Tracing the film structure of an organic semiconductor with photoemission orbital tomography

Photoemission orbital tomography (POT) is a powerful tool for investigating the orbitals and electronic band structure of oriented layers of organic molecules. In many cases, POT allows conclusions to be drawn regarding the geometric structure, but so far it has been mainly applied to (sub)monolayers and rarely to bilayers, raising the question of whether POT can also provide structure information for thicker films. Here, we use POT to analyze the band dispersion in up to eight layers of $α$-sexithiophene (6T) adsorbed on Cu(110)-p($2\times1$)O. This linear oligomer turns out to be a textbook example that exemplifies the concepts of intra- and intermolecular band dispersion in molecules. Moreover, the rich band and orbital structure information available from POT for this system enables us to trace subtle changes in the crystal structure as a function of layer thickness. Specifically, we find that the periodicity of an intermolecular band changes with film thickness, revealing an increase of the intralayer distance between the molecules with the number of layers. At the same time, the momentum distribution of photoemission from the highest occupied molecular orbital of 6T discloses a decrease of the molecular tilt angle. Following the evolution of tilt angle and lattice constant with layer thickness, we observe – purely based on electronic structure data – that the surface-templated monolayer structure relaxes into the structure of bulk 6T crystals. The experimental findings agree well with the results of density functional theory calculations.

💡 Research Summary

In this work the authors demonstrate that photoemission orbital tomography (POT), a momentum‑resolved ARPES technique that directly maps molecular orbitals in k‑space, can be extended from the usual (sub)monolayer regime to multilayer organic films and used to extract quantitative structural information. The model system is α‑sexithiophene (6T), a linear oligomer consisting of six thiophene units, deposited on a Cu(110) surface that has been reconstructed with a p(2×1) oxygen overlayer. By evaporating 6T at –5 °C the authors prepared films of well‑defined thickness ranging from one to eight molecular layers (ML) while preserving a high degree of orientational order.

The experimental set‑up employs a NanoESCA momentum microscope equipped with a He Iα (21.22 eV) photon source. Full three‑dimensional data cubes I(Eb, kx, ky) are recorded with an energy resolution of 220 meV and a momentum resolution of 0.1 Å⁻¹. Careful detector‑gain correction, secondary‑electron normalization and a parabola‑based correction of the intrinsic iso‑chromatic distortion are applied to obtain reliable momentum maps.

On the theoretical side, density‑functional theory (DFT) calculations are performed with VASP using the PBE functional, Grimme‑D3(BJ) dispersion correction and PAW potentials. Four systems are modelled: an isolated 6T molecule, a single‑layer (1 ML) of 6T on Cu(110)‑p(2×1)O, a double‑layer (2 ML) on the same substrate, and a free‑standing double‑layer in the bulk crystal geometry. For the 2 ML case the second layer is first treated as a rigid body to locate the optimal lateral shift along the molecular axis, after which a full relaxation of internal coordinates is carried out while keeping the substrate and the first layer fixed. Photoemission simulations use a damped plane‑wave final‑state model with a damping constant of 1 Å⁻¹, a photon energy of 21.2 eV and artificial broadening of 0.05 eV (energy) and 0.1 Å⁻¹ (momentum) to generate synthetic I(Eb, kx, ky) data cubes.

The experimental band maps reveal two distinct dispersive features. Along the molecular axis (ky direction) a cosine‑like band of ≈1 eV bandwidth is observed, whose minimum lies at ky = 0. This band is identified as an inter‑molecular π‑stacking band, i.e. a true electronic coupling between neighboring molecules. Its k‑space periodicity shortens as the film thickness increases, indicating that the intermolecular distance a expands from the monolayer to the thicker films. Perpendicular to the molecular axis (kx direction) a second set of bands appears: a shorter‑period cosine‑like dispersion together with a series of six nearly equally spaced “quasi‑bands” spanning 1.5–5 eV binding energy. These are interpreted as intra‑molecular features that become modulated by the multilayer stacking.

Momentum maps of the highest occupied molecular orbital (HOMO) provide a second, independent structural probe. The angular distribution of the HOMO intensity is highly sensitive to the tilt angle β of the molecular plane with respect to the substrate. By fitting the experimental maps with simulated ones, the authors extract β ≈ 38° for the monolayer, decreasing to ≈31° for the eight‑layer film. This systematic reduction mirrors the transition from the surface‑templated structure toward the bulk crystal geometry reported by Horowitz et al. (X‑ray diffraction).

Combining the two observables—k‑space periodicity of the inter‑molecular band and the HOMO tilt angle—allows the authors to reconstruct the evolution of the in‑plane lattice constant a and the tilt angle β as a function of film thickness solely from electronic‑structure data. The experimentally derived values agree within a few percent with the DFT‑optimized structures and with the bulk crystallographic parameters, confirming that POT can serve as a non‑destructive, purely electronic probe of structural relaxation in organic thin films.

The paper concludes that POT, when applied to well‑ordered multilayer organic systems, provides simultaneous access to band dispersion (hence intermolecular distances) and orbital momentum fingerprints (hence molecular orientation). This dual capability makes POT a powerful tool for studying the structural‑electronic interplay that governs charge transport in organic electronic devices such as OLEDs, organic photovoltaics, and field‑effect transistors. The authors suggest that extending this methodology to other conjugated molecules and heterostructures will enable quantitative mapping of structural transitions and charge‑transfer pathways in a broad class of organic materials.