Predicting surface anchoring: molecular organization across a thin film of 5CB liquid crystal on silicon

Anchoring, i.e. the orientation and ordering of organic molecules at a surface and its persistence away from the interface, is a key phenomenon for liquid crystal (LC) physics and technology, as well as for organic transistors and photovoltaic cells where it strongly affects transport. Recent exciting experiments have started to explore anchoring at the nanoscale, but seemingly conflicting views are emerging, suggesting either a surface order very slowly decreasing with distance, or the expected exponential decay for adhesion energies. The current theoretical description of anchoring is, however, essentially empirical, and methods for predicting the molecular structuring and orientation are conspicuously lacking. Here we establish such predictive tools by using atomistic molecular dynamics simulations and obtain the order and molecular organization of 12 and 24 nm-thick nematic and isotropic films of the popular LC 4-n-pentyl-4-cyano biphenyl (5CB) at its interfaces with an atomically flat, H-terminated, (001) crystalline silicon surface and vacuum. We show that the film adopts a hybrid configuration, with a change of the 5CB preferred orientation from planar uniform at the silicon to perpendicular at the free surface. This variation is nearly discontinuous for the thin film, and continuous with twist and bend for the thicker one. Our findings provide for the first time a full description of the interfacial silicon-LC structure at the nanoscale level.