Ant Ural

Hydrogen Sensing Using Pd-Functionalized Multi-Layer Graphene Nanoribbon Networks

www.MaterialsViews.com C O M M Hydrogen Sensing Using Pd-Functionalized Multi-Layer Graphene Nanoribbon Networks U N IC By Jason L. Johnson , Ashkan Behnam , S. J. Pearton , and Ant Ural * A IO N Sensing of gas molecules is critical in many fi elds including environmental monitoring, transportation, defense, space missions, energy, agriculture, and medicine. Solid state gas sensors have been developed for many of these applications. [ 1–3 ] More recently, chemical gas sensors based on nanoscale materials, such as carbon nanotubes and semiconductor nanowires, have attracted signifi cant research attention due to their naturally small size, large surface-to-volume ratio, low power consumption, room temperature operation, and simple fabrication. [ 4–6 ]

Electric-field-aligned growth of single-walled carbon nanotubes on surfaces

Aligned single-walled carbon nanotubes are grown onto the surfaces of SiO2/Si substrates in electric fields established across patterned metal electrodes. Calculations of the electric field distribution under the designed electrode structures, the directing ability of electric fields, and the prevention of surface van der Waals interactions are used to rationalize the aligned growth. The capability of synthesizing oriented single-walled nanotubes on surfaces shall open up many opportunities in organized architectures of nanotubes for molecular electronics.

Fractional contributions of microscopic diffusion mechanisms for common dopants and self-diffusion in silicon

An identical set of thermal oxidation and nitridation experiments has been performed for four common dopants and self-diffusion in Si. Selectively perturbing the equilibrium point-defect concentrations by these surface reactions is a powerful tool for identifying the relative importance of the various atomic-scale diffusion mechanisms. We obtain bounds on the fractional contributions of the self-interstitial, vacancy, and concerted exchange mechanisms for arsenic, boron, phosphorus, antimony, and self-diffusion in Si at temperatures of 1100 and 1000 °C. These bounds are found by simultaneously solving a system of equations making only very conservative assumptions. The validity of common approximations found in previous work and their effects on the results are also analyzed in detail. We find that B and P diffuse by a self-interstitial mechanism, whereas Sb diffusion is almost exclusively vacancy mediated. As and self-diffusion, on the other hand, exhibit evidence for a dual vacancy-interstitial mechanis...

Metal-semiconductor-metal photodetectors based on graphene/p-type silicon Schottky junctions

Metal-semiconductor-metal (MSM) photodetectors based on graphene/p-type Si Schottky junctions are fabricated and characterized. Thermionic emission dominates the transport across the junctions above 260 K with a zero-bias barrier height of 0.48 eV. The reverse-bias dependence of the barrier height is found to result mostly from the Fermi level shift in graphene. MSM photodetectors exhibit a responsivity of 0.11 A/W and a normalized photocurrent-to-dark current ratio of 4.55 × 104 mW−1, which are larger than those previously obtained for similar detectors based on carbon nanotubes. These results are important for the integration of transparent, conductive graphene electrodes into existing silicon technologies.

Effects of nanotube alignment and measurement direction on percolation resistivity in single-walled carbon nanotube films

We have used Monte Carlo simulations to study the effects of nanotube alignment and measurement direction on the resistivity in single-walled carbon nanotube films. These films consist of multiple layers of conductive nanotube networks with percolative transport as the dominant conduction mechanism. We find that minimum resistivity occurs for a partially aligned rather than a perfectly aligned nanotube film. When nanotubes are strongly aligned, the film resistivity becomes highly dependent on the measurement direction. We also find that aligning the nanotubes too strongly or measuring the resistivity in a direction which is very different from the alignment direction causes the film to approach the percolation threshold, as evidenced by the inverse power law increase in resistivity. Furthermore, the location of the resistivity minimum and the values of the inverse power law critical exponents are not universal, but depend strongly on other nanotube and device parameters. To illustrate this explicitly, we ...

Room temperature hydrogen detection using Pd-coated GaN nanowires

Multiple GaN nanowires produced by thermal chemical vapor deposition were employed as gas sensors for detection of hydrogen at concentrations from 200–1500 ppm in N2 at 300 K. Palladium coating of the wires improved the sensitivity by a factor of up to 11 at low ppm concentrations relative to uncoated controls. The GaN nanowires showed relative responses of ∼7.4% at 200 ppm and ∼9.1% at 1500 ppm H2 in N2 after a 10 min exposure. Upon removal of hydrogen from the measurement ambient, ∼90% of the initial GaN conductance was recovered within 2 min. Temperature dependent measurements showed a larger relative response and shorter response time at elevated temperature. The adsorption activation energy of the sensor was 2.2 kcal mol−1 at 3000 ppm H2 in N2. These sensors exhibit low power consumption (<0.6 mW) at 300 K.

Nitride and oxide semiconductor nanostructured hydrogen gas sensors

In this paper, we discuss the progress of nitride and oxide semiconductor nanostructures for hydrogen gas sensing. The use of catalyst metal coatings on GaN, InN and ZnO nanowires is found to greatly enhance the detection sensitivity. Pt- and Pd-coated GaN nanowires biased at small voltages show large changes in currents upon exposure to H2 gas at concentrations in the ppm range. Improvements in growth techniques for InN nanostructures have produced nanobelts and nanorods capable of hydrogen detection down to 20 ppm after catalyst coating. Functionalized ZnO nanorods were also investigated for hydrogen detection, but did not generate a relative response as high as that for the nitride-based sensors. All sensors tested exhibited no response at room temperature upon exposure to various other gases including O2, C2H5, N2O and CO2. The high surface-to-volume ratio of nanowires and the ability to use simple contact fabrication schemes make them attractive for hydrogen sensing applications.

Experimental characterization of single-walled carbon nanotube film-Si Schottky contacts using metal-semiconductor-metal structures

We demonstrate that single-walled carbon nanotube (CNT) films make a Schottky contact on silicon by experimentally characterizing metal-semiconductor-metal (MSM) structures. We find that at temperatures above 240K, thermionic emission is the dominant transport mechanism across CNT film-Si contacts, and at lower temperatures tunneling begins to dominate. At high bias voltages, the CNT film MSM devices exhibit a higher photocurrent-to-dark current ratio relative to that of metal control devices. Our results not only provide insight into the fundamental electronic properties of the CNT film-Si junction but also opens up the possibility of integrating CNT films as Schottky electrodes in conventional Si-based devices.

Resistivity scaling in single-walled carbon nanotube films patterned to submicron dimensions

The authors describe efficient patterning of transparent, conductive single-walled carbon nanotube thin films by photolithography and e-beam lithography followed by reactive ion etching, and study the transport characteristics of the films patterned down to 200nm lateral dimensions. The resistivity of the films is independent of device length, while increasing over three orders of magnitude compared to the bulk films, as their width and thickness shrink. This behavior is explained by a geometrical argument. Such “top-down” patterning of nanotube films should permit their integration into submicron device structures; however, the strong resistivity scaling will have to be taken into account.

A computational study of tunneling-percolation electrical transport in graphene-based nanocomposites

Using a tunneling-percolation model and Monte Carlo simulations, we study the resistivity of graphene-based nanocomposites as a function of both graphene sheet and device parameters. We observe an inverse power law dependence of resistivity on device dimensions and volume fraction near the percolation threshold, and find that high aspect ratio graphene sheets result in a much lower resistivity, particularly at low sheet densities. Furthermore, we find that graphene sheet area affects nanocomposite resistivity more strongly than sheet density does. These results impart important fundamental insights for future experimental investigations and applications of graphene-based conductive nanocomposites.