Photoluminescence quenching in cobalt doped ZnO nanocrystals

Abstract

Influence of cobalt doping on the luminescence properties of ZnO nanocrystals with average diameter of 3.0 nm is investigated. Time resolved measurements at 20 K show that the dark exciton luminescence is completely lost in the nanocrystals doped with cobalt, while the perturbed luminescence with slight red shift survives and exhibits a non-exponential decay curve reflecting random distribution of cobalt atoms. By analyzing the non-exponentiality, the increase of the decay rate of the band-edge luminescence induced by the inclusion of single cobalt atom is estimated to be 0.012 ps À1. V

Designing quantum dots for solotronics

“…Quenching of exciton PL by a recombination channel introduced by a magnetic dopant is an important obstacle in studying diluted magnetic semiconductors (DMS) with an energy gap larger than the magnetic ions’ internal transition energies252628. This is the case for DMS with magnetic ions other than Mn (for example, V, Cr, Fe, Co, Ni and Cu)363839.…”

“…In the case of nanostructures, quantum confinement increases the exciton energy. As a consequence, the direction of the energy transfer is reversed, leading to PL quenching 22 23 24 25 27 and a significant shortening of the exciton lifetime 21 28 .…”

“…A severe limitation of QDs doped with transition metal ions was attributed to the efficient recombination channel introduced by magnetic ions 21 22 23 24 25 26 27 28 29 when the exciton energy is higher than the intra-ionic transition energy, which should result in quenching of exciton emission. Therefore, the only QD systems with single-magnetic ions considered so far were those where the intra-ionic transition energies exceed the exciton energy, namely Mn 2+ embedded in CdTe/ZnTe and InAs/GaAs QDs 10 11 12 13 14 15 16 30 31 32 .…”

ZnO gap states investigated using magnetic circular dichroism

“…The MCD is larger for the Zn-polar film and increases slowly with energy however both spectra are relatively flat. The spectrum from the O-polar films has a weak maximum around 2.4 eV corresponding the prominent 'green line' seen in the PL spectrum [24]. Such featureless spectra have been recorded before for defect states [33] and broad PL spectra are seen in ZnO due to lattice relaxation [19,26].…”

“…The states in the gap play an important role, particularly those due to oxygen vacancies, and in pure ZnO these can be measured with photoluminescence (PL). However this technique is not always available because the PL is quenched by dopant transition metal or Al ions [24,25]. It is also very difficult to measure the direct absorption from the gap states as it is usually masked by scattering from the surfaces and crystalline imperfections.…”