Electronagativity, polarisability and band gap in Oxides

The earliest studies of conjugated molecular materials established that the mechanism behind electroluminescence was recombination of electron-hole pairs in a radiative process. This provided evidence for the existence of a band-gap within these materials, allowing comparison with inorganic semiconductors studied in the well understood eld of solid state physics. More recently, the possibility of controlling the band-gap of organic semiconductors has allowed the use of a variety of conjugated polymers in many applications, including highly sensitive chemical sensors, eld-eect transistors (OFETs), solar cells (OPVs) and high effciency organic light-emitting diodes (OLEDs).


Patric Wallace Parkinson, (Brasenose College), Ultrafast Electronic Processes at
Nanoscale Organic-Inorganic Semiconductor Interfaces, Michaelmas 2008, Doctrola Thesis

Electronagativity, polarisability and band gap in Oxides

Electronegativity, polarisability and band gap in oxides Oxygen in the oxidation state of -2 exists in oxides and oxidic compounds where the bonding is ionic, covalent or metallic. In these different types of bonding situations it has distinctly different parameters which relate to electronegativity, polarisability, etc. No other element exhibits such versatile behaviour(3,4) and it is this behaviour that imparts many of the properties to oxidic glasses. It arises, in a crude way, from oxygen being able to exist not only as bridging or nonbridging but also to exist with a degree of negative charge which can be varied, for example, by adjusting the glass composition. This negative charge should not be thought of as static but fluctuating depending on movement relative to the silicon atoms to which it is linked, and also on the proximity, movement and nature of constituent metal ions.

XA-XB=(Q/n)½

where n is the number of bonds. (For oxides, 1·1 eV must be added for each oxygen in order to correct for the double bond in the O2 molecule.)

Calculations show that in covalent oxides such as P2O5, xO is 3·5. This is the usual value quoted for oxygen in textbooks. However, for oxides where there is appreciable ionicity, there is a fall in xO. Indeed, for Cs2O, xO has fallen to 2·2. The important point that should be noted is that the oxygen atom (or rather oxide ion) has less attraction for the negative charge in the bonding as its own negative charge increases. Bearing in mind that in glass the negative charge on nonbridging oxygen atoms is greater than for bridging, it follows that there would be a decrease in oxygen electronegativity, on average throughout the network, as the proportion of basic oxide in the glass is increased.

If there is a similar trend for the band gap electronegativity, X*opt, then the smaller value of X*opt that results would have the effect of raising the top of the
valence band and this would account for the lower frequency onset of ultraviolet absorption for glasses, e.g. soda–lime–silica glasses compared with vitreous
silica.

Duffy J A, Ultraviolet transparency of glass: a chemical approach in terms of band theory, polarisability and electronegativity, Phys. Chem. Glasses, 2001, 42 (3), 151–7