The concept of e-noses - electronic devices which mimic the olfactory systems of mammals and insects - is very intriguing to researchers involved in building better, cheaper and smaller sensor devices. A better understanding of the reception, signal transduction and odor recognition mechanisms for mammals, combined with achievements in material science, microelectronics and computer science has led to significant advances in this area. Nevertheless, the olfactory system of even the simplest insects is so complex that it is still impossible to reproduce it at the current level of technology. For example, the biological receptors are regularly replaced during the life of mammals in a very reliable way so that the receptor array does not require to be recalibrated. The performance of existing artificial electronic nose devices is much more dependent on the sensor's aging and, especially, the sensor's replacement and frequently require a recalibration to account for change. Moreover, current electronic nose devices based on metal oxide semiconductors or conducting polymers that specifically identify gaseous odorants are typically large and expensive and thus not adequate for use in micro- or nano-arrays that could mimic the performance of the natural olfactory system. Nanotechnology is seen as a key in advancing e-nose devices to a level that will match the olfactory systems developed by nature. Nanowire chemiresistors are seen as critical elements in the future miniaturization of e-noses. It is now also believed that single crystal nanowires are most stable sensing elements what will result in extending of life-time of sensors and therefore the recalibration cycle. Last year we reported on a research effort Towards The Nanoscopic Electronic Nose. Scientists involved in this effort now report a second-generation, far more advanced e-nose system based on metal oxide nanowires.
Fundamental nanotechnology research in laboratories advances rapidly, as witnessed by the hundreds of new research papers that get published every month. The big bottleneck in getting these new technologies from the lab translated into commercial products is the lack of suitable large-scale fabrication techniques. Almost all laboratory experiments involve elaborate set-ups and are quite tricky processes that require a lot of skill and expertise on part of the researchers. To a large degree, nanotechnology today is more an art than a basis for industrial technologies. Think about a 15th century monk spending 10 years painstakingly writing and painting a single bible - that's where nanotechnology is today; but where we need to get to is something that resembles modern high speed printing machines where you print thousands of books an hour. Take for instance nanowires. Researchers have used nanowires to create transistors like those used in memory devices and prototype sensors for gases or biomolecules. A common approach in the lab is to grow nanowires like blades of grass on a suitable substrate, mow them off and mix them in a fluid to transfer them to a test surface, using some method to give them a preferred orientation. When the carrier fluid dries, the nanowires are left behind like tumbled jackstraws. Using scanning probe microscopy or similar tools, researchers hunt around for a convenient, isolated nanowire to work on, or place electrical contacts without knowing the exact positions of the nanowires. It's not a technique suitable for mass production. However, researchers have now developed a technique that allows them to selectively grow nanowires on sapphire wafers in specific positions and orientations accurately enough to attach contacts and layer other circuit elements, all with conventional lithography techniques. This fabrication method requires a minimum number of steps and is compatible with today's microelectronics industry.
Thin-film transistors (TFTs) and associated circuits are of great interest for applications including displays, large-area electronics and printed electronics (e.g. radio-frequency identification tags - RFID). Well-established TFT technologies such as amorphous silicon and poly-silicon are well-suited for many current applications - almost all mobile phone color screens use them - but face challenges in extensions to flexible and transparent applications. In addition, these TFTs have modest carrier mobilities, a measure of the velocity of electrons within the material at a given electric field. The modest mobility corresponds to a modest operating speed for this class of TFTs. Organic TFTs are generally better suited for flexible applications, and can be made transparent. However, mobilities in organic TFTs are generally quite low, restricting the speed of operation and requiring relatively large device sizes. Researchers at Purdue University, Northwestern University, and the University of Southern California now have reported nanowire TFTs that have significantly higher mobilities than other TFT technologies and therefore offer the potential to operate at much higher speeds. Alternatively, they can be fabricated using much smaller device sizes, which allows higher levels of integration within a given chip area. They also provide compatibility with a variety of substrates, as well as the potential for room-temperature processing, which would allow integration of the devices with a number of other technologies (e.g. for displays).
Nanowires have received a great deal of interest in recent years. As quasi one-dimensional systems they may overcome the fundamental difficulty in the electronic transport encountered when attempting to use quasi zero-dimensional structures (quantum dots) while still being able to take advantage of the effects related to quantum confinement. In a conventional semiconductor, electrons and holes typically stay in the same region after photoexcitation, which is very desirable for certain applications, light-emitting devices for instance. However, for a number of key renewable energy applications, including hydrogen generation via photoelectrochemical water splitting, dye-sensitized solar cells, and even regular solar cells, the efficient charge separation of the electron and hole after photoexcitation is instead highly preferred, although not readily available in the existing repertory of materials. In this context, researchers have explored the use of core-shell nanowires for such applications. However, for the material combinations that have been investigated, the energy alignments between the core and the shell are typically type I, and thus, the role of the shell is to either provide quantum confinement to both electrons and holes in the core or a protective cladding to the core to reduce sensitivity to the environment. Researchers at the National Renewable Energy Laboratory and Lawrence Berkeley National Laboratory designed a new class of core-shell semiconductor nanowires with strong type II band alignments. The photo-generated electron and hole in such nanowires are naturally separated in the core and shell, respectively, making them much more suitable for the solar energy applications than type I core-shell nanowires.
Tremendous progress has been made over the past few years to control the aspects of fabricating simple nanostructures such as wires, tubes, spheres, cubes etc. However, in order to build functional nanodevices, for instance for nanoelectronics or nanobiotechnology, much more complex nanoarchitectures are needed. Initially, the most common, mostly top-down, fabrication methods used for this purpose have been based on nanolithographic techniques. Unfortunately, these methods are burdened with throughput restrictions and high cost and will be of limited use for commercial mass production of nanostructures. To overcome the limitations of nanolithography, a lot of attention has been focused on self-organized bottom-up approaches, which bear good prospects for large-scale fabrication of nanostructures with controlled morphology and dimensionality, and controlled synthesis of arrays. However, the fabrication of complex nanoarchitectures requires sophisticated transfer techniques, which are far from routine, time consuming, and with low reproducibility. To add to the arsenal of scaleable bottom-up fabrication processes, researchers in Germany have developed a method for the batch fabrication of 3D-nanostructures with tunable surface properties. Resembling suspended nanowire webs, these structures have a high potential for catalytic, sensing, or fluidic applications where a high surface to volume ratio is required.
Strong and highly directional hydrogen-bonding networks are of fundamental importance in nature. Their efficiency in assisting electron-transfer processes makes them increasingly appealing for technological application inspired by biomimetic principles, i.e. the application of methods and systems found in nature to the study and design of engineering systems and modern technology. Attempting to move from microelectronics to nanoelectronics, engineers are faced with the growing difficulty of manufacturing ever tinier devices with top-down engineering approaches. They are therefore looking at possible ways for bottom-up engineering approaches with the goal of achieving the holy grail of nanotechnology - molecular self-assembly. For some time now researchers have been able to design molecules in such a way that they attach themselves to each other in alternating order, and under certain circumstances - for example on surfaces - create chains. Unfortunately the chains are not very long, because all surfaces, even extremely smooth ones, show unevenness at the atomic level. Step edges, although only a few atomic layers high, represent insurmountable hurdles to the self-assembly process, and since they are distributed randomly over the surface, the molecules form themselves into very irregular patterns. Overcoming this problem, researchers were recently able to formulate two organic molecules in such a way that they organized themselves spontaneously into long parallel chains (nanowires) on a specially prepared gold surface. Selective self-assembly on surfaces and the fundamental processes which control this phenomenon are, however, not only critical in the area of molecular electronics but also in heterogeneous catalysis - a process used in automotive catalytic converters - and in sensor technologies.
Nanowires are expected to play an important role in the emerging fields of nanoelectronics and nanooptics. In particular, the permanently growing complexity of integrated circuit designs requires a further reduction of the size of IC components that nanowires could facilitate. Nanowires are also a possible candidate for future functional nanostructures in plasmonic devices, i.e. for information (light) propagation and manipulation below the optical diffraction limit. For these purposes, cobalt disilicide (CoSi2) is a very promising contact material due to its extremely useful properties such as low resistance, its metallic behavior, its low lattice mismatch to Si of only -1.2%. the plasmon wavelength of 1.2 micrometer, and its compatibility with modern silicon technology. Many efforts have been made to fabricate silicide nanowires employing the bottom-up approach without elaborate microlithography. Researchers in Germany now have demonstrated a promising technique that allows the defect-induced formation and placing of cobalt disilicide nanowires by focused ion beam synthesis in silicon directly where it is needed.
Bacteria are ubiquitous in the earth's surface, subsurface, fresh water, and oceanic environment. Bacteria are remarkable in that they are capable of respiring aerobically and anaerobically using a variety of compounds, including metals, as terminal electron acceptors. Metal reducing bacteria can significantly affect the geochemistry of aquatic sediments, submerged soils, and the terrestrial subsurface. Microbial dissimilatory reduction of metals is a globally important biogeochemical process driving the cycling of iron and manganese, associated trace metals, and organic matte. Microbial metal reduction is of significant interest among scientists who are researching remediation of environmental contaminants. However, little is known about the biochemical or molecular mechanisms underlying bacterial metal reduction. Conducting research with toxic metal reducing bacteria, researchers discovered that bacteria produce electrically conductive nanowires in response to electron-acceptor limitation. These findings could be used to bioengineer electrical devices such as microbial fuel cells.