Atomically Thin Layers of Graphene and Hexagonal Boron Nitride Made by Solvent Exfoliation of Their Phosphoric Acid Intercalation Compounds.

The development of scalable and reliable techniques for the production of the atomically thin layers of graphene and hexagonal boron nitride (h-BN) in bulk quantities could make these materials a powerful platform for devices and composites that impact a wide variety of technologies (Nature 2012, 490, 192-200). To date a number of practical exfoliation methods have been reported that are based on sonicating or stirring powdered graphite or h-BN in common solvents. However, the products of these experiments consist mainly of few-layer sheets and contain only a small fraction of monolayers. A possible reason for this is that splitting the crystals into monolayers starts from solvent intercalation, which must overcome the substantial interlayer cohesive energy (120-720 mJ/m2) of the van der Waals solids. Here we show that the yield of the atomically thin layers can be increased to near unity when stage-1 intercalation compounds of phosphoric acid are used as starting materials. The exfoliation to predominantly monolayers was achieved by stirring them in medium polarity organic solvents that can form hydrogen bonds. The exfoliation process does not disrupt the sp2 π-system of graphene and is gentle enough to allow the preparation of graphene and h-BN monolayers that are tens of microns in their lateral dimensions.

Non-oxidative intercalation and exfoliation of graphite by Brønsted acids.

Graphite intercalation compounds are formed by inserting guest molecules or ions between sp(2)-bonded carbon layers. These compounds are interesting as synthetic metals and as precursors to graphene. For many decades it has been thought that graphite intercalation must involve host-guest charge transfer, resulting in partial oxidation, reduction or covalent modification of the graphene sheets. Here, we revisit this concept and show that graphite can be reversibly intercalated by non-oxidizing Brønsted acids (phosphoric, sulfuric, dichloroacetic and alkylsulfonic acids). The products are mixtures of graphite and first-stage intercalation compounds. X-ray photoelectron and vibrational spectra indicate that the graphene layers are not oxidized or reduced in the intercalation process. These observations are supported by density functional theory calculations, which indicate a dipolar interaction between the guest molecules and the polarizable graphene sheets. The intercalated graphites readily exfoliate in dimethylformamide to give suspensions of crystalline single- and few-layer graphene sheets.

Reversible intercalation of hexagonal boron nitride with Brønsted acids.

Hexagonal boron nitride (h-BN) is an insulating compound that is structurally similar to graphite. Like graphene, single sheets of BN are atomically flat, and they are of current interest in few-layer hybrid devices, such as transistors and capacitors, that contain insulating components. While graphite and other layered compounds can be intercalated by redox reactions and then converted chemically to suspensions of single sheets, insulating BN is not susceptible to oxidative intercalation except by extremely strong oxidizing agents. We report that stage-1 intercalation compounds can be formed by simple thermal drying of h-BN in Brønsted acids H2SO4, H3PO4, and HClO4. X-ray photoelectron and vibrational spectra, as well as electronic structure and molecular dynamics calculations, demonstrate that noncovalent interactions of these oxyacids with the basic N atoms of the sheets drive the intercalation process.

Conductive indium-tin oxide nanowire and nanotube arrays made by electrochemically assisted deposition in template membranes: switching between wire and tube growth modes by surface chemical modification of the template.

Tin-doped indium hydroxide (InSnOH) nanowires (NWs) and nanotubes (NTs) were grown from acidic aqueous solutions of inorganic precursors in a simple one-step electrochemically assisted deposition (EAD) process inside Au-plugged anodic aluminium oxide and polycarbonate membranes. When the membranes were used without any pre-treatment, InSnOH crystals nucleated on the both the Au-cathode and pore wall surfaces. By adjusting the surface chemistry of Au or the pore walls, it was possible to switch between NW and NT growth modes. InSnOH was converted into indium tin oxide (ITO) by annealing the InSnOH-filled membranes at 300 °C. The resulting wires and tubes were characterized by field emission scanning electron microscopy, transmission electron microscopy, X-ray and electron diffraction, Auger electron spectroscopy and electrical conductivity measurements. InSnOH and ITO NWs and NTs consisted of ∼25-50 nm in size crystalline grains with the cubic crystal structures of In(OH)(3) and In(2)O(3), respectively, and showed essentially the same morphological features as planar ITO films made by the same method. Separate tin oxide/hydroxide phases were not observed by any of the characterization methods. After heating in air at 600 °C, the ITO NWs had resistivity on the order of 10°Ω cm. EAD is an inexpensive and scalable solution-based technique, and allows one to grow dense arrays of vertically aligned, crystalline and conductive ITO NWs and NTs.

Ultrathin anisotropic films assembled from individual single-walled carbon nanotubes and amine polymers.

Oxidized individual single-walled carbon nanotubes and amine polymers have been assembled into 11-32-nm-thick well-ordered conductive films. The films show highly anisotropic electrical conductivity, which is dominated by the nanotubes in the horizontal plane and by polymer-mediated tunneling in the vertical direction. The ratio of the "along" to "across" conductivity is approximately 10(3). The subnanometer thick polymer layers interleaved with monolayers of nanotubes show conductivity several orders of magnitude higher than films of pristine polymers.

Coaxially gated in-wire thin-film transistors made by template assembly.

Nanowire field effect transistors were prepared by a wet chemical template replication method using anodic aluminum oxide membranes. The membrane pores were first lined with a thin SiO2 layer by the surface sol-gel method. Au, CdS (or CdSe), and Au wire segments were then sequentially electrodeposited within the pores, and the resulting nanowires were released by dissolution of the membrane. Electrofluidic alignment of these nanowires between source and drain leads and evaporation of gold over the central CdS (CdSe) stripe affords a "wrap-around gate" structure. At VDS = -2 V, the Au/CdS/Au devices had an ON/OFF current ratio of 103, a threshold voltage of 2.4 V, and a subthreshold slope of 2.2 V/decade. A 3-fold decrease in the subthreshold slope relative to that of planar nanocrystalline CdSe devices can be attributed to coaxial gating. The control of dimensions afforded by template synthesis should make it possible to reduce the gate dielectric thickness, channel length, and diameter of the semiconductor segment to sublithographic dimensions while retaining the simplicity of the wet chemical synthetic method.

Individual single-walled nanotubes and hydrogels made by oxidative exfoliation of carbon nanotube ropes.

Single-walled carbon nanotubes were oxidized by a technique previously developed for the oxidation of graphite to graphite oxide (GO). This process involves treatment with concentrated H(2)SO(4) containing (NH(4))(2)S(2)O(8) and P(2)O(5), followed by H(2)SO(4) and KMnO(4). Oxidation results in complete exfoliation of nanotube ropes to yield individual oxidized tubes that are 40-500 nm long. The C:O:H atomic ratio of vacuum-dried oxidized nanotubes is approximately 2.7:1.0:1.2. XPS and IR spectra show evidence for surface O-H, C=O, and COOH groups. The oxidized nanotubes slowly form viscous hydrogels at unusually low concentration (>or=0.3 wt %), and this behavior is attributed to the formation of a hydrogen-bonded nanotube network. The oxidized tubes bind readily to amine-coated surfaces, on which they adsorb as smooth and dense monolayer films. Thin films of the oxidized nanotubes show ohmic current-voltage behavior, with resistivities in the range of 0.2-0.5 Omega-cm.

Nanowires as building blocks for self-assembling logic and memory circuits.

The concept of assembling electronic circuits from metal nanowires is discussed. These nanowires are synthesised electrochemically by using porous membranes as templates. High aspect ratio wires, which range from 15 to 350 nm in diameter and contain "stripes" of different metals, semiconductors, colloid/polymer multilayers, and self-assembling monolayers have been made by this technique. By using the distinct surface chemistry of different stripes, the nanowires can be selectively derivatized and positioned on patterned surfaces. This allows the current-voltage properties of single and crossed nanowire devices to be measured. Nanowire conductors, rectifiers, switches, and photoconductors have been characterized. Techniques are still being developed for assembling sublithographic scale nanowires into cross-point arrays for memory and logic applications.