Sensitivity of Plasmonic Nanostructures Coated with Thin Oxide Films

Jun 18, 2010 - Refractive Index Sensing: Experimental and Theoretical Investigations ... The refractive index sensitivity for architectures with n1e n...
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J. Phys. Chem. C 2010, 114, 11769–11775

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Sensitivity of Plasmonic Nanostructures Coated with Thin Oxide Films for Refractive Index Sensing: Experimental and Theoretical Investigations Elisabeth Galopin,†,‡ Joanna Niedzio´łka-Jo¨nsson,†,‡ Abdellatif Akjouj,‡ Yan Pennec,‡ Bahram Djafari-Rouhani,‡ Adnane Noual,‡ Rabah Boukherroub,†,‡ and Sabine Szunerits*,†,‡ Institut de Recherche Interdisciplinaire (IRI, USR-3078), UniVersite´ de Lille1, Parc de la Haute Borne, 50 aVenue de Halley, BP 70478, 59658 VilleneuVe d’Ascq, France, and Institut d’Electronique, de Microe´lectronique et de Nanotechnologie (IEMN), UMR-CNRS 8520, UniVersite´ de Lille 1, Cite´ Scientifique, 59655 VilleneuVe d’Ascq, France ReceiVed: March 16, 2010; ReVised Manuscript ReceiVed: April 26, 2010

This paper reports on the use of the Lorentz-Drude model to investigate the refractive index sensitivity, S (change of nanometer per refractive index unit, in nm RIU-1) of multilayered localized surface plasmon resonance (LSPR) interfaces. The investigated interface consists of an array of gold nanostructures (Au NSs) on glass (n1 ) 1.51) coated with dielectric overlayers of different refractive index (n2 ) 1.45-2.63) and varying thickness (0-300 nm). These interfaces are in contact with solvents of different refractive index (n3 ) 1.000-1.630). The refractive index sensitivity for architectures with n1e n2 is found to be proportional to the amplitude of the oscillation of the wavelength at maximal absorption (λmax). It reaches its maximal value when the amplitude of the oscillation is maximal and its minimal value when the amplitude is null. The amplitude of the oscillations increases with an increase in refractive index of the dielectric. For n1 > n2, all the curves oscillate in phase leading quickly (ddielectric > 40 nm) to a low value of the sensitivity. Quantitatively, a LSPR architecture of glass/Au NSs/nanocrystalline diamond (n2 ) 2.4) with a thickness of 10-20 nm shows the best sensitivity. However, it has to be noted that the sensitivity is mainly affected by the thickness of the dielectric and not by the refractive index of the overcoating 1. Introduction The recent decade has witnessed enormous research efforts directed toward the understanding and use of the unique and tunable optical properties of metallic nanostructures. The development of plasmonic-based interfaces has been driven mainly by the possibility of numerous exciting applications, including sensors,1,2 and was enabled by the rapid progress in nanotechnology and nanoscale science allowing successful synthesis and characterization. Noble metal particles such as gold, silver, and copper are the metals of choice for plasmonic applications, as the excitation of surface plasmon oscillations occurs at visible frequencies.3 The influence of particle parameters such as shape, size, metal composition, and interparticle distance on the plasmonic behavior has been largely discussed and demonstrated in several reports.4 Their optical properties depend additionally markedly on the surface chemistry5,6 and the refractive index of the environment surrounding the nanostructures,7,8 which makes these interfaces suitable for sensing applications. In recent years, there have been several reports on the fabrication of nanoparticles-based plasmonic interfaces coated with different dielectric layers.5,6,9-17 The use of such multilayer structures was motivated by two factors. Poor adhesion of metal island films to the substrate induces morphological changes upon exposure to solvents and analytes, causing optical properties degradation and thus uncertainty in any detection scheme based on refractive index sensing. This poor stability has been known for silver and gold island films.7,18 Coating the nanoparticles * To whom correspondence should be addressed. † Institut de Recherche Interdisciplinaire. ‡ Institut d’Electronique, de Microe´lectronique et de Nanotechnologie.

with thin dielectric films protects the nanostructures, resulting in a reliable sensing platform. The other important feature of these multilayer plasmonic interfaces is that it widens the surface functionalization schemes for the coupling of organic or biological molecules.5,9,11,12,14 In addition to sensing, it has been shown that coating nanostructures with different thicknesses of dielectric layers allows the tuning of the plasmonic signals.10,12 Furthermore, short-19,20 and long-range plasmonic effects5,9-12,21,22 have been observed on such multilayered structures. However, until now there has been no investigation about the role of the refractive index of the dielectric coating on short- and long-range plasmonic effects. In this paper, we investigate theoretically the influence of the refractive index of the overcoating relative to the underlying substrate, supporting the metal nanostructures, on the final sensitivity of the multilayered interface for sensing. This is indeed a highly important parameter for the development of a sensitive LSPR interface. Some of the theoretical results will be compared to experimentally obtained ones. The novelty behind this study is to provide general guidelines on the influence of parameters such as the thickness of the overcoating, its refractive index together with the refractive index of solvents, where the LSPR interface might be immersed on the sensitivity of the optical sensor interface. In our case random rather than periodic gold nanostructures were formed and considered. The sensitivity of the initial uncoated interface was rather low being about 100 nm RIU-1. Using periodic nanostructures of more complex geometry such as nanorings,23,24 nanorice,25 stars,26 crescents,27 cubes,28 etc., the sensitivity of the detection may easily exceed 500 nm RIU-1. Sensitivities up to 880 nm RIU-1 were reported for example for gold nanorings.24 However, the

10.1021/jp1023839  2010 American Chemical Society Published on Web 06/18/2010

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TABLE 1: Thickness Determination of the Deposited SiOx, ITO, NCD, and a-Si0.63C0.37:H Layers Using Ellipsometry SiOx/nm

ITO/nm

NCD/nm

a-Si0.63C0.37:H/nm

0.0 4 ( 0.2 7 ( 0.3 35 ( 0.3 40 ( 0.2 55 ( 0.2 70 ( 0.2 100 ( 0.3 140 ( 0.2 155 ( 0.4 170 ( 0.2 185 ( 0.1 200 ( 0.3 230 ( 0.2 255 ( 0.2 280 ( 0.5 300 ( 0.2

0.0 15 ( 0.2 30 ( 0.2 52 ( 0.2 60 ( 0.2 88 ( 0.2 105 ( 0.2 184 ( 0.2 200 ( 0.2 220 ( 0.2 256 ( 0.2 268 ( 0.2 284 ( 0.2 300 ( 0.2

0.0 40 ( 0.2 60 ( 0.2 90 ( 0.2 120 ( 0.2 150 ( 0.2 180 ( 0.2 220 ( 0.2 240 ( 0.2 260 ( 0.2 300 ( 0.2

0.0 5 ( 0.2 10 ( 0.2 21 ( 0.2 29 ( 0.2 50 ( 0.2 76 ( 0.2 101 ( 0.2 124 ( 0.2 153 ( 0.2 175 ( 0.3 204 ( 0.2 220 ( 0.2 276 ( 0.5 300 ( 0.2

same considerations and conclusions are valid on such interfaces as obtained in this study. It is thus hoped that this systematic investigation can be used as a reference for other groups interested in similar studies. 2. Experimental Section 2.1. Formation of Gold Nanostructures on Glass. Glass slides (n ) 1.51) were cleaned in isopropanol and acetone in an ultrasound bath at room temperature, rinsed copiously with Milli-Q water, and dried under a stream of nitrogen. The clean substrates were transferred into an evaporation chamber. Gold nanostructures deposition was carried out by thermal evaporation of 4 nm thick gold films using MEB 550 S (Plassys, France). Postdeposition annealing of the Au-covered slides was carried out at 500 °C for 1 min under nitrogen atmosphere, using a rapid thermal annealer (Jipelec Jet First 100). The reproducibility of the Au evaporation was evaluated by measuring the LSPR signals of a batch of 8 samples. The standard deviation in the wavelength (λmax) and maximum absorption (Imax) is typically 2 nm and 0.02 abs units, respectively. 2.2. Deposition of Dielectric Overlayers. 2.2.1. SiOx. SiOx overlayers (Table 1) were deposited on glass coated with Au nanostructures (glass/Au NSs) by plasma-enhanced chemical vapor deposition (PECVD) in a Plasmalab 800Plus (Oxford Instruments, UK) at a pressure of 0.005 Torr for 1 h as described previously.5 The growth conditions used were as follows: substrate temperature, 300 °C; gas mixture, SiH4 (5% in N2) and N2O (the gas flow was 150 and 700 sccm for SiH4 and N2O, respectively); total pressure in the reactor, 1 Torr; and power, 20 W at 13.56 MHz. Under these experimental conditions, the deposition rate was 681 Å min-1 and the silica films display a refractive index n ) 1.45. A total thickness of 300 nm was deposited and the thinner films were obtained by etching the SiOx layer. The SiOx films are etched by a reactive ion etching (RIE) process using a gas mixture plasma in a RIE etching Plasmalab 80plus (Oxford Instruments, UK) equipped with a laser interferometer (Jobin Yvon) at 670 nm. The etching rate was calibrated by using two separate techniques. First, the etching rate was estimated within the plasma chamber by an interferometer on a SiOx layer on a planar silicon substrate. Then, the etching rate was estimated by ellipsometry measurements. The etching rate is 272 Å min-1, using the following etching parameters: gas mixture CHF3 and CF4 at 20 sccm gas flow each, 180 W forwarded power, and 50 mTorr chamber pressure.

Galopin et al. 2.2.2. ITO. ITO overcoatings (Table 1) were deposited on the glass/Au NSs interface using rf sputtering (Plassys MP 450S) at 8 × 10-8 mbar (turbomolecular rotary pump system).12,29,30 The deposition chamber contains a In2O3-SnO2 (In2O3 90% w/w; SnO2 10% w/w 99.999% purity) ceramic sputtering target (75 mm in diameter). The deposition temperature is measured with a thermocouple set behind the sample holder. ITO deposition is carried out at a rf power of 13.56 MHz under oxygen/argon atmosphere, using the following parameters: rf power ) 38 W, total pressure ) 0.012 mbar, O2/Ar ratio ) 0.051, deposition rate ) 0.6 nm min-1, and substrate temperature ) 25 °C. 2.2.3. Nanocrystalline Diamond (NCD). The growth of NCD layers on gold glass/Au NSs interfaces (Table 1) was achieved by a modified hot filament-assisted chemical vapor deposition (HFCVD) technique (Rho-BeSt process) at 800 °C, using a gas mixture of 3% methane in hydrogen. No argon or nitrogen gases have been added during the growth.9 2.2.4. Amorphous Silicon-Carbon Alloy. Amorphous silicon carbon alloy layers (a-Si0.63C0.37:H) (Table 1) were deposited onto glass/Au NSs, using plasma-enhanced chemical vapor deposition (PECVD) in a “low-power” regime.11 The following parameters were used: pressure ) 35 mTorr, temperature ) 250 °C, power density ) 0.06 W cm-2, gas flow rate ) 20 cm3 min-1, and [CH4] ) 94%. 2.3. Instrumentation. 2.3.1. UV-Vis Spectrometer. Absorption spectra were recorded with a Perkin-Elmer Lambda UV/vis 950 spectrophotometer in cuvettes of 10 mm dimension. The wavelength range was 400-800 nm. 2.3.2. Scanning Electron Microscopy (SEM). SEM images were obtained with an electron microscope ULTRA 55 (Zeiss) equipped with a thermal field emission emitter and a highefficiency In-lens SE detector. 2.3.3. Ellipsometry. Spectroscopic ellipsometry data in the visible range were obtained with a UVISEL Jobin Yvon Horiba Spectroscopic Ellipsometer equipped with DeltaPsi 2 data analysis software. The system acquired a spectrum ranging from 2 to 4.5 eV (corresponding to 300 to 750 nm) with 0.05 eV (or 7.5 nm) intervals. Data were obtained using an angle of incidence of 70° with the compensator set at 45° and fitted by regression analysis to a film-on-substrate model as described by their thickness and their complex refractive indices. The values given in Table 1 are averaged over 4 measurements taken on different spots of the surface. 2.4. Simulation Method. Calculations are performed with use of the Finite Difference Time Domain (FDTD) method, which solves Maxwell’s equations by discretizing both time and space and by replacing derivatives with finite differences.31,32 Our calculation is performed in a two-dimensional (2-D) box (along x and y axis, see Figure 1) with a propagation along the y axis. Perfect Matching Layer (PML) conditions are applied at the boundaries y of the box, in order to avoid reflections of outgoing waves.33 Along the x direction, the unit cell is repeated periodically and the structure is supposed to be infinite along the z direction. Space is discretized in both x and y directions by using a mesh interval equal to ∆x ) ∆y ) 1 nm. The equations of motion are solved with a time integration step ∆t ) ∆x/(4c), where c is the velocity of the light in vacuum and the number of time steps is equal to 2,19 which is the necessary tested time for a good convergence of the numerical calculation. The incoming pulse, having TM polarization, is generated at the bottom part of the unit cell by a current source parallel to the x axis and having a planar profile along the x direction. The

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Figure 1. Schematic representation of the structure studied in this work. The gold nanorod is characterized by the height h and the width l. The lattice parameter “a” is defined as the distance between two nearest neighboring gold nanorods. The input source is placed in the substrate and the detector in air or solvent.

Figure 2. Evolution of λmax of a glass/Au NSs interface coated with increasingly thick SiOx overlayers [air (black) water (blue), 1-butanol (green), 1,3-propanediol (orange), benzene (magenta), and CS2 (violet)]: (A) calculated curves with a ) 80 nm, l ) 30 nm, and h ) 15 nm; (B) experimentally determined curves (mean value of three measurements).

current is generated during a short period of time in such a way as to excite the electromagnetic waves in the frequency domain of interest. The transmitted signal, probed at the end of the upper part of the unit cell, is recorded as a function of time and finally Fourier transformed to obtain the transmission coefficient versus frequency. All the transmission spectra are normalized with respect to the one corresponding to a homogeneous glass structure without the array of metallic nanoparticles. The transmission is reported in dB as a function of the wavelength.

previously,10 these parameters represent the closest configuration of the experimentally formed glass/Au NSs/SiOx architectures and will be used to compare the theoretical and experimental results. For glass/Au NSs/ITO, glass/Au NSs/NCD, and glass/ Au NSs/a-Si0.80C0.20:H it was found that better fits to experimental transmission UV-vis spectra are obtained by using slightly modified geometrical parameters: l ) 25 nm, h ) 15 nm, and a ) 70 nm. These parameters result also in an average metal coating of 37%. The model used here is based on the frequency-dependent complex permittivity of metal (gold), which is described by the Lorentz-Drude model34 and used in our previous paper.10 This model allows us to fit experimental data for the frequencydependent dielectric constant of metals such as gold,34 including both the real and imaginary parts, with a good agreement in the visible wavelength range (400 < λ < 800 nm). 3.2. First System: The Refractive Index of the Substrate Is Higher than That of the Deposited Dielectric Layer (n1 > n2). Figure 2 shows theoretically and experimentally the change in λmax as a function of the thickness of the dielectric layer exhibiting a refractive index n2 ) 1.45 when covered with solvents of increasing refractive index. Representative transmission spectra for the initial glass/Au NSs LSPR interface and when coated with 140 nm of SiOx are presented in the Supporting Information. The sensitivity of the interfaces can be determined by evaluating the effect of the external dielectric medium on the position of the LSPR signal. For different solvents, the oscillations in λmax occur around a wavelength value defined by the refractive index of the solvent and the dielectric

3. Results and Discussion 3.1. Theoretical Model. Figure 1 displays schematically the interface configuration used for the theoretical study as described previously.10 It consists of a layer of gold nanostructures of diameter l, height h, and interparticle distance a deposited on glass (refractive index n1 ) 1.51) and coated with dielectric films of refractive index n2 ) 1.45-2.63 (Table 2). Three different cases can thus be distinguished: (i) the refractive index of the substrate is higher than that of the deposited dielectric layer (n1 > n2); (ii) the refractive index of the substrate is the same as that of the deposited dielectric layer (n1 ) n2); and (iii) the refractive index of the substrate is lower than that of the deposited dielectric layer (n1 < n2). The interface is furthermore covered by a nonabsorbing medium of refractive index n3 such as air, water, 1-butanol, 1,3-propanediol, benzene, and carbon disulfide (Table 1). For the calculations, the following geometrical parameters were chosen. In the case of glass/Au NSs/glass and glass/Au NSs/SiOx the used length, height, and interparticle distance used were l ) 30 nm, h ) 15 nm, and a ) 80 nm. This corresponds to an average metal coating of 37%. Indeed, as shown

TABLE 2: Refractive Indices of Dielectric Layers (n2) and Solvents (n3)a

a

dielectric

n2

solvents

n3

amorphous silica (SiOx) borosilicate glass indium tin oxide (ITO) nanocrystalline diamond (NCD) amorphous carbonated silicon (a-Si0.80C0.20:H)

1.45 1.51 2.00 2.40 2.63

air water 1-butanol 1,3-propanediol benzene carbon disulfide (CS2)

1.000 1.333 1.397 1.438 1.500 1.63

The refractive index of the glass substrate onto which the Au NSs are deposited is n1 ) 1.51.

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Figure 3. Variation of λmax with refractive index of the solvent for different SiOx overlayer thicknesses.

Figure 4. Change of refractive index sensitivity for glass/Au NSs/ SiOx as a function of the dielectric layer thickness: (A) theoretical; (B) experimental results.

coating. These oscillations around the average value of the wavelength are a result of the diverse interactions that may occur between the localized surface plasmon mode and the classical modes of the dielectric film.The periodicity of the oscillation can be calculated by the following equation: dp ) λmax/(2n2), where n2 is the refractive index of the dielectric layer [dp(SiOx) ) 580/(2 × 1.45) ) 200 nm]. This average value of the wavelength λmax and the periodicity of the oscillation are independent of the solvent used (i.e., independent of the refractive index n3). However, the amplitude of the oscillation is strongly affected by the value of the refractive index n3 of the solvent. Three behaviors can be distinguished depending of the relative value of n3 as compared to n2. When n3 increases from 1 to n2, corresponding to an index variation from air to propanediol, the amplitude of the oscillation decreases. When n3 increases from n2 to 1.63 (i.e., from benzene to CS2), the amplitude of the oscillation increases with a slight variation. Finally, the oscillation disappears when the solvent’s refractive index is in the same order as the refractive index n2 of the dielectric (n3 ≈ n2). It can also be observed that depending on the value of the

Figure 5. (A) Evolution of λmax of a glass/Au NSs interface coated with increasingly thick glass overlayers [air (black) water (blue), 1-butanol (green), 1,3-propanediol (orange), benzene (magenta), and CS2 (violet)]: (A) calculated curves using a ) 70 nm, l ) 25 nm, and h ) 15 nm; (B) comparison of the evolution of theoretically obtained refractive index sensitivity for glass/Au NSs/SiOx (n1 > n2, gray) and glass/Au NSs/glass (n1 ) n2, blue).

refractive index of the solvent, the oscillating curves start at values lower or higher than the average value of the wavelength (λmax ) 580 nm). For n3 < n2, the value of the wavelength starts (for dSiOx ) 0 nm) with its minimum value and increases gradually as the refractive index of the solvent increases. The wavelength reaches its average value for n3 ≈ n2. When n3 > n2, the behavior changes, since for dSiOx ) 0 nm the wavelength starts at its maximum value and increases gradually as the refractive index of the solvent increases. This has a direct consequence on the phase of the oscillation, which is opposite that for n3 < n2 compared to n3 > n2. Figure 3 shows the change in λmax vs the refractive index of the different solvents used to coat the substrate for some important values of the dielectric layer (SiOx) thickness. The sensitivity S (change of nanometer per refractive index unit, expressed in nm RIU-1) can be determined from the slope of the linear plot. As seen from Figure 3, three different zones can be identified, which result in slightly different sensor sensitives. For dSiOx ) 0 nm, changing water to air with a ∆n ) 0.333 results in S ) 76 nm RIU-1, while replacing water by benzene with ∆n ) 0.167 and benzene by CS2 with ∆n ) 0.13 result in S ) 91 and 140 nm RIU-1, respectively. Zone II is the

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Figure 6. Evolution of λmax of a glass/Au NSs interface coated with increasingly thick dielectric overlayers with n1 < n2 for (A) ITO, (B) NCD, and (C) a-Si0.63C0.37:H [air (black) water (blue), 1-butanol (green), 1,3-propanediol (orange), benzene (magenta), and CS2 (violet)]; calculated curves using a ) 70 nm, l ) 25 nm, and h ) 15 nm.

most representative for biological sensing and will be considered in the following. Coating the glass/Au NSs interface with 10 and 20 nm thick SiOx overlayers results in a decrease of the sensitivity to S ) 70 and 25 nm RIU-1, respectively. For a glass/Au NSs interface with a 40 nm thick SiOx coating, a negative sensitivity is found (Figure 3). This stays negative and of the same order up to a thickness of about 100 nm, and then starts to increase constantly from S ) 0.5 (120 nm) to 6 (140 nm) and 15 nm RIU-1 (160 nm), where it finds its maximum and decreases again. In summary, we notice that the sensitivity is proportional to the amplitude of the oscillation. The sensitivity reaches its maximal value when the amplitude of the oscillation is maximal and reached its minimal value when the amplitude is null (i.e., dSiOx ) 30, 100, or 220 nm). The high values of the sensitivity are also due to oscillations in opposition of phase of λmax, depending on whether n2 > n3 or n2 < n3. For n3 > n2, the reflection at the interface between the dielectric and the solvent creates a phase shift of π. This phase shift is absent for n3 < n2. Figure 4A shows in the form of a bare diagram the evolution of sensitivities as a function of the dielectric layer thickness. This demonstrates the complex interplay between the refractive index of the overlayer and the solvent refractive index. The calculated LSPR curves are corroborated mostly by the experimental LSPR curves recorded on glass/Au NSs coated with SiOx of increasing thicknesses and immersed in different solvents (Figures 2B and 4B). The main difference is found for a SiOx thickness of about 155 nm, where the experimental data show a stronger sensitivity. 3.3. Second System: The Refractive Index of the Substrate Is the Same As That of the Deposited Dielectric Layer (n1 ) n2). Figure 5A shows the calculated change in λmax as a function of the thickness of the dielectric layer with an index of refraction n2 ) 1.51, which is similar to the refractive index of the underlying glass substrate. Globally the same behavior as in the case of glass/Au NSs coated with SiOx is observed, namely unharmonic oscillation behavior, where the slope of the blue shift is steeper than in the case of a red shift. The only difference is that the average wavelength increased by 12% and

the oscillation period decreased to 2.5%. At short distances from the nanoparticles surface, the LSPR shift follows a steep slope, but as the distance from the nanoparticles increases, the curve bends over and shows a following blue shift and another red shift with a periodicity of 195 nm. Figure 5B compares the resulting sensor sensitivities depending on the thickness of the glass overcoating. At thin overlayers (d ) 10-20 nm), the sensitivity of glass/Au NSs/SiOx is slightly larger, while for thicknesses of 120-160 nm the glass/Au NSs/glass interfaces show a slightly better sensitivity. However, the values are comparable. Negative sensitivities are observed for glass overlayers between 40 and 80 nm and for glass overlayer thicknesses higher than 200 nm. 3.4. Third System: The Refractive Index of the Substrate Is Lower than That of the Deposited Dielectric Layer (n1 < n2). Figure 6 displays the plasmonic behavior of glass/Au NSs interfaces coated with dielectrics of increasingly high refractive indexes. The higher the refractive index of the overlayer, the stronger is the λmax shift initially to the blue. Indeed, in the case of a-Si0.63C0.37:H, the shift is almost 200 nm for a 10 nm thick film. This illustrates how the absorption band can be tuned with the appropriate overlayer, without changing the Au NSs on the surface. The amplitude of the oscillations increases with the refractive index of the dielectric layer. The variation of the oscillation amplitude varies from 14 nm for SiOx to 60 nm for a-Si0.63C0.37: H. But the most important modification, as compared to the previous cases (n1 e n2), comes from the behavior of the phase which behaves differently as compared for n1 > n2. All the curves oscillate now in phase that leads quickly (d g 30 nm) to very low refractive index sensitivity values. The theoretically determined sensitivities are in accordance with experimentally obtained LSPR multilayered structures (Figure 7). The match between glass/AuNSs/ITO and glass/AuNSs/a-Si0.63C0.37:H is excellent, while there is some deviation for the glass/AuNSs/ NCD system. This might be a result of the roughness of the surface. The significance of these results is that in the case of LSPR interfaces where the refractive index of the substrate is much lower than that of the deposited dielectric layer, only

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Galopin et al. multilayered LSPR interfaces with overcoatings below d ≈ 30 nm will show reasonable refractive index sensitivities. Above such dielectric thicknesses the sensitivity is regrettably small. On the other hand, thin NCD overlayers (10-20 nm) show quantitatively the best sensitivities of all the investigated interfaces (Figure 7A) and should consequently be the LSPR interface to be used for further sensing developments. Such an interface is though currently not experimentally available, as in the case of NCD no closed layers are formed below 40 nm in thickness. Diamond-like carbon (DLC) thin films show comparable chemical and physical properties to NCD. Due to the amorphous character of the coating closed films might be experimentally obtained at lower coating thicknesses and the development of glass/Au NSs/DLC LSPR with a coating of 10-20 nm should be feasible. We are in fact currently investigating the possibility of forming such stable DLC films on glass/Au NSs. 4. Conclusion The refractive index sensitivities S (in nm RIU-1) of multilayered localized plasmonic interfaces, glass/Au NSs/dielectric (n2 ) 1.45-2.63) were determined by using the Lorentz-Drude model. Three different situations were investigated. In the first case, the refractive index of the substrate is higher than that of the dielectric overcoating (n1 > n2), which is the case of glass/ Au NSs/SiOx. For this system, the sensitivity is proportional to the amplitude of the oscillation. It reaches its maximal value when the amplitude of the oscillation is maximal and its minimal value when the amplitude is null (i.e., dSiOx ≈ 30, 100, or 220 nm). The high values of the sensitivity are also due to oscillations in opposition of phase of λmax depending on whether n2 > n3 or n2 < n3. The same observations are found when n1 ) n2. It was further noticed that the amplitude of the oscillations increases with an increase in the refractive index of the dielectric layer: the oscillation amplitude varies from 14 nm for SiOx to 60 nm for a-Si0.63C0.37:H. Most importantly, when n1 < n2, all the curves oscillate in phase when the solvent was changed thus leading quickly (d > 40 nm) to a low value of the sensitivity. Thin overlayers (10-20 nm) of NCD show the best sensitivities. However, the sensibility is mainly affected by the thickness of the dielectric and not by the refractive index of the dielectric overlayer. Acknowledgment. The EU-FEDER and Interreg IV (project “Plasmobio”), the Centre National de la Recherche Scientifique (CNRS), and the Nord-Pas-de Calais region are gratefully acknowledged for financial support. The authors also want to thank L. Touahir for the deposition of the a-Si0.63C0.37:H thin films. Supporting Information Available: Transmission spectra for a glass/Au NSs interface in air and when coated with SiOx. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes

Figure 7. Change of theoretical (gray) and experimentally obtained (blue) refractive index sensitivity for (A) glass/Au NSs/ITO, (B) glass/ Au NSs/NCD, and (C) glass/Au NS/a-Si0.63C0.37:H.

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