Nanocrystals

Nanocrystals are crystalline particles with dimensions on the nanometer scale, typically ranging from about 1 to 100 nanometers. They consist of a well-defined crystal lattice and can be composed of various inorganic materials. In many semiconductor nanocrystals, quantum confinement effects cause size-dependent optical and electronic properties, most notably shifts in absorption and emission as size changes.

Common materials used to make nanocrystals include semiconductor chalcogenides such as CdSe and CdS, metal oxides like ZnO and TiO2, noble metals including gold and silver, and hybrid or perovskite compositions. Nanocrystals can be purely inorganic, though they are often capped with organic ligands or inorganic shells that stabilize the particles, control their solubility, and influence surface-related properties.

Synthesis methods for nanocrystals are typically bottom-up. Colloidal synthesis in solution uses controlled nucleation and growth to produce monodisperse particles, with ligand chemistry and reaction temperature guiding size and shape. Other routes include vapor-phase synthesis, hydrothermal methods, and sol-gel processes. Surface functionalization allows dispersion in different media and enables integration into devices or biological systems.

Key properties of nanocrystals, such as band gap, fluorescence, charge transport, and catalytic activity, depend on size, composition, shape, and surface chemistry. Their tunable optical and electronic characteristics drive applications in light-emitting diodes, solar cells, bioimaging, sensors, catalysis, and other nanotechnology fields.

Characterization typically involves transmission electron microscopy for size and morphology, X-ray diffraction for crystal structure, and spectroscopic methods to study optical behavior. Challenges include surface passivation, stability, toxicity for some materials, reproducibility, and scalable production.