In a new publication in Physics Status Solidi Rapid Research Letters project A2 (Witte) reports on the fabrication of well-defined hybrids of organic semiconductors and transition metal-dichalcogenides (TMDCs).

The two-dimensional material graphene has garnered extreme interest due to its interesting electronic properties in combination with utmost structural stability despite merely nanometer thickness. Motivated by this prominent example, further two-dimensional materials have become the focus of today’s cutting-edge research. One important class of such materials are transition metal dichalcogenides such as MoS₂, WS₂ or MoSe₂. Their interesting electronic properties have not only revealed novel physics but already enabled the fabrication of prototypical devices.

In their recent work, Tobias Breuer and Gregor Witte used such TMDC surfaces, in particular of MoS₂, to fabricate crystalline, well-defined hybrid structures with organic semiconductors (OSCs), in themselve up-and-coming technologically relevant materials. Brought together, the characteristic advantages of both material classes can be combined to potentially fabricate novel synthetic materials with superior characteristics. The authors report on the successful fabrication of such hybrids combining MoS₂ with the molecular donor and acceptor systems pentacene and perfluoropentacene. In particular, they observe that both materials are structurally compatible despite their strongly different shape symmetry and form
large crystalline islands on highly ordered MoS₂ basal planes. Interestingly, the molecules arrange in such a fashion that the contact area at the interface between TMDC and organic semiconductor is maximized, hence allowing for an efficient coupling of both constituents. Surprisingly, the OSCs are even epitaxially aligned on the substrates.

A crucial aspect for the successful fabrication of these heterostructures is the structural quality of TMDC surfaces. In previous work, it has been shown that photoluminescence efficiency is strongly increased for MoS₂ surfaces with significant defect density as compared to pristine surfaces. For the hybrid structures, the lateral dimensions of the crystalline OSC islands are reduced and the relative orientation of the molecules on the surface is inverted, hence leading to an upright molecular growth on defective MoS₂ surfaces instead of a lying configuration on pristine
MoS₂. Since the preparation of ideal, defect-free TMDC surfaces is challenging and commercially available TMDCs frequently have considerable defect densities, this aspect requires appropriate consideration. The achieved expertise on the preparation of such well-defined novel hybrid structures will enable the detailed investigation of their electronic and optical coupling mechanisms within further projects of SFB 1083.

Publication:

Tobias Breuer, Tobias Massmeyer, Alexander Mänz, Steffen Zoerb, Bernd Harbrecht, Gregor Witte
Structure of van der Waals bound Hybrids of Organic Semiconductors and Transition Metal Dichalcogenides: the Case of Acene Films on MoS₂
Physica Status Solidi – Rapid Research Letters (2016).

We congratulate Prof. Dr. Pedro M. Echenique, PI of SFB-project GP1, on his honorary doctorate received from Aalto University, Finland.

In a ceremony on Oct.7^th, Aalto University conferred an Honorary Doctorate in Technology on Pedro Miguel Echenique, Professor of the University of the Basque Country (UPV/EHU) and President of Donostia  international Physics Center (DIPC), together with another ten eminent persons in the fields of science, technology and society. The award, which is conferred every two years by Aalto University Schools of Technology, can look back on a tradition of more than 80 years, and among the recipients are renowned scientists, technologists and influencers in science and society with outstanding professional careers.
More details under Aalto University or DIPC-press release.

The German Research Foundation organizes an annual information tour for young researchers from India and Thailand on the days following their participation at the Nobel laureate meeting in Lindau. This is their second visit to the Physics department at the Philipps-Universität Marburg.

21 young researchers (students to postdocs) from India and 5 from Thailand were selected by their national committees (Department of Science & Technology of India and National Science and Technology Development Agency in Thailand) to attend the annual Nobel Laureate meeting in Lindau. This year the focus in Lindau was on Physics and Marburg was happy to host the participants on their information tour which included visits to the Peter Grünberg Institute at Forschungszentrum Jülich and the MPI for Gravitational Physics.

Professor Höfer, spokesperson of SFB 1083, gave a lecture introducing the scientific background before the delegation visited laboratories at the Renthof-location of the Physics department and had opportunity to meet and discuss their research with young research staff from SFB 1083.

Members from projects A6 and A3 successfully joined forces in the investigation and quantification of electron-vibron coupling at the interface of the organic semiconductor 1,4,5,8-naphthalene-tetracarboxylic dianhydride (NTCDA) and the Ag(111) surface.

This effect, where vibrational modes become infrared active due to electron flow across the interface, has been previously described using theoretical reasoning and heuristic models. However, the new study provides the first proof and quantitative examination thereof based on first-principles calculations. Thus, the findings provide an important insight into a key electronic property of interfaces between organic molecules and coinage metal surfaces.

The effect of electron-vibron coupling has been observed for molecular adsorbates, when a molecular orbital is partially filled upon adsorption (static charge transfer), such that its occupation changes dynamically during certain vibrations with respect to the Fermi level of the surface (dynamical charge transfer). This leads to an electron flow between metal and adsorbate, giving rise to an oscillating dipole moment perpendicular to the surface. This fulfils the selection rule for infrared activity. The effect has thus been named interfacial dynamical charge transfer (IDCT).

1,4,5,8-Naphthalenetetracarboxylic dianhydride (NTCDA), a common model compound for an important class of organic semiconductors, adsorbs on the Ag(111) surface by accepting metal electrons in the lowest unoccupied molecular orbital (LUMO), thus fulfilling the above requirement. Molecular vibrations with a specific symmetry can usually not be observed in infrared spectroscopy in the gas phase. However, strong signals were observed in the experimental spectrum of the
molecule adsorbed on the surface for the very same modes. By theoretical investigations it could be proven that this is due to strong IDCT effects.

By studying the projected density of π-symmetric electronic states, the authors verified the observation with first-principles calculations and quantified the effect with partial charge analysis. An excellent correlation between charge transfer and infrared intensity for the fully symmetric vibrational modes has been found. These findings can be applied to any metal-organic interface with a similar bonding situation and matching of energy levels.

Publication:

P. Rosenow, P. Jakob, and R. Tonner
Electron-Vibron Coupling at Metal-Organic Interfaces from Theory and Experiment
J. Phys. Chem. Lett. 7 (2016) DOI:10.1021/acs.jpclett.6b00299

Published on Apr 1, 2016 by the American Chemical Society. Image reprinted with permission. Copyright 2016, American Chemical Society.

Prof. Michael Gottfried is honored with an SCS Lectureship 2016.

The SCS Lectureship is sponsored by the Swiss Chemical Society and Prof. Gottfried will tour from June 6-10, 2016. His host during this time is Prof. Dr. Karl-Heinz Ernst at EMPA.