Left: The world's smallest company logo etched
with 35 Xenon atoms. courtesy of IBM corporation Atomic Reach
The coming revolution in engineering promised by nanotechnology will alter all our current notions of industry, computing, medicine, and manufacturing in such a way comparable only to the rise of steam power, transistors or fire. By manipulating individual atoms and molecules, building devices from the bottom-up instead of the current top-down approach of whittling blocks of materials, we will be able to manufacture any material needed, in near infinite variety, at minimal cost. Of course, such a technology is not yet within grasp, although it is not as far as we might think. There are several pathways to molecular engineering, some of which are discussed elsewhere in this issue. The three major pathways considered are protein engineering, chemosynthesis, and mechanosynthesis by physical manipulation. Scanning tunneling microscopes (STM) and atomic force microscopes (AFM) are instruments that may be adapted to mechanosynthesis, giving us a viable pathway to molecular engineering. Although some degree of advancement in technology is required to achieve this, these microscopy instruments perhaps offer the best scenario. With their manipulative elements and relative availability, these tools will provide a synthetic approach to building molecules and nanodevices.
The Scanning Tunneling Microscope
The STM was developed in 1982 by Binnig and Rohrer, winning them the
Nobel Prize in Physics in 1986. The STM is capable of resolving surface details down to
the atomic level, most notably in probing the atomic structure and organization of
important materials like silicon, gallium arsenide, and graphite. From these discoveries,
it has since found applications in industry, being used in improving the quality of
microscopic devices such as diffraction gratings, optical disk-stampers for CD production,
and magnetic recording materials. One of its major limitations, however, is that it can
only image conducting surfaces, so organic and other less-than-optimal surfaces are
unsuitable for use.
Left: The world's smallest company logo etched
with 35 Xenon atoms. courtesy of IBM corporation
The scanning tunneling microscope works by maneuvering a sharp conductive tip over a surface, allowing the sample to be resolved down to 10-8 meters. The tip of the probe, which optimally terminates with a single atom, can be moved in three dimensions with great accuracy using piezotubes. The distance between the tip and the sample surface is controlled by a voltage applied to the vertical element. This voltage is determined by a feedback circuit that measures and controls a small electrical current caused by electrons tunneling across the gap between the tip and the sample.
This tunneling current is a quantum mechanical phenomenon that has no analog in the classical world. Since electrons often operate as waves in quantum mechanics, there is a finite probability that an electron, upon encountering a voltage gap, will pass through that gap. This is equivalent to a particle encountering a surface. This probability is inversely dependent on the width of the gap, and allows the probe to utilize the sensitivity of the tunneling current to the separation between tip and sample. The tunneling current will change by a factor of two for a change in separation of 10-10 meters. With such sensitivity, the STM can resolve differences in height along contours to better than 1/100 of an atomic diameter. A full scan of the surface involves sweeping the tip back and forth across the surface to generate an image.
The final image may then be improved with standard techniques of electronic noise filtering, averaging and transform methods to enhance the image, as well as to emphasize periodic structures. Also, the size of the pattern and scanning rate may be varied so that structural information can be obtained that extends from the atomic scale into the visible range.
The Atomic Force Microscope
The atomic force microscope, or AFM, was a first generation descendent
of the STM. Developed by Binnig and others, it can image nonconducting as well as
conducting surfaces. This probe has greater potential for use in nanotechnology, as will
be described later. The AFM works by recording the interatomic forces between its tip and
the surface atoms as the tip is scanned over the surface of the sample. The AFM has two
modes of operation, attractive and repulsive. When the AFM is operated in a mode that
senses the repulsive forces between tip and sample, the tip actually touches the surface,
like a phonograph stylus on a record surface, though the tracking force is much smaller.
This allows the tip to trace over individual atoms without damaging the surface. The AFM
can also be operated so that the tip senses the attractive forces between tip and sample,
with a feedback system that prevents the tip from touching the sample.
The tip of an atomic force microscope can be made of a small diamond fragment. Eventually, considerable freedom for tip choice will be needed for molecular engineering applications. There are some proposals for molecular tips, where particular molecules, like proteins, may be used in applications requiring reactions with the tip; molecular manipulation is one such application. The tip is attached to a spring in the form a cantilever. The small repulsive forces are recorded by measuring minute movements of the cantilever. These cantilevers are made of aluminum foil, and microcantilevers are made of etched silicon dioxide. The advantages of this allow for higher resonance frequencies of tip and sample interactions, and this stronger signal decreases sensitivity of the cantilever to vibrations, resulting in greater stability.
STM and AFM in Molecular Engineering
The STM has been used by several research groups in the positioning of
atoms or molecules on surfaces, showing the possibility of molecular design. By varying
the voltage of the probe tip, Becker and Golovchencko in 1985 placed a single atom onto a
germanium surface, and Foster and others have pinned an organic molecule in a particular
location on a graphite surface. However, the most famous example of these experiments is
where STM's have been used to write the IBM corporate logo with xenon atoms.
The utility of such accomplishments must be questioned, however. Placing one, or even several atoms or molecules on a surface, is not molecular engineering. Such constructs are short-lived, as the atoms evaporate off the surface due to thermal motion. For stable structures to exist at temperatures above absolute zero, chemical bonds will need to be formed between placed atoms that will tie them to each other, to increase stability and reduce thermal motion.
Creation of these bonds and the control needed is more suited to atomic force microscopes. The AFM allows surfaces to be positioned relative to one another in a solution environment with approximately 10-11 meter positional precision and tip compressive loads as low as 10-11 Newtons. It has been proposed by researchers such as Eric Drexler, the chief authority on nanotechnology, that AFM's with molecular tips could provide an approach to achieving positional control in chemical synthesis, and thus overcome difficulties posed by the lack of reproducible, atomically-sharp tips for the probes. Arrow tips can permit screening and interchange of other tips during operation, increasing yield and reducing sensitivity to damage. They may also enable the sequential use of different tips to perform mechanosynthesis and subsequent imaging of the products.
Molecular Tips
The choice of molecular tips is pivotal in mechanosynthesis. One must
choose a reactive component that will easily manipulate the desired molecule to its
position. It is here that a wedding of different fields can be beneficial. Potential tips
may be made of proteins or particles bearing small adsorbed molecules. By combining the
approaches of organic synthesis and biotechnology, engineers may create synthetic ligand
tips (ligands are small molecules that can be bound by larger molecules), with protein
molecules as supporting structures. This has the advantage that many natural proteins bind
partially exposed ligands. Also, biotechnology advances will allow the possibility of the
use of monoclonal antibody technology that can rapidly generate proteins able to bind
almost any selected small molecule. These tip possibilities could find immediate usage in
any nanotechnology scheme in its early stages, since the field of biotechnology often, and
successfully, deals with the immobilization of molecules for reaction in vessels as fluid
flows past.
The actual mechanosynthesis of devices will be accomplished with maneuverable molecular tips on an AFM to provide positional control in sequence of synthetic steps. These tips will control local concentrations of reactants, the key to successful reactive synthesis. This technology will create receptors that serve as tool holders to bind a ligand of interest. Then tip arrays and antibody technologies will enable the use of distinctive receptors for ligands, allowing many different reactions without changing the solution constituents. A mature AFM-based mechanosynthetic technology could potentially perform hundreds of syntheses involving multiple steps in the presence of hundreds of different chemicals, with only a small probability of failure resulting from reactions with uncontrolled reagent molecules in solution. Following a molecule's creation, AFM imaging will be used to test the success of each step to insure completion.
Such a mature mechanosynthetic technology will require contributions from different fields, among them organic chemistry and protein engineering to extend tip array technology for solution-phase mechanosynthesis. One of the major objectives is the identification of useful building blocks. These components must be both accessible through conventional solution-phase synthesis (the typical chemical environment for creating molecules), and adequate for use in building molecular machine systems.
This technology is not at a working stage today, although advances are being made in protein engineering that are providing useful tip arrays. A limiting step is that the speed of mechanosynthesis is unacceptably slow using current methods. The attention-grabbing placement of individual atoms to write text may be useful for publicity, but building medical nanosubmarines or cubic micron volume supercomputers will require much more ingenuity and speed. Assembling a billion-atom device will take more time than we care to imagine at the speed of one atom per second (much less than one per hour). The union of different technologies with an AFM promises to increase efficiencies dramatically. AFM's are not particularly cheap right now, but their cost is not on a Supercollider scale either.
To realize nanotechnology and create the first nanoassemblers, a maturing technology like atomic force microscopy wedded with protein engineering could be the most useful pathway, with the ability to both position and image a device under construction with atomic precision. For now they represent the most precise tool we possess for working in the atomic realm.
Jason Wejnert is a senior studying Applied and Engineering Physics in the Engineering College.