Exciton and polarization contributions to optical transition energies in an epitaxial organic monolayer on a dielectric substrate

Optical emission and excitation spectra of the model molecule perylene-3,4,9,10-tetracarboxylic acid dianhydride adsorbed on an NaCl(100) surface were measured in the limit of isolated monomers and for two epitaxial monolayer phases (Q and HB${}^{*}$). Intermolecular dipole-dipole coupling and molecular polarization lead to distinct and structure-dependent relative shifts of the exciton energies with respect to the monomer. In combination with calculations of the exciton band structures, this gives insight into the relevant contributions to the transition energies for an interfacial organic film.

Figure 1

Models of the two monolayer phases of PTCDA on NaCl(100): (a) the commensurate quadratic (Q) structure (unit cell: 16.92 × 16.92 Å${}^2$) and (b) the incommensurate rectangular herringbone (HB${}^{*}$) structure (unit cell: 13.5 × 20.2 Å${}^2$).

Figure 2

Fluorescence (FL) (left, black) and fluorescence excitation (FLE) spectra (right, red) of the Q phase (top) and the diluted monomer phase (middle), both measured at a cryostat temperature of 20 K. The excitation wavelengths were 498 nm and 457 nm, respectively, whereas the detection wavelengths for the FLE spectra were 556 and 543 nm, respectively. In addition, the FL spectrum of single PTCDA molecules in helium nanodroplets (HND) (bottom)[17] is shown for comparison. This reference spectrum was red shifted by 1307 cm${}^{-1}$ in order to match the positions of the 0-0 transitions with those of the monomer spectra. In the inset, all relevant energy differences of the discussed phases are summarized.

Figure 3

Fluorescence spectra (right) and LEED scans (left) obtained during the transition from the metastable HB${}^{*}$ phase to the Q phase of PTCDA on NaCl(100). From (a) to (e), the fraction of the HB${}^{*}$ phase decreases at the expense of the Q phase due to stepwise thermal annealing of the sample. This can be seen from the LEED scans, where the contributions of the HB${}^{*}$ and the Q phase are indicated. For further details of the phase transition, see Ref. 7.

Figure 4

Calculated exciton band structures of the Q and HB${}^{*}$ phases of PTCDA and corresponding surface Brillouin zones. For further details, see the text.