Center for Functional Nanomaterials Seminar

The nucleation and film growth of pentacene (C22H14, Pn) and 6,13-pentacenequinone (C22H12O2, PnQ) were extensively investigated to elucidate the role of molecular and crystal packing anisotropy in nucleation and island evolution in organic molecular films. Dynamical evolution were captured in real time low-energy electron microscopy (LEEM)/photo-emission electron microscopy (PEEM) and micro-LEED patterns were recorded in LEEM experiments to obtain local crystalline structures. LEEM study was further complemented by scanning probe microscopy and density functional theory calculations.
In real-time LEEM investigation, a delayed nucleation and formation of extraordinarily large Pn grains on semiconducting 3-Bi-Si(111) and on semi-metallic Bi(0001)/Si(111) have been observed [1] in contrast to the growth of Pn on SAMs, SiO2 or wetting layer on clean silicon surfaces. This delayed and very low density nucleation and continuing growth after stopping deposition could be explained by a incorporation-limited growth processes resulted from a large energy barrier for Pn nucleation in standing-up orientation, as the molecule needs to reorient itself from a lying-down, diffusing state in order to build into the crystalline film. If the energy barrier for molecule reorientation is even higher, the growth becomes more complicated, as observed in the cases of Pn grown on fullerene layer [2] or Pn grown on device structures [3] containing differently interacting metal electrodes and dielectric channels.
The molecular tilt with respect to surface normal has been found to introduce asymmetry in LEED patterns obtained in LEEM experiments. Utilizing this asymmetry, the presence of chiral twins within a domain has been imaged in dark-field imaging mode [4]. Molecular tilt is also found to introduce asymmetric incorporation resulting chiral evolution of PnQ film [5].