Low-temperature Transmission Electron Microscopy Research Articles

Thermal stability of gold-PS nanocomposites thin films

Low-temperature transmission electron microscopy (TEM) studies were performed on polystyrene (PS, M w = 234 K) — Au nanoparticle composite thin films that were annealed up to 350°C under reduced pressure conditions. The composite thin films were prepared by wet chemical approach and the samples were then subsequently spin-coated on a carbon-coated copper grid for TEM measurements. TEM measurements were performed at liquid nitrogen temperatures to reduce the electron-beam-induced radiation damage. The results showed a marginal increase in Au nanoparticle diameter (2.3 nm–3.6 nm) and more importantly, an improved thermal stability of the polystyrene (PS) composite film much above its glass transition temperature.

Cathodoluminescence of GaInN quantum wells grown on nonpolar a plane GaN: Intense emission from pit facets

Ga x In 1 − x N quantum wells grown by metal organic vapor phase epitaxy on a plane GaN grown on r plane sapphire substrate typically show relatively large surface pits. We show by correlation of low temperature photoluminescence, cathodoluminescence, scanning and transmission electron microscopy that the different semipolar side facets of these pits dominate the overall luminescence signal of such layers.

Ice Cream: Foam Formation and Stabilization—A Review

Ice cream has been studied from ingredients to process conditions, ice crystal formation to ice crystal growth, and ingredient selection to eating quality. One of the key aspects of ice cream is air cells, as formation and acceptance of ice cream rely on foam production and stabilization. Air cells in ice cream provide a unique structure that governs many of this product's keeping and eating qualities. The air cells are dependent upon ice cream mix composition (fat, protein, and surface active ingredients) as well as processing conditions (freezer shear force, hardening time, and storage temperatures). Optical, low-temperature scanning electron microscopy and transmission electron microscopy are applied to observe the morphology and size of air cells in ice cream. Air cells are crucial in forming the product and for eating quality and enjoyment.

Self-organized structures of orbital-ordered domains inNd1−xSr1+xMnO4

Self-organizing domain structures of charge-orbital-ordered (COO) and ferro-orbital-ordered (FOO) phases were observed for tetragonal layered manganites ${\text{Nd}}_{1\ensuremath{-}x}{\text{Sr}}_{1+x}{\text{MnO}}_{4}$ ($x=0.8$ and 0.82) by low-temperature transmission electron microscopy. We revealed a marked temperature variation of coexisting COO domains with equivalent structural distortions and orbital ordering patterns at $x=0.8$. The observed competition of coexisting FOO domains at $x=0.82$ also clearly reflects the self-organizing nature of ${e}_{g}$ electrons. Furthermore, a checkered structure, in which COO domains were reproduced at the intersections of FOO domains, was found in the transition from the COO phase to the FOO phase at $x=0.8$. The observations suggest that strain energy affects the self-organizing behavior in the orbital ordering.

Magnetic properties and ferromagnetic microstructures in Al-doped La1-xSrxMnO3

Doping effect on ferromagnetic (FM) properties in perovskite manganites La1−xSrxMnO3 (x = 0.175) were investigated by low-temperature Lorentz transmission electron microscopy (TEM), in combination with the conventional magnetic measurements. It is found that La0.825Sr0.175Mn0.95Al0.05O3 exhibits a structural phase transition from the rhombohedral to orthorhombic structures at Ts~116 K. During the rhombohedral-to-orthorhombic transition, the FM domain structures changed from the macroscopic FM domains to the FM ones with the wavy stripe pattern. The ac magnetic measurements also revealed that La0.825Sr0.175Mn0.95Al0.05O3 shows unconventional magnetic behaviour at low temperature below 65 K. Partial substitution of non-magnetic Al3+ for Mn3+ has some influence on magnetic ordering and double-exchange interaction, which should induce insulator behaviour in the orthorhombic phase.

“Shaken, Not Stable”: Dispersion Mechanism and Dynamics of Protein-Dispersed Nanotubes Studied via Spectroscopy

Bundles of nanotubes have been exfoliated in solution by handful of dispersants, such as surfactants, polymers, or proteins, for efficient use in various applications. Common procedures in carbon nanotube exfoliation and dispersion are sonication followed by centrifugation. The dispersion dynamics of single-walled carbon nanotubes (SWNT) by protein were studied by UV-vis spectroscopy (enhanced with chemometric analysis) and low-temperature transmission electron microscopy (cryo-TEM). We studied the effect of both pH and BSA-to-SWNT ratio on SWNT exfoliation dynamics and recovery. We found that, indeed, BSA properties (i.e., electric charge and conformation) affect the exfoliation dynamics in a similar manner as it affects the SWNT recoveries: bulkier protein conformation-->faster exfoliation-->higher SWNT recoveries. Higher BSA-to-SWNT ratio results in lower recoveries and slower dynamics, suggesting that entropic consideration may take part in the exfoliation-stabilization process of SWNT.

CRYSTALLIZATION EXPERIMENTS ON AMORPHOUS MAGNESIUM SILICATE. II. EFFECT OF STACKING FAULTS ON INFRARED SPECTRA OF ENSTATITE

We carried out experiments of low-temperature infrared spectroscopy and transmission electron microscopy of enstatite (MgSiO3) synthesized by heating of amorphous magnesium silicate. There is a discrepancy between the infrared feature of enstatite obtained in this experiment and that of fine powdered single crystals. This reflects stacking disorder of enstatite. We show that circumstellar dust emission of enstatite is similar to the infrared feature measured in this experiment. This result strongly suggests that circumstellar enstatite has abundant stacking faults and is different from the single crystal.

Strain-induced first-order orbital flip transition and coexistence of charge-orbital ordered phases inPr0.5Ca0.5MnO3

Low-temperature transmission electron microscopy and X-ray diffraction (XRD) studies have been carried out on pellet and powder samples of Pr 0.5 Ca 0.5 MnO 3 . These studies have revealed appearance of a different type of charge-orbital ordered (COO) phase, resulting due to flipping of eg orbitals from d 3x 2 −r 2 /d 3y 2 −r 2 to d 3x 2 −r 2 /d 3z 2 −r 2 configuration. This orbital flip results in a changeover of the COO superlattice-ordering vector from (½,0,0) to (¼,½,¼) in the Pnma phase. This COO phase coexists with the conventional COO phase. Low-temperature XRD studies show that the COO phase appears only in pellet sample and not in the corresponding powder sample. The powder sample shows only conventional COO phase. Volume fractions of conventional and the other type COO phases in pellet sample of Pr 0.5 Ca 0.5 MnO 3 is estimated to be ~55% and 45%, respectively. The occurrence of orbital flip has been attributed to local strain building up in the pellet sample. The strain builds up during cooling because manganite has anisotropic thermal expansion coefficients.

Transmission Electron Microscopy and Luminescence Studies of Quantum Well Structures Resulting from Stacking Fault Formation in 4H-SiC Layers

The formation of 3C-SiC staking fault inclusions in higher band-gap other SiC polytypes provides a unique possibility to obtain quantum well (QW) structures without changing of chemical composition of epitaxial layers during growth. [1] In this report we present low temperature microphotoluminescence and transmission electron microscopy (TEM) studies of 3C-SiC stacking faults inclusions in 4H-SiC homoepitaxial layers, grown by molecular vapor deposition (MOCVD) on 4H-SiC substrates with an 8o surface miscut from the basal plane. TEM investigations showed that basal plane inclusion with cubic 3CSiC stacking faults present in the substrate propagate into the 4H-SiC layer material, forming inclined QW structures. The distance between SF was found to vary from 2 nm to 1.5 μm depending on the position on the layer. The presence of SF in the layer was confirmed by photoluminescence studies. They showed that in addition to excitonic emission typical for high quality 4H-SiC layers, well defined group of several emission lines around energy of 2.9 eV, which we attribute to QW formation induced by 3C-SiC staking faults [2]. In contrary to the emission due to nitrogen-bound excitons (observed at higher energy), which remains practically unchanged when changing position on the sample, the energy of spectral lines related to SF emission shows quasi-periodic jumps between well defined values when scanning on the sample surface (laser spot on the sample is of about 1-2 μm). The observed intensity pattern changes in a similar way. The obtained experimental data are discussed in terms of single, double and multiple quantum wells formed form neighboring 3C-SiC SF layers. It is confirmed that a concept of “structure-only” QW is promising from the point of view applications, provided precise control of FS formation during the growth of SiC based nanostructures is attainable. Our results show that, at least the formation of SF related QW structures can be successfully initiated by appropriate substrate preparation.

Occurrence of a new superlattice phase across the antiferroelectric phase transition inSr1−xCaxTiO3 (x = 0.30 and 0.40)

Low-temperature x-ray diffraction, low-temperature transmission electronmicroscopy (TEM) and dielectric permittivity studies have been carried out onSr1−xCaxTiO3 (x = 0.30 and 0.40) ceramics across its antiferroelectric phase transition. TEM has beenperformed for its structural and microstructural characterizations. Togetherwith the occurrence of the usual structural modulation along [001] of theP 21212 (andPbnm), which givesrise to the Pbcm phase, clear evidences of the occurrence of a new phase, appearing asa result of structural modulation along the [110]-type direction of thePbnm, has also been found. The temperature dependent dielectric permittivity data appears toindicate the paraelectric nature of the new phase. The coexistence of this new phase with thePbcm phase has beenattributed to Δ-point instability, which is equally probable along any of the [100]-type directions of the basicperovskite structure. It appears that irrespective of the existing perovskite distortion, which resultsin P 21212 for x = 0.3 and Pbnm for x = 0.4; the Δ-point instability follows the cubic symmetry of the very basic perovskite.

High salt stress induces swollen prothylakoids in dark-grown wheat and alters both prolamellar body transformation and reformation after irradiation

High salinity causes ion imbalance and osmotic stress in plants. Leaf sections from 8-d-old dark-grown wheat (Triticum aestivum cv. Giza 168) were exposed to high salt stress (600 mM) and the native arrangements of plastid pigments together with the ultrastructure of the plastids were studied using low-temperature fluorescence spectroscopy and transmission electron microscopy. Although plastids from salt-treated leaves had highly swollen prothylakoids (PTs) the prolamellar bodies (PLBs) were regular. Accordingly, a slight intensity decrease of the short-wavelength protochlorophyllide (Pchlide) form was observed, but no change was found in the long-wavelength Pchlide form emitting at 656 nm. After irradiation, newly formed swollen thylakoids showed traversing stromal strands. The PLB dispersal was partly inhibited and remnants of the PLBs formed an electron-dense structure, which remained after prolonged (8 h) irradiation. The difference in fluorescence emission maximum of the main chlorophyll form in salt-stressed leaves (681 nm) and in control leaves (683 nm) indicated a restrained formation of the photosynthetic apparatus. Overall chlorophyll accumulation during prolonged irradiation was inhibited. Salt-stressed leaves returned to darkness after 3 h of irradiation had, compared with the control, a reduced amount of Pchlide and reduced re-formation of regular net-like PLBs. Instead, the size of the electron-dense structures increased. This study reports, for the first time, the salt-induced swelling of PTs and reveals traversing stromal strands in newly formed thylakoids. Although the PLBs were intact and the Pchlide fluorescence emission spectra appeared normal after salt stress in darkness, plastid development to chloroplasts was highly restricted during irradiation.

極低温電顕法による電荷・軌道整列状態の層状ペロブスカイト型マンガン酸化物の超構造解析

The variation of the superstructure related to the charge/orbital ordering in layered manganites RE0.5 (Ca1-ySry) 1.5MnO4 (RE = Pr or Eu) has been systematically investigated by lowtemperature transmission electron microscopy (TEM), It is found that the superstructure is collapsed as increasing (1) the average ionic radius (rA) of A-site cations or enhancing (2) the quenched disorder by the substituting Sr for Ca. The results indicate that the electronic phase of the single layered manganites is controlled not only by the average ionic radius of A-site, but also by the quenched disorder ascribed as the variance in A-site ionic radii (σ2 = Σixiri2-rA2) .

Investigation of InAs quantum dot growth for electrical spin injection devices

AbstractThe influence of the growth conditions during molecular‐beam epitaxy on the properties of InAs/GaAs quantum dot structures were systematically investigated by low temperature photoluminescence spectroscopy and transmission electron microscopy. The circular polarization degree (CPD) of the electroluminescence was compared for two quantum‐dot spin‐injection light‐emitting diodes. The CPD depends on the position of the emission energy in the luminescence band. This correlation is similar for both samples despite the strongly different quantum dot morphologies. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Low-temperature transmission electron microscopy studies of Cd-based 2/1 approximants and quasicrystals

Transmission electron microscopy (TEM) observations on Cd-based 2/1 cubic approximants and quasicrystals were performed at low temperatures down to 20 K. For 2/1 approximants, the appearance of superlattice spots was commonly observed at temperatures below 110 K, suggesting the occurrence of a phase transition. For quasicrystals, no appreciable change in diffraction pattern was recognized at temperatures down to 20 K, which may suggest that no phase transition occurred.

Observation of spin reorientation in layered manganites La 1.2Sr 1.8(Mn 1−yRu y) 2O 7 (0.0⩽ y⩽0.2) by Lorentz transmission electron microscopy

The effect of Ru substitution for Mn in bilayered oxides La 1.2Sr 1.8(Mn 1− y Ru y ) 2O 7 ( 0 ⩽ y ⩽ 0.2 ) was investigated by magnetization measurements and low-temperature Lorentz transmission electron microscopy. It was found that the magnetic anisotropy is controlled by the Ru content y and temperature T. The easy axis changes from 〈1 1 0〉 for the y = 0 crystal to the c-axis for y = 0.2 , and it rotates away from the c-axis for the y = 0.05 and y = 0.07 crystals with decreasing temperature. Furthermore, maze-shaped magnetic domain structures were observed in the (0 0 1) thin crystals with 0.05 ⩽ y ⩽ 0.2 . Changes in domain size and structure indicate that the uniaxial magnetic anisotropy becomes stronger as Ru content y increases.

On the microstructure of the charge density wave observed in La1− x Ca x MnO3

We have used low temperature (90 K) transmission electron microscopy to investigate the ‘charge ordering’ modulation in the mixed valent manganite, La1− x Ca x MnO3. It has been stated that Mn3+ and Mn4+ ions order at low temperature to produce a structural modulation composed of supercells whose size is an integer multiple of the unmodulated unit cell. Here, we use convergent beam electron diffraction to show that the periodicity of the modulation need not be an integer multiple of the undistorted cell, even on the smallest scales. We therefore suggest that this modulation is a charge density wave with a uniform periodicity. We show that the modulation wavevector lies close to the a* axis of the crystal but need not be exactly collinear. A typical grain of size 0.5 µm in La0.48Ca0.52MnO3 had a wavevector which varied on a scale of tens of nanometres with an average of ⟨q⟩ = 0.450a * and a standard deviation Δq = 0.004 a* in its magnitude and Δθ = 0.56° in its direction at 90 K. The magnitude of the wavevector in this composition fell by 20% as the temperature was increased from 90 K to room temperature. This change occurred by nucleation and growth. Although weak, the modulation was still present at room temperature, some 30 K above the ‘charge ordering temperature’.

Structural properties and charge ordered states in RMnO 3 (R=La, Pr, Nd, Ca, Sr) and (La, Sr) 2NiO 4

Structural distortions arising from the condensations of two essential kinds of phonon modes: the triply degenerate rotational modes ( ϕ x , ϕ y, ϕ z ) of MnO 6 and the doubly degenerate Jahn–Teller active modes ( Q 1, Q 2 ) have been systematically investigated in the perovskite manganites. Microstructural features associated with certain types of distortions have been observed by transmission electron microscopy (TEM). In RMnO 3 and La(Sr) 2NiO 4, we characterize the local structure, charge ordered states and orbital ordering by means of low-temperature TEM. We present direct evidence that the stripe modulation in La(Sr) 2NiO 4 is indeed one-dimensional within each NiO 2 plane. Several typical kinds of defect structures, including antiphase boundaries and the 90°-twin domains, appear commonly in the charge-ordered states.

Charge/orbital ordering structure of Pr(1-x)Ca(x)MnO(3)(x = 3/8) examined by low-temperature transmission electron microscopy.

The structural phase transition of Pr(1-x)Ca(x)MnO(3)(x = 3/8) was investigated by means of low-temperature transmission electron microscopy. Superlattice reflection spots with a modulation wave vector q(1) = (0,1/2,0) appeared below 230 K, indicating formation of the d(3x(2-r(2))/d(3y(2)-r(2)) type of charge/orbital ordering. Below 150 K, a new series of superlattice reflection spots with a modulation wave vector q(2) = (1/4,1/4,1/2) appeared, suggesting an additional ordering of excess 1/8 Mn(3+), necessary due to the deviation of x from 1/2, with the occupation of the d(3z(2-r(2)) type of e(g) orbital.

低温電顕法によるＰｒ１‐ｘＣａｘＭｎＯ３の電荷軌道整列構造の解析

Charge- and orbital-ordered structures, induced in Pr1-xCaxMnO3 (x=3/8), are examined by low-temperature transmission electron microscopy. New type of orbital-ordered structure, containing additional Mn3+ with eg orbital along the c-direction, was proposed on the basis of the d3x2-r2/d3y2-r2 type of charge/orbital-ordered structure for x= 1/2.

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.