Engen, Paige2023-09-192023-09-192023-05https://hdl.handle.net/11299/257007University of Minnesota Ph.D. dissertation. May 2023. Major: Material Science and Engineering. Advisor: David Flannigan. 1 computer file (PDF); xxvii, 168 pages.The 1T phase of tantalum disulfide (1T-TaS2) has a rich and complex charge density wave (CDW) phase character that has been broadly studied. CDWs are a highly correlated state between electron density and lattice distortion, originally predicted to only exist in one dimension. The discovery of CDWs in two-dimensional systems spiked interest in uncovering the various phases and phase interactions. Understanding the phase interactions is of fundamental interest to better comprehend the nature of complicated CDW behaviors. 1T-TaS2 has three thermally-dependent primary CDW phases that show increased commensurability to the underlying lattice with decreasing temperature. This dissertation focuses on the room temperature nearly-commensurate (NC) phase, along with some investigation into the high temperature incommensurate (IC) phase. The work presented in this dissertation starts to elucidate the CDW stability using high-resolution and in situ transition electron microscopy (TEM).Direct observation of the CDWs allows for characterizing the interaction between the superlattice and the underlying lattice, including topological defects. Various methods have been used to visualize the CDW supercells, lattice distortion, and domain structure. However, many of these techniques lack atomic resolution, require additional image processing, or complicate the direct observation of long-range order. Here, high-resolution, bright-field TEM was used to directly observe CDW-associated moiré contrast. This methodology allows for atomic resolution of the tantalum atoms and identification of long-range CDW superlattice ordering. Investigation into the superlattice moiré contrast revealed a variety of edge dislocation topological defects within the CDW phase. Dislocations in the CDW superlattice are poorly understood, and the work presented in this dissertation is one of the first reports of edge dislocation-like topological defects. Determination of the underlying cause of the topological defect remains elusive, but preliminary analysis of the defect stability is explored. The stability and dynamics of the three primary phases have been extensively studied to extract thermal and photoexcited transitions. Defects and various specimen morphologies have been shown to destabilize and suppress the CDW phases. However, much of the previous work used techniques that spatially average out the interactions and lose the precise morphological interactions. Regionally-selective NC-IC phase transitions were investigated to determine the role of higher-order structural defects, such as crystalline step edges and ripplocations, on CDW stability. The phase transition temperature is directly related to the stability of the CDW in the various regions. Decreases in the transition temperature are correlated with a reduction in phase stability. It is shown that local variations in the identity and concentration of higher-dimensional defects will modify the NC-IC phase transition temperatures within a single specimen. It is clear that defects' local effects directly influence the CDW phase behavior and have interesting implications for understanding CDW dynamics.enThesis or Dissertation