Near-edge X-ray absorption fine structure, NEXAFS, spectroscopy
was used to investigate
the relaxations of polystyrene, a typical amorphous polymer, near a
free surface after the imposition of
a small deformation. Using synchrotron radiation, the NEXAFS dichroic
ratio was determined for both
the Auger and total electron yield processes as a function of
temperature to determine the orientation of
the polymer in the first 1 and 10 nm from the free surface,
respectively. Complete relaxation of the polymer
was not seen for temperatures less than the bulk glass transition
temperature. No evidence of enhanced
mobility at the free surface was found. A planar relaxation of the
polymer was found in the first nanometer
from the free surface, whereas in the first 10 nm, the dominant
relaxation was normal to the surface.
At present the only surface electron microscope which allows true characteristic XPEEM (photoemission electron microscopy using synchrotron radiation) and structural characterization is the spectroscopic LEEM developed at the Technical University Clausthal in the early nineties. This instrument has in the past been used mainly for LEEM studies of various surface and thin film phenomena, because it had very limited access to synchrotron radiation. Now the microscope is connected quasipermanently to the undulator beamline 6.2 at the storage ring ELETTRA, operating successfully since the end of 1996 under the name SPELEEM (Spectroscopic PhotoEmission and Low Energy Electron Microscope). The high brightness of the ELETTRA light source, together with an optimized instrument, results in a spatial resolution better than 25 nm and an energy resolution better than 0.5 eV in the XPEEM mode. The instrument can be used alternately for XPEEM, LEEM, LEED (low energy electron diffraction), MEM (mirror electron microscopy) and other imaging modes, depending upon the particular problem studied. The combination of these imaging modes allows a comprehensive characterization of the specimen. This is of particular importance when the chemical identification of structural features is necessary for the understanding of a surface or thin film process. In addition, PED (photoelectron diffraction) and VPEAD (valence photoelectron angular distribution) of small selected areas give local atomic configuration and band structure information, respectively.
Surface-sensitive and polarization-dependent near-edge X-ray
absorption fine structure
(NEXAFS) measurements clearly reveal a preferred in-plane and
out-of-plane orientation of phenyl and
CO groups at the surface of rubbed polyimide films. The
unidirectional molecular alignment at the
surface is argued to provide the template for liquid crystal (LC)
alignment of the films. Both the LC
orientation along the rubbing direction as well as the direction of the
out-of-plane LC pretilt are explained
in a simple model. In this model the LC direction follows the
preferential orientation of the phenyl rings
at the surface. The preferred phenyl orientation is explained in
terms of preferential chain segment
alignment through a pulling action of the rubbing cloth fibers.
The proposed LC alignment model is
based on the existence of a statistically significant unidirectional
bond asymmetry at the polymer surface,
and it does not require the existence of crystalline
order.
The thermal stability of amorphous hard carbon films produced by cathodic arc deposition was studied by Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy, Raman spectroscopy. and nanoindentation evaluation. Pure carbon films of
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