Article pubs.acs.org/JPCC
Probing Plasmonic Effects on the Raman Activity of Ag NanoparticleBased Nanostructures through Terphenyl Diisocyanide Adsorption Giorgio Baraldi,† Eduardo Lopez-Tobar,‡ Kenji Hara,§ Santiago Sanchez-Cortes,‡ and José Gonzalo*,† †
Laser Processing Group, Instituto de Optica, CSIC, Serrano 121, Madrid 28006, Spain Instituto de Estructura de la Materia, CSIC, Serrano 121, Madrid 28006, Spain § Catalysis Research Center, Hokkaido University, Kita-Ku, Sapporo, Hokkaido 001-0021, Japan ‡
S Supporting Information *
ABSTRACT: Nanostructures consisting on Ag nanoparticle (NP) arrays produced by pulsed laser deposition in vacuum and having different morphology are tested as substrates for surface enhanced Raman spectroscopy (SERS) of terphenyl diisocyanide (TPDI). The SERS response strongly depends on the NP size and distribution; its intensity being much stronger in the case of nanostructures containing small NPs with typical size in the range of a few tens of nm and a characteristic interparticle spacing of a few nm. This behavior is related to the existence of plasmonic hot spots between these NPs upon excitation at wavelengths close to that of their characteristic surface plasmon resonance, which favors the Raman signal of TPDI molecules having both isocyanide groups attached to Ag NPs. No changes in the spectral response are observed when the concentration of the molecule varies in the range from 10−6 to 10−3 M, which suggests the saturation of the available hot-spot sites at low concentrations. Finally, we investigate the SERS response of covered nanostructures to prevent Ag tarnishing. The SERS response decreases exponentially as the covering thickness increases from 1 to 3 nm, which is related to the progressive reduction of the uncovered Ag surface available to TPDI molecules.
■
INTRODUCTION Noble metal nanostructures consisting of arrays of Au or Ag nanoparticles (NPs) show unique optical properties such as resonant optical absorption, scattering and near-field enhancement that are related to the excitation of surface plasmon resonances (SPRs).1,2 These strong SPR-induced near-fields have been shown to play a key role in the enhancement of fluorescence signals of active molecules, nanostructures, or surface raman enhanced spectroscopy (SERS).3−7 In particular, it is expected that SERS will become a key tool in analytical chemistry for applications in areas such as molecular characterization, sensing, or detection of contaminants among others.6,8 However, a major drawback for the widespread use of SERS has been the lack of reliable methods to produce solid substrates that could be used in practical sensors. This has frequently been related to the irreproducibility associated with the methods used to immobilize colloidal solutions of NPs into solid substrates.6,8−10 Thus, turning this vision into reality requires the development of SERS substrates with optimized and reproducible response that provide a large signal increase. This is largely related to the plasmonic enhancement that takes place at wavelengths close to the SPR of metal nanostructures, which must have characteristic dimensions in the range of a few nanometers to tens of nanometers.6−8,10 In the case of metal NPs, the SERS response depends on their morphology (size, shape, spacing, and distribution of the NPs within the nanostructure) that determines the spectral features of the © 2014 American Chemical Society
SPR (i.e., the spectral region where SERS phenomena could be observed).1,8,9,11 Therefore, a precise control of the nanostructure morphology is required, and thus, different top-down and bottom-up approaches have been attempted to date to produce either randomly or periodically organized nanostructures such as thermal evaporation, sputtering, pulsed laser deposition (PLD), electrodeposition, atomic layer deposition (ALD), and lithography techniques such as electron-beam lithography or nanosphere lithography.9,10,12−17 In the case of PLD, the morphology of the NPs can be easily controlled through the deposition conditions, without the need of organic or ionic compounds that could compromise the purity of the nanostructure surface,13,14,18−20 and in fact, previous works have shown that PLD in a gas background or followed by thermal or laser annealing allows the elaboration of functional SERS substrates.12,14,21 In this work, we demonstrate that the choice of suitable experimental conditions allows the fabrication by PLD in vacuum of efficient SERS substrates having the adequate range of interparticle spacing to enhance the SERS response. One of the novelties of this work is the use of a bifunctional molecule to probe the existence of hot spots among neighboring NPs by checking the surface-enhancing activity of the fabricated Ag NP-based nanostructured substrates. To that purpose we use 4,4″-terphenyl diisocyanide Received: October 28, 2013 Revised: January 29, 2014 Published: February 12, 2014 4680
dx.doi.org/10.1021/jp410628m | J. Phys. Chem. C 2014, 118, 4680−4686
The Journal of Physical Chemistry C
Article
Figure 1. SEM images of SERS substrates: (a) S-NPs and (b) L-NPs substrates. (c) Tilted view of a L-NPs substrate.
very smooth surface with a surface roughness better than 0.5 nm. An AFM image and a surface profile corresponding to a representative a-Al2O3 buffer layer are presented in the Supporting Information. Using these experimental conditions, Ag nucleates and forms a single layer of isolated NPs in the first case, whereas it is expected to form a percolated and almost continuous layer in the second.19,20 Thus, in the later case a thermal treatment at 210 °C in air was performed in an oven for 70 min to promote the formation of NPs. Both types of SERS substrates were grown simultaneously on Si, glass, and carbon-coated mica (C-mica) substrates to allow their complete characterization. The thickness of the buffer layers guarantees that Ag NP nucleation was independent of the substrate. A Cary 5000 spectrophotometer was used to measure the extinction spectra (extinction = 100 − T, where T is the transmittance) of the SERS substrates grown on glass substrates in the 300 to 1200 nm range where the SPR of Ag NPs is expected to occur. Measurements were performed at normal incidence with unpolarized light. Size, morphology, and distribution of Ag NPs on the surface have been investigated by scanning electron microscope (FE-SEM, SU 8000, Hitachi) and transmission electron microscopy (LEO 190 operating at 120 kV) in plan-view configuration using samples grown on Si and C-mica substrates, respectively. Finally, the SERS activity of the Ag surfaces was tested using TPDI. This molecule was synthesized according to the method reported by Henderson and co-workers26 and dissolved in acetone at different concentrations. SERS spectra were collected on a Renishaw Raman InVia spectrometer equipped with an electrically cooled CCD camera. Samples were excited by means of the 532 nm laser line provided by a frequencydoubled Nd:YAG laser and a power of 50 μW at the sample. The spectral resolution was set to 2 cm−1. Acquisition time for each SERS spectrum was 10 s and consisted of only one scan. FT-Raman spectrum of a solid sample was obtained by using a MultiRam Bruker spectrometer equipped with a high-sensitivity Ge diode detector. The 1064 nm line was provided by an aircooled Nd3+:YAG laser. The laser power was 200 mW at the sample, and the resolution was set to 4 cm−1. The Raman spectra obtained were the result of 1000 accumulations.
(TPDI) as molecular probe, since its characteristics make TPDI an ideal molecule to monitor the existence of hot spots in PLD substrates: it is a bifunctional linear molecule with a total length of 2 nm, which suits very well the gaps that can exist between NPs generated by PLD and where the intensification of field is maximum,22 whereas its aromatic structure interacts strongly with metal surfaces through the two isocyanide groups (NC) existing at its edges,23,24 in contrast to the aliphatic molecules used so far to induce the formation of hot spots in colloids. Moreover, the interaction with metallic surfaces through the isocyanide groups affords important information about the interaction and coordination mechanisms,25,26 all of which makes TPDI an ideal adsorbate to probe this kind of substrate. Finally, diisocyanide molecules are promising compounds in the field of molecular electronics where these molecules could act as molecular wires to construct organic circuits due to the high electron conductance of metal−isocyanide bonds.27,28
■
EXPERIMENTAL SECTION SERS substrates consisting on a single layer of Ag NPs deposited on an amorphous Al2O3 (a-Al2O3) buffer layer have been produced by alternate PLD (a-PLD) in vacuum (10−6 mbar) using an ArF excimer laser (λ = 193 nm, τ = 25 ns fullwidth half-maximum, E = 2.5 J cm−2), which was alternatively focused on polycrystalline Al2O3 and Ag rotating targets at room temperature. The a-Al2O3 buffer layer was first deposited on the rotating substrate, placed 38 mm away from the targets and with its rotation axis shifted 10 mm with respect to that of the targets (off-axis configuration), followed by the deposition of a single layer of Ag. Below the percolation threshold, the Vollmer−Weber nucleation mechanism (i.e., island growth) that is characteristic for the growth of a metal layer on an oxide surface leads to the formation and growth of NPs, whose morphology, size, and distribution are related to the deposition conditions.18−20 In particular, a-PLD allows a precise control of the size of the Ag NPs as well as the thickness of the buffer layer through the number of laser pulses on each target,18−20 whereas the off-axis growth configuration and the rotation of the substrates ensures that the deposited layer of Ag NPs is homogeneously distributed on a ∼1.5 × 1.5 cm2 area.29 We have produced two different types of SERS substrates. In the first case the thickness of the buffer layer is 10 nm and the Ag equivalent mass thickness is 4 nm, while in the second case the a-Al2O3 buffer layer is 20 nm thick and the Ag equivalent mass thickness is 8 nm. Surface roughness was analyzed by atomic force microscopy using a Digital Instruments Nanoscope IVa microscopy and a Veeco controller operating in tapping mode. RTESP7 nanoprobe tips (Veeco) with resonance frequencies between 265 and 309 kHz and elasticity constant k ≈ 40 N/m were used in ambient air. The a-Al2O3 buffer layer presents a
■
RESULTS AND DISCUSSION Figure 1a,b shows plane view SEM images of the two types of SERS substrates in which Ag NPs can be easily identified as the brighter areas. Figure 1a corresponds to the sample containing an equivalent mass thickness of Ag of 4 nm. We observe a layer of randomly distributed, near coalescence Ag NPs that present either circular or elongated in-plane projected shapes, whereas Figure 1b shows the annealed sample that contains approximately twice the Ag content and thus presents a very 4681
dx.doi.org/10.1021/jp410628m | J. Phys. Chem. C 2014, 118, 4680−4686
The Journal of Physical Chemistry C
Article
upon partial melting of the as-deposited Ag layer during the annealing process.12 The optical extinction spectra of both types of SERS substrates are shown in Figure 4a. S-NPs present a broad
different morphology. We observe very large NPs, having circular in-plane projected shapes that were most likely formed during the annealing step, together with very elongated and irregular Ag NPs that seem to be traces of the initial percolated stage, which were not affected by annealing. From now on, we will identify the two types of substrates as S-NPs (where S stands for small) and L-NPs (where L stands for large), respectively. The NPs surface coverage as well as the NPs density decreases in the case of L-NPs with respect to S-NPs substrates: from ∼47% to ∼25% and from ∼1400 to ∼30 NPs μm−2, respectively. Finally, Figure 1c shows a tilted view of the L-NPs substrate. Very large and mid-sized Ag NPs are easily distinguishable from the background. Their average height is in the range of 40−60 and 20−30 nm, respectively, while the background has an average height of 10 nm. The size distribution of the in-plane NPs long and short axes for S-NPs obtained from plane view TEM analysis (Figure 2)
Figure 4. (a) Optical extinction spectra of S-NPs and L-NPs substrates. (b) Background Raman spectra measured for S-NPs, LNPs, and reference bare Si substrates. Excitation in panel b is at 532 nm.
extinction band peaking at λSPR ≈ 513 nm with a broad tail reaching the near-infrared region that is related to electromagnetic coupling among neighboring NPs,1,2,6,12,29 while LNPs show a more intense and narrower SPR band that peaks at 472 nm. The larger intensity of the SPR band in this case with respect to that of S-NPs substrates is related to the higher amount of deposited Ag. In addition, the narrower SPR band suggests that electromagnetic coupling is much lesser for L-NPs than in the case of S-NPs, which is in good agreement with the fact that large Ag NPs are more isolated in L-NP substrates (Figure 1b). Finally, L-NPs present a second peak at a shorter wavelength (λ ≈ 355 nm) that is most likely related to the excitation of the quadrupolar mode.1,2 It is worth noting that the position of the main SPR band beautifully fits the excitation wavelength used in SERS experiments (λexc = 532 nm). In order to test their performance as SERS substrates we have first registered the Raman spectrum for samples deposited on Si in absence of any adsorbate. Figure 4b shows a narrow and intense band at 520 cm−1 in the case of L-NPs that is assigned to the phonon band of Si as well as an emission background centered at ∼2000 cm−1, which is not seen in the case of the bare Si surface and that can be related to the presence of Ag on the substrate. In fact, this emission can be ascribed to a continuous emission of secondary photons from bare metal surfaces,30 and it has been assigned to electron−hole pair excitations in structures of submicroscopic or atomic scale roughness.31 Finally, the emission background is much higher for L-NPs than for S-NPs in good agreement with the presence of larger and irregular Ag NPs in the former case (Figure 1b,c). Figure 5 shows (a) the structure of TPDI and (b) the SERS spectra of TPDI solutions adsorbed on both substrates compared to the Raman spectrum of solid TPDI. The assignment of vibrational bands is shown in Table 1, and it has been made according to previously published SERS analysis of related isocyanides and diisocyanides.24,32−34 The SERS spectrum of a 10−6 M solution of TPDI on S-NPs surfaces resembles the Raman spectrum of the solid, although some differences can be found. The main change observed concerns the dramatic shift undergone by the band assigned to the ν(NC) vibration of the isocyanide group (NC). It shifts from 2126 cm−1 in the solid toward higher wavenumbers (∼2181 cm−1) in the S-NPs. This large blue shift indicates a strong interaction of this group through the C lone pair of
Figure 2. Plane view TEM image of a S-NPs substrate.
Figure 3. Size distribution of the NPs long axis for (a) S-NPs and (b) L-NPs substrates. The inset in panel a corresponds to the NP short axis size distribution of S-NPs substrates.
are presented in Figure 3a. Most NPs present long axis values ranging from 15 to 40 nm, with an average value of 25 nm. In addition, a very small fraction of the NPs present larger values that can be as high as 70 nm. The short axis size distribution is much narrower, with almost 90% of the NPs having short axis lengths in the range of 10 to 21 nm (average value 16 nm). This morphology is characteristic of metal NPs produced by PLD in vacuum and relates to the NP nucleation at the substrate and the growth process itself for large metal contents close to NP coalescence.12,18−20 In this case partial aggregation of smaller NPs leads to the formation of chain-like structures as it can be seen in Figures 1a and 2 for some of the NPs observed in S-NPs. In contrast, the long axis size dispersion in the case of L-NPs obtained from TEM analysis (not shown here) is much broader (Figure 3b). The NPs long axis ranges from 20 to 160 nm, which supports the coexistence of thermally formed large NPs with very elongated and irregular NPs that were formed 4682
dx.doi.org/10.1021/jp410628m | J. Phys. Chem. C 2014, 118, 4680−4686
The Journal of Physical Chemistry C
Article
making possible that both TPDI isocyanide groups can be attached to two Ag NPs.28,37 Since TPDI molecules are uniformly linked to Ag NPs, it is expected that only a small fraction of them are placed in the interparticle gaps below 3 nm. However, no vibration at 2125 cm−1, corresponding to ν(N C) of free NC groups, is observed, which suggest that only the small fraction of TPDI molecules placed in hot spots are seen in the spectra. This is most likely related to the huge intensification of the electromagnetic field occurring in such gaps that leads to a large SERS signal. This behavior is further supported by the increase of the optical absorption observed for S-NPs in the red and near-infrared regions as shown in Figure 4a that is related to electromagnetic coupling among the NPs.1,2,6,12 The other band related to the isocyanide group, which undergoes a shift as a consequence of the interaction with the metal, is that at 1191 cm−1. This band that is attributed to the ν(C−NC) motion may be slightly affected by the interaction with the metal24 and shifts to 1195 cm−1. Finally, other strong bands observed in the SERS spectrum of TPDI on S-NPs are those at 1600 and 1171 cm−1. They are attributed to in-plane vibrations of the benzene ring corresponding to ν(C C) and δ(C−H) motions, and they can be used as molecular orientation probes.32,33 Their large intensification along with that of the ν(NC) bands, and the absence of the out of plane band at 804 cm−1 indicate that the orientation of TPDI is predominantly perpendicular on the Ag NPs surface as it has been also reported for the related molecule phenyl diisocyanide.38 In the case of L-NPs, the SERS intensity of TPDI is much lower, even for a 10−4 M solution of TPDI, as it is shown in Figure 5. Two very weak ν(NC) bands are observed at 2180 and 2115 cm−1 corresponding to linked and free NC groups, respectively, together with the band at 1600 cm−1. The different position of the free ν(NC) band in relation to the solid sample can be attributed to the effect of the interaction with the metal as also reported by Swanson and co-workers.24 In order to understand this response, we must bear in mind that the NP morphology of this sample is completely different from that of S-NPs substrates. We observe very large NPs, but their density is much smaller. As a consequence the interparticle spacing is larger than 3 nm in most cases (Figure 1b), which avoids TPDI molecules to be linked to Ag NPs through both isocyanide groups, while hot spots are much less likely to occur due to the larger interparticle spacing. Thus, the overall SERS intensity is much smaller than in the case of S-NPs, and TPDI molecules appearing in the corresponding SERS spectra are bounded to the surface through only one isocyanide group. Diisocyanides, such as TPDI, induce aggregation in the case of colloidal NPs as the concentration of the adsorbate increases due to the link of both terminal isocyanide groups to different metal NPs.28,37 Thus, the adsorbate modifies the distribution of the NPs, which tends to promote the formation of hot spots, and as a consequence, the spectral position as well as the relative intensity of vibration bands of the SERS signal, such as those related to free or bounded isocyanide groups, are concentration dependent.33,37,38 One of the main differences of SERS solid substrates with respect to colloidal solutions is the absence of mobility of the NPs. Thus, we have investigated possible concentration effects on the change of the SERS signal in the case of S-NPs susbtrates. Representative normalized SERS spectra for increasing concentrations of TPDI adsorbed on S-NPs are shown in Figure 6. In none of the cases the vibration band at 2126 cm−1
Figure 5. (a) Structure of TPDI. (b) SERS spectra measured for TPDI solutions on S-NPs (10−6 M) and L-NPs (10−4 M). We include (b) the SERS spectrum (enlarged by a factor of 10) in the range 1880− 2300 cm−1 in the case of L-NPs and the Raman spectrum corresponding to solid TPDI. Spectra in panel b have been shifted vertically to ease comparison. Excitation is at 532 nm (SERS) and 1064 nm (Raman).
Table 1. Vibrational Assignments of Raman Bands of the Solid Sample and SERS Spectra of TPDI on S-NPs Substrates; Excitation at 1064 nm (FT-Raman) and at 532 nm (SERS) FT-Raman of TPDIc 2126m 1620sh 1602s 1507vw 1288sh 1283m 1191w 1172m 1007vw 826w 804w 746vw 631vw
SERS TPDI on S-NPsc (10−6 M)
assignmenta,b
2181m
νNC (bounded) νNC (free)
1600s
νCC νCC/δCH
1283m 1195w 1171m 1003vw 822vw 807vw
νC−C (inter-ring) ν(C−NC)/δCH δCH ν(Kekulé) γCH γCH ring skeletal deformation
Based on the assignments made in refs 24 and 32−34. bν, stretching; δ, in-plane deformation; γ, out-of-plane deformation. cs, strong; m, medium; sh, shoulder; w, weak; vw, very weak. a
electrons, which has an antibonding character, by means of a σdonation, with little contribution from metal-to-diisocyanide πback-donation.35 Thus, this interaction with the Ag leads to a strengthening of the NC bond responsible for the observed blue shift.36 The absence of bands corresponding to free NC groups on Ag suggests that both isocyanide groups are interacting at the indicated concentration with two different NPs through both terminal groups. As it can be deduced from Figure 2, the interparticle spacing in the case of S-NPs substrates is in many cases in the range of 2 to 3 nm, which suits the length of TPDI adsorbate, thus 4683
dx.doi.org/10.1021/jp410628m | J. Phys. Chem. C 2014, 118, 4680−4686
The Journal of Physical Chemistry C
Article
Figure 7. (a) SERS spectra of TPDI (10−5 M) on S-NPs for increasing a-Al2O3 thickness. (b) SERS intensity as a function of the nominal aAl2O3 thickness for (circles) 2181 cm−1 and (open squares) 1600 cm−1 bands. Dashed lines are fits of eq 1 to experimental data. Spectra in panel a have been shifted vertically to ease comparison. Excitation is at 532 nm.
Figure 6. Normalized SERS spectra of TPDI on S-NPs substrates at different concentrations. Spectra have been shifted vertically to ease comparison. Excitation is at 532 nm.
related to free isocyanide groups is observed, while bands at 2181 cm−1, 1600, 1283, 1195, and 1171 cm−1 are clearly visible, and their relative intensities remain similar. In addition, the peak position of these bands does not change with the TPDI concentration over the 10−6 to 10−3 M concentration range, which suggests that the adsorption scheme does not depend on the adsorbate concentration contrary to what it has been observed in the case of other related compounds such as 1,4phenylene diisocyanide.37,38 Moreover, the fact that the band at 2126 cm−1 is not observed even for a concentration as low as 10−6 M suggests that the accessible hot spot sites at the S-NPs substrates are already occupied, which is likely related to the fixed interparticle spacing in SERS solids substrates. Thus, the SERS intensity is determined by the substrate preparation method, which in turn determines the final number of possible hot spots in the substrate, in contrast to the Ag NPs in suspension where the concentration of adsorbate changes the number of hot spots in interparticle gaps. Therefore, SERS performance of solid substrates depends in the present case on the optimum choice of an interparticle spacing that fits the size of the probing molecule and is small enough to favor the existence of hot-spots among neighboring Ag NPs. As it is demonstrated in the present work, in the case of TPDI this can be achieved using SERS substrates containing smaller NPs but having a denser distribution (i.e., S-NPs substrates are preferable over L-NPs) as opposite to what it has been found in a previous work.12 However, Ag NPs present the drawback of their tarnishing when exposed to the air,15,29,39,40 which considerably reduces their SERS activity.10,15,39 The deposition of ultrathin covering layers is one of the methods proposed to protect them, although an exponential decrease of the SERS signal upon increase of the thickness of the protective layer was observed when covering Ag NPs with Al2O3 layers deposited by ALD.15,39,40 PLD allows the fabrication of covered Ag NPs with excellent chemical and optical stability in a single-step deposition process as we have demonstrated in a previous publication in which we produced Ag NP arrays with similar size and surface coverage (∼50%) to S-NPs covered by a continuous layer of a-Al2O3 as thin as ∼0.5 nm that preserved their optical properties (SPR) for more than one year.29 Thus, we have explored the SERS response of S-NPs covered with an a-Al2O3 layer deposited by PLD by using TPDI as molecular probe. Figure 7a compares the SERS spectra corresponding to uncovered S-NPs with that of S-NPs substrates covered with equivalent thicknesses of 1 and 3 nm of a-Al2O3. The SERS intensity of TPDI markedly reduces for covered substrates, although all bands are still observed for a covering
having an equivalent thickness of 1 nm, whereas only the bands at 1600 and 2181 cm−1 are detectable for 3 nm equivalent thick cover layers, and they disappear for thicker cover layers. The position of the bands does not shift in any case. From the results obtained in our previous work,29 we do not expect a complete covering of the NPs for a-Al2O3 equivalent thicknesses
