School of Chemistry - Research Publications
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ItemDirect Assembly of Large Area Nanoparticle Arrays(AMER CHEMICAL SOC, 2018-08)A major goal of nanotechnology is the assembly of nanoscale building blocks into functional optical, electrical, or chemical devices. Many of these applications depend on an ability to optically or electrically address single nanoparticles. However, positioning large numbers of single nanocrystals with nanometer precision on a substrate for integration into solid-state devices remains a fundamental roadblock. Here, we report fast, scalable assembly of thousands of single nanoparticles using electrophoretic deposition. We demonstrate that gold nanospheres down to 30 nm in size and gold nanorods <100 nm in length can be assembled into predefined patterns on transparent conductive substrates within a few seconds. We find that rod orientation can be preserved during deposition. As proof of high fidelity scale-up, we have created centimeter scale patterns comprising more than 1 million gold nanorods.
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ItemDirected Chemical Assembly of Single and Clustered Nanoparticles with Silanized Templates(AMER CHEMICAL SOC, 2018-06-26)The assembly of nanoscale materials into arbitrary, organized structures remains a major challenge in nanotechnology. Herein, we report a general method for creating 2D structures by combining top-down lithography with bottom-up chemical assembly. Under optimal conditions, the assembly of gold nanoparticles was achieved in less than 30 min. Single gold nanoparticles, from 10 to 100 nm, can be placed in predetermined patterns with high fidelity, and higher-order structures can be generated consisting of dimers or trimers. It is shown that the nanoparticle arrays can be transferred to, and embedded within, polymer films. This provides a new method for the large-scale fabrication of nanoparticle arrays onto diverse substrates using wet chemistry.
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ItemDirect Assembly of Large Area Nanoparticle Arrays( 2018-07-13)We describe the fabrication of large area arrays of single nanoparticles using electrophoretic deposition.
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ItemHot-Carrier Organic Synthesis via the Near-Perfect Absorption of Light(AMER CHEMICAL SOC, 2018-11-01)Photocatalysis enables the synthesis of valuable organic compounds by exploiting photons as a chemical reagent. Although light absorption is an intrinsic step, existing approaches rely on poorly absorbing catalysts that require high illumination intensities to afford enhanced efficiencies. Here, we demonstrate that a plasmonic metamaterial capable of near-perfect light absorption (94%) readily catalyzes a model organic reaction with a 29-fold enhancement in conversion relative to controls. The oxidation of benzylamine proceeds via a reactive iminium intermediate with high selectivity at ambient temperature and pressure, using only low-intensity visible irradiation. Control experiments demonstrated that only hot charge carriers produced following photoexcitation facilitate the formation of superoxide radicals, which, in turn, leads to iminium formation. Modeling shows that hot holes with energies that overlap with the highest-occupied molecular orbital (HOMO) of the reactant can participate and initiate the photocatalytic conversion. These results have important implications for hot-carrier photocatalysis and plasmon-hot-carrier extraction.
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ItemLarge-Area Nanofabrication of Partially Embedded Nanostructures for Enhanced Plasmonic Hot-Carrier Extraction(AMER CHEMICAL SOC, 2019-03-01)When plasmonic nanoparticles are coupled with semiconductors, highly energetic hot carriers can be extracted from the metal-semiconductor interface for various applications in light energy conversion. However, the current quantum yields for hot-electron extraction are generally low. An approach for increasing the extraction efficiency consists of maximizing the contact area between the surface of the metal nanostructure and the electron-accepting material. In this work, we developed an innovative, simple, and scalable fabrication technique that partially embeds colloidal plasmonic nanostructures within a semiconductor TiO2 layer without utilizing any complex top-down nanofabrication method. The successful embedding is confirmed by scanning electron microscopy and atomic force microscopy imaging. Using visible-pump, near-IR probe transient absorption spectroscopy, we also provide evidence that the increase in the surface contact area between the nanostructures and the electron-accepting material leads to an increase in the amount of hot-electron injection into the TiO2 layer.
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ItemLuminescence of a Transition Metal Complex Inside a Metamaterial Nanocavity(John Wiley & Sons, 2017-08-25)Modification of the local density of optical states using metallic nanostructures leads to enhancement in the number of emitted quanta and photocatalytic turnover of luminescent materials. In this work, the fabrication of a metamaterial is presented that consists of a nanowire separated from a metallic mirror by a polymer thin film doped with a luminescent organometallic iridium(III) complex. The large spin–orbit coupling of the heavy metal atom results in an excited state with significant magnetic-dipole character. The nanostructured architecture supports two distinct optical modes and their assignment achieved with the assistance of numerical simulations. The simulations show that one mode is characterized by strong confinement of the electric field and the other by strong confinement of the magnetic field. These modes elicit drastic changes in the emitter’s photophysical properties, including dominant nanocavity-derived modes observable in the emission spectra along with significant increases in emission intensity and the total decay rate. A combination of simulations and momentum-resolved spectroscopy helps explain the mechanism of the different interactions of each optical mode supported by the metamaterial with the excited state of the emitter.
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ItemNo Preview AvailablePhotoinduced Electron Transfer in the Strong Coupling Regime: Waveguide-Plasmon Polaritons(AMER CHEMICAL SOC, 2016-04)Reversible exchange of photons between a material and an optical cavity can lead to the formation of hybrid light-matter states where material properties such as the work function [ Hutchison et al. Adv. Mater. 2013 , 25 , 2481 - 2485 ], chemical reactivity [ Hutchison et al. Angew. Chem., Int. Ed. 2012 , 51 , 1592 - 1596 ], ultrafast energy relaxation [ Salomon et al. Angew. Chem., Int. Ed. 2009 , 48 , 8748 - 8751 ; Gomez et al. J. Phys. Chem. B 2013 , 117 , 4340 - 4346 ], and electrical conductivity [ Orgiu et al. Nat. Mater. 2015 , 14 , 1123 - 1129 ] of matter differ significantly to those of the same material in the absence of strong interactions with the electromagnetic fields. Here we show that strong light-matter coupling between confined photons on a semiconductor waveguide and localized plasmon resonances on metal nanowires modifies the efficiency of the photoinduced charge-transfer rate of plasmonic derived (hot) electrons into accepting states in the semiconductor material. Ultrafast spectroscopy measurements reveal a strong correlation between the amplitude of the transient signals, attributed to electrons residing in the semiconductor and the hybridization of waveguide and plasmon excitations.
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ItemNo Preview AvailableHot Carrier Extraction with Plasmonic Broadband Absorbers(American Chemical Society, 2016-04-01)Hot charge carrier extraction from metallic nanostructures is a very promising approach for applications in photocatalysis, photovoltaics, and photodetection. One limitation is that many metallic nanostructures support a single plasmon resonance thus restricting the light-to-charge-carrier activity to a spectral band. Here we demonstrate that a monolayer of plasmonic nanoparticles can be assembled on a multistack layered configuration to achieve broadband, near-unit light absorption, which is spatially localized on the nanoparticle layer. We show that this enhanced light absorbance leads to ∼40-fold increases in the photon-to-electron conversion efficiency by the plasmonic nanostructures. We developed a model that successfully captures the essential physics of the plasmonic hot electron charge generation and separation in these structures. This model also allowed us to establish that efficient hot carrier extraction is limited to spectral regions where (i) the photons have energies higher than the Schottky junctions and (ii) the absorption of light is localized on the metal nanoparticles.
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ItemFilling schemes at submicron scale: Development of submicron sized plasmonic colour filters(NATURE PORTFOLIO, 2014-09-22)The pixel size imposes a fundamental limit on the amount of information that can be displayed or recorded on a sensor. Thus, there is strong motivation to reduce the pixel size down to the nanometre scale. Nanometre colour pixels cannot be fabricated by simply downscaling current pixels due to colour cross talk and diffraction caused by dyes or pigments used as colour filters. Colour filters based on plasmonic effects can overcome these difficulties. Although different plasmonic colour filters have been demonstrated at the micron scale, there have been no attempts so far to reduce the filter size to the submicron scale. Here, we present for the first time a submicron plasmonic colour filter design together with a new challenge - pixel boundary errors at the submicron scale. We present simple but powerful filling schemes to produce submicron colour filters, which are free from pixel boundary errors and colour cross- talk, are polarization independent and angle insensitive, and based on LCD compatible aluminium technology. These results lay the basis for the development of submicron pixels in displays, RGB-spatial light modulators, liquid crystal over silicon, Google glasses and pico-projectors.
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ItemRefractive-index profiling of optical fibers with axial symmetry by use of quantitative phase microscopy(OPTICAL SOC AMER, 2002-12-01)The application of quantitative phase microscopy to refractive-index profiling of optical fibers is demonstrated. Phase images of axially symmetric optical fibers immersed in index-matching fluid are obtained, and the inverse Abel transform is used to obtain the radial refractive-index profile. This technique is straightforward, nondestructive, repeatable, and accurate. Excellent agreement, to within approximately 0.0005, between this method and the index profile obtained with a commercial profiler is obtained.