Egypt Nanotechnology Research Center

Ash Shaykh Zuwayd, Egypt

Egypt Nanotechnology Research Center

Ash Shaykh Zuwayd, Egypt
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Kasry A.,IBM | Kasry A.,Egypt Nanotechnology Research Center | Kuroda M.A.,IBM | Kuroda M.A.,University of Illinois at Urbana - Champaign | And 3 more authors.
ACS Nano | Year: 2010

Graphene is considered a leading candidate to replace conventional transparent conducting electrodes because of its high transparency and exceptional transport properties. The effect of chemical p-type doping on graphene stacks was studied in order to reduce the sheet resistance of graphene films to values approaching those of conventional transparent conducting oxides. In this report, we show that large-area, stacked graphene films are effectively p-doped with nitric acid. The doping decreases the sheet resistance by a factor of 3, yielding films comprising eight stacked layers with a sheet resistance of 90 Ω/□ at a transmittance of 80%. The films were doped either after all of the layers were stacked (last-layer-doped) or after each layer was added (interlayer-doped). A theoretical model that accurately describes the stacked graphene film system as a resistor network was developed. The model defines a characteristic transfer length where all the channels in the graphene films actively contribute to electrical transport. The experimental data shows a linear increase in conductivity with the number of graphene layers, indicating that each layer provides an additional transport channel, in good agreement with the theoretical model. © 2010 American Chemical Society.

Maarouf A.A.,IBM | Maarouf A.A.,Egypt Nanotechnology Research Center | Mele E.J.,University of Pennsylvania
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

We study the low-energy electronic properties of a junction made of two crossed metallic carbon nanotubes of general chiralities. We derive a tight-binding tunneling matrix element that couples low-energy states on the two tubes, which allows us to calculate the contact conductance of the junction. We find that the intrinsic asymmetries of the junction cause the forward- and backward-hopping probabilities from one tube to another to be different. This defines a zero-field Hall conductance for the junction, which we find to scale inversely with the junction contact conductance. Through a systematic study of the dependence of the junction conductance on different junction parameters, we find that the crossing angle is the dominant factor that determines the magnitude of the conductance. © 2011 American Physical Society.

Afify N.D.,Egypt Nanotechnology Research Center | Salem H.G.,American University in Cairo | Yavari A.,Georgia Institute of Technology | El Sayed T.,King Abdullah University of Science and Technology
Computational Materials Science | Year: 2013

Clear understanding of the superior mechanical strength of nanometer-sized metal single crystals is required to derive advanced mechanical components retaining such superiority. Although high quality studies have been reported on nano-crystalline metals, the superiority of small single crystals has neither been fundamentally explained nor quantified to this date. Here we present a molecular dynamics study of aluminum single crystals in the size range from 4.1 nm to 40.5 nm. We show that the ultimate mechanical strength deteriorates exponentially as the single crystal size increases. The small crystals superiority is explained by their ability to continuously form vacancies and to recover them. © 2013 Published by Elsevier B.V.

Afify N.D.,Egypt Nanotechnology Research Center | Salem H.G.,American University in Cairo | Yavari A.,Georgia Institute of Technology | El Sayed T.,King Abdullah University of Science and Technology
Computational Materials Science | Year: 2014

Deriving bulk materials with ultra-high mechanical strength from nanometer-sized single metalic crystals depends on the consolidation procedure. We present an accurate molecular dynamics study to quantify microstructure responses to consolidation. Aluminum single crystals with an average size up to 10.7 nm were hydrostatically compressed at temperatures up to 900 K and pressures up to 5 GPa. The consolidated material developed an average grain size that grew exponentially with the consolidation temperature, with a growth rate dependent on the starting average grain size and the consolidation pressure. The evolution of the microstructure was accompanied by a significant reduction in the concentration of defects. The ratio of vacancies to dislocation cores decreased with the average grain size and then increased after reaching a critical average grain size. The deformation mechanisms of poly-crystalline metals can be better understood in the light of the current findings. © 2013 Elsevier B.V. All rights reserved.

Wang K.,IBM | Gunawan O.,IBM | Moumen N.,IBM | Tulevski G.,IBM | And 4 more authors.
Optics Express | Year: 2010

We have developed an inexpensive and scalable method to create wire textures on multi-crystalline Si solar cell surfaces for enhanced light trapping. The wires are created by reactive ion etching, using a monolayer high self-assembled array of polymer microspheres as an etch mask. Chemical functionalization of the microspheres and the Si surface allows the mask to be assembled by simple dispensing, without spin or squeegee based techniques. Surface reflectivities of the resulting wire textured multicrystalline solar cells were comparable to that of KOH etched single crystal Si (100). Electrically, the solar cells exhibited a 20% gain in the short circuit current compared to planar multicrystalline Si control devices, and a relative increase of 7-16% in the "pseudo" efficiencies when the series resistance contributions are extracted out. © 2010 Optical Society of America.

Qin T.,University of Kent | Qin T.,Brunel University | Mountjoy G.,University of Kent | Afify N.D.,University of Kent | And 6 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

Rare-earth-doped glasses are key materials for optical technology due to the luminescent properties of 4fn ions. The crystal-field model describes the effect of local environment on transitions between 4f electrons. We present a detailed modeling study of the optical spectra of sodium disilicate glass, 33Na2O•67SiO2, doped with 0.2% and 1.0 mol% Eu 2O3. This study uses very large molecular dynamics models with up to 100 Eu3+ ions, the superposition model for covalent and overlap effects on crystal-field parameters, and realistic values for homogeneous linewidth broadening. The simulated spectra are in reasonable agreement with experiment. The trends in 7FJ energy levels across different Eu3+ ion sites have been examined and a very detailed analysis is presented that looks at how features of the spectra are related to features of the local environment of Eu3+ ions. Increasing the crystal-field strength Stotal causes the 7F0 energy level to decrease and causes the splitting of 7FJ manifolds to increase, and this is due to increasing mixing of 4f wave functions. To a reasonable approximation the crystal-field strength components Sk depend on angular positions of ligands independently of distances to ligands. The former are seen to be more significant in determining Sk, which are closely related to the rotationally invariant bond-orientational order parameters Q k. The values of S2 are approximately linear in Q 2, and the values of Q2 are higher for fivefold than sixfold coordinated rare-earth ions. These results can be of importance for efforts to enhance the local environment of rare-earth ions in oxide glasses for optical applications. © 2011 American Physical Society.

Elzatahry A.A.,King Saud University | Elzatahry A.A.,Advanced Technology and New Materials Research Institute | Abdullah A.M.,Alexandria University | Abdullah A.M.,Cairo University | And 8 more authors.
International Journal of Electrochemical Science | Year: 2012

Cobalt and nickel oxides-graphene nanocomposites have been prepared by a simple chemical route. The structure, morphology and properties were characterized using X-ray diffraction (XRD) and Transmission Electron Microscope (TEM). The electrocatalytic activity for the methanol oxidation reaction in acidic medium of these nanocomposites compared to platinum has been confirmed using cyclic voltammetry technique. © 2012 by ESG.

Chandra B.,IBM | Afzali A.,IBM | Khare N.,Indian Institute of Technology Delhi | El-Ashry M.M.,Egypt Nanotechnology Research Center | Tulevski G.S.,IBM
Chemistry of Materials | Year: 2010

Single walled carbon nanotube (SWCNT) films are candidates for use as transparent electrodes, especially where low-cost, flexible materials are desired. Chemical doping is a critical step in fabricating conductive films as doping substantially decreases the sheet resistance within SWCNTs and at tube-tube junctions. Despite the importance of chemical doping, surprisingly little effort is devoted to developing doping chemistry. Concentrated acid solutions are typically used to dope SWCNT films. Although they are effective at reducing the sheet resistance of SWCNT films, this method is plagued by two critical drawbacks. The first is that concentrated acid baths, such as HNO 3, are extremely harsh and will damage virtually any device technology. Second, the film resistance is unstable and rises dramatically over time. These drawbacks make implementation of SWCNT transparent, conducting films in technological applications extremely difficult. Here, we report an alternative doping scheme that utilizes a single-electron oxidant (triethyloxonium hexachloroantimonate) to effectively dope the SWCNT films. As evidenced by optical and electrical measurements, the compound effectively p-dopes SWCNT films. In addition to the effective doping, the resultant film resistance is stable over time. The films doped with triethyloxonium hexachloroantimonate outperform nitric acid doped films by a factor of 2.5 over time. This study introduces a new category of chemical dopants that yield stable, transparent, and conductive SWCNT films suitable for technological applications. © 2010 American Chemical Society.

Escher W.,IBM | Ghannam R.,Egypt Nanotechnology Research Center | Khalil A.,Egypt Nanotechnology Research Center | Paredes S.,IBM | Michel B.,IBM
2010 3rd International Conference on Thermal Issues in Emerging Technologies, Theory and Applications - Proceedings, ThETA 3 2010 | Year: 2010

We demonstrate an advanced packaging approach with an embedded silicon micro-channel water cooler wliere the photovoltaic cell is electrically connected by a metallization on the silicon sustrate. The backside of the silicon substrate contains the micro-machined fluidic channels thereby minimizing the thermal resistance compared to a state - of- the - an package. This leads to a reduced temperature drop between the photovoltaic cell and the coolant, allowing an increase in the temperature of recovered heat. A low-pressure drop split-flow fluid manifold is implemented to distribute the coolant from one single input to the micro-channel array and back from two outlet pons. A thermal resistance of 0.12 cm 2KW was demonstrated, which allows for the removal of 100W/cm 3 heat (>1000 suns) at a △T of I2K. Direct chip attacited silicon coolers enable higher overall concentration factor thereby reducing photovoltaic cell cost. An additional benefit of silicon is its inertness against corrosion and the matching thermal expansion coefficient which allows building of systems with a very long lifetime. The split flow configuration reduces pumping power to about 5% of the system photovoltaic output. More complex manifold micro-channel systems are proposed to minimize the pumping power to a level below 1% and to cool arrays of cells on a single large substrate.

Chandra B.,IBM | Park H.,IBM | Maarouf A.,IBM | Maarouf A.,Egypt Nanotechnology Research Center | And 2 more authors.
Applied Physics Letters | Year: 2011

Carbon nanotube thin film transistors (CNT-TFTs) are fabricated on flexible substrates using purified, surfactant-based CNT suspensions, with >95% semiconducting CNT fraction. The TFTs are made up of local bottom-gated structures with aluminum oxide as the gate dielectric. The devices exhibit high ON current densities (0.1 μA/μm) and on-off ratios (∼105) with mobility values ranging from 10-35 cm2/Vs. A detailed numerical model is used to understand the TFT performance and its dependence on device parameters such as TFT channel length, CNT density, and purity. © 2011 American Institute of Physics.

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