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Metal-catalyst-free chemical vapor deposition (CVD) of large area uniform nanocrystalline graphene on oxidized silicon substrates is demonstrated. The material grows slowly, allowing for thickness control down to monolayer graphene. The as-grown thin films are continuous with no observable pinholes, and are smooth and uniform across whole wafers, as inspected by optical-, scanning electron-, and atomic force microscopy. The sp2 hybridized carbon structure is confirmed by Raman spectroscopy. Room temperature electrical measurements show ohmic behavior (sheet resistance similar to exfoliated graphene) and up to 13% of electric-field effect. The Hall mobility is ∼40 cm2/Vs, which is an order of magnitude higher than previously reported values for nanocrystalline graphene. Transmission electron microscopy, Raman spectroscopy, and transport measurements indicate a graphene crystalline domain size ∼10 nm. The absence of transfer to another substrate allows avoidance of wrinkles, holes, and etching residues which are usually detrimental to device performance. This work provides a broader perspective of graphene CVD and shows a viable route toward applications involving transparent electrodes. [ABSTRACT FROM AUTHOR]

In this paper, we report a hierarchical simulation study of the electromigration (EM) problem in Cu-carbon nanotube (CNT) composite interconnects. This paper is based on the investigation of the activation energy and self-heating temperature using a multiscale electrothermal simulation framework. We first investigate the electrical and thermal properties of Cu-CNT composites, including contact resistances, using the density functional theory and reactive force field approaches, respectively. The corresponding results are employed in macroscopic electrothermal simulations taking into account the self-heating phenomenon. Our simulations show that although Cu atoms have similar activation energies in both bulk Cu and Cu-CNT composites, Cu-CNT composite interconnects are more resistant to EM thanks to the large Lorenz number of the CNTs. Moreover, we found that a large and homogenous conductivity along the transport direction in interconnects is one of the most important design rules to minimize the EM. [ABSTRACT FROM AUTHOR]

In this study we report on the electron transport in flexible-transparent polymer supported chemically doped chemical vapour deposited (CVD) graphene. We investigate the modified carrier transport following doping with various metal chlorides. An increase in the work function was noted for AuCl3-, FeCl3-, IrCl3-, and RhCl3-doping, whilst only SnCl2 doping showed a reduced work function. We attribute this to dopant-mediated charge transfer resulting in the formation of neutral atomic species. The neutral and near-neutral atomic populations produced metallic aggregates, with this agglomeration level critically dependent on the cohesive energy of the metallic component in each dopant. Micron-scale spatial conductivity mapping highlighted the spatially uniform low resistance of AuCl3-doped graphene. Local conductivity enhancements at grain boundaries and lattice defects within the as-synthesised polycrystalline graphene suggested that the dopant molecules tend to reside at lattice imperfections. Temperature dependent transport studies indicated that the shifted work function improved electrical conductivity due to the increase of barrier transparency between grain boundaries. Variable Range Hopping (VRH) dominated at temperatures ＜140 K in undoped graphene, whereas combined Nearest Neighbour Hopping (NNH) and diffusive transport appears to play a major role throughout the transport in all doped samples. The findings herein reveal that the underlying extended transport mechanisms associated with chemically doped CVD graphene transferred to polymer supports contrast with the highly localised transport in undoped graphene. [ABSTRACT FROM AUTHOR]

We demonstrate the growth of high-quality, continuous monolayer graphene on Cu foils using an open roll-to-roll (R2R) chemical vapor deposition (CVD) reactor with both static and moving foil growth conditions. N2 instead of Ar was used as carrier gas to reduce process cost, and the concentrations of H2 and CH4 reactants were kept below the lower explosive limit to ensure process safety for reactor ends open to ambient. The carrier mobility of graphene deposited at a Cu foil winding speed of 5 mm/min was 5270-6040 cm2V-1 s-1 at room temperature (on 50 μm ×50 μm Hall devices). These results will enable the inline integration of graphene CVD for industrial R2R production. [ABSTRACT FROM AUTHOR]

Graphene GaN-based Schottky ultraviolet detectors are fabricated. The monolayer graphene is grown by chemical vapor deposition. The graphene is much more transparent than metals, as confirmed by the fact that our devices retain their high responsivity up to 360-nm wavelength (corresponding to the band edge absorption of GaN). Importantly, by virtue of the tunable work function of graphene, the graphene GaN Schottky barrier height can be greatly enlarged. The built-in field is enhanced, and the detector performance is improved. The current ratio with and without luminescence is up to 1.6\times 10^4 . The characteristic time constants of the devices are in the order of a few milliseconds. The device open-circuit voltage and short-circuit current are also increased. At last, special type Schottky devices consisting of GaN nanorods or surface-etched GaN are prepared for complementary study. It is found although the dry etching induced surface defects lead to an increase in the dark current, and these carrier traps also greatly contribute to the photoconductivity under luminescence, resulting in extraordinarily large responsivity (up to 360 A/W at −6 V). [ABSTRACT FROM AUTHOR]

The ability to accurately design carbon nanofibre (CN) field emitters with predictable electron emission characteristics will enable their use as electron sources in various applications such as microwave amplifiers, electron microscopy, parallel beam electron lithography and advanced Xray sources. Here, highly uniform CN arrays of controlled diameter, pitch and length were fabricated using plasma enhanced chemical vapour deposition and their individual emission characteristics and field enhancement factors were probed using scanning anode field emission mapping. For a pitch of 10 mm and a CN length of 5 mm, the directly measured enhancement factors of individual CNs was 242, which was in excellent agreement with conventional geometry estimates (240). We show here direct empirical evidence that in regular arrays of vertically aligned CNs the overall enhancement factor is reduced when the pitch between emitters is less than half the emitter height, in accordance to our electrostatic simulations. Individual emitters showed narrow Gaussian-like field enhancement distributions, in excellent agreement with electric field simulations. [ABSTRACT FROM AUTHOR]

A model of the graphene growth mechanism of chemical vapor deposition on platinum is proposed and verified by experiments. Surface catalysis and carbon segregation occur, respectively, at high and low temperatures in the process, representing the so-called balance and segregation regimes. Catalysis leads to self-limiting formation of large area monolayer graphene, whereas segregation results in multilayers, which evidently "grow from below." By controlling kinetic factors, dominantly monolayer graphene whose high quality has been confirmed by quantum Hall measurement can be deposited on platinum with hydrogen-rich environment, quench cooling, tiny but continuous methane flow and about 1000 °C growth temperature. [ABSTRACT FROM AUTHOR]

Direct formation of large-area carbon thin films on gallium nitride by chemical vapor deposition without metallic catalysts is demonstrated. A high flow of ammonia is used to stabilize the surface of the GaN (0001)/sapphire substrate during the deposition at 950^\circC. Various characterization methods verify that the synthesized thin films are largely sp^2 bonded, macroscopically uniform, and electrically conducting. The carbon thin films possess optical transparencies comparable to that of exfoliated graphene. This paper offers a viable route toward the use of carbon-based materials for future transparent electrodes in III-nitride optoelectronics, such as GaN-based light emitting diodes and laser diodes. [ABSTRACT FROM PUBLISHER]

A systematic study of the Cu-catalyzed chemical vapor deposition of graphene under extremely low partial pressure is carried out. A carbon precursor supply of just \it P\bfCH_4\sim0.009 mbar during the deposition favors the formation of large-area uniform monolayer graphene verified by Raman spectra. A diluted \bf HNO_3 solution is used to remove Cu before transferring graphene onto SiO_2/Si substrates or carbon grids. The graphene can be made suspended over a \sim12 \mum distance, indicating its good mechanical properties. Electron transport measurements show the graphene sheet resistance of \sim 0.6 \bf k\Omega /\square at zero gate voltage. The mobilities of electrons and holes are \sim1800 cm^2/Vs at 4.2 K and \sim1200 cm ^2/Vs at room temperature. [ABSTRACT FROM PUBLISHER]

In this paper, we demonstrated a roughness dependent chemical vapor deposition (CVD) process to selectively grow aligned carbon nanotube bundles on silicon substrates with microscratches. Our results show that the aligned nanotubes of uniform length (around 2 μm) and diameter (20~30 nm) are assembled to nanotube bundles and in-plane vertically aligned with the microscratches. The microscratch-selective nanotube growth offers a promising approach for the fabrication of 3-D nanotube architectures in a single CVD process. [ABSTRACT FROM AUTHOR]

A noncatalytic chemical vapor deposition mechanism is proposed, where high precursor concentration, long deposition time, high temperature, and flat substrate are needed to grow large-area nanocrystalline graphene using hydrocarbon pyrolysis. The graphene is scalable, uniform, and with controlled thickness. It can be deposited on virtually any nonmetallic substrate that withstands ∼1000 °C. For typical examples, graphene grown directly on quartz and sapphire shows transmittance and conductivity similar to exfoliated or metal-catalyzed graphene, as evidenced by transmission spectroscopy and transport measurements. Raman spectroscopy confirms the sp2-C structure. The model and results demonstrate a promising transfer-free technique for transparent electrode production. [ABSTRACT FROM AUTHOR]

Large-area uniform carbon films with graphene-like properties are synthesized by chemical vapor deposition directly on Si3N4/Si at 1000 °C without metal catalysts. The as deposited films are atomically thin and wrinkle- and pinhole-free. The film thickness can be controlled by modifying the growth conditions. Raman spectroscopy confirms the sp2 graphitic structures. The films show ohmic behavior with a sheet resistance of ∼2.3-10.5 kΩ/□ at room temperature. An electric field effect of ∼2-10% (VG=-20 V) is observed. The growth is explained by the self-assembly of carbon clusters from hydrocarbon pyrolysis. The scalable and transfer-free technique favors the application of graphene as transparent electrodes. [ABSTRACT FROM AUTHOR]

We demonstrate the use of nanoimprint lithography as an alternative low-cost fabrication route for the production of ordered arrays of individual carbon nanotube field emitters. A high emission site density of 4×105 cm-2 was observed and is well within the specification of the cathode for a field emission display. The measured field enhancement values from the geometry of the nanotubes were in reasonable agreement with the values obtained through electrical measurements. We also show that the distribution of the field enhancement factor is Gaussian, indicative of the presence of well ordered arrays of carbon nanotube field emitters. [ABSTRACT FROM AUTHOR]

Remarkably stable and low noise field-emission electron sources have been obtained using individual carbon nanotubes. The maximum current fluctuation observed over 1 h was merely 0.5% and the emission frequency spectrum exhibited 1/f behavior over a bandwidth of 0.1–25 Hz, above which the shot-noise limit was reached. The average noise percentage was determined to be 0.08% (i.e., a signal-to-noise ratio of 1250) for seven CNT emitters operated at several current levels. The influence of the vacuum level was also investigated. [ABSTRACT FROM AUTHOR]

Electron emission measurements were conducted on individual carbon nanotubes. The nanotubes had a closed end and their surfaces were thoroughly cleaned. It is shown conclusively that individual carbon nanotube electron emitters indeed exhibit Fowler–Nordheim behavior and have a work function of 5.1±0.1 eV for the nanotubes under investigation, which had diameters of 1.4 and 4.9 nm. [ABSTRACT FROM AUTHOR]

We have successfully fabricated a pH-sensor array based on chemical-vapor-deposition (CVD)-synthesized graphene. As large-scale monolayer graphene is synthesized by this method, the size and the position of graphene can be controlled. Therefore, after transferring graphene onto SiO2/Si substrates, a graphene field-effect transistor (FET) array was produced. The sensing characteristics of the CVD-synthesized graphene-based device were investigated using three buffer solutions with different pH values (pHs 4.0, 6.8, and 9.3). The electrical measurements reveal that for most of the graphene FETs in the array, a similar stepwise increment in drain current was observed upon the introduction of each buffer solution with increasing pH value sequence. This will lead to the realization of the fabrication of multiplex hand-held chemical and biological sensors based on CVD-synthesized graphene. [ABSTRACT FROM AUTHOR]

An improved technique for transferring large area graphene grown by chemical vapor deposition on copper is presented. It is based on mechanical separation of the graphene/copper by H2 bubbles during H2O electrolysis, which only takes a few tens of seconds while leaving the copper cathode intact. A semi-rigid plastic frame in combination with thin polymer layer span on graphene gives a convenient way of handling- and avoiding wrinkles and holes in graphene. Optical and electrical characterizations prove the graphene quality is better than that obtained by traditional wet etching transfer. This technique appears to be highly reproducible and cost efficient. [ABSTRACT FROM AUTHOR]

To communicate, spacecraft and satellites rely on microwave devices, which at present are based on relatively inefficient thermionic electron sources that require heating and cannot be switched on instantaneously. Here we describe a microwave diode that uses a cold-cathode electron source consisting of carbon nanotubes and that operates at high frequency and at high current densities. Because it weighs little, responds instantaneously and has no need of heating, this miniaturized electron source should prove valuable for microwave devices used in telecommunications. [ABSTRACT FROM AUTHOR]