Studies of the Chemistry of Metal and Semiconductor Clusters
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STUDIES OF THE CHEMISTRY OF METAL AND SEMICONDUCTOR CLUSTERS M. F. JARROLD AND J. E. BOWER AT&T Bell Laboratories, Murray Hill, New Jersey 07974
ABSTRACT
This article summarizes recent studies of the chemistry of aluminum and silicon cluster ions containing 3-27 atoms. INTRODUCTION Atomic clusters are extremely small pieces of bulk material containing 3--1000 atoms. They are expected to display unique chemical and optical properties due to their hybrid molecular/bulk nature. In particular the chemistry of atomic clusters is expected to be a strong function of the cluster size, and for clusters smaller than some critical size the chemistry is probably quite different from that of the bulk material. In this article we describe some recent studies of the chemistry of aluminum and silicon clusters. Aluminum is a free electron metal and silicon is a semiconductor. These elements have a relatively simple electronic configuration, and it is possible to perform high quality ab initio calculations on quite large clusters (for example, Sil 0 [1]). The experiments described here were performed with positively charged cluster ions. The advantages of working with cluster ions rather than their neutral counterparts are: the clusters can be size selected before the reaction (making it possible to unambiguously determine the reactants and products); and it is relatively easy to vary the cluster ions' kinetic energy, so that studies can be performed over a wide energy range.
EXPERIMENTAL
The basic principle behind the experimental approach [2,3] used to study the reactions of size selected clusters is shown in Fig. 1. Clusters ions are generated by pulsed laser vaporization of an aluminum or silicon rod in a continuous flow of helium buffer gas. After
MASS SPECTROMETER TO SELECT CLUSTER SIZE GAS
METAL CLUSTER ION SOURCE
MASS SPECTROMETER TO ANALYZE PRODUCTS
REACTION REGION
DETECTOR
Figure 1. Schematic diagram of the experimental apparatus. Mat. Res. Soc. Syrup. Proc. Vol. 131. c1989 Materials Research Society
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exiting the source the cluster ions are focussed into a quadrupole mass spectrometer where a particular cluster size is selected. The size selected clusters are then focussed into a low energy ion beam and enter the reaction chamber. There are two possible configurations for
the reaction chamber: gas cell or drift tube. In the gas cell configuration the low energy ion beam simply passes through the gas cell which contains a reactant gas at low pressure. Reactions occur under single collision conditions and it is possible to vary the kinetic energy of the cluster ion so that the reactions can be investigated over the collision energy range 0.2-10.0eV. After exiting the gas cell the products and unreacted ions are focussed into a second quadrupole mass spectrometer where they are analyzed, and then detected. In the drift tube configuration the cluster ions are injected into a minature drift tube containing an
inert buffer gas (Ne) at a pressure of 0.4torr. The clusters are quickly thermalized and then drift acr
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