Lars Bannow and Benedikt P. Klein awarded the dissertation prize of Philipps-Universität Marburg

Congratulation to Dr. Lars Bannow and Dr. Benedikt P. Klein, PhD-students of the GRK 1782 (SW Koch) and the SFB project A4 (Gottfried), respectively, for being awarded a prize by Philipps-Universität Marburg for their excellent dissertations presented in 2019.

Lars Bannow, GRK 1782 (SW Koch) Foto: Paul Ndimande

In the thesis of Dr. Bannow with the title “Optical and Electronic Properties of Semiconductor Materials”, he investigates optical and electronic properties of novel semiconductor materials such as Ga(AsBi), In(AsBi) and the methylammonium (MA) perovskite MAPbI3. Using a combination of density functional theory calculation and semiconductor Bloch equations, a precise prediction of opto-electronic properties was possible at a minimum of experimental data. The examined materials are promising options for the fabrication of more efficient laser diodes and more economic solar cells.

Benedikt Klein, A4 (Gottfried)

In his thesis “The Surface Chemical Bond of Non-alternant Aromatic Molecules on Metal Surfaces”, Dr. Klein explores interfaces between model organic semiconductors and metals. He compares π-electron systems with alternant and non-alternant topologies and finds that the non-alternant topology leads to much stronger interfacial interactions. These studies pave the way to novel organic semiconductors with tailored properties and provide important insight into the bonding of non-alternant defect structures in graphene with metals.

See the news release of the Philipps-Universität Marburg for details of the event.

Directional ultrafast charge transfer in a TMDC heterostructure – Publication by B5 (Höfer/Mette)

In a new publication in Nanoscale Horizons, Zimmermann and coworkers introduce time-resolved SHG imaging microscopy as a new experimental method for investigating ultrafast charge-transfer processes in heterostructures of transition metal dichalcogenides.

Time- and polarization-resolved SHG microscopy in combination with pump-photon energy dependent measurements reveals ultrafast interlayer hole transfer from WSe2 to MoSe2 and vice versa. Copyright by CC BY 3.0.

Heterostructures of transition metal dichalcogenides (TMD) feature a type-II band alignment which can separate photoexcited electrons and holes into different layers through ultrafast charge transfer. While this charge transfer is essential for potential applications, the underlying mechanisms still remain elusive. Main drawbacks of previous experiments are insufficient time-resolution of the employed microscopy setups and deficiencies of linear optical spectroscopies to address individual layers of the heterostructure selectively.In their new approach, Zimmermann and coworkers have combined the advantages of time-resolved optical second-harmonic generation (SHG) with an optical microscopy setup. On the one hand, their method allows for pump-probe experiments in µm small structures with a superior time-resolution. On the other hand, the tensorial nature of the second-order nonlinear susceptibility allows them to distinguish the response from differently oriented layers to elucidate directional interlayer charge transfer as demonstrated for a rotationally mismatched WSe2/MoSe2 heterostructure. As their results show, the new approach is particularly suited to perform systematic investigations of the charge transfer in dependence of the rotational layer mismatch in TMD heterostructures.

Publication

J. E. Zimmermann, Y. D. Kim, J. C. Hone, U. Höfer, G. Mette
Directional ultrafast charge transfer in a WSe2/MoSe2 heterostructure selectively probed by time-resolved SHG imaging microscopy
Nanoscale Horizons 5 (2020) 1603 DOI: 10.1039/d0nh00396d

Contact

Dr. Gerson Mette
Philipps-Universität Marburg
SFB 1083 subproject B5
https://internal-interfaces.de/projects/B5
Tel.: +49 6421 28-24123
EMAIL

Van der Waals bound Organic Semiconductor/2D-Material Hybrid Heterosystems: Intrinsic Epitaxial Alignment of Perfluoropentacene Films on Transition Metal Dichalcogenides – Publication by A2 (Witte)

In a new publication in Chemistry of Materials project A2 (Witte) reports on the epitaxial alignment of crystalline perfluoropentacene (PFP) films on various transition metal dichalcogenides (TMDCs). This van der Waals epitaxy results in characteristic twist angles between substrate and film lattices, which are of particular interest for the optoelectronic coupling at the interface.

Epitaxial alignment of crystalline PFP films on the basal plane of MoS2 and WSe2 (Image: M. Dreher). Reprinted with permission from 2020, 32, 20, 9034-9043. Copyright 2020 American Chemical Society.

Two-dimensional (2D) materials are a subject of current research, because their different electronic properties as well as the ability to prepare films as thin as one mono­layer opens up the prospect of producing new nanoscale heterostructures and devices. Of particular interest is the stacking of such films with controlled twist angle as it critically affects the electronic interface properties. A promising extension is the combination of TMDCs with organic semi­conductors (OSC), as it allows to combine the high charge carrier mobility of the TMDCs with the OSC’s large photo-absorption cross section, which is beneficial for photovoltaic applications.

Using the example of the prototypical OSC PFP, Maximilian Dreher and coworkers analyzed in the present study the epitaxial alignment of the crystalline molecular adlayers on the basal plane of different TMDCs (MoSe2, WSe2, MoS2 MoTe2). By utilizing the optical anisotropy of PFP films, their azimuthal alignment was analyzed by means of polarization resolved reflection anisotropy. This sensitive and non-invasive method allows to characterize the epitaxial alignment even for thin films of few nanometers. The analysis yielded specific twist angles of the crystalline adlayer domains with respect to the substrate lattice, which are characteristic for the individual material combinations. Notably, the observed epitaxial order is not caused by any higher-order commensurability between substrate and adlayer, where individual molecules are bound to locally favorable adsorption sites. Instead, it results from an energetically favored alignment of the entire crystalline adlayer on the substrate surface and can be rationalized as an on-line coincidence. This peculiar epitaxy could also be theoretically modelled using a modified scheme of projection of real-space adlayer lattice points onto the substrate unit cell. In addition, the extreme sensitivity of this van der Waals epitaxy on small lattice distortions was demonstrated by films grown at slightly higher substrate temperature. Although raising the growth temperature by about 30 K yields only a small increase of the lattice constants of the PFP film due to thermal expansion in the order of a few hundredths of Angstrom, while the more rigid TMDC surface lattice is hardly affected, it causes a distinct change of the twist angle of more than 20°. The achieved epitaxial alignment and control of twist angles is an important mile stone and will be used in future studies on the optoelectronic adlayer-substrate coupling in OSC/TMDC hybrid systems within the SFB 1083.

Publication

Maximilian Dreher, Darius Günder, Steffen Zörb, and Gregor Witte
Van der Waals Bound Organic Semiconductor/2D-Material Hybrid Heterosystems: Intrinsic Epitaxial Alignment of Perfluoropentacene Films on Transition Metal Dichalcogenides
Chem. Mater. (2020) DOI:10.1021/acs.chemmater.0c03482

Contact

Prof. Dr. Gregor Witte
Philipps-Universität Marburg
SFB 1083 project A2
Tel.: 06421 28-21384
EMAIL