Energy-dispersive X-ray spectroscopy in a low energy electron microscope.

Energy-Dispersive X-Ray Spectroscopy (EDS) is a technique frequently used in Scanning and Transmission Electron Microscopes to study the elemental composition of a sample. Briefly, high energy electrons of the incident electron beam may ionize an electron from a core shell. The decay of this excited state may result in the emission of a characteristic X-ray photon or Auger-Meitner electron. A solid-state EDS detector captures the X-ray photon and determines its energy. The energy spectrum thus contains information on the elemental make-up of the sample. Low Energy Electron Microscopy (LEEM) typically utilizes incident electrons with energies in the range 0-100 eV, insufficient for the generation of elemental X-rays. In general, LEEM does therefore not allow for elemental characterization of the sample under study. Here we show how relatively simple modifications and additions to the LEEM instrument make in-situ EDS spectroscopy possible, and how high-quality EDS spectra can be obtained, thus enabling elemental analysis in LEEM instruments for the first time.

Extracting transverse electron mean free paths in graphene at low energy.

The LEEM-IV spectra of few-layer graphene show characteristic minima at specific energies, which depend on the number of graphene layers. For the same samples, low-energy TEM (eV-TEM) spectra exhibit transmission maxima at energies corresponding to those of the reflection minima in LEEM. Both features can be understood from interferences of the electron wave function in a purely elastic model. Inelastic scattering processes in turn lead to a finite, energy-dependent inelastic Mean Free Path (MFP) and a lower finesse of the interference features. Here we develop a model that introduces both an elastic and inelastic scattering parameter on the wave-function level, thus reconciling the models considered previously. Fitting to published data, we extract the elastic and inelastic MFP self-consistently and compare these to recent reports.

Gun energy filter for a low energy electron microscope.

In a Low Energy Electron Microscope (LEEM) the sample is illuminated with an electron beam with typical electron landing energies from 0-100 eV. The energy spread of the electron beam is determined by the characteristics of the electron source. For the two most commonly used electron sources, LaB6 and cold field emission W, typical energy spreads ΔE are 0.75 and 0.25 eV at full width half maximum, respectively. Here we present a design for a LEEM gun energy filter, that reduces ΔE to ∼100 meV. Such a filter has been incorporated in the IBM/SPECS AC-LEEM system at IBM. Experimental results are presented and found to be in excellent agreement with expectations.

Imaging moiré deformation and dynamics in twisted bilayer graphene.

In 'magic angle' twisted bilayer graphene (TBG) a flat band forms, yielding correlated insulator behavior and superconductivity. In general, the moiré structure in TBG varies spatially, influencing the overall conductance properties of devices. Hence, to understand the wide variety of phase diagrams observed, a detailed understanding of local variations is needed. Here, we study spatial and temporal variations of the moiré pattern in TBG using aberration-corrected Low Energy Electron Microscopy (AC-LEEM). We find a smaller spatial variation than reported previously. Furthermore, we observe thermal fluctuations corresponding to collective atomic displacements over 70 pm on a timescale of seconds. Remarkably, no untwisting is found up to 600 ∘C. We conclude that thermal annealing can be used to decrease local disorder. Finally, we observe edge dislocations in the underlying atomic lattice, the moiré structure acting as a magnifying glass. These topological defects are anticipated to exhibit unique local electronic properties.

Complementary LEEM and eV-TEM for imaging and spectroscopy.

Transmission electron microscopy at very low energy is a promising way to avoid damaging delicate biological samples with the incident electrons, a known problem in conventional transmission electron microscopy. For imaging in the 0-30 eV range, we added a second electron source to a low energy electron microscopy (LEEM) setup, enabling imaging and spectroscopy in both transmission and reflection mode at nanometer (nm) resolution. The latter is experimentally demonstrated for free-standing graphene. Exemplary eV-TEM micrographs of gold nanoparticles suspended on graphene and of DNA origami rectangles on graphene oxide further establish the capabilities of the technique. The long and short axes of the DNA origami rectangles are discernable even after an hour of illumination with low energy electrons. In combination with recent developments in 2D membranes, allowing for versatile sample preparation, eV-TEM is paving the way to damage-free imaging of biological samples at nm resolution.

Key Role of Very Low Energy Electrons in Tin-Based Molecular Resists for Extreme Ultraviolet Nanolithography.

Extreme ultraviolet (EUV) lithography (13.5 nm) is the newest technology that allows high-throughput fabrication of electronic circuitry in the sub-20 nm scale. It is commonly assumed that low-energy electrons (LEEs) generated in the resist materials by EUV photons are mostly responsible for the solubility switch that leads to nanopattern formation. Yet, reliable quantitative information on this electron-induced process is scarce. In this work, we combine LEE microscopy (LEEM), electron energy loss spectroscopy (EELS), and atomic force microscopy (AFM) to study changes induced by electrons in the 0-40 eV range in thin films of a state-of-the-art molecular organometallic EUV resist known as tin-oxo cage. LEEM-EELS uniquely allows to correct for surface charging and thus to accurately determine the electron landing energy. AFM postexposure analyses revealed that irradiation of the resist with LEEs leads to the densification of the resist layer because of carbon loss. Remarkably, electrons with energies as low as 1.2 eV can induce chemical reactions in the Sn-based resist. Electrons with higher energies are expected to cause electronic excitation or ionization, opening up more pathways to enhanced conversion. However, we do not observe a substantial increase of chemical conversion (densification) with the electron energy increase in the 2-40 eV range. Based on the dose-dependent thickness profiles, a simplified reaction model is proposed where the resist undergoes sequential chemical reactions, first yielding a sparsely cross-linked network and then a more densely cross-linked network. This model allows us to estimate a maximum reaction volume on the initial material of 0.15 nm3 per incident electron in the energy range studied, which means that about 10 LEEs per molecule on average are needed to turn the material insoluble and thus render a pattern. Our observations are consistent with the observed EUV sensitivity of tin-oxo cages.

Quantifying work function differences using low-energy electron microscopy: The case of mixed-terminated strontium titanate.

For many applications, it is important to measure the local work function of a surface with high lateral resolution. Low-energy electron microscopy is regularly employed to this end since it is, in principle, very well suited as it combines high-resolution imaging with high sensitivity to local electrostatic potentials. For surfaces with areas of different work function, however, lateral electrostatic fields inevitably associated with work function discontinuities deflect the low-energy electrons and thereby cause artifacts near these discontinuities. We use ray-tracing simulations to show that these artifacts extend over hundreds of nanometers and cause an overestimation of the true work function difference near the discontinuity by a factor of 1.6 if the standard image analysis methods are used. We demonstrate on a mixed-terminated strontium titanate surface that comparing LEEM data with detailed ray-tracing simulations leads to much a more robust estimate of the work function difference.

Reprint of Low-energy electron potentiometry.

In a lot of systems, charge transport is governed by local features rather than being a global property as suggested by extracting a single resistance value. Consequently, techniques that resolve local structure in the electronic potential are crucial for a detailed understanding of electronic transport in realistic devices. Recently, we have introduced a new potentiometry method based on low-energy electron microscopy (LEEM) that utilizes characteristic features in the reflectivity spectra of layered materials [1]. Performing potentiometry experiments in LEEM has the advantage of being fast, offering a large field of view and the option to zoom in and out easily, and of being non-invasive compared to scanning-probe methods. However, not all materials show clear features in their reflectivity spectra. Here we, therefore, focus on a different version of low-energy electron potentiometry (LEEP) that uses the mirror mode transition, i.e. the drop in electron reflectivity around zero electron landing energy when they start to interact with the sample rather than being reflected in front of it. This transition is universal and sensitive to the local electrostatic surface potential (either workfunction or applied potential). It can consequently be used to perform LEEP experiments on a broader range of material compared to the method described in Ref[1]. We provide a detailed description of the experimental setup and demonstrate LEEP on workfunction-related intrinsic potential variations on the Si(111) surface and for a metal-semiconductor-metal junction with external bias applied. In the latter, we visualize the Schottky effect at the metal-semiconductor interface. Finally, we compare how robust the two LEEP techniques discussed above are against image distortions due to sample inhomogeneities or contamination.

Space charge effects and aberrations on electron pulse compression in a spherical electrostatic capacitor.

The effects of space charge, aberrations and relativity on temporal compression are investigated for a compact spherical electrostatic capacitor (α-SDA). By employing the three-dimensional (3D) field simulation and the 3D space charge model based on numerical General Particle Tracer and SIMION, we map the compression efficiency for a wide range of initial beam size and single-pulse electron number and determine the optimum conditions of electron pulses for the most effective compression. The results demonstrate that both space charge effects and aberrations prevent the compression of electron pulses into the sub-ps region if the electron number and the beam size are not properly optimized. Our results suggest that α-SDA is an effective compression approach for electron pulses under the optimum conditions. It may serve as a potential key component in designing future time-resolved electron sources for electron diffraction and spectroscopy experiments.

Low-energy electron potentiometry.

In a lot of systems, charge transport is governed by local features rather than being a global property as suggested by extracting a single resistance value. Consequently, techniques that resolve local structure in the electronic potential are crucial for a detailed understanding of electronic transport in realistic devices. Recently, we have introduced a new potentiometry method based on low-energy electron microscopy (LEEM) that utilizes characteristic features in the reflectivity spectra of layered materials [1]. Performing potentiometry experiments in LEEM has the advantage of being fast, offering a large field of view and the option to zoom in and out easily, and of being non-invasive compared to scanning-probe methods. However, not all materials show clear features in their reflectivity spectra. Here we, therefore, focus on a different version of low-energy electron potentiometry (LEEP) that uses the mirror mode transition, i.e. the drop in electron reflectivity around zero electron landing energy when they start to interact with the sample rather than being reflected in front of it. This transition is universal and sensitive to the local electrostatic surface potential (either workfunction or applied potential). It can consequently be used to perform LEEP experiments on a broader range of material compared to the method described in Ref[1]. We provide a detailed description of the experimental setup and demonstrate LEEP on workfunction-related intrinsic potential variations on the Si(111) surface and for a metal-semiconductor-metal junction with external bias applied. In the latter, we visualize the Schottky effect at the metal-semiconductor interface. Finally, we compare how robust the two LEEP techniques discussed above are against image distortions due to sample inhomogeneities or contamination.

Quantifying electronic band interactions in van der Waals materials using angle-resolved reflected-electron spectroscopy.

High electron mobility is one of graphene's key properties, exploited for applications and fundamental research alike. Highest mobility values are found in heterostructures of graphene and hexagonal boron nitride, which consequently are widely used. However, surprisingly little is known about the interaction between the electronic states of these layered systems. Rather pragmatically, it is assumed that these do not couple significantly. Here we study the unoccupied band structure of graphite, boron nitride and their heterostructures using angle-resolved reflected-electron spectroscopy. We demonstrate that graphene and boron nitride bands do not interact over a wide energy range, despite their very similar dispersions. The method we use can be generally applied to study interactions in van der Waals systems, that is, artificial stacks of layered materials. With this we can quantitatively understand the 'chemistry of layers' by which novel materials are created via electronic coupling between the layers they are composed of.

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.

Strain-driven mound formation of substrate under epitaxial nanoparticles.

We observe the growth of crystalline SiC nanoparticles on Si(001) at 900 °C using in situ electron microscopy. Following nucleation and growth of the SiC, there is a massive migration of Si, forming a crystalline Si mound underneath each nanoparticle that lifts it 4-5 nm above the initial growth surface. The volume of the Si mounds is roughly five to seven times the volume of the SiC nanoparticles. We propose that relaxation of strain drives the mound formation. This new mechanism for relieving interfacial strain, which involves a dramatic restructuring of the substrate, is in striking contrast to the familiar scenario in which only the deposited material restructures to relieve strain.

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.

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.

Spatially-resolved structure and electronic properties of graphene on polycrystalline Ni.

We have used in situ low-energy electron microscopy (LEEM) to correlate the atomic and electronic structure of graphene films on polycrystalline Ni with nm-scale spatial resolution. Spatially resolved electron scattering measurements show that graphene monolayers formed by carbon segregation do not support the π-plasmon of graphene, indicating strong covalent bonding to the Ni. Graphene bilayers have the Bernal stacking characteristic of graphite and show the expected plasmon loss at 6.5 eV. The experimental results, in agreement with first-principles calculations, show that the π-band structure of free-standing graphene appears only in films with a thickness of at least two layers and demonstrate the sensitivity of the plasmon loss to the electronic structure.

Control of Si nanowire growth by oxygen.

Semiconductor nanowires formed using the vapor-liquid-solid mechanism are routinely grown in many laboratories, but a comprehensive understanding of the key factors affecting wire growth is still lacking. In this paper we show that, under conditions of low disilane pressure and higher temperature, long, untapered Si wires cannot be grown, using Au catalyst, without the presence of oxygen. Exposure to oxygen, even at low levels, reduces the diffusion of Au away from the catalyst droplets. This allows the droplet volumes to remain constant for longer times and therefore permits the growth of untapered wires. This effect is observed for both gas-phase and surface-bound oxygen, so the source of oxygen is unimportant. The control of oxygen exposure during growth provides a new tool for the fabrication of long, uniform-diameter structures, as required for many applications of nanowires.

A photopatternable pentacene precursor for use in organic thin-film transistors.

We demonstrate that modifying pentacene to incorporate an acid-labile moiety into its molecular structure leads to a new precursor that can be easily deposited, photopatterned, and processed via wet-chemical methods to produce organic semiconducting devices exhibiting good electrical characteristics. Acidic conditions produced by ultraviolet illumination of a co-deposited photoacid generator greatly accelerate the local conversion of this N-sulfinyl-tert-butylcarbamate pentacene adduct back to pentacene. Photopatterned thin-film transistors exhibit carrier mobilities in excess of 0.1 cm2 V-1 s-1, making this an attractive precursor for fabrication of large-area organic electronics via solution-phase methods.