Controlling the thermal conductivity of metal halide perovskites is of significance to promising applications ranging from optoelectronic devices to heat storage and conversion devices. Herein, we carry out the thermal management of bismuth halide perovskite. As measured by time-domain thermo-reflectance, the Cs3Bi2I9 thin film possesses a thermal conductivity of 0.15 W m–1 K–1 that is lower than that of the BiI3 thin film with a thermal conductivity of 0.31 W m–1 K–1. We attribute the differential thermal conductivity to the stronger phonon scattering of Cs3Bi2I9 at low frequencies as compared to that of BiI3. In addition, we fabricated photothermal detectors by employing BiI3 and Cs3Bi2I9 thin films with different heat transport characteristics. As a result, the detectors using Cs3Bi2I9 have a larger response temperature, stronger photocurrent, higher responsivity, and normalized detectivity than the detectors using BiI3. The unique heat transport characteristics provide a prospect for photothermal detection. Furthermore, the insights in this report imply opportunities to explore ultra-low thermal conductivity among metal halide perovskites.

Picosecond ultrasonics (PU), time-domain Brillouin scattering (TDBS), and time-domain thermo-reflectance (TDTR) are all in-situ, non-destructive, and non-contact experimental techniques based on the ultrafast laser pump-probe method, which can generate and detect coherent acoustic phonons (CAP) and thermal transport in thin metal film-glass substrate system. However, these techniques are generally considered different experimental methods to characterize the thermal or mechanical properties of metal nano-objects or transparent materials. Here we present a comprehensive characterization of the generation, propagation, and attenuation of high-frequency CAP and cross-plane thermal transport in the thin Cr film-glass substrate system by PU, TDBS, and TDTR. To investigate the key factors of characterizations, two kinds of thin Cr film-glass substrate systems were measured on the film side and substrate side. The measured thermal and mechanical properties show that boundary conditions and film thickness have significantly affected the characterization.

Accurate measurement of elastic constants in thin films is still an important issue to understand the scale behavior of nanosized materials. In the present study, we introduced an advanced non-destructive method, picosecond ultrasonics (PU), for measuring the out-of-plane elastic modulus of thin chromium (Cr) films. The femtosecond light pulse is focused on the Cr film to excite the longitudinal acoustic phonons (LAP), which propagate along the thickness direction and repeat reflections inside the Cr film. Then, the propagation/distribution of LAP is detected by the time-delayed probe light pulse through the photoelastic effect. Therefore, we can determine the out-of-plane modulus by measuring the periodic pulse echoes or the breathing mode vibrations within the Cr film. For most Cr films, the determined modulus is smaller than the corresponding bulk value and decreases with the decreasing thickness, while for some Cr films, it closes and may exceed the bulk value. This work describes the thickness-dependent elasticity of thin Cr films and provides evidence of the stiffness enhancement in Cr films on the Si substrate. In addition, since LAP with central frequency up to 310 GHz is excited in Cr films on the SiO2 substrate, we also demonstrate the potential of Cr films as high-frequency photoacoustic transducers.