  Graphite surfaces can be manipulated by several methods to create graphene
structures of different shapes and sizes. Scanning tunneling microscopy (STM)
can be used to create these structures either through mechanical contact
between the tip and the surface or through electro-exfoliation. In the latter,
the mechanisms involved in the process of exfoliation with an applied voltage
are not fully understood. Here we show how a graphite surface can be locally
exfoliated in a systematic manner by applying an electrostatic force with a STM
tip at the edge of a terrace, forming triangular flakes several nanometers in
length. We demonstrate, through experiments and simulations, how these flakes
are created by a two-step process: first a voltage ramp must be applied at the
edge of the terrace, and then the tip must be scanned perpendicularly to the
edge. Ab-initio electrostatic calculations reveal that the presence of charges
on the graphite surface weakens the interaction between layers allowing for
exfoliation at voltages in the same range as those used experimentally.
Molecular dynamics simulations show that a force applied locally on the edge of
a step produces triangular flakes such as those observed under STM. Our results
provide new insights towards surface modification that can be extended to other
layered materials.
