Reversible thermal conductivity in epitaxial WO3 films

PI: Caroline Ross

The ability to manipulate the thermal transport in materials is of great interest from both technological and fundamental points of view. We demonstrate how thermal conductivity in epitaxial WO3 thin films can be dynamically controlled by a factor of ~1.7 via electrolyte gating, or can be tuned over a larger range, from 7.8 to 1.1 Wm-1K-1, by adjusting the oxygen stoichiometry during film growth.

Moreover, the electrical conductivity can be also controlled independently of the thermal conductivity, enriching the potential utility of WO3 and shedding light on the design of new materials for thermoelectrics or thermal management. The unit-cell volume, instead of the defect concentration, plays the dominant role in determining the lattice thermal conductivity. Point defects, for instance intercalated hydrogen, can actually increase the lattice thermal conductivity, in contrast to the expectation that lattice defects scatter phonons and therefore lower the thermal conductivity.

WO3 is a distorted perovskite structure in which the W6+ ions occupy the centers of oxygen octahedra. Films were grown with different oxygen vacancy concentrations by adjusting growth conditions, and the oxygen vacancy and hydrogen interstitial concentrations were adjusted by ionic liquid gating (left). On the right, the thermal conductivity kL correlates with unit cell volume for all the samples, however prepared.


This finding motivates a reexamination of the mechanisms that dominate the phonon transport in epitaxial films and provides a new direction for tuning the thermal conductivity in solid-state materials.

Moreover, the independent sensitivities of the thermal and electrical conductivities to the lattice dimensions and to oxygen vacancies and hydrogen enables selective control over both conductivities within broad ranges. The tunability of properties shown in WO3 suggests strategies for development of other materials for thermoelectrics or thermal management.

Researchers in the group took part in outreach activities such as the NanoObservatory.

Students and faculty participated in the NanoObservatory, a public outreach program during Cambridge Science Week.