How Giant ZnSe-Alloyed Shell Realizes Nearly Unity Quantum Yield

Core/shell heterostructure has become one of essential formulae in various applications of colloidal quantum dots (QDs). Heteroepitaxy of wide band gap materials passivates mid-gap states derived from the surface dangling bonds of cores and confines carrier wavefunctions to the inside, allowing for realizing near unity photoluminescence (PL) quantum yield (QY). In addition, the extended shell thickness (typically, 4–10 nm), so-called giant QDs, enables us to manipulate ultrafast carrier dynamics in QDs such as nonradiative Auger recombination or energy transfer in densely packed QD films. In the early stages of giant QDs, CdSe/CdS heterostructures have been exclusively spotlighted as the only suitable pair capable of extended shell thickness due to their minimal lattice mismatch. However, recent research has successfully demonstrated giant ZnSe or its alloyed shells for CdSe cores despite the large lattice mismatch that generally deteriorates PL QY as a shell thickness increases. Unfortunately, there are no reasonable explanation for how such new giant QDs are possible.<br/>Here, we scrutinize the origin of unexpectedly high PL QY observed in giant CdSe QDs with ZnSe-alloyed shells. In the synthesis of QDs growing the graded thick Cd_xZn_1-xSe (average x = 0.3) or ZnSe shell up to ~10 nm, careful heteroepitaxy reaction accomplished nearly unity PL QY at moderate shell thickness (H ~ 4 nm) followed by the gradual reduction, understood as defect formation by compressive lattice strain of shell. However, it was found that more intense strain provided by ZnSeS or ZnS shell recovered the PL QY of giant QDs. To understand such counter-intuitive phenomenon, we investigate the development of strain in the heterostructure and consequent derivation of electronic states. Along the accumulation of compressive strain applied to CdSe core, probed from the CdSe phonon mode shift and altered exciton dynamics, we observed the emergence of low-energy tail in cryo-PL spectra regardless of shell thickness. Retarded decay dynamics at this tail states and Ab-initio calculation suggest that Zn vacancy in ZnSe shell (V_Zn) introduced during the shell growth forms shallow hole traps that is 0.05–0.1 eV higher than the 1S_h state of CdSe core. The formation of V_Zn and partial strain relaxation seems to prevent the excessive strain accumulation and consequent misfit dislocation. As long as the catastrophic strain is avoided, additional hydrostatic compressive strain lifts the 1S_h state of CdSe core and the shallow traps in the shell become deactivated. Our study proposes the bright side of defects that they can mitigate the lattice strain in the growth of thick ZnSe-based shells to prevent the destruction of QDs by the excessive stress.