Hybrid Nanostructures with Enhanced Optical Prop - American

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A Facile One Pot Synthesis of Highly Stable Graphene-Ag0 Hybrid Nanostructures with Enhanced Optical Properties Rishi Maiti, Tridib Sinha, Sayantan Bhattacharya, Prasanta Kumar Datta, and Samit K. Ray J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b04059 • Publication Date (Web): 11 Sep 2017 Downloaded from http://pubs.acs.org on September 12, 2017

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The Journal of Physical Chemistry

A Facile One Pot Synthesis of Highly Stable Graphene-Ag0

Hybrid

Nanostructures

with

Enhanced Optical Properties Rishi Maiti,† Tridib K. Sinha,† Sayantan Bhattacharya,† Prasanta K. Datta † Samit K. Ray*, †, Ұ †

Department of Physics, Indian Institute of Technology Kharagpur, Kharagpur-721302, India. Ұ S. N. Bose National Centre for Basic Sciences, Kolkata-700106, India *[email protected]

ABSTRACT: We report a facile one pot approach to synthesize graphene-Ag0 hybrid plasmonic nanostructures exhibiting superior optical properties. The Ag nanoparticles (NPs) (average particle size ~25 nm) are found to be highly stabilized within the graphene matrix probably due to the favorable d-π interaction among the vacant d-orbitals of Ag0 and the π-electrons cloud of graphene moiety. Transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) have been performed to characterize the hybrid nanostructures. The synergistic effect of plasmonic Ag and graphene in the hybrid nanostructures results enhanced Raman and photoluminescence (PL) in the visible wavelength (~520 nm). Non-linear absorption (NLA) property in femtosecond regime has been studied for this hybrid nanostructure. It is also observed that the two photon absorption (TPA) coefficient of this hybrid increases from 0.0127 cm/GW to 0.0155 cm/GW when the pulse energy is increased from 77 GW/cm2 to 170 GW/cm2. The study demonstrates enhanced optical 1

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response in the graphene-metal nanocomposite, which appears attractive for applications in graphene based advanced photonic devices. 1. INTRODUCTION Graphene exhibits several unique properties1-5 and thus bears great potential for application in various fields.6-9 It is interesting for the theoretical physicists due to its 2D Dirac fermion-like features, as well as for experimentalists for its ambipolar electric field effect, ballistic transport of charge carriers, peculiar integer and fractional quantum Hall effects and Klein tunnelling.10 In spite of all these intriguing phenomena, there are some limitations to the use of graphene for photonic devices. Although several efforts have been devoted to make graphene photoluminescent by oxygen plasma treatment,11 controlled reductions of graphene oxide12,13 or by excitation with femtosecond (30-fs) laser pulses,14 however PL emission from pristine graphene by continuous wave excitation has not been reported yet due to its zero band gap. Most of the luminescence observed so far was mainly due to the disorder induced localized states created within the π-π* band and to the non-equilibrium charge carriers (thermalized and nonthermalized hot electrons) under suitable conditions.15 On the other hand, plasmonics usually exploits noble metal nano-structures which can manipulate visible light at deep-subwavelength dimension.16,17 The ability of metal nanostructures based devices to control optical radiation at the nanoscale resulting in strong field enhancement was applied in diverse areas such as data storage, spectroscopy, sensing and others.18-20 Here, we report on the enhanced light matter interaction in stable Ag0 nanoparticles (NPs) decorated few layer graphene (FLG) sheets, synthesized using a facile and one-pot method. The proposed synthesis technique of pristine graphene is facile, less-hazardous and 2


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contamination free with respect to the other methods reported earlier.21-26 The facile synthesis technique for production of large area Ag0 nanoparticle decorated few-layer graphene is also proposed here by combining and modifying the suitable methods reported earlier.27-30 The stabilization of Ag0 NPs within graphene matrix occurs due to favorable d-π interaction31-33 between the vacant d-orbitals of Ag0 and the π-electron cloud of graphene moiety. The hybrid nanocomposite exhibits an enhanced light-matter interaction, which has been experimentally demonstrated using Raman spectroscopy and photoluminescence. Nonlinear absorption (NLA) in graphene-silver hybrid structure is found to increase from 0.0127 cm/GW to 0.0155 cm/GW with increasing on axis intensity from 77 GW/cm2 to 170 GW/cm2 using single beam open aperture zscan in the femtosecond regime. The contribution from both saturation absorption (SA) and two photon absorption (TPA) has been found to play a crucial role in the mechanism of NLA. 2. EXPERIMENTAL SECTIONS 2.1.

Synthesis of G-Ag0 hybrid

Here, we prepared stable and impurity free Ag0 nanoparticles within few layered pristine graphene dispersion. In this procedure, graphite powder (Sigma Aldrich) was exfoliated in 1 M HCl solution by ultrasonication for 25 min resulting in the formation of a black dispersion. Afterwards, Ethyl acetate (EtOAc) was added to the homogenous dispersion and the whole mixture was ultrasonicated for 10 min. In another set of experiment, dichloromethane (DCM) was used instead of EtOAc. After sonication, black coloured exfoliated graphene was observed in between organic-aqueous interfaces. It was extracted with organic solvents (EtOAc and DCM respectively) and subsequent washing with DI water for several times was accomplished. In case of DCM, the extracted product was obtained in larger quantity. After extraction, the products were dried, weighed and dispersed in N-Methyl 2-Pyrrolidone (NMP) through ultrasonication. 3



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The graphene dispersion was heated (at 150 0C) for the pretreatment of NMP. To the greyish black NMP-dispersion of graphene (8 mg/mL), 5wt% (mg/100 mL) silver nitrate salt was charged and sonicated for 10 min. The appearance of greenish-orange color was observed, suggesting the formation of Ag0 nanoparticles in graphene dispersion. 2.2.

Characterization

Graphene-Ag0 hybrid dispersion was spin-coated on Si (100) substrates. Surface morphology and crystalline quality of the hybrid nanostructures were studied by field- emission scanning electron microscopy (FESEM), scanning probe microscopy (SPM) and high resolution transmission electron microscopy (HRTEM). The phase of synthesized hybrid material was studied by X-ray diffraction (XRD) at a grazing incidence mode with Cu Kα radiation (1.54 Å). The chemical bonding and stability of Ag0 nanoparticles decorated few layer graphene sheets were studied using X-ray photo electron spectroscopy with an incident AlKα X-ray of energy 1486.6 eV. The elemental mapping was performed using an ULVAC-PHI PHI-700 scanning Auger microscope. The micro-Raman spectra of pristine graphene and hybrid films were acquired using a T64000 (Jobin Yvon Horiba) spectrometer with an Argon-Krypton mixed ion gas laser (~514 nm). UVvis absorption spectrum was obtained using a fiber-probe based UV-Vis-NIR CCD spectrometer. We acquired the room temperature steady state photoluminescence (PL) spectra using a He-Cd laser of wavelength 325 nm as the excitation source and a TRIAX-320 monochromator combined to a Hamamatsu R928 photomultiplier detector. The open aperture (OA) z-scan measurements were performed with a laser pulse of 150 fs at 1 kHz repetition rate at the wavelength of 808 nm (Coherent Libra-He). The fs laser beam was focused to 58 µm (beam diameter) with a 20 cm plano-convex lens. Ag NPs decorated graphene dispersion placed in the cuvette of 1 mm path-length was moved across the focus using a motorized translation stage 4


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(Thorlab, LTS-150). The Rayleigh range (z0) of the focused beam is 1.1 cm, which is longer than the path-length of the beam in the cuvette. The input pulse power was varied with a combination of half-wave plate and a polarizer. The reference and the transmitted laser pulses were detected using two Si-photodiodes (Thorlab, PDA10A), and the data were recorded through a lock-in amplifier for better signal-to-noise ratio. The lock-in amplifier was triggered by a 1 kHz signal and was synchronized with the femtosecond laser pulse. 3. RESULTS & DISCUSSION 3.1.

Synthesis Mechanism

We report a facile, one-pot method for synthesis of high-quality G-Ag0 hybrid nanocomposite using N-Methyl 2-Pyrrolidone (NMP) as the reducing agent and graphene itself as a stabilizer. The large scale production of pristine graphene from graphite using HCl as an intercalant has been performed as depicted in Figure 1a. Here, H+ ion occupies the interstitial sites of graphitic layers through an electrostatic interaction and weakens the existing π-π* interaction. The subsequent sonication acts as a promoting force to overcome this π-π* interaction. Highly hydrophilic HCl thus favours the formation of water processable graphene, while, lyophilisation produced light weight and spongy graphene powders. The intercalated graphene has been extracted by Ethyl acetate (EtOAc) and dichloromethane (DCM), where larger amount of graphene was obtained from the water-DCM interface maybe due the common ion (i.e.,Cl-) of HCl and DCM which leads accumulation of more HCl intercalated graphene in the interface of DCM and water. After extraction the graphene was dispersed in NMP to get a homogenous graphene-ink (greyish black) and heated at 150 0C temperature for the pretreatment of NMP.34 On addition of AgNO3 to this pre-treated graphene-dispersion, the activated NMP reduced the Ag+ to Ag0 with the subsequent greenish-orange color appearance within ~2 min as 5



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schematically presented in Figure 1b. During the extended sonication (10 min), Ag nanoparticles (NPs) in graphene are stabilized through d-π interaction, which may occurs among the vacant dorbitals of silver and filled π-electrons of graphene. During this interaction, graphene will be slightly positively polarized (as it donates the π-electron).Through electrostatic interaction, another FLG will be interacted with positively charge layer. These FLGs will also interact with other Ag0 Nps using the remenant π-electrons, leads to the decoration of Ag0 nanoparticles on few layer graphene sheets.35-38 The mechanism is supported due to the presence of delocalized πelectrons in graphene matrix, which can be treated as the integration of 2π aromatic rings that might be capable of forming composite having transition metal complex structures.39,40 Thus πbonding will have some sort of sigma-bond character in the hybrid nano-structures, resulting in enhancement of sp3 character in the hybrid structure as evidenced from XPS spectra. During Ag0 nanoparticle formation, the trapped NMP molecules are also responsible for the production of very few oxygen functional groups on graphene defect sites (edge defects), which will be discussed in details during X-ray photoelectron spectra (XPS) analysis.

6


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Graphite

(a)

Graphite in HCl

Sonication, 25 min

Lyophilisation

Water processable graphene Graphene -HCl

EtOAc

Extraction

Organic solvent processable graphene

Aqueous part

Graphene

Graphene in DCM

GrapheneHCl

(b) AgNO3

O2 O2

Graphene ink

H

O

Pre-treated Graphene ink

Ag H

N

O2

H

HO

O

O

G-Ag Dispersion

N N

O O

Figure 1. Schematic representation of (a) HCl intercalated pristine graphene preparation and purification. (b) In-situ formation and stabilization of Ag0 nanoparticles in the graphene dispersion using one-pot method.

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3.2.

Characteristics of hybrid nanostructures We have studied the surface morphology and microstructures of synthesized graphene-Ag0 hybrid composites. Figures 2a and b show typical field-emission scanning electron (FESEM) micrographs of few layer graphene sheets and Ag0 NPs decorated FLG sheets, respectively. The size distribution of Ag nanoparticles shown in the inset of Figure 2b indicates the formation of an average particle size of around 25 ± 5 nm for Ag. Figure 2c shows the high resolution transmission electron microscopy (HRTEM) image of pristine graphene and the corresponding hexagonal selected area electron diffraction (SAED) pattern (inset), indicating the growth of a few layer crystalline hexagonal graphene sheets. The SAED pattern contains multiple spots, which could be ascribed to overlapping domains of graphene layers, backfolding of edges, or intrinsic rotational stacking faults.41 The TEM image in Figure 2d shows large-area graphene sheets decorated with Ag NPs of average size ~ 25 ± 5 nm corroborating the FESEM results. The corresponding high-resolution image of Ag NPs is shown in Figure 2e, which supports the crystallinity of synthesized nanoparticles within the graphene for the (111) plane of fcc Ag with an interplanar spacing of 0.23 nm. The X-ray diffraction (XRD) pattern of graphene-Ag0 hybrid nanostructures is shown in Figure 2f. Five well-defined diffraction peaks at 38.1°, 44.4°, 64.5°, 77.4°, and 81.6° attributed respectively, to (111), (200), (220), (311), and (222) planes of fcc crystal structure of metallic silver are observed. The SAED pattern, HRTEM image and XRD results reveal that the synthesized Ag NPs embedded within the graphene matrix are highly crystalline.

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(a)

(b)

(c)

(d)

(e) (f)

Figure 2. Typical FESEM images of (a) few layer graphene (FLG) sheets. (b) graphene sheets decorated by Ag0 NPs. The inset of figure in panel (b) indicates an average Ag nanoparticle size of around 25 ± 5 nm. (c) HRTEM image of a few layers graphene with corresponding SAED pattern in the inset. (d) Ag0 NPs decorated few layer graphene sheets. (e) High resolution image of a single Ag NP showing lattice fringes for the (111) plane. (f) XRD pattern of graphene-Ag0 9



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hybrid sample showing five well-defined diffraction peaks at 38.1°, 44.4°, 64.5°, 77.4°, and 81.6°, respectively, corresponding to (111), (200), (220), (311), and (222) planes of fcc Ag.

X-ray photoelectron spectra (XPS) of graphene and hybrid nanostructures are presented in Figure 3. Figure 3a exhibits only sp2 hybridized C=C (284.6 eV) peak in pristine graphene, which is consistent with the previously reported data.42 On the other hand, the C1s spectrum of grapheneAg0 hybrid displays (Figure 3b) two small humps at a higher binding energy value (~ 286 & ~ 288 eV), suggesting the presence of a finite amount of oxygen functional groups with sp2 hybridized carbon atoms.13 The origin of these additional peaks in C1s spectrum is ascribed to the Ag0 nanoparticle formation, and its stabilization within the graphene sheets through the d-π interaction. During Ag0 nanoparticle formation, the byproduct HNO3 oxidizes the edges of graphene. The nascent Ag0 stabilizes through d-π interaction among the vacant d-orbitals of Ag0 and the π-electrons cloud of graphene moiety. So, the sp2 hybridized π-bonds may have some sp3 hybridized single bond character, which is also evident from the higher full-with-half-maximum (FWHM) of C1s peak of G-Ag0 hybrid, as compared to the pristine graphene. The trapped NMP molecule may also be responsible for showing these oxygen functionalities. Figure 3c presents the binding energy of Ag 3d electrons for G-Ag0 hybrid nanostructures for as deposited and sample after 2 months, where the peaks at 367.9 and 373.9 eV are ascribed to the Ag 3d5/2 and Ag 3d3/2 electronic states, respectively. The difference in the binding energy between the above electronic states for Ag 3d electrons is 6.0 eV remain almost unchanged over this time period, which suggests the formation and stabilization of Ag0 chemical state in G-Ag0 hybrid composite.43

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(a) (c)

(b)

Figure 3. High resolution C1s XPS spectra of (a) pristine graphene, (b) graphene-Ag0 hybrid, and (c) Binding energy of Ag 3d electrons showing stability even after 2 months. We have also performed the Auger electron Spectroscopy (AES) to investigate the nature of chemical bonding and elemental mapping with high spatial resolution in graphene-Ag0 NPs hybrid nanostructures. AES is a standard and powerful surface analytical technique based on the analysis of energetic electron emitted from an excited atom after a series of internal relaxation events.44 In Figure 4, AES spatial distribution spectra are presented for G-Ag0 hybrid nanostructures. Figure 4a shows a typical FESEM micrograph of Ag NPs decorated graphene sheets, where the AES mapping has been acquired. The AES survey spectrum showing the presence of C KLL, O KLL, Ag MNN and Si KLL transition dips in Figure 4b. The presence of C and Ag troughs confirm the formation of a G-Ag0 hybrid structure. Si and O signals arise from 11



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the SiO2/Si substrate. Figure 4c exhibits the AES spatial distributions of C KLL in the sample. The carbon-rich regions in G-Ag0 are presented by red color consistent with the SEM images of graphene sheets shown in Figure 4a. However, AES map of Ag MNN surface of G-Ag0 shown in Figure 4d are represented by green color, showing the formation of particles. Figure 4e presents the combined AES mapping of G-Ag0 hybrids, where Ag NPs are decorated on top of graphene sheets.

Figure 4. (a) Typical FESEM image of G-Ag0 hybrid nanostructure. (b) Auger electron spectra (AES) showing C KLL, O KLL, Ag MNN and Si KLL spectral lines. Auger elements maps for the different elements same location shown in figure (a) AES Elemental mapping for (c) C, (d) Ag and (e) for combined image showing where Ag0 NPs are decorated on top of the graphene sheets. 3.3.

Raman, absorption & Photoluminescence Characteristics

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Typical Raman spectra of pristine graphene and G-Ag0 hybrids are presented in Figure 5a. The spectrum of graphene exhibits a weak D-peak (~1360 cm-1) and two prominent peaks corresponding to the most prominent G (~1580 cm-1) and 2D (~2740 cm-1) bands, respectively. The G-band is assigned to the E2g phonon vibrations of sp2 carbon atoms, while the D-band is A1g breathing mode of carbon associated with structural defects and disorders. The 2D phonon bands originate from a second order double resonant process between non-equivalent K-points in graphene’s 1st brillouin zone, connecting two zone boundaries.45,46 The SERS enhancement of Gand D- peaks, up shift of G-band and downshift of 2D band with respect to graphene spectrum, broadening of G-band and enhancement of D-band suggest the formation of hybrid nanostructures, as shown in Figure 5a.47,48 In Figure 5b, UV-Vis absorption spectra are presented for pristine graphene and G-Ag0 hybrid samples. The plot shows a broad absorption band in the entire visible region with a peak at ~2.48 eV. The result in Figure 5b indicates a significant enhancement in the absorbance of G-Ag0 hybrid with respect to control graphene.27,49-50 The enhancement of Raman and absorption signal can be attributed to both the enhanced near-field electronic oscillation and scattering effect of Ag nanoparticles. Due to the localized plasmon resonance (LSPR) of metal nanoparticles, the enhance coupling of light happen around the graphene-metal interface, leading to an amplification of the electrical field that can effectively improve the optical properties of hybrid nanostructures. Figure 5c shows a typical PL spectrum of as-synthesized hybrid nanostructures drop-casted on top of the Si substrate. The PL spectrum shows a broad emission range from 1.6 to 3.2 eV (400 nm to 700 nm) with a maximum intensity at 2.38 eV (~520 nm). It may be noted that the pristine graphene does not exhibit any emission, when excited by a continuous wave source, as shown in Figure 5c. On the other hand, G-Ag0 hybrid nanostructures 13



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exhibit strong photoluminescence. The origin of this steady state room temperature strong PL is from Ag NPs which is stabilized within the graphene surface.27 The visible luminescence from hybrid is due to the excitation of electrons from occupied d-bands of Ag into states above the Fermi level. Subsequent electron-phonon and hole-phonon scattering process leads to an energy loss and finally an electron from an occupied Fermi level recombines radiatively with the s-p or d-band holes making the hybrid material a luminescent one.

(a)

(b)

(c)

Figure 5. a) Typical Raman spectrum of a pristine graphene and G-Ag0 hybrid showing SERS enhancement, broadening and upshift of G band and downshift of 2D band. (b) UV-Vis absorption spectra of pristine graphene and graphene-Ag0 hybrid samples. (c) Typical strong visible emission shown by G-Ag0 hybrid nanostructures excited by a 325 nm laser. 3.4.

Nonlinear absorption property of G-Ag0 hybrids

The nonlinear optical measurements for pristine graphene and graphene-Ag0 hybrid nanostructures were carried out by a single beam z-scan technique.51 Pristine graphene sample shows only saturable absorption (SA) whereas, Ag decorated graphene sample exhibits reverse 14


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saturable absorption (RSA) behaviour as shown in Figures 6a and b, respectively. Open aperture z-scan measurements performed on graphene-Ag0 hybrid nanostructures with on axis intensity of 77, 106, 127 and 170 GW/cm2 respectively, are shown in Figure 6b. At lower intensity, contribution from SA can be observed in OA curves whereas at higher intensity multiphoton absorption is a dominant phenomenon. Also we measured OA z scan of Graphene in water at 106 GW/cm2 to check saturation absorption behaviour. To understand the z-scan results, we consider SA and two-photon absorption (TPA) simultaneously in analysis.52 A single Gaussian beam with a beam waist of w0 can be represented as,51 I ( z, r ) 

I0 2r 2 t2 exp(  ) exp(  ) 1  ( z 2 / zR2 ) w2 ( z )  p2

(1)

where, I0 is the on-axis intensity at focus, zR is the Rayleigh length, τp is the pulse-width of input pulse, and w(z) can be defined as, w2(z) = w02(1+z2/zR2). The propagation of laser beam within the sample can be determined from the intensity equation dI   ( I ) I dz

(2)

where, α(I) is the intensity dependent absorption coefficient. For an in-homogeneously broadened two-level system with both SA and TPA, α(I) can be written as

 (I) 

0 1

I

  eff I

I Sat

(3)

where, ISat is the saturation intensity and βeff is the TPA coefficient. The transmittance of the incident laser beam is obtained by solving the equation (2) from equation (3), and by integrating it over time and radial direction. The intensity equation has been solved numerically using 15



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Crank-Nichelson method. Assuming the input beam to be Gaussian, the integration limit of t and z is varied from -∞ to +∞ and 0 to ∞, respectively. The theoretical fits of the obtained data resulted in the determination of the values of ISat and βeff. For graphene-metal hybrids, the saturation intensity (ISat) is extracted to be 14 GW/cm2 and TPA coefficients are 0.0127, 0.0136, 0.0145 and 0.0155 cm/GW respectively, for 77, 106, 127 and 170 GW/cm2 whereas for Graphene saturation intensity is 34 GW/cm2 with TPA coefficient (βeff) of 0.0012 cm/GW. As we can see from the fitted curves while graphene only shows SA which has also been reported earlier52 whereas, in graphene-Ag0, the nonlinearity can be tuned by varying input pulse energy. At sufficient excitation intensity, the photogenerated carrier density in graphene increases significantly (much larger than the intrinsic electron and hole carrier densities in graphene at room temperature).53 This causes the filling up of states near the edges of the conduction and valence bands, blocking further absorption and yielding transparency to light at photon energies just above the band-edge. If the incident light intensity becomes strong enough, because of the Pauli blocking effect, the generated carriers fill the conduction band, preventing further excitation of electrons from the valence band and allowing photons to be transmitted without loss, as shown in Figure 6a.

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Figure 6. Open aperture z-scan of (a) Graphene and (b) graphene-Ag0, where open circle denotes experimental data and the solid lines are the fitted curves. Figure 7 presents the schematic band diagram of G-Ag0 hybrid nanostructures. The Work function of Ag NPs (WAg) is found to be 4.3 eV which has been calculated by using the following eqn: WNP=W∞ + (1.08/D (nm)) eV

54

where, W∞ is the work function of bulk Ag

surface which is 4.26 eV 55 and D is the diameter of the nanoparticles in the unit of nm.

Figure 7. Schematic Energy band diagram of the G-Ag0 hybrid nanostructures and corresponding shift of graphene Fermi level (ΔEF) due to the charge transfer. The work function of free standing graphene (WG=4.48 eV) is modified due to charge transfer mechanism56 for the hybrid nanostructures as depicted in Figure 7. Calculations have also been reported that the graphene work-function in contact with Ag NPs is significantly lower (by 0.18 eV) than in the free-standing graphene. So, the graphene Fermi level, in contact with Ag shifts a little bit upwards with respect to the conical points, leading to n-type doping of graphene57. At 800 nm, the absorption spectrum of hybrid nanostructures exhibits a prominent absorption band. 17



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Therefore, in addition to intraband transitions of graphene (due to shift of the fermi level in conduction band), direct excitations to the LSPR band of Ag NPs might induce TPA at higher pump intensity. The simultaneous operation of these mechanisms results RSA as dominating phenomena for G-Ag0 hybrid nanostructures as compared to the pristine graphene.50, 58

CONCLUSIONS In summary, we have successfully synthesized Ag0 nanoparticles decorated few layer graphene sheets using a facile and one pot technique to enhance the light-matter interaction in graphene. The novel technique provides the formation of graphene sheets stabilized Ag0 NPs (average size ~ 25 nm), promoted by the d-π interaction among vacant d-orbitals of Ag0 nanoparticles and πelectrons cloud of graphene. XPS and AES analysis have indicated the formation of Ag0 chemical

state

in

hybrid

composite.

The

hybrid

nanostructures

exhibit

improved

photoluminescence and nonlinear absorption property over the pristine graphene. Compared to pristine graphene where saturable absorption is dominant, hybrid structures show significant NLA characteristics due to high TPA coefficient. The results indicate that the synthesized novel Ag0 decorated plasmonic nanostructures can be used as platform for wavelength tunable graphene based photonic devices in future. ACKNOWLEDGEMENTS Authors are thankful to SDGRI-UPM project of IIT Kharagpur (IIT/SRIC/PHY/UPM/201415/118) for providing necessary equipment for z-scan measurements. The use of XPS facility under FIST Project (SR/FST/PSII-022/2010) of Department of Physics, IIT Kharagpur is also gratefully acknowledged.

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AUTHOR INFORMATION Corresponding Authors Prof. Samit K. Ray Email : [email protected] Phone : +91-3222-283838 Fax : +91-3222-255303 Author Contributions Rishi Maiti has designed and performed the experimental work. He has prepared the overall manuscript. Tridib K. Sinha assisted in material synthesis and structural characterization. Sayantan Bhattacharya has carried out the non-linear optical measurements. Prasanta K Datta provided guidance for manuscript preparation. Samit K. Ray provided overall guidance to conduct the research and prepare the manuscript. All authors discussed the results and have given approval to the final version of the manuscript.

Notes T. K. Sinha and S. Bhattacharya equally contributed to this work. The authors declare no competing financial interest.

REFERENCES (1) Geim, A. K.; Novoselov, K. S. The Rise of Graphene. Nat. Mater. 2007, 6, 183-191. (2) Morozov, S. V.; Novoselov, K. S.; Katsnelson, M. I.; Schedin, F.; Elias, D. C.; Jaszczak, J. A.; Geim, A. K. Giant Intrinsic Carrier Mobilities in Graphene and its Bilayer. Phys. Rev. Lett. 2008, 100, 016602. (3) Lee, C.; Wei, X.; Kysar, J. W.; Hone, J. Measurement of the Elastic Properties and Intrinsic Strength of Monolayer Graphene. Science 2008, 321, 385-388. 19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 26

(4) Balandin, A. A.; Ghosh, S.; Bao, W.; Calizo, I.; Teweldebrhan, D.; Miao, F.; Lau, C. N. Superior Thermal Conductivity of Single-Layer Graphene. Nano Lett. 2008, 8, 902-907. (5) Kuzmenko, A. B.; van Heumen, E.; Carbone, F.; van der Marel, D. Universal Optical Conductance of Graphite. Phys. Rev. Lett. 2008, 100, 117401. (6) Schwierz, F. Graphene Transistor. Nat. Nanotech. 2010, 5, 487-496. (7) Bonaccorso, F.; Sun, Z.; Hasan, T.; Ferrari, A. C. Graphene Photonics and Optoelectronics. Nat. Photon. 2010, 4, 611–622. (8) Brownson, D. A. C.; Kampouris, D. K.; Banks C. E. An Overview of Graphene in Energy Production and Storage Applications. J. of Power Sources 2011, 196, 4873-4885. (9) Novoselov, K. S.; Fal′ko, V. I.; Colombo, L.; Gellert, P. R.; Schwab, M. G.; Kim, K. A Roadmap for Graphene. Nature 2012, 490, 192–200. (10) Castro Neto, A. H.; Guinea, F.; Peres, N. M. R.; Novoselov, K. S.; Geim, A. K. The Electronic Properties of Graphene. Reviews of Modern Phys. 2009, 81, 109-162. (11) Gokus, T.; Nair, R. R.; Bonetti, A.; Bohmler, M.; Lombardo, A; Novoselov, K. S.; Geim, A. K.; Ferrari, A. C.; Hartschuh, A. Making Graphene Luminescent by Oxygen Plasma Treatment. ACS Nano, 2009, 3, 3963-3968. (12) Chien, C. T.; Li, S. S.; Lai, W. J.; Yeh Y. C.; Chen, H. A.; Chen, I. S.; Chen, L.; Chen, K. H.; Nemoto, T.; Isoda, S. et al. Tunable Photoluminescence from Graphene Oxide. Angew. Chem. Int. Ed. 2012, 51, 6662–6666. (13) Maiti, R.; Midya, A.; Narayana, C.; Ray, S. K. Tunable Optical Properties of Graphene Oxide by Tailoring the Oxygen Functionalities using Infrared Irradiation. Nanotechnology, 2014, 25, 495704. (14) Lui, C.H.; Mak, K.F.; Shan, J.; Heinz, T.F. Ultrafast Photoluminescence from Graphene. Phys. Rev. Lett. 2005, 105,127404.

20


Page 21 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(15) Stoehr, R.J.; Kolesov, R.; Pflaum, J.; Wrachtrup, Jr. Fluorescence of Laser-Created Electron-Hole Plasma in Graphene. Phys. Rev. B, 2010, 82, 121408. (16) Barnes, W. L.; Dereux, A.; Ebbesen, T. W. Surface Plasmon Subwavelength Optics. Nature 2003, 424, 824-830. (17) Ozbay, E. Plasmonics: Merging Photonics and Electronics at Nanoscale Dimensions. Science 2006, 311, 189-193. (18) Liang, Z.; Sun, J.; Jiang, Y.; Jiang, L.; Chen, X. Plasmonic Enhanced Optoelectronic Devices. Plasmonics 2014, 9, 859-866. (19) Kneipp, K.; Wang, Y.; Kniepp, H.; Perelman, L. T.; Itzkan, I.; Dasari, R. R.; Feld, M. S. Single Molecule Detection Using Surface-Enhanced Raman Scattering (SERS). Phys. Rev. Lett. 1997, 78, 1667-1670. (20) Atwater, H. A.; Polman, A. Plasmonics for Improved Photovoltaic Devices. Nat. Mater. 2010, 9, 205-213. (21) Parvez, K.; Wu, Z. S.; Li, R.; Liu, X.; Graf R.; Feng X.; Müllen, K. Exfoliation of Graphite into Graphene in Aqueous Solutions of Inorganic Salts. J. Am. Chem. Soc. 2014, 136, 60836091. (22) Sampath, S.; Basuray, A. N.; Hartlieb, K. J.; Aytun, T.; Stupp, S. I.; Stoddart, J. F. Direct Exfoliation of Graphite to Graphene in Aqueous Media with Diazaperopyrenium Dications. Adv. Mater. 2013, 25, 2740-2745. (23) Wang, X.; Fulvio, P. F.; Baker, G. A.; Veith, G. M.; Unocic, R. R.; Mahurin, S. M.; Chib M.; Dai S. Direct Exfoliation of Natural Graphite into Micrometre Size Few Layers Graphene Sheets using Ionic Liquids. Chem. Commun. 2010, 46, 4487-4489. (24) Ciesielski, A.; Samorı,` P. Graphene via Sonication Assisted Liquid-Phase Exfoliation. Chem. Soc. Rev. 2014, 43, 381-398.

21



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 26

(25) Hernandez, Y.; Nicolosi, V.; Lotya, M.; Blighe, F. M.; Sun, Z.; De, S.; McGovern, I. T.; Holland, B.; Byrne, M.; Gun’Ko, Y. K. et al. High-Yield Production of Graphene by LiquidPhase Exfoliation of Graphite. Nat. Nanotechnol. 2008, 3, 563-568. (26) Martín-García, B.; Velazquez, M. M.; Rossella, F.; Bellani, V.; ́ Diez, E.; García Fierro, J. L.; Perez-Hernandez, J. A.; Hernandez-Toro, ́ J.; Claramunt, S.; Cirera, A. Functionalization of Reduced Graphite Oxide Sheets with a Zwitterionic Surfactant. ChemPhysChem. 2012, 13, 3682-3690. (27) Maiti, R.; Sinha, T. K.; Mukherjee, S.; Adhikari, B.; Ray, S. K. Enhanced and Selective Photodetection Using Graphene-Stabilized Hybrid Plasmonic Silver Nanoparticles. Plasmonics 2016, 11, 1297-1304. (28) Sinha, T. K.; Ghosh, S. K.; Maiti, R.; Jana, S.; Adhikari, B.; Mandal, D.; Ray, S. K. Graphene-Silver Induced Self-polarized PVDF Based Flexible Plasmonic Nanogenerator Towards the Realization for New Class of Self-Powered Optical Sensor. ACS Appl. Mater. & Interfaces, 2016, 8, 14986–14993. (29) Pastoriza-Santos I.; Liz-Marzan L. Formation and Stabilization ́ of Silver Nanoparticles through Reduction by N, N-Dimethylformamide. Langmuir 1999, 15, 948−951. (30) Jeon, S. –H.; Xu, P.; Mack, N. H.; Chiang, L. Y.; Brown, L.; Wang, H.−L. Understanding and Controlled Growth of Silver Nanoparticles Using Oxidized N-Methyl-pyrrolidone as a Reducing Agent. J. Phys. Chem. C 2010, 114, 36-40. (31) Muetteries, E. L.; Bleeke, J. R.; Wuchere, E. J. Structural, Stereochemical, and Electronic Features of Arene-Metal Complexes. Chem. Rev. 1982, 82, 499-525. (32) Heimel, G.; Duhm, S.; Salzmann, I.; Gerlach, A.; Strozecka, A.; Niederhausen, J.; Bürker, C.; Hosokai, T.; Fernandez Torrente, I.; Schulze, G. et al. Charged and Metallic Molecular Monolayers Through Surface-Induced Aromatic Stabilization. Nat. Chem. 2013, 5, 187-194. (33) Lindeman, S. V.; Rathore, R.; Kochi, J. K. Silver(I) Complexation of (Poly)aromatic Ligands: Structural Criteria for Depth Penetration into cis-Stilbenoid Cavities. Inorg. Chem. 2000, 39, 5707-5716. 22


Page 23 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(34) Lin, Z.; Yao, Y.; Li, Z.; Liu, Y.; Li, Z.; Wong, C.-P. Solvent-Assisted Thermal Reduction of Graphite Oxide. J. Phys. Chem. C, 2010, 114, 14819−14825. (35) Avdoshenko, S. M.; Ioffe, I. N.; Cuniberti, G.; Dunsch, L.; Popov, A. A. Organometallic Complexes of Graphene: Toward Atomic Spintronics using a Graphene Web. ACS Nano 2011, 5, 9939−9949. (36) Mingguang, C.; Xiaojuan, T; Wangxiang, L.; Bekyarova, E.; Guanghui, L.; Matthew, M. Robert, C. H. Application of Organometallic Chemistry to the Electrical Interconnection of Graphene Nanoplatelets. Chem. Mater. 2016, 28, 2260–2266. (37) Werner, H. At Least 60 Years of Ferrocene: The Discovery and Rediscovery of the Sandwich Complexes. Angew.Chem. Int. Ed. 2012, 51, 6052−6058. (38) Heinze, K.; Lang, H. Ferrocene-Beauty and Function. Organometallics 2013, 32, 56235625. (39) Ogimachi, B. Y. N.; Andrews, L. J.; Keefer, R. M. The Coördination of Polyalkylbenzenes with Silver Ion. J. Am. Chem. Soc. 1956, 78, 2210-2213. (40) Maslowsky, E. Organometallic benzene complexes. J. Chem. Educ. 1993, 70, 980. (41) Robertson, A. W.; Warner, J. H. Hexagonal Single Crystal Domains of Few-Layer Graphene on Copper Foils. Nano Lett. 2011, 11, 1182-1189. (42) Li, D.; Müller, M. B.; Gilje, S.; Kaner, R. B.; Wallace, G. G. Processable Aqueous Dispersions of Graphene Nanosheets. Nat. Nanotech. 2008, 3, 101-105. (43) Pol, V.G., Srivastava, D.N., Palchik, O., Palchik, V., Slifkin, M.A., Weiss, A.M., Gedanken A. Sonochemical Deposition of Silver Nanoparticles on Silica Spheres. Langmuir 2002, 18, 3352-3357. (44) Briggs, D.; Sea, M. P. Practical Surface Analysis. Auger and X-ray Photoelectron Spectroscopy. John Wiley & Sons: New York, USA 1984.

23



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 26

(45) Malarda, L.M.; Pimentaa, M.A.; Dresselhaus, G.; Dresselhaus, M.S. Raman spectroscopy in graphene. Phys. Rep. 2009, 473, 51-87. (46) Ferrari, A. C.; Basko, D. M. Raman Spectroscopy as a Versatile Tool for Studying the Properties of Graphene. Nat. Nanotech. 2013, 8, 235-246. (47) Maiti, R.; Haldar, S.; Majumdar, D.; Singha, A.; Ray, S. K. Hybrid Opto-Chemical doping in Ag Nanoparticle Decorated Monolayer CVD Graphene Probed by Raman Spectroscopy. Nanotechnology, 2017, 28, 075707. (48) Lee, J.; Novoselov, K. S.; Shin, H. S. Interaction Between Metal and Graphene: Dependence on the Layer Number of Graphene. ACS Nano 2011, 5, 608–612. (49) Kravets, V. G.; Schedin, F.; Jalil, R.; Britnell, L.; Novoselov, K. S.; Grigorenko, A. N. J. Surface Hydrogenation and Optics of a Graphene Sheet Transferred onto a Plasmonic Nanoarray. J. Phys. Chem. C 2012, 116, 3882-3887. (50) Kalanoor, B. S.; Bisht, P. B.; Ali S. A.; Baby T. T.; Ramaprabhu, S. Optical nonlinearity of Silver-Decorated Graphene. J. Opt. Soc. Am. B 2012, 4, 669-675. (51) Sheik-Bahae, M.; Said, A. A.; Wei, T. H.; Hagan, D. J.; Van Stryland, E. W. Sensitive Measurement of Optical Nonlinearities using a Single Beam. IEEE J. Quantum Electron.1990, 26, 760-769. (52) Bhattacharya, S; Maiti, R.; Das, A. C.; Saha, S.; Mondal, S.; Ray, S. K.; Bhaktha, S. N. B.; Datta, P. K. Efficient Control of Ultrafast Optical Nonlinearity of Reduced Graphene Oxide by Infrared Reduction. J. Appl. Phys. 2016, 120, 013101. (53) Xing, G.; Guo, H.; Zhang, X.; Sum, T. C.; Huan, C. H. A. The Physics of Ultrafast Saturable Absorption in Graphene. Opt. Exp. 2010, 18, 4564-4573. (54) Zhou, L.; Zachariah, M. R. Size Resolved Particle Work Function Measurement of Free Nanoparticles: Aggregates vs. Spheres. Chem. Phys. Lett. 2012, 77, 525-526.

24


Page 25 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(55) Michaelson, H.B. The Work Function of the Elements and its Periodicity. J. of Appl. Phys. 1977, 48, 4729-4733. (56) Bao, Q.; Zhang, H.; Wang, Y.; Ni, Z.; Yan, Y.; Shen, Z. X.; Loh, K. P.; Tang, D. Y. Atomic Layer Graphene as Saturable Absorber for Ultrafast Pulsed Laser. Adv. Funct. Mater. 2009, 19, 3077-3083. (57) Giovannetti, G.; Khomyakov, P. A.; Brocks, G.; Karpan, V. M.; Brink, J. van den; Kelly, P. J. Doping Graphene with Metal Contacts. Phys. Rev. Lett. 2008, 101, 026803. (58) Subrahmanyam, K.S.; Manna, A. K.; Pati, S. K.; Rao, C.N.R. A Study of Graphene Decorated with Metal Nanoparticles. Chem. Phys. Lett. 2010, 497, 70-75.

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Hybrid Nanostructures with Enhanced Optical Prop - American

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