  Nitrogen-vacancy (NV) centers in diamond can be used for nanoscale sensing
with atomic resolution and sensitivity; however, it has been observed that
their properties degrade as they approach the diamond surface. Here we report
that in addition to degraded spin coherence, NV centers within nanometers of
the surface can also exhibit decreased fluorescence contrast for optically
detected electron spin resonance (OD-ESR). We demonstrate that this decreased
OD-ESR contrast arises from charge state dynamics of the NV center, and that it
is strongly surface-dependent, indicating that surface engineering will be
critical for nanoscale sensing applications based on color centers in diamond.
