  An Atomic Force Microscope is used to locally manipulate a single layer
graphene sheet. Transport measurements in this region as well as in the
unmanipulated part reveal different charge carrier densities while mobilities
stay in the order of 10000 cm^2/(Vs). With a global backgate, the system is
tuned from a unipolar n-n' or p-p' junction with different densities to a
bipolar p-n junction. Magnetotransport across this junction verifies its
nature, showing the expected quantized resistance values as well as the
switching with the polarity of the magnetic field. The mixing of edge states at
the p-n junction is shown to be supressed at high magnetic fields.
