Everything you wanted to know about the TPA molecule adsorbed on Au(111)

Abstract: The electronic properties of Triphenylamine (TPA) in gas phase and adsorbed on gold(111) have been simulated with Quantum Espresso using Density Functional Theory (DFT). To better understand how the presence of a gold surface affects sunlight absorption in the system, partial Density Of States (pDOS) and Near Edge X-ray Absorption Fine Structure (NEXAFS) of the system have been calculated. To describe the electronic excitation, three different methods have been used, No Core Hole (NCH), Full Core Hole (FCH) and Half Core Hole (HCH) approximation. The excitation of the TPA molecule was made in the nitrogen (N) atom and in the four different carbon (C) atoms with different electronic environments, C-ipso, C-ortho, C-meta and C-para. When using the HCH method, the absorbing atom must be described by a pseudopotential (PP) which includes half of a hole in the 1s orbital. This PP has been generated and a detailed summary of the process is described. The TPA/gold system relaxes to a position with the central N atom of TPA above an gold (Au) atom in the second layer of the surface and at a distance of 3.66 Angstrom, to the first layer. TPA keeps its symmetry with only small differences in the length of atomic bonds when adsorbed. The most striking result of this study is how the band gap of TPA is affected by the gold layer. From the pDOS we can observe that TPA in gas phase has a clear band gap of 2.2 eV with C-ortho dominating in the valence region and the four carbons dominating in the first unoccupied states. When depositing the molecule on the surface of Au(111), the band gap is essentially gone and a number of states appear between the previous highest occupied and lowest unoccupied molecular orbital in TPA. These new states align in energy with three clusters of states of the gold suggesting an interaction between the molecule and the surface. In the generated NEXAFS of nitrogen and carbon in TPA gas phase, one can observe a small pre-peak before the first unoccupied state. This is reinforced when adsorbing the molecule, which generates a pre-peak of approximately 3 eV in width. The pre-peak is connected to the new peaks seen in pDOS, correlating with experimental results on the same system.