Exciton States and Optical Absorption in CdSe and PbS Nanoplatelets

Energy transfer in near-infrared photoluminescent PbS/CdS quantum dot-based three-dimensional networks and films

Nanocrystal (NC) assemblies, such as films or NC-based three-dimensional networks, provide insight into the nanoscopic properties of their building blocks, and are macroscopic materials. In NC assemblies in general, interactions between the building blocks have to be considered, since a decrease of the distance between the NCs allows phenomena such as energy transfer to occur. Novel quantum dot-based aerogels with optical properties in the near-infrared (NIR) region are not characterized in terms of these important properties in the literature, while the structural complexity of these networks raises questions about the interactions within. Since knowledge about the physical phenomena is vital for applications, we here investigate the photoluminescence (PL) of PbS/CdS QD-based assemblies, namely drop-cast films and aerogels with steady-state measurements at cryogenic temperatures and time-resolved measurements at room temperature. We find multiple emissive in-gap states (IGS), and a correlation between the number of nearest QD neighbors, the distance between the QDs in the assemblies and the non-radiative recombination rate by linking the observations to different energy transfer phenomena.

Possibility of Exciton Bose-Einstein Condensation in CdSe Nanoplatelets

The quasi-two-dimensional exciton subsystem in CdSe nanoplatelets is considered. It is theoretically shown that Bose-Einstein condensation (BEC) of excitons is possible at a nonzero temperature in the approximation of an ideal Bose gas and in the presence of an "energy gap" between the ground and the first excited states of the two-dimensional exciton center of inertia of the translational motion. The condensation temperature (Tc) increases with the width of the "gap" between the ground and the first excited levels of size quantization. It is shown that when the screening effect of free electrons and holes on bound excitons is considered, the BEC temperature of the exciton subsystem increases as compared to the case where this effect is absent. The energy spectrum of the exciton condensate in a CdSe nanoplate is calculated within the framework of the weakly nonideal Bose gas approximation, considering the specifics of two-dimensional Born scattering.

Colloidal 2D Lead Chalcogenide Nanocrystals: Synthetic Strategies, Optical Properties, and Applications

Lead chalcogenide nanocrystals (NCs) are an emerging class of photoactive materials that have become a versatile tool for fabricating new generation photonics devices operating in the near-IR spectral range. NCs are presented in a wide variety of forms and sizes, each of which has its own unique features. Here, we discuss colloidal lead chalcogenide NCs in which one dimension is much smaller than the others, i.e., two-dimensional (2D) NCs. The purpose of this review is to present a complete picture of today's progress on such materials. The topic is quite complicated, as a variety of synthetic approaches result in NCs with different thicknesses and lateral sizes, which dramatically change the NCs photophysical properties. The recent advances highlighted in this review demonstrate lead chalcogenide 2D NCs as promising materials for breakthrough developments. We summarized and organized the known data, including theoretical works, to highlight the most important 2D NC features and give the basis for their interpretation.