  Two-dimensional atomic crystals are extensively studied in recent years due
to their exciting physics and device applications. However, a molecular
counterpart, with scalable processability and competitive device performance,
is still challenging. 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. 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. As a result, monolayer dioctylbenzothienobenzothiophene molecular
crystal field-effect transistors on boron nitride show record-high carrier
mobility up to 10cm2V-1s-1 and aggressively scaled saturation voltage around
1V. Our work unveils an exciting new class of two-dimensional molecular
materials for electronic and optoelectronic applications.
