Hexacene on Cu(110) and Ag(110): Influence of the Substrate on Molecular Orientation and Interfacial Charge Transfer

Abstract

Hexacene, composed of six linearly fused benzene rings, is an organic semiconductor material with superior electronic properties. The fundamental understanding of the electronic and chemical properties is prerequisite to any possible application in devices. We investigate the orientation and interface properties of highly ordered hexacene monolayers on Ag(110) and Cu(110) with X-ray photoemission spectroscopy (XPS), photoemission orbital tomography (POT), X-ray absorption spectroscopy (XAS), low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), and density functional theory (DFT). We find pronounced differences in the structural arrangement of the molecules and the electronic properties at the metal/organic interfaces for the two substrates. While on Cu(110) the molecules adsorb with their long molecular axis parallel to the high symmetry substrate direction, on Ag(110), hexacene adsorbs in an azimuthally slightly rotated geometry with respect to the metal rows of the substrate. In both cases, molecular planes are oriented parallel to the substrate. A pronounced charge transfer from both substrates to different molecular states affects the effective charge of different C atoms of the molecule. Through analysis of experimental and theoretical data, we found out that on Ag(110) the LUMO of the molecule is occupied through charge transfer from the metal, whereas on Cu(110) even the LUMO+1 receives a charge. Interface dipoles are determined to a large extent by the push-back effect, which are also found to differ significantly between 6A /Ag(110) and 6A /Cu(110).

Observation of Hole States at Perylene/Au(110) and Au(111) Interfaces

“…The incident angle was 68°, and the polarization vector was in the x – z (Figure a) and y – z planes (Figure b). The 2D momentum map of photoemission is associated with the Fourier transform of the molecular orbitals in the plane wave final state approximation. − Figure shows the HOMO distribution (Figure a) and the corresponding simulated photoemission momentum maps (Figure b–i). The molecular plane is set in the x – y plane so that the short axis is directed along the y axis for α = 0° (Figure b) and tilted along the short axis every ±5 to ±30° for Figure c–h.…”

“…Recently, the relation between the 2D maps of photoelectron momentum and the Fourier transform of the molecular orbital has been emphasized, which makes the assignment of the spectra to MOs more intuitive. − This technique is called photoemission tomography (PT). PT analysis has been developed for π-conjugated molecules including 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) and polyacenes, which are flat-lying on a twofold symmetry surface such as Ag(110) and Cu(110). − The combination of perylene and Au(110) is challenging for this technique because perylene is less elongated than acenes, which may break the twofold symmetry upon adsorption, and the Au(110) surface exhibits reconstruction, which reduces the surface flatness. Nevertheless, we have collected fruitful spectroscopic results by combining these methods, which reasonably reveal both the geometric and electronic structures of the perylene monolayer and their changes upon Br doping.…”

Orientation, electronic decoupling and band dispersion of heptacene on modified and nanopatterned copper surfaces

“…For example, the adsorption and the location of the energy levels of 7 A on Cu(110) have been recently reported [22]. Due to the rectangular unit cell of the Cu(110) surface and the elongated shape of the 7 A molecule, a preferential orientation on the surface, as in the case of the shorter acenes (5 A, 6 A) [11,[25][26][27] can be expected. Indeed, in the work of Boné et al [22], STM and photoemission orbital tomography (POT) were used to find the preferential orientation of the 7 A molecule with its long molecular axis along the long axis of the Cu(110) unit cell and its aromatic plane parallel to the surface.…”

“…Research into the properties of higher acenes (n > 5, i.e. beyond pentacene) is presently attracting much interest from both the applied and basic science communities [8][9][10][11], despite the difficulties resulting from their sensitivity to air and light. The interest is driven by their expected superior properties [10,12,13] such as high lying highest occupied molecular orbital (HOMO) energy levels, an increasing carrier mobility and a decreasing band gap [14][15][16].…”

Emergence of π-Magnetism in Fused Aza-Triangulenes: Symmetry and Charge Transfer Effects

“…This requires spin-selective charge transfer: the number of α and β spin electrons transferring to and from the surface must be different. Otherwise, equal contributions would not change the net spin, as in conventional charge transfer scenarios resulting in the depopulation of frontier molecular orbitals. − Although each case has been widely reported, a better understanding of parameters influencing the emergence of magnetism vs conventional orbital depopulation by charge transfer could allow their rational tuning.…”