Evidence for Temperature-Dependent Electron Band Dispersion in Pentacene

Evidence for temperature-dependent electron band dispersion in a pentacene thin film polymorph on graphite is provided by angle- and energy-dependent ultraviolet photoelectron spectroscopy. The bands derived from the highest occupied molecular orbital exhibit dispersion of $\sim 190\mathrm{meV}$ at room temperature, and $\sim 240\mathrm{meV}$ at 120 K. Intermolecular electronic coupling in pentacene thin films is thus confirmed to be dependent on temperature and possibly crystal structure, as suggested by additional infrared absorption measurements.

Figure 1

(a) X-ray diffractogram of pentacene on HOPG for increasing thickness $\theta$. The inset shows a schematic side view of pentacene molecule orientation on the substrate, and the crystallographic unit cell. (100) is oriented along the plot $y$ axis. (b) Representative AFM image of a 12 nm thick pentacene film on HOPG ($5\times 5\mu {\mathrm{m}}^2$; $z$ scale: 30 nm).

Figure 2

Room temperature (a) angle-resolved and (b) energy-dependent photoemission spectra of the pentacene HOMO region (on HOPG). Binding energy is given relative to the Fermi level, set to zero.

Figure 3

(a) Summary of the HOMO-emission peak-maximum binding energies measured for pentacene/HOPG (spectra shown in Fig. 2) at room temperature and at 120 K. (b) Fit of Eq. (3) to the experimental data at 120 K. The error bars shown in (a) refer to the absolute error in BE, the symbol size indicates the error obtained for BE shifts of subsequent measurements.

Figure 4

(a) Infrared absorption spectra of 12 nm pentacene on HOPG and ${\mathrm{SiO}}_2$. The three labeled peaks correspond to CH out-of-plane bending modes. (b) Comparison of the pentacene HOMO region PES line shape for (i) pentacene/HOPG $h\nu =25\mathrm{eV}$, ${\theta }_e=0°$, (ii) pentacene/HOPG $h\nu =45\mathrm{eV}$, ${\theta }_e=0°$, and (iii) $\mathrm{\text{pentacene}}/{\mathrm{SiO}}_2$ $h\nu =25\mathrm{eV}$, ${\theta }_e=0°$.