Properties of fluorinated graphene films.

Graphene films grown on Cu foils have been fluorinated with xenon difluoride (XeF(2)) gas on one or both sides. When exposed on one side the F coverage saturates at 25% (C(4)F), which is optically transparent, over 6 orders of magnitude more resistive than graphene, and readily patterned. Density functional calculations for varying coverages indicate that a C(4)F configuration is lowest in energy and that the calculated band gap increases with increasing coverage, becoming 2.93 eV for one C(4)F configuration. During defluorination, we find hydrazine treatment effectively removes fluorine while retaining graphene's carbon skeleton. The same films may be fluorinated on both sides by transferring graphene to a silicon-on-insulator substrate enabling XeF(2) gas to etch the Si underlayer and fluorinate the backside of the graphene film to form perfluorographane (CF) for which calculated the band gap is 3.07 eV. Our results indicate single-side fluorination provides the necessary electronic and optical changes to be practical for graphene device applications.

Quantum linear magnetoresistance in multilayer epitaxial graphene.

We report the first observation of linear magnetoresistance (LMR) in multilayer epitaxial graphene grown on SiC. We show that multilayer epitaxial graphene exhibits large LMR from 2.2 K up to room temperature and that it can be best explained by a purely quantum mechanical model. We attribute the observation of LMR to inhomogeneities in the epitaxially grown graphene film. The large magnitude of the LMR suggests potential for novel applications in areas such as high-density data storage and magnetic sensors and actuators.

An Axially Continuous Graphene-Copper Wire for High-Power Transmission: Thermoelectrical Characterization and Mechanisms.

The demand for high-power electrical transmission continues to increase with technical advances in electric vehicles, unmanned drones, portable devices, and deployable military applications. In this study, significantly enhanced electrical properties (i.e., a 450% increase in the current density breakdown limit) are demonstrated by synthesizing axially continuous graphene layers on microscale-diameter wires. To elucidate the underlying mechanisms of the observed enhancements, the electrical properties of pure copper wires and axially continuous graphene-copper (ACGC) wires with three different diameters are characterized while controlling the experimental conditions, including ambient temperature, gases, and pressure. The study reveals that the main mechanism that allows the application of extremely large current densities (>400 000 A cm-2 ) through the ACGC wires is threefold: the continuous graphene layers considerably improve: 1) surface heat dissipation (224% higher), 2) electrical conductivity (41% higher), and 3) thermal stability (41.2% lower resistivity after thermal cycles up to 450 °C), compared with pure copper wires. In addition, it is observed, through the use of high-speed camera images, that the ACGC wires exhibit very different failure behavior near the current density limit, compared with the pure copper wires.

Functionalized Ga2O3 nanowires as active material in room temperature capacitance-based gas sensors.

We report the first evidence for functionalization of Ga(2)O(3) nanowires (NWs), which have been incorporated as the active material in room temperature capacitance gas-sensing devices. An adsorbed layer of pyruvic acid (PA) was successfully formed on Ga(2)O(3) NWs by simple room temperature vapor transport, which was confirmed by Fourier transform infrared spectroscopy. The effect of the adsorbed PA on the surface properties was demonstrated by the change in the response of the NW gas-sensing devices. Results indicate that the adsorption of PA reduced the sensitivity of the Ga(2)O(3) NW device to common hydrocarbons such as nitromethane and acetone while improving the response to triethylamine by an order of magnitude. Taking into account the simplicity of this functionalization together with the ease of producing these capacitance-based gas-sensing devices, this approach represents a viable technique for sensor development.

Evidence for Chemical Vapor Induced 2H to 1T Phase Transition in MoX2 (X = Se, S) Transition Metal Dichalcogenide Films.

Electron-donors can impart charge to the surface of transition metal dichalcogenide (TMD) films while interacting with the film via a weak physisorption bond, making them ideal for vapor and gas sensors. We expose monolayer MoS2 and MoSe2 films to strong electron-donor chemical vapor analytes. After analyzing the resultant behavior and taking into consideration doping effects, we conclude that exposure to strong electron-donors could be a method of inducing the semiconductor-metal 2H-1T TMD phase transition. We find that the conductance response to strong electron donors in both monolayer MoS2 and MoSe2 FET devices ceases after moderate exposure, with final value of the conductance being on order of that expected for the 1T phase. Full device relaxation back to a semiconducting state is accomplished by annealing in vacuum at 400 °C. We also examine chemically exposed TMD films intermittently interrogated with Raman and photoluminescence spectroscopy. We observe the appearance of weak characteristic 1T phase Raman features for MoS2 and we observed a quenching of the photoluminescence of both TMD films that is recoverable with annealing. Considering all of our data together, the effects cannot be described by doping alone. Additionally, our results suggest a mechanism for a new type of passive chemical vapor sensor.

Dynamics of chemical vapor sensing with MoS2 using 1T/2H phase contacts/channel.

Ultra-thin transition metal dichalcogenides (TMDs) films show remarkable potential for use in chemical vapor sensing devices. Electronic devices fabricated from TMD films are inexpensive, inherently flexible, low-power, amenable to industrial-scale processing because of emergent growth techniques, and have shown high sensitivity and selectivity to electron donor analyte molecules important for explosives and nerve gas detection. However, for devices reported to date, the conductance response to chemical vapors is dominated by Schottky contacts, to the detriment of the sensitivity, selectivity, recovery, and obscuring their intrinsic behavior. Here, we use contact engineering to transition the contacts in a MoS2 FET-based chemical vapor sensor to the 1T conducting phase, while leaving the channel in the 2H semiconducting state, and thus providing Ohmic contacts to the film. We demonstrate that the resultant sensors have much improved electrical characteristics, are more selective, and recover fully after chemical vapor exposure-all major enhancements to previously MoS2 sensor devices. We identify labile nitrogen-containing electron donors as the primary species that generate a response in MoS2, and we study the dynamics of the sensing reactions, identifying two possible qualitative models for the chemical sensing reaction.

Electrical stimulation of isolated retina with microwire glass electrodes.

The development of high-resolution retinal prostheses fabricated from silicon wafers presents an interesting problem: how to electrically bridge the space between the flat silicon wafer and the curved retinal surface. One potential "bridge" is a microwire glass electrode. In this paper we present our results in evaluating microwire glass electrodes. We stimulated isolated rabbit retina (n = 5) with a 0.0256 cm(2) microwire electrode. The current and pulse duration were varied from 498 to 1660 microA and 0.1 to 3 ms, respectively. We found that short pulses produced more spikes per coulomb and longer pulses produced more spikes per milliamp. The optimal pulse duration range of 0.7-1 ms was identified as a compromise between the advantages of short and long pulses. Stimulation of isolated rabbit retina with microwire glass results in consistent neuronal spike formation at safe charge density, 20.7 +/- 4.3 microC/cm(2). We also examined the response of retinas (n = 6) to stimulation with a smaller microwire electrode, 0.0002 cm(2). We found that less current was required (15 microA versus 756 microA) for a 1 ms pulse, but at the expense of greater charge density (75 microC/cm(2) versus 29.5 microC/cm(2)). Nonetheless, a 128-fold reduction in area resulted in only a 2.7-fold increase in charge density required for a 1 ms pulse duration. The results presented here indicate that microwire glass can be used as a neural stimulating electrode to bridge the gap between flat microelectronic stimulator chips and curved neuronal tissue.

Reduced graphene oxide molecular sensors.

We demonstrate reduced graphene oxide as the active material for high-performance molecular sensors. Sensors are fabricated from exfoliated graphene oxide platelets that are deposited to form an ultrathin continuous network. These graphene oxide networks are tunably reduced toward graphene by varying the exposure time to a hydrazine hydrate vapor. The conductance change of the networks upon exposure to trace levels of vapor is measured as a function of the chemical reduction. The level of reduction affects both the sensitivity and the level of 1/ f noise. The sensors are capable of detecting 10 s exposures to simulants of the three main classes of chemical-warfare agents and an explosive at parts-per-billion concentrations.

Role of defects in single-walled carbon nanotube chemical sensors.

We explore the electronic response of single-walled carbon nanotubes (SWNT) to trace levels of chemical vapors. We find adsorption at defect sites produces a large electronic response that dominates the SWNT capacitance and conductance sensitivity. This large response results from increased adsorbate binding energy and charge transfer at defect sites. Finally, we demonstrate controlled introduction of oxidation defects can be used to enhance sensitivity of a SWNT network sensor to a variety of chemical vapors.

Capacitance and conductance of single-walled carbon nanotubes in the presence of chemical vapors.

Simultaneous conductance and capacitance measurements on a single-walled carbon nanotube (SWNT) network are used to extract an intrinsic property of molecular adsorbates. Adsorbates from dilute chemical vapors produce a rapid response in both the capacitance and the conductance of the SWNT network. These responses are caused by a combination of two distinct physiochemical properties of the adsorbates: charge transfer and polarizability. We find that the ratio of the conductance response to the capacitance response is a concentration-independent intrinsic property of a chemical vapor that can assist in its identification.