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.
