News – Page 11 – www.internal-interfaces.de

New Publication by B4 (SW Koch/Kira)

20. March 2015/in News /by sfb1083

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

2015 Student Winter Seminar

17. February 2015/in News /by sfb1083

SFB-students organize their first winter seminar in the Marburger Haus, Kleinwalsertal.

From 08.02.-12.02.2015 students and postdocs associated with the SFB organized their first Winter Student Seminar in Austria (Scientific Program and Report).

Research in Marburg 2010-2014 published in English

30. October 2014/in News /by sfb1083

Philipps-Universität Marburg’s research brochure for the years 2010-2014 is now also available in English. On pages 34-35 it includes a contribution on SFB 1083.

Forschen in Marburg 2010-2014 or Research in Marburg 2010-2014

International Summer School on Semiconductor Interfaces

31. March 2014/in News /by sfb1083

Methods and Model Systems.

Group image (Photo by Pascal Hens)

July 27th – August 31st, 2014
Palacio de Miramar, Donostia – San Sebastián, Spain

More information on the scientific program and registration procedure

Kick-Off Meeting of the SFB 1083

26. November 2013/in News /by sfb1083

Official start of the Collaborative Research Centre 1083 “Structure and Dynamics of Internal Interfaces”

November 27th – 29th, 2013
Philipps-Universität Marburg – Department of Physics – Renthof 5, GrHs

More information on the SFB 1083 Kickoff meeting_online.

New Publication by A2 (Witte)

30. September 2013/in News /by sfb1083

Structure of Molecular Acceptor-Donor Interfaces: The Case of Pentacene and C60

Organic photovoltaic (OPV) cells based on conjugated polymers or small molecular weight organic compounds have attracted substantial research attention in the last decade due to their potential to provide clean and low-cost electrical energy. Unfortunately, the complex interface structure of polymer blends has so far largely hampered microscopic studies on the energetics and dissociation dynamics of photo-generated excitons formed at the involved donor-acceptor junctions. Aiming to improve the understanding of the microstructure and energetics of molecular hetero-structures, prototypical model interfaces between small molecular weight organic
compounds that form crystalline films, such as Buckminster fullerene (C₆₀) and pentacene (PEN), are of particular interest. Although mutual conformations and the exact structure of the interface between both compounds are theoretically proposed to have significant impact on the actual device characteristics, the microstructure of
these interfaces had so far hardly been addressed in previous works.

In a current study of the working group Molecular Solids (Prof. G. Witte) which was just published in the journal ACS Applied Materials & Interfaces, the interface between the organic semiconductors pentacene and fullerene has been investigated by combining microscopy (AFM, SEM) with diffraction (XRD) and x-ray absorption spectroscopy (NEXAFS) techniques. It was demonstrated that utilizing the anisotropic interaction between different molecular compounds and tuning the effective diffusion length enables structural control over nanostructures. To understand the interface formation in detail, well-defined, crystalline pentacene multilayers have been used as support, on which small amounts of C₆₀ have been deposited. By adjusting the substrate temperature during growth, the effective diffusion length was tuned and a site specific nucleation was achieved which allows or suppresses C₆₀ nucleation at step edges, or even activates diffusion along step edges yielding separated
but edge-pinned C₆₀ clusters. As a result, C₆₀-adlayer structures of different dimensionality have been realized ranging from planar films (2D) to step decorated chains (1D) to clusters (0D). Interestingly, all these different structures are found to be fully stable at room temperature. It was further demonstrated that such 1D and 0D C₆₀ structures can be overgrown by subsequent pentacene deposition, forming a crystalline cover layer of the same orientation as the bottom layer, hence enabling the formation of low dimensional buried organic heterostructures.

The present results are an important milestone towards an understanding and controlled fabrication of interfaces between organic semiconductors and will be the basis for further spectroscopic studies of the buried C₆₀ nanostructures that are explored in the collaborative research centre 1083 “Structure and Dynamics of Internal
Interfaces”.

Publication: T. Breuer and G. Witte
Diffusion-Controlled Growth of Molecular Heterostructures: Fabrication of Two-, One-, and Zero-Dimensional C₆₀ Nanostructures on Pentacene Substrates
ACS Applied Materials and Interfaces (in print, 2013), DOI: 10.1021/am402868s

Press Release: Research at Interfaces

27. May 2013/in News /by sfb1083

In a new collaborative research centre, physicists and chemists from Marburg are investigating the “structure and dynamics of internal interfaces”.

In a new collaborative research centre, physicists and chemists from Marburg are investigating the “structure and dynamics of internal interfaces”. This is the title of a “Sonderforschungsbereich” (SFB), whose installation has just been decided by the “Deutsche Forschungsgemeinschaft” (DFG). The physicist Professor Dr. Ulrich Höfer from the Philipps-Universität is the initiator and spokesman of the consortium, which will be supported by the DFG with 8.7 million Euros during the following four years. (Picture: Pressestelle der Philipps-Universität/Markus Farnung)

This is the title of a “Sonderforschungsbereich” (SFB), whose installation has just been decided by the “Deutsche Forschungsgemeinschaft” (DFG). The physicist Professor Dr. Ulrich Höfer from the Philipps-Universität is the initiator and spokesman of the consortium, which will be supported by the DFG with 8.7 million Euros during the following four years.

“With this funding decision the work of our researchers over many years receives its well-deserved acknowledgement”, says Professor Dr. Katharina Krause, president of the Philipps-Universität. “The university administration is extremely pleased, that the investments into materials sciences and semiconductor physics pay off in this way and we thank the involved scientists for their outstanding commitment.”

Interfaces are contact areas between different materials. They play a decisive role in miniaturized semiconductors, which are used, for example in electronic circuits. These semiconductors are constructed of several layers of different elements, similar to a sandwich cake. “The interfaces between the different materials frequently determine which optical and electronic characteristics such semiconductor devices possess”, explains Höfer.

The importance of internal interfaces will continue to increase further, when future hybrid materials combine the characteristics of metals, traditional inorganic semiconductors and organic materials”, predicts Höfer. Examples of such hybrid materials are novel solar cells and biosensors. “Our microscopic understanding of the structure and dynamics of internal interfaces, however, is lagging far behind their enormous importance.” The main cause of this knowledge gap is the experimental difficulty to detect the weak signals of the interface, which is often buried under many layers of other materials.

The initiators of the new CRC aim to close this gap by the collaboration between different research areas such as semiconductor physics, surface science, chemical synthesis, structural analysis and laser spectroscopy. For these efforts the University of Marburg offers a perfect environment because, according to the DFG referees, the combined expertise in these research areas is worldwide unique at this location. The list of the participating scientists includes 15 groups of the Faculties of Physics and Chemistry, of the Materials Sciences Center, and of a guest project of the Basque institution “Donostia International Physics Center”, in San Sebastián, Spain.

Initially, the investigations will not be directed towards specific functional materials as those generally consist of many, frequently not well defined interfaces. Instead, the focus will be on model systems with specially prepared internal interfaces. These interfaces will be structurally characterized on the atomic level and their optical and electronic properties will be systematically investigated.

In this way, a detailed microscopic understanding and prediction of chemical bonding, electronic coupling and energy transfer for different classes of heterointerfaces shall be achieved. “In the medium and long term this knowledge shall be used in order to tailor interfaces for new applications and construct devices with novel properties and functions”, explains Höfer.

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