  We performed infrared transmission experiment on ion-gel gated graphene and
measured carrier scattering rate g as function of carrier density n over wide
range up to n=2E13 cm-2. The g exhibits a rapid decreases along with the gating
followed by persistent increases on further carrier doping. This behavior of
g(n) demonstrates that carrier is scattered dominantly by the two scattering
mechanisms, namely, charged impurity (CI) scattering and short-range disorder
(SR) scattering, with additional minor scattering from substrate phonon (SPP).
We can determine the absolute strengths of all the scattering channels by
fitting the g(n) data and unveils the complete n-dependent map of the
scattering mechanisms g(n)=gCI(n)+gSR(n)+gSPP(n). The gCI(n) and gSR(n) are
larger than those of SiO2$-gated graphene by 1.8 times, which elucidates the
dual role of the ion-gel layer as a CI-scatterer and simultaneously a
SR-scatterer to graphene. Additionally we show that freezing of IG at low-T
(~200 K) does not cause any change to the carrier scattering.
