Metal cluster aggregation sources

Formation of metal clusters by gas aggregation, in which metal atoms are evaporated or sputtered into a cooled inert gas flow at relatively high pressure, has been well established in last decade. By repeated collisions with the carrier gas, the supersaturated metal vapor nucleates and forms clusters. The mechanism of cluster formation can be explained with homogeneous and heterogeneous nucleation theories. The gas aggregation methods have been applied extensively to produce small clusters of metals such as zinc, copper, silver etc. [23-26]. In some cases this method was used in combination with a mass filter such as a quadruple or a time-of-flight spectrometer [27, 28], The metal vapor for cluster source can be produced by either thermal evaporation [23-28] or sputter discharge [22, 29]. 

Baker SH, Thornton SC, Edmonds KW, Maher MJ, Norris C, Binns C (2000) The construction of a gas aggregation source for the preparation of size-selected nanoscale transition metal clusters. Rev Sci Instrum 71 3178... 

In the gas-aggregation source a metal is vaporized and introduced in a flow of cold inert gas in which the vapor becomes highly supersaturated. Clusters are mainly produced by successive single-atom addition in the build-up of larger species. This type of source has been used to produce continuous cluster beams of alkali elements. By using two separate ovens in the source, each containing separate materials, clusters with two elements can be produced as Ceo covered with alkali metals [83]. The limitation of this type of source is that only metals with a low melting point can be studied. 

An example of a size-selecting cluster source is depicted in Fig. 16.1 [11]. A laser ablates a metal target, generating gas-phase metal atoms. Clusters then form in an aggregation tnbe that allows rough control of cluster size due to the inert gas flow rate, pressure and length of the channel. 

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