Abstract
We report the fabrication and characteristics of a novel graphene-Ag0 hybrid plasmonic nanostructure-based photodetector exhibiting moderately high responsivity (∼28 mA/W) and spectral selectivity (∼510 nm) in the visible wavelength. The formation of highly stable Ag0 nanoparticles with an average size of 40 nm is observed within the graphene layers, resulting in n-type doping of hybrid material. The absorption peak of graphene-Ag0 hybrid is redshifted to the visible wavelength (∼510 nm) from the plasmonic Ag peak (∼380 nm) in agreement with the optical simulation results for embedded metal nanoparticles. The study demonstrates the synergistic effect of the graphene-metal nanocomposite, which appears attractive for applications in graphene-based photonic devices.
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Introduction
Two-dimensional materials including graphene [1] and transition metal dichalcogenides (TMDs) have attracted immense research interests for novel electronic and optoelectronic devices [2]. Though mono- and few-layers of TMDs exhibit tunable direct band gap useful for optical emitters and detectors [3, 4], active graphene-based photonic devices have found limited applications due to the zero band gap of pristine graphene. Therefore, an enhanced light-matter interaction in graphene is required for their use in active photonic devices. The incorporation of metal nanostructures is believed to be a promising approach to enhance the light-matter interaction [5, 6]. Recently, plasmonic nanostructures have been utilized for surface-enhanced Raman spectroscopy (SERS) [7], single-molecule spectroscopy [8], enhanced photodetection [9], photovoltaics [10], and light-emitting devices [11]. The near-field enhancement of photovoltage conversion efficiency in graphene has been demonstrated by decorating Au nanostructures on top of a graphene sheet [12, 13]. The approach has been also reported to provide a spectral selectivity that enables multicolor photodetection [14]. Although silver (Ag) is the most promising plasmonic material [15] due to its low loss at optical frequencies, the photodetection using the near-field enhancement of Ag-decorated graphene nanostructures is yet to be reported owing to the sulfurization of Ag0 to form a layer of silver sulfide (Ag2S) readily in an ambient condition [16, 17].
Here, we report the enhanced light-matter interaction of novel graphene-Ag0 hybrid nanostructures where Ag nanoparticles (NPs) are stabilized by few-layer graphene sheets. The shift of the plasmonic Ag peak to the visible region with an enhanced optical absorption of synthesized graphene-silver (G-Ag0) hybrid nanocomposite could be explained by the optical simulation using finite element method. The hybrid plasmonic nanostructures have been used to fabricate photodetectors with an enhanced and selective optical detection around ∼510 nm, making the graphene-based material attractive for selective and multicolor photodetection.
Experimental
Synthesis of Hybrid Nanostructures
Here, we prepared stable Ag0 nanoparticles embedded within a few-layered pristine graphene dispersion. In this typical procedure, we first prepared few-layer pristine graphene (FLG) from graphite using sodium chloride (NaCl) intercalation [18] and dispersed the FLG in N-methyl-2-pyrrolidone (NMP) through ultrasonication. After adding the precursor silver-salt and catalytic volume of dimethylformamide (DMF; as reducing agent) into the grayish black NMP dispersion of graphene, Ag0 nanoparticles were formed with orange coloration of the dispersion. The catalytic DMF reduced the Ag+ to Ag0 and self-oxidized to dimethylcarbamic acid [(CH3)2NCOOH]. From the precursor salt (AgNO3), the nitric acid was produced as a by-product during the red-ox process. The acidic nature of the solution was observed, which confirmed the generation of HNO3 and the formation of nanoparticles as well.
Characterization of G-Ag0
Graphene-Ag0 hybrid dispersion was spin coated on Si (100) substrates. Surface morphology and crystalline quality of the hybrid nanostructures were studied by scanning probe microscopy (SPM) and high-resolution transmission electron microscopy (HRTEM). 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). The chemical bonding and stability of Ag nanoparticles embedded within few-layer graphene sheets were studied using X-ray photoelectron spectroscopy with incident AlKα X-ray of energy 1486.6 eV. UV-vis absorption spectrum was obtained using a fiber-probe-based UV-vis-NIR CCD spectrometer.
Device Fabrication and Measurements
To fabricate the plasmon-enhanced photodetector devices, graphene-Ag0 hybrid solution was first spin coated on 300-nm-thick SiO2/Si (100) substrates. Semitransparent metal (Au) circular dots (0.2 mm2) were deposited by thermal evaporation to define top lateral contact electrodes on the film at room temperature under a base pressure of ∼1 × 10−7 Torr. The spectral photocurrent response of the device was measured by a standard lock-in detection technique using a broadband light source (xenon arc lamp). The current-voltage (I-V) characteristics of the devices were studied using a Keithley semiconductor parameter analyzer (model no. 4200-SCS).
Results and Discussions
Synthesis and Characterization
We have synthesized high-yield few-layer graphene-Ag0 hybrid nanocomposite using AgNO3 as precursor salt, catalytic amount of dimethylformamide (DMF) [19] as the reducing agent, and graphene itself as the stabilizer [20]. The stabilization of Ag0 in graphene is probably due to formation of metal-arene complex [21, 22] type G-Ag0 nanocomposite where graphene being a 2-π aromatic system [23] acts as ligand towards silver nanoparticle. The schematic synthesis mechanism is depicted in Fig. 1a, b, respectively.
Atomic force microscopy (AFM) and transmission electron microscopy (TEM) has been performed to investigate the surface morphology and microstructures of synthesized graphene-Ag0 hybrid composites. The AFM in Fig. 2a and the hight distribution in Fig. 2b of graphene-Ag0 hybrid depicts an average thickness of the NPs around ∼35–40 nm. The TEM image in Fig. 2c shows typical graphene sheets with both embedded and decorated Ag NPs of average size ∼40 ± 5 nm, corroborating the AFM results. SAED pattern in the inset of Fig. 2c exhibits concentric rings with intermittent bright diffraction spots, attributed to the (111), (200), (220), (311), and (222) planes of face-centered cubic (fcc) silver NPs. X-ray photoelectron spectra (XPS) of Ag 3d electrons shown in Fig. 2d with the characteristic peaks at 367.9 and 373.9 eV ascribed, respectively, to the Ag 3d5/2 and Ag 3d3/2 electronic states, suggesting the formation and stabilization (even after 50 days) of Ag0 chemical state in G-Ag0 hybrid composite [24].
The Raman spectra of pristine graphene (control sample) and graphene-Ag0 hybrid are shown in Fig. 3a, b, respectively. The spectrum of graphene exhibits a weak D-mode (∼1364 cm−1) and two prominent peaks correspond to the most prominent G (∼1583 cm−1) and 2D (∼2735 cm−1) bands, in accordance with the reported results of pristine graphene [25, 26]. The Raman intensity of graphene-Ag0 hybrid exhibits an enhancement by a factor of 20 over the control sample. The position of the G-band in Fig. 3b is upshifted from 1583.6 to 1586 cm−1, and 2D-band downshifted from 2738 to 2715 cm−1 for the G-Ag0 hybrid, as compared to the control graphene sample indicating Ag NPs induced n-type doping of graphene-Ag0 nanocomposites [27] due to metal-graphene interaction via electron transfer [28].
We studied the absorption spectra to investigate the light-matter interaction in graphene-Ag0 NPs hybrid. In Fig. 3c, UV-vis absorption spectra are presented for pristine graphene, Ag NPs, and G-Ag0 hybrid. The plot shows the plasmonic absorption peak of control Ag NPs at around 384 nm. However, for G-Ag0 hybrid, the broad absorption peak is found to be shifted 507 nm. The result in Fig. 3c shows a significant enhancement in the absorbance of G-Ag0 hybrid in the entire visible region with respect to control graphene. The absorption increases due to the presence of Ag NPs within the graphene sheets. The redshift of the absorption peak as compared to plasmonic Ag one occurs due to the interaction of electrons with the graphene sheets. This experimental result could be explained by optical simulation using finite element method discussed in the following.
Optical Simulation
The enhanced absorption with a large redshift of the plasmonic peak to the visible region could be explained by optical simulation using two-dimensional finite element method carried out with COMSOL MULTIPHYSICS (4.3b). The analysis provides the electric field distribution in the vicinity of graphene-Ag0 NP interface. The analytical solution has been obtained by assuming the plane wave is incident along the y-direction of propagation and the temperature and pressure kept constant at T = 297 K and P = 1 atm. We also assumed that relative permittivity is ε r = (n − ik)2, where the conductivity, σ, and relative permeability, μ r, are taken as 0 and 1 at the boundary. The simulation was done by choosing a maximum mesh size of 2 nm inside the graphene sheet, Ag nanoparticles, and in their vicinity, while it was 10 nm in the remaining area. The optical constants of Ag (n ag and k ag) have been taken from the literature [29], whereas for graphene, we used the relation [30]
where n graphene is the refractive index of graphene, c the speed of light, and λ is the incident wavelength. We assumed the real part of refractive index as 3.0 for the graphene sheet [30]. The analytical solution has been obtained by the plane wave equation given as
where E 0 = 1 V/m is the amplitude of incident wave. First, we investigated the light-matter interaction behavior considering Ag NPs on top of the graphene sheets, which results in a strong confinement of the radiation around ∼372 nm in Fig. 4a, which is close to the plasmonic Ag absorption data. Figure 4b presents the electric field distribution in resonant (∼520 nm) condition for Ag NPs embedded within a few-layered graphene sheets, showing a large redshift of the plasmonic peak and the strong confinement of the electric field energy around the vicinity of the NPs. This is in good agreement with the experimental data for G-Ag0 hybrid shown in Fig. 3c. Figure 4c presents the comparison of the estimated electric field under two different conditions. The resonant peak is found to be around ∼372 nm for Ag NPs decorated on graphene surface. On the other hand, Ag NPs embedded within the FLG sheets exhibit the resonance peak around ∼520 nm for 40-nm particle size. Though the electric field is found to be higher for Ag NPs on top of the graphene sheets, the absorption peak can be shifted to the visible range only by incorporating the metal NPs within FLG sheets, as shown in Fig. 4c. The calculated plasmon resonance behavior of Ag NPs is found to be very sensitive to the surrounding environment and the size of the NPs as shown in Fig. 4c, d, respectively. The origin of double peaks in simulation is attributed to the electric quadrupolar resonance at a shorter wavelength and a second broad band at a higher wavelength due to the lowest order (dipolar) localized plasmons [31].
Device Performance
The schematic structure of the fabricated Au/graphene-Ag0/Au lateral photodetector is shown in Fig. 5a. Figure 5b presents the current-voltage (I-V) characteristics of Au/G-Ag0/Au lateral p-n-p junction under dark and illuminated conditions. A photo to dark current ratio of ∼500 is observed at a bias of −4 V. In comparison, the inset of Fig. 5b, for the control graphene device, exhibits a negligible photocurrent. The difference in work function between the Au electrodes and the G-Ag0 hybrid nanostructures leads to charge transfer at the junction, resulting in an inhomogeneous doping profile and creates a p-n-p junction along the graphene channel laterally shown in Fig. 5c which is useful for the collection of light-induced e-h pairs efficiently.
We have studied the bias and power-dependent switching characteristics of the fabricated plasmonic photodetector using an incident wavelength 514 nm. The switching response of the detector for 80-μW incident radiation with different reverse bias voltage is shown in Fig. 6a. The device shows a higher current ON/OFF ratio at a higher bias, since a higher electric field causes efficient extraction of charge carriers in the G-Ag0 hybrid/Au junction. By varying the illuminated power at −2 V, a higher current ON/OFF ratio is observed for increasing power, as shown in Fig. 6b. Figure 6c shows an obvious enhancement of photocurrent amplitude of 11–12 times higher than those without Ag nanoparticles. A peak responsivity up to 28 mA/W is achieved for our graphene-Ag0 hybrid device (shown in Fig. 6d) at −4 V. On the other hand, the peak responsivity for control graphene device is found to be ∼1.8 mA/W at the same bias. The peak spectral responsivity has been reported to be about 0.42 and ∼200 mA/W for pristine graphene [32] and graphene oxide/n-Si heterojunction photodetector [33], respectively. This shows an enhancement of the plasmonic detector responsivity by a factor of 15 over our control device. Importantly, the spectral photocurrent shows an apparent peak around 510 nm, which are close to the absorption peak of graphene-Ag0 hybrid nanostructures. This suggests that the enhancement and selectivity of responsivity peak is indeed originated from the plasmon-enhanced absorption in hybrid graphene devices.
The enhancement in photocurrent in graphene-Ag0 NP hybrid devices can be attributed to both the enhanced near-field electronic oscillation and scattering effect of Ag nanoparticles [14, 34]. Due to the localized plasmon resonance of metal nanoparticles, light is confined around the graphene-metal interface, leading to an enhancement of the electrical field that can effectively boost the absorption of a few-layer graphene sheets. In addition, the scattering from Ag NPs can also play a vital role to the increased photocurrent, similar to the plasmonic enhancement effect observed in an optical nano-antenna-based photodetector [35, 36]. The results clearly demonstrate that plasmonic Ag NPs embedded within a few-layer graphene can be an active material for enhanced light-matter interaction. The photodetector performance shows that the novel hybrid 2D plasmonic nanostructures could be very attractive for future graphene-based optoelectronic devices.
Conclusion
In summary, we have successfully synthesized Ag0 NPs stabilized by few-layer graphene sheets to enhance the light-matter interaction in graphene. The upshift of Raman G-bands and the lowering of 2D-bands (of the hybrids) as compared to pristine graphene indicate the n-type doping induced by Ag NPs. The synthesized plasmonic hybrid nanostructures show an improved light confinement, due to embedded and decorated Ag nanoparticles, which led to the enhancement of optical absorption. The fabricated photodetector exhibits a spectral selectivity in the visible wavelength range, with the enhancement of responsivity by a factor of ∼15 over pristine graphene. From our experiments, it is apparent that the synthesized G-Ag0 hybrid nanostructures may contain (i) Ag NPs on top of graphene sheets or (ii) Ag NPs embedded within the layers of graphene sheets, (iii) or a combination of both. Our optical simulation result is very close to the second case, indicating the synthesized hybrid nanostructures mainly contain embedded Ag NPs within the FLG sheets. Theoretical analysis also indicates that the selectivity in detection can be tuned by varying the size and dielectric environment Ag NPs. Further experimental studies need to be carried out on the above issues. The study reports for the first time the use of stable Ag0 nanoparticle-embedded plasmonic structures for the realization of wavelength-tunable graphene-based photonic devices.
References
Bonaccorso F, Sun Z, Hasan T, Ferrari AC (2010) Graphene photonics and optoelectronics. Nat Photonics 4:611–622
Wang QH, Kalantar-Zadeh K, Kis A, Coleman JN, Strano MS (2012) Electronics and opto-electronics of two-dimensional transition metal dichalcogenides. Nat Nanotechnol 7:699–712
Splendiani A, Sun L, Zhang YB, Li TS, Kim J, Chim CY, Galli G, Wang F (2010) Emerging photoluminescence in monolayer MoS2. Nano Lett 10:1271–1275
Mukherjee S, Maiti R, Midya A, Das S, Ray SK (2015) ACS Photon 2:760
Ozbay E (2006) Merging photonics and electronics at nanoscale dimensions. Science 311(5758):189
Liang Z, Sun J, Jiang Y, Jiang L, Chen X (2014) Plasmonic enhanced optoelectronic devices. Plasmonics 9:859–866
Otto A, Mrozek I, Grabhorn H, Akemann W (1992) Surface-enhanced Raman scattering. J Phys Condens Matter 4:1143
Kneipp K, Wang Y, Kniepp H, Perelman LT, Itzkan I, Dasari RR, Feld MS (1997) Single molecule detection using surface-enhanced Raman scattering (SERS). Phys Rev Lett 78(9):1667
Gogurla N, Sinha AK, Santra S, Manna S, Ray SK (2014) Multifunctional Au-ZnO plasmonic nanostructures for enhanced UV photodetector and room temperature NO sensing devices. Sci Rep 4:6483
Atwater HA, Polman A (2010) Plasmonics for improved photovoltaic devices. Nat Mater 9:205
Curto AG, Volpe G, Taminiau TH, Kreuzer MP, Quidant R, van Hulst NF (2010) Unidirectional emission of a quantum dot coupled to a nanoantenna. Science 329:930
Echtermeyer TJ, Britnell L, Jasnos PK, Lombardo A, Gorbachev RV, Grigorenko AN, Geim AK, Ferrari AC, Novoselov KS (2011) Strong plasmonic enhancement of photovoltage in graphene. Nat Commun 2:458
Liu WL, Lin FC, Yang YC, Huang CH, Gwo S, Huang MH, Huang J (2013) The influence of shell thickness of Au@TiO2 core–shell nanoparticles on the plasmonic enhancement effect in dye-sensitized solar cells. Nanoscale 5:7953
Liu Y, Cheng R, Liao L, Zhou H, Bai J, Liu G, Liu L, Huang Y, Duan XF (2011) Plasmon resonance enhanced multicolour photodetection by graphene. Nat Commun 2:579
Cobley CM, Skrabalak SE, Campbell DJ, Xia Y (2009) Shape-controlled synthesis of silver nanoparticles for plasmonic and sensing applications. Plasmonics 4:171
Bennett H, Peck R, Burge D, Bennett J (1969) Formation and growth of tarnish on evaporated silver films. J Appl Phys 40:3351
Elechiguerra JL, Larios-Lopez L, Liu C, Garcia-Gutierrez D, Camacho-Bragado A, Yacaman M (2005) Corrosion at the nanoscale: the case of silver nanowires and nanoparticles. J Chem Mater 17(24):6042–6052
Parvez K, Wu ZS, Li R, Liu X, Graf R, Feng X, Müllen K (2014) Exfoliation of graphite into graphene in aqueous solutions of inorganic salts. J Am Chem Soc 136:6083
Pastoriza-Santos I, Liz-Marzán LM (1999) Formation and stabilization of silver nanoparticles through reduction by N,N-dimethylformamide. Langmuir 15:948–951
Lu L, Liu J, Hu Y, Zhang Y, Chen W (2013) Graphene-stabilized silver nanoparticle electrochemical electrode for actuator design. Adv Mater 25:1270
Muetteries EL, Bleeke JR, Wuchere EJ (1982) Structural, stereochemical, and electronic features of arene-metal complexes. Chem Rev 82:499
Heimel G, Duhm S, Salzmann I, Gerlach A, Strozecka A, Niederhausen J, Bürker C, Hosokai T, FernandezTorrente I, Schulze Winkler GS, Wilke A, Schlesinger R, Frisch J, Bro¨ker B, Vollmer A, Detlefs B, Pflaum J, Kera S, Franke KJ, Ueno N, Pascual JI, Schreiber F, Koch N (2013) Charged and metallic molecular monolayers through surface-induced aromatic stabilization. Nat Chem 5:187
Popov IA, Bozhenko KV, Boldyrev AI (2012) Nano Res 5:117
Pol VG, Srivastava DN, Palchik O, Palchik V, Slifkin MA, Weiss AM, Gedanken A (2002) Sonochemical deposition of silver nanoparticles on silica spheres. Langmuir 18:3352
Malarda LM, Pimentaa MA, Dresselhausb G, Dresselhaus MS (2009) Raman spectroscopy in graphene. Phys Rep 473:51
Ferrari AC, Basko DM (2013) Raman spectroscopy as a versatile tool for studying the properties of graphene. Nat Nanotechnol 8:235
Das A, Pisana S, Chakraborty B, Piscanec S, Saha SK, Waghmare UV, Novoslov KS, Krishnamurthy HR, Geim AK, Ferrari AC, Sood AK (2008) Monitoring dopants by Raman scattering in an electrochemically top-gated graphene transistor. Nat Nanotechnol 3:210
Shi Y, Dong X, Chen P, Wang J, Li L-J (2009) Effective doping of single-layer graphene from underlying SiO2 substrates. Phys Rev B 79:115402
Johnson PB, Christy RW (1972) Optical constants of the noble metals. Phys Rev B 6:4370
Bruna M, Borini S (2009) Optical constants of graphene layers in the visible range. Appl Phys Lett 94:031901
Le Ru EC, Etchegoin PG (2008) Principals of surface enhanced Raman spectroscopy. Elsevier
Xia F, Mueller T, Golizadeh-Mojarad R, Freitag M, Lin Y, Tsang J, Perebeinos V, Avouris P (2009) Photocurrent imaging and efficient photon detection in a graphene transistor. Nano Lett 9:1039–1044
Maiti R, Manna S, Midya A, Ray SK (2013) Broadband photoresponse and rectification of novel graphene oxide/n-Si heterojunctions. Opt Express 21(22):26034–26043
Matheu P, Lim SH, Derkacs D, McPheeters C, Yu ET (2008) Metal and dielectric nanoparticle scattering for improved optical absorption in photovoltaic devices. Appl Phys Lett 93(11):113108
Knight MW, Sobhani H, Nordlander P, Halas NJ (2011) Photodetection with active optical antennas. Science 332:702–704
Fang Z, Liu Z, Wang Y, Ajayan PM, Nordlander P, Halas NJ (2012) Graphene-antenna sandwich photodetector. Nano Lett 12:3808
Acknowledgments
This work is supported by the partial funding from CSIR-sponsored “GBH” and DST-ITPAR-sponsored “GPU” projects. The use of the XPS facility of the Department of Physics, IIT Kharagpur, is gratefully acknowledged.
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Maiti, R., Sinha, T.K., Mukherjee, S. et al. Enhanced and Selective Photodetection Using Graphene-Stabilized Hybrid Plasmonic Silver Nanoparticles. Plasmonics 11, 1297–1304 (2016). https://doi.org/10.1007/s11468-015-0175-0
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DOI: https://doi.org/10.1007/s11468-015-0175-0