Formation of Solid-State Excitons in Ultrathin Crystalline Films of PTCDA: From Single Molecules to Molecular Stacks

We directly follow the evolution of the absorption spectrum from a single molecule to a dimer and further to a one-dimensional molecular stack: We determine the optical absorption properties of ordered monolayer to multilayer films of PTCDA (3,4,9,10-perylenetetracarboxylic dianhydride) on muscovite mica(0001) surfaces by in situ differential reflectance spectroscopy. The data clearly show the transition from the single molecule to a dimer spectrum, followed by the exciton delocalization to a molecular crystal exciton. The accompanying spectral shifts compare favorably with recent model concepts.

Figure 1

Differential reflectance spectra of PTCDA on mica during the growth from 0.1 to 2.7 ML ($\pm 10\%$) film thickness. Because of the acquisition time of $\sim 40\mathrm{s}$ and a deposition rate of about $0.23\mathrm{M}\mathrm{L}/\min $, the layer thicknesses increase by 0.2 ML during the measurement. We give the mean coverage in italic style, i.e., 0.1 ML will read 0–0.2 ML. However, 2.7 ML corresponds to the total final thickness. Note the clear change in the characteristic spectral shape and the two intersection points at about 2.30 and 2.38 eV. For comparison, the absorption spectrum of PTCDA dissolved in dimethylsulfoxide is shown in the bottom (dashed line, shifted, arbitrary units).

Figure 2

The PTCDA herringbone structure: (left) top view onto the (102) plane of the PTCDA crystal, and side view (right). The two types of dimers, $A$ (in-plane) and $B$ (stacked), are indicated. The crystal data are ($\alpha$ modification) $a=3.72Å$, $b=11.96Å$, and $c=17.34Å$ [2].

Figure 3

The differential reflection spectra divided by the film thickness $d$ and the energy $E$, a quantity which is proportional to $\mathrm{I}\mathrm{m}[{\widehat{ϵ}}_A(E)]$, for several film thicknesses between 0.5–2.3 ML. Two isosbestic points at 2.27 and 2.62 eV occur, being compatible with the assumption of the growth of a linear chain. In the submonolayer regime, a weak feature at 1.87 eV is visible (see the text). In the bottom, the spectrum of a 4.1 ML PTCDA film is shown for comparison (different sample, shifted).

Figure 4

The position of the low energy feature Peak1 in DRS during film growth of PTCDA of different samples. The error bars indicate the uncertainty of the Peak1 maximum and the layer thickness. Two different slopes of the shift are obvious, indicated by arrows. The dashed line shows the fit with the model of Agranovich et al. [24] ($E_0=2.215\mathrm{e}\mathrm{V}$, $\Delta =0.115\mathrm{e}\mathrm{V}$; see the text).