1,721,001 research outputs found
Dataset for Laser-Driven Phase Segregation and Tailoring of Compositionally Graded Microstructures in Si-Ge Nanoscale Thin Films
No full textDataset supports: Aktas, Ozan, Oo, Swe, MacFarquhar, Stuart, James, Mittal, Vinita, Chong, Harold and Peacock, Anna(2020). Laser-Driven Phase Segregation and Tailoring of Compositionally Graded Microstructures in Si-Ge Nanoscale Thin Films. ACS Applied Materials & Interfaces. DOI: http://dx.doi.org/10.1021/acsami.9b22135</span
Effect of non-conformal gold deposition on SERS related plasmonic effects
No full textRecently, a comprehensive three dimensional computational model based on rigorous coupled wave analysis (RCWA) has been developed to investigate the properties of surface plasmons resident on metal coated arrays of inverted pyramidal pits used for SERS sensing applications in the form of ‘klarite’. This simulation tool allows the identification of a variety of dispersive features including propagating and localized surface plasmons as well as simple diffraction relating to the influence of geometrical features. In this paper, we investigate the influence of non-conformality of the gold coating over the internal surfaces of the inverted pyramidal pits on plasmon dispersion. Modeling reveals very strong changes in plasmon behavior as a function of gold layer conformality. Dependent upon conformality of the gold coating we find that the nano-textured metallic surface can behave either as an efficient broadband mirror-like reflector or as an efficient broadband, wide angle absorber at infrared wavelengths. Creation of a broadband wide angle absorbing surface such as this has important implications for photovoltaic cells. For sensing applications, understanding the effect of metal layer conformality on plasmon dispersion gives clear insight into how to further improve the SERS enhancement factor
Design, fabrication and optimization of large area chemical sensor based on Surface-enhanced Raman Scattering (SERS) mechanism
No full textIn recent years there has been an increasing interest in analysis and identification of complex molecules for the medical diagnostics, pharmaceutical research and homeland security applications. If these molecules are present in high concentration, a technique known as Raman spectroscopy can be utilized. Unfortunately, only one in every 1012 photon incidence on molecule undergoes Raman scattering resulting in weak Raman absorption. An efficient technique to overcome this limitation is to utilize surface-enhanced Raman scattering (SERS) whereby molecules are placed on the surface of nanostructured metallic substrate which performs the function of transducting photons into and out of the molecules. SERS extends the scope of Raman scattering to detect molecules at low concentrations to few/single molecule level. Previously the ‘KlariteTM’ substrate consisting of an inverted array of square based pyramidal nanostructures patterned onto a Silicon substrate has been demonstrated to afford highly reproducible SERS signals with approximately 107 enhancement factor.In this report, the effect of geometrical parameters associated with the inverted pyramidal array on SERS effect for sensing applications was investigated. Geometrical parameters studied include pitch length, pit size, aspect ratio of the base of pyramid and fill factor. 3D computational modelling based on Rigorous Coupled Wave Analysis (RCWA) is used to bridge with theory. From these observations, the geometrical parameters of inverted pyramid nanostructures have been optimized for better sensing ability. A test chip is fabricated for the purpose of performing a matrix experiment, allowing deconvolution of geometrical variables: lattice pitch (1000nm-3000nm), pit size (500nm-2500nm), pit aspect ratio [width to length]. Fabrication steps include electron-beam lithography, anisotropic wet etching and metallization.Computational and experimental reflectometry systems were applied to enable the identification and analysis of a variety of dispersive features including propagating surface plasmons, localized surface plasmons and diffraction dispersion. From the study of inverted pyramid, plasmonic behaviors are observed: 1. the surface plasmon polariton depends strongly on polarization. 2. Highly dispersive features arising from simple surface diffraction effects appear insensitive to polarization state. 3. As the pit size gets bigger, the diffraction efficiency decreases but the wavelength/angular position remain the same. 4. Diffractive features are relatively sharp and clearly defined (narrow bandwidth), and are highly dependent on lattice pitch. Hence they move in wavelength and angle (e.g. highly dispersive) as pitch is varied. These features relate to the coupling of light into or out of the sensor chip. 5. Localized surface plasmons have characteristic of small wavelength shift over wide angular range (low dispersion), and are generally broader in bandwidth. Plasmon features can conclusively be identified over diffractive features by making comparisons between simulations ‘with’ and ‘without’ the top metal coating.In order to derive the optimal geometry for SERS sensor, a highly stable test molecule which is known to form a monolayer coating on gold is required. For this purpose benzenethiol was used as standard in this work. Devices were tested using a Renishaw Invia Raman system. The main wavelength of interest here is 785nm where this laser is readily available and compatible with the end user Raman system. Full details of the optical and Raman measurements are carried out on the silicon test platform. Results show that the averaged SERS enhancement factor was only slightly dependent upon lattice pitch, but was highly dependent on pit size and aspect ratio. Density of the pits plays a further role simply by increasing the number of pits/unit area and so provides extra increase in SERS signal. The experimental data shows this is not simply a surface area dependent effect, but the optimal SERS signal can be obtained by close packing as tightly as possible pits of the optimal size. Minimum spacing (between adjacent pits) of 250nm is found to give the highest SERS enhancement. The optimal aspect ratio was found to be 1:1.2 and the optimal pit size determined to be 1000nm. This new optimized design shows 10-fold improvement in sensitivity compared to current available benchmark Klarite. The study has also explored the possibility of replicating the optimized design to a cost effective and disposable polymer for the purpose of mass production. This was carried out using nanoimprint lithography. The replicated plastic sensor is comparable to the benchmark silicon Klarite. As a proof of principle, the qualitative performance in two demonstrator molecules such as ibuprofen and melamine has been carried out. The disposable plastic sensor was demonstrated for the possibility of dual sensing mechanisms such as surface plasmon resonance (SPR) and surface-enhanced Raman scattering (SERS). The sensitivity of plastic sensor using SPR mechanism is 225.83nm/RIU on thiol molecule. The work has also been carried out for an alternative SERS sensor design by changing the sidewall profile to 90º angle from (100) silicon etched plane. Changing the sidewall profile makes impact on the plasmonic behaviour. The straight sidewalls are favourable to the localized plasmon mode. The structures with slope sidewalls are favourable to both localized and propagating plasmons inside the cavities. This work was conducted as part of the FP7 "PHOTOSENSE" consortium project
Optimization of SERS enhancement from nanostructured metallic substrate based on arrays of inverted rectangular pyramids and investigation of effect of lattice non-symmetry
No full textIntegration of nanostructures and waveguide core for surface enhanced Raman spectroscopy: a novel excitation method
No full textSurface Enhanced Raman Spectroscopy (SERS) allows the intensity of Raman scattering to be enhanced by a factor of 106 by placing molecules within a few nm of a rough metal surface. In this paper we investigate a completely different configuration for the excitation mechanism, incorporating an optical waveguide beneath a nano-structured precious metal surface. The pyramidal geometry projects the Plasmon field into free space, thus increasing the cross section of interaction between the analyte molecules and optical fields, thereby increasing device sensitivity. In this arrangement the excitation field comes from underneath and enters the nanostructures at the base. This allows the emission to reach the discrete sensing areas effectively and provides ideal parameters for maximum Raman interactions. Using FDTD modeling methods the waveguide coupled SERS nanostructures were analyzed and its performance at different gold thicknesses was determined. The model investigates efficiency of coupling between the waveguide and surface plasmons, but also investigates spatial localization around sharp features of the geometry. Thin films of aluminum oxide and silicon oxynitride were reactively sputtered and characterized to determine their suitability as the waveguide core material. It was found that silicon oxynitride slab waveguide losses were too high to be considered as the core. The 2D and 3D simulations were based on an aluminum oxide core
Nonlinear characterization of laser processed polysilicon waveguides for integrated photonics
No full textPolycrystalline silicon offers the full complement of functionality required for integrated optoelectronic systems, including nonlinear optical processing. Here we report low loss laser-crystallized polycrystalline silicon waveguides with nonlinear coefficients equivalent to those of crystalline silicon
Large-area 2D-MoS<sub>2</sub>/black-Si heterostructure for next-generation energy storage
No full textA global shift to clean, low-carbon technologies requires development of low-cost, highly efficient energy storage. In particular, the electrode-electrolyte interface is key in development of stable, high-efficiency batteries [1], with high surface-area electrodes in the nano-dimensions being the pinnacle [2].The usage of two-dimensional (2D) materials in energy storage is continuously expanding [3] due to efficient ion transport between the single-atomic layers, and superior, atomically smooth, surface-areas leading to increased ion adsorption and surface reactions [4]. Further surface-area enhancement of the 2D materials, using scalable industrial compatible processes, could revolutionise the energy storage sector. In this work, ultra-high surface area “2D-MoS2black-Si” heterostructures were developed by merging two large-area scalable processes. Nanoscale grass-like black-Si acts as a scaffold for direct 2D growth, avoiding costly and complex transfer processes, significantly enhancing the surface-area of 2D-MoS2. These hybrids are expected to out-perform current state-of-the-art, when used for applications such as battery anodes [5], but also lend themselves to gas sensing, water splitting and solar cells [6,7].Our two large-area processes are top-down vertical silicon nanowires (SiNWs) and a 2-step MoS<sub>2</sub> growth. The former uses AgNO<sub>3</sub> and hydrofluoric (HF) acid solution, with a cyclic process on the Si surface, caused by nucleation from Ag nanoparticles and etching from HF. This results in grass-like nanowires, whose height is controlled by etch time. The Si surfaces boast ultra-low broadband reflectivity (&lt;1%), and are typically used in photovoltaics [8]. The 2-step MoS2 technique firstly deposits MoO3 one atomic layer at a time via atomic layer deposition (ALD), followed by an anneal in hydrogen disulphide, converting the layers to MoS2. This offers significant advantage over other MoS2/SiNW heterostructure work published to date, which use electrodeposition, hydrothermal, or conventional chemical vapour depositions techniques [9-12]. Not only is our technique large-area compatible, but we are able to fine-tune the number of MoS2 layers via ALD cycles, giving us greater control and quality, whilst also using the MoO3 layer to protect the SiNW from the sulphur exposure in the anneal step. The result is a high-quality MoS2, with layer number optimisation, conformally coating large-areas of black-Si.The MoS2/black-Si stacks were characterised using Raman Spectroscopy and photoluminescence (PL) for measuring MoS2 layer number and quality, scanning electron microscopy (SEM) for assessing potential nanowire damage and transmission electron microscopy (TEM) for conformality and in-depth MoS2 analysis. The use of an alumina interfacial layer, via ALD, was also investigated.We successfully grew large-area monolayer and multilayer MoS2 directly onto black-Si, with no nanowire degradation. By integrating an alumina interfacial layer, we further increased the MoS2 quality, with fewer defects. This demonstrates the compatibility of our technique for fabricating scalable high-quality 2D-MoS2/black-Si heterostructures in a tuneable and highly controllable way. Our next step is to directly assess the impact of our 2D material on silicon nanowire electrodes for next generation batteries, by using Electrochemical Impedance Spectroscopy.References:[1] 10.3389/fchem.2020.00821 [2] 10.1016/j.ssi.2016.11.028 [3] 10.1016/j.cclet.2019.10.028 [4] 10.1002/aenm.201600025 [5] 10.1002/adfm.200601186 [6] 10.1021/acsami.8b08114 [7] 10.1039/C4CS00455H [8] 10.1016/j.solmat.2016.10.044 [9] 10.1039/C7RA13484C [10] 10.1007/s12274-014-0673-y [11] 10.1007/s12633-018-0014-y [12] 10.1016/j.matdes.2016.07.09
Integrated waveguide and nanostructured sensor platform for surface-enhanced Raman spectroscopy
No full textLimitations of current sensors include large dimensions, sometimes limited sensitivity and inherent single-parameter measurement capability. Surface-enhanced Raman spectroscopy can be utilized for environment and pharmaceutical applications with the intensity of the Raman scattering enhanced by a factor of 106. By fabricating and characterizing an integrated optical waveguide beneath a nanostructured precious metal coated surface a new surface-enhanced Raman spectroscopy sensing arrangement can be achieved. Nanostructured sensors can provide both multiparameter and high-resolution sensing. Using the slab waveguide core to interrogate the nanostructures at the base allows for the emission to reach discrete sensing areas effectively and should provide ideal parameters for maximum Raman interactions. Thin slab waveguide films of silicon oxynitride were etched and gold coated to create localized nanostructured sensing areas of various pitch, diameter, and shape. These were interrogated using a Ti:Sapphire laser tuned to 785-nm end coupled into the slab waveguide. The nanostructured sensors vertically projected a Raman signal, which was used to actively detect a thin layer of benzyl mercaptan attached to the sensors
Laser-written silicon-germanium alloy microstructures with tunable compositionally graded profiles
No full textA laser processing method is introduced for post-deposition tailoring of local composition and bandgap in amorphous silicon-germanium thin films on silicon substrates. Spatial distribution of the alloy constituents can be controlled through the scan speed
3D analysis of surface Plasmon dispersion for SERS sensor based on inverted pyramid nanostructures
No full textSurface enhanced Raman scattering (SERS) can be used to amplify the Raman cross-section of signals by several orders of magnitude, when a mixed photon-Plasmon mode (surface Plasmon polaritons) couples to molecules on a nano textured metallo-dielectric substrate. In this paper we demonstrate a comprehensive 3D computational model based on Rigorous coupled wave analysis (RCWA) for the purpose of analysing propagating and localised surface Plasmon polaritons supported by planar SERS substrates based on periodic array of metal coated inverted pyramidal nanostructures. Although studies [1, 2] have explored the optical properties of inverted square pyramidal pits using simulation and experimentation, there has yet been no investigation performed on rectangular inverted pyramidal pits. Here we perform 3D modelling and simulation on rectangular pit arrays with aspect ratio 1:1.2 over 400nm thick gold. We investigate the effect of incident polarisation and electric-field density within the pits and show that inverted rectangular pyramidal pit array can be used as highly effective SERS and Plasmonic substrates
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