Advantageous Features in Materials Probing Techniques Expected with the Light Source at Ochang in Korea
Hyun-Joon Shin, Jinjoo Ko, and Seunghwan Shin
Abstract
A fourth-generation light source (Korea-4GLS) with 4-GeV electron energy, 58-pm-rad electron emittance, and 400-mA electron current is under construction at Ochang, Korea. Here, we describe the advantages of the Korea-4GLS compared with the light source at Pohang (Pohang Light Source: PLS-II) in terms of the geometry of its probing techniques and application materials. In the case of probing laterally homogeneous specimens where an X-ray size from 10 µm to 1 mm is permissible and the sample is generally larger, then techniques such as X-ray scattering, X-ray absorption spectroscopy, and X-ray photoelectron spectroscopy benefit from flux increase. In this case, the Korea-4GLS is better than the PLS-II in the energy range of approximately 1–2.5 keV (higher energy spectrum for soft X-ray-based techniques), 2–4 keV (medium energy-based techniques), and 10–100 keV (higher energy spectrum for hard X-ray-based techniques). When probing laterally inhomogeneous specimens, the X-ray size must be below 1 µm. Here, nano-probe and spectro-nanoscopy techniques, such as coherent diffraction imaging, ptychography, scanning type transmission X-ray microscopy, or photoelectron microscopy leverage the increase in brilliance. These techniques can leverage all photon energy ranges, especially soft X-ray spectro-nanoscopes (>100-fold gain at the focused area) that require an exit slit as a virtual light source.
1. Introduction
X-rays from synchrotron radiation facilities or synchrotron light sources have been widely applied in probing various materials [17]. The principle behind synchrotron radiation-based material analysis is light-matter interaction [13], with light or X-rays in the energy range of approximately 20 eV–100 keV. Here, X-rays can be categorized as ‘soft X-rays’, ‘medium energy X-rays’, and ‘hard X-rays’ corresponding to the photon energy ranges of 20 eV–2.5 keV, 2–4 keV, and 3–100 keV. In this context, the relationship between the energy and wavelength of X-rays are given by λ ≈ 1240/E, where λ and E denote the photon wavelength in nm and photon energy in eV, respectively.
The light-matter interaction results in diverse measurable quantities. X-ray intensity distribution in space resulting from the scattering and interference of X-rays out of the specimen (or sample), energy changes in the outgoing X-rays, generation of free electrons from the specimen with specific kinetic energies, spatial distribution of ejected electrons, and X-ray intensity change through the specimen are a few examples. Accordingly, diverse types of probing techniques have been developed to measure these quantities and extract the physical and chemical properties of the specimen [17]. Some of these techniques, such as X-ray scattering (XRS), X-ray diffraction (XRD), and X-ray absorption spectroscopy (XAS) in transmission, electron yield, and fluorescence modes are illustrated in Fig. 1. XAS can be further classified into extended X-ray absorption fine structure (EXAFS) and near-edge X-ray absorption fine structure (NEXAFS). In addition, X-ray photoelectron spectroscopy (XPS), X-ray/photo-emission spectroscopy (XES), and X-ray fluorescence spectroscopy (XFS) are illustrated in Fig. 1. Furthermore, these techniques can be categorized into scattering and spectroscopy techniques, where the scattering techniques probe the lattice parameter, atomic, molecular, crystal, and geometrical structure, while spectroscopy techniques probe the valence state, oxidation number, electronic structure, electron-energy structure, and crystal structure of the specimen. In addition to the developments in these scattering and spectroscopy techniques, significant technical developments have recently been achieved in X-ray imaging and X-ray focusing (or X-ray lens) [4,5]. Consequently, X-ray imaging, X-ray microscopy, X-ray nanoscopy, or a combination of these techniques with scattering and spectroscopy techniques have become highly effective methods for probing materials [4,5]. It must also be noted that X-ray imaging is used to probe the internal structure of a specimen non-destructively, whereas spectro-nanoscopy is used to probe the local structure and chemical states of the materials within the region of interest (ROI) with submicron spatial resolution.
Because synchrotron-based X-ray techniques are widely being accepted as superior or complementary to other probing techniques, the demand for the adoption of X-ray techniques at the PLS-II has increased; hence, beamtime applications have become overbooked [7]. Furthermore, more competitive probing techniques, such as nanoprobes with improved data quality, are required for exploring new scientific avenues in the field of energy. Accordingly, the Korea- 4GLS was developed in Ochang, in which the emittance of the electron beam in the storage ring is the 100 times less than that of the PLS-II, and the electron beam energy is 4 GeV [8]. However, the electron current was identical to that in the PLS-II. Consequently, the flux is higher for certain photon energies and brilliance increases substantially (by a factor of approximately 100 or greater) for all photon energies [8]. This manuscript describes the advantages of the Korea-4GLS over the PLS-II in terms of the delivered intensity of X-rays to the ROI of the specimen.
2. Impact of flux and brilliance on probing techniques
2.1. Flux and brilliance from X-ray sources
The electron parameters inside the storage ring are summarized in [8]. Briefly, the electron beam energy, emittance, and current are 4 GeV, 58 pm-rad, and 400 mA, respectively. The circumference of the storage ring is 800 m, and the beam stabilities at the insertion device are 2.5 and 0.45 um in the horizontal and vertical directions, respectively. The root mean square value of the bunch length is 13 ps. The flux and brilliance of X-rays are estimated for the bending magnet, wiggler, and undulators, based on the parameters of these sources, as summarized in [8]. The flux and brilliance simulated by the SPECTRA code [9] for the Korea-4GLS and PLS-II are presented in Figs. 2 and 3, respectively. In these Figures, the insertion device parameters (magnetic field, insertion device length, and period) are set to identical values for both the Korea-4GLS and PLS-II.
In Fig. 2, the unit of flux is photons/s/0.1%BW. The solid and dotted lines represent PLS-II and Korea-4GLS, respectively. The flux primarily depends on the electron current and energy in the storage ring [10]. The higher electron energy (4 GeV) of the Korea-4GLS results in an effective energy shift towards a higher-energy spectrum, which manifests in the bending magnet spectrum, as well as the insertion devices spectra. Compared with those of PLS-II, the flux is increased in the energy regions marked ‘A’, ‘B’, ‘C’, and ‘D’ where the photon energy lies in a range of approximately 0.5–2.5 keV (higher energy spectrum of soft X-rays generated by undulators), 2–4 keV (medium energy Xrays from soft X-ray undulators), 10–40 keV (higher energy spectrum of hard X-rays from undulators, wigglers, and a bending magnet), and 30–100 keV (higher energy spectrum of hard X-rays from wigglers and bending magnet), respectively.
In Fig. 3, the unit of brilliance is photons/s/mm2/mrad2/0.1%BW. The solid and dotted lines represent PLS-II and Korea-4GLS, respectively. The brilliance is significantly enhanced, i.e., by a factor of approximately 10–1,000 in all of the energy ranges. This increase can be attributed to the reduced emittance of the electron beam. In conclusion, Figs. 2 and 3 demonstrate that material-probing techniques that are sensitive to brilliance will exhibit optimal advantages.
2.2. Advantages of flux gain for probing techniques
The constraints for the beamlines of synchrotron radiation probing techniques can be classified into two in terms of the delivery efficiency of X-ray intensity into the ROI of the specimen: i) the size of the X-ray is permitted to be relatively large, i.e., up to approximately 1 mm at the sample. and the specimens are laterally homogeneous or uniform and larger in size than the X-ray size, such as in powder samples. ii) the size of the X-rays must be less than 1 µm. This is typically used to perform nano-probe or spectro-nanoscopy in the case of heterogeneous samples or samples of the order of several to tens of micrometers in size.
The first case is illustrated in Fig. 4, which shows the schematic plan view of a beamline that delivers loosely focused X-rays to the sample. In the phase-space concept, all X-rays from the X-ray source are delivered to the specimen. Here, the geometry indicates differences based on the soft and hard X-ray beamlines. In the case of a soft X-ray beamline guided by blue dotted lines after the beamline optics, an exit slit is typically situated before an endstation that is equipped with probing techniques to monochromatize X-rays. To achieve the desired spectral or energy resolution (a spectral resolving power of approximately 5,000), the width of the exit slit is conventionally 10–50 µm along the direction of dispersion (vertical direction). In the horizontal direction, as illustrated in Fig. 4, the width can be several millimeters or wider. A condensing mirror set may be positioned between the exit slit and the sample position with probing techniques to reduce the soft X-ray size. In the case of hard X-ray beamlines, a pair of double crystals monochromatizes the X-rays; thus, no narrow exit slit is required for the beamline. As a result of this geometry, X-rays can be efficiently delivered to the sample position (guided by the pink dotted lines after the beamline optics). A condensing mirror set, such as the K-B mirrors set, can be placed between the double crystals and the sample to condense the X-rays. Both the soft and hard X-ray beamlines deliver X-rays in the millimeter size or of the order of several tens to hundreds of µmeters in size to the sample. This is achieved without losing any of the X-rays, provided that the beamline optics (blue box in the figure) accept X-rays from the source, regardless of their size or divergence. Therefore, the beamline of the Korea-4GLS with a smaller X-ray size and emittance exhibits a negligible advantage over a beamline with a larger X-ray size and emittance; that is, in these beamlines, all the X-rays are efficiently delivered to the sample position. Consequently, the competitiveness of the probe is determined solely by the flux. This improves the photon energy ranges indicated by the ‘A’, ‘B’, ‘C’, and ‘D’ areas of Fig. 2. It must be noted that X-ray probing techniques such as XPS, XAS, XRD, EXAFS, NEXAFS, protein crystallography (PX), small-angle X-ray scattering (SAXS), and high-resolution powder diffraction (HRPD) are applied in this case.
2.3. Advantages of brilliance gain for probing techniques
The other case is when the X-rays must be focused down to a size less than 1 µm or of the order of tens of nanometers to perform nanoprobe orhe other case is when the X-rays must be focused down to a size less th spectro-nanoscopy. A schematic view of the beamline is presented in Fig. 5, in which a typical soft X-ray nanoscope is presented on the right. A virtual source, an X-ray lens (e.g., a Fresnel zone plate), and a sample are the basic components of this nanoscope. In the case of soft X-ray nanoscopy, an exit slit in the horizontal direction is required as a virtual source, along with a vertical exit slit that serves as a monochromator component. To realize a 10-nm X-ray size or 10-nm space resolution with a moderate demagnification factor of 1,000, as illustrated in Fig. 5, the virtual source size must be 10 µm. According to current conventions, the radius of a zone plate is approximately 100 µm, and the distance from the virtual source to a zone plate is approximately 2 m, which results in a 50-µrad numerical aperture. For practical applications, the virtual source size (or the exit slit width) is required to be small in both the horizontal and vertical directions. In the vertical direction (elevation view), the exit slit width must of the order of 10 µm to provide the desired spectral resolution. The exit slit size in the horizontal direction must also be approximately 10 µm. The X-ray or electron beam size in the storage ring at PLS-II is approximately 0.5 mm in the horizontal direction of the soft X-ray energy range. Consequently, considering the beamline optics demagnification, required virtual source size, transmittance at the exit slit, and divergence acceptance at the X-ray lens, less than approximately 1 % of the X-rays are delivered within the area of the zone plate (blue dotted line in Fig. 5). This inefficiency can be overcome with a lower emittance source at the Korea-4GLS (red dotted line), where the transmittance through the exit slit (smaller size) and the delivery of X-rays to the zone plate are efficient (narrower divergence). Hence, the brilliance gain directly provides substantial advantages in soft X-ray nanoprobes. In short, it results in a 100-fold increase or greater in gain of X-ray intensity at the ROI of the specimen.
In the case of hard X-rays, a double crystal set monochromatizes X-rays. Thus, no exit slit is required, and the beamline does not need a tight virtual source (pink dotted line for the PLS-II case) between the beamline optics and the X-ray lens. The X-rays can be delivered to the focusing X-ray lens without being tightly focused through a slit. Considering the size of the hard X-ray lens, Korea-4GLS results in a gain approximately 10 times greater than that of the PLS-II at the X-ray lens. Hence, hard X-ray nano-probes also benefit from the increase in brilliance gain.
When employing a zone plate as the focusing lens, the resulting coherence improves the zone plate efficiency. The Korea-4GLS exhibits greater coherence than the PLS-II for all energy ranges [8]. Hence, practical improvements in the efficiency of the nanoprobe and data quality can be achieved. Figure 3 shows that the Korea-4GLS exhibits increased brilliance in all of the energy ranges compared with the PLS-II. Furthermore, it provides several practical advantages, especially for soft X-ray nanoprobes that deliver over approximately 100 times gain by only considering the geometry of a beamline and nanoprobe. In addition, several X-ray techniques such as coherent diffraction imaging (CDI), coherent X-ray imaging (CXI), scanning photoelectron microscopy (SPEM), nano angle resolved photoelectron spectroscopy (nano-ARPES), scanning transmission X-ray microscopy (STXM), and ptychography can leverage the features of the Korea-4GLS to achieve improved performance. Finally, projection imaging techniques at the Korea-4GLS also derive several benefits when using an X-ray energy at approximately 100 keV, owing to the increase in penetration depth and coherence, as well as the reduced source size.
3. Conclusions
The flux and brightness of X-rays from different X-ray sources, as a function of photon energy, were estimated from the electron beam parameters of the Korea-4GLS at Ochang and, subsequently, compared with those of the PLS-II. The advantages that probing techniques derive from a gain in ‘flux’ and ‘brilliance’ were described based on the X-ray delivery efficiency to the ROI of the specimen, considering the its lateral homogeneity or uniformity. In summary, i) when X-rays can be delivered to the ROI of the specimen with negligible loss, and the specimen is uniform in lateral space, then flux gain influences the performance of the probing technique used. For instance, XPS, XAS, XRD, EXAFS, XAS, PX, and SAXS techniques benefit from flux gain in the photon energy ranges denoted by ‘A’, ‘B’, ‘C’, and ‘D’ in Fig. 2. ii) When the X-ray is required to be focused down to a size of the order of submicrons or tens of nm, then brilliance is the deciding factor. For instance, nano-ARPES, nano-XPS or SPEM, STXM, and Ptychography techniques benefit considerably from brilliance gain in the soft X-ray region. In addition, CDI, CXI, Ptychography, and STXM in hard X-rays also benefit, as well as RIXS or XES that prefer to adopt an X-ray size of the order of tens of µm. iii) Finally, projection imaging techniques with high-energy (100 keV) X-rays will pave the way for broader application of X-rays. Furthermore, the Korea-4GLS exhibits greater coherence than the PLS-II [8] for all the energy ranges, owing to the decrease in X-ray size. Consequently, the techniques that are sensitive to coherence, such as CDI, CXI, ptychography, and nanoscopes with zone plates as the focusing lens, derive significant benefits. Considering these advantages, it can be concluded that 4GLS significantly enhances the competitiveness of material probing techniques. Thus, several facilities have been constructed, and more are being planned globally [1120].
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