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Solutions to Q&S related to electronic structure of materials lecture
1 a) Si+2Cl2=SiCl4
1 b) The geometric arrangement in SiCl4 consist of Si atom in a center of a
tetrahedron and Cl atoms occupying the four corners of the tetrahedron.
1 c) Since the bonding geometry described in 1b) is symmetric it implies that the
molecule has no permanent dipole moment because the centers of positive and
negative charges coincide. Intermolecular forces between SiCl4 are thus
expected to be very week (van der Waals forces). At room temperature the
material is expected to be in a gas or liquid phase but not in a solid phase.
2 a)
2 b)
Visible light contains radiation in the wavelength range 4000Å (violet)7000Å (deep red). A material that appears red in transmission light absorbs all
the wavelengths of the visible light in the range 4000-6500Å and transmits
wavelengths >6500Å. This material must belong to semiconducting class of
materials since photons with energies of red radiation can excite electrons
from valence to conduction band.
Assuming that optical absorption starts at =6500Å we get the following
value for the bandgap of the material:
I defined in my lectures semiconductors as materials with a bandgap below 1.5
eV. However I also pointed that this value is a rather “diffuse” one definitely
not a sharp one. Thus a bandgap of 1.9 eV makes a material a semiconductor.