Epitaxy of highly ordered organic semiconductor crystallite networks supported by hexagonal boron nitride

Abstract

This study focuses on hexagonal boron nitride as an ultra-thin van der Waals dielectric substrate for the epitaxial growth of highly ordered crystalline networks of the organic semiconductor parahexaphenyl. Atomic force microscopy based morphology analysis combined with density functional theory simulations reveal their epitaxial relation. As a consequence, needle-like crystallites of parahexaphenyl grow with their long axes oriented five degrees off the hexagonal boron nitride zigzag directions. In addition, by tuning the deposition temperature and the thickness of hexagonal boron nitride, ordered networks of needle-like crystallites as long as several tens of micrometers can be obtained. A deeper understanding of the organic crystallites growth and ordering at ultra-thin van der Waals dielectric substrates will lead to grain boundary-free organic field effect devices, limited only by the intrinsic properties of the organic semiconductors.

Light‐Assisted Charge Propagation in Networks of Organic Semiconductor Crystallites on Hexagonal Boron Nitride

“…Such pattern is substrate‐mediated, and follows the tri‐fold rotation and mirror symmetry of the hBN lattice. Thus, in between the chiral‐paired growth directions lies a high symmetry direction of the underlying substrate . Considering the preferred adsorption site of individual molecules on hBN (Figure e), it is expected that the long molecular axis is aligned with the zigzag direction of hBN.…”

“…In the case of vdW epitaxy, rotational commensuration is maintained in spite of the lattice mismatch . Preferred adsorption sites of individual molecules, combined with the LMA‐LNA relation, and point‐group symmetry of the substrate will result in the specific growth directions of the crystallite network . Since DHTA7 has an acene backbone, its preferred adsorption on hBN is with its LMA in a zigzag direction, as obtained by DFT calculations.…”

“…It is worth mentioning that self‐assembly and self‐alignment of elongated nanostructures of small rod‐like molecules is possible only on certain crystalline substrates, most of which are conductive. Exceptions are van der Waals dielectric surfaces, where many acene‐ and phenylene‐based molecules tend to nucleate in a flat lying orientation . On conventional dielectric surfaces (see Figure S1, Supporting Information), DHTA7 exhibits quasi layer‐by‐layer growth with the molecular backbone being almost perpendicular to the substrate plane, and therefore self‐assembly and self‐alignment of the elongated nanostructures is not possible.…”

“…While the applications above mentioned rely on extended organic semiconductor layers on hBN, an interesting alternative approach is to use self‐assembled and self‐aligned quasi‐1D nanostructures . On hBN, formation of quasi‐1D needle‐like crystallites has been demonstrated for spin‐coated oleamides and vapor deposited phenylenes …”

“…The use of organic semiconductors in vdW heterostructures offers many advantages; since the organic layers or nanostructures form through self‐assembly, there is no need for elaborate transfer procedures, and the resulting heterostructures are self‐aligned with respect to the high‐symmetry directions of their 2D material support . Furthermore, organic chemistry offers almost an infinite variety of molecular “building blocks.” Especially, adding functional groups to the backbone of the molecules has been a well‐established molecular design pathway to engineer desired structural, transport, and opto‐electronic properties .…”