Alberto F. Morpurgo

Gate-induced insulating state in bilayer graphene devices

The potential of graphene-based materials consisting of one or a few layers of graphite for integrated electronics originates from the large room-temperature carrier mobility in these systems (approximately 10,000 cm2 V(-1) s(-1)). However, the realization of electronic devices such as field-effect transistors will require controlling and even switching off the electrical conductivity by means of gate electrodes, which is made difficult by the absence of a bandgap in the intrinsic material. Here, we demonstrate the controlled induction of an insulating state--with large suppression of the conductivity--in bilayer graphene, by using a double-gate device configuration that enables an electric field to be applied perpendicular to the plane. The dependence of the resistance on temperature and electric field, and the absence of any effect in a single-layer device, strongly suggest that the gate-induced insulating state originates from the recently predicted opening of a bandgap between valence and conduction bands.

Science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems

We present the science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems, targeting an evolution in technology, that might lead to impacts and benefits reaching into most areas of society. This roadmap was developed within the framework of the European Graphene Flagship and outlines the main targets and research areas as best understood at the start of this ambitious project. We provide an overview of the key aspects of graphene and related materials (GRMs), ranging from fundamental research challenges to a variety of applications in a large number of sectors, highlighting the steps necessary to take GRMs from a state of raw potential to a point where they might revolutionize multiple industries. We also define an extensive list of acronyms in an effort to standardize the nomenclature in this emerging field.

Bipolar supercurrent in graphene

Graphene—a recently discovered form of graphite only one atomic layer thick—constitutes a new model system in condensed matter physics, because it is the first material in which charge carriers behave as massless chiral relativistic particles. The anomalous quantization of the Hall conductance, which is now understood theoretically, is one of the experimental signatures of the peculiar transport properties of relativistic electrons in graphene. Other unusual phenomena, like the finite conductivity of order 4e2/h (where e is the electron charge and h is Planck’s constant) at the charge neutrality (or Dirac) point, have come as a surprise and remain to be explained. Here we experimentally study the Josephson effect in mesoscopic junctions consisting of a graphene layer contacted by two closely spaced superconducting electrodes. The charge density in the graphene layer can be controlled by means of a gate electrode. We observe a supercurrent that, depending on the gate voltage, is carried by either electrons in the conduction band or by holes in the valence band. More importantly, we find that not only the normal state conductance of graphene is finite, but also a finite supercurrent can flow at zero charge density. Our observations shed light on the special role of time reversal symmetry in graphene, and demonstrate phase coherent electronic transport at the Dirac point.

Organic single-crystal field-effect transistors

Organic electronics constitute an innovative field, with interesting applications complementary to the silicon semiconductor technology. From a scientific perspective, there is large interest in the fundamental understanding of electrical transport in organic semiconductors. However, a well-developed microscopic description is still lacking, due to the complicated character of the many-body polaronic-type of charge carriers in molecular compounds. In this Thesis, we have experimentally studied the intrinsic charge transport properties of organic semiconductors by using organic single-crystal field-effect transistors. The electric field-effect has been frequently used to investigate thin films of organic compounds. Unfortunately, thin-film transistors are not suitable for the study of intrinsic electronic properties of organic conductors, because their characteristics are often strongly affected by imperfections of the film structure and by insufficient purity of organic materials. Thus, for a higher degree of molecular ordering and an improved quality of the FET, we fabricate devices on the surface of a free-standing single crystal of organic molecules. In short, in this work we have achieved successful fabrication of high-quality single-crystal FETs, exhibiting high mobilities and signs of intrinsic transport. Herewith, we have identified new aspects that influence charge transport in organic semiconductor FETs, and we have performed exploratory measurements in the charge density regime approaching one carrier per molecule.

Tunable Fröhlich polarons in organic single-crystal transistors

In organic field-effect transistors (FETs), charges move near the surface of an organic semiconductor, at the interface with a dielectric. In the past, the nature of the microscopic motion of charge carriers—which determines the device performance—has been related to the quality of the organic semiconductor. Recently, it was discovered that the nearby dielectric also has an unexpectedly strong influence. The mechanisms responsible for this influence are not understood. To investigate these mechanisms, we have studied transport through organic single-crystal FETs with different gate insulators. We find that the temperature dependence of the mobility evolves from metallic-like to insulating-like with increasing dielectric constant of the insulator. The phenomenon is accounted for by a two-dimensional Fröhlich polaron model that quantitatively describes our observations and shows that increasing the dielectric polarizability results in a crossover from the weak to the strong polaronic coupling regime. This represents a considerable step forward in our understanding of transport through organic transistors, and identifies a microscopic physical process with a large influence on device performance.

Controlled fabrication of metallic electrodes with atomic separation

We report a technique for fabricating metallic electrodes on insulating substrates with separations on the 1 nm scale. The fabrication technique, which combines lithographic and electrochemical methods, provides atomic resolution without requiring sophisticated instrumentation. The process is simple, controllable, reversible, and robust, allowing rapid fabrication of electrode pairs with high yield. We expect the method to prove useful in interfacing molecular-scale structures to macroscopic probes and electronic devices.

Quantitative Determination of the Band Gap of WS2 with Ambipolar Ionic Liquid-Gated Transistors

We realized ambipolar field-effect transistors by coupling exfoliated thin flakes of tungsten disulfide (WS(2)) with an ionic liquid dielectric. The devices show ideal electrical characteristics, including very steep subthreshold slopes for both electrons and holes and extremely low OFF-state currents. Thanks to these ideal characteristics, we determine with high precision the size of the band gap of WS(2) directly from the gate-voltage dependence of the source-drain current. Our results demonstrate how a careful use of ionic liquid dielectrics offers a powerful strategy to study quantitatively the electronic properties of nanoscale materials.

Metallic conduction at organic charge-transfer interfaces

The electronic properties of interfaces between two different solids can differ strikingly from those of the constituent materials. For instance, metallic conductivity-and even superconductivity-have recently been discovered at interfaces formed by insulating transition-metal oxides. Here, we investigate interfaces between crystals of conjugated organic molecules, which are large-gap undoped semiconductors, that is, essentially insulators. We find that highly conducting interfaces can be realized with resistivity ranging from 1 to 30 kohms per square, and that, for the best samples, the temperature dependence of the conductivity is metallic. The observed electrical conduction originates from a large transfer of charge between the two crystals that takes place at the interface, on a molecular scale. As the interface assembly process is simple and can be applied to crystals of virtually any conjugated molecule, the conducting interfaces described here represent the first examples of a new class of electronic systems.

Integrated nanotube circuits: Controlled growth and ohmic contacting of single-walled carbon nanotubes

Single-walled carbon nanotubes are synthesized by chemical vapor deposition of methane at controlled locations on a substrate using patterned catalytic islands. The combined synthesis and microfabrication technique presented here allows a large number of ohmically contacted nanotube devices with controllable length to be placed on a single substrate. Transport studies demonstrate ohmic contacting, giving two-terminal resistances as low as 20 kΩ at low temperatures.

Field-effect transistors on tetracene single crystals

We report on the fabrication and electrical characterization of field-effect transistors at the surface of tetracene single crystals. We find that the mobility of these transistors reaches the room-temperature value of 0.4?cm2/V?s. The nonmonotonous temperature dependence of the mobility, its weak gate voltage dependence, as well as the sharpness of the subthreshold slope, confirm the high quality of single-crystal devices. This is due to the fabrication process that does not substantially affect the crystal quality.