I prepared TEM samples of CD with 1-2 droplets carbon dots water solution in 2nm carbon layer copper grid. But I almost can not find any dots in the images. Most of the materials stay out of the thin carbon layer hole. There is nothing inside. Some of holes with thin carbon layer are covered with a totally black carbon layer. For some images, it is hard to identify whether it belongs to my samples. So is it a good choice to use copper grid with carbon layer? How should I prepare the carbon dots samples for TEM?