Institute of Solid State Physics

DE

 New design strategies for tuning electrode properties by SAMs

 

DACH Project funded by the Austrian Science Fund (FWF)

The injection of charge carriers is a crucial process in virtually all electronic device. Controlling these injection processes is particularly difficult for organic semiconductors, where doping processes are much more complex than in their inorganic counterparts. In the current project, we, therefore, developed an alternative strategy for changing the injection efficiency. This approach is based on the adsorption of so-called polar self-assembled monolayers, where the particular advantage of our approach is that the dipoles are embedded into the molecular backbones of the assembled molecules. In this way, we were able to electrostatically change contact resistances by several orders of magnitude. Notably, we showed that our approach can also is compatible with flexible electronics and it can also be efficiently applied to more complex electronic circuits. Moreover, it is useful also beyond the field of organic electronics, as demonstrated by its application to MoS2-based devices.
Of particular relevant for the project is that we did not only demonstrate a practically useful effect, but were also able to explain it at an atomistic level. To achieve that, it was necessary to draw from the complementary expertise of several groups in Germany and Austria that would cover the entire “value chain”. Starting from chemical synthesis (A. Terfort and coworkers at the University of Frankfurt), it involved spectroscopic investigation of the interfaces by means of surface-science techniques (M. Zharnikov and coworkers at the University of Heidelberg), the quantum-mechanical simulation of the electronic properties of the monolayers (E. Zojer and coworkers at the Graz University of Technology), and the fabrication of electronic devices (B. Stadlober, Joanneum Research) and their study by means of simulation (K. Zojer, Graz University of Technology).
These studies provided detailed insight into which types of molecules would be ideally suited for achieving the desired goals and which structural elements should be avoided. They also helped us to further develop the understanding of collective electrostatic effects occurring at virtually all types of interfaces and allowed us to show that these effects fundamentally impact the outcomes of x-ray photoelectron spectroscopy (XPS) experiments. Based on a combination of experimental and theoretical studies, we were able to show for a variety of interfaces that the data obtained by XPS experiments are not only determined by the chemical environment of the studied atoms (as is the common understanding in the scientific community), but are also crucially impacted by the local electrostatic potential that is significantly modified by the adsorbed polar monolayers.
The results of our research have been published in numerous papers in high-ranking journals, they were presented as (invited) talks at international conferences, and also triggered the application for a follow-up project.

Project team (group leaders):
Barbara Stadlober, Joanneum Research
Karin Zojer, Technische Universität Graz
Andreas Terfort, Universität Frankfurt
Michael Zharnikov, Universität Heidelberg
Egbert Zojer (PI), Technische Universität Graz

Publications (involving Austrian team members):
1. A. Matković et al., Interfacial Band Engineering of MoS2/Gold Interfaces Using Pyrimidine Containing Self-Assembled Monolayers: Towards Contact Resistance Free Bottom-Contacts. Adv. Elect. Mater. 6, 2000110 (2020); 10.1002/aelm.202000110.
2. M. Gärtner et al., Self-Assembled Monolayers with Distributed Dipole Moments Originating from Bipyrimidine Units, J. Phys. Chem. C 124, 504-519 (2020); 10.1021/acs.jpcc.9b08835.
3. E. Sauter et al., Dithiocarbamate Anchoring Group as Flexible Platform for Interface Engineering, Phys. Chem. Chem. Phys., 21, 22511 (2019); 10.1039/c9cp03306h.
4. E. Zojer et al., The impact of dipolar layers on the electronic properties of organic/inorganic hybrid interfaces, Adv. Mater. Interfaces (Hall of Fame Review), 1900581 (2019); 10.1002/admi.201900581; highly-downloaded article.
5. F. Ishiwari et al., Triptycene Tripods for the Formation of Highly Uniform and Densely Packed Self-Assembled Monolayers with Controlled Molecular Orientation, J. Am. Chem. Soc., 141, 5995 (2019); 10.1021/jacs.9b00950.
6. M. Krammer et al., Critical Evaluation of Organic Thin-Film Transistor Models, Crystals 2019, 9(2), 85, 10.3390/cryst9020085.
7. M. Gärtner et al., Understanding the Properties of Tailor-Made Self-Assembled Monolayers with Embedded Dipole Moments for Interface Engineering, J. Phys. Chem. C 122, 28757 (2018). 10.1021/acs.jpcc.8b09440
8. A. Petritz et al., Embedded Dipole Self-Assembled Monolayers for Contact Resistance Tuning in p-Type and n-Type Organic Thin Film Transistors and Flexible Electronic Circuits, Adv. Funct. Mater. 28, 1804462 (2018), 10.1002/adfm.201804462.
9. S. S. Harivyasi et al., Van der Waals interaction activated strong electronic coupling at the interface between chloro boron-subphthalocyanine and Cu(111), J. Phys. Chem. C, 122, 14621, (2018), 10.1021/acs.jpcc.8b03675.
10. M. L. Tietze et al., Elementary steps in electrical doping of organic semiconductors, Nat. Comm. 9, 1182 (2018), 10.1038/s41467-018-03302-z.
11. F. Glöcklhofer et al., Dicyano- and tetracyanopentacene: foundation of an intriguing new class of easy-to-synthesize organic semiconductors, J. Mater. Chem. C 5, 2603 (2017); 10.1039/c7tc00143f.
12. J. Ossowski et al., Relative stability of thiolate- and selenolate-bonded aromatic monolayers on the Au(111) substrate, J. Phys. Chem. C 121, 28031 (2017), 10.1021/acs.jpcc.7b09685.
13. O. M. Cabarcos et al., Effects of Embedded Dipole Layers on Electrostatic Properties of Alkanethiolate Self-Assembled Monolayers, J. Phys. Chem. C 121, 15815 (2017); 10.1021/acs.jpcc.7b04694.
14. I. Hehn et al., Employing X-ray Photoelectron Spectroscopy for Determining Layer Homogeneity in Mixed Polar Self-Assembled Monolayers, J. Phys. Chem. Lett. 7, 2994 (2016); 10.1021/acs.jpclett.6b01096.
15. A. Kovalchuk et al., Dipole-induced asymmetric conduction in tunneling junctions comprising self-assembled monolayers, RSC Adv.6, 69479 (2016); 10.1039/c6ra10471a.
16. M. Krammer et al., Prediction of the current across metal-organic semiconductor interfaces for device operation (in preparation)
17. M. Krammer and K. Zojer, Efficient determination of the field-dependence of the charge mobility for hopping conduction: The correction energy concept. (in preparation)

 

 

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