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El Sheikh Zayed City, Egypt

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

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. Source

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

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. Source

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

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. Source

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

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. Source

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

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. Source

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