We present a novel synthesis of ligand-free colloidal silicon nanocrystals (Si-NCs) that exhibits efficient photoluminescence (PL) in a wide energy range (0.85–1.8 eV) overcoming the bulk Si band gap limitation (1.12 eV). The key technology to achieve the wide-range controllable PL is the formation of donor and acceptor states in the band gap of Si-NCs by simultaneous doping of n- and p-type impurities. The colloidal Si-NCs are very stable in an ordinary laboratory atmosphere for more than a year. Furthermore, the PL spectra are very stable and are not at all affected even when the colloids are drop-cast on a substrate and dried in air. The engineering of the all-inorganic colloidal Si-NC and its optical data reported here are important steps for Si-based optoelectronic and biological applications.

We demonstrate the formation of a new type of surfactant-free colloidal silicon nanocrystal (Si-NC). The characteristic structural feature of the Si-NCs is simultaneous doping of phosphorus (P) and boron (B) in and on the surface of Si-NCs. The codoped Si-NCs are stable in methanol for more than a year and exhibit luminescence in the near-infrared range. We perform comprehensive studies on the structure of codoped colloidal Si-NCs and discuss the mechanism of the high solution dispersibility.

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A crystalline silicon (Si) nanoparticle (NP) of 100–200 nm in diameter exhibits a highly saturated color owing to Mie resonance, and can be a component to realize angle‐insensitive structural color covering the entire visible range. However, to date, coloring a substrate by Si nanostructures has only been achieved in a very small area by using electron beam lithography and dry etching processes. In this work, a Si NP color ink capable of coloring a flexible substrate by a painting process is developed. The sphericity of Si NPs is very high; the circularity factor obtained from a transmission electron microscope image reaches 0.97. The average diameter of Si nanospheres is controlled from 95 to 200 nm, and the polydispersity defined by the standard deviation divided by the average diameter is as small as 6%. Because of the high sphericity, high crystallinity, high size purity, and perfect dispersion in solution, the Si nanosphere solutions exhibit vivid colors recognizable by naked eye in a range of blue to orange. The Si nanosphere color inks combined with a polymer binder are capable of coloring flexible substrates by a painting process.