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Single-Nanoparticle Plasmonic Electro-optic Modulator Based on MoS2 Monolayers Bowen Li,†,§ Shuai Zu,†,§ Jiadong Zhou,‡ Qiao Jiang,† Bowen Du,† Hangyong Shan,† Yang Luo,† Zheng Liu,‡ Xing Zhu,† and Zheyu Fang*,†
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†
School of Physics, State Key Lab for Mesoscopic Physics; Academy for Advanced Interdisciplinary Studies; Collaborative Innovation Center of Quantum Matter, Peking University, Beijing 100871, China ‡ Center for Programmable Materials, School of Electrical and Electronic Engineering, Nanyang Technology University, Singapore 639798 S Supporting Information *
ABSTRACT: The manipulation of light in an integrated circuit is crucial for the development of high-speed electro-optic devices. Recently, molybdenum disulfide (MoS2) monolayers generated broad interest for the optoelectronics because of their huge exciton binding energy, tunable optical emission, direct electronic band-gap structure, etc. Miniaturization and multifunctionality of electro-optic devices further require the manipulation of light−matter interaction at the single-nanoparticle level. The strong exciton−plasmon interaction that is generated between the MoS2 monolayers and metallic nanostructures may be a possible solution for compact electro-optic devices at the nanoscale. Here, we demonstrate a nanoplasmonic modulator in the visible spectral region by combining the MoS2 monolayers with a single Au nanodisk. The narrow MoS2 excitons coupled with broad Au plasmons result in a deep Fano resonance, which can be switched on and off by applying different gate voltages on the MoS2 monolayers. A reversible display device that is based on this single-nanoparticle modulator is demonstrated with a heptamer pattern that is actively controlled by the external gates. Our work provides a potential application for electro-optic modulation on the nanoscale and promotes the development of gate-tunable nanoplasmonic devices in the future. KEYWORDS: MoS2, exciton−plasmon interaction, electro-optic modulator, Fano resonance, trions direct electronic band-gap structure,21−24 versatile optical transitions,25−27 and tunable mechanical properties.28,29 Moreover, tightly bound excitons in the MoS2 monolayers make it a good candidate for fundamental physical studies and provide a significant opportunity to realize an electro-optic device that operates in the visible spectral region.26,30−34 The combination of plasmonic nanostructures with MoS2 monolayers shows a great application potential.23,27,35 The strong electromagnetic field associated with the localized surface plasmon (LSP) resonance can be confined to the deep subwavelength space. Depositing metallic nanostructures onto the MoS2 monolayers can realize a stronger light−matter interaction36−39 and has been used to produce significant advances for the photovoltaics,38 photodetection,40,41 and sensing.42 On the other side, the tightly bound trions (quasiparticles composed of two electrons and a hole) and excitons of the MoS2 monolayers can be tuned by electrical doping, which
P
lasmonics and transition metal dichalcogenides (TMDCs) are two of the most rapidly advancing research areas in nanophononics.1−4 Surface plasmons, with properties of nanoscale light confinement5 and electromagnetic field enhancement,6 have induced great applications in plasmonic focusing,7 integrated waveguides,8 active modulation,9,10 hot electron photodetection,11 etc. Plasmonic Fano resonance, as a coherent scattering phenomenon with an asymmetric line shape, is caused by the interference between a bright broad mode and a dark narrow state,12 which has been widely used for biosensing,13 optical switching,14 and electromagnetically induced transparency.15 Recently, the Fano resonance was further investigated by using a metallic nanostructure interacting with semiconducting quantum dots, where the strong coupling between the plasmon and exciton can generate a so-called “plexciton” resonance,16,17 which provides potential applications for the quantum optical device. Meanwhile, the extraordinary properties of TMDC materials have generated great interest in the optoelectronics community.18−20 Molybdenum disulfide (MoS2) monolayers have attracted tremendous interest in recent years because of their © 2017 American Chemical Society
Received: August 1, 2017 Accepted: September 1, 2017 Published: September 1, 2017 9720
DOI: 10.1021/acsnano.7b05479 ACS Nano 2017, 11, 9720−9727
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Figure 1. (a) Schematic of the electro-optic modulator, where a single Au nanodisk with a radius of 60 nm is deposited on the MoS2 monolayers. The coupling strength between a single Au plasmon and MoS2 exciton can be effectively tuned by the gate voltage. (b) PL spectrum of the MoS2 monolayers on the SiO2/Si substrate, which shows a strong emission peak at ∼680 nm. Inset: Corresponding Raman spectrum of the MoS2 monolayers. (c) Scattering spectra of the Au nanodisk with and without the MoS2 monolayers, and the absorption spectrum of the MoS2 monolayers (green line). The black dashed line represents the exciton−plasmon coupling region at ∼660 nm. (d) The coupled oscillator model, which includes the LSP, neutral exciton, and trion as three oscillators.
between the MoS2 monolayers and electrodes was measured as 2.598 MΩ, which demonstrates a fine ohmic contact (see Figure S2). The Au electrode, far away from the sample area, is mainly used for bias voltage control. Figure 1b is the Raman and photoluminescence (PL) spectra of the tested MoS2 monolayers, where a strong PL emission appears at ∼680 nm and the in-plane (E12g) and out-of-plane (A1g) Raman modes appear at 390 and 411 cm−1, respectively. Figure 1c is the absorption spectrum of the MoS2 monolayers with its characteristic peaks of A and B excitons at ∼660 and ∼610 nm, respectively (green line). The LSP resonance of the Au nanodisk is shown in the same panel as the yellow line. When the LSP is tuned close to the MoS2 A exciton, the near-field coupling strength between the MoS2 exciton and Au nanodisk plasmon can be dramatically increased and finally results in a strong Fano resonance (orange line). At room temperature, the MoS2 A exciton usually is contributed from both the neutral exciton (A0) and trion (A−). With a negative gate bias, the MoS2 absorption is dominated by the enhanced neutral exciton resonance. The contribution of the trion emerges gradually when the applied voltage changes to zero and further increases to the positive value. Therefore, the MoS2 absorption can be continually tuned by the electrical doping and further to be used for the modulation of the plasmon−exciton coupling strength. To intuitively understand this tunability, we can consider the Au LSP and MoS2 neutral exciton and trion as three classical coupled oscillators, as shown in Figure 1d. The oscillation strength of the neural exciton and trion can be effectively modified by the electrical doping, which can induce a change in the coupling constants g1 and g2 and finally affects the intensity of the Fano resonance. More details about our oscillator model can be found in the methods of theory calculation.
has been successfully used to control the optical properties of MoS 2 with an applied gate voltage.43 Therefore, the combination of the MoS2 monolayers and designed metallic nanostructure to realize the control of exciton−plasmon coupling could potentially generate broad interest in the design of compact plasmonic electro-optic modulators. However, a nanoscale modulator with ultrahigh tunability and optical sensitivity requires exciton−plasmon coupling at the singlenanoparticle level; thus the precise control of the metallic particle size and position, as well as the electrode contacts, is quite challenging for the device design and fabrication. In this article, we experimentally demonstrate a hybrid Au nanodisk/MoS2 platform that can realize nanoplasmonic electro-optic modulation at the single-particle level in the visible spectral region. The narrow MoS2 exciton coupled with the single plasmon dipole can result in an asymmetric Fano resonance, with its intensity and spectral position ultrasensitive to the local perturbation, such as the refractive index change of the MoS2 monolayers, which can be effectively tuned by the applied gate voltage.
RESULTS AND DISCUSSION Figure 1a is the schematic of our nanoplasmonic electro-optic modulator. The chemical vapor deposition (CVD)-grown MoS2 monolayers were exfoliated onto a SiO2/Si substrate by using the poly(methyl methacrylate) (PMMA) nanotransfer method; then a Au nanodisk with a radius of 60 nm and thickness of 30 nm was fabricated on the MoS2 monolayers via e-beam lithography and following Au evaporation, as shown in the scanning electron microscopy (SEM) image in Figure S1. To improve the conductivity of the contact, the electrodes were deposited by 5 nm Ti followed by 70 nm Au and then annealed at 200 °C in an Ar environment for 1 h. The contact resistance 9721
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Figure 2. (a, b) Reflection spectra of MoS2 monolayers with the gate voltage changed from +8 V to −8 V. An obvious and continuous spectral tuning was found under either positive (a) or negative gate voltages (b). (c, d) Scattering spectra of a single Au nanodisk on the MoS2 monolayers with positive and negative gate voltages. As shown in (c), by increasing positive voltage from 0 V to 8 V, the Fano resonance is gradually weakened and finally turned off. Conversely, an enhanced Fano resonance is turned on by increasing negative voltage as shown in (d).
Figure 3. (a) Experimentally measured MoS2 A exciton intensity in the energy range of 1.78 to 1.9 eV for the gate voltage changed from −8 V to +8 V. Neutral exciton (green line) and trion (orange line) that contribute to the A exciton (yellow line) are fitted in the Lorentzian shape.44 The solid arrows represent the trend of the energy evolution of the neutral exciton and trion, respectively. (b) Measured (solid line) and calculated (dashed line) scattering spectra of the Fano resonance in the range of 1.78 to 1.95 eV under the same gate voltage as (a). (c) Calculated MoS2 A exciton intensity and its corresponding neutral exciton and trion contributions.
A series of single Au nanodisks with different radii were fabricated to systematically investigate the exciton−plasmon interaction of this Au−MoS2 hybrid nanostructure, and the generated Fano resonance based on a single nanodisk was measured by using a commercial hyperspectral dark-field
imaging system (HIS V3, CytoViva Co.) (see Figure S3a−c). The coupling efficiency of a single-nanoparticle modulator is mainly determined by the scattering cross section in Au/MoS2 hybrids (see Figure S3d−f). To achieve the best coupling of 9722
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Figure 4. (a, b) Resonance energies (a) and intensity (b) of the MoS2 A exciton, neutral exciton, and trion that are extracted from Figure 3a and c. The calculation results show an excellent agreement with experimental measurements.
exciton and trion over the spectral range from 1.78 to 1.89 eV was fitted from the experimentally measured MoS2 absorption spectra,44 as shown in Figure 3a. An overall suppression of the MoS2 A exciton absorption can be found within this spectral range when the gate voltage Vg is changed from −8 V to +8 V, where the neutral exciton diminishes rapidly and finally disappears to the background. For the trion, on the other hand, the resonance broadens gradually with the increasing of the gate voltage and its intensity remains almost unchanged. This dependency of the MoS2 absorption on the electronic doping level shows a gate-dependent tunability of the MoS2 A exciton, which is further used for the tuning of the plasmon− exciton coupling. After the Au nanodisk was fabricated on the MoS 2 monolayers, the plasmon−exciton coupling occurs under excitation of incident light. The modified coupled oscillator model that was introduced in Figure 1d now can be used to describe the plasmon−exciton coupling behavior and explain the origin of this gate-dependent Fano resonance. Because the dielectric constant ε(ω) of the MoS2 monolayers can be considered by a multi-Lorentzian model, both MoS2 absorption bands and LSP mode can be represented as multiple oscillators. As shown in Figure 1d, the LSP, neutral exciton (A0), and trion (A−) as three oscillators can have respective resonance frequencies of ωLSP, ωA0, and ωA− and damping constants of γLSP, γA0, and γA−. These three oscillators can be driven by the harmonic external forces as a1eiωt, a2eiωt, and a3eiωt. By coupling the LSP with the neutral exciton and trion, respectively, via a spring with the real coupling constants g1 and g2, the motion equation of this coupled oscillator model can be written as
incident photon, the size of the Au nanodisk needs to be carefully tailored. The Au nanodisk with a radius of 60 nm was selected for the following experiment, because its plasmon resonance overlaps with the MoS2 A exciton in the frequency domain and thus can be used for the strong plasmon−exciton coupling, which is also confirmed by the finite-difference time-domain (FDTD) simulations (see Figure S3d−f). To investigate the gate dependence of this Fano resonance, reflection spectra of the pristine MoS2 monolayers at different gate voltages (Vg) were first tested as shown in Figure 2a and b, respectively, where the characteristic A exciton peak is found at ∼668 nm when Vg = 0 V. With the Vg changed from 0 V to +8 V, the absorption intensity of the A exciton decreases gradually and its spectrum peak red-shifted from 668 nm to 673 nm. On increasing the gate bias from 0 V to −8 V, the A exciton absorption increases and a spectral blue-shifting from 668 nm to 665 nm is found. By exciting the single Au nanodisk that was fabricated on the MoS2 monolayers, an asymmetric Fano resonance was recorded, as the narrow MoS2 excitons coupled with the broad Au dipolar mode. From Figure 2c, we can see that, for the positive gates, the coupling strength between the MoS2 A exciton and the single Au plasmon dipole decreases as the gate bias increases. For the negative gates, the coupling between the exciton and plasmon was effectively enhanced when the voltage changed from 0 V to −8 V and led to a deeper Fano resonance (Figure 2d). The Fano resonance usually has a frequency difference with the absorption dip of the MoS2, and due to the broadening of the MoS2 exciton line width, the shift of the Fano resonance in Figure 2d could hardly be observed. To see more clearly the relation between the reflectance and scattering spectra, the reflectivity and scattering dip were extracted from Figure 2, and we plotted their variation as a function of the applied gate voltages in Figure S4. With the gate voltage increased from −8 V to +8 V, the reflectivity increases and the absorption of MoS2 decreases. The scattering dip of the Au/ MoS2 hybrids shows a similar tendency. These results demonstrate that this gate-dependent Fano resonance is strongly sensitive to the exciton−plasmon coupling strength. By controlling the MoS2 exciton absorption with different external gate voltages, one can realize a switching on and off of this Fano resonance, as shown in Figure 2c and d, respectively. As mentioned above, the absorption of the MoS2 monolayers for the A exciton is mainly contributed from both the neutral exciton and trion. To quantitatively investigate this gatedependent Fano resonance, the behavior of both the neutral
⎡− ω2 ⎢ ⎢ + ⎢ ⎢ ⎢ ⎢ ⎢ ⎢⎣
+ iγLSPω 2 ωLSP g1
g1 − ω2 + iγA ω A 0 0
+ ω A20 g2
0
⎤ ⎥ ⎥ ⎥⎡ x1 ⎤ ⎡ a1 ⎤ ⎥⎢ x 2 ⎥ = ⎢ a 2 ⎥ 0 ⎥⎢ ⎥ ⎢ ⎥ ⎥⎢⎣ x3 ⎥⎦ ⎢⎣ a3 ⎥⎦ ⎥ − ω2 + iγA−ω + ω A2− ⎥⎦ g2
where xi (i = 1, 2, 3) represents the oscillation amplitude of each coupled oscillator. By solving the above equation, we can further calculate the sum of the squared modulus of the 3 amplitudes |∑i = 1 xi(ω)|2 , which represents the overall light scattering and can be used to fit the line shape and intensity of the Fano resonance that was measured in the experiment. As shown in Figure 3b, there is an excellent agreement between the experimentally recorded Fano resonance and the fitted 9723
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Figure 5. (a) Measured voltage variation of the device by applying a fast pulse train (+8 V) with a period of 1 ms and 50% duty cycle. (b) Rise and (c) decay region of the electric response of MoS2 monolayers, which shows the response time is about 50 ns. (d) The 8 V high and 700 ms wide electrical voltage pulse, which was used as a “trigger” to tune the optical response of the MoS2 monolayers. (e) Time-resolved reflectance of our modulator, which indicates the device switch time is less than 200 ms.
spectrum that was based on the calculation of our coupled oscillator model. At this moment, all of the parameters for our calculation model, such as ai, ωi, γi, etc., can be obtained from the fitted line shape and intensity of the Fano resonance (see Supplementary Table S1). If the coupling constants g1 and g2 equal zero, the oscillation intensity for each of the free oscillators can be descripted as
2
ai 2
−ω + iγω i + ωi
monolayers can be effectively doped by applying a back-gate bias. As shown in Figure 5a, a fast voltage pulse train (+8 V) was applied to the modulator with a period of 1 ms and 50% duty cycle, and the oscilloscope was used to monitor the voltage variation. The response time of this MoS2-loaded capacitor was estimated as ∼50 ns by extracting the rise and decay regions as plotted in Figure 5b and c, respectively, which has the same order of magnitude as the previous reported works.46,47 A negative electrical pulse train (−8 V) was also applied and measured (see Figure S6), which shows the same electric response for our modulator. To theoretical estimate the rise and decay time of the modulator, the back gate was modeled as a parallel plate capacitor because the size of the electrode is much larger than the thickness of the SiO2 layer.18 The rise or decay time can be calculated using the expression τ = RC, where C = 1.15 × 10−11 F is the capacitance between the electrode and the back gate (C = ε0εrS/d; εr = 3.9; S = 100 × 100 μm2; d = 30 nm) and R is the resistance. Because the resistance of Au is much smaller than that of the Si substrate, R can approximately equal the resistance of Si as R = 3 kΩ (R = ρL/Ssi; ρ = 3 Ω·cm; L = 1 mm; Ssi = 1 × 1 cm2). Therefore, the calculated rise or decay time was 34.5 ns, which agrees well with experimental results. We further exploited the electro-optic response time of our modulator by recording the time-resolved reflectance of the MoS2 monolayers around the A exciton resonance with a series of squared electrical voltage pulses in milliseconds. The experimental setup is illustrated in Figure S7. A “trigger” was implemented by applying an 8 V high and 700 ms wide pulse train to detect the optical response of the device, as shown in Figure 5d. The exposure time of the detection CCD camera (Acton SP2500, Princeton Instruments Co.) is set at 200 ms, which is the limit of the spectrometer real-time sampling rate for valid signal detection. An obvious reflectance switching was observed in Figure 5e, and the electro-optic switch time of our device can be estimated as less than 200 ms. In fact, the response time of our modulator should be much faster than 200 ms, because the measured time scale is seriously limited by the integration time of the camera for the signal processing. The plasmon scattering from the nanostructure is directly related
2
, by which we
calculated the MoS2 A exciton absorption and its corresponding neutral exciton and trion contributions as Figure 3c. The gatedependent scattering and absorption spectra of the sample under gate voltages of −6, −2, 2, and 6 V are plotted in the Supporting Information (see Figure S5). In order to get a detailed comparison between the experimental and calculation results, the photon energy and spectral intensity of the neutral exciton and trion under different gate voltages are extracted respectively from Figure 3a and c and plotted in Figure 4, where we can see more clearly that as the applied voltage changed from −8 V to +8 V, the photon energy of the neutral exciton increased from 1.86 eV to 1.89 eV, while the energy of the trion remains at ∼1.84 eV (Figure 4a). For the spectral intensity, both the MoS2 A exciton and neutral exciton decrease, while the trion approximately preserves its spectral weight (Figure 4b). The calculated results from our three-oscillator model show an excellent agreement with the experimental data. In comparison with the previous two-oscillator model45 that described exciton−plasmon coupling, we consider the contribution of the MoS2 trion to the calculation, which makes our three-oscillator model more precise in describing the interaction between the MoS2 excitons and Au LSP. This excellent agreement further confirms our calculation model with the fitted parameters is more effective in describing this gate-dependent Fano resonance and can be used to explore the physics insight of this MoS2 exciton−plasmon coupling. In order to exploit the switch time of our single-nanoparticle plasmonic modulator, we first measured the electric response of the MoS2 monolayers. As illustrated in Figure 1a, the top and bottom Au electrodes form a plate capacitor, where the MoS2 9724
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Figure 6. (a) SEM image of the fabricated electro-optic display device with Au nanodisks patterned on the MoS2 monolayers. The scale bar is 10 μm. (b and c) Magnified SEM image showing a heptamer pattern with a portion of the MoS2 removed from the substrate. The Au nanodisks and MoS2 inside the heptamer pattern are isolated and cannot be easily tuned by the gate voltage. Scale bar: 3 μm (b) and 1 μm (c). (d−f) Far-field scattering images of our display device under different gate voltages. (d) As Vg = −8 V, the heptamer appears and becomes a convex image; (e) as Vg = 0 V, the heptamer pattern is invisible and hidden in the background; (f) as Vg = +8 V, the heptamer pattern becomes concave with the background intensity subtracted. (g−i) Corresponding scattering intensities along the lines that are marked in (d)−(f) to show the contrast degree of our display device.
microscope with a 660 nm bandpass filter placed at the collection optical path (more details can be found in the Materials and Methods). When the gate voltage is changed to −8 V, the coupling strength between the MoS2 A exciton and Au plasmon increases and results in a strong Fano resonance at a wavelength of ∼660 nm, which weakens the scattering of Au nanodisks that are fabricated on the surrounding MoS2 monolayers and displays a heptamer pattern as shown in Figure 6d. When the gate voltage is decreased to zero, the heptamer pattern disappears in the background, as shown in Figure 6e. For a positive gate bias of +8 V, an inverse control of heptamer pattern display by subtracting the background signal (the intensity inside the heptamer) is demonstrated, as shown in Figure 6f. By plotting the spatial distribution of the scattering intensity along the nanodisk array, the quantitative modulation of the plasmon scattering intensity is indicated as in Figure 6g− i, where the far-field scattering light of the single particle can be clearly distinguished, and the full width at half-maximum of the
with the dielectric permittivity of the MoS2 monolayers, which can be effectively tuned by the back-gate bias; thus we believe the practical switch time should be at the nanosecond scale, as shown in Figure 5b and c. With the understanding of this gate-dependent exciton− plasmon coupling, we fabricated a two-dimensional display device by patterning the single Au nanodisks as an array on the MoS2 monolayers. Figure 6a−c are SEM images of our electrooptic modulator with different zoom-in scales. The Au nanodisks with a radius of 60 nm and period of 1 μm (as the image pixel size) were deposited as a rectangular array on the MoS2 monolayers, where a portion of the MoS2 was removed by reactive ion etching, showing a heptamer pattern as in Figure 6b. Detailed information about the fabrication process of the reversible heptamer display device is shown in Figure S8. Inside of this heptamer pattern, the isolated Au nanodisks and MoS2 cannot be directly modulated by the external gate voltage. Under incident light illumination, each of the Au plasmon scattering was homogeneously recorded by a dark-field 9725
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ASSOCIATED CONTENT
scattering beam width can be calculated as 370 nm, which is approaching the optical diffraction limit. This reversible display device is different from the traditional electro-optic modulator, because our gate-voltage control is achieved at a single-nanoparticle level (∼120 nm) with an ultrahigh sensitivity, which can be explained as the reason for direct coupling between the single Au plasmon dipole and the MoS2 A exciton. The strong Fano resonance that is generated from this exciton−plasmon coupling is the key for the device tuning. The reversible heptamer pattern display in the work realized an image pixel size of 1 μm, which breaks through the resolution bottleneck of the state-of-the-art display device and benefits a more sensitive and faster electro-optic modulator prototype at the nanoscale. In theory a subwavelength resolution display with a pixel size of λLSP/2 can be further achieved based on our single-nanodisk modulator configuration.
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b05479. Additional figures and a table (PDF)
AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. ORCID
Zheyu Fang: 0000-0001-5780-0728 Author Contributions §
B. Li and S. Zu contributed equally to this work.
Notes
The authors declare no competing financial interest.
CONCLUSIONS In summary, we realized a gate-dependent electro-optic modulator with a single Au nanodisk that was fabricated on the MoS2 monolayers. The plasmon−exciton coupling in this Au/MoS2 hybrid nanostructure generates an excitonic Fano resonance, which can be effectively tuned by the applied gate voltage. This electro-optic modulation was explained and calculated by our coupled three-oscillator model, which shows an excellent agreement between the experimental measurements and the calculated results. By controlling the strength of the excitonic Fano resonance at the single-particle level, a reversible and ultrasensitive optical display device based on this nanoscale modulator was successfully demonstrated. Our proposed single-nanoparticle modulator configuration provides an effect way to actively manipulate the exciton−plasmon interaction, which offers a possible solution for ultrathin and nanoscale electro-optic devices in the future.
ACKNOWLEDGMENTS This work is supported by the National Key Research and Development Program of China (grant no. 2017YFA0206000), National Basic Research Program of China (grant nos. 2015CB932403 and 2017YFA0205700), National Science Foundation of China (grant nos. 61422501, 11674012, 11374023, 61176120, 61378059, and 61521004), and Foundation for the Author of National Excellent Doctoral Dissertation of PR China (grant no. 201420), National Program for Support of Top-notch Young Professionals. Z.F. acknowledges the open projects of key laboratory of surface physics of Fudan University, and the open projects of National Laboratory of Solid State Microstructures of Nanjing University. REFERENCES (1) Wang, Q. H.; Kalantar-Zadeh, K.; Kis, A.; Coleman, J. N.; Strano, M. S. Electronics and Optoelectronics of Two-Dimensional Transition Metal Dichalcogenides. Nat. Nanotechnol. 2012, 7, 699−712. (2) Liu, X.; Galfsky, T.; Sun, Z.; Xia, F.; Lin, E.-c.; Lee, Y.-H.; KénaCohen, S.; Menon, V. M. Strong Light-Matter Coupling in TwoDimensional Atomic Crystals. Nat. Photonics 2015, 9, 30−34. (3) Komsa, H.-P.; Kotakoski, J.; Kurasch, S.; Lehtinen, O.; Kaiser, U.; Krasheninnikov, A. V. Two-Dimensional Transition Metal Dichalcogenides under Electron Irradiation: Defect Production and Doping. Phys. Rev. Lett. 2012, 109, 035503. (4) Kim, S.; Konar, A.; Hwang, W.-S.; Lee, J. H.; Lee, J.; Yang, J.; Jung, C.; Kim, H.; Yoo, J.-B.; Choi, J.-Y. High-Mobility and LowPower Thin-Film Transistors Based on Multilayer MoS2 Crystals. Nat. Commun. 2012, 3, 1011. (5) Barnes, W. L.; Dereux, A.; Ebbesen, T. W. Surface Plasmon Subwavelength Optics. Nature 2003, 424, 824−830. (6) Kim, S.; Jin, J.; Kim, Y.-J.; Park, I.-Y.; Kim, Y.; Kim, S.-W. HighHarmonic Generation by Resonant Plasmon Field Enhancement. Nature 2008, 453, 757−760. (7) Becker, J.; Zins, I.; Jakab, A.; Khalavka, Y.; Schubert, O.; Sönnichsen, C. Plasmonic Focusing Reduces Ensemble Linewidth of Silver-Coated Gold Nanorods. Nano Lett. 2008, 8, 1719−1723. (8) Gramotnev, D. K.; Bozhevolnyi, S. I. Plasmonics Beyond the Diffraction Limit. Nat. Photonics 2010, 4, 83−91. (9) Temnov, V. V.; Armelles, G.; Woggon, U.; Guzatov, D.; Cebollada, A.; Garcia-Martin, A.; Garcia-Martin, J.-M.; Thomay, T.; Leitenstorfer, A.; Bratschitsch, R. Active Magneto-Plasmonics in Hybrid Metal-Ferromagnet Structures. Nat. Photonics 2010, 4, 107− 111. (10) Khatua, S.; Chang, W.-S.; Swanglap, P.; Olson, J.; Link, S. Active Modulation of Nanorod Plasmons. Nano Lett. 2011, 11, 3797−3802.
MATERIALS AND METHODS Optical Measurement. The Raman, reflection, and PL spectra were measured by a home-built optical microscope equipped with a 100× objective lens (MPlanFL, Olympus Co.). The white light source with fiber illumination (MI-150, Dolan-Jenner Co.) was focused onto the back focal plane of the objective for the reflection spectrum measurement. The surface plasmon resonance and reflection spectra of the Au/MoS2 hybrid structure were collected by a spectrometer (iHR550, Horiba Co.). Commonly, the Fano resonance in nanostructures was characterized by using white light illumination and detected by the scattering light. The scattering spectrum of the sample was measured by a commercial dark-field imaging system (HIS V3, CytoViva Co.), and far-field scattering images shown in Figure 6 were obtained by dark-field optical spectra mapping with a 660 nm bandpass filter (FB660-10, Thorlabs Co.) placed in the detection optical path. The Au electrode is far away from the sample area, and the reflection from the surface of the Au electrode is filtered out by the slit of the spectrometer. Numerical Simulations. Full-field electromagnetic wave simulations were performed using the finite-difference time-domain method solver (FDTD Solutions, Lumerical). The investigated structure was simulated using perfectly matched layers along all directions. A total-field scattering-field source was launched and illuminated the structure along the −z direction. In the simulations, we used Palik data for the Au and Si complex refractive index. The dielectric permittivity of the MoS2 monolayers was extracted by parametrizing experimental data into a band and exciton transition (Brunaur−Emmett−Teller) model as reported in previous works.48 9726
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Article
ACS Nano
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DOI: 10.1021/acsnano.7b05479 ACS Nano 2017, 11, 9720−9727
