Disperse the nanoparticles powder in epoxy resin (or conducting polymer) and make microtomy for geting the TEM samples.

What is the power of ultrasound machine you used? I saw different types. Maybe it is not powerful enough to destroy the agglomerates. Maybe you can also add some surfactant that will separate the particles.

An easy thing you could try is to not let it rest overnight but put the sample on the grid for half a minute or one minute and then touch the grid to a bit of filterpaper to get rid of remaining material and fluid. The drying process may be forcing the particles together as the big drop evaporates, so as long as the sonication step was sucessfull you would get a better result when blotting.

The problems is that there are three factors in your case possibly leading to particle agglomeration:

i) surface tension of the particles in solution: use a solvent with lower surface tension

ii) magnetic interaction: de-magnetise particles before preparation

iii) heating leads to sintering effects, even if temperature is low: avoid any heating as receommended above

I agree with Thomas Walther. heating leads to sintering effects. I faced the same problem, So it is better to dry the grid at room temperature itself (better keep it in vacuum desiccator) as the solvent which you used (ethanol) evaporates at room temperature after some time.

1. Magnetic samples can damage the TEM machine; to avoid any disaster, you should use some resins for the preparation of samples.

2. You can use ultrasonic probe or, any sonication set up for the dispersion of samples.

3. Cut proper size of the resin on which you would have impregnated the samples.

4. As told above, you should not provide any external heat for drying purpose.