  The nucleation and growth of pure titanium nanoparticles in a low-pressure
sputter plasma has been believed to be essentially impossible. The addition of
impurities, such as oxygen or water, facilitates this and allows the growth of
nanoparticles. However, it seems that this route requires so high oxygen
densities that metallic nanoparticles in the hexagonal aTi-phase cannot be
synthesized. Here we present a model which explains results for the nucleation
and growth of titanium nanoparticles in the absent of reactive impurities. In
these experiments, a high partial pressure of helium gas was added which
increased the cooling rate of the process gas in the region where nucleation
occurred. This is important for two reasons. First, a reduced gas temperature
enhances Ti2 dimer formation mainly because a lower gas temperature gives a
higher gas density, which reduces the dilution of the Ti vapor through
diffusion. The same effect can be achieved by increasing the gas pressure.
Second, a reduced gas temperature has a "more than exponential" effect in
lowering the rate of atom evaporation from the nanoparticles during their
growth from a dimer to size where they are thermodynamically stable, r*. We
show that this early stage evaporation is not possible to model as a
thermodynamical equilibrium. Instead, the single-event nature of the
evaporation process has to be considered. This leads, counter intuitively, to
an evaporation probability from nanoparticles that is exactly zero below a
critical nanoparticle temperature that is size-dependent. Together, the
mechanisms described above explain two experimentally found limits for
nucleation in an oxygen-free environment. First, there is a lower limit to the
pressure for dimer formation. Second, there is an upper limit to the gas
temperature above which evaporation makes the further growth to stable nuclei
impossible.

Materials Science