High-resolution Transmission Electron Microscopy Observations Research Articles

Microstructure and composition of the grain/binder interface in WC–Ni3Al composites

The microstructure and composition of WC/Ni3Al interface were studied. An orientation relationship of [100]WC//[110]Ni3Al and (001)WC//(001)Ni3Al with a good coherence, besides many random orientation relationships between WC and Ni3Al, has been repetitively found by selected area electron diffraction and high resolution TEM observations. The XRD pattern of WC–Ni3Al composites indicated that the major binder phase was Ni3Al and showed possibility of coherence between WC and Ni3Al as common interplanar spacings existed. Electron probe microanalysis results revealed that the atomic ratio of Al:Ni is close to 1:3 in binder phase and WC/Ni3Al interface in the WC–Ni3Al composites has a sharper compositional gradient and a smaller width of transition region than the WC/Co interface in WC–Co composites.

Mechanical property improvement by texture control of magnetron co-sputtered Zr-Ti films

The present work studies the effect of substrate temperature and film composition on the structural and mechanical properties evolution of magnetron sputtered Zr-Ti films. As-deposited films show a monotonically strengthening (0002) crystallographic texture ranging from ambient temperature to 523 K, while then reveal a (0002) texture to randomly orientated structure transition at higher temperature. High Resolution TEM observations reveal a competitive and reconstruction growth mechanism which is in good agreement with the well-known Structure Zone Model. Nano-indentation measurements revealed that texture strengthening contribute to the improvement of mechanical properties. These results suggest that by establishing a semi-quantitative phase diagram based on the Structure Zone Model, structure and structure-related properties modification can be easily realized and precisely controlled by modifying the TS/Tm region during deposition.

Microstructure and texture analysis of δ-hydride precipitation in Zircaloy-4 materials by electron microscopy and neutron diffraction

This work presents a detailed microstructure and texture study of various hydrided Zircaloy-4 materials by neutron diffraction and microscopy. The results show that the precipitated δ-ZrH1.66generally follows the δ(111)//α(0001) and δ[1{\overline 1}0]//α[11{\overline 2}0] orientation relationship with the α-Zr matrix. The δ-hydride displays a weak texture that is determined by the texture of the α-Zr matrix, and this dependence essentially originates from the observed orientation correlation between α-Zr and δ-hydride. Neutron diffraction line profile analysis and high-resolution transmission electron microscopy observations reveal a significant number of dislocations present in the δ-hydride, with an estimated average density one order of magnitude higher than that in the α-Zr matrix, which contributes to the accommodation of the substantial misfit strains associated with hydride precipitation in the α-Zr matrix. The present observations provide an insight into the behaviour of δ-hydride precipitation in zirconium alloys and may help with understanding the induced embrittling effect of hydrides.

Direct Evidence for Local Symmetry Breaking during a Strain Glass Transition

Strain glass transition is a unique nanoscale displacive transition with local symmetry breaking while maintaining the macroscopic symmetry or average structure unchanged. It usually occurs in the "nonmartensitic" composition range of a martensitic system. So far, only indirect evidence exists for such a transition, essentially from macroscopic measurements and low-resolution transmission electron microscopy observations, and there is a lack of direct evidence for the speculated local symmetry breaking and the sluggish nature of the glass transition. In this Letter we report in situ high-resolution transmission electron microscopy observations on a Ti50(Pd41Cr9) strain glass alloy and direct evidence for these key issues. Our results show that at temperatures well above the strain glass transition temperature (Tg), the lattice is essentially an undistorted B2 structure. With approaching Tg, the local symmetry breaking gradually occurs with the formation and growth of nanomartensite clusters with a combined stacking period of three and four plane intervals, but the average structure measured by x-ray diffraction remains B2. These nanomartensite clusters become finally frozen below Tg. Our results provide not only a microscopic basis for the macroscopic properties of strain glass, but also new insights into a range of possible applications of this unique class of materials.

Motion of 1/3⟨111⟩ dislocations on Σ3 {112} twin boundaries in nanotwinned copper

The atomic structure of Σ3 {112} ITBs in nanotwinned Cu is investigated by using aberration-corrected high resolution transmission electron microscopy (HRTEM) and in situ HRTEM observations. The Σ3 {112} ITBs are consisted of periodically repeated three partial dislocations. The in situ HRTEM results show that 1/3[111] partial dislocation moves on the Σ3 {112} incoherent twin boundary (ITB), which was accompanied by a migration of the ITB. A dislocation reaction mechanism is proposed for the motion of 1/3[111] Frank partial dislocation, in which the 1/3[111] partial dislocation exchanges its position with twin boundary dislocations in sequence. In this way, the 1/3[111] dislocation can move on the incoherent twin boundary in metals with low stacking fault energy. Meanwhile, the ITB will migrate in its normal direction accordingly. These results provide insight into the reaction mechanism of 1/3[111] dislocations and ITBs and the associated migration of ITBs.

Synthesis, characterization and optical activity of La-doped ZnWO4 nanorods by hydrothermal method

One-dimensional La doped ZnWO4 nanostructures have been synthesized by hydrothermal method. The effect of various La contents doped ZnWO4 samples on phase, morphologies and optical properties was investigated and characterized by X-ray diffraction (XRD), field scanning electron microscopy (FESEM), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), Raman and UV–visible spectroscopy, respectively. The results of XRD, SEM and SAED revealed that all products were one-dimensional monoclinic ZnWO4 nanorods while doped La3+ constants into ZnWO4 lattice. The observation of TEM and HRTEM showed that the obtained La3+ doped ZnWO4 with different La constants were homogenous nanorods shapes with grow along [021] direction. The optical property result showed that La3+ doped ZnWO4 is a red shifted absorption when comparing to pure ZnWO4.

Bandgap variation in grain size controlled nanostructured CdO thin films deposited by pulsed-laser method

Cadmium oxide (CdO) thin films were prepared by pulsed laser deposition technique. Their structure, surface morphology, optical and electrical properties have been investigated. With a decrease in the laser energy density, the average grain size of the CdO film can be adjusted from 108 to 25 nm. High-resolution TEM observation showed that more crystalline defects like lattice distortion, dislocation and amorphous structure existed in the small grained (25 nm) CdO film, and X-ray photoelectron spectroscopy analysis confirmed that the film had more oxygen vacancies. The electrical and optical properties of the films significantly depended on the grain size. With the grain size decreasing to 25 nm, the optical band gap energy of the CdO film increased obviously from 2.82 to 3.33 eV. This change in the nature of material from semimetal to a wide band gap semiconductor, combining with its higher optical transmission (92 %) in visible light region, higher carrier concentration (1.25 × 1021 cm−3) and lower electrical resistivity (2.8 × 10−4 cm−3), makes the nano-grained CdO film very useful in optoelectronic applications.

Core structure and dissociation energetics of basal edge dislocation in α-Al2O3: A combined atomistic simulation and transmission electron microscopy analysis

Using large-scale molecular dynamics (MD) simulations, we investigate the energetics and local structures/stresses of partial dislocations, 1/3〈011¯0〉 and 1/3〈101¯0〉, dissociated from the 1/3〈112¯0〉 perfect basal edge dislocation in α-Al2O3. The validity of the model adopted in the simulation is confirmed by comparing with theoretical stress/strain distributions and with those experimentally obtained from a high-resolution transmission electron microscopy (HRTEM) observation. Partial dislocation pairs have a stable inter-core distance (∼2nm), which is also a phenomenon that is observed in the HRTEM experiments. The distance between the partials can be explained quantitatively by the balance between an elastic core–core repulsion and an effective attractive force against the extension of stacking faults (SFs). A comparison is made for two types of core structures of partial dislocations: a pair of partials with Al-terminated/O-terminated extra-half planes and that with Al-/Al-terminated ones. The overall tendency of the inter-core interaction and the equilibrium distances are the same in both cases, whereas the Al–O-terminated pair is slightly favourable in energy at the equilibrium distance. A residual shear stress on the SF plane is observed in the MD results, which can be attributed to local atomic structure in the SF.

Site-selective sulfurization of bromide-capped palladium nanocubes by polysulfide and the underlying mechanism

This article describes site-selective sulfurization of Pd nanocubes capped by a monolayer of chemisorbed Br− ions. High-resolution transmission electron microscopy and high-angle annular dark-field scanning TEM observations showed that PdS was not formed until a certain quantity of polysulfide () ions had been introduced (300 μl, or 18.8 ppm in the final reaction solution). Spot energy dispersive x-ray spectroscopy and x-ray photoelectron spectroscopy analyses confirmed that the surface-chemisorbed Br− ions were completely substituted by ions before the initiation of the sulfurization reaction. In the presence of sufficient ions (>300 μl or >18.8 ppm), PdS phase was selectively developed from the highly active corners, which then moved to the edges and finally towards the center until the entire nanocube was converted into PdS. The resultant PdS was found to be amorphous by electron microscopy and powder x-ray diffraction measurements. The amorphous structure of PdS facilitated the penetration and diffusion of species and thus acceleration of the reaction kinetics. As a result, the sulfurization of 13 nm Pd nanocubes was completed within a few minutes after the addition of adequate Na2Sx, leading to a much more severe poisoning effect, compared with other noble metals such as Ag, by sulfur.

Study on solid state reactions of nanocrystalline Cu–Ge alloys upon mechanical alloying and annealing

The structural evolution of Cu–12 wt·%Ge (∼Cu–11 at·%Ge) alloy processed by means of mechanical alloying (MA) with subsequent heat treatment was studied using X-ray diffraction profiles, scanning electron microscopy, transmission electron microscopy (TEM) and high resolution TEM observations as well as differential thermal analysis(DTA). The fcc Cu(Ge) solid solution (α) was produced at early stages of MA and amorphised upon further milling. This was followed by the formation of ζ−Cu5Ge intermetallic nanocrystals after 40 h of milling. The subsequent annealing process led to the nanocrystallisation of the amorphous powder. The X-ray crystallite size data showed the exceptional thermal stability of the alloy prepared. The mechanism of the stability was also investigated. In addition, a thermodynamic analysis based on Miedema’s semiempirical model was carried out for the evaluation of the experimental results and a good agreement was found between them.

A rock magnetic study on red palaeosols in Yun-Gui Plateau (Southwestern China) and evidence for uplift of Plateau

Magnetic signals of red palaeosols from the Yun-Gui Plateau (YGP), southwestern China, are studied using rock magnetism, selective chemical dissolution, differential X-ray diffraction (DXRD) and high-resolution transmission electron microscopy (HRTEM) in order to explore the potential of red palaeosol as a proxy indicator of paleogenesis and uplift of plateau. Red palaesols are characterized by highly magnetic signals and dark red color with a hue of 5 YR (yellow-red). The low-frequency magnetic susceptibility (χlf) of topsoils is shown to vary from 1500 × 10−8 to 2500 × 10−8m3 kg−1 in a decreasing pattern from the top to bottom of the profile. Magnetic profiles reveal that the red palaeosols contain significant amount of fine-grained superparamagnetic (SP) grains, which is attributed to the higher concentration of pedogenic SP maghemite. The dithionite–citrate–bicarbonate (DCB) procedure can selectively dissolve ultrafine pedogenic magnetic minerals in the red palaeosols, as evidenced by the highly correlation between mass-specific frequency-dependent magnetic susceptibility (χfd) and χlf loss of DCB treatment. The magnetic loss after DCB treatment accounts for 87–95 per cent of the original susceptibility. Rock magnetism and DXRD reveal that the main magnetic mineral in the red palaeosols is the pedogenic SP/stable single domain (SP/SSD) maghemite. These pedogenic maghemites account for about 1 per cent of free iron oxides. HRTEM observations show the evidence of pedogenic SP (<∼20–30 nm) and pure maghemite. These magnetic particles vary from several to tens of nanometres in size and exhibit typical crystallochemical characteristics of this mineral. Magnetic evidence suggests that the red palaeosols experience a strongly pedogenic processes. Pedogenic processes result in the neoformation of hematite and maghemite, and causes a substantial increase in the magnetic susceptibility and other magnetic signals. Therefore, the rock magnetism of red palaeosols potentially yields significant palaeopedogenic information and evidence on the neotectonic movement of plateau. Results of the present work suggest that the red palaeosols could be a good tool for understanding the amplitude and the age of uplift in the YGP.

Electrochemical characteristics of ZnSe and its nanostructured composite for rechargeable Li-ion batteries

Zinc selenide (ZnSe; mineral name: stilleite) and its nanostructured composite (ZnSe/C) are prepared by simple solid-state synthetic routes, and their potential as electrode materials for rechargeable Li-ion batteries is investigated. The reaction mechanism between ZnSe and Li is demonstrated using ex situ X-ray diffraction and extended X-ray absorption fine structure analyses. The X-ray diffraction patterns and high-resolution transmission electron microscopy observations confirm that ZnSe nanocrystallites in ZnSe/C are uniformly distributed within an amorphous carbon matrix. The nanostructured ZnSe/C composite electrode exhibit excellent electrochemical performances such as a high capacity and long cycling behavior of ca. 705 mAh g−1 over 300 cycles.

On the dependence of hole concentration and its mobility on crystallization degree in p-type cuprous oxide film

P-type cuprous oxide films were prepared by magnetron sputtering with different crystallization degrees from columnar polycrystalline to amorphous phase. The film with amorphous structure possesses a hole concentration of 1.92 × 1019 cm−3, even comparable to those of MIII- doped Cu2O crystalline films and two orders of magnitude higher compared to the well crystallized Cu2O, 6.51 × 1017 cm−3. Meanwhile, Hall mobility of the amorphous film is one order of magnitude larger with respect to those of the crystalline counterparts. Combining high-resolution transmission electron microscopy observations with x-ray photoelectron spectroscopy analysis, it is revealed that the localization of holes at the oxygen 2p band can be weakened by suppressing crystallization for the films, which is believed to be attributed to the asymmetric characterizations of Cu–O chemical bonds caused by topological disorder atomic arrangement inherent to amorphous structure.

Epitaxial growth and characterization of Gd2O3-doped HfO2 film on Ge (001) substrates with zero interface layer

The GHO (Gd2O3-doped HfO2) films were epitaxially grown on Ge (001) substrates adopting cube-on-cube mode with zero interface layer using pulsed laser deposition (PLD). Reflection high-energy electron diffraction (RHEED) and high-resolution transmission electron microscopy (HRTEM) observation revealed a sharp interface of GHO/Ge and orientation relationship corresponding to (001)GHO//(001)Ge and [011] GHO//[011]Ge. The band offset for GHO/Ge stack was evaluated to be 3.92 eV for valence band and 1.38 eV for conduction band by X-ray photoelectron spectrum. Small equivalent oxide thickness (0.49 nm) and interface state density (7×1011 cm−2) were achieved from Au/Ti/GHO/Ge/Al capacitors.

Randomly interstratified illite–vermiculite from weathering of illite in red earth sediments in Xuancheng, southeastern China

Mixed-layer illite–vermiculite (I/V) clays of the Xuancheng red earth were investigated using X-ray diffraction (XRD), Fourier-transform infrared (FTIR) spectrometer, and high-resolution transmission electron microscopy (HRTEM) methods. The XRD results indicated that the mixed layer I/V clays were characterized by the broad 10–14Å peak with a maximum intensity at ~12Å in air-dried sample, which decomposed into two peaks at 10Å and 14Å peaks respectively after ethylene glycol treatment. After K+ saturation, the 14Å peak changed to 10Å, which reverted back to 14Å after saturation with Mg2+ with a notable decrease in intensity of the 10Å peak. The broad 10–14Å peak collapsed to 10Å on heating to 400°C. These suggested that the broad 10–14Å peak resulted from randomly interstratified illite–vermiculite clays together with discrete illite and also vermiculite. HRTEM observation showed that straight 10Å illite layers were interstratified with 12Å vermiculite layers, suggesting that transformation of illite to vermiculite involves an exchange of K+ by hydrated cations associated with a swelling of the illite. The (060) spacing of I/V clay is 1.50Å, indicating a dioctahedral precursor of illite and thus reinforcing the transformation from illite to vermiculite. FTIR analysis suggested that most band positions of vermiculite and illite in mixed-layer I/V did not change in comparison with those of the discrete clay species, indicating that the basic crystal structure of mixed-layer illite–vermiculite remains the same as the discrete clays. The interlayer cations in the interlayer region of vermiculite layers of the I/V clays are mainly Ca and minor Mg, Al, and Fe. The abundant Ca makes the I/V clays to behave like I/S on glycolated and thermal treatment, while the Mg, Al, and Fe complex ions in the interlayer region cause the vermiculite layer to lose its expandability and to behave like chlorite. The mixed-layer I/V clays were widely formed in late Quaternary alluvial fans in subtropical to tropical areas, where the sediments experienced synchronous pedogenic modification. The syndepositional pedogenesis process leads to an incomplete transformation of illite to vermiculite and causes the formation of metastable intermediates of mixed-layer illite–vermiculite.

Cooperative cytotoxic activity of Zn and Cu in bovine serum albumin-conjugated ZnS/CuS nano-composites in PC12 cancer cells

Series of self-assembled and mono-dispersed bovine serum albumin (BSA)-conjugated ZnS/CuS nano-composites with different Zn/Cu ratios had been successfully synthesized by a combination method of the biomimetic synthesis and ion-exchange strategy under the gentle conditions. High-resolution transmission electron microscopy observation, Fourier transform infrared spectra and zeta potential analysis demonstrated that BSA-conjugated ZnS/CuS nano-composites with well dispersity had the hierarchical structure and BSA was a key factor to control the morphology and surface electro-negativity of final products. The real-time monitoring by atomic absorption spectroscopy and powder X-ray diffraction revealed that the Zn/Cu ratio of nano-composites could be controlled by adjusting the ion-exchange time. In addition, the metabolic and morphological assays indicated that the metabolic proliferation and spread of rat pheochromocytoma (PC12) cells could be inhibited by nano-composites, with the high anti-cancer activity at a low concentration (4 ppm). What were more important, Zn and Cu in nano-composites exhibited a positive cooperativity at inhibiting cancer cell functions. The microscope observation and biochemical marker analysis clearly revealed that the nano-composites-included lipid peroxidation and disintegration of membrane led to the death of PC12 cells. Summarily, the present study substantiated the potential of BSA-conjugated ZnS/CuS nano-composites as anti-cancer drug.

A new modulated structure in α-Fe2O3 nanowires

A new modulated structure consisting of periodic (112̄0) stacking faults (SFs) in the α-Fe2O3 nanowires (NWs) formed by the thermal oxidation of Fe foils is reported, using a combination of high-resolution transmission electron microscopy (HRTEM) observations and HRTEM image simulations. The periodicity of the modulated structure is 1.53 nm, which is ten times (33̄00) interplanar spacing and can be described by a shift of every ten (33̄00) planes with 1/2 the interplanar spacing of the (112̄0) plane. An atomic model for the Fe2O3 structure is proposed to simulate the modulated structure. HRTEM simulation results confirm that the modulated structure in α-Fe2O3 NWs is caused by SFs.

Stress-induced thickening of Ω phase in Al–Cu–Mg alloys containing various Ag additions

The thickening of Ω phase in Al–Cu–Mg alloys containing various bulk Ag contents during stress aging at 200°C with a tensile stress of 240MPa was investigated by a combination of transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM) and atom probe tomography (APT). TEM characterization confirmed preferred orientation of Ω phase in all stress-aged samples. Corresponding quantitative TEM calculations revealed the thickening kinetics of Ω phase was significantly accelerated during stress aging as compared to that during stress-free aging at 200°C. HRTEM analysis on the α/Ω interfacial structure confirmed that the applied tensile stress facilitated the rapid nucleation of the growth ledge on the broad face of Ω phase, thereby resulting in the accelerated plate thickening during stress aging at 200°C. Meanwhile, quantitative TEM analysis highlighted the stress-induced thickening of Ω phase at 200°C was affected by the bulk Ag content. This was consistent with the HRTEM observation as the ledge nucleation was found to be suppressed with increasing Ag addition. Our APT analysis on different stress-aged samples further suggested the progressive enrichment of Ag atoms in the segregation layer helped to stabilize the interfacial structure and was responsible for the lowest nucleation rate of the ledge in 1.77Ag alloy as compared to that in 0.46Ag alloy.

Raman-assessed structural evolution of as-deposited few-layer graphene by He/H2 arc discharge during rapid-cooling thinning treatment

We report a novel method to reduce layer number and structural defects of few-layer graphene (FLG) in He/H2 arc discharge via rapid cooling treatment. By adjusting the parameters of He/H2 mixtures appropriately, the effects of incremental cooling rates on the formation, morphology and structural defects of the raw soot have been investigated. According to observations of transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and atomic force microscopy (AFM), such FLG affected by rapid cooling tends to be thinner than untreated counterpart. TEM and HRTEM observations show that such sample contains a large amount of FLG with a fraction of single-layer graphene (SLG). AFM results show the thickness of the thinnest flake in the soot equals 0.5nm, indicating the presence of SLG. With the increasing of cooling rate, Raman-based estimations not only show the reduction of defect density by calculating the nanocrystallite size La, but confirm the thinning trend for layer number via Raman-assessed layer number. The possible reason for thinning effect is discussed on both aspects of crystal growth and fluid dynamics.

Magnetic properties of carbon-encapsulated Fe–Ni alloy nanocomposites

Carbon-encapsulated Fe–Ni alloy nanoparticles (Fe–[email protected]) have been fabricated with different Ni/Fe ratio by an solid-state route using melamine as carbon source. The structure and morphology of Fe–[email protected] nanoparticles were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). The XRD characterization results reveal that all products are alloys with no carbide impurity. The TEM and HRTEM observations show that the alloy nanoparticles are encapsulated in carbon shells. Additionally, the reactions involved in the syntheses are speculated. The variation of magnetic properties of Fe–[email protected] with Ni/Fe has been discussed according to the room temperature VSM measurement results.