Thickness-Dependent Morphologies and Surface-Enhanced Raman

May 20, 2011 - Luo , W.; van der Veer , W.; Chu , P.; Mills , D. L.; Penner , R. M.; Hemminger , J. C. J. Phys. Chem. C 2008, 112, 11609. [ACS Full Te...
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Thickness-Dependent Morphologies and Surface-Enhanced Raman Scattering of Ag Deposited on n-Layer Graphenes Haiqing Zhou,†,‡ Caiyu Qiu,†,‡ Fang Yu,†,‡ Huaichao Yang,†,‡ Minjiang Chen,†,‡ Lijun Hu,†,‡ and Lianfeng Sun*,† † ‡

National Centre for Nanoscience and Technology, Beijing 100190, China Graduate School of Chinese Academy of Sciences, Beijing 100049, China

bS Supporting Information ABSTRACT: After thermal deposition of silver films onto n-layer graphenes, the following results have been obtained. First, the dependence of silver morphologies on the layer number is studied via controlling the sample temperature at 298, 333, and 373 K. This can be attributed to the changes in surface properties and/or surface diffusion coefficient of n-layer graphenes at different temperatures. Second, Raman scattering of n-layer graphenes is greatly enhanced after Ag deposition and the enhancement factors depend on the layer number of n-layer graphenes. Monolayer graphene has the largest enhancement factors, and the enhancement factors decrease with layer number increasing. For graphite, almost no enhancement effect has been detected. Third, the dependences of the enhancement factors on laser wavelength, thickness, and morphologies (nanoparticle size and spacing) of silver film are also studied. The Raman enhancement observed here is mainly attributed to the coupled surface plasmon resonance (SPR) absorption of silver nanoparticles.

1. INTRODUCTION Surface-enhanced Raman scattering (SERS) was unexpectedly discovered in 1974 by Fleischmann et al.1 Usually, due to the strong surface plasmon resonance (SPR) of the metal (normally gold or silver) nanoparticles, metal substrates (Au, Ag, and Cu), either with rough surfaces or in the form of nanoparticles, can be used as SERS substrates with enhancement factors as large as 1010 1015.2 4 Thus, these metal substrates play significant roles in detecting organic components at the single molecular level.5 7 Recently, with the development of nanofabrication and the understanding of the plasmonic properties of nanomaterials, SERS is expected to be one of the powerful and ultrasensitive analysis methods to be applied in various fields, such as electrochemistry, analytical chemistry, chemical physics, solid state physics, biophysics, and even medicine.1,8 10 As the enhancement may be remarkably long ranged and dependent on the substrate morphology, the Raman signal of the molecules attached to metallic nanostructures can be enhanced by many orders of magnitude, so it is very efficient and useful to apply the surface-enhanced Raman scattering to study or detect carbon-based materials using rough gold or silver surfaces.11 13 For example, after the decoration of gold nanoparticles with controlled size and interparticle distance onto surface-grown SWCNTs, every SWCNT on the substrate can be fully detected and in situ characterized.13 However, the limitation of these famous SERS substrates is that the metal substrates with smooth r 2011 American Chemical Society

surfaces are not active for Raman enhancement, and they are nearly planar and thereby have limited surface area. Compared with the ordinary SERS metal substrates, carbon-based materials have potential advantage in improving the SERS signals and the corresponding detection sensitivity because of their nanoporous surfaces with huge surface area.14 16 As a typical example of truly two-dimensional materials with only a honeycomb sheet of sp2bonded carbon atoms, graphene can provide the ideal prototype to be effectively used as SERS substrates to enhance the Raman signals of the adsorbed organic molecules, which can be attributed to the charge transfer between graphene and the molecules resulting in a chemical enhancement.17 19 That is to say, this Raman enhancement can be mainly related to the short-range or chemical mechanism (CM) regarding on the electronic structure of adsorbed molecules. Generally speaking, two classes of SERS mechanisms, electromagnetic and chemical, are widely accepted. Compared to the chemical mechanism, electromagnetic mechanism (EM) is really a long-range effect and dependent on the enhanced electromagnetic fields generated at the interface between the investigated molecules and the metal substrates with appropriate morphologies. The SERS of graphene resulting from Received: December 31, 2010 Revised: April 6, 2011 Published: May 20, 2011 11348

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The Journal of Physical Chemistry C the SPR-mediated field enhancement has been reported recently.20 22 For instance, Fred Schedin et al. have found the significant enhancement at 633 nm for graphene deposited with well-defined Au particle arrays.20 Indeed, metallic dots can be well-defined via electron beam lithography. However, due to the poor adhesion of Cr/Au dots on graphene and the possible existence of capillary forces, metallic dots are slightly shifted from previously lithography-defined positions during lift-off process. Thereby, these well-defined metallic dots cannot give us the general insights into SERS of graphene, as these dots are not directly modulated by the surface of n-layer graphenes. Actually, the morphologies of gold on pristine n-layer graphenes are dependent on the layer number,23 indicating different surface properties of graphenes. Since the surface diffusion coefficient and barrier are different among n-layer graphenes, metal film morphologies can be greatly related to the layer number.23,24 Therefore, to well understand the graphenebased SERS from EM mechanism, experimental verification of the relationship between Raman enhancement and n-layer graphenes is still necessary. In this work, we have thermally deposited 2 or 5 nm silver films onto n-layer graphenes. Using the combined techniques of scanning electron microscopy (SEM) and Raman spectroscopy, we have got the following results. First, the dependence of silver morphologies on the layer number is not obvious when the sample is kept at room temperature. However, with the increase in the sample temperature, the thickness-dependent silver morphologies become much more distinct. These can be ascribed to the change of the surface properties and/or surface diffusion coefficient of n-layer graphenes at different temperatures. Second, surface-enhanced Raman scattering of n-layer graphenes is observed after thermal deposition of silver films. The corresponding enhancement factors are dependent on the layer number with monolayer graphene having the biggest enhancement factor at the laser wavelength of 514 nm, and Raman enhancement of the G and 2D peaks is very different. Finally, further investigations prove that the observed Raman enhancement is greatly related to laser wavelength, film thickness, and film morphologies. Therefore, we attribute the observed Raman enhancement mainly to the coupled surface plasmon resonance (SPR) absorption of silver nanoparticles.

2. EXPERIMENTAL METHODS Graphene flakes, which were mechanically exfoliated25,26 from natural graphite (Alfa Aesar) using Scotch transparent tape 600 (3M), were transferred onto the silicon substrate with a 300 nm layer of SiO2.23 According to the color contrast of optical microscope (Leica DM4000) and Raman spectra of n-layer graphenes,27 the layer number of graphenes can be definitely confirmed. With a similar method to our previous work,23 SERSactive metals (Au and Ag) were then thermally deposited onto nlayer graphenes. The morphologies of silver film were characterized by SEM in detail. The Raman measurements were carried out using a microRaman spectroscopy (Renishaw inVia Raman Spectroscope) under ambient conditions with laser wavelengths of 514.5 nm (2.41 eV) and 633 nm. The laser power was kept below 0.5 mW, and a 100 objective lens with a numerical aperture of 0.90 can offer us a ∼1 μm laser spot size. Several spectra were collected to ensure the credibility and repeatability of the results.

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Figure 1. Thickness-dependent morphologies of 5 nm Ag on n-layer graphenes with the sample kept at a controlled temperature. Scale bar: 1 μm. (a, b) Silver morphologies deposited on monolayer and bilayer graphene at 298 K. (c, d) Silver morphologies on monolayer and bilayer graphene at 333 K. (e, f) Silver morphologies on monolayer and bilayer graphene at 373 K.

3. RESULTS AND DISCUSSION It is reported that the size of Ag nanoparticles and the interparticle distance can have great effects on the electromagnetic SERS enhancement.28,29 Therefore, to better investigate these effects on the SERS of n-layer graphenes, it is necessary to study the growth and nucleation of Ag nanoparticles on n-layer graphenes. With the application of a power film resistor (MP 9100), we can keep our substrate at a controlled temperature (298 448 K) higher than room temperature (Supporting Information). Here, we have thermally deposited 5 nm Ag films onto n-layer graphenes at 298, 333, and 373 K. The corresponding silver morphologies are displayed in Figure 1. When 5 nm Ag is thermally deposited onto graphenes with the substrate at 298 K, the differences in the Ag morphologies on monolayer and bilayer graphenes are not obvious (Figure 1a,b). Compared with monolayer graphene, only a very little larger interparticle distance and bigger size of Ag grains can be found for bilayer graphene or graphite (Figures S1 and S2), which can help us to understand the larger Raman enhancement found on 2 or 5 nm Ag-decorated monolayer graphene than bilayer graphene. As the temperature of the substrate increases to 333 K, after 5 nm Ag deposition, the density of Ag nanoparticles on monolayer graphene is larger than that on bilayer graphene, and the interparticle distance becomes lower, but the particle size is almost the same (Figure 1c,d). However, when the temperature 11349

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Figure 2. Raman spectra of pristine (a) and 5 nm Ag-covered (b) n-layer graphenes. From the intensities and shapes of the G (∼1580 cm 1) and 2D (∼2700 cm 1) peaks, surface-enhanced Raman scattering of n-layer graphenes can be observed obviously, which is dependent on the layer number.

comes to 373 K, not only the density of Ag particles, the particle size, but also the interparticle distance is very different among nlayer graphenes (Figure 1e,f and Supporting Information Figure S3). For example, it is easy to find that the sizes of Ag particles on monolayer graphene are much smaller than those on bilayer graphene. Consequently, by means of keeping the substrate at controlled temperatures, we have found that the silver morphologies are dependent on the layer number, which is consistent with our recently reported work.23 It is well-known that the dependence of silver morphologies on n-layer graphenes can be well explained by thermodynamic (e.g., energetics and stability) and kinetic (e.g., surface diffusion) factors as in the situation of gold.23,24 Meanwhile, the changes in the silver morphologies at different temperatures can be attributed to the varying of surface free energy or surface diffusion coefficient with temperature.23,30 To well understand this standpoint, we should consider the relationship between surface diffusion coefficient (D) and the temperature (T), which can be described by D µ exp( En/KT), and Ag particle density N µ (1/D)1/3, where K is the Boltzmann constant and En is surface diffusion barrier of Ag on n-layer graphenes. Thereby, we can get the equation N µ exp(En/3KT), which mainly depicts the connection of Ag particle density with temperature. With the rise of temperature, the surface diffusion coefficient (D) increases, which means that an arriving atom has a high probability to find an existing island, rather than form a new island, resulting in the decrease (increase) of Ag particle density (size). Therefore, with sample temperature increasing, the thicknessdependent morphologies of silver on graphenes become much more distinct. Using Raman spectroscopy, we have found different SERS effects of Ag on n-layer graphenes. In order to compare the different enhancement factors among n-layer graphenes, Raman spectra of pristine and 5 nm Ag-covered n-layer graphenes measured at 514 nm are shown in Figure 2. As is well-known, there are two prominent Raman peaks. One is the G peak at ∼1580 cm 1, while the other is the 2D peak at ∼2700 cm 1. According to these Raman spectra in Figure 2, we can find several features. On one hand, the intensities of the G and 2D band for monolayer or bilayer graphene both increase significantly after Ag decoration (Supporting Information Figure S4). Also, it is not difficult to find that the G band intensity is the largest for 5 nm Ag-decorated monolayer graphene and decreases with layer number increasing, which is very contrary to the previous report on the Raman spectra of pristine n-layer

graphenes (G peak intensity increases with the layer number increasing).31 Meanwhile, there is an obvious decrease in 2D band intensity ranging from 5 nm Ag-covered monolayer, bilayer, trilayer to four layer graphenes. Furthermore, these features are also observed in Raman spectra of Ag-deposited graphenes excited by 633 nm (Supporting Information Figure S5). Therefore, considering the previously reported increase in G band intensity with the number of pristine graphene layers,31 and nearly equivalent 2D band intensity among pristine n-layer graphenes except monolayer graphene in our experiment, it is reasonable for us to conclude that Raman scattering of monolayer graphene is enhanced with the largest enhancement factor among n-layer graphenes (Supporting Information Figure S6), indicating the strong interaction of monolayer graphene with Ag, while for graphite or very thicker graphene, the enhancement factors for G and 2D bands are nearly as large as 1 (Supporting Information Figure S7). And the enhancement factors of the G and 2D bands both decrease with the layer number increasing. On the other hand, Raman intensities of G and 2D bands are enhanced quite differently for monolayer or bilayer graphene, suggesting that the Raman enhancement factors between the G and 2D peaks are different from each other. The enhancement factor for the G band is much larger than that for the 2D band (Supporting Information Figure S6), which is discussed in the following. Furthermore, the strong surface enhancement may be greatly related to the coupled surface plasmon resonance absorption of the Ag nanoparticles decorated on graphenes. Thus, from the appearance of the D peak as well as the great change in the shape of Raman spectrum collected on Ag-covered monolayer graphene, the electromagnetic field generated at the interface between monolayer graphene and silver film is much stronger than other n-layer graphenes. One of the important issues here is why monolayer graphene interacts quite stronger with Ag than other n-layer graphenes. We should consider the layer-dependent surface diffusion of metal on graphenes, which is greatly related to the surface diffusion barrier of n-layer graphenes.23,24,32 In view of the Ag morphologies on graphenes (Figure 1 and Figures S1 S3), together with the equation N µ exp(En/3KT), we can infer that monolayer graphene has the largest surface diffusion barrier (En) and lowest surface diffusion coefficient (D) for Ag nanoparticles, indicating monolayer graphene has the largest ability to prevent Ag particles from moving along graphene surface, which means the strongest interaction between Ag and monolayer graphene.33 Moreover, compared with other n-layer graphenes, for monolayer graphene, 11350

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Figure 3. Comparison of Raman enhancements in the Raman spectra of monolayer and bilayer graphene at different laser wavelengths. (a, b) Raman enhancements of monolayer and bilayer at 514 or 633 nm, respectively. (c, d) Comparison of Raman enhancements at 514 and 633 nm observed on monolayer (c) or bilayer (d) graphene. The black- and red-marked Raman spectra are corresponding to those of pristine and Ag-decorated n-layer graphenes, respectively.

it is much more obvious to observe the change in G peak shape as well as the appearance of D peak, and almost no Raman enhancement is observed for Ag-decorated graphite or thicker graphene. Quite interesting, after 2 or 5 nm Ag is thermally deposited onto n-layer graphenes at room temperature, the morphologies of Ag film on monolayer, bilayer, trilayer, and four-layer graphene are almost the same (Supporting Information Figure S8). The Raman enhancing effects are related to several factors: the morphologies of metal (size, shape, and density), the thickness of graphenes, and their interactions. For the morphologies of Ag nanoparticles on graphenes show little difference at room temperature, the main reason for this different Raman enhancing effect is believed to originate from the different thickness of the graphenes and their interactions. On the basis of these investigations, we think Raman spectra of n-layer graphenes have been greatly enhanced by Ag deposition, and monolayer graphene interacts much stronger with Ag compared with other n-layer graphenes. So what is the mechanism of these unique features? To have a better understanding of the mechanism, controlled experiments with other laser wavelength 633 nm are carried out. The differences in Raman spectra measured at 514 or 633 nm for 2 nm Ag-covered monolayer or bilayer graphene are obvious (Figure 3a,b). More than 10 spectra on n-layer graphenes have been taken with each laser wavelength before and after Ag decoration. For clarity, we define the ratio (IAg-covered/Ipristine) as the enhancement factor to investigate the corresponding Raman enhancement. On this basis, it is distinct to observe that the

enhancement factors for the G and 2D peaks of monolayer or bilayer graphene obtained at 514 nm are much larger than those acquired at 633 nm. That is, for monolayer graphene at 514 nm excitation, the enhancement factors are 49.5 and 17.9 for G and 2D peaks, respectively, while at 633 nm, the factors are 15.9 and 8.3 for G and 2D peaks, respectively (Figure 3c and Supporting Information Table S1). Meanwhile, the enhancement factors of the G and 2D bands for bilayer graphene at 514 nm are still much larger than those at 633 nm correspondingly (Figure 3d and Supporting Information Table S1). Lastly, we still find that the G-band Raman intensity is increased much larger than the 2D band. To well account for this conclusion, we should take these two prominent features into consideration. One is the ratios between the intensities of the G and 2D peaks (I2D/IG) in Raman spectra of pristine n-layer graphenes, which are more or less 2 4 and 1 for monolayer and bilayer graphene, respectively. The other is still the corresponding ratios (I2D/IG) obtained after 2 nm Ag decoration. For monolayer graphene, the ratio is approximate to 1.4. For bilayer graphene, the ratio is 0.6, which is much smaller than 1. So we can conclude that the enhancement factor for the G band is much larger than the 2D band. As a result, the enhancement strength on the Raman scattering of n-layer graphenes is dependent on the excitation wavelength, which may be mainly attributed to the decreasing coupling absorbance of the surface plasmon resonance (SPR) on the Ag surface.34,35 To further study the effect of coupled SPR absorption of Ag grains on the Raman scattering of n-layer graphenes, we have compared the enhancement effects of 2 and 5 nm Ag on the graphene samples. Similar to the results of 2 nm Ag-decorated n-layer graphenes, Raman spectra of n-layer graphenes are still 11351

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The Journal of Physical Chemistry C obviously enhanced with 5 nm Ag deposited on them (Figure 4). However, for monolayer or bilayer graphene decorated with 5 nm Ag film, the relevant Raman spectra are enhanced at a larger scale. Thus, a greater G or 2D band Raman intensity can be obtained than those in the condition of 2 nm Ag (Supporting Information Table S2). These results can be figured out in this way: Compared with 2 nm Ag deposition, a larger density of Ag grains on n-layer graphenes can be formed after 5 nm Ag deposition. So it is much more probable for two adjacent Ag grains to keep at a low distance and the Ag surface to be much rougher, which changes the SERS hot spot distributions. This results in higher localized electric fields, and then a large enhancement factor can be induced.36,37 Therefore, it further confirms that the strong SERS enhancement of Ag-decorated n-layer graphenes mainly derives from the electromagnetic enhancement caused by the coupled SPR absorption of decorated silver nanoparticles with high density and small interparticle distance. To further investigate the EM mechanism on the Raman scattering of Ag-decorated n-layer graphenes, we have collected the relevant Raman spectra of n-layer graphenes which are deposited with 5 nm Ag at temperatures 298, 333, and 373 K (Figure 5 and Supporting Information Figure S9). With the temperature increasing, the Ag particles grow in a large size. To the best knowledge of us, the optimum size of Ag nanoparticle for

Figure 4. Surface-enhanced Raman scattering of monolayer and bilayer graphene after thermal deposition of 2 or 5 nm Ag film. After thermal deposition of 5 nm Ag film onto n-layer graphenes, the corresponding Raman scattering is enhanced much larger than that in the situation of 2 nm Ag film.

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SERS activity is around 100 nm.28,29 That is to say, when the size is below 100 nm, the SERS signal increases with an increase in the size. This is well consistent with our results: When the temperature increases from 298 to 333 K, the Ag particle size increases, and the interparticle distance is almost unchanged. Thus, at 514 nm laser excitation, a larger enhancement factor for 5 nm Agdecorated monolayer or bilayer graphene can be obtained at 333 K than those at 298 K. However, compared with the silver morphologies at 298 and 333 K, when the temperature comes to 373 K, although the size of Ag particles become much larger and still smaller than 100 nm, the corresponding enhancement factor become smaller. This may be due to the broader interparticle gaps under this condition. As is concerned, the individual or large spaced nanoparticles can seldom exhibit large SERS enhancement. Contrarily, when the interparticle gap becomes narrow, a high concentration of the so-called “hot spots” can be formed between nanoparticles. Thus, the SERS intensity can be greatly enhanced by the electromagnetic interaction or coupling between metal nanostructures.38 40 It is reported that the SERS enhancement of the interacting metal nanoparticles is about 5 6 orders of magnitude exceeding that of the isolated nanoparticles,41 which can be used to well account for the weaker Raman enhancement than that at 298 and 333 K. Meanwhile, compared with 633 nm, Raman spectra collected at 514 nm are enhanced a little larger (Figure S9). In a word, to well understand these Raman enhancements, the size and interparticle spacing of silver on n-layer graphenes should be taken into consideration. It is suggested that plasma resonance in metal nanostructures is highly sensitive to the subtle difference in their shape and size. Thereby, the silver morphologies are also of crucial importance for the demonstration of SERS enhancement. As discussed above, EM is related to the local electromagnetic field, which is greatly dependent on the species of SERS-active metals. Hence, the efficiency of Raman enhancement can be greatly affected by different metals. Here, the enhanced effects of 2 nm Ag or 2 nm Au on the Raman spectra of monolayer or bilayer graphene at 514 and 633 nm are shown in Figure 6. After thermal evaporation of 2 nm Au onto graphenes, there is almost no Raman enhancement found on these Au-deposited nlayer graphenes using 514 nm laser excitation. However, for 2 nm Ag-decorated monolayer or bilayer graphene, the relevant Raman scattering is enhanced at larger enhancement factors. That is, the G and 2D band intensities in the Raman spectra of Ag-decorated n-layer graphenes are enhanced much more

Figure 5. Surface-enhanced Raman scattering measured on monolayer (a) and bilayer (b) graphene excited by 514 nm after 5 nm Ag thermally deposited onto the sample kept at controlled temperatures (298, 333, and 373 K). Different Raman enhancements can be found in these Raman spectra, which are marked with different colors (black, red, and blue at 298, 333, and 373 K, respectively). 11352

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Figure 6. Surface-enhanced Raman scattering measured on 2 nm Ag or 2 nm Au decorated monolayer and bilayer graphene using 514 nm (a) or 633 nm (b) laser excitation. The black-, red-, and blue-marked Raman spectra correspond to those measured on pristine, 2 nm Au-decorated, and 2 nm Agdecorated n-layer graphenes, respectively.

obviously than those of Au decoration (Figure 6a). Compared with 514 nm, for 633 nm, Raman intensities of G and 2D peaks for graphenes after Au decoration are greatly enhanced, which is much more obvious than those after Ag deposition (Figure 6b). Thus, the different surface plasmon resonance (SPR) of Au and Ag at the laser wavelength of 514 or 633 nm is probably the main factor to well explain our experimental results. The second important issue here is why Raman enhanced efficiency decreases with layer number increasing and monolayer graphene has the largest Raman enhanced efficiency. First of all, surface diffusion coefficients of Ag on graphenes are different, which results in different particle size, density, and interparticle distance of Ag. These changes can lead to different behaviors on Raman enhancement. Second, surface plasmon resonance of Ag nanoparticles plays dominant role in enhancing the Raman signal of the adsorbed molecules by the long-range electromagnetic mechanism. This mechanism can be confirmed from the dependence of SERS effect of n-layer graphenes on the laser excitation (Figures 3 and 6, Supporting Information Figure S6). Third, it is necessary to consider the weak interlayer interaction among graphite layers or the thickness of graphenes and different interaction between Ag and graphenes.42 Fourthly, the screening of electromagnetic field by the top graphene layer is very weak, as the light transmittance of n-layer graphenes is proportional to 1 nπR, where R= e2/pc ≈ 1/137.43 Lastly, compared with pristine graphenes, for 2 or 5 nm Ag-decorated graphenes, the full width at half-maximum (FWHM) as well as intensity ratios of G and 2D peaks and G band shape have taken great changes, meaning that charge transfer is induced between Ag and graphene.44

4. CONCLUSIONS In present work, with the sample kept at a controlled temperature, after thermal deposition of 2 or 5 nm silver film onto n-layer graphenes, we first find that the morphologies of silver on n-layer graphenes are dependent on the layer number. With the layer number increasing, the density of Ag nanoparticles decreases, and the particle size becomes bigger. Meanwhile, compared with those of pristine n-layer graphenes, Raman enhancement has been observed obviously on the Raman spectra of Ag-decorated n-layer graphenes, which also depends on the layer number and is quite different for the G and 2D peaks. Namely, the enhancement factor of the G band is much larger

than that of the 2D band, and both become smaller as the layer number increases. In addition, for graphite, almost no enhancement effect has been detected. To figure out the origin of SERS, controlled experiments have been carried out. We find that the Raman enhancement can be greatly affected by the laser wavelength, thickness, and morphologies (nanoparticle size and spacing) of silver film. Therefore, we propose that the coupled surface plasmon resonance (SPR) absorption of silver nanoparticles decorated on n-layer graphenes contributes most to the strong surface enhancement, and layer-dependent Raman enhanced efficiency may be related to Ag morphologies, the thickness of graphenes, and their interaction with Ag. This work may give us some new insights into graphene-based microanalysis, and it is useful for exploiting and designing the best SERS substrate to characterize molecules with low concentration.

’ ASSOCIATED CONTENT

bS

Supporting Information. Modulation of the substrate temperature, morphologies of 5 nm Ag deposited on graphite, AFM images of 5 nm Ag on n-layer graphenes at 298 K, thickness-dependent morphologies of 5 nm silver on n-layer graphenes, Raman spectra of pristine and 5 nm Ag-decorated monolayer or bilayer graphene, surface-enhanced Raman spectra of 5 nm Ag-decorated n-layer graphenes at 633 nm, Raman enhancement factors of 5 nm Ag-decorated n-layer graphenes, SERS of 2 or 5 nm Ag on graphite at 514 nm, film morphologies of 2 nm Ag on n-layer graphenes, Raman enhancement factors of 2 nm Ag-decorated n-layer graphenes, comparison of the Raman enhancement between 2 and 5 nm Ag, and surface-enhanced Raman spectra of 5 nm-Ag decorated graphenes at different temperatures. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Tel: 86-10-82545584. Fax: 86-1062656765.

’ ACKNOWLEDGMENT This work was supported by National Science Foundation of China (Grants 10774032, 90921001, and 50952009). 11353

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