09-10-2012, 05:01 PM
SEMINAR ON NANOTECHNOLOGY
nanotechnology.doc (Size: 315.5 KB / Downloads: 79)
Introduction
Molecular nanotechnology or Nanotechnology is the name given to a specific sort of manufacturing technology to build things from the atom up, and to rearrange matter with atomic precision. In other words, we can say that nanotechnology is a three dimensional structural control of material and devices at molecular level. The nanoscale structures can be prepared, characterized, manipulated, and even visualized with tools.
HISTORY OF NANOTECHNOLOGY
Any advanced research carries inherent risks but nanotechnology bears a special burden. The field's bid for respectability is colored by the association of the word with a cabal of futurist who foresee nano as a pathway to a techno-utopia: unparalleled prosperity, pollution-free industry, even something resembling eternal life.
In 1986-five years after IBM researchers Gerd Binnig and Heinrich Rohrer invented the scanning tunneling microscope, which garnered them the Nobel Prize-the book Engines of Creation, by K. Eric Drexler, created a sensation for its depiction of godlike control over matter. The book describes self-replicating nanomachines that could produce virtually any material good, while reversing global warming, curing disease and dramatically extending life spans. Scientists with tenured faculty positions and NSF grants ridiculed these visions, noting that their fundamental improbability made them an absurd projection of what the future holds.
But the visionary scent that has surrounded nanotechnology ever since may provide some unforeseen benefits. To many nonscientists, Drexler's projections for nanotechnology straddled the border between science and fiction in a compelling way. Talk of cell-repair machines that would eliminate aging as we know it and of home food-growing machines that could produce victuals without killing anything helped to create a fascination with the small that genuine scientists, consciously or not, would later use to draw attention to their work on more mundane but eminently more real projects. Certainly labeling a research proposal "nanotechnology" has a more alluring ring than calling it "applied mesoscale materials science."
Less directly, Drexler's work may actually draw people into science. His imaginings have inspired a rich vein of science-fiction literature . As a subgenre of science fiction-rather than a literal prediction of the future-books about Drexlerian nanotechnology may serve the same function as Star Trek does in stimulating a teenager's interest in space, a passion that sometimes leads to a career in aeronautics or astrophysics.
The danger comes when intelligent people take Drexler's predictions at face value. Drexlerian nanotechnology drew renewed publicity last year when a morose Bill Joy, the chief scientist of Sun Microsystems, worried in the magazine Wired about the implications of nanorobots that could multiply uncontrollably. A spreading mass of self-replicating robots-what Drexler has labeled "gray goo"-could pose enough of a threat to society, he mused, that we should consider stopping development of nanotechnology. But that suggestion diverts attention from the real nano goo: chemical and biological weapons.
Among real chemists and materials scientists who have now become nanotechnologists, Drexler's predictions have assumed a certain quaintness; science is nowhere near to being able to produce nanoscopic machines that can help revive frozen brains from suspended animation. (Essays by Drexler and his critics, including Nobel Prize winner Richard E. Smalley, appear in this issue.) Zyvex, a company started by a software magnate enticed by Drexlerian nanotechnology, has recognized how difficult it will be to create robots at the nanometer scale; the company is now dabbling with much larger micromechanical elements, which Drexler has disparaged in his books.
NANOTECHNOLOGY TOOLS
What would it mean if we could inexpensively make things with every atom in the right place? For starters, we could continue the revolution in computer hardware right down to molecular gates and wires -- something that today's lithographic methods (used to make computer chips) could never hope to do. We could inexpensively make very strong and very light materials: shatterproof diamond in precisely the shapes we want, by the ton, and over fifty times lighter than steel of the same strength. We could make a Cadillac that weighed fifty kilograms, or a full-sized sofa you could pick up with one hand. We could make surgical instruments of such precision and deftness that they could operate on the cells and even molecules from which we are made -- something well beyond today's medical technology. The list goes on -- almost any manufactured product could be improved, often by orders of magnitude.
THE ADVANTAGES OF POSITIONAL CONTROL
One of the basic principles of nanotechnology is positional control. At the macroscopic scale, the idea that we can hold parts in our hands and assemble them by properly positioning them with respect to each other goes back to prehistory:
At the molecular scale, the idea of holding and positioning molecules is new and almost shocking. However, as long ago as 1959 Richard Feynman, the Nobel prize winning physicist, said that nothing in the laws of physics prevented us from arranging atoms the way we want: "...it is something, in principle, that can be done; but in practice, it has not been done because we are too big."
Before discussing the advantages of positional control at the molecular scale, it's helpful to look at some of the methods that have been developed by chemists -- methods that don't use positional control, but still let chemists synthesize a remarkably wide range of molecules and molecular structures.
SELF ASSEMBLY
The ability of chemists to synthesize what they want by stirring things together is truly remarkable. Imagine building a radio by putting all the parts in a bag, shaking, and pulling out the radio -- fully assembled and ready to work! Self assembly -- the art and science of arranging conditions so that the parts themselves spontaneously assemble into the desired structure -- is a well established and powerful method of synthesizing complex molecular structures. A basic principle in self assembly is selective stickiness: if two molecular parts have complementary shapes and charge patterns -- one part has a hollow where the other part has a bump, and one part has a positive charge where the other part has a negative charge -- then they will tend to stick together in one particular way. By shaking these parts around -- something which thermal noise does for us quite naturally if the parts are floating in solution -- the parts will eventually, purely by chance, be brought together in just the right way and combine into a bigger part. This bigger part can combine in the same way with other parts, letting us gradually build a complex whole from molecular pieces by stirring them together and shaking.
SCANNING TUNNELING MICROSCOPE (STM)
The STM is a device that can position a tip to atomic precision near a surface and can move it around. The scanning tunneling microscope is conceptually quite simple. It uses a sharp, electrically conductive needle to scan a surface. The position of the tip of the needle is controlled to within 0.1 angstrom (less than the radius of a hydrogen atom) using a voltage-controlled piezoelectric drive. When the tip is within a few angstroms of the surface and a small voltage is applied to the needle, a tunneling current flows from the tip to the surface. This tunneling current is then detected and amplified, and can be used to map the shape of the surface, such as a blind man tapping in front of him with his cane, we can tell that the tip is approaching the surface and so can "feel" the outlines of the surface in front of us.
Many different types of physical interactions with the surface are used to detect its presence. Some scanning tunneling microscopes literally push on the surface -- and note how hard the surface pushes back. Others connect the surface and probe to a voltage source, and measure the current flow when the probe gets close to the surface. A host of other probe-surface interactions can be measured, and are used to make different types of STMs. But in all of them, the basic idea is the same: when the sharp tip of the probe approaches the surface a signal is generated -- a signal which lets us map out the surface being probed.
SELF REPLICATION : MAKING THINGS INEXPENSIVELY
The requirement for low cost creates an interest I self-replicating manufacturing systems. These systems are able both to make two copies of itself, and those two make two copies each and so on. We can have trillions of nanobots in no time, each one operating independently to carry out a trillionth of the job. This system will be reasonably inexpensive, effective and time saving process.
Positional control combined with appropriate molecular tools should let us build a truly staggering range of molecular structures -- but a few molecular devices built at great expense would hardly seem to qualify as a revolution in manufacturing. How can we keep the costs down?
If we could make a general purpose programmable manufacturing device which was able to make copies of itself then the manufacturing costs for both the devices and anything they made could be kept quite low -- likely no more than the costs for growing potatoes.
Drexler called such devices "assemblers."