Magnetic Dipole Resonance and Coupling Effects Directly Enhance

Article Cite This: Chem. Mater. 2018, 30, 1472−1483

pubs.acs.org/cm

Magnetic Dipole Resonance and Coupling Effects Directly Enhance the Raman Signals of As-Grown Graphene on Copper Foil by over One Hundredfold Yi-Chuan Tseng, Tzu-Yao Lin, Yang-Chun Lee, Che-Kuei Ku, Chun-Wei Chen, and Hsuen-Li Chen* Department of Materials Science and Engineering, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan S Supporting Information *

ABSTRACT: Large-area graphene is commonly prepared through chemical vapor deposition (CVD); in situ and nondestructive methods for its characterization are desirable. In this paper, we demonstrate a practical methodexploiting magnetic dipole resonance and coupling effectswith which the Raman signals of graphene on copper (Cu) foil can be directly, faithfully, and greatly enhanced. The magnetic dipole resonance of a silicon nanoparticle (SiNP) can effectively couple its electromagnetic field with the Cu foil to induce an enormous electric field located solely at the position of the SiNP on the graphene. The coupled electromagnetic field can lead to hot spots of high electric field intensity (E2/ E02 = 123.2) within the graphene. Even when we positioned only a few SiNPs upon the graphene/Cu foil, we obtained a Raman signal enhancement (ca. 206 times) much greater than that of graphene transferred onto a 300 nm oxide film (ca. 12 times). From a series of experiments comparing the Raman signals of graphene before and after removing the SiNPs on the graphene/Cu foil, we found that the coated SiNPs had almost no effect on the quality of the as-grown graphene. Furthermore, we have used the SiNP-enhanced Raman signals to distinguish the local quality of as-grown graphene at different areas and, therefore, the quality of the underlying Cu foil. Thus, this approach based on magnetic dipole resonance and coupling appears to be very useful for in situ and nondestructive characterization of as-grown graphene on Cu foil, without the need for transfer processes or harmful processing conditions.

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number of nucleation sites for the carbon atoms.20−22 Although the techniques for preparing CVD-grown graphene are gradually maturing, developing in situ, rapid, and nondestructive methods for directly characterizing the properties of graphene on Cu substrates, without the need for any transfer processes, would be welcome. Raman scattering spectroscopy is a powerful, efficient, and nondestructive method that can provide abundant information about various materialstheir bonding and structural quality and the vibrations of their constituent molecules.23,24 Raman scattering is generally a practical method for characterizing the structural and material properties of graphene (e.g., atomic structures of disorders, defects, and edges and strain).25 Nevertheless, as-grown graphene on Cu foil provides very weak Raman signals because (i) the carbon atoms of monolayer graphene absorbs only a small portion of the incident light required to generate the Raman scattering signal26 and (ii) interference between the excitation laser and the reflected light on the Cu foil surface presenting a very low localized electric field that inhibits the Raman scattering of graphene.27 Background noise from the Cu foil also has a negative effect when characterizing CVD-grown graphene.28,29

INTRODUCTION Because graphene has excellent electronic, thermal, optical, and mechanical properties,1−4 this unique two-dimensional (2D) material is attracting great interest for its potential applications in many fields. Single-layer graphene (SLG) is commonly prepared using a micromechanical exfoliation method,5 but other methods have been developed to produce large-area and high-quality graphene. For example, the growth of uniformly ordered, large-area graphene on an insulating substrate can be performed through the thermal decomposition of silicon carbide (SiC).6,7 The production of graphene using costeffective graphite as a raw material has been proposed through the chemical reduction of graphene oxide.8,9 Obtaining graphene with smooth edges and controllable widths can be performed through the plasma cutting of carbon nanotubes.10,11 In general, large-area graphene is most effectively prepared using chemical vapor deposition (CVD) to grow crystal carbon atoms on metal surfaces.12−19 To improve the quality of CVD-grown graphene, many investigations have been made into the use of various metal substrates, including Ni,12−14 Ru,15,16 and Cu.17−19 Because carbon displays extremely low solid-solubility in Cu, CVD growth of graphene on polycrystalline Cu foil has gradually become a mainstream process. More recently, the CVD growth of graphene on polycrystalline Cu foil and single-crystal Cu substrates has been compared. High-quality graphene can be obtained by avoiding the influence of Cu grain boundaries and decreasing the © 2018 American Chemical Society

Received: June 8, 2017 Revised: February 15, 2018 Published: February 23, 2018 1472

DOI: 10.1021/acs.chemmater.7b02291 Chem. Mater. 2018, 30, 1472−1483

Article

Chemistry of Materials

the in situ, nondestructive Raman characterization of as-grown graphene on Cu foil.

A common approach toward enhancing the weak Raman signal of graphene on Cu foil is to transfer the graphene from the Cu foil onto a designed substrate [e.g., silicon dioxide (SiO2)/Si,30,31 one-dimensional photonic crystal,32,33 or nanocavity structure27] that provides a large electric field enhancement at the interface between the graphene and the substrate, thereby, increasing the light absorption and Raman scattering of the graphene. Unfortunately, such transfer processes can have unavoidable and unpredictable effects on the as-grown graphene, including the introduction of defects, impurities, and stress. As a result, the transferred graphene might not faithfully reveal the original character of the as-grown graphene on the Cu foil. The use of metal nanostructures is another major approach toward enhancing the Raman scattering of graphene.34−36 Upon integrating graphene with plasmonic metal nanostructures, the enhanced electric field can be used to significantly increase the light−graphene interaction. Furthermore, to avoid the deleterious effects of transfer processes and directly enhance the weak Raman signals of CVD-graphene on Cu foil, several methods have been proposed for integrating the metal nanostructures with the graphene/Cu foil.37,38 For instance, Zhao et al. deposited Au nanoislands onto the surface of graphene/Cu foils to enhance the Raman signals of the asgrown graphene;37 similarly, Xiang et al. observed enhanced Raman signals of as-grown graphene after thermal annealing of an Au film to form Au nanoparticles (AuNPs) on a graphene/ Cu foil.38 These approaches using metal nanostructures to enhance the Raman signals of the graphene can, however, have negative effects. For example, the metal nanostructures might seriously affect the carrier mobility, carrier concentration, and surface properties of the graphene.39 Therefore, it can be difficult to obtain the real Raman spectra of graphene without disturbances caused by metal doping.39 Moreover, after measuring the Raman spectra, it would be very difficult to remove the metal nanostructures from the surfaces of the asgrown graphene on the Cu foils. Dielectric nanostructures of high refractive index, including silicon nanoparticles (SiNPs), can generate high electromagnetic responses in the visible spectral range.40−43 The magnetic dipole resonance, which is accompanied by a displacement current, would induce a strong electromagnetic field inside the resonant SiNPs.43 In this study, we arranged SiNPs of specific dimensions upon an as-grown graphene/Cu foil to give rise to a dramatically intense electromagnetic field near the interface between the SiNPs and the graphene. Numerical simulations revealed that the strong electromagnetic field around the graphene originated from coupling between the magnetic dipole resonance of the SiNPs and the free carriers in the underlying Cu foil. The stronger electromagnetic field around the graphene could, therefore, be employed to greatly enhance its Raman signals. Indeed, the Raman signals of the as-grown graphene on the Cu foil were enhanced to a degree much higher even than that of the graphene transferred onto a 300 nm oxide/Si substrate. Notably, we observed experimentally that this approach enhanced the faithful Raman signals of as-grown graphene with almost no additional influence. Moreover, the SiNPs positioned on the CVDgrown graphene were readily removed without destroying the graphene. We have also found that this enhancement approach can be used to readily characterize the differences in quality of graphene grown on Cu foils having grain boundaries of various sizes. Therefore, this proposed method has great potential for

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EXPERIMENTAL METHODS

CVD-Grown Graphene. The conventional CVD process was used to grow graphene on Cu foils.44 Polycrystalline Cu foils (Nilaco) were placed on a hot wall furnace featuring a fused silica tube. Two procedures were used to grow graphene on Cu foils of various grain sizes. (i) The furnace was heated to 1000 °C. The polycrystalline Cu foils in the furnace were annealed for 3 h to increase the Cu grain sizes. After annealing, a reduction process was conducted in a H2 flow prior to the introduction of CH4. The single-layer graphene was synthesized through Cu-catalysis using CH4 as the carbon source.45 After 30 min of growth, the CH4 flow was turned off and the system was cooled under an Ar flow to room temperature. (ii) Polycrystalline Cu foil was used to directly grow graphene without prior annealing. Both procedures were conducted at a pressure of 500 mTorr. CVD-Grown Graphene on 300 nm Oxide/Si Substrate. The 300 nm oxide (SiO2)/Si substrates were cleaned sequentially with acetone, isopropyl alcohol (IPA), and deionized water and then dried under a flow of N2. The CVD-grown graphene was transferred onto the 300 nm oxide/Si substrate, over an area of approximately 1 cm2, through a polymer-mediated transfer process.33 SiNPs on Graphene/Cu Foil. SiNPs (diameter: ca. 120 nm) were purchased from Emaxwin Technology. The SiNPs were dispersed (concentration: 5 × 10−3 wt %) in EtOH under ultrasonication (30 min). The SiNP suspension was then spun (3000 rpm) onto the graphene/Cu foil. After measuring the Raman spectrum, the SiNPs were removed from the graphene/Cu foil by placing the SiNP/ graphene/Cu foil in EtOH under ultrasonication. AuNPs on Graphene/Cu Foil. The graphene/Cu foil was immersed in 1 mM (3-aminopropyl)trimethoxysilane (APTMS) in EtOH and incubated for 6 h. After a self-assembled monolayer (SAM) of APTMS had formed on the graphene/Cu foil surface, the graphene/Cu foil was rinsed sequentially with EtOH and deionized water to remove any nonbound APTMS. An aliquot (300 μL) of a AuNP suspension (Ted Pella) was placed on the graphene/Cu foil for 1 h. After the AuNPs had immobilized on the graphene/Cu foil surface, the samples were rinsed with deionized water and dried under a flow of N2. Measurement and Characterization. Scanning electron microscopy (SEM) images of the SiNP/graphene/Cu foils were recorded using a NOVA NANO 450 instrument. Optical images of the grain boundaries of the Cu foils were recorded using an optical microscope (Olympus), with focusing by a 20× objective having a numerical aperture of 0.4. Raman spectra of the graphene samples were recorded using a micro-Raman microscope (UniRAM, UniNanoTech) equipped with a monochromator (focal length: 75 cm). The wavelength of the excitation laser line was fixed at 532 nm. The power density of the excitation laser was ca. 300 μW/μm2. The laser beam was focused by a 100× objective having a numerical aperture of 0.95. The spot size of the excitation laser was approximately 0.4 μm2.

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RESULTS AND DISCUSSION Mie scattering results in the first and second lowest frequency resonances of dielectric particles corresponding to the terms for the magnetic and electric dipoles. The magnetic dipole resonance occurs at a value of λ/n of approximately dp, where λ is the wavelength of incident light, n is the refractive index of the particle, and dp is the particle diameter.40,43 The magnetic dipole of a SiNP having a diameter of 120 nm would resonate at a wavelength of approximately 530 nm (further details are provided in the Supporting Information). The resonant SiNPs would induce a strong electromagnetic field inside and around the SiNPs. In this study, we found that the intense electromagnetic field generated from magnetic dipole resonance of the SiNPs could be coupled to the as-grown 1473


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Figure 1. (a) Schematic representation of the simulation model: a plane wave (wavelength: 532 nm) was incident to a SiNP/graphene/Cu substrate system. (b, c) Simulated electromagnetic field intensity (b) and electric field intensity (c) and distribution of electric field intensity on the crosssection of the SiNP/graphene/Cu system. (d, e) Simulated magnetic field intensity (d), electric field intensity (e), and distribution of electric field intensity on the cross-section of the SiNP/graphene/SiO2 system. (f, g, h) Electric field intensity distributions of (f) a graphene coating with a SiNP on a Cu substrate, (g) a graphene coating with a SiNP on a SiO2 substrate, and (h) bare graphene on a Cu substrate.

graphene/Cu foil to concentrate an intense electromagnetic field around and within the graphene. In other words, the

Raman signal of as-grown graphene on a Cu foil could be enhanced significantly through a locally concentrated electro1474


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To further understand the enhancement of the electric field within graphene, we used a flat monitor along the x−y plane, located at the position of the graphene, to observe the electric field distribution. For comparison, Figure 1f−h displays the electric field distributions of a graphene coating with a SiNP on a Cu substrate, a graphene coating with a SiNP on a SiO2 substrate, and bare graphene on a Cu substrate, respectively. The electric field distribution (x−y plane) of the graphene coating with a SiNP in Figure 1f features two significant hot spots located on both sides of the SiNP, corresponding to the x−z plane simulation in Figure 1c. Remarkably, intense enhancements in the electric field intensity occurred at the locations of the hot spots (E2/E02 = 123.2). We also found (Figure 1g) two electric field hot spots within the graphene coating with a SiNP placed on a SiO2 substrate, but their intensities were much lower (E2/E02 = 12.9) than those in the SiNP/graphene/Cu system. (In the following section we discuss the ranges of the enhancements in the electric field intensities induced by the SiNPs.) In contrast, the electric field intensities within the bare graphene on the Cu substrate (Figure 1h) were distributed uniformly, over the entire range of the simulated domain. Because of interference between the incident light and its reflective light at the metal surface,27 the electric field intensity within the bare graphene on the Cu substrate was quite low (E2/E02 = 0.42). (Please see the Supporting Information for additional details regarding the oxidized surfaces of the SiNPs.) Increasing the intensity of Raman signals typically requires enhancements in the electric field of the excitation laser and the emission of Raman scattered light; in general, the enhancement factor of Raman signals approximates proportionally to the fourth power of the electric field (E4).50 To exploit the concentrated electric field within the graphene in the SiNP/ graphene/Cu foil system, we arranged SiNPs upon CVD-grown graphene on Cu foil and measured the Raman signal of the graphene. Figure 2a provides a schematic representation of the system employed for Raman spectral measurement of a SiNP/ graphene/Cu foil. SiNPs having a diameter of 120 nm were chosen to induce the magnetic dipole resonance at the wavelength of the Raman excitation laser (532 nm). Here, we deliberately arranged a low surface coverage of the SiNPs to ensure that (i) the well-dispersed SiNPs could generate their magnetic dipole resonance without interference from vicinal SiNPs; (ii) each strong electromagnetic field within the graphene would originate only from the coupling effect between the SiNP and the underlying Cu foil; and (iii) the incident excitation laser light and the spontaneous Raman scattering emission from the graphene would not be substantially absorbed by aggregated SiNPs. [An SEM image of the well-dispersed SiNPs on the surface of a graphene/Cu foil is presented later in this paper (Figure 4b).] Figure 2b displays the Raman spectra of the graphene measured under various conditions. We compared the Raman spectra of the graphene on a Cu foil, the graphene on a 300 nm oxide/Si substrate, and the graphene in a AuNP/graphene/Cu foil system (as discussed later) and in a SiNP/graphene/Cu foil system. We observed a very weak Raman signal from the asgrown graphene on the Cu foil (black line in Figure 2b). Although the 2D band of the graphene was evident, the G band was not quite as obvious. After transferring the graphene to the 300 nm oxide/Si substrate, the Raman signal of the transferred graphene was enhanced significantly, with both the 2D and G bands readily observed (blue line in Figure 2b). Interestingly,

magnetic field without the need for transfer processes or special substrates. To study the magnetic dipole resonance behavior of SiNPs placed upon graphene/Cu foil systems, we first investigated the electric and magnetic field distributions of a Raman excitation light inside a SiNP/graphene/Cu foil system. We used a threedimensional finite-difference time-domain (3D-FDTD) approach to simulate the behavior of excitation light interacting with the SiNP/graphene/Cu foil system. Figure 1a displays a schematic representation of the simulation model: an incident plane wave with an electric field polarization along the xdirection having a wavelength of 532 nm was incident to the SiNP/graphene/Cu foil system. In the system, a SiNP having a diameter of 120 nm was placed upon single-layer graphene (thickness: 0.3 nm) on a Cu substrate. According to the electric field of incident light setting an x-polarization direction, we arranged monitors at the y−z and x−z planes to observe the magnetic and electric field distributions, respectively, of the cross-section. Figure 1b displays the magnetic field distribution of the y−z cross-section of the SiNP/graphene/Cu system; the corresponding electric field distribution in the x−z plane is displayed in Figure 1c. In Figures 1b and 1c, the black round line represents the shape of the SiNP; the red dotted lines indicate the positions of the single-layer graphene; the black arrows denote the directions of the electric fields; and the colors map the intensities of the magnetic/electric fields. Figure 1b reveals an obvious and intense magnetic hot spot near the center of the SiNP. The directions of the electric fields (black arrows) inside the SiNP reveal the magnetic dipole resonance, accompanied by a displacement current loop. Figure 1c displays the electric field distribution of the SiNP/graphene/Cu system during the magnetic dipole resonance of the SiNP. A circular distribution of the electric field occurred inside the SiNP and also propagated outside the SiNP. Moreover, two remarkably concentrated electric field hot spots appeared between the SiNP and the Cu substrate. The two intense hot spots underlying the SiNP were located exactly at the position of the graphene (red dotted line). We attribute the intense electric field hot spots as originating from a strong coupling effect between the electromagnetic field around the SiNP and the free carriers inside the underlying Cu substrate. To further clarify the coupling effect, we investigated the case of a SiNP placed upon a graphene/dielectric (SiO2) substrate. We simulated a SiNP having a diameter of 120 nm (corresponding to a magnetic dipole resonance wavelength of 532 nm) on the graphene/SiO2 substrate. Figure 1d,e presents the simulated magnetic and electric field distributions, respectively, for this system. The obvious magnetic hot spot at the center of the SiNP (Figure 1d) indicates that the SiNP experiences an intense magnetic dipole resonance. In the electric field distribution (Figure 1e), we observed a clear electric field loop around the center of the SiNP and, simultaneously, an accompanying electric field outside the SiNP. Nonetheless, the electric field intensity located at the position of the graphene in this system was much lower than that in the SiNP/graphene/Cu system (Figure 1b); in other words, an electric field could not be concentrated at the position of the graphene in the SiNP/graphene/SiO2 system. For this reason, the strong electric field hot spots located at the position of the graphene in the SiNP/graphene/Cu substrate system (Figure 1b) presumably originated from a coupling effect between the SiNP and the underlying Cu substrate. 1475


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distribution of the cross-section of the AuNP/graphene/Cu system. Clear hot spots appeared at dipolar positions of the AuNP, indicating that the incident light upon the AuNP could effectively generate localized surface plasmon resonance (LSPR). The locations (dipolar position) of the hot spots were, however, away from the position of the underlying graphene. Although there was a fractional electric field intensity located at the position of the graphene, the dipolar position of the electric field intensity had a dominant effect on the electric field distribution. We arranged a monitor along the x−y plane, located at the position of the graphene, to observe the electric field distribution within the graphene. Figure 3c displays the electric field distribution on the graphene/Cu substrate coated with a AuNP. For comparison, we normalized the electric field distribution of the SiNP/graphene/Cu (Figure 3d) to the same scale as that in Figure 3c (AuNP/graphene/Cu). The intensity and area of the hot spot on the AuNP/graphene/Cu were much weaker and smaller, respectively, than those on the SiNP/graphene/Cu. To further investigate the areas of electric field enhancement generated by the SiNP and AuNP within the graphene, we used 3D-FDTD simulations to quantitatively analyze the effective ranges of the electric field. We placed a SiNP having a diameter of 120 nm (suitable for magnetic dipole resonance conditions at a wavelength of 532 nm) upon a graphene layer and simulated the distribution of the electric field within the graphene. For comparison, we also placed a AuNP having a diameter of 50 nm (suitable for LSPR conditions at a wavelength of 532 nm) upon a graphene layer and ran the same simulation. Figure 3e displays the electric field distribution within the graphene/Cu coated with a SiNP as a color map of the electric field intensity (E2): a blue region represents values of E2 of less than or equal 1; green represents values between 1 and 10; yellow represents values between 10 and 100; and red represents values between 100 and 300. Figure 3f displays the corresponding color map of the electric field intensity (E2) within the graphene presenting a coated AuNP: a blue region also represents values of E2 of less than or equal 1; green represents values between 1 and 10; and yellow represents values between 10 and 20. Within these graphene layers, the area of high electric field (green, yellow, red) generated from the SiNP was much larger than that (green, yellow) generated from the AuNP. Estimating the areas of the enhanced electric fields from the SiNP (E2 > 10) and the AuNP (E2 > 10) revealed that the former was 112 times greater than the latter. Similarly, the total area of the enhanced electric field (E2 > 1) around the SiNP was also larger (254 times) than that of the AuNP. Even more notably, the SiNP generated quite strong electric field hot spots (E2 > 100) within the underlying graphene. Therefore, the large area of the electric field generated by the magnetic dipole resonance of nonmetallic NPs can be more effective at enhancing the Raman scattering signals of underlying graphene than the LSPR effect generated by metallic NPs. We experimentally measured the Raman signals of the graphene in the AuNP/graphene/Cu foil system. Figure 3g displays the Raman spectra of bare graphene on a Cu foil and of graphene/Cu foils coated with AuNPs at various densities. Relative to the Raman spectrum of the bare graphene on the Cu foil (black line in Figure 3g), a weak Raman signal appeared from the high-density (coverage: 20.1%) AuNP/graphene/Cu foil sample (red line in Figure 3g). We attribute the weak Raman signal to the incident light and Raman scattering light

Figure 2. (a) Schematic representation of the Raman experimental arrangement; light from a Raman excitation laser (wavelength: 532 nm) was incident to a SiNP/graphene/Cu foil sample. (b) Measured Raman spectra of graphene on Cu foil, on a 300 nm oxide/Si substrate, in a AuNP/graphene/Cu foil sample, and in a SiNP/graphene/Cu foil sample.

after placing only a few SiNPs upon the graphene/Cu foil of the same batch, the Raman signals of both the 2D and G bands of the graphene were enhanced dramatically, beyond those even of the Raman signals of the graphene on the 300 nm oxide/Si substrate (magenta line in Figure 2b). Notably, the Raman spectrum of the graphene in each of these cases did not feature a D band. In general, an oxide/Si substrate has a surface electric field intensity higher than that of a Cu foil; thus, transferring CVD-grown graphene from a Cu foil onto an oxide/Si substrate is commonly employed to enhance the Raman scattering signals of the graphene.30,31 Here, we found that placing only a few SiNPs onto CVD-grown graphene/Cu foil was an even more efficient way of obtaining significantly enhanced Raman signals from the as-grown graphene. We suspect that extremely high electric fields within tiny gaps, highly localized around the graphene layer, resulted from strong coupling between the displacement current loop inside and around the SiNPs and the induced charges on the Cu foil. To compare the Raman enhancement effects of SiNPs and metal NPs placed upon the graphene/Cu foil, we investigated the behavior of AuNPs on an as-grown graphene/Cu foil. First, we simulated a AuNP arranged upon a graphene/Cu foil to observe its electric field distribution. Figure 3a provides a schematic representation of the simulation model; the plane wave with an electric field polarization along the x-direction was incident to the AuNP/graphene/Cu foil system. In this system, the AuNP (diameter: 50 nm) was placed upon a single-layer graphene on a Cu substrate. Figure 3b displays the electric field 1476


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Figure 3. (a) Schematic representation of the simulation model; a plane wave (wavelength: 532 nm) was incident to the AuNP/graphene/Cu substrate system. (b) Simulated electric field intensity distribution on the cross-section of the AuNP/graphene/Cu system. (c) Electric field intensity distribution on a graphene coating with a AuNP. (d) Electric field distribution (normalized scale) within graphene arranged in a SiNP/graphene/Cu foil system, for comparison with the AuNP/graphene/Cu system in (c). (e, f) Electric field distributions within graphene having a coated (e) SiNP and (f) AuNP. (g) Measured Raman spectra of graphene placed on a Cu foil and in AuNP/graphene/Cu foil samples at various AuNP densities.

stronger Raman signal to the enhancement in electric field arising from coupling between the AuNPs and the Cu foil; moreover, the low-density AuNPs may have absorbed only a small proportion of the incident light and Raman scattering

being largely absorbed by the high density of AuNPs. In contrast, the Raman signal of the graphene was enhanced when the AuNPs were present at low density (coverage: 13.6%) on the graphene/Cu foil (blue line in Figure 3g). We attribute the 1477


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simulations in Figurse 1f,h and 3c, we calculated the average value of E4 in each case. We considered both the wavelength of the excitation light and the emission of the Raman signal to calculate the average values of E4. From the ratios of the values of E4 for the various systems, we estimated the enhancements in the Raman signals of graphene. Relative to the Raman signal from the graphene on the Cu substrate, the Raman signals of graphene on the 300 nm oxide/Si substrate, the AuNP/ graphene/Cu system, and the SiNP/graphene/Cu system were enhanced by approximately 14, 1.9, and 258 times, respectively. From the measured spectra (Figures 2b and 3g), relative to the Raman signal of the as-grown graphene on the Cu foil, the graphene on the 300 nm oxide/Si substrate, in the AuNP/ graphene/Cu foil, and in the SiNP/graphene/Cu foil featured Raman signal enhancements of approximately 12, 3.7, and 206 times, respectively. Thus, the experimental Raman signal enhancement of the graphene in the AuNP/graphene/Cu foil was higher than that in the simulation, presumably because the beam spot of the experimental excitation laser excited several AuNPs simultaneously on the graphene/Cu foil. The experimental results for the SiNP/graphene/Cu foil were consistent with the simulation, suggesting that the substantially enhanced Raman signals of the graphene did indeed originate from coupling of the electromagnetic fields of the SiNPs and the Cu foil. Raman spectroscopy, STM measurement, and simulations have been used previously to investigate the large physisorption strain after CVD growth of graphene on metal substrates.47,48 In those studies, the Raman signals of the as-grown graphene on the metal substrates exhibited different characteristics arising from the surface strain induced by the background crystallinity of the metal substrate, even though the graphene was of the same quality as that after transfer onto the SiO2 substrate. We have demonstrated that coupling of the electromagnetic field between SiNPs and Cu foil can efficiently enhance the Raman signals of sandwiched graphene. Nevertheless, to practically characterize the Raman signals of as-grown graphene in the SiNP/graphene/Cu foil system, we had to consider the influence of the SiNPs on the graphene. Accordingly, we performed experiments to remove the SiNPs from the graphene/Cu foil and then compare the properties of the graphene before and after removing the SiNPs. To remove the SiNPs, we placed a sample of the SiNP/graphene/Cu foil (that we had used for the experimental Raman spectroscopic measurements) in EtOH and subjected it to ultrasonication for 1 h. Figure 4a displays a schematic representation of the SiNP removal process. Because EtOH was the solvent of the initial SiNP suspension, the SiNPs upon the graphene/Cu foil were amenable to removal through ultrasonic cleaning in EtOH. Figure 4b,c displays SEM images of the graphene/Cu foil before and after removing the coated SiNPs, respectively. In Figure 4b, we observe well-dispersed SiNPs, each having a diameter of approximately 120 nm, on the surface of the graphene/Cu foil. After removing the coated SiNPs, Figure 4c reveals a clear surface for the graphene/Cu foil. The two SEM images provide clear evidence for the coated SiNPs being easy to remove from the graphene/Cu foil. Next, we compared the Raman spectra of the graphene/Cu foil in its as-grown state, after coating with the SiNPs, and after removing the SiNPs. Figure 4d presents the Raman spectra of the as-grown graphene on the Cu foil (black line), the SiNP/graphene/Cu foil (blue line), and the graphene on the Cu foil after removing the coated SiNPs (red line). Similar to the situation in Figure 2b, a

light. Comparing these experimental results with those obtained using the SiNPs (Figure 2b), the signal enhancement provided by the SiNPs was better than that of the AuNPs. The LSPR phenomenon generated by AuNPs would induce strong absorption of the incident light by the AuNPs. In contrast, the low loss of SiNPs would not greatly absorb the incident light. In addition, coupling between the SiNPs and the Cu substrate would induce strong electric field hot spots located only at the position of the graphene in the SiNP/graphene/Cu substrate system. Although we observed a strong Raman signal from the graphene in the AuNP/graphene/Cu foil (red line in Figure 2b), its enhancement was lower than that in the case of the SiNP/graphene/Cu foil (purple line in Figure 2b). In addition, in comparison with the Raman spectrum of the bare graphene on a Cu foil, the Raman signal from the AuNP/graphene/Cu foil system was blue-shifted, presumably a metal doping phenomenon from the AuNPs. We also observed that the graphene provided a broadened G peak in the Raman spectra. We suspect that this broadened signal may have arisen from defects or strain that resulted from the process of coating the AuNPs on the graphene/Cu foil.46 Tables 1 and 2 provide further details regarding the enhancement factors and spectral peaks. Table 1. Simulated Average Values of Enhancements of the Fourth Power of the Electric Field (E4) and Measured Enhancements of 2D Band Intensities of Graphene on Cu Foil, on a 300 nm Oxide/Si Substrate, in a AuNP/ Graphene/Cu Foil Sample, and in a SiNP/Graphene/Cu Foil Sample

Gr./Cu Gr./300 nm oxide/Si AuNPs/Gr./ Cu SiNP/Gr./Cu

simulated E4avg.

simulated E4avg. enhancement

measured Raman signal enhancement

0.148 2.07

1 14

1 12

0.281 38.1

1.9

3.7

258

206

Table 2. Characteristics of Raman Spectra (peak positions of 2D and G bands; peak shifts of 2D band; FWHM of 2D and G bands) Used To Identify the Quality of Graphene on a Cu Foil, in a SiNP/Graphene/Cu Foil, after Removing the SiNPs from a SiNP/Graphene/Cu Foil, on a 300 nm Oxide/ Si Substrate, and in a AuNP/Graphene/Cu Foil

Gr./Cu Foil SiNPs/Gr./Cu foil Gr./Cu foil (after removing SiNPs) Gr./300 nm oxide AuNPs/Gr./Cu foil

2D peak position (cm−1)

2D peak shifting (cm−1)

G peak position (cm−1)

2D fwhm (cm−1)

G fwhm (cm−1)

2646 2646

0 0

1571 1571

38.9 38.8

15.3 16.8

2646

0

1571

38.6

14.7

2655

+9

1573

56.7

18.8

2651

+5

1572

72

34

Here, we consider just the 2D band of graphene to discuss the enhancement abilities of the various structures, determined from both the simulated and measured results. From the 1478


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obtained from the Raman spectral characteristics; for comparison, we also provide the data for the transferred graphene on the 300 nm oxide/Si substrate and the graphene in the AuNP/graphene/Cu foil. In terms of the Raman peak positions (first three columns in Table 2), the 2D and G bands of the graphene were not affected after placing the SiNPs on the graphene/Cu foil, or after removing the SiNPs. In contrast, the peak positions of the 2D and G bands were red-shifted when we transferred the graphene from the Cu foil to a 300 nm oxide/Si substrate; similarly, the peak position of the 2D band of the graphene was also red-shifted when placed in the AuNP/ graphene/Cu foil. The red-shifting in these spectra indicates that the as-grown graphene may have suffered the effects of strain during its transfer to the 300 nm oxide/Si substrate.49,49 In contrast, placing SiNPs upon the surface would not likely cause any strain on the as-grown graphene. The red-shifting in the spectra might also infer that the electron mobility of the asgrown graphene changed after doping with the AuNPs.49 In terms of the fwhm’s of the Raman peaks of the graphene (last two rows of Table 2), the coated SiNPs did not obviously affect the 2D and G bands of the as-grown graphene on the Cu foil, suggesting that the SiNPs did not induce additional doping on the as-grown graphene on the Cu foil. In contrast, the fwhm’s of both the 2D and G bands of the graphene were broadened after its transfer from the Cu foil to the 300 nm oxide/Si substrate. Graphene can undergo a change in its electron mobility after changing the substrate from Cu foil to 300 nm oxide/Si.51,52 Moreover, placing AuNPs on the graphene/Cu foil resulted in significantly broadened fwhm’s for both the 2D and G bands of the graphene. The broadened 2D peak provided evidence for the graphene being doped, and with a change in its electron mobility, after positioning the AuNPs upon it. Our observed wavelength-shift and broadening of the 2D peak of the graphene in the AuNP/graphene/Cu foil are similar to those reported previously.37,38 Relative to the transferred graphene and the AuNP/graphene/Cu foil, we believe that the quality of the graphene in the SiNP/graphene/ Cu foil sample was almost unaffected by the few coated SiNPs; in addition, the original quality of the graphene was not changed after removing the SiNPs. For this reason, we believe that coupling the electromagnetic field between coated SiNPs and underlying Cu foil is an effective, practical, and nondestructive approach for enhancing the Raman signals of asgrown graphene. (Please see the Supporting Information for more details of the estimated effects of the background crystallinity of the Cu foil and of the SiNPs on the graphene/ Cu foil.) To demonstrate the practicality of this approach, we prepared two graphene samples to discriminate their quality after enhancing their Raman signals through coating with SiNPs. We used polycrystalline Cu foils, prepared with and without annealing, to grow graphene sample of different quality. After growth of the graphene, SiNPs having a diameter of 120 nm were placed upon both samples. The crystal grains of the annealed Cu foil were coarser (larger) than those of the original Cu foil.22 Previous studies have revealed that the grain boundaries and crystallinity of the underlying Cu can have significant effects on the quality of the CVD-grown graphene.20−22 Figure 5a displays a schematic representation of the coarse and fine crystal grains of Cu foils that we used to prepare the various graphene samples. Figure 5b,c presents optical micrographs of the SiNP/graphene/Cu foils prepared without and with annealing of the Cu foil, respectively. The

Figure 4. (a) Schematic representation of the approach for SiNP removal. (b, c) SEM images of graphene/Cu foils (b) before and (c) after removing the coated SiNPs. (d) Raman spectra of graphene: asgrown on the Cu foil, in the SiNP/graphene/Cu foil, and on the Cu foil after removing the coated SiNPs. For more details, see the Supporting Information.

weak Raman signal arose from the as-grown graphene on the Cu foil, while a significantly enhanced signal was obtained from the SiNP/graphene/Cu foil. Interestingly, after removing the coated SiNPs, the Raman signal of the graphene was almost the same as that of the original as-grown graphene. Moreover, the characteristic features in the Raman spectra that identified the quality of graphene (including the positions and fwhm’s of the 2D and G peaks) were almost identical in each of these three cases. Table 2 summarizes the distinct graphene qualities 1479


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Figure 5. (a) Schematic representation of SiNP/graphene on Cu foils having fine and coarse crystal grains. (b, c) Optical micrographs of the measured SiNP/graphene/Cu foil samples having (b) fine (small) and (c) coarse (large) grains. (d, e) Measured Raman spectra of graphene grown on Cu foil having (d) fine (small) grains and (e) coarse (large) grains (before coating with SiNPs). (f, g) Measured Raman spectra corresponding to the graphene grown on the Cu foil having (f) fine (small) grains and (g) coarse (large) grains (after coating with SiNPs).

grain boundaries of the Cu foils are clearly evident in these images. The original polycrystalline Cu foil (without annealing) featured finer (smaller) grains (270 μm; Figure 5c). We have marked the measured positions that provided enhanced Raman signals of graphene in the

optical micrographs. Figure 5d,e displays the measured Raman spectra of the graphene grown on the original polycrystalline Cu foil (Figure 5b) and the annealed Cu foil (Figure 5c) before coating with SiNPs, respectively. In Figure 5d,e, we measured the Raman signals from three random points of the graphene grown on the two different Cu foils, obtaining six very similar 1480



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CONCLUSIONS We have developed a practical method, involving magnetic dipole resonance and coupling effects, through which the Raman signals of as-grown graphene on copper foil can be directly, faithfully, and greatly enhanced. The magnetic dipole resonance of SiNPs can effectively couple the electromagnetic field, generated from the displacement current loop with the Cu foil, to induce an intense electric field located solely at the position of the graphene. Remarkably, the coupled electromagnetic field resulted in hot spots of high electric field intensity (E2/E02 = 123.2) within the graphene. In addition, the range of the enhanced electric field intensity exceeded the physically projected area of a SiNP. This behavior implies that the resonant SiNPs could efficiently enhance the large-scale electric field of the hot zone within the graphene in the SiNP/ graphene/Cu foil system. When examining how the coupled electric field could enhance the Raman signals of as-growth graphene, we found that the presence of only a few SiNPs upon the graphene/Cu foil facilitated measurement of the Raman signal. The Raman signal enhancement of the SiNP/graphene/ Cu foil sample (ca. 206 times) was much higher than that of the graphene placed on a 300 nm oxide (ca. 12 times). To further investigate the influence of the SiNPs on graphene, we performed a series of experiments to compare the Raman signals of the graphene before and after removing the SiNPs from the graphene/Cu foil. The coating SiNPs had almost no effect on the quality of the as-grown graphene. Finally, we investigated the graphene grown on Cu foils of two different grain sizes. We could use the SiNP-enhanced Raman signals of the graphene to distinguish the local quality of the as-grown graphene at different sites and, therefore, the quality of the underlying Cu foil. Accordingly, we have demonstrated that locally concentrated electromagnetic fields can be used practically to identify the quality of graphene on Cu foils. We believe that this approach for Raman signal enhancement might be useful for in situ and nondestructive characterization of asgrown graphene on Cu foil, without the need for transfer processes or harmful processing conditions.

and weak Raman signals. These Raman signals featured only the 2D peaks and could not be used to differentiate the quality of the graphene on the distinct Cu substrates. In contrast, Figure 5f,g displays the measured Raman spectra of the graphene grown on the original polycrystalline Cu foil (Figure 5b) and the annealed Cu foil (Figure 5c) after coating with SiNPs, respectively. In Figure 5f, the five Raman spectra presented in the different colors were recorded from the positions marked in Figure 5b. The positions A, D, and E provided similar Raman signals: clear 2D and G peaks, with a height ratio of greater than 2:1. In addition, D bands, associated with defects in graphene, were absent in the spectra recorded at positions A, D, and E. The spectra are consistent with almost perfect single-layer graphene existing at these positions. In contrast, the Raman spectrum of graphene at position B featured a prominent defect peak (D band) in addition to the 2D and G peaks, suggesting that defected CVD-grown graphene would exist at the junction between the grain boundaries of the Cu foil. The Raman spectrum of the graphene collected from the C position featured 2D and G peaks at an intensity ratio of approximately 1:1, in addition to an obvious D peak, consistent with the graphene overlapping at this position; simultaneously, this overlapped region of graphene presumably featured defects or dislocations. (Additional details of the Raman mapping of the SiNPs/graphene/ Cu substrate are provided in the Supporting Information.) Moreover, for the graphene grown on the annealed Cu foil, we used the SiNP-enhanced Raman spectra (Figure 5g) to identify the quality of the as-grown graphene at the five positions in the image in Figure 5c. The graphene grown on a single Cu grain having a single crystal orientation provided quite homogeneous Raman signals (Figure 5g) at the five positions. The clear 2D and G peaks, the high 2D-to-G intensity ratios, and the absence of a D peak all indicate the unblemished nature of the single-layer graphene grown on the annealed Cu foil. To summarize the results for the as-grown graphene on Cu foils featuring distinct grain sizes, we make the following observations: (i) SiNP-enhanced Raman signals of graphene can be used to distinguish the quality of as-grown graphene on Cu foils; notably, the original Raman signals of the as-grown graphene on the Cu foils (Figure 5d,e) were too weak to identify the quality of the graphene, with a clear 2D peak being the only signal. (ii) The graphene grown on Cu foils having coarse and fine crystal grains possessed different qualities and uniformities. (iii) The single-layer graphene formed on a single Cu grain was highly uniform. (iv) The Cu grain boundaries resulted in defects in the graphene quality. Previous studies have suggested that the graphene grown on single-crystal Cu has been of high quality.21 We must emphasize, however, that the current data are not sufficiently statistically robust to confirm that graphene grown on large-grain Cu would be of superior quality. Large-grain Cu has more “flat areas,” which might lead to a higher probability of collecting Raman data from a smooth and high-quality graphene. We have found that locally concentrated electromagnetic fields induced through coupling between the magnetic dipole resonance of SiNPs and the underlying Cu foils can be used practically to identify the quality of as-grown graphene on Cu foils, because such systems provide greatly enhanced Raman spectra.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b02291. Details of enhancing Raman signals of graphene through magnetic dipole resonance; discussion of oxidized surface of SiNPs; separation of the effect between background crystallinity of Cu and SiNPs on the graphene/Cu foil; Raman signal mapping of SiNPs/graphene/Cu foil system; and fluctuation of Raman signal of as-grown graphene on a copper foil (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(H.-L.C.) E-mail: [email protected]. ORCID

Chun-Wei Chen: 0000-0003-3096-249X Hsuen-Li Chen: 0000-0002-7569-572X Notes

The authors declare no competing financial interest. 1481


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ACKNOWLEDGMENTS We thank the Ministry of Science and Technology, Taiwan, for supporting this study under contracts MOST 106-2221-E-002158-MY3 and MOST 106-2221-E-002-105-MY3.

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