1,720,972 research outputs found
Antiferromagnetic Single Domain L1(2) FePt(3) Nanocrystals
No full textCompositionally ordered, single domain, antiferromagnetic L1(2) FePt3 nanocrystals were synthesized by coating colloidally grown Pt-rich Fe-Pt nanocrystals (Fe0.27Pt0.73) with thermally stable SiO2 and annealing at 700 degrees C in forming gas (7% H-2 in N-2). Without the silica coating, the nanocrystals transform predominately into the L1(0) FePt phase due to interparticle diffusion of Fe and Pt atoms. Magnetization measurements of the L1(2) FePt3 nanocrystals revealed two antiferromagnetic transitions near the bulk Neel temperatures of 100 and 160 K. Combining L1(2) FePt3 nanocrystals with L1(0) FePt nanocrystals was found to produce the constriction in field-dependent magnetization loops that has been observed near zero applied field in ensemble measurements of single domain silica-coated L1(0) FePt nanocrystals [Lee, D. C.; et al. J. Phys. Chem. B 2006, 110, 11160]
High yield multiwall carbon nanotube synthesis in supercritical fluids
No full textMultiwall carbon nanotubes (MWNTs) with outer diameters of 10 - 50 nm and wall thicknesses of 5 - 20 nm were synthesized in supercritical toluene at temperatures ranging from 600 to 645 degrees C at 8.3 MPa. Nanotube formation was catalyzed by metallocenes such as cobaltocene, nickelocene, and ferrocene or cobalt or iron nanocrystals; toluene served as both the solvent and the carbon source for nanotube growth. Supplemental carbon sources, either hexane or ethanol (similar to 30 vol%), increased the yield of the carbon nanotubes relative to pure toluene, and catalytic amounts of water (0.75 vol%) minimized the formation of carbon filaments and amorphous carbon. Cobaltocene, with ethanol as a supplemental carbon source, gave the highest percentage of nanotubes in the product (similar to 70%) and the highest conversion of toluene to MWNTs (similar to 4%). The MWNTs tended to exhibit bamboo morphology and appear to grow by a folded-growth mechanism with graphitic sheets wrapped around the seed metal particles. Cobaltocene was also found to catalyze coiled nanotube formation, with the appearance of springs, hairpins, lassos, and coiled ropes
Rotational and translational diffusivities of germanium nanowires
No full textUnderstanding the rheological behavior of dilute dispersions of cylindrical nanomaterials in fluids is the first step towards the development of rheological models for these materials. Individual particle tracking was used to quantify the rotational and translational diffusivities of high-aspect-ratio germanium nanowires in alcohol solvents at room temperature. In spite of their long lengths and high aspect ratios, the rods were found to undergo Brownian motion. This work represents the first time that the effects of solvent viscosity and confinement have been directly measured and the results compared to proposed theoretical models. Using viscosity as a single adjustable parameter in the Kirkwood model for Brownian rods was found to be a facile and versatile way of predicting the diffusivities of nanowires across a broad range of length scales
Synthesis and magnetic properties of silica-coated FePt nanocrystals
No full textColloidal FePt nanocrystals, 6 nm in diameter, were synthesized and then coated with silica (SiO(2)) shells. The silica shell thickness could be varied from 10 to 25 nm. As-made FePt@SiO(2) nanocrystals have low magnetocrystalline anisotropy due to a compositionally disordered FePt core. When films of FePt@SiO(2) particles are annealed under hydrogen at 650 degrees C or above, the FePt core transforms to the compositionally ordered L1(0) phase, and superparamagnetic blocking temperatures exceeding room temperature are obtained. The SiO(2) shell prevents FePt coalescence at annealing temperatures up to similar to 850 degrees C. Annealing under air or nitrogen does not induce the FePt phase transition. The silica shell limits magnetic dipole coupling between the FePt nanocrystals; however, low temperature (5 K) and room temperature magnetization scans show slightly constricted hysteresis loops with coercivities that decrease systematically with decreased shell thickness, possibly resulting from differences in magnetic dipole coupling between particles
Catalytic solid-phase seeding of silicon nanowires by nickel nanocrystals in organic solvents
No full textColloidal nickel (Ni) nanocrystals were used to direct the synthesis of crystalline silicon (Si) nanowires in an organic solvent. The reaction temperatures ranged from 400 degrees C to 520 degrees C with pressures from 14.3 to 23.4 MPa, conditions that are well above the critical point of the solvent. The Ni nanocrystals play two roles in the synthesis: (1) Ni catalyzes the decomposition of the silicon precursors, i.e., arylsilanes, alkylsilanes, and trisilane, to silicon; (2) Ni nanocrystals induce silicon crystallization through the solid phase alloying of Si in the Ni seeds. We call this nanowire growth mechanism supercritical fluid-solid-solid (SFSS) synthesis
Synthesis and magnetic properties of colloidal MnPt3 nanocrystals
No full textThe colloidal synthesis and magnetic properties of MnPt3 nanocrystals are reported. The nanocrystal size depended on the Mn reactant used, but the Mn:Pt stoichiometry was always 1:3. As synthesized, the nanocrystals are compositionally disordered with the face-centered cubic (fcc) A1 phase. Annealing at 580 C changed the MnPt3 crystal structure to the compositionally ordered Ll(2) phase (AuCu3 structure) with higher magnetocrystalline anisotropy. Magnetization measurements showed that the A1 nanocrystals are paramagnetic and the Ll(2) MnPt3 nanocrystals are superparamagnetic
Youngs modulus and size-dependent mechanical quality factor of nanoelectromechanical germanium nanowire resonators
No full textGermanium cantilever nanoelectromechanical resonators were fabricated using chemically grown nanowires with diameters ranging from 50 to 140 nm. Single nanowires were mechanically positioned at the edge of a copper transmission electron microscope (TEM) grid and then pinned to the grid with local platinum deposition. Oscillating cantilevers were induced into electromechanical resonance with an applied AC voltage, and the frequency response of the vibrational amplitude was measured. From this data, the Young's modulus of the nanowires was determined to be insensitive to diameter in this size range with an average value of 106 GPa (with 95% confidence limits of +/- 19 GPa), which is on par with the literature values for bulk Ge (100-150 GPa). The mechanical quality factors (Q) of the nanowire cantilevers were also measured and found to decrease with decreasing diameter. The data indicate that energy dissipation from the oscillating cantilevers occurs predominantly via surface losses, which increase in magnitude with increasing surface area-to-volume ratio of the nanowires
Electrochemistry and electrogenerated chemiluminescence of films of silicon nanoparticles in aqueous solution
No full textFilms of octadecyl-capped Si nanoparticles (NPs) (diameter, 3.4 +/- 0.7 nm) prepared by drop-coating on indium tin oxide (ITO) showed electrogenerated chemiluminescence (ECL) for both cathodic and anodic potential sweeps in KOH solutions containing peroxydisulfate. The redox potentials of the Si NPs can be estimated as approximately -0.9 and +0.95 V (versus Ag/AgCl) based on the anodic potential for the onset of ECL minus the ECL peak energy. The ECL exhibits a relatively broad spectrum (FWHM = 160 nm) with a peak wavelength of 670 nm (1.85 eV), similar to the photoluminescence spectra. In electrochemical studies in KOH solution in the absence of peroxydisulfate, an anodic current peak appears at about -1 V (versus Ag/AgCl) following a scan to negative potentials. A similar peak has been observed during the etching of a bulk single crystal Si electrode in alkaline aqueous solution. Unpassivated surface sites of Si NPs seem to be etched at potentials negative of the anodic oxidation peak
Application of aberration-corrected TEM and image simulation to nanoelectronics and nanotechnology
No full textThe image quality in electron microscopy often suffers from lens aberration. As a result of lens aberrations, critical information appears distorted at the atomic scale in high-resolution transmission electron microscopy (HRTEM). In scanning TEM (STEM), the spatial resolution of images and the quality of spectroscopic data are greatly reduced. With the recent introduction of aberration-corrected lenses and monochromators, new and exciting images with sub4.1-nm spatial resolution are now recorded routinely, and electron energy loss data has been used to determine the location of a single atom in an atomic column. As a result of the decreased focal depth of an aberration-corrected lens used in STEM, the dream of three-dimensional (3-D) atomic resolution is one step closer and for HRTEM it was shown that 3-D imaging with atomic resolution is feasible. However, understanding imaging and spectroscopy in HRTEM and STEM still requires refined modeling of the underlying electron scattering processes by multislice image simulation. Since research into the physics and technology of nanoelectronic devices has already moved into sub-10-nm transistor gate lengths, the need for well-understood imaging and spectroscopy at nanoscale dimensions is already upon us. Fortunately, nanowires and other nanotechnology materials serve as useful test samples as well as being potential materials for future nanoelectronics. This enables early development of microscopy methods that will be used to investigate future generations of integrated circuits
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