We report graphene films composed mostly of one or two layers of graphene grown by controlled carbon precipitation on the surface of polycrystalline Ni thin films during atmospheric chemical vapor deposition (CVD). Controlling both the methane concentration during CVD and the substrate cooling rate during graphene growth can signifi cantly improve the thickness uniformity. As a result, one-or two-layer graphene regions occupy up to 87% of the fi lm area. Single layer coverage accounts for 5 % 11% of the overall fi lm. These regions expand across multiple grain boundaries of the underlying polycrystalline Ni fi lm. The number density of sites with multilayer graphene/graphite (>2 layers) is reduced as the cooling rate decreases. These fi lms can also be transferred to other substrates and their sizes are only limited by the sizes of the Ni fi lm and the CVD chamber. Here, we demonstrate the formation of fi lms as large as 1 in 2 . These fi ndings represent an important step towards the fabrication of large-scale high-quality graphene samples. KEYWORDS Graphene, chemical vapor deposition, nickel catalyst, carbon nanomaterialsAddress correspondence to jingkong@mit.edu Exploring ways to synthesize graphene which allow scalability, have low fabrication cost and facilitate integration with other materials, could play an important role in both fundamental research and the realization of future graphene applications. Several graphene synthesis approaches have been developed, including: (1) exfoliation methods (both mechanical [1,2] and chemical [3 8]); (2) graphitization of SiC surfaces [9,10]; (3) graphene precipitation/deposition on transition metal surfaces [11,12]; (4) gas phase/ substrate-free formation of graphene sheets [13]. Procedures such as (1) and (4) produce free-standing graphene isolated from any substrate which enables the integration of graphene with other materials. Methods like (2) and (3) produce graphene bound to a specific substrate which limits the flexibility of these approaches. Recently, it has been shown that graphene films can be grown by ambient pressure chemical vapor deposition (CVD) on thin films of transition metals and isolated from their growth Here, we present an important advance to further improve the thickness uniformity of these fi lms. We show that the area of multilayer graphene regions on the fi lm can be reduced and the regions with singleor bi-layer graphene (1 2 LG) can be increased to 87% of the overall fi lm area. The precipitation of monolayer and multilayer graphene from bulk transition metals is widely known [18,19]. It occurs due to the temperature-dependent solubility of carbon in transition metals. This concept has recently been used to produce 1 2 LG under either vacuum conditions [11,19] or in an ambient pressure CVD process [14 17]. In our process, carbon is introduced into the bulk of thin (~500 nm) Ni films by decomposing a highly diluted hydrocarbon gas (CH 4 ). Graphene precipitation is promoted on the surface of the Ni fi lms upon cooling of the Ni C ...

A quasianalytical modeling approach for graphene metal-oxide-semiconductor field-effect transistors (MOSFETs) with gapless large-area graphene channels is presented. The model allows the calculation of the I-V characteristics, the small-signal behavior, and the cutoff frequency of graphene MOSFETs. It applies a correct formulation of the density of states in large-area graphene to calculate the carrier-density-dependent quantum capacitance, a steady-state velocity-field characteristics with soft saturation to describe the carrier transport, and takes the source/drain series resistances into account. The modeled drain currents and transconductances show very good agreement with experimental data taken from the literature {Meric et al., [Nat. Nanotechnol. 3, 654 (2008)] and Kedzierski et al., [IEEE Electron Device Lett. 30, 745 (2009)]}. In particular, the model properly reproduces the peculiar saturation behavior of graphene MOSFETs with gapless channels.

Due to sampling difficulties, little is known about microbial communities associated with sinking marine snow in the twilight zone. A drifting sediment trap was equipped with a viscous cryogel and deployed to collect intact marine snow from depths of 100 and 400 m off Cape Blanc (Mauritania). Marine snow aggregates were fixed and washed in situ to prevent changes in microbial community composition and to enable subsequent analysis using catalyzed reporter deposition fluorescence in situ hybridization (CARD-FISH). The attached microbial communities collected at 100 m were similar to the free-living community at the depth of the fluorescence maximum (20 m) but different from those at other depths (150, 400, 550, and 700 m). Therefore, the attached microbial community seemed to be "inherited" from that at the fluorescence maximum. The attached microbial community structure at 400 m differed from that of the attached community at 100 m and from that of any free-living community at the tested depths, except that collected near the sediment at 700 m. The differences between the particle-associated communities at 400 m and 100 m appeared to be due to internal changes in the attached microbial community rather than de novo colonization, detachment, or grazing during the sinking of marine snow. The new sampling method presented here will facilitate future investigations into the mechanisms that shape the bacterial community within sinking marine snow, leading to better understanding of the mechanisms which regulate biogeochemical cycling of settling organic matter.T he formation of macroscopic organic aggregates such as marine snow is an important process for the removal of photosynthetically fixed carbon from the surface ocean to the deep sea (1). The export of organic matter is the main driver for the oceanic sequestering of atmospheric carbon dioxide on long time scales (2).Settling marine snow aggregates harbor diverse microbial communities that play an important role in the degradation of the organic compounds and release of dissolved nutrients and organic matter to the water column (3-5). Therefore, the attached microbial communities may play an important role in the regulation of the magnitude and efficiency of the biological carbon pump. However, little is known about bacterial community composition and dynamics within marine snow collected below the depths reached by scuba divers. This is because the fragile nature of marine snow makes the collection of individual, intact marine snow particles nearly impossible.Previous investigations have relied on size fractionation of water samples to observe and compare clustering of free-living bacteria to that of bacteria associated with particles (5-8). These studies observed that the composition of the microbial communities associated with particles differed from that of free-living communities in the surrounding water column. However, other studies have found similarities between the free-living and attached microbial communities (9-11), potentially indicating methodolo...

bIron fertilization experiments in high-nutrient, low-chlorophyll areas are known to induce phytoplankton blooms. However, little is known about the response of the microbial community upon iron fertilization. As part of the LOHAFEX experiment in the southern Atlantic Ocean, Bacteria and Archaea were monitored within and outside an induced bloom, dominated by Phaeocystis-like nanoplankton, during the 38 days of the experiment. The microbial production increased 1.6-fold (thymidine uptake) and 2.1-fold (leucine uptake), while total cell numbers increased only slightly over the course of the experiment. 454 tag pyrosequencing of partial 16S rRNA genes and catalyzed reporter deposition fluorescence in situ hybridization (CARD FISH) showed that the composition and abundance of the bacterial and archaeal community in the iron-fertilized water body were remarkably constant without development of typical bloom-related succession patterns. Members of groups usually found in phytoplankton blooms, such as Roseobacter and Gammaproteobacteria, showed no response or only a minor response to the bloom. However, sequence numbers and total cell numbers of the SAR11 and SAR86 clades increased slightly but significantly toward the end of the experiment. It seems that although microbial productivity was enhanced within the fertilized area, a successionlike response of the microbial community upon the algal bloom was averted by highly effective grazing. Only small-celled members like the SAR11 and SAR86 clades could possibly escape the grazing pressure, explaining a net increase of those clades in numbers.

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It has been shown that few-layer graphene films can be grown by atmospheric chemical vapor deposition using deposited Ni thin films on SiO(2)/Si substrates. In this paper we report the correlation between the thickness variations of the graphene film with the grain size of the Ni film. Further investigations were carried out to increase the grain size of a polycrystalline nickel film. It was found that the minimization of the internal stress not only promotes the growth of the grains with (111) orientation in the Ni film, but it also increases their grain size. Different types of SiO(2) substrates also affect the grain size development. Based upon these observations, an annealing method was used to promote large grain growth while maintaining the continuity of the nickel film. Graphene films grown from Ni films with large versus small grains were compared for confirmation.

This research considers the effects of diffraction, attenuation, and the nonlinearity of generating sources on measurements of nonlinear ultrasonic Rayleigh wave propagation. A new theoretical framework for correcting measurements made with air-coupled and contact piezoelectric receivers for the aforementioned effects is provided based on analytical models and experimental considerations. A method for extracting the nonlinearity parameter β11 is proposed based on a nonlinear least squares curve-fitting algorithm that is tailored for Rayleigh wave measurements. Quantitative experiments are conducted to confirm the predictions for the nonlinearity of the piezoelectric source and to demonstrate the effectiveness of the curve-fitting procedure. These experiments are conducted on aluminum 2024 and 7075 specimens and a β11(7075)/β11(2024) measure of 1.363 agrees well with previous literature and earlier work. The proposed work is also applied to a set of 2205 duplex stainless steel specimens that underwent various degrees of heat-treatment over 24h, and the results improve upon conclusions drawn from previous analysis.