Nanoparticles exhibit properties different from that of a bulk material and these properties have been shown to be size dependent. Particles of metals and semiconductors whose size is on the same order as the wavelength of the electron are of extraordinary interest since they behave electronically as zero-dimensional (0D) quantum dots. Therefore, the classical laws of physics are no longer valid and have to be exchanged for quantum mechanics. Because of the appearance of discrete energetic states similar to molecular orbitals, semiconductor clusters have been called ‘artificial atoms’, and provide the unique opportunity to study semiconductor properties as they evolve from atoms to small molecules to a bulk crystal. Excellent examples of size-dependent discrete optical transitions exist for clusters of direct band gap semiconductors, such as CdSe and InAs.

The key ingredient to most successful methods for producing nanocrystals has been the use of capping ligands that bind to the particles surface and provide a steric barrier to aggregation. The capping ligands tend to exhibit the properties of surfactants: one end binds strongly to the particle surface while the opposite end interacts with the solvating fluid. In a good solvent, the ligands extend from the nanocrystal surface and provide steric stabilization, which typically limits size to the nanometer range and prevents unwanted agglomeration. The highly successful wet chemical techniques used to synthesize the group II-VI and III-V semiconductors cannot be applied to silicon or germanium, which require temperatures much higher than the boiling point of capping solvents to degrade the necessary precursors. Aerosol methods have produced silicon nanocrystals, however, the size distributions are broad and a thick oxide coating has been required to stabilize their structure.