The 3 GeV ring of the Max IV facility is a source of x-rays with unprecedented average brightness B. Generally, the coherent photon flux Fcoh extractable from a source scales with the brightness as:
Fcoh ∝ B∙λ2
with λ denoting the wavelength of the emitted radiation. Obviously, the highest coherent photon flux can thus be generated at sources of high brightness and at longer wavelengths, like soft x-rays. Experiments based on coherent soft x-ray illumination are thus exploiting a unique capability of the MAX IV facility. Soft X-ray Coherent Imaging (CXI) and Scanning Transmission X-ray Microscopy (STXM) are such experiments. These methods are applicable in a wide range of fields as illustrated below.
- CXI: Understanding the structural origin of functionality in advanced materials.
In many materials, nanoscale structure is the link between atomic arrangement and macroscopic properties and resulting functionality, such as e.g. colossal magnetoresistance1,2. This connection remains to be investigated for many classes of materials and will lead to new insight in solid state physics and materials science. In particular in the area of nanomagnetism, coherent imaging has reached a level of maturity enabling first studies3 of the structure-function relationship in tailored materials. The strong X-ray magnetic dichroism contrast in conjunction with sample environment flexibility is a key advantage in this area and may be applied e.g. to the study of multiferroic materials.
- CXI: Resolving the ultrastructure of biological cells, viruses and other biological objects in 3D with spatial resolutions below 10 nm.4,5
X-ray imaging closes a gap between electron and light microscopy and allows to image biological object without artefacts from staining or fixation. Resolving structural detail at the sub-10 nm level will enable to understand e.g. cellular mechanisms ranging from DNA topology to intracellular material transport, especially when X-ray information can be correlated with optical techniques and electron techniques. Furthermore, coherent scattering from small probe beams can provide additional information on local structure via the analysis of the diffraction pattern, e.g. regarding structural anisotropy in fibrous structures.
- STXM: Soft Matter, Biomedical Materials and Life Science
Answers to many scientific questions in this area reside in the detailed chemical analysis at a nanometre spatial scale. The precise composition of microgels and biomimetic materials is crucial in order to achieve their expected function, and nanoscale phase segregation can give rise to self-organized structures in co-polymer systems or hybrid organic-inorganic systems6. Knowledge of the distribution of particular chemical species can connect metabolic pathways to cellular compartments and organelles. The nanospectroscopy capabilities of the STXM lead to new insight in these areas. In soft matter research, radiation damage is an issue and dose-efficient imaging is a particular strength of STXM as no optical elements are located downstream of the sample. Operation in the “water window” below the absorption edge of oxygen in water is highly desirable and makes the soft X-ray range unique for these experiments7.
- STXM: Earth and Environmental sciences
STXM can provide quantitative maps of chemical species at environmentally relevant concentrations (i.e., mg/kg) and has already contributed in advancing knowledge of biogeochemical processes8, mineral/organic soil compositions9 or pollutants and aerosol particles10. Given the importance of environmental questions, the ability to see the composition of nanoparticles, and their interaction with a variety of biotopes can be expected to be of continuous interest. As other examples, fossilization of species and detailed information on the chemistry of grain boundaries is of fundamental importance for “growth mechanisms” of metamorphic rocks and for the interpretation of geochronological data.
- STXM: Physics, Chemistry and Material Science
Catalysts speed up chemical reactions and are of importance in many industrial processes, including storing energy in fuel cells and batteries, and processing and refining oil and gas. STXM allows to study how a single catalyst particle works at the molecular level11 and can be expected to contribute significantly to this area in the future e.g. via the investigation of hydrogen storage materials or solid/liquid interfaces. Magnetic dynamics on a picosecond time scale has in recent year increasingly been studied by STXM in pump-probe experiments due to its compatibility with pulses of electric and magnetic fields12,13. As spintronics is a growing research area, the study of field or current induced magnetic dynamics on the nanoscale can be expected to be a strong future research field at the STXM endstation. An impressive illustration of the very special contrast mechanisms available in STXM is given by a recent study of graphene, where for the first time images of the distortions in the electron cloud surrounding this prototypical 2D-nanomaterial could be visualized14.
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Coherent X-ray Imaging (CXI)
In a spatially and temporally (i.e. monochromatic) coherent X-ray wave, all photons are in phase. To scatter such a wave from a specimen is an ideal starting point to extract information on the sample structure. In this situation, if we could measure the intensity and phase of the scattered radiation for all scattering angles, we would directly obtain a 3D image of the object under investigation. The problem, however, is that we cannot measure the phase of the x-rays, as the oscillations are too fast for any currently conceivable detector. CXI is concerned with recovering this phase information and thus generating an image of the object, ideally with diffraction limited resolution. With the increasing availability of coherent x-ray beams, these techniques are developing rapidly. Holography, coherent diffraction imaging and ptychography are schemes that are increasingly used for nanoscale x-ray imaging.
In holography for example, a reference beam is used to encode the phase via interference with the object beam1. For example, a small reference aperture next to the sample can be used to provide a reference beam. The resulting coherent diffraction pattern is a hologram, and the image of the object can be obtained by direct Fourier inversion (FT). In the figure below this is moreover done in resonance with a Co core level excitation (778 eV), providing x-ray magnetic circular dichroism (XMCD) contrast for the magnetic domain structure of the sample1. Pure magnetic domain contrast is then obtained by looking at the difference signal from opposite x-ray helicities.
Reprinted by permission from Macmillan Publishers Ltd: Nature 432, 885, copyright (2004).
Iterative phase retrieval techniques such as coherent diffraction imaging and ptychography rely on additional information to recover the phases of the scattered radiation via iterative algorithms. These boundary conditions can be quite simple (e.g. that the sample has a finite extent) and hence applicable to many classes of samples. If the phase can be recovered, spatial resolution is only limited by the maximum momentum transfer in the scattering experiment. These coherent imaging approaches can be carried out using the same contrast mechanisms known from conventional soft x-ray microscopy (elemental, chemical, magnetic etc.)
Direct, real space X-ray imaging (like STXM) with tens of nanometer resolution has been intensively used and perfected over the past few decades. However, one clear limit in achieving even better spatial resolution is set by the manufacturing quality of the X-ray optics (mirrors and lenses). If we also take into account the arrival of FEL sources, which has pushed an interest for diffractive imaging techniques and the development of extensive image reconstruction theory. Simultaneously, the computing power needed for fast, iterative image reconstruction and analysis is now a reality and within the last 15 years, lensless X-ray imaging has been able to blossom as an approach which is not affected by x-ray optics manufacturing limitations. As a result, research is currently carried out in a wide variety of fields. Pushing the spatial resolution below 10 nm and increasing information content (specific contrast; temporal resolution; 3D information; sample environment; multiplexing, etc.) are key areas.
Examples illustrating the width of this field range from the development of holographic imaging in reflection, so that even materials that can only be grown epitaxially on crystalline substrates can be imaged,2 to keyhole diffractive imaging,3,4and ptychographic tomography5, where extended objects can be investigated and the properties of the x-ray beam itself can be analyzed. So, just as light microscopy has proven to be of crucial cross-sectional importance to many areas of science and technology on the micrometer scale, now the development of coherent X-ray imaging is well on its way to offer similar significance in nanoscience.
For an insight into this burgeoning field, a good place to start is the review by Chapman and Nugent, and the references to the different CXI approaches mentioned in there.6
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- M. Dierolf et al., Nature 467, 436 (2010).
- H. N. Chapman, and K. A. Nugent, Nat. Photonics 4, 833 (2010).
Scanning transmission x-ray microscopy (STXM)
In its simplest form STXM offers chemically specific information on nm-size areas of a thin sample. The basic technique uses a coherent, monochromatic x-ray beam that is focused through a Fresnel zone plate onto the sample. An aperture which only admits first order focused light is put between the zone plate and sample. Currently, typical focal points (and thereby spatial resolutions) are of the order of 15-50 nm in diameter, which is largely determined by the width of the outer rings of the zone plate.
By monitoring the X-ray signal transmitted through the thin specimen (from 100 nm to a micrometer or so) an image of the sample is obtained as it is raster-scanned. The contrast is hence given by the elements that do/don’t absorb x-ray photons of a specific energy. In this way C can be differentiated from O, but also C-H from C-O as they have different absorption energies – the energy resolution of these type of beamlines is around 0.1 eV.
Alternatively, staying in one spot one can vary the photon energy and record the transmitted intensity to get an x-ray absorption spectrum. Combining these two procedures will then give you a full chemical map over an area of choice.
One of the main advantages of STXM is that the sample can be mounted in air (with sufficiently short X-ray path length), in a He atmosphere, or sandwiched between two X-ray transparent silicon nitride windows. The latter approach is used to study wet samples such as eg. hydrated polymers1 or biological material, but can also be used in conjunction with gas flow2. The focus on the soft x-ray regime makes elements like carbon, nitrogen and oxygen accessible for analysis by exploiting the natural absorption contrast in the water window, rather than having to stain or heavy metal-label parts of the sample, and at a lower radiation dose then in a transmission electron microscope. Using the polarisation of the light magnetic information (circular polarisation) or bond orientation (linear) is accessible, as demonstrated in the picture above3. Most current STXM set ups also provide the possibility for fluorescence measurements (materials with elements showing limited x-ray transmission) and 3D tomography scanning of samples4. Also in the STXM, some of the CXI methods, like ptychography, can be implemented.
For an introduction to STXM, read more on Adam Hitchcock’s website: STXM-intro