Surface‐enhanced Raman scattering of 4‐mercaptopyridine on sub‐monolayers of α‐Fe2O3 nanocrystals (sphere,spindle, cube)

We report surface‐enhanced Raman scattering (SERS) spectra from 4‐mercaptopyridine (4‐Mpy) adsorbed on sub‐monolayers of α‐Fe₂O₃ nanocrystals (sphere, spindle, cube). The maximum enhancement factor has been estimated to be about 10⁴ compared to that of 4‐Mpy in solution. A possible mechanism has been proposed that the charge transfer between the α‐Fe₂O₃ nanocrystals and the 4‐Mpy molecules is most likely responsible for the observed enhancement of Raman intensity of adsorbed 4‐Mpy molecules as surface plasmon resonances have not occurred. Copyright © 2009 John Wiley & Sons, Ltd.     

FT‐IR,FT‐Raman and SERS spectra of anilinium sulfate

The FT‐IR and FT‐Raman spectra of anilinium sulfate were recorded and analyzed. The surface‐enhanced Raman scattering (SERS) was recorded from a silver electrode. The vibrational wavenumbers of the compound have been computed using the Hartree‐Fock/6‐31G* basis and compared with the experimental values. The molecule is adsorbed on the silver surface with the benzene ring in a tilted orientation. The presence of amino and sulfate group vibrations in the SERS spectrum reveal the interaction between amino and sulfate groups with the silver surface. The direction of the charge transfer contribution to SERS has been discussed from the frontier orbital theory. Copyright © 2009 John Wiley & Sons, Ltd.     

Direct observation of surface‐enhanced Raman scattering in ZnO nanocrystals

We have been able to observe the surface‐enhanced Raman scattering (SERS) from 4‐mercaptopyridine (4‐Mpy) molecules adsorbed on ZnO nanocrystals, which display 10³ enhancement factors (EFs). An excitation wavelength‐dependent behavior is clearly observed. Another molecule BVPP is also observed to have surface‐enhanced Raman signals. The chemical enhancement is most likely responsible for the observed enhancement, since plasmon resonances are ruled out. The research is important not only for a better understanding of the SERS mechanism, but also for extension of the application of Raman spectroscopy to a variety of adsorption problems on a semiconductor surface. Copyright © 2009 John Wiley & Sons, Ltd.     

New insights on surface‐enhanced Raman scattering based on controlled aggregation and spectroscopic studies,DFT calculations and symmetry analysis for 3,6‐bi‐2‐pyridyl‐1,2,4,5‐tetrazine adsorbed onto citrate‐stabilized gold nanoparticles

A systematic study on the surface‐enhanced Raman scattering (SERS) for 3,6‐bi‐2‐pyridyl‐1,2,4,5‐tetrazine (bptz) adsorbed onto citrate‐modified gold nanoparticles (cit‐AuNps) was carried out based on electronic and vibrational spectroscopy and density functional methods. The citrate/bptz exchange was carefully controlled by the stepwise addition of bptz to the cit‐AuNps, inducing flocculation and leading to the rise of a characteristic plasmon coupling band in the visible region. Such stepwise procedure led to a uniform decrease of the citrate SERS signals and to the rise of characteristic peaks of bptz, consistent with surface binding via the N heterocyclic atoms. In contrast, single addition of a large amount of bptz promoted complete aggregation of the nanoparticles, leading to a strong enhancement of the SERS signals. In this case, from the distinct Raman profiles involved, the formation of a new SERS environment became apparent, conjugating the influence of the local hot spots and charge‐transfer (CT) effects. The most strongly enhanced vibrations belong to a₁ and b₂ representations, and were interpreted in terms of the electromagnetic and the CT mechanisms: the latter involving significant contribution of vibronic coupling in the system. Copyright © 2010 John Wiley & Sons, Ltd.     

Size‐dependent SERS enhancement of colloidal silver nanoplates: the case of 2‐amino‐5‐nitropyridine

Surface‐enhanced Raman scattering (SERS) spectra of 2‐amino‐5‐nitropyridine (ANP) adsorbed on colloidal silver triangular nanoplates were obtained using samples with different mean sizes and surface plasmon frequencies. The relative SERS enhancement factor for each sample was determined by the analysis of the normalized SERS excitation profiles of ANP vibrational modes for nanoplates in suspension, without aggregation. The SERS profiles are blue‐shifted in relation to the localized surface plasmon peak. The detailed characterization of both morphology and concentration of the samples in addition to a rigorous normalization of the SERS spectra allowed a quantitative correlation between the SERS profiles and the mean size of the nanoplates. This correlation indicated the existence of an optimum size of the nanoplates for maximum Raman enhancement. Copyright © 2008 John Wiley & Sons, Ltd.     

On the nature of “sers active sites”

Active sites of short range enhancement in surface enhanced Raman scattering (SERS) of adsorbates on cold deposited, porous silver films have been envisioned either as “cavity sites with local electromagnetic resonances” or as “sites of atomic scale roughness with photon driven charge transfer between metal and adsorbate”. Whereas SERS of CO and N₂ is observed at low coverages, the Raman signal of physisorbed O₂ is below the noise level, even at monolayer coverage of the “internal surface” or “filling of the pores”. On the other hand, SERS of ethylene and pyridine is quenched by some percent of a monolayer of oxygen. The Raman signal of multilayer condensed O₂ is stronger for a “compact” film than for a porous film. These observations cannot be reconciled with the first hypothesis, but they are consistent with the second.     

Raman,infrared, SERS and theoretical study of 3‐(1‐phenylpropan‐2‐ylamino) propanenitrile,fenproporex

Infrared, Raman and surface‐enhanced Raman scattering (SERS) spectra of 3‐(1‐phenylpropan‐2‐ylamino)propanenitrile (fenproporex) have been recorded. Density functional theory (DFT) with the B3LYP functional was used for optimizations of ground state geometries and simulation of Raman and SERS vibrational spectra of this molecule. Bands of the vibrational spectra were assigned in detail. The comparison of SERS spectra obtained by using colloidal silver and gold nanoparticles with the corresponding Raman spectrum reveals enhancement and shifts in bands, suggesting a possible partial charge‐transfer mechanism in the SERS effect. Information about the orientation of fenproporex on the nanometer‐sized metal structures is also obtained. Copyright © 2011 John Wiley & Sons, Ltd.     

Surface‐enhanced Raman scattering studies of carbon nanotubes using Ag‐core Au‐shell nanoparticles

Surface‐enhanced Raman scattering from carbon nanotube bundles adsorbed with plasmon‐tunable Ag‐core Au‐shell nanoparticles (Ag@Au nps) was carried out for the first time. By utilizing nanoparticles whose plasmon resonance peak (541, 642 nm) closely matches the commonly used Raman excitation sources (532, 632.81 nm), we can observe a large enhancement in the Raman signatures of carbon nanotubes. We obtain greater enhancement in the Raman signal for the above case when compared to nanotubes adsorbed with conventional Ag, Au or other ‘off resonant’ Ag@Au nps. The power‐dependent SERS experiment on single‐walled nanotubes (SWNTs) with resonant Ag@Au nps reveals a linear behavior between the G‐band intensity and the photon flux density, which is in agreement with the vibrational pumping model of SERS. The observed enhancement by resonance matching is pronounced for carbon nanotubes and may lead to insights into understanding nanotube–nanoparticle interaction. Copyright © 2009 John Wiley & Sons, Ltd.     

1,4-苯二硫醇分子的表面增强共振拉曼散射的化学增强

利用密度泛函和含时密度泛函理论方法研究了1,4-苯二硫醇分子在两个金团簇之间的表面增强拉曼散射及表面增强共振拉曼散射光谱. 采用对应四种不同形式的电荷转移激发态能量的入射光,计算了表面增强共振拉曼光谱. 结果显示,光谱增强的效果与电荷转移的形式密切相关. 不同的电荷转移形式对增强因子的贡献是有差异的.     

P-thiocresol 吸附在银纳米粒子表面系统的表面增强拉曼散射和表面增强共振拉曼散射的增强研究

文中从实验和计算两方面报道了在514.5 nm激发光下P-Thiocresol吸附在银胶表面系统的表面增强拉曼散射(SERS).文中分析了它的增强机制,发现增强主要来自于电磁场增强.如果考虑距离为2nm的两个银纳米粒子的耦舍效应,两粒子之间的SERS的电磁场增强为7.16 × 107.静态化学增强亦起到部分增强作用,它的增强倍数为6.所以,总的SERS增强,包括静态化学增强和电磁场增强,是Gtotal=Gsc ×GEM=4.4×108.我们也理论地研究了此系统的表面增强共振拉曼散射(SERRS).当激发光与P-Thiocresol-Ag3系统的激发态共振时,电荷转移机制(化学增强)也将起到重要作用,最强的增强可迭106.我们使用电荷密度将激发光下p-Thlocresol和Ag团簇问的电荷转移结果可视化,这是电荷转移的直接理论证据.对于SERRS增强,包括电荷转移和电磁场增强机制,能达到1013.     

IR,Raman and SERS spectra of ethyl salicylate

The IR and Raman spectra of ethyl salicylate were recorded and analyzed. The surface enhanced Raman scattering (SERS) spectrum was recorded in a silver colloid. The vibrational wavenumbers of the compound have been computed using the Hartree‐Fock/6‐31G* basis. The direction of charge transfer contribution to SERS has been discussed from the frontier orbital theory. The presence of methyl modes in the SERS spectrum indicates the nearness of the methyl group to the metal surface and the presence of ring vibrations and out‐of‐plane ring modes in the SERS spectrum suggests a flat orientation of the molecule on the silver surface. The first hyperpolarizability is calculated and the calculated molecular geometry has been compared with the reported similar structures. Copyright © 2009 John Wiley & Sons, Ltd.     

亚乙基硫脲分子激发拉曼虚态的研究

本文从拉曼峰强入手,求得了亚乙基硫脲(ETU)分子的"时间分辨键极化率",并讨论了该分子的激发拉曼虚态性质,发现了该分子"激发虚态电子向分子外围键流动"、"电子弛豫后的键极化率分布与基态电子的密度分布相似"、"不同激发波长下的键极化率衰减时间满足不确定关系"等特点。本文还研究了该分子的表面增强拉曼光谱,指出"电荷转移机制"的极化率弛豫时间长于"电磁增强机制"等与表面增强拉曼效应相关的结论。     

Adsorption of aniline on silver mirror studied by surface‐enhanced Raman scattering spectroscopy and density functional theory calculations

The adsorption of aniline on a silver mirror was studied by surface‐enhanced Raman scattering (SERS) spectroscopy and density functional theory (DFT) calculation methods. The normal Raman and SERS spectra of pure aniline liquid and its solutions were recorded by a micro‐Raman spectrometer with excitation at 514.5 nm. Orientation of the aniline molecule adsorbed on the Ag mirror is discussed. The results indicate that pure aniline is adsorbed on the surface of the Ag mirror with a tilted orientation. The conformer with the nitrogen atom interacting with the metal surface would be dominant. DFT calculations further confirm the experimental results that charge transfer (CT) takes place from the highest occupied molecular orbital(HOMO) of aniline to the singly occupied molecular orbital (SOMO) of the silver surface. In this paper, the frontier molecular orbital theory has been successfully used to explore the interaction between the aniline molecule and the silver surface. Copyright © 2011 John Wiley & Sons, Ltd.     

Surface‐enhanced Raman spectroscopy of 4‐tert‐butylpyridine on a silver electrode

We report the observation of large surface‐enhanced Raman scattering (SERS) (10⁶) for 4‐tert‐butylpyridine molecules adsorbed on a silver electrode surface in an electrochemical cell with electrode potential set at − 0.5 V. A decrease in electrode potential to − 0.3 V was accompanied by a decrease in relative intensities of the vibrational modes. However, there were no changes in vibrational wavenumbers. Comparison of both normal solution Raman and SERS spectra shows very large enhancement of the intensities of a₁, a₂, and b₂ modes at laser excitation of 488 nm. Enhancement of the non‐totally symmetric modes indicates the presence of charge transfer as a contributor to the enhancement. Copyright © 2011 John Wiley & Sons, Ltd.     

Spectroscopic investigations and computational study of sulfur trioxide–pyridine complex

The Fourier transform infrared (FT‐IR) and FT‐Raman spectra of sulfur trioxide‐pyridine complex were recorded and analyzed. The potential‐dependent surface‐enhanced Raman scattering (SERS) was recorded from an electrochemically roughened silver electrode. The vibrational wave numbers of the compound were computed using the Hartree–Fock/6‐31G* basis and compared with the experimental values. The presence of strong pyridine ring vibrations in the SERS spectrum reveals the interaction between the pyridine ring and the silver surface. The molecule is adsorbed on the silver surface with the pyridine ring in a tilted orientation. The direction of charge‐transfer contribution to the SERS is discussed from the frontier orbital theory. The value of the calculated first hyperpolarizability is comparable to those reported for similar structures, which makes this molecule an attractive object for future studies of nonlinear optics. The optimized geometrical parameters of the title compound are in agreement with similar reported structures. Copyright © 2011 John Wiley & Sons, Ltd.     

密度泛函理论研究黄曲霉素B1的表面增强拉曼散射效应

用密度泛函理论B3LYP方法和6-311G(d,p)/Lanl2DZ优化得到黄曲霉素B₁(AFB₁)分子及其复合物AFB₁-Ag的稳定结构,并计算了复合物的表面增强拉曼光谱和预共振拉曼光谱. 结果表明,AFB₁分子的拉曼光谱很大程度依赖于吸附位点以及入射光的激发波长. 与分子的常规拉曼光谱相比,复合物表面增强拉曼光谱中C=O伸缩振动模的增强因子约为10²～10³复合物的极化率增强而导致的静态化学增强,并分析了振动模式的振动方向与其拉曼强度的关系．选择复合物最大吸收峰附近激发光266和482 nm以及远离共振吸收波长785和1064 nm作为入射光,计算得到不同入射光激发下复合物的预共振拉曼光谱．结果表明其增强因子最大达到10⁴量级,主要是由电荷转移产生的共振增强引起的．     

Raman scattering of L‐tryptophan enhanced by surface plasmon of silver nanoparticles: vibrational assignment and structural determination

Vibrational bands of L ‐tryptophan which was adsorbed on Ag nanoparticles (∼10 nm in diameter) have been investigated in the spectral range of 200–1700 cm⁻¹ using surface‐enhanced Raman scattering (SERS) spectroscopy. Compared with the normal Raman scattering (NRS) of L ‐tryptophan in either 0.5 M aqueous solution (NRS‐AS) or solid powder (NRS‐SP), the intensified signals by SERS have made the SERS investigation at a lower molecular concentration (5 × 10⁻⁴ M ) possible. Ab initio calculations at the B3LYP/6‐311G level have been carried out to predict the optimal structure and vibrational wavenumbers for the zwitterionic form of L ‐tryptophan. Facilitated with the theoretical prediction, the observed vibrational modes of L ‐tryptophan in the NRS‐AS, NRS‐SP, and SERS spectra have been analyzed. In the spectroscopic observations, there are no significant changes for the vibrational bands of the indole ring in either NRS‐AS, NRS‐SP, or SERS. In contrast, spectral intensities involving the vibrations of carboxylate and amino groups are weak in NRS‐AS and NRS‐SP, but strong in SERS. The intensity enhancement in the SERS spectrum can reach 10³–10⁴‐fold magnification. On the basis of spectroscopic analysis, the carboxylate and amino groups of L ‐tryptophan are determined to be the preferential terminal groups to attach onto the surfaces of Ag nanoparticles in the SERS measurement. Copyright © 2008 John Wiley & Sons, Ltd.     

Surface‐enhanced Raman scattering study of Ag@PPy nanoparticles

Surface‐enhanced Raman scattering (SERS) spectra of Ag@polypyrrole (PPy) nanoparticles with both 488 and 1064 nm excitation were investigated. Experimental results as well as theoretical analysis demonstrated that electromagnetic (EM) enhancement and charge transfer (CT) both rebounded to the SERS effect of Ag@PPy nanoparticles. When near‐IR excitation (1064 nm) was used for the SERS measurements, the contribution from CT was amplified relative to that from EM because the energy of the near‐IR excitation is far from the surface plasmon resonance of the nanosized Ag particles. The increased doping level of PPy, leading to optimal energy matching between the Fermi levels of the Ag nanoparticles and the energy levels in PPy molecules, could obviouslyenhance the SERS signal. These results suggested that the SERStechnique wasan effective tool for investigating the doping effect and interface interaction in metal‐conductingpolymer composite nanoparticles. In particular, the SERS technique with near‐IR excitation could give more information regarding the contribution of the charge‐transfer mechanism to the spectral enhancement of this kind of system. Copyright © 2010 John Wiley & Sons, Ltd.     

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     

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.