The impact of substrate induced phases and crystal phase

All performed experiments conclusively show that terminal fluorination of 6P causes a transition from a 3d growth for the parent 6P to an almost 2D growth mode for 6PF4 on ZnO(1010). In the following we discuss factors, which may contribute to this pronounced change.

The fluorination of 6P in the four “corner” positions changes intermolecular elec-trostatic interactions through the introduction of two local dipole moments at both molecular termini. The crystal cohesive energy will thus be different for the two molecules. Furthermore, a flat-lying 6PF4 molecule, diffusing on the surface of an island exposing fluorine atoms at its boundary, will experience a substantially altered electrostatic field as compared to 6P, where the islands are presenting hydrogen at their periphery. As a consequence, the activation energy for molecules diffusing on islands as well as the Ehrlich-Schwoebel barrier, both critical parameters in a kinetically controlled growth process, will differ. Indeed, an increase of the Ehrlich-Schwoebel barrier with increasing crystal cohesive and surface binding energies has been predicted.^[40] Our result of 6PF4 layer-by-layer growth therefore suggests that both cohesive as well as binding energies are reduced by terminal fluorination. Fur-ther theoretical input is required to verify the sign and to assess the magnitude of the change in the energy terms.

The physical size of 6P and 6PF4 molecules is nearly identical. However, changes in the intermolecular electrostatic interactions as well as possibly weak C-H· · ·F-H bonding may cause subtle changes in the structure of the bulk phase of the films, entailing the larger molecular tilt angle of 6PF4 than for 6P. The molecular tilt angle has previously been identified to influence the magnitude of the Ehrlich-Schwoebel barrierEES,^[39,206] which is the energy barrier for interlayer diffusion. For rod-like molecules, a larger molecular tilt has been related to a smaller step edge barrier, accounting for the lower energy required to bend a molecule around an inclined island boundary during the downward motion. Consequently, the here observed molecular orientation of 6PF4 in the bulk phase implies a lowerE_ES and contributes to enhanced downward mass transport, facilitating layer-by-layer growth. Inspecting the 6PF4 crystal structure close to the interface, we, however, evaluate a surface induced structure with the molecular tilt angle being smaller compared to the bulk.

From our measurements we deduce that the tilt angle changes gradually with film thickness. We therefore observe a change from more upright standing to slightly tilted molecules in the first 3 MLs. With regard to the Ehrlich-Schwoebel barrier one would thus expect that the more upright standing surface induced phase, increases the E_ES and hinders layer-by-layer growth at the initial growth stage. However, according to our results nucleation of the second layer starts, when the ZnO surface is almost completely covered by islands. This suggests that the effect of a largerE_ES in the first 6PF4 ML might be counterbalanced by a better molecular diffusivity due to a lower surface energy. Its minimisation is the driving force behind the surface

induced structure, so that for more upright standing molecules the surface energy is apparently lower than for tilted molecules as found in the bulk phase of the film.

A lower surface energy on top of the first monolayers leads to enhanced diffusivity of admolecules and therefore contributes to better layer-by-layer growth. Similar to the present case, a surface induced phase with more upright standing molecules has been correlated with layer-by-layer growth in the similar molecular system di-indenoperylene.^[19,83] Another significant aspect, which may influence the divergent roughness evolution of the thin films, is the difference in the crystal structure of 6P and 6PF4. While in 6P theβ- andγ-phases grow in coexistence, crystal-phase purity and, therefore, the growth of only one crystal phase is observed in 6PF4 films. In 6P, the grain boundaries occurring between different phases can act as nucleation sites for succeeding layers and, thus, may contribute to the observed film roughening.

5.3.7 Summary

By applyingin situ real-time X-ray reflectivity measurements, we demonstrate how terminal fluorination of 6P drastically alters the growth mode and the resulting crystal structure of molecular thin films. In our study, fluorination enhances downward mass transport through a combination of increased surface diffusivity and a lower Ehrlich-Schwoebel barrier, leading to a smooth, almost layer-by-layer thin film growth.

Characteristic features of the thin films are summarized in Fig. 5.12.

2.72nm γ-phase

β-phase

2.60nm 2.60nm

2.60nm

2.72nm 2.72nm

2.64nm 2.46nm 2.42nm

6P

6PF4

a)

ZnO (1010) ZnO (1010)

Figure 5.12: Schematic drawing illustrating the rough 6P growth with two different polymorphs in a) and the smooth 6PF4 growth with a surface induced structure in b).

We find that both molecules grow in a highly crystalline fashion with almost upright standing molecules when deposited at room temperature on ZnO(1010). We further demonstrate that the fluorinated molecule grows in a phase pure manner with a gradually increasing molecular tilt angle in the first three MLs. In contrast 6P thin films exhibit a simultaneous growth ofβ- and γ-phases and we find no evidence

for a changing tilt angle. However, the most striking difference is the change from 3d for 6P to an almost 2d morphology for its fluorinated derivative. Our results demonstrate that selective fluorination can decisively alter growth processes and film roughness and therefore presents a viable strategy to produce crystalline organic layers with thin film morphologies as demanded for the use in opto-electronic devices.