The authors Tobias Breuer and Gregor Witte working in project A2 of SFB 1083 demonstrate a novel concept for the preparation of organic interfaces and solid heterostructures of tunable molecular orientation in a new publication in ACS Applied Materials & Interfaces.

The controlled preparation of heterostructures of various materials is an important prerequisite for the fabrication of electronic devices and sensors. For example, structural control of heterostructures of conventional, inorganic semiconductors enables electronic band engineering. In contrast with such covalently bound systems, rather little is known on appropriate strategies to facilitate organic heterostructures of precise, controlled structure. This is especially true, as organic materials are typically bound by rather weak van-der-Waals interaction only. Therefore, direct utilization of covalent bonding mechanisms which also direct the structure formation
in inorganic heterostructures is not possible. In the past, a number of strategies were developed to prepare organic films of high structural order and homogeneity. These mostly utilized enhanced adsorption energies at inorganic substrates or lattice match to gain control over film structure and molecular orientation. However, these approaches have been limited to unitary films only. The realization of organic heterostructures also requires consideration of molecular orientation, a peculiarity of molecular materials which is absent in the inorganic counterparts. Due to their anisotropic shape, the orientation of individual molecules in the films constitutes an important parameter, as opto-electronic characteristics of processed molecular solids are strongly determined by their alignment.

The authors developed the idea to employ molecular pattern recognition on the nanoscale. They show that molecules of sufficiently similar structure can transfer their orientation from a bottom layer to the top layer of the second compound and thereby “inherit” their structure. The authors also show that this process reliably takes place for all different mutual configurations (i.e. in standing or recumbent orientation) of the model-type organic donor molecules pentacene (PEN) and acceptor molecules perfluoropentacene (PFP). This has allowed them to facilitate internal interfaces in organic heterostructures in different exclusive orientations. Unlike frequently observed for unitary organic film growth, orientation of the constituents is not only controlled at the interface with the organic bottom layer but also persists for thicker top layers (thicknesses up to 30 nm), proving the stability of this concept.

This novel structuring method for precise molecular interfaces with tunable molecular orientation paves the way for a detailed characterization of the intermolecular
electronic coupling as a function of the mutual alignment which will be addressed in upcoming studies within SFB 1083.

Publication:

T. Breuer and G. Witte: Controlling Nanostructures by Templated Templates: Inheriting Molecular Orientation in Binary Heterostructures,
ACS Applied Materials & Interfaces (2015), DOI: 10.1021/acsami.5b07409

Prof. Michael Gottfried, PI of project A4, is honored with a Chinese Academy of Sciences (CAS) Award 2015.

Prof. Dr. Michael Gottfried, jointly with his research colleague Prof. Dr. Junfa Zhu of the National Synchrotron Radiation Laboratory (NSRL) at the University of Science and Technology of China (Hefei, Anhui, China), were recognized for their outstanding efforts in training the next generation of young scientists with the “International Partnership Award for Young Scientists 2015” by the Chinese Academy of Sciences (CAS). Profs. Gottfried and Zhu met at the University of Erlangen-Nürnberg several years ago and Prof. Gottfried spent in total 6 months at the NSRL as a visiting scientist and guest professor during the years 2009-2015.

Alexander Lerch, Ph.D. student in the research group on surface physics headed by Professor Dr. Ulrich Höfer, was honored with a “Best Presentation Award for Young Scientists” at the 9^th International Symposium on Ultrafast Surface Dynamics (USD9) at Lake Biwa, Japan.

USD9 was the 9^th symposium in a series of biannual meetings in the field of femtosecond dynamics at surfaces and interfaces which has become the foremost international discussion forum to outline and trigger present and future research in that field.

Poster “Time-resolved nonlinear spectroscopy at the buried GaP/Si interface” by A. Lerch, K. Brixius, A. Beyer, K. Volz, W. Stolz,
and U. Höfer (Philipps-Universität Marburg)
The 9^th International Symposium on Ultrafast Surface Dynamics (USD9) May 25-29, 2015, Lake Biwa, Japan

Members of SFB 1083 project B4 are co-authors on a new publication in PRL entitled “Coherent Terahertz Control of Vertical Transport in Semiconductor Heterostructures”.

Members of SFB 1083 project B4 on “Microscopic Theory of Optical Excitations in Interface-Dominated Material Systems” spear-headed by theoretical physicists Professors Stephan W Koch and Mackillo Kira together with their colleagues from Aalto University in Finland (including Osmo Vänskä who recently joined the workgroup as a postdoc here in Marburg) co-authored a new publiction in the journal Physical Review Letters entitled “Coherent Terahertz Control of Vertical Transport in Semiconductor Heterostructures”. It demonstrates a new principle where excitons (Coulomb-bound electron-hole pairs) are selectively moved through a semiconductor interface without moving electrons or holes (electronic vacancies) themselves. This insight
provides new principles to applications, such as computers, semiconductor lasers, and solar cells, where it is desirable to selectively move different electronic clusters through material interfaces.

Publication: Osmo Vänskä & al.: Coherent terahertz control of vertical transport in semiconductor heterostructures, Physical Review Letters 114 (2015) 116802, DOI: 10.1103/PhysRevLett.114.116802, URL: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.114.116802

Read below from the university’s press release in German:

Transport ohne Träger

Theoretische Physiker erweitern die Anwendungsmöglichkeiten von Halbleitern.

Physiker aus Marburg und dem finnischen Aalto haben eine Versuchsanleitung entwickelt, die neue nanotechnische Anwendungen von Halbleitern eröffnet. Demnach ist eine Sequenz  elektromagnetischer Pulse in der Lage, Ladungen über die inneren Grenzflächen von Halbleitern zu transportieren, die aus ungleichartigen Materialien bestehen. Das Team veröffentlicht seine Ergebnisse in der aktuellen Ausgabe der Fachzeitschrift „Physical Review Letters“, die am 20. März 2015 erscheint.

„Der Transport von Ladungen über Grenzflächen hinweg ist von entscheidender Bedeutung bei vielen Naturerscheinungen und technischen Anwendungen“, erklärt Mitverfasser Professor Dr. Mackillo Kira von der Philipps-Universität. Das gilt etwa für Solarzellen, aber auch für organische Prozesse wie die Photosynthese, mit der Pflanzen Energie aus Sonnenlicht gewinnen. Solarzellen beruhen auf Halbleitern, die aus mehreren Schichten verschiedener Materialien aufgebaut sind.

Kira und sein Marburger Kollege Professor Dr. Stephan Koch sowie ihre finnischen Partner Dr. Osmo Vänskä und Professor Dr. Ilkka Tittonen wählten als Modell ein Halbleitersystem auf der Basis von Galliumarsenid, das die Beweglichkeit eines Teilchens stark einschränkt, einen so genannten Quantentopf. Sie nutzten einen vor wenigen Jahren entwickelten Theorierahmen für die Quanten-Laserspektroskopie, um zu zeigen, wie ein effizienter Ladungstransfer über innere Grenzflächen hinweg verwirklicht werden kann. Dabei kommen Terahertz-Strahlen zur Anwendung, um den Halbleiter kontrolliert anzuregen.

„Das gelingt derart präzise, dass es sogar möglich ist, quantenmechanische Eigenschaften zu transportieren, ohne Teilchen zu bewegen“, hebt Koautor Koch hervor. Die vorgeschlagene Vorgehensweise ist den Autoren zufolge geeignet, Grenzflächen bei nanotechnologischen Anwendungen zu charakterisieren und deren Eigenschaften nutzbar zu machen.

Professor Dr. Stephan Koch und Professor Dr. Mackillo Kira lehren Theoretische Halbleiterphysik an der Philipps-Universität. Erst vor wenigen Jahren legten sie einen neuen Theorierahmen für die Quanten-Laserspektroskopie vor.

Die aktuelle Veröffentlichung wurde unter anderem von der „Suomen Akatemia“ (Akademie von Finnland) sowie durch den Sonderforschungsbereich (SFB) 1083 der Deutschen Forschungsgemeinschaft an der Philipps-Universität finanziell gefördert. Der SFB vereint mehr als 60 Forscherinnen und Forscher aus Chemie und Physik, die Grenzflächen an einer Vielzahl anorganischer und organischer Festkörper untersuchen, um anhand ihrer Modellsysteme zu einem detaillierten Verständnis der chemischen Bindung, der elektronischen Kopplung und der Energieübertragung zu gelangen.

Originalveröffentlichung: Osmo Vänskä & al.: Coherent terahertz control of vertical transport in semiconductor heterostructures, Physical Review Letters 114 (2015) 116802, DOI: 10.1103/PhysRevLett.114.116802, URL: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.114.116802

Pressemitteilung zur Theorie der quantenoptischen Spektroskopie: https://www.uni-marburg.de/aktuelles/news/2011/0918a
Pressemitteilung zum SFB 1083: https://www.uni-marburg.de/aktuelles/news/2013b/0524a