Additional Diffraction Spots Research Articles

Two-Dimensional Metal–Organic Frameworks with Unique Oriented Layers for Oxygen Reduction Reaction: Tailoring the Activity through Exposed Crystal Facets

Two-Dimensional Metal–Organic Frameworks with Unique Oriented Layers for Oxygen Reduction Reaction: Tailoring the Activity through Exposed Crystal Facets

Complex internal faults of MgZn2 in Mg[sbnd]Zn binary alloy

Complex internal planar faults in the MgZn2 phase of a binary Mg6Zn alloy were thoroughly studied using transmission electron microscopy. Two staking faults lying on {101¯0}MgZn2 and {213¯0}MgZn2 planes were observed, with the latter one found to be able to be densely arranged in the whole MgZn2 phase, resulting in a new structure with a monoclinic structure, a = 1.71 nm, b = 2.12 nm, c = 4.05 nm, and β = 116°. Subsequently, stacking faults, stacking fault boundary as well as the evolved twin boundaries lead to many additional diffraction spots and even hexagonal diffraction holes in the corresponding SAED patterns.

TEM and electron diffraction analysis of ω-Fe to cementite transformation in quenched and tempered high carbon steels

Quenching and tempering are mostly employed to tune the mechanical properties of the high-carbon steels. In the present study, transmission electron microscopy (TEM) and selected area electron diffraction (SAED) are used to examine the microstructural evolution in quenched and tempered high carbon steels. In quenched specimens, the ω-Fe(C) phase is a common substructure in twinned martensite and its diffraction spots are located at 1/3 and 2/3 (21¯1)α-Fe positions along the [011]α-Fe zone axis (ZA). When specimens are in-situ heated in TEM, few additional diffraction spots are observed at 1/6, 3/6 and 5/6 (21¯1)α-Fe positions along the [011]α-Fe ZA. Moreover, martensite decomposes into a lamellar structure and ω-Fe(C) phase transforms into θ-Fe3C cementite during tempering. The TEM and electron diffraction analysis reveals that diffraction spots of θ-Fe3C cementite phase are located at 1/6, 2/6, 3/6, 4/6 and 5/6 (222¯)α-Fe and (21¯1)α-Fe along [112]α-Fe and [011]α-Fe ZAs. Furthermore, the orientation relationships between θ-Fe3C cementite and α-Fe are indexed as: [013]θ//[112]α-Fe, [001]θ//[011]α-Fe, [1¯13]θ//[111]α-Fe and [1¯02]θ//[131]α-Fe, which are related to the transformation of ω-Fe to θ-Fe3C cementite. The current study provides a baseline to understand the microstructural evolution in high carbon steels during heat treatment processes.

The origin of striation in the metastable β phase of titanium alloys observed by transmission electron microscopy.

For the β phase of Ti-5553-type metastable β-Ti alloys, striations in transmission electron microscopy (TEM) bright- and dark-field images have been frequently observed but their origin has not been sufficiently investigated. In the present work, this phenomenon is studied in depth from the macroscopic scale by neutron diffraction to the atomic scale by high-resolution TEM. The results reveal that the β phase contains homogeneously distributed modulated structures, intermediate between that of the β phase (cubic) and that of the α phase or the ω phase (hexagonal), giving rise to the appearance of additional diffraction spots at 1/2, 1/3 and 2/3 β diffraction positions. The intermediate structure between β and α is formed by the atomic displacements on each second {110}β plane in the [Formula: see text] direction, whereas that between β and ω is formed by atomic displacements on each second and third {112}β plane in the opposite [Formula: see text] direction. Because of these atomic displacements, the {110}β and {112}β planes become faulted, resulting in the streaking of β diffraction spots and the formation of extinction fringes in TEM bright- and dark-field images, the commonly observed striations. The present work reveals the origin of the striations and the intrinsic correlation with the additional electron reflections of the β phase.

Characterization of Basal Plane Dislocations in PVT-Grown SiC by Transmission Electron Microscopy

4H- and 6H-SiC grown by physical vapor transport method were investigated by transmission electron microscopy (TEM). From the TEM diffraction patterns observed along the [11-20] zone axis, 4H- and 6H-SiC were identified due to their additional diffraction spots, indicating atomic stacking sequences. However, identification was not possible in the [10-10] zone axis due to the absence of additional diffraction spots. Basal plane dislocations (BPDs) were investigated in the TEM specimen prepared along the [10-10] zone axis using the two-beam technique. BPDs were two Shockley partial dislocations with a stacking fault (SF) between them. Shockley partial BPDs arrayed along the [0001] growth direction were observed in the investigated 4H-SiC. This arrayed configuration of Shockley partial BPDs cannot be recognized from the plan view TEM with the [0001] zone axis. The evaluated distances between the two Shockley partial dislocations for the investigated samples were similar to the equilibrium distance, with values of several hundreds of nanometers or even values as large as over a few micrometers.

Aging-time-resolvedin situmicrostructural investigation of tin films electroplated on copper substrates, applying two-dimensional-detector X-ray diffraction

Pure thin tin films, electroplated on copper substrates, were investigated byin situX-ray diffraction analysis during room-temperature aging, using a laboratory diffractometer equipped with a two-dimensional detector. Diffraction spots of single Sn grains in the Sn film could be observed and traced over time by using the diffraction method adopted. For the as-deposited specimens, striking changes of the Sn reflection spots, concerning their position and intensity, as well as sudden appearances and disappearances of additional diffraction spots, were observed. This could be ascribed to local microstructural changes in the films such as grain rotation, grain growth and grain dissolution. In contrast to the as-deposited specimens, so-called post-baked specimens (i.e.annealed at 423 K for 1 h after layer deposition) exhibited a stable microstructure. The results obtained are discussed with respect to their relevance for the Sn whisker-growth phenomenon.

Preparation and characterization of SnO2 nanoparticles by hydrothermal route

This paper demonstrates the synthesis of SnO2 nanoparticles using a simple hydrothermal route in the presence of the surfactant hydrazine at 100 °C for 12 h. X-ray diffraction (XRD), field emission scanning electron microscopy, and transmission electron microscopy (TEM) were employed to characterize the as-prepared product, and optical property was studied by UV-visible diffuse reflectance spectroscopy (DRS). The XRD pattern of the as-prepared sample is indexed to the tetragonal structure of SnO2, and the calculated particle size is 22.4 nm, which is further confirmed by TEM. The selected area electron diffraction patterns showed continuous ring patterns without any additional diffraction spots and rings of secondary phases, revealing their crystalline structure. Analysis of the DRS spectrum showed the bandgap of the synthesized SnO2 to be 3.6 eV. The anionic surfactant hydrazine plays a key role in the formation of the SnO2 nanostructures. A probable reaction for the formation of SnO2 nanoparticles is proposed.

Structural evolution of FeCO3 through decarbonation at elevated temperatures

The structural evolution of siderite (s), FeCO3, through decarbonation at elevated temperatures has been investigated by the in-situ single-crystal X-ray diffraction technique using an area detector. When the crystal was heated above 255 °C, the transparent crystal turned colour in faint black from surface, indicating that the decarbonation commenced. The spinel-type magnetite (m), Fe3O4, first appeared in coexistence with the FeCO3 parent crystal. The orientation relationship between the rhombohedral FeCO3 and the cubic Fe3O4 can be described as [111]m // [001]s, and [2-1-1]m // [120]s. On further heating, additional diffraction spots appeared at 411 °C. They were indexed on the basis of the corundum-type hematite (h), (α-Fe2O3). The rhombohedral α-Fe2O3 unit cell had the same orientation relationship with the parent rhombohedral FeCO3 one, i.e., [001]h // [001]s, and [100]h // [100]s. On further heating the parent FeCO3 phase disappeared completely at 464 °C. The formation of iron oxides in FeCO3 depended on not only temperature but also the holding time. The structural relationships among FeCO3, Fe3O4, and α-Fe2O3 are discussed.

Low‐temperature growth of InGaN/GaN nano‐islands investigated by grazing‐incidence X‐ray diffraction

AbstractGrazing‐incidence X‐ray diffraction (GIXRD) has been used to study the structural properties of nanoscale In‐GaN/GaN(0001) islands, and especially the influence of GaN low‐temperature overgrowth of such structures on their strain state, chemical composition and defect structure. Prior to overgrowth, the presence of almost completely relaxed islands with a very high In content is deduced from the diffraction data. After cap layer growth, the In content is found to be significantly reduced and a broad distribution of composition and strain observed. In case of multiple InGaN island layers separated by GaN spacer layers, additional diffraction spots appear, allowing to identify the formation of cubic nitride material due to the low growth temperature (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Pt interactions with annealed and chemically-etched Nb-doped SrTiO3(0 0 1) surfaces: Metal/oxide surface chemical effects on band bending behavior

XPS and LEED have been used to characterize the interaction of sputter-deposited Pt (maximum coverage <5 ML) with Nb-doped SrTiO3(001) surfaces prepared either by annealing in O2 and then UHV, or by chemical-etching in aqua regia. The annealed surface exhibits an ordered (1×1) LEED pattern, with additional diffraction spots and streaks indicating the presence of oxygen vacancies. Increasing Pt coverage results in the decrease of the observed Pt(4f7/2) binding energy and the uniform shift of the Sr(3d), Ti(2p) and O(1s) levels to smaller binding energies, as expected for Pt cluster growth and surface-to-Pt charge donation on an n-type semiconductor. The etched surface is disordered, and exhibits a hydroxylated surface with a contaminant C film of ∼23Ǻ average thickness. Pt deposition on the etched surface results in an immediate decrease in the intensity of the OH feature in the O(1s) spectrum, and a uniform shift of the Sr(3d), Ti(2p) and O(1s) levels to larger binding energies with increasing Pt coverage. The observed Pt(4f7/2) binding energy on the etched surface (∼72eV) is independent of Pt coverage, and indicates substantial electronic charge donation from the Pt to surface hydroxyl species. The observation of band bending towards higher binding energies upon Pt deposition (behavior normally associated with p-type semiconductors) demonstrates that sub-monolayer quantities of adsorbates can alter metal/oxide interfacial charge transfer and reverse the direction of band bending, with important consequences for Schottky barrier heights and device applications.

Phase separation and atomic ordering in In xAl yGa 1−x−yN layers

We studied the occurrence of phase separation and atomic ordering in quaternary In x Al y Ga 1− x − y N layers by transmission electron microscopy. Three layers of different compositions were examined: one lattice-mismatched In 0.10Al 0.02Ga 0.88N layer together with two lattice-matched In 0.12Al 0.29Ga 0.59N and In 0.06Al 0.18Ga 0.76N layers. The composition modulations were seen in all the layers. The wavelengths ( λ) of composition modulations resulting from phase separation were calculated using selected-area electron diffraction patterns. The smaller λ (9 nm) in In 0.10Al 0.02Ga 0.88N layer in comparison to that in In 0.12Ga 0.88N layer ( λ = 20 nm) suggests that the driving force for phase separation in In x Al y Ga 1− x − y N layers is greater than that in In x Ga 1− x N layers with similar In contents. Energy dispersive spectroscopy line profiles across In x Al y Ga 1− x − y N/GaN interfaces revealed a gradual increase in Al and In incorporation. Additional (0 0 0 1) diffraction spots in an SADP taken on an 〈 1 1 ¯ 0 0 〉 zone from the In 0.06Al 0.18Ga 0.76N layer suggests that atomic ordering can occur in quaternary layers. Arguments are developed to rationalize these experimental observations.

Single crystalline ZnS nanotubes and their structural degradation under electron beam irradiation

ZnS nanotubes were synthesized using wet-chemistry method. These nanotubes appear to be extremely unstable under electron beam irradiation. Time dependent transmission electron diffraction patterns disclose the appearance of additional diffraction spots that belong to ZnO, with the prolonged e-beam irradiation duration. The experimental results suggest that displacement damage followed by oxidation is mainly responsible for the structural degradation of these ZnS nanotubes.

INCOMMENSURATE METALLIC SURFACTANT LAYER ON TOP OF InN FILM

The surface structure of InN film heteroepitaxially grown on a GaN buffer layer by MBE is followed by low energy electron diffraction (LEED). The metallic surfactant layers on top of the InN surfaces show an incommensurate structure rather than being disordered. The metal in the incommensurate structure induces additional diffraction spots in the LEED. Based on the Auger experiments, not only In atoms but also Ga are present on the surface of the InN films.

Double-helix structure in multiwall boron nitride nanotubes

A new nanotube structural form is reported that resembles a double helix in multiwall boron nitride nanotubes (MW-BNNT) grown by a carbon-free chemical-vapor-deposition process as documented by evidence obtained by transmission electron diffraction and microscopy. The double-helix structure is found in MW-BNNTs exhibiting the same chirality in its different walls. The MW-BNNTs deviate from the structure of ideal nested coaxial cylindrical tubes. Most significantly, bright- and dark-field electron imaging reveals regular zigzag dark and bright spots on the side walls of the nanotubes. The repeating distance between the bright, or dark, spots is related to the chiral angle of the nanotube. Electron diffraction patterns recorded from individual nanotubes show additional diffraction spots belonging to the 201 zone axes, which are not allowed in a perfectly cylindrical nanotube. These additional diffraction spots become asymmetrical as smaller sections of the nanotube are probed. A series of diffraction patterns recorded along the tube axis showed that the imperfections giving rise to these spots move in a regular fashion around the circumference of the tube. It is shown that all experimental evidence supports the structure model of two helices; one is polygonal in cross section and highly crystalline and the other is circular and less ordered. It is further suggested that the double-helix structure is a result of stronger wall-wall interactions associated with the ionic bonding in boron nitride.

Physisorption of Small Molecules on Single Crystal Alkali Halide Surfaces: CO and N2 Adsorbed on KCl(100)

Abstract The adsorption of CO and N2 on KCl(100) single crystal cleavage planes has been investigated by means of low energy electron diffraction (LEED) at primary currents in the nA range as well as polarization infrared spectroscopy (PIRS) in transmittance geometry. These isoelectronic adsorbates behave very similarly, and three different adsorption phases can be distinguished as a function of surface coverage for both of them. Initially a (1×1) structure is observed and assigned to a commensurate monolayer with one molecule per KCl ion pair. The CO infrared spectrum of this ‘phase I’ is characterized by a doublet absorption of the CO stretching vibration, which is discussed in the context of a correlation field splitting. The lack of superstructure diffraction peaks is attributed to a high degree of orientational disorder in this phase. From LEED adsorption isotherms the isosteric heat of adsorption of N2 has been determined to be 11±3 kJ mol−1. Upon increase in coverage by 50% ‘phase II’ is formed which exhibits a large number of additional diffraction spots. It is assigned to a bilayer in which the second layer is only half filled. Based on the LEED experiments a structure model is proposed in which the second layer is characterized by growth in rows along the [210] direction and a high degree of roughness in the perpendicular direction. This model can explain all experimentally observed main features. Finally upon further exposure formation of three-dimensional solid is observed, which proceeds via Stransky–Krastanov growth of crystalline 3D clusters with the structure of the low-temperature cubic α-phase. A simulation of the CO cluster infrared spectra within the dipole–dipole coupling approach can reproduce all major observed vibrational features.

Transmission electron microscopy study on ferromagnetic (Ga,Mn)N epitaxial films

We report on the crystal structure of epitaxial (Ga,Mn)N films with Mn=0.2% grown by plasma-enhanced molecular beam epitaxy, exhibiting n-type conductivity and room temperature ferromagnetism (Ms=0.76 emu/cm3, Hc=90 Oe). The additional diffraction spots are found in the zone axis of B=[11̄00] for the (Ga,Mn)N epilayer due to a single twin orientation in the closed packed hexagonal matrix. High-order Laue zone measurements reveal the expansion of lattice parameter a (3.1851–3.1865 Å) by doping Mn, demonstrating that Mn ions substitute for Ga ions in the (Ga,Mn)N.

Evolution of vacancy ordering structures in epitaxial YbSi2−x thin films on (111) and (001)Si

The evolution of vacancy ordering structures in epitaxial YbSi2−x thin films on both (111) and (001)Si has been studied by transmission electron microscopy (TEM). Epitaxial YbSi2−x thin films were grown on Si by either room temperature deposition with subsequent thermal annealing or deposition at elevated temperature. Epitaxial YbSi2−x was found to form in samples annealed at 300 °C for 30 min and the appearance of additional diffraction spots is attributed to the formation of an ordered vacancy superstructure in the epitaxial YbSi2−x thin films. In other samples, the split or streaking of extra diffraction spots is attributed to the formation of out-of-step structures. The variation of out-of-step structures with the annealing temperature is correlated to a change in vacancy concentration that makes compressive stress in the Si sublattice relax. From studying plan-view and cross-sectional TEM samples in conjunction with simulated diffraction patterns, the three-dimensional structures of vacancy ordering were determined. The vacancy ordering structures are expected to exert a strong influence on the physical properties of epitaxial rare-earth silicide nanowires grown on (001)Si.

In-Situ Low-Temperature Transmission Electron Microscopy of the Structural Phase Transitions in a Thiospinel CuIr2S4 Compound

The structural phase transition in a thiospinel CuIr 2 S 4 compound which is associated with a metal-to-insulator transformation at approximately 230 K was studied by in-situ low-temperature transmission electron microscopy. It was directly revealed that the structural transition is characterized by the formation of fine {110} twin lamellae and the appearance of additional electron diffraction spots along the <100> * , <110> * and <111> * directions of the room-temperature cubic CuIr 2 S 4 . On the basis of results of electron diffraction analysis, the lattice structure and symmetry of the CuIr 2 S 4 low-temperature insulating phase were determined. Furthermore, we found that the characteristic additional diffraction spots for the insulating phase became quite faint or disappeared when the temperature decreased below approximately 50 K. This suggests a subsequent structural change in the insulating CuIr 2 S 4 at extremely low temperatures.

Growth Process and Microstructure of ε Martensite in an Fe-Mn-Si-Cr-Ni Shape Memory Alloy

Growth process and microstructure of £ martensite formed by cooling in thermomechanically-treated Fe-Mn-Si-Cr-Ni shape memory alloys have been investigated in order to clarify whether or not very fine martensite plates such as observed in the case of stress-induced transformation are also formed in the case of simple cooling. Either such fine martensite plates or the lamella structure which consists of h.c.p. and f.c.c. phase is not observed in the case of the simple cooling, both in thermomechanically and non-thermomechanically-treated specimens, but there is a tendency that thinner martensite plates are formed in the former specimens. In the present study, additional diffraction spots, besides h.c.p. spots, are formed, which may have resulted from double hexagonal structure or some kinds of long period stacking structures. Stacking fault density in the h.c.p. martensite was examined by electron diffraction and it was found that the values of fault parameter a are almost the same in both the streses-induced and thermally-induced martensites.

Evolution of vacancy ordering and defect structure in epitaxial YSi2−x thin films on (111)Si

The evolution of vacancy ordering and defect structure in epitaxial YSi2−x thin films on (111)Si have been studied by both conventional and high-resolution transmission electron microscopy. Epitaxial YSi2−x with an ordered vacancy structure was grown on (111)Si by rapid thermal annealing. In samples annealed at 500 °C for 120 s epitaxial YSi2−x was found to form. After annealing at 600 °C for 15 s, the appearance of additional diffraction spots is attributed to the formation of an ordered vacancy superstructure in the epitaxial YSi2−x thin films. In samples annealed at 600 °C for longer time or higher temperatures, the splitting of extra diffraction spots is correlated to the formation of an out-of-step structure. Streaking of the split diffraction spots in the diffraction pattern is attributed to the presence of an out-of-step structure with a range of M values. The M was found to settle down to 2 after high-temperature and/or long time annealing. Planar defects in YSi2−x films were analyzed to be stacking faults on {101̄0} planes with 1/6 〈1̄21̄3〉 displacement vectors. Computer simulation was carried out to determine the atomic structure of stacking fault. The simulated images are seen to match rather well with experimental images taken under two different values of defocus.