Molecular Beam Studies on Semiconductor Clusters: Polarizabilities and Chemical Bonding

Abstract

Schematic representation of a semiconductor cluster isolated in the molecular beam during the flight though an electric Stern-Gerlach field. The cluster flies with a velocity v parallel to the cylindrical bent electrode surfaces, which can be seen in the cross section. The field induces a dipole moment in the cluster that is proportional to the polarizability. Owing to the well-defined inhomogeneity of the field a somewhat larger force is exerted on the negative partial charge than on the positive. The resulting force F and the polarizability can be determined in the molecular beam experiment from the deflection of the cluster from the linear flight path. REVIEWS Molecular Beam Studies on Semiconductor Clusters: Polarizabilities and Chemical BondingJorg August Becker* This article discusses experiments that measure polarizabilities, permanent dipole moments, and absorption spectra of semiconductor clusters such as Si,, Ga,As,w, and Ge,Te, isolated in a molecular beam. The experiments show that the transition from the typical optoelectronic properties of macroscopic semiconductor crystals to the molecular properties of small polyhedral semiconductor molecules occurs in several steps. The bonding in nanometer-sized semiconductor colloids with approximately 103-105 atoms is similar to that in the solid state. The bonding orbitals of the semiconductor crystals, which extend over a nanometer range, are, however, somewhat more strongly localized in these clusters and are therefore already slightly modified. Over the size range from lo3 to 10 atoms, a gradual transition occurs from localized solid-statelike bonds to molecule-like bonds. Interestingly, quantum size effects are found even in relatively small particles with about 10' atoms. These effects can be observed in the form of unusually small static polarizabilities of Si, clusters or in the absorption bands of Ga,As, clusters, which are strongly blue-shifted relative to those of the solid state. For smaller clusters, these effects disappear over the size range 10'-10 atoms, which is manifested by an increasing red-shift of the absorption bands with decreasing cluster size.

Assessing the performance of ab initio methods on static (hyper)polarizability predictions for silicon clusters. Si4 as a test case

“…The optical properties of semiconductor clusters are currently a research subject of advanced scientific and technological interest. 1,2 The electric polarizability of silicon clusters in particular have been extensively studied both experimentally [3][4][5][6][7] and theoretically. [8][9][10][11][12][13][14][15][16][17][18][19][20][21] Most of the above studies focus on the dipole polarizability.…”

Density Functional Study of LinHm Clusters. Electric Dipole Polarizabilities

“…The dipole polarizability, however, represents a suitable molecular property that can take on the role of the dielectric constant and can be measured without touching the cluster. 1 The dipole polarizability comprises the information of the bonding character in the cluster, and on the other hand, through the Classius-Mosotti relation it yields information about the dielectric constant of the solid-like macroscopic particle formed with the cluster. The experimental measurement of cluster polarizabilities is relatively easy in comparison to other properties of neutral clusters.…”

“…The chemical bonding is profoundly changed in the formation of a cluster whereas the solid-state-like properties like the dielectric constant are of no significance in a cluster of few atoms. The dipole polarizability, however, represents a suitable molecular property that can take on the role of the dielectric constant and can be measured without touching the cluster . The dipole polarizability comprises the information of the bonding character in the cluster, and on the other hand, through the Classius−Mosotti relation it yields information about the dielectric constant of the solid-like macroscopic particle formed with the cluster.…”

“…The experimental measurement of cluster polarizabilities is relatively easy in comparison to other properties of neutral clusters. In the presence of an electric field the neutral clusters are deflected in linear order due to their dipole polarizabilities, − allowing their measurement. Hence, the polarizabilities can be one of the most important pieces of information about the nature of the bonding and the geometrical structure of the neutral clusters.…”

Electric dipole polarizabilities of Ni12–58

“…Such variations are due to discontinuous changes in electronic/geometric structures, and much effort has been put into correlating these structural attributes to observed physical and chemical properties. [10][11][12] The potential value of polarizability measurements for cluster isomer identification has been recognized by Moullet et al 13 In this paper, the static dipole polarizabilities of nickel clusters (N 12 -N 58 ), obtained via electric field deflection measurements, are presented. For example, molecular beam-based electric deflection measurements have shown that the polarizabilities of pure and mixed alkali metal clusters [5][6][7][8] and aluminum clusters 9 display variations that can be correlated to features in geometric and/or electronic structures.…”