SERS activity of silver and gold nanostructured thin films deposited by pulsed laser ablation

The deposition power dependence of visible transmittance and refractive index of room temperature-deposited ZnO:Al thin films by RF magnetron sputtering has been studied. All films exhibited high visible transmittance and near-complete UV absorption. The refractive index of the films decreased continuously with an increase in the RF power at all photon energies in the visible and near-IR region, which has been partially attributed to the decreased packing density of the films. For each film, the refractive index exhibited strong frequency dispersion in the weak-absorption region. The origin of optical dispersion at different RF power has been discussed in the light of a single-oscillator model.     

SERS activity of pulsed laser ablated silver thin films with controlled nanostructure

The surface‐enhanced Raman scattering (SERS) activity of silver thin films deposited by the pulsed laser ablation technique was investigated. The samples were grown in a controlled Ar atmosphere at pressures ranging between 10 and 70 Pa, and changing the number of laser pulses. Different surface morphologies, from isolated nearly spherical nanoparticles (NPs) to larger islands with smooth edges, were observed by means of scanning and transmission electron microscopies, as a function of the different deposition conditions adopted. SERS measurements were performed by soaking the samples in rhodamine 6G aqueous solutions over the concentration range between 1.0 × 10⁻⁴ and 5.0 × 10⁻⁸ M . Raman spectra were acquired using both the 632.8 and 514.5 nm excitation sources. The dependence of the SERS activity of the samples on the observed surface morphology is presented and discussed. The presence of the so called hot spots is envisaged. Copyright © 2011 John Wiley & Sons, Ltd.     

IR laser desorption of oligonucleotides

The desorption of oligonucleotides by 3 m{mu}m laser irradiation has been studied by laser induced fluorescence imaging of the resulting gas phase plumes. Fitting of the plume data has been achieved by using a modified Maxwell Boltzmann distribution which incorporates a range of stream velocities. Spatial density profiles, velocities and temperature variation have been determined from these fits indicating that the oligonucleotide plume only achieves a partial thermal relaxation. This laser desorption technique may provide a means of overcoming the limited mass range of gas phase biomolecules available from thermal evaporation techniques.     

Femtosecond laser induced desorption of water from silver nanoparticles

The photodesorption mechanism of H₂O from quartz-supported silver nanoparticles has been studied by femtosecond laser two-pulse correlation and fluence dependence measurements. With the laser wavelength close to the maximum of the (1,1) plasmon resonance of the nanoparticles, the desorption was found to be purely thermal, i.e., induced by coupling of the desorption coordinate to the nanoparticle lattice temperature, both in the low- and the high-coverage regimes. The lattice cooling times of the nanoparticles are in the range of several hundred ps, in accordance with recent time-resolved X-ray measurements. Also observed is a reversible red-shift of the nanoparticle plasmon modes with increasing H₂O coverage which is attributed to dielectric screening. PACS 78.67.-n; 82.53.-k; 65.80.+n     

Synthesis by pulsed laser ablation in Ar and SERS activity of silver thin films with controlled nanostructure

Thin silver films were deposited by pulsed laser ablation in a controlled Ar atmosphere and their SERS activity was investigated. The samples were grown at Ar pressures between 10 and 70 Pa and at different laser pulse numbers. Other deposition parameters such as laser fluence, target to substrate distance and substrate temperature were kept fixed at 2.0 J/cm², 35 mm and 297 K. Film morphologies were investigated by scanning and transmission electron microscopies (SEM, TEM). Surface features range from isolated nearly spherical nanoparticles to larger islands with smoothed edges. Cluster growth is favored by plume confinement induced by background gas. After landing on the substrate clusters start to aggregate giving rise to larger structures as long as the deposition goes on. Such a path of film growth allows controlling the surface morphology as a function of laser pulse number and Ar pressure. These two easy-to-manage process parameters control the number density and the average size of the as-deposited nanoparticles. We investigated the influence of substrate morphologies on their surface enhanced Raman scattering properties. Raman measurements were performed after soaking the samples in rhodamine 6G aqueous solutions over the concentration range between 1.0 × 10⁻⁴ and 5.0 × 10⁻⁸ M. The sensitivity of the film SERS activity on the surface features is put into evidence.     

Determination of fraction of molecules excited by laser IR radiation

To explain the observed excitation by laser IR radiation of moderate intensity (I = 1?10 MW/cm²) of the many rotational states (much greater than $\text{Ed}{}_{\mathrm{<mtext>01</mtext>}}\text{2}\text{hcB}$) it is suggested that mixing of the ground state components of the common symmetry type (“N-mixing”) take place. Estimation of the fraction of molecules excited by multiphoton processes is in satisfactory agreement with an experiment.     

IR absorption of highly vibrationally excited SF6

In a double-resonance experiment, the absorption of various CO₂ laser lines by sulfur hexafluoride was measured, before and after the SF₆ was pumped by a fixed frequency CO₂ laser to a level of 5 quanta/molecule. The absorption is substantially shifted to longer wavelengths. But the short wavelength wing of the absorption band is not completely bleached. Instead a shoulder of several cm⁻¹ width is left. This shoulder is probably important for the explanation of the infrared laser induced dissociation of SF₆.     

Tunable surface-plasmon-resonance wavelength of silver island films

This paper experimentally and theoretically investigates the effect of the underlayer medium on tuning of the surface plasmon resonance (SPR) wavelength of silver island films,and the effect of substrate temperature on the morphologies and optical properties of the films.From the absorption spectra of single Ag with various thickness and overcoated (Ag/TiO 2) films deposited on glass substrates at various substrate temperatures by RF magnetron sputtering,we demonstrate that the surface plasmon resonance wavelength can be made tunable by changing the underlayer medium,the thickness of metal layer and the substrate temperature.By varying substrate temperatures,the interparticle coupling effects on plasmon resonances of nanosilver particles enhance as the spacing between the particles reduces.When the substrate temperature is up to 500 C,the absorption peak decreases sharply and shifts to shorter wavelength side due to the severe coalescence between silver islands in the film.     

银岛膜表面胸腺嘧啶吸附行为的表面增强拉曼光谱和表面增强红外光谱研究

利用激光刻蚀法制备了具有化学纯净表面的银岛膜,该岛膜有很好的表面增强特性。利用表面增强拉曼光谱和表面增强红外光谱对胸腺嘧啶分子在银岛膜表面的吸附状态进行了对比研究。表面增强拉曼光谱中CN和C—O伸缩振动模式的出现表明胸腺嘧啶分子由原来的酮式结构变成了烯醇式结构;C(4)O伸缩振动谱带明显增强和N(3)的去质子化异构体特征峰的存在证明胸腺嘧啶分子是通过O(8)和N(3)的共同作用倾斜地吸附在银岛膜表面。对10-5 mol.L-1胸腺嘧啶在银岛膜表面上的红外光谱利用欧米采样器进行了反射法测量,发现其红外吸收增强了200倍。红外信号分析的结果支持了胸腺嘧啶分子通过O(8)与银表面发生相互作用的论断,同时也可得出胸腺嘧啶倾斜地吸附在银岛膜表面的结论。     

Fluorescence quenching in laser dyes by absorption from an excited singlet state

The effects of absorption of pump radiation from the first excited singlet state on the fluorescence quantum yield are calculated by numerical solution of simple rate equations, and compared with experiments performed earlier for the dye 7-diethylamino-4-methyl-coumarin at N₂-laser pumping. Analytical solutions for high and low pump intensities are found for the case of an optically thin solution.     

SERS study of pyridine vapor exposed to silver substrates electrochemically prepared.

Silver electrodes, previously prepared in KCl, KBr and KNO₃ electrolyte solutions, are exposed to a saturated vapor of pyridine, at room temperature. Surface complexes involving a silver halide and pyridine are detected during the gas - solid interaction. A low frequency band was observed, without exposure to pyridine, when a large amount of silver halide deposited on the electrode, is irradiated with a laser blue line. Photoreduction of the silver halide into metal aggregates occurs during the exposure to the laser.

Some hypothesis about the adsorption of pyridine at room temperature are advanced from results obtained with a silver electrode prepared in a KNO₃ solution.     

785 nm激光诱导银纳米三角片聚集表面增强拉曼散射效应

为实现表面增强拉曼散射（SERS）光谱的强信号快速检测分析,报道了通过785 nm激光诱导银纳米三角片（AgNPRs）聚集的方法。采用配体辅助化学还原法制备了AgNPRs,其边长约为80 nm,表面等离子体吸收峰出现在约774 nm处,对785 nm光产生有效吸收。在785 nm光辐照下,AgNPRs逐渐聚集,对巯基苯甲酸的SERS信号逐渐增强,其源于AgNPRs吸收的光转化为热而引起的AgNPRs聚集。其增强因子高达10⁹。为快速获得强SERS信号,激发光功率需大于250 mW。     

IR absorption enhancement for ionized p -nitrobenzoic acid on silver island films epitaxially grown on the 7 × 7 reconstructed Si(111) surface

An enhanced infrared absorption is reported for p-nitrobenzoic acid (PNBA) adsorbed on molecular-beam-epitaxially grown silver films at room temperature on 7×7 reconstructed Si(111) substrates. Reflection high-energy electron diffraction reveals the formation of Ag(111) islands even for 82-monolayer-thick silver deposition on the substrate. The NO₂ symmetric stretch of the surface-ionized species (PNBA^-) is a maximum in intensity at a thickness of 64 monolayers and is one order of magnitude stronger than the same mode of PNBA deposited at a thickness of 5 nm on a Si wafer. Based on the change in film morphology and in light transmittance with silver film thickness, this absorption enhancement is ascribed to the excitation of transverse collective electron resonance of the silver islands. The thickness dependence of the enhancement and of the film morphology will be compared with those observed for silver films deposited on oxidized Si(111) substrates. PACS 61.14.Hg; 68.37.Hk; 78.30.-J     

Light absorption by island metal films in the infrared range

This study reports on theoretical investigations of the electric and magnetic absorption of small metallic particles as functions of their shape, with the wavelength of incident e/m radiation assumed to be much greater than the size of the particles. A general expression for the absorption cross-section is derived for the cases when both the bulk and surface electron scattering are dominant. In the IR-region the absorption cross-section is shown to be extremely sensitive to the shape of particles; for particles of a constant volume, yet different in shape, it can change by several orders of magnitude. It is also shown that under dominant surface scattering the optical conductivity of an asymmetric particle is described by a tensor whose principal values are calculated for the ellipsoidal shape. The expression obtained for the electric and magnetic absorption renders ground to infer that the ratio of the contributions due to these absorption mechanisms greatly depends on the shape of particles and polarization of light.     

Nanostructured silver thin films deposited by pulsed laser ablation

Nanostructured thin films were deposited by excimer laser ablation of silver targets in controlled atmospheres of He and Ar. The film structural properties were investigated by means of scanning electron microscope and transmission electron microscope imaging. The film growth mechanism was identified as the result of coalescence of nanometric clusters formed during plume flight. Cluster formation involves plume confinement as a consequence of the increased collisional rate among plasma species. Fast photography imaging of the laser-generated silver plasma allowed to identify plasma confinement, shock wave formation and plasma stopping.     

Optical absorption of submonolayer silver films: Size dependence of ϵbound in small island particles

Optical absorption of submonolayer silver films deposited on a fused quartz ATR prism was measured for the spectral range between 1.5 and 6.0 eV. The observed spectra were interpreted well by the model of two-dimensional distribution of rotational ellipsoids applying the classical size effect to ?_bound and modifying ?_bound from the bulk value better than in the previous paper [Surface Sci. 103(1981)L97], where ?_free and ?_bound are the dielectric constants of silver contributed from free electrons and bound electrons, respectively. Electron micrographs showed the validity of the geometrical model used for the island films.     

Spectral properties of pyrazine adsorbed on silver electrodes and cold silver films from surface enhanced Raman scattering (SERS)

Surface enhanced Raman spectra of molecules adsorbed on electrode surfaces, metal-island films, colloidal particles and UHV-evaporated low-temperature substrates often show considerable differences in mode strength and positions between each other and the bulk molecular spectrum. These differences and their possible origins are discussed for pyrazine adsorbed on silver electrodes and cold silver films.     

Two-color laser desorption of nanostructured MgO thin films

Neutral magnesium atom emission from nanostructured MgO thin films is induced using two-color nanosecond laser excitation. We find that combined vis/UV excitation, for single-color pulse energies below the desorption threshold, induces neutral Mg-atom emission with hyperthermal kinetic energies in the range of 0.1-0.2 eV. The observed metal atom emission is consistent with a mechanism involving rapid electron transfer to three-coordinated Mg surface sites. The two-color Mg-atom signal is significant only for parallel laser polarizations and temporally overlapped laser pulses indicating that intermediate excited states are short-lived compared to the 5 ns laser pulse duration.