177 research outputs found

    Precise and ultrafast molecular sieving through graphene oxide membranes

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    There has been intense interest in filtration and separation properties of graphene-based materials that can have well-defined nanometer pores and exhibit low frictional water flow inside them. Here we investigate molecular permeation through graphene oxide laminates. They are vacuum-tight in the dry state but, if immersed in water, act as molecular sieves blocking all solutes with hydrated radii larger than 4.5A. Smaller ions permeate through the membranes with little impedance, many orders of magnitude faster than the diffusion mechanism can account for. We explain this behavior by a network of nanocapillaries that open up in the hydrated state and accept only species that fit in. The ultrafast separation of small salts is attributed to an 'ion sponge' effect that results in highly concentrated salt solutions inside graphene capillaries

    Impermeable Barrier Films and Protective Coatings Based on Reduced Graphene Oxide

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    Barrier films preventing permeation of gases and moistures are important for many industries ranging from food to medical and from chemical to electronic. From this perspective, graphene has recently attracted particular interest because its defect free monolayers are impermeable to all gases and liquids. However, it has proved challenging to develop large-area defectless graphene films suitable for industrial use. Here we report barrier properties of multilayer graphitic films made by chemical reduction of easily and cheaply produced graphene oxide laminates. They are found to provide a practically perfect barrier that blocks all gases, liquids and aggressive chemicals including, for example, hydrofluoric acid. In particular, if graphene oxide laminates are reduced in hydroiodic acid, no permeation of hydrogen and water could be detected for films as thin as 30 nm, which remain optically transparent. The films thicker than 100 nm become completely impermeable. The exceptional barrier properties are attributed to a high degree of graphitization of the laminates and little structural damage during reduction. This work indicates a close prospect of thin protective coatings with stability and inertness similar to that of graphene and bulk graphite, which can be interesting for numerous applications

    An alternative approach to the construction of Schur-Weyl transform

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    We propose an alternative approach for the construction of the unitary matrix which performs generalized unitary rotations of the system consisting of independent identical subsystems (for example spin system). This matrix, when applied to the system, results in a change of degrees of freedom, uncovering the information hidden in non-local degrees of freedom. This information can be used, inter alia, to study the structure of entangled states, their classification and may be useful for construction of quantum algorithms.Comment: 6 page

    Electronic structure, optical and magnetic properties of Co2_{2}FeGe Heusler alloy films

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    Optical properties of ferromagnetic half-metallic full-Heusler Co2_{2}FeGe alloy are investigated experimentally and theoretically. Co2_{2}FeGe thin films were obtained by DC magnetron sputtering and show the saturation magnetization at TT=10 K of mm\approx5.6 μB\mu_{B}/f.u., close to the value predicted by the Slater-Pauling rule. First-principles calculations of the electronic structure and the dielectric tensor are performed using the full-potential linearized-augmented-plane-wave method in the generalized gradient (GGA) and GGA+U approximations. The measured interband optical conductivity spectrum for the alloy exhibits a strong absorption band in the 1 - 4 eV energy range with pronounced fine structure, which agrees well with the calculated half-metallic spectrum of the system, suggesting a near perfect spin-polarization in the material.Comment: 6 pages, 9 figure

    Self-Tuning Wireless Network Power Management

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    Current wireless network power management often substantially degrades performance and may even increase overall energy usage when used with latency-sensitive applications. We propose self-tuning power management (STPM) that adapts its behavior to the access patterns and intent of applications, the characteristics of the network interface, and the energy usage of the platform. We have implemented STPM as a Linux kernel module—our results show substantial benefits for distributed file systems, streaming audio, and thin-client applications. Compared to default 802.11b power management, STPM reduces the total energy usage of an iPAQ running the Coda distributed file system by 21% while also reducing interactive file system delay by 80%. Further, STPM adapts to diverse operating conditions: it yields good results on both laptops and handhelds, supports 802.11b network interfaces with substantially different characteristics, and performs well across a range of application network access patterns.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/41365/1/11276_2005_Article_1768.pd

    Excitonic Effects on Optical Absorption Spectra of Doped Graphene

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    We have performed first-principles calculations to study optical absorption spectra of doped graphene with many-electron effects included. Both self-energy corrections and electron-hole interactions are reduced due to the enhanced screening in doped graphene. However, self-energy corrections and excitonic effects nearly cancel each other, making the prominent optical absorption peak fixed around 4.5 eV under different doping conditions. On the other hand, an unexpected increase of the optical absorbance is observed within the infrared and visible-light frequency regime (1 ~ 3 eV). Our analysis shows that a combining effect from the band filling and electron-hole interactions results in such an enhanced excitonic effect on the optical absorption. These unique variations of the optical absorption of doped graphene are of importance to understand relevant experiments and design optoelectronic applications.Comment: 15 pages, 5 figures; Nano Lett., Article ASAP (2011

    Graphene-protected copper and silver plasmonics

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    Plasmonics has established itself as a branch of physics which promises to revolutionize data processing, improve photovoltaics, increase sensitivity of bio-detection. A widespread use of plasmonic devices is notably hindered (in addition to high losses) by the absence of stable and inexpensive metal films suitable for plasmonic applications. This may seem surprising given the number of metal compounds to choose from. Unfortunately, most of them either exhibit a strong damping of surface plasmons or easily oxidize and corrode. To this end, there has been continuous search for alternative plasmonic materials that are, unlike gold, the current metal of choice in plasmonics, compatible with complementary metal oxide semiconductor technology. Here we show that copper and silver protected by graphene are viable candidates. Copper films covered with one to a few graphene layers show excellent plasmonics characteristics surpassing those of gold films. They can be used to fabricate plasmonic devices and survive for at least a year, even in wet and corroding conditions. As a proof of concept, we use the graphene-protected copper to demonstrate dielectric loaded plasmonic waveguides and test sensitivity of surface plasmon resonances. Our results are likely to initiate a wide use of graphene-protected plasmonics.Comment: 22 pages, 5 figure

    Ultrathin graphene-based membrane with precise molecular sieving and ultrafast solvent permeation

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    Graphene oxide (GO) membranes continue to attract intense interest due to their unique molecular sieving properties combined with fast permeation. However, their use is limited to aqueous solutions because GO membranes appear impermeable to organic solvents, a phenomenon not yet fully understood. Here, we report efficient and fast filtration of organic solutions through GO laminates containing smooth two-dimensional (2D) capillaries made from large (10-20 μm) flakes. Without modification of sieving characteristics, these membranes can be made exceptionally thin, down to â 1/410 nm, which translates into fast water and organic solvent permeation. We attribute organic solvent permeation and sieving properties to randomly distributed pinholes interconnected by short graphene channels with a width of 1 nm. With increasing membrane thickness, organic solvent permeation rates decay exponentially but water continues to permeate quickly, in agreement with previous reports. The potential of ultrathin GO laminates for organic solvent nanofiltration is demonstrated by showing >99.9% rejection of small molecular weight organic dyes dissolved in methanol. Our work significantly expands possibilities for the use of GO membranes in purification and filtration technologies
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