X射线自由电子激光在化学与能源材料科学中的应用

在过去的十多年中,X射线自由电子激光器(X-ray Free Electron Laser,XFEL)的成功研制和快速发展,极大地推动了超快X射线光谱学实验技术的发展,并且在物理、化学和生物科学等不同研究领域获得了广泛的应用。通过飞秒激光脉冲对样品的激发,X射线可以在不同时间尺度下,跟踪固态、液态和气态等各种情况下样品的微观结构动力学过程。X射线吸收和发射光谱,衍射和散射是探测激光诱导结构变化的典型工具。文章将介绍近年来飞秒X射线技术的发展,及其在化学与能源材料领域的应用。相信随着上海X射线自由电子激光器的建成,将使得飞秒X射线技术在更多的科学领域发挥作用。     

High pressures: Recent trends in materials science

Abstract

Pressure was developed during the 20th century. The most important illustration of the use of high pressure in Materials Science was the synthesis of diamond at the beginning of the fifties.

This contribution will describe the main scientific research axis developed these last years and based on high pressure (synthesis of new materials, stabilization of specific structures, crystal-growth, preparation of finely divided materials…).

In parallel some industrial developments will be analyzed.

In conclusion, the potential of high pressure will be sketched for the near future.     

Applications of nano-polycrystalline diamond anvils to X-ray absorption spectroscopy under high pressure

ABSTRACT

Recently, nano-polycrystalline diamond (NPD) anvils have been widely applied in high pressure research using X-ray absorption spectroscopy (XAS). The nanometer-sized polycrystallization in NPD anvils enables us to obtain glitch-free X-ray absorption spectra. This advantage of NPD anvils drastically improves the experimental conditions of XAS, which has previously used conventional single-crystal diamond (SCD) anvils. Distorted spectra due to the glitches from the SCD anvils have been an inevitable problem of XAS. This paper reviews recent studies of XAS and related spectroscopic techniques using the NPD anvils, which have mainly been performed on BL39XU of SPring-8. We demonstrate how NPD anvils are useful when using XAS for high pressure research.     

Laser-driven switching dynamics in phase change materials investigated by time-resolved X-ray absorption spectroscopy

Time-dependent X-ray absorption spectroscopy at the Ge L₃ edge is used to study the optical switching dynamics of epitaxial phase change materials grown on Si(111) membranes. Small differences between the static absorption spectra of the crystalline and laser-amorphized phase of GeTe are observed. Furthermore, a new approach for pump-probe studies on phase change materials is presented. However, no time-dependent change in the X-ray absorption > 0.1% has been detected so far. This is mainly attributed to sample deterioration at 3 kHz pump-probe rates.     

X-ray absorption spectroscopy and high pressure

Abstract

The difficulties of the adaptation of high pressure to x-ray absorption are presented. The advantages of the energy-dispersive geometry are discussed as well as the future improvements expected with the new synchrotron radiation sources.

Presented at the IUCr Workshop on ‘Synchrotron Radiation Instrumentation for High Pressure Crystallography’, Daresbury Laboratory 20-21 July 1991     

A new tool for nanoscale X-ray absorption spectroscopy and element-specific SNOM microscopy

Investigations of complex nanostructured materials used in modern technologies require special experimental techniques able to provide information on the structure and electronic properties of materials with a spatial resolution down to the nanometer scale. We tried to address these needs through the combination of X-ray absorption spectroscopy (XAS) using synchrotron radiation microbeams with scanning near-field optical microscopy (SNOM) detection of the X-ray excited optical luminescence (XEOL) signal. The first results obtained with the prototype instrumentation installed at the European Synchrotron Radiation Facility (Grenoble, France) are presented. They illustrate the possibility to detect an element-specific contrast and to perform nanoscale XAS experiments at the Zn K and W L(3)-absorption edges in pure ZnO and mixed ZnWO(4)/ZnO thin films.     

A new cell for X-ray absorption spectroscopy study under high pressure

X-ray absorption fine structure （XAFS） spectroscopy is a powerful technique for the investigation of the local environment around selected atoms in condensed matter. XAFS under pressure is an important method for the synchrotron source. We design a cell for a high pressure XAFS experiment. Sintered boron carbide is used as the anvils of this high pressure cell in order to obtain a full XAFS spectrum free from diffraction peaks. In addition, a hydraulic pump was adopted to make in-suit pressure modulation. High quality XAFS spectra of ZrH2 under high pressure （up to 13 GPa） were obtained by this cell.     

A new cell for X-ray absorption spectroscopy study under high pressure

X-ray absorption fine structure (XAFS) spectroscopy is a powerful technique for the investigation of the local environment around selected atoms in condensed matter. XAFS under pressure is an important method for the synchrotron source. We design a cell for a high pressure XAFS experiment. Sintered boron carbide is used as the anvils of this high pressure cell in order to obtain a full XAFS spectrum free from diffraction peaks. In addition, a hydraulic pump was adopted to make in-suit pressure modulation. High quality XAFS spectra of ZrH2 under high pressure (up to 13 Gpa) were obtained by this cell.     

Quantitative depth profiling of photoacid generators in photoresist materials by near-edge X-ray absorption fine structure spectroscopy

Near-edge X-ray absorption fine structure (NEXAFS) spectroscopy was used to quantify the surface composition and depth profiling of photoacid generators in thin film photoresist materials by varying the entrance-grid bias of a partial electron yield detector. By considering model compositional profiles, NEXAFS distinguishes the surface molar excess within the top 6 nm from the bulk. A surface enriched system, triphenylsulfonium perfluorooctanesulfonate, is contrasted with a perfluorobutanesulfonate photoacid generator, which displays an appreciable surface profile within a 6 nm segregation length scale. These results, while applied to 193-nm photoresist materials, highlight a general approach to quantify NEXAFS partial electron yield data.     

X-ray absorption spectroscopy studies in supercritical aqueous solutions

ABSTRACT

The past two decades have witnessed significant advances in the application of X-ray absorption spectroscopy (XAS) to the study of supercritical aqueous fluids. The data obtained using in situ XAS have provided insights into the stability and the structure of metal complexes that are fundamental to understanding natural and industrial hydrothermal processes. Important recent advances using XAS can be attributed to the use of new high temperature and pressure autoclaves designed specifically for the analysis of fluids at extreme temperatures and pressures, improved techniques for the acquisition of X-ray absorption spectra and molecular-level computational modelling used in association with XAS analysis. High-brilliance light sources have not only provided new opportunities for XAS investigations of supercritical fluids, but have also revealed the effects of beam-induced radiolysis of the same fluids. The advent of energy-dispersive and rapid-acquisition XAS holds promise for future studies of beam-induced radiolysis and of the kinetics associated with the formation of metal complexes in high-temperature fluids.     

Resonant elastic X-ray scattering in chemistry and materials science

The applications of anomalous scattering for locating metal atoms and discriminating between different elements has increased when optimised with synchrotron X-radiation.The on-resonance effect enhances the targeted elemental signal and allows small occupancies to be determined, including in situations of a mixed metal population at a single atomic site. Thus the applications of resonant elastic X-ray scattering in biological, inorganic and materials chemistry is being widely applied to single crystals, which is our emphasis, but also powders, fibres, solutions, amorphous and thin film states of matter. Recent developments have included the use of high photon energies (upto 100 keV) as well as softer X-rays (2 keV). The various instrument and technical capabilities have improved in the last 15 years. This ease of measurement of the resonant scattering signals along with absorption edge shifts indicates an expansion to the measurement of multiple data sets, to allow monitoring of redox changes. Whilst crystal structure determination in biological crystallography has been revolutionised by the MAD method, it is not a requirement for chemical or materials crystallography, as other phasing techniques are routine. Synchrotron source upgrades will allow nano-sized X-ray beams to be more widely available. The new X-ray lasers suggest new capabilities too.     

The use of X-ray photoelectron spectroscopy in corrosion science

Most metals in commercial use are protected from reaction with an environment at ambient temperature by an oxide layer of ~5 nm in thickness. This layer may increase in thickness, by solid state diffusion, at high (>300°C) temperature or may dissolve and be replaced by soluble species by electrochemical reactions in aqueous environments. These are the twin processes of corrosion and the problem of preventing such destructive transformations had led to a longstanding interest by corrosion scientists in the thin film: and indeed in sub-monolayers. Prior to the introduction of the techniques of surface derived ion and electron spectroscopy the chemical analyses of films, which had known physical and electronic characteristics, were unavailable. This review attempts to specify the requirements of an analytical technique for thin films and examines the present role of XPS in this context. The areas covered will include elemental analysis, evaluation of valence state, non-destructive information, elemental distribution and finally the use of ion-etch depth profiling. Examples are drawn from corrosion problems in both aqueous and gaseous environments.     

Bromine metallization studied by X-ray absorption spectroscopy

The experimental data carried out by M?ssbauer and magnetic resonances investigations of the structural phase transitions in K₂ZnCl₄ crystals are discussed by a simple electrostatic model, calculating, the lattice contributions to the local electric potential V(r), electric field intensity E(r) and electric field gradient tensor, (r) and taking into account both the fractional electric point charges and rigid lattice approximations. The validity of the model is proved by a good fit of the computing results and experimental data of quadrupole splitting parameters at K sites obtained by ³⁹K-NMR methods in high temperature incommensurate phase ( Pnam symmetry). The experimental results obtained by M?ssbauer and EPR methods in commensurate phase (Pna2₁ symmetry) of iron and copper doped K₂ZnCl₄ crystals are explained by relaxing the rigid lattice approximation. The insertion of probe ions appear to be done on not-exactly-Zn²⁺ site. Received 3 February 1999 and Received in final form 4 May 1999     

In situ soft X-ray absorption spectroscopy of flames

The feasibility of in situ soft X-ray absorption spectroscopy for imaging carbonaceous species in hydrocarbon flames is demonstrated using synchrotron radiation. Soft X-rays are absorbed by core level electrons in all carbon atoms regardless of their molecular structure. Core electron spectroscopy affords distinct advantages over valence spectroscopy, which forms the basis of traditional laser diagnostic techniques for combustion. In core level spectroscopy, the transition linewidths are predominantly determined by the instrument response function and the decay time of the core–hole, which is on the order of a femtosecond. As a result, soft X-ray absorption measurements can be performed in flames with negligible Doppler and collisional broadening. Core level spectroscopy has the further advantage of measuring all carbonaceous species regardless of molecular structure in the far-edge region, whereas near-edge features are molecule specific. Interferences from non-carbon flame species are unstructured and can be subtracted. In the present study, absorption measurements in the carbon K-edge region are demonstrated in low-pressure (P _total = 20–30 Torr) methane jet flames. Two-dimensional imaging of the major carbonaceous species, CH₄, CO₂, and CO, is accomplished by tuning the synchrotron radiation to the respective carbon K-edge, near-edge X-ray absorption fine structure (NEXAFS) transitions and scanning the burner.     

X-ray absorption spectroscopy in dispersive mode and by total reflection

The association of the total reflection scheme with the dispersive mode allows one to record in short time the EXAFS and XANES spectra of the first 30 Å of a surface. In situ time resolved observations of the surface modifications under chemical or electrochemical treatments are thus accessible. This promising technique is illustrated through two examples: (i) the thermal oxidation of a metallic copper surface and (ii) the electrochemical inclusion of Cu⁺ ions within a 5000 Å organic conducting polymer (poly-3-methylthiophene) electrochemically deposited on a platinum surface.     

Photoacoustic X-ray absorption fine-structure spectroscopy of the NiK-edge

The X-ray absorption in a 5 m thick Ni foil for energies from 8.2 keV to 8.8 keV was measured by the photoacoustic method using a gas-microphone-cell. Results are presented for both photoacoustic amplitude and phase as a function of incident photon energy. The absorption fine structure above the NiK-edge is clearly resolved. The data are compared with a simple theory for the signal generation, and discrepancies are explained by discussing the various energy dissipation channels following the X-ray absorption.     

高分辨率电子能量损失谱在材料科学中的应用

简要介绍了FEI Titan80—300STEM扫描透射电镜中装配的Wien-filter型能量单色器(monochromator).文章特别指出,装配有能量单色器的FEI Titan80—300STEM扫描透射电镜,可以直接给出高能量分辨率（~0.1eV）的电子能量损失谱.利用高分辨电子能量损失谱,在高能损失区,对于K或L能级自然宽度(natural width of energy level)小于0.5eV的元素,可以获得更细致的的近限精细结构（energy-loss near-edge structure）,更有利于解析其电子结构;在低能损失区,可以用于精确地确定半导体材料的带隙（bandgap）以及p型掺杂引起的带隙能的变化.     

Nitrogen-doped carbon spheres: an X-ray absorption near-edge structure spectroscopy study

Carbon spheres (CSs) and low content nitrogen (2.5 and 3.5 at%) doped carbon spheres (NCSs) were synthesized by a pyrolysis process at 900 °C using $\mathrm{C}_{2}\mathrm{H}_{2}$ and CH₃CN. The electronic structure and chemical bonding of CSs and NCSs were studied using Raman spectroscopy, valence-band photoemission spectroscopy (VBPES) and X-ray absorption near-edge structure spectroscopy (XANES). XANES spectroscopy showed the effect of carbon and nitrogen bonds and graphitic as well as pyridinic structures on the NCS structure. For the spheres VBPES showed the effect of nitrogen on the spectral shape of the density of states and confirmed the role played by the $\pi$ bonds in controlling the electronic and structural properties of the NCSs. VBPES also showed that both $\pi$ and $\sigma$ bands shift towards higher binding energies after N-doping which is consistent with the XANES data. The consistency of the spherical geometry, chemical purity, suitable electronic/bonding structure and the availability of macroscopic quantities of the N–CSs makes these spheres promising new materials for different applications.     

Picosecond time-resolved X-ray absorption spectroscopy of ultrafast aluminum plasmas

We have used point-projection K-shell absorption spectroscopy to infer the ionization and recombination dynamics of transient aluminum plasmas. Two femtosecond beams of the 100 TW laser at the LULI facility were used to produce an aluminum plasma on a thin aluminum foil (83 or 50 nm), and a picosecond x-ray backlighter source. The short-pulse backlighter probed the aluminum plasma at different times by adjusting the delay between the two femtosecond driving beams. Absorption x-ray spectra at early times are characteristic of a dense and rather homogeneous plasma. Collisional-radiative atomic physics coupled with hydrodynamic simulations reproduce fairly well the measured average ionization as a function of time.