Observation of dark exciton luminescence from ZnO nanocrystals in the quantum confinement regime

Abstract

Time-resolved luminescence measurements are performed on high-quality ZnO nanocrystals in a quantum confinement regime prepared by fast cooling and succeeding surface passivation. A very long luminescence lifetime of ∼0.1 μs, which is indicative of the existence of a dark exciton state, is observed. The temperature dependence of the lifetime and the luminescence peak energy is successfully accounted for by a thermal equilibrium model of excitonic dark and bright states.

Photoluminescence quenching in cobalt doped ZnO nanocrystals

“…The procedure of the lifetime estimation have been described in our previous report. 25 It was also shown from detailed temperature dependence that the Stokes shift for this component was very small and it must be interpreted as a dark exciton PL, which has a spin triplet nature and, therefore, a long radiative lifetime. 25 This effect is more prominent in nanocrystals because the quantum confinement effect enhances the exchange interaction between electron and hole.…”

“…25 It was also shown from detailed temperature dependence that the Stokes shift for this component was very small and it must be interpreted as a dark exciton PL, which has a spin triplet nature and, therefore, a long radiative lifetime. 25 This effect is more prominent in nanocrystals because the quantum confinement effect enhances the exchange interaction between electron and hole. 26 As to the low energy component, a larger Stokes shift (lower luminescence energy) and a faster decay imply that this component is affected by a kind of extra charge situated at the surface or at an internal trap because these perturbations violate the spherical symmetry of the potential and induces the mixing of triplet and singlet states and enhances the oscillator strength.…”

“…The initial fast decay is regarded as a transition from optically generated singlet state of the electron-hole pair to the optically inactive triplet state (dark exciton). 25 The inclusion of Co atoms into the nanocrystals makes the initial decay faster and the background weaker. This indicates that the non-radiative recombination caused by the Co atoms is faster than the triplet-singlet conversion.…”

Non-monotonous size-dependent photoluminescence and excitonic relaxations in nanostructured ZnO thin films

“…11 The existence of a dark exciton state was proposed to account for the very long luminescence lifetime of $0.1 ms for ZnO nanocrystals with an average size of $3 nm. 23 On the contrary, theoretical calculations disproved the possibility of a dark exciton ground state forming in ZnO QDs. 24 In addition to the intrinsic radiative decay being controlled by oscillator strength, 22,25 for a full understanding of exciton emission, we have to consider the relaxation channels due to competing radiative and defectrelated nonradiative decays.…”

“…22,25 Furthermore, the size-dependence of relative PL intensities has been modelled and ascribed to geometrical effects and size-correlated density variations of luminescent defect states in ZnO nanowires. 30,31 In view of the unambiguous identication of emission peculiarities occurring due to conned excitons, it is important to clarify the contributions to the exciton recombination via nonradiative defect states, which is challenging and can be related to discrepancies between the discussed experimental 11,23 and theoretical 22,24 studies.…”