Orbital Tomography of Organic Semiconductor Films

The frontier orbitals of molecules are the prime determinants of their chemical, optical and electronic properties. Arguably, the most direct method of addressing the (occupied) frontier orbitals is ultra-violet photoemission spectroscopy (UPS). Although UPS is a mature technique from the early 1970s on, the angular distribution of the photo emitted electrons was thought to be too complex to analysed quantitatively. With our angle resolved UPS (ARUPS) work on conjugated molecules both in ordered thick films and chemisorbed monolayers we have shown that the angular (momentum) distribution of the photocurrent from orbital emissions can be simply understood. The approach we have been developing is becoming known as orbital tomography. It takes an holistic view of the photoemission from the orbitals assuming a plane wave approximation for the final state: The ARUPS data cube is thus essentially a momentum space view of the orbital which can be related, by a Fourier transform, to the real space electron distribution. In the past few years the potential power of orbital tomography has been demonstrated and it has been shown that it can determine molecular geometries, gain insight into the nature of the surface chemical bond, unambiguously determine the orbital energy ordering in molecular homo- and heterostructures and even reconstruct orbitals in real space. The global aim of this project is to develop and consolidate orbital tomography and provide a basis whereby it could be even more generally applied. It will involve systematic studies of the polarization and energy dependence of the incident photon on the angular (momentum) distribution of photoemitted electrons from a variety of device relevant molecular adsorbate systems. It will lead to advances in understanding valence band photoemission in general and the electronic structure of organic semiconductors in particular. To this end three specific tasks will be undertaken. The first is a general systematic study of polarisation effects on the photoemission intensity distribution to explore the limits of the plane wave final state approximation. The second is a study of the photon energy dependence with the ultimate goal of reconstructing 3 dimensional real space orbitals of adsorbed molecules. The third is a systematic study of the metal phthalocyanines. On the one hand this is to explore the capability of orbital tomography for non-orbitals and the role, if any, of heavy scatters, on the other, the results will yield a definitive orbital energy assignment for the M-Pcs.

The frontier orbitals of molecules are the prime determinants of their chemical, optical and electronic properties. Arguably, the most direct method of addressing the (filled) frontier orbitals is ultra-violet photoemission spectroscopy (UPS). Although UPS is a mature technique from the early 1970s on, the angular distribution of the photo emitted electrons was thought to be too complex to analyse quantitatively. With the work of this project we have shown that the UPS angular distribution can in fact be simply understood and analysed. This has opened up a new experimental window for the study of device relevant organic films which we have come to call orbital photoemission tomograghy (PT). The results have demonstrated that PT with the plane wave final state (PWFS) description is in better agreement with experimental observations than initially thought. Particularly, the PWFS accounts well for the overall dependence of the photocurrent on the photon energy which has allowed us to reconstruct three-dimensional real space images of orbitals from experimental data. Another hypothesis formulated and tested was that PT can determine surface reaction intermediates as the frontier orbitals should be argueably the most sensitive indicator of a particular molecular species. The determination of reaction pathways and the identification of reaction intermediates are key issues in chemistry. Surface reactions are particularly challenging, since many methods of analytical chemistry are inapplicable at surfaces. One of our test studies for this was the molecule DBBA on Cu(110) which decomposes on mild heating and goes on to form graphene on annealing. The surface intermediate yielded the clearest molecular orbital momentum maps of any adsorbate we have observed. The results showed that photoemission tomography is extremely sensitive to the character of the frontier orbitals. Specifically, hydrogen abstraction at the molecular periphery was easily detected, and the precise nature of the reaction intermediates can be determined. The thermally induced reaction of dibromo-bianthracene to graphene was shown unambiguously to proceed via a fully hydrogenated bisanthene intermediate in contradiction to scanning probe studies in the literature. We anticipate that photoemission tomography will become a powerful companion to other techniques in the study of surface reaction pathways. Extensive work on molecular films on thin dielectric interlayers was begun. With PT we could demonstrate that introducing a thin insulating layers between molecule and metal can, counterintuitively, increase the amount of charge transferred to the molecule. Not only could PT quantify charge transfer to molecules it could distinguish between, and quantify, charged and uncharged species on the dielectric films. This work will resolve the decades of confusion regarding the electronic level alignment of, and charge transfer to, organic films on dielectric interlayers. In the atomically controlled study here we demonstrate that the relationships of the controlling parameters are very simple and express the pure physics at dielectric interfaces. We believe that the results and understanding of this work have significant ramifications in fields ranging from catalysis, single molecule magnets through to organic electronics.