In this talk surface-mediated self-assembly and self-alignment of fiber/needle-like nanostructures composed of small molecule organic semiconductors is presented. The nanostructures are formed on the surface of two-dimensional (2D) materials, as graphene, MoS2, and hexagonal boron nitride. To understand the epitaxial relations that governs the self-assembly, two rod-like molecules are chosen for the epitaxial growth: para-hexaphenyl (6P) and dihydrotetraaza-heptacene (DHTA7). 6P is a model molecule for understanding growth and interfacial phenomena in organic electronics [1]. In comparison to 6P with a phenylene backbone (armchair-like), DHTA7 has similar backbone length and is comparably stable to prolonged air exposure [2]. However, in contrast to 6P, DHTA7 has acene-based backbone (zigzag-like) and is not forming a herringbone crystal structure – as is the case with 6P – due to dipolar interactions and hydrogen bonding between neighboring molecules. Both molecular species grow as elongated crystallites on 2D materials [3-5], with individual molecules oriented in a way that maximizes π-network overlap with the substrate (flat-lying molecules) and therefore maintain rotational commensuration with the substrate, which is a key to self-alignment.
Organic molecules are deposited using a hot-wall epitaxy technique (near-equilibrium growth conditions), which allows a formation of over 100 µm long individual crystallites. By atomic force microscopy (AFM) analysis of the preferred growth directions and knowing the preferred adsorption sites of individual molecules obtained from density functional theory calculations, contact planes are revealed. Furthermore, several examples on charge transfer, peculiar nanomechanics, and opto-electrical properties of these nanostructured van der Waals interfaces will be presented [6,7].

[1] Hlawacek, G., et.al., Nano Lett. 11, 333 (2011).
[2] Matković, A., et.al., Surf. Sci. 678, 128 (2018).
[3] Kratzer, M., et.al., Appl. Phys. Lett. 106, p.103101 (2015).
[4] Kratzer, M. and Teichert, C., Nanotech. 27, p.292001 (2016).
[5] Matković, A., et.al., Sci. Rep. 6, p.38519 (2016).
[6] Matković, A., et.al., Sci. Rep. 7, p.9544 (2017).
[7] Vasić, B., et.al., Nanoscale 10, pp.18835 (2018).