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.     

In situ observation of oxygen deficiency occurring during electron irradiation in high Tc superconductor YBa2Cu3Oy

The decreasing process of oxygen in YBa₂Cu₃O _y is investigated through high resolution electron microscopy (HREM) and convergent beam electron diffraction (CBED). Measurements of the axial length in HREM images show that oxygen content y decreases faster near a twin boundary than at the inner part of a twin lamella. The transformation from an orthorhombic to a tetragonal phase starts at a twin boundary and the transformed region propagates to an inner region of lamella. Lattice strains are observed near boundaries between transformed and non-transformed regions. The transformation is almost complete within 30 s during observation of HREM images at 400 kV and at room temperature. A value of y was quantitatively measured by analysing observed intensities of energy-filtered CBED patterns with the dynamical theory. The value of y decreases from 6.9 to 6.5 when 200 kV electrons are irradiated for 160 s in a microscope at 108 K. More precise analysis of the intensities provides information on charge distribution along the c -axis as well as local oxygen content at a spatial resolution of several nanometres.     

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.     

Near- and far-field second-harmonic imaging of quasi-phase-matching crystals

Second-harmonic scanning near- and far-field optical microscopy of an electric-field poled KTiOPO₄ quasi-phase-matching crystal has been accomplished. This has been done in order to reveal the walls that form the intersections between inverted and non-inverted crystal domains. The domain walls are seen clearly only in images recorded by means of second-harmonic generation because of a large nonlinear contrast, and they appear as bright stripes when studied in a reflection geometry but they are dark when studied in transmission. The images show that the duty cycle of the quasi-phase-matching crystal differs from the ideal and that the walls are not completely smooth. These effects, in combination with the observed scattering from the domain walls, are expected to lower the output of the crystal when used for frequency doubling. We conclude that the wall thickness is no more than approximately 100 nm, which makes it a suitable test object for the resolution capabilities of scanning near-field optical microscopes that are used for nonlinear imaging.     

Near-field optical studies of local photomodification in nanostructured materials

Fractal aggregates of silver nanoparticles are studied experimentally using atomic force microscopy and photon scanning tunnelling microscopy. Large changes in the near-field optical response of fractal aggregates are observed after the irradiation of samples with nanosecond laser pulses. The threshold energy density for photomodification using a 532 nm laser is measured to be 9 mJ cm⁻². It is shown that photomodification-induced changes in the local optical response can be two orders of magnitude larger than changes in far-field absorption.     

Spatial distribution and polarization dependence of the optical near-field in a silicon microfabricated probe

This paper reports on the spatial distribution and polarization behaviour of the optical near-field at the aperture of a Si micromachined probe. A sub-100 nm aperture at the apex of a SiO₂ tip on a Si cantilever was successfully fabricated by selective etching of the SiO₂ tip in a buffered-HF solution using a thin Cr film as a mask. The aperture, 10–100 nm in size, can be reproducibly fabricated by optimizing the etching time. The optical throughput of several apertures was measured. For a 100 nm aperture, a throughput of 1% was approved. The probe shows a very high optical throughput owing to the geometrical structure of the tip. The spatial distribution of the near-field light is measured and simulated using a finite difference-time domain method. The polarization behaviour of apertures with different shapes was analysed using a photon counting camera system.     

Optical microcantilever consisting of channel waveguide for scanning near-field optical microscopy controlled by atomic force

We develop a novel optical microcantilever for scanning near-field optical microscopy controlled by atomic force mode (SNOM/AFM). The optical microcantilever has the bent channel waveguide, the corner of which acts as aperture with a large tip angle. The resonance frequency of the optical microcantilever is 9 kHz, and the spring constant is estimated to be 0.59 N/m. The optical microcantilever can be operated in contact mode of SNOM/AFM and we obtain the optical resolution of about 200 nm, which is as same size as the diameter of aperture. We confirm that the throughput of optical microcantilever with an aperture of 170 nm diameter would be improved to be more than 10⁻⁵.     

Three-dimensional morphometry in scanning electron microscopy: a technique for accurate dimensional and angular measurements of microstructures using stereopaired digitized images and digital image analysis

A method for accurate dimensional and angular measurements of microstructures analysed in the scanning electron microscope is described. The method considers central and parallel projections and involves (a) digital image acquisition of stereopaired images from the scanning electron microscope's photodisplay, (b) generation of 3D-image representations, (c) setting of measuring points in the digitized stereopaired images, (d) computation of exact space coordinates ( x / y / z ) from the corresponding point coordinates ( x _L/ y _L; x _R/ y _R), (e) determination of distances and angles between consecutive corresponding points using vector equations, and (f) transfer of computed data into spreadsheets of the data analysis software using dynamic data exchange with simultaneous graphical display of the frequency distribution of variables.
Measurements performed on specimens with known dimensions (grid with 10 μm wide square meshes, polystyrene beads with 0.33 μm diameter) and angles (synthetic crystals of K(Al,Cr)[SO₄], CuSO₄.5H₂O and NaCl) revealed a high accuracy in dimensional as well as angular measurements (total error 1 ± 0.5%).
In Monte Carlo experiments the overall error was found to depend strongly on the size of the measured structure relative to the size of the measurement field (field width).     

Inducing superconductivity at a nanoscale: photodoping with a near-field scanning optical microscope

The local modification of an insulating GdBa₂Cu₃O_6.5 thin film, made superconducting by illumination with a near-field scanning optical microscope (NSOM), is reported. A 100-nm aperture NSOM probe acts as a sub-wavelength light source of wavelength λ_exc = 480–650 nm, locally generating photocarriers in an otherwise insulating GdBa₂–Cu₃O_6.5 thin film. Of the photogenerated electron–hole pairs, electrons are trapped in the crystallographic lattice, defining an electrostatic confining potential to enable the holes to move. Reflectance measurements at λ = 1.55 μm at room temperature show that photocarriers can be induced and constrained to move on a ≈200 nm scale for all investigated λ_exc. Photogenerated wires present a superconducting critical temperature T _c = 12 K with a critical current density J _c = 10⁴ A cm⁻². Exploiting the flexibility provided by photodoping through a NSOM probe, a junction was written by photodoping a wire with a narrow (≈ 50 nm) under-illuminated gap. The strong magnetic field modulation of the critical current provides a clear signature of the existence of a Josephson effect in the junction.     

Towards phonon photonics: scattering-type near-field optical microscopy reveals phonon-enhanced near-field interaction

Diffraction limits the spatial resolution in classical microscopy or the dimensions of optical circuits to about half the illumination wavelength. Scanning near-field microscopy can overcome this limitation by exploiting the evanescent near fields existing close to any illuminated object. We use a scattering-type near-field optical microscope (s-SNOM) that uses the illuminated metal tip of an atomic force microscope (AFM) to act as scattering near-field probe. The presented images are direct evidence that the s-SNOM enables optical imaging at a spatial resolution on a 10 nm scale, independent of the wavelength used (λ=633 nm and 10 μm). Operating the microscope at specific mid-infrared frequencies we found a tip-induced phonon-polariton resonance on flat polar crystals such as SiC and Si₃N₄. Being a spectral fingerprint of any polar material such phonon-enhanced near-field interaction has enormous applicability in nondestructive, material-specific infrared microscopy at nanoscale resolution. The potential of s-SNOM to study eigenfields of surface polaritons in nanostructures opens the door to the development of phonon photonics—a proposed infrared nanotechnology that uses localized or propagating surface phonon polaritons for probing, manipulating and guiding infrared light in nanoscale devices, analogous to plasmon photonics.     

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.     

Spectroscopic scanning near-field optical microscopy with a free electron laser: CH2 bond imaging in diamond films

Hydrogen chemistry in thin films and biological systems is one of the most difficult experimental problems in today's science and technology. We successfully tested a novel solution, based on the spectroscopic version of scanning near-field optical microscopy (SNOM). The tunable infrared radiation of the Vanderbilt free electron laser enabled us to reveal clearly hydrogen-decorated grain boundaries on nominally hydrogen-free diamond films. The images were obtained by SNOM detection of reflected 3.5 µm photons, corresponding to the C–H stretch absorption, and reached a lateral resolution of 0.2 µm, well below the λ/2 (λ= wavelength) limit of classical microscopy.     

A study of charge distribution in Bi2Sr2CaCu2O8+δ superconductors using novel electron-diffraction and imaging techniques

We report our study of the distribution of valence electrons in Bi₂Sr₂CaCu₂O_8+δ high-temperature superconductors using novel electron-diffraction and imaging techniques. The former method was based on quantitative analyses of the diffraction intensity of many reflections as a function of crystal thickness to determine, with an unprecedented accuracy, the Fourier components of the electron distribution in Bi₂Sr₂CaCu₂O_8+δ. The latter was based on examining the effect of charge transfer on many-beam imaging by comparing the observed and calculated low- and high-resolution images of long-period displacive and charge modulation of the cuprate. Our study demonstrates that fast electrons have greater sensitivity than X-rays to valence electrons distribution at small scattering angles, and that electron microscopy is a powerful tool in revealing charge distribution in materials.     

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.     

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.     

Femtosecond near-field scanning optical microscopy study of molecular thin films

A near-field scanning optical microscope has been combined with a two-colour time-resolved pump-probe measurement system. It has a noise-equivalent transmittance change of 5.0 × 10⁻⁵ for a probe pulse with an intensity of 30 nW. The system has been used for evaluating molecular thin films that have a domain structure, particularly for observing a gate action of the single domains. The results include key features to understand an origin of the domains and suggest that the film composition is uniform over a distance of several micrometres.     

Interface study by dual-beam FIB-TEM in a pressureless infiltrated Al(Mg)–Al2O3 interpenetrating composite

This paper considers the microstructures of an Al(Mg)–Al₂O₃ interpenetrating composite produced by a pressureless infiltration technique. It is well known that the governing principle in pressureless infiltration in Al–Al₂O₃ system is the wettability between the molten metal and the ceramic phase; however, the infiltration mechanism is still not well understood. The objective of this research was to observe the metal–ceramic interface to understand the infiltration mechanism better. The composite was produced using an Al-8 wt% Mg alloy and 15% dense alumina foams at 915°C in a flowing N₂ atmosphere. After infiltration, the composite was characterized by a series of techniques. Thin-film samples, specifically produced across the Al(Mg)–Al₂O₃ interface, were prepared using a dual-beam focussed ion beam and subsequently observed using transmission electron microscopy. XRD scan analysis shows that Mg₃N₂ formed in the foam at the molten alloy–ceramic infiltration front, whereas transmission electron microscopy analysis revealed that fine AlN grains formed at the metal–ceramic interface and MgAl₂O₄ and MgSi₂ grains formed at specific points. It is concluded that it is the reactions between Al, Mg and the N₂ atmosphere that improve the wettability between molten Al and Al₂O₃ and induce spontaneous infiltration.     

A nanometre-scale non-periodic structural variation in high temperature superconducting ceramics and the implications for properties

Non-periodic structural variation has been found in the high T _c cuprates, YBa₂Cu₃O_{7- x} and Hg_0.67Pb_0.33Ba₂Ca₂Cu₃O_8+δ, by image analysis of high resolution transmission electron microscope (HRTEM) images. We use two methods for analysis of the HRTEM images. The first method is a means for measuring the bending of lattice fringes at twin planes. The second method is a low-pass filter technique which enhances information contained by diffuse-scattered electrons and reveals what appears to be an interference effect between domains of differing lattice parameter in the top and bottom of the thin foil. We believe that these methods of image analysis could be usefully applied to the many thousands of HRTEM images that have been collected by other workers in the high temperature superconductor field. This work provides direct structural evidence for phase separation in high T _c cuprates, and gives support to recent stripes models that have been proposed to explain various angle resolved photoelectron spectroscopy and nuclear magnetic resonance data. We believe that the structural variation is a response to an opening of an electronic solubility gap where holes are not uniformly distributed in the material but are confined to metallic stripes. Optimum doping may occur as a consequence of the diffuse boundaries between stripes which arise from spinodal decomposition. Theoretical ideas about the high T _c cuprates which treat the cuprates as homogeneous may need to be modified in order to take account of this type of structural variation.     

Tapping-mode tuning-fork near-field scanning optical microscopy of low power semiconductor lasers

The newly developed inverted tapping-mode tuning-fork near-field scanning optical microscopy (TMTF-NSOM) is used to study the local near-field optical properties of strained AlGaInP/Ga_0.4In_0.6P low power visible multiquantum-well laser diodes. In contrast to shear-force mode NSOM, TMTF-NSOM provides the function to acquire the evanescent wave intensity ratio | I (2ω)|/| I (ω)| image, from which the evanescent wave decay coefficient q can be evaluated for a known tapping amplitude. Moreover, we probe the near-field stimulated emission spectrum, which gives the free-space laser light wavelength λ_o and the index of refraction n _r of the laser diode resonant cavity. Once q , λ_o, and n _r are all measured, we can determine the angle of incidence θ_o of the dominant totally internally reflected waves incident on the front mirror facet of the resonator. Determination of such an angle is very important in modelling the stability of the laser diode resonator.     

Evaluation of 26Mg stable isotope as an in vivo tracer of magnesium transport for SIMS ion microscopy imaging studies

Isotopic detection with high sensitivity, one of the most important features of ion microscopy, allows the in vivo application of stable isotopes as tracers for unravelling smaller tissue structures implicated with transport capabilities. The evaluation of the mass interferences associated with a particular mass of secondary ion signals is a necessity for tracer studies with stable isotopes. We have tested the feasibility of ²⁶Mg stable isotope as a tracer of magnesium transport in the killifish. The fish were given a single intraperitoneal injection of 3 μmol ²⁶MgCl₂.6H₂O (99.5% ²⁶Mg enrichment), and the renal distribution of ²⁶Mg was examined in frozen freeze-dried cryosections with ion microscopy. High-mass resolution analyses were performed to evaluate the purity of positive secondary ion signals of the nominal masses 24, 25 and 26 in order to assess the purity of ²⁴Mg, ²⁵Mg and ²⁶Mg signals, respectively, in kidneys of control and ²⁶Mg-injected fish. In kidneys of control fish, the purities of ²⁴Mg, ²⁵Mg and ²⁶Mg signals were ≈ 97%, 82% and 90%, respectively. In fish that were injected with ²⁶Mg stable isotope, an enhancement of ²⁶Mg⁺ secondary ion signals was observed with signal purity reaching 95%. These observations indicate that ²⁶Mg can be used successfully as a tracer of magnesium transport in animal models. To uncover the distribution of tracer ²⁶Mg from the naturally abundant background of this isotope, a pixel-by-pixel digital subtraction is applied to the raw ion microscopy mass 26 image.