Recently, I have tried to take TEM image of carbon quantum dots that I prepared. The TEM imaging was performed with carbon film coated copper grid (200 mesh) under 300 keV. My sample was dissolved in aqueous solution with well dispersion. Their luminescent properties were fine. So, I just applied the solution onto the grid directly and then dried. However, I couldn't get any highly dispersed particle image at all. Most of them looked like micron-sized spherical complex. Interestingly, the CQD particles were vanished under the beam of high magnification.

I'd like to know how I should prepare the sample and what the problem is.

Thank you very much for your advise!