The charge state of the Pd surface is a critical parameter in terms of the ability of Pd nanocrystals to activate O2 to generate a species that behaves like singlet O2 both chemically and physically. Motivated by this finding, we designed a metal-semiconductor hybrid system in which Pd nanocrystals enclosed by {100} facets are deposited on TiO2 supports. Driven by the Schottky junction, the TiO2 supports can provide electrons for metal catalysts under illumination by appropriate light. Further examination by ultrafast spectroscopy revealed that the plasmonics of Pd may force a large number of electrons to undergo reverse migration from Pd to the conduction band of TiO2 under strong illumination, thus lowering the electron density of the Pd surface as a side effect. We were therefore able to rationally tailor the charge state of the metal surface and thus modulate the function of Pd nanocrystals in O2 activation and organic oxidation reactions by simply altering the intensity of light shed on Pd-TiO2 hybrid structures.

Although multiphoton-pumped lasing from a solution of chromophores is important in the emerging fields of nonlinear optofluidics and bio-photonics, conventionally used organic dyes are often rendered unsuitable because of relatively small multiphoton absorption cross-sections and low photostability. Here, we demonstrate highly photostable, ultralow-threshold multiphoton-pumped biexcitonic lasing from a solution of colloidal CdSe/CdS nanoplatelets within a cuvette-based Fabry–Pérot optical resonator. We find that colloidal nanoplatelets surprisingly exhibit an optimal lateral size that minimizes lasing threshold. These nanoplatelets possess very large gain cross-sections of 7.3 × 10−14 cm2 and ultralow lasing thresholds of 1.2 and 4.3 mJ cm−2 under two-photon (λexc=800 nm) and three-photon (λexc=1.3 μm) excitation, respectively. The highly polarized emission from the nanoplatelet laser shows no significant photodegradation over 107 laser shots. These findings constitute a more comprehensive understanding of the utility of colloidal semiconductor nanoparticles as the gain medium in high-performance frequency-upconversion liquid lasers.

Wet-chemically synthesized cesium lead halide nanoparticles have many attractive properties that make them promising as optical gain media, but generally suffer from poor stability under ambient conditions and an optical gain threshold that is widely believed to be dictated by the need for biexcitons. These conditions make it impractical for such particles to be utilized as gain media given the need to undergo repeated stimulated emission processes at above-threshold pump intensities over long periods of time. We demonstrate that the surface treatment of CsPbBr nanoparticles with a mixture of PbBr, oleic acid, and oleylamine not only raises their fluorescence quantum yield to nearly unity and prolongs their stability in air from days to months, but it also dramatically increases their trion photoluminescence lifetime from ∼0.9 to ∼1.6 ns. Via a combination of time-resolved photoluminescence and transient absorption spectroscopy, we provide evidence for trion gain at sufficiently low pump intensities in which the likelihood of predominantly biexciton-based gain is small. We then show that, in line with theoretical prediction, the amplified spontaneous emission (ASE) threshold of a thin film of surface-treated CsPbBr nanoparticles reduces to a record low of ∼1.2 μJ/cm with a corresponding average exciton occupancy per nanoparticle of 0.62. The ultralow pump threshold and increased stability allow for stable ASE over millions of laser shots, paving the way for the deployment of these nanoparticles as viable solution-processed optical gain media.

Particle size is one important parameter of nanocrystals that need to be tightly controlled, owing to its versatility for tailoring the properties and functions of nanocrystals towards various applications. In this article, oxidative etching by hydrogen chloride is employed as a tool to control the size of metallic nanocrystals. As a result of the size control, investigations into the size-dependent plasmonic and catalytic properties of metallic nanocrystals can be investigated. Given that the shape can be kept consistent when tuning the particle size in this system, it enables the systematic investigation of size-dependent properties free of the influence of other factors such as shape effect.