Inert Gas Aggregation

One of the most commonly used techniques for the preparation of clusters is inert gas aggregation [3-5]. Most of the particles described in the present review were prepared with this technique. Therefore, it is necessary to deal with specific features in the described experiments. In principle, the experimental setup for the inert gas aggregation technique

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Encyclopedia of Nanoscience and Nanotechnology Edited by H. S. Nalwa Volume 10: Pages (161-174)

is shown in Figure 1. The nucleation cell consists of three main parts: the evaporation area, the nucleation area, and the deposition area with the transfer systems and the preparation chambers. In contrast to systems where helium is used as the nucleation gas [6], the present preparations used argon as the nucleation gas. The material under investigation (generally pure metals but compounds can also be used) is evaporated from an open crucible or a Knudsen cell. The evaporation is performed by direct heating of a tantalum foil wrapped around an Al2O3 container. Sometimes, quartz or carbon containers are used. In general, the argon pressure is kept between 10-1 and 1 mbar. The condensation of nuclei and further cluster growth take place within an area cooled down by liquid nitrogen, allowing for supersaturation of the metal-argon mixture. The cluster growth is terminated by pumping off the argon gas with a highly efficient helium-cooled cryo-pump. The helium cryo-pump was the reason to use argon as the nucleation gas instead of helium. The clusters are then deposited in the deposition area at 10-610-7 mbar pressure on an amorphous carbon film of about 4-nm thickness covering a commercial copper grid as it is used for electron microscopy. The clusters are then transferred under vacuum or under argon gas by means of a transfer system from the reaction chamber to the microscope to protect the clusters against oxidation or further reactions under ambient conditions. The cluster flow is controlled with a microbalance quartzoscillator. Typical deposition rates were 0.1 nm/s. The deposition time was on the order of 1-3 s to avoid coalition either in the cluster beam or on the substrate. Several apertures, one of them variable between 1-and 45-mm diameter, influence the particle size distribution obtained with the nucleation cell. Typical dates for size distributions obtained with the nucleation cell are the following for Ag clusters: mean particle size of 4.5-nm diameter, full width at half-maximum of 1.3-nm diameter. Argon pressure and evaporation temperature also influence the particle growth. The size distribution in the case of the preparation of silver clusters is studied in detail and described elsewhere [7] in a comparison between time-of-flight mass spectroscopy and electron microscopy. With the described nucleation cell it is possible to prepare clusters consisting of a few atoms up to several thousands of atoms. For the present structural characterization of clusters by HRTEM, a region of 1- to 10-nm diameter could easily be achieved. The number N0 of atoms in a sphere of diameter D is given by

a is the lattice parameter for the cubic elementary cell. Therefore, the size region D = 1,..., 10 nm diameter corresponds for Ag and Au to N0 = 31,..., 30,800 and for Cu to N0 = 44,..., 44,514.

A typical example of an overview electron microscopic image for Cu clusters is shown in Figure 2. In order also to study reactivity during the evaporation process—prior to the nucleation of clusters—an additional gas inlet into the evaporation region is installed. In the present representation, however, only examples of the inlet of O2 will be given.

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