Open Access
Add to BibSonomyAbstract
We present a novel technique to tune the plasmon resonance of metal-coated silicon tips in the whole visible region without altering the tips original sharpness. The technique involves modification of the refractive index of silicon probe by thermal oxidization. Lowering the refractive index of silicon tip coated with metal shift the plasmon resonance of the metallic layer to shorter wavelength. Numerical simulation using FDTD agrees well with the empirical results. This novel technique is very useful in tip-enhanced Raman spectroscopy studies of various materials because plasmon resonance can tuned to a specific Raman excitation wavelength.
©2009 Optical Society of America
1. Introduction
Tip-enhanced Raman spectroscopy (TERS) has been recognized as a powerful tool for optically analyzing molecules with a nanoscale spatial resolution beyond the diffraction limit of light [1–7]. The high spatial resolution of TERS is accompanied with the chemical selectivity provided by the Raman signatures, which offers a great potential in application to the rapidly growing field of nanoscience and nanotechnology such as nano-analysis and imaging of individual carbon nanotube (CNT) [8–12], DNA base [13, 14], organic molecules [15–17], and strained silicon [18, 19].
The key principle of TERS is an excitation of the surface plasmon resonance at the tip of the metallic probe. The plasmon resonance is accompanied with an enhanced electromagnetic field that strongly excites the Raman scattering of the molecules at the vicinity of the metallic tip. The excited Raman scattering is further enhanced through the scattering process occurring on the same metallic tip. This doubly enhancing mechanism provides Raman enhancement factor of typically 103-4[20], and occasionally as much as 107-9[21–23].
In order to obtain a strong Raman enhancement effect, the plasmon resonance wavelength (PRW) of the metallic probe, the Raman excitation wavelength, and the Raman scattered wavelength need to be spectrally matched with each other. In Raman scattering, because the wavelength shift from the excitation is relatively small compared with the spectral width of typical plasmon resonance, the Raman excitation wavelength and the scattered wavelength can be treated as almost identical. Thus, matching only the peak the wavelengths of the plasmon resonance and the Raman excitation laser is more than enough to attain higher field enhancement.
There are several ways to match the PRW with excitation laser. Conventionally, a combination of tip material and metal coating is chosen so that the PRW of the probe matches the available laser wavelength. For example, electrochemically sharpened gold (Au) probe and Ag- and Au-coated tungsten probes show plasmon band typically between 600 nm to 700 nm [24, 25]. Thus, these types of probes are suitable only with red excitation laser. Similarly, silver (Ag)-coated silicon (Si) probes show large Raman enhancement factor for excitation wavelength around 800 nm [20]. Although this is practical, considering the cost and limited number of laser sources, this method sometimes suffers from a low enhancement due to the non-optimal matching between the excitation wavelength and the PRW of the TERS probe [20]. In order to obtain an optimal matching between the excitation wavelength and the PRW, more precise technique is demanded to tune the PRW of TERS probe.
The control of the PRW in a metal nanostructure can be realized by two strategies. The first is manipulating the shape, size and environment of the nanostructure to modify the PRW [26–29]. Van Duyne et al. demonstrate the shift of PRW from lithographically patterned arrays of metal nanoparticles on flat substrates having various refractive indices. The measured extinction spectra were shifted toward the longer wavelength with increasing refractive index [27]. The second strategy makes use of the split of surface plasmon mode in a core-shell structure, also known as nanoshell, to shift the PRW [30, 31]. The challenge in these techniques of controlling the PRW for use in TERS is the shrinkage and integration into the tip with nanoscale dimensions.
Zenobi et al. reported a pioneering work in modifying the refractive index of the probe base material in fabricating metal-coated TERS probe. They deposited materials with low refractive indices such as SiO2 (n = 1.5) and AlF3 (n = 1.4) onto Si (n = 4.4) probe prior to the Ag coating. Higher TERS enhancement factor were observed compared to Ag-coated Si probe for 488 nm blue excitation wavelength [32, 33]. The drawback of this technique is the increase in the tip diameter of base material resulting to lower spatial resolution. This is because at least ten nanometer-thick layer of intermediate material needs to be deposited to completely replace the high refractive index base material.
Motivated by the above method, we present novel technique to modifying the refractive index of the Si probe that will allow not only tuning the PRW throughout the visible range of the spectrum for higher enhancement, but also preserving the tip diameter for higher spatial resolution. The modification of the refractive index is done by thermally oxidizing the surface of the Si probe. The tuning of the PRW on the other hand is achieved by simply changing the thermal oxidization time to vary the thickness of the oxidized layer. The oxidized layer can be varied from several nanometers to several tens nanometers to control the effective refractive index of the Si probe [34]. With this wide-range of controlling the PRW of the metallic probe, we can selectively set the PRW to a specific wavelength in visible that will match the Raman excitation wavelength.
The advantage of thermal oxidization employed in the present method is keeping the sharpness of the tip. This is because the spatial resolution in TERS microscopy is comparable to the diameter of the TERS probe. The effect of the volume expansion of Si in the oxidization of Si tip can be neglected [34]. Another advantage of the present technique is an ability to change the refractive index of the base material from n = 4.4 of Si to n = 1.5 of SiO2 by changing the thickness of the oxidized layer. This means the PRW of the probe can be easily controlled and precisely adjusted to a desired Raman excitation wavelength. The thermal oxidization method is compatible with mass production of highly reliable and reproducible TERS probes. The technique can seamlessly be combined with the sophisticated batch processes employed in fabricating Si probes on a commercial basis.
This work is organized as follows. First, we numerically examined the possibility of the PRW shift in Ag-coated Si probe in the presence of the SiO2 layer using finite-difference time-domain (FDTD) simulations. After the simulations, we looked into the thickness dependence of SiO2 layer with oxidization time under thermal oxidization process using planar Si wafers. The oxidized wafers with varying SiO2 thicknesses were then coated with a thin layer of Ag by evaporation. The reflectance spectra were measured to observe PRW shifts associated with different SiO2 thicknesses on Si. Finally, the shift of the PRW in an actual TERS probe was demonstrated by comparing the scattering spectra of oxidized and non-oxidized Ag-coated Si probes.
2. FDTD Simulation
In order to confirm the shift of the PRW with altering thickness of the SiO2 layer, we first numerically investigated the electromagnetic field at the tip apex of oxidized probe coated with Ag using FDTD method. The tip apex was modeled as a three-layered concentric sphere consisting of Si, SiO2, and Ag as shown in Fig. 1(a). The outer radius of the SiO2 sphere was initially set at 25 nm, indicating the sphere is entirely made-up of SiO2. The outer radius does not change throughout the simulation. A 10 nm thick layer of Ag was placed outside the SiO2 sphere. In order to simulate the presence of non-oxidized material, the innermost radius is varied from 0 nm (all SiO2) to 25 nm (all Si). The near-field spectra of the electric field at the vicinity of the probe tip were then calculated with varying the thickness of the SiO2 layer, ranging from 0 nm (all Si) to 25 nm (all SiO2). The calculations were performed using XFDTD 6 software package [35].
A Gaussian pulse having a white spectrum in the visible wavelength region was directed to the concentric spheres with TM polarization. The electric field spectra were evaluated at the position one pixel away from the tip apex. The calculation space consisted of a cube with a height of 140 nm, located around the center of the model sphere. The whole calculation space was meshed at 1 nm per a pixel. Liao’s second order absorbing boundary condition was used as the outer boundary of the calculation space [36]. The iterations were continued until the value of the electric field converged at the evaluating point. The obtained time-varying electric field data was Fourier transformed to obtain the spectrum. The optical constants of SiO2 were taken from Ref. [37]. The Drude parameters for Ag and the Lorentz parameters for Si were obtained by fitting the tabulated data of the optical constants listed in Ref. [37].
The calculated spectra were shown in Fig. 1(b). When the SiO2 thickness was set to zero, i.e. the core of the sphere consisted solely of Si, the peak of the plasmon resonance appeared at 780 nm. When increasing the thickness of the SiO2 layer, the peak wavelength was gradually shifted toward the shorter wavelength. The calculated peaks of the PRWs were 600 nm, 530 nm, and 500 nm for the SiO2 thicknesses of 5 nm, 10 nm, and 15 nm, respectively. When the thickness of the SiO2 layer was further increased to 20 nm, the PRW also shifted to the shorter wavelength, however, the amount of the shift became smaller. This is attributed to the fact that the volume of the Si became too small compared with the volume of SiO2 to induce a significant change in the effective refractive index. When the thickness of the SiO2 was set at 25 nm, i.e. the Si was completely oxidized, the PRW peak appeared at around 485 nm. These results demonstrate the ability of changing SiO2 layer to control the PRW peak of metals coated onto SiO2. The PRW was shifted within the whole visible wavelength region, ranging from 780 nm (red) to 485 nm (blue) at 0 nm (all Si) to 25 nm (all SiO2) thickness.
Fig. 1. (a) FDTD Model representing the Ag-coated oxidized Si probe that consists of Si, SiO2, and Ag layers. (b) Calculated near-field spectra of the Ag-coated oxidized Si probes with different SiO2 thicknesses.
In addition to the PRW shift, the peak intensity also increased with increasing the SiO2 thickness. This is because the volume of the absorptive Si was decreased with increasing the SiO2 thickness. Thus, the energy loss from silicon is minimized, which in effect means more energy to excite surface plasmon in Ag film. This effect is an additional benefit in oxidizing Si probe. It helps in increasing scattering efficiency compared with non-oxidized probe.
3. Experiment
3.1. SiO2 thickness dependence on oxidization time
We first studied the relation between the oxidization time and the thickness of the SiO2 layer formed by the thermal oxidization method. This was done by oxidizing Si wafers instead of oxidizing Si cantilever probes.
A cleaned Si(111) wafer was cut into small pieces and oxidized at 1100°C in a vapor flow. After the temperature of the furnace was stabilized, a water vapor was fed into the furnace through a 10 mm diameter glass tube. The vapor was obtained by boiling water in a flask with an evaporating rate of 1.0 mL/min. The pieces of the Si wafer were quickly inserted into the furnace through a narrow duct to prevent a decrease of the temperature during the insertion. The oxidization time was varied from one minute to 20 minute. An ellipsometer was used to measure the thicknesses of the formed SiO2 layer.
The SiO2 thicknesses that were measured by the ellipsometer were plotted against the oxidization time and shown in Fig. 2(a). A 4 nm thick native oxidized layer was present on the pre-oxidized Si wafer. The SiO2 thickness was monotonically increased with a parabolic dependence on the oxidization time. The thickness of the oxidized layer that was immediately formed within the first two minutes is approximately 60 nm. After two minutes, the thickness of the SiO2 was increased with a constant rate against the oxidization time. Because the tip diameter of the Si probe is typically several tens of nanometers, oxidization for several minutes is enough to form a layer of SiO2 at the tip apex.
Fig. 2. (a) Oxidization time vs. SiO2 thickness measured using ellipsometer. (b) Pictures of the oxidized Si wafers with increasing thermal oxidization time. (c) Pictures of the oxidized Si wafers in (a) after the coating of a 10 nm thick Ag film by evaporation.
The pictures of the oxidized Si wafers are shown in Fig. 2(b). The surfaces of the oxidized Si wafers exhibit characteristic colors depending on the thickness of the SiO2 layer due to the multiple beam interference [38]. The thicknesses measured by the ellipsometer were consistent with the values estimated from the color of the oxidized wafer [38].
3.2. PRW shift in Ag-coated oxidized Si wafer
In order to obtain the amount of shift from Ag metal, the surface of the oxidized Si wafers were coated with a 10 nm thick Ag. The coating was done by evaporating Ag in a vacuum (~ 10-4 Pa) with a deposition rate of 0.5 Å /s. To minimize the variation in the thickness of the Ag film on all the wafers, we simultaneously coated the whole batch of oxidized wafers by fixing them on a rotating holder inside the vacuum chamber. The thickness of the deposited Ag film was calibrated by measuring the depth of scratches made on the Ag film using an atomic force microscope (AFM). The pictures of the Ag-coated wafers are shown in Fig. 2(c). The presence of the Ag film changed the color of the wafers from their original color expressed without Ag.
The reflectance spectra of the Ag-coated wafers were measured using a UV-vis spectrophotometer at normal incidence. The thin Ag film having the average thickness of 10nm is known to have an island structure. The corrugated surface of the island structure allows the normally incident light to couple with the surface plasmon on the Ag film. The excited plasmon is measured as a peak in the reflectance spectrum.
The measured reflectance spectra of the Ag-coated wafers are shown in Fig. 3. Fig. 3(a) represents the spectrum of bare Si wafer (non-oxidized) coated with Ag. It exhibits a broad spectral peak at around 550 nm. The spectral broadening is attributed to the inhomogeneous size distribution of the Ag grains formed on the wafer. Fig. 3(b) is the spectrum of the Ag coated wafer with 47 nm thick SiO2 layer. The plasmon peak shifted to the shorter wavelength, around 470 nm, relative to the peak observed in the Ag coated non-oxidized wafer. Increasing further the SiO2 thickness means shifting the peak plasmon towards the blue region. The peak wavelength was blue-shifted to 460 nm for the 66 nm thick SiO2 layer (spectrum (c)), and 430 nm for the 70 nm thick SiO2 layer (spectrum (d)). The amount of the blue-shift was larger for the thicker SiO2 layer. This result demonstrates the capability of the present technique to gradually shift the PRW to various wavelengths by controlling the thickness of the SiO2 layer.
Fig. 3. Reflectance spectra of the 10 nm thick Ag films deposited on the SiO2/Si layered structures having different SiO2 thicknesses.
The peak at 350 nm observed in spectrum (a) in Fig. 3 is attributed to the increased reflectance of Si at this wavelength. As was expected from the FDTD calculations, the scattering intensity became larger with increasing the SiO2 thickness. For the wafers having more than 100 nm thick SiO2 layer, the peak of the plasmon resonance could not be identified because of the overwhelming spectral fringe coming from the multiple beam interference caused by the SiO2 layer.
3.3. PRW shift in Ag-coated oxidized Si probe
3.3.1. Oxidization of Si cantilever probe
Si cantilever probes were oxidized using the same oxidization condition as described in Sec. 3.1. Fig. 4 compares transmission electron microscope (TEM) images of the probe tip for (a) before oxidization, (b) after 30 s oxidization, and (c) 5 min oxidization. In Fig. 4(a), the probe exhibits a contour fringe that is characteristic of the single crystalline Si. After 30 s oxidization, a thin light-grayed layer appeared on the surface of the Si probe (Fig. 4(b)). This layer is easily distinguished from Si part because of the lack of the contour fringe, and could be assigned to the SiO2 [34]. The thickness of the formed SiO2 layer at the probe after 30s of thermal oxidization was approximately 20 nm. After 5 min oxidization, the thickness of the SiO2 layer was increased to approximately 100 nm (Fig. 4(c)). The thickness of the SiO2 layer formed on the probe was consistent with the thickness of the SiO2 layer formed on the wafer at the corresponding oxidization time. This indicates that the thermal oxidization process has a satisfactory reproducibility in controlling the thickness of the oxidized layer. Using 5 min oxidization time, the tip apex of the Si probe was completely oxidized and the oxidized layer extends to approximately 500 nm inward along the probe shaft. There was no noticeable change in the diameter of the oxidized tip.
Fig. 4. TEM images of the Si probes for (a) before oxidization, (b) after 30 s oxidization, and (c) 5 min oxidization. Material compositions are schematically explained in the inset in each image. Dotted lines are guides to indicate the material boundary.
3.3.2. Measurements of scattering spectra from Ag-coated oxidized probe
In order to demonstrate the shift of the PRW in the actual TERS probe, scattering spectra from the tip of Ag-coated oxidized probes were measured. For comparison, the scattering spectrum obtained with the Ag-coated Si probe oxidized for 5 min (see Fig. 4(c)) was weigh against the spectrum obtained with the Ag-coated non-oxidized Si probe (see Fig. 4(a)). At the 5 min oxidization time, the tip of the Si probe was completely oxidized and transformed to SiO2 (Fig. 4(c)). A 20 nm thick Ag film was evaporated simultaneously onto the oxidized and non-oxidized probes in the same manner as described in Sec. 3.2. Scanning electron microscope (SEM) images of the oxidized and non-oxidized probes coated with Ag are shown in Fig. 5 (a) and (b), respectively. There was no apparent difference in the surface morphology of the Ag grains that can be observed at the tip between these two probes.
The scattering spectra from the probe tips were measured using a dark-field illumination configuration. The optical setup is schematically shown in the left panel of Fig. 6. The tip of the AFM-controlled probe was set in contact with the surface of a glass cover slip. The tip was illuminated with a white light from a halogen lamp focused onto the tip by an objective lens having a numerical aperture (NA) of 0.28. The diameter of the illuminating focused spot was approximately 1μm in the full-width at half-maximum (FWHM). The light scattered by the probe tip was collected by another objective lens having NA = 0.6. The illuminating axis was inclined at 70 degrees with respect to the detection axis. At this angle, the illuminating light can not directly enter the detection objective, and only the scattered light by the probe tip was collected by the detection objective. The spectrum measured when the tip was retracted five micrometers away from the glass surface was subtracted from the spectrum obtained when the tip was in contact with the surface of the glass cover slip. The difference in the spectra was then normalized to the spectrum of the halogen lamp, by which the scattering spectra of the probe tip were drawn. The spectrum of the halogen lamp is shown in the inset at the right panel in Fig. 6. The spectrum has adequate spectral components between 450nm and 750 nm.
Fig. 6. (Left) Schematic diagram of the experimental setup. (Right) Scattering spectra of (a) Ag-coated oxidized Si probe and (b) Ag-coated non-oxidized Si probe. Inset shows a spectrum of the halogen lamp used for the illumination.
The measured scattering spectra from the Ag-coated oxidized and non-oxidized probes are plotted in the right panel in Fig. 6. Both spectra showed broad spectral shape. The broadened spectra were resulted from the large illuminated area, i.e., the large number of silver grains illuminated at the probe tip, as suggested by the presence of the multiple plasmon resonance peaks. Because of the ensemble character in the spectral measurements, it is practically difficult to evaluate quantitatively the spectral shift of Ag grain located only at the tip apex. Nevertheless, the Ag-coated oxidized probe (spectrum (a)) has strong spectral components covering the region from 450 nm to 650 nm, while the Ag-coated non-oxidized probe (spectrum (b)) shows spectral components only between 600 nm and 750 nm. The spectral components in the blue-to-green region below 570 nm could only be observed for the Ag-coated oxidized probe. The presence of the spectral components in the blue-to-green region strongly suggests the blue-shift in PRW occurring at the individual Ag grain at the tip of the Ag-coated oxidized probe. The overall spectral feature was also shifted to the shorter wavelength for the Ag-coated oxidized probe. This further supports the conclusion that the presence of the oxidized layer shifted the PRW of the Ag-coated probe to the shorter wavelength.
In addition to the PRW shift in the oxidized probe, a large increase in the scattering intensity was also observed in the spectrum shown in Fig. 6. This result indicates the oxidization of Si probe is useful also in increasing signal intensity in TERS measurements.
4. Conclusion
We have presented a novel technique for controlling the PRW in the metal-coated TERS probe to achieve an optimized spectral matching between the PRW and the Raman excitation wavelength using thermal oxidization process. The PRW of the Ag film deposited on a Si cantilever probe is shifted to a specific wavelength by controlling the effective refractive index of the probe. Effective refractive index is easily changed by gradually varying the thickness of SiO2 in the tip apex. Because the shifting range of the PRW covered the whole visible wavelength, the present technique is applicable in optimizing the PRW for various laser sources used in TERS experiments. Moreover, extremely sharp TERS probes can be easily fabricated from commercially available Si cantilever probes because thermal oxidization process preserves the tip sharpness. This is also very useful in obtaining high resolution TERS image because the spatial resolution is dependent on the tip sharpness. We believe more precise control of the PRW is achievable by optimizing the process conditions such as vapor flow rate and oxidization time in oxidizing Si probe. In optimizing the condition, it will be necessary to measure more precisely the PRW shifts provided by different SiO2 thicknesses. In order to do this measurement, it is required to reduce the illuminating area and reduce the number of Ag grains down to a single-particle level to resolve plasmon resonance peak of Ag grain only at the tip apex. This can be done using total internal reflection (TIR) illumination technique in which only the evanescent waves interact with the tip apex [24, 25].
References and links
1. S. Kawata and V. M. Shalaev, Tip Enhancement (Elsevier, 2007).
2. N. Hayazawa, Y. Inouye, Z. Sekkat, and S. Kawata, “Metallized tip amplification of near-field Raman scattering,” Opt. Commun. 183, 333–336 (2000). [CrossRef]
3. R. M. Stöckle, Y. D. Suh, V. Deckert, and R. Zenobi, “Nanoscale chemical analysis by tip-enhanced Raman spectroscopy,” Chem. Phys. Lett. 318, 131–136 (2000). [CrossRef]
4. M. S. Anderson, “Locally enhanced Raman spectroscopy with an atomic force microscope,” Appl. Phys. Lett. 76, 3130–3132 (2000). [CrossRef]
5. N. Hayazawa, Y. Inouye, Z. Sekkat, and S. Kawata, “Near-field Raman scattering enhanced by a metallized tip,” Chem. Phys. Lett. 335, 369–374 (2001). [CrossRef]
6. B. Pettinger, G. Picardi, R. Schuster, and G. Ertl, “Surface-enhanced and STM-tip-enhanced Raman spectroscopy at metal surfaces,” Single Mol. 5, 285–294 (2002). [CrossRef]
7. B. Pettinger, B. Ren, G. Picardi, R. Schuster, and G. Ertl, “Nanoscale probing of adsorbed species by tip-enhanced Raman spectroscopy,” Phys. Rev. Lett. 92, 096101 (2004). [CrossRef] [PubMed]
8. 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, 095503 (2003). [CrossRef] [PubMed]
9. N. Hayazawa, T. Yano, H. Watanabe, Y. Inouye, and S. Kawata, “Detection of an individual single-wall carbon nanotube by tip-enhanced near-field Raman spectroscopy,” Chem. Phys. Lett. 376, 174–180 (2003). [CrossRef]
10. N. Anderson, A. Hartschuh, S. Cronin, and L. Novotny, “Nanoscale vibrational analysis of single-walled carbon nanotubes,” J. Am. Chem. Soc. 127, 2533–2537 (2005). [CrossRef] [PubMed]
11. Y. Saito, N. Hayazawa, H. Kataura, T. Murakami, K. Tsukagoshi, Y. Inouye, and S. Kawata, “Polarization measurements in tip-enhanced Raman spectroscopy applied to single-walled carbon nanotubes,” Chem. Phys. Lett. 410, 136–141 (2005). [CrossRef]
12. T. Yano, P. Verma, S. Kawata, and Y. Inouye, “Diameter-selective near-field Raman analysis and imaging of isolated carbon nanotube bundles,” Appl. Phys. Lett. 88, 093125 (2006). [CrossRef]
13. T. Ichimura, N. Hayazawa, M. Hashimoto, Y. Inouye, and S. Kawata, “Tip-enhanced coherent anti-Stokes Raman scattering for vibrational nanoimaging,” Phys. Rev. Lett. 92, 220801 (2004). [CrossRef] [PubMed]
14. A. Rasmussen and V. Deckert, “Surface- and tip-enhanced Raman scattering of DNA components,” J. Raman Spectrosc. 37, 311–317 (2006). [CrossRef]
15. N. Hayazawa, Y. Inouye, Z. Sekkat, and S. Kawata, “Near-field Raman imaging of organic molecules by an apertureless metallic probe scanning optical microscope,” J. Chem. Phys. 117, 1296–1301 (2002). [CrossRef]
16. B. Pettinger, B. Ren, G. Picardi, R. Schuster, and G. Ertl, “Tip-enhanced Raman spectroscopy (TERS) of malachite green isothiocyanate at Au(111): bleaching behavior under the influence of high electromagnetic fields,” J. Raman Spectrosc. 36, 541–550 (2005). [CrossRef]
17. B. S. Yeo, S. Mädler, T. Schmid, W. Zhang, and R. Zenobi, “Tip-enhanced Raman spectroscopy can see more: the case of cytochrome c,” J. Phys. Chem. C 112, 4867–4873 (2008). [CrossRef]
18. D. Mehtani, N. Lee, R. D. Hartschuh, A. Kisliuk, M. D. Foster, A. P. Sokolov, and J. F. Maguire, “Nano-Raman spectroscopy with side-illumination optics,” J. Raman Spectrosc. 36, 1068–1075 (2005). [CrossRef]
19. N. Hayazawa, M. Motohashi, Y. Saito, H. Ishitobi, A. Ono, T. Ichimura, P. Verma, and S. Kawata, “Visualization of localized strain of a crystalline thin layer at the nanoscale by tip-enhanced Raman spectroscopy and microscopy,” J. Raman Spectrosc. 38, 684–696 (2007). [CrossRef]
20. B. S. Yeo, W. Zhang, C. Vannier, and R. Zenobi, “Enhancement of Raman signals with silver-coated tips,” Appl. Spectrosc. 60, 1142–1147 (2006). [CrossRef] [PubMed]
21. C. C. Neacsu, J. Dreyer, N. Behr, and M. B. Raschke, “Scanning-probe Raman spectroscopy with single-molecule sensitivity,” Phys. Rev. B 73, 193406 (2006). [CrossRef]
22. K. F. Domke, D. Zhang, and B. Pettinger, “Toward Raman fingerprints of single dye molecules at atomically smooth Au(111),” J. Am. Chem. Soc. 128, 14721–14727 (2006). [CrossRef] [PubMed]
23. W. Zhang, B. S. Yeo, T. Schmid, and R. Zenobi, “Single molecule tip-enhanced Raman spectroscopy with silver tips,” J. Phys. Chem. C 111, 1733–1738 (2007). [CrossRef]
24. C. C. Neacsu, G. A. Steudle, and M. B. Raschke, “Plasmonic light scattering from nanoscopic metal tips,” Appl. Phys. B 80, 295–300 (2005). [CrossRef]
25. D. Mehtani, N. Lee, R. D. Hartschuh, A. Kisliuk, M. D. Foster, A. P. Sokolov, F. Čajko, and I. Tsukerman, “Optical properties and enhancement factors of the tips for apertureless near-field optics,” J. Opt. A 8, S183–S190 (2006). [CrossRef]
26. C. F. Bohren and D. R. Huffman, Absorption and Scattering of Light by Small Particles (Wiley, 1983).
27. M. D. Malinsky, K. L. Kelly, G. C. Schatz, and R. P. Van Duyne, “Nanosphere lithography: Effect of substrate on the localized surface plasmon resonance spectrum of silver nanoparticles,” J. Phys. Chem. B 105, 2343–2350 (2001). [CrossRef]
28. K. L. Kelly, E. Coronado, L. L. Zhao, and G. C. Schatz, “The optical properties of metal nanoparticles: The influence of size, shape, and dielectric environment,” J. Phys. Chem. B 107, 668–677 (2003). [CrossRef]
29. A. Taguchi, S. Fujii, T. Ichimura, P. Verma, Y. Inouye, and S. Kawata, “Oxygen-assisted shape control in polyol synthesis of silver nanocrystals,” Chem. Phys. Lett. 462, 92–95 (2008). [CrossRef]
30. S. J. Oldenburg, R. D. Averitt, S. L. Westcott, and N. J. Halas, “Nanoengineering of optical resonances,” Chem. Phys. Lett. 288, 243–247 (1998). [CrossRef]
31. E. Prodan, C. Radloff, N. J. Halas, and P. Nordlander, “A Hybridization Model for the Plasmon Response of Complex Nanostructures,” Science 302, 419–422 (2003). [CrossRef] [PubMed]
32. B. S. Yeo, T. Schmid, W. Zhang, and R. Zenobi, “Towards rapid nanoscale chemical analysis using tip-enhanced Raman spectroscopy with Ag-coated dielectric tips,” Anal. Bioanal. Chem. 387, 2655–2662 (2007). [CrossRef] [PubMed]
33. X. Cui, W. Zhang, B. S. Yeo, R. Zenobi, C. Hafner, and D. Erni, “Tuning the resonance frequency of Ag-coated dielectric tips,” Opt. Express 15, 8309–8316 (2007). [CrossRef] [PubMed]
34. A. Ono, K. Masui, Y. Saito, T. Sakata, A. Taguchi, M. Motohashi, T. Ichimura, H. Ishitobi, A. Tarun, N. Hayazawa, P. Verma, Y. Inouye, and S. Kawata, “Active control of the oxidization of a silicon cantilever for the characterization of silicon-based semiconductors,” Chem. Lett. 37, 122–123 (2008). [CrossRef]
36. K. S. Kunz and R. J. LuebbersThe Finite Difference Time Domain Method for Electromagnetics (CRC press, 1993).
37. E. D. Palik, Handbook of Optical Constants of Solids (Academic press, 1991).
38. S. M. Sze and K. K. Ng, Physics of Semiconductor Devices (Wiley, 2006). [CrossRef]
