(Invited) Design and Synthesis of Colloidal QDs for Quantum Emitters

Abstract

On-demand generation of quantum photons is one of the four major targets of Quantum Information Science (QIS), identified by the NSF as one of the five “industries of the future.” Colloidal semiconductor quantum dots (QDs) are viable candidates for quantum materials, owing to their tunable, bright, and stable emission. Identifying a suitable semiconductor material and engineering long exciton coherence times is key to producing single coherent photons on demand. We show that colloidal compositionally-graded CdSe/ZnS QDs designed with a high degree of asymmetric internal strain demonstrate superior quantum emission properties compared to conventional sharp-interface CdSe/ZnS QDs. In comparison to conventional colloidal CdSe/ZnS core/shell QDs, we find that, in asymmetrically strained CdSe QDs, over six times more light is emitted directly by the bright exciton. Temperature-dependent PL dynamics and fluorescence line narrowing (FLN) spectroscopy reveals the radiative recombination rates of bright and dark excitons, the relaxation rate between the two, and the energy spectra of the quantized acoustic phonons in the QDs that can contribute to relaxation processes. We attribute the six-fold improvement to the four-fold deceleration of the bright-dark relaxation and the doubled radiative recombination rate in the strained QDs.The next step in this research is to bring these improvement from the visible to the near-IR emission, where most communication bandwidths lie. This area has been traditionally dominated by III-V materials, which are chemically challenging in the colloidal state. We design, synthesize, and characterize alternative strong near-IR materials and assess them as potential quantum emitters. Last but not least, we demonstrate applications of electrochemistry in programmable hot syntheses of colloidal QDs.