Coverage-Dependent Charge Reduction of Cationic Gold Clusters on Surfaces Prepared Using Soft Landing of Mass-Selected Ions

Abstract

The ionic charge state of monodisperse multiply charged cationic gold clusters on surfaces may be controlled by selecting the coverage of mass-selected ions soft landed onto a substrate. Polydisperse diphosphine-capped gold clusters were synthesized in solution and introduced into the gas phase by electrospray ionization. Mass selection was employed to isolate a multiply charged cationic cluster species (Au 11 L 5 3+, m / z = 1409, L = 1,3-bis(diphenylphosphino)propane) which was delivered to the surfaces of four different self-assembled monolayers on gold (SAMs) at controlled coverages of 10 11 and 10 12 clusters. Employing the spatial profiling capabilities of in situ time-of-flight secondary ion mass spectrometry (TOF-SIMS), it is shown that, in addition to the chemical functionality of the monolayer (as demonstrated previously: ACS Nano 2012, 6, 573), the coverage of cationic gold clusters on the surface may be used to control the relative abundance of different charge states of the soft landed multiply charged clusters. In the case of a 1 H,1 H,2 H,2 H -perfluorodecanethiol monolayer (FSAM) almost complete retention of charge by the deposited Au 11 L 5 3+ clusters was observed at a lower coverage of 10 11 clusters. In contrast, at a higher coverage of 10 12 clusters, pronounced reduction of charge to Au 11 L 5 2+ and Au 11 L 5 + was observed on the FSAM. When soft landed onto 16- and 11-mercaptohexadecanoic acid surfaces on gold (16,11-COOH-SAMs), the mass-selected Au 11 L 5 3+ clusters exhibited partial reduction of charge to Au 11 L 5 2+ at lower coverage and additional reduction of charge to both Au 11 L 5 2+ and Au 11 L 5 + at higher coverage. On the surface of the 1-dodecanethiol (HSAM) monolayer, the most abundant charge state was found to be Au 11 L 5 2+ at lower coverage and Au 11 L 5 + at higher coverage, respectively. A coverage-dependent electron tunneling mechanism is proposed to account for the observed reduction of charge of mass-selected multiply charged gold clusters soft landed on SAMs. The results demonstrate that one of the critical parameters that influence the chemical and physical properties of supported metal clusters, ionic charge state, may be controlled by selecting the coverage of charged species soft landed onto surfaces.