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Pushing the Boundaries of Technology
By Michael Berger
The Royal Society of ChemistryCopyright © 2009 Michael Berger
All rights reserved.
How Can You 'See', 'Feel' or 'Hear' Something so Incredibly Tiny?
In 1981, the scanning tunneling microscope (STM) was invented, followed 4 years later by the atomic force microscope (AFM) — and that's when nanoscience and nanotechnology really started to take off. Various forms of scanning probe microscopes (SPM) based on these discoveries are essential for many areas of today's research. Conventional optics cannot resolve objects measuring tens of nanometers or less because the visible wavelength of light is roughly between 400 and 750 nm. With scanning probe techniques, all of a sudden the nanoworld became accessible to scientists in many countries, and these instruments have been the workhorses of nanoscience and nanotechnology research ever since.
"Today these methods are still making a tremendous impact on many disciplines that range from fundamental physics and chemistry through information technology, quantum computing, spintronics and molecular electronics, and all the way to life sciences," Christoph Gerber and Hans Peter Lang from the National Competence Center for Research in Nanoscale Science at the University of Basel in Switzerland write in an article in Nature Nanotechnology. "Indeed, some 4000 AFM-related papers were published in 2006 alone, bringing the total to 22 000 since it was invented, and the STM has inspired a total of 14000 papers. There are also at least 500 patents related to the various forms of SPM. Commercialization of the technology started in earnest at the end of the 1980s, and approximately 10 000 commercial systems have been sold so far to customers in areas as diverse as fundamental research, the car industry and even the fashion industry. There are also a significant number of home-built systems in operation. Today some 30–40 companies are involved in manufacturing SPM and related instruments, with an annual worldwide turnover of $250–300 million. Moreover, the market of SPMs is predicted to double over the next 5 years."
Unless they work in a state-of-the-art laboratory equipped with multimillion-dollar high-tech instruments, most people find it impossible to visualize nanoscale objects. The overused description that one nanometer is 50 000–100 000 times smaller than the diameter of a hair isn't really helpful either. Even scientists who work with the latest electron microscope techniques on a daily basis, and who have brought us all these amazing images from the nanoworld, often find it difficult, if not impossible, to make the mental connection between what they see with their own eyes and what the read-outs on their AFM show them. In this respect, a nanoscientist peering into the nanorealm isn't that different from an astronomer looking at the farthest reaches of the observable universe — the scales, be it nanometers or light years, overwhelm our brain's capacity for visualization.
While our five senses do a reasonably good job at representing the world around us on a macro scale, we have no existing intuitive representation of the nanoworld, ruled by laws entirely foreign to our experience. This is where molecules mingle to create proteins; where you wouldn't recognize water as a liquid; and where minute morphological changes would reveal how much 'solid' things such as the ground or houses are constantly vibrating and moving. Therefore, before we delve into the world of nanoscale probing and imaging, our first story is about an idea that could result in tools to explore the boundaries between the nanoscopic and the macroscopic worlds — touching nanoscale water, shaking hands with bacteria, crushing a virus between your fingers, playing nano-Lego. For scientists, it could also lead to a new generation of professional lab tools that allow nanoscale manipulation with precise control of tool interaction with nano-objects.
1.1 Getting all Touchy-Feely
Our sense of touch connects us to the world around us and it is an integral part of how we experience things, both physically and emotionally. In the virtual world of remote-control robots, scientific models, or computer games, users generally lack tactile, or haptic, feedback, which either makes delicate manipulative tasks difficult or keeps the subject purely visual and often inscrutable (such as an electron microscope image of a nanoscale object). The desire for natural and intuitive human machine interaction has led to the inclusion of haptics in human–machine interfaces. The user is able to control inputs to the system through hand movements and in turn receives feedback through tactile stimulation in the hands. Sophisticated, state-of-the-art haptic user-interface software is capable of adding interactive, realistic virtual touch capabilities to human–computer interactions. Among the uses are medical applications, remote vehicle or robotic control, military applications, and video games. Users are said to feel realistic weight, shape, texture, dimension, dynamics, and force effects. Applying the use of real-time virtual reality and multisensory user interface to nanoscience, scientists in France have begun to open up the otherwise only scientifically described reality of the nanoworld to a nonscientific public.
"A central challenge is how we can put our hands on scientifically explored parts of reality that cannot be reached by our senses and whose rules are completely foreign to our representation of reality," Joël Chevrier tells us. "Since science is full of abstract descriptions, it is hard to represent it in an easy way. But thanks to computer sciences and robotics we now have the necessary tools to use human senses to explore, in real time, model worlds as they are described by science, or even true reality when coupling these multisensory interfaces to real nanosensors and nanoactuators."
Chevrier, a professor at the Université Joseph Fourier in Grenoble, France, together with his collaborators hopes to open up a completely new field for our perception. This new 'playground' — using haptic, vision, and sound inter-faces — is the world we live in; but explored at scales entirely foreign to every-thing we experience around us.
"In the nanoworld simulacrum that we have begun to build, object identification will be based on the intrinsic physical and chemical properties of the probed entities, on their interactions with sensors, and on the final choices made in building a multisensory interface so that these objects become coherent elements of the human sphere of action and perception," says Chevrier.
In other words, we might be able to touch, feel, and interact with the nanoworld which otherwise is not open to our direct experience. Chevrier hopes that this will be a major step in helping nonscientists understand nanosciences and nanotechnologies. The scientifically described part of our reality — much of what mathematics, physics, or chemistry is about — is usually inaccessible to people not trained in these subjects, i.e. to most of us. Opening up this part of reality to everybody could go a long way in creating interest in science education and science careers, and help a better-informed public to lead a more objective discussion on the pros and cons of nanotechnologies.
Rather than using the abstract descriptions and experiments of a classical science education, the French team has begun to use real-time virtual reality combined with a multisensory human–machine interface to allow the direct perception of and interaction with the nanoworld.
"One way to develop this extension of the sphere where our senses are efficient can be based on nanosensors and nanoactuators," explains Chevrier. "Another approach is to use virtual environments which can bring the nano-world to us through real-time multisensory interfaces. This can dramatically enhance the possibilities for easy exploration of remote realities foreign to our senses and can trigger spontaneous motivation in users, similar to what we observe in video game players."
Chevrier and his team have built a virtual AFM and coupled it to an advanced haptic interface as well as a sonification and visualization system. The resulting instrument allows its user to experience contact of a surface at the nanoscale. About 10 000 people used this demonstrator during three exhibitions in Grenoble, Paris, and Geneva.
A central part of this concept is not a new idea. It actually goes back to the earliest days of experimental science: Galileo's use of a telescope to observe the Moon and coming to the immediate conclusion that the Moon is Earth-like. As Galileo immediately emphasized, this dramatic change in the human representation of the universe is caused by direct use of senses technically extended by an instrument, and not by a posteriori rational demonstration.
"Our proposal can be seen as a revival of this famous tale," says Chevrier. "There is a major difference, however. Two points can illustrate the need for new approaches in implementing the nanoscale in virtual environments:
(1) As we gradually approach the nanoscale, continuous description no longer stands and the molecular, discontinuous structure of matter is revealed. Atomic scale is a radical rupture with our common experience that is based on the objective existence of isolated continuous objects.
(2) Can we manage to 'see' and 'touch' an electron, a particle that has a mass and an electric charge but has no classical spatial extension in the sense of a material sphere, although it is at the root of the stability of matter? In fact, seeing or touching an electron has no intrinsic meaning. Electron-based objects can, however, be created and our interaction with these unusual objects defined."
Almost all scientific data today is represented visually. That's why we have all these amazing electron microscope images and artists' impressions of nanoscale objects. That's also why most people can't really get a grip (literally) on scientific discoveries unless they result in a better TV set or more stain-resistant shirts. Enriching the visual component with interactive tactile and sound aspects, and wrapping the whole thing into a virtual reality environment, will give us a much richer and more real experience of these objects.
At the Center for Cognitive Ubiquitous Computing (CUbiC) at Arizona State University in the USA they have developed some interesting haptic visualization schemes. Many object features are easy to invoke in human memory and are presented through tactile cueing. There are, however, some features that are not primary haptic features but may contribute to further knowledge of the object. One example is the weight of the object. At CUbiC they have developed a haptic visualization scheme for the presentation of weight. In this scheme, a user is able to bounce the virtual object off an imaginary surface. When the object hits back, it generates a vibrotactile stimulation analogous to its weight.
Even if technology will one day offer us sophisticated tools to explore the nanoworld with our senses, the question is whether we will be able to really grasp it. Imagine an atom. Chances are you are seeing a Nagaoka (Figure 1.1). In 1904, a Japanese physicist named Hantaro Nagaoka created the classic atom image with planet-like electrons orbiting around a nucleus.
This is the picture that many people have in mind — cute, but wrong. Reality at the atomic scale is much, much weirder: atoms are mostly empty space and the solid world we experience around us is an illusion. Timothy Ferris has described this nicely in his book Coming of Age in the Milky Way:
"A bar of gold, though it looks solid, is composed almost entirely of empty space. The nucleus of each of its atoms is so small that if one atom were enlarged a million billion times, until its outer electron shell was as big as greater Los Angeles, its nucleus would still be only about the size of a compact car parked downtown. The electron shells would be zones of insubstantial lightning, each a mile or so thick, separated by many miles of space. Nor, to return to the old classical metaphor, does a cue ball strike a billiard ball. Rather, the negatively charged fields of the two balls repel each other; on the subatomic scale, the billiard balls are as spacious as galaxies, and were it not for their electrical charges they could, like galaxies, pass right through each other unscathed."
So, while your 'reality' tells you that you are sitting in your chair right now as you are reading this, reality at the subatomic level means that you are not really in contact with your chair — thanks to the repulsion of the chair's electrons and your own, you are actually floating on it at a height of a fraction of a nano-meter. The point is that, even if we might have the tools one day to truly experience the nanoworld, its rules are so foreign to our human experience that we might not be able to comprehend it anyway.
Of course, this first instrument built by Chevrier's team in Grenoble is more Galileo telescope than Hubble space observatory. But it is an interesting beginning that one day might result in virtual worlds that will allow us to go all weird at the nanoscale.
1.2 Feeling our Way Through the Nanoworld
Let's come back to the AFM mentioned at the beginning and look at an example of how scientists continuously work to improve these instruments. Since the nanoscale world is accessible only with specialized — and often very expensive — tools, the ongoing improvement of these instruments, and the development of new ones, is a crucial aspect of continuous progress in nanosciences and nanotechnologies. Contrary to much of the hype surrounding the field, a large part of 'nanotechnology' today is about developing new tools, techniques, and applications to explore and understand phenomena at the nanoscale.
"Children begin to learn by seeing, hearing, tasting and, above all, by touching. In a very similar approach, we are currently learning to orient ourselves in the nanoworld by 'feeling' materials — not with our fingers, but with microscopes that allow us to probe these materials with atomic resolution."
The ability of researchers to engineer novel materials that possess superior electronic, thermal, magnetic, and mechanical properties depends on tools that can identify and characterize material components and their spatial arrangement at the nanoscale. Equally important, understanding structure–function relationships in biological systems also demands tools that can probe structural properties with molecular resolution: AFMs are the most widely used tools to image matter at the nanoscale.
The operating principle of an AFM is based on an atomically sharp tip, placed at the end of a flexible cantilever beam, that is brought into physical contact with the surface of a sample. The cantilever beam deflects in proportion to the force of interaction. Scanning across the surface, the sharp tip follows the bumps and grooves formed by the atoms on the surface. A topography of the surface can be generated by monitoring the deflections of the flexible cantilever beam.
Because of its mechanical operation, the AFM can in principle also perform nanomechanical measurements. This aspect of the AFM has been explored by researchers over the past two decades. However, current state-of-the-art techniques are very slow — it takes about a second for the AFM tip to approach, push into, and retract from the surface of a material — and rather large forces are applied during the measurement process that damage the tip and the sample.
Researchers at Harvard and Stanford universities have developed a specially designed AFM cantilever tip, the torsional harmonic cantilever (THC), which offers orders-of-magnitude improvements in temporal resolution, spatial resolution, indentation, and mechanical loading compared to conventional tools. With high operating speed, increased force sensitivity, and excellent lateral resolution, this tool facilitates practical mapping of nanomechanical properties.
Notwithstanding all the advances that have been made in the field of AFM, Ozgur Sahin from the Nanomechanical Sensing Group at the Rowland Institute at Harvard University in the USA says that so far a technique that can quantitatively map mechanical properties in detail with nanoscale resolution is missing. "Mechanical properties of matter are largely determined by the nature of chemical bonds and their spatial organization in the material," he explains. "Furthermore, materials used in everyday life exhibit a huge variation in their mechanical properties. Diamond, for example, is almost a million times stiffer than rubbery materials. The spectrum of mechanical properties of materials spans the range between these two extremes. These observations tell us that there is a lot of information in mechanical properties of materials."
Excerpted from Nano-Society by Michael Berger. Copyright © 2009 Michael Berger. Excerpted by permission of The Royal Society of Chemistry.
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