patterning of as-prepared low-dimensional materials and heterostructures with great flexibility and high precision.Scanning probe lithography based on LAO provides a robust and general nanolithography tool for a wide range of applications.Its practical use,however,has been strongly hampered due to the requirement of a prefabricated microelectrode to conduct the driving electrical current.Unlike conventional LAO driven by a direct current,the developed electrode-free LAO is driven by a high-frequency(>10 kHz)alternating current applied through capacitive coupling,which eliminates the need of a contacting electrode.Using this technique,we demonstrated flexible nanolithography of graphene,hexagonal boron nitride,and carbon nanotubes on insulating substrates with ? 10 nm precision.Such an in situ and electrode-free nanolithography with high etching quality opens up new opportunities for fabricating ultraclean nanoscale devices and heterostructures with great flexibility.