Grazing-incidence small-angle scattering

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Grazing-incidence small-angle scattering (GISAS) is a scattering technique used to study nanostructured surfaces and thin films. The scattered probe is either photons (grazing-incidence small-angle X-ray scattering, GISAXS) or neutrons (grazing-incidence small-angle neutron scattering, GISANS). GISAS combines the accessible length scales of

SAXS or SANS) and the surface sensitivity of grazing incidence diffraction
(GID).

Geometry of a GISAS experiment. 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 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.

Applications

A typical application of GISAS is the characterisation of

nanoscale in thin films. Systems studied by GISAS include quantum dot arrays,[1]
growth instabilities formed during in-situ growth,[2] self-organized nanostructures in thin films of
block copolymers,[3]
silica mesophases,[4][5] and
nanoparticles.[6][7]

GISAXS was introduced by Levine and Cohen

viruses
attached to surfaces or in lipid layers.

Interpretation

As a hybrid technique, GISAS combines concepts from transmission small-angle scattering (SAS), from grazing-incidence diffraction (GID), and from diffuse reflectometry. From SAS 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. With diffuse (off-specular) reflectometry it shares phenomena like the Yoneda/Vinyard peak at the critical angle of the sample, and the scattering theory, the

distorted wave Born approximation (DWBA).[12][13][14] However, while diffuse reflectivity remains confined to the incident plane (the plane given by the incident beam and the surface normal), GISAS explores the whole scattering from the surface in all directions, typically utilizing an area detector. Thus GISAS gains access to a wider range of lateral and vertical 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 or neutrons has to be always taken into account in the case of thin film studies,[15][16] 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 SAS, while refractive corrections apply to the scattering along perpendicular cuts of the scattering image, for instance along a scattering rod.

In the interpretation of GISAS images some complication arises in the scattering from low-Z films e.g. organic materials on silicon wafers, when the incident angle is in between 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.[16]

These complications are often more than offset by the fact that the dynamic enhancement of the scattering intensity is significant. In combination with the straightforward scattering geometry, where all relevant information is contained in a single scattering image, in-situ and real-time experiments are facilitated. Specifically self-organization during MBE growth[2] and re-organization processes in block copolymer films under the influence of solvent vapor[3] have been characterized on the relevant timescales ranging from seconds to minutes. Ultimately the time resolution is limited by the x-ray flux on the samples necessary to collect an image and the read-out time of the area detector.

Experimental practice

Dedicated or partially dedicated GISAXS beamlines exist at most

synchrotron light sources (for instance Advanced Light Source (ALS), Australian Synchrotron, APS, ELETTRA (Italy), Diamond (UK), ESRF, National Synchrotron Light Source II (NSLS-II), Pohang Light Source (South Korea), SOLEIL (France), Shanghai Synchrotron (PR China), SSRL

At

reflectometers
.

GISAS does not require any specific sample preparation other than thin film deposition techniques. Film thicknesses may range from a few nm to several 100 nm, and such thin films are still fully penetrated by the x-ray beam. The film surface, the film interior, as well as the substrate-film interface are all accessible. By varying the incidence angle the various contributions can be identified.

References

  1. ISSN 0040-6090
    .
  2. ^
    S2CID 7244337
    .
  3. ^
    S2CID 122797468
    .
  4. ISSN 1520-6106
    .
  5. S2CID 98053328
    .
  6. ISSN 0003-6951
    .
  7. PMID 17163739
    .
  8. ISSN 0021-8898
    .
  9. ^ A. Naudon in H. Brumberger (ed.): "Modern Aspects of Small-Angle Scattering", (Kluwer Academic Publishers, Amsterdam, 1995), p. 191.
  10. ISSN 0921-4526
    .
  11. ISSN 0921-4526
    .(Proceedings of SXNS–6)
  12. PMID 9946532
    .
  13. PMID 9981092
    .
  14. ISSN 0021-8898
    .
  15. ISSN 0024-9297
    .
  16. ^
    ISSN 0021-8898
    .

External links
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