Silver Nanoparticles on Porous Silicon: Approaching Single Molecule

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Silver Nanoparticles on Porous Silicon: Approaching Single Molecule Detection in Resonant SERS Regime Alessandro Virga, Paola Rivolo, Francesca Frascella, Angelo Angelini, Emiliano Descrovi, Francesco Geobaldo, and Fabrizio Giorgis J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp405117p • Publication Date (Web): 08 Sep 2013 Downloaded from http://pubs.acs.org on September 9, 2013

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The Journal of Physical Chemistry

Silver Nanoparticles on Porous Silicon: Approaching Single Molecule Detection in Resonant SERS Regime Alessandro Virga†, Paola Rivolo†, Francesca Frascella†, Angelo Angelini†, Emiliano Descrovi†, Francesco Geobaldo†, and Fabrizio Giorgis†, §,* †

Applied Science and Technology Department, Politecnico di Torino, Torino I-10129, Italy

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Istituto Italiano di Tecnologia, Center for Space Human Robotics, Torino I-10129, Italy

ABSTRACT Metal-dielectric nanostructures consisting of Ag nanoparticles synthesized within a mesoporous silicon matrix are exploited for single molecule detection by Surface-Enhanced Resonance Raman Scattering (SERRS). The morphology is controlled yielding plasmonic resonances in the visiblenear infrared range. Enhanced Raman activity of the substrates are tested using Cy3 and R6G dyes as probe molecules. Tuning the particle plasmonic resonance close to the molecule electronic resonance we demonstrate Raman enhancements larger than 1010. Time resolved Raman spectroscopy at very low molecule concentration yields intensity fluctuations which can be mainly ascribed to a charge transferenhancement mediated by the molecules diffusion between different sites on Ag particles.

Keywords: Raman spectroscopy; Nanoparticles; Porous silicon; Plasmonics; Dyes

INTRODUCTION Surface-Enhanced Raman Scattering (SERS) is a sensitive technique allowing to measure vibrational spectra from individual molecules1-3. Among single-molecule spectroscopies, it provides much more detailed information as compared to the broad fluorescence spectra (with the exception of low temperature photoluminescence for crystals). Actually, due to the almost unstructured spectra, fluorescence does not provide detailed molecular information, and photobleaching effects often inhibit single molecule analysis. Raman spectroscopy provides highly resolved vibrational information and although the molecular Raman cross sections are much smaller than the fluorescence ones, SERS mechanism can enhance the Raman efficiency making it competitive in terms of signal intensity. The increase of the Raman scattering efficiency is attributed to two fundamental effects. The main concerns with the enhancement of the electromagnetic (EM) fields localized at the edges of metallic particles after excitation of surface plasmons at resonance conditions, which can lead to giant EM enhancement up to factors of (1011-1012)4. The second contribution (much weaker with respect to the former) deals with a charge-transfer (CT) process wherein an electron can be transferred from an excited metal state to a vibrational level within the target molecule yielding an enhancement of 10÷103 5,6. In addition to such mechanisms, the possible EM resonance carried out through real (rather than virtual) electronic transitions within an analyte in contact with the metal can provide a

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further contribution to the Raman enhancement, which in combination with SERS yields the wellknown Surface-Enhanced Resonance Raman Scattering (SERRS) 7. Clusters of silver and gold nanoparticles (NP’s), generally used in colloidal solution, represent the most efficient types of SERS-active substrates exhibiting the largest enhancement effects. In such systems, the Raman efficiency is strictly correlated to the spatial arrangement of the nanoparticle clusters, characterized by ‘hot-spots’ where the co-location of nanoparticles is optimized in dimers/trimers assemblies 8-10. Besides NP’s, lithographically produced nanostructures characterized by long-range ordering with fully controlled shape and size have been used as SERS substrates 11-12. This allowed to reach a controlled electromagnetic coupling between metal particles, yielding the above mentioned hotspots, which can give a dominant contribution to the SERS efficiency. Despite the high efficiency of such substrates, the involved technology presents some drawbacks affecting costs and complexity if their production on large area is considered. In the last decade, it has been shown that efficient SERS-active substrates can be synthesized on large area taking advantage of a porous Si (p-Si) based matrix. Ag nanostructures were prepared by thermal decomposition of Ag nitrate in oxidized p-Si 13-14, by immersion plating of Ag within the pores of p-Si 15-16 and by inkjet printing17. In all the previous studies, the SERS regime established in silvered p-Si nanostructures has been approached regardless of the analyte electronic transitions, typically in off-resonant electronic excitation of dyes and biomolecules 13-19. In this work, we report on a SERRS analysis performed on Ag NP’s in p-Si obtained by immersion plating. We analyze the enhanced Raman efficiency provided by SERRS using two dyes (Cy3 and R6G), demonstrating sensitivities suitable for single molecule detection. Time dependent Raman scattering has been analyzed as a local probe for checking the enhancement mechanisms.

EXPERIMENTAL DETAILS Materials. Porous Si layers were synthesized by electrochemical etching of boron-doped silicon wafer (34mohm/cm). Anodization process was carried out in HF based producing samples with 80% of porosity with a thickness around 4-5 µm. Ag NP’s were obtained immersing freshly porous silicon in AgNO3 aqueous solution with concentration of 10-3 M, temperature of 30–60 °C and dipping time of 30-90 s. After the dip coating synthesis, the p-Si samples were rinsed in a solution of hydrochloric acid at a concentration of 1 mM for 10 s to remove any contaminant adsorption during the specimen preparation and storage. This cleaning process does not affect the Raman response of the bare silvered substrates which is almost spectrally flat in absence of any analytes. Cyanine Cy3-H was synthesized by reaction of indolenine and a polymethinic chain obtained through condensation of aniline with propane. The solid powder was dissolved in absolute ethanol. Rhodamine R6G was purchased by Sigma-Aldrich. Both the dyes were dissolved in ethanol at several molar concentrations. Characterizations. Scanning electron microscopy images of silvered p-Si samples were obtained as secondary electron images with 5 keV electrons using an in-lens detector of a Zeiss SUPRA 40 (Zeiss SMT, Germany) field emission electron microscope (FESEM). Specular reflection spectra were obtained by using an Agilent Cary 5000 (Agilent California, USA) UV–


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Visible–NIR spectrophotometer equipped with a 12.5° reflectance unit, in the range of 200–2000 nm. Silvered p-Si substrates were impregnated with the Cyanine and Rhodamine dye dispersed in ethanolic solution (10 ml) with different molar concentrations for 3 min. We verified that an increased impregnation time does not influence the intensity of the Raman spectra. Raman spectroscopy were performed by means of a Renishaw inVia Reflex (Renishaw PLC, United Kingdom) micro-Raman spectrophotometer equipped with a cooled charge-coupled device camera. Samples were excited with an Ar–Kr laser source (wavelength of 514.5 nm, and 647 nm), providing a photon flux lower than 60 W/cm2. The spectral resolution and integration time were 3 cm-1 and 10 s, respectively. The presented Raman spectra were obtained after the subtraction of the baseline represented by the dye fluorescence. At the optical probe conditions the CCD saturation has been prevented. For time resolved measurements, a spectrum was acquired for 100 s with integration time of 1 s. Raman imaging was performed on 2 x 2 mm area. All the Raman spectra excited with the same wavelength directly compared in the following section were recorded at similar conditions.

RESULTS AND DISCUSSION Morphology and optical properties of metal-dielectric nanostructures. Ag NP’s has been synthesized through the impregnation of mesoporous Si (average pore dimension ~2–20 nm). The influence of the synthesis parameters (temperature, dipping time and Ag salt conc.) on the nanostructure morphology was described elsewhere 20. The synthesis kinetics allows the growth of particles with size beyond the pore diameter, and their density and average size are strictly dependent on the redox-synthesis parameters. Actually, by fixing the temperature and the Ag salt solution concentration, the size dispersion increased towards larger particles by increasing the time of impregnation. Moreover, the process temperature growing yields a monotonic increase of the of redox reactions rate producing metallic Ag. In this regime, the NP’s are sticked on the porous silicon substrate and most of the lateral surface is surrounded by air. Figs. 1 show selected samples with morphology gradually changed.

Figure 1 FESEM images of Ag NP’s obtained by immersion of p-Si in AgNO3 solution with different temperature/dipping time/salt concentration #1: 60 °C/30 s/ 10-2 M; #2: 30 °C/60 s/ 10-2 M; #3: 50 °C/60 s/ 10-2 M; #4: 50 °C/90 s/ 10-2 M

In order to check the optical response of the metal-dielectric nanostructures, specular spectral reflectance measurements has been performed on the samples previously analyzed by FESEM. As shown in Figs. 2, reflectance spectra are characterized by dips corresponding to plasmon resonances ascribed to enhanced absorption/scattering processes. All the collected spectra are characterized by ACS Paragon Plus Environment


a specific dip around 310–320 nm due to the Ag bulk plasmon, while additional dips distributed over the visible-near IR energy range can be ascribed to either Localized Surface Plasmons (LSP) coupled to individual particles or yielded by interparticles short-range interactions. Moreover, the effect of the dielectric substrate on the plasmonic properties of the nanoparticles must be taken into account both for the resonance energies and for the efficiency of the EM field localization21,22. In the comparison of the reflectance spectra, the specimens are characterized by plasmonic resonances subjected to a typical red-shift when the average particle size is increased, as theoretically expected23. On the other hand when the morphology of the Ag nanostructures becomes more complex with lager average sizes, multiple resonances can be found.

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Wavelength (nm) Figure 2 Specular reflectance spectra of different silvered p-Si substrates whose morphology is shown in Figure 1. The interference fringes clearly observable at large wavelengths are due to the p-Si layers supporting the Ag NP’s. The spectra are vertically shifted for clarity.

It is worth to underline that the metal/dielectric substrate preparation method here presented is easy, rapid and obtainable on large area when compared to lithographic methods; the nanostructure morphology obtained is quite complex but the optical response of samples obtained with the same nominal growth conditions is very similar. In order to prove that, we checked the reflectance spectra of several silvered p-Si specimens obtained with the same synthesis conditions. Fig. 3 shows reflectance spectra featured by a dominant plasmonic dip (considered an optical fingerprint for the suitable choice of the SERS substrate vs a specific analyte and excitation energy). This is a powerful check of the repeatability of the growth process aimed to obtain Ag nanostructures with similar optical properties.


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SERRS Analysis of Cy3 and R6G. In the recent past, the metal-dielectric nanostructures here discussed has been used as efficient SERS substrates for the detection of organic molecules and short peptides20. The Raman scattering efficiency of a molecule in physical contact with the Ag particles can be optimized if the excitation/Raman scattering energies are close to a plasmon resonance identified by a spectral dip in the reflectance spectra. In order to maximize such an efficiency, we approach to the SERRS regime, where the laser excitation frequency (514.5 nm), besides the Plasmon resonance of the substrate, is also matching with the maxima of the analyte absorptions. We check the SERRS effect by analyzing the vibrational spectra of two organic dyes: Cyanine Cy3 and Rhodamine R6G, whose absorbance spectra are shown in Fig. 4 and Raman active modes are summarized in Table I.

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Absorbance spectra of Cy3 and RG6 dyes used as probe molecules in the SERRS analysis.



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Table I Assignment of vibrational modes in SERS spectra of Cy3 and R6G

Figure 5 shows SERRS spectra of Cy3 excited at 514.5 nm with several concentrations adsorbed on sample #3, which shows the plasmonic dip around such a wavelength. As clearly evidenced, similar spectra are observed for concentration ranging within 10-6 M and 10-12 M evidencing the vibrational fingerprints at 1370 cm-1 (methine chain), 1406 cm-1 and 1470 cm-1 (CH3 deformations), 1590 cm-1 (C=N stretching) 24. At a concentration as low as 10–14 M the Raman spectrum is slightly modified, but the modes ascribed to CH3 deformation and C=N stretching can be still detected. It is worth to underline that considering a micrometric laser spot size as the used optical probe, and neglecting eventual dye accumulation effects in pores or nanometric gaps of the substrate, a single molecule detection can foreseen for concentration lower than 10–11 M. In previous studies, we defined a parameter of merit related to the silvered p-Si substrates, evaluating the External Amplified Raman Efficiency (EARE), defined as the ratio between the dye concentration threshold which is detectable on the Ag NP’s, with respect to the minimum concentration of the detected analyte adsorbed on a bare p-Si substrate without Ag NP’s20. Such a parameter represents an underestimation of the well known Enhancement Factor (EF), defined as the ratio of the intensities of the scattered radiation for SERS and normal Raman scattering per molecule. The Cy3 Raman spectrum for a concentration of 10-4 M obtained on bare pSi does not show any vibrational mode, which can be accounted for an EARE, and thus a fortiori for an EF larger than 1010. In order to check the spatial distribution of the Raman active emitters with diffraction-limited resolution, Raman imaging have been performed on an area of 2 x 2 mm for some selected specimens as shown in Figure 6. For a rather high Cy3 concentrations (10–8 M) the intensity of the ACS Paragon Plus Environment

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Raman mode at 1590 cm-1 is homogeneously distributed over the whole area, whilst for the lowest concentration (10–14 M), a detectable signal is collected only on few sites likely corresponding to single molecules localized within Raman hot spots.

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Figure 5 SERRS spectra of Cy3 adsorbed on substrate #3 at different molar concentrations. As a reference, the bottom graph shows the detected Raman spectrum of 10-4M dye molecules on bare p-Si. All the spectra were obtained under 514.5 nm excitation.

Figure 6 Raman imaging performed on an area of 2 x 2 mm of substrate #3 for several Cy3 concentrations (10–8 M, 10-11 and 10–14 M respectively). The mapping concerns with the intensity of the Raman mode at 1590 cm-1.

SERS analysis of R6G at several concentrations has been performed on SERS substrate #3 in onand off-resonance conditions (excitation at 514.5 nm and 647 nm, respectively). As clearly shown in Figs. 7 a-b, for an excitation energy far from the plasmonic resonance and the analyte electronic transitions, the detection threshold is around a concentration of 10-7 M. Instead, the SERRS regime allows a detection for concentrations as low as 10-12 M (typical value for a SM detection regime), that can be explained by a EF increase of at least five order of magnitudes with respect to the electronic off-resonant excitation condition. At high molar concentrations (larger than 10-8 M in SERRS regime), the R6G Raman spectra shows the typical vibrational modes, with the higher intensity at 612 cm-1 (C-C in-plane bending in ACS Paragon Plus Environment


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xanthene/phenyl rings), 774 cm-1 (C-H out-of-plane bending), 1127 cm-1 (C-H in-plane bending in xanthene/phenyl rings), 1184 cm-1 (C-H in-plane bending in xanthene ring), 1309 cm-1 (hybrid mode concerning with NHC2H5 group and xanthene/phenyl rings), 1364 cm-1/1509 cm-1 (both ascribed to C-C ring stretching related to the xanthene ring), 1575 cm-1 (C-C stretching related to the phenyl ring) and 1650 cm-1 (C-C stretching related to the xanthene ring)25. At lower R6G concentrations, the spectrum is slightly different, as also found in other experimental findings on several dyes28,29. Actually some mode shows an abrupt intensity decrease (i.e. at 774 cm-1,1184 cm-1, 1309 cm-1, 1575 cm-1 and 1650 cm-1), while other modes occurring at 797 cm-1 (hybrid mode referring to the motion of the NHC2H5 groups and of the xanthene/phenyl rings), 1275 cm-1 (C-O-C stretching related the COOC2H5 group bonded to the phenyl ring) and 1595 cm-1 (C-H and C-C motion related to the phenyl ring)25 get stronger. Actually, the different Raman spectra of the dyes at high and low molar concentration can be ascribed to the following considerations. Both EM and chemical enhancement concur to boost the Raman signal. Obviously the former effect is much stronger than the second one, and at high molar concentration of the dyes, the EM enhancement alone (actuated at moderate and small analyte-NP distance) could be enough to allow the Raman detection. Differently, at low concentration, the Raman detection should need of both the effects. On the other hand, CT effect, occurring for molecules in intimate contact with NPs, as discussed in the following section, is strictly dependent on the specific vibrational mode and on the local Fermi level of the probed NPs systems. Thus, only the vibrational modes resonantly matching with the local Fermi level of Ag nanostructure can take advantage of chemical enhancement justifying the different intensity distribution of the Raman band at low molar concentration.

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Figure 7 SERS spectra of R6G at several concentrations adsorbed on substrate #4 in on- and offelectronic/plasmonic resonance conditions: excitation at (a) 514.5 nm and (b) 647 nm, respectively. As a reference, the bottom graph in (a) shows the detected Raman spectrum of 10-4M dye molecules on bare p-Si.

Time resolved Raman Scattering. Evidence of temporal fluctuations concerning with the Raman spectrum and intensity has emerged in single-molecule SERS analysis, thus representing a typical hallmark for this regime1,6,8,30,31. Figure 8 shows representative time-resolved Raman spectra of R6G at high and low concentrations (10-6 M and 10-12 M respectively). For a concentration of 10-6 M the Ag NP’s produce stationary SERS spectra, while for a molar concentration of 10-12 the ACS Paragon Plus Environment

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Raman spectra show a typical intensity oscillation of the vibrational bands on the time scale of a few seconds with a certain spectral wandering, in agreement with previous findings for R6G added on Ag nanocrystals in colloidal solutions8. Figure 8 b shows the intensity temporal fluctuations, where the dominant process concerns with a proper intensity oscillation of the modes at 1275 cm-1, 1370 cm-1, 1510 cm-1 and 1595 cm-1.

Figure 8 Time-dependent Raman trajectories of R6G at concentration of 10-6 M and 10-12 M. The time evolves from bottom to top. The dashed lines in b) indicate the vibrational bands at low molecular concentration discussed in the text.

In order to analyze the dynamics of such fluctuations independently on the total intensity changes, it is essential to get rid of a diffuse background well known in SERS analysis5. For this reason, we calculated the time dependent intensity of the Raman bands after subtracting the overall background. While for concentration of 10-6 M all the band intensity fluctuations can be considered negligible (fractional deviations from mean values within 1-2%, are not shown here), for 10-12 M they are within 20 % for resonances at 1370 cm-1 and 1595 cm-1 and ranging up to 50% for modes at 1275 cm-1 and 1510 cm-1 (Figure 9 a-d). With the aim to discuss the origin of the spectral fluctuations at single molecule detection regime, we need to distinguish between the EM and the chemical enhancement mechanisms yielding the increased SERS cross section. In the case of EM enhancement, the selection rules concerning with the Raman active modes are related to the polarization of the molecular oscillation with respect to the nanoparticle surface subjected to plasmonic resonance. Thus, the intensity fluctuations could be ascribed to different orientations of the molecule with respect to the nanoparticle surface, or more likely, with respect to the gap within a NP dimer or nano-crevasses among neighboring NP’s. Depending on the molecule orientation, significant enhancements for particular vibrational modes could occur, while other are expected to be quenched. Indeed, during a molecule rotation, fluctuations for vibrations within different molecular planes should be uncorrelated in time. On the other side, modes related to the same vibrational plane should show synchronous intensity fluctuations. ACS Paragon Plus Environment


To analyze the correlation among the intensity fluctuations concerning with the individual modes, we calculated the intensity ratio of the Raman bands concerning with similar vibrational symmetry. In this regard, Figure 9e shows the intensity ratio of the band at 1510 cm-1 with respect to the band at 1370 cm-1 (in-plane vibrational modes of the xanthene ring). It was observed that this intensity ratio varies by 50%, the same results are obtained by an intensity normalization with respect to the other bands. Such a behavior demonstrates that in our case, the intensities of the analyzed Raman modes oscillate independently, although some mode is characterized by the same reference vibration plane. Moreover, the expected rotational motion of a molecule around a NP adsorption site would range on the nanosecond time scale. Therefore, any intensity fluctuation observed in our analysis on the temporal scale of seconds would be averaged thus resulting in a stationary signal. These considerations likely rule out the orientational effects correlated to the EM field coupling in explaining the reversible intensity fluctuations observed for the Raman bands. δI/

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Figure 9 (a-d) Time dependent intensity of four dominant Raman bands (1275 cm-1, 1370 cm-1, -1 1510 cm and 1595 cm-1) related to the SERRS spectrum of R6G at a concentration of 10-12 M. (e) Temporal variation of ratios between vibrational band intensities related to in-plane xanthene ring vobrations. Data are presented as fractional fluctuations over the mean value, δI/ and δR/.

On the other hand, the CT-enhancement mechanism has been proposed as the main responsible for the Raman blinking effect occurring at single molecule regime6, 8. Although such a mechanism is still under debate, one of the most discussed schemes5 involves the incident radiation striking the metallic nanostructures resulting in a photon being excited within the metal to a higher energy level. From this excited state, a charge transfer process to a vibrational level of the same energy within the adsorbed analyte takes place. Variations in vibrational energy states occur resulting in the transfer of a photon of different frequency being passed back to the metallic energy levels, and returned to ACS Paragon Plus Environment

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the ground state of the metal (i.e. the Stokes Raman signal). In such a scheme, the resonant enhancement is strictly related to the local Fermi level of the metal NP’s and to the involved vibrational levels of the molecule. These modes, linked to the HOMO and LUMO levels of the adsorbed analyte, are not expected to fluctuate, whilst variations in the local work function (LWF) in correspondence to different NP sites can be foreseen. In fact, LWF oscillation to steps and point defects on metal surfaces has been theoretically and experimentally demonstrated in terms of a redistribution or smoothing of the electron cloud at surface protrusions32-34which characterize our polydispersed particles. If we assume that a lateral diffusion of the R6G molecules with respect to the NP’s surface occurs (i.e. thermally activated at room temperature), different LWF’s (that is different Fermi levels) would be probed during the motion, where the molecule could be adsorbed at different NP sites. This would activate CT-enhancements for different vibrational modes along the diffusion path, thus giving an account of the intensity oscillation of the analyzed Raman modes which takes place independently of each other.

CONCLUSIONS Efficient SERS substrates were obtained by immersion plating of porous silicon samples in AgNO3 aqueous solutions, controlling the morphology and the average particle sizes. The Ag based metal dielectric nanostructures are characterized by plasmonic resonances in the visible-near infrared energy range. For such structures, which can be synthesized on large area, we found noticeable Raman enhancements; single molecule detection of Cy3 and R6G dyes has been demonstrated in SERRS regime. Time resolved Raman spectroscopy performed on R6G at very low concentration (SM probing) yielded intensity fluctuations that are interpreted in the light of the electromagnetic and charge-transfer enhancement mechanisms, where the last seems to play a dominant rule, taking into account variations in the local work function along the NP’s surface.

AUTHOR

INFORMATION

*E-mail: [email protected] Tel. +39-011-5647354 Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS Financial support from Projects NANOMAX (Progetto Bandiera MIUR PNR (2011–2013) and NEWTON (MIUR FIRB 2011–2014) is gratefully acknowledged.



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REFERENCES (1) Nie, S.; Emory, S. R. Probing Single Molecules and Single Nanoparticles by Surface-EnhancedRaman-Scattering. Science 1997, 275, 1102-1106. (2) Kneipp, K.; Wang, Y.; Kneipp, H.; Perelman, L. T.; Itzkan, I.; Dasari, R. R.; Feld, M. S. Single Molecule Detection Using Surface-Enhanced Raman Scattering (SERS). Phys. Rev. Lett. 1997, 78, 1667-1670. (3) Hui, Z. Z.; Li, L. Surface-Enhanced Resonance Raman Scattering Spectroscopy of Single R6G Molecole. Chin. Phys. 2006, 15, 136-131. (4) Xu, H. X. ; Aizpurua, J.; Kall, M. ; Apell, P. Electromagnetic Contributions to Single-Molecule Sensitivity in Surface-Enhanced Raman Scattering. Phys. Rev. E 2000, 62, 4318-4324. (5) Otto, A.; Mrozek, I.; Grabhorn, H.; Akemann, W. Surface-Enhanced Raman Scattering. J. Phys. Condens. Matter 1992, 4, 1143. (6) Park, W. H.; Kim, Z. H. Charge Transfer Enhancement in the SERS of a Single Molecule. Nano Lett. 2010, 10, 4040-4048. (7) Mahajan, S.; Baumberg, J. J.; Russell, A. E.; Bartlett, P. N. Reproducible SERRS from Structured Gold Surfaces, Phys. Chem. Chem. Phys. 2007, 9, 6016-6020. (8) Weiss, A.; Haran, G.; Time-dependent single-molecule Raman Scattering as a Probe of a Surface Dynamics. J. Phys. Chem. B 2001, 105, 12348-12354. (9) Chuntonov, L.; Haran, G. Trimeric Plasmonic Molecules: the Role of Symmetry, Nano Lett., 2011, 11, 2440–2445. (10) Itoh, T; Iga, M; Tamaru, H.; Yoshida, K.; Biju, V.; Ishikawa, M. Quantitative Evaluation of Blinking in Surface Enhanced Resonance Raman Scattering and Fluorescence by Electromagnetic Mechanism. J. Chem. Phys. 2012, 136, 24703-24709. (11) Gunnarsson, L.; Bjerneld, E. J.; Xu, H.; Petronis, S.; Kasemo, B.; Käll, M. Interparticle Coupling Effects in Nanofabricated Substrates for Surface-Enhanced Raman Scattering. Appl. Phys. Lett. 2001, 78, 802-804. (12) Huebner, U; Boucher, R ; Schneidewind, H ; Cialla, D; Popp, J. Microfabricated SERS-Arrays with Sharp-Edged Metallic Nanostructures. Microelectronic Eng. 2008, 85, 1792-1794. (13) Chan, S.; Kwon, S.; Koo, T. W.; Lee, L. P.; Berlin, A. A. Surface-Enhanced Raman Scattering of Small Molecules form Silver Coated Silicon Nanopores. Adv. Mater. 2003, 15, 1595-1598. (14) Giorgis, F.; Virga, A.; Descrovi, E.; Chiodoni, A.; Rivolo, P.; Venturello, A.; Geobaldo, F. SERS-active Substrates Based on Silvered Porous Silicon. Phys. Stat. Sol. C 2009, 6, 1736. (15) Lin, H.; Mock, J.; Smith, D.; Gao, T.; Sailor, M. J. Surface-Enhanced Raman Scattering from Silver-Plated Porous Silicon. J. Phys. Chem. B 2004, 108, 11654-11659. (16) Giorgis, F.; Descrovi, E.; Chiodoni, A.; Froner, E.; Scarpa, M.; Venturello, A.; Geobaldo, F. Porous Silicon as Efficient Surface Enhanced Raman Scattering (SERS) Substrate. Appl. Surf. Sci. 2008, 254, 7494-7497. (17) Chiolerio, A.; Virga, A.; Pandolfi, P; Martino, P.; Rivolo, P.; Geobaldo, F.; Giorgis, F. Direct Patterning of Silver Particles on Porous Silicon by Inkjet Printing of a Silver Salt via In-situ Reduction. Nanoscale Res. Lett. 2012, 7, 502. (18) Panarin, A. Y.; Terekhov, S. N.; Kholostov, K. I.; Bondarenko, V. P. SERS-active Substrates Based on n-type Porous Silicon. Appl. Surf. Sci. 2010, 256, 6969-6976. (19) Chursanova, M. V.; Germash, L. P.; Yukhymchuk, V. O.; Dzhagan, V. M.; Khodasevich, I. A.; Cojoc, D. Optimization of Porous Silicon Preparation Technology for SERS Applications. Appl. Surf. Sci. 2010, 256, 3369-3373. (20) Virga, A.; Rivolo, P.; Descrovi, E.; Chiolerio, A; Digregorio, G.; Frascella, F.; Soster, M.; Bussolino, F.; Marchiò, S.; Geobaldo, F.; Giorgis, F. SERS Active Ag Nanoparticles in Mesoporous Silicon: Detection of Organic Molecules and Peptide–Antibody Assays. J. Raman Spectros. 2012, 43, 730–736.


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(21) Mertens, H.; Verhoeven, J.; Polman, A.; Tichelaar; F. D. Infrared Surface Plasmons in Twodimensional Silver Nanoparticle Arrays in Silicon. Appl. Phys. Lett. 2004, 85, 1317-1319. (22) Virga, A.; Gazia, R.; Pallavidino, L.; Mandracci, P.; Descrovi, E.; Chiodoni, A.; Geobaldo, F.; Giorgis, F. Metal-dielectric Nanostructures for Amplified Raman and Fluorescence Spectroscopy. Phys. Status Solidi C 2010, 7, 1196–1199. (23) Kreibig, U.; Vollmer, M. Optical properties of metal clusters. (Springer, Berlin 1995) (24) Sato, H.; Kawasaki, M.; Kasatani, K.; Katsumata, M. Raman Spectra of Some Indo-, Thia- and Selena-carbocyanine Dyes. J. Raman Spectros. 1988, 19, 120-132. (25) Watanabe, H.; Hayazawa, N.; Inouye, Y.; Kawata, S. DFT Vibrational Calculations of Rhodamine 6G Adsorbed on Silver: Analysis of Tip-enhanced Raman Spectroscopy. J. Phys. Chem. B 2005, 109, 5012-5020. (26) Hildebrandt, P.; Stockburger, M. Surface-Enhanced Resonance Raman Spectroscopy of Rhodamine 6G Adsorbed on Colloidal Silver. J. Phys. Chem. 1984, 88, 5935–5944. (27) Majoube, M.; Henry, M. Fourier-transform Raman and Infrared and Surface-enhanced Raman Spectra for Rhodamine. 6G Spectrochim. Acta Part A: Molecular Spectroscopy, 1991, 47, 1459–1466 (28) Lu, H. P. Site-specific Raman Spectroscopy and Chemical Dynamics of Nanoscale Interstitial Systems. J. Phys. Cond. Matt. 2005, 17, R333-R355. (29) Botta, R.; Upender, G.; Sathyavathi, R.; Rao, D. N.; Bansal, C. Silver Nanoclusters Films for Single Molecule Detection Using Surface Enhanced Raman Scattering (SERS). Mat. Chem and Phys. 2013, 137, 699-703. (30) Vosgröne, T.; Meixner, A. J. Surface- and Resonance-enhanced Micro-Raman Spectroscopy of Xanthenes dyes: from the Ensemble to Single Molecules. Chem. Phys. Chem. 2005, 6, 154-163. (31) Kitahama, Y.; Enogaki, A.; Tanaka, Y.; Itoh, T.; Ozaki, Y. Truncated Power Law Analysis of Blinking SERS of Thiacyanine. Molecules Adsorbed on Single Silver Nanoaggregates by Excitation at Various Wavelengths. J. Phys. Chem. C 2013, 117, 9397−9403. (32) Smoluchowski, R. Anisotropy of the Electronic Work Function of Metals. Phys. Rev. 1941, 60, 661-674. (33) Wandelt, K. The Local Work Function: Concept and Implications. Appl. Surf. Sci. 1997, 111, 1-10. (34) Jia, J. F.; Inoue, K.; Hasegawa, Y.; Yang, W. S.; Sakurai, T. Variation of the Local Work Function at Steps on Metal Surfaces Studied with STM . Phys. Rev. B 1998, 58, 1193-1196.



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Table of Contents - Graphic


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Figure 1 FESEM images of Ag NP’s obtained by immersion of p-Si in AgNO3 solution with different temperature/dipping time/salt concentration #1: 60 °C/30 s/ 10-2 M; #2: 30 °C/60 s/ 10-2 M; #3: 50 °C/60 s/ 10-2 M; #4: 50 °C/90 s/ 10-2 M



Figure 2 Specular reflectance spectra of different silvered p-Si substrates whose morphology is shown in Fig. 1. The interference fringes clearly observable at large wavelengths are due to the p-Si layers supporting the Ag NP’s. The spectra are vertically shifted for clarity.

Reflectance (arb. units)

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#1

#2 #3 #4 400

8001200

Wavelength (nm)


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Figure 3 Specular reflectance spectra of different silvered p-Si substrates synthesized with the same growth parameters: temperature 60 °C, dipping time 40 s, salt concentration 10-2 M.

40

Reflectance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


20

0

400

800 1200

Wavelength (nm)



Figure 4 Absorbance spectra of Cy3 and RG6 dyes used as probe molecules in the SERRS analysis.

-8

Cy3 2 10 M -8 R6G 2 10 M

Absorbance (arb. units)

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400

450

500

550

Wavelength (nm)


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Figure 5 SERRS spectra of Cy3 adsorbed on substrate #3 at different molar concentrations. As a reference, the bottom graph shows the detected Raman spectrum of 10-4M dye molecules on bare p-Si. All the spectra were obtained under 514.5 nm excitation.


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-6

10 M -8

10 M -9

10 M 10

-10

M

-11

10 M -12 10 M -14

10 M -4 10 M

1400

1500

1600

pSi +Cy3

-1

Wavenumber (cm )



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

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Figure 6 Raman imaging performed on an area of 2 x 2 mm of substrate #3 for several Cy3 concentrations (10–8 M, 10-11 M and 10–14 M respectively). The mapping concerns with the intensity of the Raman mode at 1590 cm-1.


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1509 1575 1595 1650

1275 1309 1364

1127 1184

774 797

612

Figure 7 SERS spectra of R6G at several concentrations adsorbed on substrate #3 in on- and off-electronic/plasmonic resonance conditions: excitation at (a) 514.5 nm and (b) 647 nm, respectively. As a reference, the bottom graph in (a) shows the detected Raman spectrum of 10-4M dye molecules on bare p-Si.

-6

10 M -7

10 M -8

10 M -9

10 M -10 10 M -11 10 M -12 10 M -4 10 M

600

800

1000

1200

1400

Wavenumber/cm

1600

pSi+R6G


a) Raman intensity (arb. units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


-6

10 M

-7

10 M

-8

10 M

600

800

-1

1000

1200

1400 -1

Wavenumber (cm )


1600


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

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Figure 8 Time-dependent Raman trajectories of R6G at concentration of 10-6 M and 10-12 M. The time evolves from bottom to top. The dashed lines in b) indicate the vibrational bands at low molecular concentration discussed in the text.


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Figure 9 (a-d) Time dependent intensity of four dominant Raman bands (1275 cm-1, 1370 cm-1, 1510 cm-1 and 1595 cm-1) related to the SERRS spectrum of R6G at a concentration of 10-12 M. (e) Temporal variation of ratios between vibrational band intensities related to in-plane xanthene ring vobrations. Data are presented as fractional fluctuations over the mean value, I/ and R/.

I/

0,5

a)

1595 cm

-1

0,0 -0,5 0

I/

0,5

20

40

b)

60

Time (s)

80

100 1510 cm

-1

0,0 -0,5 0

I/

0,5

20

40

c)

60

Time (s)

80

100 1370 cm

-1

0,0 -0,5

I/

0 0,5

20

40

d)

60

Time (s)

80

100 1275 cm

-1

0,0 -0,5 0

R/

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0,5

20

40

e)

60

Time (s)

80

100

I1510/I1370

0,0

-0,5 0

20

40

60

80

100

Time (s)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Table I Assignment of vibrational modes in SERS spectra of Cy3 and R6G


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Silver Nanoparticles on Porous Silicon: Approaching Single Molecule

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