It is shown here that deviations from a prolate ellipsoidal shape have a significant effect on the optical properties of gold nanorods. Transitions from rods to ‘dumbbell'‐ or ‘phi'‐shaped particles lead to a shift in the longitudinal plasmon peak in the blue and red directions, respectively. Development of ‘dog‐bone' shapes leads to a red‐shift and to the development of a third peak. A broad and flexible color gamut can be obtained.
The optical properties of in situ deposited gold nano-particle coatings are investigated for potential application in architectural glass. It is found that the optical properties of the coating can be controlled by the pH of the deposition solution. At a pH of 5.1 the color of the coatings develops from pink, through violet, to blue in transmission. This is due to a plasmon resonance peak at 520 nm from isolated particles, and one at about 700 nm due to near-field dipole interactions, with an intermediate zone of coexistence of the two, which produces the violet color. However, the two peaks do not coexist in the spectra of coatings produced at pH 8.0 or at pH 10.0, with the peak due to the 520 nm resonance being swamped by the development of the resonance due to particleparticle interactions. In all cases the 700 nm peak could be broadened and red-shifted by increasing the deposition time. The reasons for these differences are explored, and are shown to be attributable to the smaller, more aggregated morphology of nano-particles precipitated at the higher pHs. The In situ precipitation of gold nanoparticles onto glass for potential architectural applications Coatings of similar average density and similar nominal particle size, but different patterns of aggregation, can produce quite different optical transmission spectra, (a) shows substantially isolated particles of 60 nm diameter, and the corresponding spectrum shows a peak at 520 nm, whereas (b) is aggregated from much smaller particles and has a broad absorption peak at approximately 655 nm.4
Fax: 61-2-95147553 2 Red-shifting of the optical absorption spectra of aggregates of gold nanoparticles by dipole-dipole interactions is of considerable interest, both for theoretical reasons and because the phenomenon can be potentially exploited in various applications. A convenient and practical way to control the effect is to assemble the aggregated ensemble of n gold nanoparticles on the outer surface of larger dielectric spheres. Here we show by experiment and calculation how the spectra of these structures can be systematically morphed from that of isolated gold particles, through the regime of broad absorption dominated by particle-particle interactions and finally to the limiting case of a continuous nano-shell. The experimental data was produced using the process of deposition-precipitation, which provides a facile method to decorate polystyrene micro-spheres with gold nanoparticles.There is no need for prior functionalisation of the micro-sphere surface in our method of depositionprecipitation. Calculations were carried out using a code based on the discrete dipole approximation (DDA). The spectra were dominated by three effects. These were a peak absorption at about 540 nm produced by the conventional plasmon resonance of spherical gold nanoparticles, a broad absorption in the range 600 to 900 nm caused by diverse dipole-dipole interactions between particles which strengthened as the number of attached gold particles increased and finally, when n was large, an absorption peak due to the onset of nanoshell-like resonances. The experimental spectra could be successfully fitted by spectra calculated using combinations of these effects.3
When nanorods of Au, Ag and some other elements are aligned with a preferred orientation with respect to light, their optical extinction characteristics become dependent on the polarization and angle of incidence of the light. This effect is explored here and it is shown that it could potentially be exploited to produce a 'colour-change coating'. However, particle-particle interactions are also likely to occur in such coatings, with red shifting of extinction spectra occurring for end-on-end configurations of monodisperse rods, and blue shifting for side-by-side configurations. Surprisingly, the particle-particle interactions are attenuated if they are between rods of differing aspect ratios, and this offers a useful new means of control of the optical properties of coatings of nanorods.
The unique optical properties of gold nanorods, which exhibit tuneable absorption as a function of their aspect ratio, suggest that they might have potential applications in coatings for solar control on windows. Here we explore the properties of coatings produced by attaching gold nanorods to the surface of glass. Such coatings can attenuate solar radiation effectively, even at very low gold contents, but the figure-of-merit, T vis /T sol , of our experimental coatings was close to unity, indicating that they are not spectrally selective, However, calculations are presented to show how coatings comprised of a blend of rods with aspect ratios of greater than 3 can produce coatings with T vis /T sol of up to at least 1.4. The maximum value possible for perfectly spectrallyselective coating in sunlight is 2.08. Unfortunately, the practical realization of such coatings requires the further development of reliable methods to scale up the production of gold nanorods of longer aspect ratios. Gold Bulletin 2006 • 39/4 156
A simplified method to produce spindle-shaped particles with a hematite core and a silica shell is described. The silica shell can, in turn, serve as the substrate for an outer coating of Ag or Au nanoparticles. The resulting multi-layer core-shell particles display a flexible optical extinction spectrum, due primarily to the sensitivity of their plasmon resonance to the morphology of the precious metal outer coating. The deposition of silver or gold onto spindleshaped cores produces particles with distinctive optical properties. A silica interlayer between core and shell facilitates attachment of the precious metal. The morphology of the metal coating can be controlled by adjustment of the deposition conditions, and controls the optical extinction characteristics.
Nanoscale particles of metals such as gold can interact with light by means of a plasmon resonance, even though they are much smaller than the wavelengths of visible light. The proportions of light that are absorbed and scattered vary with wavelength. Any light that is absorbed will cause heating of the particles, and this effect may potentially be exploited for solar glazing coatings, nanoscale lithography or medical treatments. The position of maximum absorption of an isolated spherical nanoparticle is 518 nrn, but this may be significantly red-shifted by means of decreasing the symmetry to an prolate spheroid or 'nanorod', or by producing a metal 'nanoshell' on a dielectric core, or by aggregating insulated spherical particles. Absorption peaks in the vicinity of 655 nm for aggregated particles and 780 nm for prolate spheroids are demonstrated here. Absorbed energy is released as heat into the environment of the particles, and will cause a temperature rise within the particle the magnitude of which depends upon the value of the effective heat transfer coefficient between particle and environment. The latter is not known, but we show how highly localized temperature rises of some tens of Celsius might be conceivable in systems illuminated by sunlight.
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