Realizing nearly-free-electron like conduction band in a molecular film through mediating intermolecular van der Waals interactions

Xingxia Cui,Jingsi Qiao,Ding Han,Hrvoje Petek,Hongli Guo,Zhihao Cui,Limin Cao,Wei Ji,Min Feng,Linwei Zhou,Qing Liu,Yafei Li,Chungwei Lin

doi:10.1038/s41467-019-11300-y

Abstract

Collective molecular physical properties can be enhanced from their intrinsic characteristics by templating at material interfaces. Here we report how a black phosphorous (BP) substrate concatenates a nearly-free-electron (NFE) like conduction band of a C60 monolayer. Scanning tunneling microscopy reveals the C60 lowest unoccupied molecular orbital (LUMO) band is strongly delocalized in two-dimensions, which is unprecedented for a molecular semiconductor. Experiment and theory show van der Waals forces between C60 and BP reduce the inter-C60 distance and cause mutual orientation, thereby optimizing the π-π wave function overlap and forming the NFE-like band. Electronic structure and carrier mobility calculations predict that the NFE band of C60 acquires an effective mass of 0.53–0.70 me (me is the mass of free electrons), and has carrier mobility of ~200 to 440 cm2V−1s−1. The substrate-mediated intermolecular van der Waals interactions provide a route to enhance charge delocalization in fullerenes and other organic semiconductors.

Highlights

Collective molecular physical properties can be enhanced from their intrinsic characteristics by templating at material interfaces
superatom molecular orbitals (SAMOs) and their NFE bands are too high in energy to participate in charge transport unless C60 molecules are endohedrally doped with metallic atoms[23]
We have provided the experimental evidence and the theoretical understanding for an unusually dispersive NFE-like conduction band of a C60 monolayer, which we attribute to a favorable van der Waals (vdW) templating of C60 molecules that enhance π–π interactions

Summary

Introduction

Collective molecular physical properties can be enhanced from their intrinsic characteristics by templating at material interfaces. We show by scanning tunneling microscopy (STM) and theory that the BP substrate organizes C60 molecules into a compressed monolayer and imposes a favorable orientation that optimizes the intermolecular π-π couplings, resulting in a nearly-free-electron (NFE) like lowest unoccupied molecular orbital (LUMO) band in C60 monolayers. Such NFE-like LUMO band would dominate charge transport in C60 assemblies. The NFE conduction formed by π-π interactions is worthy of further exploration in C60 or other organic molecules on inert substrates, because it realizes a long sought mode for charge transport for high-performance organic electronics and optoelectronics[4]

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Conclusion