Silica Nanowires Decorated with Metal Nanoparticles for Refractive

Article pubs.acs.org/JPCC

Silica Nanowires Decorated with Metal Nanoparticles for Refractive Index Sensors: Three-Dimensional Metal Arrays and Light Trapping at Plasmonic Resonances Annalisa Convertino,*,† Massimo Cuscunà,†,‡ Faustino Martelli,† Maria Grazia Manera,§ and Roberto Rella§ †

IMM-CNR Roma, Institute for Microelectronics and Microsystems, Via del Fosso del Cavaliere 100, 00133 Roma, Italy IMM-CNR Lecce, Institute for Microelectronics and Microsystems, Strada Provinciale per Monteroni, 73100 Lecce, Italy

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ABSTRACT: We report the fabrication of silica nanowires (NWs) decorated with Au or Ag nanoparticles (NPs) by dewetting thin metal films evaporated on the NWs. The Au or Ag NPs, displaced along the NWs, form a three-dimensional (3D) ensemble of metallic NPs in a macroporous structure. Their optical behavior results from the combination of the high white-light scattering of silica NWs with the selective absorption of the localized surface plasmon resonances (LSPRs) of the NPs, causing light trapping just at the LSPR wavelengths. Such a 3D plasmonic structure shows a strong dependence of the LSPR wavelength on the refractive index of the environment in which the 3D NP ensemble is immersed, a feature that makes them morphologically and optically peculiar materials appealing for sensing applications.

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INTRODUCTION

the tiny interparticle spacing limits the propagation of the biomolecules.12,13 In this work we exploit the unique morphological and optical characteristics of a nanowire (NW) forest to develop novel plasmonic materials obtained by decorating silica NWs with metal (Au or Ag) NPs. The decoration of silica nanostructures, such as NWs or nanosprings, has been already reported in the literature16,17 and used, e.g., for surface enhanced Raman spectroscopy. In this work we show that one-dimensional silica structures offer a large surface area to attach Au or Ag NPs, providing at the same time a macroporous support framework for 3D NP assembly, easily accessible by analyte molecules. In addition, the strong light scattering typical in NW materials18−23 combined with the selective LSPR absorption resonances of Au or Ag NPs is a strategic tool to achieve efficient light absorption just at the LSPR frequencies. Indeed the light passing through a NW forest undergoes multiple scattering events from each NW. The scattering events fold the light path many times in a random walk inside the NW forest causing an enhancement of the light absorption with consequent drastic reduction of the reflectivity at the frequencies absorbed by the NW material, i.e., light trapping. As such, NWs capture and absorb significantly more photons than an equivalent volume of bulk material. Similar strategies have been recently reported,24−26 where enhanced LSPR and surface enhanced Raman spectroscopy have been found in Ag-

Noble metal nanoparticles (NPs) are of great interest in biological sensing and for fabrication of nanosensors due to their impact in several biomedical areas including proteomics, drug discovery, cancer diagnostics, and therapy.1−7 They are characterized by sharp spectral absorptions, commonly in the UV−vis region, when the incident photon frequency is resonant with the collective oscillations of the conduction electrons, the so-called localized surface plasmon resonances (LSPRs). By functionalizing their surface with bioreceptor molecules, a molecular recognition with one biomolecular counterpart can be revealed through shifts in the LSPR spectrum. Currently, LSPR sensors consist of a monolayer of metal NPs chemically immobilized onto a functionalized substrate, so-called two-dimensional (2D) chips.8−10 In these 2D chips the sensitivity is limited by the low LSPR signals due to the small active area of the metal NPs (typical diameter of 30−120 nm) and their low density in the defined surface area. Hence, the fabrication of nanostructured LSPR sensors that are sensitive to a conventional UV−vis instrument is still a challenging task. Many efforts have been devoted to improve the LSPR sensor sensitivity by exploiting the high field confinement effect in an ordered 2D array of closely packed metal NPs,1,11 or by increasing the NP density in three-dimensional (3D) assemblies of NPs.12−15 Anyway these strategies, often, require a fine control of the fabricated structure and the use of very sophisticated and expensive fabrication techniques such as electron beam lithography. Moreover, in some cases, like as for the 3D chips, © 2013 American Chemical Society

Received: November 29, 2013 Revised: December 11, 2013 Published: December 11, 2013 685

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was used to create the plasma. In these growth conditions 15−20 μm long Si NWs with average diameter of 150−200 nm at the bottom and tapered shape are obtained.19 After the growth, the Si NWs were thermally oxidized to form SiOx (silica) NWs27 via a thermal treatment in a convection oven (controlled O 2 atmosphere) at 980 °C for 8 h. The decoration of the silica NWs with Au or Ag NPs was obtained through thermal evaporation of the metallic thin films on the silica NWs, followed by thermal annealing at temperature, Ta, ranging from 400 to 900 °C for 6′. Metal films with nominal coverage thickness, tm, of 8, 12, and 20 nm were evaporated. Characterizations. The morphology of the silica NWs/NPs systems was verified by scanning electron microscopy (SEM). The optical properties were studied by measuring the angleintegrated total reflectivity in the spectral range between 200 and 1100 nm with a Lambda 35 UV−vis spectrophotometer equipped with an integrating sphere. RI studies were performed on the samples immersed in methanol (n = 1.329), acetone (n = 1.359), tetrahydrofuran (n = 1.394), and toluene (n = 1.497) by measuring the integrated reflectivity spectra with a Lambda 35 UV−vis spectrophotometer.

Figure 1. SEM images of silica NWs. In the inset is an image with greater detail.

decorated Si nanostructures. In our case in which the silica NWs are transparent, the absorption is only due to the LSPR of the NPs immobilized on the wires and the possibility of trapping the light just at LSPR wavelengths allows the detection of weak LSPR changes due to surrounding medium modifications. We hence fabricated silica NWs via oxidation of self-assembled Si NWs deposited onto Si or fused silica substrates. The silica NWs were successively decorated with metal (Au or Ag) NPs by dewetting thin metallic films evaporated on the NWs. Light trapping at the Au and Ag LSPR wavelengths was demonstrated by the drastic reduction of the reflectivity at those wavelengths. The potential of these materials as LSPR sensors was studied by investigating their sensitivity to changes of the refractive index (RI) of the surrounding environment with a conventional UV−vis instrument.

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RESULTS AND DISCUSSION Figure 1 shows SEM images of pristine silica NWs fabricated on a Si substrate. The NWs are long, tens of micrometers, with a quite tapered shape and a bottom diameter in the range of 150−200 nm, as discussed in ref 19. They are interwoven into a dense forest with a macroporous structure such as that visible in the inset. Parts a−e and f−l of Figure 2 show the dewetting pattern evolution for 12 nm of Au or Ag films, respectively, on silica NWs as a function of the annealing temperature. Initially, the as deposited metal films form large islands homogeneously covering the silica NWs (Figure 2a,f). The thermal annealing at 400 °C (Figure 2b,g), 600 °C (Figure 2c,h), 800 °C (Figure 2d,i), and 900 °C (Figure 2e,l) induces the dewetting of the metal islands. A transition from the island structures toward quasispherical NPs at 600 °C is observed: the islands indeed break-up and form NPs with a very large distribution of the size. At higher temperature, 800 and 900 °C, the distribution size seems to

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EXPERIMENTAL SECTION Materials. Silica NWs were produced by thermal annealing of Si NWs grown by plasma enhanced chemical vapor deposition (PECVD). First, gold-induced Si NWs were grown on both Si (100) and fused silica substrates. To induce the NW growth, a 1 nm thick Au film was evaporated onto the substrates prior to growth. The growth was performed using pure SiH4 as precursor at a total pressure of 1 Torr and substrate temperature of 360 °C. A 13.56 MHz radiofrequency with power density of 50 mW cm−2

Figure 2. Dewetting pattern evolution for 12 nm thick (a−e) Au and (f−l) Ag films covering silica NWs as a function of the annealing temperature: (a, f) as deposited Au and Ag films, and after thermal annealing at (b, g) 400, (c, h) 600, (d, i) 800, and (e, l) 900 °C. Although the images are 2D projections, the 3D distribution of the metal NPs is clearly envisaged. The scale bar is 1 μm. 686

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Figure 4, where the SEM images of the samples with 8 nm (Figure 4a) and 20 nm (Figure 4b) of Ag annealed at the

shrink and the metal NPs homogenously cover the silica NWs, assuming the aspect of octopus tentacles. The NP size, shape, and distribution are clearly different between Au and Ag, and these differences can certainly be ascribed to the different wettability and agglomeration properties of the two metals. In order to discuss in more detail some of these differences, in Figure 3, we compare the images obtained at the

Figure 4. SEM image of Ag-NP/silica-NWs obtained for coverage thicknesses of 8 (a) and 20 nm (b), annealed at Ta = 800 °C. The scale bar is 1 μm.

temperature of 800 °C are compared. The thicker coverage induces the formation of larger NPs, as expected. From our morphological study, some original findings, mainly important for biosensing applications, can be finally evidenced. First, we have fabricated highly dense 3D ensembles of metal NPs. Indeed the NPs lie on the NWs which form a 3D platform that, as a consequence, imparts a 3D spatial distribution to the NPs themselves. This 3D metal NP ensemble is easily accessible to gases, liquids, and biomolecules due to the microporous structure of the NW forest, a feature that makes it easy to put a large amount of NPs in contact with the environment (cells, saliva, and tissues, etc.) to be tested. Second, the interparticle separation offers sufficient space for the NP functionalization with big molecules. Figure 5 presents the integrated reflectivity spectra in the 200− 1100 nm range of pristine silica NWs (Figure 5a) and of silica NWs covered with 12 nm of Au (Figure 5b) or Ag (Figure 5c) annealed at 800 °C, together with their photographic images (Figure 5d−f). For these optical investigations, the analyzed samples were fabricated on fused silica substrates in order to minimize any signal from the substrate.19 Figure 5a compares the reflectivity spectra of the pristine NWs (purple dotted line) and the bare fused silica substrate (black line). Silica NWs achieve a much higher total reflection than the bare substrate, whose reflectivity is below 10% in the investigated spectral range: larger by more than 40% over the whole range and up to 80% at 200 nm. The NW size and tapered shape ensure indeed a large light scattering at all wavelengths in the visible range, and with the silica being transparent in the UV−vis range, the pristine NWs appear as a natural bright white mat, as shown in Figure 5d. The reflectivity spectra of pristine (purple dotted line) and of Audecorated NWs (black line) are plotted in Figure 5b. The two spectra show very similar reflectivity profiles in the IR spectral

Figure 3. (a) SEM images of silica NWs with Au NPs and (b, c) related high-magnification images; (d) SEM images of silica NWs with Ag NPs; (e and f) more detailed images. Both samples were obtained with a metal coverage of 12 nm and an annealing temperature of 900 °C.

annealing temperature of 900 °C for Au (Figure 3a−c) and Ag (Figure 3d−f). The Au NPs are randomly distributed along the silica NWs (Figure 3b) and show a weakly elongated shape (Figure 3c). Their size is quite small with a diameter ranging from 30 to 50 nm and an interparticle space of 10−30 nm. Conversely, Ag NPs show a well-defined spherical shape and are characterized by a larger size, as shown in the images of Figure 3d−f. Their diameter lies between 80 and 130 nm (Figure 3e,f) depending on both the NW size and the location on the NWs (bottom or tip): the larger the NW size, or location size, the larger the NP diameter. Ag NPs are well-aligned along the NWs forming NP chains with a quite regular interparticle distance that can achieve 50−100 nm. The very different morphological characteristics between Au and Ag NPs suggest that at high annealing temperature the Au diffuses in both axial and radial NW directions leading to the formation of densely packed small NPs, randomly distributed on the entire NWs. Conversely the Ag migration seems to be forced by the one-dimensional support only along the NW length providing the formation of large NP chains along the NW length. We have discussed the case related to a metal coverage of 12 nm, but a very similar behavior has been observed for thinner and thicker metal coverage. The metal coverage thickness is found to change the NP size as evidenced in 687

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Figure 5. Total integrated reflectivity spectra: (a) pristine silica NWs (purple dotted line) compared with bare fused silica substrate (black line); (b) AuNP/silica-NWs and (c) Ag-NP/silica-NWs(black line) and pristine silica NWs (purple dotted line). The insets give the inverse reflectivity spectra. In the lower part of the figure we show the optical image of (d) pristine silica NWs and (e) Au- and (f) Ag-decorated silica NWs.

The potential of the Au-NP/silica-NWs and Ag-NP/silicaNWs as RI sensors was investigated by measuring the integrated reflectivity spectra of these materials immersed in solvents with different RIs: air (n = 1), methanol (n = 1.329), acetone (n = 1.359), tetrahydrofuran (n = 1.394), and toluene (n = 1.496). Figure 6 shows some typical experimental results obtained by measuring a series of samples, grown on Si substrate with different coverage, tm = 8, 12, and 20 nm, and Ta varied in the range of 600−900 °C. In particular, the inverse UV−vis reflectivity (1/R) spectra of Ag-NP/silica-NWs (Figure 6a), obtained for tm = 8 nm and Ta = 700 °C, and Au-NP/silica-NWs (Figure 6b), obtained for tm = 12 nm and Ta = 800 °C, are plotted by changing the surrounding medium RI. These samples have been selected as showing the best sensitivity, and SEM analysis results showed they are characterized by NPs with similar diameters between 30 and 70 nm. The decorated NWs exhibit a red shift of the LSPR position and an increase in peak intensity as a function of the RIs of the solvents. The insets show a linear shift of the LSPR positions as functions of the solvent RIs. The sensitivities, evaluated as the shift in the LSPR peak wavelength per unit change of RI, ΔλLSPR/RIU, are of 227 and 127 nm/RIU for Ag NPs and Au NPs, respectively. In order to better define the sensing performance of our materials, we also consider the figure of merit (FOM), defined as sensitivity divided by the LSPR line width.31 This parameter is widely used to characterize nanostructures sensing potential independently of their shape or size. For our materials we find FOM values of 2.6 and 1.5 for Ag NPs and Au NPs, respectively. The sensitivity and FOM values that we found are among the highest reported in the literature, as it results from the comparison of our results with those listed in Table 1. In this table we report a summary of ΔλLSPR/RIU and FOM for several Au and Ag nanostructures, characterized by LSPR wavelengths close to those of our structures. Although our 3D NP ensembles are very disordered and characterized by a certain spread in size

range, but the presence of the Au NPs greatly modifies the reflectivity spectrum of the silica NWs at around 525 nm where a drastic reflection reduction, down to 10%, occurs. In this sample, only the red part of the spectrum is reflected, a feature that imparts the red color to the sample (see Figure 5e). A similar reflectivity spectrum can be observed in Figure 5c for Agdecorated NWs; in this case the reduction of the reflectivity occurs at around 370 nm, the reflected part of the visible spectrum is wider, and the color of the sample becomes yellow (see Figure 5f). In the two samples, the wavelength at which the reflectivity strongly decreases is that of the Au and Ag LSPR, respectively. It is worth noting that the inverse of the reflectivity profile (insets of Figure 5b,c) of the metal-decorated silica NWs has the typical feature of the Au or Ag LSPR extinction coefficient.28 This behavior can be well-described by the recent model of diffuse optical reflectors,18 which successfully described the optical behavior of Si and GaAs NWs.18−20 From that model we derive that in the spectral range where the NP/NW system is absorbing, the reflectivity, R, is inversely proportional to the wire absorption coefficient, α, according to the following relationship: R≈

1 Nαd

(1)

In our case, with the silica NWs being transparent, α can be only related to the LSPR absorption of the NPs immobilized on the wires, N is roughly the number of NPs that the propagating light encounters inside the silica structure, and d is the NP diameter. Similar spectra have been observed for all of the samples, obtained with different coverages and annealing temperatures, with the well-known LSPR wavelength dependence on the particle size.29,30 Finally, it is worth evidencing that the light trapping at plasmonic resonances in silica NW forest permits to achieve more efficient light absorption at those wavelengths. This peculiarity could be indeed exploited to increase the LSPR sensitivity. 688

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CONCLUSION In conclusion, we reported a new class of plasmonic materials consisting of silica NWs decorated with metal NPs. These materials are characterized by morphological and optical features very attractive for biosensing applications. First, they form a dense 3D ensemble of metal NPs, easily accessible to liquids, vapors, or biomolecules due to the macroporous structure of the NW forest and the large interparticle distance. Second, they offer absorption enhancement at the LSPR wavelengths because of light trapping effects in the NW forest. We have shown their potential as plasmonic sensors by monitoring their behavior as refractive-index sensors and achieving very high sensitivities. Although we have focused our investigation on silica NWs decorated with Au or Ag NPs, the proposed strategy to decorate silica NWs can be extended to other types of sensing NPs or molecules, including the fluorescent ones, the only constraint being the ability to immobilize those NPs or molecules on the silica NWs. Finally, the proposed decorated silica NWs can be grown on different substrates including fused silica, opening up the opportunity of an effective coupling of these plasmonic materials with optical fibers.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: ++ 39 06 49934581. Present Address ‡

NANO-CNR Lecce, Institute Nanoscience, Via Arnesano, 73100 Lecce, Italy. Notes

The authors declare no competing financial interest.

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Figure 6. UV−vis spectra of the selected (A) Ag-NP/silica NW (tm = 8 nm and Ta = 700 °C) and (B) Au-NP/silica-NW (tm = 12 nm and Ta = 800 °C) structures immersed in solvents with different RIs as follows: (a) methanol (n = 1.329), (b) acetone (n = 1.359), (c) tetrahydrofuran (n = 1.394), and (d) toluene (n = 1.496). In the insets are the dependencies of the LSPR wavelengths on the RIs.

ACKNOWLEDGMENTS The authors thank M. Maiani and F. Casino for technical support.

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REFERENCES

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Table 1. Summary of Refractive Index Sensitivity, ΔλLSPR/RIU, and Figure of Merit (FOM) Reported for Several Gold and Silver Nanostructures ref 15

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particle

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521 530 570 600 527 538 430 520 564 525 375

166 61.3 199 128 44 83 118 160 191 127 227

689

FOM

3.8 0.6 1.5 1.6 2.2 1.8 1.5 2.6

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