Superlattices with scattering mechanisms at multiple length scales efficiently scatter phonons at all relevant wavelengths and provide a convenient route to reduce thermal transport. Here, we show, both experimentally and by atomistic simulations, that SiGe superlattices with well-established compositional gradients and a sufficient number of interfaces exhibit extremely low thermal conductivity.
Our results reveal that the thermal conductivity of long-period (30–50 nm) superlattices with thicknesses below 200 nm is still thickness-dependent and higher than that of the corresponding alloy thin film. Increasing the number of periods up to 16 has a strong impact on heat propagation, leading to thermal conductivity values below the thin-film alloy limit. Lattice dynamics calculations confirm that the reduced thermal conductivity stems from the simultaneous effects of mass scattering, graded interface scattering, and coherent interference from the lattice periodicity. This study provides a significant step forward in understanding the role of compositional gradients in heat transport across nanostructures. The strategy of employing long-period graded superlattices with extremely low thermal conductivities has great potential for micro- and nano-thermoelectric generation and cooling of Si-based devices.
The combination of light‐emitting diodes (LEDs) and rare earth (RE) phosphors as color‐converting layers comprises the basis of solid‐state lighting. Indeed, most LED lamps include a photoluminescent coating made of phosphor material, i.e., crystalline matrix suitably doped with RE elements, to produce white light from a blue or ultraviolet LED chip.
At short length scales phonon transport is ballistic: the thermal resistance of semiconductors and insulators is quantized and length independent. At long length scales, on the other hand, transport is diffusive and resistance arises as a result of the scattering processes experienced by phonons. In many cases of interest, however, these two transport regimes coexist. Here we propose a first-principles approach to treat quasiballistic phonon transport where diffusive and ballistic phonons receive separate theoretical treatments.
The surprising optical properties of the non-π-conjugated polymer poly(phenylene methylene) (PPM) and its derivatives—that is, absorption in the 350–450 nm and photoluminescence (PL) in the 400–600 nm spectral regions—have been attributed to chromophores formed by homoconjugation along the polymer chain. The enabling role of homoconjugation, however, was hitherto ascertained primarily by excluding alternative origins of luminescence.
Transfer printing is one of the key nanofabrication techniques for the large‐scale manufacturing of complex device architectures. It provides a cost‐effective and high‐throughput route for the integration of independently processed materials into spatially tailored architectures.