High-resolution Electron Microscopy Studies Research Articles

Synthesis and photoluminescence study of poly(ethylene glycol)-capped ZnS:Mn 2+ nanoparticles

Manganese-doped zinc sulphide nanoparticles capped with poly(ethylene glycol) (PEG) had been synthesised in aqueous solution by using PEG as surface modifier. The nanoparticles were prepared by chemical precipitation method at room temperature, which were characterised by the high-resolution analytical transmission electron microscope, X-ray diffraction (XRD), UV–visible and photoluminescence spectroscopy. Photoluminescene studies for all the investigated samples displayed characteristic band edge peak at about 430 nm. The broad peak at about 590 nm was ascribed to Mn incorporation. In addition, XRD and the high-resolution analytical transmission electron microscope studies showed the formation of cubic PEG-ZnS:Mn2+ nanoparticles with an average crystallite size of 3 nm.

Nanosized Ce1−xLaxO2−δ/Al2O3 solid solutions for CO oxidation: Combined study of structural characteristics and catalytic evaluation

Nanosized lanthanum-doped ceria (Ce1−xLaxO2−δ) and the same dispersed on alumina support (Ce1−xLaxO2−δ/Al2O3) were synthesized following a simple and high yielding procedure. Formation of solid solutions and interaction with the support were monitored by X-ray diffraction (XRD), Raman spectroscopy (RS; UV-RS and Vis-RS), high-resolution electron microscopy (HREM), Brunauer–Emmett–Teller (BET) surface area, X-ray photoelectron spectroscopy (XPS), temperature programmed reduction (TPR) and diffuse reflectance spectroscopy (UV-DRS) techniques. XRD results confirmed the formation of ceria–lanthana solid solutions. HREM studies revealed well distributed and small sized Ce–La–oxide particles of ∼7nm size over the surface of alumina support. Formation of defect sites or oxygen vacancies was investigated by RS using 325 and 632.81nm excitation laser lines which confirmed their presence in the investigated samples. RS studies further deduced that evolution of oxygen vacancies (∼570cm−1) was due to different ionic valence state of dopant (La3+) from that of Ce4+, while the development of MO8-type complex (∼600cm−1) was due to different ionic radius of dopant (La3+, 0.110nm) from that of Ce4+ (0.097nm). XPS results suggested that there is no significant change in the Ce3+ amount during thermal treatments due to the absence of undesirable cerium aluminates formation. TPR data revealed that surface modification with support enhances the reducibility of the doped ceria. Catalytic evaluation showed a 50% CO conversion temperature of 634K per 66mg of CL for the CL/A catalyst, which is lower than the CL sample (636K per 100mg of CL), possibly on account of an increased dispersion, more oxygen vacancies and easy reducibility.

Epitaxial growth and metal-insulator transition of vanadium oxide thin films with controllable phases

Vanadium oxide thin films with well controlled phases such as rhombohedra V2O3 and monoclinic VO2 were synthesized on Al2O3 (0001) substrates by optimizing the processing parameters of a polymer assisted deposition technique. X-ray diffraction and high-resolution transmission electron microscopy studies revealed that both V2O3 and VO2 films can be well controlled with good epitaxial quality. The temperature dependency of electrical resistivity demonstrated sharp metal-insulator transitions (MITs) for V2O3 and VO2 films. The crystallinity and the strains in the films are believed to play critical roles in determining the MIT properties.

High-resolution electron microscopy study of the early stages of calcium sulfate formation

High-resolution electron microscopy study of the early stages of calcium sulfate formation

Epitaxial growth of CdTe films on GaAs-buffered (001) Si substrates by metal organic chemical vapor deposition

Epitaxial (001) CdTe thin film was successfully grown on a (001) Si substrate using an atomically smooth, thin (2nm) GaAs buffer layer deposited using the metal organic chemical vapor deposition method. High-resolution transmission electron microscopy studies revealed a periodic distribution of dislocations due to lattice mismatch along the interfaces between CdTe film, GaAs buffer layer, and Si substrate. Most of the dislocation cores were located inside the GaAs buffer layer or at the interface between the CdTe and GaAs layers, indicating that GaAs buffer layer originally coherent to Si, was fully relaxed during the CdTe layer growth. The thin GaAs buffer layer effectively absorbed any strain coming from the large lattice mismatch between a CdTe layer and a Si substrate by forming an array of structural defects in the GaAs layer, which permitted epitaxial growth of CdTe on the Si substrates.

Preparation and characterization of polyaniline/cerium dioxide (CeO2) nanocomposite via in situ polymerization

Crystalline ceria (CeO2) nanoparticles have been successfully synthesized by a microwave-assisted solution method. Polyaniline (PANI)/cerium dioxide (CeO2) nanocomposite was synthesized by in situ polymerization of aniline in the presence of CeO2 nanoparticles. Characterization of CeO2 and PANI/CeO2 nanomaterials are carried out using various studies such as powder X-ray diffraction, infrared spectral and UV–Vis absorption spectral analyses, scanning electron microscopic and high-resolution transmission electron microscopic (HRTEM) studies and thermal analysis. The HRTEM of the images indicate that the CeO2 nanoparticles were embedded in the PANI matrix forming the core–shell structure.

Facile thermal explosion synthesis and optical properties of Al-doped flatted 3C-SiC microcrystals with 4H-SiC quantum interlayers

A facile thermal explosion synthesis (TES) method is developed to in situ synthesize batches of high-quality Al-doped flatted 3C-SiC microcrystals. Structural and optical analysis reveals that one dimensional (1D) disordered regions, which structurally resemble wide band-gap 4H-SiC nano-interlays, are acquired and sandwiched in the 3C-SiC microcrystal matrix via Al doping. High-resolution transmission electron microscopy (HRTEM) studies demonstrate that both atomic arrangements with growth predominantly in the 〈111〉 direction and planar structural defects coexist in the Al-doped flatted 3C-SiC microcrystals. A stacking sequence model is adopted to explain the formation mechanism of 4H-SiC nano-interlayers induced via Al doping. The photoluminescence (PL) spectra analysis shows that a new intensive, sharp ultraviolet (UV) emission (3.5eV) was observed for the flatted 3C-SiC microcrystal, except for a blue emission (3.1eV) derived from the surface defects. It is speculated that the quantum confined effect induced by quantum barrier structures is responsible for the new sharp UV emission. This comprises the first report of using a TES method for the synthesis of flatted Al-doped SiC microcrystals. The present method suggests a unique technique for synthesizing quantum barrier structures in SiC microcrystal matrix, which could be extended to other doping elements, and gives a new insight to understand the PL behavior of SiC materials.

Synthesis and characterisation of the novel double perovskites La2CrB2/3Nb1/3O6, B = Mg, Ni, Cu

The novel perovskites La2CrB2/3Nb1/3O6, B=Mg, Ni, and Cu have been synthesised at 1350°C in air via the citrate route. Rietveld refinements using neutron powder diffraction (NPD) data showed that the compounds adopt the GdFeO3 type structure with space group Pbnm, and unit cell parameters a≈b≈2×ap and c≈2×ap, where ap≈3.8Å. Selected area electron diffraction (SAED) of B=Ni and Cu samples confirmed space group Pbnm. However, distinct reflections forbidden in Pbnm symmetry, but allowed in the monoclinic sub-group P21/n and unit cell parameters a≈b≈2×ap and c≈2×ap, β≈90° were present in SAED patterns of B=Mg sample. This indicates an ordering of the B-cations within the crystal structure of La2CrMg2/3Nb1/3O6. High-resolution electron microscopy (HREM) study indicating uniform, without formation of clusters, ordering of B-cations in the crystallites of La2CrMg2/3Nb1/3O6. Magnetic susceptibility measurements show that the compounds are antiferromagnetic (with some glass or spin clustering effects due to additional ferromagnetic interactions between the B-cations) with TN for La2CrB2/3Nb1/3O6, B=Mg, Ni, Cu being 90, 125 and 140K, respectively.

Transmission electron microscopic study of pyrochlore to defect-fluorite transition in rare-earth pyrohafnates

A structural transition in rare earth pyrohafnates, Ln2Hf2O7 (Ln=Y, La, Pr, Nd, Tb, Dy, Yb and Lu), has been identified. Neutron diffraction showed that the structure transforms from well-ordered pyrochloric to fully fluoritic through the lanthanide series from La to Lu with a corresponding increase in the position parameter x of the 48f (Fd3¯m) oxygen site from 0.330 to 0.375. As evidenced by the selected area electron diffraction, La2Hf2O7, Pr2Hf2O7 and Nd2Hf2O7 exhibited a well-ordered pyrocholoric structure with the presence of intense superlattice spots, which became weak and diffuse (in Dy2Hf2O7 and Tb2Hf2O7) before disappearing completely as the series progressed towards the Lu end. High resolution electron microscopic studies showed the breakdown of the pyrochlore ordering in the form of antiphase domains resulting in diffused smoke-like superlattice spots in the case of Dy2Hf2O7 and Tb2Hf2O7.

Plasma-Enabled Growth of Single-Crystalline SiC/AlSiC Core–Shell Nanowires on Porous Alumina Templates

We report the catalyst-free synthesis of the arrays of core–shell, ultrathin, size-uniform SiC/AlSiC nanowires on the top of a periodic anodic aluminum oxide template. The nanowires were grown using an environmentally friendly, silane-free process by exposing the silicon supported porous alumina template to CH4 + H2 plasmas. High-resolution scanning and transmission electron microscopy studies revealed that the nanowires have a single-crystalline core with a diameter of about 10 nm and a thin (1–2 nm) amorphous AlSiC shell. Because of their remarkable length, high aspect ratio, and very high surface area-to-volume ratio, these unique structures are promising for nanoelectronic and nanophotonic applications that require efficient electron emission, light scattering, etc. A mechanism for nanowire growth is proposed based upon the reduction of the alumina template to nanosized metallic aluminum droplets forming between nanopores. The subsequent incorporation of silicon and carbon atoms from the plasma leads to nucleation and growth from the top of the alumina template.

Structure and magnetization reversal mechanism in L10 FePt films with perpendicular magnetic anisotropy

A series of L10 Fe63Pt37 films with controlled thickness (tFM) were deposited on MgO(100) substrates for microstructure and magnetization reversal mechanism study. X-ray diffraction measurements show that face-centered tetragonal (200) peak also exists in addition to face-centered tetragonal (002) one, and becomes weak for thick films. High resolution electron microscopy study reveals the existence of periodic misfit dislocations at the FePt/MgO interface and other types of defects such as twins and antiphase boundary inside the film. Out-of-plane initial magnetization shows a slow increase responding to the external magnetic field and then follows a steep increase. The out-of-plane coercivity HC at room temperature decreases with increasing tFM and increases when the angle θH between the external magnetic field and the film normal direction increases. HC at θH=0 changes as a linear function of temperature for individual samples and the slope decreases with increasing tFM. In addition, magnetic viscosity measurements show that the fluctuation field at room temperature decreases with increasing tFM. These phenomena indicate that the magnetization reversal in the L10 FePt films should be realized by the motion of weakly pinned domain wall and thus governed by the thermal activation model. The magnetization reversal thermal activation volume and corresponding energy increase with increasing tFM, as a result of the interactions between domain walls and structural defects can be attributed to the sample microstructural characteristic evolution with tFM.

Evidence of the Griffiths phase in multiferroic BiMnO3 and BiFe0.5Mn0.5O3 films

Microstructure and magnetic properties of BiMnO3 and BiFe0.5Mn0.5O3 films, prepared by rf magnetron sputtering on LaAlO3 (001) single-crystalline substrate, are investigated. The selected-area electron diffraction analysis allows us to identify the crystal structure of the BiMnO3 film as orthorhombic, while the BiFe0.5Mn0.5O3 film has a hexagonal lattice symmetry. High-resolution electron microscopy study reveals the presence of strip-domain phase with a periodic spacing of about 3c in both films. Magnetic measurements show that in addition to the basic paramagnetic phase the films exhibit Griffiths phase behavior in a wide temperature range. We argue that the observed weak ferromagnetism is due to the strip-domain layered inclusions, rather than intrinsic physical origin of the films.

Core–shell structure in copper ferrite–polyaniline nanocomposite: Confirmation by laser Raman spectra

Laser Raman spectroscopy was used to investigate the presence of core–shell structure in copper ferrite–polyaniline nanocomposites prepared by in situ polymerization method. Laser beams with two excitation powers, 0.3mW and 3mW, were employed to study the ferrite–polyaniline interaction and its changes by the laser induced heating effects. Low energy Raman spectra distinctly displayed polyaniline bands only without bands for CuFe2O4 in all the nanocomposites. This observation attributes polyaniline shell over the CuFe2O4 core particle, since the low energy was not sufficient for the beam to penetrate the polyaniline shell. However, with higher incident energy due to the induced heating effect, there was disturbance of polyaniline shell and the Raman frequencies were shifted depending on the shell thickness. Raman spectra of copper ferrite–polyaniline nanocomposites recorded using higher energy beam were consistent with the low energy Raman spectra of polyaniline samples annealed at three different temperatures of 200, 300 and 400°C respectively for 2h. High-resolution scanning electron microscope study also supports the formation of core–shell structure. It revealed that the average particle size increases with increase of shell thicknesses.

Self-induced polarization rotation of laser beam in fullerene (C70) solutions

Nonlinear self-rotation of elliptically polarized laser pulses (λ=532nm, τFWHM~12ns) in toluene, benzene and binary mixture (toluene+ethanol) solutions of fullerene C70 has been investigated experimentally. Absolute values and signs of the nonlinear refractive indices (n2) and nonlinear optical susceptibilities χ(3)(ω,−ω,ω) of C70 solutions in toluene and benzene at different values of polarization ellipse (θ=0.2÷0.8) have been determined. High-resolution transmission electron microscope studies of C70 solutions showed that in toluene+ethanol mixtures ball-shaped C70 clusters are formed with particle sizes in the range ~100÷500nm. It has been demonstrated, that the clusters sizes depend on the C70 concentration and volume fraction of ethanol in toluene. Correlation between the processes of C70 clusters formation in solutions and the values of polarization self-rotation angle of transmitted laser beam has been demonstrated. Physical mechanisms of laser induced optical activity in fullerene solutions have been discussed.

Suzuki cross-coupling catalyzed by palladium (II) complexes bearing 1-aryl-3,4,5,6-tetrahydropyrimidine ligands

A novel highly efficient monodentate imine palladium catalyst system has been tested for Suzuki cross-coupling reaction. Under the standard conditions, a series of aryl halides, including phenyl iodide, aryl bromides and phenyl chloride, were coupled with phenylboronic or 1-naphthylboronic acid, producing the corresponding biaryls in good to excellent yields. High-resolution transmission electron microscopy (HR-TEM) study of the recycled catalyst showed the formation of palladium nano particles in the catalytic system.

Atomic layer deposition of GaN at low temperatures

The authors report on the self-limiting growth of GaN thin films at low temperatures. Films were deposited on Si substrates by plasma-enhanced atomic layer deposition using trimethylgallium (TMG) and ammonia (NH3) as the group-III and -V precursors, respectively. GaN deposition rate saturated at 185 °C for NH3 doses starting from 90 s. Atomic layer deposition temperature window was observed from 185 to ∼385 °C. Deposition rate, which is constant at ∼0.51 A/cycle within the temperature range of 250 – 350 °C, increased slightly as the temperature decreased to 185 °C. In the bulk film, concentrations of Ga, N, and O were constant at ∼36.6, ∼43.9, and ∼19.5 at. %, respectively. C was detected only at the surface and no C impurities were found in the bulk film. High oxygen concentration in films was attributed to the oxygen impurities present in group-V precursor. High-resolution transmission electron microscopy studies revealed a microstructure consisting of small crystallites dispersed in an amorphous matrix.

Synthesis of layered–layered 0.5Li2MnO3·0.5LiCoO2 nanocomposite electrode materials by the mechanochemical process and first principles study

Li2MnO3-stabilized LiCoO2 electrode materials were synthesized using the method of mechanochemical process. Li2MnO3 was prepared and the mechanochemical process was carried out with LiCoO2, which yielded the layered–layered integrated structure nanocomposites. X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, and high-resolution transmission electron microscopy studies confirmed the structural integration of 0.5Li2MnO3·0.5LiCoO2 electrode materials. We also performed the high temperature heat treatment, where our 0.5Li2MnO3·0.5LiCoO2 electrode materials showed improvement in the discharge capacity (∼180 mA h g−1) with good cycleability. To obtain a physical insight into the performance of the nanocomposite structure, we carried out first principles calculations to obtain activation energy barriers of Li+ de-/intercalation, which suggested that utilizing both Li2MnO3 and LiCoO2 components can enhance the Li+ diffusion for the layered–layered integrated structure.

Precipitation of partially coherent α-Al(Mn,Fe)Si dispersoids and their strengthening effect in AA 3003 alloy

The orientation relationship (OR) between α-Al(Mn,Fe)Si dispersoids and the Al matrix in AA 3003 alloy has been systematically studied by selected area electron diffraction patterns. New types of ORs between the cubic dispersoids and the cubic matrix phase have been found as 〈1−11〉p//〈1−11〉m and {5−2−7}p//{011}m. High-resolution transmission electron microscopy and crystal structure simulation studies on the habit planes of dispersoids show that α-Al(Mn,Fe)Si dispersoids are partially coherent with the Al matrix. Precipitation of α-Al(Mn,Fe)Si dispersoids in the alloy has been found to have a strong influence on enhancing both the yield strength and tensile strength of the alloy. The dispersion-hardening effect of the dispersoids has been explained by the Orowan mechanism.

Fabrication and Structure Characterization of Te Butterfly Nanostructures

Te nanowires and butterfly nanostructures have been fabricated by template-free electrodeposition (TFED) in aqueous solution. By high-resolution transmission electron microscopy (HRTEM) study, the favored growth directions of the nanowires and the wings of the butterfly nanostructures were determined to be along the [0001] direction of trigonal Te, and the twinning plane of the butterfly nanostructures was (11-22). The cathodoluminescence measurements carried out at different positions of the butterfly nanostructure indicated that the twin boundaries influenced the photoemission efficiency.

Control over the Vertical Growth of Single Calcitic Crystals in Biomineralized Structures

Acidic biomacromolecules frequently incorporate into biomineralized structures to control the morphology and extent of crystal growth. The study of such processes has been hindered by the scarcity of a model system that mimics the influence of acidic biomacromolecules on mineral crystal growth. A carbonic anhydrase-assisted system was developed to model CaCO3 deposition at an air/solution interface. Textured CaCO3 crystals were found to grow in a direction orthogonal (vertical) to the air/solution interface. The crystal growth anisotropy became more pronounced upon addition of an anionic polymer, and an amorphous morphology was found at sufficiently high polymer concentrations. X-ray diffraction and high-resolution transmission electron microscopy studies showed that most calcite crystals grew along the (014) and (001) planes vertically, whereas the (012) and (110) planes were oriented in the lateral direction. The added acidic polymers adsorbed predominantly onto the (012) or (110) faces of the growing ...