All existing transistors are based on junctions - obtained by changing the polarity of silicon from positive to negative. Researchers have now demonstrated a new type of transistor in which there are no junctions and no doping concentration gradients. The key to fabricating a junctionless gated resistor is the formation of a semiconductor layer that is thin and narrow enough to allow for full depletion of carriers when the device is turned off - something that was achieved by fabricating silicon nanowires with a diameter of a few dozens of atomic planes. The electrical current flows in this silicon nanowire, and the flow of current is perfectly controlled by a ring structure that electrically squeezes the silicon wire in the same way that you might stop the flow of water in a hose by squeezing it.
One of the outstanding challenges in nanotechnology generally, and in the exploitation of so-called 'bottom-up' assembly of basic nanoscale building blocks such as nanowires, is the development of techniques for assembling large numbers of such nanostructures into more complex systems and precisely specified patterns in an accurate, deterministic manner. For instance, it is possible to build transistors, optical devices, and sensors with very specific properties using nanowires. Thus many useful applications of nanowires will depend on the ability to take these building blocks and organize them in some deterministic way in order to ultimately construct and interface with a nanowire-based system. New work demonstrates the basic capability for, and elucidates some of the guiding principles in, the use of dielectrophoretic behavior to direct the placement of large numbers of nanowires on complex, pre-patterned structures as might be required for integration of nanowires with, for example, silicon-based microelectronic circuitry. It shows that a high level of placement precision can be achieved by paying careful attention to the signal frequency as well as the macroscopic electrode architecture that is employed.
The fight against infections is as old as civilization. Silver, for instance, had already been recognized in ancient Greece and Rome for its infection-fighting properties and it has a long and intriguing history as an antibiotic in human health care. Modern day pharmaceutical companies developed powerful antibiotics - which also happen to be much more profitable than just plain old silver - an apparent high-tech solution to get nasty microbes such as harmful bacteria under control. However, thanks to emerging nanotechnology applications, silver is making a comeback in the form of antimicrobial nanoparticle coatings. As even the most powerful antibiotics become less and less effective, researchers have begun to re-evaluate old antimicrobial substances such as silver and as a result, antimicrobial nano-silver applications have become a very popular early commercial nanotechnology product. Researchers in China have now further advanced the nanotechnology application of silver be developing a novel multi-action nanofiber membrane containing four active components, each playing a different role in the membrane's excellent antibacterial function.
Synthetic fibers are ubiquitous in modern society and their manufacture represents a huge, multi-billion dollar worldwide industry. Synthetic fibers - carbon fibers, nylon, polyester, kevlar, spandex, etc. - are manufactured from fossil fuels, usually from oil, but sometimes from coal or natural gas. Most of these materials are not biodegradable and, in addition to their significant carbon footprint during production, they pose environmental problems at the end of their life cycle. Natural fibers, on the other hand, such as wool and cotton, come from renewable animal or plant sources but they usually lack the high-performance characteristics of many synthetic fibers. This may change, as the new field of bio-based nanomaterials promises to deliver environmentally friendly, high-performance bio-fiber materials that can replace some of the synthetic materials.
Optical imaging of materials is full with rich physical, chemical and biological information about the sample, because the optical energies in the visible range coincide with the atomic and molecular transition energies of many materials. Apart from the topographical information, the optical image therefore contains information about intrinsic properties of a material. However, the wave nature of light prevents the light to focus in a volume smaller than half of the wavelength, which is about 200-300 nm for visible light. Therefore, it is almost impossible to image nanomaterials, which could be a few nanometers in size, using optical imaging process. A typical lens made of, for example glass, will not be sufficient to image a nanomaterial. In work that gives rise to a new concept of a lens for optical imaging, scientists in Japan have proposed a lens made of silver nanorods, rather than a curved glass surface. This metallic nanolens is capable of manipulating light in such a way that an optical image of nanoscale objects can be obtained in the visible range.
Diamonds have been known in India for at least 3000 years and are thought to have been first recognized and mined there. The most familiar usage of diamonds today is as gemstones in jewelry but, apart from being a girl's best friend, it seems that diamonds, especially nanodiamonds, are quickly becoming a scientist's best friend as well. Diamonds are the hardest natural material - the word diamond comes from the Greek term adamas, which means 'invincible' - has the lowest coefficient of thermal conductivity, is electrically insulating, chemically inert, and optically transparent. In nanoparticulate form, diamonds possess an additional property that makes them so interesting for researchers: since they are carbon-based and non-toxic they are a suitable material for drug delivery, drug diagnostics and medical imaging applications. One of the challenges in fabricating nanodiamond coatings and composite materials is the difficulty of controlling the size, texture, and crystalline quality of the diamond particles. Now, researchers in Portugal have demonstrated for the first time the facile fabrication and the conformal coating of nanocrystalline diamond onto silica nanofibers by a two-step method: synthesis of templates on silicon wafer; and coating of the silica fibers with nanocrystalline diamond.
The vision of revolutionary bottom-up nanotechnology is based on a concept of molecular assembly technologies where nanoscale materials and structures self-assemble to microscale structures and finally to macroscopic devices and products. We are a long way from realizing this vision but researchers are busily laying the foundation for nanoscale engineering. Assembling nanoscopic components into macroscopic materials is an appealing goal but one of the enormous difficulties lies in bridging approximately six orders of magnitude that separate the nanoscale from the macroscopic world. Until machinery capable of automated and industrial-scale nano-assembly can be built, the parallelism of chemical synthesis and self-assembly is necessary when controlling materials at the nanoscale. An obvious direct approach to molecular nanotechnology therefore is to start with organic molecules as building blocks. Modest from the viewpoint of molecular manufacturing visionaries, but quite fascinating to a lot of scientists, research into nanofibers, as a modification of organic crystals, is making good progress. New research results coming out of Denmark offer the basis for a novel organic-molecule-based nanotechnological concept that allows for a multitude of applications in fundamental research and in device applications. Essentially, this concept is based on three steps: 1) directed self-assembled surface growth of nanofibers from functionalized molecules; 2) transfer and manipulation of individual fibers as well as of ordered arrays; and 3) device integration.
'Smart' is the key buzz word used by materials engineers when they describe the future of coatings, textiles, building structures, vehicles and just any material that you can think of. Materials are made 'smart' when they are engineered to have properties that change in a controlled manner under the influence of external stimuli such as mechanical stress, temperature, humidity, electric charge, magnetic fields etc. Nature of course is full with 'smart' materials that are capable of adapting to new tasks, are self-healing, and can self-assemble autonomously simply out of a solution of building blocks. Duplicating this feat with man-made materials will one day become a reality thanks to nanotechnology. Scientists not only dream about self-repairing cars or building walls that turn transparent like windows, they are actively working on the first steps towards these goals. Simple smart materials (that are not nanotechnology based) are already a reality, such as piezoelectric materials and shape memory alloys. Emerging nanotechnologies are now about to give scientists the tools to take smart materials to the next performance level. For instance, the European project Inteltex is developing a new, multifunctional textile that could be used as a wallpaper to detect temperature changes or chemical leakage or that could be used in medical and protective wear to monitor body temperature and mechanical stress. MIT's Institute for Soldier Nanotechnologies works on smart surfaces that switch properties. Nanotechnology-enabled smart materials are still very early days but basic progress is being made. Another small building block towards smart materials was recently reported by Italian researchers who demonstrated photo-switchable nanofibers based on the reversible transformation between two molecular photochemical states, exhibiting different chemico-physical characteristics.