C. J. D. Hetherington

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-Ångstrom 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.

Low-dose aberration corrected cryo-electron microscopy of organic specimens

Spherical aberration (C(s)) correction in the transmission electron microscope has enabled sub-angstrom resolution imaging of inorganic materials. To achieve similar resolution for radiation-sensitive organic materials requires the microscope to be operated under hybrid conditions: low electron dose illumination of the specimen at liquid nitrogen temperature and low defocus values. Initial images from standard inorganic and organic test specimens have indicated that under these conditions C(s)-correction can provide a significant improvement in resolution (to less than 0.16nm) for direct imaging of organic samples.

Exploring the fundamental effects of deposition time on the microstructure of graphene nanoflakes by Raman scattering and X-ray diffraction

A systematic study is reported of the growth of vertically aligned few layered graphene (FLG) nanoflakes on Si (100) substrates by microwave plasma enhanced chemical vapour deposition (MPECVD) method. Asymmetric grazing incident angle X-ray diffraction (GIAXRD) studies revealed a structural transformation, from nanocrystalline graphite layers to FLG, with the increase of growth time. As the growth time increased we observed a preferred vertical orientation of FLGs accompanied by a sharp decrease in the d002 spacing. Transmission electron microscopy shows these structures have highly graphitized edge planes which terminate in a few layers (1–3) of graphene sheets. Detailed Raman studies not only support the structural transformation but also confirm that the process occurs via the sudden release of stress in nanocrystalline turbostratic graphite films. Graphical plot of all major Raman parameters (such as G peak position, ID/IG value, FWHM of D, G, and G′ peaks) vs.growth time shows a well defined trend. Using the graphical plots a tentative trajectory of the Raman parameters is proposed, which can be very useful in understanding structural transformation during growth process. Finally, a possible growth mechanism of FLGs is presented.

Electron microscopy observations on the role of twinning in the evolution of microstructures

Twinning plays an important role in phase transformations and can have significant effects on microstructural evolution. Different roles of twinning in the development of microstructures during precipitation and phase transformations are reviewed and illustrated with examples from investigations by high-resolution electron microscopy, including the effect of multiple twinning on the development of Ge precipitates in Al-Ge and Ag-Ge alloys, the twin dissociation of grain boundaries in Au, the formation of hexagonal Si at twin intersections and the effect of twin boundaries on the equilibrium shape of Pb inclusions in Al.

High-resolution TEM and the application of direct and indirect aberration correction

Aberration correction leads to a substantial improvement in the directly interpretable resolution of transmission electron microscopes. Correction of the aberrations has been achieved electron-optically through a hexapole-based corrector and also indirectly by computational analysis of a focal or tilt series of images. These direct and indirect methods are complementary, and a combination of the two offers further advantages. Materials characterization has benefitted from the reduced delocalization and higher resolution in the corrected images. It is now possible, for example, to locate atomic columns at surfaces to higher accuracy and reliability. This article describes the JEM-2200FS in Oxford, which is equipped with correctors for both the image-forming and probe-forming lenses. Examples of the use of this instrument in the characterization of nanocrystalline catalysts are given together with initial results combining direct and indirect methods. The double corrector configuration enables direct imaging of the corrected probe, and a potential confocal imaging mode is described. Finally, modifications to a second generation instrument are outlined.

Electron Image Series Reconstruction of Twin Interfaces in InP Superlattice Nanowires.

The twin interface structure in twinning superlattice InP nanowires with zincblende structure has been investigated using electron exit wavefunction restoration from focal series images recorded on an aberration-corrected transmission electron microscope. By comparing the exit wavefunction phase with simulations from model structures, it was possible to determine the twin structure to be the ortho type with preserved In-P bonding order across the interface. The bending of the thin nanowires away from the intended 110 axis could be estimated locally from the calculated diffraction pattern, and this parameter was successfully taken into account in the simulations.

The application of spherical aberration correction and focal series restoration to high-resolution images of platinum nanocatalyst particles

A JEOL 2200FS transmission electron microscope equipped with a field emission gun, an objective lens spherical aberration corrector and an in-column energy filter has been used to acquire through focal series of high-resolution images of platinum nanocatalyst particles using a small value of the spherical aberration coefficient. The degree to which spherical aberration correction provides an improvement to image quality and interpretability for such particles is discussed, both with and without the use of through-focal series restoration.

Aberration-corrected HREM/STEM for semiconductor research

Aberration correction leads to a substantial improvement in the resolution of transmission electron microscopes. The JEM-2200FS in Oxford (Begbroke site) is equipped with correctors for both TEM and STEM. Alignment of the TEM and STEM correctors is achieved through variations of the Zemlin tableaux. The microscope can be used to study the same or similar regions of a sample in both TEM and STEM modes.

Real-time in-situ Investigation of III-V Nanowire Growth using Custom-designed Hybrid Chemical Vapor Deposition-TEM

Semiconductor nanowires are an important technology for applications in electronics, light harvesting and generation, and fundamental physics investigations [1]. Their advantages lay particularly in their geometry, with the nanoscale diameter allowing for quantum confinement and enhanced surface sensitivity, and their extended (axial) dimension greatly facilitating integration of the materials into complex structures and devices. However, of even greater importance is the potential for forming new, novel materials via the bottom-up crystal growth process in one dimension, with unusual properties that can be tuned according to design. These new materials include for example metastable crystal phases that do not form in bulk semiconductors, and metastable alloys of two or more semiconductors. Although these novel materials represent a great potential of semiconductor nanowires, the ability to develop and control their formation depends on a mechanistic understanding of the transient and dynamic processes that control their growth.