Colloidal two-dimensional (2D) nanoplatelet heterostructures are particularly interesting as they combine strong confinement of excitons in 2D materials with a wide range of possible semiconductor junctions due to a template-free, solution-based growth. Here, we present the synthesis of a ternary 2D architecture consisting of a core of CdSe, laterally encapsulated by a type-I barrier of CdS, and finally a type-II outer layer of CdTe as so-called crown. The CdS acts as a tunneling barrier between CdSe- and CdTe-localized hole states, and through strain at the CdS/CdTe interface, it can induce a shallow electron barrier for CdTe-localized electrons as well. Consequently, next to an extended fluorescence lifetime, the barrier also yields emission from CdSe and CdTe direct transitions. The core/barrier/crown configuration further enables two-photon fluorescence upconversion and, due to a high nonlinear absorption cross section, even allows to upconvert three near-infrared photons into a single green photon. These results demonstrate the capability of 2D heterostructured nanoplatelets to combine weak and strong confinement regimes to engineer their optoelectronic properties.

No abstract

The application of CdSe nanoplatelets (NPLs) in the ultraviolet/ blue region remains an open challenge due to charge trapping typically leading to limited photoluminescence quantum efficiency (PL QE) and sub-bandgap emission in core-only NPLs. Here, we synthesized 3.5 monolayer core/crown CdSe/CdS NPLs with various crown dimensions, exhibiting saturated blue emission and PL QE up to 55%. Compared to core-only NPLs, the PL intensity decays monoexponentially over two decades due to suppressed deep trapping and delayed emission. In both core-only and core/crown NPLs we observe biexcitonmediated optical gain between 470 and 510 nm, with material gain coefficients up to 7900 cm −1 and consistently lower gain thresholds in crowned NPLs. Gain lifetimes are limited to 40 ps, due to residual ultrafast trapping and higher exciton densities at threshold. Our results provide guidelines for rational optimization of thin CdSe NPLs toward lighting and light-amplification applications.

Transition metal dichalcogenides (TMDs) are nanostructured semiconductors with prospects in optoelectronics and photocatalysis. Several bottom-up procedures to synthesize such materials have been developed yielding colloidal transition metal dichalcogenides (c-TMDs). Where such methods initially yielded multilayered sheets with indirect band gaps, recently, also the formation of monolayered c-TMDs became possible. Despite these advances, no clear picture on the charge carrier dynamics in monolayer c-TMDs exists to date. Here, we show through broadband and multiresonant pump−probe spectroscopy, that the carrier dynamics in monolayer c-TMDs are dominated by a fast electron trapping mechanism, universal to both MoS 2 and MoSe 2 , contrasting hole-dominated trapping in their multilayered counterparts. Through a detailed hyperspectral fitting procedure, sizable exciton red shifts are found and assigned to static shifts originating from both interactions with the trapped electron population and lattice heating. Our results pave the way to optimizing monolayer c-TMDs via passivation of predominantly the electron-trap sites.

No abstract