D. Graf

Raman imaging of doping domains in graphene on SiO2

We present spatially resolved Raman images of the G and 2D lines of single-layer graphene flakes. The spatial fluctuations of G and 2D lines are correlated and are thus shown to be affiliated with local doping domains. We investigate the position of the 2D line—the most significant Raman peak to identify single-layer graphene—as a function of charging up to ∣n∣≈4×1012cm−2. Contrary to the G line which exhibits a strong and symmetric stiffening with respect to electron and hole doping, the 2D line shows a weak and slightly asymmetric stiffening for low doping. Additionally, the linewidth of the 2D line is, in contrast to the G line, doping independent making this quantity a reliable measure for identifying single-layer graphene.

Tunable Coulomb blockade in nanostructured graphene

We report on Coulomb blockade and Coulomb diamond measurements on an etched, tunable single-layer graphene quantum dot. The device consisting of a graphene island connected via two narrow graphene constrictions is fully tunable by three lateral graphene gates. Coulomb blockade resonances are observed and from Coulomb diamond measurements, a charging energy of ≈3.5meV is extracted. For increasing temperatures, we detect a peak broadening and a transmission increase of the nanostructured graphene barriers.

Local gating of a graphene Hall bar by graphene side gates

We have investigated the magnetotransport properties of a single-layer graphene Hall bar with additional graphene side gates. The side gating in the absence of a magnetic field can be modeled by considering two parallel conducting channels within the Hall bar. This results in an average penetration depth of the side gate created field of approx. 90 nm. The side gates are also effective in the quantum Hall regime, and allow to modify the longitudinal and Hall resistances.

Local oxidation of Ga[Al]As heterostructures with modulated tip-sample voltages

Nanolithography based on local oxidation with a scanning force microscope has been performed on an undoped GaAs wafer and a Ga[Al]As heterostructure with an undoped GaAs cap layer and a shallow two-dimensional electron gas. The oxide growth and the resulting electronic properties of the patterned structures are compared for the constant and modulated voltages applied to the conductive tip of the scanning force microscope. All the lithography has been performed in noncontact mode. Modulating the applied voltage enhances the aspect ratio of the oxide lines, which significantly strengthens the insulating properties of the lines on GaAs. In addition, the oxidation process is found to be more reliable and reproducible. Using this technique, a quantum point contact and a quantum wire have been defined and the electronic stability, the confinement potential and the electrical tunability are demonstrated to be similar to the oxidation with constant voltage.

Phase-coherent transport measured in a side-gated mesoscopic graphite wire

We investigate the magnetotransport properties of a thin graphite wire resting on a silicon oxide substrate. The electric field effect is demonstrated with back and side gate electrodes. We study the conductance fluctuations as a function of gate voltage, magnetic field, and temperature. The phase coherence length extracted from weak localization is larger than the wire width even at the lowest carrier densities, making the system effectively one dimensional. We find that the phase coherence length increases linearly with the conductivity, suggesting that at

Multi-terminal transport through a quantum dot in the Coulomb-blockade regime

1.7\phantom{\rule{0.3em}{0ex}}\mathrm{K}

Coulomb oscillations in three-layer graphene nanostructures

dephasing originates mainly from electron-electron interactions.

Quantum dot with internal substructure

Three-terminal tunnelling experiments on quantum dots in the Coulomb-blockade regime allow a quantitative determination of the coupling strength of individual quantum states to the leads. Exploiting this insight, we have observed independent fluctuations of the coupling strengths as a function of electron number and magnetic field due to changes in the shape of the wave function in the dot. Such a detailed understanding and control of the dot-lead coupling can be extended to more complex systems such as coupled dots, and are essential for building functional quantum electronic systems.

Multi‐terminal transport through semi‐conductor quantum dots

We present transport measurements on a tunable three-layer graphene single electron transistor (SET). The device consists of an etched three-layer graphene flake with two narrow constrictions separating the island from source and drain contacts. Three lateral graphene gates are used to electrostatically tune the device. An individual three-layer graphene constriction has been investigated separately showing a transport gap near the charge neutrality point. The graphene tunneling barriers show a strongly nonmonotonic coupling as a function of gate voltage indicating the presence of localized states in the constrictions. We show Coulomb oscillations and Coulomb diamond measurements proving the functionality of the graphene SET. A charging energy of 0.6meV is extracted.

Spatially Resolved Raman Spectroscopy of Single- and Few-Layer Graphene

We report the fabrication and measurement of a finite‐periode lateral superlattice within a Coulomb blockaded island, thereby creating a quantum dot with a periodic internal substructure. The potential modulation introduces an additional length scale to the quantum dot state, which can be probed with a perpendicular magnetic field.