Plenary Session, Award Lecture
Nano- and single-crystals of lead halide perovskites: from bright light emission to hard radiation detection
M. V. Kovalenko1,2
1ETH Zürich, Department of Chemistry and Applied Biosciences, CH-8093, Zurich, Switzerland, 2Empa-Swiss Federal Laboratories for Materials Science and Technology, CH-8600, Dübendorf, Switzerland
Chemically synthesized inorganic nanocrystals (NCs) are considered to be promising building blocks for a broad spectrum of applications including electronic, thermoelectric, and photovoltaic devices. We have synthesized monodisperse colloidal nanocubes (4-15nm edge lengths) of fully inorganic cesium lead halide perovskites (CsPbX3, X=Cl, Br, and I or mixed halide systems Cl/Br and Br/I) using inexpensive commercial precursors [1]. Their bandgap energies and emission spectra are readily tunable over the entire visible spectral region of 410-700nm. The photoluminescence of CsPbX3 NCs is characterized by narrow emission line-widths of 12-42nm, wide color gamut covering up to 140% of the NTSC color standard, high quantum yields of up to 90% and radiative lifetimes in the range of 4-29ns. Identical synthesis methodology is perfectly suited also for hybrid perovskites CH3NH3PbX3 [2].
Post-synthestic chemical transformations of colloidal NCs, such as ion-exchange reactions, provide an avenue to compositional fine tuning or to otherwise inaccessible materials and morphologies. While cation-exchange is facile and commonplace, anion-exchange reactions have not received substantial deployment. Here we present fast, low-temperature, deliberately partial or complete anion-exchange in CsPbX3 NCs [3]. By adjusting the halide ratios in the colloidal NC solution, the bright photoluminescence can be tuned over the entire visible spectral region (410-700 nm). Furthermore, fast inter-NC anion-exchange is demonstrated as well, leading to uniform CsPb(Cl/Br)3 orCsPb(Br/I)3 compositions simply by mixing CsPbCl3, CsPbBr3 and CsPbI3 NCs in appropriate ratios.
We also present low-threshold amplified spontaneous emission and lasing from CsPbX3 NCs [4]. We find that room-temperature optical amplification can be obtained in the entire visible spectral range (440-700nm) with low pump thresholds down to 5±1 µJ cm-2 and high values of modal net gain of at least 450±30cm-1. Two kinds of lasing modes are successfully realized: whispering gallery mode lasing using silica microspheres as high-finesse resonators, conformally coated with CsPbX3 NCs, and random lasing in films of CsPbX3 NCs.
Here we also demonstrate that 0.5-1 centimeter large, solution-grown single crystals of MAPbI3 can serve as inexpensive, operating at ambient temperatures solid-state gamma detectors (e.g. for direct sensing of photons with energies as high as mega-electron-volts, MeV) [5]. Such possibility arises from extremely high room-temperature mobility(µ)-lifetime(t) product of 10-2 cm2 V-1, low dark carrier density 109 - 1011cm-3 and low density of charge traps 3 × 1010 cm–3 , and high absorptivity of hard radiation by lead and iodine atoms.

[1] L. Protesescu et al. Nano Letters 2015, 15, 3692–3696
[2] O. Vybornyi et al. Nanoscale 2016, DOI: 10.1039/C5NR06890H
[3] G. Nedelcu et al. Nano Letters 2015, 15, 5635–5640
[4] S. Yakunin et al. Nature Communications 2015, 9, 8056