Ting Guo

Catalytic growth of single-walled nanotubes by laser vaporization

Direct laser vaporization of transition-metal/graphite composite rods produced single-walled carbon nanotubes (SWT) in the condensing vapor in a heated flow tube. A much higher yield of nanotubes was found, with little of the amorphous overcoating on those produced by the metal-catalyzed arc-discharge method. A mixture of Co with Ni catalyzed about 50% of all the carbon vaporized to SWT. A model for SWT growth is presented for both the present case and the arc in which the metal particle size is limited due to the concurrent carbon condensation.

Picosecond-milliangstrom lattice dynamics measured by ultrafast X-ray diffraction

Fundamental processes on the molecular level, such as vibrations and rotations in single molecules, liquids or crystal lattices and the breaking and formation of chemical bonds, occur on timescales of femtoseconds to picoseconds. The electronic changes associated with such processes can be monitored in a time-resolved manner by ultrafast optical spectroscopic techniques, but the accompanying structural rearrangements have proved more difficult to observe. Time-resolved X-ray diffraction has the potential to probe fast, atomic-scale motions. This is made possible by the generation of ultrashort X-ray pulses, and several X-ray studies of fast dynamics have been reported,. Here we report the direct observation of coherent acoustic phonon propagation in crystalline gallium arsenide using a non-thermal, ultrafast-laser-driven plasma — a high-brightness, laboratory-scale source of subpicosecond X-ray pulses. We are able to follow a 100-ps coherent acoustic pulse, generated through optical excitation of the crystal surface, as it propagates through the X-ray penetration depth. The time-resolved diffraction data are in excellent agreement with theoretical predictions for coherent phonon excitation in solids, demonstrating that it is possible to obtain quantitative information on atomic motions in bulk media during picosecond-scale lattice dynamics.

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).

Generation of 18-fs, multiterawatt pulses by regenerative pulse shaping and chirped-pulse amplification

Transform-limited, 18-fs pulses of 4.4-TW peak power are produced in a Ti:sapphire-based chirped-pulsed amplification system at a repetition rate of 50 Hz. Regenerative pulse shaping is used to control gain narrowing during amplification, and an optimized, quintic-phase-limited dispersion compensation scheme is used to control higher-order phase distortions over a bandwidth of ~100 nm. Seed pulses are temporally stretched >100,000 times before amplification.

Ab initio theoretical predictions of C28, C28H4, C28F4, (Ti@C28)H4, and M@C28 (M=Mg, Al, Si, S, Ca, Sc, Ti, Ge, Zr, and Sn)

Recent experiments have demonstrated that C28 is the smallest fullerene cage that successfully traps elements in its inside. In this work, we have studied the electronic structures, equilibrium geometries, and binding energies of the title molecules at the self‐consistent field (SCF) Hartree–Fock level of theory employing basis sets of double‐zeta quality. The empty C28 fullerene is found to have a 5A2 open‐shell ground state and behaves as a sort of hollow superatom with an effective valence of 4, both toward the outside and inside of the carbon cage. The theoretical evidence suggests that C28H4 and C28F4 should be stable molecules. The possibility of simultaneous bonding from the inside and outside of the C28 shell, as in (Ti@C28)H4, is also explored. Our calculations show that the binding energy of the M@C28 species is a good indicator of the success in experimentally trapping the metal atoms (M) inside the fullerene cage. Based on these results, we propose that elements with electronegativities smalle...

The electronic structure of Ca@C60

Abstract The electronic structure of Ca@C60 in the gas phase was probed by ultraviolet photoelectron spectroscopy (UPS) of Ca@C−60. The electron affinity of Ca@C60 was found to be 3.0 eV. The spectrum suggests that Ca donates its two 4s electrons to the C60 t1u LUMO. Ab initio self-consistent field Hartree-Fock calculations were performed on Ca@C60 at four different symmetries, Ih, D3d, D5d, and C5v. We found that the C5v symmetry has the lowest energy with the central Ca2+ ion 0.7 A away from the center of C60 and that Ca@C60 has a 3A2 triplet electronic ground state in C5v symmetry.

CHEMICAL ENHANCEMENT BY NANOMATERIALS UNDER X-RAY IRRADIATION

We report here a new phenomenon of dynamic enhancement of chemical reactions by nanomaterials under hard X-ray irradiation. The nanomaterials were gold and platinum nanoparticles, and the chemical reaction employed was the hydroxylation of coumarin carboxylic acid. The reaction yield was enhanced 2000 times over that predicted on the basis of the absorption of X-rays only by the nanoparticles, and the enhancement was found for the first time to depend on the X-ray dose rate. The maximum turnover frequency was measured at 1 × 10(-4) s(-1) Gy(-1). We call this process chemical enhancement, which is defined as the increased yield of a chemical reaction due to the chemical properties of the added materials. The chemical enhancement described here is believed to be ubiquitous and may significantly alter the outcome of chemical reactions under X-ray irradiation with the assistance of nanomaterials.

Generation of hard x rays by ultrafast terawatt lasers

A compact, tabletop terawatt Ti:sapphire laser drive, ultrafast hard x-ray source for time-resolved x-ray diffraction studies is described. With a copper target the energy conversion efficiency from laser photons (800 nm) to copper K x-ray radiation (1.54 A) is 0.008%. The optimal laser intensity for generating these x rays is 1018 W cm−2, lower than the highest laser intensity available (5×1018 W cm−2) from the laser system. These results are consistent with a theoretical model proposed on the basis that the x rays are produced as a result of laser driven electron ionization of core level electrons of Cu atoms near room temperature. This source also provides features such as ultrashort pulse duration, extremely small source size, variable wavelengths, high peak spectral brightness, and the potential for multiple beam line experiments. X-ray diffraction patterns from GaAs single crystals and amorphous Ni films recorded with this source are presented.

Electronic shell closings in metal cluster plus adsorbate systems: Cu+7CO and Cu+17CO

The stability of CO‐chemisorbed small clusters of copper have been studied both by first principles calculations and by experiment. Evidence is found that the shell model (which predicts that clusters of 8, 18, and 20 electrons are particularly stable) is useful both for the bare metal clusters, and for these clusters with a chemisorbed CO−provided the CO is considered to have contributed two electrons. Experiments supporting this view are reported for gold clusters as well.