Structural and optical properties of organic nanostructures on dielectric substrates in the monolayer regime: recent trends
| Quand ? |
Le 13/12/2019, de 10:00 à 11:00 |
|---|---|
| Où ? | Meeting room M7 |
| Participants |
Laurent Nony |
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To meet the demand of increased performances (efficiency, energy harvesting, etc.), organic optoelectronic devices, among which are organic light-emitting diodes[i], organic solar cells[ii], and organic field effect transistors[iii], must rely on materials whose structure, adsorption, and absorption properties on solid surfaces must be tailored at best down to the nanoscale[iv]. The functionality (optical and electronic properties) of these ultrathin organic-film-based devices relies on the structural properties of the interfacial molecular layer condensed upon adsorption on the substrate[v] (epitaxy; molecule’ structure, symmetry and reactivity; charge transfer; vertical vs. lateral interactions...). Thus, exploring the relationships between structure and properties of these layers at the molecular scale is necessary.
If optical properties are targeted, one must also make sure that the molecular layers develop well-defined electronic bands for proper light absorption, while keeping molecular electronic states which remain decoupled from those of the substrate. These criteria can be fulfilled on dielectric substrates, wherein the molecules retain well-defined vertical and lateral interactions, as well as a weak charge transfer with the substrate to avoid optical quenching[vi].
Over the past ten years, our group has been studying a large set of molecular tectons and the properties of the nanostructures they formed upon adsorption on bulk alkali halides single crystals[vii] (NaCl(001), KCl(001), RbCl(001), KBr(001)..., cf. fig.1). Our characterization methods are non-contact atomic force microscopy (nc-AFM) combined with Kelvin probe force microscopy in ultra-high vacuum at room temperature, as well as Differential Reflectance Spectrometer (DRS), to get access to the optical properties (dielectric function) of the adsorbed organic layers in situ[viii].
Along with those developments, we also started activities in “on-surface polymerization” on dielectric substrates[ix]. On-surface synthesis is a promising research field aimed at creating new organic materials. Over the past ten years, many reactions pathways have been reported for molecules adsorbed on metals. But so far, none was reported on dielectrics due to their weak reactivity as compared to metallic surfaces.
During this talk, those concepts will be illustrated through three examples taken among our recent results.
Figure 1 : Examples of organic nanostructures as measured in nc-AFM in UHV and at room temperature, after8. a- diboronic acid on KCl(001). b- Hexatriphenylene on KCl(001). c- CDB on KCl(001). d- CTB on KCl(001).
[i] K. Sato et al., Phys. Rev. Lett. 110, 247401 (2013).
[ii] T. Drori et al., Phys. Rev. Lett. 101, 037401 (2008).
[iii] M. Akai-Kasaya et al., Phys. Rev. Lett. 115, 196801 (2015).
[iv] Y. Zhang et al., Phys. Rev. Lett. 116, 016602 (2016).
[v] A. Massé et al., Phys. Rev. Lett. 113, 116604 (2014).
[vi] X. Qiu et al. Science 299, 542 (2003).
[vii] R. Pawlak et al., J. Phys. Chem. C 114, 9290 (2010) ; F. Bocquet et al., Phys. Rev. Lett. 108, 206103 (2012) ; A. Amrous et al., Adv. Mater. Interf., 1400414 (2014); J. Gaberle et al., J. Phys. Chem. C 121, 4393 (2017).
[viii] F. Bocquet et al., Phys. Rev. B 97, 235434 (2018).
[ix] F. Para et al., Nature Chemistry 10, 1112–1117 (2018).