Graphene, with its unique electronic and structural qualities, has become an important playground for studying adsorption and assembly of various materials including organic molecules. Moreover, organic/graphene vertical structures assembled by van der Waals interaction have potential for multifunctional device applications. Here, we investigate structural and electrical properties of vertical heterostructures composed of C60 thin film on graphene. The assembled film structure of C60 on graphene is investigated using transmission electron microscopy, which reveals a uniform morphology of C60 film on graphene with a grain size as large as 500 nm. The strong epitaxial relations between C60 crystal and graphene lattice directions are found, and van der Waals ab initio calculations support the observed phenomena. Moreover, using C60-graphene heterostructures, we fabricate vertical graphene transistors incorporating n-type organic semiconducting materials with an on/off ratio above 3 × 10(3). Our work demonstrates that graphene can serve as an excellent substrate for assembly of molecules, and attained organic/graphene heterostructures have great potential for electronics applications.
The distribution of dopant sites in doped poly(3-hexylthiophene) (P3HT) thin films is characterized using optical absorption, grazing-incidence X-ray diffraction, and conducting atomic force microscopy (c-AFM). It is shown that dopant sites can be directly observed using c-AFM and that the solution temperature dramatically impacts phase separation and conductivity in spin-cast films.
Strongly textured organic semiconductor micropatterns made of the small molecule dioctylbenzothienobenzothiophene (C(8)-BTBT) are fabricated by using a method based on capillary force lithography (CFL). This technique provides the C(8)-BTBT solution with nucleation sites for directional growth, and can be used as a scalable way to produce high quality crystalline arrays in desired regions of a substrate for OFET applications.
In this report, the crystallization mechanisms in solution-processed, annealed thin films of semiconducting small molecule 5,6,11,12-tetraphenylnaphthacene (rubrene) blended with three different amorphous polymers (polystyrene (PS), poly(methyl methacrylate) (PMMA), and poly(4-vinylpyridine) (P4VP)) are investigated. The results show that the degree of phase separation, the exact crystal structure, and the electronic properties of the blend films depend strongly on the choice of polymer binder. While rubrene films crystallized from blends with PS and P4VP consist of crystalline spherulites in a mostly orthorhombic crystal structure, rubrene in PMMA blends contains a significant fraction of triclinic phase and is generally more disordered. Structural characterizations also reveal a high degree of vertical phase separation in PS and P4VP films, which is attributed to residual solvent effects in the case of rubrene/PS films and a preferential hydrophilic interaction with the Si/SiO 2 interface for rubrene/P4VP films. This type of phase separation is shown to be critical for crystallization and lead to improved field-effect mobilities. Finally, this processing technique easily allows for patterning of transistors using chemically modified substrates, which is useful for large-scale device fabrication.
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