How does one tell where the valence band begins and where the conduction band be

Question

How does one tell where the valence band begins and where the conduction band begins from the DOS/Energy graphs
Also I'm having trouble understanding the concept of Crystal Orbital Hamilton Population (COHP) so I'm not sure what information the graphs on the right would tell me that would be useful.

Diamond vs Graphite Diamond 1.2 Diamond 0.8 0.6 0.4 i EF 0) EF 0.2 25 -20 15 -10 10 -20 15 10 -5 10 15 20 3.5 Graphite d- 1.42 ang 2.5 d 3.36 ang CD Graphite EF 0.5 20 15 10 20 15 10-5 10 15 20 ENERGY (ev) ENERGY (ev)

Explanation / Answer

The position of Valence band and Conduction band is a very important concept for knowing the conductivity of the material. For semiconductors and insulators, a certain energy gap exists between top of the valence band and bottom of the conduction band. Which signifies that, electron can only go to higher energy level (conduction band) from lower energy level (valence band) if it gets some extra energy from outside. Although for metals, valence band and conduction band coincides. Another thing is important here is the location of hypothetical Fermi energy level EF. The probability of occupancy of an electron in Fermi level is 0.5. The energy level below Fermi level is valence band and top of this level is conduction band. So from this energy diagram we can easily see that top of the valence band is at 0 eV and bottom of the conduction band is the region over the Fermi energy level. Another thing I want to mention here, this energy diagram for diamond and graphite is very different although they both are made from same element Carbon. We can see graphite is a good conductor since there is no energy gap is present between valence and conduction band. Whereas, for Diamond we can see clearly a band gap is existing indicates why diamond is a bad conductor.

COHP provides necessary information about the bonding which we cannot get from band diagram alone (because it gives only the location of the bands). It becomes positive for bonding due to positive overlap population and becomes negative for antibonding due to negative overlap population. For lower energy, bonding and for higher energy antibonding occurs. So, by seeing the COHP diagram, we can say the region where COHP value is positive, bonding occurs and since energy is low in that region so valence bond is situated in that particular region. Similarly, we can identify the position of conduction band also by seeing negative COHP value.

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