School of Chemistry - Research Publications

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    An investigation of evanescent wave-induced fluorescence spectroscopy for exploring high refractive index media
    Chakraborty, S ; Xu, Y ; Roberts, A ; Goswami, D ; Smith, TA (IOP Publishing Ltd, 2023-01-01)
    Abstract Evanescent wave-induced fluorescence spectroscopy (EWIFS) is a widely used technique for probing the interfacial behavior of different complex media in investigations of samples in the physical, chemical, and biological sciences. This technique takes advantage of the sharply decaying evanescent field, established following total internal reflection (TIR) at the interface of two media, for spatially identifying the photoluminescence characteristics of the sample. The generation of the evanescent field requires the refractive index of the second medium to be lower than that of the first, so a major disadvantage of this increasingly widely used spectroscopic technique is the inability to exploit the advantages of EWIFS to image a sample with a higher refractive index than the incident substrate medium. A proposed configuration in which a thin, low refractive index intermediate layer is established between the TIR substrate and a high refractive index sample is investigated. We illustrate that this arrangement does not afford the desired advantages of evanescent field-induced fluorescence measurements for investigating high refractive index media.
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    Deconstructed beetles: Bilayered composite materials produce green coloration with remarkably high near-infrared reflectance
    Ospina-Rozo, L ; Priscilla, N ; Hutchison, J ; van de Meene, A ; Roberts, NW ; Stuart-Fox, D ; Roberts, A (ELSEVIER, 2023-06)
    Beetle elytra (hardened forewings) are a promising source of inspiration to develop or enhance the performance of human-fabricated composite materials. The structures responsible for optical properties in the ultra-violet to visible spectrum (300–700 nm) have been extensively characterised, but we have limited knowledge of optical properties and their physical origin in the near-infrared (NIR; 700–1700 nm). We examined the elytra of three species of green scarab beetles (Xylonichus eucalypti, Anoplognathus prasinus and Paraschizognathus olivaceus) with very high NIR reflectance. We manually separated layers in the elytra to disambiguate their contributions to the overall optical response. We show that unlike other scarabs, nanostructures within the cuticle layer do not produce notable reflectance. Instead, the cuticle resembles a pigment-based filter with 50% transmittance in the NIR and absorption in the visible spectrum contributing to the green appearance. Each species has a layer below the cuticle that appears white to the naked eye and produces broadband reflectance, particularly in the near-infrared; however, the structure of the white underlay differs markedly between the three species. In A. prasinus and P. olivaceus, the structure is disordered (no regular, repeated elements at optical length scales); whereas in Xylonichus eucalypti, the white underlay was notably thinner and comprised quasi-ordered hollow cylindrical structures embedded in a chitin matrix. We modelled the coherent scattering produced by this structure to demonstrate that it is responsible for broadband visible and NIR reflectance. We discuss biological implications and technological applications of the composite structure of beetle elytra.
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    A General Method for Direct Assembly of Single Nanocrystals
    Zhang, H ; Liu, Y ; Ashokan, A ; Gao, C ; Dong, Y ; Kinnear, C ; Kirkwood, N ; Zaman, S ; Maasoumi, F ; James, TD ; Widmer-Cooper, A ; Roberts, A ; Mulvaney, P (WILEY-V C H VERLAG GMBH, 2022-07)
    Abstract Controlled nanocrystal assembly is a pre‐requisite for incorporation of these materials into solid state devices. Many assembly methods have been investigated which target precise nanocrystal positioning, high process controllability, scalability, and universality. However, most methods are unable to achieve all of these goals. Here, surface templated electrophoretic deposition (STED) is presented as a potential assembly method for a wide variety of nanocrystals. Controlled positioning and deposition of a wide range of nanocrystals into arbitrary spatial arrangements − including gold nanocrystals of different shapes and sizes, magnetic nanocrystals, fluorescent organic nanoparticles, and semiconductor quantum dots − is demonstrated. Nanoparticles with diameters <10 nm are unable to be deposited due to their low surface charge and strong Brownian motion (low Péclet number). It is shown that this limit can be circumvented by forming clusters of nanocrystals or by silica coating nanocrystals to increase their effective size.
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    Direct Assembly of Large Area Nanoparticle Arrays
    Zhang, H ; Cadusch, J ; Kinnear, C ; James, T ; Roberts, A ; Mulvaney, P (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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    Directed Chemical Assembly of Single and Clustered Nanoparticles with Silanized Templates
    Kinnear, C ; Cadusch, J ; Zhang, H ; Lu, J ; James, TD ; Roberts, A ; Mulvaney, P (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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    Concealed Structural Colors Uncovered by Light Scattering
    Akinoglu, EM ; Song, J ; Kinnear, C ; Xue, Y ; Zhang, H ; Roberts, A ; Koehler, J ; Mulvaney, P (WILEY-V C H VERLAG GMBH, 2020-11)
    Abstract Unusual structural colors are demonstrated in thin‐film coatings due to a combination of optical interference and light scattering effects. These vivid colors are concealed under ambient illumination but can be observed when light is reflected from the film surface. The origin of the effect is explored computationally and it is shown that, in thin‐films of lossless dielectrics coated on near‐perfect conductors, incident electromagnetic waves form standing waves. Electric field intensities at the thin film interfaces are maximized for wavelengths that fulfil destructive interference conditions, while nanoscale roughness can enhance scattering at these boundaries. The interplay of these two factors yields vivid, thickness‐dependent colors. This approach increases the repertoire of optical effects and perceived colors in thin coatings. When combined with traditional thin‐film interference colours, dichromatic images with distinctly changing colors can be generated, which can function as a covert, optical security feature.
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    Direct Assembly of Large Area Nanoparticle Arrays
    Mulvaney, P ; ZHANG, H ; KINNEAR, C ; Cadusch, J ; JAMES, T ; ROBERTS, ANN ( 2018-07-13)
    We describe the fabrication of large area arrays of single nanoparticles using electrophoretic deposition.
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    Hot-Carrier Organic Synthesis via the Near-Perfect Absorption of Light
    Xiao, Q ; Connell, TU ; Cadusch, JJ ; Roberts, A ; Chesman, ASR ; Gomez, DE (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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    Large-Area Nanofabrication of Partially Embedded Nanostructures for Enhanced Plasmonic Hot-Carrier Extraction
    Ng, C ; Zeng, P ; Lloyd, JA ; Chakraborty, D ; Roberts, A ; Smith, TA ; Bach, U ; Sader, JE ; Davis, TJ ; Gomez, DE (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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    Direct Assembly of Vertically Oriented, Gold Nanorod Arrays
    Zhang, H ; Liu, Y ; Shahidan, MFS ; Kinnear, C ; Maasoumi, F ; Cadusch, J ; Akinoglu, EM ; James, TD ; Widmer-Cooper, A ; Roberts, A ; Mulvaney, P (WILEY-V C H VERLAG GMBH, 2021-02-03)
    Although many nanoscale materials such as quantum dots and metallic nanocrystals exhibit size dependent optical properties, it has been difficult to incorporate them into optical or electronic devices because there are currently no methods for precise, large‐scale deposition of single nanocrystals. Of particular interest is the need to control the orientation of single nanocrystals since the optical properties are usually strongly anisotropic. Here a method based on electrophoretic deposition (EPD) is reported to precisely assemble vertically oriented, single gold nanorods. It is demonstrated that the orientation of gold nanorods during deposition is controlled by the electric dipole moment induced along the rod by the electric field. Dissipative particle dynamics simulations indicate that the magnitude of this dipole moment is dominated by the polarizability of the solution phase electric double layer around the nanorod. The resulting vertical gold nanorod arrays exhibit reflected colors due to selective excitation of the transverse surface plasmon mode. The EPD method allows assembly of arrays with a density of over one million, visually resolvable, vertical nanorods per square millimeter.