Raman and scanning tunneling spectroscopic investigations on graphene-silver nanocomposites

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 [r]

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Original Article

Sheena S Sukumarana, C.R Rekhaa, A.N Resmib, K.B Jineshb,*, K.G Gopchandrana,**

aDepartment of Optoelectronics, University of Kerala, Kariavattom, Thiruvananthapuram, Kerala 695581, India

bDepartment 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

Article history: Received 16 April 2018 Received in revised form 20 June 2018

Accepted 23 June 2018 Available online 30 June 2018 Keywords:

Graphene-silver nanocomposites Raman spectroscopy

Scanning tunneling spectroscopy Density of states

a b s t r a c t

Graphene-silver (G-Ag) nanocomposites were prepared using liquid phase exfoliation of graphiteflakes followed by a reduction of silver nitrate The plasmonic characteristics of these nanocomposites are highly sensitive to the Ag concentration in thefilms 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 con-centration 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 nano-composites 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/)

1 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 theﬁelds of application of graphene[2] The presence of metal nanoparticles reduces the van der Waals force between the graphene sheets and the aggre-gation 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

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 theﬂuorescence 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 spec-trum 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 gra-phene-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 * 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

Contents lists available atScienceDirect

Journal of Science: Advanced Materials and Devices

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d

https://doi.org/10.1016/j.jsamd.2018.06.003

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is normally used to study the nature of graphene-silver in-teractions 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 tech-niques with the interaction between silver and graphene as a function of silver concentration

2 Experimental

Graphiteﬂakes, SDBS, silver nitrate (AgNO3) and NaBH4were

the chemicals used in the synthesis process Distilled water was used throughout in the synthesis

Graphiteﬂakes 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 theﬁnal concentration of AgNO3was 0.25 mM in

AgNO3-graphene solution To the ice cold 0.5 mM sodium

boro-hydride 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 addi-tion 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 mi-croscopy (SEM) and transmission electron mimi-croscopy (TEM) tech-niques withﬁeld 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 cm1 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 Results and discussion

The UV-visible absorption spectrum of graphene and G-Ag and the corresponding spectra are given inFig 1.Fig 1(a) shows the absorp-tion spectrum of graphene dispersion and the peak observed around

dences 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 im-ages are given inFig 2.Fig 2(a) gives SEM image of the composite which consists of grapheneﬂakes 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 aggre-gation 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 theFig 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 graphiteﬂakes, exfoliated graphene and graphene-silver composites are given inFig The prominent peaks observed in the Raman spectrum of graphite are G (~1580 cm1) and 2D (~2700 cm1) The G-band comes from doubly degenerate inplane longitudinal and transverse optical

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phonon modes (iLO&iTO), E2gphonon modes at theG-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 cm1 viz., D-peak which arises as the breathing mode of phonons with A1gsymmetry 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 G-peak (ID/IG > 1) and the correlation between full width at half

maximum (FWHM) of G-peak and ID/IGis found to be negligible

Also a well resolved shoulder, D0peak (~1620 cm1) 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 grapheneflakes 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 cm1respectively in the exfo-liated 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 inFig 3(b) The deconvoluted 2D-peak indicates presence of<5 layers in the exfoliated graphene dispersion Thus the exfoliated graphene sheets consist of few-layer

graphene with smallerﬂake size Using this dispersion G-Ag I and G-Ag II were synthesized and the corresponding Raman spectra are given inFig For both the G-Ag the intensities of D, G and 2D peaks were found enhanced as expected due to SERS arising from the presence of silver nanoparticles in the composites Metallic particles like gold and silver are known substrates for the SERS The enhancement of Raman signals can come from either enhanced electromagneticﬁeld due to the surface plasmons localised near 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

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of silver nanoparticle increased and the corresponding values are given inTable The decrease in the intensity of 2D and I2D/IGcan be

attributed to the increase in electroneelectron collision due to in-crease in carrier concentration when doped with Ag [31e33] Similar decrease in the ratio of I2D/IGwas observed by Lee et al after

depositing gold nanoparticles onto graphene sheets by thermal evaporation[21]

In addition to the enhancement in the intensities of the peaks, the position of the peaks G and 2D is found to be shifted in the composites The shift of G-peak to 1598 and 2D-peak to 2685 cm1 in G-Ag II implies that Ag addition induces n-type behaviour to the sheets [33e35] Coming to the shape of the peaks, silver nano-particles did not alter the shape of D and 2D peaks but the shape of G-band gets altered from that of the exfoliated sheets with the appearance of shoulders, depending on the concentration of silver As discussed earlier, G-peak comes from the degenerate iLO and iTO phonon modes (E2g) hence its splitting indicates the removal of

degeneracy implying the two phonons have different energies in G-Ag, resulting from change in electronic structure The electronic structure of graphene is altered by the charge transfer between silver nanoparticles and graphene sheets Thus the splitting of G-peak can be attributed to the interaction between the silver nanoparticles and the graphene sheets as proposed[36,37] The G-peak splitting was observed for monolayer, bilayer and trilayer graphene and no splitting was noted for multilayer graphene (MLG), evidencing that the sheets obtained are not MLG[21] The FWHM of G-peak is found to increase with concentration of silver The width of G-peak depends on the coupling between the electron and phonon and its broadening indicates possibility of phonon decay channels into electronehole pairs[32,38] The Raman anal-ysis shed light towards the interaction between the silver nano-particles and graphene sheets in the composites

In order to further conﬁrm the interaction between silver nanoparticles and exfoliated graphene, I-V measurements were done in the exfoliated graphene and G-Ag composites using STM in spectroscopic mode Interestingly, the I-V characteristics of G-Ag I and G-Ag II are observed to be not passing through the origin while that of exfoliated graphene passes through the origin (Fig 4) This implies that the G-Ag composites show an offset current, non-zero current at zero voltage whereas for exfoliated graphene zero offset current is observed The offset current of the order of nanoampere (nA) is observed and is found to increase with the concentration of silver nanoparticles This offset current point towards the existence of additional electronﬂow between the tip and the sample, at zero bias The surface plasmons might be the contributors to this

additional currentﬂow causing the offset current in G-Ag nano-composites[39] In order to verify the contribution from the surface plasmons to the offset current, further I-V measurements are car-ried out in presence of diode lasers (532 and 635 nm) where the surface plasmons can be excited optically Upon laser incidence, the offset current is found to increase for both G-Ag I and G-Ag II (Fig 4) This may be due to the optically excited surface plasmons which increased the offset current compared to that in the absence of lasers Moreover, it is observed that in the presence of green laser 532 nm, higher offset current is detected for both G-Ag I and G-Ag II than in presence of red laser 635 nm and this may be due to the closeness of plasmons in G-Ag I and G-Ag II with the green laser, 532 nm This clearly demonstrates that the presence of electron clouds leads to non-zero offset currents in G-Ag composites The offset currents measured for G-Ag I under dark condition, in pres-ence of green and red lasers are 1.89 ± 0.25, 2.73 ± 0.19 and 2.58± 0.16 nA, respectively and for G-Ag II the corresponding offset currents are 5.03 ± 0.76, 7.12 ± 0.85 and 6.41 ± 0.60 nA, respectively

The factor which gives rise to non-zero offset current was further investigated by measuring tunneling current as a function of distance between the tip and the sample and a typical variation is given in Fig Normally the tunneling current (I) has an

Table

Calculated ID/IG and I2D/IG ratios of graphene and grapheneesilver

nanocomposites

Sample ID/IG I2D/IG

Graphene 2.30 1.35

GeAg I 1.43 0.31

GeAg II 1.77 0.09

Fig Tunneling I-V characteristics of graphene and graphene-silver composites, G-Ag I and G-Ag II in the absence and the presence of green and red lasers Here GL and RL represent green and red laser

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exponential dependence on the distance (z) between tip and the sample and the corresponding relation is given in equation(1)

Iẳ e2Kzị (1)

where K is the decay constant and is given by K¼pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi8p2mf=h,

wherefis the work function, h is the plank's constant and m is the mass of the electron[40]

In the case of exfoliated graphene, tunneling current decreases exponentially with the z value whereas for the graphene-silver composites the I-z shows a complex relation different from the exponential relation which is in agreement with the observation made by Moller et al for silverﬁlms[39] This rule out the factors such as thermal expansion of the tip and photo voltage of the surface causing offset current which are having exponential dependence on the tunneling current with the distance[41] All these support the effect of plasmons on the tunneling current in the STM Hence the presence of surface plasmons is evident in STM investigations also, in addition to the UV-visible absorption and Raman spectroscopic investigations

Moreover to the offset current, an asymmetry is observed in the non-linear I-V characteristics, i.e current is more for positive voltages than that for negative voltages for G-Ag nanocomposites This asymmetry is attributed to variation in the density of states (DOS) of the sample due to the incorporation of the silver nanoparticles The theoretical work function of silver and graphene are 4.2 and 4.48 eV, respectively, making electron transfer from silver to graphene possible thereby varying the DOS of the sample[42] Conductance measurements (dI/dV) were carried out to investigate the change in DOS due to electron transfer, which directly gives the local density of states (LDOS) and the results are shown inFig The negative and positive region inFig 6corresponds to valence and conduction bands with regard to electronﬂow from the sample to tip or vice-versa depending on the bias applied In the LDOS of exfoliated graphene sheets given inFig 6(a), the Fermi-level appears to be closer to the valence band (VB) showing larger hole density in the exfoliated sheets conﬁrming its p-type nature and this originates from electron withdrawing nature of the surfactant used viz., SDBS [22] On introducing silver nanoparticles the Fermi-level of graphene is found to be shifted towards the conduction band (CB), showing n-type behaviour (Fig 6(b) and (c)) and this comes from the transfer of electrons from the silver to graphene due to the difference in the work function As the concentration of silver increases Fermi level shifts towards the CB implying that more electrons are transferred to the sheets, this makes DOS to change in graphene-silver composites resulting in asymmetric behaviour in the I-V characteristics Charge transfer between graphene and the silver nanoparticles was evident in the LDOS measurements and is in agreement with the observa-tions made in the Raman analysis, such as the shift of G and 2D band

and split of the G-bands The charge transfer mechanism also plays a vital role in the enhancement of Raman spectrum of graphene in the composites

4 Conclusion

Knowledge about the mechanism behind the interaction be-tween metal and graphene is vital for developing different devices out of graphene-metal hybrids Especially, G-Ag compositesﬁnd potential applications in differentﬁelds Here we have successfully synthesized G-Ag nanocomposites using liquid phase exfoliated graphene dispersion The Raman spectroscopic investigations revealed the strong interaction between silver nanoparticles and exfoliated graphene sheets as evidenced by G-band splitting Silver incorporation caused a reduction in the intensity of 2D-peak and a shift in the position of 2D and G-peaks in G-Ag nanocomposites The observed increase in the intensity of Raman signals was attributed to the SERS phenomena Interestingly, in the scanning tunneling measurements an offset current was observed for G-Ag composites and was found to increase with silver concentration It was observed that this offset current further increases when irra-diated with laser beams The offset current was attributed to the plasmons due to silver and was deduced from the non-exponential dependence of I-z curve Inherently the behaviour of these gra-phene layers were of p-type but LDOS studies showed that silver incorporation causes shift in the position of Fermi level and transforms it to behave like n-type with increase in Ag concentra-tion The charge transfer between silver nanoparticles and the graphene sheets were described together with STM studies and changes in DOS as a function of Ag concentration

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