Rudolf M. Tromp

Large-Area Graphene Single Crystals Grown by Low-Pressure Chemical Vapor Deposition of Methane on Copper

Graphene single crystals with dimensions of up to 0.5 mm on a side were grown by low-pressure chemical vapor deposition in copper-foil enclosures using methane as a precursor. Low-energy electron microscopy analysis showed that the large graphene domains had a single crystallographic orientation, with an occasional domain having two orientations. Raman spectroscopy revealed the graphene single crystals to be uniform monolayers with a low D-band intensity. The electron mobility of graphene films extracted from field-effect transistor measurements was found to be higher than 4000 cm(2) V(-1) s(-1) at room temperature.

Atomic-scale transport in epitaxial graphene

The high carrier mobility of graphene is key to its applications, and understanding the factors that limit mobility is essential for future devices. Yet, despite significant progress, mobilities in excess of the 2×10(5) cm(2) V(-1) s(-1) demonstrated in free-standing graphene films have not been duplicated in conventional graphene devices fabricated on substrates. Understanding the origins of this degradation is perhaps the main challenge facing graphene device research. Experiments that probe carrier scattering in devices are often indirect, relying on the predictions of a specific model for scattering, such as random charged impurities in the substrate. Here, we describe model-independent, atomic-scale transport measurements that show that scattering at two key defects--surface steps and changes in layer thickness--seriously degrades transport in epitaxial graphene films on SiC. These measurements demonstrate the strong impact of atomic-scale substrate features on graphene performance.

A new two-dimensional particle detector for a toroidal electrostatic analyzer

We describe a new two‐dimensional detector for the detection of ions scattered from a solid target, analyzed in energy and scattering angle by a toroidal electrostatic analyzer. The detector resolves the scattering angle with a resolution of 0.4° over a range of 25°, and the ion energy with a resolution of 120 eV over a range of 2000 eV, at 100 keV ion energy. The energy resolution of the spectrometer was improved with a factor 4 relative to its previous performance with a one‐dimensional scattering angle detector, while−at the same time−the dose efficiency (count/μC) was improved by a factor 5–10.

Low-energy electron microscopy

Low-energy electron microscopy (LEEM) is a relatively new microscopy technique, capable of high-resolution (5 nm) video-rate imaging of surfaces and interfaces. This opens up the possibility of studying dynamic processes at surfaces, such as thin-film growth, strain relief, etching and adsorption, and phase transitions in real time, in situ, as they occur. The resulting video movies contain an unprecedented amount of information that is amenable to detailed, quantitative analysis. In this paper we discuss the principles of LEEM and its application to problems in science and technology.

Elastic recoil detection for medium‐energy ion scattering

Medium‐energy ion scattering (MEIS) has been successfully applied for many years as a technique for structural analysis of solids. Advantages over competing techniques include superb depth resolution (5–10 A), quantitative information (well‐known cross sections), and ease of interpretation. A weakness of the technique is the lack of sensitivity to light elements. We have adapted the technique to detect light elements by elastic recoil detection analysis (ERDA). This has been used to analyze samples containing hydrogen and boron, with depth resolution of ≊10 A, comparable to conventional MEIS. This is an order of magnitude improvement over conventional ERDA.

Nanoscale measurements of unoccupied band dispersion in few-layer graphene.

The properties of any material are fundamentally determined by its electronic band structure. Each band represents a series of allowed states inside a material, relating electron energy and momentum. The occupied bands, that is, the filled electron states below the Fermi level, can be routinely measured. However, it is remarkably difficult to characterize the empty part of the band structure experimentally. Here, we present direct measurements of unoccupied bands of monolayer, bilayer and trilayer graphene. To obtain these, we introduce a technique based on low-energy electron microscopy. It relies on the dependence of the electron reflectivity on incidence angle and energy and has a spatial resolution ∼10 nm. The method can be easily applied to other nanomaterials such as van der Waals structures that are available in small crystals only.

Low-Energy Electron Potentiometry: Contactless Imaging of Charge Transport on the Nanoscale

Charge transport measurements form an essential tool in condensed matter physics. The usual approach is to contact a sample by two or four probes, measure the resistance and derive the resistivity, assuming homogeneity within the sample. A more thorough understanding, however, requires knowledge of local resistivity variations. Spatially resolved information is particularly important when studying novel materials like topological insulators, where the current is localized at the edges, or quasi-two-dimensional (2D) systems, where small-scale variations can determine global properties. Here, we demonstrate a new method to determine spatially-resolved voltage maps of current-carrying samples. This technique is based on low-energy electron microscopy (LEEM) and is therefore quick and non-invasive. It makes use of resonance-induced contrast, which strongly depends on the local potential. We demonstrate our method using single to triple layer graphene. However, it is straightforwardly extendable to other quasi-2D systems, most prominently to the upcoming class of layered van der Waals materials.

Temporal and lateral electron pulse compression by a compact spherical electrostatic capacitor

A novel solution for high intensity electron pulse compression in both space and time is proposed in this paper. Based on the unique properties of the central-force electrostatic field of a spherical electrostatic capacitor, the newly developed α-Spherical Deflector Analyzer (α-SDA) with 2π total deflection is utilized for the practical realization of femtosecond electron pulse compression. The mirror symmetry of the system at π deflection causes not only the cancellation of the geometrical and chromatic aberrations at 2π, but also leads to aberration-free time reversal of the electron pulse in the exit plane. As a consequence, the time-divergent electrons at the input are transformed to a time-convergent pulse at the output. In the symmetric case with the first time compression exactly at π, the shortest electron pulse behind the α-SDA analyzer is a mirror symmetric to the original electron pulse at the photocathode. It results in extremely short final electron pulses that are limited only by the duration of the laser pulse, the emittance of the electron bunch, and by imperfections of the real system.

Highly uniform step and terrace structures on SiC(0001) surfaces

Highly uniform step and termination structures on 4H- and 6H-SiC(0001) surfaces have been prepared via moderate annealing in disilane. Atomic force microscopy and dark-field low-energy electron microscopy imaging indicate single-phase terminations separated solely by half-unit-cell-height steps, driven by stacking fault energy. The atomic structure of 4H-SiC(0001)-√3 × √3R30°-Si has been determined quantitatively by nanospot low-energy electron diffraction. The topmost stacking fault at the 4H surface has been found to be between the second and third bilayers.

eV-TEM: Transmission electron microscopy in a low energy cathode lens instrument

We are developing a transmission electron microscope that operates at extremely low electron energies, 0-40 eV. We call this technique eV-TEM. Its feasibility is based on the fact that at very low electron energies the number of energy loss pathways decreases. Hence, the electron inelastic mean free path increases dramatically. eV-TEM will enable us to study elastic and inelastic interactions of electrons with thin samples. With the recent development of aberration correction in cathode lens instruments, a spatial resolution of a few nm appears within range, even for these very low electron energies. Such resolution will be highly relevant to study biological samples such as proteins and cell membranes. The low electron energies minimize adverse effects due to radiation damage.