Termin: Dienstag, 17.11.2015
Zeit: 15.30 Uhr
Ort: Hauptgebäude, Raum HG 0.20
"A robust SQUID multichannel magnetometer system for versatile applications"
Jan-Hendrik Storm
Physikalisch-Technische-Bundesanstalt Berlin
We present the prototype module of our robust multichannel SQUID magnetometer system which is intended for high-precision measurements of biomagnetism and spin precession. Further demanding applications are magnetorelaxometry and ultra-low field NMR, where pulsed magnetic fields of up to 50 mT are typically applied. The system is operated in a liquid helium dewar with a flat bottom placed inside the Berlin Magnetically Shielded Room (BMSR-2). A modular design is used consisting of 7 modules each equipped with 18 magnetometers. The magnetometers are designed with Nb wire-wound flux antennas. A total of 16 small antennas (17 mm diameter) form a regular grid with channels sensitive to all three spatial directions. Two large antennas with 71 mm diameter sensitive in z-direction surround the grid at two different heights and allow the detection of deep sources. Each antenna is connected to the input of a thin-film Nb SQUID current sensor via a detachable contact. The SQUIDs are equipped with integrated input current limiters. Feedback into the antennas is employed to minimize crosstalk between channels. The current sensor chip package involves a superconducting shield of Nb. The configuration of the SQUID magnetometers and the multichannel arrangement does not significantly degrade the superior magnetic environment inside the BMSR-2 which has been verified by simulations. The measured white noise of the small-size magnetometers were between 0.6 and 1.25 fT/ÖHz, and well below 1 fT/ÖHz for the large ones. The experimental performance of the prototype module will be discussed in detail.
Termin: Dienstag, 10.11.2015
Zeit: 15.30 Uhr
Ort: Hauptgebäude, Raum HG 0.20
"Probing the Semiconductor/Liquid Interface of a Photoelectrochemical Cell"
Matthias H. Richter
E-mail: mrichter[at]caltech.edu, richtmat[at]b-tu.de
Photoelectrochemical water splitting has a more than four decade long history [1]. A photoelectrochemical cell based on a rectifying semiconductor/liquid junction provides a method of converting solar energy to electricity or fuels. While much work has been devoted to the study of these semiconductor/liquid junctions, the direct observation of the energetics at the solid/liquid interface by X-ray photoelectron spectroscopy under applied biases has not heretofore been explored.
Operando ambient-pressure X-Ray photoelectron spectroscopy (AP-PES) has been used herein to directly characterize the semiconductor/liquid junction at room temperature under real-time electrochemical control [2, 3]. Operando AP-XPS with Tender X-ray synchrotron radiation, characterized by an enhanced elastic escape depth of the photoelectrons, has enabled simultaneous monitoring of the solid surface, the solid/electrolyte interface, and the bulk electrolyte of a PEC cell as a function of the applied potential, U. The experiment provides direct insight into the nature of the rectifying or ohmic junction in addition to quantification of defect states densities and band bending in photoelectrochemical half cells.
References
- A. Fujishima, K. Honda, Nature 238 (1972) 37
- S. Axnanda et al., Sci. Rep., 5, 9788-22 (2015).
- M. F. Lichterman et al., Energ. Environ. Sci., 8, 2409-2416 (2015).
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gez. Prof. Götz Seibold
"Polaronen in (manchen) Oxiden"
Prof. Dr. D. Schmeißer
Polaronen sind zwar alte Bekannte in der Festkörper-Physik (und -Chemie), haben aber derzeit eine neue Bedeutung gewonnen, speziell bei der Beschreibung der faszinierenden Eigenschaften von Oxiden. Der experimentelle Nachweis von oxidischen Polaronen kann durch resonante Photoelektronen-Spektroskopie erfolgen. Dies wird gezeigt für eines der Cu-Oxide (BBISCO, HT-Supraleiter) und für dünne (<3nm) Hf-Oxid Schichten (Gate-Dielektrikum, nicht-flüchtige Speicher).
Dies ist ein Bericht über die physikalischen Aktivitäten im Rahmen meines Freisemesters (WS 2014/15), anschließend gibt es Gelegenheit für heiße Diskussionen bei kühlem Umtrunk.
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gez. Prof. Dieter Schmeißer
Termin:Dienstag, 30.06.15
Zeit: 15.30 Uhr
Ort: Hauptgebäude, Raum HG 0.20
"Nonlinear laser lithographie for industrial applications"
Iaroslav Gnilitskyi
University of Modena and Reggio Emilia
Nanostructure formation through surface treatment is mostly performed with well-established techniques including lithography and laser-induced periodic surface structuring (LIPSS). However, these techniques suffer either from the limited flexibility, high-cost, complex equipment, or suffer from the low-speed, problems of material control, and lack of uniformity and repeatability over large areas. Recently, a technique called Nonlinear Laser Lithography (NLL) was introduced, which allows fabrication of extremely uniform nanostructures, with excellent long-range repeatability and at high-speeds. NLL can be applied to a variety of materials, including non-planar, even flexible surfaces. While NLL generates essentially LIPPS-type of nanostructures, it does so by utilizing nonlinear feedback mechanisms arising from the interaction of femtosecond laser pulses with the target surface, as well as from the laser-initiated chemical reaction. Key features, such as superior uniformity and ability to process non-flat surfaces are a direct consequence of the self-regulation provided by these feedback mechanisms. Applications of surface-treated nanostructures have been demonstrated in various fields including electronics, optoelectronics, photovoltaics, tribology, wettability. Although the outcomes are encouraging, because of the problems of material and process control, they are still not suitable for transfer to industrial applications. It is appears that these problems can be overcome by NLL-induced nanostructures, thanks to their aforementioned superior features, with potential for substantial impact in these and related fields.
Termin: Mittwoch, 9. Juli 2014
Zeit: 13.45 Uhr
Ort: Zentrales Hörsaalgebäude, Seminarraum 4
"Electrical discharges and plasmas at atmospheric pressure Surface modification and stability of plasma-treated polymers"
Prof. Gabriela Borcia
Faculty of Physics, Al I Cuza University Iasi, Romania
Many polymer properties rest on the surface chemical composition, material structure and surface orientation of specific chemical functionalities, all intrinsically related, and defining the material interaction with its environment. In this respect, the surface modification of polymers is a constant challenge, since it induces a perturbation of the material, which adds to the complexity of the interaction and enhances the dynamics of the material functionality.
Various types of discharges and plasmas encountered have advantages compared to the other methods when seeking to change chemically and physically the polymer surface of interest. Plasma modification techniques should be capable of modifying only the topmost layers of the chosen material, rendering the polymer concerned better suited to the post-treatment processing required without affecting its bulk properties. Particularly, dielectric barrier discharges (DBD) working at atmospheric pressure gained much interest, compared to the more common low-pressure installations, for providing short treatment times, room temperature operation, dispensing with vacuum equipment, excellent flexibility with respect to their geometrical shape, and scaling-up to large dimensions.
Also, the gaseous environment allowing for homogeneous DBD regime does not allow the same flexibility as with low-pressure discharges, often imposing the use of inert gas, at least as major constituent of the mixture fed to the discharge. The usual high cost of inert gases, combined to the high consumption of gas associated to atmospheric-pressure processing, asks for systematic optimization of the plasma source.
Recognizing the above, this work aims to provide investigation over issues related to technological applications of plasma processing, as reproducibility and uniformity of the surface properties in relation to discharge space-time characteristics, control on the surface modification processes, post-treatment dynamics and stability of modified surfaces.
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