Carbon Nanotubes in Nanoelectronics

Carbon nanotubes hold promise as basic components for nanoelectronics—they can be conductors, semiconductors and insulators. In 2001 IBM made the most basic logic element, a NOT gate, out of a single nanotube, and researchers in Holland created a variety of more complex structures out of collections of tubes, including memory elements. Recently IBM created nanotube transistors that outperformed the best silicon devices available. There are two big hurdles to overcome for nanotube-based electronics. One is connectibility—it's one thing making a nanotube transistor, it's another to connect millions of them up together. The other is the ability to ramp up to mass production. Traditional lithographic techniques are based on very expensive masks that can then be used to print vast numbers of circuits, bringing the cost per transistor down to one five-hundredth of a US cent. Current approaches to nanotube electronics are typically one-component-at-a-time, which cannot prove economical. Molecular electronics (which, strictly speaking, includes nanotubes) faces similar scaling hurdles. There are some possible solutions, however.

NANO AFTER MORE'S LAW

You may have heard of Moore's law, which dictates that, the number of transistors in an integrated circuit doubles every 12 to 24 months. This has held true for about 40 years now, but the current lithographic technology has physical limits when it comes to making things smaller, and the semiconductor industry, which often refers to the collection of these as the "red brick wall", thinks that the wall will be hit in around fifteen years (the best resource for information on these limits is the International Technical Road map on Semiconductors - see http://public.itrs.net/). At that point a new technology will have to take over, and nanotechnology offers a variety of potentially viable options.

The total potential for nanotechnology in electronics has been estimated to be about $300 billion per year within 10 years, and another $300 billion per year for global integrated circuit sales (R. Doering, “Societal Implications of Scaling to Nanoelectronics,” 2001). But it's actually much harder to predict the commercially successful technologies in the world of electronics than in the world of materials.

The assumption that continually increasing processing power will automatically slot into a computer hardware market that continues to grow at the rate it has done historically, is not necessarily sound. Most of the growth over the last decade has been driven by personal computers and some argue that this market is nearing saturation. Certainly there will be other applications. Increasing the intelligence of computers, and giving them the capability to interact verbally in a sophisticated manner, would certainly bring benefits, but increasing hardware capabilities is only half the story, with the biggest challenges being designing the software. Another area predicted to see major growth, is ubiquitous computing, whereby processors start to be incorporated in all manner of objects around us, which then communicate with each other and us. However, the requirements here are for relatively simple processors and in many cases there is no need for them to be particularly small either. Cost improvements remain a critical factor and it is the correlation of increasing transistor density with a reduction in cost per transistor that has probably kept Moore's law on track for so long. This relationship need not continue, however and several new approaches, which aren't even nanoscale, hold promise of creating simple circuits cheaply, such as using arrays of MEMS-based micro mirrors to build custom circuits, or the use of ink jet printers to churn out simple ones (interestingly, nanoparticulates figure in the potential of this probably near-market technology). These approaches also offer the ability to create low runs of circuits, or even one-off bespoke designs, at low prices, whereas photo lithographic approaches need massive production runs to achieve economies of scale. Soft lithography, too, offers cheap, micro scale, circuitry and is being pursued in the creation of flexible displays. These technologies could take a share of the existing semiconductor market and certainly future markets such as electronic tagging of goods or the processors required for ubiquitous computing.

High-production-run electronics will continue to be dominated by photo lithographic approaches for years to come, with the advent of the molecular nanotechnologies that could dramatically improve the power and density of processors while competing with photo lithography on cost still way off the radar for the investment community. The reason for this is the challenge of using such approaches to create the complex structures required for processors, an obstacle that doesn't apply to data storage, as we will see later. Soft lithography and nanoimprinting, however, are showing promise of coming into investment range in the near future. A company has already been formed to develop one flavor of soft lithography, the step and flash approach, for nanoelectronics and Stephen Chou of Princeton recently developed a variant of his nanoimprinting approach (already used to make commercially available sub wavelength optical components) to make nanoscale structures by melting silicon.

ELECTRONICS AND INFORMATION TECHNOLOGY

The impact of the information technology (IT) revolution on our world has far from run its course and will surely outstrip the impact of the industrial revolution. Some might claim it has done so already. Key to this is decades of increasing computer power in a smaller space at a lower cost.

DEVICES

MEMS. Making machines in the micro realm is something that is already well established. Microelectromechanical systems (MEMS) are generally constructed using the same photolithographic techniques as silicon chips and have been made with elements that perform the functions of most fundamental macro scale device elements - levers, sensors, pumps, rotors, etc. MEMS already represent a $4 billion industry, which is projected to grow to $11 billion by 2005.

NEMS. Moving to the nanoscale will present a host of new issues. For this reason, and possibly a lack of economic drivers for making machines smaller in general (smaller isn’t necessarily better), we shouldn't expect a vast array of products to flow out of MEMS and the nano version, NEMS, in the near future. However, there is sure to be a significant but modest evolution, especially in such areas as lab-on-a-chip type technologies, and NEMS devices have potential in the telecoms industry.

Tiny Medical Devices. MEMS and NEMS hold promise in the medical field, as little devices controlling the release of a drug, for instance, or even in the control functions of prosthetics, such as artificial hearts. However, it should be noted that where a passive system can perform the same function as an active one, the passive one would normally be less expensive and more reliable. Sometimes, however, an active device makes sense—recently a MEMS device was created that can grip and release individual blood cells without harming them. One can imagine such a device being used in a system to inject genes or other substances into cells. The use of nanotubes as syringes has even been suggested.

Advanced Lasers. Lasers constitute an area that is likely to be commercially affected by nanotechnology in the near future. Quantum dots and nanoporous silicon both offer the potential of producing tunable lasers—ones where we can choose the wavelength of the emitted light. Classic lasers, including solid-state ones, are dependent upon the physical and chemical properties of their components and are thus not tunable. Given the market for solid-state lasers, developments in this area are likely to be commercially significant.

DIP-PEN NANOLITHOGRAPHY

This technique uses atomic force microscope (AFM) tips like old-fashioned quill pens, depositing an ink on a surface, the ink usually being something that forms self-assembled monolayers. A variation uses hollow AFM tips that have a well to hold the ink.

Lines just a few nanometers across have been created and, in theory, a wide variety of different inks can be used. The approach clearly offers great flexibility but not the sort of throughput that would be required for mass production. Throughput can be increased significantly by having arrays of tips, and companies working on the technology talk of potentially hundreds of thousands.

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SOFT LITHOGRAPHY

Soft Lithography

This term covers a variety of approaches akin to traditional printing. A mold is created that can then is used to make an imprint in a material or apply ink to it, plus there are several other variations. A variant already used in creating optical components is nanoimprinting, which uses a hard mold to make an impression in a polymer. A recent variation on this uses a quartz mold, which is placed in contact with silicon. The silicon is then melted with a powerful laser, leaving an impression of the mold.

In general, no special technology is required for these techniques, nor are the fantastically clean environments required for existing silicon chip production, for example. Additionally, a wide variety of materials can be used.

Soft lithography is already used to make micro fluidic systems, such as those in labon- a-chip systems, and it scales readily down to the nanoscale—depending on the variant of the technology used, resolution can get below 10 nanometers. The attraction for nanoelectronics is clear—the technology is simple, offers a high level of parallelism (and thus economies of scale from high production runs), and can produce complex patterns with nanoscale features. As a replacement for traditional lithography for creating electronic devices, however, there is currently a major obstacle—the technique is not well suited to making the precisely aligned, multilayered structures currently used in microelectronics, although work is being done to overcome this limitation.

The alignment problem is lessened if larger feature sizes are acceptable and the approach has been investigated for making flexible displays. Additionally, the creation of the master is much cheaper than for photolithography and the process would become economical for much lower production runs, such as for device specific electronics.

TECHNIQUES FOR BUILDING NANO SCALE STRUCTURES

Self-assembly. Self-assembly is nature's favorite way of building things. Simply create materials that naturally combine with each other in desired ways. Self-assembled monolayers, which we have already mentioned, are a simple example.

Self-assembly typifies an approach that is often mentioned in writings on nanotechnology, the bottom-up approach. Assembling a car engine, say, from its components is a bottom-up approach (although not an example of self-assembly) and involves little wastage. Machining some of the components out of blocks of material is a top-down approach, and involves more wastage.

Self-assembly potentially offers huge economies, and is considered to have great potential in nanoelectronics for this reason and because it could produce just about the densest electronics feasible. It is a part of some of the promising approaches to making molecular memory that may bear fruit in a few years. Tackling processors is another matter, however, because of the greater complexity involved? In this area self-assembly will likely be combined initially with some more traditional top-down approach, for example, getting molecular components to self-assemble on a patterned substrate in some sort of hybrid system, which many believe will represent the first commercialization of nanoelectronics.

A drawback of self-assembly approaches to date is that they are not that reliable and the results have a much higher rate of variability (read flaws) way to get around this is to design software that takes account of the flaws and allows imperfect circuitry to operate reliably, through testing and selection of viable components. than we are accustomed to with lithographic approaches.

NANO SILVER BULLET

Your toothpaste may be a pesticide. So might your electric razor, your computer keyboard, and your child's teddy bear. These products, and scores of others, combine one of the world's oldest disinfectants--silver--with one of its hottest new industries: nanotechnology. The manufacturers of these products boast that they fight bacteria, molds, and fungus. Therefore, according to the U.S. Environmental Protection Agency (EPA), these products may be pesticides. Though this may sound like a Rush Limbaugh story line about paranoid eco-Nazis, the reality is that we're lacing ordinary household goods with known toxins. And until scientific testing and federal laws catch up with this new technology, we may be exposing the environment--and our own bodies--to untold harm.

The new science of nanotechnology allows manufacturers to use materials that measure between 1 and 100 nanometers. (A nanometer is a billionth of a meter, or roughly 1/100,000 the width of a human hair.) While nanoparticles can occur naturally and by accident--in diesel soot, for example--it's only in the past decade or so that scientists have widely learned to create and manipulate them. Many nanotechnologies use nano-versions of common materials, like carbon and silver. These tiny particles take on almost magical qualities: Insoluble materials can become soluble, nonconductive substances start conducting electricity. Nanomaterials can be orders of magnitude more powerful than the same substance at normal scale. Myriad green applications are in the works, and medical miracles are promised.

For now, though, nanotech is largely used in industrial and consumer products, from cosmetics to fleece to plastic food containers. Often, the benefits are more convenient than essential: White sunscreen turns clear on the skin; fabrics resist stains and static; leftovers stay fresh longer. There are over 600 nano consumer products on the market today--up from about 200 two years ago, when the Washington-based Project on Emerging Nanotechnologies (PEN) started keeping an inventory--with three to four new products added weekly.

ROLE OF NANO IN NATURE

Apr 4th, 2008:
A role for nanotechnology in capturing and storing green house gases The greenhouse effect is primarily a function of the concentration of water vapor, carbon dioxide, and other trace gases in the Earth's atmosphere that absorb the terrestrial radiation leaving the surface of the Earth. Changes in the atmospheric concentrations of these greenhouse gases can alter the balance of energy transfers between the atmosphere, space, land, and the oceans. The capture and storage of greenhouse gases could play a significant role in reducing the release of greenhouse gases into the atmosphere (read more about capture and storage of carbon dioxide here). Carbon dioxide (CO2) is the most important greenhouse gas and captures the limelight in most reports on global warming. While other greenhouse gases make up less of the atmosphere, they account for about 40 percent of the greenhouse gas radiation sent back to Earth. They can also be much more efficient at absorbing and re-emitting radiation than carbon dioxide, so they are small but important elements in the equation. In fact, molecule-for-molecule some gases containing lots of fluorine are 10,000 times stronger at absorbing radiation than carbon dioxide. A new systematic computational study shows an interesting approach of how nanotechnology, in this case the use of carbon nanotubes and other nanomaterials, could lead to effective filters for the capture and storage of greenhouse gases. ...more