  We investigate the effect of surface termination on the charge state of
nitrogen vacancy centers, which have been ion-implanted few nanometers below
the surface of diamond. We find that, when changing the surface termination
from oxygen to hydrogen, previously stable NV- centers convert into NV0 and,
subsequently, into an unknown non-fluorescent state. This effect is found to
depend strongly on the implantation dose. Simulations of the electronic band
structure confirm the dissappearance of NV- in the vicinity of the
hydrogen-terminated surface. The band bending, which induces a p-type surface
conductive layer leads to a depletion of electrons in the nitrogen vacancies
close to the surface. Therefore, hydrogen surface termination provides a
chemical way for the control of the charge state of nitrogen-vacancy centers in
diamond. Furthermore, it opens the way to an electrostatic control of the
charge state with the use of an external gate electrode.
