Nanoarea Electron Diffraction Research Articles

Fabrication and Structure Characterization of Quasi-2-Dimensional Amorphous Carbon Structures

Quasi-2-dimensional total amorphous and half-amorphous carbon structures are fabricated from single-layer or few-layer of graphene via high-energy electron beam irradiation. Sample structures before and after electron beam irradiation are recorded by high resolution imaging and coherent nano-area electron diffraction using transmission electron microscopy (TEM). The atomic pair distribution functions of the sample are obtained from electron diffraction patterns. In the quasi-2-dimensional amorphous carbon material, the carbon hexagonal ring structure is not the main structural feature anymore, and the low order nearest distances between carbon atoms are slightly different from that in perfect graphene. Although the sample shows a long-range disordered atomic arrangement, it still holds short-range order and even middle-range order extending to 0.5 nm, as many zigzag carbon chains remain after electron beam irradiation.

Folding and stacking defects of graphene flakes probed by electron nanobeam

Combining nanoscale imaging with local electron spectroscopy and diffraction has provided direct information on folding and stacking defects of graphene flakes produced by unrolled multi-walled carbon nanotubes. Structural data obtained by nanoarea electron diffraction complemented with systematic electron energy loss spectroscopy measurements of the surface plasmon losses of single flakes show the presence of flat bilayer regions coexisting with folded areas where the topology of buckled graphene resembles that of warped carbon nanostructures.

The Formation and Utility of Sub-Angstrom to Nanometer-Sized Electron Probes in the Aberration-Corrected Transmission Electron Microscope at the University of Illinois

We evaluate the probe forming capability of a JEOL 2200FS transmission electron microscope equipped with a spherical aberration (Cs) probe corrector. The achievement of a real space sub-Angstrom (0.1 nm) probe for scanning transmission electron microscopy (STEM) imaging is demonstrated by acquisition and modeling of high-angle annular dark-field STEM images. We show that by optimizing the illumination system, large probe currents and large collection angles for electron energy loss spectroscopy (EELS) can be combined to yield EELS fine structure data spatially resolved to the atomic scale. We demonstrate the probe forming flexibility provided by the additional lenses in the probe corrector in several ways, including the formation of nanometer-sized parallel beams for nanoarea electron diffraction, and the formation of focused probes for convergent beam electron diffraction with a range of convergence angles. The different probes that can be formed using the probe corrected STEM opens up new applications for electron microscopy and diffraction.

Electron energy-loss study of the electronic structure of atomic scaleSrTiO3−SrMnO3−LaMnO3superlattices

$\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}$ (STO), $\mathrm{Sr}\mathrm{Mn}{\mathrm{O}}_{3}$ (SMO), and $\mathrm{La}\mathrm{Mn}{\mathrm{O}}_{3}$ (LMO) ultrathin films with layer thicknesses of 1--4 perovskite unit cells were grown on a STO substrate. The physical and electronic structures of ultrathin films and interfaces were characterized with nanoarea electron diffraction, scanning transmission electron microscopy (STEM), and electron energy-loss spectroscopy (EELS). The diffraction patterns show that the epitaxial growth of LMO, SMO, and STO with strain is closely lattice matched to the STO substrate. High resolution $Z$-contrast STEM images clearly show the superlattice structure with atomically sharp interfaces. EELS shows changes in the density of unoccupied states of oxygen, which is present in all films, and of titanium at the STO-SMO interface. We find that the $\mathrm{O}\phantom{\rule{0.2em}{0ex}}K$ edge in layered LMO shifts depending on the thickness of the film. No peak shifts of Mn in $\mathrm{La}\mathrm{Mn}{\mathrm{O}}_{3}$ and $\mathrm{Sr}\mathrm{Mn}{\mathrm{O}}_{3}$ are observed within our instrument resolution, indicating that the valence of the Mn ion is approximately constant across the interface while the electron density of oxygen atoms accepts the local charge transfer.

Morphological Changes at a Silver Surface Resulting from Exposure to Hyperthermal Atomic Oxygen

Bulk single crystals of Ag(100) and Ag(111) at a sample temperature of 220 °C were exposed to a beam containing hyperthermal atomic oxygen with a nominal translational energy of 5.2 eV. The resultant oxide scales and interface structures were characterized by cross-sectional (scanning) transmission electron microscopy ((S)TEM) and scanning electron microscopy (SEM). The oxide scales formed on Ag(100) and Ag(111) were more than 10 microns thick and contained numerous micropores, microchannels, grain boundaries, and defects (e.g., twins), which can provide pathways for the rapid transport of oxygen atoms through the thick oxide scales to the oxide/Ag interface. Energy dispersive X-ray spectroscopy (EDS) and nanoarea electron diffraction pattern (NA-EDP) investigations revealed that the oxide scales were predominately polycrystalline silver, containing only ∼3−5 atom % of oxygen, which is mainly in the form of Ag2O. Additional low-fluence exposures of thin foils of single-crystal Ag followed by analysis with TEM and NA-EDP identified crystalline grains of Ag, with dimensions of 500−800 nm, imbedded in a porous region of nanometer-sized Ag2O grains during the initial stages of surface transformation by hyperthermal atomic oxygen. A mechanism based on the rapid oxidation of Ag by atomic oxygen followed by thermal reduction of the Ag2O at 220 °C is proposed to explain the formation of the thick and unique polycrystalline silver “oxide” scales that were observed.

Structure determinations of double-wall carbon nanotubes grown by catalytic chemical vapor deposition

We report an application of nanoarea electron diffraction for structure determination of double-wall carbon nanotubes (DWNT) grown by catalytic chemical vapor deposition. The structures of 30 tubes were determined from experimental diffraction patterns. Among these tubes, the inner and outer wall structure of 18 tubes was precisely determined by comparison with kinematic electron diffraction simulations. For the structure of the DWNTs, our experiment revealed a mixture of semiconducting-metallic (S-M), S-S and M-M tubes. The spacing between the two walls ranges from 0.335 nm to 0.384 nm. Most DWNTs are incommensurate and chiral.

Structure and chirality distribution of multiwalled boron nitride nanotubes

We report on a high-resolution electron diffraction study of the structure of individual multiwalled boron nitride nanotubes (MW-BNNTs). The tube chirality was determined by electron diffraction. Diffraction patterns were recorded from small sections of the nanotubes, ∼125nm long, using the nanoarea electron diffraction technique. Accurate measurements of the MW-BNNT chiral angles and their distribution were made from diffraction patterns. Generally, the tube chiralities within each MW-BNNT are strongly correlated; clustering around a single chirality with a dispersion of a few degrees. Multihelix nanotubes were rarely observed. Statistics based on 67 nanotubes revealed a dispersion of the chiral angles (α) with some preference of tubes in the ranges of 10°⩽α⩽15° and 25°⩽α⩽30°. Since various properties of nanotubes depend on the tube structure (diameter and chirality), the results presented here have general significances to nanotube growth and applications.

Coherent nano‐area electron diffraction

We describe the new coherent nano-area electron diffraction (NED) and its application for structure determination of individual nanostructures. The study is motivated by the challenge and the general lack of analytical techniques for characterizing nanometer-sized, heterogeneous phases. We show that by focusing electrons on the focal plane of the pre-objective lens using a 3rd condenser lens and a small condense aperture, it is possible to achieve a nanometer-sized highly parallel illumination or probe. The high angular resolution of diffraction pattern from the parallel illumination allows over-sampling and consequently the solution of phase problem based on the recently developed ab initio phase retrieval technique. From this, a high-contrast and high-resolution image can be reconstructed at resolution beyond the performance limit of the image-forming objective lens. The significance of NED for nanostructure characterization will be exemplified by single-wall carbon nanotubes and small metallic clusters. Imaging from diffraction patterns, or diffractive imaging, will be demonstrated using double-wall carbon nanotubes.

Structure determination of individual single-wall carbon nanotubes by nanoarea electron diffraction

In this letter, we report an electron diffraction determination of chiral vectors (n,m) of individual single-wall carbon nanotubes (SWNTs). Electron diffraction patterns from individual SWNTs were recorded on imaging plates using a parallel electron beam over a section of tube of ∼50 nm long. Using two tubes of 1.39 and 3.77 nm in diameter, we show that the details of electron diffuse scattering can be detected for both the small and large tubes. The quality of diffraction patterns allows the accurate measurement of both the diameters and chiral angles of SWNTs for a direct determination of chiral vectors. The electron diffraction technique is general and applicable to other forms of individual nanostructures.

Formation and Microstructure of Carbon‐Encapsulated Superparamagnetic Co Nanoparticles

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Formation and microstructure of carbon encapsulated superparamagnetic Co nanoparticles

Carbon encapsulated magnetic cobalt nanoparticles have been synthesized by the modified arc-discharge method. Both high resolution transmission electron microscopy (HREM) and powder X-ray diffraction (XRD) profiles reveal the presence of 8–15 nm diameter crystallites coated with 1–3 carbon layers. In particular, HREM images indicate that the intimate and contiguous carbon fringe around those Co nanoparticles is good evidence for complete encapsulation by carbon shell layers. The encapsulated phases are identified as hcp α-Co, fcc β-Co and cobalt carbide (Co3C) nanocrystals using X-ray diffraction (XRD), nano-area electron diffraction (SAED) and energy dispersive X-ray analysis (EDX). However, some fcc β-Co particles with a significant fraction of stacking faults are observed by HREM and confirmed by means of numerical fast Fourier transform (FFT) of HREM lattice images. The carbon encapsulation formation and growth mechanism are also reviewed.

Nano-area electron diffraction pattern reconstructed from three-dimensional Fourier spectrum.

A method to obtain a nano-area electron diffraction pattern in transmission electron microscopy (TEM) was proposed, based on three-dimensional (3D)) image formation theory. This method allows us to reconstruct an electron diffraction pattern from a 3D Fourier spectrum of high-resolution through-focus images. As a test case, an electron diffraction pattern from a tilted Si single crystal was reconstructed using the proposed method and compared with the conventional selected-area electron diffraction pattern. The intensity distribution of the reconstructed electron diffraction pattern was confirmed to be qualitatively equal to that of the selected-area electron diffraction pattern, though the degree of the equivalency between these patterns reduces at the high frequency region and the reproducibility of the intensity degrades when the number of images used in the image processing was decreased. By selecting areas in a reconstructed exit wave field, some electron diffraction patterns were obtained from the nano-areas without the influence of spherical aberration.

Analytical electron microscopy of thin films and interfaces

In this paper a brief overview is presented of some of the techniques available with transmission electron microscopy (TEM) to characterize thin films and interfaces. The critical role that sample preparation plays is discussed. Examples of the application of many of the analytical techniques used in TEM are given. These include nano area electron diffraction, energy-dispersive X-ray spectroscopy and electron energy loss spectroscopy. High resolution imaging is available at atomic resolution and new methods are being developed to analyze the images including computer simulation. Imaging of magnetic domains is important to the study of materials used in digital data storage.