Unveiling the Structural Origin of the High Carrier Mobility of a Molecular Monolayer on Boron Nitride

TL;DR

This study reveals that the record-high carrier mobility in C8-BTBT monolayer transistors on boron nitride is primarily due to the high-quality single crystal growth facilitated by substrate flatness, guiding future epitaxial film development.

Contribution

It demonstrates that substrate flatness and layer-by-layer growth are crucial for achieving high-quality molecular monolayers with exceptional charge transport properties.

Findings

Single crystal C8-BTBT monolayers exhibit record-high mobility.

Substrate flatness below 0.6 nm is essential for high-quality crystal growth.

Layer-by-layer growth on h-BN and graphene promotes large-area monolayer formation.

Abstract

Very recently, it was demonstrated that the carrier mobility of a molecular monolayer dioctylbenzothienobenzothiophene (C8-BTBT) on boron nitride can reach 10 cm2/Vs, the highest among the previously reported monolayer molecular field-effect transistors. Here we show that the high-quality single crystal of the C8-BTBT monolayer may be the key origin of the record-high carrier mobility. We discover that the C8-BTBT molecules prefer layer-by-layer growth on both hexagonal boron nitride and graphene. The flatness of these substrates substantially decreases the C8-BTBT nucleation density and enables repeatable growth of large-area single crystal of the C8-BTBT monolayer. Our experimental result indicates that only out-of-plane roughness greater than 0.6 nm of the substrates could induce disturbance in the crystal growth and consequently affect the charge transport. This information would be important in guiding the growth of high-quality epitaxy molecular film.