T. R. Ohno

Uranium Stabilization of C28: A Tetravalent Fullerene

Laser vaporization experiments with graphite in a supersonic cluster beam apparatus indicate that the smallest fullerene to form in substantial abundance is C28. Although ab initio quantum chemical calculations predict that this cluster will favor a tetrahedral cage structure, it is electronically open shell. Further calculations reveal that C28 in this structure should behave as a sort of hollow superatom with an effective valence of 4. This tetravalence should be exhibited toward chemical bonding both on the outside and on the inside of the cage. Thus, stable closed-shell derivatives of C28 with large highest occupied molecular orbital—lowest unoccupied molecular orbital gaps should be attainable either by reacting at the four tetrahedral vertices on the outside of the C28 cage to make, for example, C28H4, or by trapping a tetravalent atom inside the cage to make endothedral fullerenes such as Ti@C28. An example of this second, inside route to C28 stabilization is reported here: the laser and carbon-arc production of U@C28.

XPS probes of carbon-caged metals

Abstract X-ray photoemission spectral probes of endohedral lanthanum—fullerene complexes, La@C n , show that the central La atom is in a formal charge state close to +3, and has been effectively protected from reaction with water an oxygen by the enclosing fullerene cage. Similar results were obtained with endohedral yttrium complexes, Y @C n , and the first carbon-caged metal cluster, Y 2 @C 82 . EPR spectral results are also reported for Y @C 82 which is found to be an S = 1 2 spin system with a 0.48 G hyperfine splitting (at 9.216 GHz) centered at g =1.9999(1).

Interaction of O2 with C60: photon-induced oxidation

Abstract High resolution photoemission studies of solid O 2 in contact with multilayer films of C 60 on solid substrates showed conversion to CO, CO 2 carbonyl-like structures, and a carbon residue upon exposure to low energy and high energy photon sources. These processes are probably activated by the capture of a low energy electron to form the more reactive O − 2 species. These results reveal thermodynamically favored reaction processes but also kinetic limitations for our condensed structures. No reaction of O 2 with C 60 was observed without photoexcitation.

Formation of fullerides and fullerene-based heterostructures.

Two potassium fulleride phases, metallic K3C60 and nonmetallic K6C60, are formed when potassium is incorporated into thin C60 films under ultrahigh vacuum conditions. Phase separation is observed for intermediate stoichiometries. Results obtained for the C60-K3C60 heterostructure demonstrate that it is stable against potassium migration from the K3C60 phase. In contrast, the C60-K6C60 interface is not stable and K3C60 is formed.

Ge on Bi2Sr2−xCa1+xCu2O8+y: Reduced reactivity through cluster assembly

Photoemission studies of low‐temperature deposition of ∼30‐A‐diam Ge clusters on single‐crystal Bi2 Sr2−x Ca1+x Cu2 O8+y (100) show that an interface is produced with no evidence of substrate disruption. Analysis of the superconductor core level emission as a function of coverage indicates uniform overlayer growth and complete surface coverage. These cluster‐assembled interfaces were stable when warmed to 300 K, with only a slight reduction of Cu 2p3/2 satellite emission characteristic of the superconductor. In contrast, conventional atom‐by‐atom Ge deposition produces a Ge oxide layer and surface disruption.

Reactive metal overlayer formation on high‐temperature superconductors at 20 K

Photoemission results demonstrate that atom deposition of Ti, Cr, and Cu at 20 K on the high‐temperature superconductors (HTSs) dramatically reduces interfacial reaction relative to 300 K growth but does not completely eliminate it. Thin Ti‐O or Cr‐O layers are formed during atom deposition of ∼2 A of Ti or Cr on YBa2Cu3O7 or Bi2Sr2Ca1Cu2O8 because oxygen is withdrawn from the Bi‐O and/or Cu‐O layers. Interfacial reactions are diffusion limited at 20 K, and metal overlayers nucleate on the reacted layers. These metal layers are more uniform than those grown at 300 K because clustering is suppressed. There is no additional disruption for Cr/HTS interfaces when warmed to 300 K, but increased disruption is evident for Ti/HTS interfaces. The differences reflect the stabilities of Cr and Ti in contact with their own interfacial oxide. Cu atom deposition on Bi2Sr2Ca1Cu208(100) at 20 K also leads to much less disruption than observed for deposition at 300 K.

Nondisruptive oxide overlayer growth on GaAs(110)

Three different ways of forming oxide overlayers on GaAs(110) have been examined with x‐ray photoemission. First, Cr atoms were deposited onto cleaved GaAs(110) at 300 K, producing a disrupted region over which Cr metal grew. Subsequent exposure to O2 resulted in an inhomogeneous overlayer with areas of thick Cr2O3‐like oxides in addition to As and Ga oxides. GaAs oxidation was enhanced by Cr‐induced surface disruption, but there was no evidence of a catalytic process. Second, metallic clusters of Cr containing hundreds of atoms were condensed onto GaAs(110). In this case, no substrate disruption was observed at low temperature. O2 exposure resulted in Cr2O3 formation with small amounts of Ga2O3 and no detectable As2O3. Third, Cr atoms and O2 molecules were condensed onto a Xe buffer layer on GaAs(110) to produce Cr2O3‐like species out of contact with the semiconductor. Buffer layer desorption brought these Cr2O3 aggregates into contact with the substrate. The overlayer produced in this manner was abrupt,...