Die Bestimmung des Phosphors in Bronzen und anderen Legierungen

Ohne Zusammenfassung     

Electronic mechanism of STM-induced diffusion of hydrogen on Si(100)

We have observed a scanning tunneling microscopy (STM) induced lateral transfer of a single hydrogen atom on the Si(100) surface. The transfer rate of the hydrogen atom is proportional to the electron dose, indicating an electron-assisted transfer mechanism. Measurements of the relations between the transfer rate and the sample bias and temperature give further support for an electronic mechanism. The bias dependence of the transfer rate shows a peak, and from a first principles electronic structure calculation we show that the position of the peak is related to the energy of a localized surface resonance. We propose that the hydrogen transfer is related to inelastic hole scattering with this surface resonance. We develop a microscopic model for the hydrogen transfer, and using the experimental data we extract information on the resonance lifetime and the transfer yield per resonant electron. The transfer takes place by tunneling through a small excited state transfer barrier. The transfer rate is increased if the hydrogen atom before the resonant excitation is vibrationally excited, and this gives rise to an increasing transfer rate with increasing sample temperature.     

Parallel confocal detection of single molecules in real time

The confocal detection principle is extended to a highly parallel optical system that continuously analyzes thousands of concurrent sample locations. This is achieved through the use of a holographic laser illumination multiplexer combined with a confocal pinhole array before a prism dispersive element used to provide spectroscopic information from each confocal volume. The system is demonstrated to detect and identify single fluorescent molecules from each of several thousand independent confocal volumes in real time.     

Liquid chromatography/electrospray ionization/isotopic dilution mass spectrometry analysis of n-(phosphonomethyl) glycine and mass spectrometry analysis of aminomethyl phosphonic acid in environmental water and vegetation matrixes.

A liquid chromatography/electrospray/mass spectrometry (LC/ES/MS) method was developed for the analysis of glyphosate (n-phosphonomethyl glycine) and its metabolite, aminomethyl phosphonic acid (AMPA) using isotope-labelled glyphosate as a method surrogate. Optimized parameters were achieved to derivatize glyphosate and AMPA using 9-fluorenylmethyl chloroformate (FMOC-Cl) in borate buffer prior to a reversed-phase LC analysis. Method spike recovery data obtained using laboratory and real world sample matrixes indicated an excellent correlation between the recovery of the native and isotope-labelled glyphosate. Hence, the first performance-based, isotope dilution MS method with superior precision, accuracy, and data quality was developed for the analysis of glyphosate. There was, however, no observable correlation between the isotope-labelled glyphosate and AMPA. Thus, the use of this procedure for the accurate analysis of AMPA was not supported. Method detection limits established using standard U.S. Environmental Protection Agency protocol were 0.06 and 0.30 microg/L, respectively, for glyphosate and AMPA in water matrixes and 0.11 and 0.53 microg/g, respectively, in vegetation matrixes. Problems, solutions, and the method performance data related to the analysis of chlorine-treated drinking water samples are discussed. Applying this method to other environmental matrixes, e.g., soil, with minimum modifications is possible, assuring accurate, multimedia studies of glyphosate concentration in the environment and the delivery of useful multimedia information for regulatory applications.     

Phase equilibria for the system Mn0.394Ti0.606OS at 1380 and 1485°K

This paper reports equilibrium phase data for the manganese-containing system Mn_0.394Ti_0.606OS. The system was studied at 1380 and 1485°K by an equilibration and quench technique. The oxygen and sulfur fugacities of the equilibrating gas atmosphere were independently controlled using mixtures of the three gases hydrogen, carbon dioxide, and hydrogen sulfide. The results are presented on log f_S2 vs log f_O2 phase diagrams which are discussed in terms of the equilibria between manganese in the oxide phases and manganese in the α-MnS sulfide phase. The results are shown to be consistent with previously published data for the subsystems MnTiO, MnOS, MnS, and TiOS.     

Electronic transport at semiconductor surfaces––from point-contact transistor to micro-four-point probes

The electrical properties of semiconductor surfaces have played a decisive role in one of the most important discoveries of the last century, transistors. In the 1940s, the concept of surface states––new electron energy levels characteristic of the surface atoms––was instrumental in the fabrication of the first point-contact transistors, and led to the successful fabrication of field-effect transistors. However, to this day, one property of semiconductor surface states remains poorly understood, both theoretically and experimentally. That is the conduction of electrons or holes directly through the surface states. Since these states are restricted to a region only a few atom layers thick at a crystal surface, any signal from them might be swamped by conduction through the underlying bulk semiconductor crystal, as well as greatly perturbed by steps and other defects at the surface. Yet recent results show that this type of conduction is measurable using new types of experimental probes, such as the multi-tip scanning tunnelling microscope and the micro-four-point probe. The resulting electronic transport properties are intriguing, and suggest that semiconductor surfaces should be considered in their own right as a new class of electronic nanomaterials because the surface states have their own characters different from the underlying bulk states. As microelectronic devices shrink even further, and surface-to-volume ratios increase, surfaces will play an increasingly important role. These new nanomaterials could be crucial in the design of electronic devices in the coming decades, and also could become a platform for studying the physics of a new family of low-dimensional electron systems on nanometre scales.     

Local environments and lithium adsorption on the iron oxyhydroxides lepidocrocite (gamma-FeOOH) and goethite (alpha-FeOOH): a 2H and 7Li solid-state MAS NMR study

2H and 7Li MAS NMR spectroscopy techniques were applied to study the local surface and bulk environments of iron oxyhydroxide lepidocrocite (gamma-FeOOH). 2H variable-temperature (VT) MAS NMR experiments were performed, showing the presence of short-range, strong antiferromagnetic correlations, even at temperatures above the Néel temperature, T(N), 77 K. The formation of a Li+ inner-sphere complex on the surface of lepidocrocite was confirmed by the observation of a signal with a large 7Li hyperfine shift in the 7Li MAS NMR spectrum. The effect of pH and relative humidity (RH) on the concentrations of Li+ inner- and outer-sphere complexes was then explored, the concentration of the inner sphere complex increasing rapidly above the point of zero charge and with decreasing RH. Possible local environments of the adsorbed Li+ were identified by comparison with other layer-structured iron oxides such as gamma-LiFeO2 and o-LiFeO2. Li+ positions of Li+-sorbed and exchanged goethite were reanalyzed on the basis of the correlations between Li hyperfine shifts and Li local structures, and two different binding sites were proposed, the second binding site only becoming available at higher pH.     

Resolving the different silicon clusters in Li12Si7 by 29Si and (6,7)Li solid-state NMR spectroscopy

Structural signatures: The analysis of Si-Si and Si-Li connectivities by solid-state NMR spectroscopy allows the different types of silicon clusters to be discriminated in the model lithium silicide compound Li(12)Si(7) (see picture, Si clusters red and blue, Li ions gray). The results provide new NMR spectroscopic strategies with which to differentiate and study the structures formed in silicon-based electrode materials.