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
small-angle scattering (SAS:
SAXS or
SANS) and the surface sensitivity of
grazing incidence diffraction (GID).
Applications
A typical application of GISAS is the characterisation of
self-assembly
Self-assembly is a process in which a disordered system of pre-existing components forms an organized structure or pattern as a consequence of specific, local interactions among the components themselves, without external direction. When the ...
and
self-organization
Self-organization, also called spontaneous order in the social sciences, is a process where some form of overall order arises from local interactions between parts of an initially disordered system. The process can be spontaneous when suffic ...
on the
nanoscale
The nanoscopic scale (or nanoscale) usually refers to structures with a length scale applicable to nanotechnology, usually cited as 1–100 nanometers (nm). A nanometer is a billionth of a meter. The nanoscopic scale is (roughly speaking) a lo ...
in thin films. Systems studied by GISAS include quantum dot arrays,
growth instabilities formed during in-situ growth,
self-organized nanostructures in thin films of
block copolymers
In polymer chemistry, a copolymer is a polymer derived from more than one species of monomer. The polymerization of monomers into copolymers is called copolymerization. Copolymers obtained from the copolymerization of two monomer species are some ...
,
silica mesophases,
and
nanoparticles
A nanoparticle or ultrafine particle is usually defined as a particle of matter that is between 1 and 100 nanometres (nm) in diameter. The term is sometimes used for larger particles, up to 500 nm, or fibers and tubes that are less than 10 ...
.
GISAXS was introduced by Levine and Cohen to study the dewetting of gold deposited on a glass surface. The technique was further developed by Naudon and coworkers to study metal agglomerates on surfaces and in buried interfaces.
With the advent of
nanoscience
The nanoscopic scale (or nanoscale) usually refers to structures with a length scale applicable to nanotechnology, usually cited as 1–100 nanometers (nm). A nanometer is a billionth of a meter. The nanoscopic scale is (roughly speaking) a lo ...
other applications evolved quickly, first in hard matter such as the characterization of
quantum dots
Quantum dots (QDs) are semiconductor particles a few nanometres in size, having optical and electronic properties that differ from those of larger particles as a result of quantum mechanics. They are a central topic in nanotechnology. When the ...
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
A polymer (; Greek '' poly-'', "many" + '' -mer'', "part")
is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...
films, polymer blends,
block copolymer
In polymer chemistry, a copolymer is a polymer derived from more than one species of monomer. The polymerization of monomers into copolymers is called copolymerization. Copolymers obtained from the copolymerization of two monomer species are ...
films and other self-organized nanostructured thin films that have become indispensable for nanoscience and technology. Future challenges of GISAS may lie in biological applications, such as
proteins
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...
,
peptides
Peptides (, ) are short chains of amino acids linked by peptide bonds. Long chains of amino acids are called proteins. Chains of fewer than twenty amino acids are called oligopeptides, and include dipeptides, tripeptides, and tetrapeptides.
...
, or
viruses
A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea.
Since Dmitri Ivanovsky's ...
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). 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,
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.
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
and re-organization processes in block copolymer films under the influence of solvent vapor
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 many
synchrotron light sources (for instance
SSRL,
APS,
CHESS
Chess is a board game for two players, called White and Black, each controlling an army of chess pieces in their color, with the objective to checkmate the opponent's king. It is sometimes called international chess or Western chess to dist ...
,
ESRF
The European Synchrotron Radiation Facility (ESRF) is a joint research facility situated in Grenoble, France, supported by 22 countries (13 member countries: France, Germany, Italy, the UK, Spain, Switzerland, Belgium, the Netherlands, Denmark, ...
,
HASYLAB,
NSLS, Pohang Light Source) and also Advanced Light Source at LBNL.
At
neutron research facilities,
GISANS is increasingly used,
typically on small-angle (SANS) instruments or on
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
External links
GISAXS and GIWAXS tutorialby Detlef Smilgies - Updated Link!
GISAXS wikiby Kevin Yager
modelling/fitting software by Rémi Lazzari
FitGISAXSmodelling/fitting software by David Babonneau
BornAgainmodelling and fitting software by Scientific Computing Group of MLZ Garching
HiPGISAXSMassively Parallel GISAXS simulation code by
LBNL
Lawrence Berkeley National Laboratory (LBNL), commonly referred to as the Berkeley Lab, is a United States national laboratory that is owned by, and conducts scientific research on behalf of, the United States Department of Energy. Located in ...
{{DEFAULTSORT:Grazing-Incidence Small-Angle X-Ray Scattering
X-rays
Scattering
Synchrotron-related techniques
Scientific techniques
Neutron scattering
Nanotechnology