Stephan Friedrich

A multichannel superconducting soft x-ray spectrometer for high-resolution spectroscopy of dilute samples

We have built a high-resolution high-efficiency superconducting soft x-ray spectrometer for synchrotron-based fluorescence-detected absorption spectroscopy. The sensor is a 3×3 array of 200×200 μm2 superconducting Nb–Al–Al2O3–Al–Nb tunnel junctions with an energy resolution around 15 eV below 1 keV and a total count rate capability of ≈100 000 counts/s. This sensor array is cooled to ≈0.1 K by a two-stage adiabatic demagnetization refrigerator while held at the end of a 40-cm-long cold finger that can be inserted into an UHV sample chamber for x-ray fluorescence measurements. We present L-edge absorption spectra of dilute transition metals (≈few 100 ppm) and discuss spectrometer performance with respect to the analysis of metalloproteins.

A superconducting tunnel junction x-ray detector with performance limited by statistical effects

We have characterized a thin-film Nb/Al/AlOx/Al/Nb superconducting tunnel junction (STJ) optimized for low electronic noise as an x-ray detector in the 0.2–1 keV photon energy range. The spectra measured with this junction have high spectral purity with, to the best of our knowledge, the best energy resolution ever achieved with this type of detector in this energy band. The discrepancy between the theoretical and experimental energy resolution is only about 15%. Part of this small discrepancy may be explained by the fact that our junction has electrodes made from niobium/aluminum bilayers, while the theoretical result is for electrodes made from only one material. To the best of our knowledge, this is the first time that resolution achieved with a STJ x-ray detector is in agreement with the resolution predicted from statistical fluctuations in the creation and tunneling of quasiparticles.

Cryogenic X-ray detectors for synchrotron science.

Cryogenic detectors operated at temperatures below 1 K offer an order of magnitude higher energy resolution than conventional semiconductor-based energy-dispersive detectors, and orders of magnitude higher efficiency that grating spectrometers. Initially developed for astrophysics applications, these detectors are increasingly used in synchrotron-based research, both for detector characterization and for high-resolution X-ray spectroscopy. This article reviews current cryogenic detector technologies and their performance with respect to their use in synchrotron science. It also discusses areas of research that can benefit from improved energy resolution detectors, and outlines desirable detector developments in the context of novel science they would enable.

Measurement of Small Molecular Dopant F4TCNQ and C60F36 Diffusion in Organic Bilayer Architectures

The diffusion of molecules through and between organic layers is a serious stability concern in organic electronic devices. In this work, the temperature-dependent diffusion of molecular dopants through small molecule hole transport layers is observed. Specifically we investigate bilayer stacks of small molecules used for hole transport (MeO-TPD) and p-type dopants (F4TCNQ and C60F36) used in hole injection layers for organic light emitting diodes and hole collection electrodes for organic photovoltaics. With the use of absorbance spectroscopy, photoluminescence spectroscopy, neutron reflectometry, and near-edge X-ray absorption fine structure spectroscopy, we are able to obtain a comprehensive picture of the diffusion of fluorinated small molecules through MeO-TPD layers. F4TCNQ spontaneously diffuses into the MeO-TPD material even at room temperature, while C60F36, a much bulkier molecule, is shown to have a substantially higher morphological stability. This study highlights that the differences in size/geometry and thermal properties of small molecular dopants can have a significant impact on their diffusion in organic device architectures.

L-edge X-ray absorption spectroscopy of some Ni enzymes: Probe of Ni electronic structure

Abstract L-edge X-ray absorption spectroscopy has been used to study the electronic structure of Ni in the Ni–Fe hydrogenases and CO-dehydrogenases under a variety of conditions. The L-edge spectra are interpreted by comparison with the spectra of Ni model complexes and by ligand field multiplet simulations to examine the Ni oxidation and electronic spin states. The spectra for Ni in oxidized Desulfovibrio gigas and Pyrococcus furiosus enzymes are consistent with a covalent Ni III species. All of the reduced hydrogenases in this study exhibit a high spin Ni II spectrum, and no Ni I has been observed. In contrast to hydrogenases, the native Clostridium thermoaceticum CO-dehydrogenase has a low spin Ni II and exhibits a clearly different spectral multiplet. Spectroscopy of Ni enzymes using a 15-eV resolution STJ detector and using the new ALS beamline 4.0.2 with a 0.2 eV energy resolution show great promises for future biological L-edge spectroscopy.

High resolution tunnel junction extreme ultraviolet detectors limited by quasiparticle counting statistics

Superconducting tunnel junctions (STJs) can be used as high-resolution high-count rate photon detectors. They are based on the measurement of the excess quasiparticle tunneling current caused by the absorption of a photon in one of the junction electrodes. We have fabricated Nb-Al-AlO/sub x/-Al-Nb tunnel junction detectors with different sizes and characterized them in synchrotron experiments. We present a study of the detector performance in the energy band between 50 and 1000 eV. For photon energies below 70 eV, the intrinsic device resolution of the best STJ devices agrees with the theoretical limit set by the statistics of the charge generation and tunneling processes.

Sensitivity and S/N-ratio of superconducting high-resolution X-ray spectrometers

Superconducting tunnel junction (STJ) X-ray spectrometers have been developed for synchrotron-based high-resolution soft X-ray spectroscopy. We are quantifying the improvements in sensitivity and signal-to-noise ratio that STJ spectrometers can offer for the analysis of dilute specimens over conventional semiconductor and grating spectrometers. We present analytical equations to quantify the improvements in terms of spectrometer resolution, detection efficiency and count rate capabilities as a function of line separation and spectral background. We discuss the implications of this analysis for L-edge spectroscopy of first-row transition metals.

High work-function hole transport layers by self-assembly using a fluorinated additive

The hole transport polymer poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) derives many of its favorable properties from a PSS-rich interfacial layer that forms spontaneously during coating. Since PEDOT:PSS is only usable as a blend it is not possible to study PEDOT:PSS without this interfacial layer. Through the use of the self-doped polymer sulfonated poly(thiophene-3-[2-(2-methoxyethoxy) ethoxy]-2,5-diyl) (S-P3MEET) and a polyfluorinated ionomer (PFI) it is possible to compare transparent conducting organic films with and without interfacial layers and to understand their function. Using neutron reflectometry, we show that PFI preferentially segregates at the top surface of the film during coating and forms a thermally-stable surface layer. Because of this distribution we find that even small amounts of PFI increase the electron work function of the hole transport layer. We also find that annealing at 150 °C and above reduces the work function compared to samples heated at lower temperatures. Using near edge X-ray absorption fine structure spectroscopy and gas chromatography we show that this reduction in work function is due to S-P3MEET being doped by PFI. Organic photovoltaic devices with S-P3MEET/PFI hole transport layers yield higher power conversion efficiency than devices with pure S-P3MEET or PEDOT:PSS hole transport layers. Additionally, devices with a doped interface layer of S-P3MEET/PFI show superior performance to those with un-doped S-P3MEET.

Molybdenum X-ray absorption edges from 200 to 20,000eV: the benefits of soft X-ray spectroscopy for chemical speciation.

We have surveyed the chemical utility of the near-edge structure of molybdenum X-ray absorption edges from the hard X-ray K-edge at 20,000eV down to the soft X-ray M(4,5)-edges at approximately 230eV. We compared, for each edge, the spectra of two tetrahedral anions, MoO(4)(2-) and MoS(4)(2-). We used three criteria for assessing near-edge structure of each edge: (i) the ratio of the observed chemical shift between MoO(4)(2-) and MoS(4)(2-) and the linewidth, (ii) the chemical information from analysis of the near-edge structure and (iii) the ease of measurement using fluorescence detection. Not surprisingly, the K-edge was by far the easiest to measure, but it contained the least information. The L(2,3)-edges, although harder to measure, had benefits with regard to selection rules and chemical speciation in that they had both a greater chemical shift as well as detailed lineshapes which could be theoretically analyzed in terms of Mo ligand field, symmetry, and covalency. The soft X-ray M(2,3)-edges were perhaps the least useful, in that they were difficult to measure using fluorescence detection and had very similar information content to the corresponding L(2,3)-edges. Interestingly, the soft X-ray, low energy ( approximately 230eV) M(4,5)-edges had greatest potential chemical sensitivity and using our high-resolution superconducting tunnel junction (STJ) fluorescence detector they appear to be straightforward to measure. The spectra were amenable to analysis using both the TT-multiplet approach and FEFF. The results using FEFF indicate that the sharp near-edge peaks arise from 3d-->5p transitions, while the broad edge structure has predominately 3d-->4f character. A proper understanding of the dependence of these soft X-ray spectra on ligand field and site geometry is necessary before a complete assessment of the utility of the Mo M(4,5)-edges can be made. This work includes crystallographic characterization of sodium tetrathiomolybdate.

Fabrication of Mo/Cu multilayer and bilayer transition edge sensors

We are developing cryogenic high-resolution X-ray, Gamma-ray and neutron spectrometers based on superconducting Mo/Cu transition edge sensors. Here we discuss the sensor design for different applications, present the photolithographic fabrication techniques, and outline future detector development to increase the spectrometer sensitivity.