Tuning Radiative Recombination in Cu-Doped Nanocrystals via Electrochemical Control of Surface Trapping

Abstract

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.

Spectroelectrochemistry of Quantum Dots

“…By raising/lowering the Fermi level, the occupancy of surface trap sites can be modified. Thus, the overall emission intensity as well as the ratio between the intensities of the band edge and Cu‐related features can be reversibly tuned . SEC experiments obtained with Mn 2+ ‐doped CdS NCs provide convincing evidence against any fundamental role played by surface trap states in the electrochemical PL quenching of these NCs .…”

Engineering colloidal quantum dot solids within and beyond the mobility-invariant regime

“…Packing additional ligands onto the CQD surface or growing a complete shell to passivate the core more fully can be expected to improve overall charge carrier lifetimes 38,43,44 . In addition, unintentionally introduced impurities presented at the time of synthesis or deposition represent a little-explored potential source of midgap trap states [45][46][47][48] . Distinguishing the nature of trap states in these studies may be done spectroscopically 49 and with transmission electron microscopy 50 .…”

From Cu2S nanocrystals to Cu doped CdS nanocrystals through cation exchange: controlled synthesis, optical properties and their p-type conductivity research

“…One possible origin for this only dopant-related peak is the recombination of carriers that are bound to the impurity with those that are optically excited into the lowest quantum-confined level [7]. When Cu is in the +1 state, activation of the impurity band occurs via nonradiactive capture of a hole from the valence band, thus the Cu dopant emission results from the recombination of electrons from the conduction band and Cu-bound holes in the Cu T 2 states [12]. The schematic diagram of electronic structure responsible for dominant Cu dopant PL is summarized in the insert schematic diagram in Fig.…”