Published on Apr 02, 2024
The main impediment to molecular manufacturing today is the lack of an experimental procedure for routinely and precisely building objects, atom by atom, at the molecular scale. The key to this is molecular positional assembly, or mechanosynthesis — the formation of covalent chemical bonds using precisely applied mechanical forces.
The first and most famous experimental demonstration of molecularly precise positional assembly of individual atoms, albeit without forming lasting covalent bonds, was achieved by Eigler and Schweizer at IBM Almaden in 1989, when they used an STM to position 35 xenon atoms on a nickel surface to spell out the corporate logo “IBM”.
The first complex positionally assembled molecularly precise structure which included formation of covalent bonds was created by Lee and Ho in 1999. Using a cold scanning probe tip on a silver surface in vacuum, they picked up one carbon monoxide molecule and covalently bonded it to an adsorbed iron atom using an electric pulse, then repeated with a second CO molecule at the same site, making a positionally-assembled molecule of Fe(CO)2, a simple rabbit-ear-shaped molecularly precise structure fabricated at a specific site on the silver surface.
You start with a flat diamond surface, bring in a mechanosynthetic tool, position it precisely over the workplace, lower it down to deposit, say, a carbon dimer on the diamond surface, lift away the discharged tool, then repeat with successive dimers, resulting in the positional assembly of diamond.
Diamond has exceptional properties, such as high strength and extreme stiffness, high thermal conductivity and low frictional coefficient, and chemical inertness. Of even greater interest, previous theoretical analyses support the idea that molecular machinery can be made from stiff hydrocarbons, including diamond. CVD techniques can already make large gem-quality diamonds at a manufacturing cost of $100 per carat, up to several carats in size. But these are bulk processes which only produce a large featureless crystal of pure diamond.
Bulk-produced C(110) surface is entirely passivated by hydrogen atoms , eliminating any dangling bonds. Working with hydrogenated surfaces would require removing the H atoms, creating dangling bonds to more readily accept additional carbon atoms. One way to do this is to bake out the entire C(110) surface at a temperature above 1400 K, in vacuum [movie omitted]. This drives off the hydrogen, leaving a clean carbon-only surface with the same structure in the case of C(110). With C(111) and C(100), the surface structure changes when hydrogens are removed. The C(110) surface also stays the same afterwards, when it cools down.
Hydrogen abstraction tools, for selectively removing hydrogen atoms one at a time from a diamond surface, have also received some study. This shows a simulation done on the C(111) surface by Brenner’s group in 1994 . The hydrogen atom comes right off.
But besides removing hydrogens, we must be able to add one or more carbon atoms to a growing diamond surface in precisely chosen locations. One way would be to pick and place molecular building blocks such as whole adamantane cages. But control of simultaneous multiple bond formations between adjacent blocks is relatively difficult, and then our minimum feature size is about one block.
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