Enhanced SERS Stability of R6G Molecules with Monolayer Graphene

May 14, 2014 - Department of Applied Physics, The Hong Kong Polytechnic University, Hung Hom, Kowloon, People's Republic of China. ‡. The Hong Kong ...
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Enhanced SERS Stability of R6G Molecules with Monolayer Graphene Yuda Zhao,†,‡ Yizhu Xie,† Zhiyong Bao,† Yuen Hong Tsang,†,‡ Liming Xie,§ and Yang Chai*,†,‡ †

Department of Applied Physics, The Hong Kong Polytechnic University, Hung Hom, Kowloon, People’s Republic of China The Hong Kong Polytechnic University Shenzhen Research Institute, Shenzhen, People’s Republic of China § CAS Key Laboratory of Standardization and Measurement for Nanotechnology, National Center for Nanoscience and Technology, Beijing, People’s Republic of China ‡

S Supporting Information *

ABSTRACT: In this work, we used monolayer graphene, either underneath or on top of the R6G molecules, to enhance the stability and reproducibility of surface enhanced Raman spectroscopy (SERS). The time evolution of characteristic peaks of the organic molecules was monitored using Raman spectroscopy under continuous light irradiation to quantitatively characterize the photostability. Graphene underneath the organic molecules inhibits the substrate-induced fluctuations; and graphene on top of the organic molecules encapsulates and isolates them from ambient oxygen, greatly enhancing the photostability. Our results showed that the average lifespan of R6G molecules with graphene encapsulation can be increased by about 6-fold under high laser power density (3.67 × 106 W/cm2) and is less dependent on the power density of light irradiation.

1. INTRODUCTION Surface-enhanced Raman spectroscopy (SERS) enables singlemolecule level sensitivity by virtue of high local electric-field. However, the high local electric-field of SERS substrate leads to the photoinduced damages of the probed molecules (especially for organic molecules), results in photophysical desorption or photochemical reactions, and induces the fluctuations of the characteristic peaks in the Raman spectra.1−3 These instability and irreproducibility of SERS greatly hinder quantitative analysis for both single molecule and ensemble experiment. In order to suppress the molecule−substrate and moleculeambient interactions, researchers have attempted to make the local environment more chemically stable. For the purpose of the inhibition of molecule−substrate interaction, Li et al. demonstrated high-quality Raman spectra with the use of silica or alumina coated Au nanoparticles.4 Zhang et al. reported atomic layer deposition Al2O3 overlayer on Ag substrate for stable detection of CaDPA.5 Guo et al. showed microsized PDMS wells for enhancing the photostability of R6G molecules.6 The other straightforward aspect is to inhibit the molecule−ambient interaction. Steidtner et al. showed that the photostability can be improved greatly in ultrahigh vacuum compared with that in a pure oxygen atmosphere.7 Kantsler et al. proved that adding an oxygen scavenger can significantly increase the photostability of fluorescent molecules.8 Several groups encapsulated organic molecules in core−shell structures to prevent them from reactive oxygen and greatly increase photostability.9,10 Cohen et al. used silica-based material to encapsulate the fluorescent molecules for high stability and long lifetime.11 Zaiba et al. reported the encapsulation of organic molecules in Au nanoshells. The lifetime at excitation state is © 2014 American Chemical Society

significantly reduced, as well as the possibilities of oxygen attack on the organic molecules.12 But the core−shell structure usually involves complex fabrication processes. It is also difficult to control the thickness of metal shell that is critical for the enhancement of photostability.10 Noble metals, silica, alumina, and other traditional core−shell materials can improve the photostability of organic molecules, but they decrease the sensitivity in the meantime because of their high optical absorption in the visible spectrum. Graphene, a two-dimensional carbon atomic membrane, is chemically inert, transparent to a broad span of light, and highly impermeable to ambient oxygen.13−15 Jin Zhang and his coworkers have showed clean Raman spectroscopy with the use of the graphene-enhanced Raman spectroscopy (GERS).1−3 In this work, we used monolayer graphene to enhance the SERS stability of R6G molecules. We fabricated the test structures with R6G molecules underneath or on top of monolayer graphene and characterized the photostability with Raman spectroscopy.16,17 We find that monolayer graphene coating greatly enhances the stability and reproducibility of Raman spectra of R6G molecules under high power density illumination.

2. EXPERIMENTAL SECTION We fabricated corrugated Ag thin film (100 nm thickness) by magnetron sputtering on SiO2/Si substrate, where the Ag thin film serves as a SERS substrate. Monolayer graphene was grown Received: April 9, 2014 Revised: May 14, 2014 Published: May 14, 2014 11827

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one spot and moved a small step to a new spot and then started monitoring the time evolution of Raman spectra to avoid the zero time error.

on Cu foil by a chemical vapor deposition method and was transferred onto rough Ag films. We chose commonly used rhodamine 6G (R6G) as the probed organic molecule, which can be adsorbed on the substrate by dipping the substrates into R6G ethanol solution (2 × 10−3 mol/L) for 12 h. Figure 1a−c

3. RESULTS AND DISCUSSION SERS provides a route to examine the subtle structure changes of adsorbed molecules and their interactions with the environment and has been widely adopted to characterize the photostability.19−21 We can observe the fluctuations of intensity, position, and shape of the characteristic peaks in Raman spectra during the continuous laser irradiation process. The intensity of Raman peaks of R6G molecules decreases under the laser exposure, accompanying with the position shift and peak shape change, and the characteristic peak eventually vanishes. The decrease in the peak intensity is related to the reduced number of the R6G molecules in the collection volume of Raman spectroscopy. The peak shape change and the peak position shift are related to the change in the chemical bonding of the R6G molecules and their interactions between molecules and the environment.22,23 To exclude the possibility of photoinduced damages to the monolayer graphene, we first characterize the time evolution of Raman spectra of graphene on Ag substrate under continuous 8 min laser irradiation (power density: 3.67 × 106 W/cm2). As presented in Figure 1d, the Raman spectra of graphene show no D peak under continuous illumination, which is related to the disorder pattern in the hexagonal rings.24 The intensity, position, and shape of the 2D and G peaks in Raman spectra of graphene are stable during the 8 min light illumination, indicating that graphene is quite resistant to photodamages, even under the optical density up to 3.67 × 106 W/cm2, because graphene has high optical transmittance (up to 97%), absorbs little optical energy, and also has high thermal conductivity.25−27 We compare the characteristic Raman peaks of R6G molecules between the structures of R6G/Ag, R6G/G/Ag, and G/R6G/Ag. Figure 2a shows the time evolution of Raman spectra of R6G/Ag samples under high power-density light illumination for 8 min. Two obvious bands are observed at 1362 and 1648 cm−1, which are attributed to aromatic stretching vibration modes.3,28 The intensity of the two bands decreases greatly during 8 min laser irradiation, suggesting that the R6G molecules are photodamaged in the R6G/Ag structure. The shape of Raman peaks of R6G/Ag sample changes dramatically under continuous light illumination. The position of characteristic peaks of R6G molecules is unstable, and the relative intensity ratio of peak to peak shows large fluctuation. The interaction between Ag and R6G molecules is

Figure 1. Schematic illustration of (a) rhodamine 6G (R6G) molecules absorbed on Ag surface as a control structure (R6G/Ag). (b) R6G molecules absorbed on graphene/Ag substrate (R6G/G/Ag). (c) R6G molecules sandwiched between Ag and monolayer graphene (G/R6G/Ag). (d) Time evolution of Raman spectra of monolayer graphene on Ag substrate under continuous 8 min laser irradiation (power density: 3.67 × 106 W/cm2, every spectrum with a 5 s acquisition).

shows the schematic illustration of our three test structures. Bare Ag substrate coated with R6G molecules (R6G/Ag) is a control structure without graphene coating. By altering the sequence of the fabrication process, we can place monolayer graphene either underneath (R6G/G/Ag) or on top (G/R6G/ Ag) of R6G molecules. The detailed fabrication and transfer methods of graphene onto a metal surface have been described elsewhere.18 Raman spectroscopy (HORIBA HR800) with the excitation wavelength of 488 nm was used to measure Raman spectra of graphene and the R6G molecule. A 100× objective was used to focus the laser beam and to collect the Raman signal. The diameter of the laser spot is about 1 μm. Laser power was measured by an optical power meter. When we conducted a laser exposure test, we first focused the laser on the sample at

Figure 2. Time evolution of Raman spectra of R6G peak shape under high laser power density (3.67 × 106 W/cm2) for 8 min: (a) R6G/Ag (b) R6G/G/Ag (c) G/R6G/Ag. The “*” marked in (b) and (c) denotes the G peak (∼1588 cm−1) of monolayer graphene. 11828

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Figure 3. Schematic of R6G molecules at excitation states in the structures of (a) R6G/Ag, (b) R6G/G/Ag, and (c) G/R6G/Ag.

usually accepted as weak Ag−N bonding.29 The position shift and shape change in the Raman spectra is likely to be related to the change in the weak bonding and the orientation of R6G molecules under long-time light illumination. R6G is one kind of fluorescent molecule. The SERS instability can be resulted from photobleaching, an irreversible structure damage. The photobleaching process can be generally described according to the Jablonski diagram of the relevant states.30 A molecule absorbs a photon, and is excited from ground state S0 to excitation state S1 at the rate of kabs. There are two competing processes for the excitation state of the fluorescent molecules. In one way, the molecules at excitation states can relax to the ground state through spontaneous emission at the rate of kf or intrinsic nonradiative process at the rate of kinr. In the other way, the molecules at the excitation state can be photobleached at the rate of kb. The processes of the excitation, relaxation, and photobleaching can be quantitatively described according to the below equations.30 d S0 = −kabsS0 + k f S1 + k inrS1 dt

(1)

dS1 = kabsS0 − k f S1 − k inrS1 − k bS1 dt

(2)

states are exposed in air, they can be easily photobleached because of the reaction with oxygen in ambient.7,36 To provide a locally inert environment to the organic molecules, and enhance the photostability (the decrease of kb), we coated monolayer graphene on top of the R6G molecules. As shown in Figure 2c, the peak shape of Raman spectra of R6G molecules is very stable during the continuous light illumination, and all the characteristic peaks are apparent on the G/R6G/Ag structure, indicating that the use of monolayer graphene encapsulation can greatly improve the photostability of the R6G molecules. The monolayer graphene in the G/ R6G/Ag structure, on one hand, provides additional path for the active molecules to relax from excitation state to ground state; on the other hand, the monolayer graphene can prevent the R6G molecules from reacting with oxygen and permanently damaging the structure of the molecules. In the R6G/G/Ag structure, the R6G molecules are exposed to ambient environment; in the G/R6G/Ag structure, the highly impermeable graphene isolates molecules and Ag substrate from the oxygen in ambient environment. The high transparency of graphene allows light to transmit with negligible optical loss and keep comparable sensitivity.14 Figure 3 shows the schematic diagrams for the suggested mechanisms for the three different structures. When R6G molecules absorb photons from light, they are excited from ground state S0 to excitation state S1. The molecules at excitation state either relax to the ground state or react with ambient environment. In the R6G/G/Ag and G/R6G/Ag structures, the xanthenes ring of R6G molecules lie parallel to the graphene surface.37,38 The Fermi level of graphene (∼4.6 eV) lies between LUMO (∼3.28 eV) and HOMO (∼5.35 eV) energy level of R6G molecules.39−41 R6G molecules form π−π bonding with graphene, allowing the charge transfer between graphene and R6G molecules. Graphene provides additional path for the molecules to relax from the excitation state to the ground state, and reduces the number of molecules at the excitation state.42 The photobleaching rate is proportional to the number of molecules at excitation states. Hence, the use of graphene underneath or on top of organic molecules enhances the photostability. The oxygen in ambient environment has been proved to be one important factor for the photostability of organic molecules.7 Graphene has been demonstrated to be highly impermeable to gas and liquid.14,43 In the G/R6G/Ag structure, oxygen are less likely to react with the R6G molecules at excitation state because of graphene protection. The chemically inert environment further decreases the probability of photobleaching. To investigate the effects of graphene on enhancing the photostability resistance, we also prepared a G/ R6G/G/Ag structure, as schematically shown in Figure S1. We find that the decay rate of Raman peak intensity in the G/R6G/ G/Ag structure is close to that in the G/R6G/Ag structure. This indicates that the dominant effect of monolayer graphene in the G/R6G/Ag structure is the impermeability of oxygen.

To suppress the photobleaching in the competing processes of excitation state, one can increase kf and kinr or decrease kb. Researchers have made efforts to significantly enhance the spontaneous emission (the increase of kf) with the use of plasmonic structures.31,32 Kéna-Cohen et al. have shown that the photobleaching rate can be reduced by at least 2 orders of magnitude with the selective removal of long-lived states by the effect of plasmonic sinks.33 Figure 2b shows the time evolution of the Raman spectra of R6G/G/Ag structure, which are more stable than that of the R6G/Ag structure during continuous light illumination. Compared with Raman spectra of R6G/Ag samples, the shape of the Raman peak of R6G/G/Ag shows less fluctuation, and the characteristic peaks are obvious. The R6G molecules absorbed on the graphene show higher photostability than that directly on Ag substrate. Compared with R6G on bare Ag substrate, we also observe that the peak position of an aromatic stretching vibration mode shifts from 1648 to 1650 cm−1 with the graphene coating on Ag substrate. This is possibly due to the orientation change of the R6G molecules on substrate.28,34 We presume that the bonding between R6G molecules and graphene is stable because of the π−π bond interaction. This interaction enables the charge transfer between graphene and R6G molecules.35 The monolayer graphene in the R6G/G/Ag structure provides additional path for the molecules to relax from excitation state to ground state, and hence reduce the probability of photoreaction for the molecules at excitation states. However, the peak intensity still decreases under longtime light illumination. When the R6G molecules at excitation 11829

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decreased and the stability of R6G molecules is improved during the light illumination. The position shift of Raman peak in different structures indicates the stability of chemical bonding of R6G molecules. Figure 4b−d show the peak position shift of the three test structures around 1649 cm−1. We calculated the standard deviation of peak position at ∼1649 cm−1 in different structures. Our results show that the deviation of the peak position is relatively large for the R6G/Ag (1.51 cm−1) structure, and the deviation is reduced for the R6G/G/Ag (0.51 cm−1) and G/R6G/Ag (0.26 cm−1) structures. The position shift of ∼1649 cm−1 peak is related to the destruction of xanthenes ring stretching. If R6G molecules stably absorb on the SERS substrate, the peak position should be stable and less fluctuated; if the R6G molecules are photobleached, the peak position will fluctuate greatly. With the use of graphene encapsulation, the environment of R6G molecules is more chemically inert. Thus, the G/R6G/Ag structure shows high photostability. The number of the molecules at the excitation state is proportional to the laser power density. Low power-density laser can increase photostability but decrease the SERS sensitivity. It is also interesting to investigate the effect of power density on the photostability in our structures. We characterized the time evolution of Raman spectra with three different power densities of light illumination: 3.67 × 106, 9.18 × 105, and 3.67 × 105 W/cm2. We still chose the Raman peak of R6G at ∼1649 cm−1 as the representative peak to extract the average lifespan. As shown in Figure 5a−c, we plot the intensity of the Raman peak at ∼ 1649 cm−1 as a function of the light illumination time. The average lifespan of R6G molecules elongates with the decrease of laser power density. With the decrease of power density by one order in magnitude, the average lifespan increases 2.74 and 2.41 times in the R6G/Ag and the R6G/G/Ag structures, respectively. The average lifespan of the G/R6G/Ag structure is the largest among the three structures and R6G molecules are quite stable because of the chemically inert environment. Table S1 summarizes the lifespan of R6G molecules in the three structures under different power densities. Figure S2 show the decay rate of the peak intensity as a function of the light illumination time under different power densities. The curve is fitted according to the exponential decay. In the R6G/Ag and R6G/G/Ag structures, the decay rate is dependent on the power density of the light illumination. When the power density of the light illumination is reduced, the number of the molecules at excitation states decreases, and the photostability increases. Remarkably, in the G/R6G/Ag structure, the decay rate shows less dependent on the power density of the laser. We speculate that the

It is important to quantitatively describe the time that a R6G molecule continuously emits photons before it is photobleached. In this work, we define the average lifespan (ta) of R6G molecules as the time duration in which the peak intensity in Raman spectrum declines to half of the initial peak intensity. In order to accurately measure ta of molecules, we choose the ∼1649 cm−1 peak (characteristic of the xanthenes ring stretching) as a representative to conduct the time evolution of Raman spectra. As shown in Figure 4a, ta of R6G molecules

Figure 4. (a) Raman peak intensity of ∼1649 cm−1 peak of R6G molecules on different structures as a function of the light exposure time. Insert: decay rate of the peak (∼1649 cm−1) intensity on different structures as a function of light exposure time. Position shift of 1649 cm−1 peak of R6G molecules as a function of the light exposure time in the structures of (b) R6G/Ag, (c) R6G/G/Ag, and (d) G/R6G/Ag.

in the structures of R6G/Ag, R6G/G/Ag G/R6G/Ag can be extracted: 9.95, 15.66, and 57.91 s, respectively. We can find that ta of R6G molecules in the G/R6G/Ag structure is 5.82 times longer than that in the R6G/Ag structure and 3.67 times longer than that in the R6G/G/Ag structure. With monolayer graphene coating, the average lifespan of R6G molecules can be extended to a large scale. The inset of Figure 4a shows the decay rate of Raman peak intensity on different structures with exponential decay fitting. It clearly demonstrates that the intensity decay rate is high in the R6G/Ag structure and is much lower in the R6G/G/Ag and G/R6G/Ag structure. With monolayer graphene coating, the photobleaching rate is

Figure 5. Normalized intensity of Raman peak (∼1649 cm−1) of R6G molecules in the different structures as a function of light illumination time under different power densities: (a) R6G/Ag, (b) R6G/G/Ag, (c) G/R6G/Ag. The light illumination duration is 120 s. Every spectrum with a 5 s acquisition. 11830

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photostability is significantly enhanced with the monolayer graphene encapsulation because it isolates the molecules from the ambient oxygen. We can still observe the intensity decline of the Raman peak in the G/R6G/Ag structure at the initial stage of the light illumination. This is probably related to the oxygen adsorbed on the Ag surface that is unavoidable in the fabrication process or the photoinduced thermal effect.

4. CONCLUSIONS In summary, we used graphene to enhance the photostability of organic molecules. We find that graphene coating can greatly enhance the photostability of R6G molecules. In one way, graphene encapsulates R6G molecules from ambient environment, excluding the possibility of the reaction between R6G molecules and oxygen; in the other way, graphene and R6G molecules forms π−π bond interaction, facilitates the charge transfer process between R6G molecules and graphene and increases the relaxation rate of the excitation states of R6G molecules. Our method can be possibly extended to other organic molecule systems that are vulnerable to the photoinduced damages.

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ASSOCIATED CONTENT

S Supporting Information *

Additional supporting analytical data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +852 27665678. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge partial financial support from the Hong Kong Polytechnic University (Grant Nos.: G-UA51, G-YN02, and 1ZE14), the National Natural Science Foundation of China (Grant No.: 61302045), and Beijing Natural Science Foundation (Grant No.: 2132056).

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REFERENCES

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The Journal of Physical Chemistry C

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dx.doi.org/10.1021/jp503487a | J. Phys. Chem. C 2014, 118, 11827−11832