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Two-dimensional (2D) transition metal dichalcogenide (TMDC) monolayers are promising materials for light-emitting devices due to their excellent electric and optical properties. However, defects are inevitably introduced in the fabrication of TMDC monolayers, significantly influencing their emission properties. Although photoluminescence (PL) is considered as an effective tool for investigating the defects in TMDC monolayers. However, the PL from the defect-bound excitons is revealed only at low temperatures. Here, we show that the PL from the defect-bound excitons in a WS2 monolayer can be effectively revealed at room temperature by exploiting the transverse electric polarized wave supported by a Si3N4/Ag heterostructure. It is revealed that the defect-bound excitons in all possible positions of the WS2 monolayer can be effectively excited by the TE wave with significantly enhanced in-plane electric field localized on the surface of the Si3N4 layer. In addition, the emission from defect-bound excitons can propagate to the collection point with small attenuation. More importantly, the exciton dynamics in the WS2 monolayer can be modified by the Si3N4/Ag heterostructure, allowing the simultaneous excitation of neutral excitons, charge excitons (trions), and defect-bound excitons in the WS2 monolayer attached on the Si3N4/Ag heterostructure. We inspect the PL spectra obtained at different positions and find that the relative intensity of defect-bound excitons depends on the collection position. We also examine the dependences of the PL intensity and bandwidth on the excitation power for the three types of excitons. It is found that they exhibit different behaviors from those observed in the optical measurements by using the traditional excitation method. Our findings suggest a new way for exciting and studying the dynamics of multi-excitons at room temperature and indicate the potential applications of the TE wave in probing the defects in TMDC monolayers.

Plasma protein‐induced aggregation of nanoparticles (NPs) is a crucial issue in many applications, such as drug delivery. Although great efforts have been made to investigate the protein adsorption kinetics or protein‐induced NPs coalescence in bulk solutions, limited evidence has been uncovered for interfacial circumstances. Diet, disease, medicine, or senility could thoroughly change interfacial physicochemical properties of the inner lining of blood vessels. Implants including stents and artificial heart valves also have varied and evolutionary interfaces. Hence, there is an urgent need to understand the mechanism behind the non‐specific protein adsorption and NP‐protein aggregation in such interfacial cases. Here, we use evanescent light scattering to observe polystyrene NPs‒fibrinogen aggregation at substrates with varying surface properties. A density‐fluctuation correlation function is utilized to reveal the relaxation dynamics of the aggregates. Both time‐resolved and spatial‐correlated evidence shows that the aging process of such soft materials is out‐of‐equilibrium, where the dynamics faster and slower than exponential can coexist in one single relaxation process. Besides, corona formation, inner stress, and interconnection together determine the microstructure, local adhesion, and structural relaxation of the aggregates, which can further correspond to the protein‐to‐NP ratio as well as the surface chemistry of NPs and substrates. [ABSTRACT FROM AUTHOR]

We investigate numerically and experimentally the optical properties of the transverse electric (TE) waves supported by a dielectric-metal heterostructure. They are considered as the counterparts of the surface plasmon polaritons (i.e., the transverse magnetic (TM) waves) which have been extensively studied in the last several decades. We show that TE waves with resonant wavelengths in the visible light spectrum can be excited in a dielectric-metal heterostructure when the optical thickness of the dielectric layer exceeds a critical value. We reveal that the electric and magnetic field distributions for the TE waves are spatially separated, leading to higher quality factors or narrow linewidths as compared with the TM waves. We calculate the thickness, refractive index and incidence angle dispersion relations for the TE waves supported by a dielectric-metal heterostructure. In experiments, we observe optical resonances with linewidths as narrow as ∼10 nm in the reflection or scattering spectra of the TE waves excited in a Si3N4/Ag heterostructure. Finally, we demonstrate the applications of the lowest-order TE wave excited in a Si3N4/Ag heterostructure in optical display with good chromaticity and optical sensing with high sensitivity.

Manipulating the exciton dynamics in a heterobilayer (HB) composed of two transition metal dichalcogenides (TMDCs) is important in the development of photonic/plasmonic devices based on TMDC HBs. Here, the realization of such a manipulation in a MoS2/WS2 HB is reported by using a Si/Au hybrid nanocavity composed of a Si nanoparticle and an Au film, which is manifested in the modification in the photoluminescence (PL) of the embedded MoS2/WS2 HB. It is shown that a transition from PL quenching to PL enhancement can be achieved by adjusting the diameter of the Si nanoparticle, which modifies the plasmon resonance supported by the Si/Au nanocavity. More interestingly, it is demonstrated that the enhancement factor can be manipulated by shifting the exciton/trion resonance close to or far away from the plasmon resonance by simply increasing the laser power. It is revealed that the manipulation is realized by effectively controlling the strain and Purcell effects induced by the Si/Au nanocavity. A PL enhancement factor as large as ≈187 in the MoS2/WS2 HB at a high laser power is observed. The findings suggest the potential applications of dielectric‐metal hybrid nanocavities in the manipulation of the exciton dynamics in TMDC HBs and the development of novel plasmonic devices. [ABSTRACT FROM AUTHOR]

Transition metal dichalcogenides (TMDCs) have attracted great interest due to their excellent electronic and optical properties. Although much attention has been paid to TMDC monolayers, the high refractive index of bulk TMDCs in the lossless region implies potential applications of TMDC nanoparticles/nanostructures in the fields of nanophotonics and nonlinear optics. In this work, the second harmonic generation (SHG) in WS2 nanoparticles with different geometrical symmetries is investigated both numerically and experimentally. It is revealed that such a WS2 nanoparticle supports electric dipole (ED) mode, anapole mode, and whispering gallery mode (WGM) in which the SHG is enhanced. It is found that the SHG from a WS2 nanoparticle with C2 symmetry exhibits a strong dependence on the polarization of the excitation laser light around the ED resonance. Interestingly, it is observed that the SHG intensities at the two polarization angles perpendicular to each other (θ = 0° and θ = 90°) become equal at the ED resonance. On the short-wavelength side of the ED resonance, the SHG intensity at θ = 0° is smaller than that at θ = 90°. The situation is reversed on the long-wavelength side. The strongest SHG is achieved at the anapole resonance for θ = 0° and at the WGM resonance for θ = 90°. The polarization-dependent ED resonance manifested in the scattering spectra of the WS2 nanoparticle is responsible for this behavior. In a WS2 nanodisk, the SHG becomes insensitive to the polarization of the excitation laser light. It is shown that the SHG intensity from a WS2 nanoparticle is much larger than that from a WS2 monolayer, with an enhancement factor of up to ∼30. Our findings suggest a way to enhance and manipulate the second-order nonlinear optical responses of TMDC nanoparticles and lay a foundation for the realization of TMDC-based photonic devices. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) transition metal dichalcogenide (TMDC) monolayers are promising materials for light-emitting devices due to their excellent electric and optical properties. However, defects are inevitably introduced in the fabrication of TMDC monolayers, significantly influencing their emission properties. Although photoluminescence (PL) is considered as an effective tool for investigating the defects in TMDC monolayers. However, the PL from the defect-bound excitons is revealed only at low temperatures. Here, we show that the PL from the defect-bound excitons in a WS2 monolayer can be effectively revealed at room temperature by exploiting the transverse electric polarized wave supported by a Si3N4/Ag heterostructure. It is revealed that the defect-bound excitons in all possible positions of the WS2 monolayer can be effectively excited by the TE wave with significantly enhanced in-plane electric field localized on the surface of the Si3N4 layer. In addition, the emission from defect-bound excitons can propagate to the collection point with small attenuation. More importantly, the exciton dynamics in the WS2 monolayer can be modified by the Si3N4/Ag heterostructure, allowing the simultaneous excitation of neutral excitons, charge excitons (trions), and defect-bound excitons in the WS2 monolayer attached on the Si3N4/Ag heterostructure. We inspect the PL spectra obtained at different positions and find that the relative intensity of defect-bound excitons depends on the collection position. We also examine the dependences of the PL intensity and bandwidth on the excitation power for the three types of excitons. It is found that they exhibit different behaviors from those observed in the optical measurements by using the traditional excitation method. Our findings suggest a new way for exciting and studying the dynamics of multi-excitons at room temperature and indicate the potential applications of the TE wave in probing the defects in TMDC monolayers. [ABSTRACT FROM AUTHOR]

Transition metal dichalcogenides (TMDC), such as molybdenum disulfide (MoS2), have attracted great interest owing to their excellent electronic and optical properties. Efficient second harmonic generation (SHG) has been successfully demonstrated in the MoS2 monolayer, in sharp contrast to the negligible SHG from the bulk material. Here, the nonlinear optical responses of hexagonal‐prism‐like MoS2 nanoparticles are investigated, which support Mie resonances in the near infrared spectral range. It is revealed that the Mie resonances of such a MoS2 nanoparticle can be clarified into two types, which exhibit enhanced and suppressed SHG. It is verified that the SHG from the MoS2 nanoparticle is strongly correlated with the electric field distribution at the optical resonance. For the electric quadrupole mode with an anti‐symmetric electric field distribution, the SHG is greatly suppressed. As a result, a nanohole appears in the emission pattern of the SHG from the MoS2 nanoparticle. In sharp contrast, the SHG from the electric dipole mode with a symmetric electric field distribution can be one order of magnitude stronger than that from a MoS2 monolayer. The findings open new horizons for manipulating the nonlinear optical responses of TMDC nanoparticles and pave the way for realizing photonic devices. [ABSTRACT FROM AUTHOR]