  Down-scaling device dimensions to the nanometer range raises significant
challenges to traditional device design, due to potential current leakage
across nanoscale dimensions and the need to maintain reproducibility while
dealing with atomic-scale components. Here we investigate negative differential
resistance (NDR) devices based on atomically precise graphene nanoribbons. Our
computational evaluation of the traditional double-barrier resonant tunneling
diode NDR structure uncovers important issues at the atomic scale, concerning
the need to minimize the tunneling current between the leads while achieving
high peak current. We propose a new device structure consisting of multiple
short segments that enables high current by the alignment of electronic levels
across the segments while enlarging the tunneling distance between the leads.
The proposed structure can be built with atomic precision using a scanning
tunneling microscope (STM) tip during an intermediate stage in the synthesis of
an armchair nanoribbon. An experimental evaluation of the band alignment at the
interfaces and an STM image of the fabricated active part of the device are
also presented. This combined theoretical-experimental approach opens a new
avenue for the design of nanoscale devices with atomic precision.
