Charge Doping of Large-Area Graphene by Gold-Alloy Nanoparticles

Nov 26, 2013 - The HRTEM and EELS were performed by recording the images close to the ...... Japanese Journal of Applied Physics 2015 54, 066101 ...
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Charge Doping of Large-Area Graphene by Gold-Alloy Nanoparticles Maria Antoaneta Bratescu* EcoTopia Science Institute, Nagoya University, Furo-cho Chikusa-ku, Nagoya 464-8603, Japan

Nagahiro Saito Department of Materials, Physics and Energy Engineering, Green Mobility Collaborative Research Center, Nagoya University, Furo-cho Chikusa-ku, Nagoya 464-8603, Japan S Supporting Information *

ABSTRACT: We present a facile, one-step, and surfactant-free method for direct synthesis and loading of stable gold and gold-alloy nanoparticles (NPs) on large-area graphene using an electrical discharge in a liquid environment, termed solution plasma. We observed a charge doping of graphene by the gold NPs, which depends on the particles’ chemical composition, even if the NPs contain a few percent of trivalent sp metals, such as indium (In) or gallium (Ga). Raman and electron energy loss spectroscopy (EELS) methods show that graphene is doped with electrons (n-type) in the case of gold NPs and with holes (p-type) in the case of gold-alloy NPs. The Raman band shift indicates that the amount of the transferred electrons from the gold NPs to graphene is −2 × 10−4 electrons per unit cell. The gold-alloy NPs receive from graphene (2 and 4) × 10−5 electrons per unit cell if the gold NPs contain In and Ga, respectively. In the EELS spectra, the decrease in the intensity of the 1s-π* transition and the shift of the π* peak to higher energy confirm the depopulation of the antibonding states caused by the electron transfer from graphene to the gold-alloy NPs.

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INTRODUCTION Graphene is a good conductor with high transparency, flexibility, and strength, and it is environmentally stable.1 Nanoparticles (NPs) are useful materials especially due to their surface plasmon resonance (SPR) and catalytic activity. The NP size, composition, and dielectric surroundings can change the optical and electrical properties and the catalytic activity.2,3 Graphene decorated with NPs shows enhanced catalytic activity; for example, PtAu alloy NPs on graphene have been used for formic acid oxidation,4 and graphene-supported Pt and Pt−Ru NPs were found to be an efficient electrocatalyst for methanol and ethanol oxidation.5 In these cases, the synthesis of alloy NPs required the use of surfactants that were also involved in the catalysis process. Recently, graphene and graphene-based hybrid nanoassemblies gained considerable attention in optoelectronic devices such as displays, touch screens, light-emitting diodes, and solar cells, which require materials with low sheet resistivity and high transparency.6−8 Graphene can fulfill multiple functions in light-conversion systems as the transparent conductive window, photoactive material, channel for charge transport, and catalyst.8 For example, a single-layer graphene/n-Si Schottky junction exhibits high solar-power conversion efficiency,6 or chemical vapor deposition (CVD) graphene with indium tin oxide electrodes on polyethylene terephthalate substrate may be used as a transparent conductive electrode in an organic © 2013 American Chemical Society

photovoltaic cell, which demonstrates the great potential of the CVD graphene films for flexible optoelectronics devices.7 To be used in electronics and optics, graphene must be in contact with other materials, which can change its electrical and optical properties. The substrate, charge impurities, doping with chemical functional groups, and metal contacts can shift the position of the Fermi level of graphene.9−12 The mechanism responsible for the changes in the phonon dispersion of graphene on Ni(111) surface, that is, the suppression of the Kohn anomalies (KAs), was explained by the hybridization of the π bands of graphene with the metallic d bands.9 Raman spectroscopy showed the presence of the excess charges on graphene both under the dopants monitored in an electrochemically top-gated graphene transistor and also in the absence of intentional doping.10,13 A detailed correlation analysis of Raman G and 2D modes has demonstrated the effects of mechanical strain and charges substrate-mediated in graphene.14,15 Density functional theory (DFT) was used to explain how graphene was doped by adsorption on metal substrates, and it was found that even in the case of weak adsorption the position of the Fermi level moves away from the conical points due to the doping of graphene with electrons or holes.16 Received: September 19, 2013 Revised: November 25, 2013 Published: November 26, 2013 26804

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were carried out before loading the NPs on the Cu-graphene surface to confirm the quality of the graphene. The morphology of the gold-alloy NPs on the suspended graphene was observed by TEM (JEM − 2500SE, Jeol) with 200 kV accelerating voltage. The samples for TEM analysis were prepared by placing the graphene without or with NPs on a holey Cu TEM grid. The PMMA was carefully removed by washing the grid with acetone and vacuum drying, as previously described. The composition of the NPs on the graphene surface was analyzed by electron-dispersive spectroscopy (EDS) in bright-field mode and EELS. The HRTEM and EELS were performed by recording the images close to the Scherzer defocus, and the sample height was adjusted to keep the objects focused in the optimum lens current. A beam current density of ∼10 A cm−2 used for HRTEM observation using a CCD camera brings at most a temperature increase of a few degrees; current was reported to have little influence on the samples.21 Using a Gatan Imaging Filtering device for EELS operated by Filter Control and Digital Micrograph software, we recorded the EELS spectra with 0.2 eV per pixel and 2 mm aperture, which corresponds to a collection angle of 10 mrad. The spectra calibration was checked before and after each measurement using the zero loss peak. The crystal structure of the gold-alloy NPs on graphene transferred on glass substrates was analyzed by X-ray diffractometry (XRD, SmartLab, Rigaku), equipped with Cu Kα radiation source (λ = 0.154056 nm), using an X-ray powder diffraction method. The identification of the crystalline phases in the gold-alloy NPs structure was carried out using integrated X-ray powder diffraction software − PDXL qualitative analysis, from Rigaku.

A major issue is to understand the charge-transfer phenomenon to control the doping of graphene. Furthermore, graphene with plasmonic NPs can offer a new perspective for light conversion systems by optimization of visible-light absorption via the SPR of the NPs, followed by electron exchange between graphene and NPs and electron transport through graphene.17 We present a facile, one-step, and surfactant-free method for the synthesis and loading of stable gold and gold-alloy NPs on large-area graphene without NP deterioration using an electrical discharge in a liquid solution, termed solution plasma (SP). We investigated the charge-transfer process between graphene and gold-alloy NPs by Raman spectroscopy and electron energy loss spectroscopy (EELS) in high-resolution transmission electron microscopy (HRTEM).

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EXPERIMENTAL METHODS Graphene was synthesized on 100 mm × 400 mm × 25 μm area Cu foils (Alfa Aesar), in a quartz hot furnace at 1000 °C, in H2 and CH4 gases at 60 Pa total pressure over 30 min.18 The base pressure was 0.5 Pa. The quartz furnace was carefully cleaned of any carbon or copper traces from the previous deposition by using a diluted solution of HF (Kanto Chemicals). The experimental setup of the SP is explained in detail in ref 19. The Cu-graphene foil was introduced in the 1 mM indium or gallium nitrate (Sigma-Aldrich) solution in SP between two gold rod electrodes to synthesize the gold-alloy NPs directly on the Cu-graphene surface. The SP was maintained at 1000 V, 2 A, with a repetition rate of 15 kHz and a pulse width of 1 μs during 3 min.19−21 The gold NPs on the Cu-graphene surface were produced by gold electrode erosion in a 1 mM KNO3 (Sigma-Aldrich) solution for 3 min under the same discharge conditions. The Cu-graphene foils without and with NPs were washed with water, dried, and spin-coated with PMMA (poly(methyl methacrylate)) (MW = 950 kDa, in 4% anisole, MicroChem) at 3000 rpm during 1 min. The graphene films without and with NPs were removed from the Cu foils by slowly etching in a ∼0.2 M aqueous solution of Fe(NO3)3 (Sigma-Aldrich) for ∼24 h. The graphene films covered with PMMA were repeatedly washed in water and water−ethanol solution and carefully lifted from water and transferred to the target substrates, with the PMMA coating on top. The PMMA film was removed using acetone. The final substrates were dried in vacuum at 60 °C for 4 h. After synthesis, the colloidal solutions in a 1 cm optical absorption path length cuvette and the graphene without and with NPs on glass substrates were characterized by UV−vis spectroscopy (UV−vis−NIR 3600 spectrometer, Shimadzu) in the spectral range 400−750 nm, with 0.5 nm spectral resolution. Zeta potential of the suspended NPs in aqueous solutions was measured with Photal ELS-7300K, Otsuka Electronics. Raman spectroscopy was performed with an inVia Raman Microscope, Renishaw, with 1 cm−1 spectral resolution, 0.05 cm−1 repeatability, with a laser at 532 nm, and a spatial resolution of 1 μm for the 20× objective. Sample scanning was repeated on different places on the surface within an area of ∼400 μm2, in steps of 10 μm. The calculation of more than 300 Raman spectra for each sample in a map, that is, the fwhm, the G and 2D band position, and the ratio I2D/IG, was performed with a homemade Matlab program. The Raman measurements

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RESULTS AND DISCUSSION The gold-alloy NPs were directly synthesized and loaded on the Cu-graphene foil surface in a one-step process in an aqueous salt metal SP between two gold electrodes. The SP is a useful and simple method for the metal NPs synthesis because this nonequilibrium plasma can provide extremely rapid reactions due to the reactive chemical species, radicals, and UV radiation produced in atmospheric pressure plasma at room temperature. 20,21 The gold-alloy NPs were produced by the simultaneously processes of gold electrode erosion and the metal reduction from ion to neutral form, without any surfactant.19 In this way, the SP method offers the possibility to directly load the gold-alloy NPs on large-area graphene. The graphene with NPs was then transferred onto different substrates without deterioration, maintaining indium and gallium in the NPs composition. XRD with PDXL software determines the crystal structure and composition of the gold-alloy NPs on graphene transferred on glass and silicon substrates. The AuIn NPs contain 5 wt % Au and 95 wt % Au−In phase with 4 wt % In, and the AuGa NPs contain 3 wt % Au and 97 wt % Au−Ga phase with 12 wt % Ga (Figure S1 in the Supporting Information).19,22 This means that the AuIn and AuGa NPs contain ∼4 wt % In and ∼12 wt % Ga, respectively. The EDS maps confirm the presence of In and Ga in the NPs composition (Figure S2 in the Supporting Information). In the NP composition and on the graphene surface, nitrogen and oxygen atoms, which may induce charge doping in graphene, were not detected by EDS and EELS.11 26805

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Visible absorption spectra of a ∼1 cm2 area of graphene with NPs on glass were used to measure the SPR. The SPR peaks at 573.0 nm of the AuIn NPs and at 571.5 nm of the AuGa NPs are red-shifted when compared with the SPR peak of the gold NPs on graphene at 543.0 nm (Figure 1a−d). In solution the

Figure 2. Set of Raman maps and typical spectra of the graphene on Si substrate. The spectra were derived from the Raman map data shown on the left side of the Figure. These images contain the microscope image of the sample and two maps of the intensity distribution of the 2D band (green) and G band (red). The spectra were normalized to the 2D band intensity. (a) Raman maps and spectra of graphene without NPs. Raman maps and spectra of the graphene decorated with (b) gold NPs and gold-alloy NPs with (c) 4 wt % In and (d) 12 wt % Ga. The black vertical dashed lines indicate the position of the G band at 1585 cm−1 and the D band at 2678 cm−1 for the undoped graphene layer. The shifts of the Raman bands are indicated by the short (colored) dash lines. Statistical analysis of the position of (e) G band, (f) 2D band, and (g) ratio between the intensities of 2D and G bands for graphene without and with NPs. The standard deviation, σ, for G band position is ±2 and ±3 cm−1 for graphene without and with NPs, respectively. For 2D band position, σ is ±3 and ±5 cm−1 for graphene without and with NPs, respectively.

Figure 1. UV−vis spectroscopy of the NPs on graphene and in solution. Visible absorption spectra of graphene transferred on glass (a) without NPs and (b) with gold NPs. The SPR peak in the absorption spectra of graphene on glass with (c) AuIn NPs and (d) AuGa NPs is red-shifted when compared with Au NPs on graphene. The two peaks at 476 and 669 nm are due to the light interference on the layer, corresponding to the interference orders 2 and 3. (See the Supporting Information). Panel e represents the optical absorption spectra through 1 cm optical absorption path of the gold and goldalloy NPs in water solution after synthesis.

Figure. The analysis of more than 300 spectra over a ∼1 cm2 area, allows us to determine Raman peak positions and shifts for graphene samples decorated with gold and gold-alloy NPs. The G and 2D band frequency shifts induced by the gold NPs are opposite to those of the gold-alloy NPs on graphene. Figure 2e−g shows the statistical analysis of the position of G band, 2D band, and the ratio between the intensities of 2D and G bands, respectively. In the case of graphene without NPs, the position of the G band is at 1585 ± 2 cm−1 and the position of the D band is at 2678 ± 3 cm−1. Despite the relatively high sensitivity to charge doping, the analysis using the peak frequencies of the G and 2D bands has revealed a significant amount of discrepancies among many reported works regarding the accuracy of quantification.13,24 In addition, typical frequency variation in a given top-quality graphene can be more than several cm−1 because of the native charge doping and mechanical strain.10,14 Large-area CVD-grown graphene shows even larger variation in the Raman bands frequencies induced by transfer and interaction with a target substrate.15,25 Furthermore, a typical surface hole-doping concentration of about nini ≈ 1.5 × 1013 cm−2 obtained in experiments10,25,26 corresponds to an important Fermi energy level shift (Eini F ≈ 0.5 V). In the present experiment graphene without NPs was

Au, AuIn, and AuGa NPs have the SPR peak at 520.0, 535.0, and 560.0 nm, respectively, which are blue-shifted compared with their corresponding SPR peak on the surface. This is due to the NPs’ change in shape from spherical in solution to ellipsoidal on surface (Figure 1e).2,19 The two peaks in the absorption spectra at 476 and 669 nm are produced by light interference on the layer. (See the Supporting Information.) Around 700 nm, the absorbance of the graphene without NPs is 0.050 ± 0.002, which would correspond to a bilayer graphene because one graphene layer absorbs πα = 2.3% fraction of the incident visible light, where α (= 1/137).23 In the case of graphene with NPs, the background caused by the NPs on the surface increases the absorbance to 0.066 ± 0.002, 0.065 ± 0.002, and 0.081 ± 0.002 for Au, AuIn, and AuGa NPs, respectively (Figure 1). The SPR absorption peaks are weak (∼0.01) because the surface coverage of the gold NPs on the graphene is much smaller than the maximum possible surface coverage. Figure 2 shows a set of typical Raman spectra of graphene on a SiO2/Si substrate (a) without NPs and (b) with Au NPs and gold-alloy NPs (c) with 4 wt % In and (d) 12 wt % Ga, derived from their corresponding map shown in the left side of the 26806

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Figure 3. Low-magnification TEM images of the suspended graphene without and with the NPs on holey carbon TEM grid. (a) Suspended graphene without NPs with identification of the number of layers. One (right) and two (left) graphene layers are easily distinguished by the SAED patterns. Suspended graphene with (b) Au, (c) AuIn, and (d) AuGa NPs.

assumed to have a surface charge concentration nini and a Fermi energy level shift Eini F , which are considered the initial doping point. The NPs on the graphene produce a change of the equilibrium lattice parameter, which causes the stiffening or the softening of the phonons and a rapid modification of the phonon dispersion curve (dynamic effects), close to KAs.13,27,28 In graphene, the KAs have been proven in field-effect transistor (FET) experiments, where the applied gate voltage modulates the charge doping of graphene and the measured frequency shift of the G band obeys KAs in accordance with the gatevoltage-induced Fermi-level variations.13,27 In the present experiment, the observed frequency shift of the G band was positive and also negative, with respect to initial doping, when the gold or gold-alloy NPs were loaded on the graphene surface. The downshift of the G-band frequency is explained within the nonadiabatic approach considering nini. Using the dynamic approach in the context of the DFT calculation, the G-band frequency shift with respect to zero doping (Dirac point) (ΔωG) in dependence with Fermi energy level EF is given by the relation:24,29

experimental G-band frequency shift (Δωexp G ) relative to the ini initial G-band frequency shift (Δωini G ), which is induced by n as: ΔωGini ± |ΔωGexp| = α′(E Fini ± |ΔE F|)

The excess charge on graphene caused by transferred electrons or holes from the NPs to graphene was calculated from the exp change of the Fermi level relative to Eini F and ΔωG . In the case −1 of graphene with gold NPs, Δωexp −2 cm is , which means a G decrease in n with −3.5 × 1012 cm−2 and a movement of EF with ΔEF= −0.06 eV. The average amount of the transferred electrons from the gold NPs to the graphene unit cell is N = −2 × 10−4. −1 The Δωexp in the case of the G shifts are +0.7 and +1 cm AuIn and AuGa NPs, respectively, caused by the transfer of electrons from graphene to the gold-alloy NPs. This produces an increase in n with (1.3 and 1.9) × 1012 cm−2, which correspond to an increase in EF with ΔEF = 0.02 and 0.03 eV, respectively. The hole doping of graphene unit cell is N = (2 and 4) × 10−5 if the gold NPs contain In and Ga, respectively. The variation of the 2D band frequency with doping is mainly due to the charge transfer: an electron doping produces a down shift of the frequency, and a hole doping causes an upshift of the 2D band frequency.10 We compared the above calculated results of EF derived from Δωexp G with those obtained from the 2D band shifts (Δωexp ), using the experimental data 2D reported in refs 13 and 14. For graphene with gold NPs, Δωexp 2D is −2 cm−1, which means that EF moves downward with −0.03 eV. In the case of graphene with AuIn NPs and AuGa NPs, −1 Δωexp 2D are +7 and +6 cm , and EF moves upward with 0.07 eV. The uncertainties in the calculation of ΔEF, n, and N are difficult to assess due to the high standard deviation values in Raman measurements. In all Raman spectra, the 2D band was fitted with one Lorentz function with an average full width at half-maximum (fwhm) of 35 ± 5 cm−1, which corresponds to monolayer graphene.30,31 This result agrees with UV−vis spectroscopy measurements because the absorbance was measured over a large graphene area (∼1 cm2), which incorporates defects, edges, and folding, making the absorbance greater than 0.023.

ℏΔωG = α′|E F| + (α′ℏωa0 /4) ·ln(|(|E F|−ℏωa0 /2) /(|E F| + ℏωa0 /2)|)

where α′/(2πℏc) = 35.8 cm−1/(eV), c is the speed of light, ℏ is the reduced Planck constant, and ω0a is the adiabatic zero doping phonon frequency. In this relation, there are two logarithmic divergences for EF = ±ℏω0a /2, and for |EF| ≫ ℏω0a/ 2 the frequency shift increases as α′|EF|, as in the present experiment. The Fermi energy in graphene changes as: EF = ℏ|νF|(πn)1/2, where |vF| = 1.1 × 108 cm s−1 is the Fermi velocity and n is the change of the surface electron concentration in cm−2 with respect to zero doping graphene.13 In a simple analytical model, the number of electrons per graphene unit cell transferred from (or to) metal NPs due to the change of the Fermi level is given by: N = sign(ΔEF)D0ΔE2F/2, where D0 = 0.09 per (eV2 unit cell), is a proportionality constant factor, considering the graphene density of states to be linear with energy, within ±1 eV, and sign(x) is sign of x.16 ΔEF produced by the NPs loaded on the graphene is related to the 26807

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Figure 4. HRTEM images of the suspended graphene layer (a) without NPs, (b) with one gold NP, and with one gold-alloy NP containing (c) 4 wt % In and (d) 12 wt % Ga. The insert B represents a magnified image of the region A. The graphene hexagonal crystal structure can be distinguished. The crystal planes of the NP are identified in the panels b−d.

The Raman spectra analysis was done for those spectra that correspond to a single-layer graphene. Likewise, in the Raman spectra, the dispersion of the data for the G, 2D position, and the intensity ratio between these bands is generated by the surface defects, the edges,14,25,32,33 and the nonuniformity in the NPs distribution on the surface, as can be seen in the Raman maps from Figure 2 and in the low-magnification TEM analysis from Figure 3. In some of the Raman spectra, the D band was detected with a very low intensity, about 50 times smaller than those of the G band, especially after the loading of the NPs on the graphene surface. The D band is due to the breathing modes of sp2 atoms, requires a defect to be Raman active, and usually can be detected at the edges.10 The ratio between the 2D band and the G band intensities (I2D/IG), which is an important parameter to characterize the monolayer,30 has an average value ∼3 for the graphene without NPs and decreases to ∼1.5 to 2 after the NPs were synthesized on the graphene. A similar decrease in I2D/IG was measured when graphene was doped with electrons or holes by applying an external voltage in an electrochemical cell.13,27 The suspended graphene without and with NPs has been observed by TEM with low magnification (Figure 3). One or two graphene layers without NPs were easily distinguished by selected area electron diffraction (SAED) patterns (Figure 3a).

Figure 4 displays the HRTEM images of the suspended graphene without and with one NP, where the graphene hexagonal crystal structure can be distinguished and the crystal structure of the NPs can be identified. The gold NPs are multiple twinned particles (MTPs) with icosahedral morphology and single-nanotwinned face-centered cubic configurations (Figure 4b−d).21 In the present experiment, EELS measures the excitation of the carbon K-shell electron (1s electron) to empty antibonding π* and σ* states at ∼285 and ∼290 eV, respectively.34 The first peak is a sharp and narrow peak, while the second peak is a very broad signal. The fine structure of the EELS signal gives information about the difference in the carbon doping. The EELS carbon-edge signals of graphene without and with various NPs are shown in Figure 5. Each spectrum corresponds to an HRTEM image presented in Figure 4. The carbon edge (CE), the position of the peak corresponding to electron transition 1s-π* (π*), and the relative ratio between the signal intensities of 1s-π* and 1s-σ* transitions (r) are influenced by the charge doping of graphene by the NPs. For the suspended graphene without NPs CE and π* are at 284 and 284.8 eV, respectively, and r is 0.60. If the gold NPs are loaded on the graphene, then the CE and π* are found at lower energy, at 283.4 and 284.6 eV, respectively, and the value r 26808

dx.doi.org/10.1021/jp409368c | J. Phys. Chem. C 2013, 117, 26804−26810

The Journal of Physical Chemistry C

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Considering the experimental results from the Raman spectroscopy analysis, the calculated separation distance between the graphene and the NPs is ∼0.32 nm in the case of the gold NPs and ∼0.33 nm in the case of the gold-alloy NPs. (See Figure S3 in the Supporting Information.)16 The small amount of In or Ga in the gold-alloy NPs composition induces changes in their electrical properties due to a difference in the electron density between gold and the other metal. In a binary gold alloy, the charge transfer is an important component in the surface metal−metal bonds that involve dissimilar elements. Therefore, the variation in the coordination number of a metal or in the geometrical arrangement of its neighbors can produce changes in the orbital hybridization that increase its electronegativity.41 The magnitude of zeta potential of the suspended NPs in water at pH 7 was −40.11, +42.58, and +48.15 mV for Au, AuIn, and AuGa, respectively (Supporting Information). The increased electronegativity of the gold-alloy NPs can explain why they accept electrons from graphene differently from the gold NPs.

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CONCLUSIONS We investigated large-area graphene decorated with gold and gold-alloy NPs directly synthesized onto the graphene surface in an aqueous SP without using any surfactant. After the transfer of the graphene with NPs on different substrates, the NPs maintained indium and gallium in their composition. We observed that graphene may be doped n- or p-type. The EELS results were consistent with Raman spectroscopy results; that is, the electrons and holes are transferred from the gold and gold-alloy NPs to graphene, respectively. Raman spectroscopy is a microanalysis method and provides average values of the Fermi level shift. EELS data confirm on the nanoscale range the type of charge transferred between graphene and the NPs. We found that the average charge transferred from graphene unit cell was −2 × 10−4 e and ∼4 × 10−5 e for gold and goldalloy NPs, respectively.

Figure 5. Carbon edge of the EELS spectra of suspended (a) graphene layer, (b) graphene with gold NPs, and graphene with gold-alloy NPs containing (c) In and (d) Ga. The excitation of the carbon K-shell electron to empty antibonding π* and σ* states is at ∼285 and ∼290 eV, respectively. The carbon edge at 284 eV is shifted depending on the graphene charge doping, which indicates the sign of the Fermi level change. The black dotted vertical line indicates the position of π* peak for graphene. For each case, the position of π* is indicated by a colored arrow. The positions of CE are marked with colored dots arrows.

decreases at 0.46. In the case of the graphene with the goldalloy NPs, the CE and π* shift at higher energy, and r is less than that of the graphene without NPs. For the AuIn NPs, these values are at CE = 284.4, π* = 286.8 eV, and r = 0.50, and for the AuGa NPs, CE = 285, π* = 287.3 eV, and r = 0.45. In graphene decorated with gold-alloy NPs, the shift of π* to higher energy is due to the increase in the carbon 1s binding energy caused by the greater net charge per carbon atom. If the electrons are transferred from graphene to the gold-alloy NPs, then the π* states are depopulated and the intensity of the 1sπ* transition decreases.35,36 The transferred electrons from the gold NPs to graphene cause a decrease of the carbon 1s binding energy and consequently a shift of CE and π* energies to lower values. However, in the case of the gold NPs, a clear change of the 1s-π*transition intensity has not been observed. The EELS data confirm the type of the electron transfer between graphene and the NPs without giving a quantitative evaluation because of the low resolution of the experiment (0.2 eV per pixel). The Fermi level shift depends on the separation distance, the difference in the work functions, and the chemical interaction between graphene and the NP.16,37 The work function of the NPs differs from that of the bulk metal because it is affected by the size of the NPs38 and the presence of indium or gallium in the composition.39 The work function of the Au, AuIn, and AuGa NPs becomes 5.52, 5.45, and 5.40 eV, respectively, if the NPs size is