We have written Spotlight after Spotlight pointing out the numerous challenges that researchers are facing with regard to nanofabrication. Uncountable research papers have been written about the numerous methods available for synthesizing nanomaterials. What today is called 'nanofabrication' deals with the issues of fabricating complex and functional nano- and microstructures by integrating these synthesized nanomaterials. To complicate matters, many of these nanomaterials are fragile, either because they are composed of a limited number of atomic layers or because they are 'soft', i.e., of biological or molecular nature. This fragile nature of some materials creates a major headache: how to integrate them at an individual level into devices without altering their structure and, consequently, their properties during device fabrication. Currently, researchers use mainly e-beam lithography and, in some cases, focused ion beam to fabricate devices which incorporate nanostructures. These methods have been proven to be very useful, for example for investigations of carbon nanotubes. However, they can not be applied on fragile nanostructures, because they damage or contaminate the structures - this could result from exposure to high energy particle beams; the requirement for lift-off steps; exposure to chemicals, etc. Researchers have now shown that individual nanostructures can be integrated into functional devices using dynamic nanostenceling which allows the integration of individual nanostructures into devices using entirely scanning probe based methods and without exposure to damaging conditions, such as high-energy charged particles, heat, or resists.
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.
Some pundits writing about nanotechnology get carried away by their own hype and talk about self-assembly as if bottom-up fabrication technologies, where molecules get assembled into everyday products, are just around the corner. We took a swing at this in our Spotlight from a few days ago (Nanotechnology 'pencil sharpeners' add to researchers' nanofabrication toolbox). Today we bring you another example from the cold reality of the labs that makes clear how early stages this whole field of self-assembly really is. Today, when researchers - with both feet firmly on the ground - talk about self-assembly they mostly talk about template-assisted nanocrystal superlattices in the form of planar thin films. Bottomline is that even the controllable fabrication of highly ordered homogeneous nanostructures on surfaces remains a difficult challenge. And IBM's much touted 'self-assembling nanotechnology' (see: IBM applies self-assembling nanotechnology to conventional chip manufacturing) is nothing more than a patterning process that creates a film with trillions of holes around the on-chip wiring. Moving from a planar geometry of self-assembled nanoscale building blocks such as nanocrystals or nanotubes to a free-standing, three-dimensional multifunctional architecture is not a trivial undertaking. Researchers are just about to make the first steps to such multifunctional (still nanoscale) hierarchical architectures that both retain the properties of the nanocrystals and offer multifunctionality.
Proteins, large organic compounds made of amino acids, provide many of the most basic units of function in living systems. They make up about half of the dry mass of animals and humans. There may be as many as 1 million different types of proteins in the human body (it is estimated that the human proteome is comprised of an average of 5-7 protein isoforms per open reading frame in the human genome and a further 600 000-odd immunoglobulins present in serum at any given moment) - nobody really knows. The word protein comes from the Greek prota, meaning 'of primary importance', and they actually may become of great importance in nanoscale fabrication as well. Proteins have an amazing number of functions inside our bodies: Enzymes serve as catalysts to break down food into various components; transport proteins such as hemoglobin transport molecules (e.g. oxygen); storage proteins store molecules (e.g. iron is stored in the liver as a complex with the protein ferritin); structural proteins such as keratin or collagen are needed for mechanical support in tissues like cartilage and skin but also hair and nails; proteins are the major component of muscles and for instance actin or myosin are key to contracting muscle fibers; hormones control the growth of cells and their differentiation; antibody proteins are needed for immune protection; and toxins are, well, toxic, but in minute amounts could have beneficial medical properties. Scientists believe that this variety of natural protein functions - actuation, catalysis, structural transport and molecular sequestering - could serve as valuable and versatile building blocks for synthesis of functional materials. Researchers now have found that nanometer-scale changes in protein conformation can be translated into macroscopic changes in material properties. The result is a new class of dynamic, protein-based materials.
Animals that cling to walls and walk on ceilings owe this ability to micro- and nanoscale attachment elements. The highest adhesion forces are encountered in geckos. For centuries, the ability of geckos to climb any vertical surface or hang from ceilings with one toe has always generated considerable interest. A gecko is the heaviest animal that can 'stand' on a ceiling, with its feet over its head. This is why scientists are intensely researching the adhesive system of the tiny hairs on its feet. On the sole of a gecko's toes there are some one billion tiny adhesive hairs called setae (3-130 micrometers in length), splitting into even smaller spatulae (about 200 nanometers in both width and length) at the end. It was found that these elastic hairs induce strong van der Waals forces. This finding has prompted many researchers to use synthetic microarrays to mimic gecko feet. Recent work, mainly from A. Dhinojwala, P.M. Ajayan, M. Meyyappan, and L. Dai groups, as well as the Max Planck Institute for Metals Research in Germany (see our previous Spotlight: Gecko nanotechnology), has indicated that aligned carbon nanotubes (CNTs) sticking out of substrate surfaces showed strong nanometer-scale adhesion forces. Although carbon nanotubes are thousands of times thinner than a human hair, they can be stronger than steel, lighter than plastic, more conductive than copper for electricity and diamond for heat. Applications of such bio-inspired development of artificial dry adhesive systems with aligned carbon nanotubes could range from low-tech fridge magnets to holding together electronics or even airplane parts.
Nanotechnology enabled synthetic biology could one day lead to an artificial construct that operates like a living cell. That day might be a considerable distance off, given the difficulties scientists are still having in even understandiing the organizing principles and workings of a cell, not to mention being able to duplicate cell components and assembling them into a working whole. The large discrepancy between the functional density (i.e., the number of components or interconnection of components per unit volume) of cells and engineered systems highlights the inherent challenges posed by such a task. Just take 'simple' bacteria like Escherichia coli (which has an approx. 2 square micrometer cross-sectional area). The E. coli cell has some 4.6-million base-pair chromosome (the equivalent of a 9.2 megabit memory) that codes for as many as 4,300 different polypeptides under the inducible control of several hundred different promoters. The most advanced silicon chips will be able in a few years time to come close to this performance (on the other hand, you have several trillion E. coli in your gut; you would need to swallow a lot of computer chips to match this combined 'computing' power). Another way to look at the synthetic cell challenge is to regard the cellular environment as a highly complex synthetic medium, in which numerous multistep reactions take place simultaneously with an efficiency and specificity that scientists are not capable of duplicating at this scale. Researchers in The Netherlands have now succeeded in constructing nanoreactors that can be used to perform one-pot multistep reactions - another step towards the goal of artificial cell-like devices, but more promising in the short term for screening and diagnostic applications.
Proponents of 'atomically precise manufacturing' and 'molecular manufacturing' love to talk about the mind-boggling possibilities that these technologies would offer. These visions range from the modest, such as improved materials and more efficient production methods for chemicals (already on the horizon), to the outrageous, such as molecular desktop fabs (far, far out). Articles about revolutionary nanotechnology almost always skip the hard part, i.e. the tremendous amount of research breakthroughs that are required to get from where we are today to the promised land. It's as if some flight enthusiasts in 1903, when Orville Wright took off on the first powered flight, would have debated first class cabin design of 600-seat jet airliners flying New York - Tokyo non-stop. Nanotechnology researchers today are still struggling with very basic problems such as being able to control the synthesis of nanoparticles. It might come as a bit of a shock to the nanotechnology enthusiasts among you, but even something utterly fundamental as synthesizing metal nanoparticles today is more of an empirical trial and error approach than a predictable, fully controlled process. Let's take a look at new research coming out of a Spanish university that exemplifies the tiny, yet elementary, steps researchers need to take to master the nanoscale.
Contamination of superhydrophobic surfaces with low-surface-tension organic liquids is one of the leading reasons why superhydrophobic surfaces are not widely used in practical applications. If engineers were to succeed in creating a surface that repels any liquid the practical implications obviously would be substantial. In new work, researchers use metastable states to control wetting properties of solids. Since there is a much wider range of potentially available metastable states than thermodynamically stable states, the approach can greatly broaden the range of available control over the wetting behavior of solid surfaces.