Why the MoS2 single layer has the direct and larger band gap compare to its bulk counterparts, which has smaller but indirect band gap.
Direct band gap in 2 D Transition metal dichalcogenides (TMDs) of which MoS2 is an example depends on the localized d orbital of the Transition metal (TM) which are minimum affected by the inter layer coupling due to its location in the unit cell. While indirect band gap in these materials depends on overlap of d orbital of TM and pz orbital of the chalcogenide atoms which strongly depends on inter layer coupling. Thus as the number of layer is decreased the the intrinsic direct band gap of the material becomes more pronounced.
While the band gap broadening in single layer is due to quantum confinement effect of charge carriers which is common in nano systems.
Dear Dr. Sandip Ghosh
Greeting
Please, You should mention what are about your samples? Are they prepared to have the amorphous nature? or prepared to have nanocrysatlline structures. Note, the band gap, thickness and crystallite size of the thin films are very related to each others. So you should define the nature of your samples.
But in general, the band gap of any semiconductor varies with increasing or decreasing of the film thickness or increasing the number of layers, according to many conditions. This is attributed to that the material behaves as the bulk one. But, in your case I think that you have some thing should be clarified.
Best regards
The gap value is decreased with increases of particle size/ thickness of the films.
http://www3.ntu.edu.sg/home/ecqsun/rtf/jap-ge.pdf
This link may help you.
Dear Sandip,
In case of very thin film in the order few interatonic distances, there will be a quantum mechanical confinement of the electrons inside the material leading to the quantization of electronic energy inside the conduction band and valence band. The consequence of this quantization is increase the energy gap compared to the bulk materials. There is a direct effect of the size of the material on its energy band structure such as its dimensions decreases its energy gap increases and may turn from indirect to direct bandgap.
Best wishes
Think of a capacitor the smaller d the smaller the ionization Voltage. In the capacitor
C = epsilon A / d
- Badges
- Science topic