Search

Your search keyword '"Khakani, M. A. El"' showing total 25 results
Start Over Author "Khakani, M. A. El"
25 results on '"Khakani, M. A. El"'

1. Electronic correlations in graphite and carbon nanotubes from Auger spectroscopy

Author

Perfetto, E., Cini, M., Ugenti, S., Castrucci, P., Scarselli, M., De Crescenzi, M., Rosei, F., and Khakani, M. A. El

Subjects
Condensed Matter - Strongly Correlated Electrons, Condensed Matter - Mesoscale and Nanoscale Physics
Abstract

We have determined the screened on-site Coulomb repulsion in graphite and single wall carbon nanotubes by measuring their Auger spectra and performing a new theoretical analysis based on an extended Cini-Sawatzky approach where only one fit parameter is employed. The experimental lineshape is very well reproduced by the theory and this allows to determine the value of the screened on-site repulsion between $2p$ states, which is found to be 2.1 eV in graphite and 4.6 eV in nanotubes. The latter is robust by varying the nanotube radius from 1 to 2 nm., Comment: 5 pages, 3 figures, to appear in Phys. Rev. B

Published
2007
Full Text
View/download PDF

2. Softening of the insulating phase near Tc for the photo-induced insulator-to-metal phase transition in vanadium dioxide

Author

Hilton, D. J., Prasankumar, R. P., Fourmaux, S., Cavalleri, A., Brassard, D., Khakani, M. A. El, Kieffer, J. C., Taylor, A. J., and Averitt, R. D.

Subjects
Condensed Matter - Strongly Correlated Electrons, Condensed Matter - Materials Science
Abstract

We use optical-pump terahertz-probe spectroscopy to investigate the near-threshold behavior of the photoinduced insulator-to-metal (IM) transition in vanadium dioxide thin films. Upon approaching Tc a reduction in the fluence required to drive the IM transition is observed, consistent with a softening of the insulating state due to an increasing metallic volume fraction (below the percolation limit). This phase coexistence facilitates the growth of a homogeneous metallic conducting phase following superheating via photoexcitation. A simple dynamic model using Bruggeman effective medium theory describes the observed initial condition sensitivity., Comment: accepted for publication in Physical Review Letters

Published
2007
Full Text
View/download PDF

7. Ultra-High Oxidation Resistance of Suspended Single-Wall Carbon Nanotube Bundles Grown by an 'All-Laser' Process

Author

Khakani M A El, J H Yi, and B Aïssa

Subjects
Materials science, Macromolecular Substances, Surface Properties, Annealing (metallurgy), Scanning electron microscope, Molecular Conformation, Biomedical Engineering, Bioengineering, Activation energy, Carbon nanotube, law.invention, law, Materials Testing, Nanotechnology, General Materials Science, Graphite, Particle Size, Laser ablation, Nanotubes, Carbon, Lasers, Temperature, General Chemistry, Condensed Matter Physics, Arrhenius plot, Chemical engineering, Electrode, Crystallization, Oxidation-Reduction
Abstract

Single-wall carbon nanotubes (SWCNTs) were laterally grown on SiO2/Si substrates by means of an "all-laser" growth process. Our "all-laser" process stands out by its exclusive use of the same pulsed UV laser, first, to deposit the CoNi nanocatalyst and, second, to grow SWCNTs through the laser ablation of a pure graphite target. The "all-laser" grown SWCNTs generally self-assemble into bundles (5-15 nm-diam.) sprouting from the CoNi nanocatalyst and laterally bridging the 2 microm gap separating adjacent catalysed electrodes (in either "suspended" or "on-substrate" geometries). A comparative study of the oxidation resistance of both suspended and on-substrate SWCNTs was achieved. The "all-laser" grown SWCNTs were subjected to annealing under flowing oxygen at temperatures ranging from 200 to 1100 degrees C. Systematic scanning electron microscopy observations combined with micro-Raman analyses revealed that more than 20% of suspended nanotubes were still stable at temperatures as high as 900 degrees C under flowing O2 while the on-substrate counterpart were completely burnt out at this temperature. Accordingly, the activation energy, as deduced from the Arrhenius plot, of the suspended SWCNTs is found to be as high as approximately 180 kJ mol(-1) (approximately 9 times higher than that of the on-substrate ones). The high quality (almost defect-free) of the nanotubes synthesized by the "all-laser" approach, their protected tips into the embedded CoNi catalyst nanolayer together with their suspended geometry are thought to be responsible for their unprecedented ultra-high oxidation resistance. This opens up new prospects for the use of these suspended nanotubes into nanodevices that have to operate under highly oxidizing environments.

Published
2007
Full Text
View/download PDF

8. Single-Wall Carbon Nanotubes Synthesis by Means of UV Laser Vaporization: Effects of the Furnace Temperature and the Laser Intensity Processing Parameters

Author

MATERIALS TECHNOLOGY LAB OTTAWA (ONTARIO), Braidy, N., Khakani, M. A. El, Botton, G. A., MATERIALS TECHNOLOGY LAB OTTAWA (ONTARIO), Braidy, N., Khakani, M. A. El, and Botton, G. A.

Abstract

Carbon single-wall nanotubes (SWNTs) have been successfully synthesized by means of KrF laser vaporization of a Co-Ni-doped graphite pellet in a flowing argon atmosphere. The effects of two key processing parameters, namely the furnace temperature (in the 25-1150 deg C range) and the laser intensity (in the 0.8-4.4 x 10(exp 8) W/sq cm range), on the yield and the structural characteristics of the carbon SWNTs were investigated. By characterizing the obtained deposits by means of transmission electron microscopy and micro-Raman spectroscopy techniques, we were able to identify a threshold temperature as low as ^ 550 deg C, below which no carbon SWNTs can be grown. The increase of the furnace temperature from 550 to 1150 deg C was found to lead not only to a significant increase in the SWNTs yield but also to the formation of larger SWNTs bundles. Raman analysis have also revealed that the diameter distribution peak shifts from ^ 1.05 to ^ 1.22 nm as the temperature is raised from 550 to 1150 deg C. At the highest furnace temperature of 1150 C, we also found that a minimum laser intensity of about 1.6 x 10(exp 8) W/sq cm is required to grow carbon SWNTs by means of the KrF laser. Higher laser intensities have resulted in a higher yield of SWNTs with relatively thicker bundles. Moreover, the increase of the laser intensity was found to promote the growth of 1.22 nm-diameter nanotubes to the detriment of thinner carbon nanotubes (1.05 and 1.13 nm-diameters)., Pub in: Materials Research Society Symposium Proceedings, Volume 703. Prepared in collaboration with the Institut National de la Recherche Scientifique, Canada. This article is from ADA401575 Nanophase and Nanocomposite Materials IV held in Boston, Massachusetts on November 26-29, 2001

Published
2001

13. Mechanical properties of silicon carbide films for X-ray lithography application

Author

Jean, A., primary, Khakani, M. A. El, additional, Chaker, M., additional, Gat, E., additional, Dodier, J., additional, Papadopoullos, A., additional, Lafontaine, H., additional, Pépin, H., additional, Kieffer, J. C., additional, Ravet, M. F., additional, Madouri, A., additional, Bourneix, J., additional, Rousseaux, F., additional, and Gujrathi, S. C., additional

Published
1992
Full Text
View/download PDF

14. Electrical transport properties of single wall carbon nanotube/polyurethane composite based field effect transistors fabricated by UV-assisted direct-writing technology.

Author

Aïssa, B., Therriault, D., Farahani, R. D., Lebel, L. L., and Khakani, M. A. El

Subjects
ELECTRIC properties of carbon nanotubes, POLYURETHANES, COMPOSITE materials, FIELD-effect transistors, MICROFIBERS, SILICA, FINITE element method
Abstract

We report on the fabrication and transport properties of single-walled carbon nanotube (SWCNT)/polyurethane (PU) nanocomposite microfiber-based field effect transistors (FETs). UV-assisted direct-writing technology was used, and microfibers consisting of cylindrical micro-rods, having different diameters and various SWCNT loads, were fabricated directly onto SiO2/Si substrates in a FET scheme. The room temperature dc electrical conductivities of these microfibers were shown to increase with respect to the SWCNT concentrations in the nanocomposite, and were about ten orders of magnitude higher than that of the pure polyurethane, when the SWCNT load ranged from 0.1 to 2.5 wt% only. Our results show that for SWCNT loads ≤1.5 wt%, all the microfibers behave as a FET with p-type transport. The resulting FET exhibited excellent performance, with an Ion=Ioff ratio of 105 and a maximum on-state current (Ion) exceeding 70 μA. Correlations between the FET performance, SWCNTs concentration, and the microfiber diameters are also discussed. [ABSTRACT FROM AUTHOR]

Published
2012
Full Text
View/download PDF

15. Effect of the Microelectrode Geometry on the Diffusion Behavior and the Electroanalytical Performance of Hg-Electroplated Iridium Microelectrode Arrays Intended for the Detection of Heavy Metal Traces

Author

Drogoff, B. Le, Khakani, M. A. El, Silva, P. R. M., Chaker, M., and Vijh, A. K.

Abstract

Mercury-electroplated iridium microband arrays intended for heavy metal determination purposes were successfully fabricated by means of microelectronic processing techniques. Iridium microbands of 5 μm width each and lengths of 50 μm, 100 μm and 5 mm were investigated and their diffusion behavior and analytical performance compared with those of 5 μm microdisk arrays. The limiting current responses of disk arrays and 5 mm length microband arrays were found to be in good agreement with the radial and hemicylindrical flux expressions, respectively. In contrast, microband arrays having an aspect ratio (length/width) below 50 exhibit diffusion profiles that are characterized by a mixture of both radial and hemicylindrical diffusion components. Such a behavior is thought to be a consequence of edge effects at the extremity of the bands. To perform square-wave anodic stripping voltammetry (SWASV) analysis, the iridium microelectrode arrays were successfully Hg-electroplated and a complete surface coverage was obtained even on microbands having an aspect ratio as high as 1000. The electroanalytical performance of the various Hg-electroplated iridium microelectrode array geometries was evaluated by measuring Cd and Pb ion concentrations in synthetic solutions by means of SWASV over a concentration range as wide as 0.1 to 50 μg L–1. Their detection efficiency (as expressed by the redissolution current normalized to the microelectrodes surface) is shown to be significantly lower than that of microdisk arrays having the same critical dimension (5 μm). Moreover, not only the detection efficiency but also the detection limits were found to decrease as the length of the microbands increases.

Published
2001
Full Text
View/download PDF

24. Growth, structural and optoelectronic properties tuning of nitrogen-doped ZnO thin films synthesized by means of reactive pulsed laser deposition.

Author

Naouar, M., Ka, I., Gaidi, M., Alawadhi, H., Bessais, B., and Khakani, M. A. El

Subjects
*ZINC oxide thin films, *OPTOELECTRONICS, *NITROGEN, *PULSED laser deposition, *CRYSTAL growth, *CRYSTAL structure
Abstract

Pulsed laser deposition has been successfully used to achieve in-situ nitrogen doping of zinc oxide thin films at a temperature as low as 300°C. Nitrogen-doped zinc oxide (ZnO:N) thin films with a maximum nitrogen content of 0.7at.% were obtained by varying the nitrogen background pressure in the range of 0-150mTorr. The ZnO:N thin films were found to present hexagonal crystalline structure with dense and smooth surface. X-ray photoelectron spectroscopy analysis confirms the effective incorporation of nitrogen into ZnO thin films. Optical transmission together with room temperature photoluminescence measurements show that the band gap of the ZnO:N films shifts from 3.3eV to 3.1eV as nitrogen concentration varies in the range of 0.2-0.7at.%. The narrower band gap is obtained at an optimal nitrogen concentration of 0.22at.%. This band gap narrowing is found to be caused by both nitrogen incorporation and nitrogen-induced defects in the ZnO:N films. [ABSTRACT FROM AUTHOR]

Published
2014
Full Text
View/download PDF

25. Phase diagram of the ultrafast photoinduced insulator-metal transition in vanadium dioxide.

Author

Cocker, T. L., Titova, L. V., Fourmaux, S., Holloway, G., Bandulet, H.-C., Brassard, D., Kieffer, J.-C., Khakani, M. A. El, and Hegmann, F. A.

Subjects
*VANADIUM oxide, *METAL-insulator transitions, *TERAHERTZ spectroscopy, *FEMTOSECOND pulses, *PHASE diagrams, *MOLECULAR probes, *PHASE transitions
Abstract

We use time-resolved terahertz spectroscopy to probe the ultrafast dynamics of the insulator-metal phase transition induced by femtosecond laser pulses in a nanogranular vanadium dioxide (V02) film. Based on the observed thresholds for characteristic transient terahertz dynamics, a phase diagram of critical pump fluence versus temperature for the insulator-metal phase transition in V02 is established for the first time over a broad range of temperatures down to 17 K. We find that both Mott and Peierls mechanisms are present in the insulating state and that the photoinduced transition is nonthermal. We propose a critical-threshold model for the ultrafast photoinduced transition based on a critical density of electrons and a critical density of coherently excited phonons necessary for the structural transition to the metallic state. As a result, evidence is found at low temperatures for an intermediate metallic state wherein the Mott state is melted but the Peierls distortion remains intact, consistent with recent theoretical predictions. Finally, the observed terahertz conductivity dynamics above the photoinduced transition threshold reveal nucleation and growth of metallic nanodomains over picosecond time scales. [ABSTRACT FROM AUTHOR]

Published
2012
Full Text
View/download PDF

Catalog

Books, media, physical & digital resources
See catalog results