Hi Dr. Krishnaiah,

410% seems unrealistic though some people show QYs of 200% for CsPbX3 nanocrystals and other materials. While calculating QYs, may be, you did not take considerations of some parameters, the reason you overestimated QYs. Calculating QYs using relative methods is very tricky, anyone can easily get very erroneous results. To verify your results, you can compare your relative QYs with absolute QYs. You can measure absolute QYs using the method described in this article in details:

https://pubs.acs.org/doi/10.1021/acs.jpcc.9b05612

Article Mechanistic Exploration of Dodecanethiol Treated Colloidal C...

Best,

Aslam

Krishnaiah Damarla

Photoluminescence quantum yield more than 100% is possible.

This occurs via electron-hole pair multiplication (EHPM) i.e. generation of multiple electron–hole (e–h) pairs in semiconductors upon the absorption of a single photon. EHPM is known as multiple exciton generation (MEG) in quantum dots and impact ionization (II) in bulk semiconductor. More than one electron-hole pair (EHP) may be produced by absorption of one photon with energy at least twice the semiconductor band gap, Eg. This phenomena is reported in several quantum dots such as PbSe, PbS, PbTe, Si. Arthur J. Nozik, Matthew C. Beard, Victor I. Klimov, and M. G. Bawendi did lots of pioneering work in this field.

In your case, check the band gap of the semiconductor and find the valid explanation. Before that, I would recommend to follow the guidelines provided by Horiba UK limited for PLQY measurements. I have attached here.

Few references for MEG:

1. J. Phys. Chem. Lett. 2011, 2, 1282–1288 (Multiple Exciton Generation in Semiconductor Quantum Dots)

2. Nano Lett. 2010, 10, 3019–302 (Comparing Multiple Exciton Generation in Quantum Dots To Impact Ionization in Bulk Semiconductors: Implications for Enhancement of Solar Energy Conversion)

3. Science 2000, 287, 1011–1013 (Quantization of Multiparticle Auger Rates in Semiconductor Quantum Dots)

4. Phys. Rev. Lett. 2004, 92, 186601 (High Efficiency Carrier Multiplication in Pbse Nanocrystals: Implications for Solar Energy Conversion)

5. Nano Lett. 2013, 13, 3078−3085 (Size and Composition Dependent Multiple Exciton Generation Efficiency in PbS, PbSe, and PbSxSe1−x Alloyed Quantum Dots)

Dear Dr. Md Aslam Uddin , Thanks for the suggestion. I have taken all parameters such as refractive index, OD and emission intensities of the sample and standard . I also repeated several times for the calculation.

Even though, I am getting same result. Could you please explain the reason

Dear Dr. Atanu Jana and Md Aslam Uddin Thanks for the fruitful suggestion. I have also read some useful articles on quantum yield more than 100%. Then, I came to know that, QY upto 200% possible because of the effect of quantum cutting/quantum splitting, which eventually leads to multiple exciton generation (MEG) as you mentioned. But, I don't know on which basis they calculated (relative / absolute method) the QY. Even though, in my case it is more than 200% , i.e., approximately doubles. I need still reasonable answer for this. Please suggest me any books/calculations/analytical methods.

Thank you very much for your suggestions.

Krishnaiah Damarla

If you achieved PLQY more than 100%, it means there is generation of MEG. PLQY = the number of emitted photons/ the number of absorbed photons. I have shared some papers. Please go through it. More than 200% QY was also reported.

Absolute QY gives the more accurate value without relying the QY of a standard which is required for relative QY measurement. Absolute measurement is gaining importance as integrating sphere setups are increasingly becoming commercially available. Previously, I attached the method for relative QY measurement. Here I am sharing a link to measure the absolute QY. https://jascoinc.com/applications/quantum-yield-integrating-sphere/

If you want reasonable answer, please mention the excitation wavelength and emission peak of your as-synthesized nanomaterial.

Atanu Jana

Dear Professor, thanks for the reliable answer. Regarding my material, the excitation wavelengths are 330nm, 360nm and and emission peaks are at 526 and 541nm respectively and you can check the spectra as attached below.

Krishnaiah Damarla

Thanks for providing the data and graphs. I have divided my answers into three parts:

(i) Data analysis:

You have synthesized a nanomaterial which shows photoluminescence quantum yield (PL QY) = 410%. It means the material emitted almost 4 photons per one absorbed photon. PLQY = the number of emitted photons/ the number of absorbed photons. You mentioned excitation wavelength 360 nm and corresponding emission peak at 541 nm. It means you have excited your material with the photons having energy 3.44 eV (360 nm) and the energy of emission photons 2.92 eV (541 nm). The energy of 4 emitted photons = 4 x 2.92 eV = 9.16 eV. You have supplied excitation energy 3.44 eV and achieved 9.16 eV. This breaks the law of conservation of energy. In your case, you are providing low energy and getting high energy. So it is not possible to get PL QY as high as 410% with the excitation wavelength 360 nm (3.44 eV).

Another excitation wavelength is 330 nm (3.75 eV) and corresponding emission peak is 526 nm (2.35 eV). The energy of 4 emitted photons = 4 x 2.35 eV = 9.4 eV. You have supplied excitation energy 3.75 eV and achieved 9.4 eV. This phenomena also breaks the law of conservation of energy. So it is impossible to get PL QY as high as 410% with the excitation wavelength 330 nm (3.75 eV).

N.B. More than one electron-hole pair (EHP) may be produced by absorption of one photon with energy at least twice the semiconductor band gap, Eg.

(ii) Graph analysis:

You provided the absorbance and PL spectra. The absorbance value is very high at 330 nm and 360 nm. For PL QY calculation, you need to take a very dilute solution of the sample. It should be below 0.1 because Beer-Lambert Law is applicable only for dilute solution.

From your PL spectra, it is predicted that your sample is not properly soluble in solvent. When you have heterogeneous sample, then scattering effect is very high.

(iii) Experiment analysis:

For PL QY measurement, you took quinine sulfate as standard which emits around 460 nm. It is recommended that the standard sample should absorb at the excitation wavelength of choice for the test sample and emits in a similar region. Your samples have emission peaks at 526 nm and 541 nm. So fluorescein dyes will be better choice for this region.

In order to minimize the re-absorption effects, absorbance in the 10 mm fluorescence cuvette should never exceed 0.1 at and above the excitation wavelength. Above this level, non-linear effect will be observed because of inner filter effects and consequently, the resulting PL QY will be perturbed.

Dear Prof. Atanu Jana , Its really a nice information. I will do the same as you said.