  We show how a scanning probe microscope (SPM) can be used to image electron
flow through InAs nanowires, elucidating the physics of nanowire devices on a
local scale. A charged SPM tip is used as a movable gate. Images of nanowire
conductance vs. tip position spatially map the conductance of InAs nanowires at
liquid He temperatures. Plots of conductance vs. back gate voltage without the
tip present show complex patterns of Coulomb-blockade peaks. Images of nanowire
conductance identify multiple quantum dots located along the nanowire - each
dot is surrounded by a series of concentric rings corresponding to Coulomb
blockade peaks. An image locates the dots and provides information about their
size. The rings around individual dots interfere with each other like Coulomb
blockade peaks of multiple quantum dots in series. In this way, the SPM tip can
probe complex multi-dot systems by tuning the charge state of individual dots.
The nanowires were grown from metal catalyst particles and have diameters ~ 80
nm and lengths 2 to 3 um.
