Phonon-assisted and zero-phonon radiative transitions in nanoscale silicon quantum dots are studied using a new approach which combines a full calculation of the confined electronic eigenstates and vibration modes. We predict that the confinement, combined with the indirect bandgap of bulk silicon, must have several important consequences on the luminescence of a single silicon dot: i) a large broadening of the peaks, in the range of 10s of meV for a 3 nm dot, in spite of the atomic-like electronic structure of the dot ii) a great sensitivity of the spectrum to the size and the shape of the dot. We obtain that phonon-assisted transitions always dominate, even for size below 2 nm. Finally, we show that the radiative recombination in presence of an oxygen related surface defect (Si=O) is also assisted by optical phonons.
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