Creating the right image

X-ray imaging at the Diamond Light Source facility means visualisation of nanoscale objects

X-rays can now be used to study anything from catalytic reactions to high-speedmagnetisation dynamics to protein folding. A huge research complex in Oxfordshire, known as Diamond Light Source, will be home to some 22 unique research laboratories (simply called beamlines), each equipped with state-of-the-art x-ray optics to focus x-rays produced by a synchrotron. Currently under construction, Diamond is the largest UK scientific investment for 30 years. The facility is on schedule to start operations in early 2007.

Gerd Materlik, Diamond' chief executive

A synchrotron normally guides electrons travelling close to the speed of light on a circular path to generate high-intensity x-rays. But at Diamond, advanced magnetic arrays, called undulators, will gently wiggle the electron' path to produce high-brilliance beams of x-rays. These advances allow x-ray beams to be produced with tiny source sizes and with extremely low divergence.

Diamond Light Source, Oxfordshire, due to start
operations in early 2007

The result is a powerful x-ray source with many of the characteristics of a laboratory laser, but operating at much shorter wavelengths. The x-rays can be focused down to nanometers allowing diffraction, spectroscopy and microscopy to be deployed using tuneable wavelengths ranging from 0.01nm to 15nm-plus. The tuneability of the x-ray wavelength allows samples to be studied element by element due to the fingerprint-like resonances of the atomic core levels.

Perhaps most importantly, the synchrotron, as the name might suggest, has a time structure that produces x-rays in picosecond bursts with repetition rates of microseconds to nanoseconds allowing x-ray time-resolved studies. The x-rays will be used to image nanoscale objects using techniques including photoemission electron microscopy, microfocusing x-rays with mirrors and lenses, coherent diffraction, tomography and holography. In this article we will review some initial areas of activity in x-ray imaging at Diamond Light Source.

PhotoEmission Electron Microscopy (PEEM)
Electron microscopy traditionally uses highly energetic electrons to diffract the electrons at the sample in order to produce a final high-resolution image. At Diamond Light Source, the Nanoscience Beamline will use x-ray photons to generate the imaging electrons for an electron microscope by using the photoemission process which is inherently surface sensitive. Photoemission occurs when x-rays, with sufficient energy to overcome the binding of electrons within a material, release the electrons from the sample.

This phenomenon was correctly described by Einstein exactly 100 years ago and earned him the Nobel Prize. The electrons are collected outside the material using a series of electron lenses that form an image of the solid' surface. The PhotoEmission Electron Microscope (PEEM) at Diamond will combine a microfocus x-ray spot (10 x 3 with circular and linear polarisation to study magnetism using x-ray magnetic circular dichroism (XMCD). The resolution of the PEEM is 4nm to 40nm and is mainly limited by the spherical and chromatic aberration of the electron lenses.

XMCD is sensitive to ferromagnetism through polarisation dependent absorption resonances that, for the most technologically relevant materials, lie in the x-ray range 0.5-2nm. Tuning the wavelength of the circularly polarised x-rays to the 2p core levels leads to a huge increase in the electron yield from the material which is due to dipole allowed transitions into the unoccupied 3d states which are responsible for magnetism in transition metals.

The magnitude of this electron yield depends on the relative orientation of the sample magnetisation with the circularly polarised light and gives rise to the XMCD. This effect can be used as a contrast mechanism to image magnetic domains in ferromagnetic and ferrimagnetic nanomagnets by dividing images taken at wavelengths corresponding to the absorption resonance by images taken at slightly longer wavelengths using the opposite polarisation. Figure 1 shows a PEEM image of magnetic domains in micron-sized structures of permalloy taken using XMCD as the contrast mechanism at the Fe absorption resonance.

The tiny structures of permalloy have magnetically broken up into domains in order to lower the stray magnetic field energy of the individual squares. In complicated magnetic heterostructures this procedure can be repeated for the absorption resonances of the other constituent elements leading to an element and spatially specific breakdown of the magnetic composition in important nanomagnet devices such as spin valves.

The x-rays at Diamond are produced using electron bunches circulating close to the speed of the light in a synchrotron. The synchrotron can produce ~30ps pulses of x-ray light with a repetition rate of 2ns to 2µs making them ideal for studying picosecond magnetisation dynamics in complicated nanomagnets. Understanding magnetisation reversal dynamics at picosecond timescales is important for many technological applications of magnetic materials. PEEM combined with XMCD allows direct observation of the local magnetisation in a sample as a function of time. The x-rays are synchronised to probe the sample at different times after a picosecond magnetic pulse has perturbed the magnetic domains allowing access to the picosecond motion of the magnetic domain structure with nanometer spatial resolution.

Microfocus imaging
The refractive index of materials in the x-ray wavelength range is slightly smaller than one which means that x-ray mirrors are forced to operate at grazing angles of incidence making them as long as 1.5m and difficult to make. The final x-ray spot size at the sample is almost entirely determined by the quality of the optics used. Fortunately, advances in x-ray optics in recent years means that the high-brilliance of Diamond can be preserved all the way to the sample. X-ray microfocussing can be achieved using lenses, zone plates or mirrors depending on the spot size required.

The Microfocus Spectroscopy Beamline, covering the wavelength range 0.05-0.5nm, will use Kirkpatrick-Baez focusing mirrors combined with piezoelectric benders to produce a 1 x 1 spot at the sample. The microfocused x-ray beams will be used to determine local composition and structure, using spatially resolved x-ray spectroscopy and x-ray diffraction. The spectroscopy is again element selective due to the unique absorption resonances for each wavelengths (higher energies), so that the beam penetrates much more of the sample compared to the Nanoscience beamline which primarily studies surfaces.

The effects of environmental degradation through speciation of toxic elements in solids and plants will be one area of activity for the Microfocus beamline. Toxic metals are contained in contaminated soils and sediments, but the host phases are often unknown. Industrially contaminated land can contain a variety of crystalline, amorphous and sorbed species in soil that is chemically and mineralogically complex. The natural ability of some plants and microorganisms to absorb toxic metals and de-activate them also offers a very real prospect for an effective method of cleaning up contaminated land and riverbeds.

The Microfocus beamline will allow the quantification of such microbially mediated mineral transformations in-situ. Chemical state mapping on the scale of individual bacteria (~1micron) will be a significant step towards identifying mineral transforms under real environmental conditions. The trace elements within the soil are first located by rastering the sample across the beam while detecting the x-ray fluorescence. This gives a map of all the elements in the soil. The next step is to study a chosen area using x-ray absorption spectroscopy which gives the chemical and local co-ordination of small concentrations of elements of interest. The same area is then investigated using x-ray diffraction which identifies the dominant mineral species at the give location in the sample.

Soils containing ferromanganese nodules rich in nanocrystalline Mn oxide having very large surface areas can easily sorb trace minerals. X-ray diffraction and x-ray fluorescence has been used to identify which trace element is attached to which mineral in the soil. The analysis indicated that Ni was predominantly located at the same areas as the mineral lithiophorite. X-ray absorption at the Ni 1s absorption resonance then showed that the Ni was substituted in the MnO₂ layers of the lithiophorite instead of the Al(OH)₃ layers. This is a fixed form of Ni which makes it highly immobile and thus not very accessible to living organisms (ie, not harmful).

Coherent diffraction imaging
Coherent diffraction with visible wavelength lasers has long been used to study diffusion and relaxation processes on the submicron length scale. Coherent diffraction leads to a speckle pattern provided that the difference in path lengths of light scattered by different parts of the illuminated object are close to the coherence length of the incident beam. The coherence of an x-ray source is determined by the spatial extent of the synchrotron electron beamcross-section, so that the undulators at Diamond will provide a huge increase in the coherence of the x-ray beams.

This opens up the opportunity to extend the spatial resolution of a speckle pattern from millimetres into the nanometres lengthscale. It is partly this quality of the x-ray source that make the x-ray beams at Diamond X-ray images reconstructed from a coherent x-ray speckle pattern are aberration-free, diffraction-limited and avoid the resolution and depth-of-field limitations of lens-based imaging systems. Due to the penetrating power of the x-rays, x-ray speckle patterns can also provide three-dimensional imaging of the interior of micron sized particles, at nanometer resolution. The Magnetism and Materials beamline, covering the wavelength range 0.05-0.5nm, will combine state of the art detectors with a high coherent flux making it well positioned to exploit the high coherence of x-rays at Diamond.

A coherent diffraction experiment consists of illuminating the sample with monochromatic coherent x-rays and recording a single speckle pattern around a diffraction spot. The phases of the pattern are recovered from the measured intensities using established phase retrieval algorithms and then the unknown object is recovered by Fourier inversion. If a series of speckle patterns are recorded after tilting the sample in the x-ray beam then 3-D tomography is also possible. If we consider the energy that a speckle pattern should be recorded at, it is a general rule in x-ray coherence experiments that larger wavelengths (~2nm), available on the Nanoscience beamline, are more favourable.

Figure 1. Magnetic domains imaged with the PEEM using circularly polarised x-rays at the Fe absorption resonance for permalloy microstructures. The arrows in the left square show the direction of the magnetisation. Images courtesy of Dr C Quitmann, Swiss Light Source, Paul Scherrer Institut (Switzerland)

This is because at larger wavelengths the coherence from the synchrotron is higher, the scattering from a small sample is stronger and the x-rays can be detected more efficiently with current CCD technology. Figure 2(a) shows an example of a speckle pattern recorded at a wavelength of 2.11nm from clusters of 50nm gold spheres in a droplet of colloidal solution deposited on a 100nm thick SiN membrane.

Figure 2. (a) Coherent x-ray diffraction (ie speckle pattern)
from 50nm gold balls suspended in solution. The black disc in the middle of the image is a beam stop. (b) SEM image and (c) reconstructed coherent diffraction image of the gold balls. (d) Coherent x-ray diffraction expected around a diffraction peak along with the reconstructed nanoparticle (inset).

(a), (b) and (c) Taken from H. He et al. Phys. Rev. B 67, 174114 (2003). (d) Taken from I. A. Vartanyants and I. K. Robinson, J. Phys.:Condens. Matter 13, 10593 (2001)

Figure 2(b) and (c) show a comparison of a scanning electron microscope (SEM) image from two gold ball clusters with a reconstructed image from the speckle pattern shown in Figure 2(a). The disadvantage of using larger wavelengths is that the penetrating power of the x-rays is reduced and the resolution reduced. The Magnetism and Materials beamline covers the range for shorter wavelength activities at Diamond until a purpose built beamline is ready. Figure 2(c) shows the expected coherent diffraction pattern around a Bragg diffraction peak along with a reconstructed image of the scattering nanoparticle from the pattern. The potential of x-ray coherent diffraction imaging is largely unexplored to date, the developments at Diamond will be a timely enabling technology for industrial research.

Several examples of imaging with x-rays at Diamond have been outlined, but there will be many more activities such as 3-D tomography from bones and cells, imaging of surface chemical reactions and holography of magnetic domains. Diamond will offer scientists unrivalled access to nanoscale and molecular imaging on the picosecond timescale.

By Dr Sanjeet Dhesi, principle beamline scientist at Diamond Light Source
enquiry number 02463