Reversible Defect in Graphene Investigated by Tip ... - Springer Link

Plasmonics (2012) 7:555–561 DOI 10.1007/s11468-012-9342-8

Reversible Defect in Graphene Investigated by Tip-Enhanced Raman Spectroscopy Peijie Wang · Duan Zhang · Lilin Li · Zhipeng Li · Lisheng Zhang · Yan Fang

Received: 9 November 2011 / Accepted: 20 February 2012 / Published online: 8 March 2012 © Springer Science+Business Media, LLC 2012

Abstract In this paper, a single defect in graphene was created by an Au nanoparticle attached to atomic force tip working in tapping mode. Then it was investigated by tip-enhanced Raman spectroscopy (TERS). The TERS tip interacted with the graphene are able to induce an atomic deformation of carbonic structure which then can be recovered after retracting the tip. The reversible defect was confirmed by the iterative observation of D-band Raman signal of graphene as the tip force on and off. Further more, the Au particles as a nano-antenna can enhance the weak D-band signal from the single graphene defect significantly. These finds will give us better understanding of the origination of graphene defects and the interaction between nanoparticles and graphene. Keywords Artificial defect · Tip enhanced Raman spectroscopy (TERS) · Graphene · Surface plasmon (SP)

Introduction Graphene has attracted great attention since its discovery by Novoselov et al. [1]. This is not only because of it is an ideal test-bed for studying the fundamental two-dimensional problems [2, 3], but also it possesses

P. Wang · D. Zhang · L. Li · Z. Li · L. Zhang · Y. Fang (B) The Beijing Key Laboratory for Nano-Photonics and Nano-Structure, Department of Physics, Capital Normal University, Beijing 100048, China e-mail: [email protected] P. Wang e-mail: [email protected]

unique electronic transport properties [4] such as nearballistic transport [5, 6] at room temperature and high mobility [7–10] of charge carriers, which has great potential applications in electronic devices. However, all these interesting transport properties depend strongly on the lattice defects of graphene [11]. Hence, the study of the defects in graphene is very important and has already attracted great theoretical interest [12, 13]. In fact, graphene obtained from mechanical method has a very low density of lattice defects. But crystal deformations [14], corrugations, extrinsic ripples and curvature can produce structure distortions as lattice defects. These kinds of defects may break the hexagonal symmetry in local domains on graphene [15]. Thus, techniques which lead to the control of these topographic or morphological defects can play an important role in improving the electronic transport properties of graphene. Also the controllability of the morphological defects has great potential applications in realizing the nano-size switching, phase transformation, and recording the information by the micronano-manipulation technology. Hence the controllability and reversibility of the morphological defects will become an important research area on graphene in near future. To this aim, it is necessary to find out a method to realize the reversibility of the morphological defects. Raman spectroscopy is a fast and non-destructive method and has played an important role in the structural characterization of graphitic materials [16]. It has become a powerful tool for understanding the behavior of electron and phonon in graphene and allows monitoring of doping, defects, strain, disorder, chemical modifications and edeges [17]. Surface enhanced Raman spectroscopy (SERS) [18, 19] can enhance the intensity of the conventional Raman

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scattering and has been developed into a high sensitive, in situ and effective spectroscopic technique for detecting the interaction between the adsorbed molecules and the surface of metal nanoparticles. Recent advance in SERS study of graphene has attracted much interest in the binding of metal to graphene for the research on graphene/metal composites and the interactions between graphene and metals [20, 21]. The structure and electronic properties of graphene can also be tailored by chemical adsorption of metal atoms on the layer or by incorporation of metal atoms into its structure. Furthermore, the graphene/gold nanocomposites are potential substrates for SERS studies [22, 23]. The interactions between gold nanoparticles and graphene may confer a unique electron or energy transfer mechanism that subject to the SERS enhancement. A few papers have studied the interaction between gold adatoms and a graphitic surface [24, 25] which infer the strong bonding between metal and carbon atoms. Decoration of graphene sheets with nanoparticles has been demonstrated to reveal special features in new hybrids that can be widely utilized in catalysts [26], supercapacitors [27], biosensors [28], etc. But the requirement that metal surfaces are roughened or nanostructured is one of the most severe restrictions in the application of SERS to a wide variety of problems. Also this SERS process is irreversible. Recently, Tip-enhanced Raman spectroscopy (TERS) [29–32], is emerging as a promising spectroscopic tool which combines scanning probe microscopy (SPM), such as scanning tunneling microscopy (STM), atomic force microscopy (AFM) with Raman spectroscopy. It not only can circumvent this severe restriction of substrates in SERS, but also provide rich spectral and structural information near the metal tip due to the excitation of the localized surface plasmon in the tip-substrate [33]. Furthermore, the high spatial resolution beyond the light diffraction limits of TERS has been used to provide high-resolution images of nanoscale samples [34]. By combining the sensitivity and rich chemical information of SERS with the high spatial resolution of scanning probe microscopy, TERS has the potential applications on surface analysis and the capability of probing molecular adsorbates at specific catalytic sites with an enormous surface sensitivity and nanometer spatial resolution [35]. Furthermore, with the development of technology of SPM, the TERS tips will have further application on tailoring the graphene layers to produce well-defined graphite nanostructures [36]. The structure and electronic properties of graphene can also be tailored by means of catalytic action driven by the local distortions induced on the surface [37].

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Here, we use TERS technology to induce the reversible defects in graphene and get rich information of the interaction between the TERS tip and the surface of graphene. In this paper, firstly, the artificial defects in graphene were induced by depositing Ag nano-particles on its surface with magnetron sputtering method, then the SERS of graphene deposited by Ag nano-particles were studied. It was demonstrated that a significant enhancement in intensity and fruitful Raman modes of graphene were originated from the Ag nanopaiticleinduced defects in graphene. This further infers the interaction between graphene and Ag nanopaiticles. Then the reversible defect in graphene was induced and investigated by TERS technique. The enhancement of TERS spectra of graphene are also analyzed and simulated by FDTD method.

Experimental Methods Graphene Sample Preparation The traditional mechanical exfoliation method was used [38] to prepare the single-layer graphene on top of a SiO2 /Si wafer with a carefully chosen thickness of SiO2 (which is 315 nm )which provides a good optical contrast. The metallographs of the graphene were collected by optical image microscope (Carl Zeiss Axio). For SERS experiment, the Raman spectra were recorded with a microprobe Raman system (Renishaw H 13325). A 50× objective was used to perform an 180◦ backward scattering configuration. The excitation line is 514.5 nm of Ar+ laser. The laser power on the sample was about 2.0 mW. The entrance-slit width was 50 μm and the integral time was 20 s. Artif icial Defect of Graphene Induced by Ag Nanoparticle For inducing the artificial defect of graphene by SERS experiment, the Ag nano-particles were deposited on graphene by magnetron sputtering. The sputtering voltage is 0.3 kV; Argon pressure is 0.45 Pa; The sputtering of time is 45s. TERS Measurements For the study of reversible defect in graphene, the TERS experiment were performed by the TERS setup which is combining of the AFM instrument with the Raman spectrameters (Nanonics MV4000). For TERS measurements, the Raman microscope utilizes an Olympus 50× long working distance objective with a working distance of 15 mm and a NA of 0.45 to perform an 180◦ backward scattering configuration. An incident semiconductor laser (532 nm) is directed to the optical setup by reflecting

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Fig. 1 A schematic illustration of the TERS experimental setup by top-illuminating; the AFM is mounted on the sample stage of an upright microscope with a long-working-distance objective lens. The blow-up region shows the electric field component which is parallel to the surface on the center of Gaussian beam and titling some angle on the periphery of the beam interacting with the tip. The near field is measured when the tip is in sample contact, while the far field signal is measured when the tip is retracted from the sample

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actions between nano-particle and the carbon atom can produce structure distortions showing domains of symmetry lower than hexagonal symmetry [15]. Such domains can exhibit admixture of sp2 and sp3 C orbital character and orbital misalignment. Figure 2a shows the optical image of a typical micro-mechanical cleavage graphene. Figure 2b is the optical image of the same graphene as shown in (a) by depositing Ag nanoparticles. The angle between the two adjacent edges is 120◦ , which indicates that the two edges have same chirality such as armchair or zigzag [39], respectively. Black line in Fig. 2c show the normal Raman spectrum of graphene. The sharp and well symmetric 2D band exhibits a single Lorentzian feature with a full width at half maximum (FWHM) of 24 cm−1 , which is a characteristic feature of monolayer graphene. The most prominent features in the normal Raman spectra of monolayer graphene are the G band appearing at 1582 cm−1 and the 2D band at about 2686 cm−1 using laser excitation at 514 nm. The G band is due to the doubly degenerate (iTO and LO) E2g symmetric mode at the Brillouin zone center which is Raman active for

mirrors. The apex of the cantilever of AFM was set just under the microscopy and adjusted on the focal spot with the TERS tip axis tilting 45 ◦ along the laser beam axis, This polarized light is focused on the tip and the sample. To attain an efficient tip-enhancement effect in the reflection mode TERS experiment, the AFM is equipped with TERS probe consisting of a Au spherical nanoparticle, of less than 100 nm diameter, affixed to a glass cantilever. the AFM tip works in tapping mode. Figure 1 gives the schematic illustration of the TERS experimental setup by top-illuminating; The AFM is mounted on the sample stage of an upright microscope. The blow-up region shows the electric field component which is parallel to the surface on the center of Gaussian beam and titling some angle on the periphery of the beam interacting with the tip. The near field is measured when the tip is in contact with the sample, while the far field signal is measured when the tip is retracted from the sample.

Results and Discussion Artificial defect such as extrinsic ripples and curvature of graphene sheet can be manipulated by depositing nano-particles on the surface of graphene. The inter-

Fig. 2 a The optical image of a typical micro-mechanical cleavage graphene sheet; b The optical image of the same graphene as shown in a after depositing Ag nano-particles on it; The angle between the two adjacent edges is 120◦ c the normal Raman spectrum of graphene in black line with a laser excitation at 514 nm. the red line shows the SERS of the monolayer graphene deposited by silver nanoparticles

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sp2 carbon networks. And 2D band is the overtone of the so-called defect D-band. It is noted that here we focused the laser spot on the center of the graphene sample. For the D-band, it cannot be observed here in normal Raman spectra because there is lack of defect on the center part of the graphene. (Generally only at the edge of a graphene sample, it can be observed as the edge defect). After being deposited by the Ag nanoparticles, the artificial defect on graphene were induced efficiently and D-band was observed profoundly at 1345 cm−1 in the SERS spectrum (shown by the red line in Fig. 2c). The sharp 2D peak demonstrates that the main structure of the monolayer graphene is well preserved after the deposition of Ag nano-particles. For the local domain of deformation by the deposited nanoparticle, a very rough estimate of the defect spacing shows the empirical formula:  −1   ID −10 −3 4 (1) La = 2.4 × 10 nm λ IG where the grain size La in disordered graphite relates to the ratio of the integrated D and G band intensities I D and IG [40]. This leads to La ∼ 16nm for our case which is less than the transport mean free path of 50 nm in graphene. As to the origin (in the frequency) of the D and 2D bands of the monolayer graphene, it was interpreted by the double resonance (DR) Raman process [41, 42]. In fact, the D band originate from a second-order process involving one iTO phonon and one defect. The 2D band originate from a second-order process involving two iTO phonons near the K point. This means that even in the center of the graphene there are no defects, the 2D-

Fig. 3 a The optical image of a typical micro-mechanical cleavage graphene sheet for TERS experiment; b the AFM 3D image of the same graphene as shown in a; c Tip-enhanced Raman spectra of single layer Graphene on SiO2 /Si substrate with the excitation laser at 532 nm and measured with a Au-coated AFM TERS tip. The two Raman spectra were measured with the tip retracted from the sample by black line and with the tip in contact with the sample by red line, respectively

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band can be observed, but the D-band, will not appear. The SERS spectrum by the red line in Fig. 2c also shows that G and 2D bands were enhanced, respectively. The enhancement factor for the G band is of 4.0 times and 2D band of 1.9 times in comparison with these of the pristine graphene. The Raman shifts for the G band of SERS is blue shifted for 7.7 cm−1 and the 2D band for 4.0 cm−1 ; The width of both bands are broadened in SERS spectra either. These characters of Raman bands in SERS spectra confirm the effective interaction between the nanoparticles and the surface of graphene. The SERS technique has its advantage for inducing and detecting the artificial defect, but this SERS process is irreversible and noncontrollable. It may destruct the pure pristine sample. Considering the new merit of reversible and a noninvasive spectroscopy approach, then the TERS experiment was performed as shown below to induce the reversible defect in graphene. Figure 3a shows the optical image of graphene and Fig. 3b shows its AFM image. The triangle like single layer graphene has long side length about 10 μm and the short one about 6 μm. The angle between the two adjacent edges is 60◦ . Figure 3c shows the TERS of the graphene sample. It is noted that we focus the laser spot on the center part of the graphene, the D-band does not appear in normal far field raman spectroscopy as shown in Fig. 3c (the black line). This demonstrates that there is lack of defect in the center of graphene. When the TERS tip is in contact with the surface of graphene, as shown in Fig. 3c by the red line, the tipinduced artificial defect in graphene cause a significant intensity in the D band at 1341 cm−1 associated with

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inter-valley electron scattering. This new kind of artificial defect induced by the TERS tip is due to the interaction of the tip with the carbon atom of graphene which make new extrinsic deformation on the surface of graphene. This induced defect behavior just like the edge defect (Its Raman shift is similar to that of the edge defect of graphene)and is consistent with the dispersion of the graphene optical phonon. When the tip is retracted, the D band disappeared again as shown in Fig. 3c red line. Here it is noted that the 2D band is also composed only one Lorentzian peak with definite symmetry which demonstrates the main structure of the single-layer graphene sample is well preserved without destructive by laser or by the TERS tip. Under the local enhancement of electro-magnetic field by surface plasmon of nano-particle adhered to the TERS tip, the D band is also enhanced and observed clearly. So are the G, 2D and the other combination raman bands. The enhancement factor for the G band is 1.37 times and for the 2D band it is 1.17 times in comparison with these of the pristine graphene. These phenomena demonstrate the effective interaction between TERS tip and the surface of graphene. By contacting and retracting the tip from the surface of graphene, the artificial defect can be induced. Also this process is reversible. As to the interaction of the TERS tip with the surface of graphene, we considered three possible kinds of mechanisms: Firstly, it was reported that the graphene of freestanding [43] and graphene on the SiO2 [44] exhibit intrinsic microscopic corrugations. But this intrinsic corrugation will not break the symmetry of the grip of graphene, also keep the graphene as the perfect two dimensional single layer crystal. When the TERS

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tip is in contact with the surface of graphene, the atomic force of tip and the interaction of the tip with the carbon atom of graphene will induce the extrinsic corrugation of surface of graphene, this extrinsic corrugation will break the symmetry of the hexagon ring in locally domain on graphene and loose the selection rule such that the D band can be activated in Raman scattering. Secondly, it is difficult to observe this tiny morphological defect by normal experiment technique. TERS technique has developed into an extremely sensitive spectroscopic approach [29], can enhance the very tiny Raman signal induced by the interaction in the atomic level due to TERS enhancement. It is generally believed that TERS arises at certain “hot sites” by the Au nano-particles on the AFM apex where the optical interactions are locally enhanced due to the surface plasmon resonances (SPR) of the Au nanoparticle [19, 45]. Here, the enhancement of G and 2D bands of TERS are smaller than that of SERS. Considering the larger number of mental nanoparticles deposited on the surface of graphene in SERS experiment, and only one mental nanoparticle adhere to the AFM apex in TERS experiment, this result is reasonable. The enhancement factor of TERS can be demonstrated by the simulation of nanotip on the SiO2 substrate by FDTD simulation. Figure 4a show the scheme of the simulation. The gap between the tip and substrate is 10 nm. The incident polarization is along x axis. Figure 4b is the simulated electromagnetic field distribution around the tip. and Fig. 4c demonstrates the simulated electromagnetic field distribution on the substrate plane (xy-plane). The average enhancement of intensity is 2 ∼ 3 times by considering the area of laser spot which is about 3 μm2 .

Fig. 4 a Scheme of the simulation. The gap between the tip and substrate is 10 nm. The incident polarization is along x axis. b Simulated electromagnetic field distribution around the tip. c Simulated electromagnetic field distribution on the substrate plane (xy plane)

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Thirdly, the interaction between TERS tip and graphene’s hexagonal C-C bonds would change the bond strength. This is our next further subject, it will tell how the fine structure of the graphene carbon and the carbon bonds were affected by artificial manipulating.

Conclusion In summary, the reversible artificial defect in graphene was induced and detected by TERS technology. For comparison, the artificial defect induced by SERS technique was also studied. The SERS results show that the artificial morphological defect can be induced by the nanoparticle deposited on the surface of graphene. The enhanced D band in the SERS spectra demonstrated that there was a strong interaction between the nanoparticles and the carbon atom of graphene. This interaction breaks the symmetry of graphene and the defect Raman band are activated. However, the SERS process is irreversible and destructive to the sample. The new promising TERS technique can overcome these shortages of SERS and can be applied for realizing the reversible artificial defect and nondestructive to the sample. It not only can induce the defect in graphene but also and control this defect by contacting or retracting the TERS tip on the surface of sample. The very nature of proximal probe methods encourages exploration of the nanoworld beyond conventional microscopic imaging. Scanned probes now allow us to perform “engineering” operations on the sample surface at the ultimate limits of fabrication. Along with the in situ spectroscopy detecting, TERS tips can be used to tailor the graphene layers and even to produce graphene nanoribbons through STM manipulation [46]. It will has great potential applications for surface analysis by means of chemical modifications, controlling the doping of graphene or local distortions driven catalytic action. This new efficient and noninvasive TERS technique may open a way for the mechanical controllability of the morphology of two dimensional nano-material and nano-device which will have great potential for further fundamental research and applications.

Acknowledgements We acknowledge the support of this research by the National Natural Science Foundation of China (Grant No. 21073124 and 10904171) and Beijing Nova Program (2011079).

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