Surface-Induced Orientation Control of CuPc Molecules for the Epitaxial Growth of Highly Ordered Organic Crystals on Graphene

Abstract

The epitaxial growth and preferred molecular orientation of copper phthalocyanine (CuPc) molecules on graphene has been systematically investigated and compared with growth on Si substrates, demonstrating the role of surface-mediated interactions in determining molecular orientation. X-ray scattering and diffraction, scanning tunneling microscopy, scanning electron microscopy, and first-principles theoretical calculations were used to show that the nucleation, orientation, and packing of CuPc molecules on films of graphene are fundamentally different compared to those grown on Si substrates. Interfacial dipole interactions induced by charge transfer between CuPc molecules and graphene are shown to epitaxially align the CuPc molecules in a face-on orientation in a series of ordered superstructures. At high temperatures, CuPc molecules lie flat with respect to the graphene substrate to form strip-like CuPc crystals with micrometer sizes containing monocrystalline grains. Such large epitaxial crystals may potentially enable improvement in the device performance of organic thin films, wherein charge transport, exciton diffusion, and dissociation are currently limited by grain size effects and molecular orientation.

Van Der Waals Heterostructures between Small Organic Molecules and Layered Substrates

“…On bare glass, elongated CuPc grains, 94 ± 34 nm long and 35 ± 6 nm wide, are observed with a root mean squared film roughness of 2.0 nm, and similar to previous AFM observations of standing up oriented CuPc [46,47].Round CuPc grains are observed on a few layers of G (FLG) with a greater roughness of 4.2 nm. The 2D-GIXRD pattern in Figure 4d for CuPc on bare glass only shows one single sharp peak concentrated on the meridian (q xy = 0), implying that CuPc molecules are oriented perpendicularly to the surface of the glass substrate, similarly on Si [48]. On the contrary, the pattern in Figure 4e shows multiple new diffraction points for CuPc on FLG-covered glass.…”

“…Thus, CuPc adopts a near face-on orientation with the molecular plane tilted on average 9 • with respect to the substrate. The continued appearance of the (100) reflection along the meridian indicates that there are still some crystallites adopt the edge-on configuration in the film [48]. Conductive atomic force microscopy (C-AFM) measurements show that at a single-grain level CuPc on (G covered) PEDOT:PSS/ITO presents a hole mobility of (1.9 ± 0.2) × 10 −3 ((1.6 ± 0.2) × 10 −2 ) cm 2 /Vs.…”

Supramolecular Approaches to Graphene: From Self‐Assembly to Molecule‐Assisted Liquid‐Phase Exfoliation

“…A typical STM image of PTCDA Metallophthalocyanines (MPc) represent another class of aromatic compounds investigated on EG grown on various metal substrates such as Ru(0001), Ni(111), Pt (111) or Ir(111), to name a few. [133][134][135][136][137][138][139][140][141] EG has relatively strong interactions with substrates such as Ru(0001) or Ni (111), whereas it interacts weakly with Ir(111) and SiC(0001). [142] While graphene grown on weakly interacting substrates exhibits HOPG-like adsorption landscapes, once grown on strongly interacting substrates it displays moiré superlattice, which strongly affects the molecular physisorption.…”

Growth of Metal Phthalocyanine on Deactivated Semiconducting Surfaces Steered by Selective Orbital Coupling

“…Metal phthalocyanine (MPc) is one of the most frequently studied planar molecules in organic molecular thin film growth on a variety of insulating, semiconducting, and more commonly, metallic surfaces [8,. Of them, metallic substrates provide more flexibility in tuning the MPc-substrate interaction through orbital hybridization and charge redistribution associated with the central transition-metal (TM) ion [16,20,21].…”