Epitaxial growth of perfluoropentacene films with predefined molecular orientation: A route for single-crystal optical studies

Using atomic-force microscopy and x-ray diffraction we show that perfluoropentacene (C${}_{22}$F${}_{14}$, PFP) forms long-range ordered, epitaxial films on KCl(100) and NaF(100) cleavage planes. On both substrates the films adopt the same crystalline bulk phase, but surprisingly exhibit quite different molecular orientations, being upright oriented on NaF and recumbent oriented on KCl. Accompanied thermal desorption spectroscopy measurements indicate the absence of a stabilized seed layer, like on metals, hence suggesting that in both cases the PFP films are stabilized by an electrostatic point-in-line relationship between the outermost fluorine atoms and the alkali cations of the alkali halide surfaces. Furthermore, the transparency of both substrates was utilized to perform detailed transmission UV/Vis spectroscopy and polarized optical microscopy measurements along well-defined crystallographic directions. From these data the orientation of transition dipole moments of the various optical excitations were experimentally determined and a directional anisotropic exciton coupling was observed, which is attributed to the asymmetric molecular packing motif within the (100) plane of the PFP crystal lattice.

Figure 1

Comparison of AFM data showing the morphology of PFP films deposited on NaF(100) (left panels) and on KCl(100) (right panels). (a),(e) Deposition at $T=310$ K; (b),(f) deposition at $T=343$ K; (d),(j) statistical analysis of the island orientation of latter samples. Panel (c) reveals a magnified micrograph of (b) with accompanied line scan and panel (g) displays a magnified detail showing an uncovered KCl substrate region. Panel (h) shows an optical micrograph of a PFP film deposited at $T=363$ K on KCl together with (i) a corresponding AFM micrograph.

Figure 2

AFM data of PFP films on NaF(100) and KCl(100) showing (a),(b) the initial growth (nominal thickness 3 nm) and (c),(d) the back side of 50-nm PFP films after the substrate was dissolved in water. All samples were prepared at temperatures of 343 K.

Figure 3

Thermal desorption spectra of PFP films of different thickness deposited (a) on KCl(100) and (b) on NaF(100). All spectra were recorded for the molecule ion and a heating rate of $\beta =0.5$ K/s.

Figure 4

X-ray diffraction data of 50-nm PFP films grown on NaF(100) and KCl(100). (a) $\theta /2\theta$ scans and azimuthal $\varphi$ scans recorded for (b) the $(112){}_{\mathrm{PFP}}$ reflex on NaF and (c) the $(012){}_{\mathrm{PFP}}$ reflex on KCl together with magnified $\varphi$ scans (insets) showing the corresponding fine structure.

Figure 5

Hard-sphere model illustrating the molecular structure of epitaxial PFP films (a) on NaF(100) and (b) on KCl(100). The green balls represent the outermost fluorine atoms that are in contact with the substrate. For clarity only one rotational domain is shown. The right-handed sketches show the molecule orientation within the PFP needles (those not true to scale).

Figure 6

Summary of optical properties of PFP films. (a) UV/Vis transmission absorption spectra of 50-nm PFP grown on NaF(100) and KCl(100), compared to PFP on SiO${}_2$ (Ref. 39) (lowermost spectra). The uppermost spectra of PFP/KCl were recorded by using linear polarized light oriented along the $\langle 0\stackrel{̲}{1}1\rangle$ and $\langle 011\rangle$ azimuth directions of KCl. Panels (b) and (c) display polarized optical micrographs for PFP/KCl with polarization plane oriented along the $\langle 011\rangle$ azimuth illuminated with (b) blue light (spectral region III) and (c) red light (spectral region I). Panel (d) shows absorption intensities of PFP/KCl in spectral regions I and III as functions of polarization direction. Panel (e) displays the orientation of transition dipole moments with respect to the PFP lattice derived for the different spectral excitations. Corresponding optical micrographs with linear polarized light are shown (f),(g) for PFP/KCl and (h) for PFP/NaF.