  We used scattering-type scanning near-field optical microscopy (s-SNOM) to
characterize nickel nanoclusters grown inside single-walled carbon nanotubes
(SWCNT). The nanotubes were filled with Ni(II) acetylacetonate and the
molecules were transformed into nickel clusters via annealing. The metal
clusters give high local contrast enhancement in near-field phase maps caused
by the excitation of free charge carriers. The near-field contrast was
simulated using the finite dipole model, approximating the clusters with
elliptical nanoparticles. Compared to magnetic force microscopy, s-SNOM appears
much more sensitive to localize metal clusters inside carbon nanotubes. We
estimate the detection threshold to be ~600 Ni atoms.
