Introduction

Most members of the class of stiff hydrocarbons cannot be synthesized today, let alone economically synthesized. More broadly, almost none of the ways that physical law in principle allows us to arrange atoms can be economically achieved with present methods. The objective of the field of molecular nanotechnology is to develop methods which permit the economic synthesis of most structures permitted by physical law. As the direct attempt to achieve this objective might prove difficult, the problem is often broken down into subobjectives and subgoals. One approach is to assume that a general and inexpensive ability to manufacture complex structures with molecular precision can be achieved by using an assembler (Merkle, 1996a). The design of such a device is further broken down into subsystems, and the design and analysis of the subsystems carried out in greater detail. Various aspects of the over all system design can be considered (Drexler 1992, Merkle 1992, 1996a, 1997a, 1997b). This paper focuses on simplifying the set of molecular tools used, and the chemical reactions used to refresh those tools.

A complete analysis of the reactions by which an assembler converts incoming raw materials into reactive tools used to synthesize molecular structures is greatly simplified if we restrict ourselves to the elements hydrogen and carbon, and further restrict our attention to structures that are relatively stiff (excluding, for example, floppy polymers). The stiff hydrocarbons include a wide enough class of materials to be a very attractive goal (diamond, graphite, and structurally related materials are included in this class). Essentially all mechanical structures can be made from stiff hydrocarbons; including struts, bearings, gears, levers, etc. This can be most readily seen by noting that the strength-to-weight ratio of diamond is over 50 times that of steel or aluminium alloys -- a single part made of metal could be functionally replaced by a similarly shaped stiff hydrocarbon part. The resulting part would be lighter and stronger than the part it replaced, improving overall performance. The class of stiff hydrocarbons also includes molecular computers which, by today's standards, would be extraordinarily powerful (Drexler, 1992).

A more general assembler, able to manufacture structures which incorporate most of the elements of the periodic table, would be substantially more difficult to analyze. One approach to breaking down the task of building a relatively large and complex structure would be to consider a series of small incremental changes to an exposed surface, the cumulative effect of which would be to manufacture the whole. This implies we must analyze small changes to the exposed surface, presumably by considering small clusters of atoms on that surface. A very minimal cluster might be a single atom and the atoms to which it is bonded. If one atom is bonded to (say) three neighbors, and all four atoms can be any one of about 100 possibilities, then this gives us 100⁴ or ~100,000,000 possible clusters. This analysis is crude and likely too small because (a) atoms are often bonded to more than three other atoms and (b) understanding an incremental change to a small cluster often requires examination of atoms farther away than one bond length. Despite its shortcomings, this crude model tells us that we will need to analyze many types of incremental surface modifications before we can reasonably hope to synthesize the full range of structures accessible using this approach.

By contrast, if our structures contain only hydrogen and carbon then the design and analysis problems become much simpler. Hydrogen can only be bonded to one other atom which, if we exclude hydrogen gas, must be carbon. A carbon atom will usually be bonded to two, three, or four neighboring atoms, which can only be hydrogen or carbon. Our previous crude analysis would assign 2⁴ or 16 possible local clusters for carbon. While this can be reduced by considering isomers, it must also be increased to consider interactions that extend beyond a single bond length, e.g., aromatic rings and the like. In any event, the complexities of analyzing hydrocarbon structures with sufficient accuracy for the purposes discussed here is tractable with present capabilities. We cut short the combinatorial explosion before it begins.

While this rather drastic pruning makes the problems of designing and analyzing a hydrocarbon assembler more tractable, it does not directly address the feasibility of more general assemblers. Smalley in particular has argued (Smalley, 1997a) that a "completely universal" assembler is impossible, though he also said (Smalley, 1997b) "Most interesting structures that are at least substantial local minima on a potential energy surface can probably be made one way or another." He argues that a small set of molecular tools will be unable to catalyze all the reactions needed to synthesize the remarkably wide range of structures that are possible. Success will require the use of a great many custom-made catalytic structures.

Given the remarkable size of the combinatorial space of possible molecular structures it does indeed seem likely that at least some members of this space will resist direct synthesis by an assembler equipped with a relatively modest number of molecular tools. However, even if we assume that a substantial percentage of the space is inaccessible via this route (an assumption as yet lacking any clear support) the remaining "small" fraction would include structures of enormous value. Even the ability to manufacture only the highly restricted range of structures defined by the stiff hydrocarbons would usher in a revolution in manufacturing.

Further research aimed at clarifying the range of structures amenable to synthesis by positionally controlled molecular tools seems called for. Ideally, this would include not only the proposal and analysis of particular sets of molecular tools and the range of structures they could reasonably make, but also proposals of structures which could not be synthesized by the use of positionally controlled molecular tools.

One example of an "impossible" structure is a cubic meter of flawless diamond. Before it could be finished, background radiation would have introduced flaws. Drexler argued that it should be possible to define a structure which would be stable if complete but unstable when almost complete, a sort of molecular stone arch (Drexler, 1986, page 246). However, a specific, relatively small, stiff and stable structure that can reasonably be viewed as "impossible" to synthesize using positionally controlled tools has not yet been proposed. While it seems likely that at least some such structures must exist, our understanding of this issue would be greatly improved by specific examples.

An assembler operates in some external environment. While many environments are possible, we consider one specific environment in this paper: a feedstock solution made from the solvent acetone; butadiyne as a source of hydrogen and carbon; neon to support acoustic waves in the interior of the assembler while at the same time not reacting with the highly reactive molecular tools; and a "vitamin" which provides small amounts of elements such as silicon, tin, and one (or more) transition metals -- used basically for catalytic purposes.

Butadiyne (C₄H₂) seems attractive as a source of hydrogen and carbon for several reasons. The linear structure of butadiyne means that a simple tubular structure, such as a bucky tube, can serve as a suitable binding site to bind butadiyne from the feedstock solution (Merkle, 1997a). As bucky tubes are themselves made of hydrocarbons, a system which can synthesize most hydrocarbons should be able to synthesize the required binding sites. Second, as suggested by the following, relatively simple reactions can be used to convert butadiyne into useful molecular tools. Third, butadiyne has two carbons for every hydrogen. While it's difficult at present to make precise statements about the ratio of carbon to hydrogen in the structural elements of a hydrocarbon assembler, the presence of graphite and relatively thick diamond structures would make carbon significantly more common than hydrogen. As the present proposal uses a single molecule to provide both hydrogen and carbon, a molecule with a relatively high ratio of carbon to hydrogen is desirable (provided other constraints can be met).

By definition, an assembler can make another assembler: they are self replicating (Merkle, 1992, 1994, 1996a). To support self replication it is essential to achieve closure: it must be possible for the assembler to make everything it needs from the feedstock. In this paper, which focuses on the needed molecular tools, we must show that it is possible to generate a new set of molecular tools given only an existing set of molecular tools and a supply of butadiyne. It is not quite sufficient to show that it is possible to make each individual tool given the set of molecular tools and butadiyne. For example, if we could make one dimer deposition tool but used two hydrogen abstraction tools in the process, and we could make one hydrogen abstraction tool but required two dimer deposition tools in the process, then it would be impossible to make a new set of molecular tools from an existing set of molecular tools. The process would "run down hill" until we had exhausted our initial set of tools.

We will first consider how to make each molecular tool, and then consider "quantitative parts closure" in a later section to ensure that it is possible to manufacture a complete set of new tools without depleting an existing set.