Ultrafast ExcitonFormation in Perovskite QuantumRods

Investigating exciton formation in semiconductor materials is essential for efficient light-to-energy conversion applications. Exciton formation has been extensively studied in two-dimensional materials but is rarely observed in single quantum systems. Here, we construct one-dimensional CsPbBr₃ quantum rods (QRs) with two confined dimensions and one unconfined dimension. We observe subpicosecond ultrafast exciton formation in such a single quantum system. Exciton formation is found to occur during hot carrier cooling instead of at the band edge, in agreement with theoretical expectations. The coexistence of Mahan excitons and abundant free carriers is found in QRs when the carrier density is higher than the Mott density. Furthermore, by comparing QRs with weak-confined zero-dimensional quantum dots, the presence of the unconfined dimension of QRs is found to be the key factor for the observation of exciton formation. These results facilitate the understanding of many-body phenomena and open up new application platforms for single quantum systems.