Reduction in metal-oxide thin films has been suggested as the key mechanism responsible for forming conductive phases within solid-state memory devices, enabling their resistive switching capacity. The quantitative spatial identification of such conductive regions is a daunting task, particularly for metal-oxides capable of exhibiting multiple phases as in the case of TiOx. Here, we spatially resolve and chemically characterize distinct TiOx phases in localized regions of a TiOx?based memristive device by combining full-field transmission X-ray microscopy with soft X-ray spectroscopic analysis that is performed on lamella samples. We particularly show that electrically pre-switched devices in lowresistive states comprise reduced disordered phases with O/Ti ratios around 1.37 that aggregate in a ~100 nm highly localized region electrically conducting the top and bottom electrodes of the devices. We have also identified crystalline rutile and orthorhombic-like TiO2 phases in the region adjacent to the main reduced area, suggesting that the temperature increases locally up to 1000 K, validating the role of Joule heating in resistive switching. Contrary to previous studies, our approach enables to simultaneously investigate morphological and chemical changes in a quantitative manner without incurring difficulties imposed by interpretation of electron diffraction patterns acquired via conventional electron microscopy techniques.
Lab-on-Chip is a technology that could potentially revolutionize medical
Point-of-Care diagnostics. Considerable research effort is focused towards innovating
production technologies that will make commercial upscaling financially viable. Printed
circuit board manufacturing techniques offer several prospects in this field. Here, we present
a novel approach to manufacturing Printed Circuit Board (PCB)-based Ag/AgCl reference
electrodes, an essential component of biosensors. Our prototypes were characterized both
structurally and electrically. Scanning Electron Microscopy (SEM) and X-Ray
Photoelectron Spectroscopy (XPS) were employed to evaluate the electrode surface
characteristics. Electrical characterization was performed to determine stability and pH
dependency. Finally, we demonstrate utilization along with PCB pH sensors, as a step
towards a fully integrated PCB platform, comparing performance with discrete commercial
Bioactive sol-gel calcia-silica glasses can regenerate damaged or diseased bones due to their ability to stimulate bone growth. This capability is related to the formation of a hydroxyapatite layer on the glass surface, which bonds with bone, and the release of soluble silica and calcium ions in the body fluid which accelerates bone growth. The addition of silver ions imbues the glass with antibacterial properties due to the release of antibacterial Ag+ ion. The antibacterial activity is therefore closely dependent on the dissolution properties of the glasses which in turn are related to their atomic-level structure. Structural characterisation of the glasses at the atomic level is therefore essential in order to investigate and control the antibacterial properties of the glass. We have used neutron diffraction to investigate the structure of silver-containing calcia-silica sol-gel bioactive glasses with different Ag2O loading (0, 2, 4, 6 mol %). The presence of the silver had little effect on the host glass structure, although some silver metal nanoparticles were present. Results agreed with previous computer simulations.
A series of phosphate-based sol?gel glasses in the system P2O5?CaO?Na2O?SiO2 were synthesised
using PO(OH)32x(OC2H5)x (x = 1, 2) as a phosphorus precursor and alkoxides of sodium,
calcium and silicon in an ethylene glycol solution. It has been found that the upper limit for gel
formation is about 22 mol% phosphorus and that the gelation time increases with increasing
phosphorus content of the sol. X-ray diffraction (XRD) along with X-ray fluorescence chemical
analysis (XRF) have been performed on samples containing 45 mol% of P2O5 and 0, 10, 15 and
25 mol% of SiO2 with varying amount of modifier oxides (CaO, Na2O). All the samples are
predominantly amorphous up to 400 uC and some of them, depending on the composition, retain
their amorphous structure up to 600 and 800 uC. To the knowledge of the authors, this is the first
time that phosphate-based glasses having these compositions have successfully been synthesised
via the sol?gel method.
Glasses in the system 40(P2O5)?x(B2O3)?(60 ? x)(Na2O) (10 d x d 25 mol%) were prepared by the sol?gel technique. A mixture of mono- and diethylphosphates was used as precursor for P2O5, boric acid and sodium methoxide were used as source compounds for B2O3 and Na2O, respectively. The dried gels obtained were heat treated at 200, 300 and 400 °C. Structural development occurring during heat treatment and changes with composition were investigated using X-ray diffraction, thermal analysis, infrared spectroscopy, 11B and 31P solid state NMR. Systems with x = 20 and x = 25 mol% are amorphous up to 400 °C, whereas systems with lower B2O3 content are partially crystalline. This work extends sol?gel preparation of amorphous borophosphate systems having P2O5 as the main component.
Nanocomposites containing FeCo alloy nanoparticles dispersed in a highly ordered 3D cubic Im3m mesoporous silica (SBA-16) matrix were prepared by a novel, single-step templated-assisted sol?gel technique. Two different approaches were used in the synthesis of nanocomposites; a pure SBA-16 sample was also prepared for comparison. Low-angle X-ray diffraction, transmission electron microscopy and N2 physisorption at 77 K show that after metal loading, calcination at 500 °C and reduction in H2 flux at 800 °C the nanocomposites retain the cubic mesoporous structure with pore size not very different from the pure matrix. X-ray absorption fine structure (EXAFS) analysis at Fe and Co K-edges demonstrates that the FeCo nanoparticles have the typical bcc structure. The final nanocomposites were tested as catalysts for the production of carbon nanotubes by catalytic chemical vapour deposition and high-resolution TEM shows that good quality multi-walled carbon nanotubes are obtained.
Structural information on a MnFe2O4?SiO2 nanocomposite aerogel and on the pure silica aerogel matrix were
obtained by total X-ray scattering experiments. The total pair distribution function of the silica aerogel is in
agreement with literature data on melt-quenched silica. The total pair distribution function of the nanocomposite
contains the contribution of all the pair correlations of the atomic species making the interpretation more difficult.
The difference curve obtained by subtracting the total pair distribution function of the matrix from that of the
nanocomposite, allows to selectively study the structural environment of the nanoparticles.
© 2011 Elsevier B.V. All rights reserved
The structure of the iron oxyhydroxide called feroxyhyte (´-FeOOH), which shows an elusive X-ray
powder diffraction pattern, has been represented so far using models describing a mean structure based
on the crystalline network of the iron(III) oxide hematite (±-Fe2O3). In this paper, a novel description of
the mean structure of feroxyhyte is presented, which is based on the structure of the thermodynamically
stable iron oxyhydroxide goethite. Starting from different local arrangements present in the goethite
network, a mean structural model is determined which shows an X-ray powder diffraction pattern
almost coincident with previous studies. This outcome enables to integrate the structure of feroxyhyte
among those of other well characterized iron oxyhydroxides.
Copper-based nanoparticles, supported on either a silica aerogel or cubic mesostructured silicas obtained by using two different synthetic protocols, were used as catalysts for the water gas shift reaction. The obtained nanocomposites were thoroughly characterised before and after catalysis through nitrogen adsorption?desorption measurements at ?196 °C, TEM, and wide- and low-angle XRD. The samples before catalysis contained nanoparticles of copper oxides (either CuO or Cu2O), whereas the formation of metallic copper nanoparticles, constituting the active catalytic phase, was observed either by using pre-treatment in a reducing atmosphere or directly during the catalytic reaction owing to the presence of carbon monoxide. A key role in determining the catalytic performances of the samples is played by the ability of different matrices to promote a high dispersion of copper metal nanoparticles. The best catalytic performances are obtained with the aerogel sample, which also exhibits constant carbon monoxide conversion values at constant temperature and reproducible behaviour after subsequent catalytic runs. On the other hand, in the catalysts based on cubic mesostructured silica, the detrimental effects related to sintering of copper nanoparticles are avoided only on the silica support, which is able to produce a reasonable dispersion of the copper nanophase.
Porous monoliths of nanocomposites containing Ni (5 wt.%) and FeNi (5 wt.%) nanoparticles dispersed on an SBA-16 type matrix were prepared following a templated-gelation method based on the sol?gel process. The nanocomposites were characterized by energy dispersive X-ray spectroscopy, N2 physisorption at 77 K, X-ray diffraction and transmission and scanning electron microscopy. In particular, N2 physisorption and transmission electron microscopy analysis show that the ordered mesoporous structure and the high surface area of all the samples are preserved after calcination in air at 500 °C and also after reduction in H2 flux at 800 °C, indicating a very high thermal stability of the samples. As a result of the effective dispersion of the nanophase within the porous texture, nanocomposites containing Ni nanocrystals with an average size of 6 nm homogeneously dispersed within the pores of the amorphous silica matrix were obtained.
The structural properties of zinc ferrite nanoparticles with spinel structure dispersed in a highly
porous SiO2 aerogel matrix were compared with a bulk zinc ferrite sample. In particular, the details
of the cation distribution between the octahedral (B) and tetrahedral (A) sites of the spinel structure
were determined using X-ray absorption spectroscopy. The analysis of both the X-ray absorption near
edge structure and the extended X-ray absorption fine structure indicates that the degree of inversion
of the zinc ferrite spinel structures varies with particle size. In particular, in the bulk microcrystalline
sample, Zn2+ ions are at the tetrahedral sites and trivalent Fe3+ ions occupy octahedral sites (normal
spinel). When particle size decreases, Zn2+ ions are transferred to octahedral sites and the degree of
inversion is found to increase as the nanoparticle size decreases. This is the first time that a variation
of the degree of inversion with particle size is observed in ferrite nanoparticles grown within an
Extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure
(XANES) techniques at both Fe and Mn K-edges were used to investigate the formation of
MnFe2O4 nanoparticles embedded in a silica aerogel matrix as a function of calcination
temperature (at 450, 750 and 900 1C). Up to 450 1C, two separated highly-disordered phases of
iron and manganese are present. With increasing the temperature (to 750 and 900 1C), the
structure of aerogel nanoparticles becomes progressively similar to that of the spinel structure
MnFe2O4 (jacobsite). Quantitative determination of cations distribution in the spinel structure
shows that aerogels calcined at 750 and 900 1C have a degree of inversion i = 0.20. A pure
jacobsite sample synthesised by co-precipitation and used as a reference compound shows a much
higher degree of inversion (i = 0.70). The different distribution of iron and manganese cations in
the octahedral and tetrahedral sites in pure jacobsite and in the aerogels can be ascribed to partial
oxidation of Mn2+ to Mn3+ in pure jacobsite, confirmed by XANES analysis, probably due to
the synthesis conditions.
The formation of FeCo alloy nanoparticles embedded in a highly ordered 3D cubic mesoporous silica
matrix (SBA-16) was thoroughly studied using several techniques. In particular, the selectivity of
Extended X-ray absorption fine structure and X-ray absorption near-edge structure spectroscopy at
both the Fe and Co K-edges allowed us to determine that before reduction treatment Fe and Co are
present in a poorly crystalline environment, while after reduction treatment FeCo nanoparticles with
the typical bcc structure are formed. FeCo alloy nanoparticles are used in several applications:
biomedical (magnetic carriers for drug delivery and cell separation), magnetic (data storage) and
catalytic. In this work, FeCo nanoparticles formed in situ in the SBA-16 matrix were used for the
production of carbon nanotubes by catalytic chemical vapour deposition. Transmission electron
microscopy indicates that good quality multi-walled carbon nanotubes are obtained.
A mesoporous ordered cubic Im3m silica (SBA-16) characterized by a three dimensional cage-like structure of pores was used as a host matrix for the preparation of a series of FeCo-SiO2 nanocomposites with different alloy loading and composition by the wet impregnation method. The mesoporous structure of the SBA-16-type support, prepared according to a versatile sol-gel templated synthetic method, which makes use of n-butanol as a co-surfactant, is stable during the treatments necessary to obtain the final nanocomposites, as pointed out by low-angle X-Ray diffraction, transmission electron microscopy, and N2 physisorption at 77?K. Wide-angle X-ray diffraction shows that upon reduction at 800 °C, FeCo nanocrystals (6?7?nm) with the typical bcc structure are formed and energy-dispersive X-ray spectroscopy analysis, performed by scanning transmission electron microscopy on one of the samples, shows that the Fe/Co atomic ratio in the alloy nanoparticles is very close to the expected value of two. Electron tomography was used for the first time to gain evidence on the highly interconnected mesoporous structure of SBA-16 and the arrangement of the nanoparticles within the matrix. It was found that spherical alloy nanocrystals with narrow size distribution are homogeneously distributed throughout the mesoporous matrix and that the resulting FeCo-SiO2 nanocomposite material displays superparamagnetic behavior with high strength dipolar interactions, as expected for particles with a large magnetic moment.
A series of Fe/Co based nanocomposites where the matrix is mesoporous ordered cubic Im3m silica (SBA-16 type) characterized by a three dimensional cage-like structure of pores were obtained by two different approaches: impregnation and gelation. X-ray diffraction and transmission electron microscopy analysis show that after metal loading, calcination at 500 °C and reduction in H2 fl ux at 800 °C the nanocomposites retain the well-ordered structure of the matrix with cubic symmetry of pores. All nanocomposites prepared were tested for the production of carbon nanotubes by catalytic chemical vapour deposition. Transmission electron microscopy points out that good quality multi-walled carbon nanotubes are obtained.
Nanocomposites containing FeCo alloy nanoparticles dispersed in a highly ordered cubic mesoporous silica
(SBA-16) matrix were prepared using two different synthetic methods, co-precipitation and impregnation.
Extended X-ray Absorption Spectroscopy (EXAFS) technique at both Fe and Co K-edges was used to
investigate the structure of FeCo nanoparticles and the presence of additional disordered oxide phases. EXAFS
technique gives evidence of differences in the oxidation degree of the FeCo nanoparticles depending on the
synthetic method used.
A series of catalysts containing iron and cobalt nanoparticles supported on a highly ordered mesoporous cubic Im3m silica (SBA-16) were prepared by wet impregnation and used for the production of multi-walled carbon nanotubes (MWCNTs) by catalytic chemical vapor deposition (CCVD) of acetylene. The catalysts were characterized by low- and wide-angle X-ray diffraction, N2 physisorption analysis at 77 K and transmission electron microscopy to study the influence of different metal loading and impregnation time on the CCVD process. Quality and morphology of the MWCNTs was assessed by transmission and scanning electron microscopy, whereas thermal analysis was used to estimate the amount of CNTs produced. It was found that the nanocomposites are catalytically active with particular reference to samples with relatively high metal loading, and are stable under the conditions adopted for the CNT production by the CCVD process.
A series of novel nanocomposites constituted
of FeCo nanoparticles dispersed in an ordered
cubic Im3m mesoporous silica matrix (SBA-16) have
been successfully synthesized using the wet impregnation
method. SBA-16, prepared using the non-ionic
Pluronic 127 triblock copolymer as a structuredirecting
agent, is an excellent support for catalytic
nanoparticles because of its peculiar three-dimensional
cage-like structure, high surface area, thick
walls, and high thermal stability. Low-angle X-ray
diffraction, N2 physisorption, and transmission electron
microscopy analyses show that after metal
loading, calcination at 500 C, and reduction in H2
flux at 800 C, the nanocomposites retain the wellordered
structure of the matrix with cubic symmetry
of pores. FeCo alloy nanoparticles with spherical
shape and narrow size distribution (4?8 nm) are
homogeneoulsy distributed throughout the matrix and
they seem in a large extent to be allocated inside the
A series of phosphosilicates P2O5 (45 mol%)?CaO (20?40 mol%)?Na2O (5?20 mol%)?SiO2 (10?15 mol%)
has been synthesised by the sol?gel route using PO(OH)3?x(OC2H5)x (x = 1, 2) as the phosphorus precursor.
Evolution ofthe gels with temperature was studied using thermogravimetric analysis and differential
thermal analysis. These phosphosilicates show good stability against crystallisationremaining fully amorphous
even after calcination at 400 æC. The short-range structure of the stabilised sol?gel glasses and the
effect of adding modifier oxides to the network structure have been investigated using a combination of
techniques: FT-IR spectroscopy, 31P magic angle spinning NMR, high energy XRD and P L-edge XANES.
Fantechi E, Campo G, Carta D, Corrias A, de Julián Fernández F, Gatteschi D, Innocenti C, Pineider F, Rugi F, Sangregorio C (2012) Exploring the Effect of Co Doping in Fine Maghemite Nanoparticles, Journal of Physical Chemistry C 116 (14) pp. 8261-8270
American Chemical Society
The formation of NiFe2O4 nanoparticles dispersed in an aerogel silica matrix was investigated as a function of calcination temperature by X-ray absorption fine structure and X-ray absorption near edge structure at both the Fe and Ni K-edges. In particular, nanocomposite aerogels containing a relative NiFe2O4 amount of 10 wt % and calcined at 450, 750 (1 h and 20 h), and 900 °C were studied. A quantitative determination of the relative occupancy of iron and nickel cations in the octahedral and tetrahedral sites of the spinel structure was obtained. It has been found that nickel ferrite prepared by sol?gel has the classical inverted spinel structure found in bulk materials with nickel(II) cations fully occupying the octahedra sites and iron(III) equally distributed between octahedra and tetrahedra sites.
The atomic level structure of a series of monodisperse single crystalline nanoparticles with a
magnetic core of manganese ferrite was studied using X-ray absorption fine structure (EXAFS)
and X-ray absorption near edge structure (XANES) techniques at both the Fe and Mn K-edges,
and conventional and high resolution transmission electron microscopy (TEM and HRTEM).
In particular, insights on the non-stoichiometry and on the inversion degree of manganese
ferrite nanocrystals of different size were obtained by the use of complementary structural and
spectroscopic characterization techniques. The inversion degree of the ferrite nanocrystals, i.e. the
cation distribution between the octahedral and tetrahedral sites in the spinel structure, was found
to be much higher (around 0.6) than the literature values reported for bulk stoichiometric
manganese ferrite (around 0.2). The high inversion degree of the nanoparticles is ascribed to
the partial oxidation of Mn2+ to Mn3+ which was evidenced by XANES, leading to
non-stoichiometric manganese ferrite.
Glasses in the system 40(P2O5)?x(B2O3)?(60 x)(Na2O) (10 6 x 6 30 mol%) have been prepared by the
melt-quenching technique. Thermal properties were studied using differential thermal analysis and
the relationship between composition and thermal stability was obtained. Structural characterization
was achieved by a combination of experimental data (infrared and Raman spectroscopy, 11B and 31P solid
state NMR). In particular, variations in the phosphate network structure upon addition of B2O3 and Na2O
were investigated. Analysis of the data indicates that with increasing B2O3 content and decreasing Na2O,
the glass network shows increasing levels of cross-linking between phosphate and borate units. Evidence
of direct B?O?P bonds was observed. In the compositional range investigated, borate groups contain
boron almost exclusively in four-fold coordination.
Titanium dioxide thin films have attracted increasing attention due to their potential in next-generation memory devices. Of particular interest are applications in resistive random access memory (RRAM) devices, where such thin films are used as active layers in metal?insulator?metal (MIM) configurations. When these devices receive a bias above a certain threshold voltage, they exhibit resistive switching (RS), that is, the resistance of the oxide thin film can be tuned between a high resistive state (HRS) and a low resistive state (LRS). In the context of this work, we have used conductive atomic force microscopy (C-AFM) to identify the resistive switching thresholds of titanium dioxide thin films deposited on Si/SiO2/Ti/Pt stacks to be used in memory devices. By performing a set of reading/writing voltage scans over pristine areas of the thin films, we have identified the critical thresholds, which define a reversible operation (soft-breakdown, SB) via localized changes in electrical resistance across the film and an irreversible operation (hard-breakdown, HB) that includes both changes in local electrical resistance and thin film topography. We have also assessed the transition from SB to HB when thin films are stimulated repeatedly with potentials below the identified onsets of HB, validating a history dependent behavior. This study is therefore aimed at presenting new insights in RRAM device programmability, reliability, and eventually failure mechanisms.
Metal?insulator?metal (MIM) devices based on titanium dioxide thin films exhibit resistive switching behavior (RS); i.e., they have the ability to switch the electrical resistance between high-resistive states (HRS) and low-resistive states (LRS) by application of an appropriate voltage. This behavior makes titanium dioxide thin films extremely valuable for memory applications. The physical mechanism behind RS remains a controversial subject but it has been suggested that it could be interface-type, without accompanying structural changes of the oxide, or filament-type with formation of reduced titanium oxide phases in the film. In this work, X-ray absorption spectroscopy (XAS) at the Ti K-edge (4966 eV) was used to characterize the atomic-scale structure of a nonstoichiometric TiO2?x thin film before and after annealing and for the first time after inclusion in a MIM device based on a Cr/Pt/TiO2?x/Pt stack developed on an oxidized silicon wafer. The advantage of the XAS technique is that is element-specific. Therefore, by tuning the energy to the Ti K-edge absorption, contributions from the Pt, Cr, and Si in the stack are eliminated. In order to investigate the structure of the film after electrical switching, XAS analysis at the Ti K-edge was again performed for the first time on the Cr/Pt/TiO2?x/Pt stack in its virgin state and after switching to LRS by application of an appropriate bias. X-ray absorption near-edge structure (XANES) was employed to assess local coordination and oxidation state of the Ti and extended X-ray absorption fine structure (EXAFS) was used to assess bond distances, coordination numbers, and Debye?Waller factors. XAS analysis revealed that the as-deposited film is amorphous with a distorted local octahedral arrangement around Ti (average Ti?O distance of 1.95 Å and coordination number of 5.2) and has a majority oxidation state of Ti4+ with a slight content of Ti3+. The film remains amorphous upon insertion into the stack structure and after electrical switching but crystallizes as anatase upon annealing at 600 °C. These results do not give any indication of the appearance of conducting filaments upon switching and are more compatible with homogeneous interface mechanisms.
In this paper we review an interesting method of PET recycling, i.e. chemical recycling; it is based on the concept of depolymerizing the condensation polymer through solvolytic chain cleavage into low molecular products which can be purified and reused as raw materials for the production of high-quality chemical products. In this work our attention is confined to the hydrolysis (neutral, acid and alkaline) and glycolysis processes of PET chemical recycling; operating conditions and mechanism of each method are reported and described. The neutral hydrolysis has an auto accelerating character; two kinetic models have been proposed: an half-order and a second order kinetic model. The acid hydrolysis could be explained by a modified shrinking core model under chemical reaction control and the alkaline hydrolysis by a first-order model with respect to hydroxide ion concentration. To describe glycolysis, two different kinetic models have been proposed where EG can act or not as internal catalyst. Further experimental and theoretical investigations are required to shed light on the promising processes of PET chemical recycling reviewed in this work.
The FDA approved BioGlass® product is one of very few bone regenerative materials actually in clinical use, having been used in
The structural properties of CoFe2O4?SiO2 highly porous nanocomposite aerogels have been investigated by X-ray Absorption Spectroscopy and Transmission Electron Microscopy techniques. The aerogels are obtained by supercritical drying of composite gels obtained using a two step procedure where fast gelation is achieved using urea in the second step. The formation of CoFe2O4 nanocrystals in the silica matrix begins after calcination at 750 °C of the parent aerogel and is complete after calcination at 900 °C, while the high porosity of the sample is mostly retained.
The influence of PtRu bimetallic particle size and composition on cinnamaldehyde selective hydrogenation
has been investigated for the first time using well-defined catalysts based on carbon nanotubes support.
Very high selectivity towards cinnamyl alcohol together with high activity have been obtained
provided a high temperature treatment of the catalyst is performed. HRTEM, WAXS and EXAFS analyses
permit us to conclude that the remarkable influence of this high temperature treatment on both activity
and selectivity arises from different phenomena. First, a particle size and a structural effect have been evidence
that permits to increase the selectivity. WAXS and EXAFS point the formation of alloyed PtRu nanoparticles.
Second, the heat treatment allows the removal of oxygenated groups from CNT surface. This
may increase the cinnamaldehyde adsorption capacity and decrease the activation barrier for diffusion
of substrate and product on the CNT surface, thus contributing to an increase in the activity.
Phosphate based glasses have attracted great interest because of their application as bioresorbable materials. Because of their solubility in body fluid they can be used as degradable temporary implants for hard and soft tissue replacement and augmentation. In the present work, glasses with a composition of 45% P2O5, 35% CaO, 25% Na2O have been synthesised using the sol-gel method. Gels were obtained using OP(OR)x(OH)3?x (x=1, 2) as phosphorus precursor and alcoxides of calcium and sodium in ethylene glycol. Gels remained amorphous up to a calcination temperature of 400°C. At higher temperature, crystalline ³-Ca2P2O7 was identified. A comparison between the structure of the synthesised sol-gel glass and the structure of an analogue glass generated via the conventional melt quenching method has been performed. A broad based characterisation approach combining many techniques (x-ray diffraction, thermal analysis, infrared spectroscopy, 31P solid state NMR spectroscopy, P L-edge XANES) has been used. The structure of sol-gel, and its melt quenched analogue, is mainly formed by chains of (PO4)3? tetrahedral sharing two bridging oxygen (i.e. methaphosphate units).
Printed circuit boards (PCB) technologies are an attractive system for simple sensing and microfluidic
systems. Controlling the surface properties of PCB material is an important part of this technology and to
date there has been no study on long-term hydrophilisation stability of these materials. In this work, the
effect of different oxygen plasma input power and treatment duration times on the wetting properties
of FR-4 surfaces was investigated by sessile droplet contact angle measurements. Super and weakly
hydrophilic behaviour was achieved and the retention time of these properties was studied, with the
hydrophilic nature being retained for at least 26 days. To demonstrate the applicability of this treatment
method, a commercially manufactured microfluidic structure made from a multilayer PCB (3-layer FR-4
stack) was exposed to oxygen plasma at the optimum conditions. The structures could be filled with
deionised (DI) water under capillary flow unlike the virgin devices.
Nanocomposites made out of FeCo alloy nanocrystals supported onto pre-formed mesoporous ordered silica which features a cubic arrangement of pores (SBA-16) were investigated. Information on the effect of the nanocrystals on the mesostructure (i.e. pore arrangement symmetry, pore size, and shape) were deduced by a multitechnique approach including N2 physisorption, low angle X-ray diffraction, and Transmission electron microscopy. It is shown that advanced transmission electron microscopy techniques are required, however, to gain direct evidence on key compositional and textural features of the nanocomposites. In particular, electron tomography and microtomy techniques make clear that the FeCo nanocrystals are located within the pores of the SBA-16 silica, and that the ordered mesostructure of the nanocomposite is retained throughout the observed specimen.
Ahmed I, Abou Neel E, Valappil S, Nazhat S, Pickup D, Carta D, Carroll D, Newport R, Smith M, Knowles J (2007) The Structure and Properties of Silver-Doped Phosphate Based Glasses, Journal of Materials Science 42 (23) pp. 9827-9835
An undoped and two silver-doped (0, 3 and 5 mol% Ag) phosphate glass compositions were investigated for their structure and properties. These compositions had in a previous study been investigated for their antimicrobial properties, and were found to be extremely potent at inhibiting the micro-organisms tested. Thermal, X-ray diffraction (XRD), nuclear magnetic resonance (NMR) and X-ray absorption Near Edge Structure (XANES) studies were used to elucidate the structure of the compositions investigated, whilst degradation and ion release studies were conducted to investigate their properties. No significant differences were found between the Tg values of the silver containing glasses, while XRD analysis revealed the presence of a NaCa(PO3)3 phase. NMR showed the dominance of Q2 species, and XANES studies revealed the oxidation state of silver to be in the +1 form. No correlation was seen between the degradation and cation release profiles observed, and the P3O93? anion was the highest released anionic species, which correlated well with the XRD and NMR studies. Overall, it was ascertained that using Ag2SO4 as a precursor, and producing compositions containing 3 and 5 mol% Ag, the levels of silver ions released were within the acceptable cyto/biocompatible range.
Guerry P, Carroll D, Gunawidjaja P, Bhattacharya P, Carta D, Pickup D, Ahmed I, Abouneel E, Thomas P, Knowles J, Newport R, Smith M (2011) Solid State NMR as a probe of Inorganic Materials: Examples from Glasses and Sol-gels, Materials Research Society Symposium Proceedings 984
Cambridge University Press
Regoutz A, Gupta I, Serb A, Khiat A, Borgatti F, Lee TL, Schlueter C, Torelli P, Gobaut B, Light M, Carta Daniela, Pearce S, Panaccione G, Prodromakis T (2015) Role and optimization of the active oxide layer in TiO2-based RRAM, Advanced Functional Materials 26 (4) pp. 507-513
TiO2 is commonly used as the active switching layer in resistive random access memory. The electrical characteristics of these devices are directly related to the fundamental conditions inside the TiO2 layer and at the interfaces between it and the surrounding electrodes. However, it is complex to disentangle the effects of film ?bulk? properties and interface phenomena. The present work uses hard X-ray photoemission spectroscopy (HAXPES) at different excitation energies to distinguish between these regimes. Changes are found to affect the entire thin film, but the most dramatic effects are confined to an interface. These changes are connected to oxygen ions moving and redistributing within the film. Based on the HAXPES results, post-deposition annealing of the TiO2 thin film was investigated as an optimisation pathway in order to reach an ideal compromise between device resistivity and lifetime. The structural and chemical changes upon annealing are investigated using X-ray absorption spectroscopy and are further supported by a range of bulk and surface sensitive characterisation methods. In summary, it is shown that the management of oxygen content and interface quality is intrinsically important to device behavior and that careful annealing procedures are a powerful device optimisation technique.
The structure of aged melt-quenched sodium borophosphate glasses of composition
(P2O5)40(B2O3)x(Na2O)60?x (with x in the range 10?40) has been studied by high-energy X-ray diffraction
(HEXRD), 31P and 11B magic angle spinning (MAS) NMR. Similar to the fresh samples, both P O P and
P O B linkages are found to be present in these glasses. All three techniques show that the cross-linking
between borate and phosphate units increases with boron oxide content. Distinctively upon aging, the
glass is found to hydrolyze causing the network to degrade. At the same time, crystalline phases are now
also observed. XRD and DTA show that the samples have a higher tendency towards crystallization with
increasing boron oxide content upon exposed to moisture. 31P and 11B MAS NMR results are in agreement
with these findings. TGA data show that samples with higher boron oxide content take up more moisture
upon aging, suggesting that crystallization may be associated with glass hydrolysis. HEXRD results also
suggest that sodium ions are preferentially associated with borate units with increasing boron oxide
© 2008 Elsevier B.V. All rights reserved.
H, 13C, 17O, 29Si MAS and 93Nb static NMR is reported from a series of sol?gel prepared (Nb2O5)x(SiO2)1ýx
materials with x ¼ 0:03; 0.075 or 0.30. 13C NMR shows that by 500 1C the organic precursor fragments have been removed although
some residual carbon remains as a separate phase. The 29Si NMR typically shows three Q-species (Q2,3,4) in the initial gels, and that
with increasing heat treatment the average n of the Qn
-species increases as the organic fragments and hydroxyl groups are removed. 17O shows unequivocally that the x ¼ 0:03 and 0.075 samples are not phase separated, while at the much higher niobia-content of
x ¼ 0:30 Nb?O?Nb signals are readily detected, a definite indication of the atomic scale phase separation of Nb2O5. Th e x ¼ 0:03
and 0.075 samples heated to 750 1C are thus representative of amorphous niobium silicates. Comparison is made to other sol?gel
prepared metal silicates especially with another Group Va metal tantalum. The effects of tantalum and niobium on the silica
network are very different and it is suggested here that most of the niobium is present as NbO4, forming part of the silicate network.
The effect of O2 and Ar plasma etching on poly(chloro?p?xylylene) (Parylene C) is thoroughly studied by atomic force microscopy, X?ray photoelectron spectroscopy, and static contact angle measurements. Results indicate that O2 plasma changes the topography more drastically than Ar plasma. Furthermore, despite the fact that Ar plasma is expected to be chemically inert, both plasmas introduce O2 to the surface of the Parylene C films, while Ar plasma additionally reduces the amount of Cl present in the polymer. The effect on the viability of cultured cardiomyocytes is also examined, indicating that cells attach and survive both on Ar and O2 treated films in contrast to untreated Parylene. These observations can provide useful insight into the field of material science and tissue engineering.
Ternary phosphate-based glasses in the system P2O5?CaO?Na2O were synthesized using the sol?gel approach. Glasses in this system have the potential for use as bioactive materials. A mixture of mono- and dialkyl phosphate PO(OH)3?x(OC2H5)x (x = 1, 2) and alkoxides of sodium and calcium in an ethylene glycol solution were used as precursors. One of the compositions has also been synthesized by sonocatalysis (application of ultrasonic vibration to the sol). The systems synthesized, which remain fully amorphous even after calcination at 400 °C given the appropriate composition, have been characterized using X-ray diffraction (XRD). Thermal properties have been examined by means of thermogravimetric analysis (TGA) and differential thermal analysis (DTA). The structure of the phosphate network has been studied as a function of composition using Fourier transform infrared spectroscopy (FT-IR) and 31P MAS NMR.
Resistive random access memory (ReRAM) crossbar arrays have become one of the most promising candidates for next-generation non volatile memories. To become a mature technology, the sneak path current issue must be solved without compromising all the advantages that crossbars offer in terms of electrical performances and fabrication complexity. Here, we present a highly integrable access device based on nickel and sub-stoichiometric amorphous titanium dioxide (TiO2?x), in a metal insulator metal crossbar structure. The high voltage margin of 3 V, amongst the highest reported for monolayer selector devices, and the good current density of 104 A/cm2 make it suitable to sustain ReRAM read and write operations, effectively tackling sneak currents in crossbars without compromising fabrication complexity in a 1 Selector 1 Resistor (1S1R) architecture. Furthermore, the voltage margin is found to be tunable by an annealing step without affecting the device's characteristics.
Phosphate-based glasses have recently attracted much interest as a new generation of biomaterials because of their ability to react and
dissolve in the physiological environment and eventually to be replaced by regenerated hard or soft tissue. A series of phosphate-based
glasses containing 45 mol% P2O5 and various amounts of CaO and Na2O were synthesized by sol?gel and melt-quenching techniques.
A comparison between the structure of the sol?gel glass and the structure of the analogous melt-quenched glasses has been undertaken. A
broad-based characterization approach combining different techniques has been used to investigate the short-range structure of the
glasses and the effect of adding modifier oxides to the network structure (conventional and high energy X-ray diffraction, infra-red spectroscopy,
31P solid state magic angle spinning NMR spectroscopy). Sol?gel and melt-quenched glasses appear to have a similar structure,
showing similar Qn distributions and atomic correlations.
Sol-gel derived calcium silicate glasses may be
useful for the regeneration of damaged bone. The mechanism
of bioactivity is as yet only partially understood but
has been strongly linked to calcium dissolution from the
glass matrix. In addition to the usual laboratory-based characterisation
methods, we have used neutron diffraction with
isotopic substitution to gain new insights into the nature of
the atomic-scale calcium environment in bioactive sol-gel
glasses, and have also used high energy X-ray total diffraction
to probe the nature of the processes initiated when bioactive
glass is immersed in vitro in simulated body fluid. The
data obtained point to a complex calcium environment in
which calcium is loosely bound within the glass network
and may therefore be regarded as facile. Complex multistage
dissolution and mineral growth phases were observed
as a function of reaction time between 1 min and 30 days,
leading eventually, via octacalcium phosphate, to the formation
of a disordered hydroxyapatite (HA) layer on the glass
surface. This methodology provides insight into the structure
of key sites in these materials and key stages involved in
their reactions, and thereby more generally into the behaviour
of bone-regenerative materials that may facilitate improvements
in tissue engineering applications.
Ferrihydrite is a generic term for various poorly ordered Fe(III) oxyhydroxides which are naturally occurring
as nanocrystals and are believed to constitute the ferric core of ferritine, the main iron storage protein
in biological systems. Unlike other iron oxides, the exact structure and composition of ferrihydrite is still
a matter of debate. In this work, we have prepared and characterized the two main forms of ferrihydrite
referred to as 2-lines and 6-lines, on the basis of the number of reflections observed in the (X-ray) diffraction
pattern. Thermal and textural properties have been studied; structural characterization has been
performed by X-ray diffraction, transmission electron microscopy and X-ray absorption spectroscopy
(EXAFS and XANES). The structure of the two forms results to be quite similar. The study of the magnetic
properties indicates that the small differences between the 2-lines and 6-lines ferrihydrite samples are
mainly caused by the different weight of the magnetic spins located on the particle surface, related to the
different nanoparticles mean size.
X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) techniques at both Fe and Co K-edges were used to investigate the formation of CoFe2O4 nanoparticles embedded in a silica aerogel matrix as a function of calcination temperature and CoFe2O4 content. In particular, nanocomposite aerogels containing relative CoFe2O4 amounts of 5 and 10 wt % and calcined at 450, 750, and 900 °C were studied. The evolution of the nanophase with calcination temperatures depends on the composition. In the sample containing 10 wt % of nanophase, results indicate that CoFe2O4 nanocrystals were formed after calcination at 750 °C, whereas in the sample containing 5 wt % of nanophase, they were obtained only after calcination at 900 °C. Quantitative determination of the distribution of the iron and cobalt phase in the octahedral and tetrahedral sites of the spinel structure shows that cobalt ferrite prepared by sol?gel has a partially inverted spinel structure with a degree of inversion around 0.70.
Titanium oxide (TiOx) has attracted a lot of attention as an active material for resistive random access memory (RRAM), due to its versatility and variety of possible crystal phases. Although existing RRAM materials have demonstrated impressive characteristics, like ultra-fast switching and high cycling endurance, this technology still encounters challenges like low yields, large variability of switching characteristics, and ultimately device failure. Electroforming has been often considered responsible for introducing irreversible damage to devices, with high switching voltages contributing to device degradation. In this paper, we have employed Al doping for tuning the resistive switching characteristics of titanium oxide RRAM. The resistive switching threshold voltages of undoped and Al-doped TiOx thin films were first assessed by conductive atomic force microscopy. The thin films were then transferred in RRAM devices and tested with voltage pulse sweeping, demonstrating that the Al-doped devices could on average form at lower potentials compared to the undoped ones and could support both analog and binary switching at potentials as low as 0.9 V. This work demonstrates a potential pathway for implementing low-power RRAM systems.
The next generation of nonvolatile memory storage may well be based on resistive switching in metal oxides. TiO2 as transition metal oxide has been widely used as active layer for the fabrication of a variety of multistate memory nanostructure devices. However, progress in their technological development has been inhibited by the lack of a thorough understanding of the underlying switching mechanisms. Here, we employed high-angle annular darkfield scanning transmission electron microscopy (HAADF-STEM) combined with two-dimensional energy dispersive X-ray spectroscopy (2D EDX) to provide a novel, nanoscale view of the mechanisms involved. Our results suggest that the switching mechanism involves redistribution of both Ti and O ions within the active layer combined with an overall loss of oxygen that effectively render conductive filaments. Our study shows evidence of titanium movement in a 10 nm TiO2 thin-film through direct EDX mapping that provides a viable starting point for the improvement of the robustness and lifetime of TiO2-based resistive random access memory (RRAM).
Samples of nickel cobaltite, a mixed oxide occurring in the spinel structure which is currently extensively investigated because of its prospective application as ferromagnetic, electrocatalytic, and cost-effective energy storage material were prepared in the form of nanocrystals stabilized in a highly porous silica aerogel and as unsupported nanoparticles. Nickel cobaltite nanocrystals with average size 4 nm are successfully grown for the first time into the silica aerogel provided that a controlled oxidation of the metal precursor phases is carried out, consisting in a reduction under H2 flow followed by mild oxidation in air. The investigation of the average oxidation state of the cations and of their distribution between the sites within the spinel structure, which is commonly described assuming the Ni cations are only located in the octahedral sites, has been carried out by X-ray Absorption Spectroscopy providing evidence for the first time that the unsupported nickel cobaltite sample has a Ni:Co molar ratio higher than the nominal ratio of 1:2 and a larger than expected average overall oxidation state of the cobalt and nickel cations. This is achieved retaining the spinel structure, which accommodates vacancies to counterbalance the variation in oxidation state.
Novel nanocomposite catalysts for single step Water Gas Shift Reaction (WGSR) were prepared by deposition-precipitation and impregnation of Pt-CeO2 nanophases onto an ordered mesoporous silica support featuring a cubic arrangement of mesopores (SBA-16 type). The highly interconnected porosity of the SBA-16 developing in three-dimension (3D) provides a scaffold which is easily accessible to reactants and products by diffusion. The textural and morphological properties of the final catalyst were affected by the procedure utilized for dispersion of the nanophases onto SBA-16. Catalysts prepared by deposition-precipitation present highly dispersed nanocrystalline CeO2 on the surface of SBA-16 and retain high surface area, high thermal stability and high Pt accessibility. Catalysts prepared by impregnation show improved Pt-CeO2 interaction but a more significant decrease of surface area compared to pure SBA-16, due to the confinement of the CeO2 crystallites within the mesoporous matrix.
As a result, catalysts prepared by deposition-precipitation are effective for WGSR under working conditions in the high temperature range (around 300-350 °C), whereas catalysts prepared by impregnation are suitable for the process operative at low temperature (LT-WGSR). Our results point out that catalyst preparation procedures can be used to optimise the performance of heterogenous catalysts, by controlling the CeO2 crystallites size and optimizing Pt-CeO2 contact by embedding. Improved thermal and chemical stability was achieved using a mesoporous scaffold.