Prospective Chalcohalide Perovskites: Pursuing (and Failing) the Synthesis of CsBiSCl2 Nanocrystals.

Heavy pnictogen chalcohalides are often termed lead-free, perovskite-inspired materials. Despite theoretical predictions, incontrovertible experimental demonstrations of heavy pnictogen chalcohalides adopting a perovskite structure are lacking. Here we report our attempts to prepare CsBiSCl2 adopting a perovskite structure as colloidal nanocrystals. Synthesis of nanoscale materials can indeed rely on fast, nonequilibrium reactions and on large, eventually thermodynamically favorable surface energies, leading to the possibility of stabilizing kinetically trapped or metastable phases. However, we obtained no CsBiSCl2, but a mixture of nanocrystals of secondary phases, namely Cs3BiCl6 submicrometric polyhedra, Bi2S3 nanoscopic rods, and Cs3Bi2Cl9 nanoscopic dots, whose low polydispersity enabled an effective separation via size/shape selective precipitation. This work confirms that heavy pnictogen chalcohalides are hardly prone to adopting a perovskite structure. Nevertheless, chemistry at the nanoscale offers multiple possibilities for overcoming phase segregation and pursuing the synthesis of prospective mixed anion compound semiconductors.

Optical gas sensor based on the combination of a QD photoluminescent probe and a QD photodetector.

We report on a sensor architecture for detection of hazardous gases. The proposed device is based on the integration of a solid-state quantum dot (QD) photoluminescent probe with a QD photodetector on the same substrate. The effectiveness of the approach is demonstrated by developing a compact optical sensor for trace detection of explosives in air. The proposed architecture is very simple and consists of a silicon substrate with both surfaces coated with QD films. The upper layer acts as photoluminescent probe, pumped by a blue LED. The change of photoluminescence intensity associated to the interaction between the QDs and the target analyte is measured by the QD photodetector fabricated on the opposite side of the substrate. The sensor is mounted into a small chamber provided with the LED and the front-end electronics. The device is characterized by using nitrobenzene as representative nitroaromatic compound. Extremely low concentrations (down to 0.1 ppm) can be detected by the proposed device, with a theoretical detection limit estimated to be as low as 2 ppb. Results are repeatable and no ageing effect is observed over a 70 d period. The proposed architecture may provide a promising solution for explosive detection in air as well as other sensing applications, thanks to its sensitivity, simple fabrication process, practical usability and cost effectiveness.

Colloidal Bismuth Chalcohalide Nanocrystals.

Here we present a colloidal approach to synthesize bismuth chalcohalide nanocrystals (BiEX NCs, in which E=S, Se and X=Cl, Br, I). Our method yields orthorhombic elongated BiEX NCs, with BiSCl crystallizing in a previously unknown polymorph. The BiEX NCs display a composition-dependent band gap spanning the visible spectral range and absorption coefficients exceeding 105  cm-1 . The BiEX NCs show chemical stability at standard laboratory conditions and form colloidal inks in different solvents. These features enable the solution processing of the NCs into robust solid films yielding stable photoelectrochemical current densities under solar-simulated irradiation. Overall, our versatile synthetic protocol may prove valuable in accessing colloidal metal chalcohalide nanomaterials at large and contributes to establish metal chalcohalides as a promising complement to metal chalcogenides and halides for applied nanotechnology.

Stable Ligand Coordination at the Surface of Colloidal CsPbBr3 Nanocrystals.

Ruling over the surface chemistry of metal halide perovskite nanocrystals (NCs) is crucial to access reliable luminophores. Here, we provide an atomic-level description of the surface of colloidal CsPbBr3 NCs, achieving an effective passivation strategy that leads to near-unity photoluminescence quantum yield. To this end, we used two different types of CsPbBr3 NCs, which had been synthesized with an outer shell of either oleylammonium bromide ion pairs or Cs-oleate complexes. We perturbed the dynamic equilibria at the NCs' surface with ligands from a comprehensive library, including amines (and their conjugated acids) with different basicities, chain lengths, and steric encumbrances. We demonstrate that control of both ligand binding affinity and ligand-to-NC molar ratio is essential to attain thermodynamically stable coordination of the NC surface. We thus present a reliable protocol for managing the surface chemistry of colloidal CsPbBr3 NCs and for selectively addressing their ligand-induced morphological (and structural) transformations.

The dynamic surface chemistry of colloidal metal chalcogenide quantum dots.

The chemical species (ligands) at the surface of colloidal inorganic semiconductor nanocrystals (QDs) mediate their interactions with the surroundings. The solvation of the QDs reflects a subtle interplay between ligand-solvent and ligand-ligand interactions, which eventually compete with the coordination of the ligands at the QD surface. The QD surface coordination and solvation are indeed fundamental to preserve their optoelectronic properties and to foster the effective application of QD-based inks and nanocomposites. Here we investigate such ligand interactions by exploiting diffusion ordered NMR spectroscopy (DOSY), which is suggested as an essential complement to spectral line width analysis. To this end, we use colloidal metal chalcogenide (CdS, CdSe, and PbS) QDs with (metal-)oleate ligands at their surface in several solvents exhibiting different viscosities and polarities. We demonstrate that the ligand shell is dynamically bound to the metal chalcogenide QDs, and is thus in equilibrium between the QD surface and the surrounding solvent. Such dynamic equilibria depend on ligand-solvent interactions, which are more prominent in aliphatic, rather polar solvents that favor the solvation of the ligands and, as a consequence, their displacement from the QD surface. In addition, the ligand-ligand interactions, which are more relevant for larger QDs, contribute to the stabilization of the ligand bonding at the QD surface.