Published: 14 March, 2002

Going hand-in-hand to the edge of a diamond

What happens at the edge of a diamond? How does the carbon end?

Your question is very interesting. To answer it, let me start off with some background information, which you probably already know. Everything is made up of atoms. There are only 90 different kinds of atoms; yet taken together and in different combinations and amounts these 90 different atoms become a cup of water, a stone, a rock or a leaf.

A good analogy for this is in the food we eat. The same four ingredients: bread, tomato, cheese and meat can be turned into pizza, or a cheeseburger (with ketchup), or a taco.

The amazing thing is that very rarely do we ever see an object, which is composed of only a single type of atom. Diamonds are such objects: each one is made up of carbon atoms. The smallest diamond in a jewelry store has inside it more carbon atoms than there are people on this earth.

To answer your question, you need to know one more thing. Every atom has an environment in which it is most "happy" (the technical term for "happy" is lowest free energy, but let's not worry about that). For atoms to be happy they have to make bonds. Just as people are happy when they hold hands with someone on their left and also on their right, atoms are also happy when they are "holding hands" or joining with other atoms.

In this analogy, the carbon atom, which makes up diamond, is a monkey. Each carbon atom has two hands and two feet and it wants each of these hands and feet to hold onto other carbon atoms' hands and feet. (Please remember that this is an analogy. Atoms don't really have things on them that look like hands or feet.)

In diamond, each carbon atom holds hands and feet with four different carbon atoms. These four different atoms are all on different sides. Near the edge of a diamond, a carbon atom faces a real problem. There are carbon atoms on one side of it, but no carbon atoms on the other side. One solution for these on-the-edge carbon atoms find is that while their feet hold hands with two different carbon atoms in the interior of the diamond, the edge carbon atom's hands are both linked to the same neighboring carbon atom.

We can actually use high power microscopes to see that this is true. There is a nice example of this taken on a real diamond at the National Center for Electron Microscopy and published last year in an article by M. A. O'Keefe et. al. . They published their work in a magazine called Ultramicroscopy (volume 89, page 215). In this picture one sees white short rods. Each of these short rods is a pair of atoms. Each atom in the pair is holding both hands of the other atom in the pair. Quite frankly, it is an amazing microscope. I would like to see it someday.