Walt A. de Heer

Electronic Confinement and Coherence in Patterned Epitaxial Graphene

Ultrathin epitaxial graphite was grown on single-crystal silicon carbide by vacuum graphitization. The material can be patterned using standard nanolithography methods. The transport properties, which are closely related to those of carbon nanotubes, are dominated by the single epitaxial graphene layer at the silicon carbide interface and reveal the Dirac nature of the charge carriers. Patterned structures show quantum confinement of electrons and phase coherence lengths beyond 1 micrometer at 4 kelvin, with mobilities exceeding 2.5 square meters per volt-second. All-graphene electronically coherent devices and device architectures are envisaged.

A Carbon Nanotube Field-Emission Electron Source

A high-intensity electron gun based on field emission from a film of aligned carbon nanotubes has been made. The gun consists of a nanotube film with a 1-millimeter-diameter grid about 20 micrometers above it. Field-emission current densities of about 0.1 milliampere per square centimeter were observed for applied voltages as low as 200 volts, and current densities greater than 100 milliamperes per square centimeter have been realized at 700 volts. The gun is air-stable, easy and inexpensive to fabricate, and functions stably and reliably for long times (short-term fluctuations are on the order of 10 percent). The entire gun is only about 0.2 millimeter thick and can be produced with virtually no restrictions on its area, from less than 1 square millimeter to hundreds of square centimeters, making it suitable for flat panel display applications.

Magnetism from the atom to the bulk in iron, cobalt, and nickel clusters

Molecular beam deflection measurements of small iron, cobalt, and nickel clusters show how magnetism develops as the cluster size is increased from several tens to several hundreds of atoms for temperatures between 80 and 1000 K. Ferromagnetism occurs even for the smallest sizes: for clusters with fewer than about 30 atoms the magnetic moments are atomlike; as the size is increased up to 700 atoms, the magnetic moments approach the bulk limit, with oscillations probably caused by surface-induced spin-density waves. The trends are explained in a magnetic shell model. A crystallographic phase transition from high moment to low moment in iron clusters has also been identified.

Chemical Modification of Epitaxial Graphene: Spontaneous Grafting of Aryl Groups

The addition of nitrophenyl groups to the surface of few-layer epitaxial graphene (EG) by the formation of covalent carbon-carbon bonds changed the electronic structure and transport properties of the EG from near-metallic to semiconducting.

Nanoscale Tunable Reduction of Graphene Oxide for Graphene Electronics

Writing Conductive Lines with Hot Tips The interface within devices between conductors, semiconductors, and insulators is usually created by stacking patterned layers of different materials. For flexible electronics, it can be advantageous to avoid this architectural constraint. Graphene oxide, formed by chemical exfoliation of graphite, can be reduced to a more conductive form using chemical reductants. Wei et al. (p. 1373) now show that layers of graphene oxide can also be reduced using a hot atomic force microscope tip to create materials comparable to those of organic conductors. This process can create patterned regions (down to 12 nanometers in width) that differ in conductivity by up to four orders of magnitude. Conducting regions can be drawn on graphene oxide sheets with a heated atomic force microscope tip. The reduced form of graphene oxide (GO) is an attractive alternative to graphene for producing large-scale flexible conductors and for creating devices that require an electronic gap. We report on a means to tune the topographical and electrical properties of reduced GO (rGO) with nanoscopic resolution by local thermal reduction of GO with a heated atomic force microscope tip. The rGO regions are up to four orders of magnitude more conductive than pristine GO. No sign of tip wear or sample tearing was observed. Variably conductive nanoribbons with dimensions down to 12 nanometers could be produced in oxidized epitaxial graphene films in a single step that is clean, rapid, and reliable.

Room-temperature metastability of multilayer graphene oxide films.

Graphene oxide potentially has multiple applications. The chemistry of graphene oxide and its response to external stimuli such as temperature and light are not well understood and only approximately controlled. This understanding is crucial to enable future applications of this material. Here, a combined experimental and density functional theory study shows that multilayer graphene oxide produced by oxidizing epitaxial graphene through the Hummers method is a metastable material whose structure and chemistry evolve at room temperature with a characteristic relaxation time of about one month. At the quasi-equilibrium, graphene oxide reaches a nearly stable reduced O/C ratio, and exhibits a structure deprived of epoxide groups and enriched in hydroxyl groups. Our calculations show that the structural and chemical changes are driven by the availability of hydrogen in the oxidized graphitic sheets, which favours the reduction of epoxide groups and the formation of water molecules.

Carbon onions produced by heat treatment of carbon soot and their relation to the 217.5 nm interstellar absorption feature

Abstract A new material containing macroscopic quantities of hollow nanometric carbon onions with from 2 to about 8 graphitic shells, with outer diameters ranging from 3 to 10 nm (characterized by high-resolution electron microscopy), is produced by heat treatment of pure carbon soot. Other structures (i.e. nanotubes) are also formed. Ultraviolet spectra of suspensions of this material in water reveal an absorption band with a peak absorption at 264 nm ± 3 nm and with widths from 1.4 to 2.0 μm −1 . Several features of the absorption spectra closely resemble the interstellar absorption band at 217 nm.

Large area and structured epitaxial graphene produced by confinement controlled sublimation of silicon carbide

After the pioneering investigations into graphene-based electronics at Georgia Tech, great strides have been made developing epitaxial graphene on silicon carbide (EG) as a new electronic material. EG has not only demonstrated its potential for large scale applications, it also has become an important material for fundamental two-dimensional electron gas physics. It was long known that graphene mono and multilayers grow on SiC crystals at high temperatures in ultrahigh vacuum. At these temperatures, silicon sublimes from the surface and the carbon rich surface layer transforms to graphene. However the quality of the graphene produced in ultrahigh vacuum is poor due to the high sublimation rates at relatively low temperatures. The Georgia Tech team developed growth methods involving encapsulating the SiC crystals in graphite enclosures, thereby sequestering the evaporated silicon and bringing growth process closer to equilibrium. In this confinement controlled sublimation (CCS) process, very high-quality graphene is grown on both polar faces of the SiC crystals. Since 2003, over 50 publications used CCS grown graphene, where it is known as the “furnace grown” graphene. Graphene multilayers grown on the carbon-terminated face of SiC, using the CCS method, were shown to consist of decoupled high mobility graphene layers. The CCS method is now applied on structured silicon carbide surfaces to produce high mobility nano-patterned graphene structures thereby demonstrating that EG is a viable contender for next-generation electronics. Here we present for the first time the CCS method that outperforms other epitaxial graphene production methods.

Observing the quantization of zero mass carriers in graphene.

Resolving Landau Levels in Graphene The charge carriers in a two-dimensional conductor, when placed in a magnetic field, can develop an additional set of quantized energy levels. These Landau levels correspond to the carriers now moving in cyclotron orbits. In graphene, which consists of single-atom-thick sheets of graphite, an unusual set of Landau levels with nonequal energy spacing can develop in graphene layers that have undergone symmetry breaking caused by rotation between adjacent layers. Miller et al. (p. 924) used scanning tunneling microscopy at cryogenic temperatures to map out Landau levels in graphene grown on silicon carbide with high energy and momentum resolution, including the characteristic level in graphene that can occur at zero energy. Scanning tunneling microscopy on graphene reveals non-equally spaced Landau energy levels induced by a magnetic field. Application of a magnetic field to conductors causes the charge carriers to circulate in cyclotron orbits with quantized energies called Landau levels (LLs). These are equally spaced in normal metals and two-dimensional electron gases. In graphene, however, the charge carrier velocity is independent of their energy (like massless photons). Consequently, the LL energies are not equally spaced and include a characteristic zero-energy state (the n = 0 LL). With the use of scanning tunneling spectroscopy of graphene grown on silicon carbide, we directly observed the discrete, non-equally–spaced energy-level spectrum of LLs, including the hallmark zero-energy state of graphene. We also detected characteristic magneto-oscillations in the tunneling conductance and mapped the electrostatic potential of graphene by measuring spatial variations in the energy of the n = 0 LL.

Electronic Shell Structure and Metal Clusters

Publisher Summary The chapter presents a study on electronic shell structure and metal clusters. Unlike the “macroscopic” atoms, the metallic clusters described from both experimental and theoretical viewpoints in this chapter come far closer to having bulk properties, because they are 10–100 times bigger. The science of clusters is a rapidly growing interdisciplinary field with great promise for the production of new ideas and physical systems. Its ideas are relevant to related problems in the physics of atoms, molecules, condensed matter, and transitions among these systems. The chapter presents a review emphasizing on the electronic shell model, which is elegant and simple, avoids the complexity of elaborate quantum chemical computer calculations for clusters containing large numbers of atoms, and possesses the power of predictability. The study of the physical properties of states intermediate between the atom and the solid is called cluster physics. Lacking a precise definition, it is said that a cluster is a stable group of a few or a few hundred identical atoms or molecules. This chapter mainly discusses metal clusters. Atomic theory depends on the application of angular momentum conditions in the Coulomb field.