In cases, where the Pl comes by excitonic transitions, excitons hve a certain binding energy which ranges from 1 meV to 100-1000 meV. If the exciton binding energy is equivalent or less than the room temperature thermal energy, excitons are dissociated by this thermal energy and a certain PL intensity is low. However, when we reduce the temperature, the thermal energy reduces, thereby, reducing the dissociation of excitons and increasing the radiative decay of excitons. this increases the PL intensity at low temperatures.

What you mean by PL ?

prof.Walid Tawfik in my question PL is Photoluminiscence spectroscopy .

In cases, where the Pl comes by excitonic transitions, excitons hve a certain binding energy which ranges from 1 meV to 100-1000 meV. If the exciton binding energy is equivalent or less than the room temperature thermal energy, excitons are dissociated by this thermal energy and a certain PL intensity is low. However, when we reduce the temperature, the thermal energy reduces, thereby, reducing the dissociation of excitons and increasing the radiative decay of excitons. this increases the PL intensity at low temperatures.

Hello Lakshmi,

Just a little addition to Arunandan Kumar's response. The PL intensity is proportional to the number of radiative (excitonic in this case) transitions in the sample, and so is also proportional to the concentration of excitons in the sample. As explained by Arunandan, the concentration of excitons decreases exponentially with increase in temperature.

thank you kanand malik and arunandan kumar . I have one more doubt so if intensity of PL is directly propotional to the radiative recobinations ,so whether the photocatalytic activity of semiconductors will be high to the compounds having low PL intensity as photocatalytic activity depends on electron-hole seperation and also whether photocatalytic activity depends on temperature .

Hello Lakshmi,

Are you trying to measure photocatalytic activity of some semiconductor by PL, may be, as a function of temperature? What do you mean by electron-hole separation?

@Kanand Malik

Sir, I am not measuring photocatalytic activity of semiconductors by PL but trying to correlate defects in semiconductors by PL with Photo catalytic activity of semiconductors like TiO2 and ZnO. The mechanism proposed is oxidation of organic pollutants by photoinduced electron-holes and many attempts have been made to create traps fro electrons to prevent electron and hole pair recombination .So, if we create oxygen vacancies in metal oxide to trap electrons what will happen to the intensity of PL .According to literature PL corresponding to oxygen vacancies is at 500-530 nm, wether the intensity of PL increases corresponding to the parent compound showing higher vacancies or it decreases as reduction in recombination of electron-hole pairs. Sir, Pls clarify my doubt .

I see your problem. By the broad 500-530 nm PL you must be referring to the green luminescence band in ZnO. This occurs (most likely) due to e-h recombination at oxygen vacancies (OV) (please see J. Phys. Chem. C, 2012, 116 (40), pp 21297–21307) in ZnO. In this case, oxygen vacancies trap free electrons from the conduction band, and then holes from the valence band recombine with them. On the other hand, the recombination of free excitons (band-to-band e-h recombination) gives a sharp PL line at 375 nm.

In this context, it appears that the increase in the intensity of the PL band at low temperatures that you have observed has a different origin. It is not due to the reduced dissociation of free excitons at lower temperatures. The electron capture cross-section of OVs is likely to decrease exponentially with temperature which implies that the lifetime of trapped electrons in OVs increases exponentially with decrease in temperature. As a result, the rate of the radiative recombination of holes with trapped electrons at the OVs increases, leading to an increase in the intensity of the 500-530 nm PL band; and this does not mean that the concentration of e-h pairs (free exciton) has increased.

In my opinion, the idea of reduction of e-h recombination by using OVs as electron traps has a problem. OVs trap electrons from the conduction band, but since holes recombine with these electrons at the OVs, you do not get more free electrons and free holes in the conduction and the valence band, respectively. Hence, the photocatalytic activity of the material cannot be increased by increasing the concentration of OVs in ZnO.

Hope this helps and gives you a new line of thought.

Best wishes

In most of substances undergoing PL,the probability is directly proportional to their HOMO-LUMO energy gap which,in general,increases with temperature up to certain limit.

An important factor that affects on the temperature dependence of the PL bands as excitonic and caused by the recombination of carriers with the participation of various defect centers are the exciton (electron)-phonon interaction. The value of this interaction increases with increasing temperature, that always leads to a decrease in the intensity of PL bands. In addition, the temperature significantly changes the defective crystal structure. In particular, the neutral donor or acceptor centers at higher temperatures may become the charged centers. Scattering of excitons or charge carriers on such centers is stronger than in the case of the neutral centers. This factor will also lead to a decrease in the intensity of PL bands with increasing temperature.

@Manohar Sehgal

Hopefully, you are talking about something else, not photoluminescence. Also, the semiconductors discussed here do not have HOMO-LUMO gaps, which are more characteristics of organic molecules and nano-structured non-crystalline materials.

When it comes to excitonic photoluminescence, it is necessary to compare the energy of excitons connection with thermal energy. If these energies are close, then the thermal decay of excitons occurs, which result in reducing the intensity of photoluminescence. Another case: emission of donor-acceptor pairs. If the thermal energy is close to the ionization energy of donor or acceptor centers, they will be ionize, and therefore intensity of donor-acceptor pairs photoluminescence will decrease.

Thanks Lakshmi Prasanna V for starting this interest discussion. And thanks for all valuable comments.

agree wid Prof Walid, your question will indeed help us to provide better clarification.

Thanks to all contributors.

Hi,

Temperature is a measure of the kinetic energy of the atoms constituting a piece of material. In solids the energy associated to these atoms vibrations is quantified. Phonons are the quanta of thermal energy in solids. At 0 K there is no movement, therefore no phonon. At finite temperature, the higher this temperature the larger the number of phonons that do populate the solid.

In materials any kind of potential energy excitation, excitons, biexcitons, localized impurity states, vacancy states, Cooper pairs , etc., is coupled to the phonon bath as they interact with the atoms themselves. The coupling between the excitations and the phonon is independent of temperature.

The higher the temperature the larger the number of phonons and thus the larger the number of interactions that will de-excite the excitations. Therefore the life-time of the excitations decreases as the temperature increases. As a consequence their population density in the solid drops down with increasing temperature, but also their energy distribution measured by their spectrum broadens. So even if the population density of an excitation remains large at high temperature it may disappear do to the broadening of its spectrum that cannot be anymore extracted from the noise.

For excitons in semiconductors. The binding energy of an excitons in GaAs is only a few meV to be compared to the maximum energy of 20 meV for phonons at room temperature. the exciton in GaAs can only be seen at helium temperature. It does not mean there are no excitons at room temperature but due to a recombination time of the electron and hole in the picoseconde range it population density is negligeable.

ZnO exciton having a 60 meV binding energy larger than the maximum phonon energy at RT can be observed, despite its PL spectrum is enlarged due to the life-time decrease of the exciton.

Note. The life-time and the spectral width are conjugated quantities related by : delta nu . delta tau ~ 1/2. delta nu being the width of the spectrum and delta tau the life-time. This relation is at the origin of the uncertainty principal.

See also my answer to the question: Will an isolated quantum system relax?

https://www.researchgate.net/post/Will_an_isolated_quantum_system_relax2

how about if PL at room temperature is higher than at low temperature? Please comment on this?

Dear Dr. Kanad Malik,

Dear colleagues,

Would you please suggest me further references related closely to your interpretations above?

Best regards,

Ahmed

Interesting and from above mentioned discussions I am also got some information newly. Thanks Dr. Lakshmi to ask this question.