Samuel MacNaughton

Kelvin probe microscopy and electronic transport measurements in reduced graphene oxide chemical sensors.

Reduced graphene oxide (RGO) is an electronically hybrid material that displays remarkable chemical sensing properties. Here, we present a quantitative analysis of the chemical gating effects in RGO-based chemical sensors. The gas sensing devices are patterned in a field-effect transistor geometry, by dielectrophoretic assembly of RGO platelets between gold electrodes deposited on SiO2/Si substrates. We show that these sensors display highly selective and reversible responses to the measured analytes, as well as fast response and recovery times (tens of seconds). We use combined electronic transport/Kelvin probe microscopy measurements to quantify the amount of charge transferred to RGO due to chemical doping when the device is exposed to electron-acceptor (acetone) and electron-donor (ammonia) analytes. We demonstrate that this method allows us to obtain high-resolution maps of the surface potential and local charge distribution both before and after chemical doping, to identify local gate-susceptible areas on the RGO surface, and to directly extract the contact resistance between the RGO and the metallic electrodes. The method presented is general, suggesting that these results have important implications for building graphene and other nanomaterial-based chemical sensors.

Carbon nanotube and graphene based gas micro-sensors fabricated by dielectrophoresis on silicon

A gas sensor based upon an array of reduced graphene oxide (RGO) and single wall carbon nanotube (SWNT) micro-assemblies is presented. Due to the nature of their structure and exceptional surface-to-volume ratio, graphene sheets and carbon nanotubes demonstrate unparalleled chemisorption properties, providing greater sensitivities than a bulk material. Micro-assemblies of RGO platelets and SWNTs were created on lithographically-patterned electrode arrays utilizing dielectrophoresis. The resistivity of the assemblies can be tuned by adjusting the amplitude and frequency of the applied field. The completed array was tested in an enclosed chamber into which various gases were introduced (including common volatile organic compounds and simulants of nerve agents). Changes in the resistance of each micro-assembly due to chemisorption were monitored. Both types of assemblies (RGO and SWNT) demonstrated unique and repeatable responses to various organic compounds and other vapors. Consistent assembly and gas response results were achieved for sensor elements across many substrates.

Electronic nose based on graphene, nanotube and nanowire chemiresistor arrays on silicon

Large arrays of cross-reactive sensor elements offer the ability to identify vapors from the signature response of the ensemble. We create an array of heterogeneous gas sensing microassemblies using dielectrophoretic assembly of various nanomaterials on silicon. Gas sensitive materials include reduced graphene oxide (rGO) platelets, single wall carbon nanotubes, and copper oxide nanowires. The array was exposed to a wide range of volatile organic compounds such as isopropyl alcohol, acetone, and aromatic compounds. The entire array was monitored to supply real-time resistance data from each sensor element. The chemiresistive assemblies show fractional resistance changes exceeding 150 percent upon introduction of volatile organic compounds. The use of cross-reactive, heterogeneous assemblies of diverse nanomaterials facilitates construction of an electronic nose.

High-Throughput Heterogeneous Integration of Diverse Nanomaterials on a Single Chip for Sensing Applications

There is a large variety of nanomaterials each with unique electronic, optical and sensing properties. However, there is currently no paradigm for integration of different nanomaterials on a single chip in a low-cost high-throughput manner. We present a high throughput integration approach based on spatially controlled dielectrophoresis executed sequentially for each nanomaterial type to realize a scalable array of individually addressable assemblies of graphene, carbon nanotubes, metal oxide nanowires and conductive polymers on a single chip. This is a first time where such a diversity of nanomaterials has been assembled on the same layer in a single chip. The resolution of assembly can range from mesoscale to microscale and is limited only by the size and spacing of the underlying electrodes on chip used for assembly. While many applications are possible, the utility of such an array is demonstrated with an example application of a chemical sensor array for detection of volatile organic compounds below parts-per-million sensitivity.

Electrodeposited Copper Oxide and Zinc Oxide Core-Shell Nanowire Photovoltaic Cells

Uncertainty in energy capacity, limited fossil fuel resources, and changes in climate predicate a need for increased research and development into alternative and sustainable energy solutions. Solar energy is one solution to this problem and many variations of it exist; however, the majority of them are prohibitively expensive. We propose a low-cost solar energy generation method which is cost-effective both in materials and production. Our solution will utilize cheap, abundant materials as well as lower-cost fabrication methods to produce photovoltaic (PV) cells. Although it is unlikely that the efficiency of such cells will be record-breaking, its low cost should make its price-per-watt-produced competitive, which is one of the most important metrics for the commercialization of any solar technology. Our design consists of a radial heterojunction comprised of p-type copper oxide and n-type zinc oxide nanowires, which are oxides of earth-abundant materials. The nanowires have a core-shell design to minimize carrier travel distance and maximize junction area. Furthermore, we utilize a wet chemistry fabrication process, making the production of such cells inexpensive, easily scalable and non-demanding in terms of fabrication energy. The process involves growing copper nanowires, oxidizing, plating zinc oxide, and depositing a top contact.

In-Situ Large Area Fabrication of Metamaterials on Arbitrary Substrates Using Paint Process

This paper proposes a novel method to make large area metamaterials on arbitrary planar hard or ∞exible substrates, in-situ. The method is based on painting the desired substrate with metallic and dielectric paints through a patterned stencil mask. We demonstrate this painting approach to fabricate ultra- thin electromagnetic absorbers based on metamaterials at X-band frequencies (8{12GHz) with paper-based stencils, silver ink and latex paint. Measurement results on absorber samples made with this process show absorption of 95%{99% in close agreement with simulation results. The proposed painting approach is a simple low cost additive manufacturing process that can be used to realize metamaterials, frequency selective surfaces, radar absorbers, camou∞age screens, electromagnetic sensors and EMI protection devices.

Metal-oxide coaxial nanowire photovoltaic cells

Two of the biggest losses in conventional solar cells are reflections and carrier thermalization. The nanowire geometry presented here has the potential to mitigate both of these losses [1, 2], however it requires nonconventional and non-planer fabrication procedures. Presented here is a facile synthesis of nanowire array solar cells consisting of a metal oxide heterojunction. The nanowires consist of a core-shell geometry consisting of coaxial layers of copper, copper oxide, zinc oxide, and indium tin oxide: these layers correspond to the bottom contact, p-layer, n-layer, and top contact, respectively.

Wet chemistry based copper oxide and zinc oxide nanowire photovoltaic cells

Solar cells are a promising, green energy source; however, cost and efficiency limitations prevent widespread adoption. We propose a solar cell design that is cost-effective both in production and materials. Here, the junction is made of copper (I) oxide and zinc oxide, which are oxides of earth-abundant metals. Furthermore, we utilize a wet chemistry fabrication process, making the production of such cells inexpensive and easily scalable. The process involves growing copper nanowires, plating zinc, oxidizing, and depositing a top contact, detailed below in Figure 1. This is a greener manufacturing method of solar cells where no harmful compounds or excessive energy is used in fabrication.