  A gate induced insulating behavior at zero magnetic field is observed in a
high mobility suspended monolayer graphene near the charge neutrality point.
The graphene device initially cleaned by a current annealing technique was
undergone a thermo-pressure cycle to allow short range impurities to be
adsorbed directly on the ultra clean graphene surface. The adsorption process
generated a strong temperature and electric field dependent behavior on the
conductance of the graphene device. The conductance around the neutrality point
is observed to be reduced from around $e^2/h$ at 30 K to $\sim0.01~e^2/h$ at 20
mK. A direct transition from insulator to quantum Hall conductor within
$\approx0.4~T$ accompanied by broken-symmetry-induced $\nu=0,\pm1$ plateaux
confirms the presence of intervalley scatterers.
