Electrical nature and surface enhanced Raman spectroscopy of Ag nanoparticles decorated graphene Sheet

The interaction between graphene and silver nanoparticles (AgNPs) is investigated by studying the surface-enhanced Raman spectroscopy (SERS) that shows a large enhancement of Raman signal from the AgNPs coated graphene compared to the bare graphene. In addition, the electrical nature of the sample is examined using a two-probe probe station to acquire current-voltage (I-V) characteristics exhibiting a semi-conducting current behaviour of the AgNPs-graphene heterojunctions, in contrast to a semi-metallic electronic behaviour observed for the pristine graphene. We propose band gap engineering of graphene from semi-metallic to semi-conducting via breaking of A-B sub-lattice symmetry due to deposition of AgNPs which is believed to be associated by causing deformations at the regions where graphene is in close proximity with the AgNPs. Furthermore, there is also a possibility of doping induced by AgNPs to graphene and thereby, emergence of a bandgap due to tuning of the electronic structure. In this work, we report these investigations which possess profound implications for application of graphene as a semi-conducting material.


Introduction
Graphene, the veteran of the 2D materials family, has attracted a major attention due to its promising nanoelectronic applications.The sp 2 hybridized hexagonal carbon lattice of graphene and its atomic thickness contribute to exciting properties such as anomalous quantum hall effect, high Fermi velocity as the charge carriers mimic massless Dirac-fermions [1], Klein tunnelling [2] etc.However, a major drawback of graphene is its zero bandgap that prevents any semiconductor applications and also, lightmatter interaction of graphene is very weak owing to its atomic thickness.One of the subtle ways to investigate the light-matter interaction is to coat graphene with metal nanoparticles (NPs) of silver (Ag), gold (Au) etc. that are well known for their excellent plasmonic properties.These studies are based on excitations of the conduction electrons of metal NPs [3,4] with the incident electromagnetic field to excite the localized surface plasmons (LSPs) and induce a resonance called localized surface plasmon resonance (LSPR) [5] that significantly enhances the Raman signal.This method emerged as a powerful analytical technique known as surface enhanced Raman spectroscopy (SERS).Raman spectroscopy has played a crucial role in studying and understanding the fundamental properties of graphitic materials including graphene over years [6,7].Moreover, it has grown as a dynamic tool to characterize the electrons and phonons in graphene [8].Herein, we study the interaction of light with AgNPs coated graphene by investigating the SERS spectra.Unlike the case of graphene-Ag nanocomposites [9], AgNPs spin coated over graphene in this study shows better Raman signal with higher intensity compared to bare graphene.The changes in the Raman spectra of graphene invoke the proof for the charge transfer between graphene and AgNPs.This must be reflected in the electronic behaviour of the sample, and indeed we observe different trends in the current-voltage (I-V) characteristics acquired using a two-probe probe station.In this letter, we address these effects with a qualitative understanding of the underlying physical phenomena.

Experimental techniques
Single layer of graphene on the copper (Cu) foil was procured from Sigma Aldrich India which was grown by chemical vapour deposition (CVD) method.We adapted a polymer supported chemical process to transfer the graphene from the Cu foil [10].During the growth process of graphene by CVD, carbon was also deposited at the bottom side of the foil and as a result, removal of the residual carbon was necessary.This was carried out by an etching process using nitric acid-distilled water solution in the ratio of 1:3, followed by separating the graphene sheet from the upper side of the Cu foil through a PMMA assisted transfer method.The single layer graphene was then placed on a 2 micron thick SiO 2 /Si substrate.This transfer method is described elsewhere [11].AgNPs of diameter 40 nm and concentration 0.02 mg/ml were procured from Sigma Aldrich which were deposited by spin coating with 20 L of AgNP solution at 2500 rpm speed and 500 rpm/s acceleration on one half of the graphene sheet.The other half of the graphene was covered using a glass mask.Optical microscopy, Raman spectroscopy and I-V measurements were performed on both bare graphene and AgNPs decorated graphene layer.Optical microscopy measurements were performed using a compound microscope: model MX61-F of OLYMPUS Corporation.Raman spectra were acquired using a Renishaw Invia Raman microscope with laser excitation of 532 nm for a frequency range 100-3200 cm -1 .Raman signals of pristine graphene and AgNPs decorated graphene were measured separately.For the I-V measurements, Au electrodes were made using a thermal evaporation system via shadow mask technique.I-V curves were acquired using a two-probe probe station across bare graphene and AgNPs-graphene using an Agilent B2912A source/measure unit.

Optical Microscopy
The optical microscopy image of the sample is shown in figure 1.The transparent graphene layer and the spin coated AgNP film on the graphene sheet are marked in the figure.

Raman Scattering
The measured Raman spectra of pristine graphene (red) and AgNPs-graphene (black) are shown in figure 2. Laser excitation produces two prominent Raman peaks for monolayer graphene at around 1577 cm -1 and 2674 cm -1 .The former is known as the G band and the latter is called the 2D or G' band.There is also a defect induced D band which is expected around half the frequency of the 2D band.This band is prominent at the edges of the sample or for the specimen with disorders.We observe a very feeble signature of D peak at around 1341 cm -1 for pristine graphene, signifying a very small amount of defects or disorders present in the sample (figure 2).The small peak like feature evident at around 2450 cm -1 is the G* band representing double resonant Raman scattering in graphene involving one transverse optical (TO) and one longitudinal acoustical (LA) phonon modes [12].The Raman shifts of the AgNPs-graphene reveal a higher intensity of the bands compared to the pristine graphene which is clearly visible in figure 2. This increase in the intensity after the deposition of the AgNPs on graphene is due to surface enhanced Raman scattering (SERS).In SERS, the enhancement of the Raman peaks is mainly due to the surface plasmon resonance (SPR) of the plasmonic materials by laser excitation of the sample in the far-field and the inelastic scattering of the Raman dipole in the near field [13][14][15][16].SPR is usually associated with metal films whose dimensions are much greater than the wavelength of the incident light, allowing the plasmon resonance to propagate on the surface of the film.Excitations of conduction electron in metallic nanostructures with dimensions smaller than that of incident light (AgNP-40 nm and Laser-532 nm) is called localized surface plasmon resonance (LSPR), which can get coupled with electromagnetic fields leading to the confinement of light to very small volumes [17].Thus, the electromagnetic field gets enhanced due to the resonance between the confined light and the sharp corner of the metal nanoparticle or the very small gap between the nanoparticles called "hot spot" [18].This enhancement of the electric field, in turn, amplifies the weak Raman signal due to the small cross-sectional area in the sample.In our case, the SERS hotspots may be identified as both the sharp edges of AgNPs and the narrow gap between the AgNPs and the graphene.Comparing the Raman peaks of graphene at the areas with and without AgNPs, in addition to the enhanced intensity, we observe a spectral shift from 2674 cm -1 to 2673 cm -1 for the 2D peak.For the G line, the observed spectral shift is from 1577 cm -1 to 1579 cm -1 .A similar trend was reported earlier in a study by Alexander Urich et.al., about Ag nanoisland enhanced Raman scattering in graphene [19].
The Raman scattering process of graphene is mainly interpreted based on phonon dispersion of carbon atoms.Since the unit cell of the graphene consists of two carbon atoms, there are six identified phonon dispersion bands in graphene.The G band arises due to the first order Raman scattering processes which is associated with doubly degenerate in-plane transverse optic (iTO) phonon mode or longitudinal optic (LO) phonon mode.In fact, this is the only band that arises from the normal first order Raman scattering process in graphene.The 2D and D bands originate from the second order process.The 2D band is associated with two iTO phonons near the K-point and the D band is always defect induced.In addition there are other weak bands such as G* band which is associated with one iTO and one longitudinal acoustic (LA) phonons.These are the most prominent bands in the Raman spectrum of graphene.AgNPs decorated graphene I-V curve acquired from bare graphene reveals usual semi-metallic characteristic, whereas AgNPs coated graphene exhibits semiconducting behaviour (figure 3b).Using first principles density functional theory (DFT) calculations, K.S. Subrahmanyam et.al [9] have investigated, the changes in the structural and electronic properties of graphene through deposition of Ag, Au, Pd and Pt nanoclusters.They have reported a local breakage of A-B sublattice symmetry of graphene due to deformation at the regions where graphene is in close proximity to the metal nanocluster.This causes an opening of a band gap near the Fermi level of graphene.They have proposed the possibility of placing the Fermi level of graphene in this energy gap by appropriate tuning of the carrier concentration so that the system could behave as a semiconductor.In the I-V curve, one can also see that the current has been reduced for AgNPs-graphene possibly due to the quantum tunnelling effects induced by defects in the sample.The intensity of the D peak in the measured Raman spectrum of AgNPs-graphene is enhanced (figure 2), indicating presence of defects.This could induce an energy barrier at the AgNPs-graphene interface and a current transfer through the interface will be due to quantum tunnelling with a low yield of current.

Conclusion
In this work, we investigated the electronic and electrical nature of AgNPs coated graphene by performing SERS and two probe I-V measurements.An enhanced Raman signal from AgNPsgraphene is due to LSPs as a result of the light-matter interaction causing excitations of the conduction

Figure 3 (
a) depicts the schematic of the two probe measurement system.In figure3(b), we show the I-V characteristics of the sample measured separately for pure graphene and AgNPs decorated graphene.

Figure 3 .
Figure 3. (a) Schematic of the I-V measurements, (b) I-V characteristics of the pristine graphene and AgNPs decorated graphene