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Size and Wavelength Dependence of the Charge-Transfer Contributions to Surface-Enhanced Raman Spectroscopy in Ag/PATP/ZnO Junctions Alexander P. Richter,† John R. Lombardi,*,‡ and Bing Zhao§ Studiengang Umwelt-, Verfahrens- und Biotechnik, MCI - Management Center Innsbruck, Internationale Fachhochschulgesellschaft mbH, Egger-Lienz-Straβe 120, A-6020 Innsbruck, Austria, Department of Chemistry, The City College of New York, New York, New York 10031, and State Key Laboratory of Supramolecular Structure and Materials, Jilin UniVersity, 2699 Qianjin AVenue, Changchun 130012, People’s Republic of China ReceiVed: October 12, 2009; ReVised Manuscript ReceiVed: December 7, 2009
By use of surface-enhanced Raman spectroscopy, we observe the degree of charge-transfer for Ag/PATP/ ZnO sandwich compounds as a function of both ZnO nanoparticle size and as a function of excitation wavelength. We show that there are several likely charge-transfer resonances. The most obvious is the resonance at particle diameter of 27.7 nm for all wavelengths. In a theoretical study it has been suggested that when there is an electron acceptor on the nanoparticle surface it may form a complex with the semiconductor exciton and that this is most likely the origin of the size-dependent resonance. At the smallest size (18.2 nm) studied here, there is an increase in degree of charge-transfer (relative to adjacent sizes), indicating the possibility of another, lower-lying charge-transfer state, which also could be caused by the acceptor-exciton complex. The other resonance suggested by our data is to higher excitation energy for all particle sizes. It can be seen that the degree of charge-transfer is rising as the excitation wavelength is shortened, indicating an additional charge-transfer resonance in the ultraviolet. 1. Introduction Considerable work has recently been carried out on the surface-enhanced Raman spectroscopy (SERS) to molecules adsorbed on ZnO nanoparticles. It has been shown that the Raman spectrum of certain molecules (4-mercaptopyridine (4MPy) and 4-mercaptobenzoic acid (4-MBA)) can be enhanced by several orders of magnitude by adsorption on ZnO nanoparticles1 and further that as the nanoparticle radius is varied the enhanced Raman intensity goes through an apparent resonance maximum2 at about 14 nm. Despite the fact that this particle radius is considerably larger than the exciton Bohr radius (about 1.5 nm), and therefore is unlikely to be due to quantum confinement effects,3 this has been interpreted as due to a size dependent charge-transfer transition. Further work has shown that certain sandwich compounds of ZnO thin films with the molecule p-aminothiophenol (PATP) and Ag colloidal nanoparticles display differential ability to undergo charge-transfer, depending on the relative orientation of the PATP with respect to the metal and semiconductor.4 Specifically it was found that when the PATP is attached to the ZnO by the thiol and the Ag by the -NH2 (ZnO/PATP/Ag) considerable charge transfer was observed, while in the reverse configuration, i.e., the PATP is attached to the Ag by the thiol and the ZnO by the -NH2 (Ag/ PATP/ZnO), charge-transfer was inhibited. The molecule in such a sandwich structure thus acts like a molecular rectifier. These measurements were facilitated by the striking observation, originally by Osawa et al.,5 that intensities of the nontotally symmetric vibrational bands of symmetry b2 were measures of the amount of charge-transfer between metal and molecule. It was found, for example, that with excitation at 1064 nm for * To whom correspondence should be addressed. E-mail: lombardi@ sci.ccny.cuny.edu. † Internationale Fachhochschulgesellschaft mbH. ‡ The City College of New York. § Jilin University.
Figure 1. SERS of PATP on Ag colloids at 514.5 and 1064 nm.
PATP on Ag nanoparticles there was almost no Raman intensity in the b2 bands; it was entirely in the totally symmetric a1 modes. On the contrary, with excitation at 514.5 nm, the b2 band intensity dominates the spectrum. See Figure 1. This observation is totally consistent with the results of Osawa, and the interaction between the CT resonance and the surface plasmon resonance (SPR) responsible for SERS has been explored in a recent article.6 It was shown that the intensity of nontotally symmetric modes (such as the b2 mode in PATP) is diagnostic of contributions to the enhancement from chargetransfer transitions. In that article, we also introduced a quantitative measure of the degree to which the charge transfer contributes to the overall SERS enhancement (pCT) by exploiting the relative intensities of nontotally symmetric to totally symmetric bands. It is found that by varying the conditions (such
10.1021/jp909772e 2010 American Chemical Society Published on Web 01/05/2010
SERS in Ag/PATP/ZnO Junctions as the location of the Fermi level of the metal, the laser excitation energy as in Figure 1, or the particle size as in ref 2) wide variations in pCT can be obtained.6,7 This suggests that PATP is an ideal molecule for measuring the amount by which charge transfer contributes to the overall SERS enhancement. Note that the considerable increase in SERS intensity in the sandwich compounds when compared with those with just PATP on ZnO is most likely due to the added plasmon resonance of the Ag particles. We should also point out that for PATP on Au nanoparticles a similar phenomenon has been observed,8 with the charge-transfer responsible for selective enhancement of b2 modes. However, the contribution of charge-transfer was found to be considerably smaller than for Ag nanoparticles. The work on Ag/PATP/ZnO sandwich compounds was originally carried out using thin films of ZnO, which are composed mainly of needlelike hexagonal rods of ZnO. The Ag particles were mostly colloidal spheres. Without the Ag, only an extremely weak signal was obtained for the molecule on ZnO. However, the signal was found to be somewhat larger on ZnO spherical nanoparticles,1 and these have the added advantage that it is possible to synthesize them in a relatively monodisperse size selective fashion.9 It would therefore be of considerable interest to carry out this study using colloidal ZnO nanoparticles (rather than thin films) and monitor the degree of charge transfer as a function of both the excitation wavelength and the ZnO particle size. Note that a very valuable study of SERS at metal/PATP/metal junctions has been carried out, enabling the visualization of orbital changes with varying amounts of charge transfer.10 In that study, tunneling at the rectifier junction was found to be responsible for the movement of charge. In this work we report on both of these phenomena. By utilization of the degree of charge-transfer as a measure, we follow the effect of varying both ZnO size and excitation wavelength on pCT for a range of wavelengths (475.6 to 1064 nm) and diameters (from 18.2 to 33.1 nm). The degree of charge transfer is chosen in such a way as to be independent of the total intensity and therefore measures only the degree to which the charge-transfer resonances contribute to the Raman signal. This parameter is then able to identify clearly the location of charge-transfer resonance within the ranges studied. We choose Ag/PATP/ZnO sandwiches and are able to compare the resonances obtained in both arrangements. 2. Experimental Section We performed two main studies, a wavelength-dependence study and a size-dependence study of ZnO. In the wavelengthdependence study, the degree of charge transfer, pCT(νi), in the sample Ag/PATP/ZnO (27.7 nm) at excitation wavelengths ranging from 476.5 to 676.4 nm was compared with one another. In the ZnO size-dependence study, we investigated six ZnO nanoparticle diameters ranging from 18.2 to 33.1 nm on pCT(νi) values in the sample sequence Ag/PATP/ZnO at different wavelengths and compared the pCT(νi) value of each sample with one another. We compared the degree of charge transfer, pCT(ν9b) value, of each sample with one another. Although the pCT obtained for all the b2 modes was about the same, since the ν9b line at about 1142 cm-1 is fairly isolated (see Figure 1), while the other nontotally symmetric bands are somewhat overlapped, we felt that line gave the most reliable and reproducible results. Thus in the following we report results on only one line. The results using the other two b2 lines were comparable. In the excitation-wavelength dependence study, the sample Ag/PATP/ZnO (27.7 nm) was investigated at the excitation
J. Phys. Chem. C, Vol. 114, No. 3, 2010 1611 wavelengths of 476.5, 488.0, 496.5, and 514.5 nm, obtained from an Ar-ion laser, and at the excitation wavelength of 676.4 nm, obtained from a Kr-ion laser. Both lasers used are Spectra Physics Beam Lok 2020 lasers. For a spectrometer, a Spex Model 1401 (0.75 m) double monochromator with photon counting was used. Scattered light was focused onto the slit of the spectrometer, and the slit widths were set at 150 µm, providing a resolution of about 6 cm-1. To enable comparisons of the obtained pCT(ν9b) values, all settings, including the laser power, were held constant. All spectra were recorded at 10 mW at each excitation wavelength. The recorded range was from 900 to 1700 cm-1 Raman shift. The degree of charge transfer (pCT(νi)) of the main vibrational mode 1142 ν9b b2 with respect to 1077 ν7a (a1) was calculated. In the size-dependence study, we investigated Ag/PATP/ZnO sandwich structures with ZnO sizes 18.2, 23.8, 25.2, 27.7, 30.6, and 33.1 nm on the degree of charge transfer (pCT(ν9b)) to find a size-dependent resonance region. The spectra of all six samples were recorded at the wavelengths 488, 633, 785, and 1064 nm. The reported range of interest in cm-1 was from 900 to 1700. All spectra of the size-dependence study were recorded at the lowest possible laser power at each excitation wavelength. By doing this, the sample distortion was minimized, as was SERS spectra distortion due to laser heat. A Bruker Senterra Raman microscope was used for the excitation wavelengths of 488 (0.25 mW laser power), 633 (0.20 mW laser power), and 785 nm (1.00 mW laser power). The system employed a charge-coupled device detector with a scanning time of 30 s. A 100× objective was used, and the aperture setting was 50 µm × 1000 µm. The resulting scanning resolution at each wavelength was 3-5 cm-1. For the laser line 1064 nm (5.00 mW laser power), a Bruker Ram II FT-Raman Vertex70, Nd:YAG with a 40× objective and a liquid N2-cooled Ge-detector was utilized. The aperture diameter was 5 mm. At each recording 64 runs were performed giving a resolution of 3-5 cm-1. A baseline correction was conducted for pCT(νi) value calculations at each spectra. Baseline correction was performed in OriginlLab8.0 according to Figure S1 of Supporting Information, showing the baseline-corrected spectrum of the sample Ag/PATP/ZnO(27.7 nm) obtained at 488-nm excitation wavelength. For comparison, the baseline anchor points were set constant along the abscissa of each spectrum pertaining to each excitation wavelength. At each excitation wavelength, the baseline was adapted slightly to the occurring background patterns. Silver colloids were prepared according to the literature protocol suggested by Lee and Meisel.11 Specifically, 90 mg of AgNO3 was dissolved in 500 mL of distilled H2O. The solution was heated to boiling temperature. Subsequently, 10 mL of 1% sodium citrate was added dropwise. After boiling for one hour, the Ag colloid was brownish. The concentration of the Ag solution was 7 × 10-4 mol/L. Transmission electron microscopy (TEM) investigations show Ag spheres (80%) and rods (20%) as depicted in Figure 2. The mean diameter of the Ag spheres was 30 nm, while the average length of the rods was about 60 nm. Therefore, the approximate particle size using spheres is 30 nm. By use of the initial Ag concentration of 7 × 10-4 mol/L, the silver particle density in solution was 3.2 × 1014 particles/L. To obtain samples of various-sized ZnO nanoparticles, the following sample preparation was conducted.12 To produce Zn(OH)2, 40.0 mL of 0.5 mol/L NaOH solution was added dropwise to 100 mL of 0.1 mol/L Zn(Ac)2 solution under vigorous stirring. Then, NH4HCO3 powder was added to the Zn(OH)2 precipitate and stirred for 30 min. A zinc-carbonate hydroxide colloid was obtained. Next, the colloid was filtered
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Figure 2. TEM image of Ag solution showing Ag spheres and nanorods.
with a G3 filter under reduced pressure and dried at 80 °C. As a result, the precursor of a small crystallite Zn5(CO3)2(OH)6 was formed. In the next step, the precursor was calcined at 350, 450, 550, 650, 750, and 850 °C for 2 h to obtain five sets of samples. These were washed and rinsed with deionized water and anhydrous alcohol and filtered with a G4 filter under reduced pressure three or four times. Finally, the products were dried at 70 °C. The ZnO nanoparticle sizes were estimated using the Scherrer formula.13 ZnO nanoparticle sizes of 18.2 (350 °C), 23.8 (450 °C), 25.2 (550 °C), 27.7 (650 °C), 30.6 (750 °C), and 33.1 nm (850 °C) were obtained. PATP was obtained from Sigma-Aldrich Inc. Maryland/USA. PAPT with a concentration of 1 × 10-2 mol/L in methanol solution was used for sample preparation. Ag/PATP/ZnO Sample Preparation. Two mL Ag solution (7 × 10-4 mol/L) and 1 mL PATP (1 × 10-2 mol/L) were mixed and sonicated for 10 min and put aside for 24 h. The following day the sample was rinsed with methanol and water. The sonication time at each rinsing run was 10 min. Then 2 mg ZnO nanoparticles were added. After the Ag/PATP/ZnO sample was sonicated for an additional 10 min, the excess water was removed and the sample dried on a silicon wafer. 3. Degree of Charge Transfer
Figure 3. ZnO size-dependence study at 488 nm excitation wavelength.
For determination of pCT(νi), it is important to record SERS spectra at several different excitation wavelengths.6 Figure 1 shows SERS spectra of PATP on Ag recorded at 514.5 and 1064 nm. At 1064 nm, there is no CT contribution, since the excitation wavelength is far from charge-transfer resonance. Therefore, all b2 lines are reduced in intensity significantly. We may therefore take Ik(SPR) ) 0. A strong a1 line like the one at 1077 cm-1 can normally be used as I0(SPR). However, in our studies, charge-transfer effects appear ubiquitous, in systems with ZnO quantum dots, so there is no wavelength at which only SPR contributes to the intensity. Furthermore, we cannot utilize the previous results for I0(SPR) of PATP on Ag alone, since the conditions are not comparable. We instead must take an approximation to eq 1, in which we take Ik(SPR) ) 0 and I0(SPR) ) I0(CT). In this latter we assume that the chargetransfer contribution to the totally symmetric mode is small and does not affect its total intensity very much. This is confirmed by the fact that the intensities of the totally symmetric lines are nearly constant in all our samples. We may then take the ratio
In addition to SPR and molecular transitions, charge transfer is the third main contributor to SERS signals. The calculation of the degree of charge transfer, pCT(k), of a k-line was performed according to literature with eq 1.6 The index “k” is used to identify individual lines in the SERS spectra
R)
Ib2 Ik(CT) ) 0 Ia1 I (CT)
(2)
so that the degree of charge-transfer becomes
pCT(k) )
Ik(CT) - Ik(SPR) Ik(CT) + I0(SPR)
pCT )
(1)
In this equation Ik(CT) is the intensity of a line where CT adds to the SERS intensity caused by surface plasmon resonances (Ik(SPR)), while I0(SPR) is the intensity of a chosen totally symmetric line in a region of the spectrum where only SPR contributes to the signal. The line (k) may be either a totally symmetric or nontotally symmetric line, but in the latter case, Ik(SPR) is usually quite small, and we may take it to be zero in many cases. Note pCT has the advantage that in a region of the spectrum where there is no charge ransfer, it is zero, while if the charge-transfer contribution to the intensity in much greater than the SPR contribution pCT approaches 1. An additional advantage is that it is independent of the total intensity, and therefore corrections need not be made for variations due to sample preparation, spectrometer alignment, or other factors.
R 1+R
(3)
It is this expression which will be used throughout this work for the degree of charge transfer. 4. Results In Figure 3 we show typical spectra of Ag/PATP/ZnO as a function of ZnO particle size, taken at 488 nm. For spectra taken at other wavelengths and details of baseline subtraction, determination of integrated intensities, and calculation of pCT, please see Supporting Information. Figure 3 illustrates the variation of the relative intensities of the nontotally symmetric (b2) lines (such as that at 1144 cm-1) as compared to the totally symmetric a1 lines (such as that at 1078 cm-1). In Figure 4 we show typical results for the spectra taken with ZnO particle diameter of 27.7 nm as a function of excitation
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Figure 5. Size-dependence study of pCT(ν9b) values as a function of excitation wavelength and ZnO nanoparticle size. Figure 4. Wavelength-dependence study for Ag/PATP/ZnO(27.7 nm); SERS spectra at 476.5, 488.0, 496.5, 514.5, and 676.4 nm excitation wavelength.
sized ZnO nanoparticles. This indicates the possible onset of a further resonance at smaller ZnO particle sizes than 18.2 nm.
TABLE 1: Degree of Charge-Transfer pCT Values of the ZnO Size-Dependence Study of the Samples Ag/PATP/ZnO (18.2-33.1 nm) at 488.0-1064 nm Excitation Wavelength as Depicted in Figure 5
5. Conclusion
wavelength [nm]
18.2
23.8
25.2
27.7
30.6
33.1
488 633 785 1064
0.63 0.53 0.40 0.29
0.62 0.49 0.37 0.25
0.64 0.48 0.43 0.33
0.67 0.58 0.49 0.44
0.62 0.52 0.49 0.30
0.60 0.50 0.44 0.26
wavelength. For spectra taken at other ZnO diameters and details of baseline subtraction, determination of integrated intensities and calculation of pCT see Supporting Information. As in Figure 3, Figure 4 illustrates the variation of the relative intensities of the nontotally symmetric (b2) lines (such as that at 1144 cm-1) as compared to the totally symmetric a1 lines (such as that at 1078 cm-1). Throughout all recordings we observed no significant frequency shifts for the peaks among the spectra obtained at different excitation wavelengths for each sample compared to the assigned peaks of the PATP molecule in literature protocol5 The calculated pCT(ν9b) values to Figure 4 can be found in Supporting Information. In Table 1 we list the resultant determination of all the pCT(ν9b) values of the ZnO sizedependence at different wavelengths. Note that the pCT values range from 0.25 to 0.67. This is considerably larger that those obtained with Au,8 which are on the order of 0.11. Figure 5 shows the size-dependence study of the sample Ag/PATP/ZnO with ZnO nanoparticle sizes from 18.2 to 33.1 nm along the z-axis, excitation wavelengths from 488.0 to 1064 nm along the x axis, and pCT(ν9b) values on the y axis (vertical). All pCT(ν9b) values are observed to decrease with increasing wavelength. This indicates the likelihood that at higher energy there is a charge-transfer resonance for all particle sizes. At each wavelength the investigated the Ag/PATP/ZnO sample with ZnO nanoparticle size of 27.7 nm gives the maximum pCT(ν9b) value and therefore indicates a size-dependent charge-transfer resonance at this particle size. The pCT(ν9b) values decrease toward larger ZnO nanoparticle sizes up to 33.1 nm and toward smaller ZnO nanoparticle sizes down to 23.8 nm. For the sample 23.8 nm throughout all wavelengths the lowest pCT(ν9b) values are observed. Sample 18.2 nm shows significantly higher pCT(ν9b) values compared to the adjacent sample with 23.8 nm-
We have obtained a measure of the degree of charge-transfer for Ag/PATP/ZnO sandwich compounds as a function of both ZnO nanoparticle size and as a function of excitation wavelength. It is clear from the results (Figure 5) that there are several likely charge-transfer resonances. The most obvious is the resonance at a particle diameter of 27.7 nm for all wavelengths. This is at exactly the same particle size as was observed in sizedependent resonance in systems composed of several other molecules (namely 4-mercaptopyridine (4MPy) and 4-mercaptobenzoic acid (4-MBA)) adsorbed on ZnO nanoparticles.2 In the latter study, no Ag colloid was added, as it is in this study. Furthermore, since only the total Raman intensity was examined, it could not be firmly established that the resonance was a charge-transfer resonance. In the present study, the results are independent of the total intensity, and we measure only the degree of charge transfer so that all resonances observed must be of the charge-transfer variety. Thus in this study we have firmly established that the size dependent resonance of the previous study is indeed charge transfer in nature. In a theoretical study3 Fonoberov and Balandin suggested that when there is an electron acceptor (in our case PATP) on the nanoparticle surface it may form a complex with the semiconductor exciton. PATP, 4MPy, and 4-MBA all have a low-lying unfilled π* orbital, which can readily act as an electron acceptor. Their calculations show there exists a charge-transfer state such that the difference between lowest excited energy levels of an acceptor-exciton complex and the ground state increases with particle size. Although they did not extend their calculations beyond particle sizes of 6 nm (diameter), we suggest that these can be extrapolated to larger sizes and provide a likely explanation for our observations. Note that at the smallest size (18.2 nm) studied here there is an increase in degree of charge transfer (relative to adjacent sizes), indicating the possibility of another, lower-lying charge-transfer state, which also could be caused by the acceptor-exciton complex, although this can only be proven by continuation to still smaller particle sizes. The other resonance suggested by our data is to higher excitation energy for all particle sizes (see Figure 5). It can be seen that the degree of charge-transfer is rising as the excitation wavelength is shortened, indicating an additional charge-transfer resonance in the ultraviolet. As the ultraviolet is approached, there is a resonance with the semiconductor exciton, allowing for electron transfer from the conduction band to the unfilled
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molecular π* orbital. Note this is not the same resonance as discussed in the previous paragraph, which is due to the formation of an acceptor-exciton complex. To our knowledge, this work represents the first study in which both the size dependence and wavelength dependence of the degree of charge-transfer are measured in a semiconductor quantum dot system. We have shown that the previously observed size dependence may be ascribed to charge-transfer, and elucidated several other likely resonances which may be observed with further work. Acknowledgment. We would like to thank Dr. J. Yang, Richard Livingstone, Alexandre Cassidy, and Faiza Anwar from the Center for Analysis of Structures and Interfaces (CASI) for their assistance in the project. For financial support, we are indebted to the National Institute of Justice (Department of Justice Award No. 2006-DN-BX-K034) and the City University Collaborative Incentive Program (No. 80209). This work was also supported by the National Science Foundation under Cooperative Agreement No. RII-9353488, Grant No. CHE0091362, CHE-0345987, and Grant No. ECS0217646 and by the City University of New York PSC-BHE Faculty Research Award Program. This research was also supported by the NIH/ NIGMS/SCORE Grant NO. GM08168 and an NCSA Grant CHE050065 for computer facilities. Scientific research work at The Metropolitan Museum of Art was supported in part by grants from the Andrew W. Mellon Foundation, the David H. Koch Family Foundation, and the National Science Foundation Grant IMR 0526926 (supplied the Bruker Senterra/Ramanscope combined Dispersive Raman/FT-Raman spectrometer). We thank the Marshall Plan Foundation/Austria, which provided support to one of us (A.R.) during his stay at City College. Supporting Information Available: Baseline-corrected spectrum of the sample Ag/PATP/ZnO (27.7nm) recorded at 488 nm, pCT(νi) values in the sample Ag/PATP/ZnO (27.7 nm) at five different excitation wavelengths, ZnO size-dependence study at 488 nm excitation wavelength, ZnO sizedependence study at 633 nm excitation wavelength, ZnO size-
Richter et al. dependence study at 785 nm excitation wavelength, ZnO sizedependence study at 1064 nm excitation wavelength, excitation wavelengths 488 nm and pCT(νi) values excitation wavelength 633 nm, pCT(νi) values in the sample Ag/PATP/ZnO at six different ZnO sizes, excitation wavelengths 785 nm and pCT(νi) values in the sample Ag/PATP/ZnO at six different ZnO sizes, and excitation wavelength 1064 nm and pCT(νi) values in the sample Ag/PATP/ZnO at six different ZnO sizes. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Wang, Y.; Ruan, W.; Zhang, J.; Yang, B.; Xu, W.; Zhao, B.; Lombardi, J. R. Direct Observation of Surface-Enhanced Raman Scattering in ZnO Nanocrystals. J. Raman Spectrosc. 2009, 40, 1072. (2) Sun, Z.; Zhao, B.; Lombardi, J. R. ZnO Nanoparticle Size-dependent Excitation of Surface Raman Signal from Adsorbed Molecules: Observation of a Charge-Transfer Resonance. Appl. Phys. Lett. 2007, 91, 221106. (3) Fonoberov, V. A.; Balandin, A. A. Appl. Phys. Lett. 2004, 85, 5971. Fonoberov, V. A.; Alim, K. A.; Balandin, A. A.; Xiu, F.; Liu, J. Phys. ReV. B 2006, 73, 165317. (4) Sun, Z.; Wang, C.; Yang, J.; Zhao, B.; Lombardi, J. R. Nanoparticle Metal-Semiconductor Charge Transfer in ZnO/PATP/Ag Assemblies by Surface-Enhanced Raman Spectroscopy. J. Phys. Chem. C 2008, 112, 6093– 6098, 10.1021/jp711240a. (5) Osawa, M.; Matsuda, N.; Yoshii, K.; Uchida, I. Charge Transfer Resonance Raman Process in Surface-Enhanced Raman Scattering from p-Aminothiophenol Adsorbed on Silver: Herzberg-Teller Contribution. Phys. Chem. C. 1994, 98, 12702–12707. (6) Lombardi, J. R.; Birke, R. L. A Unified Approach to SurfaceEnhanced Raman Spectroscopy. Phys. Chem. C. 2008, 112, 5606–5617. Lombardi, J. R.; Birke, R. L. Acc. Chem. Res. 2009, 42, 734–742. (7) Chenal, C.; Birke, R. L.; Lombardi, J. R. Determination of the Degree of Charge-Transfer Contributions to Surface Enhanced Raman Spectroscopy. ChemPhysChem. 2008, 9, 1617–1623, DOI: 10.1002/ cphc.200800221. (8) Yoon, J.; Park, J. S.; Yoon, S. Langmuir 2009, 21, 12475–12480. (9) Jing, L. Q.; Xu, Z. L.; Shang, J.; Sun, X. J.; Cai, W. M.; Guo, H. C. Mater. Sci. Eng. 2002, A 332, 356. (10) Sun, M.; Xu, H. ChemPhysChem 2008, 10, 392–399. (11) Lee, P. C.; Meisel, D. J. Phys. Chem. 1982, 86, 3391–3395. (12) Jing, L. Q.; Xu, Z. L.; Shang, J.; Sun, X. J.; Cai, W. M.; Guo., H. C. Mater. Sci. Eng 2002, A 332, 356. (13) See EPPS Document No. E-APPLAB-91-041748 for XRD pattern of the ZnO NPs calcined at different temperatures.
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