7 research outputs found
Xāray Lithography on Perovskite Nanocrystals Films: From Patterning with Anion-Exchange Reactions to Enhanced Stability in Air and Water
Films
of colloidal CsPbX<sub>3</sub> (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 Cī»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<sub><i>x</i></sub>Br<sub><i>y</i></sub>Cl<sub><i>z</i></sub> compositions, starting
from a film with homogeneous CsPbX<sub>3</sub> 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 40ā50%,
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/m<sup>2</sup> for native surfactants to 8500 cd/m<sup>2</sup> 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,ā 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 publicationsā 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 reflectionāabsorption
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 Cs<sub>4</sub>PbBr<sub>6</sub> Nanocrystals into Bright Perovskite CsPbBr<sub>3</sub> through Physical and Chemical Extraction of CsBr
Perovskite-related
Cs<sub>4</sub>PbBr<sub>6</sub> nanocrystals
present a āzero-dimensionalā crystalline structure where
adjacent [PbBr<sub>6</sub>]<sup>4ā</sup> octahedra do not share
any corners. We show in this work that these nanocrystals can be converted
into āthree-dimensionalā CsPbBr<sub>3</sub> 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 (Cs<sub>4</sub>PbI<sub>6</sub>)
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 AgNO<sub>3</sub> 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 AgNO<sub>3</sub> 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/hĀ·cm<sup>2</sup> due to the release of less than 1 ppm of Ag<sup>+</sup> ions
by the foams. No bacterial regrowth was observed after further dilution
of the treated water, to arrive below the safety threshold of Ag<sup>+</sup> for drinking water (0.1 ppm), demonstrating the excellent
bactericide performance of the ME/Ag filters
Cu<sub>2</sub>Se 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, Cu<sub>2</sub>Se 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 āacceptorā phases
represented by rod- and wire-shaped CdSe nanocrystals. This thermal
treatment slowly transforms the initial CdSe nanocrystals into Cu<sub>2ā<i>x</i></sub>Se nanocrystals, through the complete
sublimation of Cd and the partial sublimation of Se atoms. Both Cu
ādonorā and āacceptorā particles were
not always in direct contact with each other; hence, the gradual transfer
of Cu species from Cu<sub>2</sub>Se 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