The direct conversion of solar energy into fuels or feedstock is an attractive approach to address increasing demand of renewable energy sources. Photocatalytic systems relying on the direct photoexcitation of metals have been explored to this end, a strategy that exploits the decay of plasmonic resonances into hot carriers. An efficient hot carrier generation and collection requires, ideally, their generation to be enclosed within few tens of nanometers at the metal interface, but it is challenging to achieve this across the broadband solar spectrum. Here the authors demonstrate a new photocatalyst for hydrogen evolution based on metal epsilon-near-zero metamaterials. The authors have designed these to achieve broadband strong light confinement at the metal interface across the entire solar spectrum. Using electron energy loss spectroscopy, the authors prove that hot carriers are generated in a broadband fashion within 10 nm in this system. The resulting photocatalyst achieves a hydrogen production rate of 9.5 µmol h cm that exceeds, by a factor of 3.2, that of the best previously reported plasmonic-based photocatalysts for the dissociation of H with 50 h stable operation.
The engineering of broadband absorbers to harvest white light in thin-film semiconductors is a major challenge in developing renewable materials for energy harvesting. Many solution-processed materials with high manufacturability and low cost, such as semiconductor quantum dots, require the use of film structures with thicknesses on the order of 1 μm to absorb incoming photons completely. The electron transport lengths in these media, however, are 1 order of magnitude smaller than this length, hampering further progress with this platform. Herein, we show that, by engineering suitably disordered nanoplasmonic structures, we have created a new class of dispersionless epsilon-near-zero composite materials that efficiently harness white light. Our nanostructures localize light in the dielectric region outside the epsilon-near-zero material with characteristic lengths of 10-100 nm, resulting in an efficient system for harvesting broadband light when a thin absorptive film is deposited on top of the structure. By using a combination of theory and experiments, we demonstrate that ultrathin layers down to 50 nm of colloidal quantum dots deposited atop the epsilon-near-zero material show an increase in broadband absorption ranging from 200% to 500% compared to a planar structure of the same colloidal quantum-dot-absorber average thickness. When the epsilon-near-zero nanostructures were used in an energy-harvesting module, we observed a spectrally averaged 170% broadband increase in the external quantum efficiency of the device, measured at wavelengths between 400 and 1200 nm. Atomic force microscopy and photoluminescence excitation measurements demonstrate that the properties of these epsilon-near-zero structures apply to general metals and could be used to enhance the near-field absorption of semiconductor structures more widely. We have developed an inexpensive electrochemical deposition process that enables scaled-up production of this nanomaterial for large-scale energy-harvesting applications.
Controlling broadband light in nanoscale volumes is a desired goal in nanophotonics. Metastructures tackle this problem by subwavelength nanostructured patterns. The current technology reaches footprints of 50 nm with plasmonic nanostructures. Scaling down these values is challenging, especially in low loss dielectrics. Here, a new class of metasurfaces is introduced, “printed” point‐to‐point by free‐electron waves and created by altering the resonant atomic transition of inexpensive photosensitive materials. With this approach it is possible to directly write a desired distribution of refractive index and extinction coefficient with a resolution equal to the focusing accuracy of the electron beam, theoretically limited to the single nanometer. An application of this technology is illustrated in structural coloration. Currently, the best results are obtained with plasmonics at 127 000 dual polarization interferometry (DPI), with 50–200 nm structures and chromaticity ranging from blue to yellow. Free‐electron metasurfaces can generate the complete spectrum of colors of the cyan, yellow, magenta, and black system with resolutions up to 256 000 DPI, and nanostructures of 10 nm radius by using a single inexpensive layer of transparent material. This platform can enable a new generation of low cost transparent media supporting ultradense optical circuitry for broadband light control.
The study of efficient mechanisms of photon conversion processes into electronic, thermal and chemical energy is an interdisciplinary research field spanning physics, chemistry and material science. In recent years, different physical mechanisms sustained by the engineering of diverse complex photonic structures have emerged to offer significant advances in the area of thermal energy generation, photocatalytic and photoelectrochemical energy transformation. The efficient behavior of these systems results from the integration, with different levels of complexity, of dielectric and metallic optical nanostructures into hierarchical disordered architectures, which have shown to significantly improve broadband light-harvesting, electronic charges extraction and light energy confinement. The review aims to concisely highlight the most recent progress in this field, with emphasis on discussing the physics and applications of complex lightwave systems for the realization of efficient processes of photon energy harvesting.
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