Chemical Physics Letters

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1 Chemical Physics Letters 474 (2009) Contents lists available at ScienceDirect Chemical Physics Letters journal homepage: Nanometer-scale size dependent imaging of cetyl trimethyl ammonium bromide (CTAB) capped and uncapped gold nanoparticles by apertureless near-field optical microscopy Yohannes Abate, Adam Schwartzberg, Daniel Strasser, Stephen R. Leone * Department of Chemistry, Department of Physics, and Lawrence Berkeley National Laboratory, University of California, Berkeley, CA 94720, USA article info abstract Article history: Received 31 December 2008 In final form 14 April 2009 Available online 17 April 2009 Apertureless near-field scanning optical microscopy (ANSOM) is used to image optical near-field light scattering from uncapped gold nanoparticles and gold nanoparticles capped with the cationic surfactant cetyl trimethyl ammonium bromide (CTAB). The measurements investigate the gold-particle size-dependent signals and the modification of those signals by the spacer layer of commonly used CTAB in the visible at k = 633 nm. Imaging of capped nanoparticles by apertureless near-field microscopy opens the possibility to predict quantitative layer thicknesses of capping agents on the surface of nanoparticles, as well as the effect of capping layers on the optical scattering properties of nanoparticles. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction * Corresponding author. Fax: address: srl@berkeley.edu (S.R. Leone). The scattering type apertureless near-field scanning optical microscope (ANSOM) is a promising optical imaging technique for microscopy and spectroscopy of nanostructures and single molecules, one that achieves lateral resolution of a few nanometers [1 3,8,9]. Being responsive to the dielectric properties of the medium, ANSOM, in addition to providing surface sensitive detection, has the ability to detect subsurface objects buried within a host medium on a sub-wavelength distance to the near-field probe [1 3]. Thus it is possible, in principle, to detect the properties of nanoparticles embedded in various media, such as polymers or capping layers. Capping layers are used ubiquitously in nanomaterials; they are essential for making nanoparticles as well as for utilizing nanoparticles for several applications. There is a central need, therefore, to characterize the structure of the capping agents on the surface of nanoparticles, as well as the effect of capping layers on the properties of nanoparticles. The capping layers may be organic-based or semiconductor-based, in the case of core shell nanostructures. Capping of semiconductor quantum dots by inorganic semiconducting materials can result in quantum confinement properties, which improves the confinement efficiency with capping thickness [4]. Applications of gold nanoparticles as biological sensors and celltargeting agents require capping with organic surfactants like cetyl trimethyl ammonium bromide (CTAB) [5]. Since CTAB and similar surfactants do not greatly perturb electron transfer reactions on the surfaces of metal nanoparticles they can be used for electroanalysis of biomolecules [5,6]. Given the versatility of capping materials it is important to characterize the uniformity of the capping layer on the surface of the nanoparticle, the capping layer thickness, how the layer binds to the nanoparticle and ultimately the effect of the capping layer on electrical and dielectric properties of the nanoparticle. In addition to offering very high spatial resolution, the ability of ANSOM to detect nanoscale features embedded in a covering medium makes it a valuable tool to characterize capping layers in a nondestructive way [1,2]. Even though studies have been reported that investigate the capability of ANSOM to image through thin extended surface layers [1,2], the application of the ANSOM technique to the study of size-dependent nanoparticles embedded in a capping medium is not yet explored. In this work, we report the imaging of CTAB capped and uncapped spherical gold nanoparticles by the ANSOM technique. The measurements investigate the gold-particle size-dependent signals and the modification of those signals by the CTAB layer in the visible at k = 633 nm. The effect of the capping agent on the near-field signal is explored by approximating the transparent capping surfactant to first order as a spacer layer. The capping agent causes displacement of the tip away from the core metal nanoparticle and also causes displacement of the metal particle from the substrate by an amount proportional to the thickness of the capping layer. In addition, comparison of capped and uncapped nanoparticles of the same height means that the gold particle is smaller in the case of the capped particle, which results in less coupling for the ANSOM signal derived. As explored below, the signal contribution of a metal particle covered in a capping layer is smaller than the signal contribution of a gold particle of the same size without the capping layer because of the increased tip sample and sample substrate distances. The relative /$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi: /j.cplett

2 Y. Abate et al. / Chemical Physics Letters 474 (2009) importance of these spacer layer effects on the near-field signal is quantified. A value of 2 5 nm is extracted for an average spacer layer dimension under realistic capping layer conditions by application of the dipole dipole coupling model to the data. Comparison of the theoretical and experimental results suggests multilayer coating of the surfactant. 2. Experimental setup The ANSOM apparatus in our laboratory is home built around a commercial tapping mode AFM scanner head (Bioscope, Veeco Instruments). Commercially available Cr Au coated Si tips with a nominal radius of curvature of less than 50 nm are used. Linearly polarized radiation (p-polarized) from a stabilized HeNe laser (633 nm, Micro-g, Inc.) is directed to the input aperture of a microscope objective lens (0.42 NA, 20X, Mitutoyo) and focused onto the tip sample junction with an angle of 30 with respect to the sample surface. The same objective lens collects the backscattered light from the tip sample junction. The tip oscillates vertically near the resonance frequency of the cantilever (X 80 khz) with full amplitude of (20 100) nm above the sample surface, which periodically scatters the near-field signal towards an optical detector. The signal directed to the photodiode contains a large far-field scattering background due to scattered light from the sample surface and tip shaft. This background is removed by demodulating the signal at harmonic frequencies (2X, 3X) of the tip oscillation frequency (X) via a lock-in amplifier (7820 model, Signal Recovery) [1,3,8]. We find that the third harmonic (3X) demodulation removes most of the background while maintaining a sufficient signal-to-noise ratio and the third harmonic is used here for all measurements. The ANSOM and AFM topography images are acquired simultaneously by raster-scanning the sample stage (Polytech PI-500) and recording both the near-field scattering signal and the height information as a function of the sample position. We demonstrate the effect of the capping layer thickness and particle size on the ANSOM contrast of nanoparticles by imaging CTAB capped and uncapped colloidal gold particles with heights (h) between 35 nm and 110 nm. Gold nanoparticles are synthesized by a modified Frens method [7]. Briefly, 100 ml of an aqueous HAuCl 4 3H 2 O solution ( M) is brought to a boil while stirring, and 1 ml of 1% aqueous sodium citrate is added. The boiling and stirring are continued for 30 min to ensure reaction completion. By using a reduced volume of sodium citrate, relatively large, and highly polydisperse gold nanoparticles are formed. Ten milliliters of these prepared gold nanoparticles are then mixed with 5 ml of a warmed (35 C) 0.8 M aqueous solution of cetyl trimethyl ammonium bromide (CTAB), and left stirring on a warm hotplate for 1 h. The solution is centrifuged, and the supernatant is removed. The particles are then re-dispersed in fresh ultrapure water. This is repeated twice to ensure complete removal of excess CTAB from solution. Particles are coated onto 1 cm 2 silicon wafers which had been ultrasonically cleaned, first in a 1% solution of Hellmanex detergent, then in ultrapure water (Millipore, Milli-Q 18 M cm). The clean wafers are soaked in a 0.1% solution of (3-aminopropyl) trimethoxysilane (APTMS) for 5 min. Upon removal, the substrates are rinsed well with ultra pure water then blown dry with nitrogen. The capped or uncapped particle solutions are placed onto the surface and allowed to react for 1 h, after which they are rinsed with water and dried. Experiments are performed with samples where either the uncapped or the capped gold nanoparticles are adsorbed on Si substrate. Several AFM analyses of the topography are done on a number of prepared samples (over 8 pairs) to select the best samples that contain a random total height distribution, h t = h +2d (where h is the height of the gold and d is the thickness of the capping layer), between 35 and 110 nm for both samples containing capped and uncapped gold particles. Selected nearly identical samples containing capped and uncapped gold nanoparticles are placed side by side on a micrometer translation stage on top of an XYZ piezo stage. Back and forth translation from one sample to another is effectively achieved using a micrometer stage while the tip is fixed under the same alignment conditions. Simultaneous topography and optical imaging is performed on two pairs of selected uncapped and capped samples with different tips used on different days. A total of over 60 nanoparticles from two pairs of samples containing capped and uncapped nanoparticles (on average 15 particles per sample) were taken for analysis. Over 10 nanoparticles were discarded due to irregular shapes and uncertain structures, which may arise due to impurities or conglomeration effects. The same tip vibration amplitude is maintained for all tips used and the same subsequent data analysis is applied on the results of both samples as described more below. 3. Results and discussion Fig. 1a and b shows AFM topographic images (T) of uncapped (U) and capped (C) gold nanoparticles, respectively, up to 110 nm in size on a flat Si substrate. The AFM images of the nanoparticles could provide information on the morphology of the nanoparticle surface and are an important tool in the analysis of our results. Based on the AFM images, we do not consider in our analysis any nanoparticles that show irregular shapes and uncertain structures as described above. We rely on the AFM height information rather than the lateral size measurement to categorize particles according to size. In order to minimize conglomeration of particles, the samples are highly dispersed, as verified by AFM measurements. The use of the CTAB capping agent also helps to minimize the van der Waals attraction between the nanoclusters, which could lead to agglomeration. ANSOM optical images (A) (third harmonic intensity) of uncapped and capped gold nanoparticles on the Si substrate at 633 nm are shown in Fig. 1c and d, respectively. Imaging in these figures is performed with the same tip and tip vibration amplitude in a soft tapping mode. Results are consistent whether the sample containing capped or uncapped particles is imaged first. Particles >35 nm in height are used for the experiment, and these show brighter scattering (bright contrast) relative to the Si surface (Fig. 1c and d) [8,9]. In the ANSOM, very small particles are expected to exhibit negative near-field contrast compared to the substrate regardless of their dielectric constants; this effect of size on the ANSOM image contrast for small particles (<80% of the radius of the probing tip) has been explored before [8]. Comparison of ANSOM data sets here is based on experiments performed under controlled experimental conditions, where imaging is done under similar alignment conditions and tip vibration amplitude, with the capped and uncapped particles. The results on Fig. 1c, d, c 0 and d 0 clearly show differences in signal strengths: the optical images obtained for uncapped gold particles reveal stronger ANSOM signals compared to the capped particles of the same total height (gold plus capping layer). We show line plots of the ANSOM images for quantitative comparison of the near-field amplitude signal and AFM image height of two particles (Fig. 1a 0 d 0 ). The AFM results for uncapped and capped gold nanoparticles of the same height are shown in Fig. 1a 0 and b 0, respectively. When we compare topography (T) of an uncapped (U T ) nanoparticle and topography of a capped (C T ) gold nanoparticle of the same total height (Fig. 1a, b, a 0 and b 0, respectively), the optical amplitude signal is generally weaker on the capped gold nanoparticles (on average weaker by 30% compared to the uncapped signal) as shown in the example given on Fig. 1c, d, c 0 and d 0. The results

3 148 Y. Abate et al. / Chemical Physics Letters 474 (2009) Fig. 1. AFM topographic image of (a) uncapped and (b) capped gold nanoparticles adsorbed on a flat Si substrate; corresponding simultaneously recorded ANSOM images of (c) uncapped and (d) capped gold nanoparticles. (a 0 ) and (b 0 ) Line profile extracted along the dashed line in (a) and (b), respectively, showing the topography of two equally high particles U T (uncapped particle topography) and C T (capped particle topography). (c 0 ) and (d 0 ) Line profile showing different optical signals for the two particles U A (uncappped particle ANSOM signal) and C A (capped ANSOM signal) in (c) and (d), respectively. The ANSOM line profile shown in (c 0 ) and (d 0 ) are normalized to the signal of a Si substrate. are normalized to the signal amplitude of a blank Si substrate (AN- SOM intensity of particle divided by ANSOM intensity of silicon substrate). The particles chosen in this example have the same height (h = 72 nm); comparison of the normalized optical signal amplitude shows a weaker optical signal (Fig. 1c 0 and d 0 ) on the capped particle. Naturally a capped particle of the same height as an uncapped one has a smaller gold nanoparticle at its center. The overall effect is due both to the reduced size of the gold nanoparticle and the displacement of the ANSOM tip and the displacement of the gold particle from the supporting substrate due to the spacer layer. These effects are investigated in a model below. To understand the effects of gold particle size and capping agent spacer layers on the near-field optical signals, we theoretically modeled the configuration shown in Fig. 2a and b. The model is based on an analytical solution of the electrostatic boundary-value problem that takes into account the effect of the substrate for a system of spherical particles [10]. For a system of two spheres above a substrate, an analytical expression for the sphere s polarizability is possible in the dipole approximation. In this approximation, the extreme end of the probe tip and the nanoparticle are considered as point dipoles interacting with their own image dipoles generated by the sample surface. The model is further based on the assumption that the line connecting the centers of the spheres is perpendicular to the substrate surface (as shown by the solid line on Fig. 2b). When a gold nanoparticle is located between the probing tip and a flat sample the near-field amplitude signal is commensurate to the effective polarizability of the system of four dipoles, probe tip and the nanoparticle considered as point dipoles and their image dipoles generated by the sample surface. This effective

4 Y. Abate et al. / Chemical Physics Letters 474 (2009) Fig. 2. (a) Sample geometry showing experimental arrangement. (b) Depiction of the model used for the numerical calculation, h is the height of the core gold nanoparticle, d is the thickness of the capping layer. Also shown are the image dipoles below the Si substrate. polarizability is the sum of the polarizability tensor of the tip and nanoparticle given by Eq. (20) in Ref. [10]. The nanoparticle dipole is given by the polarizability of a single Au sphere, given by the following equation, a AU =(h/2) 3 (e NP 1)/(e NP + 2), in units of length cubed, where h is the height of the nanoparticle as determined from AFM. The dielectric constant of the gold nanoparticle and the Si substrate used are e NP = i and e Si = i, respectively, at 633 nm [11]. In this experiment we used Au Cr coated Si tips (Mikromasch) rather than solid Au tips. The probe dipole, coated with 20 nm thick gold, is approximated by the dielectric constant and polarizability of gold, as indicated above. For the configuration shown in Fig. 2a and b where a Au nanoparticle of diameter h is encapsulated in a capping layer of thickness d, the near-field signal is modified due to the effects of the surfactant. The optical response of a capped metal nanoparticle is affected in several ways in comparison to an uncapped nanoparticle. Due to the capping layer the metal particle is displaced from the surface of the substrate by the capping layer thickness. Also the scattering tip is displaced from the embedded metal particle by the thickness of the capping layer. Comparison of capped and uncapped nanoparticles of the same height means that the gold particle (h) is smaller in the case of the capped particle. The signal contribution of a metal particle covered in a capping layer (h + 2d) is smaller than the signal contribution of a particle of the same height without the capping layer. The effect of the thin polymer capping agent on the near-field signal can be theoretically added in a first approximation by assuming the thin capping layer to be a spacer layer, the effect of which is simply to displace the tip from the nanoparticle and the nanoparticle from the substrate, causing a weaker near-field interaction [1]. This is a reasonably good assumption if the capping material is thin and has a low refractive index and low absorption, and thus the capping layer is assumed to be completely transparent to the near-field [1]. The thickness of the spacer layer can then be inferred to first order from the decay length of the near-field amplitude signal [1]. Based on this assumption, we consider in our model the tip and the nanoparticle as dipoles and the CTAB polymer layer as a spacer layer of thickness d between the probing tip and the nanoparticle and between the nanoparticle and the substrate as shown in Fig. 2b. The near-field scattering amplitude is linearly proportional to the sum of the effective polarizabilities of the tip and the gold nanoparticle. It can be easily shown that the dominant contribution to the total effective polarizability is the effective polarizability of the nanoparticle, which is directly proportional to the cube of the radius of the nanoparticle imaged. As a result, as the size of the nanoparticle increases the scattering field amplitude also increases significantly. This strong dependence of the signal on the geometry of the nanoparticle is sensitive and becomes more so as the size of the scattering tip decreases. After a small region of the contrast reversal the signal increases as shown in Fig. 3; a small change of the abscissa causes a significant change in the ordinate. The effect of the thin coating layer on the near-field coupling should be Fig. 3. Simulation results of the optical signal as a function of the total particle height, the sum of the core gold particle height (h) plus the thickness of the spacer layer (2d), [a thickness of d between the tip and the nanoparticle and a similar thickness d between the substrate and the nanoparticle, assuming a uniform capping layer]. Also shown are the schematics of a tip and a nanoparticle describing the different points on the graphs as shown.

5 150 Y. Abate et al. / Chemical Physics Letters 474 (2009) significant. This makes ANSOM a potential tool to study surface coatings, to estimate thickness, and to assess the uniformity of the coating. In Fig. 3 we show calculation results using the extended dipole model for the signal amplitude of uncapped gold nanoparticles and CTAB capped gold nanoparticles using a tip radius of 50 nm. The model results are normalized to a calculated signal amplitude of a blank Si substrate (ANSOM intensity of particle/ansom intensity of silicon substrate) and is plotted as a function of the total height of particles, which is the sum of the capping layer thickness and the core gold nanoparticle (h + 2d). The capping layer material is assumed to be uniform around the nanoparticle surface with the thickness d. As the thickness of the spacer layer (d) increases, the scattering signal amplitude decreases as shown in Fig. 3 by the solid, dotted, and dashed curves. A capping spacer layer decreases the near-field coupling signal of a capped nanoparticle due to three different reasons. First it increases the separation between the scattering tip and the nanoparticle by an amount proportional to the thickness of the layer. Second it displaces the nanoparticle from the substrate also by an amount proportional to the layer thickness. Third the gold nanoparticle embedded in the capping layer is smaller than the uncapped gold nanoparticle of the same height to which it is compared. In fact, as far as the scattering signal from the tip is concerned, the size of the capped gold nanoparticle appears smaller than it would be without the capping layer. To quantitatively describe this interesting particle size reduction effect due to the capping layer, we refer back to Fig. 3. Consider for example a 75 nm high gold nanoparticle shown in Fig. 3 by A (uncapped) and a second gold nanoparticle with a very thin capping layer (2 nm) shown in Fig. 3 by B. Due to the capping layer the optical signal of a 71 nm gold, capped particle (B) of total height 75 nm (gold + capping layer) decreases by 15% compared to the equivalent uncapped particle signal (A) of the same height 75 nm. The near-field signal of a capped particle of total height 75 nm (B) is equivalent to a 66 nm uncapped gold nanoparticle shown in (C). As far as the near-field signal contribution, the 71 nm gold appears to the tip as if it is a 66 nm gold particle (C). Its size appears smaller by 5 nm than it actually is. This size reduction effect becomes significant as the size of the capping/spacer layer increases. Consider for example the case when the capping layer is doubled (4 nm thick capping layer). The signal of the capped particle (D) of total height 75 nm (gold + capping layer) decreases by 26% compared to the equivalent uncapped particle signal (A) of the same height, 75 nm. The gold particle size embedded inside is 67 nm. However the near-field contribution of the capped particle is equivalent to a 57 nm gold particle (E). Its size appears 10 nm smaller, in this case, than it is. For bigger nanoparticles (>120 nm), this size reduction effect of the spacer layer is less pronounced. For example if we repeat the above analysis for a total height of a 150 nm particle, with 2 nm capping layer, the capped gold particle size appears only 2 nm less than it actually is (as opposed to 5 nm less for a 71 nm particle described above). So the effect of the capping layer on the near-field amplitude signal is larger for smaller particles. The scattering signal reduction of a capped nanoparticle is because the capping layer causes a separation between the tip and the gold nanoparticle and the gold nanoparticle and the substrate, which results in a weaker near-field coupling. Which of these effects is larger and is more important in causing the gold particle to appear smaller can be assessed. The relative importance of the separation between the tip and the gold nanoparticle and the gold nanoparticle and the substrate to the near-field contrast is explored in a 3-D numerical calculation (Fig. 4) using the coupled dipole formulation using a tip radius of 50 nm as in Fig. 3. Fig. 4 shows the normalized ANSOM signal plotted for a gold nanoparticle of height 100 nm versus both the separation distance between the probe tip and the gold nanoparticle (d 1 ) and the separation distance between the gold nanoparticle and the substrate (d 2 ). The near-field signal is smaller for any separation distance between the tip and the gold nanoparticle (d 1 ) than it is for the same separation distance between the gold nanoparticle and the substrate (d 2 ). From Fig. 4 we observe that d 1 is a more important effect than d 2 to the near-field reduction. It is also important to notice that a number of combinations of d 1 and d 2 could result-in the same near-field amplitude signal, as a result of this model calculation, and it is not possible to tell which particular combination results in a given near-field signal amplitude. These theoretical predictions have important implications to the analysis of the experimental data as discussed below. By treating the tip and the nanoparticle as dipoles and the CTAB as a spacer in our model, we estimated the thickness of the capping layer produced by the synthesis and nanoparticle deposition in this experiment. As noted above, the CTAB has a weak near-field interaction, due to its low dielectric constant and low absorption. This is confirmed by absorption experiments (not shown); CTAB represents a transparent spacer layer of thickness d between the probing tip and the nanoparticle and between the nanoparticle and the substrate. Based on this model, we calculate the signal amplitude of uncapped gold nanoparticles and CTAB capped gold nanoparticles normalized to the signal amplitude of a Si substrate as a function of the height of the particles (Fig. 5). The experimental data points plotted in Fig. 5 are acquired by comparing the AFM height peak of a nanoparticle at a pixel (x, y) and the ANSOM signal amplitude for the same pixel (x, y) as Fig. 4. The ANSOM signal plot versus both the separation distance between the probe tip and the gold nanoparticle (d 1 ) and the separation distance between the gold nanoparticle and the substrate (d 2 ) also defined by the schematics shown. The height of the gold nanoparticle is chosen to be 100 nm for this calculation.

6 Y. Abate et al. / Chemical Physics Letters 474 (2009) Fig. 5. Experimental near-field signal measured from the ANSOM images as a function of the total height (h + 2d) of particles measured from AFM images of samples containing uncapped gold particles (rectangles) and CTAB capped gold nanoparticles (circles) absorbed on a Si substrate. All values are normalized to that of the Si substrate. Also superimposed are the simulation results of the normalized optical signal as a function of the particles total height (h + 2d). Solid curve for d = 0 (uncapped), dotted curve is for d = 2 nm and dashed curve for d = 5 nm. described above, for both the capped and uncapped nanoparticles. We exclude pixels on particle edges which may display other complicated effects and also pixels over particles that arise due to two or more nanoparticles aggregated together (as determined by AFM measurements) or other uncertain structures in the analysis as described above. The radius of the tip obtained from the fit of the size dependent amplitude signal of the uncapped gold nanoparticle is 40 nm, in qualitative agreement with manufacturer s estimate (MikroMasch) of <50 nm nominal radius of curvature of Cr Aucoated tip. Using a fixed tip radius of 40 nm and assuming a uniform capping layer thickness d around the nanoparticle we superimpose our calculation results with the normalized experimental data and approximately estimate the thickness (d) of the capping layer to be between 2 and 5 nm. A CTAB monolayer surfactant coating is expected to be 2 nm in thickness [4,12,13]. A bilayer CTAB surfactant coating is 4 nm in thickness [4,12 15]. The molecular structure of CTAB and a possible structure of bilayer CTAB on the surface of gold nanoparticle is shown in Fig. 6. Several studies concerning the adsorption of CTAB on nanoparticle surfaces have been carried out, and a multilayer structure has been suggested to be readily possible in aqueous environment [4,13,15]. A mixture of capped gold nanoparticles with multilayer thickness of CTAB could easily be present in the sample, which could be the source of the observed larger thickness range. The theoretical prediction is reasonably good considering the simplification of the model used. However, according to the model we cannot clearly distinguish where the capping layer could be thicker/thinner on the nanoparticle surface as can be understood from the multiple combinations possible in Fig. 4. The model does not take into account the realistic condition that the capping agent is not fully transparent (due to its own dielectric constant >1). Also, the extended dipole model predicts rapidly increasing near-field amplitude signal with increasing particle size, reaching an unreasonably high value for nanoparticles larger than 100 nm, much larger than those considered here. This is accounted for by considering the Au particle to be an extended metal surface when its diameter is much larger than the tip. In which case, the nearfield amplitude signal should be computed by considering the dipole interaction of the tip with its mirror image in the metal surface. As a result, the near-field amplitude signal will not be unreasonably high but rather assumes a constant value regardless of the Fig. 6. (a) Molecular structure of CTAB. (b) Possible structure of bilayer CTAB on the surface of gold nanoparticle. increasing sizes of the nanoparticles imaged. A more complete theoretical model that takes into account the vertical composition of the sample which includes the dielectric constant of the capping layer (e > 1) would improve the quantitative prediction. It will also be important to reduce thermal and mechanical drift of the experimental setup to minimize the signal fluctuations observed in Fig. 5. Any thermal and mechanical instability that could cause inconsistent separation of the tip and the nanoparticle will produce noise in the optical signal comparable to the thickness of the capping layer due to the sensitive distance-dependent near-field coupling as described above (Fig. 3). An additional source of uncertainty in this experiment arises from lack of a reference material to compare all near-field measurements. An ideal experiment would have a clean internal reference surface, such as gold, to calibrate the amplitude of the ANSOM signal. Here the experiments were carried out on oxidized silicon. The inevitable thin oxide layer (<1 2 nm) on the silicon wafer substrate in air, to which the APTMS layer is grafted, gives rise to uncertainty in the baseline signal of Figs. 1 and 5. Moreover, by selecting different reference points to obtain the amplitude, differing results for the scale calibration can be observed. This does not alter the main conclusions of the observations or the method but could contribute to scatter observed in these figures. Also, knowledge of the precise size of the particles is a weakness in the current experiment and undoubtedly leads to scatter in the data. By measurement with AFM, a calibration can be made on the average particle size. However, in future work the sizes will be measured accurately for each particle in situ. In the future, we plan to perform experiments using a silica capping layer because of the greater potential uniformity of the desired thickness layer.

7 152 Y. Abate et al. / Chemical Physics Letters 474 (2009) Conclusion A thin capping layer can cause a significant modification of the near-field signal of a metal particle due to three mutually inclusive effects of the capping layer on the near-field coupling of the embedded metal particle and the scattering tip. For the particle range presented here, the effect of the organic capping agent can be described as a spacer layer, decreasing the near-field coupling signal of a capped nanoparticle by causing a separation between the scattering tip and the nanoparticle and the nanoparticle and the substrate by an amount proportional to the layer thickness. Comparison of capped and uncapped nanoparticles of the same height means that the gold particle is smaller and so the ANSOM signal is reduced in the case of the capped particle. We effectively estimated the spacer layer dimension by application of the modified dipole dipole coupling model to the data, which assumes the embedding organic layer to be a transparent spacer layer in a first approximation. Calculations performed using the model also indicate that the tip nanoparticle separation and nanoparticle substrate separation caused by the capping agent are each important in causing near-field signal reduction, while the tip nanoparticle separation is a stronger effect. In the future, by including the dielectric constant of the capping agent and considering higher multipoles in the theoretical model, an improved quantitative description may be possible. Acknowledgment This work was supported by the Director, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division, U.S. Department of Energy under Contract No. DE-AC02 05CH Stephen R. Leone gratefully acknowledges the generous support of a Morris Belkin Visiting Professorship at the Weizmann Institute of Science. References [1] T. Taubner, F. Keilmann, R. Hillenbrand, Opt. Express 13 (2005) [2] J. Aizpurua, T. Taubner, F.J. Abajo, M. Brehm, R. Hillenbrand, Opt. Express 16 (2008) [3] M.B. Raschke, C. Lienau, Appl. Phys. Lett. 83 (2003) [4] B.S. Zou, R.B. Little, J.P. Wang, et al., Int. J. Quantum Chem. 72 (1999) 439. [5] J.L. West, N.J. Halas, Annu. Rev. Biomed. Eng. 5 (2003) 285. [6] T. Horibe, J. Zhang, M. Oyamaa, Electroanalysis 19 (2007) 847. [7] G. Frens, Nature 241 (1973) 20. [8] Z.H. Kim, S.H. Ahn, B. Liu, et al., Nano Lett. 7 (2007) [9] A. Cvitkovic, N. Ocelic, R. Hillenbrand, Nano Lett. 7 (2007) [10] V.V. Gozhenko, L.G. Grechko, K.W. Whites, Phys. Rev. B 68 (2003) 12. [11] B. Hecht, H. Bielefeldt, Y. Inouye, et al., J. Appl. Phys. 81 (1997) [12] B. Nikoobakht, M.A. El-Sayed, Langmuir 17 (2001) [13] Z.M. Sui, X. Chen, L.Y. Wang, et al., Phys. E-Low-Dimens. Syst. Nanostruct. 33 (2006) 308. [14] N.R. Jana, L.A. Gearheart, S.O. Obare, et al., J. Mater. Chem. 12 (2002) [15] W. Wang, B.H. Gu, L.Y. Liang, et al., J. Phys. Chem. B 108 (2004)