Toward Single-Crystal Hybrid-Carbon Electronics: Impact of Graphene Substrate Defect Density on Copper Phthalocyanine Film Growth

Abstract

Graphene has long been recognized as a potential replacement for indium tin oxide as a transparent conducting substrate that may not only be cheaper to manufacture but also may provide mechanical flexibility and templating for preferential organic film growth. Here, we report the discovery that the thin film growth mode and crystal structure of copper phthalocyanine (CuPc), a prototype organic semiconductor, is extremely sensitive to even atomic-scale defects (e.g., steps) on the graphene surface and that high quality films can be grown with a well-defined crystal orientation that should be favorable for optimized solar cell applications. The initial growth involves flat-lying copper phthalocyanine molecules in a triclinic brickstone crystal with (012̅ ) orientation. Thicker films on pristine graphite, as well as thin films on lower quality graphene, show an orientational transition to the flat-lying (112̅ )-oriented brickstone, which nucleates near film defects and grows in more compact 3D islands. The thickness dependent transition between these two flat-lying crystal orientations is sensitive to substrate defect density since this determines the extent to which initial island elongation along the substrate can alleviate strain in the film. The sensitivity of copper phthalocyanine film morphology demonstrates the extreme importance of graphene substrate quality to controlling organic film growth. The high crystallinity and optimal molecular orientation achieved here implies a new driving force for high performance organic optoelectronics based on substrate-controlled crystal engineering.

Morphological, Optical, and Electronic Consequences of Coexisting Crystal Orientations in β-Copper Phthalocyanine Thin Films

“…Additionally, CuPc thin films are known to exhibit coexisting crystal structures and crystal orientations. For example, thickness-dependent morphology transitions from α-herringbone (200) to a brickstone (100) crystal structure on a ZnO(1100) substrate and from brickstone (012̅) to brickstone (112̅) on a graphite substrate have been reported. β-CuPc has been reported to adopt several crystal orientations dependent upon the substrate and fabrication method used, including (010), , (100), − (011), (1̅01), (302), (201̅), − (001), , and (202̅) orientations.…”

Growing Ultra-flat Organic Films on Graphene with a Face-on Stacking via Moderate Molecule-Substrate Interaction

“…For example, Si substrate is doped degenerately to reduce the molecule-substrate interaction such that thick face-on oriented films can be grown without significant surface roughening 16 . A few studies show that graphene is a good template for growing uniform organic films with a face-on stacking 23 24 25 26 . However, these studies mainly focus on the structural and electrical characterization of the organic layer grown on graphene.…”

Influence of the Underlying Substrate on the Physical Vapor Deposition of Zn-Phthalocyanine on Graphene

“…12,13 More recently, there has been a growing interest for integrating graphene into organic electronic devices in order to exploit its combination of flexibility, excellent conductivity, and high optical transparency. 14−16 Hence, various studies investigated the molecule−substrate interactions and the self-assembly of the MPc structure deposited on graphene epitaxially grown on SiC(0001), 17,18 Ru(0001), 19 Ni(111), 20,21 Pt(111), 22 and so forth. While these studies achieved important milestones in better understanding the interfacial coupling between MPc molecules and graphene, they mostly focused on selfassembled ultrathin (i.e., submonolayer) MPc films deposited in ultrahigh vacuum equipment.…”

“…Significant research effort has been dedicated to understanding the MPc/substrate interface in order to produce high-quality thin films with improved electron transfer and light absorption . The MPc/substrate interaction was first studied using single-crystalline metals such as Cu(111), − Au(111), ,, and Ag(111). , More recently, there has been a growing interest for integrating graphene into organic electronic devices in order to exploit its combination of flexibility, excellent conductivity, and high optical transparency. − Hence, various studies investigated the molecule–substrate interactions and the self-assembly of the MPc structure deposited on graphene epitaxially grown on SiC(0001), , Ru(0001), Ni(111), , Pt(111), and so forth. While these studies achieved important milestones in better understanding the interfacial coupling between MPc molecules and graphene, they mostly focused on self-assembled ultrathin (i.e., submonolayer) MPc films deposited in ultrahigh vacuum equipment.…”