Ultraslow Spin Relaxation Dynamics in Colloidal Copper-Doped CdSe Quantum Dots

• Published in: Journal of physical chemistry. C Vol. 124; no. 1; pp. 1042 - 1052
• Main Authors: Kłopotowski, Ł, Mikulski, J, Szymura, M, Minikayev, R, Parlińska-Wojtan, M, Kazimierczuk, T, Kossut, J
• Format: Journal Article
• Language: English
• Published: American Chemical Society 09-01-2020
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/acs.jpcc.9b08235

Bulk semiconductors doped with copper ions have been heavily studied and employed as phosphors in early display technologies. The properties of copper-doped semiconductor nanostructures are much less studied. In particular, the spin properties of photoexcited carriers in copper-doped colloidal quantum dots are virtually unknown. Understanding the spin processes in these materials can lead to applications in novel information technologies. Moreover, the spin structure of the excited states in Cu-doped dots is analogous to that of Cu­(I) molecular complexes, which are widely studied with respect to applications in organic light-emitting diodes. Thus, understanding the spin properties of Cu-doped dots can have broad ramifications for other material systems. In this work, we study the dynamics of spin relaxation processes in photoexcited Cu-doped CdSe colloidal quantum dots at helium temperatures and in magnetic fields up to 8 T. We find that a spin polarization is induced by application of the magnetic field. However, this polarization develops extremely slowly: about 2 orders of magnitude slower than in reference Cu-free CdSe dots, at time scales on the order of 1 μs at 2 K and fields below 1 T. As the magnetic field and/or temperature is increased the spin relaxation accelerates, but in the entire field and temperature range remains much slower than in CdSe dots. We discuss various mechanisms responsible for the development of the spin polarization and conclude that the slow spin dynamics is related to the nature of the photoexcited state, which limits the carrier interaction with uncompensated spins at the dot surface. Our results thus underline the role of surface states in spin relaxation processes in colloidal quantum dots in general.