High-resolution Transmission Electron Microscope Data Research Articles

Aromatic Structural Characterization of Different-Rank Vitrinites: Using HRTEM, XRD and AFM

The aromatic structure of three different-rank vitrinites, V-YJL (R o =0.61%), V-AW (R o =1.13%), and V-JC (R o =2.22%), was investigated by X-ray diffraction (XRD), high-resolution transmission electron microscope (HRTEM), and atomic force microscope (AFM). Based on the HRTEM experimental results, it was found that with vitrinite reflectance increasing, the distributions of aromatic layers changed from disordered to well-aligned. The shapes of aromatic layers changed from twisted to more straight and parallel. In addition, there were more large-sized aromatic layers in vitrinite samples with larger vitrinite reflectance. XRD experimental data also showed that the sizes (L a, L c and N) of vitrinite crystallite structures increased and the inter-layer spacing (d 002) decreased with increasing the vitrinite reflectance, which was consistent with the HRTEM data. AFM images illustrated that the surface morphologies of V-YJL, V-AW, and V-JC vitrinite samples showed as granule-like structure, compact spherical clusters, and irregular network with granular structure. Through comprehensive analysis of HRTEM data, XRD data, and AFM data, it was found that the morphology of the vitrinite samples obtained by AFM was related to the aromatic structural features of vitrinite from HRTEM and XRD. Both the aromatic chemical structure and morphological structure were coal rank dependent, and greatly affected by the arrangement of heterogeneous atoms. This study can provide helpful information on aromatic structure of vitrinite, which is important for investigation of coal and coal clean utilization.

STRUCTURAL AND OPTICAL PROPERTIES OF CD1-XZNXS ALLOY QUANTUM DOTS

Cd1-xZnxS (0≤x≤1) alloy quantum dots (QDs) were synthesized via coprecipitation method using cetyltrimethyl ammonium bromide (CTAB) as a capping material. Elemental composition of the investigated samples was determined by the energy dispersive x-ray spectroscopy (EDX). X-ray powder diffraction (XRD) and high resolution transmission electron microscope (HRTEM) were used to characterize the crystalline structure, particle size and morphology. XRD analysis indicated that the as synthesized nanomaterials, except for ZnS, have a mixture of cubical and hexagonal phases. The percentage of the hexagonal phase was very small. ZnS nanocrystal has a cubical structure. The particle size and the lattice constant decreased with the increase of Zn content. HRTEM data showed the presence of QDs and confirmed the XRD results. Ultraviolet-visible (UV-vis) absorption spectroscopy was employed to study the optical properties. Absorption band edges showed blue-shift systematically with the increase of Zn content. The results are interpreted in terms of the quantum-size confinement effect. Measurements of XRD and UV-vis absorption spectra under different annealing temperatures up to 400°C were performed. The annealed compositions showed mixture of cubical and hexagonal crystal structure. Tendency for phase transformation, from cubic to hexagonal upon annealing were observed. The degree of tendency for phase transformation increased with the decrease of Zn content and the increase of the annealing temperature. Particle sizes were increased with increasing the annealing temperature. UV- vis absorption spectra exhibited red-shift systematically with the increase of the annealing temperature. The values of the optical band gap energies decreased with increasing annealing temperature.

Variations in Crystalline Structures and Electrical Properties of Single Crystalline Boron Nitride Nanosheets.

We report the studies of (1) the basic mechanism underlying the formation of defect-free, single crystalline boron nitride nanosheets (BNNSs) synthesized using pulsed laser plasma deposition (PLPD) technique, (2) the variation in the crystalline structure at the edges of the hexagonal boron nitride (h-BN) nanosheets, and (3) the basic electrical properties related to the BNNSs tunneling effect and electrical breakdown voltage. The nanoscale morphologies of BNNSs are characterized using scanning electron microscope (SEM) and high-resolution transmission electron microscope (HRTEM). The results show that each sample consisted of a number of transparent BNNSs that partially overlapped one another. Varying the deposition duration yielded different thicknesses of sample but did not affect the morphology, structure, and thickness of individual BNNSs pieces. Analysis of the SEM and HRTEM data revealed changes in the spatial period of the B3–N3 hexagonal structures and the interlayer distance at the edge of the BNNSs, which occurred due to the limited number of atomic layers and was confirmed further by x-ray diffraction (XRD) study. The experimental results clearly indicate that the values of the electrical conductivities of the super-thin BNNSs and the effect of temperature relied strongly on the direction of observation.

Microstructure tuning and magnetism switching of ferroelectric barium titanate

Single-crystal and polycrystal BaTiO3 (BTO) materials synthesized by the physical and chemical methods, respectively, have been studied based on microstructural characterizations and magnetic measurements. The results of X-ray diffraction and Raman scatting spectra show that a single crystal tetragonal to polycrystalline pseudo-cubic structure transformation occurs in BTO ferroelectrics, dependent of growth conditions and interface effects. High-resolution transmission electron microscope data indicate that the as-prepared BTO/SrTiO3 (001) and BTO/SrRuO3/SrTiO3 (001) heterostructures are highly c-axis oriented with atomic sharp interfaces. Lattice defects (i.e., edge-type misfit dislocations and stacking faults) in the heterostructures could be identified clearly and showed tunable with the variations of interface strain. Furthermore, the effects of vacancy defects on magnetic properties of BTO are discussed, which shows a diamagnetism–ferromagnetism switching as intrinsic vacancies increase. This work opens up a possible avenue to prepare magnetic BTO ferroelectrics.

Fine Structure Evaluation of the Pair Distribution Function with Molecular Models of the Argonne Premium Coals

The pair distribution function (PDF) is one means of evaluating the atomic spatial arrangements of coals. Analysis of PDF based on X-ray diffraction data provides structural information on turbostratic crystalline parameters that can be utilized to further characterize coal structure. With coalification, expectations are of limited growth in aromatic stacking and a slight growth in the aromatic cluster size over the lignite to medium-volatile bituminous range. The PDF can be also used to construct and validate atomistic representations for carbonaceous materials including coal. Here, the PDF was evaluated with molecular slice models of several Argonne Premium coals (Beulah-Zap, Illinois No. 6, Upper Freeport, Pocahontas No. 3), also a non-Argonne Hon Gai anthracite, and compared to experimental observations. Atomistic representations were generated directly from high-resolution transmission electron microscope (HRTEM) lattice fringe images. The Fringe3D approach populates aromatic moieties matching the distributions of fringe: length, layers per stack, interlayer spacing, and orientations to produce an aromatic slice model of limited depth. Perl scripts incorporated appropriate aliphatic and heteroatom components. This approach creates atomistic representations with greater ease, improved accuracy, and reduced computational expense than other construction approaches. The constructed coal models were partially geometry-optimized to achieve realistic bond lengths but without displacement of coal molecules enabling the distribution of fringe length, stacking, and orientations to be duplicated in 3D modeling space. The resulting coal slice models, devoid of cross-links, captured a distribution of turbostratic crystalline dimensions with an average cluster size of about 1 nm, an average interlayer spacing ranging between 0.37 and 0.39 nm, and an average stacking number of ∼2–3 in accordance with HRTEM and XRD data for Argonne coals. These structural models were used to predict PDFs and to evaluate the fine detail of the frequency spectra via examination of intermolecular and intramolecular contributions. There was good agreement between predicted and experimental observations. Analysis of the simulated intermolecular PDF contribution showed strong intensities with increasing coal rank in agreement with the growth in the stacking number and stack height observed from low- to high-rank coals. The simulated intramolecular PDF contribution showed shorter peak amplitudes for low-rank coals in comparison to high-rank coals in agreement with the increase in stack width as coal rank increases. To further examine these contributions, lattice models composed of pyrene molecules were also constructed via Fringe 3D and manipulated to directly investigate the effect of aromatic orientation distributions and stacking on the simulated PDF. Peak intensities of simulated intermolecular PDFs at the average interlayer spacing increased with the degree of alignment (ϕ) according to g(r)interd002 = 0.0014ϕ2 + 0.0107ϕ + 4.2784. This result was consistent with the slight increase in the stacking number observed from low- to high-rank coals with a more dramatic transition for anthracite.

Room-temperature ferromagnetism in Fe-doped PbTiO3 nanocrystals

Room-temperature ferromagnetism has been observed in Fe-doped PbTiO3 nanocrystals. The magnetism of the nanocrystals develops from diamagnetism to ferromagnetism and the paramagnetism on increasing nominal Fe doping concentration from 0to4mol%. Transmission electron microscope (TEM) and high-resolution TEM data indicate that Fe-doped PbTiO3 nanocrystals with the size of ∼100nm are organized to form a planarlike self-assembly via oriented aggregation. These assembled nanostructures effectively improve room-temperature ferromagnetism of the sample. The exchange interaction of ferric ions via an electron trapped in a bridging oxygen vacancy (F center) is employed to explain the ferromagnetism of Fe-doped PbTiO3 nanocrystals.

A Multistep Oriented Attachment Kinetics: Coarsening of ZnS Nanoparticle in Concentrated NaOH

Crystal growth of ZnS nanoparticles during hydrothermal coarsened in 4 M NaOH occurs via a two-stage process. In the first stage, the primary particles grow into a size over hundred times of the original volume. The initial growth rate can be fitted by an asymptotic curve. High-resolution transmission electron microscope (HRTEM) data indicate that in this stage, crystal growth mainly occurs via a multistep crystallographically specific oriented attachment (OA). The higher the coarsening temperature, the earlier the first stage ends. In the second stage, an abrupt transition from asymptotic to square parabola growth kinetics occurs. The crystal growth data can be fitted by a standard Ostwald ripening (OR) model consistent with growth controlled by dissolution/precipitation of ions in solution. HRTEM data indicate that a minor amount of OA-based growth also occurs in the early period of the second stage. A new multistep OA kinetics model analogous to the reaction between molecules was proposed to illustrate the asymptotic growth in the first stage of coarsening. The effect of concentrated NaOH was discussed and proved to be the key that hindered the OR process, attributing to the almost exclusive pure OA-based growth of ZnS particles in the first stage.

Size-Dependent Phase Transformation Kinetics in Nanocrystalline ZnS

Nanocrystalline ZnS was coarsened under hydrothermal conditions to investigate the effect of particle size on phase transformation kinetics. Although bulk wurtzite is metastable relative to sphalerite below 1020 degrees C at low pressure, sphalerite transforms to wurtzite at 225 degrees C in the hydrothermal experiments. This indicates that nanocrystalline wurtzite is stable at low temperature. High-resolution transmission electron microscope data indicate there are no pure wurtzite particles in the coarsened samples and that wurtzite only grows on the surface of coarsened sphalerite particles. Crystal growth of wurtzite stops when the diameter of the sphalerite-wurtzite interface reaches approximately 22 nm. We infer that crystal growth of wurtzite is kinetically controlled by the radius of the sphalerite-wurtzite interface. A new phase transformation kinetic model based on collective movement of atoms across the interface is developed to explain the experimental results.

Two-Stage Crystal-Growth Kinetics Observed during Hydrothermal Coarsening of Nanocrystalline ZnS

Crystal growth during hydrothermal coarsening of mercaptoethanol-capped nanocrystalline ZnS occurs via a two-stage process. In the first stage, the primary particle quickly doubles in volume. The initial growth rate can be fitted by an asymptotic curve that cannot be explained by any existing power-law dependence kinetic model developed for more coarsely crystalline material. High-resolution transmission electron microscope (HRTEM) data indicate that crystal growth within spherical nanoparticle aggregates occurs via crystallographically specific oriented attachment, despite the presence of surface-bound organic ligands. The size stabilizes for a period of time that depends on the coarsening temperature. In the second stage, following the dispersal of nanoparticles, an abrupt transition from asymptotic to cubic parabola growth kinetics occurs. The crystal growth data can be fitted by a standard Ostwald ripening volume diffusion model consistent with growth controlled by the volume diffusion of ions in solution. However, HRTEM data indicate that oriented attachment-based growth occurs in the early part of the second stage, followed by a significant reduction in aggregate surface topography, probably via surface diffusion as well as volume diffusion. We propose a new kinetic model based on oriented attachment-based growth to explain the asymptotic growth in the first stage of coarsening. The presence of surface-bound organic ligands may control the aggregation state of the nanoparticles and may permit an almost exclusive crystallographically specific oriented attachment-based growth to dominate in the first stage.

Ga 2O 3 nanomaterials synthesized from ball-milled GaN powders

Ga 2O 3 nanomaterials were synthesized from mechanically ground GaN powders with thermal annealing; Ga 2O 3 nanobelts were formed in a nitrogen atmosphere, while Ga 2O 3 nanoparticles were formed in an oxygen atmosphere. The structural properties of the Ga 2O 3 nanomaterials were investigated by X-ray diffractometer (XRD) and high-resolution transmission electron microscope (HRTEM). The study of field emission scanning electron microscopy (FESEM) on the microstructures of nanomaterials revealed that the nanobelts are in the range of about 10–200 nm width and 10–50 nm thickness, and that nanoparticles are in the range of about 20–50 nm radius. On the basis of XRD and HRTEM data, we determined that the nanobelts grow toward a direction perpendicular to the (0 1 0) lattice plane and that they are enclosed by facets of the ( 1 0 1 ̄ ) and (1 0 1) lattice planes. The formation of the nanobelts may be described by the vapor–solid (VS) mechanism, and the supersaturation degree of gaseous phase may play an important role in the formation of Ga 2O 3 nanomaterials.