Silicon nanoparticles (Si-NPs) are useful phosphor materials for light emitting devices because of their multicolor luminescence, low production cost, and abundance. The luminescence properties of Si-NPs strongly depend on their surface condition. To improve their luminance and luminance stability, Si-NPs are combined with an alkyl group. We used ultraviolet irradiation to couple the surface of Si-NPs with an organic layer consisting of an alkyl group. The surface-modified Si-NPs demonstrated bright and stable luminance in air. Our findings suggest that alkyl-passivation of Si-NPs has future practical applications for light emitting devices and displays.