  The basal plane cleavage energy (CE) of graphite is a key material parameter
for understanding many of the unusual properties of graphite, graphene, and
carbon nanotubes. The CE is equal to twice the surface energy and is closely
related to the interlayer binding energy and exfoliation energy of graphite.
Nonetheless, a wide range of values for these properties have been reported and
no consensus has yet emerged as to their magnitude. Here, we report the first
direct, accurate experimental measurement of the CE of graphite using a novel
method based on the recently discovered self-retraction phenomenon in graphite.
The measured value, 0.37 +/- 0.01 J/m2 for the incommensurate state of
bicrystal graphite, is nearly invariant with respect to temperature (from
22{\deg}C to 198{\deg}C) and bicrystal twist angle, and insensitive to
impurities (from the atmosphere). The cleavage energy for the ideal ABAB
graphite stacking, 0.39 +/- 0.02 J/m2, is calculated based upon a combination
of the measured CE and a theoretical calculation. These experimental
measurements are ideal for use in evaluating the efficacy of competing
theoretical approaches.
