There are a number of possible mechanisms which give rise to fluorescence shifts from the gas phase to flat-lying molecular monolayers. Fluorescence shifts can arise from both non- resonant and resonant interactions. Non-resonant interactions have been observed in systems where interactions between a fluorescent molecule and its environment perturb the electronic states within a molecule. Such a shift was observed by the Nottingham nanoscience group, where the fluorescence of monolayer films of tetra carboxyl phenyl porphyrin (TCPP) on hBN were shifted due to the interaction of the molecule with the substrate [74].
Resonant interactions occur due to the delocalisation of excitons across a number of molecules. The fluorescence of such structures is explained by considering the coupling of transition dipole moments. In ‘The Theory of Molecular Excitations’ [75] Davydov presented a model of transition dipole coupling in molecular fluorescence which is still used to describe the shifts of both organic monolayers and crystals. Both the Sokolowski group at the University of Bonn and the Fritz group at the University of Jena have studied the optical properties of organic monolayers, particularly PTCDA on alkali halide surfaces, in great detail. The Sokolowski group at Bonn measured the fluorescence of flat-lying isolated PTCDA molecules and two packing arrangements of monolayer PTCDA on NaCl [13]. When deposited on NaCl held at 20 K, with a coverage of much less than one monolayer, molecules are flat-lying and isolated on the NaCl surface (this is referred to as the dilute phase in the literature). On NaCl, PTCDA also forms both a square and a herringbone (rectangular) packing structure. The packing of these phases and their fluorescence spectra are shown in figure 2.6.2.1.
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Figure 2.6.2.1. The structure of both the herringbone (rectangular, HB) (a) and square (Q) (b)
phases of PTCDA on NaCl are shown. The fluorescence spectra of the square and rectangular close packing arrangements and isolated PTCDA molecules (dilute phase) are shown. Taken from [13].
The Sokolowski group have also published a similar study of PTCDA on KCl substrates [76], where a 0.1215 ± 0.0001 eV red shift was measured from PTCDA doped HND to isolated molecules on the surface, with a further 0.0496 ± 0.0001 eV red shift due to aggregation into a monolayer brick wall phase. In reference [13,76], the origin of the shift of the fluorescence peak of aggregated PTCDA compared to isolated PTCDA on alkali halide surfaces is attributed, in part, to the on-surface coupling of neighbouring transition dipole moments. The coupling of transition dipole moments was modelled and used to calculate the exciton bandstructure of each PTCDA phase. In general, the fluorescence shift from gas phase measurements of a close packed organic monolayer, ∆Etot, is made up of a shift due to interactions with the
surface, ∆Eg-s, and a shift due to intermolecular interactions, ∆Eint.
∆Etot=∆Eg-s+∆Eint (2.6.2.1)
Earlier, the Fritz group at Jena had measured the fluorescence of the same brick wall phase of PTCDA monolayers on KCl using differential reflectance spectroscopy (DRS) [14]. The brick wall phase of PTCDA on KCl was observed at room temperature for sub-monolayer coverages using
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non-contact AFM, while the herringbone (rectangular) phase was observed as the coverage was increased. The narrow peak width of PTCDA on KCl, compared to monolayers on mica and PTCDA in solution, was attributed to the registry of molecules and the substrate.
In addition to conventional spectroscopy measurements, STM induced luminescence (STML) can also provide insights into the delocalisation of excitons and their influence on molecular fluorescence [77,78,79]. For example, Zhang et al [78] used STM to investigate zinc- phthalocyanine molecules adsorbed on a thin NaCl layer on silver, see figure 2.6.2.2. Clusters of two, three and four molecules were formed using sample manipulation with the STM tip. The luminescence of monomers and oligomers, induced through excitation by the STM tip, was measured, with a red-shift observed for larger clusters. The observed shifts were found to agree with descriptions involving the coupling of transition dipole moments [75,80].
Figure 2.6.2.2. A schematic representation of phthalocyanine monomers and oligomers is
shown in addition to STM images and experimental and theoretical photon images, all scale bars are equal to 1 nm (a). STML spectra, extracted from the positions indicated in the STM images are shown (b) in addition to the enhancement factor for monomers and oligomers. Taken from [78].
More recently, the Fritz group has measured DRS spectra for PTCDA on monolayer hBN on Pt (111) and Rh (111) surfaces as well as PTCDA on graphene (on silicon carbide), gold and silver surfaces [81]. In this work they observe very similar adsorption peak positions, extracted from
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fitted DRS spectra, on hBN on Pt(111), graphene on SiC and on gold and silver surfaces. In the case of PTCDA on hBN on Rh(111), they observe a different peak position which is attributed to the trapping of PTCDA monomers within pores on the substrate surface. The relative position of the peaks is explained in terms of the on-surface coupling of the transition dipole moments of PTCDA molecules, with reference to the calculated exciton bandstructure of the observed herringbone packing arrangement [13].
In their paper, the Fritz group also discuss similarities between the DRS spectra of PTCDA on substrates with different refractive indices and extinction coefficients, and go on to suggest that the dielectric properties of the substrate have little effect on the fluorescence of molecular monolayers. While chromatic shifts between different molecular packing structures and isolated molecules on surfaces can be explained in terms of resonant coupling of transition dipole moments, the role of the substrate and the mechanisms behind shifts from ‘gas phase’ (doped HND) measurements are still not fully understood.
2.7. Conclusion
This chapter has provided an overview of the growth of molecular thin films on surfaces. The adsorption of C60 was discussed as well as a brief review of hydrogen bonded arrays and
supramolecular networks based on perylene derivatives, cyanuric acid and melamine. The two-dimensional materials, graphene and hBN, were then introduced with reference to their importance to molecular self-assembly. Finally, molecular fluorescence was discussed with an emphasis on the physical interactions pertinent to monolayer organic thin films.
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