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41544_Ward's World+MGH Nanotechnology

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3 Reliable positioning is required in order for mechanosynthetic processes to construct objects with millions to billions of pre- cisely arranged atoms. Mechanosynthetic systems are intended to perform several basic functions. Their first task is to bind raw materials from an externally provided source, typically a liquid solution contain- ing a variety of useful molecular species. This process must separate molecules of different kinds and bind them reliably to specific sites. A second task (in advanced systems) is to trans- form bound molecules into highly active chemical species, such as radicals, carbenes, and strained alkenes and alkynes. Finally, mechanical devices can apply these bound, active species to a workpiece in controlled positions and orientations to deposit or remove a precise number of atoms of specific kinds at spe- cific locations. To support these functions, it is necessary to provide ap- propriate mechanisms and conditions. Binding, transforming, and moving molecules can best be accomplished by nanoscale molecular machinery. To minimize friction, contamination, and side reactions, the ideal environment is free of fluids, which require a suitable enclosure and pumping mechanisms. If a mechanosynthetic system is to build complex products (rather than performing simple, repetitive operations), a programma- ble positioning mechanism could be used, requiring a source of instructions to guide its sequence of movements. Early-gener- ation systems will sacrifice efficiency and range of products in exchange for simpler mechanisms, for example, by operating in an ambient fluid environment and using positioning mecha- nisms with fewer degrees of freedom. Successful designs must overcome several challenges to reliable operation. Both quantum-mechanical uncertainty and thermal vibration cause random displacements in the posi- tions of parts. For nanomechanical parts at room temperature, thermal vibration is overwhelmingly more important than quantum uncertainty. Because the mean-square displacement of a part is inversely proportional to the stiffness of the struc- ture that holds it in place, the amplitude of thermal vibration could be limited in some circumstances by careful design. In mechanosynthetic systems, some vibrational amplitudes could be limited to less than one-tenth of an atomic radius. Because the probability of displacement has a Gaussian distribution, transient misalignments as large as an atomic diameter could be made extremely rare. Infrequent, high-amplitude thermal vibrations can break even strong chemical bonds. Breaking a single bond in a mo- lecular machine would typically cause it to fail. In the terrestrial ambient background radiation environment, structures with reasonable thermal stability will experience bond breakage chiefly from ionizing radiation. The rate of device damage due to ionizing radiation is roughly proportional to device mass, and devices on a scale of hundreds of nanometers will last many decades in the terrestrial environment before encoun- tering radiation damage. To be reliable, larger systems either must be made of parts large enough to tolerate some damage or must be organized redundantly so that the system itself can tolerate some failed parts. Photochemical damage can be prevented by enclosing systems in opaque shields; 0.25-mi- crometer-thick aluminum is ample for long-term protection in full sunlight. Reliable molecular manufacturing systems have strong similarities to digital computers. In conventional materials processing, as in analog electronics, all operations are some- what imprecise. Each object produced has a unique size, shape, composition, and microstructure, differing both from the ideal design and from all previous objects. In productive nanosystems, though, as in digital electronics, each operation is either entirely correct or clearly wrong. In digital logic, the result of an operation is a specific pattern of ones and zeros. No stable intermediate states are possible, and physical principles enable the design of circuits that produce the correct pattern with high reliability. In productive nanosystems, the result of an operation would be a specific pattern of bonded atoms. For suitable choices of product structure, no ambiguous states are possible, and physical principles would enable the design of processes that produce the correct pattern with high reliability. Applications One of the most common applications of nanotechnology is the use of nanoparticles of zinc oxide or titanium dioxide in sunscreens. Zinc oxide- or titanium dioxide-based sunscreens work by reflecting ultraviolet rays away from the skin. Much as digital computers can produce an indefinitely large range of patterns of information, productive nanosystems could produce an indefinitely large range of patterns of matter. For both digital computers and productive nanosystems, unlike traditional devices such as an adding machine or a lathe, it is difficult to describe the range of applications. Because productive nanosystem technology could make precise, nanoscale features, these methods could be used to fabricate improved circuitry for digital logic. It is widely recog- nized that diamond would be superior to silicon for this pur- pose, if it could be fabricated with comparable ease. Advanced productive nanosystems could make this practical. Design cal- Nanotechnology (continued) + ward ' s science + ward ' s science

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