Semiconductor nanocrystals (NCs) are potential materials for verifiable demonstrations of semiconductor-based laser cooling. The key feature that makes NCs appealing for laser cooling is their near unity emission quantum yields (QYs). An unresolved issue regarding NC QYs, however, centers on the existence of an excitation energy dependent (EED) QY. Here, we study EED QYs on three NC systems, aimed at demonstrating NC-based laser cooling (CsPbBr3, CsPbI3, and CdSe/CdS core/shell NCs). We evaluate the impact of EED QYs using two approaches. The first involves direct QY measurements using an integrating sphere. The second entails photoluminescence excitation spectroscopy where changes to NC QYs with excitation energy can be assessed qualitatively.
Establishing the optical refrigeration of semiconductors remains a longstanding goal due to potential applications in optoelectronics. Apart from stringent materials requirements, required to realize condensed phase laser cooling, namely the need to have near unity emission quantum yields, a practical challenge involves accurately measuring specimen temperatures in a non-contact fashion. Common all-optical approaches developed in response to this need include: pump– probe luminescence thermometry (PPLT) and differential luminescence thermometry (DLT). In this study, we compare and contrast PPLT and DLT to a newly developed up-conversion emission thermometry to establish the most robust approach for measuring semiconductor nanocrystal (NC) temperatures. Using high external quantum efficiency CdSe/CdS core/shell NCs, we reveal that up-conversion emission thermometry possesses higher accuracy than either PPLT or DLT. Up-conversion emission thermometry can also be used on specimens such as CsPbBr3 NCs with temperature-insensitive band gaps.
Much effort has gone into realizing laser cooling with solids over the last two decades. Multiple attempts have been made with systems that include rare-earth doped glasses, GaAs heterostructures, CdS nanobelts and hybrid perovskite nanoplatelets. Here we suggest that CsPbBr3 perovskite nanocrystals may eventually lead to verifiable demonstrations of condensed phase laser cooling. The highest emission quantum yield we have realized in CsPbBr3 nanocrystal ensembles is 99.9% at room temperature. This value lies above the critical quantum yield for CsPbBr3, needed to realize laser cooling. We also find that associated CsPbBr3 nanocrystal emission up-conversion efficiencies are large and are 75% and 32% for laser detuning energies of 24 meV and 105 meV.