SCIENTIFIC HIGHLIGHTS

Inner Structure of Adsorbed Ionic Microgel Particles

Microgel particles of cross-linked poly(NIPAM-co-acrylic acid) with different acrylic acid contents are investigated in solution and in the adsorbed state. As a substrate, silicon with a poly(allylamine hydrochloride) (PAH) coating is used. The temperature dependence of the deswelling of the microgel particles was probed with atomic force microscopy (AFM). The inner structure of the adsorbed microgel particles was detected with grazing incidence small angle neutron scattering (GISANS). Small angle neutron scattering (SANS) on corresponding microgel suspensions was performed for comparison. Whereas the correlation length of the polymer network shows a divergence in the bulk samples, in the adsorbed microgel particles it remains unchanged over the entire temperature range.

 

In addition, GISANS indicates changes in the particles along the surface normal. This suggests that the presence of a solid surface suppresses the divergence of internal fluctuations in the adsorbed microgels close to the volume phase transition.

Inner Structure of Adsorbed Ionic Microgel Particles
S. Wellert, Y. Hertle, M. Richter, M. Medebach, D. Magerl, W. Wang, B. Demé, A. Radulescu, P. Müller-Buschbaum, T. Hellweg, and R. von Klitzing
Langmuir 30 (2014) 7168
DOI: 10.1021/la500390j




 

Energy-level alignment at metal/organic interfaces: Tying up the loose ends

Organic semiconductors have tremendous potential for complementing conventional, inorganic semiconductors as active materials in (opto-)electronic devices such as light-emitting diodes (OLEDs) for display and lighting applications or solar cells (OPVCs). Electrical connection of such devices to peripheral circuitry, however, is realized by metallic contacts and the resulting interfaces to the organic semiconductor have been found to play a central role: Energy barriers for injecting charge carriers into the organic (OLEDs) or energy losses upon their extraction (OPVCs) were found to detrimentally affect device performance. Attempts to minimize these energy barriers/losses are hampered by an incomplete understanding of their origin and the parameters that govern their magnitude.

Fig. 1: Schematic illustration of the electron transfer to acceptor states at a surface or interface of an n-doped semiconductor (middle and right). The development of a space-charge region induces band-bending, which brings the acceptor state closer to the Fermi level (right) In an undoped intrinsic semiconductor (left) no such electron transfer can take place resulting in an empty acceptor state in the band gap.

 

 

A multitude of sometimes conflicting views have been expressed in literature, each treating only a certain limiting case under particular assumptions for interface properties and dominant mechanism.

Now, a team including IRIS-member Norbert Koch implemented a detailed electrostatic model, which is capable of covering the full phenomenological range of interfacial energy-level alignment regimes within a single, consistent framework. Energy barriers/losses could be reproduced in both qualitative and quantitative agreement with a series of experiments. By continuously connecting the limiting cases described by previously proposed models, conflicting views in literature could be resolved. In particular, the team highlighted the key role played by the density of electronic states in the organic semiconductor: Its shape was found to determine both the minimum value of practically achievable injection barriers as well as their spatial profile, ranging from abrupt steps at the interface with the electrode to smoothly varying curves. Especially the latter – counter intuitively induced by introducing an ultrathin, electrically insulating interlayer between metal and organic – is beneficial for charge extraction in OPVCs, as illustrated below.

Organic semiconductor density of states controls the energy level alignment at electrode interfaces
M. Oehzelt, N. Koch, and G. Heimel
Nature Communications 5 (2014) 4174
DOI: 10.1038/ncomms5174

Scattering amplitudes in gauge theories – a new Lecture Notes in Physics Book by J. Henn and J. Plefka

At the fundamental level, the interactions of elementary particles are described by quantum gauge field theory. The quantitative implications of these interactions are captured by scattering amplitudes, traditionally computed using Feynman diagrams. In the past decade tremendous progress has been made in our understanding of and computational abilities with regard to scattering amplitudes in gauge theories, going beyond the traditional textbook approach. These advances build upon on-shell methods that focus on the analytic structure of the amplitudes, as well as on their recently discovered hidden symmetries. In fact, when expressed in suitable variables the amplitudes are much simpler than anticipated and hidden patterns emerge.

These modern methods are of increasing importance in phenomenological applications arising from the need for high-precision predictions for the experiments carried out at the Large Hadron Collider, as well as in foundational mathematical physics studies on the S-matrix in quantum field theory.

Bridging the gap between introductory courses on quantum field theory and state-of-the-art research, there has been a need for a focussed text book on the subject. Recently IRIS member Jan Plefka together with his former postdoc Dr. Johannes Henn, now at the Institute for Advanced Studies in Princeton (USA) have published the first monogrpahical text on this fundamental subject. The concise yet self-contained and course-tested lecture notes are well-suited for a one-semester graduate level course or as a self-study guide for anyone interested in fundamental aspects of quantum field theory and its applications.

The book contianes numerous exercises and solutions which help readers to embrace and apply the material presented in the main text.

Scattering Amplitudes in Gauge Theories
J. M. Henn and J. C. Plefka
ISBN: 978-3-642-54021-9 (Print) 978-3-642-54022-6 (Online)
(published 03-2014)


Band-bending in organic semiconductors

Band-bending in organic semiconductors, occurring at metal/alkali-halide cathodes in organic-electronic devices, is experimentally revealed and electrostatically modeled. Metal-to-organic charge transfer through the insulator, rather than doping of the organic by alkali-metal ions, is identified as the origin of the observed band-bending, which is in contrast to the localized interface dipole occurring without the insulating buffer layer.
 
Band-Bending in Organic Semiconductors: the Role of Alkali-Halide Interlayers 
This work was supported by the DGF (SFB 951).
H. Wang, P. Amsalem, G. Heimel, I. Salzmann, N. Koch, and M. Oehzelt

Adv. Mater. 26 (2014) 925
DOI: 10.1002/adma.201303467
 


WEITERE SCIENTIFIC HIGHLIGTHS