Surface defects on nanowire structured silicon (Si) surfaces prepared by metal assisted chemical etching generate trap states in the band gap and thus minimize the efficiency of the future device. Electropolishing treatment of Si surfaces results in a reduced surface defect density. Since the interband photoluminescence (PL) of Si is very sensitive towards changes of the surface defect density, electropolishing treatments have been monitored by in situ PL measurements. A strong enhancement in PL intensity during the electropolishing experiment was obtained, which correlates to a reduced amount of surface trap states. The subsequent electrochemical exchange of Si-H bonds by Si-C bonds via methylation preserved the low defect density of the surface and prevented the formation of a native silicon oxide layer. Additional PL measurements proved the long-term stability of the methyl passivated surfaces of the Si nanowires in ambient air. (C) 2016 Elsevier Ltd. All rights reserved.