SERS Enhancement and Field Confinement in Nanosensors Based

Article pubs.acs.org/JPCC

SERS Enhancement and Field Confinement in Nanosensors Based on Self-Organized Gold Nanowires Produced by Ion-Beam Sputtering C. D’Andrea,†,⊥ B. Fazio,† P. G. Gucciardi,*,† M. C. Giordano,‡ C. Martella,‡ D. Chiappe,‡,∥ A. Toma,‡,# F. Buatier de Mongeot,‡ F. Tantussi,§,⊥ P. Vasanthakumar,§ F. Fuso,§ and M. Allegrini§ †

CNR IPCF Istituto per i Processi Chimico-Fisici, Viale F. Stagno D’Alcontres 37, I-98156 Messina, Italy Dipartimento di Fisica, Università di Genova, and CNISM, Via Dodecaneso 33, I-16146 Genova, Italy § Dipartimento di Fisica “Enrico Fermi”, Università di Pisa and INO−CNR, Sezione di Pisa, Largo Bruno Pontecorvo 3, I-56127 Pisa, Italy ‡

ABSTRACT: Since its discovery, surface-enhanced Raman spectroscopy (SERS) has pushed researchers’ interest to develop different kinds of active substrates for high sensitivity molecular detection. Defocused ion beam sputtering (IBS) represents a viable route for the production of large scale, highly reproducible SERS-active substrates consisting of near-field coupled nanowires featuring localized surface plasmon resonances in the visible and the near-infrared. Here we investigate the field enhancement and spatial confinement in the visible and the near-infrared of arrays of optically resonant gold nanowires, using two complementary techniques: SERS and scanning near-field optical microscopy (SNOM). While SERS allows us to quantify the field enhancement factor, SNOM is used to image the localization of the enhanced electromagnetic fields. We show that in the visible (633 nm) the nanowires are SERS active only for excitation polarized parallel to the wire-to-wire nanocavities, yielding enhancement factors of 7 × 103. In the near-infrared (785 nm) we observe a 2-fold larger SERS enhancement (1.3 × 104) for excitation parallel to the nanocavities and detect the onset of SERS amplification for excitation polarization parallel to the nanowires long axis. Polarization-sensitive SNOM in the near-infrared (830 nm) enables the correlation of the scattered intensity with the sample morphology at the local scale. We demonstrate that the field enhancement stems from the wire-to-wire nanocavity regions when the excitation field is polarized parallel to the wire-to-wire nanocavity, while we observe more complex field confinement patterns related to the partially inhomogeneous morphology of the substrate when the polarization is parallel to the nanowires long axis. Our experiments strongly suggest IBS-fabricated nanowires as novel substrates for plasmon-enhanced spectroscopies. and tunable plasmonic functionalities.28,29 In IBS an argon ion beam impinges on a compact metal film destabilizing the surface. The latter, subsequently, rearranges itself into an array of self-organized nanoripples oriented along the ion beam direction.28,36 By increasing the ion dose, the valleys of the ripples can reach the dielectric substrate, yielding an array of parallel and laterally disconnected NWs.36 If the wire-to-wire lateral spacing is narrow enough, typically less than 10 nm, the nanowires start to optically couple in the near-field, yielding “hot spots”, i.e., zones in which the field is maximally enhanced, in the wire-to-wire gap regions.30 Such kinds of samples, together with remarkable polarization-dependent nonlinear optical properties,31,32 enable SERS molecular detection33−35 and the fabrication of transparent metal electrodes for plasmon enhanced photon harvesting applications in the visible−nearinfrared (NIR) range.36,37 Gold NWs synthesized by IBS feature lengths up to several micrometers and widths of a few hundreds nanometers and are

1. INTRODUCTION Metal nanostructures supporting localized surface plasmon resonances (LSPRs)1 offer new ways to focus, amplify, and drive optical fields at the nanoscale, acting as optical nanoantennas.2,3 Arranged in either isolated or near-field coupled architectures, these nanostructures represent the building blocks of efficient, reproducible, large-area substrates for surface enhanced Raman spectroscopy (SERS),4,5 metal enhanced fluorescence (MEF),6−8 surface-enhanced infrared absorption-scattering (SEIRA-SEIRS),9,10 and plasmonic photovoltaics.11 Nanoantennas have triggered the development of high sensitivity, multiband spectroscopic nanosensors for the label-free detection of chemical and biomolecular compounds,12−17 as well as of solar cells with enhanced light harvesting capabilities.18−20 Different top-down and bottom-up fabrication techniques have been established to obtain SERS active metal nanostructures with engineered plasmonic properties21−25 and tunable chemical reactivity.26 Among the various bottom-up approaches, defocused ion beam sputtering (IBS)24,27 represents a low cost and time-saving way for the synthesis of large-area arrays of self-organized nanowires (NWs) featuring broadband © 2014 American Chemical Society

Received: January 21, 2014 Revised: March 22, 2014 Published: March 31, 2014 8571

dx.doi.org/10.1021/jp5007236 | J. Phys. Chem. C 2014, 118, 8571−8580

The Journal of Physical Chemistry C

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organized gold NWs fabricated by IBS of a poly crystalline gold film thermally evaporated on a low-cost soda-lime glass substrate. NWs arrays are produced over large areas in a single maskless step by controlled IBS performed in vacuum conditions (base pressure in the low 10−7 mbar range) with the ion beam incident on polycrystalline metal surfaces (Au). A pristine polycrystalline Au film (thickness 150 nm) is grown by thermal evaporation on standard microscope glass slides. The flat gold film presents a uniform distribution of connected grains and a dominant population of grains with a diameter peaked around 80 nm. The root-mean-square (rms) roughness of the as-deposited film amounts to about 2.3 nm. The Au films present a predominant (111) texture as revealed by X-ray diffraction (XRD) measurements.52 The initially compact polycrystalline Au film is then exposed to defocused ion beam sputtering from a gridded multiaperture Ar source (Tectra Instruments). We choose a grazing sputtering angle θ = 82° measured with respect to the normal and a sputtering energy E = 800 eV, while the ion dose is increased at a constant flux of 5.5 × 1014 ions/cm2 (measured in a plane parallel to the sample surface). Ion bombardment is carried out until disconnection of the compact gold film is achieved, i.e., when the valleys of the ripple undulations reach the glass substrate. From this ion dose onward, the poly crystalline film decomposes into an array of Au nanowires separated by elongated gaps running parallel to the ripple ridges. During the sputtering process the sample temperature is stabilized around T = 300 K by means of a cooled sample holder. In order to prevent charge build-up during ion irradiation, electron thermoionic emission is ensured from a biased tungsten filament placed in the vicinity of the ion extraction grid. The morphology of the samples is investigated ex situ by means of an atomic force microscope (AFM, Nanosurf Mobile S) operating in tapping mode equipped with ultra-sharp Si tips (PPP-NCRL by Nanosensor). 2.2. Extinction Spectroscopy. Extinction spectroscopy has been carried out to study the LSPR of the NWs. Measurements are made by using the white light source embedded in the Horiba Jobin-Yvon Labram HR800 spectrometer, using a polarizer (Thorlabs) to control the polarization state and switch it from parallel to the wire-to-wire nanocavity axis (nx) to parallel the NWs long axis (ny). The light is focused with a condenser on a mm spot and collected through a 10× microscope objective (NA 0.10). The spectra are acquired with a CCD Peltier cooled. Measurements are taken after absorption of MB molecules on the NWs. 2.3. Binding of the Methylene Blue Molecules. We use Methylene Blue (MB) as a probe molecule for SERS. MB is a heterocyclic aromatic compound with many uses in biosensing,53 photodynamic therapies,54 and fundamental SERS experiments.33,55,56 Binding of MB to the NWs is accomplished by immerging the sample in a solution of MB (10−4 M) in deionized water for 1h, followed by immediate rinsing in ultrapure water to remove the MB in excess, and drying in air.57,58 Due to the electronic absorption at 620−670 nm,16,33 MB is resonantly excited at 633 nm. MB therefore benefits of a ∼200 times signal increase with respect to what is measured at 785 nm due to resonant Raman scattering (RRS) processes. 2.4. Surface Enhanced Raman Spectroscopy. SERS measurements are carried out with a Labram HR800 (Horiba Jobin-Yvon) coupled to a BX51 microscope, using a 600 l/mm grating and a Peltier-cooled Synapse CCD camera for detection. SERS spectra are excited at 633 nm with a HeNe

characterized by two broad LPSRs: in the visible-NIR (700− 800 nm, related to the short axis and/or to the wire-to-wire nanocavity LSPR) and at wavelengths of few micrometers (related to the long axis LSPR).28 The latter is characterized by a long tail at shorter wavelengths, in the visible-NIR range. It has already been demonstrated that NWs made by IBS are SERS active in the visible (532, 633 nm),33,34 but no results have been reported, to our best knowledge, on the SERS activity and enhancement factors provided by this class of samples in the NIR. Furthermore, it is generally assumed that IBS gives intrinsically near-field coupled NWs with hot spots located at the wire-to-wire cavity. However, no experimental proof has been given that the degree of optical coupling in such structures is sufficient to guarantee a gap-mode resonance or if, alternatively, the strong SERS emission experimentally observed arises from to the excitation of the short axis LSPR33 or to possible structural inhomogeneities,34 and these effects cannot be a-priori excluded. In this article we investigate the properties of a dense, quasiregular array of gold NWs made by IBS in the NIR range, where both LSPRs can be excited experimentally. We combine the SERS analysis with excitation at 785 nm with polarizationsensitive scanning near-field optical microscopy (SNOM) with illumination at 830 nm to both quantify the SERS enhancement as a function of the excitation polarization and solve the intriguing puzzle on the spatial localization of the high scattering sites in NWs made by IBS. Aperture SNOM, in fact, thanks to its subdiffraction spatial resolution38,39 and analytic investigation capabilities,40 has the potential to image the hot spots in SERS active substrates such as isolated dimers,41,42 aggregates with fractal or not completely defined morphologies,43,44 and self-assembled gold nanosphere arrays.45 Here we use a polarization-sensitive aperture-SNOM46−49 in collection mode to map the light scattering properties of the NWs. This configuration circumvents both the problem related to the extremely low signal levels when detecting Raman scattering, and the problem related to illumination-mode SNOM in which the actual near-field polarization coming out of the probe is not perfectly controlled. Collection-mode SNOM, in addition, keeps an experimental arrangement similar to SERS (backscattering, precise control of the excitation field polarization) that allows us to correlate the optical and the morphological properties of the NWs. We find that at 785 nm maximum SERS occurs for polarization parallel to the wire-towire nanocavities, as for the visible (633 nm), but with an enhancement factor two times higher (1.3 × 104). In addition, we observe that in the NIR the NWs are SERS active also for excitation polarized parallel to the NWs long axis, a behavior that does not occur at 633 nm. SNOM maps show that, for excitation polarized along the wire-to wire nanocavities, the light scattering is more intense and tightly confined at the nanocavities with respect to the case in which the excitation is polarized along the NWs long axis. Our study extends and clarifies the plasmonic behavior of gold NRs made by IBS, complementing previous works on colloidal nanoparticles50 and free-standing nanorods made by electrodeposition.51

2. EXPERIMENTAL METHODS 2.1. Gold Nanowires Preparation and Morphology Characterization. In previous approaches spatially distributed SERS-active substrates have been formed by controlled agglomeration of metal deposited at glancing angles on a prepatterned template.24,29,31,32,34 Here we work with self8572



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Figure 1. (a) Schematic picture of the near-field setup used in the experiment (see text for details). (b, top) Topography map of a portion of the sample acquired through the shear-force method. The contour plot represents the position, shape and size of the illumination spot used in the experiment. The dashed contour line corresponds to the full-width at half-maximum (fwhm) determined according to the method outlined in the section 2.6. The topography line profile (b, bottom) along the dashed-dotted segment superposed to the map, reveals a NWs width ∼150−200 nm. (c) Result of the FFT analysis of the topography maps, showing a peak at a spatial frequency corresponding to a periodicity of ∼175 nm.

Laser (MellesGriot 09 LHP 925, output power 17 mW) and at 785 nm with a Diode Laser (Spectra Physics Excelsior 785, output power 40 mW). Light is focused with an Olympus 100× microscope objective NA 0.9 which serves, at the same time, to collect the Raman signal in backscattering. A ∼7 times higher laser power is required for the 785 nm SERS and Raman measurements to counterbalance the much lower signal due to the absence of electronic Resonant Raman effects (section 2.3) and the reduced sensitivity of the spectrometer with respect to 633 nm laser excitation. For signal normalization purposes, we first normalize the spectra to the laser power and integration time used in each experiment (intensities are given in counts/ mW/s). Then, for each excitation wavelength, we normalize the signals to the intensity of the 446 cm−1 peak measured on the reference sample (which has, therefore, unitary intensity in the plots shown below). We chose this peak since with the 785 nm laser the signal of the 1620 cm−1 peak is strongly depleted by the reduced sensitivity of the spectrometer (gratings + CCD camera), and therefore too noisy for correct assessments. 2.5. Scanning Near-field Optical Microscopy. The nearfield optical analysis is carried out with a homemade polarization-sensitive collection-mode SNOM,59 schematically depicted in Figure 1a. Sample illumination, is accomplished by focusing the far-field radiation onto the sample from the top along an angle α = 38° off-normal (surface projection of the wave-vector k oriented along nx). In order to enable full control of the illumination spot position, a custom-made system (OZ Optics) consisting of a single-mode optical fiber coupled to a miniaturized aspheric objective is used, producing a focal waist around 2−3 μm. The objective is housed in an independently driven piezoelectric translator (PZT) of the hollow-tube kind. The Rayleigh length is estimated around 10 μm, that is the same order of the total vertical displacement of the nanopositioner moving the sample, that ensures constant illumination intensity during the scan. The sample is scanned by means

of a closed-loop 3-axis nanopositioner (Physik Instrumente PI.533.3). By synchronizing the displacement of the objective with the in-plane sample motion, spot-like quasi-diffractionlimited illumination of a selected portion of the sample is achieved, which remains fixed during the scan. Synchronization is achieved through hardware conditioning of separate outputs of the microscope controller (RHK SPM1000). A temperature stabilized diode laser (wavelength 830 nm, maximum power, 100 mW) is used as light source. Prior to be launched into the illumination fiber, the linearly polarized laser beam crosses a λ/ 4 wave-plate, leading to circular polarization, and a rotatable Glan-Thomson polarizer (Melles-Griot). Therefore, illumination with linearly polarized radiation aligned along different directions can be attained in subsequent scans. The polarization direction is measured at the focusing objective output; due to the birefringent nature of the single-mode optical fiber, extinction ratios around 1/10 (ratio between the intensity for two mutually orthogonal polarization directions) are typically obtained. The power impinging onto the sample is kept below 10 mW in order to prevent any sample damage. Such a power is independent within ±20% of the polarization direction chosen for the illumination. Collection in the near-field is accomplished by an apertured probe of the metal-coated, tapered optical fiber kind (Nanonics), with nominal apical aperture of 50 nm. The aperture size can be considered as the lower bound for the spatial resolution of the instrument. The output of the SNOM probe is coupled to a photomultiplier with red-extended photocathode (Hamamatsu R955). The signal-to-noise ratio is enhanced by amplitude modulation of the laser beam and signal demodulation through a digital lock-in amplifier (Stanford Research SRS830DSP). The probe aperture, constantly kept close to the surface, i.e., in the near-field range, by a shear-force based feedback, collects the near-field at the sample surface, whose intensity is measured by a detector connected to the probe output. During the scan 8573



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Figure 2. AFM topography (3 × 3 μm2) of the NWs sample. The inset shows the vectors representing the NWs short axis direction (nx), the NWs long axis direction (ny), and the incident field (Eexc) forming an angle θ with the short axis direction. (b) Large area AFM topography map (scale bar 2 μm) after mechanical removal of the NWs (left side) exposing the glass substrate vs the pristine NWs area (right-hand side). (c) Histogram of the height distribution with respect to the flat glass substrate (reference central peak). The peak on the right centered around 25 nm represents the average height distribution of the NWs (the red trace represents the tails of the histogram amplified by a factor 10).

have average thickness and width of ∼25 nm and ∼150 nm, respectively, while the lengths can reach several micrometers. Further details on the morphological properties of NWs arrays fabricated by IBS can be found in ref 28. The ion bombardment has been carried out until disconnection of the deposited gold layer is achieved, with the consequent formation of nanocavities among the NWs with dimensions ranging from few to more than 10−20 nm. Bifurcations and abrupt interruptions of the NWs in the long axis direction are also observed. In order to measure the height distribution of the nanostructures, a portion of NWs has been mechanically removed on a micrometric scale. The AFM image in Figure 2b shows a comparison between the underlying substrate and the adjacent zone covered by NWs (Figure 2b left to right, respectively). The height distribution histogram is plotted in Figure 2c. It shows, on top of the flat glass surface (reference peak centered at 0 nm), the distribution of the Au NWs (peak at positive heights, centered around 25 nm extending up to 40 nm) and the depth distribution of the glass erosions (broad peak at negative values). A quantitative analysis of the AFM line scans on the flat glass portions (analogous to the left side of Figure 2b) has been performed in order to derive the interwire gap distribution. In this way it is possible to minimize tip convolution effects, finding that the cumulated surface fraction comprised in small gaps below 20 nm amounts to about 0.6%, while the cumulated surface fraction covered by larger gaps above 20 nm, can reach figures around 20% or even larger if ion erosion dose is increased at higher doses. The LSPR properties have been investigated through polarized extinction spectroscopy, switching the excitation polarization from parallel to nanocavities/short-axis (θ = 0, Eexc∥nx see inset of Figure 2a) to parallel to the NWs long axis (θ = π/2, Eexc∥ny). A strong and broad peak (Figure 3) centered around 800 nm is observed for field polarized parallel to the nanocavities axis (red line). Measurements have been carried out after absorption of methylene blue (MB), the probe molecule used for SERS, on the NWs surfaces, which causes the red shift of the LSPR with respect to NWs as grown.33 The peak arises from a LSPR in which the field can be confined either along the nanowire or in the wire-to-wire nanocavity.

of the surface, the optical signal is acquired point-to-point leading to maps of the near-field intensity. Therefore, the optical near-field maps show the scattering from the illuminated portion with subdiffraction details. SNOM produces maps of the near-field intensity scattered by the bare NWs substrate as a function of illumination conditions simultaneously with topography maps, as shown in Figure 1b, enabling correlation with the sample morphology at the local scale. Contrary to other, more conventional, implementations of collection near-field microscopy, such as the photon scanning tunneling microscope (PSTM),60,61 in our configuration the plasmon resonance is excited in far-field, preventing coupling between evanescent waves and plasmons (generated by the total internal reflection illumination geometry in PSTM) and absorption of radiation from the material to play a role. 2.6. Measurements of the Illumination Spot. Shape and size of the illumination spot are determined by using two methods, leading both to practically the same results. In the first method, the investigated sample is replaced by an unstructured (flat) gold film and the near-field intensity mapped during the scan of such a blank sample. The second method involves the use of sharp knife edges mounted on the nanopositioner parallel to one or the other scan direction. By scanning their position, controlled shadowing of the illumination spot is achieved. A large area photodetector mounted below the knives measures the shadowed intensity which is used to evaluate the spot size and shape. A variant of this method is also used to calibrate the travel of the PZT translator housing the focusing objective. Both methods reveal a twodimensional Gaussian shape for the focusing spot, with waist size (2.4 ± 0.1) μm and (2.6 ± 0.1) μm along the nx and ny directions, respectively. We attribute the slightly elliptical shape of the spot to the off-normal illumination of the sample surface. Size and shape of the illumination spot do not depend on the polarization direction.

3. RESULTS AND DISCUSSION 3.1. Morphology and LSPR of the Nanowires. Figure 2a shows the AFM image of the investigated sample. The NWs 8574



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Figure 3. Extinction spectra of the NWs with field excitation parallel to the NWs long axis (ny, black line) and to the wire-to-wire nanocavity axis (nx, red line) showing, respectively, the tail of the long axis LSPR and the peak of the nanocavity/short-axis LSPR (∼800 nm). The vertical lines indicate the laser wavelengths used for: SERS in the visible (633 nm, red line), SERS in the NIR (785 nm, brown line) and SNOM in the NIR (830 nm, blue line). The dashed boxes indicate the spectral position of the SERS bands of MB excited at 633 nm (red box) and 785 nm (brown box).

Figure 4. Visible (633 nm) SERS and RRS spectra of MB deposited on the NWs sample (a, b) and on a reference gold sample (c). (a) SERS signal with excitation field polarized along the nanocavities axis (θ = 0). (b) RRS signal with excitation polarized along the NWs long axis (θ = π/2). (c) RRS signal acquired on a flat gold film prior to IBS treatment, used as reference. Experimental conditions are the following: (a) P = 6 μW, integration time T = 30s; (b, c) P = 60 μW, T = 30s. The signal intensities in panels a and b are normalized to the reference, taking into account the different excitation powers. The reference peak intensity at 445 cm−1 (c) is normalized to one.

Given the morphology of the sample, showing both tiny and large cavities, both contributions are probably observed,62 although any conclusions can be driven at this stage. For polarization field parallel to the nanowire long axis (Eexc∥ny, black line), we observe the tail of the long axis LSPR, more and more pronounced in the NIR region. For wires featuring lengths in the micrometer range the dipolar resonance is expected to be in the mid infrared range.63 We therefore attribute this extinction band in the NIR to the occurrence of multipolar resonances64 inhomogeneously broadened because of the nanowire length distribution. More complicated effects, involving, e.g., constructive and destructive interference between different multipolar modes,65 cannot be excluded due to the irregular morphology at the nanoscale possibly featuring ultranarrow gaps between self-organized nanoparticles. 3.2. Surface Enhanced Raman Scattering. SERS measurements have been carried out exciting in the visible at 633 nm and in the NIR at 785 nm. Both wavelengths are resonant with the nanocavity/short-axis LSPR (vertical lines in Figure 3). At 785 nm we also excite the tail of the LSPR relative to the NWs long axis. The Raman bands (dashed boxes in Figure 3) are also resonant with the NWs LSPRs. In Figure 4 and Figure 5 we display the spectra at 633 and 785 nm, respectively. We report the relative intensities of (a) the SERS signals measured for excitation Eexc∥nx (nanocavities) and of (b) the signals for Eexc∥ny (long axis) normalized (see section 2.4) to the signal measured on a reference flat gold film (c) that provides little or null plasmonic enhancement.16,66 As reference we have taken one of the substrates used to fabricate the NWs, prior to IBS processing, on which MB has been absorbed following the same procedure used for the NWs. The gold film features a surface roughness ∼2.3 nm and, as already shown (see the Supporting Information of ref 16), at 785 nm the enhancement provided by these kind of substrates, if any, is not larger than 2. The normalization to the Raman signal measured after absorption of the probe molecule on the gold film, as shown in ref 16, is equivalent to the more conventional procedure based

Figure 5. NIR (785 nm) SERS and Raman spectra of MB deposited on the NWs sample (a, b) and on a reference gold sample (c). (a) SERS signal with excitation field polarized along the nanocavities axis (θ = 0). (b) SERS signal with excitation polarized along the NWs long axis (θ = π/2). (c) Raman signal acquired on the reference. Experimental conditions are the following: (a, b) P = 430 μW, integration time T = 240s; (c) P = 4.3 mW, T = 600 s. The signal intensities in panels a and b are normalized to the reference, taking into account the different powers and integration times. The peak intensity at 445 cm−1 of the reference signal (c) is normalized to one. The reference signal is ca. 2 orders of magnitude smaller than the one measured at 633 nm.

on the Raman signal measured on a solution.67 In addition, it can be applied also to fluorescent molecules as MB at 633 nm, since when deposited on gold PL of MB is quenched. We see that at 633 nm (Figure 4a) the SERS signal for excitation polarized along the nanocavities is ∼60 times the reference for the peak at 446 cm−1 and ∼75 times the reference for the mode at 1620 cm−1. This behavior is somehow expected since the 8575



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energy of the 1620 cm−1 peak is closer to the LSPR peak and therefore reradiation is expected to be stronger. Rotating the polarization to parallel to the long axis (Eexc∥ny, Figure 4b) the signal reduces by ca. 30 times, getting only 2−3 times higher than the reference. Such an amplification is too small to be attributed with certitude to a SERS effect and could be due to local changes of the NWs surface molecular density. At 785 nm (Figure 5a) the SERS signal for Eexc∥nx is ∼130 times the reference (c), more than two times the amplification measured at 633 nm. This is expected, since the excitation energy is closer to the peak of the LSPR. Switching the incident polarization to parallel to the NWs long axis (Figure 5b), we measure a signal ∼25 times the reference. We attribute this signal gain to SERS from the excitation of radiative higher order LSPR modes.64,57 This latter effect at 785 nm, in particular, reduces to ∼5 the ratio between the SERS signals acquired with the two mutually orthogonal polarizations, compared to ∼30 at 633 nm where no SERS is observed for Eexc parallel to the long axis. The correspondence between the polarization-dependent SERS intensities and the LSPR properties allows us to exclude the presence of any NPs clustering effects hidden in the NWs morphology, since this would scramble the polarization dependence of the SERS effect, making the sample more similar to an array of randomly oriented nanoparticles than to a nanowire, which is definitely not our case (see also ref 33). The reproducibility of the SERS efficiency on the substrate has been checked by acquiring several spectra, for each excitation wavelength, moving from point to point. At 633 nm the SERS efficiency (445 cm−1 peak intensity) is found to be constant within 10% (standard deviation) on areas some mm2 wide, at 785 nm within 15% (slightly higher due to the lower S/ N ratio of the measurements). The SERS gain reported above is calculated as the ratio G = ISERS/IREF between the SERS signal (ISERS, normalized to laser power and integration time) and the Raman signal measured on the reference (IREF, normalized to laser power and integration time). G provides, at a glance, quantitative information on the signal gain provided by a specific SERS-active substrate with respect to a flat gold film or to a Raman measurement made in liquid at the same analyte concentration (see Supporting Information in ref 16 for further details). G is smaller than the SERS enhancement factor, defined as EF = (ISERS/NSERS)/ (IREF/NREF),68 by a factor NREF/NSERS related to the different number of molecules probed in the SERS (NSERS) and the reference (NREF) measurements. The factor NREF/NSERS can be, in particular, much higher than 1 since the molecules experiencing SERS are those located in the nanometric hot spots, i.e., much less than those illuminated by the laser spot on the reference sample.16 G has the advantage of being free from any overestimation error made when calculating the probed molecules ratio in the EF.69 Therefore, G should always be given, when characterizing a SERS nanosensor, to make explicit what is the signal amplification provided by a specific device. On the other hand, G does not give information on the SERS enhancement provided by each nanoantenna on each single molecule, as the EF does. This is an important information when we want to simulate, implement and test new architectures of SERS-based nanosensors. In order to evaluate the EF of our NWs sample we assume the number N of probed molecules to be N = ρA, where ρ is the density of molecules absorbed on the gold surface (i.e., number of molecules per unit area) and A the sample area subject to the excitation field. The molecular density on the reference film and on the NWs

can be considered the same, since the same binding procedure of MB is used in both cases. NREF will be, therefore, proportional to the laser spot area Alaser, while NSERS will be proportional to the hot spots area Ahot‑spots excited by the laser. Consequently we will have EF = GAlaser/Ahot−spots. Note that, using this calculation procedure, we only need the molecular densities on the reference and on the NWs to be the same, we do not need to assume single monolayer molecular coverage. If we assume that the SERS signal is caused by the excitation of the short axis LSPR, the hot spot area can be considered as extended all along the illuminated NWs surface,16 and therefore EF ≈ G. More reasonably, we expect that only the molecules lying in the nanocavities provide the largest contribution to the overall SERS signal.30,70 This, because when an incident radiation excites a plasmon trapped between infinitely long semicylinders, as in our case, the huge field enhancement occurs mainly at crevices or in pores only a few nm wide. If we calculate the fraction of molecules lying in nanocavities wide 1 nm or less, we find an upper limit ∼1% of the total number of molecules under the laser spot. Normalizing to this factor we obtain SERS EFs ranging from 7 × 103 at 633 nm to 1.3 × 104 at 785 nm. These values are good enough for SERS sensing applications and not so far from those obtained with NWs produced by more expensive and time-consuming electron beam lithography.55,57,71,72 3.3. Near-Field Microscopy Analysis. SNOM analysis has been carried out in order to clarify the spatial localization of the hot spots (sites of strong field scattering) on the sample surface. The ability of aperture SNOM in measuring the field intensity is limited in the case of densely nanostructured surfaces as, for instance, semicontinuous gold films produced at the limit of percolation73,74 or randomly distributed nanostructures.75 This is a consequence of the impossibility for the apertured nearfield probe to approach very close to the surface in the case of randomly distributed nanostructures consisting of nanosized holes or pores. Moreover, those measurements are typically based on setups where the illumination comes from the backside of the sample and involves excitation of surface plasmons. Such problems are circumvented in our investigation, where the morphology of the samples, featuring long and relatively wide interwire regions, allows for a reliable approach, as can be ascertained by looking at the topography maps. In addition, the setup is conceived to achieve near-field collection in experimental conditions that closely resemble those of conventional SERS investigations and, by SNOM with polarization control we retrieve in the near-field quantitative data on the enhancement factor dependent on the polarization, readily comparable to those achieved by SERS. SNOM measurements are carried out in the same regions of the sample investigated by SERS. The high homogeneity of the SERS intensity throughout the surface ensures that the SNOM maps are representative of the optical properties of the sample even if not acquired on the exact points analyzed by SERS. Figure 1b represents a topography map produced through the shear-force method on a 9.5 × 9.5 μm2 area showing that, despite the limited topographical resolution offered by opticalfiber near-field probes, the array of parallel, quasi-regular gold NWs is clearly reconstructed. The line profile drawn in the horizontal (nx) direction along the dashed-dotted segment plotted at the bottom of the map provides a measurement of the typical NWs width around 150−200 nm. By crosssectioning the map along all the lines of the scan and applying fast-Fourier transform (FFT) algorithms to the line profiles we 8576



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Figure 6. Near-field optical maps acquired on the same region of the sample whose topography is displayed in Figure 5c illuminated with two mutually orthogonal polarization directions (see insets). Line profile analysis along the dashed-dotted segments is plotted at the bottom of the maps. The intensity is normalized to the signal averaged over the whole maps. The displayed features represent the details of the scattered spot, collected in the near-field.

find, as shown in Figure 1c, that the average spatial periodicity with which the NWs are repeated is ∼175 nm. The measured thickness is ∼25−30 nm, in agreement with AFM. A relatively large number of adjacent NWs (more than ten in the fwhm range) is efficiently illuminated. Therefore, the illumination conditions resemble those of the SERS experiment but for the angle between the k-vector and the direction normal to the sample surface. In our configuration, this implies that only the projection of the electric field parallel to the sample surface is felt by the NWs array when the polarization is chosen parallel to nx, that may decrease the effects of the interaction, hence the observed dependence on the polarization direction. The nearfield optical maps produced by the scattering of this illumination spot from the NWs array are shown in Figure 6. They can be considered as representative of the whole set of scans carried out on the sample, both in terms of spatial distribution and of intensity of the collected near-field. The two polarization configurations used in the scans are shown in the inset of the maps. In order to remove any residual dependence on the configuration (due, e.g., to the illumination power or to the near-field coupling conditions achieved in different scans) maps are normalized with respect to the minimum signal detected in the illuminated spot. In one case [panel a], the electric field vector Eexc belongs to the plane of incidence (ppolarization). As a consequence, there is a component of the electric field, Eexc·cos α along nx (shown with a dashed arrow in the inset) that excites the nanocavity axis LSPR. In the other case [panel b], for s-polarization, the electric field oscillates parallel to ny, i.e., parallel to the NWs long axis. As expected, for both polarization states the scattered intensity is almost zero far from the illuminated spot, ruling out contribution of far-field diffraction effects eventually resulting in delocalized fringes. In all scans carried out in our analysis, the near-field scattering intensity gets remarkably larger values in the conditions of panel a, see, e.g., the maps in Figure 6a and the related color scales. The example reported in the figure clearly demonstrates the occurrence of distinct spots where the collected near-field assumes relatively large values (typically, we could distinguish around 10−15 of such spots within the laser illuminated area). We note that, by comparing the near-field

intensity collected in the two polarization configurations, we obtain a result (enhancement between 3 and 4) in agreement with the SERS measurements at 785 nm, which indicate a factor ∼5 enhancement for Eexc∥nx. The slightly smaller value can be due to the oblique illumination and the consequent decrease of the active electric field amplitude in the configuration of panel a. The use of SNOM enables correlating, within the limits imposed by the subdiffraction spatial resolution of the instrument, the spatial distribution of the scattered spot to the topography of the sample. Remarkably, most of the high intensity features detected in the conditions of panel (a) are localized in the wire-to-wire nanocavity regions. We have analyzed the whole set of scans, covering an area of around 200 × 200 μm2, according to the following procedure. We have first discriminated in the topography maps the regions corresponding to valleys and hills, associated with height below or above one-half of the maximum height, respectively. We have then found than more than 70% of the scattered near-field stems from the regions identified as valleys, which in turn are almost completely coincident with the wire-to-wire gaps. This demonstrates that, even in the presence of a rather irregular morphology, the wire-to-wire nanocavity region is the preferred site for field enhancement when polarization is directed along the cavity itself.76,77 The preferential orientation of the hot spots along the nanocavities, again, excludes the presence of relevant clustering effects in the NWs morphology. We remark that true operation in the near-field is demonstrated by the strong decrease of contrast and spatial resolution in optical maps as the probe tip is raised over the surface. This demonstrates negligible contribution of far-field scattering to our results. In the case of Eexc∥ny [panel b], two main differences are found. First of all, correlation between the spatial distributions of the prominent optical features with the morphology is more intricate than with the orthogonal polarization. A rather homogeneous distribution of the scattered near-field is observed among valleys and hills. However, the largest scattered intensity is typically associated with local defects of the nanowire pattern, such as, interruptions, bifurcations, and 8577



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entanglements. For instance, the relatively strong spot appearing in Figure 6b is located close to a bending of the pattern, corresponding to the merging of two adjacent NWs (such a morphology can be discerned in Figure 1b displaying the simultaneously acquired topography map). Second, the near-field optical features appear systematically broader and smeared out when polarization is parallel to the nanowire long axis, as clearly seen in the cross section shown at the bottom of Figures 6a,b. In these conditions, no evidence of subdiffraction spatial resolution is achieved, despite of the instrument capabilities. Moreover, contrary to the other polarization configuration, in this case maxima in the near-field intensity map are no longer found in preferential association with the interwire gaps. Such a finding suggests that the intensity map is dominated by scattering from the metal surface of the individual nanowires, e.g., in correspondence to local gaps or polycrystalline grain boundaries, eventually involving adjacent nanostructures in higher order plasmonic modes, as for instance predicted for spherical nanoparticles.78

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Laboratorio MDM CNR-IMM, Via C. Olivetti 2, Agrate Brianza (MB), I-20864, Italy. # Nanostructures, Istituto Italiano di Tecnologia, via Morego 30, I-16163 Genova, Italy Author Contributions ⊥

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge support by MIUR under Project PRIN 2008J858Y7. B.F., C.D’A., and P.G.G. acknowledge funding from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement No. 241818 (FP7HEALTH-F5-2009-241818-NANOANTENNA). F.B.d.M. acknowledges funding by MSE in the framework of the Operating Agreement with ENEA for Research on the Electric System and by MAE under GR Italy-Poland bilateral program. F.T., F.F., and M.A. are grateful for partial funding to NanoSci E+ program under Grant E2-PLAS (ANR-08-NSCI-007), to the project POLOPTEL funded by La Fondazione Pisa and to the exchange project between University of Pisa and University of Paris-Sud, Orsay. Fruitful discussions with Anne Debarre are gratefully acknowledged.

4. CONCLUSIONS We have investigated the SERS enhancement and field confinement properties in the NIR of near-field coupled selforganized gold NWs fabricated by IBS on glass, using a complementary set of experimental techniques. Extinction spectroscopy highlighted the presence of a plasmon resonance along the nanocavity/short-axis direction peaked at around 800 nm, together with the NIR tail of the long axis LSPR, assigned to inhomogeneously broadened higher order resonance modes. The samples are found to be SERS active in both the visible (633 nm) and the NIR (785 nm). In the NIR the SERS enhancement is found to be 1.3 × 104 for polarization parallel to the nanocavities, namely two times higher than at 633 nm, and ca. 2.5 × 101 for polarization parallel to the NWs long axis. The dependence of the substrate optical properties as a function of the polarization is well confirmed by the collectionmode SNOM analysis, used to reconstruct the actual field intensity distribution at the sample surface. Near-field scattering maps evidence that for polarization along the nanowires/shortaxis, a strong spatial confinement along the wire-to-wire nanocavities occurs, highlighting the actual origin of the hot spots. Our study confirms the superior performances and richness of application potentials of this class of samples in the NIR with respect to the visible. The combined polarized-SERS and SNOM suggests that reducing the nanocavities width and increasing their density on the surface (e.g., by making smaller NWs) can be an effective way to increase the SERS gain of these substrates. The observation at 785 nm of the onset of SERS for polarization parallel to the NWs long axis paves the way to possible applications of this class of materials for SERS sensing with excitation at 1064 nm, where the SERS background and the fluorescence of the target molecules are negligible, with consequent advantages in terms of signal-tonoise ratio of a possible nanosensor and nanobiosensor. Our investigations suggests that near-field coupled NWs fabricated by IBS, by mimicking the hot spots formation in nanoparticles of different shapes, can be enlisted among the efficient and reproducible plasmonic architectures aimed at SERS molecular and biomolecular detection.79

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C.D’A. and F.T. contributed equally to this work.

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REFERENCES

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