2025 Theses Doctoral

Probing graphene heterostructures with atomic force microscopy techniques

Hsieh, Valerie

The ability to control and probe the properties of van der Waals (vdW) heterostructures, particularly twisted graphene-based systems, has become a key area of research in the field of experimental condensed matter physics. These systems exhibit unique electronic and mechanical behaviors which are highly tunable using various parameters such as the relative rotational twist angle between layers, strain, and material combinations. When two layers of vdW materials are stacked with slight rotational misalignment or placed on substrates with differing lattice constants, they form a moiré superlattice which can dramatically alter the material's electronic band structure.

These superlattices can give rise to unique physical phenomena, including correlated electronic states, exotic phases of matter, and unusual transport properties, making the control and characterization of them an area of significant interest in condensed matter physics research. Beyond playing host to an array of tunable correlated electronic phenomena, these moiré systems are also promising for applications within quantum sensing, optoelectronics, and nanostructure engineering. However, much of the research thus far on fundamental physics and applications of moiré phenomena has been conducted under cryogenic conditions, often with high-quality, small-area samples exhibiting low levels of disorder.

This dissertation addresses the challenge of controlling moiré disorder and gaining real-time feedback while characterizing and studying these vdW systems at room temperature, where disorder and its impact on the material properties become more significant. It focuses on the development of novel experimental approaches to manipulate and characterize moiré superlattices in situ, providing new insights into how two-dimensional moiré systems can be controlled and measured. The materials studied primarily consist of graphene and hexagonal boron nitride (hBN), and the characterization was carried out using a variety of atomic force microscopy (AFM) techniques, including lateral force microscopy (LFM), piezoresponse force microscopy (PFM), and conductive AFM (CAFM).

The results underscore the atomic force microscope as an indispensable tool for the characterization of emerging two-dimensional quantum materials. Its versatility in ambient conditions demonstrate it to be powerful in not only controllably straining and manipulating moiré heterostructures, but also in effectively characterizing the complex domain structures within these systems across a range of length scales, capturing the fine details of moiré patterns, strain-relaxation processes, and domain-dependent material behaviors. The ability to study these materials in situ at room temperature opens new possibilities for understanding the relationship between disorder, structure, and electronic behavior in graphene-based moiré systems. This work contributes to the development of new strategies for engineering vdW materials with tailored properties, and offers valuable insights into the potential to correlate the structural variations of the moiré pattern with the macroscopic transport properties of the material.