In principle you can use the Tauc equation in the same way to estimate the onset of the absorption via the Tauc method. The only requirements would be that your solvent is transparent in the wavelength regime of interest. However be very careful how to interpret the Tauc data in thin films, as interference structures will dominate the transparent regime. In this case always perform T and R measurements and use T+R in the Tauc equation. For nanoparticle suspensions you also have to do Transmission measurements with an integrating sphere as otherwise you will interprete scattering as "absorption" which will lead to wrong "band gaps". always keep in mind that the Tauc plot gives you the onset of light loss in a transmission measurement. This is only equivalent to the bandgap in very specific cases (e.g. amorphous material as outlined in the original Tauc paper). The second issue with suspensions is that the dynamic range of the spectrophotometer will determine if you can pick up weak absorptions in small NP concentrations, so one should add concentration dependent studies to determine if the structure you see is that of the fundamental absorption or higher transitions.

Dear Sridhar Panday Mathu,

Have a nice time and good day.

This condition depends on the application of the produced material it is in liquid state, or thin film.

in case of thin film no problem it is the normal case. but i think if the material is dispersed through the solvent i think it is better to measure the optical parameters Transmittance, T, Reflectance, R and/or absorbance, A  first for the mixture  then for the solvent and subtract to get T, R/A for the material then you can do the following procedure to get and calculate the optical properties:

Equation No. 4 in the attached file gives  the relation between absorption coefficient , transmittance and reflectance. 

Herein fined all what you need about optical properties from A to Z.

Due to the format of the Equations, it is included in the attached file so check the Eq. No. the see it in the attached file.

First you measure transmittance, T reflectance, R and/or absrobance.

If your spectrometer is not calibrated so you need to  calibrate your measured parameters otherwise go to step 2. Herein fined how to calculate the optical gap and constants of the film also the references are mentioned and all you need to overcame the all problems of optical properties [five items]. Due to the format of Equations it is attached in a separate file, just you know the equation number please check it through the attach file.

1- How to calculate the correction of T and R?

The reflectance was measured at normal incidence with an aluminum reference mirror. The absolute values of  transmittance, T and reflectance R of the substrates after correcting the  are given by [[**]A. A. Sagade,  R. Sharma, Sensors and Actuators B: Chemical, 133 (2008) 135-143 [**] M. Dongol, M. M. El-Nahass, A. El-Denglawey, A.F. Elhady, A.A. Abuelwafa. Current Applied. Physics 12 (2012) 1178.]

Equation No.……………………………...... (1)

Ift and Iq are the intensities of the light passing through the film-quartz system and the reference quartz, respectively, and Rq is the reflectance of quartz. In addition, if the intensity of light reflected from the sample mirror reaching the detector is Ifr and that reflected from the reflectance reference mirror is Im, then

Equation No.…………………… (2)

Ifr is the intensity of light reflected from the sample reaching the detector and Rm is the mirror reflectance.

T and R spectra were performed on thin film samples with single face. Extinction coefficient, k and refractive index, n were calculated using T, R and thickness d taking into account the experimental error of   the film thickness. T and R.

2- How to calculate  Absorption and extinction coefficient?

T, R and d are used to calculate the absorption coefficient, α

according to [[**]A. El-Denglawey, M. Dongol, M.M  El-Nahass, J. Lumin 130 (2010) 801.]:

Equation No.………………………….…….…… (3)

α is given by:

Equation No. …………………………………… (4)

The calculated α is included within high absorption region (α ≥ 104 (cm)-1).  

Extinction coefficient, k of TiO2 film is calculated by using:

Equation No. ……………………………………….…. (5)

3- How to calculate  Optical gap?

            An absorption edge of semiconductors corresponds to the threshold of charge transition between the highest nearly filled band and the lowest nearly empty band. According to inter-band absorption theory, the film’s optical gap can be calculated according to [[**]J. Tauc (Ed.). Amorphous and Liquid Semiconductors, Plenum, New York, 1976].

Equation No.…………………………. (6)

 A is a parameter of transition probability, it measures the disorder of material 

A = 4πσmin / nc∆E, Where  σmin is the minimum metallic conductivity, n is the refractive index, c is the light-velocity, and  ∆E = ∆Ec - ∆Ev   represents the  band tailing

[[**]M. M. El-Nahass, M. H. Ali, A. El-Denglawey Trans. Nonferrous Met. Soc.     China 22(2012) 3003−3011].  

is the optical gap of the material, hυ is the incident photon energy and r is the transition coefficient. The reported values of r are 2 for the measurement of indirect optical gap and 1/2 for direct optical band gap.

4- To determine the value of r plot the relation.

In case you know the value of Eg from the literature you can use:  ln (αhυ) = ln B+ r ln(hυ- Egopt) and find the value of r from the slope of the line for Ex.

 May you have both direct  and indirect band gap

The indirect optical gap, is evaluated by extrapolating the straight line part of (αhν)1/2 curves with energy axes (hυ) i.e (αhυ)1/2 = 0  according to eq:

Equation No.……………………………. (7)

Direct optical gap,  is evaluated by extrapolating the straight line part of (αhν)2 curves with energy axes (hυ) i.e (αhυ)2 = 0  according to eq:

Equation No.……………………………. (8)

Both direct, and indirect, optical gaps of TiO2 films may be found.

OR

to find the value of r the relation between (αhν)^1/r and incident energy (hν) for all values of r should be figured. The value of r which release a straight line represent the value of r and the electronic transition type.

5- How to calculate refractive index and dielectric constant?

Both R, and k at different λ were used to calculate refractive index, n according to:

[[**] T. S. Moss. Optical Process in semiconductor. Butter Worths, London, 1959.]

Equation No.     ………………………….… (9)

The dispersion of refractive index of the films is analyzed by the concept of the single oscillator and can be expressed by the Wemple–DiDomenico (WDD) relationship[ [**][S. H. Wemple, M. DiDomenico. Phys. Rev. B 3 (1971) 1338-1351] as: 

Equation No.          ……………….(10)

 Eo is the single-oscillator energy and Ed is the dispersion energy which is a measure of the intensity of the inter-band optical transition, it does not depend significantly on the band gap. The oscillator parameters can be determined  by fitting a straight line to the experimental points according to [[**][A. El-Denglawey. J. Non-Cryst. Solids 357 (2011) 1757-1763].

Plotting  vs (hν)2 allows to determine the oscillator parameters by fitting a straight line to the experimental points. By extrapolating the linear part of WDD optical dispersion relationship towards the infrared spectral region (hν = 0), static refractive index n(0), could be defined by the infinite wavelength dielectric constant ε∞ or n(0)2,

The relation between the lattice dielectric constant εL, and the refractive index, n as

[[**] A. El-Denglawey. J. Non-Cryst. Solids 357 (2011) 1757-1763.]:

Equation No. ………………………………….(11)

where εL is the lattice dielectric constant, N/m* is the ratio of the carrier concentration to the effective mass, c is the speed of light, and e is the electronic charge, εo is the permittivity of free space. The linearity of the plots of n2 versus λ2, verifying of Eq. (11). The value of εL is determined from the extrapolation of these plots to λ2 = 0 and N/m* from the slope of the graph.

Please notice that there is a direct calculation of ( hu) by this relation using wave length only.

E= hu =1240/wavelength (nm). 

I wish that is useful and help you.

Don't hesitate to ask about any thing concerning optical, electrical and structural properties.

Don't hesitate to ask any thing and feel free to contact.

Good luck

Yours

A. El-Denglawey

Dear Prof. Eldenglawey,,

             Thank you so much for your brief explanation on UV-band gap measurement. It was very wonderful experience from you. thankz a lot once again.

Dear Dr.Karsten Fleischer,

                  Thank you for your valuable support as in way of theoretical explanation. As of now, i am well clear. Thank you Doctors.