Vibrational Frequency Used as Internal Clock Reference to Access Molecule-Metal Charge-Transfer Times – Publication by A3 (Jakob)

Peter Jakob and Sebastian Thussing derived ultrafast charge-transfer times at molecule-metal interfaces using the vibrational oscillation period as an internal clock reference

A3_Jakob_PRL_News

Interfacial dynamical charge transfer at the molecule-metal interface, associated with non-adiabatic electron-vibron coupling leads to vibrational bands displaying characteristic asymmetric line shapes.

Dynamical charge-transfer processes at molecule-metal interfaces proceed in the few fs timescale that renders them highly relevant to electronic excitations in optoelectronic devices. This is particularly true when electronic ground state situations are considered that implicate charge transfer directly at the fermi energy.

Prof. Jakob and Dr. Thussing showed that such processes can be accessed by means of vibrational excitations, with nonadiabatic electron-vibron coupling leading to distinct asymmetric line shapes. Thereby the characteristic timescale of this interfacial dynamical charge transfer can be derived by using the vibrational oscillation period as an internal clock reference.

Publication

P. Jakob, S. Thussing
Vibrational Frequency used as Internal Clock Reference to access Molecule — Metal Charge Transfer Times
Phys. Rev. Lett. 126 (2021) 116801 DOI:10.1103/PhysRevLett.126.116801

Contact

Prof. Dr. Peter Jakob
Philipps-Universität Marburg
SFB project A3
Tel.: 06421 28-24328
EMAIL

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

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.

PEN PFP on MoS2

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).

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)

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).

A4 OFET 2021

Photograph and force microscopy image of the vacuum-processed OFETs, which enables their electrical characterization without exposing the devices to air (Image: Y. Radiev).

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