Robust quantum point contact operation of narrow graphene constrictions patterned by AFM cleavage lithography

TL;DR

This paper introduces a simple AFM-based technique for fabricating high-quality graphene nanoconstrictions, enabling robust observation of conductance quantization at low conductance levels even on standard substrates.

Contribution

It presents a novel AFM cleavage lithography method for creating narrow graphene constrictions with reduced edge roughness, facilitating reliable quantum point contact operation.

Findings

Conductance quantization observed at low conductance quanta on standard substrates.

High-precision patterning down to 10 nm width achieved.

Robust QPC operation demonstrated without magnetic field.

Abstract

Detecting conductance quantization in graphene nanostructures turned out more challenging than expected. The observation of well-defined conductance plateaus through graphene nanoconstrictions so far has only been accessible in the highest quality suspended or h-BN encapsulated devices. However, reaching low conductance quanta in zero magnetic field, is a delicate task even with such ultra-high mobility devices. Here, we demonstrate a simple AFM-based nanopatterning technique for defining graphene constrictions with high precision (down to 10 nm width) and reduced edge-roughness (+/- 1 nm). The patterning process is based on the in-plane mechanical cleavage of graphene by the AFM tip, along its high symmetry crystallographic directions. As-defined, narrow graphene constrictions with improved edge quality enable an unprecedentedly robust QPC operation, allowing the observation of conductance quantization even on standard $SiO_2/Si$ substrates, down to low conductance quanta. Conductance plateaus, were observed at $ne^2/h$, evenly spaced by $2e^2/h$ (corresponding to n = 3, 5, 7, 9, 11) in the absence of an external magnetic field, while spaced by $e^2/h$ (n = 1, 2, 3, 4, 5, 6) in 8T magnetic field.