NanoESCA highlights


Ferroelectric domains

The objective of this work was to quantify the critical doping level and then to investigate the polarization dependence of the band structure at the surface of the doped ferroelectric domains.
J. E. Rault et al., Phys. Rev. Lett. 111, 127602 (2013).

An ideal ferroelectric (FE) must be an insulator to show a spontaneous polarization, however, recent theoretical and some experimental work suggests that a ferroelectric state can exist up to a certain critical doping level. Thus the possibility of coexistence of ferroelectricity and d electron occupancy opens the way to fascinating properties such as intrinsic multiferroicity, or field-induced metal to insulator transitions. Further progress in this field requires knowledge of the electronic structure which makes the decisive contribution to such functional properties.
The Energy-filtered PhotoEmission Electron Microscope (PEEM) at NanoESCA beamline was used to investigate in a single experiment the FE domain distribution by imaging the photoemission threshold, the momentum-resolved electronic properties using reciprocal space PEEM and the spatially-resolved chemical states micron-sized regions.

Using a combination of in situ UHV and oxygen annealing we can control the surface doping of BaTiO3 (001) which induces a paraelectric-ferroelectric phase transition in good agreement with theoretical predictions. Below the critical doping value, ferroelectric stability leads to domain formation with in- and out-of-plane polarizations reflected in the band structure symmetry. The unambiguous observation of domain ordering means that the link between band structure and polarization could be generalized to undoped ferroelectric systems.

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Polarization Sensitive Surface Band Structure of Doped BaTiO3(001); J. E. Rault, J. Dionot, C. Mathieu, V. Feyer, C. M. Schneider, G. Geneste, and N. Barrett;
Phys. Rev. Lett. 111, 127602 (2013);
doi: 10.1103/PhysRevLett.111.127602


Molecular orbitals mapping

We have demonstrated that combining lowenergy electron diffraction with angle-resolved photoelectron spectroscopy is a very powerful method to elucidate the
geometric and electronic structures of ordered molecular adsorbates. We applied this approach to the two different monolayer phases of PTCDA on Ag(110)
M. Wießner et al., Phys. Rev. B 86, 045417 (2012).


Experimental photoelectron momentum maps (PMM) determined for photoelectrons from the LUMO and HOMO regions for one ML PTCDA for the brick-wall and the herringbone phase on Ag(110).

The properties of molecular films are determined by the geometric structure of the first layers near the interface. These are in contact with the substrate and feel the effect of the interfacial bonding, which particularly, for metal substrates, can be substantial. For the model system 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) on Ag(110), the geometric structure of the first monolayer can be modified by preparation parameters. This leads to significant differences in the electronic structure of the first layer. We show that, by combining photoelectron momentum maps (PMM) with low-energy electron diffraction (LEED), we cannot only determine the electronic structure of the interfacial layer and the unit cell of the adsorbate superstructure, but also the arrangement of the molecules in the unit cell.
 

Moreover, in bilayer films, we can distinguish the first from the second layer and, thus, study the formation of the second layer and its influence on the buried interface. Additionally, momentum patterns can give information about the electronic structure
at the interface and the symmetry of molecular orbitals.

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Electronic and geometric structure of the PTCDA/Ag(110) interface probed by angle-resolved photoemission; M. Wießner, D. Hauschild, A. Schöll, F. Reinert, V. Feyer, K. Winkler, B. Krömker;
Phys. Rev. B 86, 045417 (2012);
doi: 10.1103/PhysRevB.86.045417


Multi-orbital charge transfer at highly oriented organic/metal interfaces

Our results emphasize the importance of complementary STM and μ-ARPES measurements for characterizing such systems. While the former provide information on the molecular states localized on the phenyl rings without being able to probe the porphyrin core, the latter reveal the electronic structure of the frontier orbitals located on the macrocycle. Thus, in general, a multi-technique approach, including electronic structure calculations, is necessary to develop a consistent picture of the adsorption behavior and electronic properties of interfaces between non-planar molecules and metallic surfaces.

G. Zamborlini et al., Nature Communications, 8, 335 (2017).

Organic-based device performances have been rapidly improving in the last years, making them suitable for large-scale industrial applications, involving photo-voltaic cells, light emission systems and building of larger flexible electronics. In parallel, basic research has intensively focused on the chemical and physical properties of semiconducting π-conjugated organic molecules, as they appear to be promising for organic-based device construction. In particular, in controlling the charge injection on such devices, a predominant role is played by the molecule-substrate interaction. Charge transfer at the molecule-metal interface strongly affects the overall physical and magnetic properties of the system, and ultimately, the device performance.

On the perspective of possible technological applications, such as colorimetric gas sensors, organic spin-valves, field-effect transistors, etc., porphyrin represent a class of extremely versatile molecules, allowing for tailoring a variety of electronic, magnetic and conformational properties. In particular, supramolecular multi-porphyrin arrays are considered as functional components in nanodevices. Here, we report theoretical and experimental evidence of a pronounced charge transfer involving nickel tetraphenyl porphyrin molecules (Ni-TPP) adsorbed on Cu(100).
NiTPP molecules tend to form well-ordered islands on the Cu(100) surface already at low coverages, in particular STM measurements reveal the presence of two rotational domains mirrored with respect to the [001] direction of Cu(100). These two domains are also commensurate with the substrate (see Fig. 1a). The molecule-substrate interaction strongly affects the molecular adsorption geometry and electronic structure. Indeed, hybrid functional DFT calculations suggest a significant charge transfer from Cu to the molecule, resulting in the occupation of the gas-phase LUMO/LUMO + 1 and LUMO + 3 molecular orbitals, accompanied by a back donation of charge from the molecule to the substrate. As a consequence of this strong interaction with the substrate, the porphyrin's macrocycle approaches the surface very closely (∽2 Å), forcing the phenyl ligands to bend upwards (Fig. 1b). Therefore, the STM contrast arises mainly from the electronic states of the phenyl rings preventing the STM tip to reliably probe the states related to the macrocycle.

This limitation can be overcome by molecular orbital tomography (MOT) which combines angle resolved photoelectron spectroscopy (ARPES) with DFT calculations. This approach gives a direct access to the molecular orbitals by looking at their signature in the angular distribution of the photoemitted electrons from the molecular film. 
MOT provides a relatively simple interpretation of µ-ARPES data, since the angle dependent photoemission intensity becomes proportional to the modulus squared of the Fourier transform calculated from the real space molecular orbital. On this respect, the photoelectron emission microscope (PEEM) installed at the beamline is an ideal set-up to measure µ-ARPES patterns within a single image acquisition.
Using MOT we have shown that the remarkable charge transfer takes place at NiTPP/Cu(100) interface and it leads to filling of the higher unoccupied orbitals up to LUMO+3, thereby confirming the DFT predictions.

Figure caption: (a) STM image including two Ni-TPP domains, labeled with A and B, respectively. STM image parameters: Vb = −1.5 V, It = 0.2 nA, image size 15 × 20 nm2, measured at 4.3 K. (b) . Proposed adsorption model for Ni-TPP/Cu(100), side view. (c) Valence band photoemission spectra of clean Cu(100) and Ni-TPP/Cu(100) acquired at 26 eV photon energy. (d) PDOS onto molecular orbitals for the Ni-TPP/Cu(100) system. The energy position of the corresponding gas-phase molecular orbitals, aligned with respect to the vacuum level, is indicated with colored bars on the top axis. (e) Comparison between μ-ARPES measured patterns (left) and the correspondent calculated |FT|2 of the molecular orbitals (right).

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Multi-orbital charge transfer at highly oriented organic/metal interfaces”,      G. Zamborlini, D. Lüftner, Zh. Feng, B. Kollmann, P. Puschnig, C. Dri, M. Panighel, G. Di Santo, A. Goldoni, G. Comelli, M. Jugovac, V. Feyer, C.M. Schneider,  Nature Communications, 8, 335 (2017),
doi: 10.1038/s41467-017-00402-0


 



Nonlocal electron correlations in an itinerant ferromagnet 

we show that in itinerant ferromagnets like cobalt the electron correlations are of nonlocal origin, manifested in a complex self-energy Σσ(E,k) that disperses as function of spin σ, energy E, and momentum vector k. 
Ch. Tusche et al., Nature Communications, Vol. 9 - 1, pp. 3727 (2018)

 



Measured spin-resolved Fermi surface of fcc cobalt. Spin-resolved photoemission intensities in selected sections through the three-dimensional Fermi surface of 18 ML Co/Cu(100) measured at photon energies of hv = 85 eV (a), hv = 70 eV (b), and hv = 50 eV (c). (d) The fcc Brillouin zone together with the corresponding cuts sampled by (a)–(c). According to the 2D colour code at the lower right, red and blue intensities correspond to majority and minority electronic states 

Our understanding of the properties of ferromagnetic materials, widely used in spintronic devices, is fundamentally based on their electronic band structure. However, even for the most simple elemental ferromagnets, electron correlations are prevalent, requiring descriptions of their electronic structure beyond the simple picture of independent quasi-particles. Here, we give evidence that in itinerant ferromagnets like cobalt these electron correlations are of nonlocal origin, manifested in a complex self-energy Σσ(E,k) that disperses as function of spin σ, energy E, and momentum vector k. Together with one-step photoemission calculations, our experiments allow us to quantify the dispersive behaviour of the complex self-energy over the whole Brillouin zone. 
At the same time we observe regions of anomalously large “waterfall”-like band renormalization, previously only attributed to strong electron correlations in high-TC superconductors, making itinerant ferromagnets a paradigmatic test case for the interplay between band structure, magnetism, and many-body correlations

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Nonlocal electron correlations in an itinerant ferromagnet
Tusche Christian, Ellguth Martin, Feyer Vitaliy, Krasyuk Alexander, Wiemann Carsten, Henk Jürgen, Schneider Claus M., Kirschner Jürgen, Nature Communications, Vol. 9 - 1, pp. 3727 (2018), doi: 10.1038/s41467-018-05960-5


 


Last Updated on Monday, 11 November 2019 17:28