X‑ray Lithography on Perovskite Nanocrystals Films: From Patterning with Anion-Exchange Reactions to Enhanced Stability in Air and Water

Films of colloidal CsPbX₃ (X = I, Br or Cl) nanocrystals, prepared by solution drop-casting or spin-coating on a silicon substrate, were exposed to a low flux of X-rays from an X-ray photoelectron spectrometer source, causing intermolecular CC bonding of the organic ligands that coat the surface of the nanocrystals. This transformation of the ligand shell resulted in a greater stability of the film, which translated into the following features: (i) Insolubility of the exposed regions in organic solvents which caused instead complete dissolution of the unexposed regions. This enabled the fabrication of stable and strongly fluorescent patterns over millimeter scale areas. (ii) Inhibition of the irradiated regions toward halide anion exchange reactions, when the films were exposed either to halide anions in solution or to hydrohalic vapors. This feature was exploited to create patterned regions of different CsPbI_<i>x</i>Br_<i>y</i>Cl_<i>z</i> compositions, starting from a film with homogeneous CsPbX₃ composition. (iii) Resistance of the films to degradation caused by exposure to air and moisture, which represents one of the major drawbacks for the integration of these materials in devices. (iv) Stability of the film in water and biological buffer, which can open interesting perspectives for applications of halide perovskite nanocrystals in aqueous environments

Enhancing the Performance of CdSe/CdS Dot-in-Rod Light-Emitting Diodes via Surface Ligand Modification

The surface ligands on colloidal nanocrystals (NCs) play an important role in the performance of NC-based optoelectronic devices such as photovoltaic cells, photodetectors, and light-emitting diodes (LEDs). On one hand, the NC emission depends critically on the passivation of the surface to minimize trap states that can provide nonradiative recombination channels. On the other hand, the electrical properties of NC films are dominated by the ligands that constitute the barriers for charge transport from one NC to its neighbor. Therefore, surface modifications via ligand exchange have been employed to improve the conductance of NC films. However, in LEDs, such surface modifications are more critical because of their possible detrimental effects on the emission properties. In this work, we study the role of surface ligand modifications on the optical and electrical properties of CdSe/CdS dot-in-rods (DiRs) in films and investigate their performance in all-solution-processed LEDs. The DiR films maintain high photoluminescence quantum yield, around 40–50%, and their electroluminescence in the LED preserves the excellent color purity of the photoluminescence. In the LEDs, the ligand exchange boosted the luminance, reaching a fourfold increase from 2200 cd/m² for native surfactants to 8500 cd/m² for the exchanged aminoethanethiol (AET) ligands. Moreover, the efficiency roll-off, operational stability, and shelf life are significantly improved, and the external quantum efficiency is modestly increased from 5.1 to 5.4%. We relate these improvements to the increased conductivity of the emissive layer and to the better charge balance of the electrically injected carriers. In this respect, we performed ultraviolet photoelectron spectroscopy (UPS) to obtain a deeper insight into the band alignment of the LED structure. The UPS data confirm similar flat-band offsets of the emitting layer to the electron- and hole-transport layers in the case of AET ligands, which translates to more symmetric barriers for charge injection of electrons and holes. Furthermore, the change in solubility of the NCs induced by the ligand exchange allows for a layer-by-layer deposition process of the DiR films, which yields excellent homogeneity and good thickness control and enables the fabrication of all the LED layers (except for cathode and anode) by spin-coating

Carbodiimide/NHS Derivatization of COOH-Terminated SAMs: Activation or Byproduct Formation?

COOH-terminated self-assembled monolayers (SAMs) are widely used in biosensor technology to bind different amine-containing biomolecules. A covalent amide bond, however, can be achieved only if the carboxylic acids are activated. This activation process usually consists of forming an <i>N</i>-hydroxysuccinimidyl ester (NHS-ester) by consecutively reacting carboxylic acids with a carbodiimide and NHS. Though many papers report using this method,− the experimental conditions vary greatly between them and chemical characterization at this stage is often omitted. Evidence of an efficient activation is therefore rarely shown. Furthermore, recent publications− have highlighted the complexity of this process, with the possible formation of different byproducts. In this paper, we have conducted a study on NHS activation under different conditions with chemical characterization by polarization-modulation infrared reflection–absorption spectroscopy (PM-IRRAS) and time-of-flight secondary ion mass spectroscopy (ToF-SIMS). Our results indicate that the nature of the solvent and carbodiimide and the reactant concentrations play crucial roles in activation kinetics and efficiency

Changing the Dimensionality of Cesium Lead Bromide Nanocrystals by Reversible Postsynthesis Transformations with Amines

Changing the Dimensionality of Cesium Lead Bromide Nanocrystals by Reversible Postsynthesis Transformations with Amine

Postsynthesis Transformation of Insulating Cs₄PbBr₆ Nanocrystals into Bright Perovskite CsPbBr₃ through Physical and Chemical Extraction of CsBr

Perovskite-related Cs₄PbBr₆ nanocrystals present a “zero-dimensional” crystalline structure where adjacent [PbBr₆]^4– octahedra do not share any corners. We show in this work that these nanocrystals can be converted into “three-dimensional” CsPbBr₃ perovskites by extraction of CsBr. This conversion drastically changes the optoelectronic properties of the nanocrystals that become highly photoluminescent. The extraction of CsBr can be achieved either by thermal annealing (physical approach) or by chemical reaction with Prussian Blue (chemical approach). The former approach can be simply carried out on a dried film without addition of any chemicals but does not yield a full transformation. Instead, reaction with Prussian Blue in solution achieves a full transformation into the perovskite phase. This transformation was also verified on the iodide counterpart (Cs₄PbI₆)

Antibacterial Melamine Foams Decorated with <i>in Situ</i> Synthesized Silver Nanoparticles

A new and straightforward single-step route to decorate melamine foams with silver nanoparticles (ME/Ag) is proposed. Uniform coatings of silver nanoparticles with diameters less than 10 nm are formed <i>in situ</i> directly on the struts surface of the foams, after their dipping in an AgNO₃ solution. We prove that the nanoparticles are stably adhered on the foams, and that their amount can be directly controlled by the concentration of the AgNO₃ solution and the dipping time. Following this production route, ME/Ag foams can be obtained with silver content ranging between 0.2 and 18.6 wt % and excellent antibacterial performance, making them appropriate for various applications. Herein we explore the possibility to use them as antibacterial filters for water treatment, proving that they are able to remove completely <i>Escherichia coli</i> bacteria from water when filtered at flow rates up to 100 mL/h·cm² due to the release of less than 1 ppm of Ag⁺ ions by the foams. No bacterial regrowth was observed after further dilution of the treated water, to arrive below the safety threshold of Ag⁺ for drinking water (0.1 ppm), demonstrating the excellent bactericide performance of the ME/Ag filters

Cu₂Se and Cu Nanocrystals as Local Sources of Copper in Thermally Activated <i>In Situ</i> Cation Exchange

Among the different synthesis approaches to colloidal nanocrystals, a recently developed toolkit is represented by cation exchange reactions, where the use of template nanocrystals gives access to materials that would be hardly attainable <i>via</i> direct synthesis. Besides, postsynthetic treatments, such as thermally activated solid-state reactions, represent a further flourishing route to promote finely controlled cation exchange. Here, we report that, upon <i>in situ</i> heating in a transmission electron microscope, Cu₂Se or Cu nanocrystals deposited on an amorphous solid substrate undergo partial loss of Cu atoms, which are then engaged in local cation exchange reactions with Cu “acceptor” phases represented by rod- and wire-shaped CdSe nanocrystals. This thermal treatment slowly transforms the initial CdSe nanocrystals into Cu_2–<i>x</i>Se nanocrystals, through the complete sublimation of Cd and the partial sublimation of Se atoms. Both Cu “donor” and “acceptor” particles were not always in direct contact with each other; hence, the gradual transfer of Cu species from Cu₂Se or metallic Cu to CdSe nanocrystals was mediated by the substrate and depended on the distance between the donor and acceptor nanostructures. Differently from what happens in the comparably faster cation exchange reactions performed in liquid solution, this study shows that slow cation exchange reactions can be performed at the solid state and helps to shed light on the intermediate steps involved in such reactions