Thickness and Substrate Dependent Thin Film Growth of Picene and Impact on the Electronic Structure

Abstract

Thickness and substrate dependence of film growth, morphology, unit-cell structure, and electronic structures was thoroughly investigated for picene, the zigzag connected 5-ring molecule, by employing complementary techniques of in situ real-time X-ray reflectivity/diffraction, in situ electron spectroscopies, and atomic force microscopy. A different kind of thickness dependent structural transition was observed on SiO2 and graphite, resulting in a distinct electronic structure. On SiO2 picene films with 3D crystalline domains are formed with nearly upright molecular orientation from the initial growth stage. With increasing the film thickness the in-plane dimensions of the unit cell in the initially grown domains become smaller (in other words, more compressed), and, at the same time, crystalline domains with a more relaxed structure are nucleating on top of the compressed domains. In spite of such structural changes, the electronic structure, namely energy position of the highest occupied molecular orbital and threshold ionization potential (IPT), is not significantly altered. On graphite, on the other hand, we found a transition from a 2D (layer) to a 3D (island) growth mode with a variation of the molecular orientation from flat-lying to tilted one. The IPT changes significantly between the 2D and 3D growth regime in contrast to the SiO2 system. The origin of the different IPTs of these picene thin films is discussed. The present results are compared with other planar π-conjugated compounds, in particular pentacene which is a structural isomer of picene and shows electronic properties strongly different from picene thin films.

Delayed phase separation in growth of organic semiconductor blends with limited intermixing

“…For the PIC‐dominated mixture (Fig. a) the peaks coincide with those observed for pure PIC corresponding to two different phases H and L . The peaks of the L phase are clearly more pronounced compared to the ones of the H phase for small angles of incidence where the surface of the film contributes more to the signal than for larger angles of incidence.…”

“…The peaks of the L phase are clearly more pronounced compared to the ones of the H phase for small angles of incidence where the surface of the film contributes more to the signal than for larger angles of incidence. This is comparable to pure PIC where the H domains nucleate prior to the L domains . For the PEN‐dominated mixture only peaks of the mixed phase are observed without an apparent difference for the two angles of incidence.…”

“…In both systems in the blends dominated by the shorter compound (PIC respectively 6T) phase separation occurs. Interestingly in the pure PIC and 6T thin films different polymorphs nucleate, thus this could be another reason for the phase separation.…”

“…Mixing scenarios comprise extreme cases such as continuous statistical intermixing, formation of mixed crystal phases with defined stoichiometry and phase separation depending on sterical properties and chemical composition . Recently, a study of the structural properties of the OSC picene (C 22 H 14 , PIC) was reported and showed changes of the in‐plane structure with increasing film thickness . For mixtures of PIC and the structurally quite similar isomeric (see Fig.…”

Picene thin films on metal surfaces: Impact of molecular shape on interfacial coupling

“…The IEs for 100 Å picene coverage are 5.93 eV (on Ag[111]) and 5.97 eV (on Ag[110]). For comparison, a monolayer of flat lying picene on HOPG has an IE of 6.3 eV to 6.44 eV . In multilayers on HOPG picene adopts a herringbone arrangement and has an IE 5.94 eV and the IE of standing picene in multilayers on SiO 2 is ∼5.6 eV .…”