Dynamical switching of tungsten ditelluride via ion insertion
Keywords: Materials Science and Engineering.

Advances in the dynamic control of material properties are a key component for technological progress in areas such as computation, optics, and actuation. In electrochemical systems, small voltages can reversibly drive the incorporation of guest chemical species into a host solid, modulating its chemical composition and atomic arrangement -- and, in consequence, its functionality. This work examines such electrochemical ion insertion as a pathway for dynamically tuning the structure and properties of 2D layered materials, using the example of lithium intercalated tungsten ditelluride (Li¬xWTe2 with x between 0 and 0.5). First, I describe the realization of on-chip electrochemical platforms that enable precise control over the lithium content in single, microscale LixWTe2 flakes. I demonstrate reliable electrochemical control by comparing electrochemical signatures between bulk and single flake experiments. The achieved dynamical compositional control over individual flakes enables experiments that require thin and single-crystalline samples; further, it demonstrates the feasibility of applications in the field of nanotechnology. These microscale electrochemical platforms are used throughout this work. Second, I show how inserted lithium changes the atomic structure of the WTe2 host lattice by inducing a first order phase transition. I describe the techniques of single crystal x-ray diffraction, powder x-ray diffraction, Rietveld refinement, and density functional theory used to determine the structural changes upon lithium insertion with atomistic precision. Based on these experimental and theoretical techniques, I confirm the lithium-induced transition to a novel structural phase with an exotic, highly anisotropic, atomic arrangement. This constitutes the discovery of a new polytype in the material system of transition metal dichalcogenides, which I term the Td' polytype. Third, I present the structural evolution of this novel phase during lithium insertion and removal. I describe the use of operando single-crystalline x-ray diffraction for investigating the character of a phase transition as well as for tracking of phase fractions and lattice parameters, as lithium is entering and leaving the host lattice. I show that the most dramatic structural changes occur as part of a first order phase transition. Further, I show that the lattice of the novel Td' phase can be extensively modulated by small changes to its lithium content, enabling a unique form of highly efficient uniaxial actuation. These lattice changes in the Td' solid solution correspond to the largest in-plane chemical expansion coefficient ever reported in a single-phase material and show a high degree of energy efficiency. Fourth, I present how the vibrational properties of LixWTe2 are reversibly altered, as lithium is inserted and removed. I describe the use of in situ ultrafast electron diffraction in a pump-probe regime with laser pulses to elucidate the atomistic changes to lattice vibrations, as well as Raman spectroscopy to measure changes to the overall vibrational spectrum. The altered vibrational properties of WTe2 have implications for its thermal conductivity. Finally, I quantify the kinetic limits of the lithium-induced dynamical modulations of WTe2. I describe the use of synchronized electrochemical and x-ray diffraction measurements combined with finite difference calculations to quantify the diffusion coefficient of lithium in Td'-WTe2. I also describe the use of Nudged Elastic Band density functional theory calculations to study the trajectory of lithium hopping within the WTe2 host lattice. I elaborate on the atomistic origins of the lithium transport in Td'-WTe2. The observed kinetics enable the application of ion insertion for dynamical switching on technologically relevant timescales