Unisized two-dimensional platinum clusters on silicon(111)-7×7 surface observed with scanning tunneling microscope

Abstract

Uni-sized platinum clusters (size range of 5-40) on a silicon(111)-7 x 7 surface were prepared by depositing size-selected platinum cluster ions on the silicon surface at the collision energy of 1.5 eV per atom at room temperature. The surface thus prepared was observed by means of a scanning tunneling microscope (STM) at the temperature of 77 K under an ambient pressure less than 5 x 10(-9) Pa. The STM images observed at different cluster sizes revealed that (1) the clusters are flattened and stuck to the surface with a chemical-bond akin to platinum silicide, (2) every platinum atom occupies preferentially the most reactive sites distributed within a diameter of approximately 2 nm on the silicon surface at a cluster size up to 20, and above this size, the diameter of the cluster increases with the size, and (3) the sticking probability of an incoming cluster ion on the surface increases with the cluster size and reaches nearly unity at a size larger than 20.

Experimental observation of two-dimensional charge polarization in unisized platinum cluster disk bonded to silicon(1 1 1) surface

“…This gives the following qualitative explanation for the slight variations found cluster by cluster in the HOMO-LUMO gap and the tunneling spectra (see Figs. 1-4); the Pt-Si interaction changes slightly due to multiform deconstruction of the silicon crystal under the cluster disks when they are pinned on the surface [6]. This also results in slight differences in the geometry of the unisized cluster disks with an identical cluster size, which have been observed as finite distribution widths of their apparent heights and diameters [6].…”

“…Undesired neutral impurities were removed from the Pt þ 30 beam by deflecting the beam off the original beam axis by 7 mm in front of the sample. In this procedure, the cluster disks are stuck firmly on the surface after neutralization of Pt þ 30 into Pt 30 with a sticking probability of almost unity [6].…”

“…[5,6]. Platinum cluster cations, Pt þ n , were produced in a gas-aggregation chamber equipped with a magnetron sputtering source [36] and size-selected in a quadrupole mass-filter; the current and the size of Pt þ n were typically ranged in 900-50 pA and 5-45, respectively.…”

“…For instance, the HOMO-LUMO gap of supported clusters of metal and semiconductor atoms has been reviewed by Sattler [4]. Recently, we have clearly shown by STM studies the correlation between the HOMO-LUMO gap and the Pt-Pt interatomic distance in unisized platinum clusters, Pt n (n = 5-40), fixed on a silicon(1 1 1) surface; the HOMO-LUMO gap decreases critically at n $ 20 [5] when the platinum atoms are closely packed in a monatomic-layered two-dimensional geometry [6]; a cluster having this geometry is hereafter called a cluster disk. Therefore, the smaller HOMO-LUMO gap in the range of n > 20 plausibly originates from two-dimensional Pt-Pt metallic interaction in the cluster disk [5].…”

Deformation of slow liquid and solid clusters upon deposition: A molecular-dynamics study of Al cluster impact on an Al surface

“…In a recent experiment, the deposition of size-selected Pt n clusters (n = 5-40) on a Si(1 1 1) surface with velocities of 1.5 eV/atom, corresponding to 860 m/s, was studied [35]. The impinging clusters had a temperature of around 100 K, and hence were crystalline.…”