Tuning radiative recombination in Cu-doped nanocrystals via electrochemical control of surface trapping

The incorporation of copper dopants into II-VI colloidal nanocrystals (NCs) leads to the introduction of intragap electronic states and the development of a new emission feature due to an optical transition which couples the NC conduction band to the Cu-ion state. The mechanism underlying Cu-related emission and specifically the factors that control the branching between the intrinsic and impurity-related emission channels remain unclear. Here, we address this problem by conducting spectro-electrochemical measurements on Cu-doped core/shell ZnSe/CdSe NCs. These measurements indicate that the distribution of photoluminescence (PL) intensity between the intrinsic and the impurity bands as well as the overall PL efficiency can be controlled by varying the occupancy of surface defect sites. Specifically, by activating hole traps under negative electrochemical potential (the Fermi level is raised), we can enhance the Cu band at the expense of band-edge emission, which is consistent with the predominant Cu(2+) character of the dopant ions. Furthermore, we observe an overall PL "brightening" under negative potential and "dimming" under positive potential, which we attribute to changes in the occupancy of the electron trap sites (that is, the degree of their electronic passivation) that control nonradiative losses due to electron surface trapping.