We report the one-pot synthesis of colloidal Mn-doped cesium lead halide (CsPbX 3 ) perovskite nanocrystals and efficient intraparticle energy transfer between the exciton and dopant ions resulting in intense sensitized Mn luminescence. Mn-doped CsPbCl 3 and CsPb(Cl/Br) 3 nanocrystals maintained the same lattice structure and crystallinity as their undoped counterparts with nearly identical lattice parameters at ∼0.2% doping concentrations and no signature of phase separation. The strong sensitized luminescence from d−d transition of Mn 2+ ions upon band-edge excitation of the CsPbX 3 host is indicative of sufficiently strong exchange coupling between the charge carriers of the host and dopant d electrons mediating the energy transfer, essential for obtaining unique properties of magnetically doped quantum dots. Highly homogeneous spectral characteristics of Mn luminescence from an ensemble of Mn-doped CsPbX 3 nanocrystals and well-defined electron paramagnetic resonance spectra of Mn 2+ in host CsPbX 3 nanocrystal lattices suggest relatively uniform doping sites, likely from substitutional doping at Pb 2+ . These observations indicate that CsPbX 3 nanocrystals, possessing many superior optical and electronic characteristics, can be utilized as a new platform for magnetically doped quantum dots expanding the range of optical, electronic, and magnetic functionality.

We have designed a facile synthetic strategy for the selective deposition of Au metal on all-inorganic CsPbBr perovskite nanocrystals that includes the addition of PbBr salt along with AuBr salt. PbBr is necessary because the addition of Au to solutions of CsPbBr nanocrystals otherwise results in the exchange of Au ions from solution with Pb cations within the nanocrystal lattice to produce CsAuAuBr nanocrystals with a tetragonal crystal structure and a band gap of about 1.6 eV, in addition to Au metal deposition. Including excess Pb ions in solution prevents the exchange reaction. Au metal deposits on the surface of the nanocrystals to produce the Au-CsPbBr heterostructure nanoparticles with an Au particle diameter determined by the Au ion concentration. Fluorescence quenching caused by Au deposition monotonically increases with deposition size, but the fluorescence quantum yield (QY) is significantly greater than if any cation exchange has occurred. An optimized synthesis can produce Au-CsPbBr nanoparticles with 70% QY and no evidence of cation exchange.

CsPbBr3 nanoparticles that have been treated with NH4SCN to produce essentially trap-free surfaces show an increase of one photon up-conversion quantum yield with little change to the up-conversion energy of activation, as estimated using an Arrhenius analysis. This suggests that mid-gap trap states constitute a loss pathway, and may not be integral to the one-photon excitation mechanism.

Lack of detailed understanding of the growth mechanism of CsPbBr 3 nanocrystals has hindered sophisticated morphological and chemical control of this important emerging optoelectronic material. Here, we have elucidated the growth mechanism by slowing the reaction kinetics. When 1bromohexane is used as an alternative halide source, bromide is slowly released into the reaction mixture, extending the reaction time from ∼3 s to greater than 20 min. This enables us to monitor the phase evolution of products over the course of reaction, revealing that CsBr is the initial species formed, followed by Cs 4 PbBr 6 , and finally CsPbBr 3 . Furthermore, formation of monodisperse CsBr nanocrystals is demonstrated in a bromide-deficient and lead-abundant solution. The CsBr can be transformed only into CsPbBr 3 nanocubes if additional bromide is added. Our results indicate a fundamentally different growth mechanism for CsPbBr 3 in comparison with more established semiconductor nanocrystal systems and reveal the critical role of the chemical availability of bromide for the growth reactions.

One photon up-conversion photoluminescence is an optical phenomenon whereby the thermal energy of a fluorescent material increases the energy of an emitted photon compared with the energy of the photon that was absorbed. When this occurs with near unity efficiency, the emitting material undergoes a net decrease in temperature, so-called optical cooling. Because the up-conversion mechanism is thermally activated, the yield of upconverted photoluminescence is also a reporter of the temperature of the emitter. Taking advantage of this optical signature, cesium lead trihalide nanocrystals are shown to cool during the up-conversion of 532 nm CW laser excitation. Raman thermometric analysis of a substrate on which the nanocrystals were deposited further verifies the decrease in the local environmental temperature by as much as 25 °C during optical pumping. This is the first demonstration of optical cooling driven by colloidal semiconductor nanocrystal up-conversion.