DOI 10.1140/epje/i2003-10013-y Eur. Phys. J. E 11, 169–175 (2003) T HE EUROPEAN P HYSICAL JOURNAL E Structure and polarity of 8CB films evaporated onto solid substrates I. Drevenˇ sek Olenik 1,2, a , K. Koˇ cevar 2 , I. Muˇ seviˇ c 1,2 , and Th. Rasing 3 1 Faculty of Mathematics and Physics, University of Ljubljana, Jadranska 19, SI 1000, Ljubljana, Slovenia 2 J. Stefan Institute, Jamova 39, SI 1000, Ljubljana, Slovenia 3 Research Institute for Materials, University of Nijmegen, Toernooiveld 1, 6525 ED Nijmegen, The Netherlands Received 27 January 2003 and Received in final form 6 May 2003 / Published online: 25 June 2003 – c  EDP Sciences / Societ`a Italiana di Fisica / Springer-Verlag 2003 Abstract. Brewster-angle reflection ellipsometry and surface optical second harmonic generation were used to study the growth of 4  -n-octyl-4-cyanobiphenyl (8CB) films evaporated in air onto polymeric and quartz glass substrates. The layer-by-layer growth of the films terminates after formation of two distinctive inter- facial layers. Both of these two layers are polar and tilted. In the first layer the molecules lie nearly flat on the surface, while in the second layer they point on average about 50 ◦ toward the surface normal. The dipole moment of the second layer has a lower magnitude and an opposite direction with respect to the dipole moment of the first layer. PACS. 61.30.Hn Surface phenomena: alignment, anchoring, anchoring transitions, surface-induced layer- ing, surface-induced ordering, wetting, prewetting transitions, and wetting transitions – 68.03.Fg Evap- oration and condensation – 42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation 1 Introduction Spatial alignment of liquid crystals, which is a prerequisite for operation of essentially all kinds of liquid-crystal de- vices, is conventionally attained by use of specially treated solid substrates that induce a desired surface anchoring of the liquid-crystalline material. The aligning action is transposed from the substrate to the bulk liquid crystal via a specific interfacial layer of the mesogenic molecules adjoining the substrate. Due to short-range surface forces of the substrate, the orientational and positional ordering of the molecules within the interfacial layer are generally significantly different from the bulk order [1,2]. The vicin- ity of a surface can, for instance, induce smectic liquid- crystal layering that exists far above the temperatures of the bulk smectic phase [3,4]. Although the features of the interfacial layer are crucial for understanding and also for manipulation of the liquid-crystal surface alignment, they are still far from being fully understood. Surface forces acting on mesogenic molecules are typi- cally probed by investigating submonolayer to few mono- layers thick films of liquid crystals on various substrates. Such films can be achieved, for example, by spreading a droplet of a liquid crystal on the selected substrate. In case of a complete wetting the spreading progresses via a e-mail: irena.drevensek@ijs.si a nanoscopic precursor film, the structure of which de- pends on the polarity of mesogenic molecules. For strongly polar molecules, like the well-known alkyl-cyanobiphenyls (nCB), the precursor film is composed of a contact mono- layer covered with an interdigitated bilayer [5–13]. A sim- ilar trilayer interfacial film is formed also during compres- sion of the nCB molecules at the air-water interface [14– 20]. It is generally found that the first monolayer of the interfacial film is strongly tilted and usually also highly po- lar, while the structure of the bilayer is much more unclear. Most of the investigations indicate that this bilayer is equivalent to the layers of the bulk smectic phase, i.e. has a homeotropic molecular orientation with respect to the surface and possesses no net dipole moment. Some con- tact potential measurements, however, show that also the bilayer may exhibit a considerable amount of polarity [11, 12,18]. The structural origin of this feature is not yet re- solved, nor its relation to the polarity of the substrate. The large molecular dipole moment of nCB molecules (∼ 6 D) is assumed to play a critical role also in de- termining the growth characteristics of ultrathin films of these materials by Lagmuir-Blodgett (LB) and thermal evaporation techniques. According to the recent review by Bardosova and Tredgold none of the several attempts to achieve a homogeneous multilayer nCB structure has been successful up till now, as the adsorbate always solidifies into droplets regardless of the type of the substrate [21].