Sub-Å Resolution Electron Density Analysis of the Surface of Organic Rubrene Crystals

The electron density distribution of an organic semiconductor is observed as a function of the depth from the crystal surface or interface. The surface x-ray scattering technique combined with a recently developed analyzing technique, coherent Bragg rod analysis, enables us to observe the electron density profile. The obtained near-surface electron density profile of a single crystal of rubrene, which is known as a high-mobility organic transistor material, shows not only a large positional distribution of the molecules at the surface, but also a sub-Å molecular deformation that affects the molecular orbital.

Figure 1

(a) The structure of rubrene single crystal [7]. (b) The CTR profile along the $00\zeta$ rod. Gray squares show the CTR profile from the substrate, and open circles show the CTR profile from the rubrene. The dashed curve shows the simulated curve with $B=B_S=3.0$, the (red) thin solid curve shows that with $B_S=12.0$, and the (blue) thick curve shows the result of the COBRA (see text).

Figure 2

The depth dependence of the electron density $\rho (z)$. A schematic view of the rubrene molecules is also shown to present the position and the size of the molecules.

Figure 3

(a) The electron density profile for the molecule at the first layer, Rub(1), and the third layer, Rub(3) extracted from Fig. 2 by multi-Gaussian fitting. The profile for $[\rho (z-0.25\AA )+\rho (z+0.25\AA )]/2$ for Rub(3) is also shown. (b) Molecular size defined by the sum of the peak distance between peaks 1 and 6 and the values of the half width at half maximum for them as a function of the depth. (c) Peak widths of the six peaks as functions of the depth. (d) Depth dependence of the intratetracene peak distance.