Substantial improvements of the absolute photoluminescence quantum yield (QY) for surfactant-free silicon nanocrystals (Si-ncs) by atmospheric pressure microplasma 3-dimensional surface engineering are reported. The effect of surface characteristics on carrier multiplication mechanisms is explored using transient induced absorption and photoluminescence QY. Surface engineering of Si-ncs is demonstrated to lead to more than 120 times increase in the absolute QY (from 0.1% up to 12%) within an important spectral range of the solar emission (2.3-3 eV). The Si-ncs QY is shown to be stable when Si-ncs are stored in ethanol at ambient conditions for three months.