Latest advances in soft X-ray spectromicroscopy at SSRF

Latest advances in soft X-ray spectromicroscopy at SSRF 1 first-author 1 1 1 1 1 1 1 1 1 1 1 corresp wangyong@sinap.ac.cn wangyong@sinap.ac.cn wangyong@sinap.ac.cn 1 corresp tairenzhong@sinap.ac.cn tairenzhong@sinap.ac.cn tairenzhong@sinap.ac.cn Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China The BL08U1A beamline is established as a sophisticated platform at Shanghai Synchrotron Radiation Facility (SSRF), taking advantage of its high spatial resolution (<30 nm) and high energy resolving power (>10000), for studying properties of solid, liquid, gas, film and other forms of materials at sub-micron scale. In this paper, we present a review on newly implemented techniques, such as total electron yield (TEY), dual energy contrast imaging, nano-CT, soft X-ray excited optical luminance (SXEOL), and coherent diffraction imaging (CDI) under development. Several research cases in nanomaterials, environmental science and biology are presented to demonstrate capabilities of the beamline. Scanning transmission soft X-ray microscopy Total electron yield Coherent diffraction imaging Soft X-ray excited optical luminance I. INTRODUCTION 1 The nano science and technology was developed on the basis of new experimental techniques, especially scanning probe techniques, such as scanning tunneling microscopy or atomic force microscopy (AFM) that revolutionized modern surface and interface science allowing atomic resolution [ 1 1 ], and transmission electron microscopy (TEM) that is capable of providing structural information of the samples, in full atomic resolution [ 2 2 ]. However, these are often insufficient for nanomaterials research, where electronic properties, element composition, chemical state, and—in some cases—the magnetic properties of nanoscale materials have to be explored, with detailed spectroscopic information in high spatial resolution. Based on their unique interaction with matter, soft X-rays can be used to gain insight into the electronic properties and chemical or magnetic state in condensed matter [ 3 3 ]. As an analytical technique in rapid development, zone-plate-based scanning transmission X-ray microscopy (STXM) makes use of the advantages of high brilliance synchrotron radiation to obtain spectroscopic information on the electronic structure of atoms and molecules. The first successful scanning X-ray microscope was built by Horowitz and Howell at the Cambridge Electron Accelerator in 1972 [ 4 4 ]. They focused synchrotron radiation onto a ϕ 1-2 μm pinhole to define the microprobe, and formed images of a variety of specimens detecting either X-ray fluorescence from the specimen or transmitted X-rays. Later, Bilderback et al . used tapered capillary optics to achieve 50 nm resolution [ 5 5 , 6 6 ], and Chao et al . improved the focusing capability of X-rays down to about 15 nm [ 7 7 ]. As the first third generation synchrotron in China, SSRF operates at an electron energy of 3.5 GeV and a beam current of 240 mA. The BL08U1A beamline at SSRF is a soft X-ray beamline providing 250–2000 eV photon beams. The energy range covers the absorption edges of major elements in biology, environmental studies (soil, minerals), polymer science and other fields, such as K absorption edges of C, N, O, Na and Mg in the living cells, and L absorption edges of Cl, K, Ca, Fe, Cu and Zn. The STXM combines high spatial resolution and high energy resolution of chemical states of near-edge absorption fine structure spectroscopy (NEXAFS). Up to now, the beamline has been in operation for over five years. Considerable efforts have been made in developing new methods and imaging modalities, including total electron yield (TEY), dual energy contrast imaging, nano-CT and soft X-ray excited optical luminance (SXEOL). Coherent diffraction imaging (CDI) is under development. In this paper, we describe in details the new methods and techniques, together with their applications in biological and medical sciences, physics and environmental science. II. THE BL08U1A BEAMLINE 1 A. The monochromator 2 Spatially coherent X-ray is produced by an elliptical polarized undulator (EPU), and monochromatized by a plane grating monochromator (PGM), as shown in Fig. 1 Fig. 1 . The sagittal cylinder mirror, M1, is to collimate the photon beam in the vertical direction, absorb the heat load, suppress the high order harmonics, and deflect the beam in the horizontal direction. To protect its surface from the high heat load, M1 is designed with an internal water-cooling scheme. The core instrument is a vertically dispersing PGM equipped with a double-ruling-area grating to guarantee mechanical accuracy during grating switching. Two Au-coated gratings are used: 800 l/mm (optimized for 250–750 eV) and 1200 l/mm (optimized for 275–2000 eV). The last optical element is the toroidal mirror, M3, which refocuses the beam simultaneously in horizontal and vertical directions at the exit slit. After the exit slit, the STXM is setup to refocus the monochromized beam to 30 nm with a zone plate. The energy resolution was evaluated by measuring the shell excitation spectra of Ar and N 2 . As shown in Fig. 2 Fig. 2 , the fine structure of 7d peaks can be seen clearly. By fitting the peaks with Voigt profile, the energy resolution is 10000 at Ar edge and 7400 at N edge. Also, the spatial resolution was measured by imaging a standard test sample, which has a fine pattern of 30 nm. As shown in Fig. 3 Fig. 3 , the innermost 30 nm lines can be seen clearly, demonstrating that the spatial resolution is better than 30 nm. The beamline specifications were listed in Table 1 Table 1 . The details of these measurements were described in Ref. [ 8 8 ]. 1 1 Source 1 1 Elliptically polarized undulator 1 1 Energy range 1 1 250–2000 eV 1 1 Energy resolution(Δ E/E ) 1 1 10000@244 eV, 7400@410 eV 1 1 Photon flux at sample 1 1 1.5×10 8 phs/s@244 eV@200 mA 1 1 (in 30 nm spot) 1 1 1.1×10 8 phs/s@1840 eV@200 mA, 1 1 Spatial resolution 1 1 about 30 nm B. The STXM endstation 2 The STXM endstation, purchased from Xradia, consists of a beam focusing system, a sample scanning system, a high-speed proportional detector, a sample tank, a control system and a graphical user interface. The focusing system, sample tank and detector are installed in a vacuum chamber. The working condition of STXM is either vacuum of 1.33×10 -3 Pa (d-5 Torr) or helium environment. The vacuum chamber is separated from the beamline vacuum tube by a 100 nm thick silicon nitride window. As shown in Fig. 4 Fig. 4 , a Fresnel zone plate (FZP) is used to focus the spatially coherent soft X-rays into a nano-probe, which scans the sample point by point to form a complete image. In most cases, the image resolution depends on the focus spot size. The sample is placed at the first-order focalspot of the zone plate, which is equivalent to a convergent lens. The minimum size of the focal spot depends on the outermost width of the zone plate. An FZP with a central stop in combination with an order-sorting-aperture (OSA) is designed to produce the 1st order focal spot on the sample, and minimize the higher order diffraction and direct lights on the sample. The sample is moved by the sample stage (Sample X and Sample Y ) and Piezo stage (Piezo X and Piezo Y ), and rotated by a rotation stage in sample plane. Transmitted X-rays from the sample are detected by a photomultiplier (PMT) detector. A sample can be scanned point by point by its two-dimensional movement ( X and Y directions). The scanning precision of piezo stages is about 5 nm, and the scanning range can be selected from several microns to several hundred microns. A laser interferometer system is used for high-precision position measurement of zone plate and sample in X and Y directions. The piezo stages are scanned / positioned based on the differential position of sample and zone plate, to guarantee the highest possible X-ray-optical resolution. Figure 5 Figure 5 shows the STXM chamber and components inside. III. EXPERIMENT METHODS AND TECHNIQUES 1 Energy of the monochromatic light can be tuned continuously, so the STXM is suitable for NEXAFS study. The characteristic absorption fine structure spectra of the element (or chemical composition) can be obtained by scanning the photon energy near the absorption edge of element of interest. By 3D scanning ( X , Y , and photon energy E ) of images and properly data processing, spatial distribution of the element (or chemical composition) is obtained, so as to reveal chemical nature of the sample, or as a label of the existence of special molecule. There are two basic methods for soft X-ray spectromicroscopy: measuring the X-ray absorption near-edge spectra (XANES) by point spectrum scanning, or measuring chemical composition and its spatial distribution by energy stacking method. Also, dual-energy contrast imaging, total electron yield (TEY) and nano-CT techniques were developed after SSRF was opened to users in 2009. A. Point spectrum scanning 2 The sample location is fixed. By scanning the photon energy within a certain range near the absorption edge of a specific element in the sample, one obtains the characteristic absorption fine structure of the element. And chemical state information of the elements can be obtained by comparing the sample spectra with standard spectra of various compounds or single elements with fitting analysis. There are two ways for point spectrum scanning—focus scanning and large spot scanning. 1. Focus scanning 3 Put the zone plate and OSA in the light path, adjust the sample to the zone plate focus, and find a suitable point on the sample plane to perform the energy scanning. PMT is used to detect the transmitted light intensity. Because the spectra are obtained from a small spot (several tens of nanometers) of the sample, they reflect the chemical state information of the interested element in just that spot of the sample. The intensity will fall to about 10% of the original after the beam goes through the focus of zone plate, so the sample shall not be too thick, preferably < 2 μm. 2. Large spot scanning 3 Move the zone plate and OSA out of the light path, irradiate the sample with the X-ray beam of a few hundred microns in beam spot diameter, scan the photon energy and record the transmitted light intensity using a photodiode (PD) or PMT. The spectra reflect basically the chemical state information of the specific element in the entire sample, or determine whether the sample contains the element. The sample thickness can range from several nm to several microns scale. B. Energy stack scanning 2 Two-dimensional scanning transmission imaging is performed at each energy point for a series of energies near the absorption edge of the interested element, obtaining N absorption contrast images. Post-analysis and processing are performed on the data of the N images by using principal component analysis and cluster analysis. The information of chemical constituents of the element is acquired, including composition of the near edge absorption spectra, and 2D spatial distribution of the constituents. The stack data processing software can be obtained free online (http://xray1.physics.sunysb.edu/data/idl programs/stackanalyze.savhttp://xray1.physics.sunysb.edu/data/idl programs/stackanalyze.sav; http://xray1.physics.sunysb.edu/data/idl programs/pcagui.savhttp://xray1.physics.sunysb.edu/data/idl programs/pcagui.sav). In stack data analysis and processing, the drift of images along the X/Y directions is very common, hence the use of image correlation maximization method and the multi-window correlation maximization method to perform image registration and trimming. C. Dual-energy contrast imaging 2 This experiment method for STXM was developed after SSRF’s opening to users. The main steps are as followed. For an element of interest, select the energy at the absorption edge and another energy just above the edge, and perform two-dimensional scanning transmission imaging of the sample, respectively. Comparing and processing the two images, 2D spatial distribution of the element can be determined semi-quantitatively. Details about this method were described in Ref. [ 9 9 ]. D. TEY method 2 TEY is an indirect method to measure X-ray absorption near-edge spectra by detecting the electric current caused by secondary electrons [ 10 10 ]. The advantage of this method is independent of sample thickness and its signal-to-noise ratio is much higher than the transmission mode. TEY detects interfacial properties of the sample, rather than bulk information using the transmission mode. TEY requires that the sample be conductive or prepared on conductive substrates. In addition, the magnetic field can be easily added to study the magnetic samples using this method. E. Nano-CT technique 2 Three-dimensional microscopic imaging is of importance in such research areas as cell biology and environmental science, where 3D sample imaging is needed to know more detailed structural information. For example, the ferroferric oxide distribution in human cervical carcinoma samples can be detected with STXM, but this does not tell where it is located (on the surface, or in the cell) and how it is distributed along the depths [ 11 11 ]. We combined STXM and computed tomography technique to develop the Nano-CT technique, which obtains, non-destructively, both 3D elemental mapping and chemical information. The Nano-CT technique in identification capability of 50 nm 3D structures has been opened to users. F. SXEOL technique 2 SXEOL monitors optical luminescence (UV to near IR) emitted from the sample excited by soft X-ray photons in energies across the absorption edge of interest. Combining with NEXAFS spectra, it can provide elemental, chemical and site-specified information under favorable conditions. The total (zero order) or partial (selected wavelength) photoluminescence yield (PLY) is often used to determine the origin of luminescence. BL08U1A operates in the soft X-ray regime (250–2000 eV), which covers the so-called "water window". This offers an opportunity of checking the contributions from low atomic number elements, such as carbon, nitrogen and oxygen, which are not accessible with hard X-rays. Most importantly, at these energies the X-ray attenuation length is sufficiently short, allowing probing surface, interfacial, and/or bulk regions of the sample. At present, a concurrent collection of SXEOL and TEY NEXAFS spectra has been realized at SSRF. Figure 6 Figure 6 shows the schematic diagram and photo of the SXEOL setup (top view) in the vacuum chamber. The monochromatized X-ray photons impinge on a sample at an angle of 45. The lights emitted by the sample are collected by a collimating lens mounted at the end of an optical fiber, and transferred to the spectrometer by the optical fiber. A software triggers the spectrometer to collect a SXEOL spectrum for every energy in the energy scanning. The spectrometer has replaceable slits, a composite grating HC1 with a bandwidth of 725 nm, fairly enough for the synchrotron radiation application purposes. A commissioning of the system was carried out, and measurements on single crystal ZnO were taken at oxygen K edge. Two main features were observed from the SXEOL spectra, i.e., a broad intense defect emission band at 531.3 nm and a weak band-gap peak at 380 nm. Wavelength selected (531.3 nm and 380 nm) and zero order PLY spectra were shown in Fig. 7 Fig. 7 . The PLY spectrum at 380 nm was averaged between 377 nm and 383 nm due to very weak luminescence. A TEY spectrum is shown for comparison. All the spectra were normalized to the incident photon flux collected on a gold-coated silicon wafer. The zero order PLY is the same as the PLY spectrum taken at 531.3 nm. It is interesting to note that the PLY spectra, being similar to the TEY, exhibit a decrease of the feature at 539.4 eV relative to that at 535.8 eV, agreeing well with the literatures [ 12 12 ]. G. X-ray CDI methodology 2 The CDI is a novel and powerful lensless imaging method for imaging materials and biological specimens [ 14 14 , 15 15 ], with spatial resolution achieving 3 nm [ 16 16 ]. Since its first demonstration in 1999 [ 17 17 ], this method has been advancing rapidly through the use of synchrotron radiation, X-ray free-electron lasers, and tabletop soft X-ray lasers. In CDI, the diffraction pattern of a noncrystalline specimen is measured in the far field, and its phase is then directly retrieved to obtain an image of the specimen using iterative algorithms in combination with the oversampling method [ 18 18 ]. This method overcomes the shortcoming of traditional X-ray microscopes, where the resolution is limited by the X-ray focusing element. Theoretically, resolution of this method is limited by just the X-ray wavelength and the attainable maximal diffraction angle, hence the potential possibility of reaching atom level resolution and realizing molecule imaging [ 19 19 ]. We have carried out the algorithmic and experimental study of CDI methodology, and achieved some good results in plane-wave CDI, scanning CDI and broadband CDI [ 20 20 - 22 22 20-22 ]. These pave the ways for our implementation and application of this newly emerging ultra-resolution imaging method. The newly emerging scanning CDI method (also called ptychography) is the most suitable CDI method for STXM setup [ 23 23 - 26 26 23-26 ]. It allows samples of unlimited lateral extent to be imaged by scanning a localized illuminating probe wavefront across the transmissive sample and recording the multiple diffraction patterns at a set of partly overlapping illuminated regions, overcoming the disadvantages of the classic plane-wave CDI such as the rigid size limitation and isolation requirement on samples, slow convergence, stagnation, and the nonuniqueness of the solution. Therefore we have been developing the scanning CDI as our emphasis. A scanning CDI experimental rig was installed at the STXM endstation ( Fig. 8 Fig. 8 ), and primary scanning CDI methodology experiments using a star pattern as the sample were carried out. A number of pretty good diffraction datasets were obtained, and the scanning CDI images ( Fig. 9 Fig. 9 ) were reconstructed using scanning CDI reconstruction software we developed. In the reconstructions, the spokes of star pattern can be distinguished distinctly, in both absorption (amplitude) and phase-shift images. The 30 nm spokes can be seen clearly. The probe (illumination) amplitude and phase were also reconstructed successfully. The highest resolution is about 10 nm, derived by a power spectral density analysis of the reconstructed image ( Fig. 9(b) Fig. 9(b) ). In developing the CDI reconstruction code, we implemented a number of image reconstructing algorithms of multiple CDI methods using Matlab language, including [(a)] t t (a) Plane-wave CDI : Hybrid input-output (HIO), Error reduction, Difference Map, Relax averaged alternate reverse (RAAR), Guided-HIO, Oversampling smoothness, Shrinkwrap, Noise-robust HIO, etc., and the source codes were perfected gradually to improve the efficiency and accuracy; (b) Scanning CDI : Ptychographic iterative engine (PIE), extended PIE, parallel PIE, position-correction PIE, Nonlinear optimization (NLO), etc., and tested by simulation and experiment data, in which the position-correction PIE (pcPIE) and NLO are the newest algorithms that could correct the position errors; (c) Broadband CDI algorithm was implemented and tested by simulation and experiment; (d) Fresnel CDI reconstruction algorithm was realized, and the Ewald sphere space transformation was used for large-angle CDI; (e) Data preprocessing programs were finished and tested, such as background subtraction, diffraction stitching, diffraction center finding, beam stop area determination, etc., which are necessary for disposing experimental data before CDI reconstruction; (f) Dual energy CDI ratio contrast element mapping method was implemented. Except that the detector is a CCD camera and the sample is not necessary to be placed at the zone plate focus, the setup of ptychography is almost the same as the STXM. Therefore, the sample preparation scheme and the sample thickness range are fully equal to those of STXM. This newly developed technique will be opened to the users soon after an improvement of the automatic control system of the CCD synchronized triggering and an optimization of the experimental parameters and operations. IV. APPLICATIONS 1 Soft X-ray microscopy is an established technique for spectromicroscopy of soft matter, nanomaterials, magnetic materials, environmental and geosciences, and biology [ 27 27 - 30 30 27-30 ]. With increasing number of STXM systems available, more uses of the spectromicroscopy are predicted in new fields of research. Combining high spatial with high spectroscopic and even temporal resolution allows investigations of many properties relevant for micro- and nanostructured materials [ 31 31 , 32 32 ]. In Section IV, we show several examples illustrating that a wide range of scientific issues can be investigated using STXM. A. Nanomaterials 2D imaging and absorptions 2 Using STXM technique, the sample can be imaged, while element absorption spectra can be obtained by stack analysis, i.e., analyses of a series of images at different energies near the edge of certain element. As shown in Fig. 10 Fig. 10 , Peng et al . studied graphene-templated 2D single crystal lepidocrocite nanosheets (γ-FeOOH), an efficient catalyst to treat phenol in wastewater, and confirmed the formation of γ-FeOOH2D nanosheets [ 33 33 ]. N-doping in CNTs has shown excellent performance as electrocatalyst support or even directly as metal-free electrocatalyst for oxygen reduction, which can be used in fuel cells or biosensors [ 34 34 , 35 35 ]. However, due to the complicated situation in CNTs bundles, such as variations in helicity, defects, diameter, residues, and surface modification, further explorations are needed to understand key factors affecting performance of CNTs in different applications. Using XANES spectroscopy and STXM Xie et al . investigated N-doped carbon nanotubes (NCNTs) and their electronic structures, and state of iron atoms in the catalysis. The results are shown in Figs. 11 Figs. 11 and 12 12 . The iron exists mainly in the form of ferrocene in the carbon nanotube, but it is partly transferred into oxidation state in NCNTs. Encapsulated ferrocene residues in CNTs was revealed by STXM, which may help for the N 2 sealing [ 36 36 ]. B. Biological sample imaging 2 STXM, of high spatial resolution (30 nm) and high sensitivity of chemical identification, is an excellent method to study biological samples, such as cell, virus and tissues [ 37 37 , 38 38 ]. Zhang et al . studied the biotransformation of ceria nanoparticles (NPs) in cucumber plants, and found that CeO 2 NPs were stable under environmental and biological conditions [ 39 39 ]. NEXAFS showed that the root Ce contents are CeO 2 and CePO 4 while the shoots Ce contents are CeO 2 and cerium carboxylates. As shown in Fig. 13 Fig. 13 , some CeO 2 NPs were transformed to CePO 4 on surface of cucumber roots, which might be attributed to the enhanced dissolution of NPs at the nano-bio interface. In 2014, Chen et al . studied immunological modulation of nanomaterials [ 40 40 ]. The endohedral metallofullerenol, Gd@C 82 (OH) 22 , serves as a potential nanomedicine that can efficiently inhibit the growth and invasion of tumors with low toxicity. The immunological modulation is involved in the processes but little has been revealed about the mechanism. Figure 14 Figure 14 shows the STXM-revealed uptake process of Gd@C 82 (OH) 22 and the distribution inside the macrophages, an important immune cell, and the key signal pathways involved in the immune activation and modulation by Gd@C 82 (OH) 22 . In details, macrophages can internalize a large number of Gd@C 82 (OH) 22 and their immune activity is induced by activating the pathways of TLRs//MyD88/NF-B and NLRP3 inflammasomes that promotes the secretion of IL-1β and regulates efficiently the innate and acquired immunological responses. C. Magnetic imaging and spectroscopy 2 BL08U1A provides circularly polarized or linear light for X-ray magnetic circular dichroism in X-ray absorption (XMCD) and X-ray magnetic linear dichroism in X-ray absorption (XLCD) experiments [ 28 28 , 29 29 ]. The interaction between lattice, charge, spin, and orbital degrees of freedom is related directly and crucially to performance of electronic materials, displaying a rich spectrum of exotic phenomena. Cui et al . used a gate voltage to manipulate the orbital occupancy of (La, Sr) MnO 3 films in a reversible and quantitative manner [ 41 41 ]. The variation of orbital occupancy is effectively proved by X-ray linear dichroism (XLD) ( Fig. 15 Fig. 15 ). Positive gate voltage increases the orbital occupancy proportion and magnetic anisotropy that were initially favored by strain (irrespective of tensile and compressive), while negative gate voltage reduces the concomitant preferential orbital occupancy and magnetic anisotropy. Also, using soft X-ray spectroscopy combining with X-ray magnetic circular dichroism, Sun et al . obtained STXM images of micro magnetic structures of Ni 80 Fe 20 film, in circular, triangular and vortex shapes [ 42 42 ]. As shown in Fig. 16 Fig. 16 , the quantitative analysis results of magnetic vortex of STXM agree well with the simulation results. D. Liquid samples 2 Liquid samples can be investigated by STXM. The liquid sample is confined between two silicon nitride windows. Zhang et al . imaged interfacial micro- and nano-bubbles with the scanning transmission soft X-ray microscopy [ 43 43 , 44 44 ]. They found that nanoscopic gas bubbles in heights of < 100 nm and lateral size of several tens of nm to micrometers can exist on various surfaces immersed in water, with a lifetime of hours and even days. The astonishing stability of the nanobubbles contradicts classical thermodynamics. STXM showed the first images of submicro-bubbles at solid-water interfaces ( Fig. 17 Fig. 17 ). The length scale for stable bubbles is about 2.5 μm. V. SUMMARY 1 STXM is a powerful tool for studying nano- or microscopic scale specimens. It provides resolution intermediate between visible light microscopy and electron microscopy. It is suitable for studying wet or frozen hydrated specimens in thickness of a few micrometers, whereas scanned probe microscopes (such as scanning force or near field optical) are better suited to surfaces or very thin specimens. Typical specimens studied by soft X-ray microscopy include nanomaterials, cell, films and tissue sections, etc., and elemental mapping and chemical states of major low- Z constituents of the specimens can be obtained. The BL08U1A beamline aims at elemental imaging in nano-scale in high spatial resolution and high energy resolution. 2D and 3D imaging can be performed in the STXM endstation. The STXM beamline will be further improved. This includes the CDI to offer higher spatial resolution than STXM, and in-situ support of samples with cryo environment, magnetic field or electric field, in our sincere hope to provide a more integrated STXM platform to users. References G Binnig , C F Quate and C Gerber . Atomic force microscope . Phys Rev Lett , 1986 , 56 : 930 - 933 . DOI: 10.1103/PhysRevLett.56.930 http://doi.org/10.1103/PhysRevLett.56.930 Atomic force microscope L Reimer and H Kohl . Transmission Electron Microscopy: Physics of Image Formation . New York (USA) : Springer-Verlag , 2008 . DOI: 10.1007/978-0-387-40093-8 http://doi.org/10.1007/978-0-387-40093-8 Transmission Electron Microscopy: Physics of Image Formation J Stöhr . NEXAFS Spectroscopy . Berlin Heidelberg (Germany) : Springer-Verlag , 1992 . DOI: 10.1007/978-3-662-02853-7 http://doi.org/10.1007/978-3-662-02853-7 NEXAFS Spectroscopy P Horowitz and J A Howell . A Scanning X-Ray microscope using synchrotron radiation . Science , 1972 , 178 : 608 - 611 . DOI: 10.1126/science.178.4061.608 http://doi.org/10.1126/science.178.4061.608 A Scanning X-Ray microscope using synchrotron radiation D H Bilderback , S A Höman and D J Thiel . Nanometer spatial resolution achieved in hard X-ray imaging and Laue diffraction experiments . Science , 1994 , 263 : 201 - 203 . DOI: 10.1126/science.8284671 http://doi.org/10.1126/science.8284671 Nanometer spatial resolution achieved in hard X-ray imaging and Laue diffraction experiments D H Bilderback , D J Thiel , R Pahl . et al . X-ray applications with glass-capillary optics . J Synchrotron Radiat , 1994 , 1 : 37 - 42 . DOI: 10.1107/S0909049594007259 http://doi.org/10.1107/S0909049594007259 X-ray applications with glass-capillary optics W Chao , B D Harteneck , J A Liddle , et al . Soft X-ray microscopy at a spatial resolution better than 15 nm . Nature , 2005 , 435 : 1210 - 1213 . DOI: 10.1038/nature03719 http://doi.org/10.1038/nature03719 Soft X-ray microscopy at a spatial resolution better than 15 nm C F Xue , Y Wang , Z Guo , et al . High-performance soft X-ray spectromicroscopy beamline at SSRF . Rev Sci Instrum , 2010 , 81 : 103502 . DOI: 10.1063/1.3491837 http://doi.org/10.1063/1.3491837 High-performance soft X-ray spectromicroscopy beamline at SSRF X Z Zhang , Z J Xu , R Z Tai , et al . Ratio-contrast imaging of dual-energy absorption for element mapping with a scanning transmission X-ray microscope . J Synchrotron Radiat , 2010 , 17 : 804 - 809 . DOI: 10.1107/S0909049510031250 http://doi.org/10.1107/S0909049510031250 Ratio-contrast imaging of dual-energy absorption for element mapping with a scanning transmission X-ray microscope Ph Parent , C Laffon , C Mangeney , et al . Structure of the water ice surface studied by X-ray absorption spectroscopy at the O K-edge . J Chem Phys , 117 : 10842 - 10851 . DOI: 10.1063/1.1519256 http://doi.org/10.1063/1.1519256 Structure of the water ice surface studied by X-ray absorption spectroscopy at the O K-edge D A Shapiro , Y S Yu , M Farmand , et al . Soft X-ray ptychography for nano-materials research . 12th International conference on X-ray Microscopy . Melbourne, Australia , Oct. 26-31, 2014 . Soft X-ray ptychography for nano-materials research L Armelao , F Heig , A Jürgensen , et al . X-ray excited optical luminescence studies of ZnO and Eu-doped ZnO nanostructures . J Phys Chem C , 2007 , 111 : 10194 - 10200 . DOI: 10.1021/jp071379f http://doi.org/10.1021/jp071379f X-ray excited optical luminescence studies of ZnO and Eu-doped ZnO nanostructures H N Yu , Z H Chen , X Y Meng , et al . The development of a XEOL detection system for the scanning transmission X-ray microscopy beamline at SSRF . Chin Opt Lett , 2015 , accepted . The development of a XEOL detection system for the scanning transmission X-ray microscopy beamline at SSRF H N Chapman and K A Nugent . Coherent lensless X-ray imaging . Nat Photonics , 2010 , 4 : 833 - 839 . DOI: 10.1038/nphoton.2010.240 http://doi.org/10.1038/nphoton.2010.240 Coherent lensless X-ray imaging J Miao , T Ishikawa , I K Robinson , et al . Beyond crystallography: Diffractive imaging using coherent X-ray light sources . Science , 2015 , 348 : 530 - 535 . DOI: 10.1126/science.aaa1394 http://doi.org/10.1126/science.aaa1394 Beyond crystallography: Diffractive imaging using coherent X-ray light sources D A Shapiro , Y S Yu , M Farmand , et al . Soft X-ray ptychography for nano-materials research , 12th International conference on X-ray Microscopy . Melbourne, Australia , Oct. 26-31, 2014 . Soft X-ray ptychography for nano-materials research J W Miao , P Charalambous , J Kirz , et al . Extending the methodology of X-ray crystallography to allow imaging of micrometre-sized non-crystalline specimens . Nature , 1999 , 400 : 342 - 344 . DOI: 10.1038/22498 http://doi.org/10.1038/22498 Extending the methodology of X-ray crystallography to allow imaging of micrometre-sized non-crystalline specimens J Miao , J Kirz and D Sayre . The oversampling phasing method . Acta Crystallogr D , 2000 , 56 : 1312 - 1325 . DOI: 10.1107/S0907444900008970 http://doi.org/10.1107/S0907444900008970 The oversampling phasing method J Miao , K O Hodgson and D Sayre . An approach to three-dimensional structures of biomolecules by using single-molecule diffraction images . P Natl Acad Sci USA , 2001 , 98 : 6641 - 6645 . DOI: 10.1073/pnas.111083998 http://doi.org/10.1073/pnas.111083998 An approach to three-dimensional structures of biomolecules by using single-molecule diffraction images H G Liu , Z J Xu , X Z Zhang , et al . Effects of missing low-frequency information on ptychographic and plane-wave coherent diffraction imaging . Appl Optics , 2013 , 52 : 2416 - 2427 . DOI: 10.1364/AO.52.002416 http://doi.org/10.1364/AO.52.002416 Effects of missing low-frequency information on ptychographic and plane-wave coherent diffraction imaging H G Liu , Z J Xu , R Z Tai , et al . Influence of central beamstop on ptychographic coherent diffractive imaging . Acta Phys Sin , 2013 , 62 : 150702 . (in Chinese) DOI: 10.7498/aps.62.150702 http://doi.org/10.7498/aps.62.150702 Influence of central beamstop on ptychographic coherent diffractive imaging J W Zhu , Z J Xu , H G Liu , et al . Three-dimension elemental mapping based on coherent X-ray diffraction imaging . Nucl Tech , 2012 , 35 : 245 - 250 . (in Chinese) Three-dimension elemental mapping based on coherent X-ray diffraction imaging H M L Faulkner and J M Rodenburg . Movable aperture lensless transmission microscopy: A novel phase retrieval algorithm . Phys Rev Lett , 2004 , 93 : 023903 . DOI: 10.1103/PhysRevLett.93.023903 http://doi.org/10.1103/PhysRevLett.93.023903 Movable aperture lensless transmission microscopy: A novel phase retrieval algorithm J M Rodenburg , A C Hurst , A G Cullis , et al . Hard-X-ray lensless imaging of extended objects . Phys Rev Lett , 2007 , 98 : 034801 . DOI: 10.1103/PhysRevLett.98.034801 http://doi.org/10.1103/PhysRevLett.98.034801 Hard-X-ray lensless imaging of extended objects P Thibault , M Dierolf , A Menzel , et al . High-resolution scanning X-ray diffraction microscopy . Science , 2008 , 321 : 379 - 382 . DOI: 10.1126/science.1158573 http://doi.org/10.1126/science.1158573 High-resolution scanning X-ray diffraction microscopy D A Shapiro , Y S Yu , T Tyliszczak , et al . Chemical composition mapping with nanometer resolution by soft X-ray microscopy . Nat Photonics , 2014 , 8 : 765 - 769 . DOI: 10.1038/nphoton.2014.207 http://doi.org/10.1038/nphoton.2014.207 Chemical composition mapping with nanometer resolution by soft X-ray microscopy G Tzvetkov , B Graf , P Fernandes , et al . In situ characterization of gas-filled microballoons using soft X-ray microspectroscopy . Soft Matter , 2008 , 4 : 510 - 514 . DOI: 10.1039/B715392A http://doi.org/10.1039/B715392A In situ characterization of gas-filled microballoons using soft X-ray microspectroscopy X Zhu , A P Hitchcock , T Tyliszczak , et al . Probing magnetic polarities of magnetotactic bacteria by X-ray magnetic circular dichroism in a scanning transmission X-ray microscope . Microsc Microanal , 2014 , 20 : 1176 - 1177 . DOI: 10.1017/S1431927614007612 http://doi.org/10.1017/S1431927614007612 Probing magnetic polarities of magnetotactic bacteria by X-ray magnetic circular dichroism in a scanning transmission X-ray microscope S S Farvid , T Sabergharesou , L N Hutfluss , et al . Evidence of charge-transfer ferromagnetism in transparent diluted magnetic oxide nanocrystals: switching the mechanism of magnetic interactions . J Am Chem Soc , 2014 , 136 : 7669 - 7679 . DOI: 10.1021/ja501888a http://doi.org/10.1021/ja501888a Evidence of charge-transfer ferromagnetism in transparent diluted magnetic oxide nanocrystals: switching the mechanism of magnetic interactions T Zhu , X Lu , H Liu , et al . Quantitative X-ray photoelectron spectroscopy-based depth profiling of bioleached arsenopyrite surface by Acidithiobacillus ferrooxidans . Geochim Cosmochim Ac , 2014 , 127 : 120 - 139 . DOI: 10.1016/j.gca.2013.11.025 http://doi.org/10.1016/j.gca.2013.11.025 Quantitative X-ray photoelectron spectroscopy-based depth profiling of bioleached arsenopyrite surface by Acidithiobacillus ferrooxidans R D Arrua , A P Hitchcock , W B Hon , et al . Characterisation of polymer monoliths containing embedded nanoparticles by scanning transmission X-ray microscopy (STXM) . Anal Chem , 2014 , 86 : 2876 - 2881 . DOI: 10.1021/ac403166u http://doi.org/10.1021/ac403166u Characterisation of polymer monoliths containing embedded nanoparticles by scanning transmission X-ray microscopy (STXM) C Zhou , J Wang and J A Szpunar . X-ray chemical imaging and electronic structure of a single nanoplatelet Ni/Graphene composite . Chem Commun , 2014 , 50 : 2282 - 2285 . DOI: 10.1039/c3cc47008c http://doi.org/10.1039/c3cc47008c X-ray chemical imaging and electronic structure of a single nanoplatelet Ni/Graphene composite C Peng , B W Jiang , Q Liu , et al . Graphene-templated formation of two-dimensional lepidocrocite nanostructures for high-efficiency catalytic degradation of phenols . Energy Environ Sci , 2011 , 4 : 2035 - 2040 . DOI: 10.1039/c0ee00495b http://doi.org/10.1039/c0ee00495b Graphene-templated formation of two-dimensional lepidocrocite nanostructures for high-efficiency catalytic degradation of phenols Y F Tang , B L Allen , D R Kauffman , et al . Electrocatalytic activity of nitrogen-doped carbon nanotube cups . J Am Chem Soc , 2009 , 131 : 13200 - 13201 . DOI: 10.1021/ja904595t http://doi.org/10.1021/ja904595t Electrocatalytic activity of nitrogen-doped carbon nanotube cups L Yang , S Jiang , Y Zhao , et al . Boron-doped carbon nanotubes as metal-free electrocatalysts for the oxygen reduction reaction . Angew Chem Int Ed , 2011 , 50 : 7132 - 7135 . DOI: 10.1002/anie.201101287 http://doi.org/10.1002/anie.201101287 Boron-doped carbon nanotubes as metal-free electrocatalysts for the oxygen reduction reaction T Xie , Y Zhao , J Zhong , et al . Scanning transmission X-ray microscopy and X-ray absorption near-edge structure studies of N-doped carbon nanotubes sealed with N2 gas . J Appl Phys , 2012 , 111 : 124318 . DOI: 10.1063/1.4729903 http://doi.org/10.1063/1.4729903 Scanning transmission X-ray microscopy and X-ray absorption near-edge structure studies of N-doped carbon nanotubes sealed with N2 gas J Kirz , C Jacobsen and M Howells . Soft X-ray microscopes and their biological applications . Q Rev Biophys , 1995 , 28 : 33 - 130 . DOI: 10.1017/S0033583500003139 http://doi.org/10.1017/S0033583500003139 Soft X-ray microscopes and their biological applications J Miot , K Benzerara , G Morin , et al . Iron biomineralization by anaerobic neutrophilic iron-oxidizing bacteria . Geochim Cosmochim Ac , 2009 , 73 : 696 - 711 . DOI: 10.1016/j.gca.2008.10.033 http://doi.org/10.1016/j.gca.2008.10.033 Iron biomineralization by anaerobic neutrophilic iron-oxidizing bacteria P Zhang , Y H Ma , Z Y Zhang , et al . Biotransformation of ceria nanoparticles in cucumber plants . ACS Nano , 2012 , 6 : 9943 - 9950 . DOI: 10.1021/nn303543n http://doi.org/10.1021/nn303543n Biotransformation of ceria nanoparticles in cucumber plants Z Y Chen , Y Liu , B Y Sun , et al . Polyhydroxylated metallofullerenols stimulate IL-1β secretion of macrophage through TLRs/MyD88/NF-κB pathway and NLRP3 inflammasome activation . Small , 2014 , 10 : 2362 - 2372 . DOI: 10.1002/smll.201302825 http://doi.org/10.1002/smll.201302825 Polyhydroxylated metallofullerenols stimulate IL-1β secretion of macrophage through TLRs/MyD88/NF-κB pathway and NLRP3 inflammasome activation B Cui , C Song , G A Gehring , et al . Electrical manipulation of orbital occupancy and magnetic anisotropy in manganites . Adv Funct Mater , 2015 , 25 : 864 - 870 . DOI: 10.1002/adfm.201403370 http://doi.org/10.1002/adfm.201403370 Electrical manipulation of orbital occupancy and magnetic anisotropy in manganites L Sun , H Yan , C Zhou , et al . STXM observation and quantitative study of magnetic vortex structure . Acta Phys Sin , 2015 , accepted. (in Chinese) STXM observation and quantitative study of magnetic vortex structure L J Zhang , B Y Zhao , L Xue , et al . Imaging interfacial micro- and nano-bubbles by scanning transmission soft X-ray microscopy . J Synchrotron Radiat , 2013 , 20 : 413 - 418 . DOI: 10.1107/S0909049513003671 http://doi.org/10.1107/S0909049513003671 Imaging interfacial micro- and nano-bubbles by scanning transmission soft X-ray microscopy L J Zhang , J Wang , Y Luo , et al . A novel water layer structure inside nanobubbles at room temperature . Nucl Sci Tech , 2014 , 25 : 060503 . DOI: 10.13538/j.1001-8042/nst.25.060503 http://doi.org/10.13538/j.1001-8042/nst.25.060503 A novel water layer structure inside nanobubbles at room temperature 10.13538/j.1001-8042/nst.26.040101 Supported by National Natural Science Foundation of China (Nos. 11079050, 11290165, 11305252 and 11205232), the Programs of the Chinese Academy of Sciences (Nos. KJCX2-EW-W09 and KJZD-EW-M03), the Open Research Project of the Large Scientific Facility of the Chinese Academy of Sciences: Study on Self-assembly Technology and Nanometer Array with Ultra-high Density, the National Natural Science Foundation for Outstanding Young Scientists (No. 11225527) and the Shanghai Academic Leadership Program (No. 13XD1404400) 2015-06-27 2015-08-12 2015-08-20 2015-08-20 2021-04-11 2015 Nuclear Science and Techniques 4 26