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Solar redox flow batteries (SRFB) have received much attention as an alternative integrated technology for simultaneous conversion and storage of solar energy. Yet, the photocatalytic efficiency of semiconductor-based single photoelectrode, such as hematite, remains low due to the trade-off between fast electron hole recombination and insufficient light utilization, as well as inferior reaction kinetics at the solid/liquid interface. Herein, we present an {\alpha}-Fe2O3/CuxO p-n junction, coupled with a readily scalable nanostructure, that increases the electrochemically active sites and improves charge separation. Thanks to light-assisted scanning electrochemical microscopy (Photo-SECM), we elucidate the morphology-dependent carrier transfer process involved in the photo-oxidation reaction at a {\alpha}-Fe2O3 photoanode. The optimized nanostructured is then exploited in the {\alpha}-Fe2O3/CuxO p-n junction, achieving an outstanding unbiased photocurrent density of 0.46 mA/cm2, solar-to-chemical (STC) efficiency over 0.35% and a stable photocharge-discharge cycling. The average solar-to-output energy efficiency (SOEE) for this unassisted {\alpha}-Fe2O3-based SRFB system reaches 0.18%, comparable to previously reported DSSC-assisted hematite SRFBs. The use of earth-abundant materials and the compatibility with scalable nanostructuring and heterojunction preparation techniques, offer promising opportunities for cost-effective device deployment in real-world applications.

Circular Dichroism (CD) can distinguish the handedness of chiral molecules. However, it is typically very weak due to vanishing absorption at low molecular concentrations. Here, we suggest Thermal Circular Dichroism (TCD) for chiral detection, leveraging the temperature difference in the chiral sample when subjected to right and left-circularly polarized excitations. The TCD combines the enantiospecificity of circular dichroism with the higher sensitivity of thermal measurements, while introducing new opportunities in the thermal domain that can be synergistically combined with optical approaches. We propose a theoretical framework to understand the TCD of individual and arrays of resonators covered by chiral molecules. To enhance the weak TCD of chiral samples, we first use individual dielectric Mie resonators and identify chirality transfer and self-heating as the underlying mechanisms giving rise to the differential temperature. However, inherent limitations imposed by the materials and geometries of such resonators make it challenging to surpass a certain level in enhancements. To overcome this, we suggest nonlocal thermal and electromagnetic interactions in arrays. We predict that a combination of chirality transfer to Mie resonators, collective thermal effects, and optical lattice resonance could, in principle, offer more than 4 orders of magnitude enhancement in TCD. Our thermonanophotonic-based approach thus establishes key concepts for ultrasensitive chiral detection.

Exploring nonequilibrium hot carriers from plasmonic metal nanostructures is a dynamic field in optoelectronics, driving photochemical reactions such as solar fuel generation. The hot carrier injection mechanism and the reaction rate are highly impacted by the metal/molecule interaction. However, determining the primary type of the reaction and thus the injection mechanism of the hot carriers has remained elusive. In this work, we reveal an electron injection mechanism deviating from a purely outersphere process for the reduction of ferricyanide redox molecule in a gold/p-type gallium nitride (Au/p- GaN) photocathode system. Combining our experimental approach with ab initio simulations, we discover that the efficient inner-sphere transfer of low-energy electrons leads to a continuous enhancement in the photocathode device performance in the interband regime. These findings provide important mechanistic insights, showing our methodology as a powerful tool for analyzing and engineering hot-carrier-driven processes in plasmonic photocatalytic systems and optoelectronic devices.

Scattering-type scanning near-field optical microscope (s-SNOM) has become an essential tool to study polaritons - quasiparticles of light coupled to collective charge oscillations - via direct probing of their near field with a spatial resolution far beyond the diffraction limit. However, extraction of the polariton complex propagation constant from the near-field images requires subtle considerations that have not received necessary attention so far. In this study, we discuss important yet overlooked aspects of the near-field analysis. First, we experimentally demonstrate that the sample orientation inside the s-SNOM may significantly affect the near-field interference pattern of mid-infrared polaritons, leading to an error in momentum measurement up to 7.7% even for the modes with effective index of 12.5. Second, we establish a methodology to correctly extract the polariton damping rate from the interference fringes depending on their origin - the s-SNOM nano-tip or the material edge. Overall, our work provides a unified framework for the accurate extraction of the polariton momentum and damping from the near-field interference fringes., Comment: 15 pages, 4 figures

Applications in photodetection, photochemistry, and active metamaterials and metasurfaces require fundamental understanding of ultrafast nonthermal and thermal electron processes in metallic nanosystems. Significant progress has been recently achieved in synthesis and investigation of low-loss monocrystalline gold, opening up opportunities for its use in ultrathin nanophotonic architectures. Here, we reveal fundamental differences in hot-electron thermalisation dynamics between monocrystalline and polycrystalline ultrathin (down to 10 nm thickness) gold films. Comparison of weak and strong excitation regimes showcases a counterintuitive unique interplay between thermalised and non-thermalised electron dynamics in mesoscopic gold with the important influence of the X-point interband transitions on the intraband electron relaxation. We also experimentally demonstrate the effect of hot-electron transfer into a substrate and the substrate thermal properties on electron-electron and electron-phonon scattering in ultrathin films. The hot-electron injection efficiency from monocrystalline gold into TiO2, approaching 9% is measured, close to the theoretical limit. These experimental and modelling results reveal the important role of crystallinity and interfaces on the microscopic electronic processes important in numerous applications., Comment: 9 pages, 4 figures

CdTe is the most commercially successful thin-film photovoltaic technology to date. The recent development of Se-alloyed CdSe$_{x}$Te$_{1-x}$ layers in CdTe solar cells has led to higher device efficiencies, due to a lowered bandgap improving the photocurrent, improved voltage characteristics and longer carrier lifetimes. Evidence from cross-sectional electron microscopy is widely believed to indicate that Se passivates defects in CdSe$_{x}$Te$_{1-x}$ solar cells, and that this is the reason for better lifetimes and voltages in these devices. Here, we utilise spatially resolved photoluminescence measurements of CdSe$_{x}$Te$_{1-x}$ thin films on glass to study the effects of Se on carrier recombination in the material, isolated from the impact of conductive interfaces and without the need to prepare cross-sections through the samples. We find further evidence to support Se passivation of grain boundaries, but also identify an associated increase in below-bandgap photoluminescence that indicates the presence of Se-enhanced luminescent defects. Our results show that Se treatment, in tandem with Cl passivation, does increase radiative efficiencies. However, the simultaneous enhancement of defects within the grain interiors suggests that although it is overall beneficial, Se incorporation may still ultimately limit the maximum attainable efficiency of CdSe$_{x}$Te$_{1-x}$ solar cells.

Evaporation-driven fluid flow in porous or nanostructured materials has recently opened a new paradigm for renewable energy generation. Despite recent progress, major fundamental questions remain regarding the interfacial phenomena governing these so-called hydrovoltaic (HV) devices. Together with the lack of modelling tools, this limits the performance and application range of this emerging technology. By leveraging ordered arrays of Silicon nanopillars (NP) and developing a quantitative multiphysics model to study their HV response across a wide parameter space, this work reveals the complex interplay of surface-charge, liquid properties, and geometrical parameters, including previously unexplored electrokinetic interactions. Notably, we find that ion-concentration-dependent surface charge, together with ion mobility, dictates multiple local maxima in open circuit voltage, with optimal conditions deviating from conventional low-concentration expectations. Additionally, assessing the HV response up to molar concentrations, we provide unique evidence of ion adsorption and charge inversion for a number of monovalent cations. This effect interestingly enables the operation of HV devices even at such high concentrations. Finally, we highlight that, beyond electrokinetic parameters, geometrical asymmetries in the device structure generate an electrostatic potential that augments HV performance. Overall, our work, which lies in between single nanochannel studies and macro-scale porous system characterization, demonstrates that evaporation-driven HV devices can operate across a wide range of salinities, with optimal operating conditions being dictated by distinct interfacial phenomena. Thus it offers crucial insight and a design tool for enhancing the performance of evaporation-driven HV devices and enables their broader applicability across the salinity scale of natural and processed waters.

Harnessing non-equilibrium hot carriers from plasmonic metal nanostructures constitutes a vibrant research field. It promises to enable control of activity and selectivity of photochemical reactions, especially for solar fuel generation. However, a comprehensive understanding of the interplay of plasmonic hot carrier-driven processes in metal/semiconducting heterostructures has remained elusive. In this work, we reveal the complex interdependence between plasmon excitation, hot carrier generation, transport and interfacial collection in plasmonic photocatalytic devices, uniquely determining the charge injection efficiencies at the solid/solid and solid/liquid interfaces. Interestingly, by measuring the internal quantum efficiency of ultrathin (14 to 33 nm) single-crystalline plasmonic gold (Au) nanoantenna arrays on titanium dioxide substrates, we find that the performance of the device is governed by hot hole collection at the metal/electrolyte interface. In particular, by combining a solid- and liquid-state experimental approach with ab initio simulations, we show a more efficient collection of high-energy d-band holes traveling in [111] orientation, resulting in a stronger oxidation reaction at the {111} surfaces of the nanoantenna. These results thus establish new guidelines for the design and optimization of plasmonic photocatalytic systems and optoelectronic devices.

Luminescence constitutes a unique source of insight into hot carrier processes in metals, including those in plasmonic nanostructures used for sensing and energy applications. However, being weak in nature, metal luminescence remains poorly understood, its microscopic origin strongly debated, and its potential for unravelling nanoscale carrier dynamics largely unexploited. Here, we reveal quantum-mechanical effects emanating in the luminescence from thin monocrystalline gold flakes. Specifically, we present experimental evidence, supported by first-principles simulations, to demonstrate its photoluminescence origin when exciting in the interband regime. Our model allows us to identify changes to the measured gold luminescence due to quantum-mechanical effects as the gold film thickness is reduced. Excitingly, such effects are observable in the luminescence signal from flakes up to 40 nm in thickness, associated with the out-of-plane discreteness of the electronic band structure near the Fermi level. We qualitatively reproduce the observations with first-principles modelling, thus establishing a unified description of luminescence in gold and enabling its widespread application as a probe of carrier dynamics and light-matter interactions in this material. Our study paves the way for future explorations of hot-carriers and charge-transfer dynamics in a multitude of material systems., Comment: Main text 21 pages and 4 figures. Supplemental Information 33 pages and 17 figures

Thermonanophotonics, i.e. the study of photothermal effects in optical nanoantennas, has recently attracted growing interest. While thermoplasmonic structures enable a broad range of applications, from imaging and optofluidics devices to medical and photochemical systems, dielectric nanoantennas open new opportunities for thermo-optical modulation and reconfigurable metasurfaces. However, computing both photo-thermal and thermo-optical effects in large arrays of nanoantennas remains a challenge. In this work, we implement a fast numerical method to compute the temperature increase of multi-dimensional arrays of optical antennas embedded in a uniform medium, accounting for self-heating, collective heating as well as thermo-optical effects. In particular, we demonstrate scalable computation of temperature in 3D networks with $10^5$ particles in less than 1 hour. Interestingly, by explicitly considering the role of discrete nanoparticles on light attenuation and photothermal conversion, this approach enables the optimization of complex temperature profiles in 3D arrays. Importantly, we compute for the first time the impact of thermo-optical effects beyond the single nanoantenna. Our results show that collective heating contributions amplify these effects in multi-dimensional arrays of both Silicon and Gold nanospheres, highlighting the importance of considering them in photothermal calculations. Overall, the proposed method opens new opportunities for the rapid assessment of complex photothermal effects in arrays of optical nanoantennas, supporting the development of advanced thermonanophotonic functionalities.

The use of nanoplasmonics to control light and heat close to the thermodynamic limit enables exciting opportunities in the field of plasmonic catalysis. The decay of plasmonic excitations creates highly nonequilibrium distributions of hot carriers that can initiate or catalyze reactions through both thermal and nonthermal pathways. In this Perspective, we present the current understanding in the field of plasmonic catalysis, capturing vibrant debates in the literature, and discuss future avenues of exploration to overcome critical bottlenecks. Our Perspective spans first-principles theory and computation of correlated and far-from-equilibrium light-matter interactions, synthesis of new nanoplasmonic hybrids, and new steady-state and ultrafast spectroscopic probes of interactions in plasmonic catalysis, recognizing the key contributions of each discipline in realizing the promise of plasmonic catalysis. We conclude with our vision for fundamental and technological advances in the field of plasmon-driven chemical reactions in the coming years.

Thanks to the compact design and multi-functional light-manipulation capabilities, reconfigurable metalenses, which consist of arrays of sub-wavelength meta-atoms, offer unique opportunities for advanced optical systems, from microscopy to augmented reality platforms. Although poorly explored in the context of reconfigurable metalens, thermo-optical effects in resonant silicon nanoresonators have recently emerged as a viable strategy to realize tunable meta-atoms. In this work, we report the proof-of-concept design of an ultrathin (300 nm thick) and thermo-optically reconfigurable silicon metalens operating at a fixed, visible wavelength (632 nm). Importantly, we demonstrate continuous, linear modulation of the focal-length up to 21% (from 165 $\mu$m at 20$\deg$C to 135 $\mu$m at 260$\deg$C). Operating under right-circularly polarized light, our metalens exhibits an average conversion efficiency of 26%, close to mechanically modulated devices, and has a diffraction-limited performance. Overall, we envision that, combined with machine-learning algorithms for further optimization of the meta-atoms, thermally-reconfigurable metalenses with improved performance will be possible. Also, the generality of this approach could offer inspiration for the realization of active metasurfaces with other emerging material within field of thermo-nanophotonics.

Plasmonic nanojunctions, consisting of adjacent metal structures with nanometre gaps, can support localised plasmon resonances that boost light matter interactions and concentrate electromagnetic fields at the nanoscale. In this regime, the optical response of the system is governed by poorly understood dynamical phenomena at the frontier between the bulk, molecular and atomic scales. Here, we report ubiquitous spectral fluctuations in the intrinsic light emission from photo-excited gold nanojunctions, which we attribute to the light-induced formation of domain boundaries and quantum-confined emitters inside the noble metal. Our data suggest that photoexcited carriers and gold adatom - molecule interactions play key roles in triggering luminescence blinking. Surprisingly, this internal restructuring of the metal has no measurable impact on the Raman signal and scattering spectrum of the plasmonic cavity. Our findings demonstrate that metal luminescence offers a valuable proxy to investigate atomic fluctuations in plasmonic cavities, complementary to other optical and electrical techniques.

Harvesting non-equilibrium hot carriers from photo-excited metal nanoparticles has enabled plasmon-driven photochemical transformations and tunable photodetection with resonant nanoantennas. Despite numerous studies on the ultrafast dynamics of hot electrons, to date, the temporal evolution of hot holes in metal-semiconductor heterostructures remains unknown. An improved understanding of the carrier dynamics in hot-hole-driven systems is needed to help expand the scope of hot-carrier optoelectronics beyond hot-electron-based devices. Here, using ultrafast transient absorption spectroscopy, we show that plasmon-induced hot-hole injection from gold (Au) nanoparticles into the valence band of p-type gallium nitride (p-GaN) occurs within 200 fs, placing hot-hole transfer on a similar timescale as hot-electron transfer. We further observed that the removal of hot holes from below the Au Fermi level exerts a discernible influence on the thermalization of hot electrons above it, reducing the peak electronic temperature and decreasing the electron-phonon coupling time relative to Au samples without a pathway for hot-hole collection. First principles calculations corroborate these experimental observations, suggesting that hot-hole injection modifies the relaxation dynamics of hot electrons in Au nanoparticles through ultrafast modulation of the d-band electronic structure. Taken together, these ultrafast studies substantially advance our understanding of the temporal evolution of hot holes in metal-semiconductor heterostructures and suggest new strategies for manipulating and controlling the energy distributions of hot carriers on ultrafast timescales., Comment: 12 pages, 4 figures

Harnessing short-lived photoexcited electron-hole pairs in metal nanostructures has the potential to define a new phase of optoelectronics, enabling control of athermal mechanisms for light harvesting, photodetection and photocatalysis. To date, however, the spatiotemporal dynamics and transport of these photoexcited carriers have been only qualitatively characterized. Plasmon excitation has been widely viewed as an efficient mechanism for generating non-thermal hot carriers. Despite numerous experiments, conclusive evidence elucidating and quantifying the full dynamics of hot carrier generation, transport, and injection has not been reported. Here, we combine experimental measurements with coupled first-principles electronic structure theory and Boltzmann transport calculations to provide unprecedented insight into the internal quantum efficiency, and hence internal physics, of hot carriers in photoexcited gold (Au)-gallium nitride (GaN) nanostructures. Our results indicate that photoexcited electrons generated in 20 nm-thick Au nanostructures impinge ballistically on the Au-GaN interface. This discovery suggests that the energy of hot carriers could be harnessed from metal nanostructures without substantial losses via thermalization. Measurements and calculations also reveal the important role of metal band structure in hot carrier generation at energies above the interband threshold of the plasmonic nanoantenna. Taken together, our results advance the understanding of excited carrier dynamics in realistically-scaled metallic nanostructures and lay the foundations for the design of new optoelectronic devices that operate in the ballistic regime., Comment: 9 pages, 4 figures

Plasmonic hot carrier devices extract excited carriers from metal nanostructures before equilibration, and have the potential to surpass semiconductor light absorbers. However their efficiencies have so far remained well below theoretical limits, which necessitates quantitative prediction of carrier transport and energy loss in plasmonic structures to identify and overcome bottlenecks in carrier harvesting. Here, we present a theoretical and computational framework, Non-Equilibrium Scattering in Space and Energy (NESSE), to predict the spatial evolution of carrier energy distributions that combines the best features of phase-space (Boltzmann) and particle-based (Monte Carlo) methods. Within the NESSE framework, we bridge first-principles electronic structure predictions of plasmon decay and carrier collision integrals at the atomic scale, with electromagnetic field simulations at the nano- to mesoscale. Finally, we apply NESSE to predict spatially-resolved energy distributions of photo-excited carriers that impact the surface of experimentally realizable plasmonic nanostructures at length scales ranging from tens to several hundreds of nanometers, enabling first-principles design of hot carrier devices., Comment: 10 pages, 5 figures

We report experimental measurements for ultrathin (< 15 nm) van der Waals heterostructures exhibiting external quantum efficiencies exceeding 50%, and show that these structures can achieve experimental absorbance > 90%. By coupling electromagnetic simulations and experimental measurements, we show that pn WSe2/MoS2 heterojunctions with vertical carrier collection can have internal photocarrier collection efficiencies exceeding 70%., Comment: ACS Nano, 2017. Manuscript (25 pages, 7 figures) plus supporting information (7 pages, 4 figures)

We demonstrate near unity, broadband absorbing optoelectronic devices using sub-15 nm thick transition metal dichalcogenides (TMDCs) of molybdenum and tungsten as van der Waals semiconductor active layers. Specifically, we report that near-unity light absorption is possible in extremely thin (< 15 nm) Van der Waals semiconductor structures by coupling to strongly damped optical modes of semiconductor/metal heterostructures. We further fabricate Schottky junction devices using these highly absorbing heterostructures and characterize their optoelectronic performance. Our work addresses one of the key criteria to enable TMDCs as potential candidates to achieve high optoelectronic efficiency., Comment: Manuscript plus supplement. 5 figures in manuscript