X-ray holography with atomic resolution

The most used tool for resolving the position of atoms in crystalline structures is x-ray diffraction. Structural information is deduced from the intensities of sharp Bragg peaks. Both the quality and the dimensions of a crystal are crucial for successful application of this technique. Unfortunately, substances in nature often do not form crystalline structures, or in many cases good quality crystal can not be grown in the laboratory. With the perspective of a formidable jump in the flux provided by future x-ray sources, attention is drawn to new methods which would make it possible to resolve the structure of small samples without long-range translational periodicity. [1]

One promising method for structural studies is holography with atomic resolution. [2,3] Holography does not suffer from the “phase problem” inherent in diffraction methods and allows direct imaging of the local environment of a selected element. Although the entire hologram can be recorded in one shot [4], the solution of the twin images problem requires the collection of multiple-energy holograms [5] and thus the internal detector concept of holography [6], which requires the collection of fluorescence intensity measurements for thousands of sample orientations. This drawback, together with a very low signal-to-background ratio (approximately 0.01–0.1 %), makes holographic experiments time-demanding and thus the problem of radiation damage much more severe than in the case of diffraction.

Our aim is to develop a holography method which makes it possible to make the crucial step from demonstration experiment to real applications. From this point of view, the use of diffuse scattering holography [7] instead of fluorescence holography seems to be very promising.

[1] R. Neutze, R. Wouts , D. Van der Spoel, E. Weckert, and J.  Hajdu, Nature 406, 752 (2000).

[2] A. Szöke in Short Wavelength Coherent Radiation: Generation and Application, edited by Attwood, D. T. & J. Boker, (American Institute of Physics, New York, 1986). p. 361–367.

[3] G. Faigel and M. Tegze, Rep. Prog. Phys. 62, 355 and references therein (1999).

[4] M. Kopecký, E. Busetto, A. Lausi, M. Miculin, and A. Savoia, Appl. Phys. Lett. 78, 2985 (2001).

[5] J. J. Barton, Phys. Rev. Lett. 67, 3106 (1991).

[6] T, Gog, P. M. Len, G. Materlik, D. Bahr, C. S. Fadley, and C. Sanchez-Hanke, Phys. Rev. Lett. 76, 3132 (1996).

[7] M. Kopecký, J. Appl. Cryst. 37, 711 (2004).

Research team

• Miloš KOPECKÝ, Department of Laser Plasma
• Jan FÁBRY, Department of Dielectrics
• Jiří KUB, ROTAN Laboratory

Collaborations

Sincrotrone Trieste, Italy: Edoardo BUSETTO, Andrea LAUSI

Selected publications

E. Busetto, M. Kopecký, A. Lausi, R. H. Menk, M. Miculin, A. Savoia: X-ray fluorescence holography: A different approach to data collection, Phys. Rev. B62 (2000), 5273.

The images of nearest neighbors of gallium atoms in a GaAs crystal were obtained by the x-ray fluorescence holography technique. The fluorescence from gallium atoms was selected by means of a thin zinc foil filter that made possible the use of an x-ray silicon photodiode detector without energy resolution. This method makes possible the detection of a much higher signal with respect to all previous experiments, thus reducing drastically measuring times, that is a basic and essential step from contemporary demonstration experiments to possible practical applications of x-ray holography in structure analysis.

M. Kopecký, E. Busetto, A. Lausi , M. Miculin , A. Savoia: Recording of x-ray holograms on a position-sensitive detector, Appl. Phys. Lett. 78 (2001), 2985.

An unconventional x-ray fluorescence holography experiment was carried out by using an area detector in combination with an absorption filter. The high angular resolution and the very precise detection of intensities allowed the reconstruction of images of distant, as well as light, atoms. The simultaneous recording of the full hologram opens the possibility of one-shot imaging at atomic resolution. The hologram of a CoO single crystal was recorded on the imaging plate and the images of atoms located up to more than 7 Angstrom far from the emitter were obtained.

M. Kopecký, A. Lausi , E. Busetto , J. Kub , A. Savoia: X-ray absorption holography, Phys. Rev. Lett. 88 (2002), 185503.

The transmission of monochromatic x rays through a CoO single crystal was measured for different orientations of the sample. The small variations in the linear absorption coefficient were considered as a hologram and the real-space image of the local atomic environment was successfully reconstructed. The holographic signal constituted about 1% of the detected intensity. Besides other benefits, the use of the absorption holography can increase the signal-to-background ratio by more than 1 order compared with the fluorescence holography.

M. Kopecký, E. Busetto, A. Lausi: Reconstruction of diffraction patterns by using the Helmholz-Kirchhoff integral theorem, J. Appl. Cryst. 36, (2003), 1368.

A new method to obtain three-dimensional information on atomic arrangement from a monochromatic Laue pattern based on the Helmholz-Kirchhoff integral theorem is presented and experimentally proved by applying the algorithm to the thermal diffuse scattering from a single crystal. The advantage given by the possibility of collecting all the required data on a position-sensitive detector in one shot opens new perspectives for studies of fast physical or chemical processes in three dimensions. The reduced exposure time can also avoid radiation damage of organic specimens, and, in conjunction with an ultra-bright beam from the next generation of X- ray free-electron lasers, makes the method suitable for structural studies with individual atomic clusters. This approach can also be used, by observing the thermal diffuse scattering or order diffuse scattering from both non-crystalline samples and ‘imperfect’ crystals, for the investigation of short-range ordered arrangements of atoms.

M. Kopecký: X-ray diffuse scattering holography, J. Appl. Cryst. 37, (2004), 711.

It is shown that anomalous X-ray diffuse scattering can be treated as a hologram providing information on the local environment of the anomalous scatterer. In contrast with standard holography with atomic resolution, holographic oscillations of several percent of the total measured signal can be achieved by choosing suitable photon energies. The problem of virtual images can be solved very easily in the case of both centrosymmetric and non-centrosymmetric structures. Moreover, these holograms are not overlapped by dense and strong Kossel line patterns.