van der Waals Epitaxy of Organic Semiconductor Thin Films on Atomically Thin Graphene Templates for Optoelectronic Applications

Abstract

Conspectus Organic semiconductors (OSCs) offer unique advantages with respect to mechanical flexibility, low-cost processing, and tunable properties. The optical and electrical properties of devices based on OSCs can be greatly improved when an OSC is coupled with graphene in a certain manner. Our research group has focused on using graphene as a growth template for OSCs and incorporating such high-quality heterostructures into optoelectronic devices. The idea is that graphene’s atomically flat surface with a uniform sp2 carbon network can serve as a perfect quasi-epitaxial template for the growth of OSCs. In addition, OSC–graphene heterostructures benefit from graphene’s unique characteristics, such as its high charge-carrier mobility, excellent optical transparency, and fascinating mechanical durability and flexibility. However, we have often found that OSC molecules assemble on graphene in unpredictable manners that vary from batch to batch. From observations of numerous research systems, we elucidated the mechanism underlying such poor repeatability and set out a framework to actually control the template effect of graphene on OSCs. In this Account, we not only present our scientific findings in this spectrum of areas but also convey our research scheme to the readers so that similar heterostructure complexes can be systematically studied. We began with experiments showing that the growth of OSCs on a graphene surface was driven by van der Waals interactions and is therefore sensitive to the cleanliness of the graphene surface. Nonetheless, we noted that, even on similarly clean graphene surfaces, the OSC thin film still varied with the underlying substrate. Thanks to the graphene-transfer method and in situ gating methods that we developed, we discovered that the decisive parameter for molecule–graphene interaction (and, hence, for the growth of OSCs on graphene) is the charge density in the graphene. Thus, to prepare a graphene template for high-quality graphene–OSC heterostructures, we controlled the charge density in the graphene to minimize the molecule–graphene interaction. Moreover, the possible charge transfer between OSC molecules and graphene, which induces additional molecule–graphene interactions, should also be taken into account. Eventually, we demonstrated a wide range of optoelectronic applications that benefitted from high-quality OSC–graphene heterostructures fabricated using our proof-of-concept systems.

Heteroepitaxy in Organic/TMD Hybrids and Challenge to Achieve it for TMD Monolayers: The Case of Pentacene on WS2 and WSe2

“…This recumbent orientation is seemingly a general phenomenon as it was also observed before for PEN films grown on the basal plane of graphite, MoS 2 or h BN ,,, and can be understood by a substrate-mediated template effect. At low (i.e., submonolayer) coverage, acene molecules adsorb lying flat as this adsorption geometry maximizes the interaction of the molecular π-system with the substrate of 2D materials, which was also found for other π-conjugated molecules . However, the situation changes with increasing film thickness, since the mutual intermolecular interaction in the crystalline molecular film can exceed the molecule–TMD interaction, as shown experimentally in previous works for the case of PEN on graphite and MoS 2 . , Nevertheless, with the further growth of PEN and its per-fluorinated counterpart perfluoropentacene, the formation of epitaxial crystalline multilayer films with recumbent molecular orientation has been observed when these are deposited on highly ordered basal levels of crystalline 2D materials, hence suggesting that the epitaxial order stabilized such films. , It is noteworthy that such orientations are only possible for such large acene molecules, while for smaller acenes, which have a smaller lattice energy and a higher diffusivity, films with a recumbent molecular orientation are not possible.…”

“…This recumbent orientation is seemingly a general phenomenon as it was also observed before for PEN films grown on the basal plane of graphite, MoS 2 or hBN 21,22,29,32 and can be understood by a substrate-mediated template effect. At low (i.e., submonolayer) coverage, acene molecules adsorb lying flat as this adsorption geometry maximizes the interaction of the molecular π-system with the substrate of 2D materials, 33 which was also found for other π-conjugated molecules. 34 However, the situation changes with increasing film thickness, since the mutual intermolecular interaction in the crystalline molecular film can exceed the molecule−TMD interaction, as shown experimentally in previous works for the case of PEN on graphite and MoS 2 .…”

Role of Interfacial Interactions in the Graphene-Directed Assembly of Monolayer Conjugated Polymers

“…In addition to the existence of weak van der Waals interactions, the structural similarities between the π-conjugated core of OSCs and graphene result in strong bonding between the two materials through π–π interactions. Exploiting strong electronic coupling allows the epitaxial growth of OSCs on graphene substrates. − Moreover, graphene modulates the orientation of OSCs, often forcing the molecules to arrange co-facially on graphene; this has been evident on both thermally evaporated , and solution-processed OSC films . Similar to small molecules, graphene induces a face-on orientation or increases the molecular tilt of conjugated polymers. − As a wetting-transparent template, , the surface energy of graphene is also tuned by modulating the chemistry of the underlying substrates, thus providing unique opportunities to further engineer the templating effect of graphene.…”

Solution-Processed Isoindigo- and Thienoisoindigo-Based Donor–Acceptor–Donor π-Conjugated Small Molecules: Synthesis, Morphology, Molecular Packing, and Field-Effect Transistor Characterization

“…Over the past few decades, the synthesis of π-conjugated semiconductors has received significant attention compared to their inorganic counterparts because of the increasing demand for solution-processed and flexible organic electronics. − Many efforts have been directed toward the rational molecular design of π-conjugated electroactive and side-chain solubilizing units together with the development of novel processing strategies and device architecture/interface, which demonstrate the significant increase in charge mobility of organic field-effect transistors (OFETs) beyond that of amorphous silicon (0.5–1 cm 2 V –1 s –1 ). − An interesting challenge facing solution-processed organic semiconductors is the complex relationship between the chemical structures, thin-film morphologies, and the resulting electronic properties. − Therefore, exploring how chromophore structures and processing techniques impact the formation of the organic semiconductor film for order solid-state packing and specific charge transport behavior need to be further investigated.…”