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In this work, we report the hot-injection synthesis of Cs 3 ZnCl 5 colloidal nanocrystals (NCs) with tunable amounts of Cu + and Mn 2+ substituent cations. All the samples had a rodlike morphology, with a diameter of ∼14 nm and a length of ∼30-100 nm. Alloying did not alter the crystal structure of the host Cs 3 ZnCl 5 NCs, and Cu ions were mainly introduced in the oxidation state +1 according to X-ray photoelectron and electron paramagnetic resonance spectroscopies. The spectroscopic analysis of unalloyed, Cu-alloyed, Mn-alloyed, and Cu, Mn coalloyed NCs indicated that (i) the Cs 3 ZnCl 5 NCs have a large band gap of ∼5.35 eV; (ii) Cu(I) aliovalent alloying leads to an absorption shoulder/peak at ∼4.8 eV and cyan photoluminescence (PL) peaked at 2.50 eV; (iii) Mn(II) isovalent alloying leads to weak Mn PL, which intensifies remarkably in the coalloyed samples, prompted by an energy transfer (ET) process between the Cu and Mn centers, favored by the overlap between the lowest ( 6 A 1 → 4 T 1 ) transition for tetrahedrally coordinated Mn 2+ and the PL profile from Cu(I) species in the Cs 3 ZnCl 5 NCs. The efficiency of this ET process reaches a value of 61% for the sample with the highest extent of Mn alloying. The PL quantum yield (QY) values in these Cu, Mn coalloyed NCs are lower at higher Mn contents. The analysis of the Mn PL dynamics in these samples indicates that this PL drop stems from inter-Mn exciton migration, which increases the likelihood of trapping in defect sites, in agreement with previous studies., Competing Interests: The authors declare no competing financial interest., (© 2022 The Authors. Published by American Chemical Society.)

We report here the synthesis of undoped and Cu-doped Cs 2 ZnCl 4 nanocrystals (NCs) in which we could tune the concentration of Cu from 0.7 to 7.5%. Cs 2 ZnCl 4 has a wide band gap (4.8 eV), and its crystal structure is composed of isolated ZnCl 4 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 Cs 2 ZnCl 4 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 Cs 2 ZnCl 4 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., Competing Interests: The authors declare no competing financial interest.

Partial Hg 2+ → Cd 2+ cation exchange (CE) reactions were exploited to transform colloidal CdTe nanocrystals (NCs, 4-6 nm in size) into CdTe@HgTe core@shell nanostructures. This was achieved by working under a slow CE rate, which limited the exchange to the surface of the CdTe NCs. In such nanostructures, when annealed at mild temperatures (as low as 200 °C), the HgTe shell sublimated or melted and the NCs sintered together, with the concomitant desorption of their surface ligands. At the end of this process, the annealed samples consisted of ligand-free CdTe sintered films containing an amount of Hg 2+ that was much lower than that of the starting CdTe@HgTe NCs. For example, the CdTe@HgTe NCs that initially contained 10% of Hg 2+ , after being annealed at 200 °C were transformed to CdTe sintered films containing only traces of Hg 2+ (less than 1%). This procedure was then used to fabricate a proof-of-concept CdTe-based photodetector exhibiting a photoresponse of up to 0.5 A/W and a detectivity of ca. 9 × 10 4 Jones under blue light illumination. Our strategy suggests that CE protocols might be exploited to lower the overall costs of production of CdTe thin films employed in photovoltaic technology, which are currently fabricated at high temperatures (above 350 °C), using post-process ligand-stripping steps., Competing Interests: The authors declare no competing financial interest.

In this work, we demonstrate that a preferential Ga-for-Zn cation exchange is responsible for the increase in photoluminescence that is observed when gallium oleate is added to InZnP alloy QDs. By exposing InZnP QDs with varying Zn/In ratios to gallium oleate and monitoring their optical properties, composition, and size, we conclude that Ga 3+ preferentially replaces Zn 2+ , leading to the formation of InZnP/InGaP core/graded-shell QDs. This cation exchange reaction results in a large increase of the QD photoluminescence, but only for InZnP QDs with Zn/In ≥ 0.5. For InP QDs that do not contain zinc, Ga is most likely incorporated only on the quantum dot surface, and a PL enhancement is not observed. After further growth of a GaP shell and a lattice-matched ZnSeS outer shell, the cation-exchanged InZnP/InGaP QDs continue to exhibit superior PL QY (over 70%) and stability under long-term illumination (840 h, 5 weeks) compared to InZnP cores with the same shells. These results provide important mechanistic insights into recent improvements in InP-based QDs for luminescent applications.

Synthesis approaches to colloidal Cu 3 P nanocrystals (NCs) have been recently developed, and their optical absorption features in the near-infrared (NIR) have been interpreted as arising from a localized surface plasmon resonance (LSPR). Our pump-probe measurements on platelet-shaped Cu 3- x P NCs corroborate the plasmonic character of this absorption. In accordance with studies on crystal structure analysis of Cu 3 P dating back to the 1970s, our density functional calculations indicate that this material is substoichiometric in copper, since the energy of formation of Cu vacancies in certain crystallographic sites is negative, that is, they are thermodynamically favored. Also, thermoelectric measurements point to a p-type behavior of the majority carriers from films of Cu 3- x P NCs. It is likely that both the LSPR and the p-type character of our Cu 3- x P NCs arise from the presence of a large number of Cu vacancies in such NCs. Motivated by the presence of Cu vacancies that facilitate the ion diffusion, we have additionally exploited Cu 3- x P NCs as a starting material on which to probe cation exchange reactions. We demonstrate here that Cu 3- x P NCs can be easily cation-exchanged to hexagonal wurtzite InP NCs, with preservation of the anion framework (the anion framework in Cu 3- x P is very close to that of wurtzite InP). Intermediate steps in this reaction are represented by Cu 3- x P/InP heterostructures, as a consequence of the fact that the exchange between Cu + and In 3+ ions starts from the peripheral corners of each NC and gradually evolves toward the center. The feasibility of this transformation makes Cu 3- x P NCs an interesting material platform from which to access other metal phosphides by cation exchange.