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1 Graphene Large-Area Graphene Nanodot Array for Plasmon-Enhanced Infrared Spectroscopy Kai Zhang, Lei Zhang, Fung Ling Yap, Peng Song, Cheng-Wei Qiu, and Kian Ping Loh * Fourier transform infrared spectroscopy (FTIR) is a powerful spectrophotometric technique for the assay of materials on the basis of their chemical bonds. [1 ] However, its moderate sensitivity limits its applications in the analysis of ultrathin films or low-volume samples. The sensitivity of this technique can be enhanced either by the use of intense optical sources such as synchrotron radiation or by the multiplication of absorption paths in an attenuated total reflectance mode. [2 ] In addition to these techniques, resonance absorption due to the presence of an enhanced electric field, as in surfaceenhanced Raman spectroscopy (SERS), [3 ] provides a convenient and powerful method for signal enhancement. Noble metals have been used extensively for plasmon-induced SERS. However, the plasmonic resonances of noble metal are typically in the visible region, their weaker confinement and increased loss at lower frequencies made them less appealing for applications in mid-ir vibrational spectroscopy. [4 ] Graphene, with its tunable IR plasmonic frequencies and strong optical field confinement, [5 ] and its ability to undergo noncovalent binding with a wide range of chemical and biological molecules, [6 ] is an attractive template for plasmon-enhanced IR spectroscopy (PEIRS). It has been Dr. K. Zhang, P. Song, Prof. K. P. Loh Department of Chemistry National University of Singapore 3 Science Drive 3, Singapore , Singapore chmlohkp@nus.edu.sg Dr. K. Zhang i-lab Suzhou Institute of Nano-Tech and Nano-Bionics Chinese Academy of Science Suzhou , China Dr. L. Zhang, Prof. C.-W. Qiu Department of Electrical and Computer Engineering National University of Singapore 4 Engineering Drive 3, Singapore , Singapore Dr. F. L. Yap Institute of Materials Research and Engineering (IMRE) Agency for Science Technology and Research (A*STAR) 3 Research Link, Singapore , Singapore Prof. K. P. Loh Centre for Advanced 2D Materials and Graphene Research Centre National University of Singapore 6 Science Drive 2, Singapore , Singapore DOI: /smll demonstrated that the plasmon polaritons and associated optical fields in graphene can be tuned by varying the carrier density or patterning periodic structures. [7 ] In particular, by using periodic graphene nanostructures to compensate for the momentum mismatch between plasmons and light in free space, graphene plasmons can be extended to the mid-ir regime. [8 ] A strong electric field empowered by plasmonic resonance plays a crucial role in the enhancement of IR spectroscopy. [8c ] Plasmon-vibrational mode coupling has been studied previously for graphene nanoribbon plasmon excitation on the vibrational spectra of surface-absorbed polymers. [9 ] Giant plasmonic enhancement of infrared phonon absorption, along with mid-ir plasmonic response, has been observed in bernal-stacked bilayer graphene. [10 ] Herein, we report a block copolymer self-assembly method [11 ] for the efficient fabrication of a graphene nanodot array (GNDA) that is cost-effective and readily scalable to large areas (centimeter scale demonstrated here), which is highly favorable to practical applications. This has advantages over electron beam lithography (EBL) in terms of large-scale patterning and high-throughput. GNDAs fabricated with this method exhibit mid-ir plasmonic resonances with excellent optical field enhancement. As a proof of concept, the detection of self-assembled monolayer (SAM) molecules and single-stranded DNA (ssdna) oligomers at picomolar concentrations, as discrimination of metal ions in water solutions, has been demonstrated with GNDA-based PEIRS. In top-down lithography techniques, the mask module is critical for achieving the required geometric features. We present an approach for the formation of a nanofeature mask defined by block copolymer self-assembly that enables the efficient fabrication of GNDA over a large area. As shown schematically in Figure 1 a, the fabrication consists of four steps: i) surface modification of graphene with polymer brush molecules, ii) self-assembly of the block copolymer, iii) oxygen plasma etching, and finally iv) removal of the protective poly mer film atop the GNDA. The graphene films were grown by chemical vapor deposition (CVD) and transferred onto silicon substrates (refer to Figure S1 in the Supporting Information for the characterization of graphene film). The graphene film was modified with dopamine hydrochloride to change its wetting property, [11b ] which was essential for the self-assembly of the block copolymer on its surface. A thin film of spherical reverse micelles of polystyrene- block-poly- (2-vinylpyridine) (PS- b -P2VP) was obtained by spin-coating a dilute solution of the block copolymer in a solvent such as Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

2 Figure 1. Fabrication of GNDA by block copolymer lithography. a) Schematic fabrication processes of GNDA including polymer brush (dopamine hydrochloride) immobilization, self-assembly of PS-b-P2VP copolymer reverse micelles, oxygen plasma etching, and removal of the polymeric mask by vacuum annealing. b d) AFM images recorded during the fabrication processes of block copolymer assembly, after oxygen plasma and the final GNDA. e g) Corresponding line-scan profiles of panels (b) (d). h) Typical SEM image of a GNDA. i) Representative photograph of one tray of GNDA samples fabricated on silicon substrates shows the high throughput, batch production capability. xylene, which is selective for polystyrene block. The quasihexagonal periodic array of the spherical micelles served as a mask for oxygen plasma etching to form a discrete nanodot array. Graphene without the protection of polymeric nanodots was etched away by oxygen plasma. More details on the fabrication of GNDA are mentioned in the Experimental Section. Figure 1b d shows the atomic force microscopic (AFM) images of the nanodot array with the corresponding AFM height profiles (Figure 1e g) at different fabrication stages. A quasi-hexagonal array of PS-b-P2VP reverse micelles was formed on surface-modified graphene, with a diameter of around 60 nm (Figure 1b). The height difference between the micelles ( 25 nm), as shown in Figure 1e, was sufficient to serve as an etch mask for the underlying graphene. In Figure 1c, the polymeric bumps were slightly reduced in size to 50 nm in diameter and 15 nm in height (Figure 1f) after 30 s of oxygen plasma etching. The polymeric mask was removed by vacuum annealing at a temperature below 500 C, which left a monolayer GNDA on the substrates (Figure 1d). Figure 1i shows a photograph of GNDA fabricated on thirty-six 1 1 cm2 pieces of silicon substrate in a one-batch process. A large-area scanning electron microscopic (SEM) image of the GNDA from one of these samples is shown in Figure 1h. The dimensions of the graphene nanodots can be controlled by varying the exposure time to oxygen plasma. Figure 2a c indicates that the dot diameters can be altered to 55, 45, or 40 nm with oxygen plasma durations of 25, 30, and 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

3 Figure 2. Control of feature dimensions of graphene nanodot array and plasmonic resonances. a c) Variations in the diameter of the graphene nanodots with duration of oxygen plasma etching. d,e) Tuning of graphene plasmonic resonances by chemical doping and nanodot size, respectively. 35s, respectively. Prominent plasmonic resonances appeared in the mid-ir region for these nanodot arrays in which dipole oscillations both within a single dot and between dots contribute to the plasmon excitation (Figure S2, Supporting Information). For nanodot arrays fabricated on silicon substrates, the graphene plasmonic resonance split into two peaks around 950 and 1300 cm 1 due to coupling of the graphene plasmon with the substrate s Fuchs Kliewer surface optical phonons. [8b ] As demonstrated previously for localized far-ir graphene plasmons, [7a,b ] the mid-ir plasmonic resonance peak underwent a blueshift when the density of the graphene carrier was increased by chemical doping using nitric acid (Figure 2 d). Along with the blueshift, the peak intensity decreased as the dot diameter decreased (Figure 2 e), which is in agreement with the numerically calculated results (Figure S3, Supporting Information). Numerical simulations were performed to simulate the electric field profile around these graphene nanodots. The simulations showed that the localized electric field intensity was enhanced significantly by three orders of magnitude at the fundamental mode of k = 1044 cm 1 for a nanodot diameter of 45 nm ( Figure 3 a), which is favorable for sensing and signal enhancement measurements. In addition, three higher-order harmonic resonance modes were present at the frequency range of interest for the same GNDA design (Figure 3 b d). Surprisingly, more field antinodes appeared for these higher-order modes, which were revealed clearly in the z -component of the electric field distributions (Figure S4, Supporting Information). Although the enhancement factor decreased as the mode order increased, an enhancement factor of more than 50 was achieved even for the fourthorder mode at k = 3817 cm 1. This promises good field enhancement at detuning frequencies. Due to imperfections in fabricated GNDAs, such as nonuniformity of dot size, rough edge of single dot, etc., the linewidth of each resonance will be inevitably broadened. Therefore, a strong field enhancement will cover most of the frequency range of interest, which facilitates broadband sensing. Furthermore, GNDAs with diameters ranging from 30 to 50 nm were also calculated, as shown in Figure 3 e h. The results revealed that smaller dots supported greater field confinement and a larger enhancement factor. Detection experiments were carried out by examining the IR extinction spectra of a SAM molecule, which is a derivative of perchlorotriphenylmethyl (PTM) radicals. Due to the ultrathin morphology of the SAM molecule (Figure S5, Supporting Information), only weak peaks at wave numbers of 2850 and 2917 cm 1 could be recorded for the SAM molecule adsorbed on silicon or continuous graphene substrate, as shown in Figure 4 a. In contrast, sharp vibration peaks that were about one order of magnitude stronger were observed for the SAM molecule assembled on GNDAs under similar Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

4 Figure 3. Normalized distributions of electric field intensity at the surface of the GNDAs. a d) Normalized distribution of electric field intensity of the fundamental and higher-order harmonic modes of a GNDA with diameter of 45 nm. e h) Normalized distributions of electric field intensity at the fundamental mode for GNDAs with diameters of 50, 40, 35, and 30 nm, respectively. measurement conditions. This result presents improved detection sensitivity compared to EBL-defined graphene nanoribbon array for the sensing of surface adsorbed layers, which gave a factor of about 5 with 100 nm ribbons.[9] It is worth noting that the enhancement of the vibrational signals was also achieved at 3000 cm 1, far away from the plasmonic resonance at 1200 cm 1, which echoes the results of electric field enhancement at higher order modes in the finiteelement time-domain (FDTD) simulations. Strong enhancement can also be obtained for ssdna oligomers stacked on GNDAs (Figure 4b). The resonance peaks arise from the bond vibrations in the DNA nucleotide. An obvious intensity enhancement accompanied the red shift of the plasmonic resonance peaks due to the doping effect of the electronegative DNA. As observed in the spectra, the carbonyl groups at 1470 and 1730 cm 1 (close to the plasmonic resonances) yielded intensity enhancement more than 10 times greater than the vibrational peaks of methyl groups Figure 4. Molecular detection with GNDA-based PEIRS. Detection of a) SAM and b) ssdna with and without GNDAs, in the latter case continuous graphene film (ssdna on Si with G film) and bare silicon substrate (ssdna on Si W/O GDNA) were used as controls. c) Measurements of ssdna of various concentrations show a detection limit of M (corresponding to surface concentration of pmol mm 2; covering an area with a diameter of 5 mm; volume 50 μl) Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

5 at 2850 and 2918 cm 1. This result is attributable to the frequency-dependent attenuation of the plasmon-enhanced absorption. [9 ] The detection limit of GNDA plasmonenhanced FTIR reaches m for ssdna oligomer (Figure 4 c), which proves its ultrahigh sensitivity. The GNDAs were also applied to detect metal ions by making use of molecular cages which can bind with ions. Meta-5,10,15,20-tetrakis(1-methyl-4-pyridinio)porphyrin tetra( p-toluenesulfonate) (TMPyP), [12 ] a water-soluble porphyrin derivative that can form coordination complexes with different metal ions (shown schematically in Figure 5a), was adsorbed on the GNDA for the binding of ions. Electrostatic and p p stacking interactions between TMPyP molecules and graphene flattened the molecules, leading to improved p conjugation of the porphyrin ring, which facilitates its complexation with metal ions. [6a ] After binding on the GNDAs, the vibration peaks of the TMPyP molecules ( m ), such as sulfonate groups, alkyl C-H and N-H in pyrrole, were strongly enhanced (Figure S6, Supporting Information). As shown in Figure S7 (Supporting Information), the absorption spectra of the reagent often overlapped with those of metal chelates in ultraviolet visible spectroscopy. [12,13 ] The advantage of PEIRS is that the spectral shift is quite distinct and sensitive to different metal coordinations. Upon the binding of the porphyrins with Cd 2+ ( m, cadmium acetate), pronounced changes in intensity occurred for the sulfonate groups and alkyl C-H symmetric and antisymmetric bond deformation. More importantly, a blueshift of the N-H stretching bond in pyrrole, from 3225 to 3390 cm 1 (Figure 5 b), was seen. The spectral shift of the N-H stretching bond in pyrrole was related to the changes in the electronic structure of the porphyrin ring, which was quite similar to the frequency shifts of d(ch) and p(ch) observed for coordination between the aromatic ligand and metal ions. [14 ] As shown in Figure 5 c, as the binding of Cd 2+ ions increased, the stretching vibration of the N-H bond in pyrrole at 3225 cm 1 gradually attenuated and was overshadowed by an emerging peak at 3390 cm 1. PEIRS was able to discriminate between different metal ions (Figure 5 d). The incorporation of metal ions in porphyrins can be explained by a dissociativeinterchange mechanism. [13a, 15 ] Metal-specific changes in bond lengths and bond angles during the porphyrin deformation contribute to the peak shifts and attenuations. In control experiments, we used various metal ions of different sizes, electronegativity and binding affinity with porphyrin. A higher electronegativity of the metal ion or a reduced ionic character of the metal-porphyrin bond will induce larger frequency shift, [14 ] which can be arranged in the order of Pb 2+ >Ca 2+ >Cd 2+ >Cu 2+ >Zn 2+. In the case of large metal ions such as Pb 2+ and Cd 2+, they cannot fit into the porphyrin cage and sit on top of the molecule, thus the frequency shifts are reduced. [13a ] In summary, we have demonstrated a cost-effective way of producing large-scale GNDA with lateral dimension up to centimeter scale using block copolymer self-assembly technique. Using this macroscopically large array for sensing chemical analytes, we observed a more than ten-fold enhancement of the vibrational peaks of SAM and DNA molecules due to mid-ir plasmonic resonances. Interestingly, the enhancement can be achieved at regions far from the plasmonic resonance, Figure 5. Metal ion detection with GNDA-based PEIRS. a) Schematic illustration of metal ion binding with probe molecule TMPyP. b) Extinction spectra of TMPyP before and after binding Cd 2+ with GNDA-based PEIRS. Extinction spectra of TMPyP recorded in the presence of c) Cd 2+ at various concentrations and d) different metal ions Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

6 where FDTD simulations reveal electric field enhancement due to the presence of higher order harmonic resonances. Our work demonstrates convincingly that graphene mid-ir plasmonics can be applied for ultrasensitive chemical and biological detection. Future work can explore the enhancement of optical field by increasing the graphene carrier concentration, achieved by either chemical doping or electrostatic gating. Finally, these graphene nanostructures can also be fabricated as waveguides or resonators, paving the way towards ultrafast active graphene-based plasmonic devices. Experimental Section GNDA Synthesis : The graphene films for making graphene nanodots were synthesized by low-pressure chemical vapor deposition on copper foil and transferred onto the desired substrates using a wet-etching transfer method. The graphene film on the substrate was immersed in a dopamine hydrochloride solution (1 mg ml 1 in M Tris HCL, ph 8) for 1 h. A 0.5% (w/w) PS- b-p2vp (57000-b g mol 1 ) reverse micelle dissolved in m-xylene was then deposited on the graphene-modified surface by spin-coating at 5000 rpm. Pattern transfer of the reverse micelle topography to graphene was then carried out using oxygen plasma (30 W, 65 mt, 20 sccm). The chips were then annealed in a vacuum chamber (base pressure, 1e-7 torr) at 500 C for 2 h to remove the protective polymer layer atop the graphene nanodots. Simulations of Electric Field Distribution : All simulations were performed using LUMERICAL, an FDTD code. In these simulations, periodic boundary conditions were adopted along the x- and y -axes, with a perfectly matched layer set along the z-axis. The incident wave was normally illuminating on the surface with the electric field polarized along the x -axis. For more details within the simulations, please refer to the simulation methods in the Supporting Information. Sample Preparation in Molecule and Metal Ion Detection Experiments. To prepare the SAM molecules, a silicon substrate with GNDA was immersed in a radical (derivative of PTM radical) solution dissolved in toluene at a concentration of 10 5 mol L 1 for 90 min and then rinsed gently with toluene and dried with N 2. ssdna oligomer (sequence: 5-CAT GAA CCG-3) was used for the DNA detection experiments. The DNA fragments were diluted in phosphate-buttered saline solution by a serial dilution. A solution of 50 μl ssdna was incubated on GNDA for detection. A gentle rinse in deionized water was then performed to remove ionic salts in the phosphate-buffered saline solution and ungrafted DNA. For the metal ion incorporation and detection experiments, cadmium (Cd 2+ ), copper (Cu 2+ ), lead (Pb 2+ ), zinc (Zn2+ ), and calcium (Ca 2+ ) ions were prepared in various concentrations with cadmium acetate, copper chloride, lead acetate, zinc chloride, and calcium chloride, respectively, dissolved in pure water. Supporting Information Supporting Information is available from the Wiley Online Library or from the author. Acknowledgements K.P.L. acknowledges MOE Tier 2 grant From in situ observation to the growth scaling of graphene quantum dots (Grant No. R ) as well as the National Research Foundation funded CRP program Plasmonic-Electronics: New Generation of Devices to Bypass Fundamental Limitations (award No. NRF-CRP ) and NRF-POC grant NRF 2014NRF-POC-001 Industrially scalable approach for the growth and transfer of large area G. K.Z. acknowledges financial support from the National Natural Science Foundation of China ( ) and the Natural Science Foundation of Jiangsu province (BK ). [1] P. R. Griffiths, J. A. de Haseth, Fourier Transform Infrared Spectroscopy, John Wiley & Sons, Inc., Hoboken, NJ 2007, p. 3. [2] a) K. D. Moller, D. Scardino, T. Sears, D. Carlson, C. J. Hirschmugl, G. P. Williams, E. Chang, H. T. Liu, Int. J. Infrared Millimeter Waves 1992, 13, 275 ; b) F. N. Fu, M. P. 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