Jane S. Rigden

An atomic-scale study of the role of titanium in TiO2:SiO2 sol-gel materials

Abstract Neutron diffraction data is used to reveal details of the effect of adding titania to a silica-based sol-gel, including a first direct observation of the TiO correlation. At low Ti contents the data are consistent with the 4-fold oxygen coordination derived from NMR. Even at these concentrations the silica network is shown to have been affected significantly, with bond-length distributions being narrowed, and with a high level of network-OH indicating a qualitative change in the mesoscopic structure associated with interfacial surfaces. Data on a phase separated sample supports the conclusion that the network-OH is associated with the phase separation process.

A neutron and X-ray diffraction study of the influence of deposition conditions on the structure of a-C:H

Abstract The atomic scale structure of amorphous hydrogenated carbon (a-C:H) films prepared from an acetylene precursor by plasma enhanced chemical vapour deposition (PECVD) and a fast-atom source (FAS) have been studied by neutron and X-ray diffraction. The effect of beam energy on the structure of the film is investigated, and comparison is made to samples prepared using at fast atom (neutral particle) source, also using acetylene as the precursor. The results show that, in both deposition methods, increasing the beam energy produces a lower total sp2 hybridised carbon content in the film with evidence for a shift from pure olefinic to some aromatic/graphitic bonding in the FAS samples. The high resolution real-space neutron diffraction data allows a direct determination of the single:double bond ratio, and also shows the presence of sp1 hybridised carbon bonding environments.

The structural characterization of amorphous thin films and coatings in their as-deposited state using x-rays at shallow angles of incidence

We demonstrate the method of x-ray diffraction at shallow angles of incidence, using the intrinsically highly collimated x-ray beam generated by a synchrotron source, to study the atomic-scale structure of amorphous thin films and coatings in their as-deposited (i.e., on-substrate) state. As the incident angle is decreased, scattering from the film/coating can be isolated as contributions from the substrate are reduced. Systems studied include chemical vapor deposition (CVD) diamond films deposited onto both silicon and steel substrates, where evidence of an interfacial region between the film and silicon wafer has been observed, but we focus on a range of amorphous films/coatings (mixed TiO 2 : SiO 2 sol-gel spun films, hydrogenated carbon films and “glassy” carbon coatings, silicon: germanium semiconducting films and alumina coatings). The data are used both to comment upon the systems studied and to elucidate the potential, and the limitations, of the experimental method.

An X-ray absorption study of doped silicate glass, fibre optic preforms

Optical fibre preforms, which have their germanosilicate core regions doped with small quantities of the rare-earth element erbium, have been studied using Extended X-ray Absorption Fine Structure Spectroscopy (EXAFS) at the germanium K absorption edge. These studies were performed using a circular X-ray beam of 100 μm diameter, allowing information to be gathered as a radial function of position across the core region of the preform. This positioning was accomplished by means of a motorized pinhole collimator and sample stage developed for use on the focused X-ray beamline 8.1, at the SRS, Daresbury Laboratory, UK. The EXAFS results are consistent with the germanium sites coordinated to surrounding oxygen atoms at a mean distance of 0.17 nm. Absorption maps of the rare-earth and germanium distribution across the core region of the preforms have also been obtained, showing a correlation between the distributions of the two atom types.

The effect of temperature on the structure of amorphous hydrogenated carbon

The results of complementary inelastic neutron scattering and infrared spectroscopy studies on the structure of amorphous hydrogenated carbon (a-C: H) as a function of temperature are presented, up to a maximum temperature of 1000 °C. These complementary data show that changes in the network structure occur on heating to only 200 °C, with the amorphous network becoming progressively aromatic even before significant hydrogen loss occurs at temperatures above 400 °C. Reference is also made to earlier, diffraction-based studies of the effects of heating a-C: H).

Inhomogeneities in acid-catalyzed titania–silica and zirconia–silica xerogels as revealed by small-angle x-ray scattering

The small-angle x-ray scattering (SAXS) technique was used to investigate inhomogeneities on the scale of 10 to 600 Angstrom in acid-catalyzed titania-silica and zirconia-silica xerogels. SAXS of (TiO2)(SiO2)(1-x) and (ZrO2)(x)(SiO2)(1-x) xerogels with x 0.3 showed the presence of phase-separated regions of metal oxide, which were initially amorphous and crystallized at higher temperatures. A (TiO2)(0.18)(SiO2)(0.82) xerogel that was not initially phase separated became phase separated after heat treatment at 750 degrees C due to reduced solubility of Ti in the silica network.

A Reverse Monte Carlo Modeling Study of Diamond‐like Carbon

Reverse Monte Carlo (RMC) modeling is a novel method of obtaining 3D information on atomic structure by combining complementary data from various experiments. X-ray and neutron diffraction data, nuclear magnetic resonance results, and chemical considerations have been used as constraints in the RMC process to model diamond-like carbon, an amorphous hydrogenated form of carbon with unusual properties. The results have given us new insight into this interesting material.

Role of titanium in TiO2 : SiO2 sol–gels: an X-ray diffraction study

Transmission X-ray diffraction has been used to study a series of powdered silica: titania sol–gel glasses with titania contents ranging from a ‘pure’ silica sample through to high titania levels where phase separation is predicted to occur. Analysis of the data reveals a change in second- and third-neighbour coordination numbers with increasing Ti content and confirms that for low titanium contents the sol–gels are atomically mixed. The lower titanium content sol–gels have also been studied as spun thin films using shallow-angle X-ray diffraction. Comparison with the transmission studies shows an increase in disorder in the silica network when the material is in the form of a coating rather than in the bulk. An increase in the number of Si–O–H bonds is also suggested.

Development of ultrasmall‐angle scattering for studies of colloidal systems (abstract)a)

Ultrasmall‐angle x‐ray scattering (USAXS) using the Bonse–Hart twin‐crystal diffractometer system1 on a synchrotron radiation source (SRS) is being developed as a technique for studying the structure of colloidal dispersions and other inhomogeneous materials with features in the size range 500 A to 1 μm. The use of well‐characterized colloidal samples in this range highlights the problems which must be resolved in order to obtain reliable data from the system. This paper addresses the specific problems of attenuation, absorption, detector dead time, and smearing for the USAXS configuration. The system has been used to study a variety of materials and some previously unobserved features have been found in the scattering profile of a colloidal latex. This serves to indicate the value of the improved system in the field of colloidal science.

Nanoscale Heterogeneities in Amorphous Semiconductor x metal 1– x Alloys: A Small-angle X-ray Scattering Study

A series of small-angle x-ray scattering (SAXS) experiments has been conducted in order to probe further the X-ray absorption fine structure (EXAFS)-derived nanoscale structure of amorphous hydrogenated silicon(x)tin(1-x), hydrogenated silicon(x)nickell(1-x), and germanium,gold(1-x), materials as a function of metal content. The SAXS results reveal information on cluster formation within these reactively radio-frequency-sputtered amorphous thin films. The data are considered within the context of EXAFS data and lend support to a model in which the degree and nature of the heterogeneities depend primarily on the metal species, with the level of metal content inducing additional effects. In particular, the results support a percolation model for the metal:nonmetal transition in amorphous semiconductor,transition metal(1-x), alloys, the conducting volume elements comprising metal or metal compound-rich regions within the amorphous tetrahedral host network.