Am working in RE doped tellurite glasses with different concentrations. I couldn't get a monotonic increase/decrease in band gap values. can anyone help to find the reason behind that? I need physical phenomenon behind it.
How you define the gap? What technique are you using to calculate the gap?
If it is optical transmission experiments, you have to see Urbach edge. For bulk materials, the gap may be defined as energy at which the absorption coefficient equals 10^2. When the sample contains inhomogeneities, the absorption edge is not sharp. If the concentration of dopants increases, this inhomogeneity increases, then the slope of the Urbach edge decreases. In this case, there will be a non-monotonic change in the gap with the concentration.
There are several other reason but I need to know better your experiments and nature of dopants.
Increasing doping contents may allow the interaction of two doping species in the matrix. Moreover, the matrix can be change of crystallographic phase.
The energy bandgap of glassy materials is in fact a pseudogap, i.e. a gap appearing in the density of states of this highly disordered system and separating localized states (because of disorder) from extended states lying higher than a mobility threshold energy.
This energy bandgap does not have the same meaning as in ordered crystalline materials where it is defined from the energy band structure and where the energy bandgap separates bonding valence band states from antibonding conduction band states.
In crystalline materials, EG is sharp and well defined.
In noncrystalline or glassy materials EG is broadened and depends not only on the nature of the material (atoms species involved) but also on the degree of disorder (which results from interactions/correlations between atoms species involved).
A change in doping or composition of a glassy material will first of all change the disorder (degree and nature or range of disorder). Generally, the degree of disorder increases which is reflected by an optical energy bandgap increase.
In this case of glassy materials, the energy badgap increase with increase of doping species may be not monotonic because of disorder.
This can be easily tracked from Tauc and Urbach formalisms as done in following references:
1. Preparation and characterization of sulfide, selenide
and telluride glasses
D. Lezal ,*, J. Pedlıkov, J. Zavadil b, P. Kostka , M. Poulain
Journal of Non-Crystalline Solids 326&327 (2003) 47–52
2. Physical properties and optical band gap of new tellurite glasses within the TeO2–Nb2O5–Bi2O3 system
Author links open overlay panelYanlingWangShixunDaiFeifeiChenTiefengXuQiuhuaNie
Materials Chemistry and Physics
Volume 113, Issue 1, 15 January 2009, Pages 407-411
3. Article Effect of Bi2O3 content on the optical band gap, density and...
4. Article Optical band gap studies on lithium aluminum silicate glasse...
Best regards,
Prof A. Kadri
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