Tuning the electronic properties of self-assembled monolayers by embedded molecular dipoles
Final Report Abstract
The major goal of the project was an atomic-level understanding of the electronic and structural properties of alkylthiolate (AT) based self-assembled monolayers (SAMs) that contain embedded (di)polar functional groups. These systems represent a promising alternative to the standard design of polar monomolecular films, where dipolar groups are attached as tail moieties to the molecular backbone, forming the film-ambient interface and affecting the morphology of adjacent organic films. Addressing the issue in a broad context, we designed, fabricated, and studied not only aliphatic SAMs with embedded (di)polar functional groups (ester) but also analogous aromatic monolayers with embedded pyrimidine moiety. The latter systems have the advantages of lesser conformational flexibility, upright orientation of the embedded dipole, structural similarity to typical organic semiconductors, and better conductive properties. The two former factors simplify theoretical simulations while the two latter factors are important in context of potential applications. For both aromatic and aliphatic SAMs we varied the orientation of the embedded dipolar group in the molecular backbone; for the aliphatic SAMs we have additionally varied the position of this group in the backbone. The electronic and structural properties of the embedded-dipole SAMs were thoroughly analyzed using a number of complementary characterization techniques combined with quantum-mechanical modeling. Characterization of the electronic properties of these SAMs reveals that the embedded dipoles significantly shift the substrate work function (WF) in a systematic manner: a dipole moment pointing towards the Au substrate increases the WF, while a dipole moment pointing away decreases the WF compared to the non-sunbstituted benchmark system. The accessible WF range spans ~1 V in both aliphatic and aromatic monolayers (with the possibility of fine tuning of WF; as demonstrated), while at the same time maintaining the chemical identity and structural properties of the SAM-ambient interface. This implies that the strategy of embedded dipoles in interfacial, monomolecular layers opens up the possibility to decouple control over charge-injection from control over the nucleation and growth of organic semiconductor layers. Thus, such films have high potential in organic electronics, where they can be used for the interface engineering. Also in molecular electronics they allow the study of electrostatic effects independently from the (top) contact properties, as was demonstrated by a dedicated study correlating electric transport properties of molecular films with their intrinsic properties. Along with WF modification, the introduction of embedded dipolar group results in interesting effects in photoemission, calling into question the commonly applied concept that chemical shifts are the only cause for shifts in core-level binding energies. As shown, X-ray photoemission spectroscopy can serve as a highly sensitive probe to study electrostatic effects in molecular films but also provide information on their morphology on molecular level. The experimental results were well reproduced and explained by theoretical simulations which demonstrated that the dipolar groups induce a potential discontinuity inside the monolayer electrostatically shifting the energy levels in the regions above and below the dipoles relative to each other. This paves the way for electrostatic design of molecular films. The obtained results are both of significant scientific value and of relevance for applications. We believe that the concept of electrostatically designed molecular films can induce a paradigm shift in interfacial nanoengineering. We have already started some activities in this direction as well as specific cooperation with industry relying on concept of embedded and distributed dipolar groups.
Publications
- Transition voltages respond to synthetic reorientation of embedded dipoles in self-assembled monolayers, Chem. Sci.
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) - Nanoscale electrical investigation of layer-by-layer grown molecular wires, Adv. Mater. 26, 1688–1693 (2014)
C. Musumeci, G. Zappala, N. Martsinovich, E. Orgiu, S. Schuster, S. Quici, M. Zharnikov, A. Troisi, A. Licciardello and P. Samori
(See online at https://doi.org/10.1002/adma.201304848) - The effects of embedded dipoles in aromatic self-assembled monolayers, Adv. Funct. Mater. 25, 3943–3957 (2015)
T. Abu-Husein, S. Schuster, D. A. Egger, M. Kind, T. Santowski, A. Wiesner, R. Chiechi, E. Zojer, A. Terfort, and M. Zharnikov
(See online at https://doi.org/10.1002/adfm.201500899) - Understanding chemical versus electrostatic shifts in X-ray photoelectron spectra of organic self-assembled monolayers. J. Phys. Chem. C, 2016, 120 (6), pp 3428–3437
T. Taucher, I. Hehn, O. T. Hofmann, M. Zharnikov, and E. Zojer
(See online at https://doi.org/10.1021/acs.jpcc.5b12387)
