Silver nanoparticle coverage dependence of surface-enhanced Raman scattering

We report surface-enhanced Raman scattering (SERS) from 4-mercaptopyridine adsorbed on nanotextured silver surfaces as the coverage of silver is varied. The degree of surface enhancement is strongly dependent on silver coverage and correlated to the extinction of the surface at the Raman excitation wavelength, that extinction being determined by multiparticle surface plasmon resonances. The coverage dependence of the Raman intensity is consistent with signals being dominated by molecules at junctions inside nanoparticle aggregates where electromagnetic energy is localized into “hot spots” by interactions of the incident and scattered fields with the surface plasmons. The Raman intensity drops precipitously near the conductivity percolation threshold because these hot spots are destroyed when conducting paths allow plasmons to propagate. Our approach to substrate preparation provides clean surfaces with average enhancements ≥10⁷, an order of magnitude larger than typical for SERS. PACS 78.67.-n; 78.68.+m; 33.20.Fb     

Silver fractal networks for surface-enhanced Raman scattering substrates

Based on diffusion-limited aggregation process, a convenient nanotechnique is demonstrated to obtain large silver fractal networks for a surface-enhanced Raman scattering (SERS)-active substrate. The silver fractal networks are of high SERS enhancement factor, large dynamical range. The observed SERS efficiency can be explained in terms of strongly localized plasmon modes relative to the single particle plasmon resonance.     

Single-site surface-enhanced Raman scattering beyond spectroscopy

Recent progress in the observation of surface-enhanced Raman scattering (SERS) is reviewed to examine the possibility of finding a novel route for the effective photoexcitation of materials. The importance of well-controlled SERS experiments on a single molecule at a single site is discussed based on the difference in the information obtained from ensemble SERS measurements using multiple active sites with an uncontrolled number of molecules. A single-molecule SERS observation performed at a mechanically controllable breaking junction with a simultaneous conductivity measurement provides clear evidence of the drastic changes both in the intensity and in the Raman mode selectivity of the electromagnetic field generated by localized surface plasmon resonance. Careful control of the field at a few-nanometer-wide gap of a metal nanodimer results in the modification of the selection rule of electronic excitation of an isolated single-walled carbon nanotube. The examples shown in this review suggest that a single-site SERS observation could be used as a novel tool to find, develop, and implement applications of plasmon-induced photoexcitation of materials.     

Chemical contribution to surface-enhanced Raman scattering

We present a new mechanism for the chemical contribution to surface-enhanced Raman scattering (SERS). The theory considers the modulation of the polarizability of a metal nanocluster or a flat metal surface by the vibrational motion of an adsorbed molecule. The modulated polarization of the substrate coupled with the incident light will contribute to the Raman scattering enhancement. We show that for a metal cluster and for a flat metal surface this new chemical contribution may enhance the Raman scattering intensity by a factor of approximately 102 and approximately 104, respectively. The new SERS process is determined by the electric field parallel to the surface of the metal substrate at the molecular binding site.     

Long-range manifestation of surface-enhanced Raman scattering

The spatial scale at which the effect of surface-enhanced Raman scattering by planar Ag nanostructures manifests itself is investigated experimentally by direct measurements of the dependence of the enhancement factor on the distance between the surface of the Ag nanostructure and a layer of test organic molecules. It is found that the enhancement factor remains almost constant up to distances as large as 30 nm and drops abruptly at larger distances. The obtained dependence is universal for all kinds of organic molecules investigated. The fact that the surface enhancement of Raman scattering manifests itself on such a long spatial scale sharply contradicts the broadly accepted model assuming that the surface-enhanced Raman scattering decreases rapidly at distances as short as 2–3 nm.     

A facile synthesis of branched silver nanowire structures and its applications in surface-enhanced Raman scattering

We report a facile method of preparing novel branched silvernanowire structures such as Y-shaped, K-shaped and other multi-branchednanowires. These branched nanostructures are synthesized by reducingsilver nitrate (AgNO₃) in polyethylene glycol(PEG) with polyvinglpyrrolidone (PVP) as capping agent. Statisticaldata indicate that for the “y” typed branched nanowire,the branches grow out from the side of the trunk nanowire in a preferentialorientation with an angle of 55? between the branch and the trunk.Transmission electron microscopy (TEM) studies indicate that the defectson silver nanowires could support the growth of branched nanowires.Conditions such as the molar ratio of PVP/AgNO₃, the reaction temperature, and the degree of polymerization of reducingagent and PVP play important roles in determining the yield of thesilver branches. Due to the rough surface, these branched nanostructurescan be used as efficient substrates for surface-enhanced Raman scatteringapplications.     

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。     

Single-molecule surface-enhanced Raman scattering: Current status and future perspective

The single-molecule surface-enhanced Raman scattering （smSERS） has been extensively studied after the initial observation in 1997, yet there still exist unsettled issues in the fundamental mechanism of smSERS. In this review, we survey some of the recent breakthroughs in the mechanism of smSERS and its application.     

双酚A拉曼光谱的密度泛函理论计算及表面增强拉曼光谱研究

以密度泛函理论(DFT),RB3LYP/6-311G(d)方法计算得到的双酚A(BPA)分子振动光谱为依据,对BPA分子常规拉曼光谱进行了详细的指认,对其振动模式进行了归属.研究了BPA在金胶体系中的表面增强拉曼光谱,对其吸附方式进行了分析:BPA分子在酸性pH下,分子以=CO-吸附到金溶胶上,-OH键的振动消失,苯环以直立方式垂直于金胶上.     

Facile preparation of Au/Ag bimetallic hollow nanospheres and its application in surface-enhanced Raman scattering

In this paper, an Au/Ag bimetallic hollow nanostructure was obtained by using SiO₂ nanospheres as sacrificial templates. The nanostructure was fabricated via a three steps method. SiO₂@Au nanospheres were first synthesized by the layer-by-layer technique, and then they were coated with a layer of Ag particles, finally, the Au/Ag bimetallic hollow nanospheres were obtained by dissolution of the SiO₂ core by exposure in HF solution. Several characterizations, such as transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX) and UV visible absorption spectroscopy were used to investigate the prepared nanostructures. The effectiveness of these Au/Ag bimetallic hollow nanospheres as substrates toward surface-enhanced Raman scattering (SERS) detection was evaluated by using rhodamine 6G (R6G) as a probe molecule. We show that such Au/Ag bimetallic hollow nanospheres structure films which consisting of larger interconnected aggregates are highly desirable as SERS substrates in terms of high Raman intensity enhancement. The Au/Ag bimetallic hollow nanostructured aggregate, interconnected nanostructured aggregate and nanoscale roughness are important factors responsible for this large SERS enhancement ability.     

Computer simulation of surface-enhanced Raman scattering in nanostructured metamaterials

The simulation of local field fluctuations and surface-enhanced Raman scattering in percolation systems at the percolation threshold is described. An approximate real-space renormalization group method was used in the simulation. It allows one to radically reduce the computation time compared to an exact calculation and to obtain detailed information about the electromagnetic field. The local fields in real macroscopic systems can be calculated by using this approximation. A computer simulation of the local fields in metal island (percolation) films has been performed by the developed method. The calculation has confirmed the existence of giant local field fluctuations. In turn, the local electric field excites Raman scattering. The local fields of surface-enhanced Raman scattering have been calculated for the first time. The dependence of the Raman scattering enhancement factor on the reference frequency and Stokes shift has been derived. An experimental observation of this dependence could be considered as a confirmation of the electromagnetic nature of the enhancement.     

Nanostructured optical fiber with surface-enhanced Raman scattering functionality

A powerful method for the production of reproducible surface-enhanced Raman scattering (SERS) substrates is described based on the scaling properties of glass rods when drawn into fibers. The fabrication process involves chemically eroding the cleaved tips of drawn silica imaging fibers and then coating them with silver. For an appropriate choice of final diameter the drawn and eroded tips show clearly defined and regular triangular formations on a scale of approximately 80 nm. The favorable SERS properties of these structures have been demonstrated by the observation of enhancement factors of approximately 10(6).     

Laser Raman scattering applications

The advent of laser sources, better optical instruments, and photon-counting techniques has put Raman scattering within the range of a number of practical uses. The measurement of densities and temperatures has been demonstrated in numerous laboratory and field situations. In this paper, we consider the factors that determine the feasibility of Raman scattering measurements. The compromise between accuracy and time resolution is discussed in terms of desirable applications to short-lived processes (turbulence, kinetics etc.).     

Air stable colloidal copper nanoparticles: Synthesis,characterization and their surface-enhanced Raman scattering properties

Air stable colloidal copper nanoparticles are synthesized by a simple chemical reduction method using octadecylsilane as a reducing agent and octadecylamine as a stabilizing agent in toluene without any inert gas. The formation of nanosized copper was confirmed by its characteristic surface plasmon absorption peaks in UV–visible spectra. The transmission electron microscopic (TEM) images show that the resulting copper nanoparticles are distributed uniformly with a narrow size distribution. The X-ray diffraction (XRD) demonstrated that the obtained copper nanoparticles are single crystalline nanoparticles. Fourier transform infra-red (FT-IR) spectroscopic data suggested that the silane Si–H is responsible for the reduction of copper ions. And also the resulting colloidal copper nanoparticles exhibit large surface-enhanced Raman scattering (SERS) signals.     

Finding electromagnetic and chemical enhancement factors of surface-enhanced Raman scattering

The authors report two methods to determine electromagnetic and chemical enhancement factors in surface-enhanced Raman scattering (SERS), which are based on saturation property and decay dynamics of photoluminescence and concurrent measurements of photoluminescence and resonance Raman scattering intensities. Considerations for experimental implementation are discussed. This study is expected to facilitate the understanding of SERS mechanisms and the advancement of the usage of SERS in chemical and biological sensor applications.     

Femtosecond and nanosecond laser fabricated substrate for surface-enhanced Raman scattering

We report a simple and repeatable method for fabricating a large-area substrate for surface-enhanced Raman scattering. The substrate was processed by three steps: (i) femtosecond (fs) laser micromachining and roughening, (ii) thin-film coating, and (iii) nanosecond laser heating and melting. Numerous gold nanoparticles of various sizes were created on the surface of the silicon substrate. The 3D micro-/nanostructures generated by the fs laser provide greater surface areas with more nanoparticles leading to 2 orders of magnitude higher of the enhancement factor than in the case of a flat substrate. Using an He-Ne laser with a 632.8?nm excitation wavelength, the surface-enhanced Raman scattering enhancement factor for Rhodamine 6G was measured up to 2×10(7).     

Near-field scanning optical microscopy based on surface-enhanced Raman scattering

We describe a near-field optical microscopy technique based on the interaction of a probe molecule with the sample surface (e.g., with a flat metal surface) in the field of external optical radiation and consider the spontaneous Raman scattering characterized, in the presence of a metal surface, by the effective polarizability of the probe molecule, depending on the frequency and the distance to the sample surface. At certain distances from the probe molecule to the surface, the effective polarizability of this molecule (determined with allowance for the polarizing influence of the surface of a semi-infinite medium) at the Stokes frequency sharply increases in comparison to the quantum polarizability of an isolated molecule, which is indicative of the formation of optical near-field resonances. It is shown that the proposed method of near-field optical microscopy is characterized by high sensitivity and high spatial resolution (on the order of 1 Å).     

Synthesis of gold nanostars with fractal structure: application in surface-enhanced Raman scattering

Multi-branched gold nanostars with fractal feature were synthesized using the Triton X-100 participant seed-growth method. By increasing the amount of ascorbic acid, the branch length of gold nanostars could be greatly increased. It has been interesting to find that the secondary growth of new branches takes place from the elementary structure when the aspect ratio of the branches is greater than 8.0 and the corresponding plasmon absorption wavelength is greater than 900 nm. Raman activity of the gold nanostar films has been investigated by using the 4-mercaptobenzoic acid (4-MBA) as Raman active probe. Experimental results show that the surface-enhanced Raman scattering (SERS) ability of the gold nanostars could be efficiently improved when the fractal structure appears. The physical mechanism has been attributed to the intense increased secondary branch number and the increased “hot spots”. These unique multi-branched gold nanostars with fractal feature and great SERS activity should have great potential in sensing applications.