Grazing-incidence small-angle X-ray scattering

Grazing-incidence small-angle X-ray scattering (GISAXS) is a scattering technique most commonly done at synchrotron radiation facilities. A related technique also exists for neutron scattering (GISANS).

300px|thumb|right|Scheme 1: GISAXS scattering geometry. The incident beam strikes the sample under a small angle close to the critical angle of total external x-ray reflection. The intense reflected beam as well as the intense scattering in the incident plane are attenuated by a rod-shaped beam stop. The diffuse scattering from the sample (red arrow) is recorded with an area detector. As an example the scattering from a PSPB block copolymer film with perpendicular lamellae is shown in the detector plane. The two lobes of scattering correspond to the lateral lamellar period of about 80 nm.

GISAXS applications

GISAXS is a powerful tool to study nanostructured surfaces and thin films, combining the accessible length scales of Small-angle X-ray scattering (SAXS) [Glatter, O. & Kratky, O., eds. Small Angle X-ray Scattering. Academic Press, 1982.] and the surface sensitivity of grazing incidence diffraction (GID) [Als-Nielsen and D. McMorrow: "Elements of modern X-ray physics", (John Wiley & Sons, New York, 2001).] . Application range from the characterization of quantum dot arrays [T. H. Metzger, I. Kegel, R. Paniago, A. Lorke, J. Peisl, J. Schulze, I. Eisele, P. Schittenhelm, and G. Abstreiter: "Shape, size, strain and correlations in quantum dot systems studied by grazing incidence X-ray scattering methods", Thin Solid Films 336,1-8 (1998).] and growth instabilities formed during in-situ growth [G. Renaud et al.: "Real-Time Monitoring of Growing Nanoparticles", Science 300, 1416 (2003). ] , as well as self-organized nanostructures in thin films of block copolymersDetlef-M. Smilgies, Peter Busch, Dorthe Posselt, and Christine M. Papadakis: "Characterization of Polymer Thin Films with Small-Angle X-ray Scattering under Grazing Incidence (GISAXS)", Synchrotron Radiation News, Issue 15(5), p. 35-42, 2002.] , silica mesophases [A. Gibaud et al.: "Evaporation-Controlled Self-Assembly of Silica Surfactant Mesophases", J. Phys. Chem. B 107, 6114-6118 (2003).] , and nanoparticles [Aaron E. Saunders, Ali Ghezelbash, Detlef-M. Smilgies, Michael B. Sigman Jr., and Brian A. Korgel: "Columnar Self-Assembly of Colloidal Nanodisks", Nano Letters 6, 2959-2963(2006).] .

GISAXS was introduced by Levine and Cohen [J. R. Levine, J. B. Cohen, Y. W. Chung and P. Georgopoulos:" Grazing-incidence small-angle X-ray scattering: new tool for studying thin film growth" , J. Appl. Cryst. 22, 528-532 (1989).] to study the dewetting of gold deposited on a glass surface. The technique was further developed by Naudon [A. Naudon in H. Brumberger (ed.): "Modern Aspects of Small-Angle Scattering", (Kluwer Academic Publishers, Amsterdam, 1995), p. 191. ] and coworkers to study metal agglomerates on surfaces and in buried interfaces. With the advent of nanoscience other applications evolved quickly, first in hard matter such as the characterization of quantum dots on semiconductor surfaces and the in-situ characterization of metal deposits on oxide surfaces. This was soon to be followed by soft matter systems such as ultrathin polymer films [J. S. Gutmann, P. Müller-Buschbaum, D. W. Schubert, N. Stribeck, D. Smilgies, and M. Stamm: "Roughness Correlations in Ultrathin Polymer Blend Films" (Proceedings of SXNS–6), Physica B 283, 40 (2000).] , polymer blends, block copolymer films and other self-organized nanostructured thin films that have become indispensible for nanoscience and technology. Future challenges of GISAXS may lie in biological applications, such as proteins, peptides, or viruses attached to surfaces or in lipid layers. Nowadays dedicated or partially dedicated GISAXS beamlines exist at many synchroton radiation sources (for instance APS, CHESS, ESRF, HASYLAB, NSLS, Pohang Light Source).

GISAXS interpretation

As a hybrid technique, GISAXS combines concepts from transmission SAXS and from GID. From SAXS it uses the form factors and structure factors. From GID it uses the scattering geometry close to the critical angles of substrate and film, and the two-dimensional character of the scattering, giving rise to diffuse rods of scattering intensity perpendicular to the surface. GISAXS also shares elements of the scattering technique of diffuse reflectivity such as the Yoneda/Vinyard peak at the critical angle of the sample, and the scattering theory, the so-called distorted wave Born approximation (DWBA) [S. K. Sinha, E. B. Sirota, S. Garoff, and H. B. Stanley: "X-ray and neutron scattering from rough surfaces", Phys. Rev. B 38, 2297-2311 (1988).] [M. Rauscher, T. Salditt, and H. Spohn: "Small-angle X-ray scattering under grazing incidence: the cross section in the distorded-wave Born approximation", Phys. Rev. B, 52(23), 16855-16863, (1995). ] [R. Lazzari: "IsGISAXS: a program for grazing-incidence small-angle X-ray scattering analysis of supported islands", J. Appl. Cryst. 35, 406-421 (2002).] . However, while diffuse reflectivity remains confined to the incident plane (the plane given by the incident beam and the surface normal), GISAXS explores the whole scattering from the surface in all directions, typically utilizing an area detector. Thus GISAXS gains access to a wider range of lateral structures and, in particular, is sensitive to the morphology and preferential alignment of nanoscale objects at the surface or inside the thin film.

As a particular consequence of the DWBA, the refraction of x-rays has to be always taken into account in the case of thin film studies [Byeongdu Lee, Insun Park, Jinhwan Yoon, Soojin Park, Jehan Kim, Kwang-Woo Kim, Taihyun Chang, and Moonhor Ree: "Structural Analysis of Block Copolymer Thin Films with Grazing Incidence Small-Angle X-ray Scattering", Macromolecules 38, 4311-4323(2005).] P. Busch, M. Rauscher, D.-M. Smilgies, D. Posselt, and C. M. Papadakis: "Grazing-incidence small-angle x-ray scattering (GISAXS) as a tool for the investigation of thin nanostructured block copolymer films - The scattering cross-section in the distorted wave Born approximation", J. Appl. Cryst. 39, 433-442 (2006).] , due to the fact that scattering angles are small, often less than 1 deg. The refraction correction applies to the perpendicular component of the scattering vector with respect to the substrate while the parallel component is unaffected. Thus parallel scattering can often be interpreted within the kinematic theory of SAXS, while refractive corrections apply to the scattering along perpendicular cuts of the scattering image, for instance along a scattering rod.

Another complication arises in the scattering from low-Z films e.g. organic materials on silicon wafers, when the incident angle is inbetween the critical angles of the film and the substrate. In this case, the reflected beam from the substrate has a similar strength as the incident beam and thus the scattering from the reflected beam from the film structure can give rise to a doubling of scattering features in the perpendicular direction. This as well as interference between the scattering from the direct and the reflected beam can be fully accounted for by the DWBA scattering theory.

These complications are often more than offset by the fact that the dynamic enhancement of the scattering intensity is significant, and thus in-situ and real-time experiments are facilitated .

ee also

*Nanotechnology
*Small-angle X-ray scattering
*Biological small-angle scattering

References

External links

* GISAXS tutorial: http://staff.chess.cornell.edu/~smilgies/gisaxs/GISAXS.php
* More GISAXS information: http://www.gisaxs.de/index.html
* IsGISAXS home page: http://www.insp.jussieu.fr/axe2/Oxydes/IsGISAXS/isgisaxs.htm