The optical properties of monolayer transition-metal dichalcogenides (1L-TMDs) are predominantly governed by excitonic effects even in room temperature because of two-dimensional confined nature. As the result of strong coulomb interaction in 1L-TMDs, non-radiative exciton-exciton annihilation (EEA) is one of key influence to their light emission at nominal excitation density. Therefore, the modulation of EEA can help to make higher photoluminescence (PL) quantum yeild and develop optoelectric devices using 1L-TMDs.
Here, we observed reduced EEA rate in mechanically exfoliated monolayer tungsten disulfide (1L-WS2) by laser irradiation with improved light emission at the saturating optical excitation level. PL efficiency of 1L-WS2 in irradiated region increased with increasing the excitation intensity and finally it was 3 times higher at high excitation level compared to that in non-irradiated region, while the laser irradiated regions in 1L-WS2 have lower PL intensity at low excitation level than non-irradiation region. This kind of the excitation density dependence was confirmed by time-resolved PL measurement and EEA rate was reduced about 3 times by laser irradiation. Sulfur vacancies and lattice distortion might be formed by laser irradiation which can give rise to lower PL and shorter lifetime in laser irradiated region of 1L-WS2. But, we attribute these laser induced defects or adsorption of oxygen molecules in air to the origin of reduced EEA by hindering exciton diffusion. Our results could provide an idea for high performance opto-electric devices.
The limited Photoluminescence (PL) quantum yield of monolayer Transition metal dichalcogenides (1L-TMDs) are surprisingly shown to increase up to ~ 100 % by defect passivation mechanism i.e. suppressing the exciton quenching caused by the structural defects by simple chemical treatment. However, the mechanism behind it is in veil due to lack of experimental results in atomic level. In this work, we carried out bis(trifluoromethane) sulfonimide (TFSI) treatment of 1L-MoS2 and 1L-WS2 with different defects domains grown by chemical vapor deposition (CVD) and found drastically enhanced PL intensity in case of 1L-MoS2 while about 5 fold enhancement in case of 1L-WS2. Similarly the Raman intensity of both 1L-TMDs were increased and the intensity ratio of 2LA(M) to A1g peaks for 1L-WS2 were increased in different defect domains after TFSI treatment which are the strong evidence of defect passivation. We directly observed the atomic healing of 1L-TMDs by TFSI molecules under scanning transmission electron microscopy (STEM) analysis of pristine and TFSI treated 1L-TMDs and found that about ~ 90 % sulfur vacancies of 1L-TMDs were filled after treatment. The direct anchoring of dissociated sulfur atoms from TFSI molecules to the sulfur vacancies of 1L-TMDs was found to be energetically favorable by density functional theory calculations. Our observation shed light on the mechanism of intriguing healing process of lattice defects of 1L-TMDs and suggests that 1L-TMDs can be made defect-free which widens and prompts the practical uses of 1L-TMDs in nanophotonics applications. Furthermore, correlated experimental results and details will be presented.
Transition dichalcogenide monolayer (1L-TMD) such as MoS2, MoSe2, WS2 and WSe2 are promising 2D semiconductors with visible or near-IR wavelength emission, suitable for the nanophotonics applications including quantum optical emitters. 1L-TMDs offer advantages such as direct growth on wafers, the ability to tune the properties of the material by controlling the layer thickness, electrostatic doping, and hetero-stacking. Atomically thin, flat geometry of two-dimensional (2D) semiconductors provides the ideal coupling configuration between plasmons and excitons, leading to a new realm of light-matter interaction. It has been shown that the simple hybrid of placing Ag nanowires on MoS2 monolayers forms a highly efficient plasmon emitter and detector. Optoelectronic applications in two-dimensional (2D) transition-metal dichalcogenides (TMDs) are still limited by the weak light absorption, and moreover peak positions and shapes of exciton complexes are sensitively perturbed by varying excitation conditions, inherent from nature of atomically thin layers. Here we show that coupling of excitons with plasmons can spectrally refine the exciton emission of 1L-TMDs, maintaining contribution of only neutral excitons even with high excitation power.
Two-dimensional (2D) transition metal dichalcogenides (TMDs) play important role for optoelectronic applications such as photovoltaics, photodetectors, and field-effect transistors (FETs). However, there are still limited by several problems such as structural defects during the chemical vapor deposition (CVD) growth process, poor photoluminescence (PL) quantum yield (QY) and deeply understanding of exciton dynamics of TMDs.
Recently, it was reported that treatment using the superacid bis (trifluoromethane) sulfonamide (TFSI) resulted in a PL QY near 100% in exfoliated 1L-MoS2 monolayers. One of main reason of improved PL QY is due to repair defects induced sulfur vacancies. however, the effects of these chemical treatments varied greatly depending on the synthesis method and the type of 1L-TMD; therefore, the exact origin of the emission enhancement is still challenge.
Here, we perform detailed optical characterization of TFSI and 7,7,8,8-tetracyanoquinodimethane (TCNQ) treaded with CVD-grown 1L-MoS2 by using near-field scanning optical imaging and spectroscopy with nanoscale spatial resolution (~80nm). NSOM is optical imaging technique beyond the diffraction limit using narrow aperture that has aperture size much less than the wavelength of light. We found that 1L-MoS2 of systematic variation of the spectral weights among neutral excitons, trions indicated that p-doping by TFSI treatment. However, the PL enhancement was attributed mainly to the reduction of structural defects caused by TFSI treatment. Our results suggest that 1L-MoS2 helps to clarify the mechanism by which chemical treatment enhances the optical properties of 1L-TMDs.
We performed Finite Difference Time Domain (FDTD) calculation to investigate the enhancement of optical properties such as light scattering and absorption of Au-hybridized TiO2 core-shell structures which can lead to the improvement of photocatalytic and solar cell performance. The results showed that by hybridization of Au as core and TiO2 as shell provides the significant enhancement of light scattering and absorption. Furthermore, the tuning of scattering resonance wavelength may be achieved by varying the diameter of Au core. Our result suggests that hybridization Au and TiO2, with proper introduction of interband states in TiO2, can increase and color-tune the photocatalytic efficiency and solar cell performance of TiO2 nanostructures.
We demonstrate a simple method of stretching DNA to its full length, suitable for optical imaging and atomic force microscopy (AFM). Two competing forces on the DNA molecules, which are the electrostatic attraction between positively charged dye molecules (YOYO-1) intercalated into DNA and the negatively charged surface of glass substrate, and the centrifugal force of the rotating substrate, are mainly responsible for the effective stretching and the dispersion of single strands of DNA. The density of stretched DNA molecules could be controlled by the concentration of the dye-stained DNA solution. Stretching of single DNA molecules was confirmed by AFM imaging and the photoluminescence spectra of single DNA molecule stained with YOYO-1 were obtained, suggesting that our method is useful for spectroscopic analysis of DNA at the single molecule level.
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