Optical spectroscopy of Mn-doped perovskite nanocrystals

Abstract

In the field perovskite nanocrystals new synthesis strategies for doped and undoped NCs are required to realize stable and efficiently luminescent NCs. The aim of this thesis is to provide further insight into the synthesis and the optical properties of Mn2+-doped perovskite nanocrystals. In Chapter 2 a facile and fast one-pot room temperature synthesis method of CsPbX3 (X=Cl, Br, I) NCs is reported. In toluene, strong luminescent CsPbX3 NCs are obtained within 10 min by addition of a small volume of a concentrated HX (X= Cl, Br and I) solution to a clear Cs+ and Pb2+ precursor solution. By varying the halide composition in the NCs almost the full visible range (violet to red) can be covered by highly efficient CsPbX3 NCs. The NCs have improved stability in the presence of water because of the formation of a stable ligand shell. In Chapter 3 a room temperature method for Mn2+ doping into CsPbCl3 NCs is developed. By addition of a small amount of concentrated HCl acid to a clear solution containing Mn2+, Cs+ and Pb2+ precursors, Mn2+ doped CsPbCl3 NCs with strong orange luminescence of Mn2+ at ~ 600 nm is obtained. To enhance the Mn2+ emission intensity and to improve the stability of the doped NCs, isocrystalline shell growth was applied. Growth of an undoped CsPbCl3 shell greatly enhances the emission intensity of Mn2+ and results in lengthening the radiative lifetime of the Mn2+ emission to 1.4 ms. The core-shell NCs also show superior thermal stability and no thermal degradation up to at least 110 oC which is important in applications. In Chapter 4 we investigate the evolution of the exciton-to-Mn2+ ET efficiency as function of composition (Br/Cl ratio) and temperature in CsPbCl3-xBrx: Mn2+ NCs. The results show a strong dependence of the transfer efficiency on Br- content. An initial fast increase in the relative Mn2+ emission intensity with increasing Br- content is followed by a decrease for higher Br- contents. The results are explained by a reduced exciton decay rate and faster exciton-to-Mn2+ ET upon Br- substitution. Further addition of Br- and narrowing of the host bandgap makes back transfer from Mn2+ to the CsPbCl3-xBrx host possible and leads to a reduction in Mn2+ emission. Temperature dependent measurements provide support for the role of back transfer. In Chapter 5 the temperature dependence of exciton life times is investigated for CsPbCl3 NCs doped with Mn2+ (0~41 at%). The exciton emission life time increases upon cooling from 300 to 75 K. Upon further cooling a strong and fast sub-ns decay component develops. However, the decay is strongly bi-exponential and also a weak slow decay component is observed with a ~40-50 ns life time below 20 K. The slow component has a much stronger relative intensity in Mn-doped NCs compared to undoped CsPbCl3 NCs. The temperature dependence of the slow decay component resembles that of CdSe and PbSe QDs with an activation energy of ~19 meV for the dark-bright state splitting. Based on our observations we propose that slow bright–dark state relaxation at cryogenic temperatures gives rise to almost exclusively bright state emission. Incorporation of Mn2+ or high magnetic fields enhances the bright-dark state relaxation and allow for the observation of the long-lived dark state emission at cryogenic temperatures.