Arnhild Jacobsen

Selective Chemical Modification of Graphene Surfaces: Distinction Between Single- and Bilayer Graphene

Graphene modifications with oxygen or hydrogen are well known in contrast to carbon attachment to the graphene lattice. The chemical modification of graphene sheets with aromatic diazonium ions (carbon attachment) is analyzed by confocal Raman spectroscopy. The temporal and spatial evolution of surface-adsorbed species allows accurate tracking of the chemical reaction and identification of intermediates. The controlled transformation of sp(2) to sp(3) carbon proceeds in two separate steps. The presented derivatization is faster for single-layer graphene and allows controlled transformation of adsorbed diazonium reagents into covalently bound surface derivatives with enhanced reactivity at the edge of single-layer graphene. On bilayer graphene the derivatization proceeds to an adsorbed intermediate, which reacts slower to a covalently attached species on the carbon surface.

Graphene single-electron transistors

Graphene, a single layer of carbon atoms forming a perfectly stable and clean two-dimensional crystal with very few defects, has been proclaimed to be a new revolutionary material for electronics. These hopes rest mainly on the unique band structure properties of graphene. Although living essentially on the surface, electron mobilities in this material do not suffer extensively from surface contaminations and are surprisingly high even at room temperature. In comparison to extremely high quality semiconducting materials, such as Silicon and GaAs, the understanding of electronic transport in graphene is still in its infancy. Research on nanoscale transistors switching with only a single electron exemplifies that there are a number of unresolved problems that material scientists should tackle in the future for making the graphene dreams come true.

Transport gap in side-gated graphene constrictions

We present measurements on side-gated graphene constrictions of different geometries. We characterize the transport gap by its width in back-gate voltage and compare this to an analysis based on Coulomb blockade measurements of localized states. We study the effect of an applied side-gate voltage on the transport gap and show that high side-gate voltages lift the suppression of the conductance. Finally we study the effect of an applied magnetic field and demonstrate the presence of edge states in the constriction.

Electronic properties of graphene nanostructures

In this review, recent developments in the fabrication and understanding of the electronic properties of graphene nanostructures are discussed. After a brief overview of the structure of graphene and the two-dimensional transport properties, the focus is put on graphene constrictions, quantum dots and double quantum dots. For constrictions with a width below 100 nm, the current through the constriction is strongly suppressed for a certain back gate voltage range, related to the so-called transport gap. This transport gap is due to the formation of localized puddles in the constriction, and its size depends strongly on the constriction width. Such constrictions can be used to confine charge carriers in quantum dots, leading to Coulomb blockade effects.

Transport through graphene quantum dots

We review transport experiments on graphene quantum dots and narrow graphene constrictions. In a quantum dot, electrons are confined in all lateral dimensions, offering the possibility for detailed investigation and controlled manipulation of individual quantum systems. The recently isolated two-dimensional carbon allotrope graphene is an interesting host to study quantum phenomena, due to its novel electronic properties and the expected weak interaction of the electron spin with the material. Graphene quantum dots are fabricated by etching mono-layer flakes into small islands (diameter 60-350 nm) with narrow connections to contacts (width 20-75 nm), serving as tunneling barriers for transport spectroscopy. Electron confinement in graphene quantum dots is observed by measuring Coulomb blockade and transport through excited states, a manifestation of quantum confinement. Measurements in a magnetic field perpendicular to the sample plane allowed to identify the regime with only a few charge carriers in the dot (electron-hole transition), and the crossover to the formation of the graphene specific zero-energy Landau level at high fields. After rotation of the sample into parallel magnetic field orientation, Zeeman spin splitting with a g-factor of g ≈ 2 is measured. The filling sequence of subsequent spin states is similar to what was found in GaAs and related to the non-negligible influence of exchange interactions among the electrons.

Transport through graphene double dots

We present Coulomb blockade measurements in a graphene double dot system. The coupling of the dots to the leads and between the dots can be tuned by graphene in-plane gates. The coupling is a nonmonotonic function of the gate voltage. Using a purely capacitive model, we extract all relevant energy scales of the double dot system.

Energy and transport gaps in etched graphene nanoribbons

We report transport measurements on etched graphene nanoribbons. We show that two distinct voltage (i.e. energy) scales can be experimentally extracted for characterizing the parameter regions of suppressed conductance at low charge density in graphene nanoribbons. The energy scales are related to the charging energy of localized states and to the strength of the disorder potential. We discuss the scaling behaviour of these two energy scales as a function of the minimum width w for a number of different devices. Finally, we present a model based on Coulomb blockade, due to quantum dots forming inside the nanoribbon, explaining the observed energy scales.

Rectification in three-terminal graphene junctions

Nonlinear electrical properties of graphene-based three-terminal nanojunctions are presented. Intrinsic rectification of voltage is observed up to room temperature. The sign and the efficiency of the rectification can be tuned by a gate. Changing the charge carrier type from holes to electrons results in a change in the rectification sign. At a bias <20 mV and at a temperature below 4.2 K the sign and the efficiency of the rectification are governed by universal conductance fluctuations.

Observation of excited states in a graphene double quantum dot

We study a graphene double quantum dot in different coupling regimes. Despite the strong capacitive coupling between the dots, the tunnel coupling is below the experimental resolution. We observe additional structures inside the finite-bias triangles, part of which can be attributed to electronic excited dot states, while others are probably due to modulations of the transmission of the tunnel barriers connecting the system to source and drain leads.

Chemical modification of graphene characterized by Raman and transport experiments

A chemical approach to modify the electronic transport of graphene is investigated by detailed transport and Raman spectroscopy measurements on Hall bar shaped samples. The functionalization of graphene with nitrobenzene diazonium ions results in a strong p-doping of the graphene samples and only slightly lower mobilities. Comparing Raman and transport data taken after each functionalization step allowed the conclusion that two preferential reactions take place on the graphene surface. In the beginning a few nitrobenzene molecules are directly attached to the graphene atoms creating defects. Afterwards these act as seeds for a polymer like growth not directly connected to the graphene atoms. The effects of solvents were excluded by thorough control measurements.