Anomalous Defect Dependence of Thermal Conductivity in Epitaxial WO₃ Thin Films

Abstract

According to the Boltzmann transport theory of phonons, the lattice thermal conductivity (κ L ) depends mainly on structure-related parameters including the specific heat, the velocity, and the scattering of phonons. One common strategy for the manipulation of phonon transport has been to introduce lattice imperfections in materials which alters the mean free path of phonons, particularly for applications in thermoelectric energy conversion. [1] Based on the well-established Abeles theory of interactions between phonons and defects in alloys or semiconductors, [2] point defects, including substitutionals, [3] interstitials, [4] and vacancies, [5] are considered to cause additional scattering and therefore lower κ L due to the mass differences and local strain that they introduce. However, in addition to localized mass and strain effects, point defects can also yield significant structural changes, particularly in complex oxides. [6][7][8][9][10] In this case, the role that defects play in the phonon transport could be more subtle, which merits re-evaluation of the typical expectation that defects lower the thermal conductivity.In this work, we focus on epitaxial tungsten trioxide (WO 3 ), which exhibits an ABO 3 perovskite structure with the A-sites vacant and can accommodate a range of lattice distortions via octahedral tilts and rotations. [11][12][13] We find that the roomtemperature (RT) thermal conductivity of the WO 3 thin films evolves significantly upon electrolyte gating, accompanied by reversible structure changes caused by the intercalation or deintercalation of hydrogen (H i ), enabling a reversible control of thermal conductivity. The modulation in thermal conductivity is comparable with that reported in LiCoO 2 [14] and MoS 2 [15] by electrochemically tuning the degree of lithiation, but works in a much simpler configuration. What is more, the variation of thermal conductivity is highly dependent on the substrate on which the epitaxial WO 3 thin films are grown. An unusual increase of κ L is observed after gating with the intercalation of hydrogen, which contradicts the expectation that point defects cause additional thermal resistance and therefore reduce κ L . [16] We further investigate how another common type of point defect, the oxygen vacancy (V O ), influences the lattice structure and κ L in WO 3 thin films. The results collectively indicate that the dominant factor determining κ L is the lattice dimension, i.e., the unit-cell volume. κ L increases as the lattice contracts and Lattice defects typically reduce lattice thermal conductivity, which has been widely exploited in applications such as thermoelectric energy conversion. Here, an anomalous dependence of the lattice thermal conductivity on point defects is demonstrated in epitaxial WO 3 thin films. Depending on the substrate, the lattice of epitaxial WO 3 expands or contracts as protons are intercalated by electrolyte gating or oxygen vacancies are introduced by adjusting growth conditions. Surprisingly, the observed lattice volume, ins...

Coexistence of High Electron Conduction and Low Heat Conduction in Tungsten Oxide Epitaxial Films with 1D Atomic Defect Tunnels

“…4(b)]. Very similar decreasing tendency is also seen in the κ of WOx epitaxial films grown on LaAlO3, which is reported by Ning et al 21 The κ of WO2.97 was ~7 W m −1 K −1 and it decreased to ~2 W m −1 K −1 with decreasing x to 2.92, then gently decreased with decreasing x (Figs. S3 and S4).…”

“…WOx can be epitaxially grown on perovskite oxide single crystals coherently. [17][18][19][20][21] Therefore, we expected that WOx with 1D tunnel structure can be stabilized on a substrate. In fact, Ning et al 21 reported that the thermal conductivity of WOx films grown on (001) LaAlO3 substrate by modulating the oxygen atmosphere during the growth decreased dramatically with decreasing the x whereas the electrical conductivity increased oppositely.…”

“…To verify this hypothesis experimentally, we selected oxygen-deficient tungsten oxide (WO x , 2.7 ≤ x ≤ 3) epitaxial films as the candidate. The bulk crystal structure of oxygen-deficient WO x contains a 1D tunnel structure. , WO x can be epitaxially grown on perovskite oxide single crystals coherently. − Therefore, we expected that WO x with the 1D tunnel structure can be stabilized on a substrate. In fact, Ning et al reported that the thermal conductivity of WO x films grown on the (001) LaAlO 3 substrate by modulating the oxygen atmosphere during the growth decreased dramatically with decreasing x , whereas the electrical conductivity increased oppositely.…”

“…The bulk crystal structure of oxygen-deficient WO x contains a 1D tunnel structure. , WO x can be epitaxially grown on perovskite oxide single crystals coherently. − Therefore, we expected that WO x with the 1D tunnel structure can be stabilized on a substrate. In fact, Ning et al reported that the thermal conductivity of WO x films grown on the (001) LaAlO 3 substrate by modulating the oxygen atmosphere during the growth decreased dramatically with decreasing x , whereas the electrical conductivity increased oppositely. They also showed that the thermal conductivity of WO x decreased with increasing electrical conductivity.…”

Voltage Control of Magnetism above Room Temperature in Epitaxial SrCo1–xFexO3−δ

“…Compared with polycrystalline oxide films ( e.g ., Co 3 O 4 ), in which the response is slow even with high voltages, epitaxial complex oxides are of greater interest. This arises from two main reasons: The ordered channels present in the crystal lattice allow easier migrations of oxygen or hydrogen ions, and the strong correlations among the lattice, spin, charge and orbitals enable a concomitant control of various properties besides magnetic, for example, optical, electrical, and thermal. ,− Many strongly correlated oxides exhibit fascinating topotactic phase transitions, ,,, but obtaining reversible control of the magnetic moment above room temperature in such ordered oxides has remained challenging.…”