Synthesis, Self-assembly and Optoelectronic Properties of Monodisperse ZnO Quantum Dots

Quantum dot (QD), also called artificial atom, is a semiconductor nanocrystal with the size on the order of a few nanometers. By modifying its composition, size and shape, its density of electronic states can be engineered and thus many physical properties can be widely and easily adjusted. Strong quantum confinement of electronic carriers at nanometer scale makes band-gap and luminescence energies size and shape dependent. These dots have now been widely employed as targeted fluorescent labels for biomedical applications. Zinc oxide (ZnO) is a wide-band-gap semiconductor that presents interesting luminescent properties, which are seen in the recent demonstration of ultraviolet lasing from nanowires. These properties have stimulated the search for new synthetic methodologies for wellcontrolled ZnO nanostructures. However, many applications require the organization of nanoparticles into twoor three-dimensional (2D or 3D) superlattices. This very efficient organization was obtained by physical methods, whereas chemical approaches toward such organizations would also be of interest since they are easy to perform and allow a facile scale-up procedure. For the interest of size dependent physical properties, intensive research has been focused on fabricating ZnO QDs with ultra small size that gives strong quantum confinement. By far, various kinds of synthetic approaches have been realized in fabricating such small ZnO QDs, which can be roughly divided into two categories, chemical and physical. Chemical methods involves sol-gel method, hydrothermal growth, thermal decomposition, electrochemical method, while physical methods involves pulsed laser ablation (PLA or PLD), metalorganic chemical vapor deposition (MOCVD), radio frequency (RF) sputtering, flame spray pyrolysis (FSP) method, vapor phase transport (VPT) deposition, etc.. Chemical process takes advantage of the very exothermic reaction of the organometallic precursor bis(cyclohexyl) zinc with water to produce crystalline zinc oxide on one hand, and on the other hand the kinetic control of the decomposition by using long-alkyl-chain amine ligands. However, if the use of amine ligands allows access to ZnO nanoparticles with low size dispersity, these particles are not monodispersed, and consequently no 2D organization