Observation of a magic discrete family of ultrabright Si nanoparticles

We demonstrate that electrochemically etched, hydrogen capped ${\mathrm{Si}}_n{\mathrm{H}}_x$ clusters with $n$ larger than 20 are obtained within a family of discrete sizes. These sizes are 1.0 $({\mathrm{Si}}_{29}\mathrm{),}$ 1.67 $({\mathrm{Si}}_{123}\mathrm{),}$ 2.15, 2.9, and 3.7 nm in diameter. We characterize the particles via direct electron imaging, excitation and emission optical spectroscopy, and colloidal crystallization. The band gaps and emission bands are measured. The smallest four are ultrabright blue, green, yellow and red luminescent particles. The availability of discrete sizes and distinct emission in the red, green and blue (RGB) range is useful for biomedical tagging, RGB displays, and flash memories.