UC Berkeley

Charge density waves (CDWs) are periodic distortions in a crystal lattice which arise due to the instability of low-dimensional systems, including two-dimensional (2D) van der Waals (vdW) materials. 2D materials can be tuned through a variety of techniques: the creation of vdW heterostructures by stacking together different 2D materials can finely tune properties of the constituent materials and lead to novel behaviors; and electrochemical intercalation, whereby ions are inserted into the vdW gap, can allowed for controlled tuning of a material’s charge carrier density. Both of these techniques for modifying 2D materials offer potential routes towards manipulation of their emergent CDW phases.Chapter 1 of this dissertation contextualizes the study of CDW materials in terms of broader motivation, theoretical background, and a brief literature review. The integration of CDW materials into computer hardware could someday offer a solution to challenges in computing efficiency. Additionally, manipulation of the correlated electronic phenomena observed in CDW materials could also offer a route to high temperature superconductivity. The mechanisms underlying CDW formation include electron-electron coupling and electron-phonon coupling, which lead to periodic atomic displacements. CDWs can be seen in a variety of 1D and 2D materials, and external perturbations to these materials can be used to manipulate and suppress CDW transitions. Chapter 2 discusses the creation of van der Waals heterostructures which interface the CDW material 1T-TaS2 and and the non-CDW material 1H-WSe2. Temperature-dependent electronic transport measurements show a suppression of the CDW transition temperature in 1H-WSe2/1T-TaS2 heterostructures compared to 1T-TaS2 and reveal the TaS2 thickness- dependence of this effect. Raman spectroscopy measurements demonstrate that interfacial interactions in 1T-TaS2/1H-WSe2 lead to a significant decrease in intensity of signal from vibrational modes associated with the CDW lattice of 1T-TaS2, as well as non-uniform shifts in vibrational frequencies in sufficiently thin samples. Along with Raman spectroscopy, photoluminescence spectroscopy is carried out with the aim of uncovering the mechanisms behind this CDW suppression, and indicates that charge transfer between the materials likely occurs; however, the exact nature of the interfacial interaction that leads to CDW suppression remains somewhat elusive. Chapter 3 explores the modification of rare-earth tritelluride materials RTe3, particularly HoTe3 by electrochemical intercalation. HoTe3 is synthesized, and characterization by Raman spectroscopy and electronic transport corroborate previous studies of RTe3 materials. Electrochemical studies are carried out, allowing the intercalation of HoTe3 by Li+ to be observed in-situ by optical microscopy, revealing a pattern of several distinct visual changes that occur in HoTe3 flakes upon intercalation. A concurrent optical microscopy and electronic transport measurement during intercalation suggests that these visual stages are accompanied by a transition to an increasingly insulating phase and/or possible sample degradation. Together, these chapters demonstrate two distinct methods to modify 2D CDW materials and the use of multiple experimental techniques to analyze the effects of these modifications. The studies presented here contribute to a broader understanding of tunable properties in 2D CDW materials.