We report here the synthesis of undoped and Cu-doped Cs₂ZnCl₄ nanocrystals (NCs) in which we could tune the concentration of Cu from 0.7 to 7.5%. Cs₂ZnCl₄ has a wide band gap (4.8 eV), and its crystal structure is composed of isolated ZnCl₄ tetrahedra surrounded by Cs⁺ cations. According to our electron paramagnetic resonance analysis, in 0.7 and 2.1% Cu-doped NCs the Cu ions were present in the +1 oxidation state only, while in NCs at higher Cu concentrations we could detect Cu(II) ions (isovalently substituting the Zn(II) ions). The undoped Cs₂ZnCl₄ NCs were non emissive, while the Cu-doped samples had a bright intragap photoluminescence (PL) at ∼2.6 eV mediated by band-edge absorption. Interestingly, the PL quantum yield was maximum (∼55%) for the samples with a low Cu concentration ([Cu] ≤ 2.1%), and it systematically decreased when further increasing the concentration of Cu, reaching 15% for the NCs with the highest doping level ([Cu] = 7.5%). The same (∼2.55 eV) emission band was detected under X-ray excitation. Our density functional theory calculations indicated that the PL emission could be ascribed only to Cu(I) ions: these ions promote the formation of trapped excitons, through which an efficient emission takes place. Overall, these Cu-doped Cs₂ZnCl₄ NCs, with their high photo- and radio-luminescence emission in the blue spectral region that is free from reabsorption, are particularly suitable for applications in ionizing radiation detection.

We developed a hot-injection synthesis approach to prepare colloidal double perovskite Cs₂NaBiCl₆ nanocrystals (NCs) having a cubic shape and average size around 10 nm. We also examined the effects of replacing Na⁺ with Ag⁺ cations inside such NCs by preparing and characterizing a series of Cs₂Na_1-xAg_xBiCl₆ alloy NC samples with x ranging from 0 to 1. While pure Cs₂NaBiCl₆ NCs did not show any photoluminescence (PL) emission, Cs₂Na_1-xAg_xBiCl₆ NCs featured broad PL bands peaked at around 690nm and characterized by large Stokes shifts (≥1.5 eV). Interestingly, the PL quantum yield of these compounds was maximized for low amounts of Ag⁺ incorporated, reaching >3% for the Cs₂Na_0.95Ag_0.05BiCl₆ composition. Our density functional theory calculations coupled with spectroscopic investigations revealed that Cs₂Na_1-xAg_xBiCl₆ NCs are characterized by a complex photophysics stem-ming from the interplay of: i) the radiative recombination via trapped excitons localized in spatially connected AgCl₆-BiCl₆ octahedra; ii) surface traps, located on undercoordinated surface Bi centers, behaving as phonon-assisted nonradiative decay channel; iii) a thermal equilibrium between trapping and detrapping channels. Ultimately, we found that the system that showed the most efficient radiative emission was also the one having efficient concomitant non-radiative pathways, thus exhibiting an overall reduced PL performance. These results offer insights into developing double perovskite NCs with enhanced optoelectronic efficiency.

A simple heating-up colloidal approach has been developed to synthesize quaternary Cu–In–Zn–S (CIZS) nanocystals with different emission colors, which can be tuned by varying the Cu : In : Zn precursor ratios. The as-obtained products have a quasi-triangular shape with a small size distribution, and their crystal phase can be varied from cubic zinc-blende to hexagonal wurtzite structure with an increase of Cu stoichiometry. The inductively coupled plasma optical emission spectrometry (ICP-OES) results reveal that the as-obtained CIZS nanocrystals are Cu-deficient, and the Cu-vacancies have a significant effect on their optical properties. Moreover, the photoluminescence (PL) spectra of the CIZS nanocrystals recorded at different growth periods indicate that the partial cation exchange of Cu⁺ and In³⁺ with Zn²⁺ takes charge of the growth process. After coating a ZnS shell over the CIZS nanocrystals, the PL quantum yield (PLQY) is improved greatly and a blue-shift of the PL peak is observed as compared to that of plain CIZS nanocrystals. More interestingly, the highly luminescent CIZS/ZnS core/shell nanocrystals with different colors have been incorporated into poly(dimethylsiloxane) (PDMS) to be fabricated on an engraved “BJTU” glass substrate, and the as-obtained PDMS membranes exhibit a bright emission under UV light, which also show good stability when they are bent arbitrarily or immersed in water. This heating-up method has also been extended to the preparation of five-component Cu–Ag–In–Zn–S and Cu–Mn–In–Zn–S nanocrystals, which possess a cubic zinc-blende crystal structure and have bright luminescence. Our work may shed light on the synthesis of multi-component semiconductor nanocrystals.