PDF《Article》

2017 Accepted: September 1,

2017 Published: September 1,

2017 Article www.acsnano.org ?

2017 American Chemical Society

9720 DOI: 10.1021/acsnano.7b05479 ACS Nano 2017, 11, 9720?9727 Cite This: ACS Nano 2017, 11, 9720-9727 has been successfully used to control the optical properties of MoS2 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 single- nanoparticle 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 e?ectively 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 between the MoS2 monolayers and electrodes was measured as 2.598 MΩ, which demonstrates a ?ne 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 (E1 2g) 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-?eld coupling strength between the MoS2 exciton and Au nanodisk plasmon can be dramatically increased and ?nally 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 e?ectively modi?ed by the electrical doping, which can induce a change in the coupling constants g1 and g2 and ?nally a?ects the intensity of the Fano resonance. More details about our oscillator model can be found in the methods of theory calculation. Figure 1. (a) Schematic of the electro-optic modulator, where a single Au nanodisk with a radius of