Real‐Time Microscopy of Reorientation Driven Nucleation and Growth in Pentacene Thin Films on Silicon Dioxide

Abstract

The role of molecular reorientation processes in the self‐assembly of anisotropic molecules, such as pentacene (Pn) is studied utilizing a unique capability of low‐energy electron microscopy (LEEM) for the real‐time investigation of the film growth. In Pn film on SiO2, a layer‐by‐layer growth is observed, albeit different from the expected Volmer–Weber growth mode typical for the systems with lower adhesion (weak interfacial interaction). The observed growth mechanism is also different than conventional concept of layer‐by‐layer, or Frank van der Merwe growth. In the Pn/SiO2 system the nucleation density decreases in each consecutive layer, at least up to four monolayers. This growth mechanism is hereafter named inverse Stranski‐Krastanov growth. Furthermore, in this growth system the second layer islands nucleate preferentially at the domain boundaries formed by the interconnections of the bottom (first layer) domains. The top layer overgrows bottom layer with its own, initial in‐plane crystal orientation, regardless of the in‐plane orientations in underlying Pn domains. The dark‐field LEEM imaging allows us to distinguish between Pn domains having different azimuthal direction of molecular tilt. LEEM intensity versus start voltage (LEEM I–V) curves taken in the vicinity of mirror potential from the first and second layer Pn islands show that the surface potential of the second layer is higher by about 0.05 eV than that of the first layer, while the surface potentials for the epitaxial and non‐epitaxial parts of the second layer island are identical.

Interface energetics in zinc phthalocyanine growth on Ag(100)

“…This introduces an attachment barrier E at . In our previous reports on growth of films made of anisotropic molecules [20,34], where the molecules prefer to be at different orientations at diffusive state (lying down) than that in crystalline islands (standing up), we found that the energy barrier for molecule reorientation causes a delay in the nucleation of crystalline islands. In that experimental system the incorporation-limited growth model was proposed.…”

“…The shape anisotropy of a planar molecule such as phthalocyanine is expected to make its aggregation processes complex, in a similar fashion to film growth of other anisotropic molecules [17][18][19][20]. Moreover, the metal center is accessible from both sides of the molecular plane.…”

“…The MPc is a planar, rigid molecule, consisting of large number of atoms (49), strongly interacting with Ag surfaces [1]. Upon incorporation into a crystalline film, the molecule must assume a specific orientation and thus we expect that the anisotropy in rigid atomic arrangements and atomic bonding configuration within the molecule will result in a reorientation limited mechanism governing the growth of crystalline film [17][18][19][20].…”

Pentacene thin films on ferromagnetic oxide: Growth mechanism and spintronic devices

“…While the LSMO surface looks completely covered when a 3 nm pentacene film is deposited at 30 C, the coverage is not complete and the wetting layer disappears when pentacene is deposited above 40-45 C. The thermal energy at 40-45 C is sufficient to overcome the interaction of pentacene molecules with the substrate, while on SiO 2 a temperature of at least 80 C is needed. 24 The flat morphology over a large area shown in Figures 3(b)-3(e) has been previously observed only on SiO 2 , and only when the growth was assisted by a supersonic gas flow that gave molecules a high kinetic energy and hence a high surface diffusivity. 30 The analysis of the effective coverage, obtained by measuring the volume of the islands, 15 shows that it decreases from 75% at a substrate temperature of 45 C to 55% at 75 C: at this temperature desorption could start to play a significant role.…”

“…23 ES barrier-controlled growth leads to a three dimensional growth which results in a roughening of the film. 24 This roughening is called "rapid" because it grows with the film thickness faster than in the random deposition model (RD), where molecules attach where they arrive, without diffusion. 25 In general, it is valid that R rms $ t b , where t is the nominal film thickness and b is the growth exponent; in the RD model b ¼ 0.5.…”