  We have constructed a scanning tunneling potentiometry system capable of
simultaneously mapping the transport-related electrochemical potential of a
biased sample along with its surface topography. Combining a novel sample
biasing technique with a continuous current-nulling feedback scheme pushes the
noise performance of the measurement to its fundamental limit - the Johnson
noise of the STM tunnel junction. The resulting 130 nV voltage sensitivity
allows us to spatially resolve local potentials at scales down to 2 nm, while
maintaining angstrom scale STM imaging, all at scan sizes of up to 15 um. A
mm-range two-dimensional coarse positioning stage and the ability to operate
from liquid helium to room temperature with a fast turn-around time greatly
expand the versatility of the instrument. By performing studies of several
model systems, we discuss the implications of various types of surface
morphology for potentiometric measurements.
