  Raman spectroscopy on carbon nanotubes (CNT) yields a rich variety of
information owing to the close interplay between electronic and vibrational
properties. In this paper, we review the properties of double wall carbon
nanotubes (DWCNTs). In particular, it is shown that SWCNT encapsulating
C$_{60}$, so-called peapods, are transformed into DWCNTs when subject to a high
temperature treatment. The inner tubes are grown in a catalyst free environment
and do not suffer from impurities or defects that are usually encountered for
as-grown SWCNTs or DWCNTs. As a consequence, the inner tubes are grown with a
high degree of perfection as deduced from the unusually narrow radial breathing
mode (RBM) lines. This apostrophizes the interior of the SWCNTs as a nano-clean
room. The mechanism of the inner nanotube production from C$_{60}$ is
discussed. We also report recent studies aimed at the simplification and
industrial scaling up of the DWCNT production process utilizing a low
temperature peapod synthesis method. A splitting of the RBMs of inner tubes is
observed. This is related to the interaction between the two shells of the
DWCNTs as the same inner tube type can be encapsulated in different outer ones.
The sharp appearance of the inner tube RBMs allows a reliable assignment of the
tube modes to (n,m) indexes and thus provides a precise determination of the
relation between the tube diameter and the RBM frequencies.
