Ultrafast Control of the Optical Transition in Type-II Colloidal Quantum Wells

Abstract

Manipulating the optical transition in semiconductors at ultrashort timescales is of both fundamental interest and central importance for emerging photonic applications. Traditionally, this manipulation is realized by electrostatic gating via Stark effects or band-gap renormalizations. Here, we report an ultrafast and all-optical route to engineer an indirect transition in core− crown colloidal quantum wells (CQWs), namely, CdSe/CdTe, with a type-II band alignment. Following the intense laser pulse excitation, the indirect band transition energy exhibits a pronounced blueshift−redshift crossover on the picosecond timescale, stemming from the formation and dissipation of the transient electric field (E-field) that forms upon photoexcitation to compensate for the driving force provided by the band offsets. Both the energy shift and dynamics of the transient E-field can be modulated optically by tuning the laser pulse excitation fluence. Our finding demonstrates a strong analogy between the type-II heterojunction and a p−n junction with respect to carrier equilibrium processes, which holds promise to facilitate the integration of CQWs within optical switching networks.

Optical Control of Directional Exciton Migration in Porphyrin‐Based Metal−Organic Frameworks

“…The crystalline porphyrin‐based surface‐supported MOF nanofilms (SURMOFs) by the self‐assembling of cobalt ions and porphyrin linkers ([5,10,15,20‐(4‐carboxyphenyl) porphyrin], TCPP) on quartz substrates are prepared via the layer‐by‐layer liquid‐phase epitaxial technique [ 10,24,25 ] (see details in Experimental Section). As schematically illustrated in Figure a, the Co‐TCPP SURMOF is based on the established SURMOF‐2 structure, [ 26 ] in which the carboxylic groups of TCPP linkers are coordinately connected with Co‐oxo trinuclear clusters, yielding a highly oriented 3D topological network.…”