Nadi Braidy

Size and shape effects on the order -> disorder phase transition in CoPt nanoparticles

Chemically ordered bimetallic nanoparticles are promising candidates for magnetic-storage applications. However, the use of sub-10 nm nanomagnets requires further study of possible size effects on their physical properties. Here, the effects of size and morphology on the order-disorder phase transition temperature of CoPt nanoparticles (T(C)(NP)) have been investigated experimentally, using transmission electron microscopy, and theoretically, with canonical Monte Carlo simulations. For 2.4-3-nm particles, T(C)(NP) is found to be 325-175 degrees C lower than the bulk material transition temperature, consistent with our Monte Carlo simulations. Furthermore, we establish that T(C)(NP) is also sensitive to the shape of the nanoparticles, because only one dimension of the particle (that is, in-plane size or thickness) smaller than 3 nm is sufficient to induce a considerable depression of T(C)(NP). This work emphasizes the necessity of taking into account the three-dimensional morphology of nano-objects to understand and control their structural properties.

Single-wall carbon nanotubes synthesis by means of UV laser vaporization

Abstract We report on the synthesis of carbon single-wall nanotubes (SWNTs) produced for the first time by means of pulsed UV laser vaporization. High resolution transmission electron microscopy (HRTEM) and Raman spectroscopy have revealed that the produced nanotubes have a narrow diameter distribution and are self-organized into bundles having diameters in the (15–20) nm range. It is also shown that carbon SWNTs can be grown at a furnace temperature as low as 550 °C. At this latter temperature, the SWNTs yield is lower than at 1150 °C and the nanotubes were found to be characterized by a broader diameter distribution and a smaller bundle diameter. Finally, the increase of the laser repetition rate from 30 to 150 Hz was found to lead to a higher yield of SWNTs and larger bundles.

Performances of an 80–200 kV microscope employing a cold-FEG and an aberration-corrected objective lens

The performances of a newly developed 80-200 kV cold field emission gun (CFEG) transmission electron microscope (TEM) integrating a spherical aberration corrector for a TEM image-forming lens have been evaluated. To begin, we show that the stability of both emission and probe currents makes use of this new CFEG much friendlier. The energy spread of electrons emitted from the CFEG has been measured as a function of emission current and shows a very last 0.26 eV energy resolution at 200 kV and even 0.23 eV at 80 kV. The combination of the CFEG and the CEOS™ aberration corrector, associated with enhanced mechanical and electrical stabilities of this new microscope, allows reaching an information transfer below 75 pm at 200 and 80 pm at 80 kV. This unseen resolution at 200 kV has allowed us to study the structure of CoPt nanoparticles by observing direct images of their atomic arrangement along the high indexes zone axis. We have evidenced the presence of defects in these nanostructures that are not parallel to the electron beam. The precise stoichiometry of two iron oxides, FeO and Fe2O3, has been determined from an analysis of iron valence state that was obtained from a direct analysis of EELS fine structures spectrum of the two oxides.

Effect of coiling on the electronic properties along single-wall carbon nanotubes

Straight and coiled single-wall carbon nanotubes (SWCNTs) synthesized by laser vaporization were dispersed on highly oriented pyrolitic graphite. Their morphology and electrical properties were investigated by scanning tunneling microscopy (STM). STM images revealed that the SWCNTs (either straight or coiled) often self-organize into bundles of two or more tubes and are rarely found alone. The conductance measured along a periodically coiled CNT was found to increase at locations where the CNT is squeezed, while it decreases significantly in unsqueezed regions characterized by an unperturbed hexagonal network. This provides experimental evidence of significant conductance modulation along a one-dimensional system on the nanometer scale.

Multiple quantum well AlGaAs nanowires.

This letter reports on the growth, structure, and luminescent properties of individual multiple quantum well (MQW) AlGaAs nanowires (NWs). The composition modulations (MQWs) are obtained by alternating the elemental flux of Al and Ga during the molecular beam epitaxy growth of the AlGaAs wire on GaAs (111)B substrates. Transmission electron microscopy and energy dispersive X-ray spectroscopy performed on individual NWs are consistent with a configuration composed of conical segments stacked along the NW axis. Microphotoluminescence measurements and confocal microscopy showed enhanced light emission from the MQW NWs as compared to nonsegmented NWs due to carrier confinement and sidewall passivation.

Carbon nanotubular structures synthesis by means of ultraviolet laser ablation

We report on the synthesis of carbon nanotubular structures produced for the first time by means of pulsed KrF laser ablation of a graphite pellet at high temperature (1150 °C), under high argon gas pressure (500 torr), and at relatively high ultraviolet (UV) laser intensities (8 x 10 8 W/cm 2 ). The carbon nanotubular structures were directly observed by transmission electron microscopy and characterized by micro-Raman spectroscopy. Nanohorns (-2.5 nm diameter and -10 nm long), a few single-wall nanotubes (1.2 to 1.5 nm diameter), and other nanotubular structures (such as graphitic nanocages and low-aspect-ratio nanotubules) were clearly observed in the carbon deposit. Raman spectra in the low-frequency range confirmed a population of tubular structures with diameters ranging from 0.7 to 2.0 nm. It is shown that the relatively high UV laser intensity used here favors the growth of various nanotubular structures to the detriment of single-wall nanotubes.

Correcting scanning instabilities from images of periodic structures

A method for measuring and correcting the row displacement errors in lattice images acquired using scanning based methods is presented. This type of distortion is apparent in lattice-resolved images acquired using scanning-based techniques such as scanning transmission electron microscopy (STEM) and translates to vertical streaks convolving every feature in Fourier space. This paper presents a method to measure and correct the distortion based on the phase analysis of the streaks in Fourier space. The validity and the precision of the method is demonstrated using a model image and two experimental STEM images of Si <110> thin film and a 5 nm CoPt disordered nanocrystal. The algorithm is implemented in a freely available Digital Micrograph™ script.

Evolution of wurtzite CdTe through the formation of cluster assembled films

An approach has been developed to produce CdTe films with a high proportion of the metastable wurtzite phase. It involves the use of pulsed laser deposition to produce CdTe nanoparticles which are then collected on a substrate. Electron microscopy indicates that the nanoparticles produced have a relatively minor fraction of the wurtzite phase coexisting with the stable zinc blende phase. If these same nanoparticles arrive at a heated substrate they form a relatively dense cluster assembled film with an obvious wurtzite signature. Quite remarkable is that higher temperatures promote the metastable structure at the expense of the stable zinc blende phase.

From Nanoparticles to Process: An Aberration-Corrected TEM Study of Fischer-Tropsch Catalysts at Various Steps of the Process

χThe nanostructure of Fischer-Tropsch (FT) Fe carbides are investigated using aberration-corrected high-resolution transmission electron microscopy (TEM). The plasma-generated Fe carbides are analyzed just after synthesis, following reduction via a H2 treatment step and once used as FT catalyst and deactivated. The as-produced nanoparticles (NPs) are seen to be abundantly covered with graphitic and amorphous carbon. Using the extended information limit from the spherical aberration-corrected TEM, the NPs could be indexed as a mixture of NPs in the θ-Fe3C and χ–Fe5C2 phases. The reduction treatment exposed the NPs by removing most of the carbonaceous speSubscript textcies while retaining the χ–Fe5C2. Fe-carbides NPs submitted to conditions typical to FT synthesis develop a Fe3O4 shell which eventually consumes the NPs up to a point where 3-4 nm residual carbide is left at the center of the particle. Subscript textVarious mechanisms explaining the formation of such a microstructure are discussed.

Controlled synthesis of nickel ferrite nanocrystals with tunable properties using a novel induction thermal plasma method

Nickel ferrite spinel nanopowders were synthesised using a solution spray radio-frequency inductively coupled plasma reactor over a wide range of compositions (NixFe3-xO4, x ≤ 1), with metastable powders produced for x = 0, 0.25, and 0.5. X-ray fluorescence and X-ray diffraction coupled to Rietveld refinement show that this synthesis technique offers an excellent level of control over both the chemical and crystallographic composition of the nanopowder through the control of the input Fe/Ni ratio. The technique produces highly crystalline nanopowders without the need for post-synthesis annealing. A bulk Fe/Ni ratio ≥2 yields a pure spinel NixFe3-xO4 phase, whereas Fe/Ni ratio <2 results in the excess Ni partitioning to a secondary bunsenite (Nix,Fe1-x)O phase. Morphological analysis using transmission electron microscopy shows that two types of particles are produced in different parts of the reactor: a highly faceted powder with the truncated octahedron morphology and a smaller-sized random agglomerate. ...