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Abstract
A wafer-scale, hybrid metasurface consisting of randomly-distributed silver nanoparticles and silver substrate separated by a dielectric spacer is reported to realize the enhancement of optical absorption and Raman spectroscopy of graphene. Induced by the surface plasmon resonance, strong localized optical field and broadband absorption from the designed hybrid metasurface are achieved at visible frequencies. As a result, with graphene sheet transferred on the top of the metasurface, the absorption of graphene is remarkably improved across the entire visible region and its signal of surface enhanced Raman scattering is boosted more than 50-fold than that of single layer silver nanoparticles without silver substrate. We envision this lithography-free, hybrid metasurface would be a promising candidate as a platform for the optoelectronic applications of ultrathin two-dimensional materials.
© 2017 Optical Society of America
1. Introduction
Graphene, a two-dimensional thin material of carbon atoms, are promising for use in the next generation of high performance optoelectronic devices due to its outstanding electric and optical properties [1–3]. For instance, the broadband optical absorption [4] and ultrahigh charge mobility [5] of graphene have led to ultrafast photon detectors [6–12]. Its unique electronic band structure also makes graphene be a good transparent electrode in light-emitter diodes [13] and LCD screens [14, 15]. However, for a mono-layer graphene, the optical absorption is only 2.3% in the visible range [4], which makes its optical response be extremely weak. This is unfavorable for the application of graphene in active optoelectronic devices, such as solar energy conversion [16] and broadband photoelectric modulator [17]. Therefore, the request for enhancing light-graphene interaction and optical absorption is essential.
Several approaches have been proposed to enhance the optical absorption of graphene, such as integrating graphene with periodical plasmonic [18–22] or photonic nanostructures [23–27] that can generate strong localized optical fields on the structure surface. Although above nanostructures can improve the optical absorption of graphene, enhancement only appears at specific resonant wavelength. In addition, these nanostructures are always fabricated by the top-down approach including electron beam lithography, which significantly limits the device footprint and increases the manufacture cost.
In this letter, we propose and experimentally demonstrate a hybrid metasurface to efficiently enhance the light-graphene interaction at visible frequencies. The hybrid metasurface is consisted of randomly-distributed silver nanoparticles and silver mirror substrate separated by a dielectric spacer, forming a metal-dielectric-metal (MDM) geometry [28, 29]. The surface plasmon resonance in the hybrid MDM metasurface generates strong localized optical field and broadband absorption at visible frequencies, which greatly enhances the interaction between light and graphene sheet transferred on the top of metasurface. As a result, the hybrid metasurface not only boosts the optical absorption of atomic-thin graphene layer, but also remarkably enhances its pristine Raman signal. In compared to the case of transferring the graphene directly on the top of silver nanoparticles [30], the Raman signal from hybrid MDM metasurface is enhanced more than 50-fold. The lithography-free, large-scale wafer manufacturing process and excellent performance makes the MDM hybrid metasurface be a potential platform for the application in the high performance optoelectronic devices incorporating two-dimensional materials.
2. Structural characterization of the MDM metasurface
The graphene-coated MDM hybrid metasurface structure is schematically illustrated as Fig. 1(a). A flat, thick silver film and a SiO2 spacer are deposited on the Si substrate by sputtering, respectively. Top layer consisting of randomly-distributed silver nanoparticles is fabricated by sputtering at a very low deposition rate. The size of silver nanoparticles is controlled by varying the sputtering time. The graphene film is prepared on the Cu foils by low pressure chemical vapor deposition (CVD) in a tubular quartz reactor using methane as the carbon source under a gas mixture of H2 and Ar [31]. Then the Cu foils underneath the graphene is removed by wet chemical etching [32] and the graphene film is transferred on the top of the MDM hybrid metasurface. Figure 1(b) shows the image of prepared MDM metasurface with a diameter of 1 inch. Here, the sample size is only limited by the size of the holders in our sputter system.
Fig. 1 (a) Schematic of the graphene-coated MDM hybrid metasurface structure. The graphene film is covered on the top of the MDM metasurface. Silver nanoparticles are deposited on a SiO2 spacer above a 200 nm-thick silver film. Silicon wafer is used as the supporting substrate. (b) Image of prepared MDM metasurface with a diameter of 1 inch.
Inset of Fig. 2(a) shows the scanning electron microscopy (SEM) images of hybrid MDM metasurface. As displayed in the SEM images, the discrete silver nanoparticles are formed on the SiO2 layer, and the morphology of silver nanoparticles can be tuned by varying the sputtering time. Guided by the numerical optimizations, silver nanoparticles with average effective particle diameters about 18 nm are deposited on the top surface of 30 nm-thick SiO2 spacer on the silver substrate. Figure 2(a) shows the Gaussian distribution fitting of the diameter of the deposited silver nanoparticles. The corresponding sputtering time is 45 s and deposition rate is 1 Å/s. To investigate the performance of the hybrid MDM metasurface, we experimentally measure the absorption spectrum of the sample using UV-VIS-NIR microspectrometer (CRAIC PV20/30), as shown in Fig. 2(b). The optical absorption of the fabricated hybrid MDM metasurface is about 80% and the spectrum distribution is relatively flat across the entire visible frequency range, which agrees well with the photo of the MDM metasurface showing low surface reflection under ambient light given in Fig. 1(b). This would further benefit the broadband enhancement of optical absorption for the graphene sheet at visible frequency.
Fig. 2 (a) Histogram of effective diameter of silver nanoparticle of the hybrid metasurface. Insert: Scanning electron microscope (SEM) image of the metasurface. Scale bars is 300 nm. (b) Experimentally measured optical absorptions spectra of the hybrid metasurface.
3. Simulation and analysis
To better understand the mechanism of high absorption and associated local field enhancement of the hybrid MDM metasurface, we perform the finite-difference time-domain (FDTD) numerical simulations on a simplified structure consisting of a cylindrical silver nanoparticle. Figures 3(a) and 3(b) show that cross-section of simulated electric field intensity and electric displacement (black arrows) distribution for silver nanoparticle with and without silver substrate at the central wavelength of 532 nm. The diameter and height of the particle are 18 nm and 10 nm, respectively. For the structure with silver substrate, the thickness of SiO2 spacer is 30 nm. It can be clearly seen that compared with the single layer nanoparticle without silver substrate (Fig. 3(a)), the electric field intensity around the nanoparticle with silver substrate is much higher (Fig. 3(b)). This originates from the fact that electric displacement current in MDM geometry forms a loop (red and blue arrows shown in Fig. 3(a)) and generates inductance (L). Combined with capacitance (C) induced by the gap between silver nanoparticle and substrate, the resultant LC oscillation model results in a strong magnetic resonance [33, 34]. Therefore, the optical absorption and near-field intensity for MDM structure are much higher than those of single layer nanoparticle structure. Besides the simulations performed on the hypothetical cylindrical nanoparticle structures, we also pick an 800 nm × 800 nm region from the actual SEM image shown in Fig. 2(a) and directly import it into the simulation software. As expected, at the incident wavelength of 532 nm, the normalized electric field intensity on the top surface of hybrid MDM metasurface is also much higher than that of same-distributed single layer silver nanoparticles without metal substrate (Figs. 3(c) and 3(d)). Although here we only show the simulation results at the wavelength of 532 nm, the randomly distributed nanoparticles with a wide range of different diameters make the MDM metasurfaces possess broadband localized optical field enhancement effect.
Fig. 3 Electric field intensity and electric displacement (black arrows) distribution at the cross-section plane for single cylindrical silver nanoparticle (a) with and without silver substrate. The electric field intensity distribution on the top surface for mass silver nanoparticles (c) with and (d) without silver substrate. The shape and size of silver nanoparticles in (c) and (d) are obtained from the actual SEM image in an 800 nm × 800 nm region.
4. Enhancing absorption and Raman spectroscopy of graphene
The broadband optical absorption and huge local field enhancement make the hybrid MDM metasurface be beneficial for improving the absorption of graphene. Therefore, we carry out absorption measurements of the graphene sheet transferred on the hybrid MDM metasurface. The transferred graphene size is about 1 cm × 1 cm. Figure 4(a) shows the measured Raman spectrum of the graphene sheet transferred on the SiO2 substrate from Cu foils by wet chemical etching. The G peak (1585 cm−1) originates from a first order Raman scattering process and is a doubly degenerate phonon mode at the Brillouin zone center, while the 2D peak (2690 cm−1) originates from a two phonon double resonance Raman process [35]. The weak D peak (1350 cm−1) indicates that the transferred graphene has few defects. The intensity of the 2D peak is higher than that of the G peak, which implies that graphene with 1-2 layer are dominant in our sample [36].
Fig. 4 (a) The Raman spectrum of graphene film on the SiO2/Si substrate. Insert: The SEM image of graphene sheet transferred on top of MDM metasurface. (b) Measured optical absorption spectra of graphene sheet on metasurface. The reference is the measured optical absorption of graphene sheet on SiO2 substrate.
Inset of Fig. 4(a) shows the SEM image of the graphene film transferred on the hybrid MDM metasurface, where the boundary of graphene sheet can be clearly seen. The hybrid MDM metasurfaces has silver nanoparticles with averaged diameters ~18 nm and a SiO2 spacer with thickness of 30 nm. Figure 4(b) shows the experimentally measured optical absorption of graphene sheet transferred on metasurface, which is obtained by extracting the absorption difference between the metasurface samples with and without the graphene sheet. Here, the absorption of the graphene sheet on the SiO2 substrate is also measured as a reference. It can be seen that compared with the graphene on SiO2 substrate, the energy absorbed by graphene sheet on hybrid MDM metasurface is significantly enhanced across the entire visible spectral range. The average absorption of graphene at visible frequency (400 nm to 750 nm) increase from about 2.62% to 7.35%, corresponding to an enhancement factor of 2.81. The results imply that we can readily obtain the huge enhancement of optical absorption of graphene at visible frequency with hybrid MDM metasurface structure, which would be advantageous for many potential applications.
Besides the absorption enhancement, the strong light-graphene interaction is also beneficial for improving the surface-enhanced Raman spectroscopy (SERS) signals of the graphene. Here, a 532 nm laser with a 10 μm focusing spot size is used in the Raman scattering measurements. As a comparison, we also characterize the Raman spectrum of the graphene sheet transferred on the same distributed silver nanoparticles (effective diameter ~18 nm) without metal substrate as a reference. Figure 5(a) shows Raman spectra of the graphene transferred on the hybrid MDM metasurface and reference single layer silver nanoparticles. The result shows that the Raman signal of the graphene is much larger for the hybrid MDM metasurface than that of the single layer silver nanoparticles without metal substrate. By comparison of the Raman peak intensities of graphene on the hybrid MDM metasurface and reference structure, we obtain enhancement factor up to 50 for G peak and 20 for the 2D peak in hybrid MDM metasurface. The lager SERS enhancement factor is consistent with the stronger field intensity enhancement that is induced by the hybrid MDM metasurface structure.
Fig. 5 (a) Raman spectra of the graphene transferred on the hybrid MDM metasurface. The reference sample has same distributed silver nanoparticles without metal substrate. The signal from reference sample is multiplied by 10 for ease of visualization. (b) Raman mapping image of 1585 cm−1 Raman line of graphene on metasurface structure.
To evaluate the uniformity in SERS enhancement with position on the hybrid MDM metasurface, we perform scanning measurement of Raman signals over a 300 μm × 300 μm region of the hybrid MDM metasurface with a step size of 30 μm. The Raman mapping image of 1585 cm−1 Raman line of graphene on hybrid MDM metasurface is shown in Fig. 5(b). The background is subtracted as a function of stage position and the relative standard deviation is smaller than 6% of the mean. This indicates that the SERS enhancement spectra have a good uniformity and repeatability, which is consistent with the uniform field intensity enhancement produced by the hybrid MDM metasurface.
5. Conclusions
In summary, a hybrid MDM metasurface consisting of randomly-distributed silver nanoparticles is proposed and experimentally demonstrated to achieve strong light-graphene interactions at visible frequency. The surface plasmon resonances induced by the MDM geometry not only enhance the optical absorption of the graphene sheet, but also significantly boost its surface-enhanced Raman spectroscopy signals. The wafer-scale, lithography-free processing and excellent optical performances make the hybrid MDM metasurface be a potential platform for future application of high performance optoelectronic devices incorporating two-dimensional materials.
Funding
National Natural Science Foundation of China (61575092).
Acknowledgments
The authors acknowledge support from the Thousand Talents Program for Young Professionals, Collaborative Innovations Center of Advanced Microstructures.
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