Exciton–polaritons (EPs) can be formed in transition metal dichalcogenide (TMD) multilayers sustaining optical resonance modes without any external cavity. The self-hybridized EP modes are expected to depend on the TMD thickness, which directly determines the resonance wavelength. Exfoliated WS2 flakes were prepared on SiO2/Si substrates and template-stripped ultraflat Au layers, and the thickness dependence of their EP modes was compared. For WS2 flakes on SiO2/Si, the minimum flake thickness to exhibit exciton–photon anticrossing was larger than 40 nm. However, for WS2 flakes on Au, EP mode splitting appeared in flakes thinner than 10 nm. Analytical and numerical calculations were performed to explain the distinct thickness-dependence. The phase shifts of light at the WS2/Au interface, originating from the complex Fresnel coefficients, were as large as π/2 at visible wavelengths. Such exceptionally large phase shifts allowed the optical resonance and resulting EP modes in the sub-10-nm-thick WS2 flakes. This work helps us to propose novel optoelectronic devices based on the intriguing exciton physics of TMDs.
We fabricated hybrid nanostructures consisting of MoS2 monolayers and Au nanopillar (Au-NP) arrays. The surface morphology and Raman spectra showed that the MoS2 flakes transferred onto the Au-NPs were very flat and nonstrained. The Raman and photoluminescence intensities of MoS2/Au-NP were 3- and 20-fold larger than those of MoS2 flakes on a flat Au thin film, respectively. The finite-difference time-domain calculations showed that the Au-NPs significantly concentrated the incident light near their surfaces, leading to broadband absorption enhancement in the MoS2 flakes. Compared with a flat Au thin film, the Au-NPs enabled a 6-fold increase in the absorption in the MoS2 monolayer at a wavelength of 615 nm. The contact potential difference mapping showed that the electric potential at the MoS2/Au contact region was higher than that of the suspended MoS2 region by 85 mV. Such potential modulation enabled the Au-NPs to efficiently collect photogenerated electrons from the MoS2 flakes, as revealed by the uniform positive surface photovoltage signals throughout the MoS2 surface.
Due to their extraordinarily large optical absorption coefficients, transition metal dichalcogenides (TMDs) are gaining more and more attention for photovoltaic applications. Improving the device performance of a TMD solar cell requires an optimal device architecture and reliable fabrication processes. Metal/WS2‐multilayer/metal heterojunctions are fabricated using lithography‐free processes. 20 nm thick WS2 flakes are exfoliated on template‐stripped Ag bottom electrodes, and then 10 nm thick Au top electrodes with a diameter of 2 µm are evaporated on the WS2 surface using holey carbon films as shadow masks. Current‐sensing atomic force microscope measurements reveal that the Au/WS2/Ag devices exhibit prominent rectifying characteristics, indicating the formation of Schottky diodes. The power conversion efficiency of the Schottky junction is as high as 5.0%, when illuminated by a light‐emitting diode with a peak wavelength of 625 nm and a power density of 2.5 mW cm−2. These devices also possess broadband and incident‐angle‐insensitive absorption capability due to the very large refractive indices and extremely small thickness of the WS2 flakes. The simple fabrication procedures proposed in this work demonstrate high‐performance and high‐yield TMD photovoltaic devices.
WS2 flakes with a number of layers (NWS2) ranging from 1 to 10 are exfoliated on ultraflat template-stripped Au and Ag layers. The apparent colors of WS2/Au and WS2/Ag strongly depend on theunderlying metal layers as well as NWS2. The measured and calculated optical reflectance spectra are in good agreement, confirming the identification of NWS2 for each flake. The absorption in the WS2 flake for WS2/Au (AWS2-Au) and WS2/Ag (AWS2-Ag) is calculated: the maximum value of AWS2-Ag for NWS2 = 10 (~0.93) is much larger than that of AWS2-Au (~0.5). As expected, the local maxima of AWS2-Au and AWS2-Ag for each NWS2 are found near the exciton resonance wavelengths of WS2. The largest peak of AWS2-Ag is located close to the C exciton resonance wavelength, and the peak position shows a redshift with increasing NWS2. Despite the extremely small flake thickness, the resonant resonant modes can appear in WS2/Au and WS2/Ag, according to the anticipated phase shifts of light. These resonant modes can explain how NWS2 and the metal layer affect the optical characteristics of the WS2/metal structures.
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