Augmented Quantum Yield of a 2D Monolayer Photodetector by

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Augmented Quantum Yield of a 2-D Monolayer Photodetector by Surface Plasmon Coupling Seungho Bang, Ngoc Thanh Duong, Jubok Lee, Yoo Hyun Cho, Hye Min Oh, Hyun Kim, Seok Joon Yun, Chul Ho Park, Min-Ki Kwon, Ja-Yeon Kim, Jeongyong Kim, and Mun Seok Jeong Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b05060 • Publication Date (Web): 21 Mar 2018 Downloaded from http://pubs.acs.org on March 22, 2018

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Nano Letters

Augmented Quantum Yield of a 2-D Monolayer Photodetector by Surface Plasmon Coupling Seungho Bang†, ‡, Ngoc Thanh Duong†, Jubok Lee†, ‡, Yoo Hyun Cho§, ∥, Hye Min Oh†, Hyun Kim†, ‡, Seok Joon Yun†, ‡, Chul Ho Park†, Min-Ki Kwon§, Ja-Yeon Kim∥, Jeongyong Kim†, ‡, and Mun Seok Jeong†, ‡,* †

‡

Department of Energy Science, Sungkyunkwan University, Suwon 16419, Republic of Korea Center for Integrated Nanostructure Physics, Institute for Basic Science, Sungkyunkwan

University, Suwon 16419, Republic of Korea §

Department of Photonic Engineering, Chosun University, Gwangju 61452, Republic of Korea

∥

LED Convergence Research Center, Korea Photonics Technology Institute (KOPTI), Gwangju

61007, Republic of Korea * Address correspondence to [email protected]. ABSTRACT. Monolayer (1L) transition metal dichalcogenides (TMDCs) are promising materials for nano-scale optoelectronic devices, because of their direct band gap and wide absorption range (ultraviolet to infrared). However, 1L-TMDCs cannot be easily utilized for practical optoelectronic device applications (e.g., photodetectors, solar cells, and light-emitting diodes) because of their extremely low optical quantum yields (QYs). In this investigation, a

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high-gain 1L-MoS2 photodetector was successfully realized, based on surface plasmon (SP) of the Ag nanowire (NW) network. Through systematic optical characterization of the hybrid structure consisting of a 1L-MoS2 and the Ag NW network, it was determined that a strong SP and strain relaxation effect influenced a greatly enhanced optical QY. The photoluminescence (PL) emission was drastically increased by a factor of 560, and the main peak was shifted to the neutral exciton of 1L-MoS2. Consequently, the overall photocurrent of the hybrid 1L-MoS2 photodetector was observed to be 250-times better than that of the pristine 1L-MoS2 photodetector. In addition, the photoresponsivity and photodetectivity of the hybrid photodetector were effectively improved by a factor of ~1000. This study provides a new approach for realizing highly efficient optoelectronic devices based on TMDCs. KEYWORDS. transition metal dichalcogenide, silver nanowire network, surface plasmon, strain relaxation, plasmon induced photocurrent, photogain effect


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Two-dimensional TMDCs are layered materials containing group 6 transition metals (Mo, W) and chalcogens (S, Se, Te)1 and have many advantages (e.g., high transparency, atomically thin, and high flexibility).2,3 Their electronic band structure is converted to a direct bandgap at the K-point of the Brillouin zone, by the quantum confinement effect at the monolayer.4 The unique physical properties of 1L-TMDCs make them attractive for applications in extremely thin optoelectronic devices (e.g., heterojunction solar cells,5 ultra-sensitive photodetectors,6,7 light emitting diodes,8 and single photon emitters).9 However, the low QY of 1L-TMDCs due to their small absorption cross section, limits their application in optoelectronic devices.4 Recent attempts to improve the QY of 1L-TMDCs have exploited the coupling between the exciton of TMDCs and the SP of various noble metal nanostructures.10 A tightly coupled exciton system with a SP, can improve absorbance and spontaneous emission (Purcell effect).11 Ag nanowire (NW),12,13 Ag nanoparticles,14 Ag nanodisks,15-17 Au nanoparticles,18,19 and Au nanorods20,21 have been utilized to increase the optical QY of 2-D nanomaterials for optoelectronic devices. Moreover, the PL of WSe2 is significantly enhanced using trenched Au substrates.22 However, it is difficult to fabricate optoelectronic devices based on these materials, due to the complicated process sequence. Despite the enhanced QY, the efficiency of 1L-TMDC optoelectronic devices based on SPs is much smaller than expected. Recent studies have shown that the photocurrent of 1L-MoS2 using Au nanoparticles18,23 and nanostructures24 was enhanced two-fold and three-fold, respectively.

In this research, the optical QYs of 1L-TMDCs (e.g., MoS2, WS2, and WSe2) were significantly enhanced using a simply coated Ag NW network on a glass substrate. Also, hybrid 1L-MoS2 photodetectors were successfully fabricated on this network. The overall photocurrents were

significantly

enhanced

(250-fold

enhancement).

The

photoresponsivity

and


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photodetectivity of the hybrid photodetectors were 59.60 A W-1 and 4.51 × 1010 Jones, respectively. Our hybrid device has the potential for realizing high-gain 1L-TMDC optoelectronic devices by exciton-plasmon coupling.

RESULTS AND DISCUSSION

Figure 1. (a) Experimental process to construct the hybrid structure consisting of 1L-MoS2 and Ag NWs on a glass substrate. (b) Schematic illustration of the hybrid structure. Raman scattering of (c) 1L-MoS2 on the glass substrate and (d) encapsulated Ag NW with a thin PVP layer. The inset shows a transmission electron microscope image of a Ag NW fully covered by PVP.

The hybrid 1L-MoS2/Ag NW network structure was fabricated using a simple procedure as outlined in Figure 1a. First, the Ag NWs were spin-coated onto a clean glass substrate. Then, 1L-MoS2 was wet-transferred onto the Ag NW network. After removal of the polymethyl methacrylate (PMMA), a 1L-MoS2 and Ag NW hybrid structure was formed. Figure 1b


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illustrates a schematic of both the hybrid structure and pristine 1L-MoS2. A 532-nm excitation laser source was used to measure the optical emission of 1L-MoS2. Interestingly, the spontaneous emission wavelength of this structure is red-shifted to a neutral exciton (A°)25 in the hybrid regions. Figure 1c shows the Raman scattering of pristine 1L-MoS2. The gaps of the inplane mode (E12g) at 386 cm-1 and out-of-plane mode (A1g) at 403 cm-1 are 17 cm-1, which signifies the presence of 1L-MoS2.26 Figure 1d shows the Raman scattering of Ag NWs encapsulated with a thin polyvinylpyrrolidone (PVP) layer. The vibrational mode of Ag-O (228 cm-1) indicates that the PVP and the Ag NWs are well bonded.27 The inset of Figure 1d shows a TEM image of the PVP-encapsulated Ag NWs with an average thickness of 35 nm. The insulating PVP prevents charge transfer28 and bandgap pinning, 29 which may interfere with the SP. When a Ag NW is not encapsulated, a PL change was observed due to the charge transfer effect, as shown in Figure S1. In this research, the Ag NW encapsulated with PVP have been used for all experiments. The same flake of 1L-MoS2 was separated into a pristine region (lower area) and hybrid region (upper area) to verify the optical QY, as shown in Figure S2e. None of the flakes on the Ag NW network were damaged by the strain or inherent roughness of each Ag NW (refer to the scanning electron microscope (SEM) image in Figure S2). Figure 2a presents the PL intensity mapping, which is distributed by A° (~1.9 eV) of 1L-MoS2. The overall PL intensity of the hybrid region is significantly enhanced with respect to the pristine region. Figure 2b shows PL spectra for regions (i) and (ii) of Figure 2a. The PL enhancement factor (EF) (Iwith Ag NWs x Alaser spot

/ Iw/o AgNWs x AAg NWs)30 of 1L-MoS2 is estimated as 560 (detailed information in Figure S7).

To understand these spectral changes and the PL enhancement, each PL spectrum of the hybrid structure (top) and 1L-MoS2 (bottom) was decomposed by the B exciton (~2.04 eV), Axx (multi


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exciton, ~1.85 eV), AT (trion, ~1.87 eV), and A°, using Lorentzian fitting functions, as shown in Figure 2c.31 Only


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Figure 2. Evolution of confocal PL of 1L-MoS2 and the hybrid structure. (a) PL mapping of 1L-MoS2 and the hybrid region using a 532-nm excitation source (λex). (b) PL spectra of 1LMoS2 and the hybrid structure from (i) and (ii) in (a), respectively. (c) Each fitted PL spectrum of 1L-MoS2 (bottom) and the hybrid region (top) by Lorentzian functions for the B exciton, multi exciton (AXX), trion (AT), and neutral exciton (A°) peaks. (d) Scattering spectrum of one Ag NW with different polarization angles (E// and E⊥). Dark field images of one Ag NW with different polarization angles of incident light (insets). (e) PL mappings of one Ag NW on 1L-MoS2 with different polarization angles of a 532-nm excitation source (E// and E⊥). (f) PL spectra of Ag NW on 1L-MoS2 with changing incident polarization angle of laser (pristine 1L-MoS2, 90°, 60°, 30° and 0°). (g) Schematic diagram of the coupling between an exciton of 1L-MoS2 and the strong SP of the Ag NW network. Power-dependent PL spectra of (h) 1L-MoS2 and (i) the hybrid structure (0.001-1 mW).


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A° intensity in the hybrid region is drastically improved with respect to the B exciton, Axx, and AT. In order to confirm the plasmonic effect of the Ag NW, polarization dependent optical measurements were performed. The localized plasmon resonance of the Ag NW has a different response, depending on the incident polarization angles.32 Figure 2d shows the scattering spectra of a Ag NW by vertically (E⊥) and horizontally (E//) polarized light. Furthermore, the insets represent dark field images, which have obvious dependence on the vertically (left) and horizontally (right) polarized light. Enhanced longitudinal resonance in the visible range is also observed when the horizontally polarized light is aligned to the direction of the Ag NW. For measuring the PL tendency of different polarization angles of excitation source, the half-wave plate was utilized for rotating the polarization of the linearly polarized beam. In order to keep the polarization direction of the signal, second half-wave plate was placed in front of the spectrometer. We have utilized the excitation source of 2.33 eV (~532 nm) for PL measurement, which also can generate strong plasmonic resonance with broad wavelength range of longitudinal mode of the Ag NW. In order to observe the exciton-plasmon coupling regime, PL mappings of a Ag NW on 1L-MoS2 were performed with vertically (left) and horizontally (right) polarized light, as shown in Figure 2e. The PL intensity of neutral exciton of 1L-MoS2, is enhanced by the horizontally polarized 532-nm excitation light. Figure 2f shows the PL spectra of the Ag NW on 1L-MoS2 with different polarization angles (1L- MoS2 without Ag NW, 90, 60, 30 and 0°). It is regarded as the evidence of exciton-plasmon coupling. The main PL emission is gradually governed by neutral exciton and the PL intensity is also enhanced when incident-polarized beam is aligned to the direction of a Ag NW. A schematic is presented to explain the enhanced optical QY in Figure 2g. When the exciton of 1LMoS2 is coupled with the strong SP of the Ag NW network, the spontaneous emission rate is amplified by SP (the Purcell enhancement effect).11 The radiative decay rate of a neutral exciton


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(ГA°) is significantly amplified by dipole-dipole interactions, while the radiative decay rate of a trion and biexciton (ГA') effectively decreases owing to the reduced band-filling effect.12 As a result, the PL emission of 1L-MoS2 is governed by A° in the hybrid region. For systematic comparisons, we measured the power-dependent PL using a wide range of laser powers (0.001-1 mW). Figure 2h shows that as the excitation laser power is increased, the major PL emission of pristine 1L-MoS2 is dominated by B exciton, Axx, and AT.33 In contrast, the major PL emission of 1L-MoS2 on the Ag NW network is dominated by A°, as shown in Figure 2i. Because the band filling effect34 is effectively reduced by associating the exciton with the SP of the Ag NW network in the hybrid region.12

Figure 3. Shift of confocal Raman scattering of 1L-MoS2 and the hybrid structure. (a) Raman mapping of 1L-MoS2 and the hybrid region (λex = 532 nm). Raman peak shift mappings for (b) the E12g mode of 1L-MoS2 and (c) A1g mode of 1L-MoS2. (d) Raman scattering of 1LMoS2 (bottom) and the hybrid region (top) from (i) and (ii) in (a), respectively. (e) Scanning electron microscope (SEM) image of the supported and the suspended 1L-MoS2. (f) Tendencies of the Raman peak shift of the E12g (pink) and A1g mode (brown) of 1L-MoS2. Insets are the SEM images as the Ag NW density increases. (g) Schematic diagram for the phenomena when 1L-MoS2 is transferred onto one Ag NW, several Ag NWs and a high-density Ag NW network.


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To identify the clear reason for the observed spectral changes in the hybrid structure, we performed confocal Raman mapping, and the results are shown in Figure 3a. Raman scattering of 1L-MoS2 is effectively enhanced in the hybrid region by surface-enhanced Raman scattering.35 Figure 3b and 3c show the distribution mappings for the peak shift of the E12g and A1g modes of 1L-MoS2, respectively. The inherent surface roughness of a general SiO2 substrate can lead to deformation effects (e.g., tensile and compressive strains). It is reported that the optical QY was significantly enhanced by suspending 1L-MoS2 on a hole because of strain relaxation effect.36 In here, the E12g mode of the suspended 1L-MoS2 is red-shifted with respect to the supported case, due to strain relaxation. In our experiment, red shifted E12g mode is also induced by strain relaxation, because 1L-MoS2 is transferred onto the Ag NW network to eliminate the optical shielding effect of metal nanostructures placed on the TMDCs.21 On the other hand, the peak positions of the A1g mode of 1L-MoS2 are blue-shifted in the hybrid region as shown in Figure 3c. The strongly localized electromagnetic field of the Ag NW network could strengthen the vertical oscillations of S atoms in MoS2 and thereby affect the blue shift of the A1g mode.37 Figure 3d shows the local Raman scattering of the hybrid structure (top) and the 1L-MoS2 (bottom), which were extracted from regions (i) and (ii) respectively of Figure 3a. The peak position of the E12g mode is red-shifted from 386.0 cm-1 to 383.5 cm-1, and the peak position of the A1g mode is blue-shifted from 403.0 cm-1 to 404.5 cm-1. The completely suspended regions of 1L-MoS2 can be observed in the SEM image shown in Figure 3e. A difference in the image contrast of pristine 1L-MoS2 is observed in the high-density region of the Ag NWs (blue region). Figure 3f shows the peak shift for the A1g and E12g modes of 1L-MoS2, as the Ag NW density increases (inset SEM image). The A1g mode of 1L-MoS2 is linearly blue-shifted by the strong SP


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(brown). This linear blue shift of the A1g mode suggests that the out-plane mode of 1L-MoS2 is constantly activated as the plasmon resonance increases,37 whereas the E12g mode of 1L-MoS2 is no longer shifted beyond a certain threshold density of the Ag NWs (pink). The saturated red shift (383.5 cm-1) of the E12g mode indicates that the strain of 1L-MoS2 is completely relaxed beyond a certain density of the Ag NWs.36 As a result, the exciton-plasmon coupling seems to be more dominant than the strain relaxation effect at high Ag NWs densities. Figure 3g includes comprehensive diagrams which illustrate the overall phenomena for improving the optical QY. Integrated hot spots are steadily generated by the complex gap plasmon resonance as the Ag NW density increases.32,38 For accurate comparisons, we prepared flakes of 1L-MoS2 at different densities of Ag NW, as shown in Figure S3. The E12g modes of 1L-MoS2 are not shifted above a certain threshold density of Ag NWs with same phenomena, as shown in Figure 3f. On the other hand, if we utilize the circularly polarized excitation source for PL measurement, it is expected that PL enhancement factor will be further increased as compared with using linearly polarized excitation source due to strong SP resonance of the Ag NW network and the valley-spin polarization of TMDCs.39 Additionally, we observed strain-induced regions (iii) in Figure 3b. The direct band gap of 1L-MoS2 changes to an indirect bandgap because of the high tensile strain40, as shown in Figure S4. Micro absorption mapping confirmed the light-matter interaction of the hybrid structure in Figure 4a. The overall absorption intensity of 1L-MoS2 is enhanced in the Ag NW network region. The average absorption spectra of 1L-MoS2 (blue) and the hybrid region (red) are shown in Figure 4b. The overall absorption of the hybrid structure is enhanced in the visible range. SP resonance produces an electromagnetic field which is significantly enhanced in the near-field region close to the metal surface.41 This strongly enhanced electromagnetic field due to


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integrated gap plasmon can improve the absorption of 1L-MoS2. To clarify the absorption of the Ag NW network caused

Figure 4. Analysis of confocal absorption and relative QY of 1L-MoS2 and the hybrid structure. (a) Absorption mapping of 1L-MoS2 and the hybrid region. (b) Average absorption spectrum of 1L-MoS2 (blue) and the hybrid region (red), respectively. (c) Absorption spectra with increasing density of Ag NWs. (d) The relative QY according to density of Ag NWs.

by strong plasmon resonance, the absorption was measured with increasing Ag NW densities, as shown in Figure 4c. As the Ag NW density increases, the absorption efficiency is drastically enhanced by the strong SP from multiple integrated hot spots.32,38 Figure 4d shows the relative QYs of the hybrid structure and 1L-MoS2 (calculated as the PL intensity (s-1 µW-1) divided by the absorbance of each sample).14 The relative optical QY of 1L-MoS2 is gradually increased as the Ag NW density increases (relative PL spectra in Figure S8). The highest relative optical QY (red dot) of the hybrid structure is detected ~ 200-times larger than the pristine 1L-MoS2. In


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addition, the PL emissions by the neutral exciton of the other TMDCs (e.g., 1L-WS2 and 1LWSe2) are significantly improved (~ 100-times) in the Ag NW network, as shown in Figure S5).

Figure 5. Device configuration and photocurrent of 1L-MoS2 and the hybrid photodetector. (a) Schematic diagram of device configuration of the hybrid photodetector. Upper diagrams are a simple schematic for the pristine 1L-MoS2 and the hybrid photodetector on a glass substrate. (b) Dark field image of the hybrid photodetector (channel length = 9 µm). (c) Current-voltage (I-V) curve of pristine 1L-MoS2 photodetectors (blue) and hybrid photodetectors (red) (λex = 532 nm). The inset shows the I-V characteristic on a logarithmic scale. (d) Time-dependent photocurrent of pristine 1L-MoS2 (blue) and hybrid photodetectors (red) (on/off time = 4 min).

Finally, to realize the optoelectronic device using the hybrid structure, we fabricated hybrid photodetectors consisting of 1L-MoS2 and Ag NW network, onto a transparent glass substrate, as shown in Figure 5a). The hybrid photodetectors were accurately fabricated by etching the area of 1L-MoS2 and Ag NWs. Figure 5b shows a dark field image of a hybrid


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photodetector fabricated using a 1L MoS2 with Ag NW density of 60%. We measured currentvoltage (I-V) curves for the pristine 1L-MoS2 photodetectors and hybrid photodetectors under ambient conditions. Figure 5c represents the I-V curve of the hybrid photodetectors (red) and pristine 1L-MoS2 photodetectors (blue), respectively, when a voltage of -5 V to +5 V was applied. The overall photocurrents of the hybrid photodetectors increased significantly. Figure 5d shows the time-dependent photocurrent, which is the average of five measurements with different structures. A dc voltage (Vsd) of +5 V was applied to the hybrid and the pristine 1LMoS2 photodetectors, with an on/off time of 4 min. The average photocurrent greatly improves as indicated by the results (Ion/Ioff of the hybrid photodetector is 1,242, which is significantly higher than that of the pristine 1L-MoS2 photodetectors, at 42). The photoresponsivity of the hybrid photodetector is ~ 0.15 A W-1, which is greater than the pristine detector (0.40 mA W-1). Additionally, the photodetectivity of the hybrid photodetector is effectively enhanced from 2.32 × 107 to 8.93 × 109 Jones. To facilitate a more detailed comparison, the hybrid and the pristine photodetectors were fabricated on a SiO2 substrate, as shown in Figure S6. At the SiO2 substrate, the on/off current ratios under light and dark states (Ion/Ioff) are 1.39 × 104 for the hybrid photodetector and 62.97 for the pristine 1L-MoS2 photodetector. The photocurrent of the hybrid photodetectors was 250 times higher than that of the pristine 1L-MoS2 photodetectors. The photoresponsivity and photodetectivity of the hybrid photodetectors are ~ 59.60 A W-1 and 4.51 × 1010 Jones, which is also greater than the pristine values (0.05 A W-1 and 4.11 × 107 Jones). The EFs of photoresponsivity and photoresponsivity are observed approximately 1,000-times in the hybrid photodetector. For the pristine 1L-MoS2 photodetector, the photocurrent flow is dominant on the generation of e-h pairs by the excitation source (λex). On the other hand, in the hybrid


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photodetector, the enhanced absorbance of 1L-MoS2 by the strong plasmonic effect (the electromagnetic field enhancement and the resonant photon-scattering mechanism) of the Ag NW network induces the multiplication of photo-excited carriers. The multiple photo-excited carrier can improve the photocurrent of 1L-MoS2 hybrid photodetector.42 However, the increase in absorption by SP is not sufficient to explain the 250 times increased photocurrent as compared with the pristine 1L- MoS2. Recently, it is reported that the PVP layer can play an important role as the trap site for capturing charges.43 We believe that it can cause a photogain effect which induce huge increasement of photocurrent.44,45 The spatial separation of the multiple photoexcited carriers caused by trapping electrons in the PVP layer reduces electron-hole recombination and increases the lifetime of the carriers. Consequently, the carrier-collection efficiency can be improved in a hybrid photodetector. As a result, highly enhanced photocurrent is generated by the absorption enhancement due to strong SP coupling and the photogain effect which is caused by charge-trapping site in the outer PVP layer of the Ag NW.

CONCLUSION We presented a hybrid structure that was fabricated by wet-transferring 1L-MoS2 onto a Ag NW network. The detected PL EF was 560, and the relative QY of 1L-MoS2 was enhanced by a factor of ~200 in the hybrid structure. The enhanced optical QY originated from: 1. coupling between the excitons of 1L-MoS2 and the strong SPs of the Ag NW network and 2. the strain relaxation effect which resulted from the suspension of 1L-MoS2 onto the Ag NW network. Experimentally, we confirmed that the main reason for the enhanced optoelectronic QYs was the strong SP coupling at multiple integrated hot spots. Finally, we fabricated high quality 1L-MoS2 photodetectors by exploiting SP coupling. The hybrid photodetectors showed greatly enhanced


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photocurrents with a 250-fold enhancement over the pristine 1L-MoS2 photodetectors. The photoresponsivity and photodetectivity of the hybrid photodetectors were approximately 59.60 A W-1 and 4.51 × 1010 Jones, respectively, which were higher than those of the pristine 1L-MoS2 photodetectors (0.05 A W-1 and 4.11 × 107 Jones), respectively. Our hybrid structure can significantly enhance the optoelectronic properties of TMDCs within a strong SP coupling regime. A high-gain hybrid 1L-TMDCs photodetector based on a strong SP coupling is a promising candidate for the next-generation optoelectronic devices.

METHODS Sample Fabrication. Ag NWs were encapsulated within a thin PVP layer (~6 nm). Long Ag NW (> 35 µm) of an average diameter 35 nm were synthesized by reduction of silver nitrate (AgNO3) in ethylene glycol, known as polyol method, is the most widely used chemical method to synthesize Ag NWs.46 PVP is used as the precursor molecule of preferential growth.47 The AgNO3, PVP and ethylene glycol was purchased from Sigma-Aldrich. The Ag NWs were coated onto a clean glass substrate. The average diameter of the encapsulated Ag NW with PVP was 35 µm, as shown in Figure S2. 1L-MoS2 was obtained using a general thermal chemical vapor deposition (CVD) method on a SiO2 substrate.48 The average flake size was 50-100 µm. To fabricate the hybrid structure, CVD grown 1L-MoS2 was wet-transferred onto the Ag NW network, as shown in Figure 1a. For exact electrical measurements, the hybrid photodetectors were etched using SF6 and HNO3. The source and drain electrodes were deposited Cr (5 nm)/Au (50 nm) using an E-beam evaporator system. The Ag NW network-arrays were not attached to the electrodes. 1L-MoS2 was simultaneously used as a channel and a light absorbing material.


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Measurements. A multifunctional microscope system (NTEGRA, NT-MDT) was used for confocal PL and Raman scattering measurements. A 532-nm excitation source and high magnification objective lens (100×, NA = 0.7) were used. Throughout the experiments, gratings with 150 and 1800 grooves were used. For confocal dark field scattering and absorption measurements, a halogen lamp excitation source with a visible range spectrum and a high NA (0.9) objective lens were utilized (Nikon microscope system) together with the 150 grating. The electrical characteristics were measured using a conventional probe station system (2400 and 4200, Keithley). For the photocurrent measurements, 532 and 458-nm excitation sources were utilized. Time-dependent photocurrent measurements were facilitated using a homemade laser shutter system.


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Supporting Information The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interests. ACKNOWLEDGEMENT This work was supported by the Institute for Basic Science (IBS-R011-D1), the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (2016R1A2B2015581) and Basic Science Research Program through the National Research Foundation

of

Korea

(NRF),

funded

by

the

Ministry

of

Education

(NRF-

2016R1A6A3A11936024).


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Figure for Table of Contents


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Augmented Quantum Yield of a 2D Monolayer Photodetector by

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