Zinc Chalcogenide Seed-Mediated Synthesis of CdSe Nanocrystals: Nails, Chesses and Tetrahedrons

Systematically shape-controlled synthesis of colloidal semiconductor nanocrystals (NCs) has attracted increasing attention recently for both fundamental and technological interest. The study on the synthesis of colloidal CdSe NCs has given rise to well-developed methods for producing diverse shapes such as rods, wires, cubs and discs. In the current study, we demonstrate the shape evolution of CdSe NCs by using a seed-mediated approach by control reaction temperature and injection methods. The synthesis utilizes small (2.0–3.0 nm) zinc chalcogenide NCs with zincblende structure as seeds for subsequent growth, which results in distinct shapes of nail-shaped, tetrahedron-shaped, chess piece-shaped, and Y-shaped CdSe NCs with high yield and good uniformity. The morphologies and crystal structures of the prepared CdSe NCs were well characterized by transmission electron microscopy (TEM), high resolution TEM, and X-ray diffraction measurements. This wide variation of shapes provides important information on the growth of CdSe NCs and promotes the shape-controlled synthesis of other NCs by seed-mediated synthetic method

Dot–Wire–Platelet–Cube: Step Growth and Structural Transformations in CsPbBr₃ Perovskite Nanocrystals

While the classical mechanism for the growth of colloidal chalcogenide nanocrystals is largely understood, fundamental insights for the growth of perovskite nanocrystals still remain elusive. Using nanoclusters of ∼0.6 nm diameter as monomers and growing to more than 25 nm in a single reaction, herein, the step growth process of perovskite CsPbBr₃ nanocrystals is reported. This is performed with a step-rise of the reaction temperature with correlating annealing time. The growth is so precise that ∼0.6 nm (nearly one unit cell) increments were successively monitored in parallel with the conversion of clusters to nanowires and then to thickness tunable platelets and finally to size-tunable cube-shaped nanostructures. The entire reaction was monitored optically and microscopically, and their step growths were correlated. From these observations, the possible growth mechanism for perovskite nanocrystals along with their shape transformations was proposed

Correlation of CdS Nanocrystal Formation with Elemental Sulfur Activation and Its Implication in Synthetic Development

Formation of CdS nanocrystals in the classic approach (with octadecene (ODE) as the solvent and elemental sulfur and cadmium carboxylate as the precursors) was found to be kinetically dependent on reduction of elemental sulfur by ODE, which possessed a critical temperature (∼180 °C). After elemental sulfur was activated by ODE, the formation reaction of CdS followed closely. 2-tetradecylthiophene from the activation of S by ODE and fatty acids from the formation reaction of CdS were found to be the only soluble side products. The overall reaction stoichiometry further suggested that oxidation of each ODE molecule generated two molecules of H₂S, which in turn reacted with two molecules of cadmium carboxylate molecules to yield two CdS molecular units and four molecules of fatty acids. In comparison to alkanes, octadecene was found to be substantially more active as a reductant for elemental sulfur. To the best of our knowledge, this is the first example of quantitative correlation between chemical reactions and formation of high-quality nanocrystals under synthetic conditions. To demonstrate the importance of such discovery, we designed two independent and simplified synthetic approaches for synthesis of CdS nanocrystals. One approach with its reaction temperature at the critical temperature of S activation (180 °C) used the same reactant composition as the classic approach but without any hot injection. The other approach performed at an ordinary laboratory temperature (≤100 °C) and in a common organic solvent (toluene) was achieved by addition of fatty amine as activation reagent of elemental sulfur

Turn-on Fluorescent InP Nanoprobe for Detection of Cadmium Ions with High Selectivity and Sensitivity

We reported a “turn-on” fluorescent InP nanoprobe for detection of cadmium ions in hydrophobic and hydrophilic media. The method based on the turn-on fluorescence detection of cadmium ions has shown its high selectivity and sensitivity, which are independent of the pH of the tested samples. Also, this approach exhibits an immediate response to cadmium ions, and visualized detection of cadmium ions has further been demonstrated under a UV lamp

Turn-on Fluorescent InP Nanoprobe for Detection of Cadmium Ions with High Selectivity and Sensitivity

We reported a “turn-on” fluorescent InP nanoprobe for detection of cadmium ions in hydrophobic and hydrophilic media. The method based on the turn-on fluorescence detection of cadmium ions has shown its high selectivity and sensitivity, which are independent of the pH of the tested samples. Also, this approach exhibits an immediate response to cadmium ions, and visualized detection of cadmium ions has further been demonstrated under a UV lamp

Syntheses and Characterization of Nearly Monodispersed, Size-Tunable Silver Nanoparticles over a Wide Size Range of 7–200 nm by Tannic Acid Reduction

Nearly monodispersed spherical silver nanoparticles (Ag NPs) were synthesized by using tannic acid (TA) as both reductant and stabilizer in a 30 °C water bath. The size of the as-prepared Ag NPs could be tuned in a range of 7–66 nm by changing the molar ratio of TA to silver nitrate and pH of the reaction solutions. UV–vis spectra, TEM observations, and temporal evolution of the monomer concentrations for the reactions carried out at different experimental conditions showed that the improved size distribution and size tunability of the Ag NPs were mainly attributed to the use of TA, which could promote the balance of nucleation and growth processes of the NPs effectively. The size of the Ag NPs was extendable up to 200 nm in one-pot fashion by the multi-injection approach. The size-dependent surface-enhanced Raman scattering (SERS) activity of the as-prepared Ag NPs was evaluated, and the NPs with size around 100 nm were identified to show a maximum enhanced factor of 3.6 × 10⁵. Moreover, the as-prepared TA-coated Ag NPs presented excellent colloidal stability compared to the conventional citrate-coated ones

Mapping the Space of Inorganic and Hybrid Halides and Their Optical Properties Using Mechanochemistry and First-Principles Calculations

Inorganic and hybrid metal halides (MHs) are a class of ionic compounds that attract growing interest due to their richness of structure, properties, and resulting applications. These are largely ionic in nature and hence dominantly follow solid-state synthesis reactions rather than the solution approach. Keeping the importance of these materials in mind, herein, combination reactions of compounds via mechanochemistry is considered as a universal synthetic approach for the synthesis of MHs, and a library of MHs, including all inorganic MHs, ternary (A–B–X) MHs, enormous number of quaternary MHs based on representative 10 double perovskites (A–B–B′–X), and most of the hybrid ones based on randomly selected 49 samples as representative from the 1300 ones, are reported. The fundamental structure–property relationships are well revealed, where most of the MHs exhibit bright photoluminescence and/or magnetic properties for a few materials. Hence, the adopted concept of material design and related with their crystal structure and material properties for such a large number of halide materials not only help in building a library but also provide fundamental guidance to develop new MH materials with selective optoelectronic and magnetic properties.

Large Scale Synthesis of Air Stable Precursors for the Preparation of High Quality Metal Arsenide and Phosphide Nanocrystals as Efficient Emitters Covering the Visible to Near Infrared Region

Large Scale Synthesis of Air Stable Precursors for the Preparation of High Quality Metal Arsenide and Phosphide Nanocrystals as Efficient Emitters Covering the Visible to Near Infrared Regio

Shape Control of Ternary Sulfide Nanocrystals

Synthesis of semiconductor nanocrystals with a definite shape is the foundation of their anisotropy properties investigation; however, it is more challenging in ternary metal sulfides than that of noble metal and binary sulfides. In this paper, we report a solvent polarity control strategy to prepare a family of ternary sulfide (Ag₃SbS₃) semiconductor nanocrystals with tunable polyhedral shapes. The crystal growth speed along different directions was confined by the capping effect of the polarity of solvents that was defined by reaction temperature. Crystal shape of Ag₃SbS₃ nanocrystals could be tailored as a sphere, hexagonal plate, and prism. A shape-controllable growth mechanism was analyzed based on the Bravais–Friedel–Donnay–Harker theory by taking crystal structure characteristics and the polarity of solvents into consideration. The semiconductor nanocrystals show a near value of the band gaps for different shaped samples and facet-dependent photocatalytic water-splitting activities, which may result from the discrimination of the terminal surface structure and binding energy of Sb and S for the three different shaped nanocrystals. Thus, we provide a new crystal shape tunable strategy for ternary sulfide nanocrystal synthesis, which is important for optimizing properties and applications of sulfide semiconductor nanocrystals

Ultrafast Carrier Dynamics and Hot Electron Extraction in Tetrapod-Shaped CdSe Nanocrystals

The ultrafast carrier dynamics and hot electron extraction in tetrapod-shaped CdSe nanocrystals was studied by femtosecond transient absorption (TA) spectroscopy. The carriers relaxation process from the higher electronic states (CB₂, CB₃₍₂₎, and CB₄) to the lowest electronic state (CB₁) was demonstrated to have a time constant of 1.04 ps, resulting from the spatial electron transfer from arms to a core. The lowest electronic state in the central core exhibited a long decay time of 5.07 ns in agreement with the reported theoretical calculation. The state filling mechanism and Coulomb blockade effect in the CdSe tetrapod were clearly observed in the pump-fluence-dependent transient absorption spectra. Hot electrons were transferred from arm states into the electron acceptor molecules before relaxation into core states