Tungsten Oxide Electronic Transition

The conduction band inside the tungsten oxide structure occupies an electronic state. Its band gap is small. If tungsten oxide is excited by the proper energy (such as thermal energy, light energy and electric energy), its valence electrons can acquire the energy and be excited into the conduction band.

At the same time, the electric current is generated due to the electronic transition of tungsten oxide, so that the tungsten trioxide valence band will have positively charged holes. This characteristic of tungsten oxide plays an extraordinary role in optoelectronic devices, sensors, photocatalysis, dye-sensitized solar cells and the like.

According to the type of tungsten oxide electronic transition, it can be divided into direct band gap semiconductor and indirect band gap semiconductor. When tungsten oxide is transferred from the top of the valence band to the bottom of the conduction band, only the energy larger than the forbidden band width is required, and the semiconductor that can be transferred is called the direct band gap semiconductor. This feature makes the electron produce a vertical transition, without having to sacrifice momentum, so there is no phonon participation. Conversely, if the electrons are recombined from the conduction band back to the valence band, they are recombined directly.

Almost all of this direct recombination energy is released as light energy without releasing or sacrificing phonons. So, it is very suitable for preparing luminescent materials. When tungsten oxide is transferred from the top of the valence band to the bottom of the conduction band, in addition to the energy transfer larger than the forbidden band width, a certain momentum transfer semiconductor is needed, which is called an indirect band gap semiconductor.

This property makes the electron non-vertical transition, the electron needs momentum transfer. If the magnitude of the momentum transfer is very large, it may appear as phonons and eventually as heat. When electrons from the conduction band to the valence band will produce indirect compound, the need for a certain amount of momentum transfer to produce compound. So, the electrons in the crystal will need phonon participation, so it is hard to produce photons.

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