EXAFS study on ultrafine Ni-Co-B amorphous catalysts

\chem{\chemvar{A}C_{60}} with , , . From the temperature dependent line shift of we have extracted information about the electron density distribution on the molecules. The data also support the existence of an like bonding type between the molecules. The magnetic phase transition observed at low temperature in and is monitored by an increase of the second moment as well as an increase of the spin lattice relaxation rates of the and alkali nuclei. The results are compared to NMR data of where no signs of a magnetic ordering are found. The magnetically ordered phases are discussed in terms of antiferromagnetic spin density wave. We also report on NMR measurements of orthorhombic one dimensional and rhombohedral two dimensional polymers of obtained under high pressure. By spinning the samples up to , we are able to identify two resonances at and for the orthorhombic polymer and six resonances at , , , , and for the rhombohedral one. The static distortion of the molecules induced by the transformation under pressure must be at the origin of the observed inequivalent carbons in both systems. The NMR lineshape simulations of the obtained spectra are compatible with the suggested polymeric structures where the molecules are connected by cycloadditions. Received: 10 October 1996/Accepted: 13 November 1996     

Slow positron annihilation studies of vacancy-type defects in the near-surface region of Cu and Nb before and after wear

Received: 22 June 1998 / Accepted: 9 November 1998     

Do Mie plasmons have a longer lifetime on resonance than off resonance?

The dynamics of laser-excited electrons in elliptically shaped silver nano-particles is studied using time-resolved two-photon photo-emission. By changing the polarization of the laser pulse at a fixed frequency, a different dynamics is observed near and far from resonance. To analyze these data, the effect of radiation damping on Mie plasmons of Ag nano-particles is studied as a function of laser frequency and polarization. It is shown that the off-resonance lifetime can be shorter or longer than the lifetime at resonance, depending on the incidence conditions of the laser light and on the depolarization factors characterizing the optical response of the nano-particles. Received: 28 August 2001 / Published online: 15 October 2001     

Atomistic processes and the strain distribution in the early stages of thin film growth

Structural relaxations in small Co islands on the Cu(001) surface are investigated performing atomistic calculations. We demonstrate that the strain relief at the metal interface in the early stages of heteroepitaxy is more complicated than suggested by simple considerations based on the small mismatch between the Co and Cu bulk metals. We found that the strain distribution in the surface region near the islands varies strongly on an atomic scale. The effect of strain on the shape of the Co islands is revealed. Diffusion on the top of strained islands and edge diffusion are considered. Received: 10 April 2000 / Accepted: 15 May 2000 / Published online: 7 March 2001     

A Raman study of polymerised C60

60 are reported. Although prepared according to different routes the Raman spectra of the two polymeric phases of C₆₀ show a quantitative agreement with respect to mode positions and intensity. We conclude from this that both materials have the same structure at least in the short range order, i.e. the same type of bonding and co-ordination between neighbouring C₆₀ molecules. An investigation of the time dependence of the thermal decomposition of high pressure polymerised C₆₀ is also presented. The rate of decomposition of the polymeric phase is found to be multi-exponential at all temperatures investigated. From an Arrhenius-type analysis of the short time data and the long time data, respectively, the activation energy for thermal dissociation of polymeric bonds was found to increase with time. Received: 20 September 1996/Accepted: 11 November 1996     

Causes of different catalytic activities of metals in formation of single-wall carbon nanotubes

When single-wall carbon nanotubes (SWNTs) were formed by pulsed Nd:YAG laser ablation or arc discharge, the yield depended on the metal catalyst: NiCo> Ni∼NiFe≫Co∼Fe>Pd∼Pt. It appears that an effective catalyst for SWNT growth must satisfy three conditions: it must be a good graphitization catalyst, have low solubility in carbon, and have a stable crystallographic orientation on graphite. NiCo, Ni, and NiFe satisfy these three conditions. The poor catalytic activities of Co, Fe, Pd, and Pt for SWNT formation would be explained by the ineffectiveness of Pt and Pd as graphitization catalysts, crystallographic orientation instability of Co crystals on graphite, and high solubility of Fe in graphite. Received: 29 October 2001 / Accepted: 7 November 2001 / Published online: 23 January 2002     

Bi nanocrystals embedded in an amorphous Ge matrix grown by pulsed laser deposition

Received: 1 September 1997/Accepted 8 September 1997     

NMR study of the magnetic properties of the polymerized phase of Cs1C60

\chem{Cs_{1}C_{60}} . The NMR spectrum allows, when the Magic Angle Spinning (MAS) technique is used, to get information about the electronic structure of this phase, which points out that the spin density is not uniformly distributed on the ball. An estimation of the strength of the quadrupolar effects on is then given, which shows that it is possible to analyse the spin-lattice relaxation time in terms of magnetic effects. A comparison with the of is then justified and allows us to point out the importance of low dimensional antiferromagnetic fluctuations in this compound. We also investigate the nature of the magnetic ground state in : the broadening of the spectra states unambiguously its magnetic nature and a detailed analysis of their shape gives further indications on the antiferromagnetic nature of the magnetic order, which seems characterized by a large amount of disorder. Received: 19 November 1996/Accepted: 19 December 1996     

Carbon nanotubes formed in graphite after mechanical grinding and thermal annealing

Multi-walled carbon nanotubes with cylindrical and bamboo-type structures are produced in a graphite sample after mechanical milling at ambient temperature and subsequent thermal annealing up to 1400 °C. The ball milling produces a precursor structure and the thermal annealing activates the nanotube growth. Different nanotubular structures indicate different formation mechanisms: multi-wall cylindrical carbon nanotubes are probably formed upon micropores and the bamboo tubes are produced because of the metal catalysts. A two-dimensional growth governed by surface diffusion is believed to be one important factor for the nanotube growth. A potential industrial production method is demonstrated with advantages of large production quantity and low cost. Received: 17 May 2002 / Accepted: 12 September 2002 / Published online: 4 December 2002 RID="*" ID="*"Corresponding author. Fax: +61-2/6125-8338, E-mail: ying.chen@anu.edu.au     

Kinetics of nanocrystallization in FeCoNbB(Cu) alloys

HITPERM alloys (FeCoMBCu; M=Nb, Zr, Hf...) have been recently developed and proposed as competitive soft magnetic materials for high-temperature applications. To our knowledge, this work contains the first results on nanocrystallization isothermal kinetics for these alloys. Analysis of nanocrystallization of the studied FeCoNbB(Cu) alloys in the frame of the Johnson–Mehl–Avrami theory shows a slowing-down of the kinetics and anomalously low values of the Avrami exponent, in a similar way to that reported for FeSiBNbCu (FINEMET)-type alloys. Compositional effects of Co substitution and Cu addition are considered. A more realistic kinetic model developed by Hermann et al. accounts for the experimental data. Received: 14 November 2001 / Accepted: 24 June 2002 / Published online: 4 December 2002 RID="*" ID="*"Corresponding author. Fax: +34-95/461-2097, E-mail: conde@us.es     

Deposition and diffusion of size-selected (Ag400+) clusters on a stepped graphite surface

400 clusters on a stepped graphite surface by a combination of scanning electron microscopy experiments and computer simulations (molecular dynamics and Monte Carlo methods). We find that the shape of the clusters is only partially deformed by the impact with the surface, moreover the clusters do not create surface defects upon landing, and so are able to diffuse freely over the surface. Many clusters are found to become trapped at surface steps, where their mobility is reduced by the higher binding energy. Exploring the 1-D diffusion of clusters along the steps reveals the low mobility for larger islands, as well as the importance of defects on the step (for example kinks), which trap the mobile clusters. Received: 9 April 1998/Accepted: 25 August 1998     

The phase diagram of charge- and spin-density waves in polymeric C60

\chem{(C_{60})_{\chemindex{x}}} at interfullerene distances larger than . At smaller ball separation, a special charge-density-wave ground state occurs, which exhibits polarized spheres. At around experimental bond distances, both CDW and SDW are present with rather small amplitudes. Received: 13 September 1996/Accepted: 11 November 1996     

Observation of strain relaxation in Si1-xGex layers by optical and electrical characterisation of a Schottky junction

We have studied the effect of the strain relaxation on the band-edge alignments in a Pt/p-Si_1-xGe_x Schottky junction with x=0.14 by internal photoemission spectroscopy and current–voltage measurements. We have shown that the variations in the band-edge alignments can be observed directly by measuring the optical and electrical properties of a simple Schottky junction. The strain in the Si_1-xGe_x layer has been partially relaxed by thermal treatments at two different temperatures. The degree of relaxation and other structural changes have been determined by a high-resolution X-ray diffractometer. Both optical and electrical techniques have shown that the barrier height of the Pt/Si_0.86Ge_0.14 junction increases with the amount of relaxation in the Si_1-xGe_x layer. This shows that the valence-band edge of the Si_1-xGe_x layer moves away from the Fermi level of the Pt/Si_1-xGe_x junction. The band-edge movement results from the increase in the band gap of the Si_1-xGe_x layer after the strain relaxation. This result agrees with the theoretical predictions for the strain-induced effects on the Si_1-xGe_x band structure. Received: 18 October 2000 / Accepted: 19 December 2000 / Published online: 23 March 2001     

Computer simulation of intrinsic defects in YAlO3 single crystal

Computer simulation techniques were used to investigate intrinsic defects in YAlO₃ single crystal. A set of short-range potential parameters were derived using a relaxed fitting procedure incorporating with the known crystal properties. These parameters were then applied within the framework of the shell model. The simulation results reveal that oxygen Frenkel disorder and the antisite defect of Al ion substituting the Y ion dominate the intrinsic defects in YAlO₃. An analysis of redox reactions corroborate that the oxidation is most likely to occur via forming interstitial oxygen, while the oxidation via filling oxygen vacancies and reduction reaction may predominate at high temperature. The activation energy of oxygen vacancy migration on conduction was also studied.     

Theory of spectral hole burning for the study of ultrafast electron dynamics in metal nanoparticles

In order to study the ultrafast relaxation dynamics of surface plasmon excitation in metal nanoparticles in the presence of inhomogeneous line broadening and investigate the influence of the reduced dimensions on the dephasing time T₂ in the size regime below about 10 nm, we have recently demonstrated a novel technique based on persistent spectral hole burning [1]. Here, we describe a theoretical model that has been developed for evaluation of the experimental data and precise determination of T₂ for particles of different size and shape. Comparison of the model to experimental data for Ag nanoparticles on sapphire shows that the theoretical treatment does not only reproduce the shape of the generated holes but also the dependence of their widths on the applied laser fluence. As a result, we have a reliable and versatile tool at hand making possible systematic studies of the ultrafast electron dynamics in small metal particles, and the dependence of the femtosecond dephasing time on their size, shape and surrounding dielectric. Received: 12 September 2001 / Published online: 15 October 2001     

Lattice dynamics of RbC60, RbC70 and phase transitions in A1C60: A neutron scattering study

1 C₆₀ (A = Rb, Cs, K) and RbC₇₀ have been investigated by neutron scattering. The studies include measurements in the high temperature rotator-, the polymer-, the dimer- and the low temperature isolator-phase and include investigations of the particular phase transitions. It is found that each of the phases can be characterized by specific features of its lattice dynamics which can be used for detailed studies of the dynamics of the rotator-polymer and the dimer-polymer transitions. Particular insight in the lattice dynamics is obtained from model calculations for the RbC₆₀ polymer and dimer phase. Received: 13 September 1996/Accepted: 11 November 1996     

Nanotube growth during annealing of mechanically milled Boron

Boron powder, finely ground in a tungsten carbide ball mill in an ammonia atmosphere, has been annealed at 1200 °C in flowing nitrogen to produce small quantities of cylindrical BN nanotubes, both as isolated individuals and grouped into ropes. Thick-walled conical BN tubes are abundant in specimens annealed for longer times, and their growth was catalysed once WC debris was converted into W metal particles. Some catalytic effect of small W nanoparticles could be necessary for nanotube formation, though no tip particles have been imaged here. Given the low temperature of mechanical milling and annealing, BN growth must involve surface diffusion and solid-state reconfiguration. It could be possible to engineer desirable physical and chemical properties by exploiting the variation in cylindrical versus conical BN structures as a function of annealing time. Received: 19 December 2001 / Accepted: 3 April 2002 / Published online: 19 July 2002 RID="*" ID="*"Corresponding author. Fax: +61-2/6125-8253, E-mail: john.fitzgerald@anu.edu.au     

Visible photoluminescence from Ge+-implanted SiO2 films thermally grown on crystalline Si

+ -implanted SiO₂ films is studied as a function of different fabricating conditions (implantation dose, annealing temperature and time). The SiO₂ films containing Ge nanocrystals exhibit two photoluminescence (PL) bands peaked at 600 nm and 780 nm. There are two excitation bands in the PL excitation (PLE) spectra. With variation in Ge nanocrystal size, the PL and PLE peak energies show no appreciable shift. The PL and PLE spectral analyses suggest that during the PL process, electron–hole pairs are generated by the E(l) and E(2) direct transitions inside Ge nanocrystals, which then radiatively recombine via luminescent centers in the matrix or at the interface between the nanocrystal/matrix. Received: 27 January 1998/Accepted: 18 March 1998     

Self-assembling of gold nanoparticles in one, two, and three dimensions

In the present work, we report the growth of ordered arrays of gold nanoparticles passivated with n-alkylthiol molecules using crystallization in toluene vapor. We kept constant the average particle size and the length of the passivating molecule (1-dodecanethiol). The temperature and time of growth were varied. We show that in the initial stages of the growth, nano-chains of particles are produced. These nano-chains aggregate to form two-dimensional arrays. In the initial stages the nano-chains form a two-dimensional square lattice which then relaxes to a close-packed structure. In latter stages of growth a three-dimensional supercrystal is produced. It is found that the packing of nanoparticles corresponds to an average FCC lattice. However, large variations on the local parameters of the lattice are observed. Near the edges of the supercrystal the anomalous packing reported by Zanchet et al. [20] and Fink et al. [21] was observed. The energy of the observed structures is analyzed using molecular dynamics simulation. It is concluded that using the vapor growth method, it is possible to produce controlled ordered structures from one to three dimensions. Received: 24 February 2000 / Accepted: 28 March 2000 / Published online: 13 July 2000     

Silicon nanowires grown from Au-coated Si substrate

Amorphous Si nanowires were grown on an Au-coated Si substrate by heat treatment at 1000 °C under an H₂ atmosphere. The nanowires have a length of several tens of a micron and a diameter of 10–20 nm. The growth mechanism of the nanowires was investigated and explained with a solid–liquid–solid model. Received: 11 July 2002 / Accepted: 7 July 2002 / Published online: 4 December 2002 RID="*" ID="*"Corresponding author. Fax: +86/10-62751615, E-mail: yudp@pku.edu.cn