Surface-Sensitive X-ray and Neutron Scattering

Basic Principles

Surface-sensitive scattering experiments using X-rays or neutrons are done in different geometries:

Reflectivity (specular and diffuse)

Reflectivity (in specular scattering geometry, i.e. with incident and exit angle being equal) is used to obtain information on the electron density (in the case of X-rays) or nuclear scattering density (in the case of neutrons) along the surface normal. Specifically, this means surface and interface roughness, film thickness, and density. Beyond the angle of total external reflection, the resulting specular reflectivity curves are essentially a combination of two factors:

  • the so-called "Fresnel reflectivity" falling off roughly with 1/qz4 as an "envelope"
  • the interference pattern ("Kiessig fringes") from the scattering from different interfaces (e.g., substrate-film and film-air or film-solvent).

The diffuse part of the reflectivity (typically measured with the exit angle differing from the incident angle) offers information about lateral correlations of the roughness, which provides insight into, e.g., the growth process of a film or the capillary wave spectrum of a liquid surface. Further details can be found in this more quantitative tutorial.

Grazing-incidence diffraction (GID)

If the incident X-ray or neutron beam illuminates the surface at an angle smaller than the critical angle (approx. 0.2 deg.) a so-called "evanescent" wave is created, i.e. an electric field with exponentially decaying amplitude. This wave which parallel to the surface still has oscillating character, can give rise to Bragg diffraction from periodic structures in the plane. Using this technique of "grazing incidence diffraction" (GID), the structure in the x-y-plane can be studied in the near surface regime (down to z = 5nm).

Experimental Setup

Depending on the requirements of the experiment different sample environments are used. For X-ray diffraction studies on organic thin films we use a portable vacuum chamber.

This UHV system has been developed for preparation, in situ, and real-time X-ray diffraction measurements of organic thin films, multilayers and superlattices. The chamber is optimized for small size, while still incorporating all features necessary to achieve thin film growth under molecular beam epitaxy conditions. It can be used for thin film growth or can be carried and mounted on standard diffractometers.
Our X-ray diffraction measurements are performed at different synchrotron facilities and in-house using a dedicated two-circle X-ray reflectometer.


Reflectivity (specular and diffuse)

In a recent project out at beamline ID10B of the ESRF we studied the growth of diindenoperylene (DIP) - a large organic molecule - using X-ray reflectivity and grazing incidence diffraction. As can be seen from the well-defined Bragg reflections and associated Laue oscillations thin films of high crystalline order can be prepared. Due to the finite film thickness we also observe Kiessig fringes at low q values.

Many details about the growth kinetics of this molecule can be derived from real-time X-ray reflectivity.
Further examples for the use of X-ray or neutron scattering can be found in our list of publications.


[1] J. Als-Nielsen and D. McMorrow, Elements of Modern X-ray Physics, John Wiley & Sons (2000)
[2] M. Tolan, X-Ray Scattering from Soft-Matter Thin Films, Springer (1999)
[3] H. Dosch, Critical Phenomena at Surfaces and Interfaces, Springer (1999)
[4] K. A. Ritley, B. Krause, F. Schreiber, and H. Dosch, Rev. Sci. Instr. 72 (2001) 1453