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This paper presents a convenient and reliable method to prepare gold nanoparticles (AuNPs) on graphene. Photo-assisted synthesis (PAS) was employed to grow AuNPs in AuCl4− electrolyte on graphene. The size of AuNPs could be as large as 130 nm. This optical method had a steady growth rate of AuNPs. The distribution of AuNPs was well controlled by focusing the laser for PAS. The minimum diameter of the distribution was approximately 1 μm. Surface-enhanced Raman scattering of graphene due to AuNPs was observed. Electrical fields near AuNPs calculated by the finite-difference time-domain algorithm ensured that the Raman enhancement was attributed to the localized surface plasmons of AuNPs.
©2013 Optical Society of America
1. Introduction
Surface-enhanced Raman scattering (SERS), activated by localized surface plasmons (LSPs) of plasmonic nanostructures [1], have been widely used to enhance the sensitivity of electrochemical sensing or biosensing [2,3]. SERS has recently been applied to push the sensitivity of graphene-based sensing by an enhancement of 103 times [4]; graphene has been considered a promising base material for electrochemical sensing and biosensing [5,6]. Hong et al. presented a method to self-assembled gold nanoparticles (AuNPs) on functionalized graphene for biosensing [7]. However, to place AuNPs on graphene without chemical functionalization is difficult because the perfect lattice of graphene has no dangling bonds to cause chemical adsorption of AuNPs.
Chloroauric acid (HAuCl4) aqueous solution is used for electroless Au plating [8] and AuNP synthesis [9]. In addition, electroless plating was employed to grow Au layer/nanostructures on chemical-vapor-deposited graphene [10,11]. One emerging approach for producing AuNPs with AuCl4− solutions is by photochemical photo-assisted synthesis (PAS) [12], where AuNPs are synthesized in the AuCl4− solution through UV light irradiation. In this article, we report the deposition of AuNPs with various sizes (under 130 nm in diameter) on pristine graphene at specific locations by PAS in AuCl4− solution. PAS is an easy and reliable method, which requires only the AuCl4− solution and has a steady volume growth rate of AuNPs. By atomic force microscopy (AFM), AuNPs were revealed to be produced by PAS but not electroless plating. In addition to synthesized AuNPs, a 2 nm thick Au layer was deposited on graphene by electroless plating simultaneously. The chemical composition of the plated Au thin layer was characterized by X-ray photoelectron spectroscopy (PES). The Raman signals of graphene with synthesized AuNPs were observed both in situ and ex situ. The Raman enhancement of graphene with a single 100 nm AuNP was as high as 103 times. From the calculation of the plasmonic electromagnetic (EM) field by the three-dimensional (3D) finite-difference time-domain (FDTD) algorithm, the Raman enhancement was confirmed to be attributed to the LSP at the AuNPs.
2. Experimental and simulation modeling details
The graphene samples were prepared by mechanical exfoliation on the SiO2/Si substrates to make the graphene visible under an optical microscope. The number of graphene layers was less than three layers, verified by Raman spectroscopy. The graphene samples were placed in a Petri dish, which was filled with deionized water. AuCl4− solution was injected into the Petri dish to make an electrolyte of 4.3 mM concentration. The schematic of the experimental setup to prepare the AuNPs on gaphene by PAS and perform in situ Raman spectroscopy is shown in Fig. 1. The morphology of Au on the graphene sheet was characterized by scanning electron microscopy (SEM) and tapping-mode AFM (Agilent 5500 AFM). The chemical structures of graphene and Au were characterized with micro-PES (μ-PES) taken by scanning photoelectron microscopy (SPEM). Details of the SPEM system are described in Ref. 13. The Raman spectra of graphene were taken by a confocal optical system with a 633 nm He-Ne laser, the laser power to expose the sample was 3 mW and the spot size was 500 nm in diameter. The operating principle of the 3D FDTD algorithm to calculate the induced EM field of PAS of AuNP was based on the resolution of the discrete Curl-Maxwell's equations by iteration over time [[14]]. The Drude-Lorentz dispersion model and dielectric function measured by Johnson and Christy were employed in this calculation [15].
Fig. 1 Schematic of photo-assisted synthesis of AuNPs on graphene in AuCl4− electrolyte and in situ micro Raman measurement (not to scale).
3. Results and discussion
Figure 2(a) shows the SEM images of synthesized AuNPs distributed on the graphene sheet and the inset displays a single AuNP obtained with a growth time (t) of 20 min. It is interesting to observe that no AuNP was produced on the bare SiO2 surface during PAS. The dependence of the diameter D(t) of AuNPs with t (i.e., the duration of laser irradiation to AuCl4− solution) is shown in Fig. 2(b). AuNPs grew as large as 130 nm in diameter. The standard deviation of the diameter ranged between 10% and 25%. The increase of the diameter of AuNPs was well fitted by a cube root function of growth time,
where the onset time of the nucleation of the AuNPs (t0) = 1.8 min and the volume growth rate (α) = 2.35 × 104 nm3/min were obtained. Thus, the volume of the AuNPs increased linearly with t. The AuNPs were suggested to be produced on graphene by PAS, where the laser light supplied the energy to facilitate the reduction of Au ions to AuNPs [11,12] byFig. 2 (a) SEM images of synthesized AuNPs distributed on graphene. The inset is the image of a 100 nm Au NP on graphene after 20 min of PAS. (b) Growth of synthesized AuNPs with the time of PAS. The red curve shows the fitting with Eq. (1).
The volume growth rate was constant, so PAS of AuNPs was a reaction-limited process.
Typically, AuNPs are synthesized and deposited separately in chemical methods [7]. By PAS, synthesis and deposition of AuNPs on graphene were achieved by only one process with a single chemical (AuCl4− electrolyte). Therefore, PAS is a convenient, efficient and reliable method of producing AuNPs with a diameter under 130 nm [predictable by Eq. (1)] with good uniformity. Compared with other physical methods for preparation of AuNPs (e.g., thermal evaporation and post-annealing), PAS is a low thermal-budget and less aggressive method, which is critical for many applications in biosensing. Moreover, PAS is activated by a laser light, so it is possible to pattern AuNPs on graphene, similar to laser direct writing [16]. The photochemical PAS can be controlled by the vector scanning of laser to achieve the patterning. The resolution of PAS patterning is related with the size of laser spots. Figure 3(a) shows the distribution of AuNPs varied with focusing of laser. As the laser spot was “in focus”, two AuNPs were deposited within the area of a diameter < 1 μm, which is close to the diffraction limit. AuNPs scattered wider, if the laser was out of focus. The further the “out of focus” was, the wider the AuNPs scattered. Figure 3(b) shows the Raman mapping of G band over the area of “out of focus.” The SERS of graphene due to LSP of AuNPs was observed. The SERS will be discussed later. Therefore, the distribution of AuNPs could be controlled by focusing the laser and the best resolution was about 1 μm. Note that the heat of laser power might cause the local disturbance of electrolyte, which had AuNPs distribute wider. Such a resolution is sufficient for many applications of biosensing. In addition, no photoresist has to be applied during PAS. The residues of photoresist on graphene are a contamination for the applications of graphene in biosensing and electronics.
Fig. 3 (a) AFM image of AuNPs on graphene produced by different focusing of laser. The circles mark the areas of AuNPs under “in focus” (1), “out of focus” (2), and “further out of focus” (3) after 3 min irradiation each. The dashed line indicates the edge of graphene. (b) Raman mapping of G band (1560 cm−1 to 1620 cm−1) taken over the area of “out of focus”.
To verify that the mechanism for producing AuNPs was PAS but not electroless plating, electroless plating of Au layers was conducted on graphene. The graphene sheet was immersed into AuCl4− electrolyte without laser irradiation. After immersion for certain periods, the graphene sample was removed from the electrolyte for AFM measurement in ambience. Figure 4(a) shows the AFM images of the pristine graphene and the morphological evolution of the electroless-plated Au layer on graphene after 40 s, 80 s, and 10 min of immersion. Before immersion, the graphene was a single layer with a clean surface. After 40 s, a trace of Au seeds was observed on the surface of graphene. The coverage of Au on graphene increased with the immersion time until the Au was grown laterally into a continuous layer, which covered the entire surface of graphene, as shown in the displayed image taken after 10 min immersion. Figure 4(b) shows the height profiles near the graphene edge before and after immersion. Au seeds ceased growing vertically when their height reached 2 nm, indicating that the growth was a self-limited process.
Fig. 4 (a) Morphologic evolution of plated Au on graphene with the immersion duration in AuCl4− electrolyte. The insets in (a) is to illustrate the details of plated Au of 40 s and 80 s immersion. (b) The corresponding cross-sectional profiles along the dash lines in (a).
The chemical composition of the electroless-plated Au layer was characterized by μ-PES with a photon energy of 380 eV. The SiO2/Si substrate was coated with a transparent conductive indium tin oxide thin layer to avoid charging effect during measurement. A bi-layer graphene (BLG) sheet was prepared on the substrate. The BLG was then subjected to electroless plating for 10 min. In Fig. 5(a), the spectrum shows metallic Au photoelectron peaks of 4f7/2 and 4f5/2 at 84.0 eV and 87.6 eV, respectively, which indicated the existence of Au thin layer. The signal was weak because of the slim amount of Au. Figure 5(b) shows the C 1s spectra of pristine graphene and Au-plated graphene. The C 1s spectra were deconvoluted into carbon sp2 and sp3 peaks. The binding energy of the sp2 peak of the Au-covered graphene (284.25 eV) was approximately 0.11 eV lower than that of the pristine graphene (284.36 eV). The decrease of binding energy of the Au-plated graphene was attributed to the lowering of its Fermi energy, resulting from the charge transferring from graphene to Au [17]. The higher sp3 peak intensity of pristine graphene could be attributed to damage from the longer exposure to the synchrotron X-ray, because the pristine graphene had a low visibility in the SPEM system, and thus, it took longer to be observed for μ-PES measurement.
Fig. 5 (a) Au 4f core-level photoelectron (PE) spectrum taken from Au-plated BLG, and (b) C 1s core-level spectra of pristine BLG and Au-plated BLG.
The formation of Au can be expressed by the reduction of AuCl4− on the graphene:
The reduction of AuCl4− is spontaneous because the reduction potential of the electrolyte is 1.0 V, which is higher than that of graphene (0.22 V) [10,11]. Here, the graphene was a reductant, giving electrons to AuCl4−. During the reduction of Au, electrons transferred from graphene to electrolyte, resulting in the increase of the work function of graphene, as well as reduction potential of graphene. The reaction stopped automatically as the reduction potential of graphene met that of electrolyte. Except for the 2 nm plated Au thin layer on graphene, no AuNP was observed on the surface of graphene. Hence, the formation mechanism of AuNPs was the PAS rather than electroless plating. Note that, during the growth of AuNPs on graphene by PAS, the 2 nm thick Au layer was also deposited by electroless plating. No Au seed was formed on the insulated SiO2. We suggest that PAS of AuNPs on graphene was a seed-mediated process [18]. To apply PAS of AuNPs on typical optical components, one has to prepare graphene on the components first.
To observe the Raman enhancement with the increase of the size of the synthesized AuNPs, the Raman spectra of graphene were taken in situ during PAS. Figure 6(a) shows the Raman intensities of the G-band and 2D-band with growth time t. Both G-band and 2D-band Raman intensities began to increase after 1 min, which was nearly coincidental with the time that AuNPs grew on graphene. Therefore, the Raman signals were considered to have been enhanced by the LSP of AuNPs. Figure 6(b) shows the ex situ Raman spectra of the pristine graphene and graphene with a single 100 nm AuNP (in ambience). A remarkable D-band signal was observed for the graphene with synthesized AuNPs but no D-band was detected with the pristine graphene. The D-band signal was attributed to the lattice deformation caused by the local intensive plasmonic EM field of AuNPs (see the results of 3D FDTD calculation shown in below). Compared with the Raman spectrum of pristine graphene, the G-band and 2D-band Raman peaks of graphene with an AuNP exhibited a considerable enhancement by 40 and 16 times, respectively. The area covered by a 100 nm AuNP was only approximately 1/25 of the area irradiated by the 500 nm spot. Thus, the effective enhancement of the G-band caused by LSP could be as high as 103 times. The enhancement was suppressed in solution because of the damping effect of the solution on LSP and light scattering at the air/solution interface. In addition, no enhancement was observed to the Raman signals of graphene with the 2 nm electroless-plated Au layer, suggesting that the thin Au layer may not contribute to the Raman enhancement of graphene with AuNPs.
Fig. 6 (a) Enhancement of G-band and 2D-band Raman intensity taken in situ during PAS. The arrow indicated when synthesized AuNPs were deposited on graphene. (b) Ex situ Raman spectra taken on pristine graphene and graphene with 100 nm AuNP in ambience.
To model the plasmonic EM field by 3D FDTD simulation, a single AuNP placed on a 2 nm thick Au film (in air), a layer of 1 nm thick graphene, and an infinite SiO2 substrate were considered, as shown in Fig. 7(a). The substrate lay on the X-Y plane. The incident EM field with X-polarization propagated along the −Z direction. In the calculations, the total-field scattered-field source was applied. Figure 7(b) exhibits the simulated electrical field distributions along the X-Z plane and X-Y plane with the AuNP size of 25 and 100 nm, respectively.
Fig. 7 (a) Illustration of a synthesized AuNP and a plated Au layer on graphene for modeling of EM field by 3D FDTD simulation. (b) Representative resultant absolute electric field distributions of 150 nm × 150 nm along the X-Z plane (across the center of AuNP) and X-Y plane (at the upper surface of graphene), respectively. The dashed lines denote where the graphene and plated Au layer locate, and the arrows indicate where the hot spots are. In the color scale, “blue” presents the zero field intensity and “red” the maximum field intensity. (c) The enhancement factor at the upper surface of graphene with respect to the diameter of AuNPs, where the dotted lines indicate the EF with a 100 nm AuNP is approximately 103.
The strong field intensity appears in the vicinity of the AuNP surface because the LSP. The field were coupled into the Au layer and graphene sheet. There was an additional strong field was observed at the gap between the AuNP and Au layer near the bottom of the AuNP, which was attributed to the hot spot effect. The resultant field intensified with the increase of particle size and the coupled field became dominant for the 100 nm AuNP at the graphene surface. To calculate the field enhancement by LSP, the field intensity was integrated over a circular area of laser spot size at the upper surface of the graphene with and without the existence of an AuNP. The enhancement factor (EF) of SERS can be obtained from the expression |INP|/|I0|, where INP and I0 are respectively the integrated power with and without an AuNP [19]. Figure 7(c) shows the relationship between the particle size and EF. The EF increased largely with respect to the particle size. The calculated EF with a 100 nm AuNP is consistent with the effective enhancement of the G-band Raman signal in Fig. 6(b), indicating the Raman signals were enhanced by LSP. Accordingly, the D-band Raman signal of graphene along with AuNPs is suggested to be attributed to the lattice deformation caused by the local intensive plasmonic EM field induced by the synthesized AuNPs.
The SERS of AuNP-loaded graphene can be applied to stress sensors, because the Raman G peak shifts with the uniaxial strain in graphene by a shift rate of −5ω0−1 (cm−1MPa−1), where ω0 is the G peak position at zero stress for graphene [20]. The speed of stress sensing will be improved due to the enhancement of Raman signals of graphene. Furthermore, PAS can produce the SERS active substrates for ultrasensitive detections of microbes [21], cellular components [22], and biomolecules [23].
4. Conclusion
The preparation of AuNPs on graphene by PAS has been demonstrated in the AuCl4− electrolyte. Because PAS of AuNPs requires only the AuCl4− electrolyte and has a steady volume growth rate of AuNPs, it is a convenient and reliable method for producing AuNPs on graphene. AuNPs can be grown on selective areas and the distribution of AuNPs can be controlled by focusing the laser. In addition to AuNPs by PAS, a 2 nm thick Au thin layer was simultaneously formed on graphene by electroless plating. SERS of graphene was observed, and the Raman enhancement by a single 100 nm AuNP was 103 times. The 3D FDTD calculation indicates that the enhancement is attributed to the LSPs of AuNPs.
Acknowledgments
This work was supported by National Science Council, Taiwan and the Project of Global Research & Education on Environment and Society, Tunghai University.
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