We fabricated hybrid nanostructures consisting of MoS2 monolayers and plasmonic Au nanogratings. The polarization dependence of optical reflectivity showed a clear feature of surface plasmon polariton excitation. The MoS2 monolayers on Au nanogratings exhibited significantly enhanced photoluminescence intensity, compared with those on flat SiO2/Si substrates and Au thin films. Polarization-dependent surface photovoltage mapping was also obtained by Kelvin-probe force microscopy to study the exciton-plasmon coupling in the samples. In this presentation, we will discuss the interplay among photons, excitons, and surface plasmons in the MoS2-metal nanostructures.
The coupling between surface plasmons (SPs) and excitons in 2D transition metal dichalcogenide (TMD) materials has been attracted growing research attention in recent days. Strong electric field confinement and absorption enhancement could be expected, as a result of the SP-excition couping. We prepared exfoliated flakes of MoS2, a representative TMD material, on Au nanogratings fabricated by electron beam lithography. We studied influences of propagating SP on optical properties of the MoS2 flakes on the Au nanogratings, based on both experimental measurements and numerical calculations. Local surface potential maps of the samples suggested that the strain states in the MoS2 flakes and the dipoles formed at the MoS2/ Au interface could cause spatial modulation of the bandgap energies of the MoS2 flakes. The surface potential measurements were carried out using Kelvin probe force microscopy in dark and under TM/TE-mode light illumination. Band diagrams of the MoS2/Au nanogratings were proposed to explain all the experimental results. This study can help us to understand and control the physical characteristics of the TMD/metal nanostructures.
Understanding the band alignment at metal/2D semiconductor (SC) contacts is essential for electrical characterizations of 2D SC materials and for fabrication of high performance 2D SC devices. Many researchers have attempted to understand the electrical properties of metal/2D SC contacts and have revealed that they have unique features distinct from those of 3D SC counterparts. In this work, we investigated the surface potential (Vsurf) of exfoliated MoS2 flakes on bare and Au-coated SiO2/Si substrates using Kelvin probe force microscopy. The Vsurf of MoS2 single layers was larger on the Au-coated substrates than on the bare substrates; our theoretical calculations indicate that this may be caused by the formation of a larger electric dipole at the MoS2/Au interface leading to a modified band alignment. Vsurf decreased as the thickness of the flakes increased until reaching the bulk value at a thickness of ~20 nm on the bare and ~80 nm on the Au-coated substrates, respectively. This thickness-dependence of Vsurf was attributed to electrostatic screening in the MoS2 layers. Thus, a difference in the thickness at which the bulk Vsurf appeared suggests that the underlying substrate has an effect on the electric-field screening length of the MoS2 flakes. This work provides important insights to understand the band alignment and the charge transport at the metal/2D SC interfaces.
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