Tin-carbon clusters and the onset of microscopic level immiscibility: Experimental and computational study.

Abstract

We report the experimental observation and computational analysis of the binary tin-carbon gas phase species. These novel ionic compounds are generated by impact of C60(-) anions on a clean tin target at some kiloelectronvolts kinetic energies. Positive Sn(m)C(n)(+) (m = 1-12, 1 ≤ n ≤ 8) ions were detected mass spectrometrically following ejection from the surface. Impact induced shattering of the C60(-) ion followed by sub-surface penetration of the resulting atomic carbon flux forces efficient mixing between target and projectile atoms even though the two elements (Sn/C) are completely immiscible in the bulk. This approach of C60(-) ion beam induced synthesis can be considered as an effective way for producing novel metal-carbon species of the so-called non-carbide forming elements, thus exploring the possible onset of molecular level miscibility in these systems. Sn2C2(+) was found to be the most abundant carbide cluster ion. Its instantaneous formation kinetics and its measured kinetic energy distribution while exiting the surface demonstrate a single impact formation/emission event (on the sub-ps time scale). Optimal geometries were calculated for both neutral and positively charged species using Born-Oppenheimer molecular dynamics for identifying global minima, followed by density functional theory (DFT) structure optimization and energy calculations at the coupled cluster singles, doubles and perturbative triples [CCSD(T)] level. The calculated structures reflect two distinct binding tendencies. The carbon rich species exhibit polyynic/cummulenic nature (tin end capped carbon chains) while the more stoichiometrically balanced species have larger contributions of metal-metal bonding, sometimes resulting in distinct tin and carbon moieties attached to each other (segregated structures). The Sn2C(n) (n = 3-8) and Sn2C(n)(+) (n = 2-8) are polyynic/cummulenic while all neutral Sn(m)C(n) structures (m = 3-4) could be described as small tin clusters (dimer, trimer, and tetramer, correspondingly) attached to a nearly linear carbon chain. For example, the 1:1 (Sn:C) Sn3C3 and Sn4C4 clusters are composed of all-tin triangle and rhombus, correspondingly, with a short carbon chain (C3, C4) attached on top. The cationic Sn3C(n)(+) (n = 1-5) and Sn4C(n)(+) (n = 1-4) species exhibit various intermediate geometries. Structure calculations at the CCSD(T) level are essential since the segregation effect is not as easily evident based on the most stable structures calculated by DFT alone. Dependences of bond energies (per atom) reflect the evolution of the segregation effect. The mass spectral abundances could be reasonably rationalized in terms of calculated stabilities of the cluster ions with respect to various dissociation channels.