Carbon atoms at the edge of a diamond

Question

It is well known (the simplest textbook example) that a diamond has a well-defined arrangement of sp3 carbon atoms, as each atom is connected to four others in a tetrahedral structure.

But what about the last carbon atoms at the edge? For each of these, three bonds are missing; it has a single bond with one carbon atom inside the structure. What is the hybrid and atomic (electron) structure of the carbon atoms at the edge?

I would appreciate some references explaining this in detail (books or research articles).

Answer 1

Atoms at the edge of a crystal that have an unsatisfied valence are said to have "dangling bonds." Many elements, in addition to carbon, can have dangling bonds. Dangling bonds is a subject of current interest because of the impact these structures can have on semiconductor properties.

These dangling bonds are very similar to free radicals, except since they are immobilized in a solid, they are somewhat less reactive than free radicals in solution. Nonetheless, they can react with whatever materials they are exposed to, such as hydrogen, water vapor, oxygen, etc. In addition, if there is a neighboring dangling bond then they can both react with one another to form a bond and satisfy there valence.

When carbon or silicon surfaces are prepared under clean room conditions, the dangling bonds can persist. In the semiconductor industry this clean room preparation technique is followed by bringing in a doping gas in order to purposefully alter the electronic band structure of the substrate material.

Since unpaired electrons have magnetic properties, in carbon (or any other element) nanostructures where there is a lot more surface area to volume, the concentration of dangling bonds is much higher. Consequently, the unpaired electrons in the dangling bonds confer magnetic properties on these materials that are large enough to be easily detectable and to manipulate.

Finally, since dangling bonds represent a non-equilibrium situation, surfaces containing dangling bonds undergo a relaxation or reshaping that is referred to as "surface reconstruction."

Here is a full-text article that should help you get started: 1) Structure of the diamond 111 surface: Single-dangling-bond versus triple-dangling-bond face

Answer 2

You asked a question, belonging to surface chemistry. It is a relatively new area of research, as it relies heavily on atomic-resolved microscopy and computational methods.

Generally, the answer depends on prehistory of the surface and its environment. In case you crack a diamond, making new surface, two processes happens.

• so-known reconstruction, or reorganization of bonds on the surfaces. I'm not aware of articles on reconstruction of diamond, but silicon has diamond-like structure, so we can speculate from there.

• reaction of the surface with environment. In case of air, oxygen and water may insert into strained bonds, and react from there. It may result in all kinds of surface species up to carboxy-groups, ketone fragments, alkyl fragments and so on.

Answer 3

To add to the current answers about the individual electrons, there is also a global effect at the lattice level.

Assuming you have a monocrystal with no (or low density of) defects, the lattice will be regular until around 100 atoms from the surface. The reason is obvious: the atoms at the surface have neighbours only at one side, so they have to be displaced to balance the forces, which in turn forces a smaller displacement on the second layer, and so on.

All this has powerful implications in the study of nanostructures, like nanotechnology, as well as suspensions and colloids; as they are structures that are all close to the boundary, with no proper bulk. Also,they are fundamental in crystal growth, and, in Chemistry in catalystic reactions.

Further reading:

Note: they are excelent classical books, but they are old, and there has been quite a lot of research in surfaces in the late year. Unfortunately, I am not aware of any modern reference.

• Solid State Physics, Ashcroft and Mermin.
• Introduction to Solid State Physics, Charles Kittel (I'd avoid the parts of introduction to SS, as I find them extremely boring, but some other people like it).