  Feedback-controlled electric breakdown of graphene in air or vacuum is a
well-established way of fabricating tunnel junctions, nanogaps, and quantum
dots. We show that the method is equally applicable to encapsulated graphene
constrictions fabricated using hydrogen silsesquioxane. The silica-like layer
left by hydrogen silsesquioxane resist after electron-beam exposure remains
intact after electric breakdown of the graphene. We explore the conductance
switching behavior that is common in graphene nanostructures fabricated via
feedback-controlled breakdown, and show that it can be attributed to
atomic-scale fluctuations of graphene below the encapsulating layer. Our
findings open up new ways of fabricating encapsulated room-temperature
single-electron nanodevices and shed light on the underlying physical mechanism
of conductance switching in these graphene nanodevices.
