Is there any relation between energy band gap in group4 elements and their atomic numbers? It might be just a trivial observation but I noticed How from Carbon to Germanium the band gap went from 5.3eV to 1.1eV to 0.7eV. Is there a reason for this? Or am I reading too much in between the data?
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4$\begingroup$ Note that the electronic properties are just as much a function of bonding as they are molecular composition. Different forms of carbon, graphite and diamond for example, have significantly different properties, including band gap. See phycomp.technion.ac.il/~david/thesis/node3.html $\endgroup$– eepperly16Commented Jun 3, 2015 at 20:53
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1$\begingroup$ And the same can be said of the allotropes of Tin (metal vs semi-metal). $\endgroup$– Jon CusterCommented Jun 3, 2015 at 23:34
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Yes, there is a relationship. In the diamond structure, group IV elements have sp3 hybrid bonding. Roughly speaking, the highest of the valence bands look like "bonding" orbitals and the lowest energy of the conduction bands look like anti-bonding orbitals - in the sp3 interpretation. Covalent bonding is much stronger in the first rows of the periodic table so the difference between bonding and anti-bonding is wider, and this is what drives the band gap difference between carbon and silicon/germanium. It is also fair to say that it plays a role in making silicon narrower than germanium - but at that point crystalline effects are relatively more important and the band structure is richer.
