Phase evolution in cholesterol/DPPC monolayers: atomic force microscopy and near field scanning optical microscopy studies

A combination of atomic force microscopy (AFM) and near field scanning optical microscopy has been used to study domain formation in dipalmitoylphosphatidylcholine (DPPC)/cholesterol monolayers with cholesterol concentrations ranging from 0 to 50%. The results show a clear evolution from a mixture of liquid expanded and liquid condensed phases for cholesterol concentrations < 10% to a mixture of liquid expanded and two cholesterol‐containing phases at intermediate concentrations, and finally to a single homogeneous liquid ordered phase for 33% cholesterol. Mixtures of the various phases are clearly identified by height differences in AFM and in some cases by fluorescence imaging for samples containing 0.5% BODIPY dye, which localizes preferentially in the more fluid phase. Note that fluorescence imaging, at least with the dye used here, is unable to distinguish between the cholesterol‐rich and cholesterol‐poor phases detected at intermediate cholesterol concentrations. The combination of fluorescence and AFM imaging provides a more complete picture of the phase evolution for cholesterol/DPPC monolayers than could be obtained by either technique alone, and presents substantial advantages over conventional fluorescence microscopy in that submicrometre‐sized domains can be readily detected.     

HREM and STEM of intergranular films at zinc oxide varistor grain boundaries

Grain boundaries in model ZnO–Bi₂O₃ and ZnO–Bi₂O₃–CoO varistors and a commercial multicomponent varistor have been characterized by high-resolution electron microscopy (HREM) and scanning transmission electron microscopy (STEM), in order to determine the relationship between Bi grain boundary segregation and formation of thin intergranular films. By controlling Bi₂O₃ content, applied pressure and temperature, the grain boundary Bi excess has been systematically varied from nearly zero to Γ_Bi = 1 × 10¹⁵ cm⁻² (≈ 1 monolayer), as measured by HB 603 STEM using an area-scan method. HREM shows that intergranular amorphous films are clearly distinguishable in samples with Γ_Bi > 8 × 10¹⁴ cm⁻². These films range in thickness, depending on the Bi excess, from 0.6 to 1.5 nm. Similar films of ≈ 1 nm thickness are widely observed in the commercial varistor. The composition of the films is a ZnO–Bi₂O₃ solid solution, which is in all cases more enriched in ZnO than the bulk eutectic liquid. The Bi-doped grain boundaries in ZnO varistors therefore contain an intergranular amorphous film which has not only an equilibrium thickness, but also a distinct equilibrium composition.     

Element specific imaging with high lateral resolution: an experimental study on layer structures

Planar defects and individual layers in ceramic material are chemically imaged by high resolution energy-filtering TEM using a post-column imaging electron energy filter. Objects are barium layers in the cuprate superconductor NdBa₂Cu₃O_7−δ (isostructual to YBa₂Cu₃O_7−δ) as well as planar defects and precipitates of β-WB in tungsten- and chromium-doped TiB₂. The barium layers with a spacing of 0.42 nm in the cuprate are resolved in jump-ratio images using the Ba_N edge. In the boride system the β-WB precipitates with thickness of 0.8 nm can be chemically imaged in elemental maps of B_K, Ti_L ,Cr_L and W_M. The B as well as the Ti map show a decrease in intensity at the precipitates, whereas in the W map an increase in intensity is observed. The boron-deficient layers with a spacing of 0.38 nm in the β-WB precipitate can be resolved in boron jump-ratio images. Additionally, defects containing single boron-deficient layers are chemically imaged. Hence structures in the dimension of interatomic distances can be imaged with respect to their elemental constituents. Although high resolution electron spectroscopic images contain strong interference contrast from elastic scattering, after normalization or background subtraction the element specific images are dominated by chemical contrast.     

Studies of cell division (mitosis and cytokinesis) by dynamic secondary ion mass spectrometry ion microscopy: LLC-PK1 epithelial cells as a model for subcellular isotopic imaging

The feasibility of the renal epithelial LLC-PK₁ cell line as a model for cell division studies with secondary ion mass spectrometry (SIMS) was tested. In this cell line, cells undergoing all stages of mitosis and cytokinesis remained firmly attached to the substrate and could be cryogenically prepared. Fractured freeze-dried mitotic cells showed well-preserved organelles as revealed by fluorescence imaging of rhodamine-123 and C₆-NBD-ceramide by confocal laser scanning microscopy. Secondary electron microscopy analysis of fractured freeze-dried dividing cells revealed minimal surface topography that does not interfere in isotopic imaging of both positive (³⁹K, ²³Na, ²⁴Mg, ⁴⁰Ca, etc.) and negative (³¹P, ³⁵Cl, etc.) secondaries with a CAMECA IMS-3f ion microscope. Mitotic cells revealed well-preserved intracellular ionic composition of even the most diffusible ions (total concentrations of ³⁹K⁺ and ²³Na⁺) as revealed by K : Na ratios of approximately 10. Structurally damaged mitotic cells could be identified by their reduced K : Na ratios and an excessive loading of calcium. Quantitative three-dimensional SIMS analysis was required for studying subcellular calcium distribution in dividing cells. The LLC-PK₁ model also allowed SIMS studies of M-phase arrested cells with mitosis-arresting drugs (taxol, monastrol and nocodazole). This study opens new avenues of cell division research related to ion fluxes and chemical composition with SIMS.     

Structure of electrodeposited graded composite coatings of Ni–Al–Al2O3

The structure of electrodeposited composite coatings of Ni–Al–Al₂O₃, with Ni as the matrix and Al and Al₂O₃ as second-phase particles, was investigated using light microscopy and scanning electron microscopy. Ni coatings with no particles, which were used as reference samples, had progressively coarser structures with increasing current density. Co-deposition with Al resulted in refinement of the Ni matrix structure at high (>10 A dm⁻²) current densities. For single-particle baths, the co-deposition of Al₂O₃ was more strongly affected by current density and bath particle content than was the co-deposition of Al. However, for baths containing both Al and Al₂O₃ the amount of incorporated Al₂O₃ no longer depended on current density. With the choice of appropriate conditions, coatings of Ni with up to 39 vol.% Al₂O₃ were made. Similar experiments with Al yielded a maximum of 17.5 vol.% only. Uniform and graded mixed-particle coatings were also produced. When coatings containing Al were annealed, the reaction of the two elements resulted in the formation of either single-phase γ or two-phase γ–γ' alloys, in agreement with the equilibrium phase diagram.     

A study of the initial growth of PtxCo(1−x) thin films on Si3N4

Pt _x Co_{(1− x )} thin films where x  =0.24 have been deposited onto Si₃N₄ windows and studied using transmission electron microscopy. The films are used in ultrahigh-density recording studies and it was found that the surface of the substrate had a strong influence on the microstructure, crystallography and magnetic properties of the film. An investigation of the early growth of the film was made by studying films of different thickness between 100 and 300 Å. It was found that the grains were hexagonal in structure with a strong c -axis orientation perpendicular to the surface.     

Microfabricated silicon dioxide cantilever with subwavelength aperture

We have developed a microfabricated SiO₂ cantilever with subwavelength aperture for scanning near-field optical microscopy (SNOM), to overcome the disadvantages of conventional optical fibre probes such as low reproducibility and low optical throughput. The microcantilever, which has a SiO₂ cantilever and an aperture tip near the end of the cantilever, is fabricated in a reproducible batch process. The circular aperture with a diameter of 100–150 nm is formed by a focused ion-beam technique. Incident light is directly focused on the aperture from the rear side of the cantilever using a focusing objective, and high optical throughput (10⁻² to 10⁻³) is obtained. The microcantilever can be operated as a SNOM probe in contact mode or in dynamic mode.     

Imaging the three orientation variants of the DO22 phase by 3D atom probe microscopy

Three-phase NiAlV alloys were investigated using a three-dimensional atom probe. Ageing at 800 °C gives rise to the precipitation of two ordered phases within the supersaturated FCC solid solution, namely Ni₃Al (L1₂ structure) and Ni₃V (DO₂₂ structure). The DO₂₂ phase has three orientation variants which need to be identified in 3DAP images. It is shown that an appropriate choice of analysis site enables us to image the chemical order within both L1₂ and DO₂₂ ordered phases and to distinguish the three orientation variants of the DO₂₂ phase in reconstructed images. The lateral resolution of 3DAP in these experimental conditions was estimated through simple considerations to be less than 0.3 nm.     

Formation process of β-FeSi2 from amorphous Fe-Si synthesized by ion implantation: Fe concentration dependence

Formation processes of β-FeSi₂ from amorphous Fe-Si layers have been investigated using transmission electron microscopy (TEM). Si(111) substrates were irradiated with 120 keV Fe ions at −150°C to fluences of 1.0 × 10¹⁷ and 4.0 × 10¹⁷ cm^–2. An amorphous Fe-Si layer embedded in an amorphous Si was formed in the low-fluence sample, whereas an amorphous Fe-Si surface layer on an amorphous Si was obtained in the high-fluence one. The amorphous Fe-Si layers were crystallized to β-FeSi₂ after thermal annealing at 800°C for 2 h. Cross-sectional and plan-view TEM observations revealed that, prior to the formation of β-FeSi₂, the amorphous Fe-Si layers crystallized to α-FeSi₂ in the low-fluence sample and to ɛ-FeSi in the high-fluence one. The absence of metastable γ-FeSi₂ which is considered as a precursor of epitaxially grown β-FeSi₂ on Si was attributed to the instability of γ-phase in an amorphous matrix.     

Energy-filtering TEM and electron energy-loss spectroscopy of double structure tabular microcrystals of silver halide emulsions

Composite Ag(Br,I) tabular microcrystals of photographic emulsions were studied by the combination of energy-filtering electron microscopy (EFTEM) and electron energy-loss spectroscopy (EELS) in conjunction with energy-dispersive X-ray (EDX) microanalysis. The contrast tuning under the energy-filtering in the low-loss region was used to observe more clearly edge and random dislocations, {11⁻1} stacking faults in the grain shells parallel to {11⁻2} edges and bend and edge contours. Electron spectroscopic diffraction patterns revealed numerous extra reflections at commensurate positions in between the Bragg reflections and diffuse honeycomb contours; these were assigned to the number of defects in the shell region parallel to the grain edges and polyhedral clusters of interstitial silver cations, respectively. Inner-shell excitation bands of silver halide were detected and confirmed by EDX analyses, i.e. the Ag N_2,3 edge at 62 eV (probably overlapped with the weak I N_4,5 edge at 52 eV and the Br M_4,5 edge at 70 eV), the I M_4,5 edge at about 620 eV, and the Br L_2,3 edge at about 1550 eV energy losses. Energy-loss near-edge structure of the Ag M_4,5 edge at about 367 eV energy losses and low-loss fine structure arisen as a result of interband transitions and excitons, possibly superimposed with many electron effects, have been revealed. The crystal thickness was determined by a modified EELS log-ratio technique in satisfactory agreement with measurements on grain replicas.