Journal of Science: Advanced Materials and Devices (2018) 353e358 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original Article Raman and scanning tunneling spectroscopic investigations on graphene-silver nanocomposites Sheena S Sukumaran a, C.R Rekha a, A.N Resmi b, K.B Jinesh b, *, K.G Gopchandran a, ** a b Department of Optoelectronics, University of Kerala, Kariavattom, Thiruvananthapuram, Kerala 695581, India Department of Physics, Indian Institute of Space Science and Technology (IIST), Valiamala, Thiruvananthapuram, Kerala 695547, India a r t i c l e i n f o a b s t r a c t Article history: Received 16 April 2018 Received in revised form 20 June 2018 Accepted 23 June 2018 Available online 30 June 2018 Graphene-silver (G-Ag) nanocomposites were prepared using liquid phase exfoliation of graphite flakes followed by a reduction of silver nitrate The plasmonic characteristics of these nanocomposites are highly sensitive to the Ag concentration in the films Raman spectroscopic investigations indicated that the effect of the surface-enhanced Raman scattering is present significantly in the Raman spectra of G-Ag nanocomposites The intensity of the D, G, and 2D Raman bands is found to increase with silver concentration The interaction between graphene and silver nanoparticles causes the G-band to split depending on the concentration of Ag Scanning tunneling spectroscopic investigations showed that silver causes the Fermi level of graphene to shift towards the conduction band indicating the n-type doping and interestingly the existence of an offset current in the I-V characteristics of G-Ag nanocomposites differing from the zero value observed for graphene © 2018 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Keywords: Graphene-silver nanocomposites Raman spectroscopy Scanning tunneling spectroscopy Density of states Introduction Graphene, the first reported two-dimensional material has found its applications in diverse fields owing to the unusual properties coming from the hexagonal lattice and the delocalized p-electrons [1] Its large surface area and easiness for anchoring metal nanoparticles open up possibilities of synthesizing graphene based nanocomposites, which can broaden the fields of application of graphene [2] The presence of metal nanoparticles reduces the van der Waals force between the graphene sheets and the aggregation of metal nanoparticles can get reduced by the graphene [3,4] Numerous graphene based composites have been synthesized by many researchers which find applications in electronics, supercapacitors [5], fuel cells [4], optics [6], light emitting diodes [7] etc In order to develop different devices out of graphene-metal hybrids, knowledge about the mechanism behind the interaction between metal and graphene is vital Theoretical studies have been carried out in this regard, whereas only limited experimental studies are reported [8] Raman spectroscopy is an important tool used for the characterization of carbon based materials especially * Corresponding author ** Corresponding author E-mail addresses: kbjinesh@iist.ac.in (K.B Jinesh), gopchandran@yahoo.com (K.G Gopchandran) Peer review under responsibility of Vietnam National University, Hanoi graphene in which the number of layers, defects, doping, strain etc can be analyzed [9] The incorporation of metals like gold and silver into graphene will modify the Raman peaks of graphene due to surface enhanced Raman scattering (SERS) In addition, to study the interaction via SERS activity of silver/gold, the graphene-hybrid SERS substrates have a lot of advantages over the conventional metallic SERS substrates like improved stability, biocompatibility and reproducibility [10], and they eliminate the fluorescence back ground even in resonance Raman scattering [11] Thus obtained vibrational spectrum is devoid of the direct interaction between metal and the probe molecules, thereby obtaining cleaner spectrum of the desired species In addition, photo-induced damages like photo carbonization and bleaching in metallic SERS substrates can be avoided using graphene-metal hybrid substrates [12] Out of metallic nanoparticles silver has highest SERS activity, but it is prone to oxidation under normal conditions [12e14] Using graphene-silver hybrid as SERS substrates, it reduces the possibility of oxidation with improved colloidal stability as discussed earlier Graphene-silver (G-Ag) composite also has potential applications in antibacterial activity [15], catalysis [16,17] including photocatalysis [18], and electro-catalysis [19], biosensors [17] and as electrode for energy storage/conversion, supercapacitors [20] In most of the previous works on graphene based silver nanoparticles, graphite oxide/GO was used as the precursor for graphene, where simultaneous chemical reduction of both the silver nitrate and graphite oxide/GO takes place [10,19,20] In https://doi.org/10.1016/j.jsamd.2018.06.003 2468-2179/© 2018 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) 354 S.S Sukumaran et al / Journal of Science: Advanced Materials and Devices (2018) 353e358 mechanically exfoliated and CVD grown graphene samples silver nanoparticles are deposited using thermal evaporation or by means of RF sputtering to obtain G-Ag composites [3,12,21] In this work G-Ag composites are synthesized by means of liquid phase exfoliation with sodium dodecylbenzene sulfonate (SDBS) as surfactant followed by the reduction of silver nitrate in the graphene dispersion using sodium borohydride (NaBH4) Most of the peculiar properties of G-Ag nanocomposites arise from the interactions between the graphene and silver nanoparticles This makes the study of plasmons and G-Ag interactions important SERS spectrum of graphene obtained from the silver nanoparticles is normally used to study the nature of graphene-silver interactions Herein, a different approach is followed Specially, in addition to Raman spectrum, Scanning tunneling microscopy (STM) and spectroscopy (STS) investigations are also carried out to study the local effect of plasmons in the G-Ag composites in presence of different optical excitations In this article, attempts are made to correlate the observations made in these two techniques with the interaction between silver and graphene as a function of silver concentration Experimental Graphite flakes, SDBS, silver nitrate (AgNO3) and NaBH4 were the chemicals used in the synthesis process Distilled water was used throughout in the synthesis Graphite flakes at a concentration of 10 mg/mL was sonicated for h in 2.5 mg/mL of SDBS in water, followed by centrifugation at 2000 rpm for 90 as explained elsewhere [22] The supernatant solution containing exfoliated graphene sheets was collected and used for the synthesis of composites The graphene dispersion was stirred with silver nitrate for 10 so that the final concentration of AgNO3 was 0.25 mM in AgNO3-graphene solution To the ice cold 0.5 mM sodium borohydride solution, AgNO3-graphene solution was added drop wise with continuous stirring for The volume of AgNO3-graphene and sodium borohydride solution was kept at the ratio of 1:3 The colour of the dispersion turned to brownish yellow after the addition of AgNO3-graphene solution, and the composite was named as G-Ag I In order to study the effect of concentration of silver nanoparticles on the properties of G-Ag nanocomposites the above experiment was repeated with fold increase in molarity of both AgNO3 (0.5 mM in AgNO3-graphene) and NaBH4 (1 mM), keeping the molarity ratio same Thus obtained graphene-silver dispersion was named as G-Ag II The absorption measurements were done in the 200e900 nm region using Jasco 550UV-Vis spectrophotometer The morphology of the synthesized particles was analysed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques with field emission-SEM (FE-SEM) from FEI-Nova Nano SEM and high resolution TEM (HRTEM) from Joel JEM 2100 respectively Raman spectra were taken using Horiba Jobin Yvon LabRAM with 514 nm laser excitation from argon ion laser and a grating of 1800 cmÀ1 was used to record the spectra Scanning tunneling microscopic (STM) measurements were carried out using NanoRev STM from Quazar Technologies under ambient conditions using Pt/Ir tip and diode lasers of wavelength 532 and 635 nm were used to irradiate the samples for in situ STM measurements The diode lasers were focussed to the sample through the viewing port in the STM ~260 nm corresponds to the transition of p-electrons in the graphene [23] In addition to the peak corresponding to p-electrons, a peak at ~400 nm was observed for the G-Ag dispersions The peak in the visible region is the characteristic peak of silver nanoparticles and is attributed to the collective oscillation of the conduction band electrons, known as surface plasmon resonance (SPR) of silver nanoparticles [24] The observed single peak ~400 nm in the absorption spectrum of the composite signifies the presence of isotropic silver nanoparticles and conrms the reduction of Agỵ to form silver nanoparticles [25] As the concentration of AgNO3 increases to 0.5 mM in AgNO3-graphene solution, the SPR band dominates which evidences the increased concentration of silver nanoparticles in G-Ag II samples The formation of silver nanoparticles was also witnessed as the colour changed from black to brownish yellow The morphology of nanoparticles formed is studied using scanning and transmission electron micrographs and the corresponding images are given in Fig Fig 2(a) gives SEM image of the composite which consists of graphene flakes decorated with white dots and these are the silver nanoparticles found in the composite Fig 2(b) represents the TEM image of G-Ag II, where the graphene is observed as transparent sheet with the edges folded and the black spots seen on the sheet are the formed spherical silver nanoparticles The aggregation of silver nanoparticle was not found in the micrographs The average size of the silver nanoparticles is measured to be 19.68 ± 4.27 nm in most cases The HRTEM image shows the lattices of both graphene and silver nanoparticles From the Fig 2(c), d-spacings of planes from silver nanoparticles and graphene sheets are measured to be 0.236 and 0.33 nm corresponding to the (111) plane of the face centred cubic (fcc) crystal structure of the silver nanoparticles and that of (002) crystal plane of graphene, respectively The selected area electron diffraction (SAED) pattern of the composite consists of inner ring coming from (002) graphene (jcpds no 75-1621) and the outer rings from the (111) and (220) planes of fcc of the silver nanoparticles (jcpds 04-0783) having a d-spacing of 0.33, 2.35 and 1.43 nm, respectively Hence, the morphology consisting of graphene layer decorated with silver nanoparticles is found to be in good agreement with the lattice plane measurements carried out from the two different fringe spacings of high resolution image and the diffraction rings observed in the SAED pattern, which evidenced the G-Ag composite growth and formation The Raman spectra of the starting graphite flakes, exfoliated graphene and graphene-silver composites are given in Fig The prominent peaks observed in the Raman spectrum of graphite are G (~1580 cmÀ1) and 2D (~2700 cmÀ1) The G-band comes from doubly degenerate inplane longitudinal and transverse optical Results and discussion The UV-visible absorption spectrum of graphene and G-Ag and the corresponding spectra are given in Fig Fig 1(a) shows the absorption spectrum of graphene dispersion and the peak observed around Fig UV-visible absorption spectrum of (a) graphene dispersion, (b) G-Ag I and (c) GAg II with a concentration of 0.25 and 0.5 mM AgNO3e-graphene solution S.S Sukumaran et al / Journal of Science: Advanced Materials and Devices (2018) 353e358 355 Fig Microscopic images of G-Ag II samples (a) SEM; (b&c) TEM images showing spherical silver nanoparticles on graphene sheets; (d) selected area electron diffraction pattern phonon modes (iLO&iTO), E2g phonon modes at the G-point (zone centre) which signifies sp2 hybridisation The iTO phonons with opposite wave vectors undergo inelastic scattering near to K-point (zone boundary) to get the double resonance 2D-peak In addition to the characteristic peaks G and 2D, Raman spectrum of exfoliated graphene (Fig 3(b)) contains one more peak at around 1350 cmÀ1 viz., D-peak which arises as the breathing mode of phonons with A1g symmetry at the K(K0 )-point and it appears only when defects are present [26] Hence, the presence of D peak indicates the breakdown of translational symmetry in the lattice, thereby defects and it may be either bulk defects in the basal plane or edge defects [27] It is found that the intensity of D-peak is higher than that of Gpeak (ID/IG > 1) and the correlation between full width at half maximum (FWHM) of G-peak and ID/IG is found to be negligible Also a well resolved shoulder, D0 peak (~1620 cmÀ1) is present along with G-peak which arise as intra valley transition [9] Hence the presence of D peak is attributed to edge defects, arising from the smaller size of exfoliated graphene flakes as evident in the SEM image (Fig 2(a)) The finding of D-peak with large intensity is similar to the observation made by other researchers in liquid phase exfoliated graphene [27,28] In addition to the observation of high intense D-peak, there is a shift in the peak position of G and 2D peak in the exfoliated graphene sheet The G and 2D-peak is observed at around 1586 and 2694 cmÀ1 respectively in the exfoliated graphene The shift in peak positions may be either due to doping effects or decrease in the number of layers present in the exfoliated sheets Hence, in order to confirm the decrease in the number of layers present in the sample, the shape of 2D peak has been deconvoluted and is given in Fig 3(b) The deconvoluted 2Dpeak indicates presence of