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Coherent optical control of multi-functional nano-scale hybrid units

Periodic Reporting for period 2 - hyControl (Coherent optical control of multi-functional nano-scale hybrid units)

Reporting period: 2019-03-01 to 2020-08-31

In the physics and chemistry of materials science, an intense focus of forefront research is the search for ever-smaller and ever-faster building blocks for information and communication technology (ICT) applications. The realization of next-generation devices, in ICT fields such as spintronics, spin-orbitronics and plasmonics, will depend decisively on our ability to generate new functionalities that can be actively controlled on the shortest length and time scales.The groundbreaking idea of hyControl is to develop a conceptually new class of active ICT nano-scale materials by building functionality into the nano-scale object that naturally forms when an organic molecule is hybridized on a metallic surface: a nano-scale hybrid unit (NHyU). NHyUs will be realized by depositing selected organic molecules onto three classes of inorganic systems: transition metals; spin-textured materials such as Rashba systems and topological insulators; and magneto-plasmonic nano-structures. By tuning optical excitation to specific resonances, we will control the hybridization strength with ultrashort laser pulses, and thereby induce a coherent response in the spin, orbit, and/or electron degrees of freedom of the NHyU. Thereby we will achieve coherent control - at the molecular scale - of technologically important parameters, such as magnetization, plasmonic resonances, and spin texture. This hyControl concept will be implemented using a novel experimental method, based on pump-probe photoemission spectroscopy, that is capable of resolving the transient electronic structure of precisely those valence band electrons which mediate the hybridization in a single NHyU. While inspired by the latest achievements in molecular spintronics, hyControl will open the way to new technologies in various ICT applications, three of which - spintronics, spin-orbitronics, and plasmonics - have been selected to demonstrate the ability and versatility of optically controlled NHyUs.
The main goal of the project hyControl is to realize nano-scale hybrid units (NHyUs) that can be controlled coherently by light by modifying the hybridization at the relevant interfaces. The NHyUs should be applied in the fields of spintronics, spin-orbitronics and plasmonics, each of which requires the development of specific material systems.
In order to realize this goal, hyControl is organized in four aims: aim 1 is the development of the required methodology; while the conceptual aims 2, 3 and 4 require first the design of specific NHyUs and, on a later stage (the second half of the project), the control of physical properties that are related respectively to the fields of spintronics, spin-orbitronics and plasmonics.
In the first half (30 months) of hyControl we have successfully realized Aim 1, that requires to set up the methodology that will be used throughout the whole project. In particular, we realized a setup for pump-probe photoemission spectroscopy that is designed to give access to the time-resolved evolution of the electronic properties of molecular orbitals after optical excitation on the femtosecond time-scale. The setup consists of a momentum-microscope combined with a femtosecond-laser setup. The momentum microscope is based on a photoemission electron microscopy optics, that enables an efficient way to perform angle-resolved photoemission spectroscopy. The femtosecond-laser setup is based on a 20W PHAROS laser system (Light Conversion), that is coupled to two independent optical parametric amplifiers (OPAs). In combination with a second harmonic generation system, the OPAs generate photons in the energy range from 0.5 eV up to 3.5 eV, while keeping the pulse width below 100 fs. Additional fourth and fifth harmonic generation extend the available photon energies to 4.8eV and 6.0eV. Combined with the momentum microscope, the laser setup allows to perform pump-probe photoemission with photon energy tunable in a wide range, from 0.5 eV to 6 eV. Thanks to such unique combination, we can access both occupied and unoccupied electronic states of NHyUs in time-resolved photoemission experiments and, at the same time, tune the pump photon energy to specific optical resonances of the molecular components building the multifunctional NHyUs.
Regarding the remaining aims 2, 3 and 4, each of them contains two tasks (a and b). During the first half of hyControl, the work plan foresees for all three aims the realization of task a, i.e. the design and characterization of specifically designed NHyUs.
The three tasks (2a,3a and 4a) have been successfully accomplished. In particular, NHyUs have been prepared by depositing selected molecular components (M) on the surface of different inorganic materials (I) in ultra-high vacuum (UHV). After preparation, the systems have been characterized using a variety of surface science techniques, chosen depending on the specific physical properties of the selected systems.
As molecular components, we have identified fullerenes (C60, Me3N@C80) and porphyrins (NiTTP, CoTTP) as the main materials that will be used in the second half of the project.
As inorganic materials, we have studied noble metal surfaces, eg. Cu(100), as well as the surfaces of 3d-ferromagnets, eg. Fe(100), covered by an additional oxygen layer. We found that adding an additional oxygen interlayer between molecule and substrate is an extremely reliable method to enable self-assembly of the molecular components and at the same time to achieve sufficient decoupling between molecule and substrate to allow an efficient optical control. As spin-textured surfaces, we have characterized various 2D arrangements of Sn/Pb/Bi atoms on (111)-indexed noble metal surfaces that display non-trivial spin properties, while as magneto plasmonic nano-structures we have studied Au gratings deposited on Cr2O3, a system that will allow us to tune the coupling between coherent magnons (i.e. magnetic excitations in Cr2O3) and coherent surface plasmons excited in the Au.
In the following we summarize the scientific highlights achieved in the first half of hyControl.
Using momentum microscopy, we were able to reveal the unusual band structure of a C60 crystal made of only a few molecular layers [1]. We have generated non-interacting excitons in such system and found that they induce a substantial energetic redistribution of all transport levels of the non-excited molecules [2], which demonstrates an intrinsic correlation between the optical and transport properties of C60 molecular films. At the same time, we have also achieved the control of the transport properties of C60 by using a three-terminal device structure called in-device molecular spectroscopy [3]. Thanks to the control of the injection energy of hot carriers in C60, that is possible using the three-terminal device geometry, we were able to induce an effective negative differential resistance state that is a direct consequence of an unconventional transport regime, called Marcus inverted region [4].
Recently, using a very broad multi-techniques approach, we have demonstrated the high stability of nickel tetraphenylporphyrins (NiTPP) molecules on a Cu(001) surface, which is related to the high charge transfer taking place at the interface and causes the activation of the Ni(I) oxidation state. We have also found an intriguing strategy to induce an on-surface spin transition in this system. The strategy is based on the functionalization of the Ni metal ion with NO2, which restores the Ni(II) oxidation state while changing its configuration from low to high spin. Surprisingly, this has only a minor impact on the electronic structure of the macrocycle backbone, maintaining the occupation of the former lowest unoccupied molecular orbitals (LUMOs), up to the LUMO+3. We could also show that the interface can be completely restored to its pristine condition simply by annealing the system. These (still unpublished) results constitute very important milestones that will guide us in the design of optically controllable NHyUs based on porphyrins as molecular component in the second half of hyControl.
In [5] we have studied a reliable method to enable self-assembly of molecular films on a Cu(100) surface and at the same time to reduce the interaction strength at the interface: adding an additional oxygen interlayer at the interface. Recently, we have followed this extremely promising approach and performed a complete investigation of the electronic and geometrical structure of NHyUs composed of CoTTP molecules adsorbed on passivated Fe(100)-p(1×1)O.
In [6,7,8] we have reported the formation of 2D arrangements of Sn/Bi/Pb atoms on noble metal surfaces with non-trivial spin properties. In particular, in [6] we prepared a novel Sn/Au(111) structure that shows linearly dispersing bands close to the Fermi level, with a non-trivial spin polarization, that resemble those expected from free-standing stanene. In [7] we prepared a SnAu2/Au(111) surface alloy with Rashba-type spin-split bands that are caused by the significant mixing of Au with Sn bands together with the strong atomic spin-orbit coupling of Au. In [8], we show that deposition of molecular components on a Pb/Ag(111) quantum well system can be used to generate NHyUs with tailored band structure, especially concerning the states above the Fermi level that are extremely important for hyControl. Finally, we studied the plasmonic response of Au gratings deposited on Cr2O3. We have performed a complete characterization of the plasmon excitation in the transparency region of Cr2O3, and detected a substantial influence of magnetic order on the plasmon excitation (still unpublished results).

All the described results, constitute very important milestones that will guide us, in the second half of hyControl, in the design of optically controllable NHyUs based on these selected molecular and inorganic components. We expect a successful realization of the remaining tasks 2b, 3b and 4b, that require the optical control of physical properties of such selected NHyUs.



References:

[1] N. Haag et al. “Signatures of an atomic crystal in the band structure of a C60 thin film”. Physical Review B 101:16 (Apr. 2020). DOI: 10.1103/physrevb.101.165422.
[2] B. Stadtmüller et al. “Strong modification of the transport level alignment in organic materials after optical excitation”. Nature Communications 10:1 (Apr. 2019). DOI: 10.1038/s41467-019-09136-7.
[3] A. Atxabal et al. “Molecular spectroscopy in a solid-state device”. Materials Horizons 6:8 (2019). DOI: 10.1039/c9mh00218a.
[4] A. Atxabal et al. “Tuning the charge flow between Marcus regimes in an organic thin-film device”. Nature Communications 10:1 (May 2019). DOI: 10.1038/s41467-019-10114-2.
[5] I. Cojocariu et al. “Evaluation of molecular orbital symmetry via oxygen-induced charge transfer quenching at a metal-organic interface”. Applied Surface Science 504: (Feb. 2020). DOI: 10.1016/j.apsusc.2019.144343.
[6] M. Maniraj et al. “A case study for the formation of stanene on a metal surface”. Communications Physics 2:1 (Feb. 2019). DOI: 10.1038/s42005-019-0111-2.
[7] M. Maniraj et al. “Structure and electronic properties of the (3×3)R30 SnAu2/Au(111) surface alloy”. Physical Review B 98:20 (Nov. 2018). DOI: 10.1103/physrevb.98.205419.
[8] B. Stadtmüller et al. “Modification of Pb quantum well states by the adsorption of organic molecules”. Journal of Physics: Condensed Matter 31:13 (Feb. 2019). DOI: 10.1088/1361-648x/aafcf5.