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In semiconductor,it is always said that the sample is neutral and has 4 covalent bonds. But can the atoms at the edge of crystal satisfy this condition ? If not,then how can they become stable with 3 co-valent bond and 1 electron?

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  • $\begingroup$ Like any surface of a solid, they can't be in a position just like an atom in the solid. So, they do the best they can, which often means having positions somewhat different than on the interior, or having a 'surface reconstruction' which helps minimize the surface energy. $\endgroup$
    – Jon Custer
    Aug 24, 2017 at 16:15
  • $\begingroup$ I know that diamond, which has the same crystalline structure as silicon, usually has its surfaces naturally terminated by a monolayer of oxygen or hydrogen atoms. Don't know if the same is true of silicon or not, but a quick google search does seem to bring up a lot of links mentioning hydrogen-terminated silicon surfaces. $\endgroup$
    – user93237
    Aug 24, 2017 at 16:45
  • $\begingroup$ If exposed to moist air, layers of oxyde naturally grow on the surface. $\endgroup$
    – user154997
    Aug 24, 2017 at 16:54
  • $\begingroup$ still neutral but one bond will be "dangling". $\endgroup$
    – BeauGeste
    Aug 24, 2017 at 17:08
  • $\begingroup$ What is dangling bond? $\endgroup$ Aug 24, 2017 at 17:12

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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. 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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