  We report on the formation of lobe-lobe (bi-lobed) Au-Ge nanostructures under
ultra high vacuum (UHV) conditions (\approx 3\times 10^{-10} mbar) on clean
Si(100) surfaces. For this study, \approx 2.0 nm thick Au samples were grown on
the substrate surface by molecular beam epitaxy (MBE). Thermal annealing was
carried out inside the UHV chamber at temperature \apprx 500{\deg}C and
following this, nearly square shaped Au_{x}Si_{1-x} nano structures of average
length \approx 48 nm were formed. A \approx 2 nm Ge film was further deposited
on the above surface while the substrate was kept at a temperature of \approx
500{\deg}C. Well ordered Au-Ge nanostructures where Au and Ge residing side by
side (lobe-lobe structures) were formed. In our systematic studies, we show
that, gold-silicide nanoalloy formation at the substrate (Si) surface is
necessary for forming phase separated Au-Ge bilobed nanostructures. Electron
microscopy (TEM, STEM-EDS, SEM) studies were carried out to determine the
structure of Au - Ge nano systems. Rutherford backscattering Spectrometry
measurements show gold inter-diffusion into substrate while it is absent for
Ge.
