Most people when they hear the word semiconductor will think about their role in computers. However, semiconductors also absorb light, some absorb in the visible, thus appearing colored, e.g. gray silicon, and others in the UV, such as titanium dioxide, thus appearing white (when in microparticulate form) or colorless (when in nanoparticulate form). This light-absorbing feature is used to drive electrons around a circuit in photovoltaic cells, such as the silicon solar cell, but it can also be used to drive chemical reactions at the surface. A good example of the latter is the use of thin (15 nm) titanium dioxide film coatings on self-cleaning glass. These films upon absorbing UV light in sunlight are able to reduce oxygen, present in air, to water and oxidize any organic material on its surface to its minerals, thereby keeping the surface clean. Researchers in the UK have used this oxidation feature to developed an irreversible solvent-based blue ink, which upon activation with UV light, loses all its color and becomes oxygen sensitive; it will only gain its original color upon exposure to oxygen. A major application area for this oxygen ink is in food packaging where it could be used to detect a modified atmosphere inside food containers.
One statement of the second law of thermodynamics is that the efficiency of any heat engine or other thermodynamic process is always less that 100%. There will always be some type of friction or other inefficiency that will generate waste heat. The useful work that a heat engine can perform will therefore always be less than the energy put into the system. Engines must be cooled, as a radiator cools a car engine, because they generate waste heat. While there is no way around the second law of thermodynamics, the performance of today's power generation technology is quite appalling. The average efficiency today for fossil-fired power generation, 35% for coal, 45% for natural gas and 38% for oil-fired power generation. By the way, be skeptical when people tell you that nuclear power is good in the fight against global warming - nuclear power plants have a worse thermal efficiency (30-33%) than fossil-fired plants. Approximately 90% of the world's power is generated in such a highly inefficient way. In other words: every year some 15 billion kilowatts of heat is dumped into the atmosphere during power generation (talk about fueling global warming...). This is roughly the same amount as the total power consumption of the world in 2004. Reducing these inefficiencies would go a long way in solving the coming energy and climate problems. Thermoelectric materials - which can directly convert heat into electricity - could potentially convert part of this low-grade waste heat. Problem is that good thermoelectric materials are scarce and so far solid-state heat pumps have proven too inefficient to be practical. Two papers in this week's Nature describe how silicon devices could in principle be adapted and possibly scaled up for this purpose.
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.
Ever since the nanoworld got excited over carbon nanotubes there has been great interest, and progress, in the development of new nanotubes based on metal oxides, sulfides, nitrides, elemental species and others. The characteristic that all these tubular structures have in common is a hollow morphology which may possess circular, or square-like or hexagonal-like cross section. In a standard tubular structure, a cavity is located at the center and extends over the entire length, so that the tube cavity and the tube wall have the same symmetry axis. Structures in which an internal cavity strongly deviates from the center of symmetry towards one side are rather rare. Researchers have now synthesized novel, unconventional nanotubes that are distinctly different from any previously reported nano- and microtubes. These tubes display flattened and thin belt- or ribbon-like morphologies, which are not common for any known tubular structures. This may represent a new, interesting growth phenomenon for tubular crystal structures.
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.
The UK government has published its second research report on nanotechnology risks, outlining progress on its research agenda to address the potential risk posed by the products of nanotechnology. The report places the UK research program in an international context. The Nanotechnology Research Coordination Group (NRCG) is collaborating with international partners, particularly through the Organization for Economic Co-operation and Development (OECD) and the International Standards Organization (ISO), to share data and experiences. In this way they hope to be able to maximize the effectiveness and speed with which potential risks may be identified and managed. The report also responds to the recommendations made by the Council for Science and Technology (CST) review (March 2007) on the UK research program and the activities of the NRCG.
For centuries, man has searched for miracle cures to end suffering caused by disease and injury. Many researchers believe nanotechnology applications in medicine may be mankind's first 'giant step' toward this goal. According to Freitas nanomedicine is "...(1) the comprehensive monitoring, control, construction, repair, defense, and improvement of all human biological systems, working from the molecular level, using engineered nanodevices and nanostructures; (2) the science and technology of diagnosing, treating, and preventing disease and traumatic injury, of relieving pain, and of preserving and improving human health, using molecular tools and molecular knowledge of the human body; (3) the employment of molecular machine systems to address medical problems, using molecular knowledge to maintain and improve human health at the molecular scale." Nanomedicine not only has the potential to change medical science dramatically but to open a new field of human enhancements that is poised to add a profound and complex set of ethical questions for health care professionals. For instance, there is a fine line between medical and non-medical uses of nanotechnology for diagnostic, therapeutic and preventive purposes (e.g. non-medical implants in soldiers). The question of whether nanotechnology should be used to make intentional changes in or to the body when the change is not medically necessary is just one hot topic in a long list of concerns. The good news is that these questions are being asked, but there is still much work to be done, but despite the enormous promise of nanomedicine, and the considerable funding going into the field, the research into the ethical, legal and social implications of nanomedicine is comparatively minute. As Peter Singer wrote in his 2003 tutorial Mind the gap: science and ethics in Nanotechnology: 'The science leaps ahead, the ethics lags behind.' As with nanotechnology in general, there is danger of derailing nanomedicine if the study of ethical, legal and social implications does not catch up with scientific developments.