Article pubs.acs.org/JPCC
Few-Layer Graphene-Encapsulated Metal Nanoparticles for SurfaceEnhanced Raman Spectroscopy Youming Liu, Yue Hu, and Jin Zhang* Center for Nanochemistry, Beijing National Laboratory for Molecular Sciences, Key Laboratory for the Physics and Chemistry of Nanodevices, State Key Laboratory for Structural Chemistry of Unstable and Stable Species, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China S Supporting Information *
ABSTRACT: Shell-isolated surface-enhanced Raman scattering (shell-isolated SERS), where the isolating shell prevents metal−molecule interactions and improves the stability of the metal nanoparticles, has recently drawn a tremendous amount of attention. However, obtaining an ultrathin, seamless, and chemically stable isolating shell for shell-isolated SERS is still in its infancy. Graphene, with a high optical transparency and chemical inertness, is an ideal candidate to serve as an isolating shell. In the present work, graphene with a controlled number of layers is grown on the surface of metal nanoparticles via chemical vapor deposition, creating graphene-encapsulated metal nanoparticles (M@G, where M = Cu, Ag, and Au) suitable for shell-isolated SERS. Ultraviolet−visible spectroscopy of Ag@G, Cu@G, and their corresponding nanoparticles indicates that graphene can prevent the surface oxidation of Ag and Cu nanoparticles after exposure to ambient air, giving a SERS-active substrate with a long lifetime. The Raman spectra of cobalt phthalocyanine and rhodamine 6G on M@G substrates show that Au@G can dramatically suppress photobleaching and fluorescence of the probe molecules, resulting in an enhanced Raman signal. Hence, M@G is a promising material for applications in chemical and biological detection because of its long-term stability and superior SERS performance. shell provides effective transfer of the electromagnetic field from the metal core through the shell to the probe molecule, while a stable and pinhole-free shell can protect the inner metal core and prevent direct metal−molecule contact. However, a self-assembled monolayer is not quite stable, the synthesis of amorphous carbon is not controllable, and the preparation of an ultrathin and pinhole-free silica shell is very difficult. Up to now, it remains a challenge to obtain a stable, ultrathin, and pinhole-free isolating shell via a simple and controllable method. Graphene, composed of sp2 hybridized carbon atoms, has drawn much attention because of its fascinating properties. Graphene possesses the remarkable features of high optical transparency and chemical inertness, making it an ideal material for the isolating shell. Moreover, graphene also exhibits excellent performance in Raman spectroscopy. As illustrated in our previous work, graphene itself can work as an effective substrate for Raman enhancement, which we named grapheneenhanced Raman scattering (GERS).17−19 Observation of the first-layer effect,20 the molecular orientation effect,21 and the electric field modulation effect22−24 implies that the GERS performance can be attributed to a chemical mechanism. Owing to the fact that GERS spectra have a limited enhancement
1. INTRODUCTION Surface-enhanced Raman scattering (SERS) is a powerful tool for ultrasensitive, nondestructive, and real-time detection and has attracted considerable attention since its discovery in 1974.1−3 The commonly used SERS-active substrates are silver, gold, and copper in the form of rough surfaces or nanostructures. Surface roughness or curvature of the metal substrate gives rise to a dramatic enhancement of the local electromagnetic field through a light-excited surface plasmon resonance, known as the electromagnetic mechanism (EM).4−6 The enhancement of EM is roughly proportional to |E|,4 where E is the intensity of the electromagnetic field and which can reach 108 or more. In addition to EM, the chemical mechanism (CM) also contributes to the SERS effect. The CM, with an enhancement factor of 10−102, comes from a charge transfer between the probe molecule and the substrate.7,8 Although SERS has developed rapidly because of its outstanding performance characteristics, it is also facing challenges, such as instability of the metal substrate (especially for silver-based substrates), distortion of SERS spectra arising from metal− molecule interactions, and low reproducibility of the signal intensity caused by photoinduced damage. To overcome these problems, an isolating shell on the metal substrates was introduced, such as SiO2,9−11 amorphous carbon,12−14 or polymers.15,16 For shell-isolated SERS, the key issue is to develop a simple and controllable approach to prepare a stable, ultrathin, and pinhole-free isolating shell. An ultrathin isolating © 2014 American Chemical Society
Received: January 22, 2014 Revised: April 4, 2014 Published: April 7, 2014 8993
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factor, we further developed graphene-mediated SERS (GSERS) by fabricating a graphene−metal island composite. A monolayer of graphene successfully eliminated the metal− molecule charge transfer and made SERS analysis possible on an atomically flat surface.25,26 Other studies on graphenerelated SERS have used graphene derivative/graphene-based composites as a substrate, where a graphene shell served as an additional chemical enhancer,27−30 molecule enricher,31−33 fluorescence (FL) quencher,26,34 and protective cover.29,35 At present, the common method for preparing graphenecontaining SERS substrates is either by transferring graphene or the graphene derivative onto a metal surface or via electrostatic assembly or in-situ growth in solution, which is uncontrollable and unsuitable for large-scale synthesis. With these processes, however, the metal nanoparticles (NPs) are not entirely enclosed in a sealed graphene shell, creating a high probability that the analytes will penetrate into the interspace between the graphene and the metal and will cause spurious SERS signals. Therefore, it is of great importance to find a technique that will encapsulate metal NPs in an ultrathin and hermetically sealed graphene shell via a controllable and massproduction method. In the present work, few-layer graphene-encapsulated metal NPs (M@G, where M = Cu, Ag, and Au) are prepared, and their stability and SERS performance are studied. The graphene isolating shell encapsulated on the surface of metal NPs is grown via chemical vapor deposition (CVD). This approach allows fine control of the thickness of the graphene shell from a few layers to multilayers by tuning the growth parameters during the CVD process. The ultraviolet−visible (UV−vis) spectra are acquired to determine the stability of Ag@G and Cu@G. Compared with the bare NPs, Ag@G and Cu@G are more stable after exposure in ambient air because of the chemical inertness of the graphene shell. In this way, the graphene shell endows metal nanoparticles with a prolonged lifetime and makes them suitable for potential applications in harsh conditions. The SERS measurement on Au@G and Au NPs is carried out using cobalt phthalocyanine and rhodamine 6G as probe molecules. The graphene isolating shell is found to effectively suppress the photobleaching effect and FL as well as to contribute extra enhancement to the overall signal intensity via a chemical mechanism, giving rise to a greatly improved stability and sensitivity of the SERS signal.
Figure 1. Production process for the M@G to serve as a SERS-active substrate.
HR800; two laser lines via a 632.8 nm line from a He−Ne laser and a 514.5 nm line from an Ar+ laser) were employed to characterize the as-grown M@G. Bare-metal NPs for control experiments were prepared under the same conditions as the M@G except that no carbon source was introduced during the growth process. A monolayer Langmuir−Blodgett (LB) film of cobalt phthalocyanine (CoPc; Alfa-Aesar) was prepared on an LB trough (NIMA Technology), and Rhodamine 6G (R6G; Sigma-Aldrich) was deposited on the substrate using a standard thermal evaporation process.
3. RESULTS AND DISCUSSION The M@G are successfully prepared according to the process shown in Figure 1. As illustrated in the SEM images in Figure 2a, the M@G nanoparticles with different metal cores have different diameters and surface coverage because of the distinct melting points and surface energies of these metals. The tilted view SEM image also shows the spherical shape of M@G (Figure S1a of the Supporting Information). The graphene shell can be distinguished by TEM as shown in Figure 2b. The distance of 0.35 nm between coating layers, which is consistent with the interlayer spacing of graphite, suggests the formation of a few-layer graphene shell. Moreover, the NPs are entirely enclosed in the graphene shell as shown in Figure S1b of the Supporting Information. The Raman spectra give further information about the graphene shell, and the existence of G and 2D bands (features of graphitic sp2 materials) in the spectra of these samples confirms the formation of a graphene shell (Figure 2c). The Raman spectra also show a high D band, which is related to the defects or disorder of the crystalline structure. During the CVD process, different graphene grains coalesce into a complete graphene shell. The large curvature of the nanoparticle surface results in a small grain size and plenty of boundaries in the shell, which gives rise to the D band in the Raman spectra.36−39 Furthermore, the surface plasmon resonance (SPR) absorption bands, which are an important optical property of noble-metal nanomaterials, are measured using UV−vis spectroscopy. The UV−vis spectral peak at ∼270 nm is assigned to the absorption by the graphene shell, while the bands at 576, 372, and 543 nm are correlated with the SPR absorption bands of Cu, Ag, and Au NPs, respectively (Figure 2d). For CVD growth of the graphene shell, a prereduction treatment is found to be of great importance. Raman spectra of the as-grown products on copper with and without a prereduction treatment are shown in Figure 3a. For the product without a reductive pretreatment, only D and G bands
2. EXPERIMENTAL SECTION The preparation process for M@G is shown in Figure 1. We deposited films consisting of 20 nm of Cu, 5 nm of Ag, or 5 nm of Au onto a 300 nm SiO2/Si substrate using a thermal evaporation system (Oerlikon Leybold Vacuum UNIVEX-300) with a deposition rate of 0.1−0.3 Å·s−1. The substrate was then placed in the quartz tube (1 in.) for the graphene shell growth process. First, metal films were evolved into NPs under an Ar atmosphere at elevated temperatures after which H2 was introduced into the CVD furnace to remove metal oxides and other impurities when the set temperature was reached. Subsequently, CH4 was introduced for the growth of the graphene shell. After 30 min of growth, the flow of CH4 was switched off and the temperature was cooled to room temperature with the protection of Ar and H2. Scanning electron microscopy (SEM, Hitachi S4800), transmission electron microscopy (TEM, TEKNAI F20; 200 kV acceleration voltage), UV−visible spectroscopy (UV−vis, PerkinElmer Lambda 950), and Raman spectroscopy (Horiba 8994
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Figure 2. (a) SEM images, (b) TEM images, (c) Raman, and (d) UV−vis spectra of as-grown M@G for M = Cu, Ag, and Au.
Figure 3. (a) Raman spectra of the as-grown products with and without reductive pretreatment. (b) Raman spectra of the as-grown products at different temperatures. (c) (left) D and G bands of M@G with different CH4/H2 ratios and (right) the ID/IG as a function of the CH4/H2 ratio. The spectra in c are normalized with the G band intensity. (d−f) TEM images of Cu@G with different ratios of CH4/Ar.
are observed, implying that an amorphous carbon film rather than a graphene shell is obtained. However, D, G, and 2D peaks are measured from products with a pretreatment, which indicates the formation of a graphene shell. NPs with a graphene shell show a much lower FL background than those with an amorphous carbon coating, which suggests that the graphene shell suppresses the FL background. This will be discussed in more detail later in this section. In addition, NPs encapsulated in a graphene shell possess more uniform diameters than those with an amorphous carbon coating (Figure S1a and S1b of the Supporting Information ) because of the protection of the graphene shell. These results confirm that the prereduction treatment is essential for the formation of a graphene isolating shell and that the as-grown graphene shell can contribute a stabilization effect and an FL quenching effect. The influence of the prereduction process is also studied via X-
ray photoelectron spectroscopy (XPS) (Figure S1c of the Supporting Information). As shown in the Cu 2p XPS spectra, the copper film deposited on the SiO2/Si substrate is composed of cuprous and copper oxides after long-term storage in ambient conditions. After the pretreatment, however, cuprous and copper oxides are reduced into copper, implying that the role of H2 during the prereductive process is to remove the metal oxide and other impurities to activate the metal catalysts. Other growth parameters during the CVD process are also investigated. Figure 3b shows the influence of growth temperature upon the CVD-generated products. Only an amorphous carbon film is obtained at temperatures below 920 °C, while a graphene shell grows when the temperature is above 970 °C. Moreover, a reduced ratio of the D to G peak intensities (ID/IG) is obtained when the temperature is increased from 970 to 1020 °C. This means that the formation 8995
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Figure 4. Comparison of UV−vis spectra of (a) Ag NPs and (b) Ag@G exposed in ambient environment for different time.
which broadens the application of M@G to a wide variety of environments. Photobleaching arises from the decomposition or carbonization of the probe molecules and can cause a significant decrease of the SERS intensity and can create an adverse effect on the stability of SERS spectra.44−46 Time-dependent SERS measurements are taken on the Au@G and Au NP samples with CoPc as the probe molecule to investigate the stability of the SERS intensity (Figure 5a and b). During the 160 s
of graphene favors a higher temperature, which promotes carbon source decomposition and metal activation. The CH4/ H2 ratio also plays an important role in the graphene shell growth. As a reducing agent, H2 can remove the oxygen and other oxidizing contaminants in the gas feed and on the metal surface. On the other hand, it can also etch the as-grown graphene shell. Figure 3c illustrates the influence of the CH4/ H2 ratio upon the ID/IG of the Raman spectra. The ID/IG decreases with an increasing CH4/H2 ratio, suggesting a reduction in the boundaries or defects in the graphene shells grown with a high CH4 fraction. Because a CH4/H2 ratio that is too high leads to an increased amount of amorphous carbon deposition on the CVD tube, an optimum CH4/H2 ratio is chosen for further experiments. Remarkably, we find that the thickness of the graphene shell can be finely tuned by controlling the ratio of CH4/Ar. As shown in Figure 3d−f, the thickness of the graphene shell changes from a multilayer to a few layers as the CH4/Ar ratio is reduced, though it is difficult to characterize the thinnest graphene shells because of the difficulty in identifying the metal−graphene interface. Therefore, using low CH4/Ar ratios, we have successfully prepared ultrathin graphene isolating shells in these experiments. The surface plasmon resonance (SPR) phenomenon, caused by the collective oscillation of conduction electrons in nanostructures under incident light, is an important optical property of metal nanomaterials. However, metal substrates used in SERS detection, especially silver-based substrates, are easily oxidized when exposed to ambient conditions. Oxidation of the metal NPs results in significant changes in their plasmonic properties and thus will greatly affect their SERS performance and will limit their practical application. Oxidation of Ag NPs produces a redshift of the SPR peak position and can be identified by the UV−vis spectra.40,41 As shown in Figure 4a, the UV−vis spectra of the Ag NPs exhibit an SPR peak centered at 372 nm and a broad band below 300 nm, which are assigned to the surface and volume plasmon resonance of the NPs, respectively.42,43 Another peak located at 270 nm in Figure 4b is the absorption band of the graphene shell as discussed previously. Here, we focus on the change of the SPR absorption bands centered at 372 nm. Over a 20 h period, the SPR peak of bare Ag NPs exhibits an obvious redshift (Figure 4a), while that of the Ag@G shows no change (Figure 4b). These results suggest that the graphene shell serves as a protective shell and prevents oxidation of the Ag NPs. The same protective effect can also be observed in Cu@G and Cu NP samples (Figure S2 of the Supporting Information). Therefore, the encapsulating graphene isolating shell on the surface of metal NPs creates an extremely stable and longlifetime substrate. The graphene isolating shell serves as a barrier preventing reactive agents from reaching the metal core,
Figure 5. Stability of SERS signals of monolayer CoPc LB films on (a) Au and (b) Au@G. SERS signals of a (c) CoPc LB film and (d) 3 Å R6G deposited via vacuum evaporation on Au@G and Au NPs.
measurement, the SERS intensity of CoPc at 1534 cm−1 decreases dramatically for Au NPs, though no obvious change in signal intensity is observed for Au@G, which indicates that the graphene shell effectively suppresses photobleaching. By encapsulating the surface of metal NPs, the graphene shell suppresses the catalytic activity of metal NPs and provides a passive surface. Meanwhile, the graphene shell also stabilizes the probe molecule via π−π interactions between the graphene shell and the molecules. Both the passivation and the stabilization effects of the graphene isolating shell allow M@ G to eliminate the problem of photobleaching and have thereby greatly improved the stability of the SERS spectra. Moreover, we also find that the signal intensity of CoPc on Au@G is stronger than that on Au NPs (Figure 5c), suggesting that the graphene shell can contribute extra enhancement to the SERS intensity. Meanwhile, the enhancement factor of different peaks is different (Table S1 of the Supporting Information), which 8996
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(6) Le Ru, E. C.; Etchegoin, P. G. Principles of Surface-Enhanced Raman Spectroscopy and Related Plasmonic Effects; Elsevier: Amsterdam, The Netherlands, 2009. (7) Udagawa, M.; Chou, C. C.; Hemminger, J. C.; Ushioda, S. Raman Scattering Cross Section of Adsorbed Pyridine Molecules on a Smooth Silver Surface. Phys. Rev. B 1981, 23, 6843−6846. (8) Jiang, X. D.; Campion, A. Chemical Effects in Surface-Enhanced Raman Scattering: Pyridine Chemisorbed on Silver Adatoms on Rh (100). Chem. Phys. Lett. 1987, 140, 95−100. (9) Liz-Marzán, L. M.; Giersig, M.; Mulvaney, P. Synthesis of Nanosized Gold−Silica Core−Shell Particles. Langmuir 1996, 12, 4329−4335. (10) Mulvaney, S. P.; Musick, M. D.; Keating, C. D.; Natan, M. J. Glass-Coated, Analyte-Tagged Nanoparticles: A New Tagging System Based on Detection with Surface-Enhanced Raman Scattering. Langmuir 2003, 19, 4784−4790. (11) Li, J. F.; Huang, Y. F.; Ding, Y.; Yang, Z. L.; Li, S. B.; Zhou, X. S.; Fan, F. R.; Zhang, W.; Zhou, Z. Y.; Wu De, Y.; et al. Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy. Nature 2010, 464, 392− 395. (12) Jacob, D. S.; Genish, I.; Klein, L.; Gedanken, A. Carbon-Coated Core Shell Structured Copper and Nickel Nanoparticles Synthesized in an Ionic Liquid. J. Phys. Chem. B 2006, 110, 17711−17714. (13) Li, J.; Liu, C. Y. Carbon-Coated Copper Nanoparticles: Synthesis, Characterization and Optical Properties. New J. Chem. 2009, 33, 1474−1477. (14) Shen, A. G.; Chen, L. F.; Xie, W.; Hu, J. C.; Zeng, A.; Richards, R.; Hu, J. M. Triplex Au−Ag−C Core−Shell Nanoparticles as a Novel Raman Label. Adv. Funct. Mater. 2010, 20, 969−975. (15) Qian, X. M.; Peng, X. H.; Ansari, D. O.; Yin-Goen, Q.; Chen, G. Z.; Shin, D. M.; Yang, L.; Young, A. N.; Wang, M. D.; Nie, S. M. In Vivo Tumor Targeting and Spectroscopic Detection with SurfaceEnhanced Raman Nanoparticle Tags. Nat. Biotechnol. 2008, 26, 83− 90. (16) Yang, M. X.; Chen, T.; Lau, W. S.; Wang, Y.; Tang, Q. H.; Yang, Y. H.; Chen, H. Y. Development of Polymer-Encapsulated Metal Nanoparticles as Surface-Enhanced Raman Scattering Probes. Small 2009, 5, 198−202. (17) Ling, X.; Xie, L. M.; Fang, Y.; Xu, H.; Zhang, H. L.; Kong, J.; Dresselhaus, M. S.; Zhang, J.; Liu, Z. F. Can Graphene Be Used as a Substrate for Raman Enhancement? Nano Lett. 2009, 10, 553−561. (18) Huh, S.; Park, J.; Kim, Y. S.; Kim, K. S.; Hong, B. H.; Nam, J. M. UV/Ozone-Oxidized Large-Scale Graphene Platform with Large Chemical Enhancement in Surface-Enhanced Raman Scattering. ACS Nano 2011, 5, 9799−9806. (19) Yu, X. X.; Cai, H. B.; Zhang, W. H.; Li, X. J.; Pan, N.; Luo, Y.; Wang, X. P.; Hou, J. G. Tuning Chemical Enhancement of SERS by Controlling the Chemical Reduction of Graphene Oxide Nanosheets. ACS Nano 2011, 5, 952−958. (20) Ling, X.; Zhang, J. First-Layer Effect in Graphene-Enhanced Raman Scattering. Small 2010, 6, 2020−2025. (21) Ling, X.; Wu, J. X.; Xu, W. G.; Zhang, J. Probing the Effect of Molecular Orientation on the Intensity of Chemical Enhancement Using Graphene-Enhanced Raman Spectroscopy. Small 2012, 8, 1365−1372. (22) Xu, H.; Chen, Y. B.; Xu, W. G.; Zhang, H. L.; Kong, J.; Dresselhaus, M. S.; Zhang, J. Modulating the Charge-Transfer Enhancement in GERS Using an Electrical Field under Vacuum and an N/P-Doping Atmosphere. Small 2011, 7, 2945−2952. (23) Xu, H.; Xie, L. M.; Zhang, H. L.; Zhang, J. Effect of Graphene Fermi Level on the Raman Scattering Intensity of Molecules on Graphene. ACS Nano 2011, 5, 5338−5344. (24) Hao, Q. Z.; Morton, S. M.; Wang, B.; Zhao, Y. H.; Jensen, L.; Jun Huang, T. Tuning Surface-Enhanced Raman Scattering from Graphene Substrates Using the Electric Field Effect and Chemical Doping. Appl. Phys. Lett. 2013, 102, 011102. (25) Xu, W. G.; Ling, X.; Xiao, J. Q.; Dresselhaus, M. S.; Kong, J.; Xu, H. X.; Liu, Z. F.; Zhang, J. Surface Enhanced Raman Spectroscopy on
implies the presence of chemical enhancement. Actually, this extra enhancement is attributed to the charge transfer between the graphene shell and CoPc, which results in a chemical enhancement. Hence, graphene can contribute extra enhancement to the overall signal intensity via a chemical mechanism and can create SERS detection with improved sensitivity.20−24 Fluorescence, typically appearing in the same spectral region as the Raman scattering, has an adverse effect upon SERS detection. As shown in Figure 5d, the SERS spectra of R6G collected on Au NPs exhibit a strong FL background, making it difficult to observe the R6G signal. However, the FL background is greatly reduced with Au@G, suggesting that graphene effectively suppresses the FL background. This graphene-induced FL suppression is mainly caused by energy and electron transference and creates SERS spectra with a high signal-to-noise ratio.
4. CONCLUSION In summary, a graphene shell with controllable thickness is grown on the surface of metal NPs via a CVD process. The graphene shell effectively protects metal NPs from oxidation, dramatically suppresses the photobleaching effect and FL background, and contributes chemical enhancement, thereby producing an M@G with long-term stability and excellent SERS performance. In addition, the graphene shell exhibits biocompatibility and can prevent direct contact between the analytes and metal NPs, enabling M@G to be used in practical applications in chemical and biological detection.
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ASSOCIATED CONTENT
S Supporting Information *
Tilted view SEM image and TEM image of Cu@G. SEM images of as-grown products with and without reductive pretreatment. Cu 2p XPS spectra of as-deposited copper film before and after prereduction. Comparison of UV−vis spectra of Cu NPs and Cu@G heated in air at 150 °C for different times. Enhancement factor of different Raman peaks of Au@G. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS This work was supported by NSFC (21233001, 21129001, 51272006, and 51121091) and MOST (2011YQ0301240201 and 2011CB932601).
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REFERENCES
(1) Fleischmann, M.; Hendra, P. J.; McQuillan, A. J. Raman Spectra of Pyridine Adsorbed at a Silver Electrode. Chem. Phys. Lett. 1974, 26, 163−166. (2) Albrecht, M. G.; Creighton, J. A. Anomalously Intense Raman Spectra of Pyridine at a Silver Electrode. J. Am. Chem. Soc. 1977, 99, 5215−5217. (3) Jeanmaire, D. L.; Van Duyne, R. P. Surface Raman Spectroelectrochemistry: Part I. Heterocyclic, Aromatic, and Aliphatic Amines Adsorbed on the Anodized Silver Electrode. J. Electroanal. Chem. Interfacial Electrochem. 1977, 84, 1−20. (4) Otto, A.; Mrozek, I.; Grabhorn, H.; Akemann, W. SurfaceEnhanced Raman Scattering. J. Phys.: Condens. Matter 1992, 4, 1143. (5) Schatz, G. C.; Young, M. A.; Duyne, R. P. Electromagnetic Mechanism of SERS. Top. Appl. Phys. 2006, 103, 19−45. 8997
dx.doi.org/10.1021/jp500751a | J. Phys. Chem. C 2014, 118, 8993−8998
The Journal of Physical Chemistry C
Article
a Flat Graphene Surface. Proc. Natl. Acad. Sci. U.S.A. 2012, 109, 9281− 9286. (26) Xu, W. G.; Xiao, J. Q.; Chen, Y. F.; Chen, Y. B.; Ling, X.; Zhang, J. Graphene-Veiled Gold Substrate for Surface-Enhanced Raman Spectroscopy. Adv. Mater. 2013, 25, 928−933. (27) Sun, S. T.; Wu, P. Y. Competitive Surface-Enhanced Raman Scattering Effects in Noble Metal Nanoparticle-Decorated Graphene Sheets. Phys. Chem. Chem. Phys. 2011, 13, 21116−21120. (28) Hao, Q. Z.; Wang, B.; Bossard, J. A.; Kiraly, B.; Zeng, Y.; Chiang, I. K.; Jensen, L.; Werner, D. H.; Huang, T. J. SurfaceEnhanced Raman Scattering Study on Graphene-Coated Metallic Nanostructure Substrates. J. Phys. Chem. C 2012, 116, 7249−7254. (29) Kim, Y. K.; Han, S. W.; Min, D. H. Graphene Oxide Sheath on Ag Nanoparticle/Graphene Hybrid Films as an Antioxidative Coating and Enhancer of Surface-Enhanced Raman Scattering. ACS Appl. Mater. Interfaces 2012, 4, 6545−6551. (30) Dutta, S.; Ray, C.; Sarkar, S.; Pradhan, M.; Negishi, Y.; Pal, T. Silver Nanoparticle Decorated Reduced Graphene Oxide (Rgo) Nanosheet: A Platform for SERS Based Low-Level Detection of Uranyl Ion. ACS Appl. Mater. Interfaces 2013, 5, 8724−8732. (31) Ren, W.; Fang, Y. X.; Wang, E. K. A Binary Functional Substrate for Enrichment and Ultrasensitive SERS Spectroscopic Detection of Folic Acid Using Graphene Oxide/Ag Nanoparticle Hybrids. ACS Nano 2011, 5, 6425−6433. (32) Lu, G.; Li, H.; Liusman, C.; Yin, Z. Y.; Wu, S. X.; Zhang, H. Surface Enhanced Raman Scattering of Ag or Au NanoparticleDecorated Reduced Graphene Oxide for Detection of Aromatic Molecules. Chem. Sci. 2011, 2, 1817−1821. (33) Liu, X. J.; Cao, L. Y.; Song, W.; Ai, K. L.; Lu, L. H. Functionalizing Metal Nanostructured Film with Graphene Oxide for Ultrasensitive Detection of Aromatic Molecules by Surface-Enhanced Raman Spectroscopy. ACS Appl. Mater. Interfaces 2011, 3, 2944−2952. (34) Wang, Y. Y.; Ni, Z. H.; Hu, H. L.; Hao, Y. F.; Wong, C. P.; Yu, T.; Thong, J. T. L.; Shen, Z. X. Gold on Graphene as a Substrate for Surface Enhanced Raman Scattering Study. Appl. Phys. Lett. 2010, 97, 163111/1−163111/3. (35) Li, X. Y.; Li, J.; Zhou, X. M.; Ma, Y. Y.; Zheng, Z. P.; Duan, X. F.; Qu, Y. Q. Silver Nanoparticles Protected by Monolayer Graphene as a Stabilized Substrate for Surface Enhanced Raman Spectroscopy. Carbon 2014, 66, 713−719. (36) Yu, Q. K.; Jauregui, L. A.; Wu, W.; Colby, R.; Tian, J. F.; Su, Z. H.; Cao, H. L.; Liu, Z. H.; Pandey, D.; Wei, D. G.; et al. Control and Characterization of Individual Grains and Grain Boundaries in Graphene Grown by Chemical Vapour Deposition. Nat. Mater. 2011, 10, 443−449. (37) Kim, K.; Lee, Z.; Regan, W.; Kisielowski, C.; Crommie, M. F.; Zettl, A. Grain Boundary Mapping in Polycrystalline Graphene. ACS Nano 2011, 5, 2142−2146. (38) Huang, P. Y.; Ruiz-Vargas, C. S.; van der Zande, A. M.; Whitney, W. S.; Levendorf, M. P.; Kevek, J. W.; Garg, S.; Alden, J. S.; Hustedt, C. J.; Zhu, Y.; et al. Grains and Grain Boundaries in Single-Layer Graphene Atomic Patchwork Quilts. Nature 2011, 469, 389−392. (39) Pimenta, M. A.; Dresselhaus, G.; Dresselhaus, M. S.; Cancado, L. G.; Jorio, A.; Saito, R. Studying Disorder in Graphite-Based Systems by Raman Spectroscopy. Phys. Chem. Chem. Phys. 2007, 9, 1276−1290. (40) Henglein, A. Colloidal Silver Nanoparticles: Photochemical Preparation and Interaction with O2, Ccl4, and Some Metal Ions. Chem. Mater. 1998, 10, 444−450. (41) Yin, Y. D.; Li, Z. Y.; Zhong, Z. Y.; Gates, B.; Xia, Y. N.; Venkateswaran, S. Synthesis and Characterization of Stable Aqueous Dispersions of Silver Nanoparticles through the Tollens Process. J. Mater. Chem. 2002, 12, 522−527. (42) Sun, Y. G.; Yin, Y. D.; Mayers, B. T.; Herricks, T.; Xia, Y. N. Uniform Silver Nanowires Synthesis by Reducing Agno3 with Ethylene Glycol in the Presence of Seeds and Poly(Vinyl Pyrrolidone). Chem. Mater. 2002, 14, 4736−4745. (43) Inoue, M.; Hayashi, Y.; Takizawa, H.; Suganuma, K. Formation Mechanism of Nanostructured Ag Films from Ag2O Particles Using a Sonoprocess. Colloid Polym. Sci. 2010, 288, 1061−1069.
(44) Goncher, G. M.; Harris, C. B. Enhanced Photofragmentation on a Silver Surface. J. Chem. Phys. 1982, 77, 3767−3768. (45) Goncher, G. M.; Parsons, C. A.; Harris, C. B. Photochemistry on Rough Metal Surfaces. J. Phys. Chem. 1984, 88, 4200−4209. (46) Suh, J. S.; Moskovits, M.; Shakhesemampour, J. Photochemical Decomposition at Colloid Surfaces. J. Phys. Chem. 1993, 97, 1678− 1683.
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