  We present a simple technique to fabricate graphene quantum dots in a
cryostat. It relies upon the controlled rupture of a suspended graphene sheet
subjected to the application of a large electron current. This results in the
in-situ formation of a clean and ultra-narrow constriction, which hosts one
quantum dot, and occasionally a few quantum dots in series. Conductance
spectroscopy indicates that individual quantum dots can possess an addition
energy as large as 180 meV and a level spacing as large as 25 meV. Our
technique has several assets: (i) the dot is suspended, thus the electrostatic
influence of the substrate is reduced, and (ii) contamination is minimized,
since the edges of the dot have only been exposed to the vacuum in the
cryostat.
