Lukasz Klopotowski1,Malgorzata Szymura1,Magdalena Duda1,Miriam Karpinska1,Tomasz Kazimierczuk2,Roman Minikayev1,Magdalena Parlinska-Wojtan3
Institute of Physics, Polish Academy of Sciences1,Faculty of Physics, University of Warsaw2,Institute of Nuclear Physics Polish Academy of Sciences3
Lukasz Klopotowski1,Malgorzata Szymura1,Magdalena Duda1,Miriam Karpinska1,Tomasz Kazimierczuk2,Roman Minikayev1,Magdalena Parlinska-Wojtan3
Institute of Physics, Polish Academy of Sciences1,Faculty of Physics, University of Warsaw2,Institute of Nuclear Physics Polish Academy of Sciences3
We present experimental evidence that the photoluminescence (PL) in copper indium sulphide (CuInS<sub>2</sub>) quantum dots (QDs) occurs through a recombination of a delocalized electron and a localized hole (i.e., a free-to-bound mechanism). More specifically, we study the PL dynamics in a temperature range between 2 and 300 K and in magnetic fields up to 10 T and demonstrate that the results cannot be explained by assuming a recombination of band-edge excitons. On the other hand, the results are consistent with the free-to-bound mechanism in which the hole is localized at the d-shell of copper ions.<br/><br/>CuInS<sub>2</sub> QDs (and I-III-VI compound QDs in general) attract significant attention as more environmentally friendly alternatives to cadmium and lead chalcogenide QDs. Despite years of research, the optical properties of these nanostructures are not fully understood. Although most experimental results point to the free-to-bound process as the underlying mechanism of electron-hole recombination, some theoretical and experimental reports claim that the PL can be due to an excitonic process.<br/><br/>In this work, we find that the PL decays are strongly non-exponential underlining a strong inhomogeneity of the recombination rates. The extracted average PL lifetimes shorten with increasing the temperature. Crucially, when increasing the temperature from 2 K, we find that the short lifetime components are less sensitive to temperature change. We argue that this effect is incompatible with the excitonic mechanism while expected for the free-to-bound mechanism, where the position of the Cu ions within a QD governs the recombination rates. Furthermore, we investigate the PL decays recorded in two circular polarizations as a function of the magnetic field. This allows us to evaluate the spin relaxation rates. The rates are two orders of magnitude smaller than for CdSe QDs, which exhibit excitonic PL. On the other hand, the rates are on the same order as those evaluated for Cu-doped CdSe, in which the free-to-bound mechanism is responsible for the PL. We argue that the low spin relaxation rates originate from a limited interaction of the electron-hole pairs with the QD surface. Finally, we show that the PL polarization exhibits a particular temporal dependence: the equilibrium polarization decays with the time delay from the excitation laser. This effect can be interpreted as originating from a negative correlation between the fine structure splitting of the luminescent excited state and the PL lifetime. Crucially, this correlation is expected for the free-to-bound mechanism, while an opposite, positive correlation, is well established for excitonic photoluminescence. Thus, our results strongly support the free-to-bound recombination process as responsible for the PL of CuInS<sub>2</sub> QDs.