Qi-Fan Xiao

Monolithic polycapillary focusing optics and their applications in microbeam x‐ray fluorescence

Monolithic polycapillary focusing optics can collect a large solid angle of x rays from a point source and form intense focused beams for microbeam x‐ray fluorescence (MXRF) analysis. Such an optic was tested and characterized in a MXRF setup. For the Cu Kα line, the measured focal spot size of the optic was 43 μm full width at half‐maximum (FWHM). With the 16 W microfocusing x‐ray source (50 μm×10 μm), the average Cu Kα intensities over the focal spot was measured to be 8.9×104 photons/s/μm2. When compared to a straight single channel capillary optic with approximately the same output beam size, an intensity gain of 44 was obtained. The optic was applied to the MXRF setup to analyze trace elements in various samples and a minimum detection limit (MDL) of about 2 pg was achieved for the transition elements (V, Cr, Mn, and Fe). The optic was also used to map the distributions of trace elements in various samples.

Complete characterization by Raman spectroscopy of the structural properties of thin hydrogenated diamond-like carbon films exposed to rapid thermal annealing

We have demonstrated that multi-wavelength Raman and photoluminescence spectroscopies are sufficient to completely characterize the structural properties of ultra-thin hydrogenated diamond-like carbon (DLC:H) films subjected to rapid thermal annealing (RTA, 1 s up to 659 °C) and to resolve the structural differences between films grown by plasma-enhanced chemical vapor deposition, facing target sputtering and filtered cathodic vacuum arc with minute variations in values of mass density, hydrogen content, and sp3 fraction. In order to distinguish unequivocally between films prepared with different density, thickness, and RTA treatment, a new method for analysis of Raman spectra was invented. This newly developed analysis method consisted of plotting the position of the Raman G band of carbon versus its full width at half maximum. Moreover, we studied the passivation of non-radiative recombination centers during RTA by performing measurements of the increase in photoluminescence in conjunction with the analysis of DLC:H networks simulated by molecular dynamics. The results show that dangling bond passivation is primarily a consequence of thermally-induced sp2 clustering rather than hydrogen diffusion in the film.

Microstructure and properties of ultrathin amorphous silicon nitride protective coating

The effect of N content on the structure and properties of rf reactively sputtered amorphous silicon nitride (a-SiNx) has been studied by Rutherford backscattering spectrometry, x-ray reflectivity, ellipsometry, and nano-indentation. The N content in the film increased with the N2 concentration in the sputtering gas until the Si3N4 stoichiometry was reached. The hardness of a-SiNx increased with density, which in turn increased with the N content. The maximum hardness of 25 GPa and density of 3.2 g/cm3 were attained at the stoichiometric Si3N4 composition. With the application of a protective overcoat for magnetic disks in mind, thin a-SiNx films were deposited on CoPtCr media to examine their coverage, pinhole density, and wear resistance. According to x-ray photoelectron spectroscopy, the minimum thickness of a-SiNx required to protect the CoPtCr alloy from oxidation was 10 A, which was 10 A thinner than that of the reference amorphous nitrogenated carbon (a-CNx). A statistic model showed this lower thi...

Enhancement of microbeam x-ray fluorescence analysis using monolithic polycapillary focusing optics

A monolithic polycapillary focusing optic was tested in the microbeam x-ray fluorescence system at the Oak Ridge Center for Manufacturing Technology. The optic was designed to cover a wide energy range from 4 keV to 20 keV. The focal spot size of the output beam at 17.4 keV (Mo Kα) was measured to be 21 μm full width of half maximum. An average beam intensity of 1.5×105 photon/s/μm2 was obtained at the focus for Mo Kα line using a 12 W microfocus x-ray source. This intensity is about 2400 times over that of a direct beam at 100 mm away from the x-ray source. The small, intense x-ray beam obtained was used to analyze and map the compositions of different elements in industrial samples.

Measurements of capillary x-ray optics with potential for use in mammographic imaging.

Capillary optic arrays are bundles of hollow glass capillaries which guide x rays in a manner similar to the way fiber optics guide light. Focused postpatient capillary optic arrays have the potential to significantly improve both contrast and resolution of mammographic images compared to conventional antiscatter grids. Contrast can be improved by the nearly total scatter rejection of the optic. Effective resolution can be improved by geometric magnification without increased focal spot blurring. The best results were found for borosilicate glasses, with transmissions in excess of 60% for 22-cm-long fibers. To evaluate the scatter rejection properties, the transmission of off-axis radiation was measured. Transmission drops to < 1% at an angular displacement of 2.7 mrad. Transmission of a bulk capillary array dropped to near zero if the source was at an angle of 2.5 mrad. This implies excellent scatter rejection capabilities. To evaluate whether unchanneled photons might still reach the detector, absorption measurements were also performed on fibers and arrays. Absorption was found to be adequate for scatter rejection. All of the data agreed well with numerical simulations. Performance calculations for two potential optics geometries gave promising results.

Ultrathin TiSiN overcoat protection layer for magnetic media

TiSiN ultrathin films (10–50 A) deposited by reactive magnetron sputtering from TixSiy targets were used as anticorrosion overcoats to protect Co-containing recording media. Films’ growth, structure, composition, resistance against hydrolysis, and anticorrosion performances were studied by spectroscopy (x-ray reflectivity and diffraction, ellipsometry, x-ray photoelectron spectroscopy, Fourier transform infrared reflection–absorption spectroscopy) and simulated by molecular dynamics (using modified Tersoff-type interatomic interactions). TiSiN ultrathin overcoats were found to be dense amorphous oxynitrides containing Ti–O–Si linkages. The conversion of SiNx into SiOx by hydrolysis was prevented by introducing less than 20 at. % of Ti in the films. Thanks to the formation of Ti–O–Si linkages which densify the films and reduce oxygen diffusion, good corrosion protection of the magnetic media was achieved down to 28 A TiSiN overcoat thickness.

Numerical simulations for capillary-based x-ray optics

A numerical simulation for the transmission of x rays through glass capillaries of different geometries is described. The simulation includes corrections for surface roughness and absorption losses of the glass. These calculations are compared with experimental measurements of the transmission and exit divergence of the x rays. By fitting the experimental results with the calculations, the surface roughness of a capillary is estimated.

A multidentate lubricant for use in hard disk drives at sub-nanometer thickness

We describe a second generation of multidentate lubricant structures for use on a magnetic media in a hard disk drive. Building on earlier work where a perfluoropolyether (PFPE) chain with hydroxyl bonding moieties were placed in the middle of the chain as well as on chain ends, creating a structure with two PFPE sub-units for enhanced tribological performance under very low head-disk spacing, this paper focuses on a PFPE chain composed of three, even shorter PFPE sub-units. Experimental data focusing on surface characterization of sub-nanometer thickness films, as well as tribological performance, are presented that confirm the high confinement level achieved with the lubricant structure. Molecular dynamics calculations are also discussed, that are consistent with a molecular film of high stiffness, leading to a denser, more compact structure. This approach could pave the way to achieving the sub-nanometer head-disk clearance level, presumed necessary for storage densities exceeding the terabit per squar...

Capillary neutron optics for prompt-gamma activation analysis

A neutron lens has been constructed to focus cold neutrons from the exit of a58Ni neutron guide, which delivers a beam to the Prompt-Gamma Activation Analysis (PGAA) station at the NIST Cold Neutron Research Facility. The lens compresses a neutron beam of cross section 50 mm× 45 mm onto a focal spot of diameter 0.53 mm (fwhm) wich an average gain of 80 in neutron current density. PGAA measurements have been performed to demonstrate the enhanced sensitivity and detection limits for various elements and the spatial resolution in one transverse dimension. For the two test particles (a gadolinium glass bead and cadmium metal of sizes less than 0.5 mm), the gain in the γ-count rate with the lens is a factor of 60, and the detection limit is improved by a factor of 20. The system can be used for two-dimensional mapping of samples on a sub-millimeter scale to complement other analytical techniques such as neutron depth profiling (NDP).

Low surface energy and corrosion resistant ultrathin TiSiC disk overcoat

Ultra-thin films of titanium silicon carbide (TiSiC) were deposited by magnetron sputtering (using Ti2SiC3 targets) to form protection overcoats (OCs) onto magnetic recording media of hard disk drives. The chemico-physical properties (composition, optical constants, electrical resistivity, mass density, and surface energy) of titanium silicon nitride (TiSiN) films were measured and correlated to their OC performances in terms of protection against Si oxidation, Co corrosion, and Co diffusion. Performances of TiSiC OCs were compared to those of silicon carbide (SiC), silicon nitride (SiN), and TiSiN OCs. It was found that Ti incorporation into SiC and SiN considerably densifies the films, reduces their surface energy, and renders them more metallic. 25 A thick TiSiC OC forms stable protecting barriers than can sustain hydrolysis conditions without growth of surface silicon oxide or cobalt diffusion or oxidation in the underlying recording magnetic medium. Overall, TiSiC OCs outperformed TiSiN, SiC, and SiN...