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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 21 — Oct. 10, 2011
  • pp: 20865–20875
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Nanoscale subsurface- and material-specific identification of single nanoparticles

Zachary Nuño, Brandon Hessler, Jerry Ochoa, Young-Seok Shon, Codi Bonney, and Yohannes Abate  »View Author Affiliations


Optics Express, Vol. 19, Issue 21, pp. 20865-20875 (2011)
http://dx.doi.org/10.1364/OE.19.020865


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Abstract

We report on high resolution subsurface and material specific differentiation of silica, Au and silica-capped Au nanoparticles using scattering-type scanning near-field optical microscopy (s-SNOM) in the visible (λ=633 nm) and mid-infrared (λ=10.7 μm) frequencies. Strong optical contrast is observed in the visible wavelength, mainly because of the dipolar plasmon resonance of the embedded Au nanoparticles which is absent in the infrared. We show that the use of small tapping amplitude improves the apparent image contrast in nanoparticles by causing increased tip-particle and reduced tip-substrate interactions. Experimental results are in excellent agreement with extended dipole model calculations modified to include the capping layer characterized by its refractive index.

© 2011 OSA

1. Introduction

Herein, we employ s-SNOM to directly investigate the near-field optical contrast mechanism in coated nanoparticles by imaging three types of individual particles (silica, Au and silica-capped AuNPs) and mixed particles adsorbed on Si substrate. The widely used coating layer, namely, silica [23

23. T. Nann and P. Mulvaney, “Single quantum dots in spherical silica particles,” Angew. Chem. Int. Ed. Engl. 43(40), 5393–5396 (2004). [CrossRef] [PubMed]

, 24

24. S. O. Obare, N. R. Jana, and C. J. Murphy, “Preparation of polystyrene- and silica-coated gold nanorods and their use as templates for the synthesis of hollow nanotubes,” Nano Lett. 1(11), 601–603 (2001). [CrossRef]

], was chosen since it protects nanoparticles from aggregation and helps to maintain a greater layer uniformity [1

1. S. K. Basiruddin, A. Saha, N. Pradhan, and N. R. Jana, “Advances in coating chemistry in deriving soluble functional nanoparticle,” J. Phys. Chem. C 114(25), 11009–11017 (2010). [CrossRef]

]. Identification of capped and uncapped nanoparticles with nanoscale spatial resolution is demonstrated. An increased optical contrast between silica and silica-capped AuNPs is observed at 633 nm compared to that at 10.7 μm. The difference is mainly from the on-and off- plasmon resonances in visible and mid-infrared frequencies: AuNPs and silica-capped AuNPs have dipolar plasmon resonance near 633 nm whereas they are off-resonant in 10.7 um [25

25. S. Eustis and M. A. el-Sayed, “Why gold nanoparticles are more precious than pretty gold: noble metal surface plasmon resonance and its enhancement of the radiative and nonradiative properties of nanocrystals of different shapes,” Chem. Soc. Rev. 35(3), 209–217 (2006). [CrossRef] [PubMed]

]. In the visible wavelength the strong plasmon- polariton-resonant tip-substrate coupling allow to distinguish silica-capped AuNPs from bare silica particles [26

26. J. Aizpurua, T. Taubner, F. J. García de Abajo, M. Brehm, and R. Hillenbrand, “Substrate-enhanced infrared near-field spectroscopy,” Opt. Express 16(3), 1529–1545 (2008). [CrossRef] [PubMed]

]. The plasmon-enhanced near-field coupling between the tip’s near-field and the Au core through the silica cover layer enhances the weak near-field interaction of the silica capping layer [21

21. T. Taubner, F. Keilmann, and R. Hillenbrand, “Nanoscale-resolved subsurface imaging by scattering-type near-field optical microscopy,” Opt. Express 13(22), 8893–8899 (2005). [CrossRef] [PubMed]

]. This additional signal strength, offered by the coupling to the Au core, results in a brighter contrast (larger amplitude) of the capped particle at lower tapping amplitude of the probing tip compared to a bare silica nanoparticle. As the tapping amplitude surpasses the particle diameter, there is an increased interaction with the silicon substrate on which the particles are adsorbed. Hence, the difference in near-field amplitude contrast between the capped particles and the uncapped particles is reduced. To support our interpretation, we also performed an extended dipole model calculation that includes the refractive index of the capping layer [27

27. V. V. Gozhenko, L. G. Grechko, and K. W. Whites, “Electrodynamics of spatial clusters of spheres: Substrate effects,” Phys. Rev. B 68(12), 125422 (2003). [CrossRef]

]. The unique combined high resolution material specific imaging15 and subsurface imaging [21

21. T. Taubner, F. Keilmann, and R. Hillenbrand, “Nanoscale-resolved subsurface imaging by scattering-type near-field optical microscopy,” Opt. Express 13(22), 8893–8899 (2005). [CrossRef] [PubMed]

] capabilities of s-SNOM provide a nondestructive, highly detailed method for mapping and chemical characterization of coated nanoparticles and complex core-shell nanostructures.

2. Experimental Method

Near-field scattering measurements in the visible (He-Ne laser at 633 nm) and infrared (CO2 laser at 10.7 µm) frequencies are performed using a commercial s-SNOM setup (NeaSNOM, neaspec.com) shown in Fig. 1
Fig. 1 Schematics of the experimental setup showing optical beam paths of the s-SNOM for visible (left side of tip) and infrared frequencies (right side of tip). M = mirror, OI = optical isolator, BE = beam expander, and BS = beam splitter.
. Development and details of s-SNOM are well reviewed in various references [11

11. F. Keilmann and R. Hillenbrand, “Near-field microscopy by elastic light scattering from a tip,” Phil. Trans. R. Soc. Lond. A 362(1817), 787–805 (2004). [CrossRef] [PubMed]

, 12

12. R. Hillenbrand and F. Keilmann, “Complex optical constants on a subwavelength scale,” Phys. Rev. Lett. 85(14), 3029–3032 (2000). [CrossRef] [PubMed]

, 28

28. N. Ocelic, A. Huber, and R. Hillenbrand, “Pseudoheterodyne detection for background-free near-field spectroscopy,” Appl. Phys. Lett. 89(10), 101124 (2006). [CrossRef]

, 29

29. R. Hillenbrand and F. Keilmann, “Material-specific mapping of metal/semiconductor/dielectric nanosystems at 10 nm resolution by backscattering near-field optical microscopy,” Appl. Phys. Lett. 80(1), 25–27 (2002). [CrossRef]

]. Briefly, s-SNOM is based on a tapping mode atomic force microscope (AFM) operated in the intermittent contact mode. S-SNOM is capable of simultaneous topography, amplitude and phase contrast imaging by recording scattered laser light from commercial PtIr-coated cantilevered Si tips with a vertical oscillation frequency of 240 kHz and amplitude of ~20 nm. Near-field scattering data is acquired using a combination of demodulation of the detector signal at higher harmonics of the resonance frequency, (demodulation order n>1) and a pseudoheterodyne interferometric signal detection scheme for greater background suppression [28

28. N. Ocelic, A. Huber, and R. Hillenbrand, “Pseudoheterodyne detection for background-free near-field spectroscopy,” Appl. Phys. Lett. 89(10), 101124 (2006). [CrossRef]

].

Nanoparticle samples were prepared on a Si substrate that was first cleaned by sonication in methanol and dried with nitrogen gas. The substrate was then dipped in a 1% APTMS (3-aminopropyltrimethoxysilane) solution in ethanol. Nanoparticle solutions (nanoComposix Inc., nanocomposix.com) were drop-casted on the wafer, allowed to react for 12.5 minutes. The substrate was then rinsed with ethanol and dried with nitrogen gas. The samples containing mixed particles of three kinds (silica, silica-capped Au and pure Au nanoparticles) were prepared by diluting each solution with aqueous isopropyl alcohol (IPA) solution (3:1 IPA: H2O) and drop-casting the solutions in the order Au, silica-capped gold, and then silica particles allowing each solution to react for 10 minutes.

Figures 2(a)
Fig. 2 Transmission electron microscope (TEM) images and size statistics for monodisperse nanoparticles. (a-b) Silica-capped AuNPs, showing uniformity of shape and thickness of the silica capping layer and (c-d) silica nanoparticles. N is the number of particles measured.
and 2(c) are transmission electron microscopy (TEM) images of monodispersed silica-capped AuNPs and bare silica nanoparticles prepared by drop-casting nanoparticle solutions (nanoComposix, nanocomposix.com) on a Si wafer. TEM images shown in Fig. 2(a) indicate that the shapes of the composite silica-coated AuNPs and the thickness of the capping silica layer (average size of ~10 nm) are highly uniform. Nevertheless, slight non-uniformities of shape and size of the AuNP core and the capping layer should be considered in modeling near-field interactions of these particles and extracting quantitative information. Size statistics performed on samples of over 153 capped particles [Fig. 2(b)] and 275 bare silica particles [Fig. 2(d)] demonstrate particle size distributions of 10-20%, typical values for many colloidal samples.

3. Results and discussion

Figure 3
Fig. 3 Near-field images of a mixture of 3 types of nanoparticles: silica, Au, and silica-capped AuNPs absorbed on a Si substrate. (a) AFM topographic image of the mixture and (b) third harmonic s3 near field amplitude image. (c-d) High resolution zoom in scan results of the 3 particle types shown in green circle in (a) in a 500 nm x 500 nm area, (c) topography, and (d) third harmonic s3 near-field amplitude. (e) Line profiles of the topography of nanoparticles (red and blue dashed lines) showing similar height of the two particles. (f) Different optical signal line profiles for the two particles shown in red and blue dashed lines. Amplitude signal line profiles shown in (f) are normalized to the signal of a Si substrate.
shows topography and near-field optical images of a mixture of three types of particles (silica, Au and silica-capped AuNPs) on a Si substrate at a laser wavelength of, λ=633 nm. In the topography image [Fig. 3(a)] all particles look identical while the optical amplitude image [Fig. 3(b)] shows that the nanoparticles exhibit three different image contrasts compared to the substrate. Particles appear either bright, dark or medium compared to the Si substrate. In Fig. 3(d) a zoomed-in image of the three contrast types captured in a 500 nm x 500 nm area is shown. Line plots of AFM heights [Fig. 3(e)] and optical amplitude signals [Fig. 3(f)] of two selected particles in Fig. 3(c) show that the heights of the two particles are the same ~50 nm [Fig. 3(e)]. However, the amplitude signals differ [Fig. 3(f)], which suggests that the two particles are not the same type in the mixture composed of silica, Au and silica-capped AuNPs [15

15. A. Cvitkovic, N. Ocelic, and R. Hillenbrand, “Material-specific infrared recognition of single sub-10 nm particles by substrate-enhanced scattering-type near-field microscopy,” Nano Lett. 7(10), 3177–3181 (2007). [CrossRef] [PubMed]

]. Furthermore, in Fig. 3(d) the brightest central particle which has a normalized amplitude signal of s (s=sAu/sSi) ~3:1, is easily identified as Au as it is well known [11

11. F. Keilmann and R. Hillenbrand, “Near-field microscopy by elastic light scattering from a tip,” Phil. Trans. R. Soc. Lond. A 362(1817), 787–805 (2004). [CrossRef] [PubMed]

, 12

12. R. Hillenbrand and F. Keilmann, “Complex optical constants on a subwavelength scale,” Phys. Rev. Lett. 85(14), 3029–3032 (2000). [CrossRef] [PubMed]

]. Near-field interaction of the probing tip is expected to be stronger with the silica-capped AuNP than with a bare silica particle due to a strong plasmon enhanced coupling of the tip near-field with the buried AuNP below the coating [21

21. T. Taubner, F. Keilmann, and R. Hillenbrand, “Nanoscale-resolved subsurface imaging by scattering-type near-field optical microscopy,” Opt. Express 13(22), 8893–8899 (2005). [CrossRef] [PubMed]

]. These observations could suggest that the red line represents a silica-capped AuNP and the blue line a bare silica particle.

To validate this hypothesis we performed a controlled experiment on two samples that contained only either silica nanoparticles or silica-capped AuNPs adsorbed on Si substrate. The images used for analysis on these samples were acquired using the same scanning parameters, the same scanning tip, and the same tip vibrational amplitude. We performed pixel-by-pixel analysis of the data on each sample by correlating topography and near-field signal [15

15. A. Cvitkovic, N. Ocelic, and R. Hillenbrand, “Material-specific infrared recognition of single sub-10 nm particles by substrate-enhanced scattering-type near-field microscopy,” Nano Lett. 7(10), 3177–3181 (2007). [CrossRef] [PubMed]

]. This is done by plotting the optical signal amplitude s3(x,y) measured at a pixel (x,y) versus the height h(x,y) extracted from the corresponding topography image on each nanoparticle. The result of the analysis is plotted in Fig. 4
Fig. 4 Pixel-by-pixel correlation of measured near-field third harmonic signal contrast as a function of the topographic pixel height at 633 nm wavelength recorded for two samples, pure silica (blue data points) and silica-capped AuNPs (red data points) adsorbed on a Si substrate. Signal values on Si substrate are used to normalize all optical data.
which shows the bare silica containing sample in blue and the capped sample in red. For capped particles (red points), amplitude signal contrast values recorded at pixels near the center of the particles, which correspond to larger height values, are brighter than those farther from the center. These results support our hypothesis that the signal contrast on capped particles are brighter than those on bare silica particles, thus allowing identification of particles based on their material contrast.

A further confirmation of our hypothesis comes from a theoretical calculation using an extended dipole–dipole coupling model. The model is based on the solution of the electrostatic boundary-value problem (Laplace’s equation) for a system of many interacting dipoles in the presence of a substrate [27

27. V. V. Gozhenko, L. G. Grechko, and K. W. Whites, “Electrodynamics of spatial clusters of spheres: Substrate effects,” Phys. Rev. B 68(12), 125422 (2003). [CrossRef]

]. In this model, the extended structure of the probing tip is approximated by a point dipole located at the extreme end of the tip. The orientation of this dipole is along the direction of the external excitation laser which is assumed to be along the axis of the tip (perpendicular to the sample surface, z-axis in Fig. 1). The nanoparticles are also considered as point dipoles. The dipole orientation of the nanoparticles is also taken to be along the z-axis, similar to the tip dipole orientation [27

27. V. V. Gozhenko, L. G. Grechko, and K. W. Whites, “Electrodynamics of spatial clusters of spheres: Substrate effects,” Phys. Rev. B 68(12), 125422 (2003). [CrossRef]

]. Both the tip dipole and the nanoparticle dipole interact with their own image dipoles generated by the sample surface. The effective polarizability, described by the sum of the polarizability of the tip dipoles and the nanoparticle dipoles, allows determination of both the amplitude s and the phase φ of the near-field interaction [12

12. R. Hillenbrand and F. Keilmann, “Complex optical constants on a subwavelength scale,” Phys. Rev. Lett. 85(14), 3029–3032 (2000). [CrossRef] [PubMed]

]. The model is further improved by taking into account signal harmonic demodulation in which the 3rd harmonic signal is numerically calculated by Fourier transforming the modulated effective polarizability [14

14. Z. H. Kim and S. R. Leone, “High-resolution apertureless near-field optical imaging using gold nanosphere probes,” J. Phys. Chem. B 110(40), 19804–19809 (2006). [CrossRef] [PubMed]

].

The dielectric function (εc) of the hybrid core-shell nanoparticle (silica-coated Au) is included in the simulation as [30

30. G. P. Wiederrecht, G. A. Wurtz, and J. Hranisavljevic, “Coherent coupling of molecular excitons to electronic polarizations of noble metal nanoparticles,” Nano Lett. 4(11), 2121–2125 (2004). [CrossRef]

, 31

31. N. T. Fofang, T. H. Park, O. Neumann, N. A. Mirin, P. Nordlander, and N. J. Halas, “Plexcitonic nanoparticles: plasmon-exciton coupling in nanoshell-J-aggregate complexes,” Nano Lett. 8(10), 3481–3487 (2008). [CrossRef] [PubMed]

]
 εc=(ε2εm)(ε1+2ε2)+2(ε1ε2)(r1r2)3(ε1+2ε2)(ε1ε2)(r1r2)3
(1)
where ε1 and r1 are the dielectric constant and radius of the metal core respectively, ε2 and r2 are the dielectric constant and radius of the silica shell, respectively, and εm is the dielectric constant of the surrounding medium taken to be air. For visible wavelength experiments (at 633 nm) the dielectric constants for AuNPs, silica particles/shell and the Si substrate are taken to be ε1=−11.8 + 1.25i and ε2 =2 and εSi= 15.1 + 0.16i, respectively [32

32. E. W. Palik, Handbook of Optical Constants of Solids (Academic Press, Berlin, 1985).

]. To simulate experimental data, we assumed a constant thickness of the capping layer and varied the diameter of the core AuNP. Therefore each point in the calculation represents a spherical AuNP covered uniformly by constant thickness of silica shell.

Figure 5
Fig. 5 Calculated results of the normalized optical signals for silica-capped AuNPs (dashed line) and silica nanoparticles (solid line) as a function of particle height. Superimposed on the theoretical curves are experimental signal average values of pixels near the center of particles.
shows the result of the calculation for the bare silica (solid line) and the silica-capped AuNP (broken line). Superimposed on the theoretical graphs are experimental data points acquired on the two samples: bare silica (blue data points) and capped (red data points). Each experimental data point plotted in Fig. 5 represents an average signal value of pixels taken near the center of a particle. The theoretical calculation agrees excellently with the experimental average data points. In the calculation the only adjustable free parameter is the thickness of the capping layer. For the fit shown in Fig. 5, the thickness is taken to be 13 nm, in very good agreement to the average thickness value of ~10 nm estimated from the synthesis of the nanoparticles. As noted above the calculations assume constant thickness of the capping layer. If, on the other hand, a constant core size and a varying capping thickness are assumed in the simulation, a fit to experimental data cannot be achieved. Capped and uncapped particles are identical in the size range when the height of the capped particle is less than the total thickness of capping layer (13 nm), and result in an overlapping amplitude signal until up to 26 nm particle height. As the core AuNP increases in size, the amplitude signal of a capped silica particle becomes stronger than a bare silica nanoparticle of the same size. The broad plasmon resonance of AuNPs in the visible [25

25. S. Eustis and M. A. el-Sayed, “Why gold nanoparticles are more precious than pretty gold: noble metal surface plasmon resonance and its enhancement of the radiative and nonradiative properties of nanocrystals of different shapes,” Chem. Soc. Rev. 35(3), 209–217 (2006). [CrossRef] [PubMed]

] (expressed by the negative real part of the dielectric constant of Au) [33

33. R. Hillenbrand, T. Taubner, and F. Keilmann, “Phonon-enhanced light matter interaction at the nanometre scale,” Nature 418(6894), 159–162 (2002). [CrossRef] [PubMed]

] results in resonant tip-substrate interaction that enhances the near field contrast of the capped particles [26

26. J. Aizpurua, T. Taubner, F. J. García de Abajo, M. Brehm, and R. Hillenbrand, “Substrate-enhanced infrared near-field spectroscopy,” Opt. Express 16(3), 1529–1545 (2008). [CrossRef] [PubMed]

]. The increasing signal level with size is due to the increasing plasmon-enhanced near-field coupling between tip’s near-field and the AuNP core below the silica capping layer. The weak near-field interaction of the silica capping layer is therefore augmented by the strong near-field interaction of the embedded AuNP. This additional signal strength offered by the plasmonic Au core results in a brighter contrast (larger amplitude) of the capped particle compared to a bare silica nanoparticle of the same size.

To study the near-field amplitude contrasts in silica-capped metal particles and bare silica particles in a spectral region where the AuNPs do not possess plasmon resonance, we perform s-SNOM imaging in the infrared (λ=10.7 µm). The same two samples used in the visible study, silica particles and silica-capped AuNPs adsorbed on silicon, were utilized. The results of analysis and calculation in the infrared (λ=10.7 µm), similar to what was carried out for visible scans is shown in Figs. 6(a)
Fig. 6 (a) Pixel-by-pixel correlation of measured near-field second harmonic signal s2 as a function of the topographic pixel height at λ=10.7 µm wavelength recorded for two samples, silica (blue data points) and silica-capped AuNPs (red data points) adsorbed on a Si substrate. (b) Calculated results of the normalized optical signals for silica-capped AuNPs (dashed line) and silica nanoparticles (solid line) as a function of particle height. Superimposed on the theoretical curves are shown experimental signal average values of pixels near the center of particles.
and 6(b). The amplitude signal contrast of the silica-capped AuNPs (red dots) overlaps with amplitude signal contrast of bare silica particles (blue dots) with a small branching near the center (larger pixel height) of the particles. At 10.7 µm, AuNPs of size considered in this work are not expected to show plasmon resonance and cannot lead to signal enhancement based on a resonant near-field interaction. However, the embedded highly reflecting AuNP below the coating layer can enhance the near-field signal of a thin sample layer based mainly on the effect of reflection of the substrate as shown by the narrower and more overlapping amplitude contrast branching at 10.7 µm. In addition, discrimination between capped and uncapped particles in the infrared is made more difficult because of amplitude signal fluctuation as documented before for materials showing weak dielectric contrast [15

15. A. Cvitkovic, N. Ocelic, and R. Hillenbrand, “Material-specific infrared recognition of single sub-10 nm particles by substrate-enhanced scattering-type near-field microscopy,” Nano Lett. 7(10), 3177–3181 (2007). [CrossRef] [PubMed]

]. An improved image analysis will certainly minimize this fluctuation allowing better material specific discrimination in the infrared.

The strong optical contrast in the visible (633 nm) region demonstrated experimentally and theoretically in Figs. 4 and 5 above, can be used to identify each nanoparticle with material specificity in a mixed sample [see Figs. 7(a)
Fig. 7 Identification of particles based on their near-field amplitude contrast. (a) Topography of a mixed sample (b) Near-field amplitude contrast (c) Experimental signal average values of pixels near the center of particles red (capped), blue (uncapped) (d) Particles labeled according to their material contrast as capped (red) and uncapped (blue).
and 7(b)] composed of silica, Au and silica-capped AuNPs adsorbed on Si substrate. To achieve this goal we plotted the average (using ~35 pixels per particle) near-field amplitude signal contrast at the center of a particle as a function of the height of the nanoparticles. A clear separation of the average amplitude data points between capped particles (red data points) and uncapped particles (blue points) is shown in Fig. 7(c) which is qualitatively similar to Fig. 5. These particles are marked by a colored circled (red (capped) or blue (uncapped)) on the topography image [Fig. 7(d)] depending on whether they belong to the red or blue points on Fig. 7(c) thereby allowing us to identify particles as capped or uncapped.

To explore the role of the tapping amplitude of the probing tip on the near-field contrast of nanoparticles we carried out experimental study and showed the result in Fig. 8
Fig. 8 Third harmonic near-field amplitude signals versus tapping amplitude measured at λ=633 nm. Red dots represent data points on silica-capped Au and blue dots on pure silica nanoparticles. Signal values on Si substrate are used to normalize all optical data.
. We used the third harmonics signal normalized to the signal value on Si substrate at a laser wavelength, λ=633 nm. The normalized amplitude signal increases monotonically, until approximately the tapping amplitude is comparable to the particle diameter (~75 nm), and then approaches saturation. A similar saturation has been reported on extended samples but explanations differ [34

34. S. Amarie, T. Ganz, and F. Keilmann, “Mid-infrared near-field spectroscopy,” Opt. Express 17(24), 21794–21801 (2009). [CrossRef] [PubMed]

]. The saturation observed at higher tapping on extended samples is because the near-field extent is a fixed length, equal to about the tip diameter. Therefore, much larger tapping amplitude will not increase the signal further and leads to saturation [34

34. S. Amarie, T. Ganz, and F. Keilmann, “Mid-infrared near-field spectroscopy,” Opt. Express 17(24), 21794–21801 (2009). [CrossRef] [PubMed]

, 35

35. S. Amarie and F. Keilmann, “Broadband-infrared assessment of phonon resonance in scattering-type near-field microscopy,” Phys. Rev. B 83(4), 045404 (2011). [CrossRef]

]. However for small particles, as the tapping amplitude surpasses particle diameter, the near-field interaction is more and more with the silicon substrate on which the particles are adsorbed rather than with the particles. This leads to a signal saturation which approaches a value unity (s3 (particle)/s3 (Si) ~1) as can be seen on Fig. 8. We note that the saturation behavior is universal in nature for nanoparticles. It is not affected by the change in dielectric constants and therefore material compositions of the nanoparticles as shown by the similar trend for both the capped and uncapped particles.

Interestingly, for capped particles higher tapping doesn’t improve contrast, i.e. the contribution of the core AuNP diminishes as tapping increases [Fig. 8]. These results contrast with what was documented for layered thin films [21

21. T. Taubner, F. Keilmann, and R. Hillenbrand, “Nanoscale-resolved subsurface imaging by scattering-type near-field optical microscopy,” Opt. Express 13(22), 8893–8899 (2005). [CrossRef] [PubMed]

]. In case of thin films, there is a ‘see-through effect’ of the near-field, which means that the substrate under the thin coating layer contributes significantly to the signal [21

21. T. Taubner, F. Keilmann, and R. Hillenbrand, “Nanoscale-resolved subsurface imaging by scattering-type near-field optical microscopy,” Opt. Express 13(22), 8893–8899 (2005). [CrossRef] [PubMed]

]. At increasing tapping amplitude, the interaction probes a deeper overall depth, therefore the influence of the material under the coated film increases. Thus, in a thin film coated Au sample, Au should have increasing influence; any low near-field contrast should increase as the tapping amplitude increases. But for small particles, the thin-film approximation is not valid. At larger tapping amplitudes, the contrast becomes more and more similar between the capped and uncapped particles at larger tapping amplitudes. These results suggest that in small particle near-field imaging, the use of lower tapping amplitude allows enhanced material specific identification of particles.

In conclusion, we successfully imaged and identified silica, Au and silica-capped AuNPs adsorbed on Si substrate using s-SNOM in the visible and infrared frequencies. Subsurface near-field interaction between tip and plasmon resonant or even a reflecting core nanoparticle combined with material specific optical contrast imaging of nanoparticles makes s-SNOM very attractive for many applications. Examples of these possible applications include quality control in pharmaceutical or nanochemistry, photothermal therapeutic applications and detecting dopants and defect sites in single semiconductor nanoparticles.

Acknowledgments

The authors thank Dr. Fritz Keilmann and Dr. Zee Hwan Kim for many fruitful discussions. Financial support provided by the Department of Physics and Astronomy and the College of Natural Science and Mathematics (CNSM), California State University, Long Beach (startup grant), the American Chemical Society, Petroleum Research Fund (ACS PRF) under grant PRF# 50461-UNI10 and from Research Corporation for Science Advancement Award are all gratefully acknowledged.

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A. Cvitkovic, N. Ocelic, and R. Hillenbrand, “Material-specific infrared recognition of single sub-10 nm particles by substrate-enhanced scattering-type near-field microscopy,” Nano Lett. 7(10), 3177–3181 (2007). [CrossRef] [PubMed]

16.

A. Cvitkovic, N. Ocelic, J. Aizpurua, R. Guckenberger, and R. Hillenbrand, “Infrared imaging of single nanoparticles via strong field enhancement in a scanning nanogap,” Phys. Rev. Lett. 97(6), 060801 (2006). [CrossRef] [PubMed]

17.

Z. H. Kim, S. H. Ahn, B. Liu, and S. R. Leone, “Nanometer-scale dielectric imaging of semiconductor nanoparticles: size-dependent dipolar coupling and contrast reversal,” Nano Lett. 7(8), 2258–2262 (2007). [CrossRef] [PubMed]

18.

A. Hartschuh, E. J. Sánchez, X. S. Xie, and L. Novotny, “High-resolution near-field Raman microscopy of single-walled carbon nanotubes,” Phys. Rev. Lett. 90(9), 095503 (2003). [CrossRef] [PubMed]

19.

M. Brehm, T. Taubner, R. Hillenbrand, and F. Keilmann, “Infrared spectroscopic mapping of single nanoparticles and viruses at nanoscale resolution,” Nano Lett. 6(7), 1307–1310 (2006). [CrossRef] [PubMed]

20.

J.-S. Samson, R. Meißner, E. Bründermann, M. Böke, J. Winter, and M. Havenith, “Characterization of single diamondlike and polymerlike nanoparticles by midinfrared nanospectroscopy,” J. Appl. Phys. 105(6), 064908 (2009). [CrossRef]

21.

T. Taubner, F. Keilmann, and R. Hillenbrand, “Nanoscale-resolved subsurface imaging by scattering-type near-field optical microscopy,” Opt. Express 13(22), 8893–8899 (2005). [CrossRef] [PubMed]

22.

Y. Abate, A. Schwartzberg, D. Strasser, and S. R. Leone, “Chem. “Nanometer-scale size dependent imaging of cetyl trimethyl ammonium bromide (CTAB) capped and uncapped gold nanoparticles by apertureless near-field optical microscopy,” Phys. Lett. 474, 146–152 (2009).

23.

T. Nann and P. Mulvaney, “Single quantum dots in spherical silica particles,” Angew. Chem. Int. Ed. Engl. 43(40), 5393–5396 (2004). [CrossRef] [PubMed]

24.

S. O. Obare, N. R. Jana, and C. J. Murphy, “Preparation of polystyrene- and silica-coated gold nanorods and their use as templates for the synthesis of hollow nanotubes,” Nano Lett. 1(11), 601–603 (2001). [CrossRef]

25.

S. Eustis and M. A. el-Sayed, “Why gold nanoparticles are more precious than pretty gold: noble metal surface plasmon resonance and its enhancement of the radiative and nonradiative properties of nanocrystals of different shapes,” Chem. Soc. Rev. 35(3), 209–217 (2006). [CrossRef] [PubMed]

26.

J. Aizpurua, T. Taubner, F. J. García de Abajo, M. Brehm, and R. Hillenbrand, “Substrate-enhanced infrared near-field spectroscopy,” Opt. Express 16(3), 1529–1545 (2008). [CrossRef] [PubMed]

27.

V. V. Gozhenko, L. G. Grechko, and K. W. Whites, “Electrodynamics of spatial clusters of spheres: Substrate effects,” Phys. Rev. B 68(12), 125422 (2003). [CrossRef]

28.

N. Ocelic, A. Huber, and R. Hillenbrand, “Pseudoheterodyne detection for background-free near-field spectroscopy,” Appl. Phys. Lett. 89(10), 101124 (2006). [CrossRef]

29.

R. Hillenbrand and F. Keilmann, “Material-specific mapping of metal/semiconductor/dielectric nanosystems at 10 nm resolution by backscattering near-field optical microscopy,” Appl. Phys. Lett. 80(1), 25–27 (2002). [CrossRef]

30.

G. P. Wiederrecht, G. A. Wurtz, and J. Hranisavljevic, “Coherent coupling of molecular excitons to electronic polarizations of noble metal nanoparticles,” Nano Lett. 4(11), 2121–2125 (2004). [CrossRef]

31.

N. T. Fofang, T. H. Park, O. Neumann, N. A. Mirin, P. Nordlander, and N. J. Halas, “Plexcitonic nanoparticles: plasmon-exciton coupling in nanoshell-J-aggregate complexes,” Nano Lett. 8(10), 3481–3487 (2008). [CrossRef] [PubMed]

32.

E. W. Palik, Handbook of Optical Constants of Solids (Academic Press, Berlin, 1985).

33.

R. Hillenbrand, T. Taubner, and F. Keilmann, “Phonon-enhanced light matter interaction at the nanometre scale,” Nature 418(6894), 159–162 (2002). [CrossRef] [PubMed]

34.

S. Amarie, T. Ganz, and F. Keilmann, “Mid-infrared near-field spectroscopy,” Opt. Express 17(24), 21794–21801 (2009). [CrossRef] [PubMed]

35.

S. Amarie and F. Keilmann, “Broadband-infrared assessment of phonon resonance in scattering-type near-field microscopy,” Phys. Rev. B 83(4), 045404 (2011). [CrossRef]

OCIS Codes
(240.6680) Optics at surfaces : Surface plasmons
(160.4236) Materials : Nanomaterials
(180.4243) Microscopy : Near-field microscopy

ToC Category:
Optics at Surfaces

History
Original Manuscript: June 22, 2011
Revised Manuscript: August 21, 2011
Manuscript Accepted: August 28, 2011
Published: October 5, 2011

Virtual Issues
Vol. 6, Iss. 11 Virtual Journal for Biomedical Optics

Citation
Zachary Nuño, Brandon Hessler, Jerry Ochoa, Young-Seok Shon, Codi Bonney, and Yohannes Abate, "Nanoscale subsurface- and material-specific identification of single nanoparticles," Opt. Express 19, 20865-20875 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-21-20865


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References

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  15. A. Cvitkovic, N. Ocelic, and R. Hillenbrand, “Material-specific infrared recognition of single sub-10 nm particles by substrate-enhanced scattering-type near-field microscopy,” Nano Lett. 7(10), 3177–3181 (2007). [CrossRef] [PubMed]
  16. A. Cvitkovic, N. Ocelic, J. Aizpurua, R. Guckenberger, and R. Hillenbrand, “Infrared imaging of single nanoparticles via strong field enhancement in a scanning nanogap,” Phys. Rev. Lett. 97(6), 060801 (2006). [CrossRef] [PubMed]
  17. Z. H. Kim, S. H. Ahn, B. Liu, and S. R. Leone, “Nanometer-scale dielectric imaging of semiconductor nanoparticles: size-dependent dipolar coupling and contrast reversal,” Nano Lett. 7(8), 2258–2262 (2007). [CrossRef] [PubMed]
  18. A. Hartschuh, E. J. Sánchez, X. S. Xie, and L. Novotny, “High-resolution near-field Raman microscopy of single-walled carbon nanotubes,” Phys. Rev. Lett. 90(9), 095503 (2003). [CrossRef] [PubMed]
  19. M. Brehm, T. Taubner, R. Hillenbrand, and F. Keilmann, “Infrared spectroscopic mapping of single nanoparticles and viruses at nanoscale resolution,” Nano Lett. 6(7), 1307–1310 (2006). [CrossRef] [PubMed]
  20. J.-S. Samson, R. Meißner, E. Bründermann, M. Böke, J. Winter, and M. Havenith, “Characterization of single diamondlike and polymerlike nanoparticles by midinfrared nanospectroscopy,” J. Appl. Phys. 105(6), 064908 (2009). [CrossRef]
  21. T. Taubner, F. Keilmann, and R. Hillenbrand, “Nanoscale-resolved subsurface imaging by scattering-type near-field optical microscopy,” Opt. Express 13(22), 8893–8899 (2005). [CrossRef] [PubMed]
  22. Y. Abate, A. Schwartzberg, D. Strasser, and S. R. Leone, “Chem. “Nanometer-scale size dependent imaging of cetyl trimethyl ammonium bromide (CTAB) capped and uncapped gold nanoparticles by apertureless near-field optical microscopy,” Phys. Lett. 474, 146–152 (2009).
  23. T. Nann and P. Mulvaney, “Single quantum dots in spherical silica particles,” Angew. Chem. Int. Ed. Engl. 43(40), 5393–5396 (2004). [CrossRef] [PubMed]
  24. S. O. Obare, N. R. Jana, and C. J. Murphy, “Preparation of polystyrene- and silica-coated gold nanorods and their use as templates for the synthesis of hollow nanotubes,” Nano Lett. 1(11), 601–603 (2001). [CrossRef]
  25. S. Eustis and M. A. el-Sayed, “Why gold nanoparticles are more precious than pretty gold: noble metal surface plasmon resonance and its enhancement of the radiative and nonradiative properties of nanocrystals of different shapes,” Chem. Soc. Rev. 35(3), 209–217 (2006). [CrossRef] [PubMed]
  26. J. Aizpurua, T. Taubner, F. J. García de Abajo, M. Brehm, and R. Hillenbrand, “Substrate-enhanced infrared near-field spectroscopy,” Opt. Express 16(3), 1529–1545 (2008). [CrossRef] [PubMed]
  27. V. V. Gozhenko, L. G. Grechko, and K. W. Whites, “Electrodynamics of spatial clusters of spheres: Substrate effects,” Phys. Rev. B 68(12), 125422 (2003). [CrossRef]
  28. N. Ocelic, A. Huber, and R. Hillenbrand, “Pseudoheterodyne detection for background-free near-field spectroscopy,” Appl. Phys. Lett. 89(10), 101124 (2006). [CrossRef]
  29. R. Hillenbrand and F. Keilmann, “Material-specific mapping of metal/semiconductor/dielectric nanosystems at 10 nm resolution by backscattering near-field optical microscopy,” Appl. Phys. Lett. 80(1), 25–27 (2002). [CrossRef]
  30. G. P. Wiederrecht, G. A. Wurtz, and J. Hranisavljevic, “Coherent coupling of molecular excitons to electronic polarizations of noble metal nanoparticles,” Nano Lett. 4(11), 2121–2125 (2004). [CrossRef]
  31. N. T. Fofang, T. H. Park, O. Neumann, N. A. Mirin, P. Nordlander, and N. J. Halas, “Plexcitonic nanoparticles: plasmon-exciton coupling in nanoshell-J-aggregate complexes,” Nano Lett. 8(10), 3481–3487 (2008). [CrossRef] [PubMed]
  32. E. W. Palik, Handbook of Optical Constants of Solids (Academic Press, Berlin, 1985).
  33. R. Hillenbrand, T. Taubner, and F. Keilmann, “Phonon-enhanced light matter interaction at the nanometre scale,” Nature 418(6894), 159–162 (2002). [CrossRef] [PubMed]
  34. S. Amarie, T. Ganz, and F. Keilmann, “Mid-infrared near-field spectroscopy,” Opt. Express 17(24), 21794–21801 (2009). [CrossRef] [PubMed]
  35. S. Amarie and F. Keilmann, “Broadband-infrared assessment of phonon resonance in scattering-type near-field microscopy,” Phys. Rev. B 83(4), 045404 (2011). [CrossRef]

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