Biogenic calcium carbonate forms the inorganic component of seashells, otoliths, and many marine skeletons, and its formation is directed by an ordered template of macromolecules. Classical nucleation theory considers crystal formation to occur from a critical nucleus formed by the assembly of ions from solution. Using cryotransmission electron microscopy, we found that template-directed calcium carbonate formation starts with the formation of prenucleation clusters. Their aggregation leads to the nucleation of amorphous nanoparticles in solution. These nanoparticles assemble at the template and, after reaching a critical size, develop dynamic crystalline domains, one of which is selectively stabilized by the template. Our findings have implications for template-directed mineral formation in biological as well as in synthetic systems.

The initial stages of template-controlled CaCO3 formation revealed by Cryo-TEM

Electromagnetic Response and Energy Conversion for Functions and Devices in Low‐Dimensional Materials

We report the first observation of stable single photon sources in an electronic and photonic device-friendly material, silicon carbide (SiC). SiC is a viable material for implementing quantum communication, computation and photonic technologies.

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A room temperature single photon source in silicon carbide

Recent progress in synthesis, properties and potential applications of SiC nanomaterials

The syntheses of colloidal silicon nanocrystals (Si-NCs) with dimensions in the 3–4 nm size regime as well as effective methodologies for their functionalization with alkyl, amine, phosphine, and acetal functional groups are reported. Through rational variation in the surface moieties we demonstrate that the photoluminescence of Si-NCs can be effectively tuned across the entire visible spectral region without changing particle size. The surface-state dependent emission exhibited short-lived excited-states and higher relative photoluminescence quantum yields compared to Si-NCs of equivalent size exhibiting emission originating from the band gap transition. The Si-NCs were exhaustively characterized using transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Fourier transformed infrared spectroscopy (FTIR), and their optical properties were thoroughly investigated using fluorescence spectroscopy, excited-state lifetime measurements, photobleaching experiments, and solvatochromi...

Size vs surface: tuning the photoluminescence of freestanding silicon nanocrystals across the visible spectrum via surface groups.

Quantum dots are superior to dye molecules in many aspects from size-tunable fluorescence and resistance to photobleaching and they have thus been widely used in biology as fluorescent probes. However, the cytotoxicity of some quantum dots limits their use in biological systems, and exploiting green nanoparticles with low cytotoxicity has become one major concern in this field. Silicon carbide, one well-known power electronic semiconductor material, is considered one of the best biocompatible materials, especially to blood. In addition, it has superior properties such as low density, high hardness, high strength, and chemical inertness. In recent years, much effort has been made to synthesize nanocrystalline SiC and study its photoluminescence (PL) properties. Some synthesized SiC nanostructures showed emission in the blue-to-UV range with their properties depending sensitively on the fabrication method and even on specific experiments. Although some variations have been reported, in general the observed emissions can be ascribed to some surface or defect states in the SiC nanostructures. However, owing to their relatively large size, low emission intensity, lack of controlled synthesis, and variable optical properties, these interconnected SiC nanostructures can hardly be used as fluorescent biological labels. Kassiba and co-workers synthesized SiC nanoparticles with diameters of tens of nanometers

3C-SiC nanocrystals as fluorescent biological labels.

We investigated the structural and infrared spectral properties of porous polycrystalline 3C-SiC and 3C-SiC nanoparticles produced via electrochemical method. The porous sample consisted of parallel nanowires with periodic beadlike structures. It exhibited infrared spectral features quite different from that of single crystal. The 3C-SiC crystallites with an average size of 4 nm showed simple surface chemistry with the surfaces well passivated by dissociation of surrounding water molecules. Our result explains the distinctive optical properties in porous polycrystalline and nanocrystalline 3C-SiC and reveals the crucial conditions for quantum confinement photoluminescence to arise.

Microstructure and infrared spectral properties of porous polycrystalline and nanocrystalline cubic silicon carbide

Inorganic lead halide perovskite nanocrystals (NCs) have attracted great interest owing to their superior luminescence and optoelectronic properties. In comparison to cubic CsPbX3 (X = Cl, Br, or I) that has visible luminescence, trigonal Cs4PbX6 has a much larger bandgap and distinct optical properties. Little has been known about the luminescence properties of the Cs4PbX6 NCs. In this study, we synthesize the well-crystallized Cs4PbCl6 NCs with sizes of 2.2–11.8 nm, which exhibit stable and near-UV luminescence (with a lifetime of 19.7–24.2 ns) with a remarkable quantum confinement effect at room temperature. In comparison to the negligible Stokes shift in the CsPbCl3 NCs, the Stokes shift of the Cs4PbCl6 NCs is very large (0.91 eV). The experimental results in combination with the first-principles calculations reveal that the near-UV luminescence of the Cs4PbCl6 NCs stems from the Frenkel excitons self-trapped in the isolated PbCl64– octahedrons. This is different from the CsPbCl3 NCs whose luminescence originates from the free Wannier excitons. The theoretical model based on the lattice relaxation is proposed to account for the large Stokes shift and its abnormal decrease with the decreasing particle size.

Quasi-self-trapped Frenkel-exciton near-UV luminescence with large Stokes shift in wide-bandgap Cs4PbCl6 nanocrystals

Quantum confinement luminescence of trigonal cesium lead bromide quantum dots

We observed nonclassical crystal growth of the sodium fluorosilicate nanowires, nanoplates, and hierarchical structures through self-assembly and aggregation of primary intermediate nanoparticles. Unlike traditional ion-by-ion crystallization, the primary nanoparticles formed first and their subsequent self-assembly, fusion, and crystallization generated various final crystals. These findings offer direct evidences for the aggregation-based crystallization mechanism.

Hongxia Li

Papers

3C-SiC nanocrystals as fluorescent biological labels.

Microstructure and infrared spectral properties of porous polycrystalline and nanocrystalline cubic silicon carbide

Quasi-self-trapped Frenkel-exciton near-UV luminescence with large Stokes shift in wide-bandgap Cs4PbCl6 nanocrystals

Quantum confinement luminescence of trigonal cesium lead bromide quantum dots

Nanoparticle-mediated nonclassical crystal growth of sodium fluorosilicate nanowires and nanoplates