School of Physics - Research Publications
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ItemMultilevel nanoimprint lithography with a binary mould for plasmonic colour printing(ROYAL SOC CHEMISTRY, 2020-05-01)Pigment-free colouration based on plasmonic resonances has recently attracted considerable attention for potential in manufacturing and other applications. For plasmonic colour utilizing the metal-insulator-metal (MIM) configuration, the generated colour is not only dependent on the geometry and transverse dimensions, but also to the size of the vertical gap between the metal nanoparticles and the continuous metal film. The complexity of conventional fabrication methods such as electron beam lithography (EBL), however, limits the capacity to control this critical parameter. Here we demonstrate the straightforward production of plasmonic colour via UV-assisted nanoimprint lithography (NIL) with a simple binary mould and demonstrate the ability to control this gap distance in a single print by harnessing the nanofluidic behaviour of the polymer resist through strategic mould design. We show that this provides a further avenue for controlling the colour reflected by the resulting plasmonic pixels as an adjunct to the conventional approach of tailoring the transverse dimensions of the nanostructures. Our experimental results exhibit wide colour coverage of the CIE 1931 XY colour space through careful control of both the length and periodicity and the resulting vertical gap size of the structure during the nanoimprinting process. Furthermore, to show full control over the vertical dimension, we show that a fixed gap size can be produced by introducing complementary microcavities in the vicinity of the nanostructures on the original mould. This demonstrates a simple method for obtaining an additional degree of freedom in NIL not only for structural colouration but also for other industrial applications such as high-density memory, biosensors and manufacturing.
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ItemDirect Assembly of Vertically Oriented, Gold Nanorod Arrays(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.