We show that the substrate flatness and weak vdW interaction are crucial for high-performance monolayer organic transistors because of the minimal disturbance in both crystal growth and charge transport. Therefore, the initial stage of epitaxy relies on the vdW interaction between the molecules and substrate 16. Graphene and BN are also ideal substrates as they are atomically flat without dangling bonds 15. We select C 8-BTBT because it is among the highest mobility small-molecule organic materials 12, 13, 14. Here, we report on the epitaxial growth and OFETs of 2D dioctylbenzothienobenzothiophene (C 8-BTBT) molecular crystals down to monolayer on graphene and hexagonal boron nitride (abbreviated as BN) substrates. Therefore, 2D molecular crystals can open a new paradigm in the applications of layered materials and heterostructures. Furthermore, both carrier injection and modulation would become more efficient to greatly improve the performance of organic field-effect transistors (OFETs). Compared with bulk materials, monolayers could effectively eliminate interlayer screening, thus offer an ideal system to directly probe the effect of disorders 8, 9 and interfaces 10, 11on charge transport. Organic molecular crystals represent an important class of materials for electronic and photonic applications 4, 5, 6, 7. 2D molecular crystals held by van der Waals (vdW) forces, on the other hand, are rarely reported. Most of the 2D materials studied so far, including graphene, chalcogenides and oxides, are inorganic atomic crystals with strong in-plane chemical bonds 3.
The research on two-dimensional (2D) layered materials has been thriving for a decade now since the discovery of graphene 1, 2, 3. Our work unveils an exciting new class of two-dimensional molecular materials for electronic and optoelectronic applications. As a result, monolayer dioctylbenzothienobenzothiophene molecular crystal field-effect transistors on boron nitride show record-high carrier mobility up to 10 cm 2 V −1 s −1 and aggressively scaled saturation voltage ~1 V. The crystalline layers are atomically smooth and effectively decoupled from the substrate due to weak van der Waals interactions, affording a pristine interface for high-performance organic transistors. Here, we demonstrate that high-quality few-layer dioctylbenzothienobenzothiophene molecular crystals can be grown on graphene or boron nitride substrate via van der Waals epitaxy, with precisely controlled thickness down to monolayer, large-area single crystal, low process temperature and patterning capability. However, a molecular counterpart, with scalable processability and competitive device performance, is still challenging. Two-dimensional atomic crystals are extensively studied in recent years due to their exciting physics and device applications.
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