Did you use the same excitation wavelength? The emission varies with the excitation. That said many papers can't be reproduced exactly since other minor variables not discussed in the paper can matter. But you should be able to adjust the emission to where you want it.

Dear Prof. Nicholas Edward Schlotter,

Thank you very much for your kind response.

To clarify, after the hydrothermal synthesis, the product I obtained was in solid (powder) form. I then dispersed the powder in water and tested the emission under a 365 nm UV lamp. However, the results remained the same as I mentioned earlier: the red-CQDs appeared red in daylight but showed blue emission under UV, while the yellow-CQDs emitted green instead of yellow. Do you think this might still be related to excitation wavelength dependence, or could other factors, such as surface oxidation state, incomplete carbonization, or solvent-related effects, be responsible for this unexpected shift?

The paper shows emission shifts as a function of excitation wavelength. Check it out. Other factors are always possible as well, but I would make sure I was using the same parameters the paper used to start with....

Dear Prof. Nicholas Edward Schlotter,

Thank you for your explanation. I understand now that the paper shows excitation-dependent emission at different wavelengths (390–500 nm), even though the photos display distinct colors under a 365 nm UV lamp. In my case, with 365 nm excitation, the red-CQDs emitted blue and the yellow-CQDs emitted green. Do you think this discrepancy indicates that my synthesis did not produce the correct surface states, or should I also try other excitation wavelengths to confirm the emissions? Could you kindly suggest what experimental adjustments or checks I could perform to obtain the expected red and yellow emissions?

The best thing is to find your parameters that give you what you need. Exactly duplicating someone else's experiment isn't always possible. You have positive results so the paper is pretty good. Now make it your own.

Hello Muhammad,

Have you been able to take a fluorescence emission spectrum? That would be a more precise/information-rich way to compare your materials with the published data.

The synthesis conditions in the paper are fairly minimal, so it is difficult to assess issues in your procedure. A few things jump out to me immediately.

1) No mention of post-synthesis treatment. You mention that your material was collected as a powder and re-dispersed, there is no mention of such in the publication. Subtle details of re-dispersal (e.g. sonication) may be important for reducing aggregation which affects optical properties.

2) The solvent used for PL spectra is not mentioned - Many CDs are solvatochromic (i.e. their emissions may vary according to solvent polarity/proticity). Solvents may also affect aggregation and tautomer/protonation states. You might find a better match with the published results using different solvents.

Take some more measurements (Optical absorption/emission is a good place to start IMO), you probably need more information to properly diagnose any issues.

Finally, carbon dot synthesis can just be a bit finicky, you may need to introduce variations to suit your equipment/reagents. A little trial and error may be unavoidable.