Revealing the Self-Doped Nature of CdxAgySe Quantum Dots under Cation Exchange Control

Metal-chalcogenide nanocrystals inevitably experience a state with a nonstoichiometric ratio during their growth. Considering that a few atoms function as dopants, the nonstoichiometric composition can determine the fundamental properties of the materials. Such a stoichiometry change in real time is extremely fast, and thereby, the product lies in an energetically stable state with a stoichiometric integral ratio. The phenomenon becomes dramatic for the cation-exchange reaction. Thus, the cation-exchange reaction with a controllable exchange rate can be an excellent way to generate the unexplored metastable state with a nonstoichiometry where interesting optical and electrical properties appear. Here, we report cation-exchanged Cd_xAg_ySe quantum dots (QDs) at a metastable self-doped state exhibiting intraband transition in the mid-wavelength infrared (mid-IR) regime. The state is successfully captured by controlling the reaction rate of the cation exchange and analyzed using short-wavelength and mid-IR photoluminescence spectroscopy under cryo-temperature, transmission electron microscopy, nuclear magnetic resonance, X-ray diffraction analysis, and energy-dispersive X-ray spectroscopy. The electronic mid-IR intraband transition of the cation-exchanged self-doped quantum dots is further confirmed by measuring the infrared photocurrent spectrum of the QD-based infrared sensor.