  Evolution of electronic inhomogeneities with back-gate voltage in graphene on
SiO$_2$ was studied using room temperature scanning tunneling microscopy and
spectroscopy. The reversal of local contrast in some places in the STS maps and
sharp changes in cross-correlations between topographic and conductance maps,
when graphene Fermi energy approaches its Dirac point, are attributed to change
in charge-state of interface defects. The spatial correlations in the
conductance maps, described by two different length scales and their growth
during approach to Dirac point, show a qualitative agreement with the
predictions of the screening theory of graphene. Thus a sharp change in the two
length-scales close to the Dirac point, seen in our experiments, is interpreted
in terms of the change in charge state of some of the interface defects. A
systematic understanding and control of the charge state of defects will help
in memory applications of graphene.
