Momentum Microscopy at Work

November 11th, 2021

Scientists of the University of Graz succeeded in visualising the electronic structure of long-chain organic semiconductors and altering it by means of temperature effects in a controlled manner. More specifically, they demonstrated that monolayers of heptacene on Cu(110) behave markedly different compared to all previously studied oligoacene monolayers on coinage metal surfaces and are dissimilar to any other polycyclic aromatic hydrocarbons adsorbed on metal substrates studied so far. Photoemission tomography measurements were performed using the NanoESCA system and the STM and LEED measurements were performed in a two-chamber system with a variable-temperature Scanning Tunnelling Microscopy (VT STM) both from Scienta Omicron.

A team of researchers of the University of Graz (groups of Prof. Sterrer, experimental physics and of Prof. Puschnig, theory) in collaboration with the University of Tübingen (groups of Prof. Bettinger and Prof. Chasse) constructed a previously unexplored molecule-metal system to investigate the properties and abilities of organic semiconductors. The study was carried out by momentum microscopy, a highly sophisticated method to explore the electronic structure of nanomaterials. It was conducted at the momentum microscope of the NAWI Graz Core Facility "Photoemission Electron Microscopy for Nanomaterials" in cooperation with researchers from the Institute for Experimental Physics (group Prof. Schultze) at the TU Graz. This allowed the detection and controlled modification of the unique electronic properties of the studied metal-organic semiconductor system.

The NanoESCA Core Facility Graz represents a concentration of highly sophisticated analysis methods for creation and study of nanostructures. (Image: Thomas Boné University of Graz).

Complex Preparation of Long-chain Acenes

Acenes belong to the group of polycyclic aromatic hydrocarbons. They consist of linearly fused benzene rings, imaginable as a one-dimensional graphene chain. The here studied molecule heptacene (7A) consists of seven benzene rings and was synthesized at the University of Tübingen, Germany in a complex routine. The researchers in Graz were able to prepare the synthesized molecule into monomolecular thick layers applying an evaporation process and study its advantageous semiconducting properties. This exemplarily shows the strength of the NanoESCA high-performance infrastructure, in which the pulse microscopy system is combined with an ultra-high vacuum chamber equipped with state-of-the-art manufacturing and analysis methods and exhibits advantageous semiconductor properties.

The NanoESCA Core Facility Graz represents a concentration of highly sophisticated analysis methods for creation and study of nanostructures. (Image: Uni Graz/Boné).

Momentum Microscopy Reveals the Electronic Structure in 3D

Momentum microscopy operates by constructing three-dimensional data cubes that can be used to unambiguously assign molecular orbitals. The scientists in Graz found out that the 7A molecules on a copper surface experience strong change of their electronic states. The measurements show that electrons are transferred in previously unoccupied states, altering the key electronic parameters. The amount of transferred charge was unusually large und was confirmed by concomitant theoretical calculations. The molecule-metal system is characterised by its high degree of adaptability. The scientists proved that the molecules can orientate in two directions on the surface.

The amount of molecules in each direction can be regulated by altering the surface temperature during the creation of the molecule films. Momentum microscopy was able to corroborate that dependent on the orientation of the molecule the electronic properties and amount of transferred charge are different.

Reference

"Demonstrating the Impact of the Adsorbate Orientation on the Charge Transfer at Organic–Metal Interfaces", Thomas Georg Boné, Andreas Windischbacher, Marie S. Sättele, Katharina Greulich, Larissa Egger, Thomas Jauk, Florian Lackner, Holger F. Bettinger, Heiko Peisert, Thomas Chassé, Michael G. Ramsey, Martin Sterrer, Georg Koller, and Peter Puschnig. The Journal of Physical Chemistry C, 2021 DOI: 10.1021/acs.jpcc.1c01306

Authored by Thomas Boné. The news was originally published by University of Graz, Austria.