Abstract
In this article, we analyze the electronic structure modifications of triphenylamine (TPA), a well-known electron donor molecule widely used in photovoltaics and optoelectronics, upon deposition on Au(111) at a monolayer coverage. A detailed study was carried out by synchrotron radiation-based photoelectron spectroscopy, near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, scanning tunneling microscopy (STM), and ab initio calculations. We detect a new feature in the pre-edge energy region of the N K-edge NEXAFS spectrum that extends over 3 eV, which we assign to transitions involving new electronic states. According to our calculations, upon adsorption, a number of new unoccupied electronic states fill the energy region between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the free TPA molecule and give rise to the new feature in the pre-edge region of the NEXAFS spectrum. This finding highlights the occurrence of a considerable modification of the electronic structure of TPA. The appearance of new states in the HOMO–LUMO gap of TPA when adsorbed on Au(111) has crucial implications for the design of molecular nanoelectronic devices based on similar donor systems.
Highlights
In recent years, organic semiconductors and metal interfaces have been the focus of a large number of studies aiming to conveniently engineer innovative materials with tunable energy levels that can be exploited in molecular- and nanoelectronics
The structure of the molecule is only weakly affected by the adsorption, the main difference being that the phenyl rings are rotated by approximately 6° less with respect to the central plane of TPA as compared to the gas phase, making the molecule slightly flatter when adsorbed on the surface
Through the comparison with our previous investigations of TPA in the gas phase[5] and ab initio calculations, we could shed light on the considerable electronic structure modifications connected to the molecule−surface interactions
Summary
Organic semiconductors and metal interfaces have been the focus of a large number of studies aiming to conveniently engineer innovative materials with tunable energy levels that can be exploited in molecular- and nanoelectronics. Of high interest in this field are the π-conjugated molecules adsorbed onto non-reactive metal surfaces, such as noble metals, which are often used in optoelectric and photovoltaic devices. In this respect, it would be natural to expect that the adsorption of πconjugated molecules on these surfaces would be characterized by a weak interaction or, in case, by an interaction of strength between physisorption and weak chemisorption.[1] In the present work, we show that a combination of spectroscopic methods and theoretical calculations represents a unique and powerful tool to investigate adsorbate systems. We show evidence of a significant molecule/substrate interaction, practically abating the molecular energy gap and questioning the suitability of employing the studied materials in electronic devices
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