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Dr. Michael Hollerer

PhD Student Karl-Franzens-Universität GrazInstitute of Physics – Surface Science
Website: Surface Science

Biographical Info

2022

8.

P. Hurdax, C. S. Kern, T. G. Boné, A. Haags, M. Hollerer, L. Egger, X. Yang, H. Kirschner, A. Gottwald, M. Richter, F. C. Bocquet, S. Soubatch, G. Koller, F. S. Tautz, M. Sterrer, P. Puschnig, M. G. Ramsey

Large Distortion of Fused Aromatics on Dielectric Interlayers Quantified by Photoemission Orbital Tomography Journal Article

In: ACS Nano, vol. 16, pp. 17435-17443, 2022.

Abstract | Links | BibTeX

@article{Hurdax2022,
title = {Large Distortion of Fused Aromatics on Dielectric Interlayers Quantified by Photoemission Orbital Tomography},
author = {P. Hurdax and C. S. Kern and T. G. Boné and A. Haags and M. Hollerer and L. Egger and X. Yang and H. Kirschner and A. Gottwald and M. Richter and F. C. Bocquet and S. Soubatch and G. Koller and F. S. Tautz and M. Sterrer and P. Puschnig and M. G. Ramsey},
doi = {10.1021/acsnano.2c08631},
year = {2022},
date = {2022-01-01},
journal = {ACS Nano},
volume = {16},
pages = {17435-17443},
abstract = {Polycyclic aromatic compounds with fused benzene rings offer an extraordinary versatility as next-generation organic semiconducting materials for nanoelectronics and optoelectronics due to their tunable characteristics, including charge-carrier mobility and optical absorption. Nonplanarity can be an additional parameter to customize their electronic and optical properties without changing the aromatic core. In this work, we report a combined experimental and theoretical study in which we directly observe large, geometry-induced modifications in the frontier orbitals of a prototypical dye molecule when adsorbed on an atomically thin dielectric interlayer on a metallic substrate. Experimentally, we employ angle-resolved photoemission experiments, interpreted in the framework of the photoemission orbital tomography technique. We demonstrate its sensitivity to detect geometrical bends in adsorbed molecules and highlight the role of the photon energy used in experiment for detecting such geometrical distortions. Theoretically, we conduct density functional calculations to determine the geometric and electronic structure of the adsorbed molecule and simulate the photoemission angular distribution patterns. While we found an overall good agreement between experimental and theoretical data, our results also unveil limitations in current van der Waals corrected density functional approaches for such organic/dielectric interfaces. Hence, photoemission orbital tomography provides a vital experimental benchmark for such systems. By comparison with the state of the same molecule on a metallic substrate, we also offer an explanation why the adsorption on the dielectric induces such large bends in the molecule.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

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Polycyclic aromatic compounds with fused benzene rings offer an extraordinary versatility as next-generation organic semiconducting materials for nanoelectronics and optoelectronics due to their tunable characteristics, including charge-carrier mobility and optical absorption. Nonplanarity can be an additional parameter to customize their electronic and optical properties without changing the aromatic core. In this work, we report a combined experimental and theoretical study in which we directly observe large, geometry-induced modifications in the frontier orbitals of a prototypical dye molecule when adsorbed on an atomically thin dielectric interlayer on a metallic substrate. Experimentally, we employ angle-resolved photoemission experiments, interpreted in the framework of the photoemission orbital tomography technique. We demonstrate its sensitivity to detect geometrical bends in adsorbed molecules and highlight the role of the photon energy used in experiment for detecting such geometrical distortions. Theoretically, we conduct density functional calculations to determine the geometric and electronic structure of the adsorbed molecule and simulate the photoemission angular distribution patterns. While we found an overall good agreement between experimental and theoretical data, our results also unveil limitations in current van der Waals corrected density functional approaches for such organic/dielectric interfaces. Hence, photoemission orbital tomography provides a vital experimental benchmark for such systems. By comparison with the state of the same molecule on a metallic substrate, we also offer an explanation why the adsorption on the dielectric induces such large bends in the molecule.

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  • doi:10.1021/acsnano.2c08631

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2021

7.

L. Egger, M. Hollerer, C. S. Kern, H. Herrmann, P. Hurdax, A. Haags, X. Yang, A. Gottwald, M. Richter, S. Soubatch, F. S. Tautz, G. Koller, P. Puschnig, M. G. Ramsey, M. Sterrer

Charge-promoted self-metalation of porphyrins on an oxide surface Journal Article

In: Angew. Chem. Int. Ed., vol. 60, pp. 5078-5082, 2021.

Abstract | Links | BibTeX

@article{Egger2020,
title = {Charge-promoted self-metalation of porphyrins on an oxide surface},
author = {L. Egger and M. Hollerer and C. S. Kern and H. Herrmann and P. Hurdax and A. Haags and X. Yang and A. Gottwald and M. Richter and S. Soubatch and F. S. Tautz and G. Koller and P. Puschnig and M. G. Ramsey and M. Sterrer},
doi = {10.1002/anie.202015187},
year = {2021},
date = {2021-01-01},
journal = {Angew. Chem. Int. Ed.},
volume = {60},
pages = {5078-5082},
abstract = {Metalation and self-metalation reactions of porphyrins on oxide surfaces have recently gained interest. The mechanism of porphyrin self-metalation on oxides is, however, far from being understood. Herein, we show by a combination of results obtained with scanning tunneling microscopy, photoemission spectroscopy, and DFT computations, that the self-metalation of 2H-tetraphenylporphyrin on the surface of ultrathin MgO(001) films is promoted by charge transfer. By tuning the work function of the MgO(001)/Ag(001) substrate, we are able to control the charge and the metalation state of the porphyrin molecules on the surface.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

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Metalation and self-metalation reactions of porphyrins on oxide surfaces have recently gained interest. The mechanism of porphyrin self-metalation on oxides is, however, far from being understood. Herein, we show by a combination of results obtained with scanning tunneling microscopy, photoemission spectroscopy, and DFT computations, that the self-metalation of 2H-tetraphenylporphyrin on the surface of ultrathin MgO(001) films is promoted by charge transfer. By tuning the work function of the MgO(001)/Ag(001) substrate, we are able to control the charge and the metalation state of the porphyrin molecules on the surface.

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  • doi:10.1002/anie.202015187

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2020

6.

P. Hurdax, M. Hollerer, P. Puschnig, D. Lüftner, L. Egger, M. G. Ramsey, M. Sterrer

Controlling the Charge Transfer across Thin Dielectric Interlayers Journal Article

In: Adv. Mater. Interfaces, vol. 7, pp. 2000592, 2020.

Abstract | Links | BibTeX

@article{Hurdax2020,
title = {Controlling the Charge Transfer across Thin Dielectric Interlayers},
author = {P. Hurdax and M. Hollerer and P. Puschnig and D. Lüftner and L. Egger and M. G. Ramsey and M. Sterrer},
doi = {10.1002/admi.202000592},
year = {2020},
date = {2020-01-01},
journal = {Adv. Mater. Interfaces},
volume = {7},
pages = {2000592},
abstract = {Whether intentional or unintentional, thin dielectric interlayers can be found in technologies ranging from catalysis to organic electronics. While originally considered as passive decoupling layers, recently it has been shown that they can actively promote charge transfer from the underlying metal to adsorbates. This charging can have profound effects on the surface chemistry of atoms, atomic clusters, and molecules, their magnetic moments, and charge injection at the contacts of organic devices. Yet, controlled studies required to understand the charge transfer process in depth are still lacking. Here, a comprehensive analysis of the phenomenon of charge transfer using the atomically controlled system of pentacene on ultrathin MgO(100) films on Ag(100) is presented. It is shown that the charge transfer process is governed by the charged and uncharged molecular species with distinct energy levels in the first monolayer. The experimental approach applied in this work allows to observe and control their ratio through direct tuning of either the work function or the thickness of the dielectric interlayer.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

Close

Whether intentional or unintentional, thin dielectric interlayers can be found in technologies ranging from catalysis to organic electronics. While originally considered as passive decoupling layers, recently it has been shown that they can actively promote charge transfer from the underlying metal to adsorbates. This charging can have profound effects on the surface chemistry of atoms, atomic clusters, and molecules, their magnetic moments, and charge injection at the contacts of organic devices. Yet, controlled studies required to understand the charge transfer process in depth are still lacking. Here, a comprehensive analysis of the phenomenon of charge transfer using the atomically controlled system of pentacene on ultrathin MgO(100) films on Ag(100) is presented. It is shown that the charge transfer process is governed by the charged and uncharged molecular species with distinct energy levels in the first monolayer. The experimental approach applied in this work allows to observe and control their ratio through direct tuning of either the work function or the thickness of the dielectric interlayer.

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  • doi:10.1002/admi.202000592

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5.

P. Hurdax, M. Hollerer, L. Egger, G. Koller, X. Yang, A. Haags, S. Soubatch, F. S. Tautz, M. Richter, A. Gottwald, P. Puschnig, M. Sterrer, M. G. Ramsey

Controlling the electronic and physical coupling on dielectric thin films Journal Article

In: Beilstein J. Nanotechnol., vol. 11, pp. 1492-1503, 2020.

Abstract | Links | BibTeX

@article{Hurdax2020a,
title = {Controlling the electronic and physical coupling on dielectric thin films},
author = {P. Hurdax and M. Hollerer and L. Egger and G. Koller and X. Yang and A. Haags and S. Soubatch and F. S. Tautz and M. Richter and A. Gottwald and P. Puschnig and M. Sterrer and M. G. Ramsey},
doi = {10.3762/bjnano.11.132},
year = {2020},
date = {2020-01-01},
journal = {Beilstein J. Nanotechnol.},
volume = {11},
pages = {1492-1503},
abstract = {Ultrathin dielectric/insulating films on metals are often used as decoupling layers to allow for the study of the electronic properties of adsorbed molecules without electronic interference from the underlying metal substrate. However, the presence of such decoupling layers may effectively change the electron donating properties of the substrate, for example, by lowering its work function and thus enhancing the charging of the molecular adsorbate layer through electron tunneling. Here, an experimental study of the charging of para-sexiphenyl (6P) on ultrathin MgO(100) films supported on Ag(100) is reported. By deliberately changing the work function of the MgO(100)/Ag(100) system, it is shown that the charge transfer (electronic coupling) into the 6P molecules can be controlled, and 6P monolayers with uncharged molecules (Schottky–Mott regime) and charged and uncharged molecules (Fermi level pinning regime) can be obtained. Furthermore, it was found that charge transfer and temperature strongly influence the orientation, conformation, and wetting behavior (physical coupling) of the 6P layers on the MgO(100) thin films.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

Close

Ultrathin dielectric/insulating films on metals are often used as decoupling layers to allow for the study of the electronic properties of adsorbed molecules without electronic interference from the underlying metal substrate. However, the presence of such decoupling layers may effectively change the electron donating properties of the substrate, for example, by lowering its work function and thus enhancing the charging of the molecular adsorbate layer through electron tunneling. Here, an experimental study of the charging of para-sexiphenyl (6P) on ultrathin MgO(100) films supported on Ag(100) is reported. By deliberately changing the work function of the MgO(100)/Ag(100) system, it is shown that the charge transfer (electronic coupling) into the 6P molecules can be controlled, and 6P monolayers with uncharged molecules (Schottky–Mott regime) and charged and uncharged molecules (Fermi level pinning regime) can be obtained. Furthermore, it was found that charge transfer and temperature strongly influence the orientation, conformation, and wetting behavior (physical coupling) of the 6P layers on the MgO(100) thin films.

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  • doi:10.3762/bjnano.11.132

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2019

4.

M. Hollerer

Charge transfer through thin dielectric films: organic molecules on MgO(001)/Ag(001) PhD Thesis

2019.

Abstract | Links | BibTeX

@phdthesis{Hollerer2019b,
title = {Charge transfer through thin dielectric films: organic molecules on MgO(001)/Ag(001)},
author = {M. Hollerer},
url = {https://resolver.obvsg.at/urn:nbn:at:at-ubg:1-137820},
year = {2019},
date = {2019-02-01},
abstract = {The investigation of epitaxially grown, ultra-thin dielectric films on metal substrates is of great interest in the fields of catalysis and organic electronics. Here, we explore the energy alignment of adsorbates on top of dielectric/metal systems and their resulting charge state. Finally, we demonstrate the impact of charge transfer on fundamental physical and chemical. Organic and inorganic adsorbates are therefore systematically investigated on pristine silver single crystals (Ag(001)) and on Ag(001) supported magnesium oxide (MgO) ultra-thin films.We demonstrate integer charge transfer to the Lowest Unoccupied Molecular Orbital of pentacene (5A) adsorbed on MgO(001)/Ag(001) and fractional charge transfer of 0.7 electrons per molecule on pristine Ag(001). We identify the individual energy level alignment contributions and roughly quantify them. By varying the initial workfunction of the MgO(001)/Ag(001) substrate the equilibration mechanism, leading to the final, constant workfunction, is investigated.Charge transfer to 2-dimensional gold (Au) islands on MgO(001)/Ag(001) enables the catalytic formation of oxalates from carbon-dioxides. Here, we find a morphological transition from 2D to 3D Au-islands upon annealing to elevated temperatures that coincides with a significant decrease of the oxalate formation We conclude that the catalytic reactivity is facilitated by excess charges located on the rim of 2-dimensional Au-islands. In a related topic, we demonstrate that charge transfer controls the self-metalation reaction of free-base porphyrin (2H-TPP) to Mg-TPP on MgO(001)/Ag(001). We achieve this by tuning of the initial substrate workfunction, either enabling or inhibiting charge transfer, and monitoring of the N 1s XPS signal of the porphyrins. Furthermore, we present a scanning tunneling microscopy study on the monolayer formation of water on MgO(001)/Ag(001) and a combined-method growth study on Para-Sexiyphenyl (6P) on pristine Ag(001).},
keywords = {},
pubstate = {published},
tppubtype = {phdthesis}
}

Close

The investigation of epitaxially grown, ultra-thin dielectric films on metal substrates is of great interest in the fields of catalysis and organic electronics. Here, we explore the energy alignment of adsorbates on top of dielectric/metal systems and their resulting charge state. Finally, we demonstrate the impact of charge transfer on fundamental physical and chemical. Organic and inorganic adsorbates are therefore systematically investigated on pristine silver single crystals (Ag(001)) and on Ag(001) supported magnesium oxide (MgO) ultra-thin films.We demonstrate integer charge transfer to the Lowest Unoccupied Molecular Orbital of pentacene (5A) adsorbed on MgO(001)/Ag(001) and fractional charge transfer of 0.7 electrons per molecule on pristine Ag(001). We identify the individual energy level alignment contributions and roughly quantify them. By varying the initial workfunction of the MgO(001)/Ag(001) substrate the equilibration mechanism, leading to the final, constant workfunction, is investigated.Charge transfer to 2-dimensional gold (Au) islands on MgO(001)/Ag(001) enables the catalytic formation of oxalates from carbon-dioxides. Here, we find a morphological transition from 2D to 3D Au-islands upon annealing to elevated temperatures that coincides with a significant decrease of the oxalate formation We conclude that the catalytic reactivity is facilitated by excess charges located on the rim of 2-dimensional Au-islands. In a related topic, we demonstrate that charge transfer controls the self-metalation reaction of free-base porphyrin (2H-TPP) to Mg-TPP on MgO(001)/Ag(001). We achieve this by tuning of the initial substrate workfunction, either enabling or inhibiting charge transfer, and monitoring of the N 1s XPS signal of the porphyrins. Furthermore, we present a scanning tunneling microscopy study on the monolayer formation of water on MgO(001)/Ag(001) and a combined-method growth study on Para-Sexiyphenyl (6P) on pristine Ag(001).

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  • https://resolver.obvsg.at/urn:nbn:at:at-ubg:1-137820

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3.

M. Hollerer, D. Prochinig, P. Puschnig, E. Carrasco, H. -J. Freund, M. Sterrer

Scanning Tunneling Microscopy of the Ordered Water Monolayer on MgO(001)/Ag(001) Ultrathin Films Journal Article

In: J. Phys. Chem. C, vol. 123, pp. 3711-3718, 2019.

Abstract | Links | BibTeX

@article{Hollerer2019,
title = {Scanning Tunneling Microscopy of the Ordered Water Monolayer on MgO(001)/Ag(001) Ultrathin Films},
author = {M. Hollerer and D. Prochinig and P. Puschnig and E. Carrasco and H. -J. Freund and M. Sterrer},
doi = {10.1021/acs.jpcc.8b12256},
year = {2019},
date = {2019-01-01},
journal = {J. Phys. Chem. C},
volume = {123},
pages = {3711-3718},
abstract = {Two-dimensionally ordered monolayers of water on MgO(001) have been extensively studied in the past using diffraction and spectroscopic and computational methods, but direct microscopic imaging has not been reported so far. Here, we present a scanning tunneling microscopy (STM) study, supported by infrared and X-ray photoelectron spectroscopy, of the c(4 × 2)-10H2O and p(3 × 2)-6H2O structures prepared on ultrathin MgO(001)/Ag(001) films. For the applied tunneling conditions, the contrast in the STM images originates from the hydroxyl groups, which result from water dissociation within the monolayer. The observed periodicities match the structures for the energetically most favorable c(4 × 2) and p(3 × 2) monolayer phases obtained from density functional calculations. Although the molecular water species within the monolayers, which are essential for the stabilization of the hydroxyl groups, could not be resolved, the STM results presented in this study provide further confirmation of the predicted structural models of the c(4 × 2)-10H2O and p(3 × 2)-6H2O monolayers.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

Close

Two-dimensionally ordered monolayers of water on MgO(001) have been extensively studied in the past using diffraction and spectroscopic and computational methods, but direct microscopic imaging has not been reported so far. Here, we present a scanning tunneling microscopy (STM) study, supported by infrared and X-ray photoelectron spectroscopy, of the c(4 × 2)-10H2O and p(3 × 2)-6H2O structures prepared on ultrathin MgO(001)/Ag(001) films. For the applied tunneling conditions, the contrast in the STM images originates from the hydroxyl groups, which result from water dissociation within the monolayer. The observed periodicities match the structures for the energetically most favorable c(4 × 2) and p(3 × 2) monolayer phases obtained from density functional calculations. Although the molecular water species within the monolayers, which are essential for the stabilization of the hydroxyl groups, could not be resolved, the STM results presented in this study provide further confirmation of the predicted structural models of the c(4 × 2)-10H2O and p(3 × 2)-6H2O monolayers.

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  • doi:10.1021/acs.jpcc.8b12256

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2018

2.

X. Yang, I. Krieger, D. Lüftner, S. Weiß, T. Heepenstrick, M. Hollerer, P. Hurdax, G. Koller, M. Sokolowski, P. Puschnig, M. G. Ramsey, F. S. Tautz, S. Soubatch

On the decoupling of molecules at metal surfaces Journal Article

In: Chem. Commun., vol. 54, pp. 9039-9042, 2018.

Abstract | Links | BibTeX

@article{Yang2018,
title = {On the decoupling of molecules at metal surfaces},
author = {X. Yang and I. Krieger and D. Lüftner and S. Weiß and T. Heepenstrick and M. Hollerer and P. Hurdax and G. Koller and M. Sokolowski and P. Puschnig and M. G. Ramsey and F. S. Tautz and S. Soubatch},
doi = {10.1039/C8CC03334J},
year = {2018},
date = {2018-01-01},
urldate = {2018-01-01},
journal = {Chem. Commun.},
volume = {54},
pages = {9039-9042},
abstract = {We report a method to achieve physical and electronic decoupling of organic molecules from a metal surface. Oxygen adsorbed on the Cu(100) surface immobilizes the surface electrons in the Cu–O covalent bonds. This results in electronic surface hardening and prevents charge transfer from the metal into perylene-tetracarboxylic dianhydride molecules subsequently deposited on this surface.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

Close

We report a method to achieve physical and electronic decoupling of organic molecules from a metal surface. Oxygen adsorbed on the Cu(100) surface immobilizes the surface electrons in the Cu–O covalent bonds. This results in electronic surface hardening and prevents charge transfer from the metal into perylene-tetracarboxylic dianhydride molecules subsequently deposited on this surface.

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  • doi:10.1039/C8CC03334J

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2017

1.

M. Hollerer, D. Lüftner, P. Hurdax, T. Ules, S. Soubatch, F. S. Tautz, G. Koller, P. Puschnig, M. Sterrer, M. G. Ramsey

Charge Transfer and Orbital Level Alignment at Inorganic/Organic Interfaces: The Role of Dielectric Interlayers Journal Article

In: ACS Nano, vol. 11, pp. 6252-6260, 2017.

Abstract | Links | BibTeX

@article{Hollerer2017,
title = {Charge Transfer and Orbital Level Alignment at Inorganic/Organic Interfaces: The Role of Dielectric Interlayers},
author = {M. Hollerer and D. Lüftner and P. Hurdax and T. Ules and S. Soubatch and F. S. Tautz and G. Koller and P. Puschnig and M. Sterrer and M. G. Ramsey},
doi = {10.1021/acsnano.7b02449},
year = {2017},
date = {2017-01-01},
journal = {ACS Nano},
volume = {11},
pages = {6252-6260},
abstract = {It is becoming accepted that ultrathin dielectric layers on metals are not merely passive decoupling layers, but can actively influence orbital energy level alignment and charge transfer at interfaces. As such, they can be important in applications ranging from catalysis to organic electronics. However, the details at the molecular level are still under debate. In this study, we present a comprehensive analysis of the phenomenon of charge transfer promoted by a dielectric interlayer with a comparative study of pentacene adsorbed on Ag(001) with and without an ultrathin MgO interlayer. Using scanning tunneling microscopy and photoemission tomography supported by density functional theory, we are able to identify the orbitals involved and quantify the degree of charge transfer in both cases. Fractional charge transfer occurs for pentacene adsorbed on Ag(001), while the presence of the ultrathin MgO interlayer promotes integer charge transfer with the lowest unoccupied molecular orbital transforming into a singly occupied and singly unoccupied state separated by a large gap around the Fermi energy. Our experimental approach allows a direct access to the individual factors governing the energy level alignment and charge-transfer processes for molecular adsorbates on inorganic substrates.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

Close

It is becoming accepted that ultrathin dielectric layers on metals are not merely passive decoupling layers, but can actively influence orbital energy level alignment and charge transfer at interfaces. As such, they can be important in applications ranging from catalysis to organic electronics. However, the details at the molecular level are still under debate. In this study, we present a comprehensive analysis of the phenomenon of charge transfer promoted by a dielectric interlayer with a comparative study of pentacene adsorbed on Ag(001) with and without an ultrathin MgO interlayer. Using scanning tunneling microscopy and photoemission tomography supported by density functional theory, we are able to identify the orbitals involved and quantify the degree of charge transfer in both cases. Fractional charge transfer occurs for pentacene adsorbed on Ag(001), while the presence of the ultrathin MgO interlayer promotes integer charge transfer with the lowest unoccupied molecular orbital transforming into a singly occupied and singly unoccupied state separated by a large gap around the Fermi energy. Our experimental approach allows a direct access to the individual factors governing the energy level alignment and charge-transfer processes for molecular adsorbates on inorganic substrates.

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  • doi:10.1021/acsnano.7b02449

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