Total electronic structure analysis realised with soft X-rays
Physical Sciences
ArticleDetailDownload PDF
Designing new materials requires a detailed understanding of their chemical composition, atomic arrangement, and chemical bonding properties. Powerful spectroscopic techniques – like electron energy-loss spectroscopy – can provide much of this information, but cannot probe valence states, which are responsible for chemical bonding and thus the stability of a material. Professor Masami Terauchi at Tohoku University, Japan, pioneers the use of soft X-ray emission spectroscopy in combination with electron microscopy. His non-destructive approach provides an accurate analysis of chemical bonding states in a wide range of materials.
The macroscopic properties of a material, including its stability and hardness, electrical conductivity, and response to light and other forms of radiation, are intimately connected to its microscopic structure. It is the precise arrangement of atomic nuclei and electrons inside a sample and their ability to respond to specific perturbations – like heat or an applied electric voltage – that make a new material potentially useful for technological applications. In addition, we often require new materials to maintain their desired performance under stress or in miniaturised devices, like, for instance, optical and opto-electronic components. Developing quick, non-destructive, and accurate methods to examine the microscopic structure and electronic properties of a sample during material production is a vital goal in materials discovery.
Electron energy-loss spectroscopy
Transmission electron microscopy (TEM) is one of the most powerful and widely used techniques for obtaining information on the electronic states of extended systems. TEM exploits the interaction of beams of electrons with a known kinetic energy with a sample. During this interaction, some of the electrons are scattered inelastically and lose energy. Measuring this energy change provides direct information on the processes responsible for electron excitation (Figure 1b and c), which include transitions of electrons between different quantum–mechanical states within the sample.
7th Sun/Shutterstock.com
Chemical analysis
Electronic transitions involving low-energy (or core) states are particularly useful for working out the nature of the atoms in a sample (Figure 1c). Electron energy-loss spectroscopy (EELS) is a powerful analytical tool for determining the chemical composition of a material very accurately. This technique is coupled with electron microscopy, which uses electron beams rather than visible light, to create images of very small sample areas with resolution far superior to conventional optical microscopes. This approach can characterise materials with nanometre-scale accuracy.
Valence and conduction states
EELS can also be used to study transitions of bonding electrons (the ‘valence state’, see Figure 1b), and can probe the extended valence states responsible for electric conduction within a sample, known as ‘conduction bands’. In this case, a quantum–mechanical property known as ‘joint density of states’ is measured, which contains information concerning all possible transitions between occupied (or ‘valence’) and unoccupied (or ‘conduction’) electronic states.
“Soft X-ray emission spectroscopy coupled with electron microscopy provides detailed information on the composition and chemical bonding in bulk materials.”
Professor Terauchi at Tohoku University, Japan, explains, ‘EELS experiments provide a detailed representation of the relative energies of occupied and unoccupied electronic states, with accuracies as high as 0.1 to 1 electron-volts (eV). They also provide a means to measure dielectric properties of materials. However, they lack the ability to yield direct information about the energies of valence bands, ie, the electronic states that are ultimately responsible for the stability and chemical bonding within a sample.’
Figure 1. Schematic diagram of electronic transitions related to cathodeluminescence (a); valence electron excitation of EELS (b); inner-shell excitation spectrum of EELS (c); X-ray emission including valence electron information (d); and X-ray emission mainly used for elemental analysis (e).
Soft X-ray emission
Together with the density of states of the conduction bands, the density of states of the valence bands is necessary for the complete characterisation of the electronic structure of a material. To disentangle the contribution of valence states to joint densities of states, Terauchi has been developing techniques based on X-ray emission processes. X-ray emission occurs after electrons of core states are promoted to unoccupied states along with a de-excitation process (Figure 1d and e) of electron transitions from the valence bands to core states, or from shallow core states to deeper core states. Energy is released in these processes in the form of photons, with energies ranging from 15 to 6,000 eV or from extreme ultraviolet to soft X-ray range. Among those energies, below 2,000 eV is really promising for probing the chemical bonding states of elements in a sample.
Probing chemical states
The intensity profile of a soft X-ray emission spectrum along with transitions from valence bands to core states represents a partial density of states of the valence bands. Electronic recombination processes involving shallow and deep core states can also give rise to X-ray emission lines, and these transitions are very sensitive to the local chemical environment of the core states. ‘Soft X-ray emission spectroscopy,’ says Terauchi, ‘not only provides a suitable method for probing valence states, but also a means to obtain compositional information and even to characterise the charge states of the atoms within a sample. These features make it a very sensitive tool for chemical characterisation, which can be applied to wide classes of problems.’
Coupling soft X-ray emission and electron microscopy
Unlike competing approaches for estimating band energies and density of states, such as photoelectron emission spectroscopy (which measures the kinetic energy of the electrons emitted from a sample under irradiation), soft X-ray emission spectroscopy does not need to operate under ultra-high vacuum conditions, because the escape depth of soft X-rays of a few tens of nanometres exceeds a sample’s surface thickness. This makes it possible to combine the spectroscopy with conventional electron microscopes.
Figure 2. Soft X-ray emission spectroscopy in combination with electron microscopy provides a non-destructive approach for accurate analysis of chemical bonding states in a wide range of materials.
Another advantage of soft X-ray emission is that no net electrical charge is generated within a sample along the emissions, as all X-ray emission events originate from electronic transitions between states within the sample. Conversely, in the case of photoelectron spectroscopy, the continuous emission of negatively charged electrons creates a positive charge, which can cause ambiguities in the determination of the band energies. Soft X-ray emission spectroscopy thus shares the ability of electron energy-loss spectroscopy to be used alongside electron microscopy. One drawback of the former technique is the low emission efficiency, which calls for longer acquisition times (from ten minutes or more in case of TEM, and one-minute order in SEM) compared to EELS experiments.
“Scanning electron microscopy can be used to enhance the sensitivity of soft X-ray instrumentation for quick feedback during materials production.”
Achieving high-energy resolution
Early examples of the application of soft X-ray emission spectroscopy were mainly devoted to phase identification or ‘elemental analysis’ of materials. The energy resolution of these early devices (no less than 10 eV in the case of electron-probe micro-analyser) was insufficient to determine the density of states for the valence bands. Furthermore, this instrumentation required moving mounting optics to span specific energy ranges, making them unsuitable for transmission electron microscopes. Although resolutions as low as 0.1 eV can now be achieved using large synchrotron radiation sources, which are sufficient to resolve the density of state for the valence bands, this approach cannot be used alongside electron microscopy, owing to their massive size mismatch. The goal of Terauchi and his collaborators has therefore been to create soft X-ray emission instruments of small size (less than about 1m) and without a moving mechanism. This makes it possible to create very compact X-ray devices, which can be connected to conventional electron microscopes.
Innovative soft X-ray emission spectrometers
Terauchi has been developing a series of technical improvements over early soft X-ray emission devices, which are now bringing this technique to a virtually unmatched level of accuracy and versatility. The instruments developed and commercialised by his group do not require a motorised mechanism to move the spectrometer. They rely on innovative flat-field grazing-incidence optics with varied-line-spacing gratings, providing very compact devices, along with high-resolution area detectors. To address the low efficiency of the soft X-ray emission processes, Terauchi has also designed and commercialised spectrometers that can be coupled to conventional scanning electron microscopes and electron-probe micro-analysers. These devices have higher beam currents than transmission electron microscopes and can therefore dramatically improve the signal-to-noise ratio and acquisition time of this technique. They afford the power of large synchrotron facilities, but, because of their much-reduced size, can easily be installed and used for routine materials analysis in conventional laboratories.
Figure 3. (a) crystal structure of CaB6. (b) secondary electron (SE) image, and (c) back-scattered electron (BSE) image of Na-doped CaB6. Spectral intensity maps of (d) Ca-Lℓ,η intensity (150–154 eV), (e) B-K intensity (170–188 eV), and (f) the top part of the B-K intensity (187–188 eV). Areas A and B have different chemical bonding states.
Non-destructive chemical state mapping
The techniques and instrumentation developed by Terauchi have been applied to wide classes of materials, including simple metals (Li, Be, Al, Mg), transition metals, carbon-based materials (diamond, graphite and fullerenes), lanthanides and Sr- and B-based materials for thermoelectric devices, to name but a few. It has also been used to make predictions concerning macroscopic properties like electrical conductivity, and their dependence on sample composition, eg, in the case of carbon nitride films with variable nitrogen content. Soft X-ray emission spectroscopy coupled with electron microscopy has been shown to provide a fast, efficient, and comprehensive tool for analysing chemical bonding in complex materials. It offers a powerful and non-destructive technique to probe the electronic structure of bulk materials in real time and during the production process, and to aid the optimisation of new materials for technological applications.
Soft X-ray emission spectroscopy coupled with electron microscopy offers a powerful tool to probe the electronic structure of complex materials. In which fields of materials discovery are the techniques you have developed having the largest impact?
The largest impact may be in research and development related to electric vehicles; Li-ion batteries, solid state batteries, fuel cells, electric motors, and high tensile strength steel sheets. Those contribute to a reduction of CO2 emission and realise a sustainable society with automobiles. These instruments are also considered a powerful tool in the field of mineral resources.
Terauchi, M, Hatano, T, Koike, M, et al (2020) Recent developments in soft X-ray emission spectroscopy microscopy. IOP Conf. Series: Materials Science and Engineering, 891, 012022.
Ishii, S, Terauchi, M, Sato, Y, et al (2018) Soft X-ray emission spectroscopy study of characteristic bonding states and its distribution of amorphous carbon-nitride (a-CNx) films. Microscopy, 67 (4), 244–249.
Terauchi, M, (2014) Valence Electron Spectroscopy for Transmission Electron Microscopy. In: CSSR Kumar (ed), Transmission Electron Microscopy Characterization of 287 Nanomaterials. Springer-Verlag, Berlin, Heidelberg, 2014.
Terauchi, M, Kawana, M, (2006) Soft-X-ray emission spectroscopy based on TEM—Toward a total electronic structure analysis. Ultramicroscopy, 106, 1069–1075.
Professor Masami Terauchi has developed an analysis method of chemical bonding based on electron microscopy (EM), using soft X-ray emission spectroscopy (SXES).
Funding
This research was funded by the research programme ‘Dynamic Alliance for Open Innovation Bridging Human, Environment and Materials’ of the Ministry of Education, Culture, Sports, Science and Technology, Japan, and ‘Network Joint Research Centre for Materials and Devices’, Japan Society for the Promotion of Science KAKENHI [Grant numbers 15H02299; 19K21838]. Commercial instrument development was supported by the Collaborative Development of Innovative Seeds (practicability verification stage), Japan Science and Technology Agency.
Collaborators
Development of SXES electron microscope
Dr Masato Koike: National Institute for Quantum and Radiological Science and Technology
Dr Hideyuki Takahashi and Mr Takanori Murano: JEOL Ltd
Mr Masaru Koeda and Mr Tetsuya Nagano: Shimadzu Corporation
Bio
Masami Terauchi completed his PhD at the Faculty of Science, Tohoku University. He then moved to developing a high-energy resolution electron energy-loss spectroscopy EM, which triggered commercialisation projects of monochromator TEM around the world. Since 1999, he has been developing soft X-ray emission spectroscopy (SXES) instruments for EM.
Masami Terauchi
Contact
2–1–1 Katahira, Aoba-ku, Sendai 980–8577, Japan
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Japan
