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We report that homogeneous colloidal suspensions of chemically modified graphene sheets were readily produced in a wide variety of organic solvent systems. Two different sets of solubility parameters are used to rationalize when stable colloidal suspensions of graphene oxide sheets and, separately, of reduced graphene oxide sheets in a given solvent type are possible and when they are not. As an example of the utility of such colloidal suspensions, "paperlike" materials generated by very simple filtration of the reduced graphene oxide sheets had electrical conductivity values as high as 16,000 S/m.

In arthropods, evolution has created a remarkably sophisticated class of imaging systems, with a wide-angle field of view, low aberrations, high acuity to motion and an infinite depth of field. A challenge in building digital cameras with the hemispherical, compound apposition layouts of arthropod eyes is that essential design requirements cannot be met with existing planar sensor technologies or conventional optics. Here we present materials, mechanics and integration schemes that afford scalable pathways to working, arthropod-inspired cameras with nearly full hemispherical shapes (about 160 degrees). Their surfaces are densely populated by imaging elements (artificial ommatidia), which are comparable in number (180) to those of the eyes of fire ants (Solenopsis fugax) and bark beetles (Hylastes nigrinus). The devices combine elastomeric compound optical elements with deformable arrays of thin silicon photodetectors into integrated sheets that can be elastically transformed from the planar geometries in which they are fabricated to hemispherical shapes for integration into apposition cameras. Our imaging results and quantitative ray-tracing-based simulations illustrate key features of operation. These general strategies seem to be applicable to other compound eye devices, such as those inspired by moths and lacewings (refracting superposition eyes), lobster and shrimp (reflecting superposition eyes), and houseflies (neural superposition eyes).

Step-by-step controllable thermal reduction of individual graphene oxide sheets, incorporated into multiterminal field effect devices, was carried out at low temperatures (125-240 degrees C) with simultaneous electrical measurements. Symmetric hysteresis-free ambipolar (electron- and hole-type) gate dependences were observed as soon as the first measurable resistance was reached. The conductivity of each of the fabricated devices depended on the level of reduction (was increased more than 10(6) times as reduction progressed), strength of the external electrical field, density of the transport current, and temperature.

We report the production of aqueous suspensions of chemically modified graphene sheets, and electrically conductive “paperlike” material made from filtering such suspensions.

Transparent and electrically conductive composite silica films were fabricated on glass and hydrophilic SiOx/silicon substrates by incorporation of individual graphene oxide sheets into silica sols followed by spin-coating, chemical reduction, and thermal curing. The resulting films were characterized by SEM, AFM, TEM, low-angle X-ray reflectivity, XPS, UV-vis spectroscopy, and electrical conductivity measurements. The electrical conductivity of the films compared favorably to those of composite thin films of carbon nanotubes in silica.

Compound semiconductors like gallium arsenide (GaAs) provide advantages over silicon for many applications, owing to their direct bandgaps and high electron mobilities. Examples range from efficient photovoltaic devices to radio-frequency electronics and most forms of optoelectronics. However, growing large, high quality wafers of these materials, and intimately integrating them on silicon or amorphous substrates (such as glass or plastic) is expensive, which restricts their use. Here we describe materials and fabrication concepts that address many of these challenges, through the use of films of GaAs or AlGaAs grown in thick, multilayer epitaxial assemblies, then separated from each other and distributed on foreign substrates by printing. This method yields large quantities of high quality semiconductor material capable of device integration in large area formats, in a manner that also allows the wafer to be reused for additional growths. We demonstrate some capabilities of this approach with three different applications: GaAs-based metal semiconductor field effect transistors and logic gates on plates of glass, near-infrared imaging devices on wafers of silicon, and photovoltaic modules on sheets of plastic. These results illustrate the implementation of compound semiconductors such as GaAs in applications whose cost structures, formats, area coverages or modes of use are incompatible with conventional growth or integration strategies.

A simple optical method is presented for identifying and measuring the effective optical properties of nanometer-thick, graphene-based materials, based on the use of substrates consisting of a thin dielectric layer on silicon. High contrast between the graphene-based materials and the substrate is obtained by choosing appropriate optical properties and thickness of the dielectric layer. The effective refractive index and optical absorption coefficient of graphene oxide, thermally reduced graphene oxide, and graphene are obtained by comparing the predicted and measured contrasts. 2Identifying and characterizing a single nanometer-scale layer, or a small number of layers, of materials such as graphite, any of a number of clays, or metal dichalcogenides such as WS 2 , is challenging, but is critical for the study of such materials. 1,2 Scanning probe microscopy methods, such as atomic force microscopy (AFM), can both identify the presence of such thin sheets and determine their lateral and vertical dimensions, 3 but are somewhat time-consuming; as well, in order to obtain accurate values of height, thereby discriminating between a single layer of material and a bilayer, a relatively small area must be scanned. Scanning electron microscopy can also, in principle, be used for identification of individual layers versus multilayer sheets, but this imaging typically induces the formation of a layer of contaminant in the exposed region. 4 Optical methods, on the other hand, offer the potential for rapid, non-destructive characterization of large-area samples. Ellipsometry, for example, is widely used to determine the optical constants and thicknesses of thin films. Standard ellipsometers, though, require samples with lateral dimensions well over a millimeter. By contrast, imaging ellipsometry can have sub-micrometer resolution, and may be useful for probing optical constants and thicknesses. 5,6 Investigations into this method are ongoing, and will be reported elsewhere. For the past two years, we have focused on simpler methods that allow the use of standard confocal microscopy for rapid identification and characterization of the optical response of thin sheets. 7,8 In particular, we have investigated the use of substrates designed to interferometrically enhance the visibility of thin sheets. Interference techniques have been used for over half a century to allow for the imaging of low-contrast and transparent samples. 9,10 Over the past decade, microscopy of fluorescent monolayers has been enhanced by incorporating a thin dielectric layer between the material and a reflective substrate. 11Fabry-Perot interference in the dielectric layer modulates the fluorescence intensity, allowing the determination of the thicknesses of surface layers with nanometer precision. 12Recently, a similar method has been used for the identification of single graphene sheets. 1,2Graphene monolayers and multilayers were deposited on substrates consisting of a silicon wafer with an 3 intermediate, 300-nm thick silicon dioxide...