New design strategies for tuning electrode properties by SAMs
Solid State and Surface Chemistry, Material Synthesis
Theoretical Chemistry: Molecules, Materials, Surfaces
Final Report Abstract
The projects aimed at development and practical realization of new design strategies for tuning electrode properties by dipolar self-assembled monolayers (SAMs), accompanied by an atomic-level understanding of the electronic, electrostatic, and structural properties of the respective films. The emphasis was put on the concept of "embedded dipole" but several other concepts were tested and realized as well. The project work was performed within the DACH framework, in tight cooperation with several research groups from Austria, and with assistance of other partners from Germany, Netherlands, USA, Canada, and Taiwan. The embedding of a dipolar group into molecular backbone allows interfacial dipole engineering independent of the docking chemistry and, most importantly, without modifying the SAM-ambient interface. In view of this advantage, we have designed and studied a variety of aliphatic and aromatic SAMs on gold substrates (a popular electrode material), using polar ester and pyrimidine groups, respectively, for the mid-chain substitution. These SAMs exhibited pronounced electrostatic effects in photoemission and work function (WF) behavior. The former effects were understood in very detail; for polar SAMs they were found to be as important for the photoemission spectra interpretation as the standard chemical shift effects. The orientation of the embedded dipole either upward or downward with respect to the substrate allowed WF variation within a range of ~1 eV. This variation could also be achieved gradually by mixing molecules with the oppositely oriented, embedded polar groups. An additional option was WF tuning by electron irradiation, possible for pyrimidine- and pyridine-substituted SAMs. The concept of embedded dipole, working especially well for the aromatic SAMs, was also adapted to other substrates, taking Ag(111), ITO, and Al2O3, as test materials. The electric transport experiments on the embedded dipole SAMs showed that they represent an important model system in this context. As a further step, we designed a set of highly conductive molecules (purely aromatic and short) that exploits the concept of embedded dipoles in a fashion specifically suitable for applications in organic electronic devices. The respective interfacial layers allowed tuning the contact resistance of organic thinfilm transistors (OTFTs) over three orders of magnitude with minimum values well below 1 kΩ cm. This not only permitted the realization of highly competitive p-type (pentacene-based) devices on rigid as well as flexible substrates, but also enabled the realization of n-type (C60- based) transistors with comparable characteristics utilizing the same electrode material (Au). As prototypical examples for the high potential of the presented SAMs in more complex device structures, flexible organic inverters and a 5-stage ring-oscillator operating below 4 V with a stage frequency in the range of the theoretically achievable maximum were fabricated. Other concepts addressed and realized by us included (i) molecules with dipolar backbone ("distributed dipole") represented by a bipyrimidine unit, in which both pyrimidine groups were oriented equally, (ii) hybrid systems, in which such a bipyrimidine unit was combined with a terminal polar group, and (iii) SAMs with a polar anchoring group, such as dithiocarbamate. A surprising result was a comparably small impact of the second pyrimidine unit on the WF value, with a factor of ~1.3 with respect to the single pyrimidine group. The obtained results are both of significant scientific value and of relevance for applications. We believe that the concept of electrostatically designed molecular films, such as those with embedded and distributed dipolar groups or polar anchoring group, can induce a paradigm shift in interfacial nanoengineering. These films can also be directly utilized in organic electronics and photovoltaics devices, as was demonstrated by us by the example of p-type and n-type OTFTs and related electronic circuits on solid and flexible substrates.
Publications
- Employing X-ray photoelectron spectroscopy for determining layer homogeneity in mixed polar self-assembled monolayers, J. Phys. Chem. Lett. 7, 2994−3000 (2016)
I. Hehn, S. Schuster, T. Wächter, T. Abu-Husein, A. Terfort, M. Zharnikov, and E. Zojer
(See online at https://doi.org/10.1021/acs.jpclett.6b01096) - Transition Voltages Respond to Synthetic Reorientation of Embedded Dipoles in Self-Assembled Monolayers, Chem. Sci. 7, 781-787 (2016)
A. Kovalchuk, T. Abu-Husein, D. Fracasso, D. A. Egger, E. Zojer, M. Zharnikov, A. Terfort, and R. C. Chiechi
(See online at https://doi.org/10.1039/c5sc03097h) - Understanding chemical vs. electrostatic shifts in X-ray photoelectron spectra of organic self-assembled monolayers, J. Phys. Chem. C 120, 3428–3437 (2016)
T. C. Taucher, I. Hehn, O. T. Hofmann, M. Zharnikov, and E. Zojer
(See online at https://doi.org/10.1021/acs.jpcc.5b12387) - Adjustment of the work function of pyridine and pyrimidine substituted aromatic self-assembled monolayers by electron irradiation, J. Phys. Chem. C 121, 12834–12841 (2017)
E. Sauter, C. Yildirim, A. Terfort, and M. Zharnikov
(See online at https://doi.org/10.1021/acs.jpcc.7b03302) - Effects of embedded dipole layers on electrostatic properties of alkanethiolate self-assembled monolayers, J. Phys. Chem. C 121, 15815–15830 (2017)
O. M. Cabarcos, S. Schuster, I. Hehn, P. P. Zhang, M. M. Maitani, N. Sullivan, J.-B. Giguère, J.-F. Morin, P. S. Weiss, E. Zojer, M. Zharnikov, and D. L. Allara
(See online at https://doi.org/10.1021/acs.jpcc.7b04694) - Mixed aliphatic self-assembled monolayers with embedded polar group, J. Phys. Chem. C 121, 23017–23024 (2017)
E. Sauter, C.-O. Gilbert, J. Boismenu-Lavoie, J.-F. Morin, and M. Zharnikov
(See online at https://doi.org/10.1021/acs.jpcc.7b08671) - Characterization & compact modeling of self-aligned short channel organic transistors, IEEE Trans. Electron Devices 65, 4563-4570 (2018)
M. Torres-Miranda, A. Petritz, E. Karner-Petritz, C. Prietl, E. Sauter, M. Zharnikov, H. Gold and B. Stadlober
(See online at https://doi.org/10.1109/TED.2018.2867364) - Embedded dipole self-assembled monolayers for contact resistance tuning in p- and n-type organic thin film transistors and flexible electronic circuits, Adv. Funct. Mater. 28, 1804462 (2018)
A. Petritz, M. Krammer, E. Sauter, M. Gärtner, G. Nascimbeni, B. Schrode, A. Fian, H. Gold, A. Cojocaru, E. Karner-Petritz, R. Resel, A. Terfort, E. Zojer, M. Zharnikov, K. Zojer, and B. Stadlober
(See online at https://doi.org/10.1002/adfm.201804462) - Mixed monomolecular films with embedded dipolar groups on Ag(111), J. Phys. Chem. C 122, 19514–19523 (2018)
E. Sauter, C.-O. Gilbert, J.-F. Morin, A. Terfort, and M. Zharnikov
(See online at https://doi.org/10.1021/acs.jpcc.8b04540) - Understanding the properties of tailor-made self-assembled monolayers with embedded dipole moments for interface engineering, J. Phys. Chem. C 122, 28757–28774 (2018)
M. Gärtner, E. Sauter, G. Nascimbeni, A. Petritz, A. Wiesner, M. Kind, T. Abu-Husein, M. Bolte, B. Stadlober, E. Zojer, A. Terfort, and M. Zharnikov
(See online at https://doi.org/10.1021/acs.jpcc.8b09440)
