affilitated laboratories

Fuel cells are expected to be widely used in future automotive applications and small-scale power plants. For this to happen, several challenges must be overcome to make them cost-competitive. A reasonable consumer target entry point for these devices is thought to be around $30/kW per stack in a cell. This target cost will be driven primarily by materials innovations and system optimization. One of the key materials whose cost must be reduced and efficiency increased is the catalyst (for both hydrogen- and methanol-based fuels). CNXT researchers are working on new electro-catalyst materials that would minimize loading but increase efficiency for methanol fuels.New methods for the fabrication of Pt-Ru, and other non-noble metal catalysts that exhibit high surface areas (> 500 m2/gram) are being pursued. Specifically, catalysts with nanotube configurations are being fabricated using template and chemical filtration methods to obtain randomly oriented nanotubes of equi-atomic compositions of Pt-Ru. Lithographic techniques are also being used to produce nanotubes of specific sizes and separations. The figure below shows the basic structure of a fuel cell; the accompanying scanning electron micrograph shows a preliminary structure of some Pt-Ru nanotubes that could eventually be integrated into the fuel cell electrode system as components of a high-efficiency, minimal loading electro-catalyst.

Generation of electricity from the sun provides clean, pollution-free energy from a free and abundant source. Furthermore, it provides some measure of energy security and control. Solar photovoltaic power generation is still in its infancy. For the most part, silicon has been the dominant material used for the fabrication of photovoltaic cells. These cells typically have conversion efficiencies of less than 18% for fielded systems (and higher for research prototypes and high-end applications). The current end-user cost for solar power generated using silicon photovoltaic cells is still higher than what would be acceptable for widespread adoption. Innovations in new materials, device design concepts, and system integration will be required if the Photovoaltaic Industry is to meet its goal of supplying at least 10% of the US peak electricity generation capacity by 2030 at reasonably acceptable cost. Researchers at CNXT are exploring nanostructured materials such as conducting polymers and quantum dots as potential active media for photovoltaic conversion. The new media promise solar conversion over a broad spectrum at lower cost. The figure below illustrates a generic structure of a polymer and quantum-dot based photovoltaic cell; this is one of several basic structures being explored by researchers at CNXT. The key issues being investigated include appropriate ohmic contacts for polymer materials, control of the size and spatial distribution of the dots in the device active region, a search for suitable dopant impurities, and new device architectures for high efficiency cells.

Nanostructured materials offer unique approaches to the design and fabrication of sensors for detection of chemical and biological species. Sensing and identification of biological and chemical species is often important in many areas of healthcare and in the life sciences. For harmful gaseous chemicals, for example, nanostructures can provide large surface area-to-volume ratios that could be useful in the amplification of trace concentrations of the species before transduction into electrical, optical, or other signal form that is useful in ultra-sensitive sensor systems. In these types of sensors, one often needs to innovate on both the materials used in the transduction, as well as on the device structure and architecture. In one such project, CNXT researchers are exploring novel polythiophene nanostructures as gas sensing transducers. The basic idea behind this sensing scheme is to use a polymer in the channel of an otherwise normal, but inverted, field-effect transistor (FET) structure. The novelty is in the polymers: the specially synthesized polythiophene polymers in the channel are composed of semiconducting nanometer-scale chains. The conductivity of these chains is a function of the ambient. Adsorbed gas molecules affect the electrical transport properties of such Chem-FET devices. Thus, depending on the composition of the molecular species of the ambient (for example, when gas or vapor molecules such as benzene, toluene, methanol, hexane, methylene chloride, etc. are present), these devices could be used to detect the molecular constituents in air. The electrical response of each polythiophene chemistry differs for each vapor, and so the combination of information from a number of different Chem-FET sensor chemistries enables one to determine the vapor composition. The different polythiophene chemistries are precisely ink-jet deposited on arrays of FET electrodes, resulting in a single integrated device composed of an array of many chemical sensor varieties. The figure below shows an atomic force microscope image of the nanometer-scale polythiopene chains that are used in the channel of the gas sensing FET, along with the configuration of the FET.

In biological sensing, it is always the rule, rather than the exception, that transducers in contact with biological interfaces not foul or significantly affect the tissue environment in which the sensing is carried out. This is especially true in sensors used in humans. Sensor bio-chips that monitor or detect certain conditions in human tissue, for example, must be provided with an interface that protects the chip but admits biological molecules for analysis in the transduction chamber. One project that investigates such interfaces at CNXT is exploring the use of nanoporous semi-permeable silicon carbide membranes as potential components of future bio-chips. This project is investigating silicon carbide membranes rather than silicon because silicon carbide has a greater degree of bio-compatibility (which would lead to minimal fouling of the membrane). To construct an entire bio-chip, these membranes must be integrated with silicon micro-fluidic devices. The figure below is an illustration of the nanoporous semi-permeable membrane, along with a depiction of a potential location in a body where such a sensor could be implanted.

Over the past several decades, society has come to recognize and value the quality of a clean environment to the enjoyment of life. Nanotechnology can provide significant advantages over conventional technologies in the sensing and remediation of, for example, toxic contaminants in groundwater. Chlorinated solvents present in the sub-surface as dense non-aqueous phase liquids (DNAPL) are a consistent long-term source of groundwater contamination; they degrade environmental quality, and pose significant health risks. Remediation using conventional techniques poses significant technical challenges; lifecycle treatment costs are usually unacceptably high. One of the interdisciplinary projects that CNXT researchers are engaged in explores the design, characterization, and evaluation of the targeting of properties of Programmable Inorganic-Organic Nanodevices (PIONs) that can be delivered to subsurface contaminants and rapidly degrade them to non-toxic products. In this case, the nano-device is a highly reactive Fe0/Fe3O4 nanoparticle (~30nm diameter) that is modified with designer polymer assemblies that enable them to preferentially locate at the contaminant-water interface (see figure). Their nanometer-scale size allows them to be effectively transported in water saturated with porous media, and reach the contaminant trapped in small hard to reach pores; these devices are highly reactive due to their high surface-area-to-volume ratio. This approach derives inspiration from targeted drug delivery methods in that the particles are injected non-specifically into a flow field and migrate passively until they reach their target (the contaminant-water interface) where the hydrophobic block bonded onto them swells and causes them to remain at the target location.

The novelty of this approach is the nanostructured polymer architecture. The polymer assemblies are synthesized by a technique known as “atom transfer radical polymerization,” where the degree of polymerization, type, and location of the polymer blocks is highly controllable and can provide the desirable characteristics, such as affinity for contaminant-water interfaces. The ability to synthesize polymers with highly controllable properties makes the targeting approach very flexible. This method could also be used to accurately locate sub-surface barriers, deliver iron oxides for magnetic labeling as sensitizers for remote sensing/imaging of subsurface contaminants, or to deliver biosensors to specific locations. The general theme of endowing nanometer-scale devices with the ability to target specific locations in a system (for example, in the natural environment or elsewhere), using passive thermodynamic affinity targeting should find broad utility in other areas.

© 2007 CNXT, Carnegie Mellon University