Nanocluster

A nanocluster is a small aggregate of atoms whose dimensions place it in the nanometer scale and whose properties often differ markedly from bulk materials because of quantum confinement and a high surface-to-volume ratio. In many references, nanoclusters contain tens to thousands of atoms and measure roughly 1 to 10 nanometers, though terminology varies and some authors use the term for even smaller, molecule-like species or for somewhat larger clusters that are not strictly single molecules.

Nanoclusters exhibit size-dependent electronic, optical, and magnetic properties. For metal clusters such as gold, silver, or copper, discrete energy levels and molecule-like behavior may emerge when the size falls below about 2–3 nanometers, which can lead to fluorescence in some cases and the absence of a continuous surface plasmon resonance. The outer atoms, often stabilized by ligands or other stabilizers such as thiols, phosphines, or amines, play a major role in stability and reactivity.

Synthesis commonly employs bottom-up approaches, including chemical reduction of metal precursors in the presence of stabilizing ligands, colloidal routes, or gas-phase synthesis. Characterization methods include transmission electron microscopy, scanning transmission electron microscopy, mass spectrometry for precise cluster size determination, ultraviolet-visible spectroscopy, X-ray photoelectron spectroscopy, and fluorescence spectroscopy.

Applications span catalysis, sensing, nanoelectronics, and bioimaging. Gold nanoclusters, for example, can act as fluorescence probes and as catalysts; ligand choice and cluster size strongly influence activity, selectivity, and stability.