Guys,

Critical Science yields critical ways of expression.

Face the facts - other than that, there's no reason to get offended.

This is not FB.

Dear prof. Peter

I know the difference between the two types of transition, not fully understood but it is OK.

Now, I want to know how to define CdS and CuS transition types, So can you help me?

All semiconductors have direct and indirect band gaps. Usually Tauc's equation is used to fit experimental curves i.e. (alpha*h*frequency)^1/2 for direct and (alpha*h*frequency)^2  Vs Energy for indirect transitions. Make the fits for both cases and after analyze what is better.

Best regards,

Pedro Grima

Thanks all of you for your valuable answers

It was very important to me to know this information

My dear Dr. Amr

I missed you and all our colleagues very much. 

With respect of Your enquiry:

The relation between the absorption coefficient (α) and the incident photon energy (h*Neo) can be determined by using Tauc's relationship in the high absorption region of semiconductor, as follows:

                               Alpha*h*Neo = C ((h*Neo) - Eg)^n

where C is a constant and sometimes called the band tailing parameter and it is an energy independent constant, and Eg is the optical energy gap, which situated between the localized states near the mobility edges according to the density of states model proposed by Mott and Davis. In the given equation, the constant (n) is called the power factor of the transition mode. It is dependent upon the nature of the material, whether it is crystalline or amorphous and the photon transition. The values of (n) for direct allowed, indirect allowed, direct forbidden and indirect forbidden transitions are n = 1/2, 2, 3/2 and 3, respectively.

So, You can substitute by the  given values of (n) to know the type of the transition after plotting a graphical  representation between h*Neo and  ( Alpha*h*Neo).

For more details, You can see my attached papers. 

Best regards,

Yours 

Dr. Ahmed Saeed

Guys,

Critical Science yields critical ways of expression.

Face the facts - other than that, there's no reason to get offended.

This is not FB.

to distinguish between direct and indirect transition you must study the optical dielectric loss parameter see the following articles:

1- A novel polymer composite with a small optical band gap: New approaches for photonics and optoelectronics

2- Optical properties of pure and doped PVA: PEO based solid polymer blend electrolytes: two methods for band gap study

3- Fabrication of polymer blend composites based on [PVA-PVP](1− x):(Ag2S) x (0.01≤ x≤ 0.03) with small optical band gaps: Structural and optical properties

4- Synthesis of Polymer Nanocomposites Based on [Methyl Cellulose](1− x):(CuS) x (0.02 M≤ x≤ 0.08 M) with Desired Optical Band Gaps

5- Morphological and Optical Characteristics of Chitosan (1− x): Cuox (4≤ x≤ 12) Based Polymer Nano-Composites: Optical Dielectric Loss as an Alternative Method for Tauc’s Model

6- From Insulating PMMA Polymer to Conjugated Double Bond Behavior: Green Chemistry as a Novel Approach to Fabricate Small Band Gap Polymers

7- New method for the development of plasmonic metal-semiconductor interface layer: polymer composites with reduced energy band gap