  Silicon nanoparticles possess unique size-dependent optical properties due to
their strong electric and magnetic resonances in the visible range. However,
their widespread application has been limited, in comparison to other (e.g.
metallic) nanoparticles, because their preparation on monodisperse colloids
remains challenging. Exploiting the unique properties of Si nanoparticles in
nano- and micro-devices calls for methods able to sort and organize them from a
colloidal suspension onto specific positions of solid substrates with
nanometric precision. Here, we demonstrate that surfactant-free Silicon
nanoparticles of a predefined and narrow ($\sigma$ < 10 nm) size range can be
selectively immobilized on a substrate by optical printing from a polydisperse
colloidal suspension. The size selectivity is based on differential optical
forces that can be applied on nanoparticles of different sizes by tuning the
light wavelength to the size-dependent magnetic dipolar resonance of the
nanoparticles.
