Synthesis of semiconductor nanocrystals in organic solvents

Colloidal semiconductor nanocrystals (NCs) are crystalline particles with diameters ranging typically from 1 to 10 nm, comprising some hundreds to a few thousands of atoms. The inorganic core consisting of the semiconductor material is capped by an organic outer layer of surfactant molecules (“ligands”), which provide sufficient repulsion between the crystals to prevent them from agglomeration. In the nanometer size regime many physical properties of the semiconductor particles change with respect to the bulk material. Examples of this behavior are melting points and charging energies of NCs, which are, to a first approximation, proportional to the reciprocal value of their radii. At the origin of the great interest in NCs was yet another observation, namely the possibility of changing the semiconductor band gap — that is the energy difference between the electron-filled valence band and the empty conduction band — by varying the particle size. In a bulk semiconductor an electron e− can be excited from the valence to the conduction band by absorption of a photon with an appropriate energy, leaving a hole h+ in the valence band. Feeling each other’s charge, the electron and hole do notmove independently from each other because of the Coulomb attraction. The formed e−-h+ bound pair is called an exciton and has its lowest energy state slightly below the lower edge of the conduction band. At the same time its wave function is extended over a large region (several lattice spacings), i.e. the exciton radius is large, since the effective masses of the charge carriers are small and the dielectric constant is high [1]. To give examples, the Bohr exciton radii in bulk CdS and CdSe are approximately 3 and 5 nm.