© 1999 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Nanomedicine, Volume I: Basic Capabilities, Landes Bioscience, Georgetown, TX, 1999

9.3.5.3.5 Molecular Mechanodecomposition

Molecular mechanosynthesis can be used to construct nanoscale objects, using either precise moiety-by-moiety placement or mechanical part-by-part placement of individual nanoscale parts (Chapters 2 and 19). Following the same process in reverse, diamondoid objects may be decomposed atom by atom into their constituent elemental components. This molecular mechanodecomposition may proceed rapidly using many tools working simultaneously, if no attempt is made to preserve or record the encoded structural pattern information.

Many of the radical-based reaction mechanisms for diamond mechanosynthesis described by Drexler¹⁰ may be reversible with the application of mechanical energy, since at elevated temperatures surface groups and radical vacancies apparently can migrate after placement on growing diamond surfaces.¹²⁹³ M. Krummenacker [personal communication, 1997] suggests a speculative mechanochemical decomposition process in which a few surface hydrogen atoms are first abstracted from an edge or corner of the diamond surface to minimize steric congestion at the disassembly site. Then a radical-based tool, possibly with an oxygen radical at the tip, bonds to an exposed carbon atom and pulls it upward far enough to insert another reactive moiety such as a nitrene (an analog to carbene) into the gap, replacing another C-C lattice bond and allowing the first carbon atom to be pulled further out of the matrix. This process is repeated on an adjoining carbon atom, resulting in a two-carbon unit with four bonds to the manipulator tool, which can then be pulled the rest of the way out by mechanically disrupting the two remaining C-C bonds to the diamond lattice without exceeding the tensile strength of the tool tip. (A great deal of sensing and chemical inference remains to be described in this scenario.) Krummenacker's process might also be used to record structural information, if required (Chapter 19).

Evans¹²⁷⁷ estimates that 1760 zJ/atom of energy are required to break three C-C bonds and detach a single carbon atom from the {111} diamond crystal surface into the gas phase. With 176 carbon atoms/nm³ in diamond, mechanodecomposition of bulk diamond would require ~300 nJ/micron³ . Generously assuming only ~5 manipulator arm motions to remove each carbon atom and a manipulator operating frequency of ~1 MHz, a single manipulator can mechanically disassemble diamond at a volumetric rate of ~1000 nm³/sec-manipulator. The volume of each mechanosynthetic manipulator described in Section 9.3.1.4 is ~10⁵ nm³, so ~1000 manipulators disassembling in parallel occupy ~10% of a 1 micron³ work space and can collectively mechanically decompose ~0.001 micron³/sec of diamond crystal while drawing a total of ~300 pW of continuous power. Actual power usage may be far less, since in any reasonable decomposition process some energy will be recovered when the liberated carbon atom forms new bonds; high energies will only be encountered in the transition state, as in any chemical reaction.

Last updated on 21 February 2003