Session Y23: X-rays

Effectively Pin-hole Collimated USAXS as a Premier Tool to Quantitatively Analyze Anisotropic Hierarchical Porous Microstructures

The 2D collimated Bonse-Hart USAXS instrument was used to study the anisotropic and hierarchical pore microstructures in freeze-cast lithium titanate (Li₄Ti₅O₁₂). USAXS data with scattering vector q=0.0001Å^-1 to 0.02Å^-1 were collected at sample orientations from -10^o to 190^o. I vs. azimuthal angle polar data at q values of 0.0003Å^-1, 0.0006Å^-1, 0.001Å^-1, and 0.005Å^-1 were also collected. Six Li₄Ti₅O₁₂ samples differing by salt and gel additives and lithium titanate particle size were measured. Samples are strongly anisotropic at low q (aspect ratio of 0.3 to 0.4) for intercolumnar porosity and mildly anisotropic at high q (aspect ratio of 0.8) for interparticle pores. Models suggests the pore microstructures are comprised of two large anisotropic populations: 1μm (Φ=0.77; SA=2.88m²/cm³) and 0.5μm (Φ=0.05; SA=0.35m²/cm³) and two small, relatively isotropic populations: 38nm (Φ=0.02; SA=1.38m²/cm³) and 14nm (Φ=0.16; SA=35.3m²/cm³).   

High Magnetic Field X-ray Diffraction at the National High Magnetic Field Laboratory (NHMFL)

We develop a custom high magnetic field x-ray diffraction system for measurements in the 25 T Florida split coil magnet at the DC Field Facility of NHMFL, Tallahassee, FL, to probe spin-lattice coupling at extreme steady-state magnetic fields. This allows thermal cycling of materials with first order transitions and thermal relaxation of highly conductive or magnetocaloric materials. The system utilizes a Mo rotating anode X-ray source and PILATUS 300K-W X hybrid pixel array detector customized to tolerate the fringe fields of the split coil magnet. We plan to further modify the diffractometer with an elliptical mirror system which will enhance intensity and create a convergent x-ray beam configuration. We present a study of the magnetocaloric material, AlFe₂B₂, highlighting the evolution of the unit cell volume in field and a study of the magnetostrictive material, GdNi, highlighting the evolution of the crystal axes at high field.   

High Resolution Digital X-Ray Detection via Optical Interrogation of Electro-Optic Materials

We report on the development of a high resolution (greater than 30 lp/mm), high quantum efficiency x-ray detector in the 40 to 60 keV range using non-linear optics. Typical indirect x-ray detectors use a scintillation material to convert high energy x-ray photons into a larger number of lower energy visible photons. Indirect x-ray detectors with the previously stated specifications have a low quantum efficiency due to the combination of a thin scintillation material, which has little x-ray stopping power, and a limited ability of the optical train and CMOS sensor to collect the visible photons. Instead of using a scintillator to convert incident x-rays into lower energy visible photons, we explore the use of non-linear optics to convert incident x-rays into a localized change in the index of refraction, which can be measured with optical interferometry. The non-linear optic consists of a biased Pockels Cell. When x-rays interact with the cell they release space charges which modify the electric field within the cell and thus changes its index. With this new design of x-ray detector, we explore the tradeoffs between x-ray stopping power, transverse spatial resolution, temporal resolution, and signal to noise ratio.   

Ultrafast resonant X-ray scattering investigation of non-thermal melting using XFELs

Ultrafast melting transitions in noble metals and semiconductors irradiated by femtosecond IR laser have been reported with signatures of lattice disorder in less than a picosecond timescale, which is faster than electron-phonon energy transfer rates. With this mysteriously fast reaction of crystal lattice, the interest in unveiling the fundamental energy transfer mechanism has been intense with extensive research effort using ultrafast electron diffraction, time resolved X-ray diffraction, etc. By employing time-resolved resonant X-ray scattering, we have investigated the ultrafast lattice melting with femtosecond IR laser irradiation synchronized to the X-ray laser pulses from PAL-XFEL. Experimental evidence on the electron transition driven lattice disorder has been obtained, supporting the bond softening explanation on the nonthermal melting in covalent bonding crystals. IR laser fluence dependence of the melting time suggests essential role of electrons in bonding orbitals in causing the crystal melting.   

Three dimensional structure determination of a core-shell nanoparticle by single particle 3D imaging at PAL-XFEL

Single particle imaging is a novel x-ray imaging technique that utilizes the extremely high brilliance and spatial coherence of X-ray Free Electron Laser (XFEL) to capture the complete 3D structure of non-crystalline sample. It enables high-resolution 3D imaging of non-crystalline biological specimens without staining, fixation, and sectioning. We established a hard x-ray fixed-target coherent diffraction imaging set-up at PAL-XFEL NCI beamline and recorded 2D diffraction patterns from sphere core-shell nanoparticles with trisoctahedral gold core. We reconstructed 3D image with two different approaches. One is to reconstruct the 3D reciprocal volume from the 2D diffraction patterns via EMC algorithm, and acquire 3D structure from phasing of reciprocal volume. The other is construct the 2D images first by phasing the single 2D diffraction patterns, followed by 3D image reconstruction from the 2D image sets which is a common scheme in cryo-EM. Clear core-shell structure in nanoscale and heterogeneity was observed in the reconstruction. Detailed analysis and comparison of both reconstruction would be discussed.   

Macro-scale structural homogeneity and mass density of bulk metallic glasses revealed by their rough surfaces and ultra-small angle neutron scattering (KIST-USANS)

It has been known that bulk metallic glasses (BMGs) are homogeneous at the atomic level. We have questioned whether the homogeneity of metallic glasses confirmed at the atomic can be sustained at a much larger scale. Ultra-small angle neutron scattering (USANS) enables us to measure the structure up to tens of micrometers.
Bulk metallic ribbons have inherent rough surfaces. The non-trivial surface scatterings could mislead USANS data as the homogeneous materials have inner heterogeneity. We demonstrate that the rough surface of melt-spun amorphous ribbons can be utilized to determine their homogeneity and mass density using the USANS. The scattering of the Cu₅₀Zr₅₀ amorphous alloy disappeared under the surface neutron contrast-matched environment, indicating that the scattering originated not from its internal structure but from the surface. This confirms the homogeneity at the atomic level sustains to a micrometer. On the other hand, the crystallized alloy showed a strong scattering under the matching environment due to the structural heterogeneity inside the alloy. This technique can apply to the bulk samples when the transmission is high enough, not causing multiple scattering.   

A high quality multilayer x-ray monochromator for third generation synchrotron sources

The ability to provide high spectral resolution with high dynamic photon flux is a hallmark of third generation synchrotron sources. However, under most circumstances, only two modes of operation are currently available for user studies, monochromatic and polychromatic modes. In polychromatic operation, the spectral bandwidth can exceed 1keV, which is often too large for accurate structural reconstruction in solid-state systems. In contrast, monochromatic operation comes at significant cost to photon flux, making the study of transient structural changes difficult to measure.

In this work we descibe the design and performance of a new multilayer monochromator for use at hard x-ray synchrotrons. This monochromator can provide users with both a tuneable bandwidth and increased photon flux sufficient for rapid structural determination. Our results demonstrate that this multilayer monochromator can provide widely tunable hard x-ray, possessing energy bandwidth of 20-100eV, and photon fluxes that significantly exceed standard double crystal monochromators. This capability not only has implications for time-domain materials research, but could be utilized on a variety of x-ray scattering/spectroscopic techniques.   

Critical-Dimension Grazing-Incidence Small Angle X-Ray Scattering: Enhancing the latent signal using Bragg scattering

The semiconductor industry is continuously pushing the limits of photolithography, with feature sizes now under 10 nm. X-ray scattering¹ has emerged as a possible contender to determine the average shape of a line grating with a sub-nanometer precision. However, to fulfill its promise, faster algorithms must also be developed to interpret and extract metrics from reciprocal space scattering data. We are presenting a novel, fast, and accurate X-ray technique and analysis algorithm: Critical Dimension Grazing Incidence X-ray Scattering, CD-GISAXS

The CD-GISAXS technique operates in grazing incidence configuration with a continuous azimuthal rotation of the sample, thus does not require high-energy X-rays to penetrate the wafer and greatly reduces the data acquisition times, permitting analysis within the framework of the DWBA. The Bragg rods coming from the line gratings, intersect with the momentum transfer vector of the elastic X-ray scattering at a single point above the horizon. The Bragg rods can be scanned by rotating the momentum transfer vector, and therefore the sample.

[1] D. Sunday et al, Journal of Micro/Nanolithography, MEMS, and MOEMS, 2013   

Applications of Thin Film Atomic Layer Deposition Superconducting Titanium Nitride to Astronomical Measurements

Microwave Kinetic Inductance Detectors (MKIDs) are an appealing option for conducting millimeter-wave astronomy and measuring the Cosmic Microwave Background due to their ease of fabrication and multiplexing. MKIDs make detections when a microwave photon breaks Cooper pairs in the superconducting metal, modifying the kinetic inductance of the metal and the resonant frequency of the on-chip resonator. A material of interest for fabricating MKIDs is Atomic Layer Deposition titanium nitride (ALD TiN) due to its high kinetic inductance and low critical temperature. We can precisely control these characteristics of the TiN by tuning deposition parameters such as thickness, stage temperature, and nitrogen flow. This method also admits high quality factors, allowing for increased multiplexing, and wafer-level uniformity with film thicknesses in the range of 4-18 nm. We present measurements of the critical temperature and internal quality factor of ALD TiN resonators under the variation of various ALD parameters as it relates to fabricating MKIDs.   

The status of the ultra-high resolution spectroscopy at the HFIR

To measure the crystal lattice distortion or the lifetime of weak interactions among quasiparticles, such as phonons, electrons and magnons, with high resolution, the key is to break the inverse relationship between the resolution and useable flux. By using the Larmor precession of the neutron spin inside a given magnetic field, its momentum or energy change during the interactions with sample can be measured with ultra-high resolution. Therefore, this unique property of neutron provides us with another approach to overcome some of the limitations of conventional neutron scattering instruments. Also, it can make the best use of all the available neutrons by allowing the use of large divergent beams. The progress on upgrading the HB-1 polarized triple axis spectrometer at the High Flux Isotope Reactor of ORNL with superconducting magnetic Wollaston prisms will be presented. For neutron diffraction, the achievable resolution of the absolute peak splitting and relative lattice distortion (Δd/d) can be 2×10^-4 and 1×10^-6 relatively. While for inelastic scattering, for example phonon linewidth measurements, the resolution can be <10µeV.   

Oxidation state sensitive 3D atomic mapping of oxide-supported catalysts through X-ray Standing Wave (XSW) excited X-ray photoelectron spectroscopy (XPS)

XSW excited XPS was used to study the hydroxylated surface of rutile TiO₂ (110) single crystal, where the excess unpaired electrons of the hydroxyls play a critical role in surface chemistry and photocatalytic processes. The experiments were performed at APS DND-CAT 5IDC using a 7 keV incident beam. The hydroxylated O 1s binding energy was 2.4 eV higher than that of the bulk-like O atoms and was found to have a different XSW coherent fraction and position. Summation of these Fourier components for 3 symmetry inequivalent hkl Bragg peaks generate a chemical state sensitive 3D atomic map of oxygen. We also studied monolayer coverages of vanadium oxide (catalytic material) supported on the same surface. Collecting V, O and Ti 1s XPS signals as we scanned across a Bragg peak at different stages of a redox reaction allowed tracking of surface sites for chemically distinct V atoms, important for understanding their catalytic behaviour. These results coupled with DFT calculations will give unique insights into this as well as other related oxide supported catalysts.   

Testing for the Continuous Spectrum of X-Rays Predicted to Accompany the Photoejection of an Atomic Inner Shell Electron

For decades, experimental evidence for an elegant prediction of quantum electrodynamics has been sought: the detection of a broad spectrum of X-rays at low energies expected upon sudden ejection of an inner shell electron following absorption of an incident high energy photon. We report here on a recent attempt that provides a significantly reduced upper limit on the process that is consistent with contemporary theory and contradicts earlier observations. The Cornell High Energy Synchrotron Radiation Source (CHESS) was employed to deliver a 46 keV incident beam on solid copper film targets. To suppress extraneous events detection of scattered photons was performed in coincidence with the fluorescent decay of the K shell hole produced by photoejection. Through variable thickness targets we assessed secondary processes.