PCSI2025 Session PCSI-SuE1: Organic and Hybrid Semiconductor Materials

The combination of a scanning probe microscope with a photon detector and a Raman spectrometer, referred to as a tunneling-electron-induced light emission (STM-LE) and a tip-enhanced Raman scattering spectroscopy (STM-TERS), is quite attractive and useful to explore the optical properties and chemical analysis of nanomaterials beyond the optical diffraction limit.

In this study, we performed the STM observation to elucidate structural configuration of chiral molecules adsorbed on the metal substrates accompanied with their optical and vibrational properties using our laboratory-built STM-LE and STM-TERS systems^1-4. As for the chiral molecules, we usedracemic-mixture and enantiopure molecules of thechiral PTCDI(N,N'-Di-n-octyl-3,4,9,10-perylenetetracarboxylic Diimide) andthiaheterohelicene and their derivatives. We clearly observed the high resolution STM images for the chiral molecular assemblies, leading to the important perspective of stereochemical chiral recognition based on the formation of ordered molecular structures combined with their optical and vibrational characterizations. Theoretical calculation based on a density functional theory and a molecular dynamics simulation successfully identified the formation mechanism of the molecular self-assemblies in the chiral recognition schemes.⁵ Moreover, we attempted to investigate the optical dissymmetry of chiral PTCDI molecules using STM-LE measurement, and that of helicene derivatives using the STM-TERS measurements for evaluating the optical activity and identifying the enantiomers of the chiral molecules at a molecular scale.

References

[1] P. Krukowski, Y. Kuwahara, et al., J. Phys. Chem. C 120, 3964–3977 (2016).

[2] S. Chaunchaiyakul, Y. Kuwahara et al., J. Phys. Chem. C 121, 18162–18168 (2017).

[3] P. Krukowski, Y. Kuwahara, et al., Appl. Surf. Sci., 589, 152860 (2022).

[4] S. Kimura, Y. Kuwahara, et al., Phys. Chem. Chem. Phys. 26(43), 7658-7663 (2024).

[5] Changqing Ye, Y. Kuwahara et al., J Phys.Chem. C127(43), 21305-21312(2023)

⁺Author for correspondence: kuwahara@prec.eng.osaka-u.ac.jp [mailto:kuwahara@prec.eng.osaka-u.ac.jp]

Organic overlayers have emerged into a promising surface modification or functionalization tool and N-Heterocyclic carbenes (NHCs) and their close relatives, the N-heterocyclic olefins (NHOs), were found to be particularly suitable in this regard forming highly ordered self-assembled monolayers (SAMs) on metal surfaces that exhibit superior properties to the widely used thiol-based SAMs. As silicon remains the material of choice in today’s semiconductor technology a functionalization of its surfaces is of highest interest.

Here, a comprehensive scanning tunnelling microscopy (STM), density functional theory, and X-ray photoemission spectroscopy study of the growth of NHC and NHO monolayers on clean and modified silicon surfaces is presented [1-3]. We demonstrate the formation of highly ordered monolayers binding to the surface via a single Si–C bond (see Fig. 1). The monolayers exhibit high thermal stabilities and show large work function reductions. By systematically varying the sidegroups and the backbone of the molecules and investigating two different molecule classes (NHCs and NHOs), we obtain a profound understanding of the bonding, structure, and assembly of NHCs and NHOs on Si offering guiding rules for a targeted modification of Si surfaces.

[1] M. Franz et al., Nat. Chem. 13, 828-835 (2021).
[2] R. Zielinski et al., J. Mater. Chem. C 11, 7377 (2023).
[3] M. Das et al., Angew. Chem. Int. Ed. 62, e202314663 (2023).

Graphene and its derivates have been widely investigated with the exotic electronic properties. Doping is one of the most important ways to tune the properties of graphene-based nanostructures and materials.

The formation of self-assembled layers of organic molecules on solid surfaces is an important subject because of their possible application in advanced (opto)electronic devices.While the formation of self-assembled layers is well established on metal surfaces, the growth on silicon surfaces, which are still the material of choice in today’s semiconductor technology, is much less studied. On pure silicon, the formation of ordered organic films is usually hindered by their high dangling bond density, so that an appropriate surface modification is required. Here we demonstrate the high potential of a rare earth modification for promoting an ordered growth.

Using scanning tunneling microscopy, we show that a terbium modified Si(111) surface enables the formation of highly ordered molecular monolayers, on the example of cobalt phthalocyanine (CoPc). CoPc belongs to the transition metal phthalocyanines being a class of organic semiconductors, which has been already employed e.g. in organic light emitting diodes, photovoltaic cells, and field-effect transistors.

Our data demonstrate that single CoPc molecules stay highly mobile at the TbSi₂/Si(111) surface at room temperature. By reaching a coverage of a full monolayer, a highly ordered film consisting of large domains of flat-lying CoPc molecules in an almost fourfold symmetry with different orientations is formed

[1] M. Kubicki, M. Franz, and M. Dähne, “A Rare Earth Modified Silicon Surface as a Template for Ordered Organic Growth”, J. Phys. Chem. C 128, 13347 (2024).

⁺Author for correspondence: martin.franz@physik.tu-berlin.de