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.

Fig. 1
figure 1

a Schematic formation of Ag nanoparticles using the catalytic DMF into the graphene dispersion of NMP. b Stabilization mechanism of Ag nanoparticles embedded in few-layer graphene sheets due to the formation of metal-arene type complex through d-π interaction among the vacant d-orbitals of Ag0 and the π-electron cloud of graphene moiety

Full size image

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].

Fig. 2
figure 2

a Typical AFM image of Ag NPs embedded within graphene sheets with a total thickness of ∼45 nm. Some Ag NPs are also found to be decorated on the graphene surface. b The height distribution of the corresponding AFM image for Ag NPs. c HRTEM image of Ag NP-decorated few-layer graphene sheets with SAED pattern in the inset. d Binding energy of Ag 3d electrons showing an excellence stability of graphene-Ag0 hybrid even after 50 days

Full size image

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].

Fig. 3
figure 3

Typical Raman spectrum of a pristine graphene and b graphene-Ag0 hybrid showing the intensity enhancement by a factor of 20. The G-band position 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 sample. c UV-vis absorption spectra of control graphene, Ag nanoparticles, and graphene-Ag hybrid. The plasmon peak of graphene-Ag hybrid material is redshifted as compared to control Ag NPs, showing the increased absorption due to the enhancement of local electromagnetic field

Full size image

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]

$$ {n}_{\mathrm{graphene}}=3+\frac{i.c.\lambda }{3} $$
(1)

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

$$ {E}_y={E}_0.{e}^{-\frac{2\pi i}{\lambda}\left(y-c.t\right)} $$
(2)

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].

Fig. 4
figure 4

a Simulated figure showing the enhancement of electric field in graphene-metal interfaces when Ag NPs (40 nm) are on top of the graphene sheets. b The electric field for Ag NPs (40 nm) embedded in multilayer graphene. c Plots showing the enhancement and redshift of the plasmon peak for three different cases for Ag NPs (40 nm). d Electric field enhancement due to different-sized Ag NPs embedded within few-layer graphene sheets. The resonance band at a shorter wavelength dominates for smaller-sized (10 nm) particles, when extinction of the particle is governed by only absorption. The spectra for 40- and 80-nm Ag particles are characterized by a sharp resonance peak corresponding to the electric quadrupolar resonance and a second broad band at a higher wavelength due to the lowest order (dipolar) LSP

Full size image

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.

Fig. 5
figure 5

a Schematic diagram of Au/G-Ag0 hybrid/Au lateral photodetector device. b Current-voltage (I-V) characteristics of the fabricated device under dark and illuminated conditions. I-V characteristics of the controlled device are shown in the inset. c Cross-sectional view of the photodetector (top) and schematic energy band diagram (bottom). The red and blue lines denote the conduction and valence band of graphene, respectively. The dashed blue line shows the Fermi level, and the blue solid line represents the energy at the Dirac point of the graphene

Full size image

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.

Fig. 6
figure 6

Current transient characteristics of the G-Ag0 plasmonic photodetector for different a bias voltages at 80μW illuminated power and b different illuminated powers at a reverse bias of 2 V. c Generated Photocurrent as a function of laser (514 nm) power at −2V bias voltage. The red and blue lines indicate the response of a typical device with and without Ag nanoparticles, respectively. d Spectral photoresponse of the fabricated device showing spectral selectivity around 510 nm with a peak responsivity of ∼28 mA/W at −4V bias

Full size image

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.