Lanthanide-doped nanophosphors have emerged as promising optical labels for high-resolution, "multicolor" electron microscopy. Here, we develop a library of eleven unique lanthanide-doped nanophosphors with average edge lengths of 15.2 ± 2.0 nm (N = 4284). These nanophosphors consist of an electron stable BaYF 5 host lattice doped at 25% atomic concentration with the