Photolysis experiments on SiC mixed clusters: From silicon carbide clusters to silicon-doped fullerenes

摘要

Silicon carbon binary clusters are generated in a laser vaporization source from Si_xC_(1 - x) mixed targets (x = 0 to 50%). We have first analyzed stoichiometric (SiC)_n (n <= 40) clusters grown from a silicon carbide target (x = 50%). Both high fluence photoionization of (SiC)_n neutral clusters and photofragmentation of size-selected (SiC)_n~+ natural positive ions show that silicon-doped fullerenes emerge as stable photoproducts through the laser induced annealing of these clusters. They are detected as stable species as soon as a sufficient amount of silicon is eliminated through unimolecular processes involving the sequential losses of Si_2C and Si_3C neutral molecules in the earliest evaporation steps. This result is in favor of an efficient substitution of silicon atoms (about 12) into stable "cagelike" carbon networks. We will also show that an efficient doping of carbon fullerenes with silicon atoms can be obtained in carbon-rich mixed clusters directly grown as positive ions from nonstoichiometric targets (x < 25%). Mass abundance spectroscopy gives a clear signature of cagelike structures where silicon atoms are substituted for carbon ones. The results on the favored stability of even-numbered C_(2n - q)Si_q~+ clusters with q = 0, 1, 2 are presented here in the size range: 2n = 32 - 80. More largely doped species (q >= 3) cannot be evidenced in abundance mass spectroscopy because of unavoidable mass coincidences. A careful analysis of the photofragmentation behavior of selected sizes relative to the laser fluence nevertheless succeeds in indicating the contribution to the photofragmentation spectra of largely doped heterofullerenes C_(2n - q)Si_q~+ (q = 7 at least) that mainly dissociate by the loss of small even-numbered mixed molecules such as Si_2, Si_3C,.... Both approaches are consistent with the surprising capability of substituting a large number of silicon atoms into fullerenes without destabilizing their cage structure too much. In this respect, a value close to 12 seems to be an upper limit.