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Materials having an anisotropic crystal structure often exhibit anisotropy in the electrical conductivity. Compared to complex transition-metal oxides (TMOs), simple TMOs rarely show large anisotropic electrical conductivity due to their simple crystal structure. Here, we focus on the anisotropy in the electrical conductivity of a simple TMO, oxygen-deficient tungsten oxide (WOx) with an anisotropic crystal structure. We fabricated several WOx films by the pulsed laser deposition technique on the lattice-matched (110)-oriented LaAlO3 substrate under a controlled oxygen atmosphere. The crystallographic analyses of the WOx films revealed that highly dense atomic defect tunnels were aligned one-dimensionally (1D) along [001] LaAlO3. The electrical conductivity along the 1D atomic defect tunnels was similar to 5 times larger than that across the tunnels. The present approach, introduction of 1D atomic defect tunnels, might be useful to design simple TMOs exhibiting anisotropic electrical conductivity.

Controlling the oxygen concentration in metal oxides is one of the most effective ways to modulate their optoelectronic properties. However, such redox control is difficult for metal oxide epitaxial films due to serious damages induced in the lattice, especially around the film/substrate interface during the large volume change upon the redox treatment. For overcoming this problem, the use of metal oxides with a stress-resistant crystal structure can be effective. Here, we show a reversible redox control in the optoelectronic properties of hexagonal tungsten oxide (h-WOx) epitaxial films with a honeycomb structure. We fabricated highly c-axis-oriented h-WOx epitaxial films on the (111) yttria-stabilized zirconia (YSZ) single-crystal substrate. Upon electro-chemical redox treatment at 300 degrees C with the application of +/- 3 V to the YSZ solid electrolyte, the oxygen content x of h-WOx was reversibly controlled in the range of 2.93 <= x <= 2.99 without inducing damage to the crystal lattice. Simultaneously, the electrical conductivity was controlled from similar to 400 S cm(-1) to an insulator, and the optical transmission at 1.5 mu m in the wavelength was controlled in the range of 35-70%. The present results would be useful for developing electrochemical optoelectronic devices based on metal oxide epitaxial films.

Controlling the oxygen content can manipulate the electrical conductivity of transition metal oxides (TMOs). Although the superconductor-metal-insulator transition is useful for functional devices, an electrical path must be developed to manipulate the oxygen deficiency (delta) while maintaining the solid state. YBa2Cu3O7-delta (YBCO, 0 <= delta <= 1) is a high transition temperature (T-c) TMO that can be modulated from a superconductor (T-c approximate to 92 K when delta = 0) to an insulator (delta approximate to 1). Here, we show a simple and efficient way to manipulate delta in YBCO films using a solid-state electrochemical redox treatment. Applying a negative voltage injects oxide ions to the YBCO films, increasing T-c. Employing a positive voltage suppresses the superconducting transition and modulates the electrical conductivity. The present results demonstrate that the superconductor-metal-insulator transition of YBCO is modulated electrochemically in the solid state, opening possibilities of superconducting oxide-based device applications.

Materials showing unusual electron and heat conduction such as the coexistence of high electron conduction and low heat conduction are essentially important to realize efficient thermal management systems. Although introducing point defects or layers is known as an effective way to reduce the thermal conductivity, the coexistence of high electron conduction and low heat conduction is still challenging because of the fact that electrons and phonons are scattered simultaneously by impurities, defects, and boundaries. Although oxygen-deficient tungsten oxide (WOx) films (2.7 < x < 3.0) show the desired properties, the origin is still unclear. Here, we report that one-dimensional (1D) atomic defect tunnels give rise to the coexistence of low thermal conductivity and high electrical conductivity of WOx films. We fabricated WOx epitaxial films on LaAlO3 substrates under a precisely controlled oxygen atmosphere. Crystallographic analyses revealed that 1D atomic defect tunnels are formed randomly along the rectangular-shaped grains in the in-plane direction. The cross-plane thermal conductivity of the WOx films dramatically decreased with decreasing x, while the electrical conductivity drastically increased because of an increase of carrier electrons, and high electron conduction and low heat conduction coexist when x < 2.9. The present finding would be useful to design efficient thermal management materials.