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.
We fabricated stand-alone trilayers (3L) and their heterostructures of MoS2 and WS2 on Al2O3 substrates. In particular, two kinds of heterostructures with different stacking sequences (i.e., 3L-MoS2/3L-WS2 and 3L-WS2/3L-MoS2) were prepared for comparison. The light-induced surface potential change suggested that the internal electric field along the thickness direction was present in the stand-alone (MoS2 and WS2) and heterostructure samples. Relative peak shifts of the Raman spectra supported the presence of the internal field in our samples. Physical origins to induce the internal electric field will be discussed in the presentation.
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.
MoS2 and WS2 trilayers were grown on p-type Si wafers using atmospheric pressure plasma processes. Current-voltage measurements of MoS2/Si and WS2/Si heterojunctions showed rectifying behaviors, indicating formation of diodes. It should be noted that the very large shunt resistance indicated uniform MoS2 layer formation on the Si wafers. Relatively large dark current of the MoS2 and WS2 heterojunctions under reverse bias indicated the band to band tunneling and avalanche multiplication processes. Temperature dependence of the diode ideality factor was also studied to reveal the major recombination processes, based on conventional 3D semiconductor models. Photocurrent characteristics of the junctions were studied using green lasers (wavelength: 532 nm). Large photocurrent was observed under reverse bias, whereas photocurrent was negligibly small under forward bias. The measured photocurrent was linearly proportional to the laser power. This suggested that trapping and detrapping of the photo-generated carriers at interface defects and surface adsorbates did not much limit the collection of photo-carriers. Both MoS2/Si and WS2/Si heterojunctions showed fast photoresponse: the rising and decaying time constants were less than 0.1 ms. All these results showed that our processes could prepare high quality 2D/3D hybrid semiconductor heterojunctions with clean interfaces.