Electronic Properties of Tetraazaperopyrene Derivatives on Au(111): Energy-Level Alignment and Interfacial Band Formation.

Arnulf Stein,Daniela Rolf,Christian Lotze,Lutz H Gade,Oliver T Hofmann,Katharina J Franke,David Gerbert,Johannes J Cartus,Sascha Feldmann,Andreas Jeindl,Petra Tegeder,Benjamin Günther

doi:10.1021/acs.jpcc.1c04217

Abstract

N-heteropolycyclic aromatic compounds are promising organic electron-transporting semiconductors for applications in field-effect transistors. Here, we investigated the electronic properties of 1,3,8,10-tetraazaperopyrene derivatives adsorbed on Au(111) using a complementary experimental approach, namely, scanning tunneling spectroscopy and two-photon photoemission combined with state-of-the-art density functional theory. We find signatures of weak physisorption of the molecular layers, such as the absence of charge transfer, a nearly unperturbed surface state, and an intact herringbone reconstruction underneath the molecular layer. Interestingly, molecular states in the energy region of the sp- and d-bands of the Au(111) substrate exhibit hole-like dispersive character. We ascribe this band character to hybridization with the delocalized states of the substrate. We suggest that such bands, which leave the molecular frontier orbitals largely unperturbed, are a promising lead for the design of organic–metal interfaces with a low charge injection barrier.

Highlights

Organic electron-transporting (n-channel) semiconductors are of particular interest for their implementation in field-effect transistors.[1]
We first focus on the TAPP-CF3/Au(111) system and elucidate the adsorption structure by means of scanning tunneling microscopy (STM) and density functional theory (DFT) since it strongly influences the interfacial electronic structure
We analyze the electronic properties of amonolayer coverage of TAPP-CF3 molecules on the Au(111) surface in detail using STS and 2PPE, and complement these findings with DFT calculations

Summary

Introduction

Organic electron-transporting (n-channel) semiconductors are of particular interest for their implementation in field-effect transistors.[1] Replacing carbon atoms by nitrogen atoms in a πconjugated aromatic molecular backbone typically leads to an energetic stabilization of the frontier orbitals, i.e., the electron affinity (EA) and the ionization potential (IP) increase, while the optical gap size is almost uninfluenced.[2−5] N-heteropolycyclic aromatic molecules are expected to be promising candidates for n-channel semiconductors. Regardless of n- or p-channel semiconducting behavior in a transistor, the energetic positions of the electron affinity level or the ionization potential are the relevant quantities for device performance. The difference between the IP and EA is the transport gap (IP − EA = Etransp.), which is different from the optical gap (highest occupied molecular orbital−lowest unoccupied molecular orbital (HOMO− LUMO) transition leading to exciton formation).[17] The latter can be determined in surface-adsorbed molecules with

Methods

Results

Conclusion