Periodic Reporting for period 4 - hyControl (Coherent optical control of multi-functional nano-scale hybrid units)
Période du rapport: 2022-03-01 au 2023-02-28
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, 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). The goal is to use the hybridization strength as a control parameter to induce a response in the electronic properties of the NHyU. 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 the design of suitable NHyUs and the control of their physical properties related to the chosen fields of application (spintronics, spin-orbitronics and plasmonics). This hyControl concept has been implemented using a novel experimental method based on pump-probe photoemission spectroscopy, that can resolve the transient electronic structure of precisely those valence band electrons which mediate the hybridization in a single NHyU. The findings of hyControl will open the way to new technologies in various ICT applications, ranging from molecular spintronics to molecular electronics, optoelectronics, and photovoltaics.
Aim 1. We have successfully implemented the methodology that was proposed in hyControl, i.e. a setup for time- and angle-resolved photoelectron spectroscopy.
Aims 2, 3 and 4 have been successfully accomplished. Main achievements:
1. We have studied the electronic and transport properties of ordered C60 and Me3N@C80 molecular films and revealed the band structure of a C60 crystal. We studied non-interacting excitons in C60 and found that they induce a substantial energetic redistribution of all transport levels of the non-excited molecules. We followed the exciton dynamics in the excited states of C60 and found that charge-transfer excitons play a crucial role for the excited-state dynamics of C60. Crucially, these dynamics can be significantly modified upon alkali metal doping. This is a novel approach to enhance the light-to-charge carrier conversion efficiency in photovoltaic molecular materials.
2. We have demonstrated the high stability of NiTTP molecules on the 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. Based on these findings, we have shown that the functionalization of the Ni with NO2 restores the Ni(II) oxidation state while changing its configuration from low to high spin. The NHyU can be completely restored to its pristine condition simply by annealing the system, which is very useful for possible applications. Similar results were found for Fe-Phthalocyanines.
We have studied the Co/Alq3 interface as a model NHyU using a scheme for coherent optical control. We established that second layer molecules in NHyUs exhibit well-defined and rather homogeneous internal electronic transitions and that inhomogeneous line broadening in a disordered Alq3 layer and pure dephasing have only a minor impact on optical transitions in individual molecules. This opens interesting opportunities for future coherent control schemes in NHyUs.
3. We proposed a reliable method to enable self-assembly on a Cu(100) surface and at the same time to achieve decoupling of molecule and substrate: adding an additional oxygen interlayer between molecule and substrate. We have extended this approach to ferromagnetic 3d-metal surfaces. The passivated Fe(100)-p(1×1)O surface turned out to be also a model system to understand how the adsorption of atoms or molecules on a 3d-metal surface strongly influences electron correlation in the metal. Overall, adatom adsorption can be used to vary the material parameters of transition metal surfaces to access different intermediate electronic correlated regimes, which will otherwise not be accessible.
4. We have reported the formation of 2D arrangements of Sn/Bi/Pb atoms on noble metal surfaces with non-trivial spin properties. In particular, we prepared a novel Sn/Au(111) structure that shows linearly dispersing bands close to the Fermi level that resemble those expected from free-standing stanene. We also prepared a SnAu2/Au(111) surface alloy with Rashba-type spin-split bands. We have shown 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. Furthermore, we have studied NHyUs composed of PTCDA on Pb/Ag(111) using time-resolved two-photon photoelectron spectroscopy. We could show the isotropic nature of the momentum-dependent scattering processes of metallic bilayer systems and uncovered a new possibility to selectively tune and control scattering processes by selecting suitable NHyUs. Finally, we studied the excited state dynamics in the the topological insulator Bi2Se3.
Aims 2, 3 and 4 have been successfully accomplished. Main achievements:
1. We have studied the electronic and transport properties of ordered C60 and Me3N@C80 molecular films and revealed the band structure of a C60 crystal. We studied non-interacting excitons in C60 and found that they induce a substantial energetic redistribution of all transport levels of the non-excited molecules. We followed the exciton dynamics in the excited states of C60 and found that charge-transfer excitons play a crucial role for the excited-state dynamics of C60. Crucially, these dynamics can be significantly modified upon alkali metal doping. This is a novel approach to enhance the light-to-charge carrier conversion efficiency in photovoltaic molecular materials.
2. We have demonstrated the high stability of NiTTP molecules on the 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. Based on these findings, we have shown that the functionalization of the Ni with NO2 restores the Ni(II) oxidation state while changing its configuration from low to high spin. The NHyU can be completely restored to its pristine condition simply by annealing the system, which is very useful for possible applications. Similar results were found for Fe-Phthalocyanines.
We have studied the Co/Alq3 interface as a model NHyU using a scheme for coherent optical control. We established that second layer molecules in NHyUs exhibit well-defined and rather homogeneous internal electronic transitions and that inhomogeneous line broadening in a disordered Alq3 layer and pure dephasing have only a minor impact on optical transitions in individual molecules. This opens interesting opportunities for future coherent control schemes in NHyUs.
3. We proposed a reliable method to enable self-assembly on a Cu(100) surface and at the same time to achieve decoupling of molecule and substrate: adding an additional oxygen interlayer between molecule and substrate. We have extended this approach to ferromagnetic 3d-metal surfaces. The passivated Fe(100)-p(1×1)O surface turned out to be also a model system to understand how the adsorption of atoms or molecules on a 3d-metal surface strongly influences electron correlation in the metal. Overall, adatom adsorption can be used to vary the material parameters of transition metal surfaces to access different intermediate electronic correlated regimes, which will otherwise not be accessible.
4. We have reported the formation of 2D arrangements of Sn/Bi/Pb atoms on noble metal surfaces with non-trivial spin properties. In particular, we prepared a novel Sn/Au(111) structure that shows linearly dispersing bands close to the Fermi level that resemble those expected from free-standing stanene. We also prepared a SnAu2/Au(111) surface alloy with Rashba-type spin-split bands. We have shown 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. Furthermore, we have studied NHyUs composed of PTCDA on Pb/Ag(111) using time-resolved two-photon photoelectron spectroscopy. We could show the isotropic nature of the momentum-dependent scattering processes of metallic bilayer systems and uncovered a new possibility to selectively tune and control scattering processes by selecting suitable NHyUs. Finally, we studied the excited state dynamics in the the topological insulator Bi2Se3.
The impact of this project is mostly scientific but sets the seed also for an impact into society. hyCotrol has contributed to a deeper understanding of the electronic properties of nano-scale hybrid units, and more in general of interfaces formed between inorganic materials and molecules. For example, we introduced a strategy to increase the local spin of NHyUs in a reversible way, which could be very important for using NHyUs as a platform for the realization of multifunctional devices where the magnetism and the optical/transport properties can be controlled simultaneously by independent stimuli.
We also demonstrated that controlling charge transfer allows to modify correlations on 3d-metal surfaces at will, which has the potential to generate completely new classes of materials.We also opened the way for the application of optical coherent control schemes to NHyUs, which could lead to the implementation of this new class of materials for novel quantum-enabling technologies.
As such, hyControl provides a solid basis for the implementation of nano-scale functional units into a variety of ever-smaller and ever-faster building blocks for future ICT applications.
In addition, the setup developed in the framework of aim 1 is a novel methodology in the field of photoelectron spectroscopy. It not only matches the requirements to perform the experiments panned in hyControl but is a unique experimental setup for photoemission spectroscopy that could constitute a benchmark for the whole solid-state community. Indeed, it allows to record the emission pattern of the photoemitted electrons in a very large portion of momentum space in pump-probe experiments with photon energy tunable in a wide optical as well as XUV range.
We also demonstrated that controlling charge transfer allows to modify correlations on 3d-metal surfaces at will, which has the potential to generate completely new classes of materials.We also opened the way for the application of optical coherent control schemes to NHyUs, which could lead to the implementation of this new class of materials for novel quantum-enabling technologies.
As such, hyControl provides a solid basis for the implementation of nano-scale functional units into a variety of ever-smaller and ever-faster building blocks for future ICT applications.
In addition, the setup developed in the framework of aim 1 is a novel methodology in the field of photoelectron spectroscopy. It not only matches the requirements to perform the experiments panned in hyControl but is a unique experimental setup for photoemission spectroscopy that could constitute a benchmark for the whole solid-state community. Indeed, it allows to record the emission pattern of the photoemitted electrons in a very large portion of momentum space in pump-probe experiments with photon energy tunable in a wide optical as well as XUV range.