Michael L. Odlyzko

Identifying Hexagonal Boron Nitride Monolayers by Transmission Electron Microscopy

Multislice simulations in the transmission electron microscope (TEM) were used to examine changes in annular-dark-field scanning TEM (ADF-STEM) images, conventional bright-field TEM (BF-CTEM) images, and selected-area electron diffraction (SAED) patterns as atomically thin hexagonal boron nitride (h-BN) samples are tilted up to 500 mrad off of the [0001] zone axis. For monolayer h-BN the contrast of ADF-STEM images and SAED patterns does not change with tilt in this range, while the contrast of BF-CTEM images does change; h-BN multilayer contrast varies strongly with tilt for ADF-STEM imaging, BF-CTEM imaging, and SAED. These results indicate that tilt series analysis in ADF-STEM image mode or SAED mode should permit identification of h-BN monolayers from raw TEM data as well as from quantitative post-processing.

Atomic bonding effects in annular dark field scanning transmission electron microscopy. II. Experiments

Quantitatively calibrated annular dark field scanning transmission electron microscopy (ADF-STEM) imaging experiments were compared to frozen phonon multislice simulations adapted to include chemical bonding effects. Having carefully matched simulation parameters to experimental conditions, a depth-dependent bonding effect was observed for high-angle ADF-STEM imaging of aluminum nitride. This result is explained by computational predictions, systematically examined in the preceding portion of this study, showing the propagation of the converged STEM beam to be highly sensitive to net interatomic charge transfer. Thus, although uncertainties in experimental conditions and simulation accuracy remain, the computationally predicted experimental bonding effect withstands the experimental testing reported here.

Atomic bonding effects in annular dark field scanning transmission electron microscopy. I. Computational predictions

Annular dark field scanning transmission electron microscopy (ADF-STEM) image simulations were performed for zone-axis-oriented light-element single crystals, using a multislice method adapted to include charge redistribution due to chemical bonding. Examination of these image simulations alongside calculations of the propagation of the focused electron probe reveal that the evolution of the probe intensity with thickness exhibits significant sensitivity to interatomic charge transfer, accounting for observed thickness-dependent bonding sensitivity of contrast in all ADF-STEM imaging conditions. Because changes in image contrast relative to conventional neutral atom simulations scale directly with the net interatomic charge transfer, the strongest effects are seen in crystals with highly polar bonding, while no effects are seen for nonpolar bonding. Although the bonding dependence of ADF-STEM image contrast varies with detector geometry, imaging parameters, and material temperature, these simulations pred...

Structural Rearrangement of 2-D Zeolite Nanosheets under Electron Beam

Two-dimensional (2-D) zeolites and zeolite nanosheets are porous silicate frameworks desirable for catalytic uses involving bulky molecules [1], thin film separation membranes [2], and low-k dielectric materials [3]. Functionality of such zeolites is highly dependent on their crystal structure, thickness and pore dimensions. Low-dose transmission electron microscopy (TEM) studies at an optimum accelerating voltage have proven particularly useful in crystallographic structure determination of these electron beam sensitive materials [4]. However, it is known that zeolites undergo amorphization through radiolysis at low accelerating voltages, as well as both sputtering and amorphization through knock-on at high accelerating voltages [5]. Being inherently destructive, the examination of zeolites in TEM cannot perfectly reveal as-synthesized structure.

Including Thermal Vibrations and Bonding in HAADF-STEM Image Simulation

Conventional implementations of multislice TEM image simulation [1,2] model the electrostatic potential of a solid as that of a collection of unbonded independent atoms; this approximation is known as the “independent atom model” (IAM). It is acknowledged that IAM-based calculations introduce errors in the low-order atomic scattering factor [3], and accordingly several studies have shown that valence charge redistribution due to bonding significantly affects bright-field image contrast [4,5]. In a previous study, the authors showed that charge redistribution due to bonding alters the high-angle annular dark field (HAADF) scanning TEM (STEM) image contrast in light-element polar crystals by altering probe channeling [6]. However, that study neglected to consider the effect of thermal diffuse scattering, which quantitatively affects HAADF-STEM image contrast [7].

Simplifying Electron Beam Channeling in STEM

The channeling behavior for electron probes in conventional transmission electron microscopy (TEM) and scanning TEM (STEM) remains an active area of research because it influences the quantitative interpretation of high-resolution images and spectroscopies. Electron beam channeling causes oscillations in the STEM probe intensity during specimen propagation, which leads to differences in the number of electrons incident at different depths [1]. Understanding these short-range oscillations and the parameters that influence them is critical due to the short depths of focus of probes in modern aberration-corrected STEMs. Although sophisticated mathematical descriptions have modeled beam channeling accurately [2, 3], a less rigorous approach can provide a more accessible understanding of how this complex phenomenon affects STEM results acquired from experiments.

Simplifying Electron Beam Channeling in Scanning Transmission Electron Microscopy (STEM)

Sub-angstrom scanning transmission electron microscopy (STEM) allows quantitative column-by-column analysis of crystalline specimens via annular dark-field images. The intensity of electrons scattered from a particular location in an atomic column depends on the intensity of the electron probe at that location. Electron beam channeling causes oscillations in the STEM probe intensity during specimen propagation, which leads to differences in the beam intensity incident at different depths. Understanding the parameters that control this complex behavior is critical for interpreting experimental STEM results. In this work, theoretical analysis of the STEM probe intensity reveals that intensity oscillations during specimen propagation are regulated by changes in the beams angular distribution. Three distinct regimes of channeling behavior are observed: the high-atomic-number (Z) regime, in which atomic scattering leads to significant angular redistribution of the beam; the low-Z regime, in which the probes initial angular distribution controls intensity oscillations; and the intermediate-Z regime, in which the behavior is mixed. These contrasting regimes are shown to exist for a wide range of probe parameters. These results provide a new understanding of the occurrence and consequences of channeling phenomena and conditions under which their influence is strengthened or weakened by characteristics of the electron probe and sample.

Challenges of Oversimplifying Z-contrast in Atomic Resolution ADF-STEM

Using thickness or atomic number (Z) to interpret variations in the intensity of atomic columns has been a trademark of annular dark-field scanning transmission electron microscopy (ADF-STEM) images. As widely accepted theory, higher Z elements, in general, show increase scattering compared to lower Z elements [1]. For experiments involving a predetermined binary compound (BN, GaAS, AlN, etc), Z contrast, alone, is often times enough to conclude the chemical identity of an atomic column. This interpretation is accurate under ideal electron beam behavior where the STEM probe propagates with the majority, if not all, of its intensity along a single atomic column. However, Odlyzko [2], among others, established electron beam channeling can spatially spread the intensity of a propagating electron beam beyond a single atomic column. Under these conditions, even when a STEM probe is localized to an atomic column, a non-negligible amount of probe intensity could transfer to neighboring columns. This behavior makes image interpretation more complex, and oversimplifying conventional Z-contrast can potentially lead to wrong conclusions from ADF-STEM images.

Chemical Bonding Effects in HAADF-STEM Imaging of Light-Element Ceramics

Chemical bonding not only determines most of the useful properties of solids; it also alters the electrostatic potentials of bonded atoms, thereby also altering electron scattering from those same atoms. Despite this, it is standard to conduct multislice TEM image simulation [1,2] by modeling the electrostatic potential of a solid as that of a collection of unbonded neutral atoms, which is a computationally convenient approximation known as the “independent atom model” (IAM). IAM simulation has proven especially successful for modeling high-angle annular dark field scanning TEM (HAADF-STEM) imaging [3,4]. However, in two previous studies the authors found that charge redistribution due to chemical bonding can measurably affect HAADF-STEM imaging of polar crystals, because interatomic charge transfer alters probe channeling [5,6].

Channeling of aberration-corrected STEM probes at the sub-atomic scale

The phenomenon of fast electron channeling in atomic crystals has long been appreciated as an important factor in TEM characterization, being particularly critical for consideration in scanning transmission electron microscopy (STEM). As such, electron channeling effects have been examined to understand the thickness-dependence of annular-dark-field (ADF) STEM image contrast [1], the emergence of atomic-scale core-loss electron-energy-loss spectroscopy (EELS) spectrum image contrast [2], the orientation-dependence [3] and spatial localization [4] of X-ray energy dispersive spectroscopy (XEDS) signals, and the detectability of dopant atoms [5]; STEM imaging has, in turn, recently been used to experimentally measure electron channeling behavior [6].