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Foresight Update 9 (page 4)

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Table of Contents - Foresight Update 9

Recent Progress: Steps Toward Nanotechnology

by Russell Mills

Scanning probe devices

The scanning tunneling microscope is less than 10 years old but has already given rise to at least nine other kinds of microscopes, each designed to collect a different kind of information from a sample. Robert Pool has provided a nice overview of these devices, which all share the the same general mechanical and imaging technology: samples are placed on a surface that can be moved by the expansion and contraction of piezoelectric crystals; images are made by scanning with a pointed probe brought very close to the sample.

The devices differ in what they measure or do to the sample. The STM sends an electric current through the sample and, in effect, measures the electrical resistance. The atomic force microscope measures the longitudinal force between the probe tip and the sample (i.e., the force along the line between probe and sample), whereas the friction force microscope measures the transverse force. The magnetic force microscope measures the magnetic field around the sample; the electric force microscope measures the electric field. The scanning thermal microscope measures the temperature variations along a sample. The optical absorption microscope does spectroscopic measurements. The scanning ion-conductance microscope maps ion flows passing through the sample. The scanning near-field optical microscope uses an optical probe to make images with visible light at molecular-scale resolution. The scanning acoustic microscope uses sound waves to image samples, permitting a view beneath the surface. Some of these devices can be used to alter specimens as well as to view them.

[Science 247:634-247, 9Feb90]

Rather suddenly, science finds itself able to obtain a wealth of information at atomic or near-atomic resolution. This should bring on an avalanche of useful structural information, much of it immediately applicable to problems in molecular engineering. The direct impact should be felt first in biology and materials science; and soon after in fields that depend on these, such as biotechnology, electronics, medicine, and chemistry.

Table of Contents - Foresight Update 9

Measuring with crystal lattices

Although scanning probe devices (like the scanning tunneling microscope) can image objects of atomic size, they lack reliable methods for positioning and measuring samples. Researchers at the University of Tokyo are trying to remedy the situation by using the lattices of crystals as a calibration standard. A dual-probe scanning device is being developed in which one probe scans the sample while the other scans a reference crystal. Rough positioning of the sample would be performed by an impact drive mechanism in which a piezoelectric element strikes the block holding the sample, knocking it forward several nanometers. An impact rate of 80 impacts/sec is being investigated.

[Paper by Hideki Kawakatsu, et al., IEEE document #CH2832-4/90:197-201]

Scanning probe devices are thus being transformed from novelty items into reliable instruments for routine research.

Table of Contents - Foresight Update 9

Synthesis of shaped structures

The Diehls-Alder reaction, discovered in the mid-1800s, is a chemical reaction with broad application for building larger molecules from smaller ones. In the past few years chemists have begun using it for molecular construction on the basis of shape--to make molecules designed to look like gears, for example. In an intriguing review of the subject, Franz H. Kohnke at Univ. di Messina suggests that the next few years will see rapid development of such "structure-directed synthesis," giving rise to molecules "that look like ball bearings, beads and threads, belts, cages, chains, chimneys, clefts, coils, collars, knots, ladders, nets, springs, stacks, strips, washers, and wires--and concurrently and subsequently for molecules with function--that work like abacuses, capacitors, catalysts, circuits, clocks, conductors, dynamos, membranes, motors, nuts and bolts, resistors, screws, semiconductors, sensors, shuttles, superconductors, and switches."

[Angew. Chem. Int. Ed. Engl. Adv. Mater. 28:1103-1110, 1989]

To view atoms as construction materials and molecules as machines goes hand in hand with progress toward nanotechnology. Clearly, this viewpoint is now taking hold among chemists.

Table of Contents - Foresight Update 9

Self-replicating molecules

Julius Rebek and co-workers at MIT have designed and built a replicator: a molecule that produces copies of itself, given appropriate raw materials. The raw materials are quite specialized: one of them is a variant of adenosine (one of the four building blocks of DNA); the other is a fluorinated ester having features complementary to the adenosine. The ester component includes a catalytic group that promotes bonding between adenosine and ester.

This primitive replicator contains about two hundred atoms. The initial copies were made by ordinary chemical synthesis. When a solution of adenosine and the fluorinated ester was seeded with replicators, the replicator molecules paired up with adenosine and ester molecules and catalyzed bond formation between them, forming more replicators.

[Article by I. Amato in Science News, 3Feb90: 69]

So, structures that foster their own formation need not be very complicated. Replication appears to derive from the complementarity of the components along with the inclusion of an appropriately situated catalytic group.

An interesting question left unanswered by this research is whether it is easier to satisfy the conditions for replication with a single molecule or with a set of molecules that catalyze each others' formation.

Table of Contents - Foresight Update 9

Protein folding

The "folding problem" for proteins is to figure out what spatial configuration will be taken by a given chain of amino acids. Researchers at MIT started with the repressor protein from the lambda phage (a virus that attacks bacteria). They made numerous versions of the repressor, each with different amino acid substitutions at one or several locations, and tested the activity of phages containing these. It was found that at certain locations almost any substitution can be made with little or no effect on activity. At other locations, the protein is intolerant to any change at all. Generally speaking, amino acids located in the core of the protein could often be substituted, but only with other amino acids of similar (hydrophobic) type. Surface amino acids were usually tolerant of a wider variety of substitutions except at a few functionally important sites.

[Paper by James U. Bowie, et al. in Science 247:1306-247, 16Mar90]

Designing proteins for given roles may require less computation than one might think, since many different amino acid sequences can give rise to the same functionality.

Table of Contents - Foresight Update 9

Stability of small diamonds

Small diamonds (3 to 5 nm in diameter) constitute about 20% by weight of soot formed during detonations, and are also found in meteorites and in nucleation experiments. A group of investigators has shown by thermodynamic arguments that in this size range, diamond is as stable as graphite and its relatives, and that extremely high pressure is not necessarily required to make diamond.

[Paper by P. Badziag at Univ. of South Africa, et al., in Nature 343:244-245,18Jan90]

Diamond is being considered as a construction material for assemblers and other nanotechnological devices. Badziag's work supports the thesis that small structures can be built with diamond at low pressure. It is also comforting to know that such devices would be unlikely to disintegrate or turn spontaneously into graphite.

Table of Contents - Foresight Update 9

Observations of molecular vibration and rotation

Vibrations in chemical bonds occur on timescale of about 10-13 sec, and rotations in about 10-10 sec. Laser observations with a resolution of a few tens of femtoseconds have enabled researchers to follow these processes in detail. To study vibrations, a "pump" pulse at a wavelength of 620 nm prepared iodine molecules by sending them to higher energy. A probe pulse at 310 nm then sent these excited molecules into a still higher energy state from which they decayed by fluorescence. By varying the delay between the pump and probe pulses, the fluorescence intensity was seen to vary as the chemical bond stretched and contracted.

Rotations were followed with pulses of polarized laser light. Only those molecules with axes aligned with electric field of the pulse were excited. As the delay between pump and probe pulses was increased, the observed fluorescence declined since the molecules' axes were rotating out of alignment with the probe's electric field. The results closely matched the predictions made from quantum theory.

[Paper by M. Dantus at Cal Tech, et al., in Nature 343:737-739, 22Feb90]

Assemblers and other nanomachines will have to deal with vibrational and rotational effects every time a bond is made or broken. Polarized femtosecond spectroscopy has now provided a direct source of information about these processes.

Table of Contents - Foresight Update 9

Quantum electronics

Electrons behave more like particles when free to move in regions much larger than their wavelength, and more like waves when confined into regions comparable to their wavelength. Fabrication techniques now being developed will enable electronics to exploit the wave properties of electrons.

Today's most advanced commercial transistors have features as small as .75 micrometers (a hundredth of the diameter of a hair); quantum effect devices being experimented with today are substantially smaller, having features about 25 nanometers (about 100 atoms) across--and researchers are aiming for 10 nanometers in the near future. Devices of this scale can be made by x-ray lithography, analogous to the photolithographic technique that the electronics industry has relied on for a quarter century.

To study electron wave effects, researchers have etched electron waveguides about 30 nanometers across into a block of aluminum gallium arsenide (AlGaAs). Transistor-like behavior can be obtained from a block of AlGaAs after etching a rectangular pattern of holes.

One intriguing way to utilize quantum devices would be to arrange them as arrays of "quantum dots" on a surface, each dot storing a small amount of information and interacting with its neighbors according to prescribed rules. Such arrays, called "cellular automata," can perform computations without a network of wires to shunt information around. Arrays of 200 million quantum dots per cm2 have been made, but are not yet programmable.

[Review article by Henry I. Smith at MIT, et al., in Technology Review Apr90:26-40]

While this kind of electronics exploits only a few basic quantum mechanical effects in simple crystals, it nevertheless promises major advances in speed and miniaturization. Even greater improvements will come from more sophisticated materials, in which complex structures process information at the molecular level. The following two reports provide hints of what is to come.

Table of Contents - Foresight Update 9

Superlattices

Molecular sieves are lattice structures containing regular patterns of "host" cavities--channels, cups, or cages. A variety of molecular sieves can be made, each with its own characteristic size, shape, and pattern of host cavities.

The host cavities can be used to trap and hold "guests"--small molecules or clusters of semiconductor atoms--forcing them into regular arrays called "superlattices" and constraining their internal motions.

Molecular sieves are attracting the interest of researchers in a number of fields because they provide new ways to control and obtain information about guest molecules. Semiconductor scientists are interested in them because confinement and regular spacing of clusters of semiconductor atoms gives rise to quantum phenomena not seen in ordinary crystals or solutions of the same substances.

Superlattices should find early application in optoelectronics because of the ease with which their optical properties can be manipulated. Some superlattices, for example, undergo color changes as the temperature is varied; others respond to pressure, humidity, light, pH, or electric fields.

[Review article by Galen D. Stucky at UC Santa Barbara, et al., in Science 247:669-678, 9Feb90]

Since superlattices are generalizations of the crystalline state, they have a wider range of bulk properties than conventional crystals do, and offer more opportunities for the control of these properties. They are early examples of the kind of atomically precise manmade materials we will see more of when molecular assembly machines become available.

Table of Contents - Foresight Update 9

Artificial photosynthetic device

Ordinary photosynthesis takes place within a complex structure embedded in a membrane of a plant or bacterium. In this process a photon is captured by "antenna" molecules and its energy transferred to a pigment molecule where it is absorbed by an electron. The electron moves quickly to nearby quinone molecules, leaving behind a positive charge. From the quinone the electron passes along a chain of other structures to the outer side of the membrane and is transferred to other molecules. Meanwhile, the positive charge left behind is passed to the inner side of the membrane where it is neutralized by an electron pulled from a suitable electron donor. In plants, the electron donors are water molecules which are converted to oxygen and protons; the protons are used in the manufacture of adenosine triphosphate (ATP), the basic energy coinage of the biological realm.

At Arizona State University researchers have made a vastly simplified version of a significant part of this photosynthetic apparatus. They have designed and synthesized an impressive-looking molecule of approximately two hundred atoms that can absorb photons, transfer the energy to electrons, and send the electrons down one arm of the molecule and the positively charged "hole" down the other arm. The charge-separated state has a lifetime of about 55 microseconds and preserves 83% of the original photon energy.

[Paper by Devens Gust, et al., in Science 248:199-201, 13Apr90]

The design of this artificial photosynthetic device was no exercise in trial and error--it proceeded from a detailed understanding of how energy is transferred between quantum states of molecules. The methods used by Gust's team provide a glimpse of the awesome capabilities that will soon be routinely available to molecular engineers.

Table of Contents - Foresight Update 9

Foresight thanks Dave Kilbridge for converting Update 9 to html for this web page.

From Foresight Update 9, originally published 30 June 1990.
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