The electron-phonon interaction controls the intrinsic mobility of charges in metal halide perovskites, and determines the rate at which carriers lose energy. Here, the carrier mobility and cooling dynamics were directly examined using a combination of ultrafast transient absorption spectroscopy and optical pump, THz probe spectroscopy, in perovskites with different lead and tin content, and for a range of carrier densities. Significantly, the carrier mobility in the “hot phonon bottleneck” regime, where the LO phonon bath keeps carriers warm, was found to be similar to the mobility of cold carriers. A model was developed that provides a quantitative description of the experimental carrier cooling dynamics, including electron-phonon coupling, phonon-phonon coupling and the Auger mechanism. In the Pb and Sn alloy the duration of the hot carrier regime was extended as a result of the slower decay of optical phonons. The findings offer an intuitive link between macroscopic properties and the underlying microscopic energy transfer processes, and suggest new routes to control the carrier cooling process in metal halide perovskites to optimize optoelectronic devices.

Terahertz (THz) in vivo reflection imaging can be used to assess the water content of the surface of the skin. This study presents the results of treating 20 subjects with aqueous, anhydrous and water-oil emulsion samples and observing the changes induced in the skin using THz sensing. These regions were also measured with a corneometer, the present gold standard for skin hydration assessment within the cosmetics industry. We find that THz sensing is effective at observing the presence of oil and water on the surface of the skin, these results can be verified with the measurements of capacitance taken by the corneometer. The THz measurements reveal a distinction between the responses of subjects with initially dry or well hydrated skin, this observation is particularly noticeable with the oil-based samples. Additionally, moderate correlation was found between the THz reflected amplitude and capacitance of untreated skin with a correlation coefficient of r = −0.66, suggesting THz sensing has promising potential for assessing skin hydration.

The noninvasive and water‐sensitive characteristics of terahertz (THz) light make it highly attractive for in vivo studies, especially for skin applications. However, THz instrumentation has not been developed sufficiently to fully explore all the potential applications arising: current systems cannot obtain uncorrelated reflections from multiple configurations to determine the complicated structure of living tissues. Herein, this bottleneck is overcome by implementing a novel ellipsometry configuration able to efficiently provide four complementary sets of spectral ratios, significantly enhancing characterization capabilities. An accurate model of the skin is established and validated. The anisotropy of the stratum corneum (SC) caused by its cellular structure is verified both theoretically and experimentally. The in vivo response of skin on the volar forearm to occlusion is observed by the dynamic changes in the SC and the epidermis. In addition, the THz dispersion and birefringence sensitively probe the level of hydration and the cellular inhomogeneity, producing results in good agreement with microscope images and the biological processes of the SC. The presented technique offers a brand‐new functionality in extracting insightful structural information from complex systems, significantly extending the versatility of THz spectroscopy.

This roadmap on Nanotechnology for Catalysis and Solar Energy Conversion focuses on the application of nanotechnology in addressing the current challenges of energy conversion: 'high efficiency, stability, safety, and the potential for low-cost/scalable manufacturing' to quote from the contributed article by Nathan Lewis. This roadmap focuses on solar-to-fuel conversion, solar water splitting, solar photovoltaics and bio-catalysis. It includes dye-sensitized solar cells (DSSCs), perovskite solar cells, and organic photovoltaics. Smart engineering of colloidal quantum materials and nanostructured electrodes will improve solar-to-fuel conversion efficiency, as described in the articles by Waiskopf and Banin and Meyer. Semiconductor nanoparticles will also improve solar energy conversion efficiency, as discussed by Boschloo et al in their article on DSSCs. Perovskite solar cells have advanced rapidly in recent years, including new ideas on 2D and 3D hybrid halide perovskites, as described by Spanopoulos et al 'Next generation' solar cells using multiple exciton generation (MEG) from hot carriers, described in the article by Nozik and Beard, could lead to remarkable improvement in photovoltaic efficiency by using quantization effects in semiconductor nanostructures (quantum dots, wires or wells). These challenges will not be met without simultaneous improvement in nanoscale characterization methods. Terahertz spectroscopy, discussed in the article by Milot et al is one example of a method that is overcoming the difficulties associated with nanoscale materials characterization by avoiding electrical contacts to nanoparticles, allowing characterization during device operation, and enabling characterization of a single nanoparticle. Besides experimental advances, computational science is also meeting the challenges of nanomaterials synthesis. The article by Kohlstedt and Schatz discusses the computational frameworks being used to predict structure–property relationships in materials and devices, including machine learning methods, with an emphasis on organic photovoltaics. The contribution by Megarity and Armstrong presents the 'electrochemical leaf' for improvements in electrochemistry and beyond. In addition, biohybrid approaches can take advantage of efficient and specific enzyme catalysts. These articles present the nanoscience and technology at the forefront of renewable energy development that will have significant benefits to society.