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We report the growth of multi-walled carbon nanotube forests employing an active-active bimetallic Fe-Co catalyst. Using this catalyst system, we observe a synergistic effect by which--in comparison to pure Fe or Co--the height of the forests increases significantly. The homogeneity in the as-grown nanotubes is also improved. By both energy dispersive spectroscopy and in-situ x-ray photoelectron spectroscopy, we show that the catalyst particles consist of Fe and Co, and this dramatically increases the growth rate of the tubes. Bimetallic catalysts are thus potentially useful for synthesising nanotube forests more efficiently [ABSTRACT FROM AUTHOR]

The fabrication of Mn-doped SiC materials by thermal reaction is an interesting method for obtaining SiC-based ferromagnetic semiconductors. Here we used x-ray photoemission and absorption measurements to study the reaction and diffusion of Mn in a 6 H-SiC single crystal substrate induced by a thermal treatment. In particular we have detected the formation of Mn-Si alloys completely covered by a graphene layer. The temperature dependence of the magnetization curves reveals the presence of two distinct magnetic phases. Interestingly, one of these phases displays ferromagnetic character at temperatures close to room temperature. [ABSTRACT FROM AUTHOR]

Hydrogen plasma pretreatment is used to enforce the growth of vertically-aligned carbon nanotube forests on TiN substrates. The evolution of the substrate, catalyst, and nanotubes are studied by in situ and ex-situ photoemission and X-ray diffraction in order to understand the growth mechanism. We find that TiN retains its crystallographic structure and its conductivity during plasma pretreatment and nanotube growth, which is confirmed by electrical measurements. Plasma pretreatment is found to favor the growth of nanotube forests by root growth, as it binds the catalyst nanoparticles more strongly to the substrate than thermal pretreatment. We find that plasma pretreatment time should be limited, otherwise poor or no growth is found. [ABSTRACT FROM AUTHOR]

Core level and valence band photoemission measurements combined with near edge x-ray absorption fine structure measurements were performed on a single C70 layer adsorbed on polycrystalline Al (1 ML-C70/Al) (ML—monolayer), pure and doped with sodium atoms. The data obtained from the pure ML chemisorbed on Al surface show a semiconducting behavior of the system, which is characterized by a covalent bond between the adsorbate and the substrate. The same data show also that the C70 molecules tend to orient themselves with the C5v axis perpendicular to the surface in analogy to what observed for 1 ML-C70/Cu(111). By doping the sample with sodium atoms a charge transfer from the alkali atoms to the lowest unoccupied molecular orbital (LUMO) of the C70 molecules takes place, as underlined by the gradual increasing intensity of the C70 LUMO peak as a function of doping. Nevertheless, no metallic phases are observed for any doping step. © 2005 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We investigated the interaction of C[sub 70] with the Cu(111) surface using x-ray photoelectron spectroscopy and x-ray absorption spectroscopy. The data point to a net charge transfer from the Cu substrate to the C[sub 70] molecules directly bonded to the Cu atoms, providing a metallic character for a single layer of C[sub 70] (monolayer) adsorbed on this surface. A strong dependence of the x-ray absorption spectra from the light polarization is also observed. By comparing these experimental data with density-functional calculations of the contribution to the x-ray absorption spectral line shape of the five inequivalent carbon atoms in the C[sub 70] cage, we observed that the molecules are mainly oriented with the C[sub 5]V axis perpendicular to the Cu surface. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We present a comprehensive study on the binding and ordering of C[sub 60] on a Pd(110) surface employing scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), x-ray photoelectron spectroscopy (XPS) and x-ray photoelectron diffraction (XPD). Three well-ordered structures evolving at elevated temperatures were identified, with (&sup7; [sub -/+] &sup-1; [sub ±5), (4 × 5), (4 × 8) unit cells and a coverage of 1, 0.82 and 0.77 physical monolayers, respectively. The rearrangement of Pd substrate atoms turns out to play a crucial role in the evolution of thin C[sub 60] films. In STM images alternating bright and dark molecular rows are found for the regular structures. This height difference is attributed to a restructuring of the Pd substrate. Whereas the C[sub 60] molecules of the bright molecular rows are embedded in one layer deep microscopic pits, C[sub 60] accommodated in two layer deep pits account for the dark molecular rows. XPD results complete the understanding of the film structure. They reveal that the orientation of the C[sub 60] cages is unique in the regular structures; the molecules are facing with a 5–6 bond towards the substrate. [ABSTRACT FROM AUTHOR]

We report a comprehensive study of ordered MnSi films grown on Si(111) which provides clear proofs that these MnSi films have the same magnetic and electronic properties of bulk MnSi compound, so closing a long-standing question. Furthermore, our measurements show the presence of a room-temperature ferromagnetic transition consistent with the ferromagnetic ground state predicted for Mn atoms with reduced coordination near surfaces and interfaces of silicon by recent calculations of Hortamani et al. [Phys. Rev. B 78, 104402 (2008)]. The possibility of growing layers on semiconductors which are ferromagnetic at room temperature (RT) is of paramount importance for nonvolatile memories and spintronic devices based on the injection of spin-polarized current from a ferromagnetic metal into a semiconductor. In this perspective MnSi films grown on Si substrates represent an interesting case study. [ABSTRACT FROM AUTHOR]

The magnetic and structural properties of InAs:Mn self-organized diluted magnetic quantum dots grown by low-temperature (∼270 °C), solid-source molecular-beam epitaxy using a very low InAs growth rate (<0.1 ML/s) are investigated. A Curie temperature (TC) of ∼350 K is measured in a sample grown with a Mn/In flux ratio of 0.15. Electron energy-loss spectroscopy confirms that most of the Mn remains within the InAs quantum dots. We propose as a possible explanation for this high TC the effects of magnetic and structural disorder introduced by a random incorporation and inhomogeneous distribution of Mn atoms amongst the InAs quantum dots. [ABSTRACT FROM AUTHOR]

Ammonia (NH3) plasma pretreatment is used to form and temporarily reduce the mobility of Ni, Co, or Fe nanoparticles on boron-doped mono- and poly-crystalline silicon. X-ray photoemission spectroscopy proves that NH3 plasma nitrides the Si supports during nanoparticle formation which prevents excessive nanoparticle sintering/diffusion into the bulk of Si during carbon nanotube growth by chemical vapour deposition. The nitridation of Si thus leads to nanotube vertical alignment and the growth of nanotube forests by root growth mechanism. [ABSTRACT FROM AUTHOR]

Core level and valence band photoemission measurements combined with near edge x-ray absorption fine structure measurements were performed on a single C(70) layer adsorbed on polycrystalline Al (1 ML-C(70)/Al) (ML-monolayer), pure and doped with sodium atoms. The data obtained from the pure ML chemisorbed on Al surface show a semiconducting behavior of the system, which is characterized by a covalent bond between the adsorbate and the substrate. The same data show also that the C(70) molecules tend to orient themselves with the C(5v) axis perpendicular to the surface in analogy to what observed for 1 ML-C(70)/Cu(111). By doping the sample with sodium atoms a charge transfer from the alkali atoms to the lowest unoccupied molecular orbital (LUMO) of the C(70) molecules takes place, as underlined by the gradual increasing intensity of the C(70) LUMO peak as a function of doping. Nevertheless, no metallic phases are observed for any doping step.