Magnetic properties of encapsulated nanoparticles in nitrogen-doped multiwalled cabon nanotubes embedded in SiOx matrices.

We report the production, characterization, thermal transformations (400-1000 degrees C), and magnetic properties of nanoparticles encapsulated in nitrogen-doped multiwall carbon nanotubes (CNx-MWNT), which were embedded in silicon oxide (SiOx) matrices via sol-gel techniques. The vapor chemical deposition (CVD) method with ferrocene-benzelamine mixtures was used to synthesize Fe and Fe3C nanoparticles inside CNx-MWNTs. Composites consisting of CNx-MWNTs (filler) and SiOx (matrix) were fabricated and thermally treated to different temperatures and exposure times (t). All samples were characterized using scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM), thermogravimetic analysis (TGA), and magnetometry (vibrating sample). We found that upon thermal treatment, the ferromagnetic nanoparticles modify their morphology, composition and aspect ratio, thus resulting in drastic changes in the magnetic and structural properties. In particular, as produced encapsulated nanoparticles mainly consisting of Fe and Fe3C phases were thermally modified into magnetite (Fe3O4). We have also observed that the hysteresis loops are very sensitive to the thermal treatment of the sample. Thus we can control the magnetic properties of the samples using thermal treatments.

Metallic and ferromagnetic edges in molybdenum disulfide nanoribbons.

The magnetic and electronic properties of MoS(2) nanoribbons with zigzag and armchair edges are investigated using LSDA-DFT. We found that the properties of the nanoribbons are very different from bulk MoS(2) due to edge states. Armchair nanoribbons could be metallic and exhibit a magnetic moment; however, when passivating with hydrogen, they become semiconducting. Zigzag nanoribbons are metallic and exhibit unusual magnetic properties regardless of passivation. Our results could explain the recent evidence of ferromagnetism in flat MoS(2) clusters, and motivate the synthesis of novel MoS(2) nanosystems.

Synthesis, electronic structure, and Raman scattering of phosphorus-doped single-wall carbon nanotubes.

Substitutional phosphorus doping in single-wall carbon nanotubes (SWNTs) is investigated by density functional theory and resonance Raman spectroscopy. Electronic structure calculations predict charge localization on the phosphorus atom, generating nondispersive valence and conduction bands close to the Fermi level. Besides confirming sustitutional doping, accurate analysis of electron and phonon renormalization effects in the double-resonance Raman process elucidates the different nature of the phosphorus donor doping (localized) when compared to nitrogen substitutional doping (nonlocalized) in SWNTs.

Creation of helical vortices during magnetization of aligned carbon nanotubes filled with Fe: theory and experiment.

We report a novel magnetic phenomenon consisting of the formation of helical spin configurations during the magnetization of densely packed ferromagnetic nanowires encapsulated inside carbon nanotubes. We studied the hysteresis loops when the magnetic fields are applied parallel and perpendicular to the nanotubes axes. We also performed theoretical calculations on aligned nanowire arrays that clearly indicate the creation of helical spin vortices in the hysteresis loops. The latter are caused by the presence of strong dipolar interactions among neighboring wires.

Production and characterization of single-crystal FeCo nanowires inside carbon nanotubes.

We describe the synthesis of novel monocrystalline FeCo nanowires encapsulated inside multiwalled carbon nanotubes (MWNTs). These FeCo nanowires exhibit homogeneous Fe and Co concentrations and do not contain an external oxide layer due to the presence of insulating nanotube layers. The method involves the aerosol thermolysis of toluene-ferrocene-cobaltocene solutions in inert atmospheres. The materials have been carefully characterized using state-of-the-art high-resolution transmission electron microscopy (HRTEM), electron-energy-loss spectroscopy (EELS), scanning electron microscopy (SEM), energy-dispersive X-ray analysis (EDX), electron diffraction, HREELS-STM elemental mapping, X-ray powder diffraction, and SQUID magnetometry. We noted that the formation of FeCo alloys occurs at relatively low pyrolytic temperatures (e.g., 650-750 degrees C). These single-crystal nanowires, which have not been reported hitherto, always exhibit the FeCo (110) plane parallel to the carbon nanotube axis. The FeCo nanomaterials have shown large coercive fields at room temperature (e.g., 900 Oe). We envisage that these aligned ferromagnetic nanowires could be used in the fabrication of high-density magnetic storage devices and magnetic composites.