Nanoclusters

Metal nanoclusters consist of a small number of atoms, at most in the tens. These nanoclusters can be composed either of a single or of multiple elements, and typically measure less than 2 nm. Such nanoclusters exhibit attractive electronic, optical, and chemical properties compared to their larger counterparts. Materials can be categorized into three different regimes, namely bulk, nanoparticles or nanostructures and atomic clusters. Bulk metals are electrical conductors and good optical reflectors, while metal nanoparticles display intense colors due to surface plasmon resonance. When the size of metal nanoclusters is further reduced, to 1 nm or less, in other words to just a few atoms, the band structure becomes discontinuous and breaks down into discrete energy levels, somewhat similar to the energy levels of molecules.?=E/N Metal nanoclusters consist of a small number of atoms, at most in the tens. These nanoclusters can be composed either of a single or of multiple elements, and typically measure less than 2 nm. Such nanoclusters exhibit attractive electronic, optical, and chemical properties compared to their larger counterparts. Materials can be categorized into three different regimes, namely bulk, nanoparticles or nanostructures and atomic clusters. Bulk metals are electrical conductors and good optical reflectors, while metal nanoparticles display intense colors due to surface plasmon resonance. When the size of metal nanoclusters is further reduced, to 1 nm or less, in other words to just a few atoms, the band structure becomes discontinuous and breaks down into discrete energy levels, somewhat similar to the energy levels of molecules. Therefore, a nanocluster behaves like a molecule and does not exhibit plasmonic behavior; nanoclusters are known as the bridging link between atoms and nanoparticles. The concept of atomic nanoclusters dates to prehistoric times. The formation of stable nanoclusters such as Buckminsterfullerene (C60) has been suggested to have occurred during the creation of the universe. The first set of experiments to form nanoclusters can be traced back to 1950s and 1960s. During this period, nanoclusters were produced from intense molecular beams at low temperature by supersonic expansion. The development of laser vaporization technique made it possible to create nanoclusters of a clear majority of the elements in the periodic table. Since 1980s, there has been tremendous work on nanoclusters of semiconductor elements, compound clusters and transition metal nanoclusters. According to the Japanese mathematical physicist Ryogo Kubo, the spacing of energy levels can be predicted by where E is Fermi energy and N is the number of atoms. For quantum confinement ? can be estimated to be equal to the thermal energy (? = ??), where K is Boltzmann's constant and T is temperature. Inserting the value for the Fermi energy of gold (5.5 eV) into the equation gives the critical number of gold atoms obtained for quantum confinement as 220 atoms. This implies that a cluster can have only a certain maximum number of atoms and thus has certain upper limitations on dimensions. Not all the clusters are stable. The stability of nanoclusters depends on the number of atoms in the nanocluster, valence electron counts and encapsulating scaffolds. In the 1990s, Heer and his coworkers used supersonic expansion of an atomic cluster source into a vacuum in the presence of an inert gas and produced atomic cluster beams. Heer's team and Brack et al. discovered that certain masses of formed metal nanoclusters were stable and were like magic clusters. The number of atoms or size of the core of these magic clusters corresponds to the closing of atomic shells. Certain thiolated clusters such as Au25(SR)18, Au38(SR)24, Au102(SR)44 and Au144(SR)60 also showed magic number stability. Häkkinen et al explained this stability with a theory that a nanocluster is stable if the number of valence electrons corresponds to the shell closure of atomic orbitals as (1S2, 1P6, 1D10, 2S2 1F14, 2P6 1G18, 2D10 3S2 1H22.......). Molecular beams can be used to create nanocluster beams of virtually any element. They can be synthesized in high vacuum by with molecular beam techniques combined with a mass spectrometer for mass selection, separation and analysis. And finally detected with detectors. Seeded supersonic nozzle Seeded supersonic nozzles are mostly used to create clusters of low-boiling-point metal. In this source method metal is vaporized in a hot oven. The metal vapor is mixed with (seeded in) inert carrier gas. The vapor mixture is ejected into a vacuum chamber via a small hole, producing a supersonic molecular beam. The expansion into vacuum proceeds adiabatically cooling the vapor. The cooled metal vapor becomes supersaturated, condensing in cluster form. Gas aggregation Gas aggregation is mostly used to synthesize large clusters of nanoparticles. Metal is vaporized and introduced in a flow of cold inert gas, which causes the vapor to become highly supersaturated. Due to the low temperature of the inert gas, cluster production proceeds primarily by successive single-atom addition.