It was shown that a 2-peak green photoluminescence from porous material could be obtained from a continuous blue-shift of a red photoluminescence by positron irradiation. Samples were irradiated in air, at room temperature, with energetic positrons from 22Na. With increasing positron irradiation time, an originally red photoluminescence shifted continuously to green. A 2-peak photoluminescence then appeared, with a weak high-energy emission band (529nm) and a low-energy dominant band (562nm). The intensity of this high-energy band was enhanced by prolonged positron irradiation. The electron spin resonance signal, combined with infra-red absorption, showed that positron irradiation created dangling bonds and stimulated oxide growth in porous Si. The results were explained in terms of quantum confinement and the atom-like nature of very small nano-crystallites.

Y.M.Huang: Applied Physics Letters, 1997, 71[26], 3850-2