Mechanisms of enhancement of light emission in nanostructures of II–VI compounds doped with manganese

Abstract

Intra-shell transitions of transition metal and rare earth ions are parity forbidden processes. For Mn 2+ ions this is also a spin forbidden process, i.e., light emission should be inefficient. Surprisingly, it was reported that in nanostructures of ZnMnS the 4 T 1 to 6 A 1 intra-shell transition of Mn 2+ results in a bright photoluminescence characterized by a short PL decay time. The model of a quantum confined atom was introduced to explain the observed experimental results. It was later claimed that this model is incorrect. Based on the results of our photoluminescence, photoluminescence kinetics, time-resolved photoluminescence, electron spin resonance and optically detected magnetic resonance investigations we confirm photoluminescence enhancement and decrease of photoluminescence lifetime and relate these effects to spin dependent magnetic interactions between localized spins of Mn 2+ ions and spins/magnetic moments of free carriers. This mechanism is active in both bulk and in low-dimensional structures, but is significantly enhanced in nanostructure samples.

Doped nanoparticles for optoelectronics applications

“…Such lifetime shortening was observed together with an increase in emission efficiency, i.e., the effect could not be related to enhanced rate of a nonradiative decay [14]. Even though the proposed model of quantum confined atom turned out to be wrong, our following works confirmed the appearance of short components of the PL decay in TM/RE doped nanoparticles [5][6][7][8][9][10][11][12][13]. The most important result of these studies was observation of the co-existence of fast and slow components of the PL decay in all the samples studied, with the fast component of the PL decay being very enhanced in nanocrystals.…”

“…We found that the fast PL decay is only seen if free carriers are co-excited. Once free carriers decay, are trapped etc... the mechanism of the 4 T 1 to 6 A 1 PL enhancement is deactivated and the slow PL decay of the Mn 2+ intra-shell PL should be observed [5][6][7][8][9][10][11][12][13]. This however, turned out not to be the case.…”

“…For example, the 4 T 1 to 6 A 1 intra-shell transition of Mn 2+ ions is a parity and spin (transition between spin quartet and sextet) forbidden process. Thus, this transition is fairly inefficient in bulk samples, especially in sulfides and selenides [5][6][7][8][9][10][11][12].…”

Enhanced energy pumping to Tb3+ ions in manganese-doped ZnS nanoparticles

“…We propose that efficient host-to-Mn 2+ energy pumping is another consequence of the confinement enhanced Mn-free carriers interactions, which are responsible for the increased quantum efficiency of the 4 T 1 to 6 A 1 intra-shell PL in nanoparticles and the appearance of ultra-fast PL decay components, as studied by us separately [5][6][7][8][9]. We observed that the fastest component of the PL decay of the Mn 2+ intra-shell emission correlates with a lifetime of free carriers (due to carrier trapping, exciton binding, etc.)…”

“…Further PL, PLE, PL kinetics and magnetic resonance investigations are described elsewhere [5][6][7][8][9]. …”