This phenomenon looks like the formation of a ring that a drop of coffee leaves on the coffee table after evaporation. I remember that this old observation has been explained rather recently by physicists, but I can't remember the explanation. For avoiding this, you might decrease the concentration of the suspension or at least the volume of the drop, and dry it faster. Proabbly specialists of TEM will have a more accurate answer.

Alain

Are there any charge effects? you could ozone treat the grids previously to add the solution.... then the grid gets a negative surface charge and you will get electrostiatic repulsion from your nanoparticles (given they are also negatively charged). This may then decrease the accumulation at the grid walls. 

If the particles at the edge seemed agglomerated, and uniform then  - to Alain's point - the evapoartion phenomenon appears to be driving this effect.  However, I'd go the other way and increase the viscosity of the suspension (i.e. concentration) to minimize the convective flow during evaporation.  I don't know the mechanical limitations you have in depositing the suspension or how thick you need the final layer, so some experimentation will probably be required.  Bonne chance.

From my experience, the surface tension of the liquid in which the nano particles are suspended in, prevents the liquid to drain through the micro holes in the film on the TEM grid.  The liquid either drain over the side, trapping most of the particles close to the edg.  If you place a drop onto the grid, the slow evaporation let gravity feed the particles to the edge of the droplet rather than an even distribution of particles.  The best results I got was using a micropippette dropping smaller multiple droplets, say 2-3 onto the grid while the grid sits on tissue paper to facilitate a faster drainage.  I also move the grid to a unused area on the tissue paper after each droplet.

Perhaps try using a grid with film supports. I use lacey carbon grids for all dispersions and sometimes see agglomerates. Also, to slow the evaporation rate, I recommend storing the grid in a covered glass/petri dish to slow down the rate of evaporation. Good luck!

In you case, try to increase the concentration and dissolve your particles in ethanol.

Then do like previously mentioned by Stephan, use a tissue paper (or watman paper) place the grid on the surface, drop your droplet. The ring evaporation effect should be minimized and you will have more nanoparticles over the grid area to image besides the corners/edges of the grid. 

Please concentrate your suspension and add aliquot of your sample on a tem grid. Then wait for five to ten seconds. After that adsorb the extra sol on your grid using soft cloth edge. I hope this will hep you to tackle your problem.

Some years ago we studied  Ag NP and we obtain significative less agglomeration and clustering of NP using formvar coating instead of carbon ones. As it was recommended before, the smallest size is the droplet the best results you will obtain. When we started the study of Ag NP we were using carbon coatings and we suppressed the problem of accumulation of particles at the edge of the ring by using not a pipet but a vaporizator.  

Prepare a carbon-coated mica. Cut a piece from it (about 3 x 3 mm) and float the carbon off the mica by slowly dipping the carbon-coated mica sheet, with the mica at its upper side, from the side into a drop of your sample sitting on a plastic surface. The carbon will float off. Wait for 10 to 30 seconds (depending on the concentration of your sample) and pick up the floating carbon film either from inside the drop by lifting the grid upwards, or from outside the drop. Probably, at least in one of these variations you may avoid the effect you described because your particles are already sticking to the carbon surface

With newly designed and patented grids, it is possible to view specimens using the grid itself as a support. No carbon or former support is needed! M-Bond glue may be used to facilitate adhesion for specimens that are not flat. see examples here: www.EXpressLO.com, www.YouTube.com/LAGiannuzzi/videos