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Richard P. Feynman once said "when we have some control of the arrangement of things on a small scale we will get an enormously greater range of possible properties that substances can have, and of different things that we can do". This foresight has now been realized in the regime of two-dimensional (2D) materials, which could be artificially assembled into van der Waals (vdW) heterostructures with on-demand properties. The development of ingenious experimental techniques has created a new paradigm for electronic devices with dramatic performance and enabled exploration of novel physical phenomena. This work is dedicated to advanced vdW technologies for probing novel electronic properties of graphene based heterostructures. The work consists of two themes. In the first theme, we focus on rhombohedral graphite, which is rarely studied yet with intrinsic fascinating electronic properties. The lack of control of the stacking sequence limits most research to the more stable Bernal form of graphite. This work presents the directional encapsulation of rhombohedral graphite crystallites with hexagonal boron nitride (hBN) along the graphite zigzag edges. Facilitated by this improved vdW technique, we obtained high-quality rhombohedral graphite devices and studied their transport properties. We observed the gap opening and quantum Hall effect of the topologically protected surface states. In the quantum Hall regime, rhombohedral graphite exhibits transition between a gapless semimetallic phase and a gapped quantum spin Hall phase with giant Berry curvature. For thinner graphite (< 4nm), an insulating state at the neutrality point emerges with pronounced hysteretic behaviour in both electric and magnetic fields, which we attribute to strong electronic correlations. In the second theme, we look into the increasingly hot research topic, the twistronics, where twist angle between successive layers in a heterostructure offers a new degree of freedom to tune its electronic properties, through the modification of moiré superlattice. This work demonstrates an experimental technique that can achieve in situ dynamical rotation and manipulation of 2D materials in vdW heterostructures, thus realizing control of twist angle. The technique is mediated by a polymer patch, which is easy to operate and reproduce. Using this technique, we fabricated a heterostructure where graphene is subjected to double moiré superlattice at the two interfaces. We observed the evidence of the formation of super-moiré structures, which proves the perfect alignment of graphene with its encapsulating layer hBN and the effectiveness of the technique.

Topological insulator (TI) materials do not conduct electricity in the bulk, but instead conduct along the surface via topological surface states (TSS). To fully utilize these materials, it is critically important to understand their surface atomic and electronic structures, both of which can be sensitively probed by low energy ion scattering (LEIS). Bi2Se3 is a TI that consists of stacked quintuple layers ordered as Se-Bi-Se-Bi-Se. The surfaces are expected to show a Se-termination after mechanical cleaving, but instead various surface terminations are found. Time-of-flight (TOF) LEIS spectra show that Bi2Se3 surfaces prepared by in situ cleaving or by Ar+ ion bombardment and annealing (IBA) are always Se-terminated. Impact collision ion scattering spectroscopy (ICISS) finds no differences in the geometric structure of the top three atomic layers of IBA-prepared surfaces and in situ cleaved surfaces. Note that the annealing temperature is critical in preparing surfaces with IBA. If it’s too low, then surface will not be well-ordered. If it’s too high, then surface Se vacancies will form. Ex situ cleaved Bi2Se3 surfaces can be either Se-terminated or Bi-rich, which might be caused by contaminants that adsorb at surface defects and induce a chemical reaction. After exposure to air, IBA-prepared surfaces show less surface contamination than cleaved surfaces and maintain their Se-termination. Cs is adsorbed on Bi2Se3 as a tool to investigate the surface electronic properties of TIs. Scattered alkali ions exchange charge with surfaces through a non-adiabatic resonant charge transfer (RCT) process that depends on the local electrostatic potential (LEP) above the target atom. It is found that the neutralization probability for Na+ scattered from Se sites is unexpectedly larger than from Bi sites. This can be explained by the spatial distribution of the TSS carriers in which the electron density accumulates below the first layer Se atoms and above the second layer Bi. This spatial distribution leads to an upward pointing dipole on Se sites that reduces the LEP and a downward dipole above Bi that increases the LEP.