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."
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1) Structure of the diamond 111 surface: Single-dangling-bond versus triple-dangling-bond face