Unilamellar colloids of graphite oxide (GO) were prepared from natural graphite and were grown as monolayer and multilayer thin films on cationic surfaces by electrostatic selfassembly. The multilayer films were grown by alternate adsorption of anionic GO sheets and cationic poly(allylamine hydrochloride) (PAH). The monolayer films consisted of 11-14 Å thick GO sheets, with lateral dimensions between 150 nm and 9 µm. Silicon substrates primed with amine monolayers gave partial GO monolayers, but surfaces primed with Al 13 O 4 -(OH) 24 (H 2 O) 12 7+ ions gave densely tiled films that covered approximately 90% of the surface. When alkaline GO colloids were used, the monolayer assembly process selected the largest sheets (from 900 nm to 9 µm) from the suspension. In this case, many of the flexible sheets appeared folded in AFM images. Multilayer (GO/PAH) n films were invariably thicker than expected from the individual thicknesses of the sheets and the polymer monolayers, and this behavior is also attributed to folding of the sheets. Multilayer (GO/PAH) n and (GO/ polyaniline) n films grown between indium-tin oxide and Pt electrodes show diodelike behavior, and higher currents are observed with the conductive polyaniline-containing films. The resisitivity of these films is decreased, as expected, by partial reduction of GO to carbon.

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.

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slightly changed after their incorporation in polymer matrices, the optical properties of the resulting composites are, in principle, determined by the properties of the initial CdTe nanocrystals in solution. Therefore, this new synthetic route opens up an alternative method to produce highly fluorescent CdTe±polymer composites with predictable properties. Moreover, the fluorescent microbeads of nanocrystal±polymer composite could find application in biological labeling. [8b] Further attempts are underway to produce micrometer-sized CdTe±PS beads by swelling crosslinked PS beads in a solvent mixture containing OVDAC-coated CdTe and styrene, and followed by the polymerization of composite CdTe nanocrystals in the swollen PS beads. ExperimentalA series of aqueous colloidal CdTe solutions were prepared by adding freshly prepared NaHTe solution to 1.25 10 ±3 N N 2 -saturated CdCl 2 solutions at pH 9.0 in the presence of 3-mercaptocarboxylic acid (MPA) as a stabilizing agent [5,19]. The molar ratio of Cd 2+ /MPA/HTe ± was fixed at 1:2.4:0.5. The resulting mixture was then subjected to reflux that controlled the growth of the CdTe nanocrystals. Octadecyl-p-vinylbenzyldimethylammonium chloride (OVDAC) was synthesized according to the procedure reported by Aoyagi et al. [23] and dissolved in styrene at a concentration of 2 mg mL ±1 . 10 mL of OVDAC styrene solution was added to 20 mL of CdTe aqueous solution under vigorous stirring. The styrene phase was then separated and polymerized in a glass tube in an oil bath at 72~74 C for 30 h using 0.2 wt.-% azobisisobutyronitrile (AIBN) as initiator. Transparent CdTe±polystyrene composites resulted. For more quantitative syntheses, chloroform was used instead of styrene. Powders of the OVDAC-coated CdTe was obtained after evaporation of chloroform and a drying process. Then the composite nanocrystals were redispersed quantitatively (from 0.1 wt.-% to 10 wt.-%) in styrene, which was followed by freeradical polymerization initiated by 0.2 wt.-% AIBN in an oil bath. CdTe±poly-styrene/poly(methyl methacrylate) (PS/PMMA) composites were obtained in a similar way by dispersing CdTe±OVDAC in the mixture of methyl methacrylate and styrene with 10:1 volume ratio. The concentration of AIBN is 0.1 wt.-% in the following polymerization which was realized by a programmed heating process from 50 to 80 C.The relative PL quantum yields of all CdTe samples were estimated using Rohodamine 6G as PL reference [5,19]. For the measurement of PL quantum yield of CdTe in PS composites, composite plates were fabricated using a planar chamber as the polymerizing mode. The concentration of the inorganic nanocrystals in the polymer composites was characterized by thermogravimetric analysis (TGA), which was performed on a Mettler Netzsch STA 449C thermoanalysis instrument (Analyses were carried out under N 2 flow in the range of 30±800 C with heating rate of 20.0 C min ±1 ). For the CdTe±PS composite samples containing 10 wt.-% CdTe±OVDAC composites, the concentration of inorganic CdTe was around 3 wt....

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.

Alumina membranes containing 200 nm diameter pores were replicated electrochemically with metals (Au and Ag) to make free-standing nanowires several microns in length. Wet layer-by-layer assembly of nanoparticle (TiO2 or ZnO)/polymer thin films was carried out in the membrane between electrodeposition steps to give nanowires that contained rectifying junctions. Concentric structures with similar properties were prepared by first coating the membrane walls with multilayer films, and then growing nanowires inside the resulting tubules, or by growing films on the exposed surface of the nanowires after dissolution of the membrane. The I−V characteristics of nanowires prepared by either technique show current rectifying behavior. The electronic properties of Au(MEA)/(ZnO/PSS)19ZnO/Ag (MEA = mercaptoethylamine) devices indicate that rectification is determined by charge injection at the metal/ZnO/PSS-film interface rather than by a tunneling mechanism. In the case of Ag(TiO2/PSS)9TiO2/Au devices, switching behavior and hysteresis that could not be described by Schottky or Fowler−Nordheim characteristics was found. The combined replication/layer-by-layer synthetic approach allows one to control both the geometry and the chemical composition of diode nanowire devices.

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.

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 V DS = −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.