Synthesis, Optical and Structural Properties, and Charge Carrier Dynamics of Cu-Doped ZnSe Nanocrystals

Stable and luminescent Cu-doped ZnSe nanocrystals (NCs) were synthesized in organic solvents with octadecylamine (ODA) as the capping ligand and characterized using a combination of optical and structural characterization techniques. Successful doping was achieved by adding Cu during the growth phase of the NCs when their size was similar to 4 nm. The appearance of red-shifted, intense photoluminescence (PL) peak with doping indicates the incorporation of Cu in the NCs, and stability of dopant emission infers the internal doping of NCs. Extended X-ray absorption fine structure (EXAFS) studies revealed that Cu is surrounded by four neighbors in the lattice but is very close to the NC surface and gets oxidized when NCs are precipitated from the solution. For the undoped sample, time-resolved PL studies using time-correlated single photon counting (TCSPC) reveal the luminescence decay lifetimes of about 1.1, 12, and 60 ns that we attribute to near-bandedge, shallow trap (ST) state, and deep trap (DT) state emissions, respectively. In addition to these decay components, the Cu-doped sample was found to have a long-lived component with a lifetime of 630 ns. Luminescence decay lifetimes of near-bandedge and ST state emissions were slightly shortened by doping (1 and 10 ns, respectively), suggesting that the Cu dopant competes with these states in trapping the charge carriers from the conduction band (CB) or near-bandedge states. However, the presence of Cu was found to increase the lifetime of DT states from 60 to 100 ns probably due to a decrease in coupling of electron and hole states involved in this emission upon Cu doping. Synthesis of such stable, doped samples along with a better understanding of charge carrier dynamics is significant for emerging optical applications of these NCs.