An introduction for the non-specialist+
BIMR, McMaster University
Hamilton, ON Canada L8S 4M1
+ This document is intended to be an informal introduction of a tutorial nature, rather than an historical or comprehensive overview. Please see the articles referenced for more complete presentations of the technique and its many applications.
X-ray absorption spectroscopy has advanced rapidly in the last decades, particularly with synchrotron radiation based techniques [stohr 92]. The high brightness of the synchrotron X-ray source has allowed tremendous improvements in spectral resolution and the development of X-ray microscopies which apply X-ray absorption spectroscopy to heterogeneous materials at high spatial resolution. Novel characterisation techniques are being developed [ade 98] which allow both spectromicroscopy - imaging with spectral sensitivity, and microspectroscopy - recording spectra from very small spots. We choose to call these novel analytical tools X-ray spectromicroscopy, to emphasize that it is the combination of spectroscopy and microscopy that is the essence of these techniques. This article describes one of these techniques, scanning transmission x-ray microscopy (STXM) [ade 97, warwick 97, warwick 98], and illustrates its capabilities through two selected applications in polymer analysis [ade 92, ade 93, ade 95, smith 96]. Authoritative reviews of STXM have been published which stress materials [ade 98, ade00] and biological [kirz 95] applications.
There are many problems in polymer physics and chemistry which require detailed chemical analysis at a sub-micron spatial scale - phase segregation; determination of the morphology and interface chemistry of blends and co-polymer systems; nano-patterned structures, self assembly; and many others. ‘Traditional’ chemical spectroscopies used for polymer studies such as infrared and nuclear magnetic resonance can differentiate chemical species but they do not have the necessary sub-micron spatial resolution. Analytical transmission and scanning transmission electron microscopy have excellent spatial resolution and are very useful to visualise the structure, but electron microscopy typically does not have sufficient chemical sensitivity for quantitative chemical mapping beyond the elemental level. In many cases it is ambiguous whether structure observed by electron (or optical) microscopy arises from chemical differences or if it is simply caused by density/thickness variations or reflectivity changes (optical). In addition, electron microscopy of polymers is experimentally difficult on account of high rates of radiation damage by energetic electron beams [Egerton 87].
2. Near Edge X-ray Absorption Fine Structure
As an x-ray passes through matter, it is absorbed to an extent which
depends on the nature of the substance, the thickness of the sample, and
the density of the sample. The absorbed photons cause excitation of the
inner shell electrons of the atoms in the substance. These excited inner
shell (core) electrons can be promoted to unoccupied energy levels to
form a short lived excited state or they can be removed completely
to form an ionized state. Traditionally, X-ray absorption spectroscopy
was described in terms of absorption edges which are the onsets
of inner-shell ionization (Figure 1). There is an absorption edge
associated with each inner shell energy level of an atom, such that all
elements have an X-ray absorption edge in the soft X-ray energy range
(100-1200 eV). The amount of a particular element can be determined quantitatively
from the difference in the x-ray absorption just above and just below
its absorption edge.
The unoccupied electronic structure and thus the inner-shell excited states are determined by the geometric and electronic (bonding) structure of the sample. NEXAFS spectra differ significantly even for rather similar molecular structures, as shown in Figure 3 for some common polymers [Ade 97b]. This means that the NEXAFS spectrum of each polymer can be used as a fingerprint. In many cases, enough is known about how chemical structure and X-ray absorption spectral features are related to allow one to identify unknown species from measured NEXAFS spectra. Individual spectral features, particularly the low energy p * features, are often sufficient for qualitative identification in reasonably well characterized systems, and they can serve as useful energies for selective chemical contrast in X-ray microscopy.
There is characteristic NEXAFS structure at the absorption edge of each element in a sample. Thus combined studies of C 1s, N 1s and O 1s NEXAFS is a very powerful tool in polymer analysis. Finally, comparisons with NEXAFS spectra of pure standards provides a means to derive quantitative composition maps of the components of a complex material from a series of X-ray microscopy images.
The STXM at the Advanced Light Source (ALS) which we use can record images with less than 50 nm spatial resolution, and NEXAFS spectra from 150 - 1250 eV on stxm701 (210 - 650 eV on stxm5.3.2) with an energy resolution of about 100 meV [warwick 97, warwick 98]. Samples must be partly transparent at the X-ray energy of interest, which means a sample thickness of 50 to 300 nm for carbon studies, ranging up to 1 - 2 microns thick for higher energy edges or low density samples such as hydrated polymer gels or biological samples. The sample preparation challenges are similar to those encountered for analytical transmission electron microscopy. Microtomed sections about 100 nm thick which are mounted on 3 mm metal grids are a common form of sample. We use silicon wafers with X-ray transparent, 100 nm thick silicon nitride windows (of up to 4 mm lateral size) to study samples in water. One can even do N 1s spectroscopy through these windows. A strength of STXM relative to electron microscopy is that radiation damage is two orders of magnitude less than in electron beam imaging techniques [rightor 97], making this technique ideally suited for radiation sensitive polymers.
The transmitted X-ray signal is measured by single photon counting using counting periods (dwell times) of 0.2 - 0.5 ms per pixel for imaging. A 300x300 pixel image takes about 30 seconds to acquire. Typically, many problems can be solved by measuring a small number of images at selected, chemically sensitive images. In cases where one needs a very detailed chemical analysis of a specific region it is useful to take a systematic series of images at a fine mesh of energies around the absorption edge of interest. These image sequences - which might involve one hundred 100x200 pixel images, or about 8 Mb of raw data - can be processed off-line in a variety of ways for both qualitative species identification and to derive quantitative composition maps. Examples of these approaches are shown below.
For quantitative analysis, the transmitted signal is converted to an optical density (OD) according to
OD = ln(Io/I)
where for a given X-ray energy I0 is the incident x-ray flux, I is the transmitted flux through the sample, and ln is the natural logarithm. The OD is related to the sample properties by
OD = µ(E).r .t
where m(E) is the mass absorption coefficient at X-ray energy E, r is the density and t is the sample thickness. The mass absorption coefficient is derived from measurements of the NEXAFS spectra of the pure material. Practically, spectra are obtained by first recording an energy scan I from the spot of interest and subsequently the incident flux Io measured with the same detector and optical path but with the sample out of the beam. Typically, this takes a few minutes.
4. Example 1: Qualitative polymer analysis
This is part of a study [Hitchcock 01] of reinforcing filler particles
which are used in moulded compressed polyurethane foams in the automotive
and furnishing industries to achieve higher hardness and load bearing
capability. STXM was been used to study two types of copolymer polyol
(CPP) fillers – hard polymer particles dispersed in a polyether polyol
- present in a toluene di-isocyanate-based polyurethane. One CPP is a
copolymer styrene and acrylonitrile (SAN). The other CPP is an aromatic-carbamate
rich poly-isocyanate poly-addition product
(PIPA), derived from methylene diisocyanate. Both particles are chemically
indistinguishable by transmission electron microscopy (Figure 5)
although the size and spatial distribution are clearly revealed.
5. Image processing and advanced results
Recording sequences of images with short dwell time and fine energy spacing
can be very useful [jacobsen 00]. For instance, post-processing of the
images series can be used to extract spectra at any subset of pixels in
the sampled region, long after the measurement is finished. This approach
can provide serendipitous discoveries. An example of this for the PIPA-SAN
polyurethane filler particle system is shown in Figure 8. Relative
to multiple single point measurements, image sequence procedures also
minimize radiation damage. At the ASL point spectra cannot be acquired
without exposing the sample to ~200 msec per energy point, or say, 20
seconds for a 100 point energy scan. In contrast, image sequences which
can provide statistical precision of 1-2% in a 0.2x0.2 m
m region can be acquired with dwell times per pixel of 0.5 msec, or 50
msec total for the full 100-point energy spectrum. Of course damage is
not restricted solely to the point sampled but even so, there is a reduction
in dose of several orders of magnitude relative to point scan mode.
Singular value decomposition
The optical density is determined by the formula OD = µ.r .t, where µ is the energy-dependent mass absorption coefficient (cm2/g), r is the density (g/cm3), and t is the sample thickness (cm). If the mass absorption coefficients of each components are known at the energies sampled, the problem can be converted to a simple superposition and linear algebra can be used to convert sets of images into equivalent thickness (rt(x,y)) or composition maps. In principle, the decomposition problem can be expressed as a matrix equation Ax=d, where x is a vector describing the unknown distribution of each component (rt), d are the measured images (converted to OD scale), and A is the matrix of the mass absorption coefficients (µ) obtained from reference spectra for each component, converted to mass absorption scale. The advantage of the singular value decomposition procedure is that, once the absorption coefficients for a set of materials and energies are known, one can calculate a priori the matrix which will optimally invert an over-determined sampling of energies into the best possible component maps. It can be shown that the SVD solution is the least squares solution [strang 88, press 92].
Image sequence fits
Image sequence fitting [hitchcock 02] computes a least squares fit of the intensity at each pixel(j,k) to a linear combination of reference spectra for each component (converted to mass absorption scale). An energy-independent (constant) term (ao) is also usually included to accommodate unexpected backgrounds:
6. Example 2; Quantitative chemical mapping at sub-micron resolution
The second example illustrates an application of image sequence analysis
[Croll00]. Polymer capsules and particles with tailored walls,
and core-shell structures are attractive for a wide number of applications,
including separation science applications, adhesives, coating, chemical
delivery, etc. [Li99]. In order to optimise for a particular application,
e.g. designing the wall properties for controlled release of a specific
chemical, one needs to perform accurate quantitative analysis of the chemical
structure of the wall at high spatial resolution. The polyurea capsules
are made by dispersing a mixture of aromatic isocyanate and xylene in
an aqueous solution of a polyamine. Interfacial polymerisation reactions
of both the amine and water with the isocyanate occur at the surface of
the dispersed hydrophobic, organic droplets. Competition among amine-isocyanate
and water-isocyanate reactions is controlled by differences in kinetics
and diffusion rates. This can lead to chemically structured capsule walls.
The amine-isocyanate reaction forms an asymmetric (aromatic-aliphatic)
urea whereas the water-isocyanate forms a symmetric di-aromatic urea.
There are only a few STXM microscopes in the world. The instrumentation was originally developed by Kirz and collaborators (SUNY-Stony Brook) and the Kirz and Jacobsen groups operate three STXM at the X1 undulator beamline at the National Synchrotron Light Source in Brookhaven. At the Advanced Light Source, there are two operating STXM microscopes - one on beamline 7.0 (to be moved to beamline 11.0 in fall 2002); the other on bending magnet beamline 5.3.2 which is dedicated to polymer and soft materials analysis. A third centre for STXM is the King’s College instrument at beamline 5U2 in Daresbury, England. Other STXM instruments are under construction at BESSY II in Berlin, at the Swiss Light Source, at PLS, the Korean synchrotron, and at the Canadian Light Source.
If you are interested in analytical studies of polymers or other materials using STXM or other X-ray microscopy techniques, please contact
Hamilton, ON L8S 4M1 Canada
Tel: 905 525-9140 ext 24749
We thank Harald Ade for sharing beam time, results and a common perspective. The contributions of many people who helped develop, optimise and maintain the ALS STXM have been critical - we note in particular the contributions of Tony Warwick and George Meigs. The ALS BL 7.0 STXM was developed by T. Warwick (ALS), B.P. Tonner and collaborators, with support from the U.S. DOE under contract DE-AC03-76SF00098. The research described in this article is funded by strategic and research grants from the Natural Sciences and Engineering Research Council (NSERC) of Canada.
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