About Diamond Synthesis
The following is an excerpt from Chapter 8 of Nanosystems: Molecular Machinery, Manufacturing, and Computation (Drexler 1992), which discusses the value of diamond as a difficult test case to address in analyzing advanced mechanosynthesis. I’ve posted it for reasons discussed here. (Nanosystems describes more directly accessible objectives in Section III.)
8.6 Mechanosynthesis of diamondoid structures
Fundamental physical considerations (strength, stiffness, feature size) favor the widespread use of diamondoid structures in [advanced!] nanomechanical systems. In chemical terms, diamondoid structures comprise a wide range of polycyclic organic molecules consisting of fused, conformationally rigid cages. This section considers the synthesis of such structures by mechanochemical means, based on reagents and processes of sorts described in the preceding section, and using diamond itself as a n example of a target for synthesis.
8.6.1 Why examine the synthesis of diamond?
Diamond is an important material in its own right, but here serves chiefly as a test case in exploring the feasibility of more general synthesis capabilities. It is impractical at present to examine in detail the synthesis of numerous large-scale structures. Accordingly, it is important to choose a few appropriately challenging models.
Diamond has several advantages in this regard, as can be seen by a series of comparisons. Synthetic challenges often center around the framework of a molecule, and diamond is pure framework. In general, higher valence and participation in more rings make an atom more difficult to bond correctly. At one extreme is hydrogen placement on a surface; at the other is the formation of multiple rings through tetravalent atoms (Divalent and trivalent atoms such as oxygen and nitrogen are intermediate cases.) Solid silicon and germanium present the same topological challenges as diamond, but atoms lower in the periodic table are more readily subject to mechanochemical manipulation owing to their larger sizes and lower bond strengths and stiffnesses. Thus, a structure built entirely of rings of sp3 carbon atoms appears to maximize the basic challenges of bond formation, and diamond is such a structure. Further, diamond has the highest atom and covalent-bond density of any well-characterized material at ordinary pressures, maximizing problems of steric congestion. Although diamond is a relatively low-energy structure, lacking significant strain or unusual bonds, existing achievements (Section 8.2.3) suggest that the later features need not be barriers to synthesis, even in solution-phase processes.
Finally, diamond is a simple and regular example of a diamondoid structure. Accordingly, the description of a small synthetic cycle can suffice to describe the synthesis of an indefinitely large object; this avoids the dilemma of choosing between (1) syntheses too complex to describe in the available space, and (2) syntheses that might in some way be limited to small structures.
