Herein, we report on a room-temperature anion exchange reaction of highly emitting, all-inorganic CsPbBr3 nanocrystals (NCs) taking place entirely in the solid state. A fast exchange from Br to I and Br to mixed Br/Cl without exertion of additional energy is observed within minutes to hours, taking place by immobilization of the perovskite NCs on pure potassium halide salts (KCl, KBr, and KI). Via adjustment of the halide ratios of the embedding salt matrix, the bright fluorescence of the CsPbX3 (X = Cl, Br, or I) NCs can be tuned over a wide spectral range (400–700 nm) while maintaining the initial photoluminescence quantum yields of ∼80% and narrow full widths at half-maximum. We found that combinations of different initial CsPbX3 NCs and KX matrices result in different final halogen contents of the NCs. This is explained with a host-lattice limiting exchange mechanism. The anion exchange rate can be accelerated by pressing the soft, NC-loaded salts under pressure of 2.2 GPa. Because of the "cold flow" behavior of the potassium salts during the pressing, a complete embedding of the NCs into transparent salt pellets is achieved. This strategy allows for an easy adjustment of the NC loading as well as the form and thickness of the resulting composite. An encapsulation of the NC−salt pellets with silicone yields robustness and stability of the embedded NCs under ambient conditions. The ease of handling and the superior stability make the resulting perovskite composite materials attractive for various photonic and optoelectronic applications as demonstrated in a proof-of-concept color-converting layer for a light−emitting diode.
Three-dimensional (3D) porous metal nanostructures have been a long sought-after class of materials due to their collective properties and widespread applications. In this study, we report on a facile and versatile strategy for the formation of Au hydrogel networks involving the dopamine-induced 3D assembly of Au nanoparticles. Following supercritical drying, the resulting Au aerogels exhibit high surface areas and porosity. They are all composed of porous nanowire networks reflecting in their diameters those of the original particles (5-6 nm) via electron microscopy. Furthermore, electrocatalytic tests were carried out in the oxidation of some small molecules with Au aerogels tailored by different functional groups. The beta-cyclodextrin-modified Au aerogel, with a host-guest effect, represents a unique class of porous metal materials of considerable interest and promising applications for electrocatalysis.
Colloidal nanocrystals of metals, semiconductors, and other functional materials can self-assemble into long-range ordered crystalline and quasicrystalline phases, but insulating organic surface ligands prevent the development of collective electronic states in ordered nanocrystal assemblies. We reversibly self-assembled colloidal nanocrystals of gold, platinum, nickel, lead sulfide, and lead selenide with conductive inorganic ligands into supercrystals exhibiting optical and electronic properties consistent with strong electronic coupling between the constituent nanocrystals. The phase behavior of charge-stabilized nanocrystals can be rationalized and navigated with phase diagrams computed for particles interacting through short-range attractive potentials. By finely tuning interparticle interactions, the assembly was directed either through one-step nucleation or nonclassical two-step nucleation pathways. In the latter case, the nucleation was preceded by the formation of two metastable colloidal fluids.
A size-selected series of water-soluble luminescent Ag–In–S (AIS) and core/shell AIS/ZnS quantum dots (QDs) were produced by a precipitation technique. Up to 10–11 fractions of size-selected AIS (AIS/ZnS) QDs emitting in a broad color range from deep-red to bluish-green were isolated with the photoluminescence (PL) quantum yield reaching 47% for intermediate fractions. The size of the isolated AIS (AIS/ZnS) QDs varied from ∼2 to ∼3.5 nm at a roughly constant chemical composition of the particles throughout the fractions as shown by the X-ray photoelectron spectroscopy. The decrease of the mean AIS QD size in consecutive fractions was accompanied by an increase of the structural QD imperfection/disorder as deduced from a notable Urbach absorption "tail" below the fundamental absorption edge. The Urbach energy increased from 90–100 meV for the largest QDs up to 350 meV for the smallest QDs, indicating a broadening of the distribution of sub-bandgap states. Both the Urbach energy and the PL bandwidth of the size-selected AIS QDs increased with QD size reduction from 3–4 to ∼2 nm, and a distinct correlation was observed between these parameters. A study of size-selected AIS and AIS/ZnS QDs by UV photoelectron spectroscopy on Au and FTO substrates revealed their valence band level EVB at ∼6.6 eV (on Au) and ∼7 eV (on FTO) pinned to the Fermi level of conductive substrates resulting in a masking of any possible size-dependence of the valence band edge position.
The n‐type semiconductor system Bi 2 Te 3 Bi 2 Se 3 is known as a low‐temperature thermoelectric material with a potentially high efficiency. Herein, a facile approach is reported to synthesize core/shell heterostructured Bi 2 Te 2 Se/Bi 2 Te 3 nanosheets (NSs) with lateral dimensions of 1–3 μm and thickness of about 50 nm. Bi 2 Te 3 and Bi 2 Se 3 , as well as heterostructured Bi 2 Te 2 Se/Bi 2 Te 3 NSs are obtained via colloidal synthesis. Heterostructured NSs show an inhomogeneous distribution of the chalcogen atoms forming selenium and tellurium‐rich layers across the NS thickness, resulting in a core/shell structure. Detailed morphological studies reveal that these structures contain nanosized pores. These features contribute to the overall thermoelectric properties of the material, inducing strong phonon scattering at grain boundaries in compacted solids. NSs are processed into nanostructured bulks through spark plasma sintering of dry powders to form a thermoelectric material with high power factor. Electrical characterization of our materials reveals a strong anisotropic behavior in consolidated pellets. It is further demonstrated that by simple thermal annealing, core/shell structure can be controllably transformed into alloyed one. Using this approach pellets with Bi 2 Te 2.55 Se 0.45 composition are obtained, which exhibit low thermal conductivity and high power factor for in‐plane direction with zT of 1.34 at 400 K.
