The photocurrents measured by a scanning tunneling microscope have been used to analyze the electronic properties of nanoscale-modified WSe2 semiconductor surfaces. On uncovered crystals in ambient air, space charges along steps can be analyzed. On copper-modified semiconductor surfaces, the space charge zones around pulse-deposited metal particles and their dependence on the size of the metal particle can be determined and provide a first direct proof of size-dependent barrier heights of nanosized Schottky contacts. Also, the time evolution of the electronic properties of a metal-modified WSe2 surface under corrosion conditions was followed by photocurrent measurements. By combination of photocurrent measurements and local current/voltage curves, the influence of recombination and of charges on the photocurrent can be separated.