Notes

We have been able to prepare graphitic
sheets of thicknesses down to a few atomic
layers (including single-layer graphene), to
fabricate devices from them, and to study
their electronic properties. Despite being
atomically thin, the films remain of high
quality, so that 2D electronic transport is
ballistic at submicrometer distances. No
other film of similar thickness is known to
be even poorly metallic or continuous under
ambient conditions. Using FLG, we demon￾strate a metallic field-effect transistor in
which the conducting channel can be
switched between 2D electron and hole gases
by changing the gate voltage.
Our graphene films were prepared by
mechanical exfoliation (repeated peeling) of
small mesas of highly oriented pyrolytic
graphite (15). This approach was found to
be highly reliable and allowed us to prepare
FLG films up to 10 6m in size. Thicker films
(d Q 3 nm) were up to 100 6m across and
visible by the naked eye. Figure 1 shows
examples of the prepared films, including
single-layer graphene Esee also (15)^. To
study their electronic properties, we pro￾cessed the films into multiterminal Hall bar
devices placed on top of an oxidized Si
substrate so that a gate voltage Vg could be
applied. We have studied more than 60
devices with d G 10 nm. We focus on the
electronic properties of our thinnest (FLG)
devices, which contained just one, two, or
three atomic layers (15). All FLG devices
exhibited essentially identical electronic
properties characteristic for a 2D semimetal,
which differed from a more complex (2D
plus 3D) behavior observed for thicker,
multilayer graphene (15) as well as from
the properties of 3D graphite.
In FLG, the typical dependence of its sheet
resistivity D on gate voltage Vg (Fig. 2)
exhibits a sharp peak to a value of several
kilohms and decays to È100 ohms at high Vg
(note that 2D resistivity is given in units of
ohms rather than ohms
cm as in the 3D
case). Its conductivity G 0 1/D increases
linearly with Vg on both sides of the resistivity
peak (Fig. 2B). At the same Vg where D has its
peak, the Hall coefficient RH exhibits a sharp
reversal of its sign (Fig. 2C). The observed
behavior resembles the ambipolar field effect
in semiconductors, but there is no zero￾conductance region associated with the Fermi
level being pinned inside the band gap.
Our measurements can be explained
quantitatively by a model of a 2D metal
with a small overlap &( between conductance
and valence bands (15). The gate voltage
induces a surface charge density n 0 (0(Vg/te
and, accordingly, shifts the position of the
Fermi energy (F. Here, (0 and ( are the
permittivities of free space and SiO2, respec￾tively; e is the electron charge; and t is the
thickness of our SiO2 layer (300 nm). For