Graphene Quantum Dots Probed by Scanning Tunneling Microscopy

Abstract

Scanning tunneling spectroscopy results probing the electronic properties of graphene quantum dots are reviewed. After a short summary of the study of squared wave functions of graphene quantum dots on metal substrates, we firstly present data where the Landau level gaps caused by a perpendicular magnetic field are used to electrostatically confine electrons in monolayer graphene, which are probed by the Coulomb staircase revealing the consecutive charging of a quantum dot. It turns out that these quantum dots exhibit much more regular charging sequences than lithographically confined ones. Namely, the consistent grouping of charging peaks into quadruplets, both, in the electron and hole branch, portrays a regular orbital splitting of about 10meV. At low hole occupation numbers, the charging peaks are, partly, additionally grouped into doublets. The spatially varying energy separation of the doublets indicates a modulation of the valley splitting by the underlying BN substrate. We outline that this property might be used to eventually tune the valley splitting coherently. Afterwards, we describe graphene quantum dots with multiple contacts produced without lithographic resist, namely by local anodic oxidation. Such quantum dots target the goal to probe magnetotransport properties during the imaging of the corresponding wave functions by scanning tunneling spectroscopy.

Large tunable valley splitting in edge-free graphene quantum dots on boron nitride

“…3c, e) exhibits a valley tunability of about 7 meV without inversion of the valley ordering. On the electron side, the additional charging of defects within the h-BN 45 complicates the analysis 46 , but some ring-like structures indicating valley inversion can also be spotted 38 . Data recorded with another microtip at two different B fields exhibit very similar features (Fig.…”

Graphene quantum blisters: A tunable system to confine charge carriers

“…1. Its electronic spectrum can be probed using STM 31 , but in contrast to other experiments the electric field of the STM tip is not necessary to confine electrons 19,23 . As a result of the deformation, the inter-layer coupling strength γ 1 is strongly reduced and practically zero inside the blister.…”

“…1. Its electronic spectrum can be probed using STM 31 , but in contrast to other experiments the electric field of the STM tip is not necessary to confine electrons 19,23 . As a result of (b) Schematic representation of a cross section of the GQB depicting the position of the different atoms.…”

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

“…Besides e-beam lithography, scanning probe microscopy-based techniques can also be employed for patterning graphene. For instance, AFM-based local anodic oxidation lithography has been applied on graphene to form nanoribbons, quantum dots or nanorings, by etching, down to 30-nm-wide, insulating trenches [26][27][28] . The precise direct mechanical cutting of graphene by AFM turned out quite difficult, as graphene tends to tear and fold along various directions during the scratching 29 .…”