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.     

Frequency-domain fluorescence microscopy with the LED as a light source

We describe a frequency-domain lifetime fluorometer based on a microscope and a modulated light-emitting diode (LED) excitation source (370/460 nm), which operates in the frequency range 120 Hz–250 MHz. We collected multifrequency phase and modulation fluorescence responses from cellular areas as small as 10–15 µm in diameter. We also collected fluorescence lifetime data from cells stained by a lipophilic coumarin sensitized europium fluorophore, Coum-Eu, with a millisecond lifetime, and Ru(bpy)₂phe-C₁₂, with microsecond lifetime. Nanosecond lifetimes from native nuclei stained with SYTO 14 and SYTO 16 probes were measured as well. We demonstrate that a simple LED excitation source can, for many applications, successfully replace complex and expensive laser systems, which have been used for cellular frequency-domain lifetime measurements. As the LEDs are very stable with low noise, it will be possible to image even smaller sample areas using brighter LEDs. With availability of modulated LEDs emitting at several wavelengths covering almost the entire visible spectrum it is easy to assemble a system for the fluorophore of choice. The ability to select an excitation source for a given fluorophore and low price make such an excitation source even more practical.     

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.     

The correlation between polarization modulated near‐field optical images and the anisotropy of the probe

The true anisotropic images taken with the polarization modulation near‐field optical microscope are often influenced by the linear dichroism of the tapered fibre probe. In this paper, we develop a new method to separate the anisotropic image form probe's dichroism . Our calculations show that the near‐field optical image is simply a vector sum of the sample's dichroism and the probe's dichroism, when the probe's anisotropy is small. With this result, we demonstrate the true anisotropic images of poly(phenylene vinylene) (PPV) thin films. The PPV films show non-uniform mesoscale dichroic domains with average domain size, ∼ 0.3 mm, and the coefficient for linear dichroism is 1.25 × 10⁴ cm⁻¹.     

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.     

Scanning force images through the 'Milliscope'– a probe microscope with very wide scan range

The effectiveness and adequacy of a home-built scanning force microscope (SFM) able to cover a volume of ∼1.2 × 1.2 × 0.13 mm³ (X × Y × Z) were tested on calibrating objects, as well as on cytological and histological samples. The instrument was designed for matching the magnification range of an optical microscope (∼ 20–1200×) but its dynamics were one or two orders of magnitude higher, thanks to a lateral resolution of about 10 nm. Images ranging in size from 1.2 × 1.2 mm² to 1 × 1 µm² showed a quality comparable to that given by other SFMs on similar materials. The 'Milliscope' is a curious but effective imaging tool whose operating range overlaps at one extreme with a goldsmith's eyepiece, and at the other with an electron microscope. The intrinsic limits of scanning probe techniques and of the available SFM cantilevers prevented us taking complete advantage of the wide height range of our scanner. However, our results show that an instrument having a very wide scan area, obtained through simple, inexpensive and intrinsically linear techniques, can give a good performance even at small scan sizes. This encourages us to develop wide scan instruments, which could further increase the already extensive use of scanning force microscopy in biology.     

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.     

Identification of intergranular phases in ceramic nanocomposites

Analytical FEG-TEM was used for nanostructural and nanochemical characterization of Al₂O₃–TiN (composite I) and Si₃N₄–TiN (composite II) ceramic composite systems. The presence of vitreous intergranular phases in pockets at multiple grain junctions and in thin films (≈ 0.8 nm thick) at grain boundaries was revealed by high resolution and Fresnel fringe imaging techniques. The existence of a Ti-rich thin intergranular film at alumina grain boundaries was revealed by EDS line-scanning across internal interfaces at the 1.5 nm lateral resolution level. Extracting interface specific information at subnanometre levels by means of quantitative spatial difference EELS allowed an identification of intergranular phases. Ti sub-oxide existed in thin films at Al₂O₃ and TiN grain boundaries, whereas a mixed Al–Ti–O–N glassy phase was observed in pockets at triple grain junctions in composite I. In composite II, residual siliceous oxide and oxynitride glass phases were identified in thin films at Si₃N₄ grain boundaries and multiple grain junctions, respectively. These observations indicated that the chemistry of the intergranular phase in thin grain boundary films is notably different from that in larger pockets at multiple grain junctions.     

The interaction of surface plasmon polaritons with a silver film edge

A prism coupling arrangement is used to excite surface plasmons at the surface of a thin silver film and a photon scanning tunnelling microscope is used to detect the evane-scent field above the silver surface. Excitation of the silver/air mode of interest is performed at λ₁ = 632.8 nm using a tightly focused beam, while the control of the tip is effected by exciting a counter-propagating surface plasmon field at a different wavelength, λ₂ = 543.5 nm, using an unfocused beam covering a macroscopic area. Propagation of the red surface plasmon is evidenced by an exponential tail extending away from the launch site, but this feature is abruptly truncated if the surface plasmon encounters the edge of the silver film — there is no specularly reflected 'beam'. Importantly, the radiative decay of the surface mode at the film edge is observable only at larger tip–sample separations, emphasizing the importance of accessing the mesoscopic regime.     

Moulded photoplastic probes for near-field optical applications

The inexpensive fabrication of high-quality probes for near-field optical applications is still unsolved although several methods for integrated fabrication have been proposed in the past. A further drawback is the intensity loss of the transmitted light in the 'cut-off' region near the aperture in tapered optical fibres typically used as near-field probes. As a remedy for these limitations we suggest here a new wafer-scale semibatch microfabrication process for transparent photoplastic probes. The process starts with the fabrication of a pyramidal mould in silicon by using the anisotropic etchant potassium hydroxide. This results in an inverted pyramid limited by < 111 > silicon crystal planes having an angle of ∼ 54°. The surface including the mould is covered by a ∼ 1.5 nm thick organic monolayer of dodecyltrichlorosilane (DTS) and a 100-nm thick evaporated aluminium film. Two layers of photoplastic material are then spin-coated (thereby conformal filling the mould) and structured by lithography to form a cup for the optical fibre microassembly. The photoplastic probes are finally lifted off mechanically from the mould with the aluminium coating. Focused ion beam milling has been used to subsequently form apertures with diameters in the order of 80 nm. The advantage of our method is that the light to the aperture area can be directly coupled into the probe by using existing fibre-based NSOM set-ups, without the need for far-field alignment, which is typically necessary for cantilevered probes. We have evidence that the aluminium layer is considerably smoother compared to the 'grainy' layers typically evaporated on free-standing probes. The optical throughput efficiency was measured to be about 10⁻⁴. This new NSOM probe was directly bonded to a tuning fork sensor for the shear force control and the topography of a polymer sample was successfully obtained.     

Accurate structure determinations of very small (4–20 nm) areas, using refinement of dynamic electron diffraction data

It is shown that accurate structure refinements are possible from electron diffraction data obtained using areas 4–20 nm in diameter and 4–20 nm thick. To obtain accurate atomic positions it is essential to take the dynamic diffraction fully into account, for which new software was developed. Examples (La₃Ni₂B₂N₃, Ba₂Ca₃Cu₅O_10+δ and Ce₅Cu₁₉P₁₂) are given of the structure refinements of known and unknown structures with R -values in the range 2–6%. The procedure reported in this paper opens the way to structure determination of particles in many materials of industrial and scientific interest which cannot be solved by conventional structure determination (e.g. because of small size, heavily twinned materials or small fractions in polyphase materials).     

Picosecond time-resolved microspectrofluorometry in live cells exemplified by complex fluorescence dynamics of popular probes ethidium and cyan fluorescent protein

Time‐resolved microspectrofluorometry in live cells, based on time‐ and space‐correlated single‐photon counting, is a novel method to acquire spectrally resolved fluorescence decays, simultaneously in 256 wavelength channels. The system is calibrated with a full width at half maximum (FWHM) of 90 ps for the temporal resolution, a signal‐to‐noise ratio of 10⁶, and a spectral resolution of 30 (Δλ/Λ). As an exemple, complex fluorescence dynamics of ethidium and cyan fluorescent protein (CFP) in live cells are presented. Free and DNA intercalated forms of ethidium are simultaneously distinguishable by their relative lifetime (1.7 ns and 21.6 ns) and intensity spectra (shift of 7 nm). By analysing the complicated spectrally resolved fluorescence decay of CFP, we propose a fluorescence kinetics model for its excitation/desexcitation process. Such detailed studies under the microscope and in live cells are very promising for fluorescence signal quantification.     

Soft-X-ray damage to biological samples

X-ray damage to biological samples was investigated in the wavelength region of 2.7–5 nm, which overlaps the so-called 'water window', the wavelength range of 2.4–4.3 nm usually used in X-ray microscopy. Yeast cells and myofibrils were chosen as representatives of whole cell samples and motile protein systems, respectively. The samples were exposed to X-rays using an apparatus composed mainly of a laser-plasma X-ray source, a Wolter mirror condenser, and a sample cell. The yeast cells lost their dye exclusion ability when the X-ray flux was higher than 1 × 10⁶ photons μm⁻², while the myofibrils lost contractility when the X-ray flux was higher than 4 × 10⁵ photons μm⁻². These X-ray fluxes are lower than the flux required for the X-ray microscope observation of biological samples at a resolution higher than that of light microscopes.     

Towards metabolic mapping of the human retina

Functional alterations are first signs of a starting pathological process. A device that measures parameter for the characterization of the metabolism at the human eye-ground would be a helpful tool for early diagnostics in stages when alterations are yet reversible. Measurements of blood flow and of oxygen saturation are necessary but not sufficient. The new technique of auto-fluorescence lifetime measurement (FLIM) opens in combination with selected excitation and emission ranges the possibility for metabolic mapping. FLIM not only adds an additional discrimination parameter to distinguish different fluorophores but also resolves different quenching states of the same fluorophore. Because of its high sensitivity and high temporal resolution, its capability to resolve multi-exponential decay functions, and its easy combination with laser scanner ophthalmoscopy, multi-dimensional time-correlated single photon counting was used for fundus imaging. An optimized set up for in vivo lifetime measurements at the human eye-ground will be explained. In this, the fundus fluorescence is excited at 446 or 468 nm and the time-resolved autofluorescence is detected in two spectral ranges between 510 and 560 nm as well as between 560 and 700 nm simultaneously. Exciting the fundus at 446 nm, several fluorescence maxima of lifetime t1 were detected between 100 and 220 ps in lifetime histograms of 40 degrees fundus images. In contrast, excitation at 468 nm results in a single maximum of lifetime t1 = 190 +/- 16 ps. Several fundus layers contribute to the fluorescence intensity in the short-wave emission range 510-560 nm. In contrast, the fluorescence intensity in the long-wave emission range between 560 and 700 nm is dominated by the fluorescence of lipofuscin in the retinal pigment epithelium. Comparing the lateral distribution of parameters of a tri-exponential model function in lifetime images of the fundus with the layered anatomical fundus structure, the shortest component (t1 = 190 ps) originates from the retinal pigment epithelium and the second lifetime (t2 = 1,000 ps) from the neural retina. The lifetime t3 approximately 5.5 ns might be influenced by the long decay of the fluorescence in the crystalline lens. In vitro analysis of the spectral properties of expected fluorophores under the condition of the living eye lightens the interpretation of in vivo measurements. Taking into account the transmission of the ocular media, the excitation of NADH is unlikely at the fundus.     

Transmission electron microscopy studies of squeeze cast Al-AlN composites

Aluminium-matrix composites containing ∼45 vol.% AlN particles were fabricated by melt infiltration of aluminium into an AlN preform under a pressure up to 130 MPa. Three types of aluminium alloy (2024, 6060 and 5754) were used. The as-prepared composites were studied by light microscopy, scanning and transmission electron microscopies, and energy-dispersive X-ray spectroscopy. As a result of the melt infiltration process, the composites are very dense and the microstructure shows a homogeneous distribution of the reinforcement. The interfaces are clean with very little porosity. Composites with 2024 and 6060 matrices were carefully studied by transmission electron microscopy (TEM) and high resolution electron microscopy (HREM) after heat treatments. Dislocation density in the matrix of the reinforced material increases due to the difference in thermal expansion coefficients of aluminium alloys and AlN. This can induce an accelerated ageing response of the coherent and semicoherent precipitations of age-hardened matrices. This behaviour has been studied in the 2024 and 6060 composites by using microhardness measurements and TEM. Reactions between the AlN reinforcement and aluminium matrices (6060 and 5754) were observed and analysed by TEM. Matrices containing some of magnesium display a MgAl₂O₄ spinel formation at the AlN/matrix interface. The spinel formation is probably due to the reaction between magnesium of the matrix and the thin Al₂O₃ layer on the AlN surfaces. This reaction can affect the mechanical behaviour of the composite infiltrated with the 5754 matrix. This has been confirmed by overageing some samples at high temperatures (300 °C and 550 °C) for 10 days in order to emphasize the interfacial reactions.     

Structural studies of pulsed-laser deposited nanocomposite metal-oxide films

Pulsed laser deposition in vacuum has been used to develop metal-oxide nanocomposite films with well controlled structural quality. Results for the copper–aluminium oxide (Cu:Al₂O₃) system are used to illustrate the main morphological and structural features of these films. High resolution transmission electron microscopy (TEM) analysis shows that the films consist of Cu nanocrystals with average dimensions that can be controlled between 2 nm and 10 nm embedded in an amorphous Al₂O₃ matrix. It is observed that the in-plane shape of the nanocrystals evolves from circular to elongated, and the number of nanocrystals per unit area decreases as their size increases. This evolution is explained in terms of nucleation at the substrate surface and coalescence during the later stages of growth. The thermal stability of the films has been studied by in situ TEM annealing and no transformation could be observed up to about 800 °C when partial crystallization of the Al₂O₃ starts.     

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.     

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.     

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.     

Direct observation of a Ti/Cu interface with atomic displacement under electron irradiation

A Ti film was deposited onto a Cu substrate by means of a radio frequency magnetron sputtering method. Cross-sectional thin foils for TEM observation were prepared using a focused ion beam. Electron irradiation was carried out using a high-resolution high-voltage electron microscope operated at 1.25 MV . The Cu/Ti interface of the foils was irradiated at 623 K. In-situ observation images during electron irradiation were recorded by a CCD camera with a digital video cassette. The (020)_Cu plane on the Cu/Ti interface preferentially moved towards the Ti film with irradiation. Composition analysis of the diffused region showed that its composition corresponded to Ti₃Cu₂.