A-Si/SiO2 nanolaminates for tuning the complex refractive index and band gap in optical interference coatings.

A-S i/S i O 2 nanolaminates are deposited by magnetron sputtering and show a decreasing absorption when the a-Si single-layer thickness is reduced from 2.4n m to 0.7n m. Moreover, an increase of the Tauc band gap by 0.18e V is measured. Experimental Tauc band gaps are compared to calculated effective band gaps, utilizing a numerical Schrödinger solver. Further, it is demonstrated that the refractive index can be controlled by adjusting the a-Si and S i O 2 single-layer thicknesses in the nanolaminates. The nanolaminates are optically characterized by spectroscopic ellipsometry, transmittance, and reflectance measurements. Additionally, TEM images reveal uniform, well-separated layers, and EDX measurements show the silicon and oxygen distribution in the nanolaminates.

Sample preparation for analytical scanning electron microscopy using initial notch sectioning.

A novel method for broad ion beam based sample sectioning using the concept of initial notches is presented. An adapted sample geometry is utilized in order to create terraces with a well-define d step in erosion depth from the surface. The method consists of milling a notch into the surface, followed by glancing-angle ion beam erosion, which leads to preferential erosion at the notch due to increased local surface elevation. The process of terrace formation can be utilized in sample preparation for analytical scanning electron microscopy in order to get efficient access to the depth-dependent microstructure of a material. It is demonstrated that the method can be applied to both conducting and non-conducting specimens. Furthermore, experimental parameters influencing the preparation success are determined. Finally, as a proof-of-concept, an electron backscatter diffraction study on a surface crystallized diopside glass ceramic is performed, where the method is used to analyze orientation dependent crystal growth phenomena occurring during growth of surface crystals into the bulk.

Optical bandgap control in Al2O3/TiO2 heterostructures by plasma enhanced atomic layer deposition: Toward quantizing structures and tailored binary oxides.

Atomically thin heterostructures and superlattices are promising candidates for various optoelectronic and photonic applications. Different combinations of Al2O3/TiO2 composites are obtained by plasma enhanced atomic layer deposition (PEALD). Their growth, composition, dispersion relation, and optical bandgap are systematically studied by means of UV/VIS spectrophotometry, spectroscopic ellipsometry (SE), x-ray reflectometry (XRR), scanning transmission electron microscopy(STEM) and x-ray photoelectron spectroscopy (XPS). Besides, an effective medium approximation (EMA) approach is applied to model the heterostructures theoretically. The refractive index and the indirect bandgap of the heterostructures depend on the ratio of the two oxides, while the bandgap is very sensitive to the thicknesses of the barrier and quantum well layers. A large blue shift of the absorption edge from 400 nm to 320 nm is obtained by changing the TiO2 (quantum well) thickness from ~2 nm to ~0.1 nm separated by ~2 nm of Al2O3 (barrier) layers. PEALD unfolds the possibility of achieving optical quantizing effects within complex heterostructures enabling control of their structures down to atomic scale. It enables a path towards atomic scale processing of new 'artificial' materials with desired refractive indices and bandgap combinations by precise control of their compositions.

Nucleation efficacy and flexural strength of novel leucite glass-ceramics.

OBJECTIVES: To optimize the nucleation mechanism in leucite glass-ceramics to allow more efficacious glass-ceramic manufacture and improvements in microstructure and mechanical reliability. MATERIALS AND METHODS: An alumino-silicate glass was designed and synthesized using melt quench methods. The glass was crushed and milled using various milling times (48-93h) and spray drying. Nucleation and growth heat treatment schedules were applied to synthesize glass-ceramics. Glass/glass-ceramic powders and frit specimens were characterized using differential scanning calorimetry (DSC), transmission electron microscopy (TEM), energy-dispersive X-ray spectrometry (EDX), magic angle spinning nuclear magnetic resonance spectroscopy (MAS-NMR) and X-ray diffraction analyses (XRD). Glass-ceramic specimens were tested using the biaxial flexural strength test (BFS). RESULTS: Application of defined nucleation heat treatments resulted in the synthesis of Na/Ca titanates. NMR indicated changes to the 23Na glass spectra in the nucleated glass and TEM/EDX the presence of Na/Ca/Ti domains (<200nm) within the leucite crystals and associated with the nucleation of the leucite phase. XRD confirmed the presence of a bulk leucite phase in the glass-ceramics. SEM/TEM confirmed the crystallization of the leucite phase (65.5-69.3%) in a thermally matched glass, in conjunction with the nano Na/Ca titanate phase. The leucite glass-ceramics resulted in a high BFS (255-268MPa), with reduction in powder milling time prior to heat treatments having no significant effect on flexural strength and reliability (p>0.05). SIGNIFICANCE: Na/Ca titanates were synthesized in leucite glass-ceramics for the first time and associated with its nucleation and efficacious growth. This nucleation optimization provides opportunities for more efficient manufacturing and microstructural/mechanical reliability improvements. Improved synthesis of high strength/reliable leucite glass-ceramics is useful for construction of esthetic minimally invasive restorations.

Enhanced Magnetoelectric Coupling in BaTiO3-BiFeO3 Multilayers-An Interface Effect.

Combining various (multi-)ferroic materials into heterostructures is a promising route to enhance their inherent properties, such as the magnetoelectric coupling in BiFeO3 thin films. We have previously reported on the up-to-tenfold increase of the magnetoelectric voltage coefficient α ME in BaTiO3-BiFeO3 multilayers relative to BiFeO3 single layers. Unraveling the origin and mechanism of this enhanced effect is a prerequisite to designing new materials for the application of magnetoelectric devices. By careful variations in the multilayer design we now present an evaluation of the influences of the BaTiO3-BiFeO3 thickness ratio, oxygen pressure during deposition, and double layer thickness. Our findings suggest an interface driven effect at the core of the magnetoelectric coupling effect in our multilayers superimposed on the inherent magnetoelectric coupling of BiFeO3 thin films, which leads to a giant α ME coefficient of 480 V c m -1 Oe-1 for a 16 × (BaTiO3-BiFeO3) superlattice with a 4 . 8 nm double layer periodicity.

Sr[Li2Al2O2N2]:Eu2+-A high performance red phosphor to brighten the future.

Innovative materials for phosphor converted white light-emitting diodes are in high demand owing to the huge potential of the light-emitting diode technology to reduce energy consumption worldwide. As the primary blue diode is already highly optimized, the conversion phosphors are of crucial importance for any further improvements. We report on the discovery of the high performance red phosphor Sr[Li2Al2O2N2]:Eu2+ meeting all requirements for a phosphor's optical properties. It combines the optimal spectral position for a red phosphor, as defined in the 2016 Research & Development-plan of the United States government, with an exceptionally small spectral full width at half maximum and excellent thermal stability. A white mid-power phosphor-converted light-emitting diode prototype utilising Sr[Li2Al2O2N2]:Eu2+ shows an increase of 16% in luminous efficacy compared to currently available commercial high colour-rendering phosphor-converted light-emitting diodes, while retaining excellent high colour rendition. This phosphor enables a big leap in energy efficiency of white emitting phosphor-converted light-emitting-diodes.

Impact of magnetization and hyperfine field distribution on high magnetoelectric coupling strength in BaTiO3-BiFeO3 multilayers.

Correlations were established between the hyperfine field distribution around the Fe atoms, the multiferroic properties, and the high magnetoelectric coefficient in BaTiO3-BiFeO3 multilayer stacks with variable BiFeO3 single layer thickness, down to 5 nm. Of key importance in this study was the deposition of 57Fe - enriched BiFeO3, which enhances the sensitivity of conversion electron Mössbauer spectroscopy by orders of magnitude. The magnetoelectric coefficient αME reaches a maximum of 60.2 V cm-1 Oe-1 at 300 K and at a DC bias field of 2 Tesla for a sample of 15 × (10 nm BaTiO3-5 nm BiFeO3) and is one of the highest values reported so far. Interestingly, the highest αME is connected to a high asymmetry of the hyperfine field distribution of the multilayer composite samples. The possible mechanisms responsible for the strong magnetoelectric coupling are discussed.

The effect of TiO2 on nucleation and crystallization of a Li2O-Al2O3-SiO2 glass investigated by XANES and STEM.

Glass ceramics based on Li2O/Al2O3/SiO2 are of high economic importance, as they often show very low coefficients of thermal expansion. This enables a number of challenging applications, such as cooktop panels, furnace windows or telescope mirror blanks. Usually, the crystallization of the desired LAS crystal phases within the glasses must be tailored by a careful choice of crystallization schedule and type of nucleation agents to be used. The present work describes the formation of nanocrystalline TiO2 within an LAS base composition that contains solely TiO2 as nucleating agent. Using a combination of scanning transmission electron microscopy as well as X-ray absorption spectroscopy, it is found that a mixture of four- and six-fold coordinated Ti4+ ions exists already within the glass. Heating of the glass to 740 °C immediately changes this ratio towards a high content of six-fold coordinated Ti, which accumulates in liquid-liquid phase-separation droplets. During the course of thermal treatment, these droplets eventually evolve into nanocrystalline TiO2 precipitations, in which Ti4+ is six-fold coordinated. Thus, it is shown that the nucleation of nanocrystalline TiO2 is initiated by a gradual re-arrangement of the Ti ions in the amorphous, glassy matrix, from a four-fold towards a six-fold coordination.

Redox effects and formation of gold nanoparticles for the nucleation of low thermal expansion phases from BaO/SrO/ZnO/SiO2 glasses.

Glasses in the system BaO/SrO/ZnO/SiO2 containing 0.01 and 0.1 mol% gold were used to study the formation of gold nanoparticles with the aim to use them as nucleation agents. In order to promote gold clustering, the glasses were additionally doped with 0.5 mol% Sb2O3. Depending on the heat treatment schedule, Au particle sizes were in the range from 6 to above 50 nm. In contrast to many other gold ruby glass systems, the clustering is completely prevented by the absence of antimony; then the glasses remain colorless. Surprisingly, at higher temperatures, a re-dissolution of gold clusters was also observed, which now allows the formulation of a more comprehensive model concerning the redox and clustering behavior. This growth model is completed by the fact that a high gold concentration enables the stabilization of much smaller Au clusters. Mie theory with the aid of quantum confined size-dependent dielectric functions was successfully used to describe the optical behavior of the gold nanoparticles also for sizes below 10 nm. These results were confirmed using high resolution scanning transmission electron microscopy, including energy dispersive X-ray spectroscopy. It could also be shown that small gold particles up to a size of 50 nm are not effective as nucleating agents.

The formation of nanocrystalline ZrO2 nuclei in a Li2O-Al2O3-SiO2 glass - a combined XANES and TEM study.

The high economic importance of glass ceramics based on Li2O/Al2O3/SiO2 (LAS) is mainly due to their low coefficients of thermal expansion (CTE), which make these materials suitable candidates for a number of applications. The exact mechanism of the crystallization processes in LAS glasses is still not fully understood. The present work focuses on the formation and development of nanocrystalline ZrO2 within an LAS base composition which contains only ZrO2 as nucleating agent. Using a combination of transmission electron microscopy and X-ray absorption spectroscopy techniques, the temporal evolution of the ZrO2 nanocrystal formation is described. It is found that the formation of ZrO2 is initiated by liquid-liquid phase separation droplets with high Zr content, which eventually evolve into the nanocrystalline ZrO2 precipitations. This process is accompanied by a gradual change of the coordination of the tetravalent Zr ions from sixfold in the glass to eightfold in the crystals. The diameters of the ZrO2 crystals stay well below 4 nm, even at late stages. The degree of crystallization at each step of the crystallization process is deduced, and from that, the Avrami coefficient n is determined to be n ≈ 1, which describes a barrier-limited crystal growth process.

Correlation of Interface Impurities and Chemical Gradients with High Magnetoelectric Coupling Strength in Multiferroic BiFeO3-BaTiO3 Superlattices.

The detailed understanding of magnetoelectric (ME) coupling in multiferroic oxide heterostructures is still a challenge. In particular, very little is known to date concerning the impact of the chemical interface structure and unwanted impurities that may be buried within short-period multiferroic BiFeO3-BaTiO3 superlattices during growth. Here, we demonstrate how trace impurities and elemental concentration gradients contribute to high ME voltage coefficients in thin-film superlattices, which are built from 15 double layers of BiFeO3-BaTiO3. Surprisingly, the highest ME voltage coefficient of 55 V cm-1 Oe-1 at 300 K was measured for a superlattice with a few atomic percent of Ba and Ti that diffused into the nominally 5 nm thin BiFeO3 layers, according to analytical transmission electron microscopy. In addition, highly sensitive enhancements of the cation signals were observed in depth profiles by secondary ion mass spectrometry at the interfaces of BaTiO3 and BiFeO3. As these interface features correlate with the ME performance of the samples, they point to the importance of charge effects at the interfaces, that is, to a possible charge mediation of ME coupling in oxide superlattices. The challenge is to provide cleaner materials and processes, as well as a well-defined control of the chemical interface structure, to push forward the application of oxide superlattices in multiferroic ME devices.

Surface Crystallization of a MgO/Y2O3/SiO2/Al2O3/ZrO2 Glass: Growth of an Oriented ß-Y2Si2O7 Layer and Epitaxial ZrO2.

The crystallization behavior of a glass with the composition 54.7 SiO2·10.9 Al2O3·15.0 MgO·3.4 ZrO2·16.0 Y2O3 is studied using X-ray diffraction (XRD), scanning electron microscopy (SEM) including electron backscatter diffraction (EBSD) and (scanning) transmission electron microscopy [(S)TEM] including energy-dispersive X-ray spectrometry (EDXS). This glass shows the sole surface crystallization of four different yttrium silicates of the composition Y2Si2O7 (YS). The almost simultaneous but independent nucleation of α-, ß-, δ-, and ε-YS at the surface is followed by growth into the bulk, where ε-YS quickly dominates a first crystallized layer. An accumulation of Mg at the growth front probably triggers a secondary nucleation of ß-YS, which forms a thin compact layer before fragmenting into a highly oriented layer of fine grained crystals occupying the remaining bulk. The residual glass between the YS growth structures allows the crystallization of indialite, yttrium stabilized ZrO2 (Y-ZrO2) and very probably µ-cordierite during cooling. Hence, this glass basically shows the inverted order of crystallization observed in other magnesium yttrium alumosilicate glasses containing less Y2O3. An epitaxial relationship between Y-ZrO2 and ε-YS is proven and multiple twinning relationships occur in the YS phases.

Mechanical, structural, and optical properties of PEALD metallic oxides for optical applications.

Structural, optical, and mechanical properties of Al2O3, SiO2, and HfO2 materials prepared by plasma-enhanced atomic layer deposition (PEALD) were investigated. Residual stress poses significant challenges for optical coatings since it may lead to mechanical failure, but in-depth understanding of these properties is still missing for PEALD coatings. The tensile stress of PEALD alumina films decreases with increasing deposition temperature and is approximately 100 MPa lower than the stress in thermally grown films. It was associated with incorporation of -OH groups in the film as measured by infrared spectroscopy. The tensile stress of hafnia PEALD layers increases with deposition temperature and was related to crystallization of the film. HfO2 nanocrystallites were observed even at 100°C deposition temperature with transmission electron microscopy. Stress in hafnia films can be reduced from approximately 650 MPA to approximately 450 MPa by incorporating ultrathin Al2O3 layers. PEALD silica layers have shown moderate stress values and stress relaxation with the storage time, which was correlated to water adsorption. A complex interference coating system for a dichroic mirror (DCM) at 355 nm wavelength was realized with a total coating thickness of approximately 2 µm. Severe cracking of the DCM coating was observed, and it propagates even into the substrate material, showing a good adhesion of the ALD films. The reflectance peak is above 99.6% despite the mechanical failure, and further optimization on the material properties should be carried out for demanding optical applications.

Characterizing the residual glass in a MgO/Al2O3/SiO2/ZrO2/Y2O3 glass-ceramic.

The non-isochemical crystallization of glasses leads to glass-ceramics in which the chemical composition of the amorphous matrix differs from that of the parent glass. It is challenging to solely analyse the properties of these residual glassy phases because they frequently contain finely dispersed crystals. In this study, the composition of the residual glass matrix after the crystallization of a glass with the mol% composition 50.6 SiO2 · 20.7 MgO · 20.7 Al2O3 · 5.6 ZrO2 · 2.4 Y2O3 is analysed by scanning transmission electron microscopy (STEM) including energy dispersive X-ray analysis (EDXS). A batch of the residual glass with the determined composition is subsequently melted and selected properties are analysed. Furthermore, the crystallization behaviour of this residual glass is studied by X-ray diffraction, scanning electron microscopy including electron backscatter diffraction and STEM-EDXS analyses. The residual glass shows sole surface crystallization of indialite and multiple yttrium silicates while bulk nucleation does not occur. This is in contrast to the crystallization behaviour of the parent glass, in which a predominant bulk nucleation of spinel and ZrO2 is observed. The crystallization of the residual glass probably leads to different crystalline phases when it is in contact to air, rather than when it is enclosed within the microstructure of the parent glass-ceramics.

Bulk Crystallization in a SiO2/Al2O3/Y2O3/AlF3/B2O3/Na2O Glass: Fivefold Pseudo Symmetry due to Monoclinic Growth in a Glassy Matrix Containing Growth Barriers.

A glass with the mol% composition 17 Y2O3·33 Al2O3·40 SiO2·2 AlF3·3 Na2O·2 CeF3·3 B2O3 is heat treated at 1000 °C for 6-24 h. This results in the surface nucleation and growth of YAG. Nucleation and growth of star-shaped alumina and later of monoclinic ß-Y2Si2O7 and orthorhombic δ-Y2Si2O7 are additionally observed in the bulk. Phase identification and localization are performed by electron backscatter diffraction (EBSD) as well as TEM analysis. The monoclinic ß-Y2Si2O7 observed in the bulk occurs in the form of large, crystal agglomerates which range from 50 to 120 µm in size. The individual crystals are aligned along the c-axis which is the fastest growing axis. Ten probability maxima are observed in the pole-figures illustrating the rotation of orientations around the c-axes indicating a fivefold symmetry. This symmetry is caused by multiple twinning which results in a high probability of specific orientation relationships with rotation angles of ~36°, ~108° (also referred to as the pentagon angle) and ~144° around the c-axis. All these rotation angles are close to the multiples of 36° which are required for an ideal fivefold symmetry. This is the first report of a fivefold symmetry triggered by the presence of barriers hindering crystal growth.

Coupling of metals and biominerals: characterizing the interface between ferromagnetic shape-memory alloys and hydroxyapatite.

Durable, mechanically robust osseointegration of metal implants poses one of the largest challenges in contemporary orthopedics. The application of biomimetic hydroxyapatite (HAp) coatings as mediators for enhanced mechanical coupling to natural bone constitutes a promising approach. Motivated by recent advances in the field of smart metals that might open the venue for alternate therapeutic concepts, we explore their mechanical coupling to sputter-deposited HAp layers in a combined experimental-theoretical study. While experimental delamination tests and comprehensive structural characterization, including high-resolution transmission electron microscopy, are utilized to establish structure-property relationships, density functional theory based total energy calculations unravel the underlying physics and chemistry of bonding and confirm the experimental findings. Experiments and modeling indicate that sputter-deposited HAp coatings are strongly adherent to the exemplary ferromagnetic shape-memory alloys, Ni-Mn-Ga and Fe-Pd, with delamination stresses and interface bonding strength exceeding the physiological scales by orders of magnitude.

Variation of Zr-L2,3 XANES in tetravalent zirconium oxides.

Zr-L2,3 XANESs of tetravalent zirconium oxides with different coordination numbers and local symmetries are systematically investigated by ab initio multiplet calculations using fully relativistic molecular spinors for model clusters. Experimental Zr-L2,3 XANESs are obtained for SrZrO3, m-ZrO2 (monoclinic) and t-ZrO2 (tetragonal). The theoretical spectra are in good agreement with the experimental data. The multiplet effects are found to play essential roles in determining the peak shape. The shapes of L3- and L2-edges are systematically different. The intensity ratios of the doublet peaks at both L3- and L2-edges are found to be sensitive to the coordination number of Zr. The ratio can therefore be used to estimate the coordination number of Zr in such oxides.

Silicon nanocolumns on nanosphere lithography templated substrates: effects of sphere size and substrate temperature.

Glancing angle ion beam sputter deposition was used to grow regular arrays of Si nanocolumns with a nominal height of 650 nm at room temperature on polystyrene nanospheres with sphere diameters between 260 nm and 3550 nm, and at elevated temperatures on SiO2 nanospheres with a sphere diameter of 360 nm. Top view and cross sectional scanning electron microscopy reveals that the Si nanocolumns resemble cylinder-like structures, terminated by a hemispherical cap. Diameter, height and inter-column-spacing are found to depend linearly on the nanosphere diameter, thus giving the possibility to grow arrays of vertical Si columns with distinct porosities. For the growth at elevated temperatures, it was found that while on non-patterned substrates diffusion effects lead to broadening and finally merging of initially separated nanocolumns, on nanosphere patterned substrates this broadening effect is only moderate. No merging of columns is observable in this case, but a decrease of the column height due to a temperature-driven inter-column densification.

Effect of the LT-alpha (+250 G/A) polymorphism on markers of inflammation and clinical outcome in critically ill patients.

BACKGROUND: Tumor necrosis factor alpha as a central mediator of the inflammation cascade is correlated to sepsis outcome. Tumor necrosis factor beta (LT-alpha) binds the same cell receptor and polymorphisms in both genes have been described. To evaluate the importance of the LT-alpha (+250 G/A) polymorphism for the clinical outcome of patients developing postsurgical sepsis, 85 patients were consecutively included into this study. METHODS: Blood samples were obtained for analysis of the biallelic LT-alpha (+250 G/A) polymorphism and for determination of serum levels of sTNF-R1, TNF-alpha, IL-6, IL-8, IL-10, procalcitonin, and neopterin. Cytokine levels were measured repeatedly until the patients' discharge from the ICU. RESULTS: The allele frequency was 0.28 for TNFB1 and 0.72 for TNFB2. The genotype distribution was TNFB1 homozygotes 4/79 (5.1%), TNFB1/TNFB2 heterozygotes 37/79 (46.8%), and TNFB2 homozygotes 38/79 (48.1%). Fifty-four out of 80 (67.5%) fulfilled the criteria for severe sepsis; 36/80 (45.0%) developed septic shock. Multiple organ failure occurred in 60/80 patients (75.0%), and the overall mortality was 26/80 (32.5%). Concerning the LT-alpha-genotypes, there was no difference in the frequency of severe sepsis or shock or in the development of multi-organ failure or death between the three subgroups. The peak plasma TNF-alpha levels were similar for all genotype subgroups. CONCLUSIONS: There was no correlation between the biallelic LT-alpha (+250 G/A) polymorphism and the outcome of critically ill patients. Genotyping this locus does not seem to be useful in predicting sepsis outcome.