Gobind Das*, Manola Moretti, Bruno Torre, Marco Allione, Andrea Giugni and Enzo Di Fabrizio
Physical Sciences and Engineering (PSE), King Abdullah University of Science and Technology (KAUST), Kingdom of Saudi Arabia
Received date: December 19, 2016; Accepted date: March 02, 2017; Published date: March 06, 2017
Citation: Das G, Morett M, Torre B, Allione M, Giugni A, et al. (2017) Graphene: A Building Foundation for Efficient Plasmonic SERS Device. Biochem Anal Biochem 6: 310. doi: 10.4172/2161-1009.1000310
Copyright: © 2017 Das G, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
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In this paper graphene over SiO2/Si substrate was used as a building base to fabricate SERS device. A thin film of silver was deposited over graphene and annealed at 250°C. The substrate was examined after chemisorption of two molecules; rhodamine 6G which is a fluorescent dye, and 3-mercaptobenzoic acid, a thiol molecule. SERS enhancement factor for the proposed device is estimated to be 2.1 × 106 with respect to the flat silver substrate deposited over Si. The experimental results were compared in two ways: a) by using electromagnetic field calculation and b) quantum mechanical calculation derived from density function theory. The experimental findings were consistent with the theoretical observations.
Graphene, a planar atomic layer of carbon atoms organized hexagonally, is the center of research efforts because of its exceptional electrical, mechanical, and optical properties [1-5]. The optical and mechanical properties keep the graphene in the front line of research associated to thin-film transistors, photonics, micro-/nanomechanical system [6-8]. In the field of photonics, graphene gained a remarkable importance due to its applications ranging from analytical sensor to light-emitting devices and lasers to photodetectors . Herein, we demonstrate the graphene based Surface enhanced Raman scattering (SERS) device as an efficient analytical sensor.
SERS is an analytical technique, which provides the fingerprint of molecular bonds despite the intrinsic weak Raman signal . It is being widely employed in order to have high surface sensitivity and giant signal enhancement . There are two main factors associated to the increase in Raman signal: localization of electric field (in the range of 106 to 108) due to the presence of plasmonic hot-spots and the chemical effect (usually in the range of 102) which involves a charge transfer between metal and the chemical compound in its close proximity. When an electromagnetic radiation is exposed over it, results a huge enhancement in Raman signal. SERS effect, in combination with proper designed SERS substrate and Raman spectroscopy, augments the sensitivity of the analytical measurements .
SERS devices were fabricated by means of different techniques: e.g. electron beam lithography (EBL), optical lithography, electrochemical methods, anodic porous alumina template assisted, or a combination of different techniques (EBL and electroless, focused ion beam lithography and optical lithography), etc. [13-17]. For manufacturing the abovementioned devices, one needs different sophisticated instruments and many fabrication steps. All these processes and multistep production techniques result in an increase in the price tag of the overall devices. The optical properties of the nanostructures (size and shape) determine the efficiency of the device and the final experimental results, i.e., changing the shape and size of the nanostructure leads to a variation in the optical response [18,19]. It is always a bottleneck demand to have a uniform, reproducible, highly sensitive and cost effective SERS substrate available [20,21]. Few efforts were made in the past utilizing graphene surface for SERS applications [22-24]. In the current work, we mainly focus on the nanotailoring or nanoassembly of devices with a uniform distribution of metal nanoparticles exploiting a graphene based SERS design. The clear, sharp and well-studied Raman fingerprints of graphene (as a function of number of layers, laser excitation frequency, etc.) facilitates the molecular analysis as the Raman spectrum contains only one intense Raman band at around 1600 cm-1 in the region of importance of 400-2000 cm-1 [25,26].
In this work, a fast and simple technique is employed to fabricate SERS devices. A graphene layer is used as a building base for SERS devices. Single-layer graphene (SLG) coated with silver is used here to employ a simplified fabrication process for plasmonic application. The devices were thereafter annealed at sintering temperature (Ts) of 250°C, leads to an increase in average Ag nanoparticle size [27-31]. The morphology of as prepared (AsPrep) and annealed (As250) samples were analyzed by means of atomic force microscopy (AFM) technique. The SERS activity of devices was investigated using rhodamine 6G (Rd6G) and 3-mercapto benzoic acid (3-MBA) molecules. The SERS substrate (graphene and silver nanoparticles) behaves as an active biosensor, providing an enhanced Raman intensity and, at the same time, a fluorescence quenching substrate [32,33].
SLG on SiO2/Si substrate was purchased from Fisher Scientific – USA. Silver metal of 8 nm thickness was deposited by means of benchtop sputtering technique (Quorum Technologies, model: Q150T). The deposition rate was fixed to 0.5A/s. The sample is thus termed as AsPrep_GAg. AsPrep_GAg sample was then annealed at the sintering temperature of 250°C for 1h dwell time under controlled N2 atmosphere, thus labeled As250_GAg. Two kinds of samples (AsPrep_GAg and As250_GAg) were then analyzed by AFM techniques. The schematic representation of sample preparation for spectroscopic analysis is shown in Figure S1 (Supplementary information: S1).
The devices were, thereafter, immersed in the molecular solution (Rd6G in water and 3-MBA in ethanol from Sigma) and let chemisorb for 30 minutes at room temperature. The devices were then gently rinsed with the respective solvent to wash out excess molecules that are not directly attached to the metal surface. Afterwards, the sample was dried under nitrogen flow. Now the sample is ready to investigate the molecules of interest.
AFM measurements on AsPrep_GAg and As250_GAg samples were carried out by means of Asylum Research Instrument (model: MFP_3D-Bio AFM). The samples (before and after annealing) were measured in tapping mode by means of Mikromasch probes (HQXSC11, tip c,) of 170 kHz nominal resonance frequency and nominal spring constant 2 N/m) for a topography scanning areas of 1 × 1 um2. The following statistical quantities were calculated: for AsPrep_GAg, rms roughness of 2.06 nm and average particle radius of 10.1 ± 5.7 nm; and for As250_GAg, rms rougness of 9.12 nm and average particle radius of 18.6 ± 9.1 nm. AFM images of these two devices are shown in Figure 1.
Micro-probe Raman measurements were performed by WiTec Alpha 300RA. The system was equipped with 600 grooves/mm, objective 100 X (NA: 0.9), EMCCD detector to achieve the well dispersed intense Raman signal. The whole system was placed on active vibration platform to avoid any interference from surrounding movements. The molecules were excited by 532.0 nm laser wavelength in backscattering configuration. The samples were investigated before and after measurements to assure no conformational changes induced by laser exposure. Spectral analysis was carried out using WiTec Project 4, commercial software.
Density function theory (DFT) calculation was performed for a free molecule and for the molecule attached to silver for SERS configuration. In case of Ag attached molecule, the metal particle was attached to the thiol group releasing H atom . The calculations were carried out using the commercially available ADF2016 package. The molecule was firstly optimized in its geometry and thereafter Raman bands were estimated using generalized gradient approximations (GGA) of Perdew-Burke-Emzerhof (PBE) exchange functional and TZP basis set.
The Raman spectra of graphene sample, before and after Ag metal deposition, are shown in Figure 2. Micro-Raman spectrum of SLG on SiO2/Si substrate shows clear contribution from two characteristic G and 2D bands, centred at 1585.2 and 2674 cm-1, respectively . No D band is found in Raman spectrum of virgin graphene sample. A broad band is observed in the range of 900-1050 cm-1 which can be attributed to the second order of Si substrate. After metal sputtering, the appearance of few spectral features in the spectral range below 1500 cm-1 (in the range of 1300-1400 cm-1) and at the same time an increase in the band width of G-band can be attributed to the introduction of defects sites due to the sputtering. Liu et al. observed similar behavior, when the researchers deposited Aluminum (Al) by means of sputtering technique . It is plausible that silver atoms, in our case, during the sputtering process retain high kinetic energy, transferred to the graphene sheet. This leads to the creation of carbon vacancies or defect site. Raman band due to disorder can be observed in Figure 2c in the spectral region of 1300-1480 cm-1. However, the G and 2D bands of graphene start emerging (and disappearance of D band) as observed in Figure 2d after annealing AsPrep-GAg sample at 250°C. It could be possible because the sample may be recovering the defects after annealing at high temperature. In the past it is reported that the defect sites could be recovered when annealed at around 900°C  but in our case the annealing temperature is lower. It could be due to the presence of Ag nanoparticles on graphene, which could be playing a vital role to recover the defect sites. A sharp ‘G’ band and minimum disorder ‘D’ band contribution return into effect. Micro-Raman mapping measurements were performed on all the samples, shown in Figure 2. The set of Figures 2b-2d is related to the virgin Graphene, AsPrep_GAg, and As250_GAg samples, respectively. From lower to top panel, the optical image of the sample, the mapping analysis of sample centered around 2674 cm-1 and the Raman spectrum of three associated points mentioned in middle panel were shown in the Figure. The mapping analysis shows a similar behavior as shown in Figure 2a.
Figure 2: Raman spectra of Graphene, Graphene coated with silver metal, and Graphene coated with silver annealed at 250°C. The filling pattern under the curve symbolizes the particle’s size and density. In addition, three sets, b), c) and d), are related to the virgin Graphene, AsPrep_GAg and As250_GAg, respectively. From bottom to top panel: the optical image of the sample overlapped with the mapping analysis, the mapping analysis for the vibration band centered at 2674 cm-1, and the Raman spectra at three different locations (marked in mapping analysis image). The measurements were performed with the exposure time 0.003 sec, 0.05 sec and 0.15 sec, respectively. Laser power was kept constant for all the measurements. The spectra were baseline corrected.
Two different molecules (Rd6G, a fluorescent molecule, and 3-MBA, a thiol molecule) were extensively examined for SERS activity after deposition over silver coated substrates. The step size during the mapping measurement was maintained around 300 nm.
Rhodamine 6G, a fluorescent molecule, is being often used to investigate the functionality of SERS device because of two major reasons: to inspect the fluorescence quenching that will diminish the fluorescence background of the Raman spectrum; and to illustrate the chemical fingerprint of the molecule with enhanced Raman intensity of the vibrational bands. SERS measurements for Rd6G (50 nM), deposited over silver coated SLG samples and over silver coated Si substrate, were performed in the spectral range of 450 – 3000 cm-1, shown in Figure 3a. The spectra were baseline corrected using the Project 4 (WiTec software). The fingerprint vibrational bands of Rd6G were clearly visible at around 1650, 1575, 1513, 1364, 1285, and 1208 cm-1 [38,39]. One sharp band, observed at around 521.5 cm-1 for FlatAg_Si sample, is due to the Si substrate underneath of Ag surface. The comparative spectra show an increase in Rd6G SERS intensity though the measurement parameters (integration time, laser power and Raman active area) were reduced remarkably with respect to the reference Raman spectrum of FlatAg-Si substrate. The mapping measurements were also performed to inspect the distribution of plasmonic hot-spots through out the mapping area (18 × 18 um2). The optical image, mapping analysis for Raman band centred at 1655 cm-1, and SERS spectra of Rd6G at three points (points are indicated by numbers), deposited over AsPrep_GAg and As250_GAg samples, were shown in Figures 3b, 3b’, 3b’’ and 3c, 3c’, 3c’’, respectively. In case of AsPrep_GAg device, the plasmonic hot-spot distribution can be revealed throughout the mapping area whereas in case of As250_GAg, the hot-spots are very much confined at limited space. This exhibits the presence of small gap (of around 5 nm) between two small nanoparticles over the substrate. The annealing of sample at sintering temperature leads to an increment in nanoparticle size and the reduction in the number of hot-spots. SERS enhancement factor for Ag coated over graphene surface samples, AsPrep_GAg and As250_GAg, is found to be 2.1 × 106 and 7.2 ×105, respectively, with respect to the flat Ag surface over Si substrate (see supplementary information: S2).
Figure 3: a) SERS spectra of Rd6G deposited over silver coated Si substrate (t=25 sec, a reference spectrum), AsPrep_GAg (t=0.003 sec) and As250_GAg (t=0.1 sec) (from bottom to top). Red curves of SLG are inserted for comparison. Optical image of mapping area, SERS mapping analysis of [email protected] cm-1 of Rd-AsPrep_GAg sample (b, b’, b’’) and of Rd-As250_GAg sample (c, c’, c’’). The integration time for Rd-AsPrep_GAg and Rd-As250_GAg is 0.003 and 0.1 sec, respectively. Laser power was fixed for all the measurements.
To confirm the above statement of high electric field confinement, finite difference time domain (FDTD) simulation was carried out using Lumerical commercial package. The design of fabricated device, shown in Figure 4a, was made in such a way that it resembles the experimental one. The comparative study was made for device; dimer consisting of 12 nm radius Ag nanoparticles with gap of 8 nm, and the Ag nanoparticles dimer with radius and gap of 25 and 15 nm, respectively. The mesh resolution of the device simulation was kept 2 × 2 × 2 nm3. A linearly polarized light of 532 nm was employed for both simulations. The maximum electric field in case of 25 nm radius was found much lower with respect to the device with radius of 12 nm, as shown in Figures (4b and 4c). The theoretical findings were found in complete accordance with the experimental findings.
Furthermore, 3-MBA molecule was examined over different devices; flat Ag, AsPrep_GAg and As250_GAg. Various intense Raman bands, finger-print of 3-MBA, were observed in the spectral range of 350 – 3000 cm-1 as shown in Figure 5a [40,41]. The vibrational band at 520 cm-1 is related to the Si substrate underneath Ag coating. Intense peak centred at 420, 1001, 1084 and 1152 cm-1 can be attributed to the C-S stretching, in-plane symmetry ring breathing, the combination of C-S stretching and in-plane ring deformation, and in-plane C-H deformation, respectively. The vibration band around 1580 cm-1 corresponds to the combination of G-band of graphene and the C-C stretching of benzene ring, whereas the peak at around 2670 cm-1 is related to the 2D-band of graphene. Since 2D band (zone-boundary phonon) in graphene is always very intense with respect to G band (first order graphene band) [26,42], the presence of intense peak at around 1580 cm-1 in case of sputtered samples is due to the MBA rather than graphene placed underneath the molecule.
Figure 5: a) SERS spectra of 3MBA chemisorbed over 3MBA-As250_GAg, 3MBA-AsPrep_GAg and 3MBA-FlatAg-Si substrate, respectively (from top to bottom). Red curves of SLG are inserted for direct comparison; b) theoretical and experimental Raman spectra of 3MBA: free and chemisorbed over Ag. Two calculated Raman spectra were reported; one excited with 532 nm and other static Raman.
The occurrence of plasmonic hot-spots was revealed throughout the mapping area, shown in 2D mapping analysis for 2 bands centred at 1585 and 1001 cm-1 (see supplementary information: S3 Figure S2). The band centred at around 1000 cm-1 is only due to 3-MBA while the band around 1585 cm-1 has dominant contribution from 3-MBA with respect to the graphene. However, it is observed that the intensity of major bands of 3MBA in case of As250_GAg is much pronounced than what is observed for AsPrep_GAg. This could be due to the possible shift in resonance peak resulting from convolution of molecular resonance and localized surface plasmon resonance.
Density functional theory (DFT) calculation
Amsterdam Density Functional (ADF) package was used to carry out all the calculations for interested molecules. DFT calculation was made for 3MBA as a free molecule and when attached to the silver surface. Optimized geometry of both with and without chemisorption to silver is shown in Figure S3 (see supplementary information: section S4). Generalized gradient approximations (GGA) of Perdew- Burke-Emzerhof (PBE) exchange functional in addition to TZP basis set was used for all the calculations. In addition, highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) were also evaluated to explain the enhancement mechanism of SERS spectra (see supplementary information: S4). The energy gap between HOMO to LUMO for molecule with and without Ag was found to be 1.28 eV and 3.17 eV, respectively. Raman spectra of 3MBA with and without silver attached were also calculated, shown in Figure 5b. The molecular dynamics calculations, with (red colored spectrum) and without (pink colored spectrum) using an excitation laser wavelength, were made for 3MBA attached to silver. We have employed the laser wavelength fixed to 532 nm in case of red colored spectrum. Experimental SERS results for the samples, 3MBA-As250_ GAg and 3MBA-AsPrep_GAg, were compared with calculated Raman spectra for free molecule and for silver attached molecules. Calculated Raman peaks can be observed coherent with the SERS spectra of 3MBA molecule attached with silver. In the experimental observation, C=O and S-H vibrations were suppressed. Various vibrational Raman bands can be observed at around 1585, doublet 1430-1470, 1296, 1150, 1076, 1000, doublet 880-840, 670, and doublet at around 492-421 cm-1 which can be attributed to aromatic C-C vibration, C-H bending, combination of C-C bending and C-H bending, symmetric CCO-, C-C (aromatic) bending, combination of symmetric C-S and C-C, aromatic breathing, combination of asymmetric C-S and C-C, bending aromatic, combination of O-H and C-S bending, respectively.
The above experimental and calculated findings confirm the graphene coated with silver metal functions as a plasmonic SERS device. However, it has been found that AsPrep device works in a superior manner when chemisorbed with Rd6G whereas the annealed device for 3-MBA. This could be due to the variation of the combined effect of plasmonic and molecular resonance.
Graphene template is employed to fabricate fast and effective SERS device after depositing 6 nm of silver metal by means of sputtering technique. Two devices were prepared; AsPrep and annealed at 250°C. The devices were then chemisorbed with Rd6G (a fluorescent molecule) and 3-MBA (thiol molecule) to investigate the functioning as a SERS substrate. SERS enhancement factor was estimated to be 2.1 × 106. DFT calculation was also made to compare the SERS result of 3-MBA molecule. The results show the consistency in experimental and calculated one. Further investigation is needed to optimize efficient SERS samples by varying the annealing temperature.
GD acknowledges KAUST research computing system for their valuable support for ADF calculations.