Understanding the metal-carbon interface in FePt catalyzed carbon nanotubes.

Any tip functionalization of carbon nanotubes, for which the relative orientation between their (metallic) catalyst particle and the nanotube axis is essential, requires a detailed knowledge of the nature of the internal interface between the particle and the outgrown tube. In the present work, this interface is characterized with atomic precision using state-of-the-art low-voltage aberration-corrected transmission electron microscopy in combination with molecular dynamics simulations for the case of hard-magnetically terminated carbon nanotubes. Our results indicate that the physical principle based upon which the interfacial metal facet is chosen is a reduction of the desorption energy for carbon.

Compositional analysis of mixed-cation-anion III-V semiconductor interfaces using phase retrieval high-resolution transmission electron microscopy.

Employing exit-plane wave function (EPWF) reconstruction in high-resolution transmission electron microscopy (HRTEM), we have developed an approach to atomic scale compositional analysis of III-V semiconductor interfaces, especially suitable for analyzing quaternary heterostructures with intermixing in both cation and anion sub-lattices. Specifically, we use the focal-series reconstruction technique, which retrieves the complex-valued EPWF from a thru-focus series of HRTEM images. A study of interfaces in Al(0.4)Ga(0.6)As-GaAs and In(0.25)Ga(0.75)Sb-InAs heterostructures using focal-series reconstruction shows that change in chemical composition along individual atomic columns across an interface is discernible in the phase image of the reconstructed EPWF. To extract the interface composition profiles along the cation and anion sub-lattices, quantitative analysis of the phase image is performed using factorial analysis of correspondence. This enabled independent quantification of changes in the In-Ga and As-Sb contents across ultra-thin interfacial regions (approximately 0.6 nm wide) with true atomic resolution, in the In(0.25)Ga(0.75)Sb-InAs heterostructure. The validity of the method is demonstrated by analyzing simulated HRTEM images of an InAs-GaSb-InAs model structure with abrupt and graded interfaces. Our approach is general, permitting atomic-level compositional analysis of heterostructures with two species per sub-lattice, hitherto unfeasible with existing HRTEM methods.

A quantitative procedure to probe for compositional inhomogeneities in InxGa1-xN alloys.

The distribution of indium in a GaN/InxGa1-xN/AlyGa1-yN quantum well with x+/-Deltax=0.24+/-0.07 is quantitatively investigated by extraction of displacement fields from lattice images. Simulations accurately describe the measured strain relaxation across a wedge-shaped sample for a sample thickness up to 150nm. The proportionality between indium concentration and resulting lattice constant cx is approximated by cx=0.5185+0.111xnm. In general, it is challenging to discriminate the effects of random alloying against clustering. In InxGa1-xN this is particularly true at low indium concentrations x<0.2. For an accurate quantitative analysis, sample preparation and imaging were developed such that radiation damage can be recognized if present. Further, an analysis of detection limits and knowledge of the sample thickness are crucial for obtaining reproducible results. Data averaging is necessary to reach sufficient precision. Consequently, the size of the indium-rich clusters is poorly known if x is small. Beyond the interest in physical properties of InxGa1-xN alloys, the analysis of strain and its relaxation exemplifies how quantitative analysis is possible at an atomic level and is in excellent agreement with theoretical predictions.

3-D reconstruction of the atomic positions in a simulated gold nanocrystal based on discrete tomography: prospects of atomic resolution electron tomography.

A novel reconstruction procedure is proposed to achieve atomic resolution in electron tomography. The method exploits the fact that crystals are discrete assemblies of atoms (atomicity). This constraint enables us to obtain a three-dimensional (3-D) reconstruction of test structures from less than 10 projections even in the presence of noise and defects. Phase contrast transmission electron microscopy (TEM) images of a gold nanocrystal were simulated in six different zone axes. The discrete number of atoms in every column is determined by application of the channelling theory to reconstructed electron exit waves. The procedure is experimentally validated by experiments with gold samples. Our results show that discrete tomography recovers the shape of the particle as well as the position of its 309 atoms from only three projections. Experiments on a nanocrystal that contains several missing atoms, both on the surface and in the core of the nanocrystal, while considering a high noise level in each simulated image were performed to prove the stability of the approach to reconstruct defects. The algorithm is well capable of handling structural defects in a highly noisy environment, even if this causes atom count "errors" in the projection data.

Electron channelling based crystallography.

Electron channelling occurs when the incident electron beam is parallel to the atom columns of an object, such as a crystal or a particular crystal defect. Then, the electrons are trapped in the electrostatic potential of an atom column in which they scatter dynamically. This picture provides physical insight and explains why a one-to-one correspondence is maintained between the exit wave and the projected structure, even in case of strong dynamical scattering. Moreover, the theory is very useful to invert the dynamical scattering, that is, to derive the projected structure from the exit wave. Finally, it can be used to determine the composition of an atom column with single atom sensitivity or to explain dynamical electron diffraction effects. In this paper, an overview of the channelling theory will be given together with some recent applications.

Prospects for atomic resolution in-line holography for a 3D determination of atomic structures from single projections.

It is now established that the 3D structure of homogeneous nanocrystals can be recovered from in-line hologram of single projections. The method builds on a quantitative contrast interpretation of electron exit wave functions. Since simulated exit wave functions of single and bilayers of graphene reveal the atomic structure of carbon-based materials with sufficient resolution, we explore theoretically how the approach can be expanded beyond periodic carbon-based materials to include non-periodic molecular structures. We show here theoretically that the 3D atomic structure of randomly oriented oleic acid molecules can be recovered from a single projection.

Detecting structural variances of Co3O4 catalysts by controlling beam-induced sample alterations in the vacuum of a transmission electron microscope.

This article summarizes core aspects of beam-sample interactions in research that aims at exploiting the ability to detect single atoms at atomic resolution by mid-voltage transmission electron microscopy. Investigating the atomic structure of catalytic Co3O4 nanocrystals underscores how indispensable it is to rigorously control electron dose rates and total doses to understand native material properties on this scale. We apply in-line holography with variable dose rates to achieve this goal. Genuine object structures can be maintained if dose rates below ~100 e/Å2s are used and the contrast required for detection of single atoms is generated by capturing large image series. Threshold doses for the detection of single atoms are estimated. An increase of electron dose rates and total doses to common values for high resolution imaging of solids stimulates object excitations that restructure surfaces, interfaces, and defects and cause grain reorientation or growth. We observe a variety of previously unknown atom configurations in surface proximity of the Co3O4 spinel structure. These are hidden behind broadened diffraction patterns in reciprocal space but become visible in real space by solving the phase problem. An exposure of the Co3O4 spinel structure to water vapor or other gases induces drastic structure alterations that can be captured in this manner.

In-line three-dimensional holography of nanocrystalline objects at atomic resolution.

Resolution and sensitivity of the latest generation aberration-corrected transmission electron microscopes allow the vast majority of single atoms to be imaged with sub-Ångstrom resolution and their locations determined in an image plane with a precision that exceeds the 1.9-pm wavelength of 300 kV electrons. Such unprecedented performance allows expansion of electron microscopic investigations with atomic resolution into the third dimension. Here we report a general tomographic method to recover the three-dimensional shape of a crystalline particle from high-resolution images of a single projection without the need for sample rotation. The method is compatible with low dose rate electron microscopy, which improves on signal quality, while minimizing electron beam-induced structure modifications even for small particles or surfaces. We apply it to germanium, gold and magnesium oxide particles, and achieve a depth resolution of 1-2 Å, which is smaller than inter-atomic distances.

Nonlinear imaging using annular dark field TEM.

Annular dark field TEM images exhibit a dominant mass-thickness contrast that can be quantified to extract single atom scattering cross sections. On top of this incoherent background, additional lattice fringes appear with a nonlinear information limit of 1.2A at 150 kV. The formation of these fringes is described by coherent nonlinear imaging theory and good agreement is found between experimental and simulated images. Calculations furthermore predict that the use of aberration corrected microscopes will improve the image quality dramatically.

3D reconstruction of nanocrystalline particles from a single projection.

This paper describes an approach to retrieve the three-dimensional atomic structure of a nanocrystalline particle from the reconstructed electron exit wave function in a single projection direction. The method employs wave propagation to determine the local exit surface of each atomic column together with its mass. The exit wave in between colums is used as internal calibration so as to remove the background noise and improve the precision to the level of single atom sensitivity. The validity of the approach is tested with exit wave functions of a gold wedge reconstructed from simulated images containing different levels of noise.

Maintaining the genuine structure of 2D materials and catalytic nanoparticles at atomic resolution.

The recent development of atomic resolution, low dose-rate electron microscopy allows investigating 2D materials as well as catalytic nano particles without compromising their structural integrity. For graphene and a variety of nanoparticle compositions, it is shown that a critical dose rate exists of <100 e(-)/Å(2) s at 80 keV of electron acceleration that allows maintaining the genuine object structures including their surfaces and edges even if particles are only 3 nm large or smaller. Moreover, it is demonstrated that electron beam-induced phonon excitation from outside the field of view contributes to a contrast degradation in recorded images. These degradation effects can be eliminated by delivering electrons onto the imaged area, only, by using a Nilsonian illumination scheme in combination with a suitable aperture at the electron gun/monochromator assembly.

Observing gas-catalyst dynamics at atomic resolution and single-atom sensitivity.

Transmission electron microscopy (TEM) has become an indispensable technique for studying heterogeneous catalysts. In particular, advancements of aberration-corrected electron optics and data acquisition schemes have made TEM capable of delivering images of catalysts with sub-Ångström resolution and single-atom sensitivity. Parallel developments of differentially pumped electron microscopes and of gas cells enable in situ observations of catalysts during the exposure to reactive gas environments at pressures of up to atmospheric levels and temperatures of up to several hundred centigrade. Here, we outline how to take advantage of the emerging state-of-the-art instrumentation and methodologies to study surface structures and dynamics to improve the understanding of structure-sensitive catalytic functionality. The concept of using low electron dose-rates in TEM in conjunction with in-line holography and aberration-correction at low voltage (80 kV) is introduced to allow maintaining atomic resolution and sensitivity during in situ observations of catalysts. Benefits are illustrated by exit wave reconstructions of TEM images of a nanocrystalline Co3O4 catalyst material acquired in situ during their exposure to either a reducing or oxidizing gas environment.

Instrumental requirements for the detection of electron beam-induced object excitations at the single atom level in high-resolution transmission electron microscopy.

This contribution touches on essential requirements for instrument stability and resolution that allows operating advanced electron microscopes at the edge to technological capabilities. They enable the detection of single atoms and their dynamic behavior on a length scale of picometers in real time. It is understood that the observed atom dynamic is intimately linked to the relaxation and thermalization of electron beam-induced sample excitation. Resulting contrast fluctuations are beam current dependent and largely contribute to a contrast mismatch between experiments and theory if not considered. If explored, they open the possibility to study functional behavior of nanocrystals and single molecules at the atomic level in real time.

Imaging columns of the light elements carbon, nitrogen and oxygen with sub Angstrom resolution.

It is reported that lattice imaging with a 300 kV field emission microscope in combination with numerical reconstruction procedures can be used to reach an interpretable resolution of about 80 pm for the first time. A retrieval of the electron exit wave from focal series allows for the resolution of single atomic columns of the light elements carbon, nitrogen, and oxygen at a projected nearest neighbor spacing down to 85 pm. Lens aberrations are corrected on-line during the experiment and by hardware such that resulting image distortions are below 80 pm. Consequently, the imaging can be aberration-free to this extent. The resolution enhancement results from increased electrical and mechanical stability of the instrument coupled with a low spherical aberration coefficient of 0.595 + 0.005 mm.

Sub-Angstrom high-resolution transmission electron microscopy at 300 keV.

Sub-Angstrom transmission electron microscopy has been achieved at the National Center for Electron Microscopy (NCEM) by a one-Angstrom microscope (OAM) project using software and enhanced hardware developed within a Brite-Euram project (Ultramicroscopy 64 (1996) 1). The NCEM OAM provides materials scientists with transmission electron microscopy at a resolution better than 1 A by using extensive image reconstruction to exploit the significantly higher information limit of an FEG-TEM over its Scherzer resolution limit. Reconstruction methods chosen used off-axis holograms and focal series of underfocused images. Measured values of coherence parameters predict an information limit of 0.78 A. Images from a [1 1 0] diamond test specimen show that sub-Angstrom resolution of 0.89 A has been achieved with the OAM using focal series reconstruction.

Recording low and high spatial frequencies in exit wave reconstructions.

Aberration corrected Transmission Electron Microscope (TEM) images can currently resolve information at significantly better than 0.1 nm. Aberration corrected imaging conditions seek to optimize the transfer of high-resolution information but in doing so they prevent the transfer of low spatial frequency information. To recover low spatial frequency information, aberration corrected images must be acquired at a large defocus which compromises high spatial frequency information transfer. In this paper we present two a posteriori solutions to this problem in which the information bandwidth in an exit wave reconstruction is increased. In the first we reconstruct the electron exit wavefunction from two focal series datasets, with different, uniform focal steps, experimentally demonstrating that the width of the transfer interval can be extended from 0.2 nm⁻¹ (∼5 nm) to better than 10 nm⁻¹ (0.1 nm). In the second we outline the use of a focal series recorded with a non-uniform focal step to recover a wider range of spatial frequencies without the need for a large number of images. Using simulated data we show that using this non-uniform focal step the spatial frequency interval for a five image data set may be increased to between 0.25 nm⁻¹ (4 nm) and 8.3 nm⁻¹ (0.12 nm) compared to between 0.74 nm⁻¹ (1.4 nm) and 8.3 nm⁻¹ (0.12 nm) for the standard focal series geometry.

Atomically perfect torn graphene edges and their reversible reconstruction.

The atomic structure of graphene edges is critical in determining the electrical, magnetic and chemical properties of truncated graphene structures, notably nanoribbons. Unfortunately, graphene edges are typically far from ideal and suffer from atomic-scale defects, structural distortion and unintended chemical functionalization, leading to unpredictable properties. Here we report that graphene edges fabricated by electron beam-initiated mechanical rupture or tearing in high vacuum are clean and largely atomically perfect, oriented in either the armchair or zigzag direction. We demonstrate, via aberration-corrected transmission electron microscopy, reversible and extended pentagon-heptagon (5-7) reconstruction at zigzag edges, and explore experimentally and theoretically the dynamics of the transitions between configuration states. Good theoretical-experimental agreement is found for the flipping rates between 5-7 and 6-6 zigzag edge states. Our study demonstrates that simple ripping is remarkably effective in producing atomically clean, ideal terminations, thus providing a valuable tool for realizing atomically tailored graphene and facilitating meaningful experimental study.

Using a monochromator to improve the resolution in TEM to below 0.5Å. Part I: Creating highly coherent monochromated illumination.

Chromatic aberration limits the resolution in spherical-aberration corrected Transmission Electron Microscopy to approximately 0.7Å at 300 kV. The energy spread in the beam is the main contribution to the chromatic aberration. This spread can be reduced with a monochromator. Another limitation to the resolution in TEM can be the finite brightness of the source and the consequent partial spatial coherence of the illumination. This limitation becomes important when spherical aberration and/or defocus are present such as in uncorrected TEM or in focal-series reconstruction in TEM. We used a monochromator optimized for minimum brightness loss and a prototype 'high-brightness' gun, and obtained brightness after monochromation comparable to that of the standard Schottky FEG before monochromation. The images were acquired on the prototype TEAM 0.5 microscope, which was developed on a Titan platform by increasing its electrical and mechanical stability.

Using a monochromator to improve the resolution in TEM to below 0.5 Å. Part II: application to focal series reconstruction.

We apply monochromated illumination to improve the information transfer in focal series reconstruction to 0.5 Å at 300 kV. Contrary to single images, which can be taken arbitrarily close to Gaussian focus in a C(S)-corrected microscope, images in a focal series are taken at a certain defocus. This defocus poses limits on the spatial coherence of the illumination, and through this, limits on the brightness of the monochromated illumination. We derive an estimate for the minimum spatial coherence and the minimal brightness needed for a certain resolution at a certain defocus and apply this estimate to our focal series experiments. We find that the 0.5 Å information transfer would have been difficult and probably impossible to obtain without the exceptionally high brightness of the monochromated illumination.