Second harmonic generation response by gold nanoparticles at the polarized water/2-octanone interface: from dispersed to aggregated particles

Gold nanoparticles with a diameter of approximately 20 nm have been observed at the polarized water/2-octanone interface by the nonlinear optical technique of second harmonic generation. Electric field induced adsorption of the gold particles at this liquid/liquid interface is clearly observed and confirms that these are negatively charged. The process is quasi-reversible at high potential sweep rates, but aggregation at the interface is observed at slower sweep rates through the loss of the nonlinear optical signal. The time evolution of the second harmonic signal is also reported during potential step experiments. After a rapid increase due to adsorption, a continuous decrease in the nonlinear optical signal intensity is observed due to aggregation of the particles into large islands at the interface. Diffusion of these large islands at the interface was observed for a longer timescale through large signal fluctuations

Reflectance and SERS from an ordered array of gold nanorods

The optical properties of arrays of Au nanorods were studied by specular reflectance spectroscopy. The spectra were dominated by the surface plasmon modes of the Au nanoarrays superimposed on the effects of interference through the films. The longitudinal plasmon resonance moved to longer wavelength as the aspect ratio of the nanorods increased. The reflectance spectra were modelled by applying the Maxwell-Garnett approximation to a uniaxial thin film (composite Au/alumina) and this yielded a good match to the experimental data. SERS spectra on the Au nanorod arrays were recorded at different externally applied potentials and significant differences with respect to an electrochemically roughened Au electrode were revealed. These have been attributed to the nature of the composite nanoarrays, both their nanostructuring into rods and the regular arrangement of these rods

Transverse and longitudinal surface plasmon resonances of a hexagonal array of gold nanorods embedded in an alumina matrix

The specular reflectance from a hexagonal array of gold nanorods embedded in an alumina matrix supported on an aluminum substrate is reported. The rods were grown by electrodeposition of gold in an alumina template and were oriented with their long axis perpendicular to the film surface. Optical reflectance measurements performed with an incident light beam of S polarization only exhibited the transverse surface plasmon resonance whereas the measurements obtained with P polarization exhibited both transverse and longitudinal resonances. A model for the reflectance from a thin anisotropic film was developed and shown to be in agreement with the experimental data

Enhancement of the second harmonic signal from Hg1−xCdxTe (MCT) in the presence of an anodic oxide film

Second harmonic generation (SHG) is now widely regarded as a valuable tool for investigating electrode surfaces. Typically, most studies have been limited to substrates which lack bulk symmetry and monitoring events such as sub-monolayer formation and surface reconstruction. Here, the development of a model that can be used to quantitatively describe the enhanced SH signal observed in the presence of an anodic oxide film on a non-centrosymmetric substrate, Hg1xCdxTe (MCT), is described. The aim is to further expand the utility of SHG for probing different electrode systems. The growth of the high quality oxide films was first followed by in-situ ellipsometry. For thin films (<100 nm) grown at a constant current density of 150 A cm2, an effectively uniform oxide layer is found with a refractive index n of ~2.15 0.05 and exhibiting no absorption of the incident radiation at 632.8 nm (1.96 eV). In the presence of such an oxide film of 58 nm thickness, the second harmonic (SH) signal intensity measured in reflection is found to be significantly enhanced in both the PIN-POUT and PIN-SOUT polarization configurations. To quantify the changes observed, each layer in the model is assigned its own symmetry and optical constants (at the fundamental, and harmonic (= 2) frequencies and a defined thickness. Modeling of the SH rotational anisotropy experiments carried out at different angles of incidence indicated that most of this increase could be accounted for by multiple reflections of the fundamental wave = 1064 nm (1.17 eV) in the composite ambient/oxide/MCT layer, with little contribution from charge accumulation at the buried MCT/oxide interface for this oxide thickness

Optical characterisation of anodic sulphide films on hg1-xCdxTe (MCT) grown by the potential step method

The growth of native sulphide films on Hg1−xCdxTe (MCT) by potential steps to two different electrochemical growth regions has been studied by in-situ ellipsometry, photocurrent spectroscopy and second harmonic generation (SHG) rotational anisotropy. Films grown at −0.4 V versus SCE were porous and consisted mainly of cubic close packed (ccp) CdS. The photocurrent spectrum of the film exhibited a cut-off which closely conformed to the expected bandgap of CdS. The gradual increase in SH intensity as the sulphide film thickened was due to a contribution to the overall SH signal from the CdS film itself. No evidence of any hcp species was apparent in the rotational anisotropy patterns, with the four-fold pattern expected for the vicinal MCT surface being maintained even after film growth. When the potential was stepped to −0.3 V, the film grown absorbed the ellipsometer radiation once a thickness of 86 nm was reached. Photocurrent measurements showed a tail extending into the red region of the spectrum. These phenomena have been attributed to the incorporation of ccp HgS into the sulphide film as at this potential, the electrochemical reaction of the HgTe component of MCT can occur. SHG rotational anisotropy confirmed the four-fold symmetry of the ccp surface film

Measurement of the d(36) coefficient of mercury cadmium telluride by reflection second harmonic generation

The second order nonlinear coefficient (d36) of the narrow band gap semiconductor, mercury cadmium telluride (MCT), is measured. Because MCT is strongly absorbing at a 1.06 μm wavelength, the measurement was performed by comparing the second harmonic intensity reflected from the material surface to the second harmonic intensity measured for a quartz sample in transmission. The analysis depends on the derivation of comparable expressions for the reflected and transmitted intensities. Using this approach a value of d36=350±40 pm/V is obtained, a value much larger than those reported for similar zinc-blende type materials. The large magnitude of the MCT d36 is attributed to an electronic resonance enhancement

Monoamide and Diamide Nitric Acid Extraction at the Water/Dodecane Monitored by Second Harmonic Generation

International audienc

Combined roughness and electric polarization for the local-field enhancement of the second harmonic response from a silver-electrolyte interface

A nanoscale rough silver film is electrodeposited from a silver cyanide aqueous solution and its growth is monitored by second harmonic generation (SHG). For growth by potential step into the diffusion controlled region, a maximum in the SHG intensity is always obtained for an equivalent silver film thickness of ∼60 nm, based on the charge consumed. The SH intensity decreases for thicker but smoother films. A study as a function of an applied static electric field for an already grown film shows that the combination of nanoscale roughness and electric polarization is able to yield large enhancement of SH signal intensities. From a microscopic analysis of the data, it is shown that the nonlinear surface current perpendicular to the interface exhibits a nonlinear dependence with the applied potential, owing to the combination of roughness and electric polarization

PalmitateLuciferin: A Molecular Design for the Second Harmonic Generation Study of Ion Complexation at the Air-Water Interface

International audienc