Shedding Light on Host-to-Yb³⁺ Energy Transfer in Cs₂AgBiBr₆:Yb³⁺ (nano)crystals

The optical properties of Cs2AgBiBr6 double perovskite nanocrystals have attracted considerable attention as lead-free alternatives to lead halide perovskites. A promising strategy to create additional flexibility in the emission color is doping lanthanide ions into Cs2AgBiBr6. Incorporating Yb3+ in the lattice has been shown to give rise to near-infrared (NIR) emission, but the energy transfer mechanism remained unclear. Here, we report on the luminescence and sensitization mechanism of Yb3+ in Cs2AgBiBr6 nano- and microcrystals. We observe that the incorporation of Yb3+ in the host lattice does not strongly affect the broadband red emission of the Cs2AgBiBr6 host but does give rise to an additional and characteristic ∼1000 nm NIR line emission from Yb3+. Temperature-dependent and time-resolved photoluminescence studies of undoped and Yb-doped Cs2AgBiBr6 reveal that the energy transfer does not take place through the red emissive state of the Cs2AgBiBr6 host. Instead, there is a competition between relaxation to the red-emitting state and trapping of the photoexcited charge carriers on Yb3+. Trapping on Yb3+ subsequently results in a charge transfer state that relaxes to the 2F5/2 excited state of Yb3+, followed by NIR narrow line f–f emission to the 2F7/2 ground state

High-Throughput Characterization of Single-Quantum-Dot Emission Spectra and Spectral Diffusion by Multiparticle Spectroscopy

In recent years, quantum dots (QDs) have emerged as bright, color-tunable light sources for various applications such as light-emitting devices, lasing, and bioimaging. One important next step to advance their applicability is to reduce particle-to-particle variations of the emission properties as well as fluctuations of a single QD’s emission spectrum, also known as spectral diffusion (SD). Characterizing SD is typically inefficient as it requires time-consuming measurements at the single-particle level. Here, however, we demonstrate multiparticle spectroscopy (MPS) as a high-throughput method to acquire statistically relevant information about both fluctuations at the single-particle level and variations at the level of a synthesis batch. In MPS, we simultaneously measure emission spectra of many (20-100) QDs with a high time resolution. We obtain statistics on single-particle emission line broadening for a batch of traditional CdSe-based core-shell QDs and a batch of the less toxic InP-based core-shell QDs. The CdSe-based QDs show significantly narrower homogeneous line widths, less SD, and less inhomogeneous broadening than the InP-based QDs. The time scales of SD are longer in the InP-based QDs than in the CdSe-based QDs. Based on the distributions and correlations in single-particle properties, we discuss the possible origins of line-width broadening of the two types of QDs. Our experiments pave the way to large-scale, high-throughput characterization of single-QD emission properties and will ultimately contribute to facilitating rational design of future QD structures.ChemE/Opto-electronic Material