Resonant Optical Properties of Single Out-Diffused Silver Nanoislands

Article pubs.acs.org/JPCC

Resonant Optical Properties of Single Out-Diffused Silver Nanoislands Fabian Heisler,†,‡ Ekaterina Babich,§ Sergey Scherbak,§,∥ Semen Chervinskii,§,⊥ Mehedi Hasan,† Anton Samusev,†,# and Andrey A. Lipovskii*,§,∥ †

The International Research Centre for Nanophotonics and Metamaterials, ITMO University, Kronverksky Pr. 49, St. Petersburg 197101, Russia ‡ Abbe Center of Photonics, Friedrich-Schiller-Universtät, Max-Wien-Platz 1, 07743 Jena, Germany § Institute of Physics, Nanotechnology and Telecommunications, Peter the Great St. Petersburg Polytechnic University, Polytechnicheskaja str., 29, St. Petersburg, 195251 Russia ∥ St. Petersburg Academic University RAS, Khlopina str., 8/3, St. Petersburg, 194021 Russia ⊥ Institute of Photonics, University of Eastern Finland, P.O. Box 111, Joensuu, 80101 Finland # Ioffe Physical-Technical Institute, Polytechnicheskaja 26, St. Petersburg, 194021 Russia ABSTRACT: An ensemble of hemiellipsoidally shaped silver nanoislands was grown on a soda-lime glass substrate using the combination of ion exchange, thermal poling, and out-diffusion techniques. The nanoislands were characterized using scanning electron and atomic force microscopy, and their dipole and quadrupole plasmonic properties were studied using dark-field spectroscopy and numerical modeling using discrete dipole approximation and finite element analysis. Numerically simulated scattering spectra demonstrate good correspondence with the single-particle dark-field measurements. The maps of the dipole surface plasmon resonance (SPR) wavelength and intensity are plotted in nanoisland height-diameter coordinates. It is shown that SPR shifts to the red spectral region with the increase of nanoisland width and with the decrease in the nanoisland base diameter. The quadrupole resonance observed at shorter wavelengths demonstrates higher Q-factor, and stronger enhancement and localization in the near-field. permittivity of the substrate and surrounding media.19,20 This tunability of the SPR wavelength of nanoparticles and nanoislands allows for using different excitation sources optimized for given types of objects in the spectroscopic characterizations, e.g., different Raman analytes. By varying the shape and mutual position of the plasmonic particles in a group along with the control of the polarization of the incident light, domains of extremely high local electromagnetic field, so-called hotspots, can be obtained.8,21 The resonant wavelength corresponding to the maximum field enhancement depends on both the distance and the shape of individual nanoparticles.22 To design structures formed by multiple nanoislands with the demanded plasmonic properties, an understanding of how the optical properties of a single nanoisland can be tuned with the change of its dimensions is of great importance. During recent decades, experimental studies of optical13,14,23 properties were performed for randomly arranged arrays of nanoislands that are nanoisland films and lithographically patterned nanoparticles.24,25 A method allowing growth of single out-diffused silver nanoislands as well as selfassembled nanoislands groups at predefined positions on a glass

1. INTRODUCTION Plasmonic nanostructures are currently undergoing deep investigation as structures which are able to strongly enhance the local electromagnetic field.1 This enhancement takes place in the spectral vicinity of surface plasmon resonance (SPR) and opens the road to a variety of applications in medicine, biology, photovoltaics, and characterization techniques.2,3. In particular, it results in an increased resolution and intensity of various microscopic and spectroscopic methods,4 such as surface enhanced Raman spectroscopy (SERS)5,6 or surface enhanced fluorescence,7 and is prospective in single molecule detection and live cell investigation.8,9 Silver (Ag), gold (Au), and copper (Cu) nanoparticles are of particular interest in the context of SPR in the optical spectral range due to the high plasma frequencies of these metals.10 The resonant local enhancement of the electromagnetic field can be provided by a variety of nanostructures,11 including corrugated metal surfaces, single or arranged in groups of colloidal metal nanoparticles12 and nanoislands.13 The latter can be fabricated using metal deposition or out-diffusion techniques, and their dimensions can be precisely controlled and tuned via varying the technological growth parameters.14,15 The resonant optical properties of colloidal plasmonic nanoparticles are well-studied and known to be extremely sensitive to their size and shape,16−18 as well as to the © 2015 American Chemical Society

Received: September 16, 2015 Revised: November 6, 2015 Published: November 10, 2015 26692

DOI: 10.1021/acs.jpcc.5b09051 J. Phys. Chem. C 2015, 119, 26692−26697

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The Journal of Physical Chemistry C substrate has recently been proposed.26,27 This technological approach opens a convenient pathway to experimentally investigate the optical properties of single nanoislands. This work is aimed at a comprehensive study of the optical properties of single out-diffused silver nanoislands of different sizes and aspect ratios.

atomic force microscope AIST-NT SmartSPM and the scanning electron microscope Leo 1550 Gemini. The positions of the grown nanoislands corresponded to the positions of the holes on the electrode surface. However, in spite of using the same array of the nanoholes of 300 × 300 nm2 and the same mode of the growth, the shape/size of fabricated nanoislands slightly varied. These differences in the self-arrangement of silver atoms under the holes could be induced by tiny nonuniformities in the electrode−glass distance during the thermal poling and glass defects. The images of the electrode and the array of the nanoislands grown on the glass slide surface are presented in Figure 1a,b. Additionally, this

2. EXPERIMENTAL SECTION 2.1. Fabrication of Single Out-Diffused Nanoislands. To grow separated silver nanoislands, we used silver−sodium ion exchanged soda-lime glass slides. The slides were thermally poled using a structured glassy carbon electrode as described elsewhere,27 and the poling was followed by thermal processing of these slides in hydrogen atmosphere. In the poling, we shifted the movable ions from the anodic surface of the glass28 deeper into the bulk. Here we discuss only the concentration of silver ions. The anodic electrode with a set of holes on the surface, as it was used in the poling, provided a noticeable shift of the silver ions away from the glass surface everywhere except the regions beneath these holes. The silver ions under the holes in the electrode remained close to the glass surface because of the weaker electric field in these regions. Subsequent annealing of the samples in the hydrogen atmosphere resulted in the reduction of silver ions in the glass and, due to the low solubility of the neutral silver in silicate glasses, nucleation and growth of the silver nanoparticles in the bulk of the glass and silver nanoislands on the glass surface.29 Generally, thermal processing of the poled glass regions in hydrogen provides the reduction of silver ions in the same way as the processing of an unpoled ion-exchanged glass. However, the dynamics of silver nanoparticle formation in the bulk and nanoislands on the surface of glasses differ in poled and unpoled glass regions. In particular, the growth of nanoislands and nanoparticles in the poled regions starts later because the silver ion reduction to atoms is delayed by the time necessary for hydrogen to penetrate through the silver-depleted subsurface layer, and the nucleation of silver islands is delayed by the time necessary for the silver atoms to diffuse through this layer. Moreover, in the poled regions after the reduction of silver ions deepened in the poling, bulk nucleation of the nanoparticles prevails over the nucleation of nanoislands on the glass surface.27 Thus, poling of an ion-exchanged glass with a structured electrode results in a delayed or prevented formation of silver nanoislands on the surface in the poled regions of the glass, and a proper choice of the thermal poling and the hydrogen annealing condition allows the formation of nanoislands only in the regions corresponding to the positions of the holes in the used anodic electrode. In our experiments, the silver−sodium ion exchange was performed in the melt containing (in weight %) 95% of NaNO3 and 5% of AgNO3 during 20 min at 325 °C. This processing, according to former studies,30 results in the replacement of 75 wt % of sodium ions by silver ones at the glass surface, the depth of the silver concentration profile (0.5 of the maximal concentration) being equal to 3.5 μm. The glassy carbon electrode was manufactured using electron beam lithography and reactive ion etching. An array of 300 × 300 nm2 holes on the surface of the glassy carbon plate was formed, the depth of the holes is 400 nm. The glass poling with this anodic electrode was carried out in the air at the temperature of 300 °C under the voltage of 500 V during 4 min 30 s. Then we annealed the poled slides in hydrogen atmosphere for 30 min at 250 °C. This annealing resulted in the growth of silver nanoislands on the glass surface, as it was evidenced with the

Figure 1. SEM images of the glassy carbon electrode used for thermal poling of the ion-exchanged glass slide (a); AFM image of silver nanoislands grown on the anodic surface of poled glass, inset: SEM image of a single nanoisland (b); dark-field images of the nanoislands, different colors of scattered light can be seen, inset: dark field image of a single nanoisland (c), and the schematic of the setup used in the dark field experiments (d) with the excitation beam (gray) and the collection channel (yellow).

technique allows for easy multiplication of nanoisland structures via repeated use of the same electrode,31 while varying the electrode configuration and the mode of the processing in our experiments resulted in sets of silver nanoislands with different sizes and shapes as well as in dimers, trimers, and other groups of nanoparticles.32 2.2. Single-Particle Dark-Field Spectroscopy Setup. The dark-field spectroscopy technique is advantageous for the studies of the optical properties of isolated nanoscale systems with small scattering cross sections, since the incident and reflected from the substrate radiation is not collected and does not contribute to the measured signal.33 To study optical properties of single out-diffused nanoislands as a function of the wavelength of the incident beam, we used a dark-field spectroscopy setup with two independent optical channels realized on the basis of the AIST-NT TrIOS system34 shown in Figure 1(d). The incident polarized white light is weakly focused with an objective (Mitutoyo MPlanApo 10X, NA = 0.28) on the sample surface at oblique incidence (22° to the sample surface). The light scattered by the nanoisland is collected by a second objective (Mitutoyo MPlanApo 100X, NA = 0.7) and analyzed with a spectrometer in a confocal arrangement. Contrary to commercial dark-field microscopes, this setup allows for precise control over the polarization and the direction of the incidence of the probing beam. A typical 26693

DOI: 10.1021/acs.jpcc.5b09051 J. Phys. Chem. C 2015, 119, 26692−26697

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The Journal of Physical Chemistry C

Figure 2. First row: SEM (left, outlined) and AFM (right) images of three nanoislands (#57, 22, and 42) of different shape and size. The direction and polarization of the exciting beam in the dark-field experiments are denoted by arrows. Second row: dark-field scattering spectra of the nanoislands. Third row: scattering spectra for these nanoparticles simulated with FEM (red) and DDA (green).

of scattering can be solved exactly. Thus, the only approximation of this technique was the replacement of a solid object, in our case a nanoisland with a known shape, by the set of N cells. The polarizability of each cell is defined by its interaction with other N-1 dipoles and with the incident wave. The error of this approximation depends on the discretizing step and, according to recent studies,39 it can be easily reduced to level of 4−5%. The problem was solved for a set of wavelengths to calculate the scattering spectrum in the geometry corresponding to our experiment. In the numerical simulations performed using COMSOL Multiphysics environment (Radio frequency module: electromagnetic waves, frequency domain), the Helmholtz equation was solved throughout the defined three-dimensional space and for the defined external electric field using the finite elements method (FEM). We considered an s-polarized plane wave incident at a glancing angle of 21.7° on a hemiellipsoidal silver particle located on a dielectric substrate with dielectric permittivity of 2.5. The solution gave electric field values throughout the defined space. To obtain the near-field spectra the square of the electric field modulus was integrated over a region encircling the particle and being 1.5 times larger than the nanoparticle itself. For the comparison with the experiment farfield spectra at an angular position corresponding to the scattered field collector accepting the light from the cone above the nanoparticle (Figure 1d) were calculated using the StrattonChu formula40 describing near-field to far-field transformation. The far-field pattern was defined by the square of the absolute

dark-field image of the sample acquired using the Sentech STCN63CJ CCD camera is shown in Figure 1(c). The variableradius confocal pinhole placed at the image plane in the optical path was used to collect a signal from regions as small as several microns, thus allowing us to measure scattering spectra of single nanoislands patterned in an array with a period of 5 μm. For the measurements of reference spectra, the sample was substituted by an inclined Semrock ultrabroadband dielectric mirror reflecting the incident beam directly to the collection channel. 2.3. Numerical Methods. We used two complementary approaches to numerically model the near- and far-field light scattering by single nanoislands: the discrete dipole approximation (DDA)35,36 in the Matlab environment developed by V. Loke with coauthors37 and the finite element method (FEM) performed in the Comsol Multiphysics package. Both of these methods allow us to take the substrate into account, which essentially influences the position of the SPR in colloidal particles,17 and is even more important for the case of nanoislands,15,38 as the field for the latter is localized closer to the substrate. The modeling has been performed for the nanoislands of different hemiellipsoidal shapes, as they were characterized using scanning electron microscopy (SEM) in the lateral plane and atomic force microscopy (AFM) in the direction perpendicular to the glass surface. In the DDA approximation, every hemiellipsoidal silver particle was replaced by a set of interacting point dipoles that was discretized into an array of elements, for which the problem 26694

DOI: 10.1021/acs.jpcc.5b09051 J. Phys. Chem. C 2015, 119, 26692−26697

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The Journal of Physical Chemistry C value of electric field in the far-field, while the integration of the pattern for each of the considered wavelengths provided the spectrum. The results of the modeling obtained using these two techniques were in good agreement but did not, however, coincide completely for several of the 82 modeled nanoparticles. One possible reason for this discrepancy was the difference in the consideration of the edges of the hemiellipsoids which, generally, corresponds to the pole of the Helmholtz solution. It is noted in ref 41 that even below 1% smoothing of a hemisphere’s edges results in noticeable changes of the solution. This means that results of the simulation are essentially mesh-dependent. From the other side, there is no direct account for existing peculiarities of electric field at the edges of hemiellipsoids in the DDA approach because, contrary to FEM supporting adaptive mesh, the DDA approach uses a uniform lattice of dipoles.37 We suppose that the most correct solution should be placed in-between. It is noteworthy that for the vast majority of nanoislands, the scattering spectra simulated using these two approaches coincided pretty well.

2. Comparing the calculated and experimental spectra, one should note that some discrepancies are induced by the difference in the measured and real height of the nanoislands which were covered with 5 nm gold film (magnetron sputtering) for SEM measurements. Figure 2 confirms that the spectral positions of the SPR and the overall shape of the scattering spectra are indeed very sensitive to both the base diameter and the height of the nanoisland. For the nanoisland #57 with the height of 180 nm and the base diameter measured along incident wave polarization of approximately 140 nm (ratio of the longest and the smallest diameter of the ellipsoidal cross-section base ∼1.39) the central resonance wavelength is about λres = 615 nm. For nanoisland #22, with the height decreased down to 170 nm and the diameter increased to ∼165 nm (aspect ratio ∼1.12), the resonance shifts to a wavelength of λres = 635 nm. For nanoisland #42 with further decrease of the height down to 155 nm and an approximately equal base diameter (aspect ratio ∼1), the resonance is red-shifted toward λres = 680 nm. The behavior observed is similar to a known one for resonant properties of prolate plasmonic ellipsoids embedded in a homogeneous medium. For such a nanoparticle, the spectral position of transversal plasmon resonance becomes red-shifted with a decrease in the aspect ratio of major to minor axis.17 To demonstrate the relationship of the nanoparticles’ shape and their resonant properties, we plotted calculated maps of the spectral position (Figure 3a) and intensity (Figure 3b) of

3. RESULTS AND DISCUSSION 3.1. Surface Plasmon Resonance Dependence on Nanoisland Size. According to ref 15, the absorption spectra of nanoisland films composed of hemispherical particles demonstrate strong polarization dependence, and two resonances are observed for p-polarized light. The authors of ref 15 attribute this splitting to a broken symmetry of the hemispherical particles and strong interactions with the substrate. Since SERS experiments are mainly performed with excitation at normal incidence,11 the in-plane resonances of single silver nanoislands with different sizes and shapes are of the most interest. In our dark-field spectroscopy setup, we used a relatively low numerical aperture objective (NA = 0.7, the halfcone angle is 44.4°) in the collection channel (see Figure 1(d)), therefore the scattering originating from in-plane dipole moments induced in the nanoparticle dominates in the collected signal.36 This follows from the emission pattern of a point-like dipole.42 In the dark-field spectroscopy experiments, we used s-polarized incident light to efficiently excite in-plane electric resonances in the nanoislands. The experimental and simulation results for three of the 82 characterized single nanoislands are shown in Figure 2. The chosen typical nanoislands allow us to clearly follow the influence of their shape on the SPR spectral position. The lateral sizes of the nanoislands were obtained from the SEM images, while the heights were obtained from the AFM measurements (Figure 2, first row). In the simulations, each nanoisland was approximated by a hemiellipsoid with an elliptical base. The mutual orientation of the nanoisland and the incident wave direction has also been taken into account. The measured scattering spectra (Figure 2, second row) are in good agreement with the results of the numerical simulations (Figure 2, third row). Still, the scattering spectra simulated using the FEM and DDA approaches demonstrate some discrepancies. This is likely to be concerned with the nanoparticles’ volume discretization, which does not allow for a proper accounting of the sharp edges of the nanoisland even for a large number of dipoles. Nevertheless, the DDA method, not being as timeconsuming as the FEM, allows reproduction of the SPR spectral position with a reasonable precision, as can be seen from Figure

Figure 3. Maps of SPR wavelength (a) and relative intensity (b) plotted in nanoisland’s height−diameter coordinates.

plasmon resonance peaks in the nanoislands’ height−diameter coordinates. The calculations have been performed using DDA, which has been used for wide-scale numerical study of the SPR in single nanoislands depending on their height and base size because of the smaller time-consumption of this computation approach. The verified similarity of DDA and Comsol modeling results allows us to rely on these calculations. In the mapping, we did not consider the configurations corresponding to high pillar-like and flat disk-like nanoislands. Both of these configurations are out of interest in our consideration, as resonant properties of disks and pillars have recently been discussed,3 and their modeling requires an essential increase of the computation power. This is because of the essential difference in height and diameter, which results in the necessity of either decreasing the grid in Comsol calculations or increasing the number of dipoles in DDA modeling. The maps in Figure 3 demonstrate strong sensitivity of the SPR spectral position and intensity to both height and base 26695

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than for the ED (the main) resonance. In our experiments, the EQ resonance was well observed in nanoparticles with relatively large base diameter (about 150 nm and more), the shape of which did not strongly differ from the circular or elliptic. It also appeared in the large particles with some shape deviations, like truncated ones, but in this case the EQ peak seemed to be less intense. The comparison of near-field (NF) and far-field (FF) spectra calculated for the same nanoparticle (see Figure 4) shows some decrease of the ratio between amplitudes of EQ and ED resonances. This provides additional support to the fact that the electric field is confined more strongly at the EQ resonance and that the contribution of evanescent fields to the resonant spectra in the NF region can be essential.44 Similarly to the results presented in ref 44, the whole spectra modeled in the NF region are red-shifted relative to FF spectra because of the contribution of evanescent fields. The indication of less-pronounced EQ can also be supposed in both the experimental and FEM modeled spectra presented in Figure 2. Generally, the EQ features are more manifest for the nanoparticles that are large enough. In our FEM modeling of hemispherical nanoparticles on a glass substrate, we observed EQ in nanoparticles exceeding 40−50 nm in diameter. We believe that the EQ is sufficiently strong in nanoparticles that are more “perfect” (in the present case close to hemispherical); however, the strength of EQ should definitely depend on tiny features of the nanoparticles, which should be the subject of further studies.

diameter. Generally, the map in Figure 3a exhibits the behavior similar to the dependence of the SPR wavelength on the aspect ratio for prolate and oblate ellipsoids embedded in a homogeneous medium.17,43 Similarly, the intensity of the SPR grows with the increase of both height and diameter, the diameter effect being stronger. Therefore, by varying the base diameter and height of a hemiellipsoidal nanoisland, one can independently govern the SPR intensity and position. 3.2. Quadrupole Optical Response and near-Field Resonant Properties of Isolated Nanoislands. In a set of dark-field measurements of several nanoislands, in particular #61 (see the first row in Figure 4), we registered a two-peak

4. CONCLUSIONS We have both experimentally and numerically studied the resonant properties of single silver out-diffused nanoislands patterned on a glass substrate. It has been demonstrated that the surface plasmon resonance of single nanoisland deposited on a glass substrate can be tuned in a broad spectral range. The resonant wavelength is red-shifted with the increase of the base diameter and blue-shifted with the increase of the height of the nanoisland. Meanwhile, the resonance intensity increases with the increase of both geometrical parameters. We have also studied the distributions of the electric field in the vicinity of the nanoisland at dipole and quadrupole resonances. The quadrupole resonance observed at shorter wavelengths demonstrates higher Q-factor and stronger enhancement and localization of the near field. The presented results can be applied for optimization of nanoisland-based SERS substrates for different excitation sources and Raman analytes.

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Figure 4. First row: SEM (left) and AFM (right) images of the single nanoisland #61; second row: dark-field scattering spectrum of the nanoisland; third row: FEM-calculated quadrupole (left) and dipole (right) electric field intensity distributions for the nanoisland; and fourth row: FEM-calculated near-field (NF) and far-field (FF) scattering spectra.

AUTHOR INFORMATION

Corresponding Author

*Tel: 8-812-4488591; fax: 8-812-5435802; e-mail: lipovsky@ spbau.ru (A.A.L.). Notes

The authors declare no competing financial interest.

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behavior of the scattering spectra as presented in the second row in Figure 4, which was supposed to correspond to an electric quadrupole (EQ) resonance in the nanoparticle, while the main peak corresponds to the electric dipole (ED) resonance. Calculations of the electric field distribution (see the third row in Figure 4) confirms the quadrupole character of this peak, and our modeling of the spectra has also shown the existence of the second less intensive peak which was blueshifted with respect to the SPR peak (Figure 4). It is noteworthy that the Q-factor is essentially higher for the EQ

ACKNOWLEDGMENTS The experimental part of this work was carried out at ITMO University was supported by the Russian Science Foundation (project #15-12-20028); the technological part and modeling carried out at Peter the Great St. Petersburg Polytechnic University were supported by the Ministry for Education and Science of Russia, project #16.1233.2014/K. E.B. is additionally thankful for support from the Foundation for Assistance to 26696

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Small Innovative Enterprises, and A.A.L. is thankful to the Academy of Finland (grant #288151).

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DOI: 10.1021/acs.jpcc.5b09051 J. Phys. Chem. C 2015, 119, 26692−26697