  Few-layer graphene sheets are prepared by splitting the expanded graphites
using a high-power sonication. Atomic-level quantitative scanning transmission
electron microscopy (Q-STEM) is employed to carry out the efficient layer
statisticsm, enabling global optimization of the experimental conditions. A
two-step splitting mechanism is thus revealed, in which the mean layer number
was firstly reduced to less than 20 by heating to 1100{\deg}C and then tuned to
the few-layer region by a 5-minute 104W/litre sonication. Raman spectroscopic
analysis confirms the above mechanism and demonstrates that the sheets are
largely free of defects and oxides.
