Jason L. Johnson

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 ]

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.

Experimental study of graphitic nanoribbon films for ammonia sensing

We fabricate and study the ammonia sensing properties of graphitic nanoribbon films consisting of multi-layer graphene nanoribbons. These films show very good sensitivity to parts-per-million (ppm) level concentrations of ammonia, which is further enhanced by platinum functionalization, resulting in a relative resistance response of ∼70% when exposed to 50 ppm ammonia. In addition, the sensing response exhibits excellent repeatability and full recovery in air. We also study in detail the dependence of the sensing response on ammonia concentration and temperature. We find that the relative resistance response of the graphitic nanoribbon films shows a power-law dependence on the ammonia concentration, which can be explained based on the Freundlich isotherm. The activation energy obtained from an Arrhenius plot of the temperature-dependent measurements is ∼50 meV, which is consistent with the theoretical calculations of the adsorption energies of ammonia on large graphene sheets and nanoribbons. Their simple...

Metal-semiconductor-metal photodetectors based on single-walled carbon nanotube film-GaAs Schottky contacts

We demonstrate the Schottky behavior of single-walled carbon nanotube (CNT) film contacts on GaAs by fabricating and characterizing metal-semiconductor-metal (MSM) photodetectors with CNT film electrodes. We extract the Schottky barrier height of CNT film contacts on GaAs by measuring the dark I-V characteristics as a function of temperature. The results show that at temperatures above ∼260 K, thermionic emission of electrons with a barrier height of ∼0.54 eV is the dominant transport mechanism in CNT film–GaAs junctions, whereas at lower temperatures, tunneling begins to dominate suggested by the weak dependence of current on temperature. Assuming an ideal MS diode, this barrier height corresponds to a CNT film workfunction of ∼4.6 eV, which is in excellent agreement with the previously reported values. Furthermore, we characterize the effect of device geometry on the dark current and find that dark currents of the MSM devices scale rationally with device geometry, such as the device active area, finger ...

Electronic Transport in Graphitic Nanoribbon Films

We fabricate, pattern, and analyze thin films composed of multilayer graphene nanoribbons. These films are conductive at room temperature but depict noticeable insulating behavior at low temperatures (<20 K) due to their disordered structure. We study the transport in this strong localization regime by analyzing the dependence of resistivity on temperature and electric and magnetic fields in the framework of the variable range hopping theory. Resistivity dependence on the magnetic field confirms the insulating behavior of the films and can be fitted effectively by forward interference scattering and wave function shrinkage models at low and high magnetic field regimes, respectively. We extract large localization lengths in the range of ∼45-90 nm from both the magnetic and electric field dependence of resistivity and relate these values to the high conductance in the nanoribbons and/or good contact between them. By revealing the fundamental structural and transport properties of graphitic nanoribbon films, our results help devise methods to further improve these films for electronic and photonic device applications.

GaN nanowire and Ga2O3 nanowire and nanoribbon growth from ion implanted iron catalyst

The authors experimentally demonstrate a simple and efficient approach for nucleating the catalytic chemical vapor deposition (CVD) growth of GaN nanowires, Ga2O3 nanowires, and Ga2O3 nanoribbons by using ion implantation of Fe+ into thermally grown SiO2 layers and subsequent annealing to form the catalyst nanoparticles. This work shows that ion implantation can be used as a versatile method to create catalyst nanoparticles for wide band gap nanowire/nanoribbon growth. They also demonstrate that ion implanted catalyst nanoparticles prepared under identical conditions can be used to grow different types of nanowires/nanoribbons containing Ga by simply changing the gas types and flow rates during CVD growth. Furthermore, they systematically characterize the structural properties of the as-grown nanomaterials, and find that the distance between the Ga source and the substrate, growth temperature, growth time, and flow rates are all critical parameters for nanowire growth. They explain the growth of single-crystal wurtzite GaN and monoclinic β-Ga2O3 nanowires by the vapor-liquid-solid (VLS) growth model, whereas the growth of monoclinic β-Ga2O3 nanoribbons by a combination of the VLS and vapor-solid (VS) mechanisms. This work opens up the possibility of controlling the origin of wide band gap nanowires/nanoribbons at the nanometer scale using the technique of catalyst ion implantation through a lithographically defined mask, of integrating nanowires/nanoribbons into nonplanar three-dimensional device structures, and of growing different Ga-based wide band gap semiconductor nanostructures on the same substrate by simply changing the gas types and flow rates.The authors experimentally demonstrate a simple and efficient approach for nucleating the catalytic chemical vapor deposition (CVD) growth of GaN nanowires, Ga2O3 nanowires, and Ga2O3 nanoribbons by using ion implantation of Fe+ into thermally grown SiO2 layers and subsequent annealing to form the catalyst nanoparticles. This work shows that ion implantation can be used as a versatile method to create catalyst nanoparticles for wide band gap nanowire/nanoribbon growth. They also demonstrate that ion implanted catalyst nanoparticles prepared under identical conditions can be used to grow different types of nanowires/nanoribbons containing Ga by simply changing the gas types and flow rates during CVD growth. Furthermore, they systematically characterize the structural properties of the as-grown nanomaterials, and find that the distance between the Ga source and the substrate, growth temperature, growth time, and flow rates are all critical parameters for nanowire growth. They explain the growth of single-cr...

Patterned growth of silicon oxide nanowires from iron ion implanted SiO2 substrates

We demonstrate experimentally a simple and efficient approach for silicon oxide nanowire growth, by implanting Fe(+) ions into thermally grown SiO(2) layers on Si wafers and subsequently annealing in argon and hydrogen to nucleate the nanowires. We study the effect of implantation dose and energy, growth temperature, H(2) gas flow, and growth time on the silicon oxide nanowire growth. We find that sufficiently high implant dose, high growth temperature, and the presence of H(2) gas flow are crucial parameters for silicon oxide nanowire growth. We also demonstrate the patterned growth of silicon oxide nanowires in localized areas by lithographic patterning and etching of the implanted SiO(2) substrates before growth. We propose a simple physical model to explain the growth results. This works opens up the possibility of growing silicon oxide nanowires directly from solid substrates, controlling the location of nanowires at the submicron scale, and integrating them into nonplanar three-dimensional nanoscale device structures.

Field-emission properties of individual GaN nanowires grown by chemical vapor deposition

Single crystalline GaN nanowires were synthesized using chemical vapor deposition. Devices containing individual GaN nanowires were fabricated using contact printing. The local turn-on electric field at the tip of the GaN nanowires was compared to that of other nanomaterials. The quality of contact between GaN nanowires and metal electrodes was found to affect the field-emission behavior significantly. It was also observed that the field-emission behavior of individual GaN nanowires follows the conventional Fowler-Nordheim model in the range of applied electric fields.