Obtaining Ligand-Free Aqueous Au-Nanoparticles Using Reversible CsPbBr3 ↔ Au@CsPbBr3 Nanocrystal Transformation.

Water-hexane interfacial preparation of photostable Au@CsPbBr3 (Au@CPB) hybrid nanocrystals (NCs) from pure CsPbBr3 (CPB) NCs is reported, with the coexistence of exciton and localized surface plasmon resonance with equal dominance. This enables strong exciton-plasmon coupling in these plasmonic perovskite NCs where not only the photoluminescence is quenched intrinsically due to ultrafast charge separation, but also the light absorption property increases significantly, covering the entire visible region. Using a controlled interfacial strategy, a reversible chemical transformation between CPB and Au@CPB NCs is shown, with the simultaneous eruption of larger-size ligand-free aqueous Au nanoparticles (NPs). An adsorption-desorption mechanism is proposed for the reversible transformation, while the overgrowth reaction of the Au NPs passes through the Au aggregation intermediate. This study further shows that the plasmonic Au@CPB hybrid NCs as well as ligand-free Au NPs exhibit clear surface enhanced Raman scattering (SERS) effect of a commercially available probe molecule. Overall, the beautiful interfacial chemistry delivers two independent plasmonic materials, i.e., Au@CPB NCs and ligand-free aqueous Au NPs, which may find important implications in photocatalytic and biomedical applications.

Exciton-plasmon coupling and giant photoluminescence enhancement in monolayer MoS2through hierarchically designed TiO2/Au/MoS2ternary core-shell heterostructure.

Enhancing the light coupling efficiency of large-area monolayer molybdenum disulfide (1L-MoS2) is one of the major challenges for its successful applications in optoelectronics and photonics. Herein, we demonstrate a dramatically enhanced photoluminescence (PL) emission from direct chemical vapor deposited monolayer MoS2on a fluorine-doped TiO2/Au nanoparticle plasmonic substrate, where the PL intensity is enhanced by nearly three orders of magnitude, highest among the reported values. The formation of TiO2/Au/1L-MoS2ternary core-shell heterojunction is evidenced by the high-resolution transmission electron microscopy and Raman analyses. Localized surface plasmon resonance induced enhanced absorption and improved light coupling in the system was revealed from the UV-vis absorption and Raman spectroscopy analyzes. Our studies reveal that the observed giant PL enhancement in 1L-MoS2results from two major aspects: firstly, the heavy p-doping of the MoS2lattice is caused by the transfer of the excess electrons from the MoS2to TiO2at the interface, which enhances the neutral exciton emissions and restrains the trion formation. Secondly, the localized surface plasmon in Au NPs underneath the 1L-MoS2film initiates exciton-plasmon coupling between excitons of the 1L-MoS2and surface plasmons of the Au NPs at the MoS2/Au interface. The PL and Raman analyses further confirm the p-doping effect. We isolate the contributions of plasmon enhancement from the theoretical calculation of the field enhancement factor using the effective medium approximation of plasmonic heterostructure, which is in excellent agreement with the experimental data. This work paves a way for the rational design of the plasmonic heterostructure for the effective improvement in the light emission efficiency of 1L-MoS2, and may enable engineering the different contributions to enhance the optoelectronic performance of 2D heterostructures.

Ultrahigh Brightening of Infrared PbS Quantum Dots via Collective Energy Transfer Induced by a Metal-Oxide Plasmonic Metastructure.

We demonstrate that a solution-processed heterojunction interface formed via the addition of a thin buffer layer of CdSe/ZnS quantum dots (QDs) to a functional metal oxide plasmonic metastructure (FMOP) can set up a collective interquantum dot energy-transport process, significantly enhancing the emission of infrared PbS quantum dots. The FMOP includes a Schottky junction, formed via deposition of a Si layer on arrays of Au nanoantennas and a Si/Al oxide charge barrier. We show when these two junctions are separated from each other by about 15 nm and the CdSe/ZnS quantum dot buffer layer is placed in touch with the Si/Al oxide junction, the quantum efficiency of an upper layer of PbS quantum dots can increase by about 1 order of magnitude. These results highlight a unique energy circuit formed via collective coupling of the CdSe/ZnS quantum dots with the hybridized states of plasmons and diffraction modes of the arrays (surface lattice resonances) and coupling between such resonances with PbS QDs via lattice-induced photonic modes.

Exciton-Plasmon Coupling and Electromagnetically Induced Transparency in Monolayer Semiconductors Hybridized with Ag Nanoparticles.

Exciton-plasmon coupling in hybrids of a monolayer transition metal dichalcogenide and Ag nanoparticles is investigated in the weak and strong coupling regimes. In the weak coupling regime, both absorption enhancement and the Purcell effect collectively modify the photoluminescence properties of the semiconductor. In the strong coupling regime, electromagnetically induced transparency dips are displayed, evidencing coherent energy exchange between excitons and plasmons.

Dual Transient Bleaching of Au/PbS Hybrid Core/Shell Nanoparticles.

We examined the optical response of hybrid Au/PbS core/shell nanoparticles (NPs) using transient absorption spectroscopy. Finite-difference time-domain (FDTD) calculations and transient absorption measurements show that Au/PbS NPs have unique two extinction peaks: the peak at the longer wavelength (∼700 nm) is originated from the plasmon, and that at the shorter wavelength (550 nm) is from the local maximum of the refractive index of PbS. The transient absorption dynamics of Au/PbS NPs excited at 400 nm have clear oscillation behavior, which is assigned to the breathing mode of whole particle. We observed a weak excitation-wavelength dependence of the plasmon band. The time constant of electron-phonon coupling of Au/PbS NPs was obtained by changing the excitation intensity. We show that spectral properties of Au/PbS NPs are strongly altered by the hybrid formations, while their dynamics differ only minimally compared with those of Au NPs.