Udo Geckle

Neutron reflectometry studies on the lithiation of amorphous silicon electrodes in lithium-ion batteries

Neutron reflectometry is used to study in situ the intercalation of lithium into amorphous silicon electrodes. The experiments are done using a closed three-electrode electrochemical cell setup. As a working electrode, an about 40 nm thick amorphous silicon layer is used that is deposited on a 1 cm thick quartz substrate coated with palladium as a current collector. The counter electrode and the reference electrode are made of lithium metal. Propylene carbonate with 1 M LiClO4 is used as an electrolyte. The utility of the cell is demonstrated during neutron reflectometry measurements where Li is intercalated at a constant current of 100 μA (7.8 μA cm(-2)) for different time steps. The results show (a) that the change in Li content in amorphous silicon and the corresponding volume expansion can be monitored, (b) that the formation of the solid electrolyte interphase becomes visible and (c) that an irreversible capacity loss is present.

A Facile Route to Boronic Acid Functional Polymeric Microspheres via Epoxide Ring Opening

Boronic acid-functionalized microspheres are prepared for the first time via mild epoxide ring opening based on porous cross-linked polymeric microspheres (diameter ≈ 10 μm, porosity ≈ 1000 Å). Quantitative chemical analysis by XPS and EA evidences that there is a greater functionalization with boronic acid when employing a sequential synthetic method [1.7 atom% boron (XPS); 1.12 wt% nitrogen (EA)] versus a one-pot synthetic method [0.2 atom% boron (XPS); 0.60 wt% nitrogen (EA)] yielding grafting densities ranging from approximately 2.5 molecules of boronic acid per nm(2) to 1 molecule of boronic acid per nm(2), respectively. Furthermore, the boronic acid-functionalized microspheres are conjugated with a novel fluorescent glucose molecule demonstrating a homogeneous spatial distribution of boronic acid.

Polyphthalaldehyde-block-polystyrene as a nanochannel template

A degradable polyphthalaldehyde-polystyrene block copolymer generated by modular ligation is reported for the first time serving as a nanochannel template for the formation of nanostructured materials. The polyphthalaldehyde-b-polystyrene copolymer was spin-coated onto a surface with subsequent polyphthalaldehyde block removal. Block conjugation and block removal were confirmed by H-NMR, SEC, AFM, and SEM.

Comparative study of trap-limited hydrogen diffusion in amorphous SiC, Si0.66C0.33N1.33, and SiN1.33 films

Hydrogen tracer diffusion is studied in amorphous SiC, Si0.66C0.33N1.33, and SiN1.33 films which were produced by rf magnetron reactive sputtering and which contain about 0.5?at.% of hydrogen. The diffusion experiments were carried out in the temperature range between 700 and 1000??C with ion-implanted deuterium tracer isotopes and secondary ion mass spectrometry. Effective diffusivities are derived which are nearly identical for all three types of material and which obey Arrhenius behaviours with activation energies of ?E = 3.0?3.4?eV and pre-exponential factors of the order of D0 = 10?4?10?5?m2?s?1. These results can be explained with a trap-limited diffusion mechanism of hydrogen where the tracer atoms are temporarily trapped by carbon and nitrogen dangling bonds, which have approximately the same binding energy to hydrogen.

Structural relaxation and self-diffusion in covalent amorphous solids: Silicon nitride as a model system

Neutron reflectometry and isotope multilayers were used to investigate self-diffusion in covalent amorphous solids during isothermal annealing and its correlation to structural relaxation. Amorphous silicon nitride was chosen as a model system. Neutron reflectometry is a superior method to measure very low self-diffusivities, occurring in covalent solids, by applying only short time anneals. This allows one to determine time dependent changes of diffusivity over a broad temperature range before crystallization of the metastable solid occurs. The measured nitrogen diffusivities decrease by more than one order of magnitude during annealing between 950 and 1150 °C, while at the same time also a decrease of the atomic density is observed. We interpret this behavior as a structural relaxation of the amorphous network structure that is governed by annihilation of interstitial-like defects.

Tailored stoichiometries of silicon carbonitride thin films prepared by combined radio frequency magnetron sputtering and ion beam synthesis

Homogenous Si–C–N films of 120 nm thickness have been successfully fabricated by means of rf magnetron sputtering combined with ion implantation. These means are capable of producing various tailored stoichiometries of so far unequaled nitrogen concentration and high purity (O<0.2at.%, H<0.5at.%). The achieved compounds Si2CN4,SiCN2, and SiC2N2.2 consist of SiN4 tetrahedron layers interconnected by carbodiimid bridges. Stoichiometry, chemical binding states, and homogeneity of these layers as well as the reproducibility have been investigated by means of x-ray photoelectron spectroscopy, Fourier transform infrared, non-Rutherford back scattering spectroscopy, and resonant nuclear reaction analyses. Furthermore, sputter induced effects on the Si–C–N system during surface analytical characterization using Ar+ ions at 250 and 3250 eV for sputter cleaning have been studied carefully.

Fe-doped sodium aluminosilicate thin films: conductivity, microstructural organization and sensor properties

Abstract In order to get an interfacial layer providing reversible and fast ionic and electronic exchange between metallic contact and Na + ion-conducting sensor membrane, thin films of Fe-doped sodium aluminosilicate glass prepared by RF co-sputtering of the host glass and metallic iron have been investigated. It was found that non-reactive (Ar + ) and reactive (Ar + /O + 2 ) sputtered layers exhibit drastically different transport and sensor properties in accordance with 57 Fe conversion electron Mossbauer spectroscopic study of the local environment of iron in the films obtained. The main part of iron in the non-reactive sputtered material forms small Fe particles or clusters of 2 to 4 nm in diameter. These particles dispersed in the insulating glassy matrix cause an enormous increase of the conductivity by 9 to 10 orders of magnitude with increasing Fe content. On the other hand, room-temperature conductivity of reactive sputtered films is by a factor of 10 5 to 10 7 less than that of non-reactive sputtered samples. Both as-prepared and annealed non-reactive sputtered layers with an iron content from 3 to 12 at.% exhibit fast and reproducible redox response comparable with that of a Pt electrode. At smaller Fe concentration, redox response is hindered by low electronic conductivity. At higher iron content, oxidative and corrosion-induced phenomena affecting redox response were observed. As-prepared films reveal no Na + sensitivity even after conditioning in NaCl solutions for at least two weeks. Annealed non-reactive sputtered layers with 3–4 at.% Fe exhibit fast and reproducible sodium ion response but only in concentrated NaCl solutions and with strongly reduced slope (20–30 mV/pNa). Small concentrations of iron do not disturb sodium ion-exchange between solution and thin film. 22 Na tracer measurements of sodium uptake and loss for the obtained samples are in accordance with the sensor properties observed. It can be concluded that properly prepared and annealed films with comparable ionic and electronic conductivity, and ionic and electronic exchange current density at the interfaces are promising materials for application as an intermediate layer of an all-solid-state potentiometric sodium sensor.

Access to Intrinsically Glucoside‐Based Microspheres with Boron Affinity

Intrinsically glucoside-based microspheres are prepared in olive oil via a water in oil inverse suspension polymerization. The microspheres are characterized by scanning electron microscopy (SEM), Fourier transform infrared (FTIR) microscopy, and X-ray photoelectron spectroscopy (XPS), evidencing the intrinsic glucose character of the spheres. A novel boronic acid fluorescent molecule was subsequently conjugated to the microspheres in an aqueous environment, exhibiting the spatial and uniform distribution of glucoside as well as the affinity of the microspheres to bind with boron, evidenced via fluorescence spectroscopy measurements.

Crystallisation of magnetron sputtered amorphous Si1− x C x films (x = 1/3) studied by grazing incidence X-ray diffractometry

The crystallisation of amorphous Si1− x C x films (x = 1/3) produced via magnetron sputtering on silicon substrates was investigated. Grazing incidence X-ray diffractometry was used to analyse the crystalline precipitates obtained after annealing at temperatures between 1200°C and 1350°C. After annealing times of 15 h at 1200°C and 15 h at 1350°C, crystallisation of SiC is complete. The average crystallite size, d, was determined using the Scherrer equation. The rate constants for the initial growth of the crystallites were determined by straight line fits in the d − t diagrams (t being the annealing time), which obey the Arrhenius law. The activation enthalpy of 4.0 ± 0.7 eV is, within error limits, the same as that found for the growth of silicon carbide crystallites in magnetron sputtered Si1− x C x films (x = 1/2).

Nitrogen self-diffusion in magnetron sputtered Si-C-N films

Self-diffusion was studied in magnetron sputtered nitrogen-rich amorphous compounds of the system Si-C-N by using nitrogen as a model tracer. As shown by infra-red spectroscopy a transient metastable region exists, where the structure of the material can be visualized as silicon nitride tetrahedra which are connected by carbo-diimide (-N=C=N-) bonds to a three dimensional amorphous network. In this region diffusion studies are carried out by neutron reflectometry and isotope multilayers as a function of annealing time, temperature and chemical composition. Low diffusivities between 10−20 and 10−24 m2/s were found. In the metastable region, diffusion is faster than diffusion in amorphous silicon nitride by 1 to 2 orders of magnitude, while the activation enthalpies of diffusion between 3.1 and 3.4 eV are the same within error limits. This can be explained by the fact that the diffusion mechanism along SiN4 tetrahedra is identical to that in amorphous silicon nitride, however, the carbo-diimide bonds seem t...