Dear Dr. Kumar:

The optical gap or bandgap can be derived from ultraviolet spectroscopy, only a bit of material can be manipulated in a liquid suspension. But, depending on the material.

kind regards.

Marcos Nobre (M. A.L. Nobre)

One simple way( caution: I might be wrong) is, probably, to use a powder amount enough te render the transmittance=0. Then, the absorbance= 1-the reflectance. Putting this into Tauc's equation would yield a familiar linear plot giving the band gap from the verticle-axis intercept..

By two ways , you can find the band gaps,

1. if you are using in solid form, you can take UV-DRS(diffuse reflectance spectrum) and use Kubelka-Munk equations to obtain the band gap

2. if you are able to diffuse the material in a solvent then, take UV-Vis absorp tion/ transmission spectra, and draw Taucs plot  to obtain the band gap. You must be very careful to select whether your material possess direct or indirect band gap, according to that you must select the value of n

There are DFT calculations of the bangaps in small clusters. Consult, for example, the studies of Dixon et al.

Thanks for nice ideas. i well about kubelka munk equations, but Taucs plot  & DFT is new to mine. Here i interested to mentioned other kubelka munk methods for bandgap TiO2.

To

Prof. Marcos, What is the difference between optical gap and bandgap ? 

Prof. Abdul kader, and Mansor 

Can we explain absorption peak of metal complex through Tauc's plot ?

How to choose whether my material possess direct or indirect bandgap?which one is best? 

Dear Dr. Kumar:

Nowaday, measurements of  “gap” has been strategic for building a set of functional materials as solar-photovoltaic cell, photocatalysts and another. The “gap” or "band-gap" is a type of synonymous of forbidden band, that is correlated to the valence band and conduction band. See, metals have not defined a forbidden band for electrons (gap), in other words has not a “gap” positioned between both valence band and conduction band. The question of semimetals can be viewed in similar way that metals, only that a very narrow “gap” is further considered. In a broad sense, the existence of a gap is associated to the promotion of electrons from valence band to the conduction band. Depending on the topic in which this question is inseted, the phenomenon of electronic promotion between bands can be called of excitation. This phenomenon is possible at semiconductors and insulators (very good linear dielectrics). Excitations phenomena are complex exhibiting interesting diversity but two of them are relevant here, in specific the excitation from photon absorption and from application of a non large electric field.

Optical band-gap:

In a broad sense, optical band-gap is correlated to photon interaction with the matter. In fact the word “optical” supposes that an entire part of the electromagnetic spectrum is further considered ranging from visible (photon with energy between 1.8 to 3.1 eV), near infrared up to ultraviolet. A priori, in physical terms, a photon incident is absorbed if gap has a value in terms of energy (typically in eV) equal or higher than that the value of the energy of this photon. Furthermore, a type of band-structure distortion as a function of photon absorption is any absent. This concept is fundamental, since a specific wavenumber from solar spectra can be absorbed, in a semiconductor by a kind of tailoring of the gap of semiconductor ( development of extrinsic features, which energy levels are created in the gap allowing that electron be localized neither in valence band nor conduction band, as an example the level termed of electron-trap that exert small influence on the electric conductivity but results in great changing of optical properties), giving an advanced functional photocatalyst.

Band-gap:

Conventional electrical measurement directed to characterize semiconductors (intrinsics and extrinsics) involving the application of an external electric potential, leads to the distortion of the band-structure with relation to the origin of the electric field. Thus, in a general way, both values of “optical band-gap” and band-gap differ in a slight way, despite of that from physic-chemical viewpoint such values should be equals.

Kind regards.

Marcos Nobre (M. A. L. Nobre)

Dear Raj kumar

In the answer of Zamin Mamiyev, the tauc's equation is provided to you. In which n is type of transition. Put n=1/2 for direct transition and n=2 for indirect transition and try to have 2 tauc plots respectively. Have a closer watch. Now you can fix is it direct or indirect

Here you can find good explanation and band gap calculator,

http://www.instanano.com/characterization/theoretical/uv-vis-spectroscopy-band-gap-calculation/

hello Dr Sheik Mohammed Abdul kader@ how we can select if the band gap is direct or indirect ?