School of Physics - Research Publications
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ItemA General Method for Direct Assembly of Single Nanocrystals(WILEY-V C H VERLAG GMBH, 2022-05-28)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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ItemDirect Assembly of Large Area Nanoparticle Arrays(AMER CHEMICAL SOC, 2018-08-01)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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ItemOptical Janus Effect in Large Area Multilayer Plasmonic Films(Wiley, 2022)Plasmonic and other nanoparticles have attracted considerable interest for their role in structural coloration. The optical “Janus” effect, where the color of light reflected from a partially transmitting film depends on whether the device is viewed from the substrate or the coating side, is observed using a variety of nanostructured films. Herein, the optical Janus effect produced by homogeneous thin-film structures comprising only four layers of three different materials with a total thickness less than 300 nm is demonstrated. An asymmetric Fabry–Perot (FP) nanocavity is formed with a dielectric film bounded by two different metal films of nanoscale thickness. The semitransparent device has a transmitted color that is independent of the viewing direction. A broad color palette is available through the selection of various thicknesses and film materials. In addition to the directional optical effect, the device possesses iridescence properties and can generate images by selective removal of regions of one of the metallic films using simple photolithography. From a manufacturing perspective, this device is scalable and holds significant promise for applications in architecture, producing decorative features, and the development of overt and covert security features.
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ItemNo Preview AvailableVivid plasmonic color under ambient light(Optical Society of America (OSA), 2021-12-06)We report a novel nanoimprinted polarization-independent plasmonic pixel device utilizing different metals (Ag, Al or Au) exhibiting fade-resistant, vivid coloration under unpolarized light achieved with symmetric cross-shaped nanoantenna-hole structures. The spectral and color responses show minimal sensitivity to the polarization state of the incident light, both in reflection and transmission. The devices also have good tolerance to variations in viewing angle. Various colors are generated by simply adjusting the armlength of the cross and through choice of metal. Among all the devices, those fabricated using Ag demonstrated the best performance with 80% reflection and 12% transmission efficiencies and the production of brighter colors. With the ease of fabrication using a high-throughput NIL method, the plasmonic color devices have significant potential in sensing technology, high-resolution color printing and product-branding applications.
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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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ItemScalable and Consistent Fabrication of Plasmonic colors via Nanoimprint Lithography(SPIE-INT SOC OPTICAL ENGINEERING, 2019-01-01)We utilised thermal and UV-assisted Nanoimprint Lithography (NIL) i.e. thermal and UV-assisted to produce plasmonic coloration, and compare their ability for scalable fabrication. Several designs are presented and we show the generated colors are dependent on their geometry and the direction of polarisation of incident illumination. Finally, we demonstrated UV-NIL for consistent production of large-area (0.6×0.4 cm2) plasmonic color with extended color gamut.
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ItemPlasmonics-enabled metal-semiconductor-metal photodiodes for high-speed interconnects and polarization sensitive detectors(SPIE-INT SOC OPTICAL ENGINEERING, 2017-01-01)Metal-semiconductor-metal (MSM) photodiodes are commonly used in ultrafast photoelectronic devices. Re- cently it was shown that localized surface plasmons can su_ciently enhance photodetector capabilities at both infrared and visible wavelengths. Such structures are of great interest since they can be used for fast, broadband detection. By utilizing the properties of plasmonic structures it is possible to design photodetectors that are sensitive to the polarization state of the incident wave. The direct electrical readout of the polarization state of an incident optical beam has many important applications, especially in telecommunications, bio-imaging and photonic computing. Furthermore, the fact that surface plasmon polaritons can circumvent the di_raction limit, opens up signi_cant opportunities to use them to guide signals between logic gates in modern integrated circuits where small dimensions are highly desirable. Here we demonstrate two MSM photodetectors integrated with aluminum nanoantennas capable of distinguishing orthogonal states of either linearly or circularly polarized light with no additional _lters. The localized plasmon resonances of the antennas lead to selective screening of the un- derlying silicon from light with a particular polarization state. The non-null response of the devices to each of the basis states expands the potential utility of the photodetectors while improving precision. We also demonstrate a design of waveguide-coupled MSM photodetector suitable for planar detection of surface plasmons.
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ItemUltracompact Camera Pixel with Integrated Plasmonic Color Filters(WILEY-V C H VERLAG GMBH, 2019-09-17)Photodetector size imposes a fundamental limit on the amount of information that can be recorded by an image sensor. Compact, high-resolution sensors are generally preferred for portable electronic devices such as mobile phones and digital cameras, and as a result, a significant effort has been invested in improving the image quality provided by small-area image sensors. Reducing photodetector size, however, still faces challenges in implementation requiring improvements in current technology to meet the demand for ultracompact imaging systems such as cameras. An issue with a decrease in size is associated with photodetectors utilizing color filters. In most commonly used camera designs these filters are made of dyes or pigments and incompatible with the complementary metal-oxide-semiconductor fabrication process. They are, therefore, fabricated in two different technological processes and require subsequent alignment. As the pixel size decreases, the alignment of these layers becomes challenging. Furthermore, dye-based filters need to have a thickness of the order of micrometers to ensure sufficient absorption. Here a compact, low-cost color sensor is proposed and experimentally demonstrated utilizing monolithically integrated plasmonic antennas that have a nanoscale thickness and are fabricated in the same technological process with photodetector matrix.
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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.