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By introducing tin-doped indium oxide nanocrystals into niobium oxide glass, a new transparent material is produced with tunable and spectrally selective optical switching properties. Windows upgrade: managed transmission of heat and light Glass windows let in light and keep out extreme heat and cold, but with the advance of the field of electrochromism — the reversible change of optical transmittance in response to electrochemical charge and discharge — they could do much more. Delia Milliron and co-workers have made a step towards producing windows that can greatly reduce lighting and heating costs with the demonstration of a composite film composed of tin-doped indium oxide nanocrystals embedded in a niobium oxide glass matrix. By varying an applied voltage over a 2.5-volt range, the charged nanocrystals selectively block near-infrared light, while the glass strongly modulates visible light owing to its reconstructed bonding near the nanocrystal–glass interface. Practical 'smart windows' and other devices are still some way off, but this solution-processed nanocrystal-in-glass approach is modular and permits extensive tuning of structure and composition, and so provides a solid platform for further refinement. Amorphous metal oxides are useful in optical 1 , 2 , electronic 3 , 4 , 5 and electrochemical devices 6 , 7 . The bonding arrangement within these glasses largely determines their properties, yet it remains a challenge to manipulate their structures in a controlled manner. Recently, we developed synthetic protocols for incorporating nanocrystals that are covalently bonded into amorphous materials 8 , 9 . This ‘nanocrystal-in-glass’ approach not only combines two functional components in one material, but also the covalent link enables us to manipulate the glass structure to change its properties. Here we illustrate the power of this approach by introducing tin-doped indium oxide nanocrystals into niobium oxide glass (NbO x ), and realize a new amorphous structure as a consequence of linking it to the nanocrystals. The resulting material demonstrates a previously unrealized optical switching behaviour that will enable the dynamic control of solar radiation transmittance through windows. These transparent films can block near-infrared and visible light selectively and independently by varying the applied electrochemical voltage over a range of 2.5 volts. We also show that the reconstructed NbO x glass has superior properties—its optical contrast is enhanced fivefold and it has excellent electrochemical stability, with 96 per cent of charge capacity retained after 2,000 cycles.

Graphene is a single-layer crystal of carbon, the thinnest two-dimensional material. It has unique electronic and photonic properties.

Localized surface plasmon resonances of an individual silver nanocube are reconstructed in three dimensions using electron energy-loss spectrum imaging, resulting in a better understanding of the optical response of noble-metal nanoparticles. Observing surface excitations for nano-optics Metal nanoparticles exhibit a range of striking and useful optical properties thanks to the excitation of localized surface plasmon resonances (LSPRs). But the precise relationship between the three-dimensional structure of the nanoparticles and the resulting LSPRs can be hard to determine. Paul Midgley and colleagues have developed a spectrally sensitive imaging technique, based on electron energy-loss spectroscopy, that permits three-dimensional visualization of many of the key features associated with these LSPRs. With this technique, the interplay between the LSPRs, nanoparticle structure and substrate–nanoparticle interactions can be directly probed. This study focuses on silver nanocubes, but the method demonstrated is applicable to similar plasmonic phenomena across all metal nanoparticles. The remarkable optical properties of metal nanoparticles are governed by the excitation of localized surface plasmon resonances (LSPRs). The sensitivity of each LSPR mode, whose spatial distribution and resonant energy depend on the nanoparticle structure, composition and environment, has given rise to many potential photonic, optoelectronic, catalytic, photovoltaic, and gas- and bio-sensing applications 1 , 2 , 3 . However, the precise interplay between the three-dimensional (3D) nanoparticle structure and the LSPRs is not always fully understood and a spectrally sensitive 3D imaging technique is needed to visualize the excitation on the nanometre scale. Here we show that 3D images related to LSPRs of an individual silver nanocube can be reconstructed through the application of electron energy-loss spectrum imaging 4 , mapping the excitation across a range of orientations, with a novel combination of non-negative matrix factorization 5 , 6 , compressed sensing 7 , 8 and electron tomography 9 . Our results extend the idea of substrate-mediated hybridization of dipolar and quadrupolar modes predicted by theory, simulations, and electron and optical spectroscopy 10 , 11 , 12 , and provide experimental evidence of higher-energy mode hybridization. This work represents an advance both in the understanding of the optical response of noble-metal nanoparticles and in the probing, analysis and visualization of LSPRs.

Blinking mechanism revealed The phenomenon of fluorescence intermittency (blinking between ON/OFF states) has been observed for both naturally occurring fluorophores (such as organic dyes and biomolecules) and artificial nanostructures (such as carbon nanotubes and semiconducting nanocrystal quantum dots). This study aims to resolve the long-standing controversy surrounding the origin of photoluminescent blinking in semiconductor nanocrystals, also known as quantum dots. Researchers usually evoke the Auger, or A-type, mechanism in which a separation of charges yields to the OFF state, but recent observations have raised doubts about this explanation. Galland et al . describe a second mechanism (called B-type) in which an excited, or hot, electron becomes trapped in the shell for a time before being released to the emitting core. By controlling various parameters, such as applied voltage potential and shell thickness, the authors can control the frequency of blinking, or suppress it completely. Photoluminescence blinking—random switching between states of high (ON) and low (OFF) emissivities—is a universal property of molecular emitters found in dyes 1 , polymers 2 , biological molecules 3 and artificial nanostructures such as nanocrystal quantum dots, carbon nanotubes and nanowires 4 , 5 , 6 . For the past 15 years, colloidal nanocrystals have been used as a model system to study this phenomenon 5 , 6 . The occurrence of OFF periods in nanocrystal emission has been commonly attributed to the presence of an additional charge 7 , which leads to photoluminescence quenching by non-radiative recombination (the Auger mechanism) 8 . However, this ‘charging’ model was recently challenged in several reports 9 , 10 . Here we report time-resolved photoluminescence studies of individual nanocrystal quantum dots performed while electrochemically controlling the degree of their charging, with the goal of clarifying the role of charging in blinking. We find that two distinct types of blinking are possible: conventional (A-type) blinking due to charging and discharging of the nanocrystal core, in which lower photoluminescence intensities correlate with shorter photoluminescence lifetimes; and a second sort (B-type), in which large changes in the emission intensity are not accompanied by significant changes in emission dynamics. We attribute B-type blinking to charge fluctuations in the electron-accepting surface sites. When unoccupied, these sites intercept ‘hot’ electrons before they relax into emitting core states. Both blinking mechanisms can be electrochemically controlled and completely suppressed by application of an appropriate potential.

Ocean colour is recognised as an Essential Climate Variable (ECV) by the Global Climate Observing System (GCOS); and spectrally-resolved water-leaving radiances (or remote-sensing reflectances) in the visible domain, and chlorophyll-a concentration are identified as required ECV products. Time series of the products at the global scale and at high spatial resolution, derived from ocean-colour data, are key to studying the dynamics of phytoplankton at seasonal and inter-annual scales; their role in marine biogeochemistry; the global carbon cycle; the modulation of how phytoplankton distribute solar-induced heat in the upper layers of the ocean; and the response of the marine ecosystem to climate variability and change. However, generating a long time series of these products from ocean-colour data is not a trivial task: algorithms that are best suited for climate studies have to be selected from a number that are available for atmospheric correction of the satellite signal and for retrieval of chlorophyll-a concentration; since satellites have a finite life span, data from multiple sensors have to be merged to create a single time series, and any uncorrected inter-sensor biases could introduce artefacts in the series, e.g., different sensors monitor radiances at different wavebands such that producing a consistent time series of reflectances is not straightforward. Another requirement is that the products have to be validated against in situ observations. Furthermore, the uncertainties in the products have to be quantified, ideally on a pixel-by-pixel basis, to facilitate applications and interpretations that are consistent with the quality of the data. This paper outlines an approach that was adopted for generating an ocean-colour time series for climate studies, using data from the MERIS (MEdium spectral Resolution Imaging Spectrometer) sensor of the European Space Agency; the SeaWiFS (Sea-viewing Wide-Field-of-view Sensor) and MODIS-Aqua (Moderate-resolution Imaging Spectroradiometer-Aqua) sensors from the National Aeronautics and Space Administration (USA); and VIIRS (Visible and Infrared Imaging Radiometer Suite) from the National Oceanic and Atmospheric Administration (USA). The time series now covers the period from late 1997 to end of 2018. To ensure that the products meet, as well as possible, the requirements of the user community, marine-ecosystem modellers, and remote-sensing scientists were consulted at the outset on their immediate and longer-term requirements as well as on their expectations of ocean-colour data for use in climate research. Taking the user requirements into account, a series of objective criteria were established, against which available algorithms for processing ocean-colour data were evaluated and ranked. The algorithms that performed best with respect to the climate user requirements were selected to process data from the satellite sensors. Remote-sensing reflectance data from MODIS-Aqua, MERIS, and VIIRS were band-shifted to match the wavebands of SeaWiFS. Overlapping data were used to correct for mean biases between sensors at every pixel. The remote-sensing reflectance data derived from the sensors were merged, and the selected in-water algorithm was applied to the merged data to generate maps of chlorophyll concentration, inherent optical properties at SeaWiFS wavelengths, and the diffuse attenuation coefficient at 490 nm. The merged products were validated against in situ observations. The uncertainties established on the basis of comparisons with in situ data were combined with an optical classification of the remote-sensing reflectance data using a fuzzy-logic approach, and were used to generate uncertainties (root mean square difference and bias) for each product at each pixel.

Mapping electromagnetic hotspots It is well known that hotspots can appear on rough metallic surfaces exposed to light, where the incident light is concentrated on the nanometre scale to produce an intense electromagnetic field. This 'surface enhancement' effect can be used, for example, to detect molecules, because weak fluorescence signals are strongly enhanced by the hotspots. Such hotspots are associated with localized electromagnetic modes, caused by the randomness of the surface texture, but the detailed profile of the local electromagnetic field is so far unknown. Cang et al . now describe an ingenious experiment that exploits the Brownian motion of single molecules to probe the local field. They succeed in imaging the fluorescence enhancement profile of single hotspots on the surface of aluminium thin-film and silver nanoparticle clusters with accuracy down to 1 nm, and find that the field distribution in a hotspot follows an exponential decay. On rough metallic surfaces hotspots appear under optical illumination that concentrate light to tens of nanometres. This effect can be used to detect molecules, as weak fluorescence signals are strongly enhanced by the hotspots. Such hotspots are associated with localized electromagnetic modes, caused by the randomness of the surface texture, but the detailed profile of the local electromagnetic field is unknown. Here, an ingenious approach is described, making use of the Brownian motion of single molecules to probe the local field. The study succeeds in imaging the fluorescence enhancement profile of single hotspots on the surface of aluminium thin-film and silver nanoparticle clusters with accuracy down to one nanometre, and finds that the field distribution in a hotspot follows an exponential decay. When light illuminates a rough metallic surface, hotspots can appear, where the light is concentrated on the nanometre scale, producing an intense electromagnetic field. This phenomenon, called the surface enhancement effect 1 , 2 , has a broad range of potential applications, such as the detection of weak chemical signals. Hotspots are believed to be associated with localized electromagnetic modes 3 , 4 , caused by the randomness of the surface texture. Probing the electromagnetic field of the hotspots would offer much insight towards uncovering the mechanism generating the enhancement; however, it requires a spatial resolution of 1–2 nm, which has been a long-standing challenge in optics. The resolution of an optical microscope is limited to about half the wavelength of the incident light, approximately 200–300 nm. Although current state-of-the-art techniques, including near-field scanning optical microscopy 5 , electron energy-loss spectroscopy 6 , cathode luminescence imaging 7 and two-photon photoemission imaging 8 have subwavelength resolution, they either introduce a non-negligible amount of perturbation, complicating interpretation of the data, or operate only in a vacuum. As a result, after more than 30 years since the discovery of the surface enhancement effect 9 , 10 , 11 , how the local field is distributed remains unknown. Here we present a technique that uses Brownian motion of single molecules to probe the local field. It enables two-dimensional imaging of the fluorescence enhancement profile of single hotspots on the surfaces of aluminium thin films and silver nanoparticle clusters, with accuracy down to 1.2 nm. Strong fluorescence enhancements, up to 54 and 136 times respectively, are observed in those two systems. This strong enhancement indicates that the local field, which decays exponentially from the peak of a hotspot, dominates the fluorescence enhancement profile.