Photodoping through local charge carrier accumulation in alloyed hybrid perovskites for highly efficient luminescence

Metal halide perovskites have emerged as exceptional semiconductors for optoelectronic applications. Substitution of the monovalent cations has advanced luminescence yields and device efficiencies. Here, we control the cation alloying to enhance optoelectronic performance through alteration of the charge carrier dynamics in mixed-halide perovskites. In contrast to single-halide perovskites, we find high luminescence yields for photoexcited carrier densities far below solar illumination conditions. Using time-resolved spectroscopy we show that the charge carrier recombination regime changes from second to first order within the first tens of nanoseconds after excitation. Supported by microscale mapping of the optical bandgap, electrically gated transport measurements and first-principles calculations, we demonstrate that spatially varying energetic disorder in the electronic states causes local charge accumulation, creating p- and n-type photodoped regions, which unearths a strategy for efficient light emission at low charge-injection in solar cells and light-emitting diodes

Adsorbate-induced curvature and stiffening of graphene

The adsorption of the alkane tetratetracontane (TTC, C44H90) on graphene induces the formation of a curved surface stabilized by a gain in adsorption energy. This effect arises from a curvature-dependent variation of a moiré pattern due to the mismatch of the carbon−carbon separation in the adsorbed molecule and the period of graphene. The effect is observed when graphene is transferred onto a deformable substrate, which in our case is the interface between water layers adsorbed on mica and an organic solvent, but is not observed on more rigid substrates such as boron nitride. Our results show that molecular adsorption can be influenced by substrate curvature, provide an example of two-dimensional molecular self-assembly on a soft, responsive interface, and demonstrate that the mechanical properties of graphene may be modified by molecular adsorption, which is of relevance to nanomechanical systems, electronics, and membrane technology

Reactive Dynamics in Confined Liquids: Interfacial Charge Effects on Ultrafast Torsional Dynamics in Water Nanodroplets

The excited-state dynamics of a reactive dye molecule, auramine O, have been studied in nanoscale water droplets stabilized by a nonionic surfactant. Spectral dynamics were measured as a function of the radius of the water nanodroplet with 50 fs time resolution using time-resolved fluorescence up-conversion method. Qualitatively, the effect of confinement is to dramatically slow the rate of the reaction compared to that of bulk water. Data were quantitatively analyzed using the one-dimensional generalized Smoluchowski equation assuming a time-dependent diffusion coefficient. The results were contrasted with our earlier analysis of auramine O in aqueous nanodroplets stabilized by the ionic surfactant AOT. The excited-state reaction is slower in the nonionic surfactant, showing that interfacial charge is not required to suppress reactions in nanoscale water droplets. The location of the dye in the heterogeneous micelle is investigated by comparing the absorption spectra of AO in the micelle with those of a water- polyethyleneglycol mixture (to mimic the surfactant head group). The results suggest that the charged dye is located in the water phase