Epitaxial Growth and Electronic Structure of Sub-Stoichiometric WO_x Thin Films with Tunable Atomic Defect Tunnels

Sub-stoichiometric tungsten oxides (WO_x, x= 2.625 to 2.92), also known as Magnéli phases, exhibit unique physiochemical properties due to structural deviations from the pseudocubic WO₃ structure. These deviations, characterized by the incorporation of additional edge-sharing WO₆ octahedra, lead to the formation of complex defect structures and tunnel-like pathways. The unique and highly anisotropic arrangement of the WO₆ octahedra not only introduces additional conduction channels for charge carriers but also affects their overall electronic structure, contributing to the diverse and tunable electronic, optical, and electrochemical properties observed in Magnéli WO_x compounds.
In this study, we successfully synthesized a series of high-quality, epitaxial WO_x films with tunable one-dimensional aligned atomic-scale defect tunnels on (110)-oriented LaAlO₃ substrates using pulsed laser deposition. By adjusting growth temperature and oxygen partial pressure, we systematically controlled the type and density of defects in the WO_x films. Notably, in-plane strain played a critical role in the growth of Magnéli phases on LAO substrates. The lattice mismatch between the WO_x film and the LAO substrate induces strain along one direction, while relaxation occurs along the other direction. Scanning transmission electron microscopy (STEM) studies confirmed the presence of W₁₈O₄₉-like phases with highly oriented hexagonal defect tunnels along LAO [001] orientation. Using combined X-ray photoemission spectroscopy and X-ray absorption spectroscopy, we revealed significant influences of oxygen vacancies and lattice distortions on the electronic structure of WO_x. Specifically, oxygen vacancies in WO_x resulted in the partial reduction of W⁶⁺ to W⁵⁺ and W⁴⁺, introducing 5d electrons at the bottom of the conduction band and contributing to its metallic nature. Defect-induced lattice distortions led to the localization of electrons at partial W sites, which contributed to optical absorptions at lower photon energies. Further density functional theory (DFT)calculations revealed the atomic distribution of electronic states at W atoms with different chemical environment. Our findings provide valuable insights into the relationship between the intrinsic structural characteristics and the opto-electronic properties of Magnéli WO_x.