Optical properties of pentacene thin films and single crystals

The optical properties of pentacene single crystals and thin films are systematically investigated by variable angle spectroscopic ellipsometry. In combination with x-ray diffraction and atomic force microscopy the method allows us to distinguish between the complex optical constants ($n$ and $k$) of the different polymorphs formed in thin films during the growth process. On the biaxial anisotropic single crystal it is possible to determine the optical constants along the respective crystallographic orientations. Temperature-dependent measurements in the range from $T=5\mathrm{K}\text{to}350\mathrm{K}$ confirm the assignment of the optical spectra. The results are in good agreement with ab initio calculations of the optical properties of solid pentacene by Tiago et al. [Phys. Rev. B. 67, 115212 (2003)].

Figure 1

X-ray diffraction data from thin pentacene films prepared at different substrate temperatures. With increasing substrate temperature the amount of the so-called bulk phase in the film increases.

Figure 2

AFM images of two pentacene films grown at (a) $T=295\mathrm{K}$ and (b) $331\mathrm{K}$, respectively. (a) At low substrate temperatures the flat thin-film phase is built. (b) At higher substrate temperatures three-dimensional islands (bulk phase) are formed leading to a very rough surface.

Figure 3

Scheme of the layer structure used for the ellipsometric analysis.

Figure 4

Real and imaginary parts of the refractive index, $n_1$ and $k_1$, of the pentacene thin-film phase. The optical properties vary only slightly with the substrate temperature.

Figure 5

$n_2$ and $k_2$ of the pentacene bulk phase. The overall optical behavior is clearly different from the thin-film phase.

Figure 6

Optical constants of pentacene single crystals parallel to the $x$ and $y$ principal axes as obtained by generalized spectroscopic ellipsometry.

Figure 7

Temperature-dependent ellipsometric measurement on a film containing nearly exclusively the thin-film phase. As expected for a molecular crystal, the optical band gap decreases and the Davydov splitting of around $0.1\mathrm{eV}$ at $T=300\mathrm{K}$ increases with decreasing temperature up to $0.125\mathrm{eV}$.