Optimisation of absorption efficiency for varying dielectric spherical nanoparticles

Abstract-In this paper we compare the optical absorption for nanospheres made from a range of transition and alkali metals from Li (A=3) to Au (A=79). Numerical solutions to Mie theory were used to calculate the absorption efficiency, Q abs , for nanospheres varying in radii between 5 nm and 100 nm in vacuum. We show that, although gold is the most commonly used nanoparticle material, its absorption efficiency at the plasmon resonance is not as strong as materials such as the alkali metals. Of all the materials tried, potassium spheres with a radius of 21 nm have an optimum absorption efficiency of 14.7. In addition we also show that, unlike gold, the wavelength of the plasmon peak in other materials is sensitive to the sphere radius. In potassium the peak position shifts by 100 nm for spheres ranging from 5 nm to 65 nm, the shift is less than 10 nm for gold spheres

Shape-selective formation of monodisperse copper nanospheres and nanocubes via disproportionation reaction route and their optical properties

Synthesis of stable and monodisperse Cu nanocrystals of controlled morphology has been a long-standing challenge. In this Article, we report a facile disproportionation reaction approach for the synthesis of such nanocrystals in organic solvents. Either spherical or cubic shapes can be produced, depending on conditions. The typical Cu nanospheres are single crystals with a size of 23.4 ± 1.5 nm, and can self-assemble into three-dimensional (3D) nanocrystal superlattices with a large scale. By manipulating the chemical additives, monodisperse Cu nanocubes with tailorable sizes have also been obtained. The probable formation mechanism of these Cu nanocrystals is discussed. The narrow size distribution results in strong surface plasmon resonance (SPR) peaks even though the resonance is located in the interband transition region. Double SPR peaks are observed in the extinction spectra for the Cu nanocubes with relative large sizes. Theoretical simulation of the extinction spectra indicates that the SPR band located at longer wavelengths is caused by assembly of Cu nanocubes into more complex structures. The synthesis procedure that we report here is expected to foster systematic investigations on the physical properties and self-assembly of Cu nanocrystals with shape and size singularity for their potential applications in photonic and nanoelectronic devices. © 2014 American Chemical Society

Thin films of PtAl2 and AuAl2 by solid-state reactive synthesis

The intermetallic compounds AuAl2 and PtAl2 are colored purple and yellow respectively. In the past they have been prepared by bulk melting techniques or by co-deposition in a magnetron sputterer. Here, however, we investigate films of AuAl2, PtAl2 and (Au,Pt)Al2 prepared by sequential physical vapor deposition of the elements, followed by in situ solid-state reaction. The microstructure, dielectric functions, optical properties and thermal stability of the resulting films are characterized and compared to those prepared by bulk melting or codeposition. The (Au,Pt)Al2 films show a color gamut that stretches from purple to brassy yellow depending on composition and microstructure. High temperature synchrotron X-ray diffraction experiments show that the (Au,Pt)Al2 phase is metastable, decomposing when heated above 420 °C. In contrast, the pure AuAl2 or PtAl2 phases are stable to about 580 °C before they oxidize or decompose. The alternative possibility of producing the purple-to-yellow color gamut by depositing optical stacks of very thin films of AuAl2 and PtAl2 is also assessed. Either scheme will provide a range of colors lying between those of the binary compound endpoints. Calculations predict that deposition of AuAl2 onto PtAl2 will produce more intense colors than vice versa, an unexpected finding that is worth further investigation.Royal Thai Scholarship. Part of this research was undertaken on the Powder Diffraction beamline at the Australian Synchrotron, Victoria, Australia.http://www.elsevier.com/locate/tsf2016-08-31hb201

Comparison of Single- and Mixed-Sized Gold Nanoparticles on Lateral Flow Assay for Albumin Detection

The sensitivity and reproducibility of the lateral flow assay can be influenced by multiple factors, such as the size of gold nanoparticles (GNPs) employed. Here, we evaluated the analytical performance of single-sized and mixed-sized GNPs using a simple lateral flow assay (LFA) platform. This platform was used as a model assay to diagnose albumin levels and demonstrate the analytical performance of single-sized and mixed-sized GNPs in LFA tests. Two sizes of GNPs@anti-bovine serum albumin (BSA) conjugate proteins were mixed at different ratios. The unique optical properties of the GNPs induced a distinguishing color-shedding effect on the single- and mixed-sized GNPs@anti-BSA conjugates interacting with the target analyte BSA spotted on the test line. The use of mixed-sized GNPs@anti-BSA conjugates enhanced signal relative to the 20 nm GNPs, and provided superior stability compared with solely employing the large GNPs (50 nm). The proposed platform in this study could provide an efficient BSA detection mechanism that can be utilized as a model biomarker for confronting chronic kidney disease

Thermal Effect during Laser-Induced Plasmonic Heating of Polyelectrolyte-Coated Gold Nanorods in Well Plates

We examined the generation and transfer of heat when laser irradiation is applied to water containing a suspension of gold nanorods coated with different polyelectrolytes. The ubiquitous well plate was used as the geometry for these studies. The predictions of a finite element model were compared to experimental measurements. It is found that relatively high fluences must be applied in order to generate biologically relevant changes in temperature. This is due to the significant lateral heat transfer from the sides of the well, which strongly limits the temperature that can be achieved. A 650 mW continuous-wave (CW) laser, with a wavelength that is similar to the longitudinal plasmon resonance peak of the gold nanorods, can deliver heat with an overall efficiency of up to 3%. This is double the efficiency achievable without the nanorods. An increase in temperature of up to 15 °C can be achieved, which is suitable for the induction of cell death by hyperthermia. The nature of the polymer coating on the surface of the gold nanorods is found to have a small effect

Percolation in nanoporous gold and the principle of universality for two-dimensional to hyperdimensional networks

Percolation in nanoporous gold can be achieved with as little as 8% by volume of gold. Samples of nanoporous gold of various morphologies are analyzed with a combination of electrical and optical data. Growing thin films and complex multiply connected three-dimensional networks both display nonuniversal character. Growing films have two-dimensional morphology but a three-dimensional percolation threshold and nonuniversal critical coefficients, yet similar silver films percolate as expected with universal coefficients. Growing gold however regresses to two-dimensional resistive behavior between 65% to 100% gold, and this regime lies along a single power-law curve shared by the hyperdimensional networks of gold, suggesting underlying symmetry governed by diffusion-limited aggregation. Models of data imply either hyperdimensionality or major internal property changes as density shifts. The distinctive flat spectral signature found near the percolation threshold is common to all highly porous samples and is explained quantitatively in terms of effective plasmonic response. Parameters from fits of effective medium models to optical and resistivity data are in close agreement, especially at the highest porosities. They imply an effective dimension which increases continuously as porosity grows via the increased branching needed for structural integrity

Preparation of nanoscale gold structures by nanolithography

Gold is the material of first choice for the realisation of a large number of interesting nanoscale devices and structures due to its unique chemical and optical properties. However, conventional photolithographic processes cannot be used to manufacture such tiny structures in gold (or any other material) due to limitations imposed by the diffraction of light. New methods of lithography have been developed to overcome this limitation. In this article we review these new nanolithographic techniques, describe how they have been used to produce nanoscale precious metal artefacts, and briefly survey some of the existing and potential applications for these structures

Charging of gold/metal oxide/gold nanocapacitors in a scanning electron microscope

Triangular parallel-plate nanocapacitors were fabricated by a combination of microsphere lithography and physical vapor deposition. The devices were comprised of a 20 nm layer of dielectric material sandwiched between two 20 nm layers of gold. Dielectric materials with a range of relative permittivities were investigated. Charging of the capacitors was probed in a scanning electron microscope (SEM) by monitoring the change in brightness of the images of the devices as a function of time. The time constants, RC, associated with the charging of the capacitors, were extracted from the SEM grayscale data. The resulting average RC values were 248 ± 27 s for SiO2, 70 ± 8 s for Al2O3, 113 ± 80 s for ZnO and 125 ± 13 s for HfO2. These values are consistent with the anticipated RC values based on the resistivities and permittivities of the materials used in the devices and importantly, were measured without the need to attach any wires or leads.Australian Research Council (DP0877539-DP0984354