UC Berkeley

This thesis describes the synthesis and fabrication of various two-dimensional (2D) crystals and heterostructures that exhibit long-range charge and spin ordering. By combining intercalation with the precise design of 2D interfaces, we tune and engender targeted magnetic and charge density wave (CDW) phases in 2D crystals. Furthermore, by marrying transmission electron microscopy (TEM) imaging, optical probes, and electron transport measurements, we establish robust structure-property relationships that may guide the development of 2D crystals for miniaturized electronic devices.

Chapter 1 presents a broad overview of the field of 2D magnets and CDWs. We discuss the fundamental hindrances in realizing magnetism in the atomically thin limit and build upon these insights to outline the design principles for 2D magnets. Further, intercalated transition metal dichalcogenides (TMDs) are introduced as promising material platforms for realizing 2D crystals with long-range spin order. This chapter also provides insights into the mechanisms underlying CDW formation through an orbital-based approach. Lastly, we give a brief introduction to the CDW properties of 1T-TaS2, a layered material that is extensively studied in this thesis.

Chapter 2 describes the first synthesis and magnetic characterization of iron-intercalated tantalum disulfide (FexTaS2) in the two-dimensional (2D) limit. The synthesized 2D FexTaS2 crystals exhibit hard ferromagnetic behavior down to the thinnest limit of this intercalation compound (Fe-intercalated bilayer TaS2). Magnetic properties of this system are highly tunable and depend on the interplay between magnetocrystalline anisotropy, intercalation amount, homogeneity, and dimensionality.

Chapter 3 describes a synthetic framework for realizing heterostructures comprising intercalation compounds of TMDs through directed topotactic intercalation of van der Waals (vdW) assembled layers. The developed topochemistry method enables the creation of previously inaccessible magnetic TMD heterostructures that demonstrate strong interfacial magnetic exchange effects only possible with atomically clean heterointerfaces.

Chapter 4 describes the topotactic transformation of nanothick 1T-TaS2 crystals into verti-lateral heterostructures of 1T-TaS2 and H-TaS2. These polytype heterostructures exhibit reliable multi-step resistance and chirality switching behavior encoded by the number of H-TaS2 layers. The potential of strain engineering for realizing multifunctional phase change materials is also highlighted.

Chapter 5 describes the synthesis of Fe-intercalated polytype heterostructures comprising 1T-TaS2 and H-TaS2. In these materials, modulating the intercalant amounts leads to simultaneous changes in long-range spin and charge ordering, which coexist in these materials. This chapter explores the competition and coexistence of structural and electronic phases, offering insights into designing multifunctional materials.

Chapter 6 describes the electronic and optical properties of moiré heterostructures comprising 2D 1T-TaS2 and monolayer 1H-WSe2. Interfacial charge transfer and moiré strain encode the global CDW ordering in 1T-TaS2 while simultaneously modulating the optoelectronic behavior of 1H-WSe2. This study outlines how local perturbations can be engineered to design ensemble electronic behavior.

Chapter 7 provides a brief summary of the tuning knobs used in this thesis and outlines possible avenues for future research.