We present the results of a photoluminescence study in naturally grown filament‐like silicon crystals. It is shown that with the decrease of filament diameter from few tens to 0.1 μm, the photoluminescence spectra transform from the ones of bulk silicon to completely new spectra. The spectra at 4.2 K consist of the narrow 1.139 eV band and several very wide visible (red, yellow, green, and blue) bands with the maximum position ranging from 1.7 to 2.9 eV. The bands of visible photoluminescence do not markedly change with the increase in temperature up to 300 K, while the 1.139 eV band disappears. The most striking result is the appearance of visible light emission from crystals with transverse size much larger than the Bohr radius. The results obtained are discussed together with our previous ones obtained for these crystals, and a possible model of the filament crystal structure as well as the origin of visible photoluminescence are suggested.

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