Recently discovered double gamma/beta (γ/β) polymorph Ga2O3 structures constitute a class of novel materials providing an option to modulate functional properties across interfaces without changing the chemical compositions of materials, in contrast to that in conventional heterostructures. In this work, for the first time, we investigate thermal transport in such homo-interface structures as an example of their physical properties. In particular, the cross-plane thermal conductivity (k) was measured by femtosecond laser-based time-domain thermoreflectance with MHz modulation rates, effectively obtaining depth profiles of the thermal conductivity across the γ-/β-Ga2O3 structures. In this way, the thermal conductivity of γ-Ga2O3 ranging from 1.84 to 2.11 W m−1 K−1 was found to be independent of the initial β-substrates orientations, in accordance with the cubic spinel structure of the γ-phase and consistently with the molecular dynamics simulation data. In turn, the thermal conductivity of monoclinic β-Ga2O3 showed a distinct anisotropy, with values ranging from 10 W m−1 K−1 for [−201] to 20 Wm−1 K−1 for [010] orientations. Thus, for double γ-/β-Ga2O3 polymorph structures formed on [010] β-substrates, there is an order of magnitude difference in thermal conductivity across the γ/β interface, which can potentially be exploited in thermal energy conversion applications.

This study investigates spatially resolved phonon-mediated thermal transport across nano-sized grains and grain boundaries (GBs) in 3C-SiC using molecular dynamics (MD) simulations. This investigation involves controlling the complete range of inter-grain misorientation tilt angles (θ = 0°–90°) and nanoscale grain sizes (d = 2.18–130.77 nm). The grain boundary energy and interfacial thermal transport are found to be highly θ-sensitive and asymmetric with respect to θ = 45° due to the low symmetry associated with two interpenetrating diatomic SiC fcc lattices. When adjacent grains are tilted at θ = 14.25°, the interfacial heat conduction is highly suppressed compared to other θ values, especially for larger grains. The most stable atomic configuration of the GB region associated with the minimal GB energy results in the highest suppression of heat conduction across the GB interface. Spatially resolved thermal anisotropy reveals a strong GB-mediated nanoscale hydrodynamic phonon Poiseuille effect when heat flows parallel to the GB planes, as shown by our perturbed MD study. With the reduction of d, the intra-grain and inter-GB thermal conductivities decrease due to the enhanced phonon scattering from interfaces, but the difference between these conductivities becomes negligible for the heat flow normal to the GB planes. It is envisioned that nanoscale spatially resolved control of thermal energy will provide useful guidance to engineer nanocrystalline ceramics with tunable interfacial thermal properties.

β-Ga2O3 is a promising material for power electronics due to its wide bandgap and high breakdown field, but its low thermal conductivity poses challenges for heat dissipation. To address this, we employed ion beam exfoliation to fabricate β-Ga2O3 nanomembranes integrated with highly thermally conductive Si substrates. To do this, chromium ion implantation was used to induce stress and strain, forming rolled-up microtubes on (100)-oriented β-Ga2O3 single crystals. After successfully transferring these tubes onto Si substrates and performing thermal annealing, these microtubes were unrolled into nanomembranes. X-ray diffraction and Raman measurements revealed the high quality of the samples. Time-domain thermoreflectance was used to study thermal transport in these structures, confirming uniform thermal conductivity across three fabricated samples. A Debye-based thermal transport model was implemented to validate experimental results and define the main phonon scattering mechanisms. Non-equilibrium molecular dynamics simulations revealed that a thin amorphous SiO2 interlayer significantly enhanced the thermal boundary conductance (TBC) across the β-Ga2O3/Si interface by bridging the vibrational mismatch between β-Ga2O3 and Si. However, further increasing the interlayer thickness led to phonon scattering and reduced TBC, emphasizing the importance of precise interface thickness control. This study highlights ion beam exfoliation as a scalable approach for integrating β-Ga2O3 with thermally conductive substrates, providing a pathway to improved thermal management in β-Ga2O3-based power electronics.

Anti-Stokes photoluminescence (ASPL), which is an up-conversion phonon-assisted process of the radiative recombination of photoexcited charge carriers, was investigated in methylammonium lead bromide (MALB) perovskite nanocrystals (NCs) with mean sizes that varied from about 6 to 120 nm. The structure properties of the MALB NCs were investigated by means of the scanning and transmission electron microscopy, X-ray diffraction and Raman spectroscopy. ASPL spectra of MALB NCs were measured under near-resonant laser excitation with a photon energy of 2.33 eV and they were compared with the results of the photoluminescence (PL) measurements under non-resonant excitation at 3.06 eV to reveal a contribution of phonon-assisted processes in ASPL. MALB NCs with a mean size of about 6 nm were found to demonstrate the most efficient ASPL, which is explained by an enhanced contribution of the phonon absorption process during the photoexcitation of small NCs. The obtained results can be useful for the application of nanocrystalline organometal perovskites in optoelectronic and all-optical solid-state cooling devices.