Table of contents

This special issue of Journal of Physics: Condensed Matter contains a selection of invited and contributed papers presented at Symposium J: Synchrotron Radiation for Advanced Materials Analysis and Processing, part of the International Conference on Materials for Advanced Technologies 2003 (ICMAT 2003) and the International Union of Materials Research Societies International Conference in Asia 2003 (IUMRS-ICA 2003) which was held on 7–12 December 2003 in Singapore. ICMAT 2003 attracted more than 1500 participants and was graced by the presence of Nobel laureates Klaus von Klitzing and Sir Harry W Kroto who gave plenary and public lectures.

One of 16 symposia, Symposium J brought together 54 researchers with various backgrounds from Asia, Australia, Europe and North America. In a relaxed and comfortable atmosphere, participants were presented 1 keynote and 17 invited papers, and 15 contributed oral talks as well as 21 posters covering diverse cutting-edge work made possible by exploiting synchrotron radiation. The relaxed atmosphere encouraged lively discussions.

This special issue contains a variety of topics which are briefly described here with reference to the keynote and invited papers. H-D Nuhn paints a broad picture of the successful development of third-generation storage-ring-based synchrotron light sources and the ongoing projects aimed at building x-ray free electron lasers. Then, using photoemission, L I Johansson discusses the nature of the SiO₂/SiC interface that is attracting keen interest for its significance for semiconductor manufacturing. Presenting photoelectron diffraction, E G Michel takes the same material and shows the solution of the atomic structure in the case of the (3×2) surface reconstruction of cubic SiC. S Lizzit describes the production of high-quality thin films of the inter-metallic superconductor MgB₂ by molecular beam epitaxy and their characterization by photoemission, absorption spectroscopy, and low-energy electron diffraction. Using data obtained by simultaneous wide- and small-angle scattering as well as x-ray absorption spectroscopy, N Greaves gives a detailed explanation of how zeolites collapse when pressurized or heated. Moving on to polymers, H J Kim investigates surface capillary waves on polymer films via x-ray photon correlation spectroscopy. The growth of biological cells on polymer scaffolds is studied by P Thurner using micro-computed tomography with synchrotron radiation. S Cabrini gives an overview of the use of x-ray lithography to fabricate micro- and nano-devices for various applications. Finally, N Umesaki reports examples of applications of synchrotron radiation to characterize important industrial samples and processes.

It is a special pleasure to acknowledge the strong and comprehensive help of co-chairs O Wilhelmi and P Yang and of Ms K L Chong. Many thanks as well to the members of the Scientific Committee who gave their advice, to the members of the Local Organizing Committee, the Session Chairmen, and the Exhibitors and Sponsors. Last, but not least, the effort of authors and referees as well as of the publisher to produce this special issue is greatly appreciated.

Herbert O Moser

Singapore Synchrotron Light Source, National University of Singapore e-mail: moser@nus.edu.sg

Chair: Symposium J, ICMAT 2003

Scientific Committee

H Dosch (Chair), MPI for Metal Research, Germany

S Baik, Pohang Light Source, Korea

G L Carr, NSLS, USA

R J Cernik, Daresbury Laboratory, UK

G N Greaves, University of Wales, Aberystwyth, UK

S Kikuta, JASRI, Japan

M Kovalchuk, RAS, Russia

Z H Mai, Beijing Synchrotron Radiation Facility, China

G Margaritondo, EPFL, Switzerland

N Mårtensson, MAX Lab and University of Uppsala, Sweden

T Matsushita, Photon Factory, Japan

H O Moser, SSLS, Singapore

H Ohno, SPring-8, Japan

H A Padmore, ALS, USA

P Soukiassian, CEA and Université Paris-Sud, France

A Stevenson, CSIRO, Australia

W G Stirling, ESRF, France

M Taniguchi, HISOR, Japan

Local Organizing Committee

H O Moser (Chair), SSLS/NUS

K L Chong, SSLS/NUS

W S Sim, NUS/Chemistry

A T S Wee, NUS/Physics

O Wilhelmi, SSLS/NUS

P Yang, SSLS/NUS

Exhibitors

FEI

Hecus M Braun

M-O-T

Nanofilm Technologies International

Raith

Sponser

PRIMA Research and Technologies

The intensity of x-ray sources has increased at a rapid rate since the late 1960s by ten orders of magnitude and more through the use of synchrotron radiation produced by bending magnets, wigglers and undulators. Three generations of radiation sources have been identified depending on amplitude and quality of the radiation provided. While user facilities of the third generation were being constructed, a new concept of radiation generating devices was being developed that offers an even larger increase in peak and average brightness than had been achieved till then. The new concept of the x-ray free electron laser based on the principle of self-amplified spontaneous emission will be the basis of fourth generation x-ray source user facilities of this century.

The paper will start with a brief history of the development of x-ray sources, it will then discuss some of the differences between storage ring and free electron laser based approaches, and will close with an update of the present development of x-ray free electron laser user facilities.

Two questions thought to have a significant effect on SiC-MOS device characteristics are treated. The existence of carbon clusters or carbon containing by-products and the existence of sub-oxides at the SiO₂/SiC interface. Results of photoemission studies using synchrotron radiation of the interface of the Si-terminated surface of n-type SiC(0001) crystals are presented. The results show that no carbon clusters or carbon containing by-product can be detected at the interface of in situ or ex situ grown samples with an oxide layer thickness larger than  Å. The presence of sub-oxides at the SiO₂/SiC interface was predicted in a theoretical calculation and has been revealed in Si 2p core level data by several groups. These results were not unanimous; significant differences in the number of sub-oxide and shifts were reported. A study also including the Si 1s core level and Si KLL Auger transitions was therefore made. These data show the presence of only one sub-oxide at the interface, assigned to Si¹⁺ oxidation states. The SiO₂ chemical shift is shown to exhibit a dependence on oxide thickness, similar to but smaller in magnitude than the thickness dependence earlier revealed for SiO₂/Si.

Detailed studies of the effects induced on the  R 30° 4H-SiC(0001) surface after different NO exposures, at a substrate temperature of 800 °C, have been made. Photoemission experiments using synchrotron radiation were performed in order to study the properties of the interface formed after gas exposures. Recorded Si 2p spectra show three shifted components, besides the bulk SiC peak. These are assigned as originating from Si₃N₄ or Si¹⁺ sub-oxide, N–Si–O and SiO₂. It was concluded that SiO₂ does grow on top of N–Si–O and that Si₃N₄/Si¹⁺ is located at the interface. Two N 1s components are observed after NO exposures. The main one, located at around 398.05 eV, is assigned as originating from Si₃N₄ and the weaker one is suggested to correspond to N–Si–O bonding. The assignments are made with the aid of Si 2p and N 1s spectra collected after NH₃ and O₂ exposures under similar conditions. No graphite-like carbon or carbon by-product at the interface can be detected after large NO or O₂ exposures.

The photoelectron diffraction method is a local structural technique related to angle resolved photoemission. Its chemical sensitivity allows one to study local environments, both at surfaces and in thicker layers; this leads to a large range of applications. The origin of photoelectron diffraction is the interference of the photoelectron that exits from the crystal directly and the waves multiply scattered in the lattice. The interference pattern detected provides structural information that depends on the kinetic energy of the photoelectron. This technique can be an extremely useful tool for studying the atomic structure of advanced materials. We will show how to obtain an atomic structure in the case of the (3 × 2) reconstruction of cubic silicon carbide.

Ordered MgB₂ thin films have been successfully grown on Mg(0001) by molecular beam epitaxy (MBE). By simply depositing B on the Mg substrate a maximum film thickness of about 4 ML can be achieved, while thicker layers can be obtained by co-deposition or alternated deposition of Mg and B. Thick films show hexagonal low energy electron diffraction patterns and the photoemission and x-ray absorption features typical of MgB₂.

Zeolites collapse under modest pressure or temperature, their microporous structures transforming into glasses of conventional density. Using in situ synchrotron radiation diffraction methods we show how pressure and temperature-induced amorphization are equivalent processes and that these are mirrored by changes in the local structure of charge compensating cations. Evidence for a low density amorphous phase and a high density amorphous phase present during zeolite collapse emerges from small angle scattering experiments. Combining powder diffraction with increasing temperature or pressure, we find that the thermobaric characteristics for zeolite collapse have negative d T/d P slopes, consistent with increasing density during amorphization. However, this is not confined to a single melting curve but, instead, the regime extends over a significant region of T–P space. Moreover, zeolite amorphization involves depressurization and cavitation effects which can be used to set empirical boundaries for the stability of the low density amorphous phase. Within the region of zeolite instability the pressure or temperature of amorphization is found to be governed by the rate at which the stress is introduced—the more rapid this is, the higher the pressure or temperature the zeolite structure survives to. The temperature dependence of the rate of collapse is Arrhenian, suggesting that the initial low density amorphous phase has the characteristics of a superstrong liquid in contrast to the fragility of a conventionally melt quenched glass. Possibilities for creating 'perfect glasses' from the collapse of microporous crystals are discussed.

The characteristics of the richness in silica and the high porosity of rice husk enable its application as a good, yet cheap, heavy metal adsorbent from wastewater. This study used rice husk to sorb Cr(VI) from wastewater that contained 2000 mg Cr(VI) l⁻¹. Results of a scanning electron microscopy (SEM) experiment indicate a considerable morphology alteration of the rice husk after the sorption experiments. ¹³C-nuclear magnetic resonance (NMR) measurements show significant decrease in intensity of all the following peaks: carbonylic, carboxylic, aromatic, polysaccharides, carbohydrates and aliphatics. The X-ray absorption near edge structure (XANES) result indicates that about 70% and 90% of the Cr(VI) sorbed on the rice husk after the 12 and 48 h sorption experiments, respectively, were in Cr(III) forms. In the 12 h sorbed rice husk sample, the chromium species distribution is 41% organic Cr(III) +27%Cr(OH)₃+32%CrO₃, while in the 48 h sorbed one, it is 57% organic Cr(III) +31%Cr(OH)₃+12%CrO₃.

CuO and Cu₂O powders were reacted by hydrogen to study the reduction mechanism and the ionic states of Cu (Cu²⁺, Cu¹⁺ and Cu⁰). The reduction behaviour and the existence of an intermediate state were investigated by using synchrotron-based high-resolution in situ x-ray diffraction (S-XRD) techniques. The results revealed that CuO directly reduces to metallic Cu (), under a normal flow of hydrogen. There was no hint of an intermediate state (Cu¹⁺) during the reduction. In certain conditions, however, an intermediate phase (Cu₂O) appears under either very small flow of hydrogen (<1 cm³ min⁻¹) or very fast temperature ramping (>20 °C min⁻¹), suggesting that the reduction kinetics strongly depends on reduction conditions.

Thermal treatment has recently been emerging as a promising environmental technology to stabilize heavy metal-containing industrial sludge. This study used x-ray absorption spectroscopy (XAS) to identify the species of copper contaminant contained in aluminium oxide that is one of the main compositions of sludge and soil. Results indicate that the originally loaded copper nitrate was transformed into Cu(OH)₂ after its dissolution in the aluminium oxide slurry. Extended x-ray absorption fine structure (EXAFS) fitting indicates that the main copper species in the 105 °C dried Cu(NO₃)₂-loaded aluminium oxide is Cu(OH)₂ which accounts for ca. 75% of the loaded copper. After thermal treatment at 500 °C for 1 h, both x-ray absorption near-edge structure (XANES) and EXAFS fitting results show that CuO became the prevailing copper species (about 85%); the rest of the copper consisted of  Cu(OH)₂ and a negligible amount of Cu(NO₃)₂. It was found that most Cu(OH)₂ and Cu(NO₃)₂ decomposed into CuO at 500 °C. Further increase of the heating temperature from 500 to 900 °C resulted in more decomposition of Cu(OH)₂ and Cu(NO₃)₂; therefore CuO remained as the main copper species. However, it was suggested that about 15% of the loaded copper formed CuAl₂O₄ through the chemical reaction between CuO and Al₂O₃ at 900 °C.

X-ray photon correlation spectroscopy, an emerging technique for studying the slow dynamics of condensed matter, has been employed to probe surface fluctuations on thin supported polymer films as a function of in-plane wavevector, film thickness, temperature, and molecular weight of the polymer. The measurements were performed on thin polystyrene (PS) films of thicknesses ranging from 84 to 333 nm above the glass transition temperature. The lateral length scales probed are at least ten times smaller than those accessible in conventional dynamic light scattering. We find excellent agreement between the measured surface dynamics and the theory of overdamped thermal capillary waves on thin viscoelastic films. The values of the viscosity obtained from these data show good agreement with those of bulk PS.

Advances in cell biology and tissue engineering rely heavily on performing 2D cell culture experiments. Analysis of these is conventionally done with 2D imaging techniques such as light (LM) or electron microscopy (SEM), since they are readily available. Cells, however, might act significantly differently when cultured in 2D or 3D environments. In order to analyse cells in a 3D arrangement, new imaging techniques are necessary not only in order to visualize the periphery of the cell culture in reflection mode but also to perform qualitative and quantitative investigations of the inner parts. Synchrotron radiation micro-computed tomography (SRμCT) using hard x-rays was shown to be a promising tool that can be used for 3D cell culture visualization. SRµCT allows not only visualization of cell cultures in their native 3D environment but also use of the volumetric nature of this imaging procedure to evaluate the cells quantitatively. In our approach, cells were seeded on polymer yarns, stained and measured with SRμCT in absorption and in differential absorption contrast mode. A new segmentation procedure was developed and the measured volumetric data were quantitatively assessed. Quantification parameters included total cell volume, total yarn volume, cell volume density, which is the ratio of cell to yarn volume, and the radial cell mass distribution. The applied variation of the staining parameter of gold enhancement incubation time was shown to have significant influence on the cell volume density. Differential absorption contrast mode was found to provide similar but no additional information on the investigated sample. Using novel approaches of hierarchical volumetric imaging allows closure of the gap between imaging of whole organs and single cells and might be expanded to even higher resolutions, offering investigation of the cell machinery in closer detail.

Fatigue crack closure has been identified as an important factor in determining crack growth rates. However, the methods of measurement of crack closure remain the subject of ongoing controversy. To date, computed finite element models, analytical models and widely established compliance-based experimental methods have offered limited micromechanical insight and/or direct information on the active crack tip region within bulk material. To understand the absolute contributions of crack closure mechanisms, such as plasticity-induced and roughness-induced closure, to fatigue properties, an internal, three-dimensional insight into crack behaviour during loading and unloading is clearly of value. In this work, synchrotron radiation x-ray microtomography is carried out at a high resolution of 0.7 µm to provide unique three-dimensional in situ observation of steady state plane strain fatigue crack growth in a 2024-type Al alloy (Al–Cu–Mg–Mn). Using such high resolution imaging (additionally exploiting the phase contrast effect in interface imaging), the details of fatigue cracks are readily observed, along with the occurrence of closure. A novel microstructural crack displacement gauging method is used to quantify the mixed mode character of crack opening displacement and the closure effect. A liquid gallium grain boundary wetting technique is used in conjunction with the microtomography to visualize the correlation between the three-dimensional structure of the grains and fatigue crack behaviour. Subsequently, electron backscattering diffraction assessment of the grain orientation on the samples provides a uniquely complete 3D description of crack–microstructure interactions.

ELETTRA (http://www.elettra.trieste.it/index.html) is a third generation synchrotron radiation source facility operating at Trieste, Italy, and hosts a wide range of research activities in advanced materials analysis and processing, biology and nano-science at several various beam lines. The energy spectrum of ELETTRA allows x-ray nano-lithography using soft (1.5 keV) and hard x-ray (10 keV) wavelengths. The Laboratory for Interdisciplinary Lithography (LIILIT) was established in 1998 as part of an Italian national initiative on micro- and nano-technology project of INFM and is funded and supported by the Italian National Research Council (CNR), INFM and ELETTRA. LILIT had developed two dedicated lithographic beam lines for soft (1.5 keV) and hard x-ray (10 keV) for micro- and nano-fabrication activities for their applications in engineering, science and bio-medical applications. In this paper, we present a summary of our research activities in micro- and nano-fabrication involving x-ray nanolithography at LILIT's soft and hard x-ray beam lines.

SPring-8 is one of the third-generation large synchrotron radiation facilities. The industrial application of synchrotron radiation carried out at SPring-8 has recently produced outstanding research results in the field of materials science. We have achieved the in situ observation of a galvannealing process for steel sheet, the structural analysis of amorphous thin SiO_x films, analysis of the precise crystal structure of a hydrogen storage alloy, x-ray imaging analyses of aluminium foam and snow-tyres and XAFS analysis of a catalyst for automobiles as examples of the industrial applications of synchrotron radiation.