Self-assembly and degradation of CsPbBr3 perovskite nanocrystals

Abstract

Many applications have been envisioned for colloidal nanocrystals of perovskite CsPbBr3 , a novel nanomaterial showing extremely bright photoluminescence in the visible range of the spectrum [1]. CsPbBr3 nanocrystal based light emitting diodes have already been demonstrated [2] and several types of laser action were observed in fi?lms [3] and capillaries [4]. Here, we study several aspects of these cubic shaped nanocrystals. Firstly, we show that the nanocrystals degrade under ultra-high vacuum (UHV, 10e-13 mbar), making it hard to do scanning tunnelling microscopy (STM) and spectroscopy on these particles. With high-angle annular dark-fi?eld scanning transmission electron microscopy (HAADF-STEM) we observed that the sharp corners of the cubic particles round off and the particles fuse together with neighbouring particles to form irregularly shaped crystals. The stability of the CsPbBr3 nanocrystals is further investigated by applying heat under ambient conditions. This revealed that the nanocrystals start to degrade at temperatures above 125 deg C. At 200 deg C the nanocrystals are completely destroyed and the luminescent properties are lost. Finally we study the self-assembly of these cubic shaped nanocrystals into supraparticles and the resulting optical properties of these large aggregates. Through HAADF-STEM measurements we are able to study the supraparticles and show that the constituent nanocrystals form a non-covalently bonded crystalline 3D array of nanocrystals upon self-assembly. Moreover, we perform electron tomography to study the entire threedimensional structure of the supraparticles, identifying all nanocrystal positions within the supraparticle. These positions are used to calculate the radial distribution function of the supraparticle, and compare this to theoretical models for several crystal structures and types of disorder. The formation of supraparticles was induced by increasing the concentration of the nanocrystal dispersion or by adding anti-solvent to the nanocrystals in dispersion. We were able to controllably grow supraparticles by addition of methylacetate. Over the time of one week these supraparticles become single crystals, in contrast to the supraparticles formed upon increasing the concentration, which where stable over months while gradually increasing in size. The optical properties of individual supraparticles were studied through confocal microscopy and in solution on an ensemble level with time-resolved emission spectroscopy (TRES). During the confocal measurements, a red-shift of ca. 10 meV was observed for the supraparticle spectrum compared to a monolayer of nanocrystals. A similar red-shift was observed in the TRES measurements for a dispersion containing supraparticles compared to a solution containing an equal amount of nanocrystals. The photoluminesence (PL) decay dynamics of both the nanocrystals and the supraparticles is modelled with a double log-normal distribution of decay rates, which gives proper ?fitting results over the full measured delay time of one microsecond. The long delay times at which emission is observed indicate that delayed emission processes take place. The PL quantum yield of the supraparticles in dispersion and in a thin fi?lm on a glass slide is respectively 46% and 26%, which is remarkably high for supraparticles. This can be attributed to the high quantum yield of ca. 90% of the constituent nanocrystals.