Dark State Concentration Dependent Emission and Dynamics of CdSe Nanoplatelet Exciton-Polaritons.

The recent surge of interest in polaritons has prompted fundamental questions about the role of dark states in strong light-matter coupling phenomena. Here, we systematically vary the relative number of dark states by controlling the number of stacked CdSe nanoplatelets confined in a Fabry-Pérot cavity. We find the emission spectrum to change significantly with an increasing number of nanoplatelets, with a gradual shift of the dominant emission intensity from the lower polariton branch to a manifold of dark states. Through accompanying calculations based on a kinetic model, this shift is rationalized by an entropic trapping of excitations by the dark state manifold, while a weak dark state dispersion due to local disorder explains their nonzero emission. Our results point toward the relevance of the dark state concentration to the optical and dynamical properties of cavity-embedded quantum emitters with ramifications for Bose-Einstein condensate formation, polariton lasing, polariton-based quantum transduction schemes, and polariton chemistry.

Theory predicts UV/vis-to-IR photonic down conversion mediated by excited state vibrational polaritons.

This work proposes a photophysical phenomenon whereby ultraviolet/visible (UV/vis) excitation of a molecule involving a Franck-Condon (FC) active vibration yields infrared (IR) emission by strong coupling to an optical cavity. The resulting UV/vis-to-IR photonic down conversion process is mediated by vibrational polaritons in the electronic excited state potential. It is shown that the formation of excited state vibrational polaritons (ESVP) via UV/vis excitation only involve vibrational modes with both a non-zero FC activity and IR activity in the excited state. Density functional theory calculations are used to identify 1-Pyreneacetic acid as a molecule with this property and the dynamics of ESVP are modeled. Overall, this work introduces an avenue of polariton chemistry where excited state dynamics are influenced by the formation of vibrational polaritons. Along with this, the UV/vis-to-IR photonic down conversion is potentially useful in both sensing excited state vibrations and quantum transduction schemes.

Voltammetric Analysis of Redox Reactions and Ion Transfer in Water Microdroplets.

We report a set of voltammetric experiments for studying redox reactions and ion transfer in water microdroplets emulsified in 1,2-dichloroethane (DCE). The electrochemistry of microdroplets (rdrop ∼ 700 nm) loaded with either ferrocyanide ([Fe(CN)6]4-) or ferricyanide ([Fe(CN)6]3-), chosen due to their hydrophilic nature, was tracked using cyclic voltammetry. These heterogeneous reactions necessitated ion transfer at the droplet interface to maintain charge balance in the two liquid phases during oxidation or reduction, which was facilitated by the tetrabutylammonium perchlorate ([TBA][ClO4]) salt in the DCE phase. Experiments were performed with (1) a single macrodroplet (10-7 L) on a macroelectrode (r ∼ 1.5 mm), (2) millions of microdroplets (10-15 L) adsorbed on to a macroelectrode (r ∼ 1.5 mm), and (3) at the single microdroplet level via observing individual microdroplet collisions at an ultramicroelectrode (r ∼ 5 µm). We demonstrate that when millions of microdroplets are adsorbed onto a macroelectrode, there are two surprising observations: (1) the half-wave potential (E1/2) for the [Fe(CN)6]3-/4- redox couple shifts by +100 mV, which is shown to depend on the number of droplets on the electrode surface. (2) The reduction of [Fe(CN)6]3-, which is assisted by the transfer of TBA+ into the water droplet, displays two waves in the voltammogram. This dual-wave behavior can be explained by the formation of TBAxK3-xFe(CN)6, which is soluble in DCE. Additionally, we demonstrate that the adsorption of microdroplets onto an electrode surface offers significant amplification (×103) of the water/oil/electrode three-phase boundary when compared to the adsorption of larger macrodroplets, permitting a rigorous evaluation of heterogeneous chemistry at this distinct interface. In combination, these experiments provide new energetic and mechanistic insights for droplet systems, as well as reactivity differences between microscale and bulk multiphase systems.

Coupled Electron- and Phase-Transfer Reactions at a Three-Phase Interface.

Coupled electron- and phase-transfer reactions are fundamentally important in electrochemical energy conversion and storage, e.g., intercalation of Li+ in batteries and electrochemistry at the three-phase boundary in fuel cells. The mechanism, energetics, and kinetics of these complex reactions play an important role in device performance. Herein, we describe experimental methodology to quantitatively investigate coupled electron- and phase-transfer reactions at an individual, geometrically well-defined, three-phase interface. Specifically, a Pt-Ir wire electrode is placed across a H2O/1,2-dichloroethane (DCE) interface, creating a Pt-Ir/H2O/DCE boundary that is defined mathematically by a line around the surface of the wire. We investigated the oxidation of ferrocene (Fc), initially present in DCE (but essentially insoluble in water), at the three-phase boundary, and the transfer of its charged reaction product ferrocenium (Fc+) across the interface into the aqueous phase. In cyclic voltammetry, a reversible wave at E1/2 ∼ 0.58 V is observed for Fc oxidation in DCE on the first scan. The Fc+ produced near the H2O/DCE interface transfers into the aqueous phase. On the second and subsequent cycles, a second reversible wave at more negative potentials, E1/2 ∼ 0.33 V, is observed, corresponding to the reduction of Fc+ (and reoxidation back to Fc) in the aqueous phase. Finite-element simulations quantitatively capture the voltammetric response of coupled electron and phase transfer at the three-phase interface and indicate that the electrochemical response observed in the aqueous phase occurs within ∼200 µm of the Pt-Ir/H2O/DCE boundary. Finally, we demonstrate that the rate of transfer of Fc+ is strongly dependent on the concentration of supporting electrolyte, reaching a maximum at an intermediate electrolyte concentration, suggesting a critical role of the electric field distribution in determining the reaction rates at the three-phase interface.