Real-Time Observation of Structural and Orientational Transitions during Growth of Organic Thin Films

We study kinetically controlled orientational and structural transitions of molecular thin films during growth in situ and in real time, using diindenoperylene (DIP) as an example. By time-resolved surface-sensitive x-ray scattering (out of plane and in plane), we follow the organic molecular beam deposition of DIP on silicon oxide, on stepped sapphire, and on rubrene as an organic model surface. We identify transitions for the few-monolayer (ML) regime, as well as for thick (several 10’s of ML) films. We show that the differences in the interaction of DIP with the substrate change the thickness as well as temperature range of the transitions, which include (transient) strain, subtle changes of the orientation, as well as complete reorientation. These effects should be considered rather general features of the growth of organics, which, with its orientational degrees of freedom, is qualitatively different from growth of inorganics.

Figure 1

(a) The DIP molecule and a schematic of standing ($\sigma$) and lying ($\lambda$) structures. (b) Evolution of the real-time x-ray reflectivity as a function of time (i.e., film thickness) during growth on silicon oxide (at 130 °C). (c) Layer coverages $\theta (t)$ as obtained from fits to the above real-time data. A change from layer-by-layer growth to rapid roughening is clearly discernible after $\sim 10\mathrm{ML}$ (see arrow).

Figure 2

In-plane structure: Real-time GID scans during growth of DIP on silicon oxide (130 °C). The in-plane (11)-reflection of the $\sigma$ structure shifts during growth of the second ML, showing that the first ML has to expand (time axis scaled to be comparable to Fig. 1b.

Figure 3

GID for a $d=310\AA$ DIP film grown on silicon oxide at $-5°\mathrm{C}$ (post-growth). The dependency of the (001) $\lambda$ reflection on x-ray penetration depth $\Lambda$ shows that the $\lambda$ structure is growing on top of the $\sigma$ structure.

Figure 4

(a) Evolution of the characteristic (100) $\lambda$ and (11) $\sigma$ reflections as a function of time (film thickness), for growth on rubrene (10 °C), silicon oxide, and stepped sapphire (both at 35 °C). Stronger interactions with the substrate promote earlier nucleation of the $\lambda$ structure. (b) Schematic showing influence of substrate temperature and strength of interaction with substrate on the orientational transition from $\sigma$ to $\lambda$ structures.