  We demonstrate cooling of the center of mass motion of charged graphene
nanoplatelets levitated in a quadrupole ion trap in high vacuum down to
temperatures of 20 K. Parametric feedback based on optical measurements of
particle motion was used to achieve the particle cooling at pressure
$p<10^{-6}$ Torr, and cooling along all three axes of motion was observed.
Dependence of cooling on the electric fields was measured by varying DC
voltages on a set of auxiliary electrodes used to spatially shift the trap
minimum. Methods to calibrate mass and charge of the nanoplatelet by measuring
its motion frequency dependence on discharge were also explored.
