Engineering of TMDC-OSC Hybrid Interfaces: The Thermodynamics of Unitary and Mixed Acene Monolayers on MoS2 – Publication by A2 (Witte) and A4 (Gottfried)

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In a new publication in Chemical Science, projects A2 (Witte) and A4 (Gottfried) report on the intricate desorption characteristics of pentacene (PEN) and perfluoropentacene (PFP) monolayers on the MoS2 surface. Unitary molecular monolayers are thermally stabilized by entropy due to their high mobility rather than the organic/inorganic interface bond, which hampers the formation of close-packed and well-ordered monolayers.

 

Schematic representation of characteristic TPD traces and the temperature-dependent molecular diffusivity of unitary PEN films (left hand side) and the stabilized mixed PEN:PFP monolayer (right hand side) on MoS2 (Image: P. Dombrowski). Copyright by CC-BY-NC 3.0.

Van der Waals (vdW) bound hybrid heterosystems of inorganic two-dimensional materials (2DMs) and OSCs are currently receiving great attention due to their promising characteristics for the fabrication of flexible electronics and ultra-thin devices. While prototypical devices with 2DM-OSC hybrid heterostructures have been realized, the fundamental understanding of the 2DM-OSC interface is lacking. In their new study, the authors were able to thoroughly unravel the interplay of interface and intermolecular interactions and their effect on the structure and thermal stability of molecular monolayers.

Through temperature-programmed desorption (TPD) experiments, it was shown that the first molecular layers of PEN and PFP on MoS2 are thermally more stable than subsequent molecular layers in spite of an interface bond that is weaker than the molecular interlayer bond. This is possible due to an entropic stabilization that can only occur if the molecular adlayer is highly mobile. Thus, if the first molecular layer is only stabilized by its mobility, it cannot be well-ordered and close-packed even at low temperatures as low as 100 K. The high molecular diffusivity in the gas-like unitary PEN and PFP monolayers was identified by Monte-Carlo simulations: Intermolecular repulsion of intrinsic molecular electrostatic quadrupole moments, in combination with a weak substrate interaction, prohibits the formation of a condensed molecular monolayer.

By introducing attractive intermolecular interactions, condensation of the molecular films is favored. This was achieved in mixed monolayers of PEN and PFP that adopt a well-ordered stoichiometric 1:1 intermixture. In spite of a reduced mobility, the mixed monolayer is thermally stabilized with respect to the bulk substance due to the attractive intermolecular forces and can therefore be fabricated by selective multilayer desorption. This provides a promising prospect for the fabrication and subsequent study of well-defined 2DM-OSC interfaces for future studies within SFB 1083.

 

Publication

S.R. Kachel, P.-M. Dombrowski T. Breuer, J.M. Gottfried, G. Witte
Engineering of TMDC–OSC hybrid interfaces: the thermodynamics of unitary and mixed acene monolayers on MoS2
Chem Sci. (2021) DOI:10.1039/D0SC05633B

 

Contact

Prof. Dr. Gregor Witte
Philipps-Universität Marburg
SFB 1083, project A2
Tel.: +49 6421 28-21 384
EMAIL

Prepare with Care: Low Contact Resistance of Pentacene Field-Effect Transistors with Clean and Oxidized Gold Electrodes – Publication by A2 (Witte)

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In a new publication in Organic Electronics, project A2 (Witte) reports on the establishment of a complete high vacuum-based manufacturing and electronic characterization of organic field effect transistors (OFET).

Photograph and force microscopy image of the vacuum-processed OFETs, which enables their electrical characterization without exposing the devices to air (Image: Y. Radiev). Reprinted from Radiev et al, Prepare with Care: Low Contact Resistance of Pentacene Field-Effect Transistors with Clean and Oxidized Gold Electrodes, Org. Electron. 89 (2021) 106030, Copyright 2021, with permission from Elsevier.

The electronic coupling between OSC and metallic electrodes is of key importance for the efficiency of charge carrier injection in organic electronic devices, such as OFETs or photovoltaic cells, as it determines their idle power. Surface science-based model studies have mainly focused on the energy level alignment at such metal-organic interfaces without measuring real contact resistances, while device studies are typically performed without any microscopic structural and electronic interface characterization. In the present work, the authors introduced a high vacuum-based manufacturing of bottom contact OFETs that enables cleaning and controlled modification of metal contacts before the organic film deposition. This approach not only excludes any exposure to air, it also allows to examine the influence of controlled exposure to air on the device characteristics.

Using the example of the prototypical OSC pentacene it is demonstrated that FET structures with thoroughly cleaned gold electrodes reveal a remarkably low contact resistance. This can be further improved if the electrodes are O2 plasma treated before the pentacene film growth, which results in a thin gold oxide layer and yields one of the lowest contact resistances ever reported for this system. It is shown that this not only causes an improved energy level alignment at the metal-organic interface but also suppresses a pronounced dewetting. In addition, it was demonstrated that controlled exposure to air – even for a short time – significantly affects the device performance.

The present study is an important milestone as it enables detailed electronic transport measurements through metal-OSC interfaces with poly- and single crystal organic semiconductors. This work paves the way for a knowledge transfer about the properties of idealized model interfaces to real electronic devices applications.

Publication                                     

Yurii Radiev, Felix Widdascheck, Michael Göbel, Alrun Aline Hauke and Gregor Witte,
Prepare with Care: Low Contact Resistance of Pentacene Field-Effect Transistors with Clean and Oxidized Gold Electrodes
Org. Electron. 89 (2021) 106030 DOI:10.1016/j.orgel.2020.106030

Contact

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

Prof. Dr. F.S. Tautz won a price at „Falling Walls – The World Science Summit“

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At „Falling Walls – The World Science Summit“ during the Berlin Science Week, Prof. Dr. F.S. Tautz, PI of SFB project A12 (Tautz/Bocquet/Kumpf), was declared the winner in the category „Engineering and Technology“ with his contribution „Breakting the Wall of Building with Molecules“.

Tautz Portait

„Falling Walls“ is an event that brings together some of the best researchers of the world, discussing and celebrating the latest breakthroughs in science and society since over 10 years. The breakthroughs represent significant advances, groundbreaking developments and innovative ideas stretching over ten different categories.

 

Falling Walls 2020 during the Berlin Science Week. Copyright by Falling Walls Foundation.

Prof. Tautz (SFB project A12) submitted his 5-min long nomination film (link see below) in the category “Engineering and Technology” titled “Breaking the Wall of Building with Molecules”. In this video, Prof. Tautz gives a short insight into his research. Although manipulating atoms on surfaces is already performed for several years, researchers are struggling with moving and arranging molecules. He explains how an artificial intelligence was trained by reinforcement learning in reality and in model systems at the same time. Consequently, the agent got highly adaptive and become able to successfully manipulate molecules.

With this technique, targeted fabrication of molecular machines can be achieved giving rise to new promising technologies with high-level functionalities. Winning this price is not only a great honor for Prof. Tautz, but also represents the high importance and appreciation of the SFB’s research.

For further details, see the nomination film.

 

Contact

Prof. Dr. Stefan Tautz
FZ Jülich, SFB project A12
PGI, Experimental Physics
Tel.: +49 (0)2461 61-4561
EMAIL