Insertion/Ejection of Dopant Ions in Composition Tunable Semiconductor Nanocrystals

A new doping strategy to incorporate transition metal dopant ions in semiconductor hosts has been reported here where dopant ions are incorporated in the host lattice following a feasible cation exchange process. Composition variable CdxZn1–xSe alloy nanocrystals have been chosen as an appropriate host which is formed by the cation exchange reaction of Cd with Zn ions in ZnSe nanocrystals. Concomitant insertion and ejection of dopant Cu ions are studied. It has been observed that the composition of Zn and Cd in the nanocrystals decides how much dopant Cu ions will be retained in the host lattice. The entire doping process with evolution, wide range tuning, and quenching of dopant emission as a consequence of inclusion/exclusion of dopant ions is in situ measured during the alloying process. From the retained and expelled dopant amounts, their compatibility in the host lattice is correlated

Single-Precursor, One-Pot Versatile Synthesis under near Ambient Conditions of Tunable, Single and Dual Band Fluorescing Metal Sulfide Nanoparticles

We present a simple and versatile method for the synthesis of high-quality size-controlled metal sulfide nanoparticles. A single compound (metal xanthate) is the precursor. A Lewis-base solvent is used to achieve a low reaction temperature of 50−150 °C, usually in air. Demonstrated with CdS, the precise control over the particle size (by regulating the temperature or the concentration) enables tuning the absorption and emission spectra of the particles. We also can control the relative intensity of the narrow (30−35 nm wide) excitonic emission (tunable in the range 430−480 nm with ∼2% fluorescence quantum efficiency) and the broad emission associated with deep surface traps (in the range 550−700 nm). Using the same precursor CdS/ZnS core/shell particles are produced with a high PL yield (∼14%)

Efficient and Color-Tunable Mn-Doped ZnSe Nanocrystal Emitters: Control of Optical Performance via Greener Synthetic Chemistry

Formation of Mn-doped ZnSe quantum dots (Mn:ZnSe d-dots) using nucleation-doping strategy was studied systematically and optimized through greener approaches. The resulting d-dots were with high (∼50%) photoluminescence (PL) quantum yield (QY), which was achieved by the controlled formation of small-sized MnSe nanoclusters as the core and a diffused interface between the nanocluster core and the ZnSe overcoating layers. Synthesis of the d-dots under high temperatures (240−300 °C) was achieved by varying the structure of the metal carboxylate precursors, concentration of the inhibitors, free fatty acid, and concentration of the activation reagents, fatty amines. Highly emissive d-dots synthesized under desired conditions were found to be extremely stable upon thermal treatment up to the boiling point of the solvent (about 300 °C), which was quantitatively studied using in situ measurements. The PL peak of the d-dots was controllably tuned in a surprisingly large optical window, from 565 to 610 nm. These highly emissive and stable d-dots possess characteristics of practical emissive materials, especially for applications requiring high power, high concentration of emitters, and under tough conditions

Single-Precursor, One-Pot Versatile Synthesis under near Ambient Conditions of Tunable, Single and Dual Band Fluorescing Metal Sulfide Nanoparticles

We present a simple and versatile method for the synthesis of high-quality size-controlled metal sulfide nanoparticles. A single compound (metal xanthate) is the precursor. A Lewis-base solvent is used to achieve a low reaction temperature of 50−150 °C, usually in air. Demonstrated with CdS, the precise control over the particle size (by regulating the temperature or the concentration) enables tuning the absorption and emission spectra of the particles. We also can control the relative intensity of the narrow (30−35 nm wide) excitonic emission (tunable in the range 430−480 nm with ∼2% fluorescence quantum efficiency) and the broad emission associated with deep surface traps (in the range 550−700 nm). Using the same precursor CdS/ZnS core/shell particles are produced with a high PL yield (∼14%)

Impact of Thermal Annealing on Facet-Directed Epitaxial and Decorated Nonepitaxial Pd-CsPbBr₃ Nanocrystal Heterostructures

Halide perovskite nanocrystals are the subject of in-depth research because of their bright color-tunable optical emissions. Synthesis of these nanocrystals and their phase stability in different halide compositions and reaction environments are also extensively studied. However, going beyond these nanocrystals, the chemistry of epitaxial growth of metallic crystals on selective facets of ionic crystals has not been largely established yet. For the formation of heteronucleations as well as retaining the epitaxial relation at the junction, a high reaction temperature remains a key factor. However, with an increase in temperature, there are possibilities for cross nucleations and also detachment of metal particles from the host. Keeping these in mind, herein, the thermal impacts on different structural combinations of Pd-CsPbBr3 nanocrystal heterostructures are reported. Following the high-temperature reaction protocol, two types of heterostructures, facet-directive epitaxial and nonepitaxial decorated Pd-CsPbBr3, are first obtained, and these are further explored to study the annealing impact. In these cases, smaller size Pd particles are first grown epitaxially and form a one-to-one heterostructure, and then with time, Pd cubes are observed to be decorated randomly. However, the formation and growth of Pd particles remained sensitive to both the reaction temperature and the time as these did not grow to beyond a certain size in the heterostructure, rather facilitated cross nucleation formation. This provides strong support that annealing does not hamper the epitaxial junction and it only helps the successive growth of nonepitaxially connected metal nanocrystals

Colloidal CdSe Quantum Wires by Oriented Attachment

We report here a relatively low temperature (100−180 °C) synthetic route to high-quality and single-crystalline CdSe nanowires using air-stable and generic chemicals. The diameter of nanowires was controlled and varied in an exceptionally small size regime, between 1.5 and 6 nm. This was achieved by using alkylamines, a single type or a mixture of two different types of amines, with different chain lengths and varying the reaction temperature. The experimental results suggest the coexistence of two types of fragments in the prewire aggregates, known as “pearl-necklace” or “string-of-pearls” in the literature, which are loosely associated and chemically fused sections

Chemically Spiraling CsPbBr₃ Perovskite Nanorods

Light emitting lead halide perovskite nanocrystals are currently emerging as the workhorse in quantum dot research. Most of these reported nanocrystals are isotropic cubes or polyhedral; but anisotropic nanostructures with controlled anisotropic directions still remain a major challenge. For orthorhombic CsPbBr3, the 1D shaped nanostructures reported are linear and along either of the axial directions ⟨100⟩. In contrast, herein, spiral CsPbBr3 perovskite nanorods in the orthorhombic phase are reported with unusual anisotropy having (101) planes remaining perpendicular to the major axis [201]. While these nanorods are synthesized using the prelattice of orthorhombic Cs2CdBr4 with Pb­(II) diffusion, the spirality is controlled by manipulation of the compositions of alkylammonium ions in the reaction system which selectively dissolve some spiral facets of the nanorods. Further, as spirality varied with facet creation and elimination, these nanorods were explored as photocatalysts for CO2 reduction, and the evolution of methane was also found to be dependent on the depth of the spiral nanorods. The entire study demonstrates facet manipulation of complex nanorods, and these results suggest that even if perovskites are ionic in nature, their shape could be constructed by design with proper reaction manipulation

Subnanometer Thin β‑Indium Sulfide Nanosheets

Nanosheets are a peculiar kind of nanomaterials that are grown two-dimensionally over a micrometer in length and a few nanometers in thickness. Wide varieties of inorganic semiconductor nanosheets are already reported, but controlling the crystal growth and tuning their thickness within few atomic layers have not been yet explored. We investigate here the parameters that determine the thickness and the formation mechanism of subnanometer thin (two atomic layers) cubic indium sulfide (In₂S₃) nanosheets. Using appropriate reaction condition, the growth kinetics is monitored by controlling the decomposition rate of the single source precursor of In₂S₃ as a function of nucleation temperature. The variation in the thickness of the nanosheets along the polar [111] direction has been correlated with the rate of evolved H₂S gas, which in turn depends on the rate of the precursor decomposition. In addition, it has been observed that the thickness of the In₂S₃ nanosheets is related to the nucleation temperature

Chemically Sculpturing the Facets of CsPbBr₃ Perovskite Platelet Nanocrystals

The facet chemistry of lead halide perovskite nanocrystals is critically important for determining their shape and interface ligand binding. In colloidal nanocrystals, these are mostly controlled by adopting specific synthetic strategies with a selection of the appropriate reactants. However, using selected ligands, the surface of preformed nanocrystals can be reconstructed without altering the crystal phase and lattice structure of their core. This has been shown here for hexagonal-shaped orthorhombic CsPbBr3 platelet nanocrystals. When oleylammonium bromide was added to these postsynthesized platelets, all six edges and two planar facets are transformed from flat to wavy structures. With a variation in concentration, the crest-to-crest distance of these wavy platelets are also tuned. These became possible because of the oleylammonium ions, which changed the {200}, {012} and {020} facets of orthorhombic phase of CsPbBr3 to the more compatible {110} and {002} facets simply by surface atom dissolution. This was also observed for multisegmented platelets having multiple junctions and even for platelets having a size of more than 200 nm. While shape modulations in ionic halide perovskite nanocrystals still face synthetic challenges, these results of surface reconstruction provide strong evidence of the possibility of sculpturing surface facets and shape changes in these nanostructures