Publications

@article{Mearini2025,

title = {Substrate Stabilized Charge Transfer Scheme In Coverage Controlled 2D Metal Organic Frameworks},

author = {Simone Mearini and Dominik Brandstetter and Yan Yan Grisan Qiu and Daniel Baranowski and Iulia Cojocariu and Matteo Jugovac and Pierluigi Gargiani and Manuel Valvidares and Luca Schio and Luca Floreano and Andreas Windischbacher and Peter Puschnig and Vitaliy Feyer and Claus Michael Schneider},

url = {https://onlinelibrary.wiley.com/doi/10.1002/smll.202500507?af=R},

doi = {10.1002/smll.20250050},

year  = {2025},

date = {2025-02-17},

urldate = {2025-02-17},

journal = {Small},

volume = {2500507},

abstract = {Recently, 2D metal-organic frameworks (2D MOFs), characterized by complexcharge transfer mechanisms, have emerged as a promising class of networksin the development of advanced materials with tailored electronic andmagnetic properties. Following the successful synthesis of a 2D MOF formedby nickel (Ni) linkers and 7,7,8,8-tetracyanoquinodimethane (TCNQ) ligands,this work investigates how the Ni-to-ligand ratio influences the electroniccharge redistribution in an Ag(100)-supported 2D MOF. The interplaybetween linker-ligand and substrate-MOF charge transfer processes leads to astable equilibrium, resulting in a robust electronic structure that remainsindependent of stoichiometric ratios. This stability is primarily based on theelectron transfer from the metal substrate, which compensates for chargeimbalances introduced by the metal-organic coordination across differentMOF configurations. Despite minor changes observed in the magneticresponse of the Ni centers, these findings emphasize the robustness of theelectronic structure, which remains largely unaffected by structural variations,highlighting the potential of these 2D MOFs for advanced applications inelectronics and spintronics.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Nakayama2025,

title = {Anisotropic dispersion of excitonic bands of the single-crystal pentacene (001) surface as measured by low-energy angle-resolved high-resolution electron energy-loss spectroscopy},

author = {Y. Nakayama and F. C. Bocquet and R. Tsuruta and S. Soubatch and F. S. Tautz},

url = {https://www.sciencedirect.com/science/article/pii/S0368204825000015},

doi = {10.1016/j.elspec.2025.147514},

year  = {2025},

date = {2025-01-31},

urldate = {2025-01-31},

journal = {Journal of Electron Spectroscopy and Related Phenomena},

volume = {279},

issue = {147514},

abstract = {Low-energy high-resolution electron energy-loss spectroscopy (HREELS) is a useful technique for the charac-

terization of various excitation processes at solid surfaces. However, no successful work has been reported on

molecular single-crystal samples yet. In the present study, low-energy angle-resolved HREELS measurements

were conducted on single-crystal pentacene, an organic semiconductor. The results confirmed the excitonic

bands exhibiting energy–momentum dispersion and anisotropy of these depending on the surface crystallo-

graphic directions, corroborating the occurrence of exciton delocalization, contrary to the ordinary notion of the

Frenkel exciton for weakly interacting van der Waals molecular solids. The present results demonstrate that low-

energy angle-resolved HREELS is applicable to the precise examination of the excitonic characteristics of solid-

state surfaces, even for molecular semiconductor single crystals.},

keywords = {EELS, exciton, organic semiconductor, single crystal, vibration},

pubstate = {published},

tppubtype = {article}

}

@article{Qiu2025,

title = {Conformation-Driven Nickel Redox States and Magnetism in 2D Metal–organic Frameworks},

author = {Yan Yan Grisan Qiu and Dominik Brandstetter and Simone Mearini and Daniel Baranowski and Iulia Cojocariu and Matteo Jugovac and Giovanni Zamborlini and Pierluigi Gargiani and Manuel Valvidares and Andreas Windischbacher and Peter Puschnig andVitaliy Feyer and Claus Michael Schneider},

url = {https://advanced.onlinelibrary.wiley.com/doi/10.1002/adfm.202418186?af=R},

doi = {10.1002/adfm.202418186},

year  = {2025},

date = {2025-01-29},

journal = {Adv. Funct. Mater.},

volume = {2418186},

abstract = {2D metal–organic frameworks (2D MOFs) attract considerable attention because of their versatile properties and as potential candidates for single-atom catalysis, high-density information storage media or molecular electronics and spintronics devices. Their unique characteristics arise from an intricate interplay between the metal center, the surrounding ligands and the underlying substrate. Here, the intrinsic magnetic and electronic properties of a single-layer MOF on graphene is investigated with a combination of spectroscopic techniques and theoretical modeling. Taking advantage of the weak interaction between the MOF and graphene substrate, it is specifically focused on the influence of the coordination environment on these properties. Notably, two distinct coordination configurations are observed for the transition metal centers within the 2D MOF, and clarify how axial distortions in the ligand field affect the hybridization between the Ni 3d states and the π-symmetric molecular orbitals of 7,7,8,8-tetracyanoquinodimethane ligands, leading to the coexistence of two Ni redox states with different spin configurations. Furthermore, the transition from a nearly free-standing MOF is examined to metal-supported frameworks, elucidating the impact of substrate interactions on the electronic and magnetic properties. The findings advance the understanding of MOFs and offer insights into developing functional materials with tailored magnetic and electronic properties.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{arXiv:2501.05287,

title = {Tomographic identification of all molecular orbitals in a wide binding energy range},

author = {A. Haags and D. Brandstetter and X. Yang and L. Egger and H. Kirschner and A. Gottwald and M. Richter and G. Koller and F. C. Bocquet and C. Wagner and M. G. Ramsey and S. Soubatch and P. Puschnig and F. S. Tautz},

url = {https://arxiv.org/abs/2501.05287},

year  = {2025},

date = {2025-01-09},

urldate = {2025-01-09},

journal = {ArXiv},

abstract = {In the past decade, photoemission orbital tomography (POT) has evolved into a powerful tool to investigate the electronic structure of organic molecules adsorbed on surfaces. Here we show that POT allows for the comprehensive experimental identification of all molecular orbitals in a substantial binding energy range, in the present case more than 10 eV. Making use of the angular distribution of photoelectrons as a function of binding energy, we exemplify this by extracting orbital-resolved partial densities of states (pDOS) for 15 π and 23 σ orbitals from the experimental photoemission intensities of the prototypical organic molecule bisanthene (C28H14) on a Cu(110) surface. In their entirety, these experimentally measured orbital-resolved pDOS for an essentially complete set of orbitals serve as a stringent benchmark for electronic structure methods, which we illustrate by performing density functional theory (DFT) calculations employing four frequently-used exchange-correlation functionals. By computing the respective molecular-orbital-projected densities of states of the bisanthene/Cu(110) interface, a one-to-one comparison with experimental data for an unprecedented number of 38 orbital energies becomes possible. The quantitative analysis of our data reveals that the range-separated hybrid functional HSE performs best for the investigated organic/metal interface. At a more fundamental level, the remarkable agreement between the experimental and the Kohn-Sham orbital energies over a binding energy range larger than 10,eV suggests that -- perhaps unexpectedly -- Kohn-Sham orbitals approximate Dyson orbitals, which would rigorously account for the electron extraction process in photoemission spectroscopy but are notoriously difficult to compute, in a much better way than previously thought. },

keywords = {},

pubstate = {forthcoming},

tppubtype = {article}

}

@article{Mearini2024,

title = {Band Structure Engineering in 2D Metal–Organic Frameworks},

author = {S. Mearini, D. Baranowski, D. Brandstetter, A. Windischbacher, I. Cojocariu, P. Gargiani, M. Valvidares, L. Schio, L. Floreano, P. Puschnig, V. Feyer, C. M. Schneider},

url = {https://onlinelibrary.wiley.com/doi/10.1002/advs.202404667},

doi = {10.1002/advs.202404667},

year  = {2024},

date = {2024-08-09},

urldate = {2024-08-09},

journal = {Advanced Science},

volume = {11},

number = {2404667},

issue = {38},

abstract = {The design of 2D metal–organic frameworks (2D MOFs) takes advantage ofthe combination of the diverse electronic properties of simple organic ligandswith different transition metal (TM) centers. The strong directional nature ofthe coordinative bonds is the basis for the structural stability and the periodicarrangement of the TM cores in these architectures. Here, direct and clearevidence that 2D MOFs exhibit intriguing energy-dispersive electronic bandswith a hybrid character and distinct magnetic properties in the metal cores,resulting from the interactions between the TM electronic levels and theorganic ligand𝝅-molecular orbitals, is reported. Importantly, a method toeffectively tune both the electronic structure of 2D MOFs and the magneticproperties of the metal cores by exploiting the electronic structure of distinctTMs is presented. Consequently, the ionization potential characteristic ofselected TMs, particularly the relative energy position and symmetry of the 3dstates, can be used to strategically engineer bands within specificmetal–organic frameworks. These findings not only provide a rationale forband structure engineering in 2D MOFs but also offer promisingopportunities for advanced material design.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Emergence of Band Structure in a Two- Dimensional Metal−Organic Framework upon Hierarchical Self-Assembly},

author = {D. Baranowski and M. Thaler and D. Brandstetter and A. Windischbacher and I. Cojocariu and S. Mearini and V. Chesnyak and L. Schio and L. Floreano and C. Gutiérrez Bolaños and P. Puschnig and L. L. Patera and V. Feyer and C. M. Schneider},

url = {https://pubs.acs.org/doi/10.1021/acsnano.4c04191#},

doi = {10.1021/acsnano.4c04191},

year  = {2024},

date = {2024-07-17},

urldate = {2024-07-17},

journal = {ACS Nano},

volume = {18},

pages = {19618−19627},

abstract = {Two-dimensional metal−organic frameworks (2D-MOFs) represent a category of atomically thin materials that combine the structural tunability of molecular systems with the crystalline structure characteristic of solids. The strong bonding between the organic linkers and transition metal centers is expected to result in delocalized electronic states. However, it remains largely unknown how the band structure in 2D-MOFs emerges through the coupling of electronic states in the building blocks. Here, we demonstrate the on-surface synthesis of a 2D-MOF exhibiting prominent π-conjugation. Through a combined experimental and theoretical approach, we provide direct evidence of band structure formation upon hierarchical self-assembly, going from metal−organic complexes to a conjugated two-dimensional framework. Additionally, we identify the robustly dispersive nature of the emerging hybrid states, irrespective of the metallic support type, highlighting the tunability of the band structure through charge transfer from the substrate. Our findings encourage the exploration of band-structure engineering in 2D-MOFs for potential applications in electronics and photonics.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Bennecke2024,

title = {Disentangling the multiorbital contributions of excitons by photoemission exciton tomography},

author = {W. Bennecke and A. Windischbacher and D. Schmitt and J. P. Bange and R. Hemm and C. S. Kern and G. D’Avino and X. Blase and D. Steil and S. Steil and M. Aeschlimann and B. Stadtmüller and M. Reutzel and P. Puschnig and G. S. M. Jansen and S. Mathias},

doi = {10.1038/s41467-024-45973-x},

year  = {2024},

date = {2024-02-28},

urldate = {2024-02-28},

journal = {Nature Communications},

volume = {15},

number = {1804},

pages = {10},

abstract = {Excitons are realizations of a correlated many-particle wave function, specifi-cally consisting of electrons and holes in an entangled state. Excitons occurwidely in semiconductors and are dominant excitations in semiconductingorganic and low-dimensional quantum materials. To efficiently harness thestrong optical response and high tuneability of excitons in optoelectronics andin energy-transformation processes,access to the full wavefunction of theentangled state is critical, but has so far not been feasible. Here, we show howtime-resolved photoemission momentum microscopy can be used to gainaccess to the entangled wavefunction and to unravel the exciton’s multiorbitalelectron and hole contributions. For the prototypical organic semiconductorbuckminsterfullerene (C60), we exemplify the capabilities of exciton tomo-graphy and achieve unprecedented access to key properties of the entangledexciton state including localization, charge-transfer character, and ultrafastexciton formation and relaxation dynamics.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Klein2024,

title = {Peri-Tetracene from 1,1′-Bitetracene: Zipping up Structurally Defined Graphene Nanoribbons},

author = {M. Klein and J. B. Bauer and N. Kainbacher and M. S. Wagner and K. Greulich and P. Haizmann and E. Giangrisostomi and R. Ovsyannikov and P. Puschnig and T. Chassé and H. F. Bettinger and H. Peisert},

doi = {10.1021/acs.jpcc.3c08182},

year  = {2024},

date = {2024-02-22},

journal = {J. Phys. Chem. C},

volume = {128},

pages = {4048-4059},

abstract = {Polycyclicaromatichydrocarbons(PAHs) are promising molecules for a manifold of applications in organic electronics, spintronics, or energy storage devices. Among PAHs, particular attention has been focused on the synthesis and study of acenes and fused acenes, peri-acenes, allowing tuning of the highest occupied molecular orbital−lowest unoccupied molecular orbital (HOMO−LUMO) gap with the size of the conjugated system. As a starting point for the surface synthesis of larger PAHs, we synthesized a 1,1′-bitetracene for the first time. This precursor molecule consists of two tetracene units connected via the 1,1′-position with a torsion angle of 70°.  The interface properties of the molecule before and after annealing on a Cu(111) surface are investigated. Using X-ray photoemission spectroscopy(XPS), angle-resolved photoelectron spectroscopy (ARPES), low-energy electron diffraction (LEED), and scanning-tunneling microscopy (STM), it is experimentally demonstrated that the tetracene units zip up with the help of heat forming peri-tetracene. These results and the exact adsorption geometry are in excellent agreement with calculations using density functionaltheory (DFT). Moreover, the calculations enable the identification of newly formed valence band states at the interface to Cu(111).},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Adamkiewicz2023,

title = {Coherent and Incoherent Excitation Pathways in Time-Resolved Photoemission Orbital Tomography of CuPc/Cu(001)-2O},

author = {A. Adamkiewicz and M. Raths and M. Stettner and M. Theilen and L. Münster and S. Wenzel and M. Hutter and S. Soubatch and C. Kumpf and F. C. Bocquet and R. Wallauer and F. S. Tautz and U. Höfer},

doi = {10.1021/acs.jpcc.3c04859},

year  = {2023},

date = {2023-10-09},

urldate = {2023-10-09},

journal = {J.Phys. Chem. C},

volume = {127},

pages = {20411},

abstract = {Time-resolved photoemission orbital tomography (tr-POT) offers unique possibilities for tracing molecular electron dynamics. The recorded pump-induced changes of the angle-resolved photoemission intensities allow one to characterize unoccupied molecular states in momentum space and to deduce the incoherent temporal evolution of their population. Here, we show for the example of CuPc/Cu(001)-2O that the method also gives access to the coherent regime and that different excitation pathways can be disentangled by a careful analysis of the time-dependent change of the photoemission momentum pattern. In particular, we demonstrate by varying photon energy and polarization of the pump light how the incoherent temporal evolution of the LUMO distribution can be distinguished from coherent contributions of the projected HOMO. Moreover, we report the selective excitation of molecules with a specific orientation at normal incidence by aligning the electric field of the pump light along the molecular axis.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Kern2023,

title = {Photoemission orbital tomography for excitons in organic molecules},

author = {C. S. Kern and A. Windischbacher and P. Puschnig},

doi = {https://doi.org/10.1103/PhysRevB.108.085132},

year  = {2023},

date = {2023-08-22},

urldate = {2023-08-22},

journal = {Phys. Rev. B},

volume = {108},

pages = {085132},

abstract = {Driven by recent developments in time-resolved photoemission spectroscopy, we extend the successful method of photoemission orbital tomography (POT) to excitons. Our theory retains the intuitive orbital picture of POT, while respecting both the entangled character of the exciton wave function and the energy conservation in the photoemission process. Analyzing results from three organic molecules, we classify generic exciton structures and give a simple interpretation in terms of natural transition orbitals. We validate our findings by directly simulating pump-probe experiments with time-dependent density functional theory.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Ito2023,

title = {Build-up and dephasing of Floquet-Bloch bands on subcycle timescales},

author = {S. Ito and M. Schüler and M. Meierhofer and S. Schlauderer and J. Freudenstein and R. Reimann and D. Afanasiev and K. A. Kokh and O. E. Tereshchenko and J. Güdde and M. A. Sentef and U. Höfer and R. Huber

},

doi = {10.1038/s41586-023-05850-x},

year  = {2023},

date = {2023-04-12},

urldate = {2023-04-12},

journal = {Nature},

volume = {616},

pages = {pages 696–701},

abstract = {Strong light fields have created opportunities to tailor novel functionalities of solids. Floquet–Bloch states can form under periodic driving of electrons and enable exotic quantum phases. On subcycle timescales, lightwaves can simultaneously drive intraband currents and interband transition, which enable high-harmonic generation and pave the way towards ultrafast electronics. Yet, the interplay of intraband and interband excitations and their relation to Floquet physics have been key open questions as dynamical aspects of Floquet states have remained elusive. Here we provide this link by visualizing the ultrafast build-up of Floquet–Bloch bands with time-resolved and angle-resolved photoemission spectroscopy. We drive surface states on a topological insulator with mid-infrared fields—strong enough for high-harmonic generation—and directly monitor the transient band structure with subcycle time resolution. Starting with strong intraband currents, we observe how Floquet sidebands emerge within a single optical cycle; intraband acceleration simultaneously proceeds in multiple sidebands until high-energy electrons scatter into bulk states and dissipation destroys the Floquet bands. Quantum non-equilibrium calculations explain the simultaneous occurrence of Floquet states with intraband and interband dynamics. Our joint experiment and theory study provides a direct time-domain view of Floquet physics and explores the fundamental frontiers of ultrafast band-structure engineering.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Freudenstein2022,

title = {Attosecond clocking of correlations between Bloch electrons},

author = {J. Freudenstein and M. Borsch and M. Meierhofer and D. Afanasiev and C. P. Schmid and F. Sandner and M. Liebich and A. Girnghuber and M. Knorr and M. Kira and R. Huber},

doi = {10.1038/s41586-022-05190-2},

year  = {2022},

date = {2022-10-12},

urldate = {2022-10-12},

journal = {Nature},

volume = {610},

pages = {290--295},

abstract = {Delocalized Bloch electrons and the low-energy correlations between them determine key optical, electronic and entanglement functionalities of solids, all the way through to phase transitions. To directly capture how many-body correlations affect the actual motion of Bloch electrons, subfemtosecond (1 fs = 10−15 s) temporal precision is desirable. Yet, probing with attosecond (1 as = 10−18 s) high-energy photons has not been energy-selective enough to resolve the relevant millielectronvolt-scale interactions of electrons near the Fermi energy. Here, we use multi-terahertz light fields to force electron–hole pairs in crystalline semiconductors onto closed trajectories, and clock the delay between separation and recollision with 300 as precision, corresponding to 0.7% of the driving field’s oscillation period. We detect that strong Coulomb correlations emergent in atomically thin WSe2 shift the optimal timing of recollisions by up to 1.2 ± 0.3 fs compared to the bulk material. A quantitative analysis with quantum-dynamic many-body computations in a Wigner-function representation yields a direct and intuitive view on how the Coulomb interaction, non-classical aspects, the strength of the driving field and the valley polarization influence the dynamics. The resulting attosecond chronoscopy of delocalized electrons could revolutionize the understanding of unexpected phase transitions and emergent quantum-dynamic phenomena for future electronic, optoelectronic and quantum-information technologies.},

keywords = {Selected Preliminary Work},

pubstate = {published},

tppubtype = {article}

}

@article{Yang2022,

title = {Momentum-selective orbital hybridisation},

author = {X. Yang and M. Jugovac and G. Zamborlini and V. Feyer and G. Koller and P. Puschnig and S. Soubatch and M. G. Ramsey and F. S. Tautz},

doi = {10.1038/s41467-022-32643-z},

year  = {2022},

date = {2022-01-01},

urldate = {2022-01-01},

journal = {Nat. Commun.},

volume = {13},

pages = {5148},

abstract = {When a molecule interacts chemically with a metal surface, the orbitals of the molecule hybridise with metal states to form the new eigenstates of the coupled system. Spatial overlap and energy matching are determining parameters of the hybridisation. However, since every molecular orbital does not only have a characteristic spatial shape, but also a specific momentum distribution, one may additionally expect a momentum matching condition; after all, each hybridising wave function of the metal has a defined wave vector, too. Here, we report photoemission orbital tomography measurements of hybrid orbitals that emerge from molecular orbitals at a molecule-on-metal interface. We find that in the hybrid orbitals only those partial waves of the original orbital survive which match the metal band structure. Moreover, we find that the conversion of the metal’s surface state into a hybrid interface state is also governed by momentum matching constraints. Our experiments demonstrate the possibility to measure hybridisation momentum-selectively, thereby enabling deep insights into the complicated interplay of bulk states, surface states, and molecular orbitals in the formation of the electronic interface structure at molecule-on-metal hybrid interfaces.},

keywords = {Selected Preliminary Work},

pubstate = {published},

tppubtype = {article}

}

@article{Haags2021,

title = {Momentum-space imaging of σ-orbitals for chemical analysis},

author = {A. Haags and X. Yang and L. Egger and D. Brandstetter and H. Kirschner and F. C. Bocquet and G. Koller and A. Gottwald and M. Richter and J. M. Gottfried and M. G. Ramsey and P. Puschnig and S. Soubatch and F. S. Tautz},

doi = {10.1126/sciadv.abn0819},

year  = {2022},

date = {2022-01-01},

urldate = {2022-01-01},

journal = {Sci. Adv.},

volume = {8},

pages = {eabn0819},

abstract = {Tracing the modifications of molecules in surface chemical reactions benefits from the possibility to image their orbitals. While delocalized frontier orbitals with π character are imaged routinely with photoemission orbital tomography, they are not always sensitive to local chemical modifications, particularly the making and breaking of bonds at the molecular periphery. For such bonds, σ orbitals would be far more revealing. Here, we show that these orbitals can indeed be imaged in a remarkably broad energy range and that the plane wave approximation, an important ingredient of photoemission orbital tomography, is also well fulfilled for these orbitals. This makes photoemission orbital tomography a unique tool for the detailed analysis of surface chemical reactions. We demonstrate this by identifying the reaction product of a dehalogenation and cyclodehydrogenation reaction.},

keywords = {Selected Preliminary Work},

pubstate = {published},

tppubtype = {article}

}

@article{Stallberg2022,

title = {Ultrafast Exciton Dynamics and Charge Transfer at PTCDA/Metal Interfaces},

author = {K. Stallberg and A. Namgalies and S. Chatterjee and U. Höfer},

doi = {10.1021/acs.jpcc.2c04696},

year  = {2022},

date = {2022-01-01},

journal = {J. Phys. Chem. C},

volume = {126},

pages = {12728--12734},

abstract = {The functionality of organic electronic devices is governed by the dynamics of charge carriers and excited states in organic semiconductors. In particular, the relaxation of excitons and the transfer of charge carriers at metal electrodes crucially determine the performance of organic optoelectronic devices. In a combined experimental study we apply time-resolved photoluminescence and two-photon photoemission to reveal the ultrafast exciton dynamics and charge transfer at prototype organic/metal contacts comprising thin molecular films on single-crystalline noble-metal surfaces. On the basis of experiments with systematically varied film thicknesses, we relate the strong quenching of Frenkel excitons and charge-transfer excitons to the wave function overlap with the metal, indicating charge transfer as the dominant relaxation pathway. Moreover, the presence of an electronic interface state is found to facilitate the transfer of excited carriers across the organic/metal interface.},

keywords = {Selected Preliminary Work},

pubstate = {published},

tppubtype = {article}

}

@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 = {Selected Preliminary Work},

pubstate = {published},

tppubtype = {article}

}

@article{Wallauer2020,

title = {Tracing orbital images on ultrafast time scales},

author = {R. Wallauer and M. Raths and K. Stallberg and L. Münster and D. Brandstetter and X. Yang and J. Güdde and P. Puschnig and S. Soubatch and C. Kumpf and F. C. Bocquet and F. S. Tautz and U. Höfer},

doi = {10.1126/science.abf3286},

year  = {2021},

date = {2021-01-01},

urldate = {2021-01-01},

journal = {Science},

volume = {371},

pages = {1056-1059},

abstract = {Frontier orbitals determine fundamental molecular properties such as chemical reactivities. Although electron distributions of occupied orbitals can be imaged in momentum space by photoemission tomography, it has so far been impossible to follow the momentum-space dynamics of a molecular orbital in time, for example, through an excitation or a chemical reaction. Here, we combined time-resolved photoemission using high laser harmonics and a momentum microscope to establish a tomographic, femtosecond pump-probe experiment of unoccupied molecular orbitals. We measured the full momentum-space distribution of transiently excited electrons, connecting their excited-state dynamics to real-space excitation pathways. Because in molecules this distribution is closely linked to orbital shapes, our experiment may, in the future, offer the possibility of observing ultrafast electron motion in time and space.},

keywords = {Selected Preliminary Work},

pubstate = {published},

tppubtype = {article}

}

@article{Wallauer2021,

title = {Momentum-Resolved Observation of Exciton Formation Dynamics in Monolayer WS2},

author = {R. Wallauer and R. Perea-Causin and L. Münster and S. Zajusch and S. Brem and J. Güdde and K. Tanimura and K. -Q. Lin and R. Huber and E. Malic and U. Höfer},

doi = {10.1021/acs.nanolett.1c01839},

year  = {2021},

date = {2021-01-01},

journal = {Nano Lett.},

volume = {21},

pages = {5867--5873},

abstract = {The dynamics of momentum-dark exciton formation in transition metal dichalcogenides is difficult to measure experimentally, as many momentum-indirect exciton states are not accessible to optical interband spectroscopy. Here, we combine a tunable pump, high-harmonic probe laser source with a 3D momentum imaging technique to map photoemitted electrons from monolayer WS2. This provides momentum-, energy- and time-resolved access to excited states on an ultrafast time scale. The high temporal resolution of the setup allows us to trace the early-stage exciton dynamics on its intrinsic time scale and observe the formation of a momentum-forbidden dark KΣ exciton a few tens of femtoseconds after optical excitation. By tuning the excitation energy, we manipulate the temporal evolution of the coherent excitonic polarization and observe its influence on the dark exciton formation. The experimental results are in excellent agreement with a fully microscopic theory, resolving the temporal and spectral dynamics of bright and dark excitons in WS2.},

keywords = {Selected Preliminary Work},

pubstate = {published},

tppubtype = {article}

}

@article{Reimann2018,

title = {Subcycle observation of lightwave-driven Dirac currents in a topological surface band},

author = {J. Reimann and S. Schlauderer and C. P. Schmid and F. Langer and S. Baierl and K. A. Kokh and O. E. Tereshchenko and A. Kimura and C. Lange and J. Güdde and U. Höfer and R. Huber},

doi = {10.1038/s41586-018-0544-x},

year  = {2018},

date = {2018-01-01},

journal = {Nature},

volume = {562},

pages = {396--400},

abstract = {Harnessing the carrier wave of light as an alternating-current bias may enable electronics at optical clock rates1. Lightwave-driven currents have been assumed to be essential for high-harmonic generation in solids2–6, charge transport in nanostructures7,8, attosecond-streaking experiments9–16 and atomic-resolution ultrafast microscopy17,18. However, in conventional semiconductors and dielectrics, the finite effective mass and ultrafast scattering of electrons limit their ballistic excursion and velocity. The Dirac-like, quasi-relativistic band structure of topological insulators19–29 may allow these constraints to be lifted and may thus open a new era of lightwave electronics. To understand the associated, complex motion of electrons, comprehensive experimental access to carrier-wave-driven currents is crucial. Here we report angle-resolved photoemission spectroscopy with subcycle time resolution that enables us to observe directly how the carrier wave of a terahertz light pulse accelerates Dirac fermions in the band structure of the topological surface state of Bi2Te3. While terahertz streaking of photoemitted electrons traces the electromagnetic field at the surface, the acceleration of Dirac states leads to a strong redistribution of electrons in momentum space. The inertia-free surface currents are protected by spin–momentum locking and reach peak densities as large as two amps per centimetre, with ballistic mean free paths of several hundreds of nanometres, opening up a realistic parameter space for all-coherent lightwave-driven electronic devices. Furthermore, our subcycle-resolution analysis of the band structure may greatly improve our understanding of electron dynamics and strong-field interaction in solids.},

keywords = {Selected Preliminary Work},

pubstate = {published},

tppubtype = {article}

}

@article{Armbrust2017,

title = {Model potential for the description of metal/organic interface states},

author = {N. Armbrust and F. Schiller and J. Güdde and U. Höfer},

doi = {10.1038/srep46561},

year  = {2017},

date = {2017-01-01},

journal = {Sci. Rep.},

volume = {7},

pages = {46561},

abstract = {We present an analytical one-dimensional model potential for the description of electronic interface states that form at the interface between a metal surface and flat-lying adlayers of π-conjugated organic molecules. The model utilizes graphene as a universal representation of these organic adlayers. It predicts the energy position of the interface state as well as the overlap of its wave function with the bulk metal without free fitting parameters. We show that the energy of the interface state depends systematically on the bond distance between the carbon backbone of the adayers and the metal. The general applicability and robustness of the model is demonstrated by a comparison of the calculated energies with numerous experimental results for a number of flat-lying organic molecules on different closed-packed metal surfaces that cover a large range of bond distances.},

keywords = {Selected Preliminary Work},

pubstate = {published},

tppubtype = {article}

}

@article{Cocker2016,

title = {Tracking the ultrafast motion of a single molecule by femtosecond orbital imaging},

author = {T. L. Cocker and D. Peller and P. Yu and J. Repp and R. Huber},

doi = {10.1038/nature19816},

year  = {2016},

date = {2016-01-01},

urldate = {2020-06-09},

journal = {Nature},

volume = {539},

pages = {263--267},

abstract = {Watching a single molecule move calls for measurements that combine ultrafast temporal resolution with atomic spatial resolution; this is now shown to be possible by combining scanning tunnelling microscopy with lightwave electronics, through a technique that involves removing a single electron from the highest occupied orbital of a single pentacene molecule in a time window shorter than an oscillation cycle of light.},

keywords = {Selected Preliminary Work},

pubstate = {published},

tppubtype = {article}

}

@article{Weiss2015,

title = {Exploring three-dimensional orbital imaging with energy-dependent photoemission tomography},

author = {S. Weiß and D. Lüftner and T. Ules and E. M. Reinisch and H. Kaser and A. Gottwald and M. Richter and S. Soubatch and G. Koller and M. G. Ramsey and F. S. Tautz and P. Puschnig},

doi = {10.1038/ncomms9287},

year  = {2015},

date = {2015-01-01},

journal = {Nat. Commun.},

volume = {6},

pages = {8287},

abstract = {Recently, it has been shown that experimental data from angle-resolved photoemission spectroscopy on oriented molecular films can be utilized to retrieve real-space images of molecular orbitals in two dimensions. Here, we extend this orbital tomography technique by performing photoemission initial state scans as a function of photon energy on the example of the brickwall monolayer of 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) on Ag(110). The overall dependence of the photocurrent on the photon energy can be well accounted for by assuming a plane wave for the final state. However, the experimental data, both for the highest occupied and the lowest unoccupied molecular orbital of PTCDA, exhibits an additional modulation attributed to final state scattering effects. Nevertheless, as these effects beyond a plane wave final state are comparably small, we are able, with extrapolations beyond the attainable photon energy range, to reconstruct three-dimensional images for both orbitals in agreement with calculations for the adsorbed molecule.},

keywords = {Selected Preliminary Work},

pubstate = {published},

tppubtype = {article}

}

@article{Lueftner2014,

title = {Imaging the wave functions of adsorbed molecules},

author = {D. Lüftner and T. Ules and E. M. Reinisch and G. Koller and S. Soubatch and F. S. Tautz and M. G. Ramsey and P. Puschnig},

doi = {10.1073/pnas.1315716110},

year  = {2014},

date = {2014-01-01},

journal = {PNAS},

volume = {111},

number = {2},

pages = {605-610},

abstract = {The basis for a quantum-mechanical description of matter is electron wave functions. For atoms and molecules, their spatial distributions and phases are known as orbitals. Although orbitals are very powerful concepts, experimentally only the electron densities and -energy levels are directly observable. Regardless whether orbitals are observed in real space with scanning probe experiments, or in reciprocal space by photoemission, the phase information of the orbital is lost. Here, we show that the experimental momentum maps of angle-resolved photoemission from molecular orbitals can be transformed to real-space orbitals via an iterative procedure which also retrieves the lost phase information. This is demonstrated with images obtained of a number of orbitals of the molecules pentacene (C22H14) and perylene-3,4,9,10-tetracarboxylic dianhydride (C24H8O6), adsorbed on silver, which are in excellent agreement with ab initio calculations. The procedure requires no a priori knowledge of the orbitals and is shown to be simple and robust.},

keywords = {Selected Preliminary Work},

pubstate = {published},

tppubtype = {article}

}

@article{Puschnig2011,

title = {Orbital tomography: Deconvoluting photoemission spectra of organic molecules},

author = {P. Puschnig and E. M. Reinisch and T. Ules and G. Koller and S. Soubatch and M. Ostler and L. Romaner and F. S. Tautz and C. Ambrosch-Draxl and M. G. Ramsey},

doi = {10.1103/PhysRevB.84.235427},

year  = {2011},

date = {2011-12-01},

urldate = {2011-12-01},

journal = {Phys. Rev. B},

volume = {84},

pages = {235427},

publisher = {American Physical Society},

abstract = {We study the interface of an organic monolayer with a metallic surface, i.e., PTCDA (3,4,9,10-perylene-tetracarboxylic-dianhydride) on Ag(110), by means of angle-resolved photoemission spectroscopy (ARPES) and ab initio electronic structure calculations. We present a tomographic method that uses the energy and momentum dependence of ARPES data to deconvolute spectra into individual orbital contributions beyond the limits of energy resolution. This provides an orbital-by-orbital characterization of large adsorbate systems without the need to invoke a sophisticated theory of photoemission, allowing us to directly estimate the effects of bonding on individual orbitals. Moreover, these experimental data serve as a most stringent test necessary for the further development of ab initio electronic structure theory.},

keywords = {Selected Preliminary Work},

pubstate = {published},

tppubtype = {article}

}

@article{Puschnig2009,

title = {Reconstruction of Molecular Orbital Densities from Photoemission Data},

author = {P. Puschnig and S. Berkebile and A. J. Fleming and G. Koller and K. Emtsev and T. Seyller and J. D. Riley and C. Ambrosch-Draxl and F. P. Netzer and M. G. Ramsey},

doi = {10.1126/science.1176105},

year  = {2009},

date = {2009-01-01},

urldate = {2009-01-01},

journal = {Science},

volume = {326},

number = {5953},

pages = {702-706},

abstract = {Photoemission spectroscopy is commonly applied to study the band structure of solids by measuring the kinetic energy versus angular distribution of the photoemitted electrons. Here, we apply this experimental technique to characterize discrete orbitals of large π-conjugated molecules. By measuring the photoemission intensity from a constant initial-state energy over a hemispherical region, we generate reciprocal space maps of the emitting orbital density. We demonstrate that the real-space electron distribution of molecular orbitals in both a crystalline pentacene film and a chemisorbed p-sexiphenyl monolayer can be obtained from a simple Fourier transform of the measurement data. The results are in good agreement with density functional calculations.},

keywords = {Selected Preliminary Work},

pubstate = {published},

tppubtype = {article}

}