Session A36: Focus Session: X-ray and Electron Optics and Microscopy

Optimal conditions for combining a transmission x-ray microscope with a grating interferometer

Transmission x-ray microscope (TXM) is a powerful imaging tool that can provide resolution down to 15$\sim $20 nm. Grating interferometer (GI) is a recently established imaging technique with which both phase and attenuation information of an arbitrary specimen can be extracted in a straightforward way. The achievable resolution of a GI is limited by either the grating analyzer period or the detector pixel size, which is currently at about micron level. It is natural to imagine that combing a TXM with a GI (TXMGI) will give ability to image a weak-absorbing specimen with high resolution. However, it is not trivial to obtain reliable structure information from a TXMGI. In this presentation, we will discuss the dependence of the interferogram on three key parameters in a TXMGI, i.e. the coherence of the illumination beam, the numerical aperture of the TXM, and the grating period. Based on that result, the optimal conditions and the limits on the achievable resolution are obtained.   

Consideration in the Reconstruction of 3D atomic Structure from X-Ray

X-Ray Holography is a promising technique for recovery of the three dimensional structure of materials. With the advent of high flux sources and fast x-ray detectors this method is under serious consideration as main stream technique. Simulations based on spherical atomic scattering factors method are performed to estimate the effects of wave front curvature by application to specific systems. An assessment is made of the distortions which arise if artifacts such as errors in the position of the origin exists. Simulations are performed to determine the influence of counting rates on the image reconstruction.   

Synchrotron topographic studies of stacking faults in type-IIa diamond crystals

High-quality diamond is an ideal material for various synchrotron x-ray optical applications. However, diamond crystals generally contain various defects, among which stacking faults (SFs) are more detrimental since they are planar defects extended up to square centimeters. In this presentation, we will introduce monochromatic synchrotron topographic studies of SFs (as well as other defects) in type-IIa diamond crystals. SFs show strong contrast at the tails of the rocking curve, thus broadening the rocking curve width. The outcrops of SFs on the crystal surface still show sharp white-line contrast at the Bragg peak, indicating strong strains or lattice misorientations near the outcrops. From the variation of SF contrast with the rocking angle, we obtain a detailed picture how the extended SFs influence the diffraction performance of diamond crystals. A straightforward diffraction contrast mechanism of SFs will also be presented in addition to the dynamical theory description.   

Design, nanofabrication and testing of silicon and diamond hard X-ray optics

We have designed, fabricated and tested silicon (Si) and diamond based X-ray kinoform lenses. In design and nanofabrication of such X-ray optics elements, surface roughness and wall verticality are among the tasks of critical importance for achieving as-designed performance. Our cyclic cryogenic RIE method [1], developed to deal with such stringent nanofabrication requirements, is comparable in performance to licensed Bosch process, as established in measurements of the surface roughness, and the etch rate ($>$2 microns/min for Si, $\sim $100 nm/min for diamond) and verticality ($<$1deg over 100 microns) parameters. We compare nanofabrication procedures for Si and diamond lenses, and discuss relative merits of Si and diamond as materials for X-ray lenses. We also show sub-100 nm spot size tests of kinoform Si-based X-ray focusing optics, determined via knife edge measurements at APS 8-ID, and a preliminary results of tests of diamond-based kinoform lens at NSLS, performed at X13B and APS 8-ID. [1] A. F. Isakovic et al., submitted for publication.   

X-ray Reflectivity and Power Spectral Density of Smoothly Polished Silicon

Silicon polished by means of chemical-mechanical-polishing has been studied. A finely crevaced top surface was seen in the AFM data. The power-spectral-density was measured by means of interferometry and by atomic force microscopy, and a roughness value of 0.21- 0.23 nm rms was found by integration. X-ray reflectivity data for 10 keV x-rays were obtained at the Advanced Photon Source, and a roughness of 0.22 - 0.30 nm was found to be roughly consistent with these data. A surface layer with a slightly higher density than that of crystalline silicon was needed to model the x-ray reflectivity. Crevaces 3.6 nm deep and resulting in land areas having 85{\%} coverage were invoked for the modeling. A total layer thickness of 7.4 nm was invoked for the modeling . That is, the crevaces penetrated roughly half way through the total layer thickness. Due to the overall agreement between the two very different techniques for measuring roughness, namely, PSD and x-ray reflectivity data, we consider these results to accurately quantify roughnesses for a silicon surface that is near the state-of-art for smoothness.   

Wide-angle incidence x-ray waveguides prepared by micro-/nano-technology using crystal surface diffraction

Grazing incidence x-ray waveguides have been most studied because of its simple geometry and its applicability for all photon energies. However, wide-angle incidence waveguides are also essential for modern x-ray optics, as far as coupling/guiding x-ray beams into given directions are concerned. To investigate this possibility we have prepared waveguides on silicon wafers by x-ray lithographic technique. The waveguides are 100$\mu $m high and 1cm long with different widths and the distance between the adjacent waveguides is 2.5 mm. Both the top and bottom surface of a waveguide are plated with gold. With this type of waveguides we have actually observed the effects of guiding x-rays in both lateral and vertical directions using (113) surface diffraction in Au/Si waveguide systems.   

Fluctuation electron microscopy studies of complex structured materials

Fluctuation electron microscopy (FEM) is a hybrid imaging-diffraction technique. This technique is particularly sensitive to paracrystalline structures of dimension 0.5-2 nm, which are difficult to detect by either imaging or diffraction techniques alone. It has been successfully deployed to study paracrystalline structures in amorphous silicon, germanium thin film. This technique has also been used to study metallic glasses and oxide glasses. Until now, FEM has not been used to study disordered geological materials. In this talk we present our FEM studies of shungite, a naturally occurring disordered carbonaceous material, reveal that trace quantities of tightly curved graphene structures such as C60, or fragments of C60, is present in shungite. We also present results from our study of metamict zircon, whose crystal structure is destroyed by self-radiation during naturally occurring $\alpha $ decay events. Work is in progress to study the structural evolution during the metamictization process.   

Holography with Low Energy Electrons - a New Tool for Structural Biology

Holography is widely used for three-dimensional imaging of macroscopic objects using visible light. The same principle can also be applied for imaging of individual molecules like DNA or larger objects, for instance viruses. The holograms are recorded with coherent low energy electrons with wavelength, and thus potential resolution, in the sub-nanometer regime. The experimental setup together with holograms of individual biological molecules and their numerical reconstructions shall be presented. Current experimental and theoretical challenges of holography with low energy electrons will also be addressed. Strong forward scattering of electron waves has been taken into account for the reconstruction process. Since most biological molecules exhibit phase shifting rather than absorbing properties, the retrieval of the phase parallel to the absorbing properties of an object has been realized. On the experimental side, a method towards a significantly improved signal to noise ratio in holograms has been established by acquiring several hundred short pulsed holograms followed by a cross correlation alignment. Finally, the solution to the twin image problem in holography will be presented and reconstructed twin-image free experimental holograms will be shown.   

Theory of ultrafast electron diffraction: the role of the pulse properties.

We present a general formalism for scattering of electron bunches used in ultrafast electron diffraction (UED) experiments that incorporates characteristic parameters of the incident electron bunch. To perform the scattering calculation, we associate the classical distribution function, which describes the electron bunch just before scattering, with the asymptotic-in Wigner distribution. Using single-scattering and far-field approximations appropriate for typical UED experimental conditions, and considering the effects of the bunch parameters on the scattered signal, we derive two diffraction expressions. We derive a Fraunhofer type expression suitable for scattering from small samples, such as molecules, and a Fresnel type expression appropriate for scattering from large targets such as thin films. In our analysis we also identify the coherence length of an electron bunch. We present sample numerical calculation for scattering by nanosize particles based on our model, and discuss the effects of bunch and scattering target parameters on the diffraction signal.   

Determination of Adsorbed C$_{60}$ Nanostructures by Low-Energy Electron Diffraction

We have recently extended to nanostructures the basic theoretical capabilities of surface structure determination by Low Energy Electron Diffraction (LEED), by adopting a non-periodic cluster approach and substantially accelerating the computation time for complex structures. In this contribution, we describe two further theoretical enhancements and their application to experimental data for buckyballs adsorbed on a Cu(111) surface. One enhancement addresses occasional situations where strong multiple scattering causes poor convergence: this is solved by treating all scattering within subclusters of a few atoms with accurate matrix inversion. Secondly, for the structure determination of complex nanostructures, an efficient search method is essential: for that purpose a modified version of tensor LEED is adapted to nanostructures, called NanoTensorLEED. We exhibit the resulting ability to analyze detailed nanostructures with the case of buckyballs adsorbed on a Cu(111) surface.   

RHEED-TRAXS as a tool for in-situ stoichiometry control.

RHEED-total reflection x-ray spectroscopy (-TRAXS) is an in-situ chemical and structural characterization technique which is highly surface sensitive. This consists of a grazing-angle electron beam from which characteristic x-rays from the sample are measured also at grazing angles. We have demonstrated that monolayer sensitivity in Y and Mn films on GaN can be achieved. We have also developed a theoretical model for the angular dependence of the x-ray K\textit{$\alpha $} peaks for the thin films, based on Parratt's formalism for x-ray reflectivity and the electron trajectory simulation software CASINO, to correct for grazing angle electron beam as a source for x-rays. As the angular dependence is highly dependent upon the film thickness and the smoothness of the film, it can be used to determine the deposition rate of individual elements as well as the interface chemical roughness   

First background free measurement of the inelastic tail of the Auger electron spectrum down to 0 eV

Background free measurements of the inelastic tail of the Auger electron energy spectrum were performed by using very low energy positrons to excite Auger transitions in Au and Cu via positron-electron annihilation. The kinetic energy of the incident positrons (1.5eV) was set below the energy threshold required to excite secondary electrons resulting in a Auger spectra that was completely free of collision induced secondary electrons. The measured spectra contain contributions solely from either annihilation induced Auger electrons or annihilation induced Auger electrons that have lost energy on the way out. By using the time of flight technique it was possible to measure the inelastic tail of the Auger electron energy spectrum down to 0 eV.