Christoph Stampfer

Energy Gaps in Etched Graphene Nanoribbons

Transport measurements on an etched graphene nanoribbon are presented. It is shown that two distinct voltage scales can be experimentally extracted that characterize the parameter region of suppressed conductance at low charge density in the ribbon. One of them is related to the charging energy of localized states, the other to the strength of the disorder potential. The lever arms of gates vary by up to 30% for different localized states which must therefore be spread in position along the ribbon. A single-electron transistor is used to prove the addition of individual electrons to the localized states. In our sample the characteristic charging energy is of the order of 10 meV, the characteristic strength of the disorder potential of the order of 100 meV.

Ultrahigh-mobility graphene devices from chemical vapor deposition on reusable copper

A novel dry transfer technique opens the door to large-scale CVD graphene with carrier mobilities of up to several 100,000 cm2 V−1 s−1. Graphene research has prospered impressively in the past few years, and promising applications such as high-frequency transistors, magnetic field sensors, and flexible optoelectronics are just waiting for a scalable and cost-efficient fabrication technology to produce high-mobility graphene. Although significant progress has been made in chemical vapor deposition (CVD) and epitaxial growth of graphene, the carrier mobility obtained with these techniques is still significantly lower than what is achieved using exfoliated graphene. We show that the quality of CVD-grown graphene depends critically on the used transfer process, and we report on an advanced transfer technique that allows both reusing the copper substrate of the CVD growth and making devices with mobilities as high as 350,000 cm2 V–1 s–1, thus rivaling exfoliated graphene.

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.

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.

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.

Raman spectroscopy as probe of nanometre-scale strain variations in graphene.

Confocal Raman spectroscopy has emerged as a major, versatile workhorse for the non-invasive characterization of graphene. Although it is successfully used to determine the number of layers, the quality of edges, and the effects of strain, doping and disorder, the nature of the experimentally observed broadening of the most prominent Raman 2D line has remained unclear. Here we show that the observed 2D line width contains valuable information on strain variations in graphene on length scales far below the laser spot size, that is, on the nanometre-scale. This finding is highly relevant as it has been shown recently that such nanometre-scaled strain variations limit the carrier mobility in high-quality graphene devices. Consequently, the 2D line width is a good and easily accessible quantity for classifying the crystalline quality, nanometre-scale flatness as well as local electronic properties of graphene, all important for future scientific and industrial applications.

Franck–Condon blockade in suspended carbon nanotube quantum dots

Understanding the influence of vibrational motion of the atoms on electronic transitions in molecules constitutes a cornerstone of quantum physics, as epitomized by the Franck–Condon principle1, 2 of spectroscopy. Recent advances in building molecular-electronics devices3 and nanoelectromechanical systems4 open a new arena for studying the interaction between mechanical and electronic degrees of freedom in transport at the single-molecule level. The tunnelling of electrons through molecules or suspended quantum dots5, 6 has been shown to excite vibrational modes, or vibrons6, 7, 8, 9. Beyond this effect, theory predicts that strong electron–vibron coupling strongly suppresses the current flow at low biases, a collective behaviour known as Franck–Condon blockade10, 11. Here, we show measurements on quantum dots formed in suspended single-wall carbon nanotubes revealing a remarkably large electron–vibron coupling that, owing to the high quality and unprecedented tunability of our samples, allow a quantitative analysis of vibron-mediated electronic transport in the regime of strong electron–vibron coupling. This enables us to unambiguously demonstrate the Franck–Condon blockade in a suspended nanostructure. The large observed electron–vibron coupling could ultimately be a key ingredient for the detection of quantized mechanical motion12, 13. It also emphasizes the unique potential for nanoelectromechanical device applications based on suspended graphene sheets and carbon nanotubes.

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.

Observation of excited states in a graphene quantum dot

We demonstrate that excited states in single-layer graphene quantum dots can be detected via direct transport experiments. Coulomb diamond measurements show distinct features of sequential tunneling through an excited state. Moreover, the onset of inelastic cotunneling in the diamond region could be detected. For low magnetic fields, the positions of the single-particle energy levels fluctuate on the scale of a flux quantum penetrating the dot area. For higher magnetic fields, the transition to the formation of Landau levels is observed. Estimates based on the linear energy-momentum relation of graphene give carrier numbers of the order of 10 for our device.