School of Physics - 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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ItemAlgorithm-Designed Plasmonic Nanotweezers: Quantitative Comparison by Theory, Cathodoluminescence, and Nanoparticle Trapping(WILEY-V C H VERLAG GMBH, 2021-10)Abstract Plasmonic apertures permit optical fields to be concentrated into sub‐wavelength regions. This enhances the optical gradient force, enabling the precise trapping of nanomaterials such as quantum dots, proteins, and DNA molecules at modest laser powers. Double nanoholes, coaxial apertures, bowtie apertures, and other structures have been studied as plasmonic nanotweezers, with the design process generally comprising intuition followed by electromagnetic simulations with parameter sweeps. Here, instead, a computational algorithm is used to design plasmonic apertures for nanoparticle trapping. The resultant apertures have highly irregular shapes that, in combination with ring couplers also optimized by algorithm, are predicted to generate trapping forces more than an order of magnitude greater than those from the double nanohole design used as the optimization starting point. The designs are realized by fabricating precision apertures with a helium/neon ion microscope and are studied them by cathodoluminescence and optical trapping. It is shown that, at every laser intensity, the algorithm‐designed apertures can trap particles more tightly than the double nanohole.
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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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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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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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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.
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ItemPlasmonic Metasurface-Enabled Differential Photodetectors for Broadband Optical Polarization Characterization(AMER CHEMICAL SOC, 2016-10)
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ItemIn-Plane Detection of Guided Surface Plasmons for High-Speed Optoelectronic Integrated Circuits(WILEY, 2018-01)Abstract Constrains on the speed of modern digital integrated circuits are dominated by the metallic interconnects between logic gates. Surface plasmon polaritons have potential to overcome this limitation and greatly increase the operating speed of future digital devices. Nevertheless, an ongoing issue is the compatibility of modern planar microelectronic circuits with current methods for detecting surface plasmons. Here, a new approach to in‐plane surface plasmon polariton detection is proposed and experimentally demonstrated. The design is based on metal–semiconductor–metal photodetectors that are acknowledged as having one of the best speed characteristics among photodetectors. In the design, the photodetector structure also plays a dual role as the outcoupling grating for surface plasmons, significantly reducing the footprint of the resulting device. The technique has the potential to enable the integration of surface plasmons as signal carriers in future high‐speed optoelectronic integrated circuits.
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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.