Magic-Sized Stoichiometric II-VI Nanoclusters

Metal chalcogenide nanomaterials have gained widespread interest in the past two decades for their potential optoelectronic, energy, and catalytic applications. The colloidal growth of various forms of these materials, such as nanowires, platelets, and lamellar assemblies, proceeds through certain thermodynamically stable, ultrasmall (<2 nm) intermediates called magic-sized nanoclusters (MSCs). Due to quantum confinement and its resultant intriguing properties, isolation or direct synthesis of MSCs and their structure characterization, which is very much challenging, are current topics of fundamental and applied scientific research. By comprehensive understanding of the structure-activity relationships in MSCs, the nucleation and growth processes can be manipulated, resulting in the synthesis of novel metal chalcogenide materials for various applications. This review focuses on recent advances in the chemical synthesis, characterization, and theoretical calculations of CdSe and its related II-VI nanoclusters. It highlights the studies of photophysical and magneto-optical properties as well as heteroatom doping of MSCs followed by their chemical transformation to high-dimensional nanostructures. At the end of the review, future directions and possible ways to overcome the challenges in the research of semiconductor MSCs are also presented.

Quantum Dots of [Na4 Cs6 PbBr4 ]8+ , Water Stable in Zeolite X, Luminesce Sharply in the Green

Nanocrystals (NCs) of CsPbX₃ , X = Cl, Br, or I, have excellent photoluminescent properties: high quantum yield, tunable emission wavelengths (410-700 nm), and narrow emission band widths. CsPbBr₃ NCs show high promise as a green-emitting material for use in wide color gamut displays. CsPbBr₃ NCs have, however, not been commercialized because they are sensitive to moisture and heat. To avoid these problems, this work attempts to introduce CsPbBr₃ into five zeolites. The zeolite X product, Pb,Br,H,Cs,Na-X, shows superior stability toward moisture, maintaining its initial luminescence properties after being under water for more than a month. Its structure, determined using single-crystal X-ray crystallography, shows that quantum dots (QDs) of [Na₄ Cs₆ PbBr₄ ]⁸⁺ (not of CsPbBr₃ ) have formed. They are tetrahedral PbBr₄ ^2- ions (Pb-Br = 3.091(11) Å) surrounded by Na⁺ and Cs⁺ ions. Each fills the zeolite's supercage with its Pb²⁺ ion precisely at the center, a position of high symmetry. The peaks in the emission spectra of Pb,Br,H,Cs,Na-X and the CsPbBr₃ NCs are both at about 520 nm. The FWHM of Pb,Br,H,Cs,Na-X, however, is narrower than any previously reported for any of the CsPbBr₃ NCs, and for zeolite Y and the various mesoporous materials treated with CsPbBr₃ .

Diameter- and Length-controlled Synthesis of Ultrathin ZnS Nanowires and Their Size-Dependent UV Absorption Properties, Photocatalytical Activities and Band-Edge Energy Levels

Benefiting from their ultra-small diameters and highly structural anisotropies, ultrathin semiconductor nanowires (USNWs) are well-known for their fascinating physical/chemical properties, as well as their promising applications in various fields. However, until now, it remains a challenge to synthesize high-quality USNWs with well-controlled diameters and lengths, let alone the exploration of their size-dependent properties and applications. To solve such a challenge, we report herein a ligand-induced low-temperature precursor thermolysis route for the controlled preparation of ultrathin ZnS nanowires, which is based on the oriented assembly of the in-situ formed ZnS clusters/tiny particles. Optimized synthetic conditions allowed the synthesis of ZnS nanowires with a diameter down to 1.0 nm and a length approaching 330 nm. The as-prepared ultrathin ZnS nanowires were then intensively examined by morphological, spectroscopic and electrochemical analytical means to explore their size-dependent optical absorption properties, photocatalytic activities and band-edge energy levels, as well as their underlying growth mechanism. Notably, these USNWs, especially for the thinnest nanowires, were identified to possess an excellent performance in both the selective absorption of ultraviolet (UV) light and photocatalytic degradation of dyes, thus enabling them to serve as longpass ultraviolet filters and high-efficiency photocatalysts, respectively. For the ultrathin ZnS nanowires with a diameter of 1.0 nm, it was also interesting to observe that their exciton absorption peak positions were kept almost unchanged during the continuous extension of their lengths, which has not been reported previously.

The Zn12O12 cluster-assembled nanowires as a highly sensitive and selective gas sensor for NO and NO2

Motivated by the recent realization of cluster-assembled nanomaterials as gas sensors, first-principles calculations are carried out to explore the stability and electronic properties of Zn₁₂O₁₂ cluster-assembled nanowires and the adsorption behaviors of environmental gases on the Zn₁₂O₁₂-based nanowires, including CO, NO, NO₂, SO₂, NH₃, CH₄, CO₂, O₂ and H₂. Our results indicate that the ultrathin Zn₁₂O₁₂ cluster-assembled nanowires are particularly thermodynamic stable at room temperature. The CO, NO, NO₂, SO₂, and NH₃ molecules are all chemisorbed on the Zn₁₂O₁₂-based nanowires with reasonable adsorption energies, but CH₄, CO₂, O₂ and H₂ molecules are only physically adsorbed on the nanowire. The electronic properties of the Zn₁₂O₁₂-based nanowire present dramatic changes after the adsorption of the NO and NO₂ molecules, especially their electric conductivity and magnetic properties, however, the other molecules adsorption hardly change the electric conductivity of the nanowire. Meanwhile, the recovery time of the nanowire sensor at T = 300 K is estimated at 1.5 μs and 16.7 μs for NO and NO₂ molecules, respectively. Furthermore, the sensitivities of NO and NO₂ are much larger than that of the other molecules. Our results thus conclude that the Zn₁₂O₁₂-based nanowire is a potential candidate for gas sensors with highly sensitivity for NO and NO₂.