Due to their exceptional electronic and mechanical properties, as well as their nanoscale size, carbon nanotubes (CNTs) could become the active electronic elements in addressing next generation electronic requirements for which silicon is not a solution. Already, it has been shown extensively that semiconducting CNTs can be made into electronic components such as transistors and switches. Their thermal, mechanical, chemical stability, and large current-carrying capacity make CNTs attractive for applications not only in in electrical interconnects but also field-effect transistors, cold cathode field emitters, and sensors. Some of the problems in developing CNT-based electronic devices have to do with building reliable interconnections between CNTs and external electrical and mechanical systems and in developing a reliable fabrication batch process to allow for industrial-scale mass production that supports the direct manipulation and placement/growth of CNTs at specific locations. Pointing to a possible solution, researchers now have demonstrated controllable and simultaneous wafer-scale assembly of CNT networks by dielectrophoresis.
Carbon nanotubes (CNTs) have shown promise as an important new class of multifunctional building blocks and innovative tools in a large variety of nanotechnology applications, ranging from nanocomposite materials through nanoelectronics to biomedical applications. The exploration of CNTs in biomedical applications is well underway and exploratory uses have included CNT-coated implants, drug delivery and CNTs as components of biosensors. Notwithstanding the still not satisfactorily addressed issue of toxicity, CNTs' properties such as high strength, high electrical and thermal conductivities, and high specific surface area render them particularly useful in the fabrication of nanocomposite-derived biomedical devices. In one particular area - biomaterials applied to bone - CNTs are anticipated to improve the overall mechanical properties for applications such as high-strength arthroplasty prostheses expected to remain in the body for a long time, or fixation plates and screws that will not fail or impede healing of bone. In addition, CNTs are expected to be of use as local drug delivery systems or scaffolds to promote and guide bone tissue regeneration. A new study by Japanese scientists clearly demonstrates that multi-walled CNTs (MWCNTs) have good bone-tissue compatibility, permitting bone repair and becoming closely integrated with bone tissue. Furthermore, under certain circumstances, their results indicate that MWCNTs accelerate bone formation.
Electron microscopy is a delicate balancing act between using the highest energy electrons possible in order to obtain the best image while avoiding destruction of the sample under investigation. Trouble is, the electron beam does not just observe, it also interacts with the structure that is observed. This can be critical for fragile samples such as nanostructures: the transfer of energy from the electron beam can be sufficient that atoms are knocked right out of the material. In the worst case the sample is destroyed in front of your eyes. Scientists in Orsay, France have turned this problem to their advantage. They have developed a new approach to shaping nanomaterials atom by atom, using a scanning transmission electron microscope (STEM) as a nanometrically precise cutting tool. In this way they 'carve' their materials, such as carbon nanotubes, by removing individual atoms from specifically chosen locations. This technique has the added advantage that scientists can observe what they are doing at the same time. The new technique demonstrates a 'nanoelectron- lithography' of single walled nanotubes, a top down approach to locally control their nanostructures.
More and more carbon nanotube (CNT) applications are moving from the research lab into commercial products. For example, CNTs can be found already in tennis rackets and bicycles, displays and TV screens, and numerous resins used by aerospace, defense, health care, and electronics companies. Not surprisingly, CNT production is growing by hundreds of metric tons a year. One of the large suppliers alone, Bayer, is talking about having 3,000 metric tons of production capacity in place by 2012. As a result of the increasing supply, prices are dropping fast. While a kilogram of multi-walled CNTs (MWCNTs) sold for tens of thousands of dollars just a few years ago (and single-walled CNTs still do), the price for some types of MWCNTs has fallen to hundreds of dollars per kg. Recent market analyses forecast sales of all nanotubes to reach $1 billion to $2 billion annually within the next four to seven years. In terms of dollar value, electronics devices will be the largest end-use category, although composite materials in automotive applications may account for greater volumes. These volumes are expected to approach several thousand metric tons per year. This means that the exposure to CNTs, especially by factory workers, will increase substantially over the next few years. Since the jury is still out as to the toxicity of nanotubes it appears prudent to at least develop suitable sensor technology to detect CNTs, especially in the workplace.
Two of the major challenges of our modern, mobile society are the shrinking of available fossil energy resources on one hand and climate change associated with global warming on the other. Continuing population growth multiplied by the increase in consumption and living standards, especially in developing countries, will require more and more oil, coal and natural gas to 'power' humanity. Notwithstanding efforts like the Kyoto Protocol - which wasn't signed by the two major CO2 polluters China and the U.S. - an ever increasing rate of fossil fuel usage means that the increasing emission of CO2 is likely to cause an acceleration of the climate change that is in progress already. Transportation, in particular passenger cars, is one of the areas where new technology could lead to environmental beneficial change. Never mind that GM is still selling 15-20,000 Hummers a year, or that Tata is planning to sell millions of its new Nano car. One of the much touted technological solutions is to substitute fossil hydrocarbon based energy with the energy from carbon-free sources like the sun, nuclear energy, or the hot interior of the Earth and use hydrogen as an energy carrier. Hydrogen can be produced from water using energy from carbon-free sources and can serve as fuel in fuel cells to generate electricity, either stationary or on board of vehicles. Considerable research efforts are going into the evaluation of various nanostructures, such as carbon nanotubes, to find the most suitable hydrogen storage materials.
Transistors are the key elements of many types of electronic (bio)sensors. Since the discovery that individual carbon nanotubes (CNTs) can be used as nanoscale transistors, researchers have recognized their outstanding potential for electronic detection of biomolecules in solution, possibly down to single-molecule sensitivity. To detect biologically derived electronic signals, CNTs are often - but not always - functionalized with (conductive) linkers such as proteins and peptides to interface with soluble biologically relevant targets (linkers need not be conductive as long as they are capable of localizing the target molecule in close vicinity of the tube). Although CNT transistors have been used as biosensors for some years now, the ultimate single-molecule sensitivity, which is theoretically possible, has not been reached yet. One of the reasons that hampers the full exploitation of these promising nanosensors is that the sensing mechanism is still not well understood. Although a variety of different sensing mechanisms has been suggested previously, various studies contradict one another, and the sensing mechanism remained under debate. Researchers in The Netherlands - through modeling and specific control experiments - now have succeeded in identifying the sensing mechanism. They found that the majority of their experiments can be explained by a combination of electrostatic gating and Schottky barrier effects. Because these two mechanisms have different gate-potential dependence, the choice of gate potential can strongly affect the outcome of real-time biosensing experiments.
If you grew up in the 80's, chances are you are familiar with the addictive game 'Pipe Dream' in which a plumber tries to lay pipe before water flowing through the pipe can overwhelm him. Get ready to play this at the nanoscale (well, kind of). Although the possibility of connecting carbon nanotubes has been an intriguing one for nanotechnology researchers, realizing this feat had proven to be difficult. Since a carbon nanotube (CNT) is made of graphite, basically a rolled up sheet of graphene, the tubes have been believed to be hard and brittle. Therefore there was not much interest in trying to shape or form them. Scientists in Japan have taken a novel approach to this problem and indeed succeeded in shaping and connecting carbon nanotubes like water pipes. Their simple method will allow longer and multi-branched CNTs with serial junctions to be made by repeated joining, and may have uses for different applications. Not only does the bottom-up engineering of nanotube structures become possible (e.g. simply increasing their aspect ratio) but it could pave the way to an entirely new class of bottom-up-engineered nanostructures and integrated carbon nanotube devices.
A memory chip is an integrated circuit made of millions of transistors and capacitors. In the most common form of computer memory, dynamic random access memory (DRAM), a transistor and a capacitor are paired to create one memory cell, which represents a single bit of data. The capacitor holds the bit of information, either a 0 or a 1. The transistor acts as a switch that lets the control circuitry on the memory chip read the capacitor or change its state. Because each bit stored in a chip is controlled by one transistor, memory capacities tend to expand at the same pace as the number of transistors per chip - which still follows Moore's Law and therefore currently doubles every 18 months. The problem is that the capacitor - consisting of two charged layers separated by an insulator - can shrink only so far. The thinner insulators get the more they allow charges to tunnel through. Tunneling increases the leakage current, and therefore the standby power consumption. Eventually the insulator will break down. Researchers have been trying to develop electromechanically driven switches that can be made small enough to be an alternative to transistor-switched silicon-based memory. Electromechanical devices are suitable for memory applications because of their excellent ON-OFF ratios and fast switching characteristics. With a mechanical switch there is physical separation of the switch from the capacitor. This makes the data leakage problem much less severe. Unfortunately they involve larger cells and more complex fabrication processes than silicon-based arrangements and therefore have not been so far an alternative to scaling down beyond semiconductor transistors. Researchers now have reported a novel nanoelectromechanical (NEM) switched capacitor structure based on vertically aligned multiwalled carbon nanotubes (CNTs) in which the mechanical movement of a nanotube relative to a carbon nanotube based capacitor defines ON and OFF states.