Colloidal CdSe nanoplatelets are 2D semiconductor nanocrystals with opto-electronic properties that are similar to quantum wells. Being synthesized via bottom-up approaches, CdSe nanoplatelets typically have a thickness less than 2 nm, leading to strong confinement of the electron and hole wave functions in the vertical direction, while larger lateral dimensions give rise to a weak-to-intermediate confinement regime. They are often suspended in organic solvents or embedded in polymer films, enhancing Coulomb interactions through a low-dielectric environment. In addition, a variety of heterostructures can be synthesized with core/shell and core/crown geometries. In this talk, I will focus on the synthesis of two types of 2D heteronanoplatelets, both based on CdSe nanoplatelet cores.

First, we synthesized CdSe/ZnS core/shell nanoplatelets using a novel single-source precursor route. Here the ZnS shell is grown predominately on CdSe top and bottom planes. Despite a type-I band alignment, optical spectroscopy revealed that the band edge red shifts significantly upon shell growth. Starting from 515 nm-emitting CdSe nanoplatelets, we obtained a continuous shift of the emission peak toward 611 nm for the final CdSe/ZnS core/shell nanocrystals. K·p calculations revealed contributions to the red shift from both exciton delocalization, as well as a reduced electron-hole attraction due to a modified surface polarization induced by the ZnS shell.

Second, type-II CdSe/CdTe core/crown nanocrystals were synthesized with a CdS interfacial barrier between core and crown. Here the CdS and CdTe region were grown on the side facets of the CdSe core. The cascaded CdSe/CdS/CdTe band offsets allow for electron relaxation into the CdSe region, while the low conduction band energy of CdS yields a tunneling barrier between core and crown for the hole. Compared to CdSe/CdTe nanoplatelets without a barrier, we observed an enhanced CdSe emission and even two-photon upconversion, both enabled through the restricted hole relaxation induced by the CdS barrier.

The results highlight that the opto-electronic properties and carrier dynamics in 2D materials can be carefully steered via both quantum confinement and Coulomb interactions, by targeted material synthesis that takes advantages of both strong and weak confinement regimes. It allows for optimal design of 2D nanoplatelet heterostructures toward different photonic applications, such as light-emitting or energy harvesting devices.

Acknowledgments. This project has received funding from the Ministero degli Affari Esteri e della Cooperazione Internazionale (IONX-NC4SOL) and the European Union’s Horizon 2020 research and innovation program under Grant Agreement No. 696656 (GrapheneCore1).