James L. Fye

Structures of medium-sized silicon clusters

Silicon is the most important semiconducting material in the microelectronics industry. If current miniaturization trends continue, minimum device features will soon approach the size of atomic clusters. In this size regime, the structure and properties of materials often differ dramatically from those of the bulk. An enormous effort has been devoted to determining the structures of free silicon clusters. Although progress has been made for Sin with n < 8, theoretical predictions for larger clusters are contradictory and none enjoy any compelling experimental support. Here we report geometries calculated for medium-sized silicon clusters using an unbiased global search with a genetic algorithm. Ion mobilities determined for these geometries by trajectory calculations are in excellent agreement with the values that we measure experimentally. The cluster geometries that we obtain do not correspond to fragments of the bulk. For n = 12–18 they are built on a structural motif consisting of a stack of Si9 tricapped trigonal prisms. For n ⩾ 19, our calculations predict that near-spherical cage structures become the most stable. The transition to these more spherical geometries occurs in the measured mobilities for slightly larger clusters than in the calculations, possibly because of entropic effects.

Annealing c60+: synthesis of fullerenes and large carbon rings.

Laser vaporization of graphite generates C60+ cluster ions that are fullerenes and a mixture of roughly planar polycyclic polyyne ring isomers. Experimental studies of the annealing of the non-fullerene C60+ ions indicate that they can be converted (in the gas phase) into the fullerene and an isomer that appears to be a large monocyclic ring. Some fragmentation is associated with conversion to the fullerene geometry, but the majority of the non-fullerene C60+ isomers are cleanly converted into an intact fullerene. The emergence of the monocyclic ring (as the clusters are annealed) suggests that this is a relatively stable non-spheroidal form of these all carbon molecules. The estimated activation energies for the observed structural interconversions are relatively low, suggesting that these processes may play an important role in the synthesis of spheroidal fullerenes.

Annealing and dissociation of carbon rings

Laser vaporization of graphite generates carbon clusters which possess a number of different geometric structures ranging from planar ring systems to fullerenes. We have employed a multicollision excitation scheme combined with injected ion drift tube techniques to examine the dissociation and annealing of carbon cluster ions. In this paper, we focus on clusters containing 30–50 atoms. Nonfullerene cluster ions in this size range can be annealed in the gas phase to just two dominant isomers—the fullerene and an isomer which appears to be a large monocyclic ring. Fragmentation by loss of C2 units is associated with annealing into the fullerene. This process occurs at low injection energies and rapidly becomes more important with increasing cluster size. However, relatively few clusters follow this path in the size range examined here (less than 3% for C40+). Most of the nonfullerene isomers anneal into a large monocyclic ring which (for clusters in the size range examined here) does not appear to convert i...

Ion mobility studies of metal-coated fullerenes

Abstract Ion mobility measurements have been used to examine the structures and fragmentation processes of Nb x C 60 + complexes with up to five niobium atoms. Our results confirm the observations of Martin and collaborators that loss of C 3 is an important fragmentation process for complexes with x ≥ 3. However, ion mobility measurements for Nb x C 60 + complexes and their Nb x C 60−3 n + fragments show that they are all fullerenes. It is likely that the Nb x C 60−3 n + fragments with an odd number of carbon atoms are stabilized by a niobium atom occupying the defect site in the fullerene cage, and that this stabilization makes C 3 loss energetically favored over the usual fullerene dissociation process of C 2 loss. Detailed analysis of the ion mobility measurements shows that the niobium atoms in the Nb x C 60 + complexes are clustered together on the fullerene surface. At high injection energies a structural transformation occurs for Nb 2 C 60 + and one of the niobium atoms apparently moves to an endohedral position. This process does not occur for NbC 60 + , but an analogous structural transformation appears to occur for all complexes with x > 2.

Isomerization of pure carbon cluster ions: From rings to fullerenes

Laser vaporization of graphite generates medium size carbon cluster ion (30–100 atoms) which are either fullerenes or a mixture of polyyne ring isomers. Studies of the isomerization of the polyyne ring isomers have been performed using injected ion drift tube techniques. These experiments reveal the existence of giant carbon rings (monocyclic rings containing up to 90 atoms) and show that for some clusters sizes it is possible to convert the polycylic polyyne rings into spheroidal fullerenes. The mechanism of this remarkable structural transformation is discussed.