Surface reconstruction
From Wise Nano
Atoms at surfaces constantly change position in an attempt to reach their lowest possible energy. This sounds like trying to herd a bag of fleas. How can you construct anything with atoms when a surface suddenly looks a lot different than it should?
Short answer
Surfaces of covalently bonded materials cannot significantly reconstruct without chemical bonds being broken and remade. For strong covalent materials this can require substantial energy, and thus doesn't happen spontaneously except at elevated temperatures or on some "unhappy" (unterminated) surfaces. Adding or removing an atom from the surface can stir things up a bit, but this should be controllable by any of several methods.
Long answer
The surface of metals is generally in constant flux. But the surface of covalent solids--like diamond--is not: each atom is fixed in place by strong bonds to its neighbors. Reconstruction would require the simultaneous breaking of several bonds.
In an unimproved diamond crystal, each carbon atom on the surface has one or two hydrogens taking the place of the carbons that would have been above the surface atom if it were an interior atom. This kind of surface is described as "terminated."
In order to add atoms covalently to a surface, the surface must have dangling bonds: it must be at least partially unterminated. This makes it reactive and "unhappy." Some surfaces will in fact reconstruct, if they are left entirely unterminated. This should be avoidable by leaving most of the hydrogens in place, and just removing a few hydrogens in the area we want to deposit atoms onto. (Single, targeted hydrogen atoms have been removed from silicon by scanning tunneling microscope at room temperature.)
Cooling the material should also avoid reconstruction, since reconstruction (like almost any chemical reaction) shouldn't happen unless the atoms can get over an energy barrier between original and new state--just as gasoline requires a spark to ignite.
Depositing new atoms might seem to add energy, but if it's done slowly enough and the deposition tool is stiff enough, it should be possible to limit the amount of energy that's deposited at any one instant. Depositing atoms also takes the nearby surface atoms through a range of configurations, and if any of those configurations is partway to reconstruction then the reconstruction might happen. However, there are many trajectories that can be taken to add the atoms, and it's also possible to stick a few other atoms near the surface to control the configuration. And of course, if the surface is left mostly terminated, it will have a lot fewer configurations that "want" to reconstruct.
