Comment on "Dependence of the Fluorescent Lifetime τ on the Concentration at High Dilution"

Design Rules for Obtaining Narrow Luminescence from Semiconductors Made in Solution

Solution-processed semiconductors are in demand for present and next-generation optoelectronic technologies ranging from displays to quantum light sources because of their scalability and ease of integration into devices with diverse form factors. One of the central requirements for semiconductors used in these applications is a narrow photoluminescence (PL) line width. Narrow emission line widths are needed to ensure both color and single-photon purity, raising the question of what design rules are needed to obtain narrow emission from semiconductors made in solution. In this review, we first examine the requirements for colloidal emitters for a variety of applications including light-emitting diodes, photodetectors, lasers, and quantum information science. Next, we will delve into the sources of spectral broadening, including "homogeneous" broadening from dynamical broadening mechanisms in single-particle spectra, heterogeneous broadening from static structural differences in ensemble spectra, and spectral diffusion. Then, we compare the current state of the art in terms of emission line width for a variety of colloidal materials including II-VI quantum dots (QDs) and nanoplatelets, III-V QDs, alloyed QDs, metal-halide perovskites including nanocrystals and 2D structures, doped nanocrystals, and, finally, as a point of comparison, organic molecules. We end with some conclusions and connections, including an outline of promising paths forward.

Reply to "Comment on 'Dependence of the Fluorescent Lifetime τ on the Concentration at High Dilution'": Extended Interpretation

In "Comment on 'Dependence of the Fluorescent Lifetime τ on the Concentration at High Dilution'" by Anne Myers Kelley and David F. Kelley, the newly found concentration dependence of the fluorescence lifetime was attributed to the reabsorption and the delay of re-emission of fluorescence light. Accordingly, a comparably high optical density is required for a damping of the optically exciting light beam, causing a special profile for the re-emitted light with partial multiple reabsorption. However, an extended recalculation and re-examination on the basis of experimental spectra and the initially published data indicated an only static filtering effect by some reabsorption of fluorescent light. The resulting dynamic refluorescence is isotropically emitted in all room directions where only a small share of 0.006-0.6% adds to the measured primary fluorescence so that an interference in the measurement of fluorescent lifetimes becomes unimportant. Thus, the initially published data were further supported. The discrepancy between the two controversial papers could be resolved in terms of the different considered optical densities: a comparably high optical density can explain the interpretation of Kelley and Kelley, whereas low optical densities enabled by the use of the highly fluorescent perylene dye support our interpretation of the concentration dependence of the fluorescent lifetime.