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Air-stable layered semiconductor Bi2O2Se has attracted extensive interest recently for applications in electronics, optoelectronics, ferroelectrics, and thermoelectrics. For many of these applications, thermal transport in Bi2O2Se is of great importance but the understanding remains elusive. Here, we perform a combined experimental and theoretical study on the anisotropic thermal conductivity of single-crystalline Bi2O2Se in comparison with its parent compound Bi2Se3 over the temperature range of 80-300 K using the time-domain thermoreflectance measurements and ab initio phonon Boltzmann transport calculations. Compared with Bi2Se3, Bi2O2Se exhibits relatively higher thermal conductivity along the through-plane direction but it is lower along the in-plane direction, resulting in substantially smaller thermal anisotropy. We find the smaller thermal anisotropy of Bi2O2Se mainly originates from its stronger interlayer electrostatic interaction compared to the typical van der Waals coupling in layered materials, which makes the phonon isoenergy surfaces less anisotropic and thus weakens phonon focusing along the in-plane directions. Our study advances the fundamental understanding of thermal anisotropy in layered materials with various interlayer interactions and will facilitate the applications of Bi2O2Se in electronics and thermoelectrics.

Summary The air-stable layered semiconductor Bi2O2Se has recently attracted extensive interest because of its potential application in electronics, optoelectronics, ferroelectrics, and thermoelectrics. For many of these applications, thermal transport in Bi2O2Se is of great importance, but a complete understanding of the process remains elusive. Here, we perform a combined experimental and theoretical study of the anisotropic thermal conductivity of single-crystalline Bi2O2Se in comparison with Bi2Se3. Bi2O2Se exhibits relatively higher through-plane thermal conductivity but lower in-plane thermal conductivity, resulting in substantially smaller thermal anisotropy. This behavior originates from the stronger interlayer electrostatic interaction in Bi2O2Se compared with the typical van der Waals coupling in layered materials, making the phonon isoenergy surfaces less anisotropic and, thus, weakening phonon focusing in the in-plane directions. Our study advances the fundamental understanding of thermal anisotropy in layered materials with various interlayer interactions and will facilitate application of Bi2O2Se in electronics and thermoelectrics.

Super-aligned carbon nanotube (CNT) films have intriguing anisotropic thermal transport properties due to the anisotropic nature of individual nanotubes and the important role of nanotube alignment. The exact relation between the alignment and the anisotropic thermal conductivities, however, was not well understood due to the challenges in both the preparation of high-quality super-aligned CNT samples and the thermal characterization of highly anisotropic and porous thin films. Here, super-aligned CNT films with different alignment configurations are fabricated and their anisotropic thermal conductivities are measured using an elliptical-beam approach of time-domain thermoreflectance (TDTR). The results suggest that the alignment configuration can tune the cross-plane thermal conductivity k z from 6.4 to 1.5 W/(mK), and the in-plane anisotropic ratio from 1.2 to 13.5. This work confirms the important role of CNT alignment in tuning the thermal transport properties of super-aligned CNT films and provides an efficient way to design thermally anisotropic films for thermal management.