Atomic-resolution Transmission Electron Microscopy Images Research Articles

GaAs ablation with ultrashort laser pulses in ambient air and water environments

Water-assisted ultrashort laser pulse processing of semiconductor materials is a promising technique to diminish heat accumulation and improve process quality. In this study, we investigate femtosecond laser ablation of deep trenches in GaAs, an important optoelectronic material, using water and ambient air environments at different laser processing regimes. We perform a comprehensive analysis of ablated trenches, including surface morphological analysis, atomic-resolution transmission electron microscopy imaging, elemental mapping, photoluminescence, and Raman spectroscopy. The findings demonstrate that GaAs ablation efficiency is enhanced in a water environment while heat-accumulation-related damage is reduced. Raman spectroscopy reveals a decrease in the broad feature associated with amorphous GaAs surface layers during water-assisted laser processing, suggesting that a higher material quality in deep trenches can be achieved using a water environment.

Quantitative assessment of nanoparticle size distributions from HRTEM images

Abstract Aiming to determine the size distribution and surface area per unit mass of catalyst nanoparticles for fuel cells, we have developed a novel method of digital image processing that quantitatively determines the size of individual nano-particles from atomic-resolution transmission electron microscopy images. After introducing the basic concept of this new method, we demonstrate its efficiency using platinum nanoparticles on carbon support as a model system. Special attention is given to the error that can be introduced by estimating the surface area per unit mass from the distribution of particle diameters in the presence of asymmetry of the particle size about the mean.

Synergic grain boundary segregation and precipitation in W- and W-Mo-containing high-entropy borides

The structures of W- and W-Mo-containing high-entropy borides (HEBs) are systematically studied by combining atomic-resolution transmission electron microscopy imaging, electron diffraction, and chemical analysis. We reveal that W or W-Mo addition in HEBs leads to segregation of these elements to the grain boundaries (GBs). In the meantime, W- or W-Mo-rich precipitates also form along the GBs. Crystallographic analysis and atomic-scale imaging show that the GB precipitates in both W- and W-Mo-containing HEBs have a cube-on-cube orientation relationship with the matrix. With further strain analysis, the coherency of the precipitate/matrix interface is validated. Nanoindentation tests show that the simultaneous GB segregation and coherent precipitation, as a supplement to the grain hardening, provide additional hardening of the HEBs. Our work provides an in-depth understanding of the GB segregation and precipitation behaviors of HEBs. It suggests that GB engineering could be potentially used for optimizing the performance of high-entropy ceramics.

Multiple polarization orders in individual twinned colloidal nanocrystals of centrosymmetric HfO2

Spontaneous polarization is essential for ferroelectric functionality in non-centrosymmetric crystals. High-integration-density ferroelectric devices require the stabilization of ferroelectric polarization in small volumes. Here, atomic-resolution transmission electron microscopy imaging reveals that twinning-induced symmetry breaking in colloidal nanocrystals of centrosymmetric HfO2 leads to the formation of multiple polarization orders, which are associated with sub-nanometer ferroelectric and antiferroelectric phases. The minimum size limit of the ferroelectric phase is found to be ~4 nm3. Density functional theory calculations indicate that transformations between the ferroelectric and antiferroelectric phases can be modulated by lattice strain and are energetically possible in either direction. The results of this work provide a route towards applications of HfO2 nanocrystals in information storage at densities that are more than an order of magnitude higher than the scaling limit defined by the nanocrystal size.

Contrast Transfer Function-Based Exit-Wave Reconstruction and Denoising of Atomic-Resolution Transmission Electron Microscopy Images of Graphene and Cu Single Atom Substitutions by Deep Learning Framework.

The exit wave is the state of a uniform plane incident electron wave exiting immediately after passing through a specimen and before the atomic-resolution transmission electron microscopy (ARTEM) image is modified by the aberration of the optical system and the incoherence effect of the electron. Although exit-wave reconstruction has been developed to prevent the misinterpretation of ARTEM images, there have been limitations in the use of conventional exit-wave reconstruction in ARTEM studies of the structure and dynamics of two-dimensional materials. In this study, we propose a framework that consists of the convolutional dual-decoder autoencoder to reconstruct the exit wave and denoise ARTEM images. We calculated the contrast transfer function (CTF) for real ARTEM and assigned the output of each decoder to the CTF as the amplitude and phase of the exit wave. We present exit-wave reconstruction experiments with ARTEM images of monolayer graphene and compare the findings with those of a simulated exit wave. Cu single atom substitution in monolayer graphene was, for the first time, directly identified through exit-wave reconstruction experiments. Our exit-wave reconstruction experiments show that the performance of the denoising task is improved when compared to the Wiener filter in terms of the signal-to-noise ratio, peak signal-to-noise ratio, and structural similarity index map metrics.

Study of the Q′ (Q)-phase precipitation in Al–Mg–Si–Cu alloys by quantification of atomic-resolution transmission electron microscopy images and atom probe tomography

The precipitation mechanism of the Q phase in Al–Mg–Si–Cu alloys has long been the subject of ambiguity and debate since its metastable phase (Q′) has the same crystal structure and similar lattice parameters as its equilibrium counterparts. In the present work, the evolution of the Q′ (Q) phase during aging is studied by combination of quantitative atomic-resolution scanning transmission electron microscopy and atom probe tomography. It was found that the transformation from the Q′ to the Q phase involves changes of the occupancy of Al atoms in atomic columns of the Q′ (Q) phase. The Al atoms incorporated in the Cu, Si and Mg columns are gradually released into the Al matrix, while mixing between Cu and Si atoms occurs in the Si columns. This transformation process is mainly attributed to the low lattice misfit of the equilibrium Q phase. Besides, the formation of various compositions of the Q phase is due to the different occupancy in the atomic columns of the Q phase. The occupancy changes in the columns of the Q phase are kinetically controlled and are strongly influenced by the alloy composition and aging temperature.

Atomic-resolution transmission electron microscopy of electron beam-sensitive crystalline materials.

High-resolution imaging of electron beam-sensitive materials is one of the most difficult applications of transmission electron microscopy (TEM). The challenges are manifold, including the acquisition of images with extremely low beam doses, the time-constrained search for crystal zone axes, the precise image alignment, and the accurate determination of the defocus value. We develop a suite of methods to fulfill these requirements and acquire atomic-resolution TEM images of several metal organic frameworks that are generally recognized as highly sensitive to electron beams. The high image resolution allows us to identify individual metal atomic columns, various types of surface termination, and benzene rings in the organic linkers. We also apply our methods to other electron beam-sensitive materials, including the organic-inorganic hybrid perovskite CH3NH3PbBr3.

Effects of dry oxidation treatments on monolayer graphene

Ultraviolet–ozone (UVO) and oxygen plasma are widely used to modify the surface of materials because these processes are facile and accessible. These dry oxidation treatments are also commonly applied to 2D graphene and are presumed to induce similar oxidation effects on the graphene surface. However, in this work, these treatments are revealed to induce the formation of different types of defects on the surface of graphene because the UVO treatment causes a chemical reaction, whereas the oxygen plasma treatment causes both physical and chemical reactions. The oxygen plasma treatment results mainly in topological defects, which effectively induce the attachment of oxygen atoms onto the treated surface; by contrast, the UVO treatment induces only the attachment of oxygen atoms onto the treated surface, without inducing lattice distortion. These results are confirmed mainly by atomic-resolution transmission electron microscopy imaging and electron energy loss spectroscopy. Using such facile dry oxidation treatments, we experimentally modified the surface states of graphene at the atomic scale.

In-situ investigation of the hydrogen release mechanism in bulk Mg2NiH4

Abstract Hydrogen storage is an important aspect to enable the so-called hydrogen economy. Mg-Ni alloys are among the most promising candidates for solid-state hydrogen storage systems yet many questions remain unanswered regarding the hydriding/dehydriding mechanism of the alloys. Mg2NiH4 particularly has received much attention both for its potential as a hydrogen storage medium and also exhibits interesting properties relating to its different polymorphs. Here, the dehydriding mechanism in bulk Mg2NiH4 is investigated using in-situ ultra-high voltage transmission electron microscopy (TEM) combined with Synchrotron powder X-ray diffraction (XRPD) and differential scanning calorimetry (DSC). We find that the hydrogen release is based on a mechanism of nucleation and growth of Mg2NiHx (x∼0–0.3) solid solution grains and is greatly enhanced in the presence of crystal defects occurring as a result of the polymorphic phase transformation. Also importantly, with atomic resolution TEM imaging a high density of stacking faults is identified in the dehydrided Mg2NiHx (x∼0–0.3) lattices.

Understanding and Controlling Nucleation and Growth of TiO2 Deposited on Multiwalled Carbon Nanotubes by Atomic Layer Deposition

Controlled deposition of thin conformal oxide films on carbon nanotubes (CNTs) by atomic layer deposition (ALD) for applications in solar energy and photocatalysis is still challenging, as the early stages of nucleation and subsequent growth are not yet well understood. In this work, we employed ALD to grow TiO2 on multiwalled carbon nanotubes (MW-CNTs). The effects of deposition temperature (120–240 °C), number of ALD cycles (20–750), and surface pretreatment of the MW-CNTs with oxygen plasma on the morphology and crystallinity of the TiO2 were systematically studied using transmission electron microscopy (TEM). By tuning the deposition conditions, controllable nucleation and growth of TiO2 on CNTs was achieved. In particular, high-quality crystalline anatase conforming to the CNTs was obtained with an ALD growth temperature as low as 200 °C. Direct observation using aberration-corrected atomic-resolution TEM imaging at 120 keV revealed an island structure of crystalline TiOx at the very early stage of nucleation, followed by coalesced growth of crystalline anatase at this temperature. The study also paves the way to understand the interface between the two materials on an atomic level.

Reactive ion etching: Optimized diamond membrane fabrication for transmission electron microscopy

Commonly used preparation method for thin diamond membranes by focused ion beam (FIB) techniques results in surface damage. Here, the authors introduce an alternative method based on reactive ion etching (RIE). To compare these methods, cross-sectional samples are produced in single crystal diamond, a material that has generated growing interest for a variety of applications. The samples are examined by Raman spectroscopy and high-resolution transmission electron microscopy (TEM). Raman spectra indicate that the crystalline structure of the RIE-processed diamond is preserved, while the FIB-processed diamond membrane has a broad-background sp2 feature. Atomic-resolution TEM imaging demonstrates that the RIE-based process produces no detectable damage, while the FIB-processed sample has an amorphous carbon layer of about 11 nm thick. These findings show that the RIE-based process allows the production of diamond TEM samples with reduced near-surface damage and can thus enable direct examination of growth defects and crystallographic damage induced by processes such as ion implantation and bombardment.

General framework for quantitative three-dimensional reconstruction from arbitrary detection geometries in TEM

In Phys. Rev. Lett. 109, 245502 (2012), a method for retrieving the object's three-dimensional potential distribution by inverting the dynamical scattering was presented and validated by a reconstruction from simulated atomic resolution transmission electron microscopy (TEM) images. In this paper, an extension to ptychography and scanning confocal electron microscopy is demonstrated and validated with simulations. Ultimately, this will make it possible to operate the microscope in the mode that yields the best reconstruction instead of accommodating the microscope settings to the linear approximation to the specimen-electron interaction used in most reconstruction algorithms. Furthermore, simultaneous estimation of the object and the unknown defocus from simulated atomic resolution TEM images is attained. This is an important step towards experimental reconstructions.

Graphene Oxide: Structural Analysis and Application as a Highly Transparent Support for Electron Microscopy

We report on the structural analysis of graphene oxide (GO) by transmission electron microscopy (TEM). Electron diffraction shows that on average the underlying carbon lattice maintains the order and lattice-spacings of graphene; a structure that is clearly resolved in 80 kV aberration-corrected atomic resolution TEM images. These results also reveal that single GO sheets are highly electron transparent and stable in the electron beam, and hence ideal support films for the study of nanoparticles and macromolecules by TEM. We demonstrate this through the structural analysis of physiological ferritin, an iron-storage protein.

Quantitative assessment of nanoparticle size distributions from HRTEM images

Aiming to determine the size distribution and surface area per unit mass of catalyst nanoparticles for fuel cells, we have developed a novel method of digital image processing that quantitatively determines the size of individual nanoparticles from atomic-resolution transmission electron microscopy images. After introducing the basic concept of this new method, we demonstrate its efficiency using platinum nanoparticles on carbon support as a model system. Special attention is given to the error that can be introduced by estimating the surface area per unit mass from the distribution of particle diameters in the presence of asymmetry of the particle size about the mean.

Freely Accessible Internet Resources for Nanoscience and Nanotechnology Education and Research at Portland State University's Research Servers

ABSTRACTBecause a great deal of nanoscience and nanotechnology relies on crystalline nanometer sized or nanometer structured materials, crystallographers have to provide their specific contributions to the National Nanotechnology Initiative. Here we review two open access internet-based crystallographic databases, the Crystallography Open Database (COD) and the Nano-Crystallography Database (NCD), that store information in the Crystallographic Information File (CIF) format. Having more than ten thousand crystallographic data sets available on the internet in a standardized format allows for many kinds of internet-based crystallographic calculations and visualizations. Examples for this that are dealt with in this paper are interactive crystal structure visualizations in three dimensions (3D) and calculations of theoretical lattice-fringe fingerprint plots for the identification of unknown nanocrystals from their atomic-resolution transmission electron microscopy images.

Strain-induced shift in photoluminescence energy in In0.2Ga0.8As/GaAs quantum wires

Quantum, wires, ranging in width from 900 to 42 nm, were patterned onto a 10-nm-thick In0.2Ga0.8As quantum well in GaAs cladding then regrown by migration-enhanced epitaxy. Atomic-resolution transmission electron microscopy images of two of the quantum wires, one 400 nm wide and the other 42 nm wide, show lattice deformation of the quantum wires due to compression by the cladding. The lattice constant in the growth direction varies with horizontal position inside each wire, from largest in the wire center to smallest at the sidewalls. In the 400 nm wire, the lattice constant in the growth direction fully reaches the pseudomorphically strained value of 5.83 Å at a distance of 165 Å from the sidewall, while the lattice constant in the 42 nm wire reaches only 5.79 Å, at 75 Å from the sidewall. From the value of the compressed lattice constant in the center of the 42 nm wire, the amount of strain in the center of the wire is inferred and, from this strain, the expected strain-induced band-gap energy shift is calculated. Photoluminescence measurements are made on the wires, showing a strain-induced increase in peak emission energy with decreasing wire size. That this energy shift is strain induced is verified by comparing it to the far smaller energy shift of an unregrown but, otherwise, identical sample, which has no regrowth-induced compressive strain. For the 42 nm quantum wire, after the calculated contribution due to increased quantum confinement is accounted for, the energy shift measured by photoluminescence is consistent with the calculated value to within the experimental error.

Tuning High- T c Superconductors via Multistage Intercalation

Multistage intercalation has been used to tune the interaction between adjacent blocks of CuO(2) sheets in the bigh-T(c) (high superconducting transition temperature) superconductor Bi(2)Sr(2)CaCu(2)Ox. As revealed by atomic-resolution transmission electron microscopy images, foreign iodine atoms are intercalated into every nth BiO bilayer of the host crystal, resulting in structures of stoichiometry IBi2nSr2nCanCu2nOx with stage index n up to 4. An expansion of 3.6 angstroms for each intercalated BiO bilayer decouples the CuO(2) sheets in adjacent blocks. A comparison of the superconducting transition temperatures of the pristine host material and intercalated compounds of different stages suggests that the coupling between each pair of adjacent blocks contributes approximately 5 K to T(c) in Bi(2)Sr(2)CaCu(2)Ox.