Nanostructured Silver Substrates With Stable and Universal SERS Properties: Application to Organic Molecules and Semiconductor Nanoparticles
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Nanostructured silver films have been prepared by thermal deposition on silicon, and their properties as SERS substrates investigated. The optimal conditions of the post-growth annealing of the substrates were established. Atomic force microscopy study revealed that the silver films with relatively dense and homogeneous arrays of 60–80-nm high pyramidal nanoislands are the most efficient for SERS of both organic dye and inorganic nanoparticles analytes. The noticeable enhancement of the Raman signal from colloidal nanoparticles with the help of silver island films is reported for the first time.
KeywordsSERS Ag/Si nanoisland films Post-growth annealing Rhodamine 6G CdSe Nanoparticles
The field of physical and chemical phenomena related to the interaction of molecules and other nanoobjects with plasmons localized in or propagating over specially designed noble metal nanostructures has received increasing interest in recent years [1, 2, 3, 4, 5, 6, 7, 8]. The great progress achieved in both the technology of metal nanostructures with necessary parameters and in theoretical modelling of many particular problems has led to a broad involvement of plasmon-mediated phenomena in the fields of optoelectronics, medical diagnostics and treatment, sensor technologies, etc. [5, 7]. Among these phenomena surface-enhanced Raman scattering (SERS) and photoluminescence are believed to have great perspectives in single-molecule chemical and bio-sensing and in vivo medical diagnostics [5, 6].
Surface enhanced Raman scattering (SERS) has proven to be one of the most powerful analytical tools. This phenomenon represents strong increase of the Raman signal from analyte molecules deposited on nanostructured metallic surface.
There are two main mechanisms of SERS, long-range electromagnetic effect and short-range chemical effect. The part of electromagnetic mechanism in the resulting intensity is dominating (about 104–107), while chemical mechanism contributes only about 10–102. Chemical mechanism acts only for the first layer of analyte in direct contact with metallic surface, when charge transfer between surface and adsorbate molecule can occur. Electromagnetic mechanism is caused by electric field enhancement by excitation of surface plasmon resonance in proximity with nanoscale roughness .
One of the main prerequisites for a wide range of applications of surface enhanced spectroscopy is the availability of non-expensive nanostructured metal substrates with a morphology optimized for a maximum enhancement. The main approaches towards the preparation of such substrates are colloidal synthesis, use of templates, etching, and self-assembled formation of rough (island-like) metal surfaces . Colloidal metal nanoparticles (NPs) are relatively inexpensive, and a narrow size dispersion of NPs can be obtained, which allows spectrally narrow plasmon peaks to be obtained . While even better homogeneity of the nanostructures and related plasmon characteristics can be achieved by etching and using templates, these kinds of methods are more expensive. Self-assembled growth of island metal films on dielectric or semiconductor substrates can be a good candidate for obtaining SERS substrates with an acceptable ratio of price/quality, provided that the necessary homogeneity of the island size is achieved [5, 9, 10, 11]. Furthermore, investigations of the SERS effect on various kinds of metal nanostructures and analyte will contribute to the understanding of the role of electromagnetic and chemical contributions to this effect—an issue intensively discussed due to both application importance and fundamental interest [12, 13].
Here, we report a study of the relation between the preparation conditions of island-like silver substrates, their morphology, and SERS properties. Rhodamine 6G (Rh6G) was chosen as an analyte due to its broad application as fluorescent labelling and sensing reagent [14, 15], as well as due to its common use as an analyte in testing SERS substrates . In addition, the SERS spectra of the nanoparticles of an inorganic semiconductor, CdSe, are also obtained and analysed. In contrast to the huge work done on SERS experiments with molecules as analyte [1, 2, 3], only a few attempts succeeded in observing surface enhancement of Raman signal from semiconductor nanoparticles as analyte [17, 18, 19, 20, 21, 22, 23]. At the same time, the effect of interaction with the plasmon onto the optical and electrical properties of semiconductor NPs is presently an intense field of research . The reported successful SERS experiments on semiconductor NPs were realized using colloidal Ag NPs [17, 21, 22], Ag–CdS composites formed by the Langmuir–Blodgett technique , or by thermal deposition of silver onto the epitaxialy grown nanostructures [19, 20]. In the present work, SERS of colloidal semiconductor NPs deposited on silver island film is reported for the first time.
The initial Ag/Si nanoisland film was prepared by thermal evaporation of silver onto a cleaned silicon substrate at room temperature. The nominal thickness of the silver deposited was adjusted to (10 ± 2) nm, as estimated from the known time and rate of deposition. A series of the samples was produced by annealing of the initial Ag/Si films under ambient or nitrogen atmosphere. The actual dimensions of both the initial surface roughness and of the islands obtained after thermal treatment were derived from atomic force microscopy (AFM) images of the surface. AFM images were taken using a NanoScope IIIa set-up (Digital Instruments) operating in the tapping mode. The SERS experiments were performed using Rhodamine 6G, Cu porphyrin CuTMPy, or colloidal CdSe nanoparticles as analytes. Raman spectra of the samples were excited with the 514.5-nm line of an Ar+ion laser with a power on sample of 5 mW and registered with using a Renishaw Ramascope 2000.
The synthesis of CdSe and CdSe/ZnS NPs was undertaken following a modified procedure of Peng et al. . Cadmium oxide (CdO, 99.5%), 1-octadecene (ODE), oleic acid (OA), trioctylphosphine oxide (TOPO, 99%), Zinc acetate (ZnAc2, 99.99%), Sulphur powder (S, 99.98%) and all organic solvents were purchased from Aldrich. Hexadecylamine (HDA, >99%), octadecylamine (ODA, >90%) and n-trioctylphosphine (TOP) were obtained from Fluka and Selenium powder (Se, 99.99%) was purchased from ChemPur. All chemicals were used without further treatment, except TOP which was purified by distillation. In a typical synthesis of CdSe nanocrystals, a mixture of 0.4 mmol of CdO, 2 mmol of OA in 9.5 g of ODE was heated to 100°C under vacuum for 30 min to remove any residual oxygen and moisture. Under inert (Ar) atmosphere, this mixture was further heated to 300°C until a clear, colourless solution was obtained. After cooling this solution to 100°C, 5 mmol of TOP and 8 mmol of HDA were added and the resulting mixture was kept at 100°C under vacuum for 30 min. After heating to 270°C under inert atmosphere, a mixture of 0.4 mmol of Se in 4.5 mmol TOP and 1.5 g ODE was swiftly injected and the solution temperature held at 245°C until the desired particle size was obtained. The resulting particles were then purified by precipitation with an iso-propanol/methanol mix and redissolved in hexane or toluene.
The shell growth was carried out using the SILAR technique . An aliquot containing 10−4 mmol of CdSe nanocrystals in hexane was combined with 1.5 g of ODA and 5.0 g of ODE and heated to 100°C under vacuum for 30 min to remove any residual oxygen and moisture. Subsequently the system was switched to inert atmosphere and heated to 240°C for the shell growth. The shell growth was achieved by a series of alternating injections of a solution of ZnAc2/OA/ODE (0.04 M, OA:ODE = 1:3) and S/ODE (0.04 M). The purification steps for the core/shell nanoparticles were as previously described for the uncoated CdSe nanocrystals.
Results and Discussion
The growth of silver on silicon is known to follow the Stranski–Krastanov mechanism, i.e. formation of a continuous Ag wetting layer with three-dimensional pyramid-like islands on it. However, with annealing at high temperatures Tann in the range of 300–600°C, the interaction energy between silver atoms becomes stronger than the interaction energy between silver and silicon substrate and ripening of 3D silver islands is enhanced.
Based on a relatively strong intensity of the peaks at 613 and 775 cm−1, which are markers for charge transfer (chemical) component of the SERS on Rh6G (Fig. 2, inset) , we can conclude that the chemical mechanism also contributes to the enhancement in our case.
Special attention in the present work was paid to the stability of the prepared SERS substrates to environment during long storage or multiple use. It was found that the enhancement factor of the Ag/Si substrates does not change significantly after storage for months under ambient atmosphere. For example, the spectra shown in Figs. 2 and 4 were taken from freshly deposited analyte on Ag/Si nanoisland films after a 5-month storage. This clearly indicates the stability of the surface morphology of such nanostructured films. Probably, one of the factors which provide such stability is the (thin) layer of silver oxide formed during annealing in air. The presence of such oxide layer was revealed in the Raman spectra and will be discussed in the following paragraphs. Moreover, the role of the oxide in the SERS activity of such substrates is further confirmed by the fact that the substrates annealed in inert (nitrogen) atmosphere did not possess noticeable SERS activity.
We also investigated the stability of the substrates prepared with respect to multiple use for one and the same analyte, as well as for different analyte. Immediately after deposition of an analyte and its spectra registration, the nanoisland films were kept and rinsed in ethanol for 1 h and then another analyte was deposited on them again and the next measurements were carried out. The subsequent measurements gave Raman signals of the same order of magnitude.
At low signal intensities from the analyte molecule, like in Fig. 5, we could observe two additional features centred near 1,350 and 1,600 cm−1, correspondingly. These features can be attributed to scattering by vibrational modes of silver oxide, Ag2O , formed on the surface of the island film during high-temperature annealing. Interestingly, these two Raman peaks change in intensity almost synchronously with the analyte peak (Fig. 5). We believe that the Ag2O vibrations also undergo surface enhancement and the variation of their intensity with temperature of the Ag layer annealing is not a result of different Ag2O volume. If the latter were the case, we would expect an increase of the Ag2O-related Raman signal for the substrate obtained at 600°C (Fig. 5). Instead we observe negligible peak intensity for this Tann, which correlates with the low intensity of the analyte peak and indicates an enhancement for both features via the SERS effect.
Growing fundamental and applied interest in manipulating the optical and electronic properties of semiconductor NPs by plasmon fields generated in adjacent nanostructured metal has been observed in recent years [3, 4, 5, 6, 7]. In view of this interest, we investigated the SERS effect on colloidal CdSe NPs. Two kinds of NPs were investigated. The NPs of the first kind were homogenous (bare) CdSe NPs capped by organic ligands to preclude their aggregation. The CdSe NPs of the second kind were core-shell systems consisting of a CdSe core capped with a thin ZnS shell. The diameter of the core was about 3 nm, the shell thickness was approximately 0.5–1 nm. The diameter of the CdSe core was determined from the spectral position of the first absorption maximum, based on the relations derived in .
One of the inherent (i.e. not from the shell) surface-related vibrational modes reported for semiconductor NPs, which might be expected to be enhanced in SERS spectrum, is the so-called surface optical (SO) phonon [32, 33, 34]. The frequency of the SO mode is usually 15–25 cm−1 below that of the corresponding longitudinal optical (LO) phonon related to the internal (bulk-like) undistorted part of the NPs. The SO mode is usually observed as a more or less distinct shoulder on the lower frequency side of the LO peak (Fig. 7) [32, 33, 34]. By comparing the lineshape of the SERS and the ordinary spectra in the region of SO and LO modes (Fig. 7), we can conclude that the enhancement is of the same magnitude for LO and SO modes. Our results qualitatively agree with those obtained in Refs. [17, 18, 23]. The latter results may indicate that the enhancement of Raman scattering by (bulk-like) LO phonons occurs via enhanced absorption of the exciting light in NPs, mediated by the plasmon excitation. Therefore, the SERS effect in this case occurs not locally but for the NPs as a whole. This fact can be an indication of the spatial extension of the plasmon enhancement being noticeable even at the distance of several nm from the metal surface, in accordance with Otto etal. [35, 36]. The concentration of the exciting light energy near the apex of silver nanoislands of special morphology leads to the generation of a larger number of excitons (electron-hole pairs) inside the adjacent semiconductor NPs. The latter means a larger number of the phonon scattering events. The probability of the scattering on the strength of the electron- (exciton-) phonon coupling (EPC) may thus be the same as in the ordinary (resonant) RS. The value of the EPC strength can be estimated from the intensity ratio of the LO peak and its overtone (near 415 cm−1) , and this value is very close in both the SERS and RRS spectra (Fig. 7).
We have studied the dependence of the SERS efficiency of nanostructured silver films, prepared by thermal deposition on silicon, on the parameters of their post-growth annealing. It was found that the annealing under ambient but not inert atmosphere gives the remarkable enhancement factor. The optimal conditions of a post-growth annealing of the substrates have been established to be Tann = 400–550°C and duration of 15–20 min. Atomic force microscopy study revealed the relatively dense and homogenous array of 60–80-nm high pyramidal nanoislands, formed at such annealing conditions, to be most efficient for SERS of both organic dye and inorganic nanoparticles analyte. The noticeable enhancement of the Raman signal from colloidal semiconductor nanoparticles with help of silver island films is reported for the first time (in opposite to using of colloidal Ag in  or Langmuir–Blodgett technique in ).
Authors are grateful to D. Cojoc from CNR-INFM, Laboratorio Nazionale TASC, Area Science Park—Basovizza, Trieste, Italy for the help with carring out Raman spectra measurements and useful advice. V. Dzhagan is grateful to the Alexander von Humboldt Foundation for financial support of his work.
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