2020 Theses Doctoral

Engineering and Probing Two-dimensional Materials and Heterostructures

Zhang, Changjian

In this thesis, the development of a new technique to produce dynamically twistable van der Waals heterostructures with tunable interlayer rotational angle is introduced in details. Such devices offer great controllability of the lattice orientations in van der Waals heterostructures and in particular enabled us to study moiré superlattices at different twist angles in a single device. Encapsulated graphene/hBN moiré superlattice devices were used to demonstrate the technique. Microscopic Raman spectrum, electrical transport and interlayer mechanical resistance were measured in the devices. Results were found consistent with previous studies in multiple samples with fixed twist angles. New observations benefiting from the elimination of sample-to-sample variance were also made on the transport gap sizes, satellite peak asymmetry, periodic interlayer friction and Raman peak position of graphene/hBN moiré superlattices. In addition, great efforts of making dynamically twistable devices with thin hBN handles for near-field optical spectroscopy were made. Ultrathin hBN handles were able to move on etched graphene. Two ways of making graphite split gate were described to make dynamically twistable devices with split gate. Besides these, a few other things used throughout the research were also introduced such as growth of aligned and suspended carbon nanotubes and marking their positions using p-nitrobenzoic acid.