# To test exciton binding energy

1. Jun 23, 2011

### Bauer Yee

Hi,
I want to test exciton binding energy of some particles, what should i do ?

2. Jun 24, 2011

### Bauer Yee

supplement to the problem:
it's semiconductor powders, what dictate the exciton binding energy ? how could it be caculated ? how could it be tested ?

3. Jun 25, 2011

### Kholdstare

I know how exciton binding energy can be calculated. Though I know only for Mott-Warnier excitons. I also have an idea of how it could be tested.

4. Jul 15, 2011

### Bauer Yee

So what is binding energy equation for the Mott-Warnier excitons ? Would you please give further reference ? thanks

5. Jul 21, 2011

### Kholdstare

Mott-Warnier exciton binding energy is given by,

Eex = $\frac{-m^{*}_{r}e^{4}}{2(4\pi\epsilon)^2\hbar^{2}}$$\cdot\frac{1}{n^{2}}$

Where $m^{*}_{r}$ is effective mass of the particles in the material (for eg GaAs).

$m^{*}_{r}$ = $\frac{m^{*}_{e}m^{*}_{hh}}{m^{*}_{e}+m^{*}_{hh}}$

$m^{*}_{e}$ and $m^{*}_{hh}$ are effective mass for electrons and heavy holes in the material. $\epsilon$ is the permittivity of the material. n signifies nth excitonic orbital, which corresponds to nth excitonic binding energy. nth excitonic energy level is obtained by subtracting the nth binding energy from the conduction band.

For GaAs considering heavy holes we get Eex $\approx -\frac{4.6}{n^{2}}$ meV

BTW i was talking about 3D materials. In 2D materials it is a little different. To measure the excitonic binding energy experimentally you have to get the absorption spectra of the sample. It will show fine absorption values below the cut-off light frequency. those are caused by absorption of photon by an electron to jump from valance band to an excitonic energy level (where it forms an exciton with the hole left by it). The difference between the forbidden energy gap and excitonic energy gap (i. e. exciton absorption peak frequency multiplied by Plank's constant) will be the binding energy for that exciton.

Sorry for the late reply. Hope this helps.

6. Jul 24, 2011

### Bauer Yee

Thank you very much!

7. Jul 24, 2011

### Bauer Yee

To measure the exciton binding energy, whether is it not enough to get absorption spectra ? for example, in an absorption spectra of UV-vis spectra under room temperature, there is no fine absorption values. Does the fine absorption values appear at low temperature, or other conditions ?

8. Jul 25, 2011

### Kholdstare

In fact I dont know how practically you will get the spectra, but I think you should get some absorption in the visual spectra. However the intensity of absorption may be very small. In that case you have to look for the way to intensify it.

Theoritically I can guide you to follow some derivation and calculation from the idea that electromagnetic radiations are created by accelerating electric charge (Here oscillating electric dipole). Thus consider the two states : ground state wavefunction of the electron in the valance band and excited state wavefunction of the electron in the excitonic state. During the photon absorption process the electronic wavefunction will start from ground state, oscillate between these two states with frequency corresponding to the photon energy, and settle down to the excited state. The Rate of absorption of energy can be calculated from the amplitude and frequency of the oscillation. This rate will be immediately translated as the intensity of absorption.

I guess I've given a very vague idea of electronic transition. For a detailed analysis you can consider reading book (Quantum Physics by Resnick & Eisberg, Chapter 8.7 and Appdx B).

9. Jul 25, 2011

### Cthugha

That depends. Do you know the "pure" band gap without any excitonic effects your material has? While the band gap is well known for GaAs and many other materials, it is less well known for some materials commonly used for colloidal QDs. Have a look at R. W. Meulenberg et al., "Determination of the Exciton Binding Energy in CdSe Quantum Dots", ACS Nano, 2009, 3 (2), pp 325–330 http://pubs.acs.org/doi/abs/10.1021/nn8006916" [Broken] to check how exciton binding energies are measured in detail.

Well, low temperatures always make it easier to see the excitonic absorption. What kind of material do you have? For most II-VI materials you would expect excitonic absorption in the blue, green or yellow range, while for many III-V materials, it is in the red or infrared. Of course this is strictly valid only for self-assembled QDs, for colloidal ones there is also a huge size dependence.

Last edited by a moderator: May 5, 2017
10. Jul 26, 2011