X-ray techniques using synchrotron radiation in materials analysis

The extraordinary properties of SR have opened up new horizons for all techniques which use vacuum ultraviolet light and X-rays. In this article, some applications of SR in materials science have been discussed. X-ray absorption spectroscopy is a local, atom specific probe of the geometrical and electronic structure of the absorbing species. As an example, the lattice site location and the valence of Co in a doped high-T_c, superconductor YBa₂Cu_2–5Co_0–5O_7–4 has been discussed.     

Detailed studies of molecular conductance using atomic resolution scanning tunneling microscopy

Atomically resolved scanning tunneling microscopy and spectroscopy (STM/STS) have been used to carefully examine the relationship between molecular conductivity and the adsorption state of various organic molecules on silicon surfaces. We show that the particular configuration of styrene and cyclopentene molecules on silicon affects the conductivity of the molecules. Detailed correlation of STM images with point specific current-voltage spectroscopy reveal that observed negative peaks are due to random configuration changes driven by inelastically scattered electrons and not due to tuned alignment of molecule and electrode levels. These random processes, which include molecular rearrangement, desorption, and/or decomposition occur with increasing frequency at larger voltage and current settings.     

Studies of photon induced gas desorption using synchrotron radiation

In view of finalizing the design of the vacuum system of the Large Electron and Positron Storage Ring (LEP) we have studied synchrotron radiation induced neutral gas desorption. A 3 m section of an aluminum vacuum chamber has been exposed to the photon beam emerging from the electron storage ring DCI in Orsay, under conditions closely simulating the environment in a particle acceletor. In order of importance the gases desorbed were H₂, CO₂, CO and CH₄ with H₂O practically absent. Under the experimental conditions of an unbaked chamber and 11 mrad glancing incidence of the photons, the initial molecular desorption yields for these gases were typically 0.5, 8 × 10^?2, 2 × 10^?2 and 8 × 10^?3 molecules per photon respectively. These values could be reduced by about 1 to 2 orders of magnitude during continued photon exposure and most cases without evidence that this ‘beam cleaning action’ would be limited. After exposure to air and pumpdown of the previously cleaned chamber, we observe a significant memory effect. The dependence of the photon desorption on the angle of incidence has been studied down to a glancing angle of 11 mrad showing a definite deviation from the previously assumed 1/sin ø scaling. The implications of the results in terms of the expected beam-gas lifetime in LEP are discussed.     

Energy dispersive X-ray fluorescence analysis with synchrotron radiation

Energy dispersive X-ray fluorescence measurements were made for trace elements using synchrotron radiation from a dedicated electron storage ring at the Photon Factory in Japan. Monochromatization of the excitation beam by a crystal monochromator is effective in improving the signal to background ratio, while the synchrotron continuum excitation attains an absolute detectability of better than pg. Trace elements in a thin sample down to 60 ppb are detected by monochromatic excitation when there is no line interference. The experimental geometry optimal for a high signal to background ratio is investigated experimentally. The practical advantages of monochromatic excitation for the selective excitation of a particular element are demonstrated; these include enhanced sensitivity, suppression of the effects of the matrix element, and removal of overlapping peaks. The effects of higher order reflections and the residual tail part of the diffraction curve are also discussed in connection with quantitative analysis.     

Creation of stable molecular junctions with a custom-designed scanning tunneling microscope

The scanning tunneling microscope break junction (STMBJ) technique is a powerful approach for creating single-molecule junctions and studying electrical transport in them. However, junctions created using the STMBJ technique are usually mechanically stable for relatively short times (<1?s), impeding detailed studies of their charge transport characteristics. Here, we report a custom-designed scanning tunneling microscope that enables the creation of metal-single molecule-metal junctions that are mechanically stable for more than 1?minute at room temperature. This stability is achieved by a design that minimizes thermal drift as well as the effect of environmental perturbations. The utility of this instrument is demonstrated by performing transition voltage spectroscopy-at the single-molecule level-on Au-hexanedithiol-Au, Au-octanedithiol-Au and Au-decanedithiol-Au junctions.     

Measurement of the dimensions of microscopic objects with a scanning tunneling microscope

Translated from Izmeritel'naya Tekhnika, No. 1, pp. 23–24, January, 1992.     

Direct measurement of standing evanescent waves with a photon-scanning tunneling microscope

We present a detailed analysis of a standing evanescent wave that is caused by total internal reflection of an Ar-ion laser beam on a glass prism and investigate the coupling to a subwavelength dielectric tip of a photon-scanning tunneling microscope that is raster scanned at a close distance over the prism surface. The intensity of the evanescent field is spatially modulated with a period of 239.2 nm. It decays exponentially with a constant of 103.9 nm with increasing distance from the prism surface. Precise measurements of the standing evanescent wave can be used to calibrate the scanner and permit one to determine the spatial resolution and the coupling efficiency of the tip.     

An atomic resolution scanning tunneling microscope that applies external tensile stress and strain in an ultrahigh vacuum

We have developed an ultrahigh vacuum scanning tunneling microscope with an in situ external stress application capability in order to determine the effects of stress and strain on surface atomistic structures. It is necessary to understand these effects because controlling them will be a key technology that will very likely be used in future nanometer-scale fabrication processes. We used our microscope to demonstrate atomic resolution imaging under external tensile stress and strain on the surfaces of wafers of Si(111) and Si(001). We also successfully observed domain redistribution induced by applying uniaxial stress at an elevated temperature on the surface of a wafer of vicinal Si(100). We confirmed that domains for which an applied tensile stress is directed along the dimer bond become less stable and shrink. This suggests that it may be feasible to fabricate single domain surfaces in a process that controls surface stress and strain.     

Characterization of an Al-STJ-based X-ray detector with monochromatized synchrotron radiation

The characterization of an Al-STJ-based detector with Pb absorber was performed with monochromatized synchrotron radiation. Detector response was measured in the energy range from 3 to 10 keV. A small non-linearity of the signal pulse height was detected, probably due to the escape of recombination phonons from the detector. The non-linearity can be described by a second order polynomial function. Additionally, detector signals were recorded while an X-ray beam of 50 μm diameter was directed to several locations on and near the absorber. For a well-aligned beam, detector artefacts are of at least two orders of magnitude lower intensity than the absorber events.     

Nanoscale control of reversible chemical reaction between fullerene C60 molecules using scanning tunneling microscope

The nanoscale control of reversible chemical reactions, the polymerization and depolymerization between C60 molecules, has been investigated. Using a scanning tunneling microscope (STM), the polymerization and depolymerization can be controlled at designated positions in ultrathin films of C60 molecules. One of the two chemical reactions can be selectively induced by controlling the sample bias voltage (V(s)); the application of negative and positive values of V(s) results in polymerization and depolymerization, respectively. The selectivity between the two chemical reactions becomes extremely high when the thickness of the C60 film increases to more than three molecular layers. We conclude that STM-induced negative and positive electrostatic ionization are responsible for the control of the polymerization and depolymerization, respectively.     

Investigations of local electrical properties using tunneling/atomic force microscope with a quartz tuning fork nearfield sensor

Ability to determine local electric surface properties with a high resolution is a key issue in many modern industrial applications. In this article, authors will describe low-cost and reliable methods for investigations of electrical surface properties with a nanoscale resolution using a homebuilt modular tunneling/atomic force microscope with a quartz tuning fork as a probe. We will present the architecture of the designed system and the calibration method of the applied sensor. In our work, the usage of the tunneling atomic force microscope in the high-resolution investigations of the surface topography and identification of local spots where the tunneling current is observed will be demonstrated. We will also present current-voltage (I-V) spectroscopy performed on a gold thin film sputtered on silicon substrate and a highly oriented pyrolitic graphite (HOPG) surface, which we obtained in air ambient and at room temperature.     

Residual stress characterization of Al/SiC nanoscale multilayers using X-ray synchrotron radiation

Nanolayered composites are used in a variety of applications such as wear resistant coatings, thermal barrier coatings, optical and magnetic thin films, and biological coatings. Residual stresses produced in these materials during processing play an important role in controlling their microstructure and properties. In this paper, we have studied the residual stresses in model metal-ceramic Al/SiC nanoscale multilayers produced by physical vapor deposition (magnetron sputtering). X-ray synchrotron radiation was used to measure stresses in the multilayers using the sin²Ψ technique. The stresses were evaluated as a function of layer thicknesses of Al and SiC and also as a function of the number of layers. The stress state of Al in the multilayer was largely compressive, compared to single layer Al stresses. This is attributed to a peening mechanism due to bombardment of the Al layers by SiC and Ar neutrals during deposition. The stress evolution was numerically modeled by a simplified peening process to qualitatively explain the Al thickness-dependent residual stresses.     

新型大扫描范围原子力显微镜的研究

研制了一种大扫描范围原子力显微镜(AFM)。设计了新的扫描驱动电路,使单幅图像的扫描范围大幅度提高;用步进电机和扫描器配合扫描,得到序列图像,序列图像拼接后获得大范围样品图像。实验结果表明,采用这一方法,在±150V 的电压驱动下,AFM 的扫描范围可增大到10 ìm?1 mm 的量级,同时保持 1 nm 量级的测试分辨力。     

Direct visualization and identification of biofunctionalized nanoparticles using a magnetic atomic force microscope

Because of its outstanding ability to image and manipulate single molecules, atomic force microscopy (AFM) established itself as a fundamental technique in nanobiotechnology. (1) We present a new modality that distinguishes single nanoparticles by the surrounding magnetic field gradient. Diamagnetic gold and superparamagnetic iron oxide nanoparticles become discernible under ambient conditions. Images of proteins, magnetolabeled with nanoparticles, demonstrate the first steps toward a magnetic analogue to fluorescence microscopy, which combines nanoscale lateral resolution of AFM with unambiguous detection of magnetic markers.     

Direct observation of isolated ultrananodimensional diamond clusters using atomic force microscopy

Isolated ultrananodimensional diamond (UND) particles obtained by means of detonation synthesis have been studied using atomic force microscopy (AFM). The UND particles were deposited onto the surface of highly oriented pyrolytic graphite from a suspension based on organic compounds. The deposited UND particles were deaggregated using a two-stage treatment with ultrasound and high-dynamic-pressure pulses. The isolated UND particles were stabilized in suspension by a benzene additive. AFM images of individual UND particles have been obtained, and the phenomenon of their alignment along atomic steps on the substrate surface has been observed.     

High-performance permanent magnetic circuit designed for small-angle X-ray scattering using synchrotron radiation source

A new type of permanent magnetic circuit with several prominent characteristics was developed and applied to the studies of oriented macromolecular assemblies by small-angle X-ray scattering using synchrotron radiation source.     

Photodiode array for position-sensitive detection using high X-ray flux provided by synchrotron radiation

Synchrotron radiation provides a high intensity source over a large range of wavelengths. This is the prominent quality that has laid the foundations of the EXAFS development (Extended X-ray Absorption Fine Structure). EXAFS data can be collected in different ways. A full scan requires 5 to 10 min, compared to the one-day data collection of a conventional Bremsstrahlung X-ray tube.Recently, by using the new photodiode array (R 1024 SFX) manufactured by Reticon, it has been possible to reduce the data collection time to less than 100 ms. The key elements of this new EXAFS method are a dispersive optics combined with a position sensitive detector able to work under very high flux conditions. The total aperture of 2500 μm × 25 μm for each pixel is well suited to spectroscopic applications. Besides its high dynamic range (> 10⁴) and its linearity, the rapidity of the readout allows a flux of 10⁹–10¹⁰ photons/s over the 1024 sensing elements.     

Estimation of tunneling-barrier width in scanning tunneling microscope from noise characteristics

A method is proposed for estimation of the potential-barrier width of the tunneling junction of a scanning tunneling microscope (STM). The method is based on measurement of the fluctuations of the tunneling-barrier height and their correlation with the flicker noise of the tunneling current. Measurements with the STM feedback enabled and disabled are considered. Experimental results are presented for a tunneling junction formed by a platinum-iridium tip and a graphite specimen. The principal sources of experimental error are examined. Russia. Translated from Izmeritel’naya Tekhnika, No. 12, pp. 55–59, December, 1999.     

Real time X-ray scattering study of the formation of ZnS nanoparticles using synchrotron radiation

We investigate the growth of ZnS nanoparticles by a real-time simultaneous small and wide angle X-ray scattering (SAXS, WAXS) study using synchrotron radiation. Zinc chloride and elemental sulfur were dissolved in oleylamine. The formation of nanoparticles was induced by heating to 170 °C and 215 °C. The influence of temperature, reaction time, and sulfur concentration was investigated. After a short phase of rapid growth, saturation in size and a slower growth is observed depending on the temperature. The final size of the nanoparticles ranges between 2 and 6 nm for the investigated growth conditions and increases with the reaction temperature and sulfur concentration. SAXS analysis allows for determination of the size of the nanoparticles and proves also the existence of an organized layer of oleylamine molecules covering the nanoparticles' surfaces, which, however, appears only for diameters of the nanoparticles larger than approximately 2.8 nm. The investigation of the measured structure factor of the nanoparticle assemblies showed that the distance of an attractive interaction is 2.5 nm, which was interpreted as a consequence of the ordered oleylamine surface layer.     

Direct imaging of molecular orbitals of metal phthalocyanines on metal surfaces with an O2-functionalized tip of a scanning tunneling microscope

High-resolution scanning tunneling microscope images of iron phthalocyanine and zinc phthalocyanine molecules on Au(111) have been obtained using a functionalized tip of a scanning tunneling microscope (STM), and show rich intramolecular features that are not observed using clean tips. Ab initio density functional theory calculations and extended Hückel theory calculations revealed that the imaging of detailed electronic states is due specifically to the decoration of the STM tip with O₂. The detailed structures are differentiated only when interacting with the highly directional orbitals of the oxygen molecules adsorbed on a truncated, [111]-oriented tungsten tip. Our results indicate a method for increasing the resolution in generic scans and thus, have potential applications in fundamental research based on high-resolution electronic states of molecules on metals, concerning, for example, chemical reactions, and catalysis mechanisms.