Leandro Merces1,2, Rafael F. de Oliveira1, Davi H. S. de Camargo1, Carlos C. Bof Bufon1*
1Brazilian Nanotechnology National Laboratory (LNNano), Campinas, SP, Brazil
2University of Campinas (UNICAMP), Campinas, SP, Brazil
In molecular systems, the charge transport is governed by a series of carrier-molecule quantum interactions responsible for several chemical and biological phenomena. From a technological perspective, the understanding of how charge carriers are transported in molecules and molecular layers has paved the way to the development of novel device concepts, including organic transistors, organic light-emitting diodes, and biosensors. At the nanoscale, the electronic transport is commonly described as either direct tunneling (DT) for travelling distances in the 1-10 nm range or activated hopping for longer pathways. The continuous transition from one mechanism to the other, however, remains unknown, especially for the class of physisorbed molecules – building blocks for organic electronics. This work describes the first experimental evidence of an intermediate, long-range (> 10 nm) coherent mechanism, herein called sequential tunneling (ST), occurring in physisorbed copper-phthalocyanine (CuPc) thin films. The ST is an activationless, multi-step process intrinsically distinct from both DT and hopping conduction. By investigating the electrical characteristics of CuPc films using a versatile, nanomembrane-based platform, the boundaries of the dominant transport mechanism were determined as a function of distance (film thickness), temperature, and electric field. The evidence for such coherent charge transport (ST) across long distances (> 10 nm) has implications for a variety of quantum effects never previously verified for molecular pathways longer than a few nanometers. Therefore, these findings contribute to bridge the gap between molecular and organic electronics for the elementary set of physisorbed molecules.
This work was conducted in the Brazilian Nanotechnology National Laboratory (LNNano), and featured in the cover of the Journal of Physical Chemistry C, vol. 121, issue 31 (doi: 10.1021/acs.jpcc.7b02528).
LNNano/CNPEM (Functional Devices and Systems group, Laboratory for Surface Science), CAPES, SISNANO and FAPESP (2014/ 25979-2)