  Electronic properties of surface areas decoupled from graphite are studied
using scanning tunnelling microscopy and spectroscopy. We show that it is
possible to identify decoupled graphene monolayer, Bernal bilayer, and Bernal
trilayer on graphite surface according to their tunnelling spectra in high
magnetic field. The decoupled monolayer and bilayer exhibit Landau quantization
of massless and massive Dirac fermions, respectively. The substrate generates a
sizable band gap, ~35 meV, in the Bernal bilayer, therefore, the eightfold
degenerate Landau level at the charge neutrality point is split into two
valley-polarized quartets polarized on each layer. In the decoupled Bernal
trilayer, we find that both massless and massive Dirac fermions coexist and its
low-energy band structure can be described quite well by taking into account
only the nearest-neighbor intra- and interlayer hopping parameters. A strong
correlation between the Fermi velocity of the massless Dirac fermions and the
effective mass of the massive Dirac fermions is observed in the trilayer. Our
result demonstrates that the surface of graphite provides a natural ideal
platform to probe the electronic spectra of graphene layers.
