# 2016-2020

##### T. Jasper-Toennies, M. Gruber, S. Johannsen, T. Frederiksen, A. Garcia-Lekue, T. Jäkel, F. Roehricht, R. Herges, R. Berndt, ACS Nano 14, 3907 (2020)

Abstract:
Molecular rotors have attracted considerable interest for their prospects in nanotechnology. However, their adsorption on supporting substrates, where they may be addressed individually, usually modifies their properties. Here, we investigate the switching of two closely related three-state rotors mounted on platforms on Au(111) using low-temperature scanning tunneling microscopy and density functional theory calculations. Being physisorbed, the platforms retain important gas-phase properties of the rotor. This simplifies a detailed analysis and permits, for instance, the identification of the vibrational modes involved in the rotation process. The symmetry provided by the platform enables active control of the rotation direction through electrostatic interactions with the tip and charged neighboring adsorbates. The present investigation of two model systems may turn out useful for designing platforms that provide directional rotation and for transferring more sophisticated molecular machines from the gas phase to surfaces.

###### [BibTeX]
@article{TJT_ACSN20,
author = {Jasper-Toennies, Torben and Gruber, Manuel and Johannsen, Sven and Frederiksen, Thomas and Garcia-Lekue, Aran and Jäkel, Torben and Roehricht, Fynn and Herges, Rainer and Berndt, Richard},
title = {Rotation of Ethoxy and Ethyl Moieties on a Molecular Platform on Au(111)},
journal = {ACS Nano},
volume = {14},
number = {4},
pages = {3907-3916},
year = {2020},
note = {PMID: 32073820},
url = {https://doi.org/10.1021/acsnano.0c00029},
doi = {https://doi.org/10.1021/acsnano.0c00029}
}

##### T. Jasper-Tönnies, M. Gruber, S. Ulrich, R. Herges, R. Berndt, Angew. Chem. Int. Ed. 59, 7008 (2020)

Abstract:
The competition between honeycomb and hexagonal tiling of molecular units can lead to large honeycomb superstructures on surfaces. Such superstructures exhibit pores that may be used as 2D templates for functional guest molecules. Honeycomb superstructures of molecules that comprise a C3 symmetric platform on Au(111) and Ag(111) surfaces are presented. The superstructures cover nearly mesoscopic areas with unit cells containing up to 3000 molecules, more than an order of magnitude larger than previously reported. The unit cell size may be controlled by the coverage. A fairly general model was developed to describe the energetics of honeycomb superstructures built from C3 symmetric units. Based on three parameters that characterize two competing bonding arrangements, the model is consistent with the present experimental data and also reproduces various published results. The model identifies the relevant driving force, mostly related to geometric aspects, of the pattern formation.

###### [BibTeX]
@article{TJT_ACIE20,
author = {Jasper-Tönnies, Torben and Gruber, Manuel and Ulrich, Sandra and Herges, Rainer and Berndt, Richard},
title = {Coverage-Controlled Superstructures of C3-Symmetric Molecules: Honeycomb versus Hexagonal Tiling},
journal = {Angew. Chem. Int. Ed.},
volume = {59},
number = {18},
pages = {7008-7017},
year = {2020},
url = {https://onlinelibrary.wiley.com/doi/abs/10.1002/anie.202001383},
doi = {https://doi.org/10.1002/anie.202001383}
}

##### M. Mohr, M. Gruber, A. Weismann, D. Jacob, P. Abufanger, N. Lorente, R. Berndt, Phys. Rev. B 101, 075414 (2020)

Abstract:
The current I through nickelocene molecules and its noise are measured with a low temperature scanning tunneling microscope on a Cu(100) substrate. Density functional theory calculations and many-body modeling are used to analyze the data. During contact formation, two types of current evolution are observed, an abrupt jump to contact and a smooth transition. These data along with conductance spectra (dI/dV ) recorded deep in the contact range are interpreted in terms of a transition from a spin-1 to a spin-1/2 state that is Kondo screened. Many-body calculations show that the smooth transition is also consistent with a renormalization of spin excitations of a spin-1 molecule by Kondo exchange coupling. The shot noise is significantly reduced compared to the Schottky value of 2eI. The noise can be described in the Landauer picture in terms of the spin polarization of the transmission of ≈ 35% through two degenerate dπ-orbitals of the Nickelocene molecule.

###### [BibTeX]
@article{MMO_PRB20,
author = {Mohr, Michael and Gruber, Manuel and Weismann, Alexander and Jacob, David and Abufanger, Paula and Lorente, Nicolas and Berndt, Richard},
title = {Spin dependent transmission of nickelocene-copper contacts probed with shot noise},
journal = {Phys. Rev. B},
volume = {101},
pages = {075414},
year = {2020},
doi = {https://doi.org/10.1103/PhysRevB.101.075414}
}

##### A. Köbke, F. Gutzeit, F. Röhricht, A. Schlimm, J. Grunwald, F. Tuczek, M. Studniarek, D. Longo, F. Choueikani, E. Otero, P. Ohresser, S. Rohlf, S. Johannsen, F. Diekmann, K. Rossnagel, A. Weismann, T. Jasper-Toennies, C. Näther, R. Herges, R. Berndt, M. Gruber, Nat. Nanotechnol. 15, 18 (2020)

Abstract:
Molecular spin switches are attractive candidates for controlling the spin polarization developing at the interface between molecules and magnetic metal surfaces, which is relevant for molecular spintronics devices. However, so far, intrinsic spin switches such as spin-crossover complexes have suffered from fragmentation or loss of functionality following adsorption on metal surfaces, with rare exceptions. Robust metal-organic platforms, on the other hand, rely on external axial ligands to induce spin switching. Here we integrate a spin switching functionality into robust complexes, relying on the mechanical movement of an axial ligand strapped to the porphyrin ring. Reversible interlocked switching of spin and coordination, induced by electron injection, is demonstrated on Ag(111) for this class of compounds. The stability of the two spin and coordination states of the molecules exceeds days at 4 K. The potential applications of this switching concept go beyond the spin functionality, and may turn out to be useful for controlling the catalytic activity of surfaces.

###### [BibTeX]
@article{AKO_NNAT20,
author = {Köbke, Alexander and Gutzeit, Florian and Röhricht, Fynn and Schlimm, Alexander and Grunwald, Jan and Tuczek, Felix and Studniarek, Michał and Longo, Danilo and Choueikani, Fadi and Otero, Edwige and Ohresser, Philippe and Rohlf, Sebastian and Johannsen, Sven and Diekmann, Florian and Rossnagel, Kai and Weismann, Alexander and Jasper-Toennies, Torben and Näther, Christian and Herges, Rainer and Berndt, Richard and Gruber, Manuel},
title = {Reversible coordination-induced spin-state switching in complexes on metal surfaces},
journal = {Nature Nanotechnology},
volume = {15},
number = {1},
pages = {18--21},
year = {2020},
url = {https://doi.org/10.1038/s41565-019-0594-8},
doi = {https://doi.org/10.1038/s41565-019-0594-8}
}

##### T. Wu, L. Liu, Y. Zhang, Y. Wang, Z. Shen, N. Li, R. Berndt, S. Hou, Y. Wang, Chem. Commun. 56, 968 (2020)

Abstract:
Defining the axis of a molecular rotation is vital for the bottom-up design of molecular rotors. The rotation of tin-phthalocyanine molecules on the Ag(111) surface is studied by scanning tunneling microscopy and atomic/molecular manipulation at 4 K. Tin-phthalocyanine acts as a molecular rotor that binds to Ag adatoms and the substrate. Four different rotation axes are constructed at positions from the center to the periphery of the molecule. Furthermore, using the asymmetric appearance of the modified molecule, the rotation direction of the molecules is identified. This work provides a new approach for designing molecular rotors or motors with definable rotation radii and functions.

###### [BibTeX]
@article{TWU_CCO20,
author = {Wu, Tianhao and Liu, Liwei and Zhang, Yajie and Wang, Yifan and Shen, Ziyong and Li, Na and Berndt, Richard and Hou, Shimin and Wang, Yongfeng},
title = {Tuning rotation axes of single molecular rotors by a combination of single-atom manipulation and single-molecule chemistry},
journal = {Chem. Commun.},
volume = {56},
pages = {968-971},
publisher = {The Royal Society of Chemistry},
year = {2020},
url = {http://dx.doi.org/10.1039/C9CC07440F},
doi = {https://doi.org/10.1039/C9CC07440F}
}

##### N.A. Franchina Vergel, A. Tadjine, V. Notot, M. Mohr, A. Kouassi N'Guissan, C. Coinon, M. Berthe, L. Biadala, K.K. Sossoe, M.M. Dzagli, J.-C. Girard, G. Rodary, L. Desplanque, R. Berndt, D. Stiévenard, X. Wallart, C. Delerue, B. Grandidier, Phys. Rev. Materials 3, 094604 (2019)

Abstract:
A steplike density of states is the hallmark of a two-dimensional electron gas. Here, scanning tunneling spectroscopy is used to investigate this property in the differential conductance of an In 0.53Ga0.47As quantum well grown on p-type doped InP(001). We show that the appearance of a steplike function in the conduction band strongly depends on both the hole concentration in the quantum well and the current injected into the quantized states. Based on four-point probe transport measurements and tunneling-induced light-emission spectroscopy, we discuss the physical mechanisms at the origin of the current and unveil the significant contribution of midgap surface states in the relaxation of the tunneling electrons. These surface states, via pinning of the Fermi level, also affect the potential landscape across the quantum well, as demonstrated by tight-binding calculations of the quantum well electronic structure.

###### [BibTeX]
@article{FVE_PRM19,
author = {Franchina Vergel, N. A. and Tadjine, A. and Notot, V. and Mohr, M. and Kouassi N'Guissan, A. and Coinon, C. and Berthe, M. and Biadala, L. and Sossoe, K. K. and Dzagli, M. M. and Girard, J.-C. and Rodary, G. and Desplanque, L. and Berndt, R. and Stiévenard, D. and Wallart, X. and Delerue, C. and Grandidier, B.},
title = {Influence of doping level and surface states in tunneling spectroscopy of an In_0.53Ga_0.47As quantum well grown on p-type doped InP(001)},
journal = {Phys. Rev. Materials},
volume = {3},
pages = {094604},
publisher = {American Physical Society},
year = {2019},
doi = {https://doi.org/10.1103/PhysRevMaterials.3.094604}
}

##### S. Rohlf, J. Grunwald, T. Jasper-Toennies, S. Johannsen, F. Diekmann, M. Studniarek, R. Berndt, F. Tuczek, K. Rossnagel, M. Gruber, J. Phys. Chem. C 123, 17774 (2019)

Abstract:
The bistability of spin-crossover complexes on surfaces is of great interest for potential applications. Using X-ray absorption spectroscopy, we investigated the properties of [Fe(pypyr(CF3)2)2(phen)] (pypyr = 2-(2′-pyridyl)pyrrolide, phen = 1,10-phenanthroline), a vacuum-evaporable Fe(II) complex, in direct contact to a set of substrates. The electronic properties of these substrates range from metallic to semiconducting. While dissociation is observed on metal surfaces, efficient light-induced switching is realized on semimetallic and semiconducting surfaces. This indicates that the density of states of the substrate at the Fermi level plays a role for the integrity and functionality of the adsorbed compound. In an intermediate case, namely, [Fe(pypyr(CF3)2)2(phen)] on graphene/Ni(111), functional and dissociated species are found to coexist. This result indicates that some previous studies may deserve to be reconsidered because the possibility of coexisting intact and fragmented spin-crossover complexes was neglected.

###### [BibTeX]
@article{SRO_JPCC19,
author = {Rohlf, Sebastian and Grunwald, Jan and Jasper-Toennies, Torben and Johannsen, Sven and Diekmann, Florian and Studniarek, Michał and Berndt, Richard and Tuczek, Felix and Rossnagel, Kai and Gruber, Manuel},
title = {Influence of Substrate Electronic Properties on the Integrity and Functionality of an Adsorbed Fe(II) Spin-Crossover Compound},
journal = {J. Phys. Chem. C},
volume = {123},
number = {29},
pages = {17774-17780},
year = {2019},
url = {https://doi.org/10.1021/acs.jpcc.9b02845},
doi = {https://doi.org/10.1021/acs.jpcc.9b02845}
}

##### M. Mohr, T. Jasper-Toennies, A. Weismann, T. Frederiksen, A. Garcia-Lekue, S. Ulrich, R. Herges, R. Berndt, Phys. Rev. B 99, 245417 (2019)

Abstract:
The shot noise of the current I through junctions to single trioxatriangulenium cations (TOTA+) on Au(111) is measured with a low-temperature scanning tunneling microscope using Au tips. The noise is significantly reduced compared to the Poisson noise power of 2eI and varies linearly with the junction conductance. The data are consistent with electron transmission through a single spin-degenerate transport channel and show that TOTA+ in a Au contact does not acquire an unpaired electron. Ab initio calculations reproduce the observations and show that the current involves the lowest unoccupied orbital of the molecule and tip states close to the Fermi level.

###### [BibTeX]
@article{MMO_PRB19,
author = {Mohr, Michael and Jasper-Toennies, Torben and Weismann, Alexander and Frederiksen, Thomas and Garcia-Lekue, Aran and Ulrich, Sandra and Herges, Rainer and Berndt, Richard},
title = {Conductance channels of a platform molecule on Au(111) probed with shot noise},
journal = {Phys. Rev. B},
volume = {99},
pages = {245417},
publisher = {American Physical Society},
year = {2019},
doi = {https://doi.org/10.1103/PhysRevB.99.245417}
}

##### T. Jasper-Tönnies, A. Garcia-Lekue, T. Frederiksen, S. Ulrich, R. Herges, R. Berndt, J. Phys.: Condens. Matter 31, 18LT01 (2019)

Abstract:
The conductances of molecules physisorbed to Au(111) via an extended π system are probed with the tip of a low-temperature scanning tunneling microscope to maximize the control of the junction geometry. Inert hydrogen, methyl, and reactive propynyl subunits were attached to the platform and stand upright. Because of their different reactivities, either non-bonding (hydrogen and methyl) or bonding (propynyl) tip-molecule contacts are formed. The conductances exhibit little scatter between different experimental runs on different molecules, display distinct evolutions with the tip-subunit distance, and reach contact values of 0.003-0.05 G0. For equal tip-platform distances the contact conductance of the inert methyl is close to that of the reactive propynyl. Under further compression, the inert species, hydrogen and methyl, are found to be better conductors. This shows that the current flow is not directly correlated with the chemical interaction. Atomistic calculations for the methyl case reproduce the conductance evolution and reveal the role of the junction geometry, forces and orbital symmetries at the tip-molecule interface. The current flow is controlled by orbital symmetries at the electrode interfaces rather than by the energy alignment of the molecular orbitals and electrode states. Functionalized molecular platforms thus open new ways to control and engineer electron conduction through metal-molecule interfaces at the atomic level.

###### [BibTeX]
@article{TJT_JPCM19,
author = {T. Jasper-Tönnies and A. Garcia-Lekue and T. Frederiksen and S. Ulrich and R. Herges and R. Berndt},
title = {High-conductance contacts to functionalized molecular platforms physisorbed on Au(111)},
journal = {Journal of Physics: Condensed Matter},
volume = {31},
number = {18},
pages = {18LT01},
publisher = {IOP Publishing},
year = {2019},
doi = {https://doi.org/10.1088/1361-648X/ab0489}
}

##### T. Knaak, C. González, Y.J. Dappe, G.D. Harzmann, T. Brandl, M. Mayor, R. Berndt, M. Gruber, J. Phys. Chem. C 123, 4178 (2019)

Abstract:
Spin-crossover complexes are attractive for their spin-switching functionality. However, only a few compounds have been found to remain intact in direct contact to metal surfaces. For the design of new spin-crossover complexes, it is important to understand the mechanisms leading to fragmentation. Here, we investigate, using low-temperature scanning tunneling microscopy along with density functional theory calculations, two Fe(terpyridine)2 complexes deposited on Au(111) by electrospray ionization with in-line mass selection. Only fragments of the first compound are observed on the surface, while the second compound is strongly flattened. On the basis of a detailed analysis of the adsorbates on the surface, possible mechanisms for the fragmentation and molecular distortion are proposed.

###### [BibTeX]
@article{TKN_JPCC19,
author = {Knaak, Thomas and González, César and Dappe, Yannick J. and Harzmann, Gero D. and Brandl, Thomas and Mayor, Marcel and Berndt, Richard and Gruber, Manuel},
title = {Fragmentation and Distortion of Terpyridine-Based Spin-Crossover Complexes on Au(111)},
journal = {J. Phys. Chem. C},
volume = {123},
number = {7},
pages = {4178-4185},
year = {2019},
url = {https://doi.org/10.1021/acs.jpcc.8b11242},
doi = {https://doi.org/10.1021/acs.jpcc.8b11242}
}

##### N. Noei, A. Weismann, R. Berndt, Beilstein J. Nanotechnol. 9, 3048 (2018)

Abstract:
Spatially resolved measurements of the apparent tunneling barrier height Φapp in scanning tunneling microscopy have been used to estimate variations of the local work function Φ of surface structures. We experimentally show that Φapp can fail as a measure of Φ. The discrepancies are attributed to a kinetic-energy contribution to Φapp. This contribution depends on the lateral extent of the tunneling current filament and, consequently, on the local surface structure.

###### [BibTeX]
@article{NNO_BJN18,
author = {Neda Noei and Alexander Weismann and Richard Berndt},
title = {Apparent tunneling barrier height and local work function of atomic arrays},
journal = {Beilstein Journal of Nanotechnology},
volume = {9},
pages = {3048--3052},
publisher = {Beilstein Institut},
year = {2018},
doi = {https://doi.org/10.3762/bjnano.9.283}
}

##### M.-L. Bocquet, N. Lorente, R. Berndt, M. Gruber, Angew. Chem. (2018)

Abstract:
Depositing intact organic radicals on various substrates can be challenging. Generally, inert metal surfaces present more chances of hosting the intact radicals, but the large amounts of delocalized electronic states favor charge transfer and ultimately spin quenching. Reducing the moleculeâ€substrate interaction is a usual strategy to stabilize radicals on surfaces. To this end, thin insulating layers have been introduced to provide a controllable degree of electronic decoupling. Recently, retinoid molecules adsorbed on gold have been manipulated with a scanning tunneling microscope (STM) to exhibit a localized spin. Previous calculations have failed to find a radical derivative of the molecule on the surface. Here, we propose the formation of a neutral radical that is spatially localized in a tilted and lifted cyclic end of the molecule. The spin is created in a two-step process: a [1,3] sigmatropic hydrogen shift yielding an allene motif and the abstraction of the shifted hydrogen atom. The allene function provokes a perpendicular tilt of the cyclic end with respect to the rest of the conjugated chain and thus localizes the spin of the dehydrogenated allene in its lifted subpart. The results of density functional theory (DFT) calculations and additional STM manipulations lend support to the proposed mechanism. This work indicates that the controlled generation of molecular spins directly on a metallic surface may be achieved with a range of closed-shell molecules.

###### [BibTeX]
@article{MBO_ACH18,
author = {Marie-Laure Bocquet and Nicolas Lorente and Richard Berndt and Manuel Gruber},
title = {Spin in a closed-shell organic molecule on a metal substrate generated by a sigmatropic reaction},
journal = {Angew. Chem.},
publisher = {Wiley},
year = {2018},
doi = {https://doi.org/10.1002/ange.201812121}
}

##### J. Kröger, N. Néel, R. Berndt, Y. Wang, T. Gopakumar, in Encyclopedia of Interfacial Chemistry: Surface Science and Electrochemistry, K. Wandelt (Ed.) , 81-98 (Elsevier, 2018)

Abstract:
This article reviews some of the works of the authors on phthalocyanine adsorption on surfaces. Using a scanning tunneling microscope (STM), structural, electronic, and vibrational properties as well as single-molecule chemical reactions and electron transport across molecular junctions have been studied. The findings are relevant to general phenomena at the organic-inorganic interface.

###### [BibTeX]
@inbook{JKR_ELS17,
author = {J. Kröger and N. Néel and R. Berndt and Y.F. Wang and T.G. Gopakumar},
chapter = {Exploring the Organic-Inorganic Interface With a Scanning Tunneling Microscope},
title = {Encyclopedia of Interfacial Chemistry: Surface Science and Electrochemistry},
booktitle = {Encyclopedia of Interfacial Chemistry: Surface Science and Electrochemistry - Reference Module in Chemistry, Molecular Sciences and Chemical Engineering},
editor = {K. Wandelt},
pages = {81--98},
publisher = {Elsevier},
year = {2018},
doi = {https://doi.org/10.1016/b978-0-12-409547-2.14144-7},
isbn = {978-0-12-809894-3}
}

##### J. Kröger, A. Weismann, R. Berndt, S. Altenburg, T. Knaak, M. Gruber, A. Burtzlaff, N. Néel, J. Schöneberg, L. Limot, T. Uchihashi, J. Zhang, in Atomic- and nanoscale magnetism, R. Wiesendanger (Hrsg.) , 25-53 (Springer, 2018)

Abstract:
The Kondo effect of adatoms on surfaces may to some extent be controlled by manipulating their electronic and geometric environment. Results are presented from artificial structures like quantum well systems, from arrangements of single atoms made with a scanning tunneling microscope, and from custom-made molecules. Spin-orbit coupling at single adatoms is probed via measurements of the anisotropic magnetoresistance, in particular in single atom contacts. Such junctions are also investigated with respect to the current shot noise, which is influenced by the electron spin.

###### [BibTeX]
@inbook{JKR_SPR18,
author = {Jörg Kröger and Alexander Weismann and Richard Berndt and Simon Altenburg and Thomas Knaak and Manuel Gruber and Andreas Burtzlaff and Nicolas Néel and Johannes Schöneberg and Laurent Limot and Takashi Uchihashi and Jianwei Zhang},
chapter = {Scanning tunneling spectroscopies of magnetic atoms, clusters, and molecules},
title = {Atomic- and nanoscale magnetism},
booktitle = {Atomic- and Nanoscale Magnetism},
editor = {R. Wiesendanger},
pages = {25--53},
publisher = {Springer},
year = {2018},
doi = {https://doi.org/10.1007/978-3-319-99558-8_2}
}

##### T. Jasper-Tönnies, I. Poltavsky, S. Ulrich, T. Moje, A. Tkatchenko, R. Herges, R. Berndt, The Journal of Chemical Physics 149, 244705 (2018)

Abstract:
Trioxatriangulenium (TOTA) platform molecules were functionalized with methyl, ethyl, ethynyl, propynyl and hydrogen and sublimated onto Au(111) surfaces. Low-temperature scanning tunneling microscopy data reveal that > 99% of ethyl-TOTA and methyl-TOTA remain intact whereas 60% of H-TOTA and > 99% of propynyl-TOTA and ethynyl-TOTA decompose. The observed tendency towards fragmentation on Au(111) is opposite to the sequence of gas-phase stabilities of the molecules. Although Au(111) is the noblest of all metal surfaces, the binding energies of the decomposition products to Au(111) destabilize the functionalized platforms by 2 to 3.9 eV (190-370 kJ/mol) and even render some of them unstable as revealed by density functional theory calculations. Van der Waals forces are important, as they drive the adsorption of the platform molecules.

###### [BibTeX]
@article{TJT_JCP18,
author = {Jasper-Tönnies, Torben and Poltavsky, Igor and Ulrich, Sandra and Moje, Tobias and Tkatchenko, Alexandre and Herges, Rainer and Berndt, Richard},
title = {Stability of Functionalized Platform Molecules on Au(111)},
journal = {The Journal of Chemical Physics},
volume = {149},
number = {24},
pages = {244705},
publisher = {AIP Publishing},
year = {2018},
doi = {https://doi.org/10.1063/1.5059344}
}

##### H. Zheng, Y. Xu, G.-C. Dong, J. Kröger, R. Berndt, Surf. Rev. Lett. 0, 1850204 (2018)

Abstract:
Room temperature scanning tunneling microscopy was used to investigate the adsorption of a dye molecule, iron-phthalocyanine (FePc), on ZnO(0001). Submolecular resolution reveals the orientation of molecules with respect to crystallographic directions of the surface. Upon adsorption, the molecular symmetry is reduced. First-principles calculations trace these observations to a strong molecule-substrate bond, which induces deformations of the molecule.

###### [BibTeX]
@article{HZH_SRL18,
author = {Zheng, Hao and Xu, Yong and Dong, Guo-Cai and Kröger, Jörg and Berndt, Richard},
title = {STRUCTURE OF A PHTHALOCYANINE DYE ON ZnO},
journal = {Surf. Rev. Lett.},
volume = {0},
number = {0},
pages = {1850204},
year = {2018},
url = {https://doi.org/10.1142/S0218625X18502049},
doi = {http://doi.org/10.1142/S0218625X18502049}
}

##### M. Gruber, A. Weismann, R. Berndt, J. Phys. Cond. Mat. 30, 424001 (2018)

Abstract:
In the scanning tunnelling microscope, the many-body Kondo effect leads to a zero-bias feature of the differential conductance spectra of magnetic adsorbates on surfaces. The intrinsic line shape of this Kondo resonance and its temperature dependence in principle contain valuable information. We use measurements on a molecular Kondo system, all-trans retinoic acid on Au(111), and model calculations to discuss the role of instrumental broadening. The modulation voltage used for the lock-in detection, noise on the sample voltage, and the temperature of the microscope tip are considered. These sources of broadening affect the apparent line shapes and render diffcult a determination of the intrinsic line width, in particular when variable temperatures are involved.

###### [BibTeX]
@article{MGR_JPCM18,
author = {Gruber, Manuel and Weismann, Alexander and Berndt, Richard},
title = {The Kondo Resonance Line Shape in Scanning Tunneling Spectroscopy: Instrumental Aspects},
journal = {J. Phys. Cond. Mat.},
volume = {30},
number = {42},
pages = {424001},
year = {2018},
}

##### S. Rohlf, M. Gruber, B.M. Flöser, J. Grunwald, S. Jarausch, F. Diekmann, M. Kalläne, T. Jasper-Toennies, A. Buchholz, W. Plass, R. Berndt, F. Tuczek, K. Rossnagel, J. Phys. Chem. Lett. 9, 1491 (2018)

Abstract:
Understanding and controlling the spin-crossover properties of molecular complexes can be of particular interest for potential applications in molecular spintronics. Using near-edge X-ray absorption fine structure spectroscopy, we investigated these properties for a new vacuum-evaporable Fe(II) complex, namely [Fe(pypyr(CF3)2)2(phen)] (pypyr = 2-(2'-pyridyl)pyrrolide, phen = 1,10-phenanthroline). We find that the spin-transition temperature, well above room temperature for the bulk compound, is drastically lowered for molecules arranged in thin films. Furthermore, while within the experimentally accessible temperature range (2 K < T < 410 K) the bulk material shows indication of neither light-induced excited spin-state trapping nor soft X-ray-induced excited spin-state trapping, these effects are observed for molecules within thin films up to temperatures around 100 K. Thus, by arranging the molecules into thin films, a nominal low-spin complex is effectively transformed into a spin-crossover complex.

###### [BibTeX]
@article{SRO_JPCL18,
author = {Rohlf, Sebastian and Gruber, Manuel and Fl\"oser, Benedikt M. and Grunwald, Jan and Jarausch, Simon and Diekmann, Florian and Kall\"ane, Matthias and Jasper-Toennies, Torben and Buchholz, Axel and Plass, Winfried and Berndt, Richard and Tuczek, Felix and Rossnagel, Kai},
title = {Light-Induced Spin Crossover in an Fe(II) Low-Spin Complex Enabled by Surface Adsorption},
journal = {J. Phys. Chem. Lett.},
volume = {9},
number = {7},
pages = {1491-1496},
year = {2018},
note = {PMID: 29510617},
url = { https://doi.org/10.1021/acs.jpclett.8b00338},
doi = {http://doi.org/10.1021/acs.jpclett.8b00338}
}

##### J. Schöneberg, P. Ferriani, S. Heinze, A. Weismann, R. Berndt, Phys. Rev. B 97, 041114 (2018)

Abstract:
Pb dimers on a ferromagnetic surface are shown to exhibit large tunneling anisotropic magnetoresistance (TAMR) due to molecular π orbitals. Dimers oriented differently with respect to the magnetization directions of a ferromagnetic Fe double layer on W(110) were made with a scanning tunneling microscope. Depending on the dimer orientations, TAMR is absent or as large as 20% at the Fermi level. General arguments and first-principles calculations show that mixing of molecular orbitals due to spin-orbit coupling, which leads to TAMR, is maximal when the magnetization is oriented parallel to the dimer axis.

###### [BibTeX]
@article{JSC_PRB18,
author = {Schöneberg, Johannes and Ferriani, Paolo and Heinze, Stefan and Weismann, Alexander and Berndt, Richard},
title = {Tunneling anisotropic magnetoresistance via molecular \pi orbitals of Pb dimers},
journal = {Phys. Rev. B},
volume = {97},
pages = {041114},
publisher = {American Physical Society},
year = {2018},
doi = {http://doi.org/10.1103/PhysRevB.97.041114}
}

##### S. Karan, C. García, M. Karolak, D. Jacob, N. Lorente, R. Berndt, Nano Lett. 18, 88 (2018)

Abstract:
The ability of molecules to maintain magnetic multistability in nanoscale-junctions will determine their role in downsizing spintronic devices. While spin-injection from ferromagnetic leads gives rise to magnetoresistance in metallic nanocontacts, nonmagnetic leads probing the magnetic states of the junction itself have been considered as an alternative. Extending this experimental approach to molecular junctions, which are sensitive to chemical parameters, we demonstrate that the electron affinity of a molecule decisively influences its spin transport. We use a scanning tunneling microscope to trap a meso-substituted iron porphyrin, putting the iron center in an environment that provides control of its charge and spin states. A large electron affinity of peripheral ligands is shown to enable switching of the molecular S = 1 ground state found at low electron density to S = 1/2 at high density, while lower affinity keeps the molecule inactive to spin-state transition. These results pave the way for spin control using chemical design and electrical means.

###### [BibTeX]
@article{SKA_NLE18,
author = {Karan, Sujoy and García, Carlos and Karolak, Michael and Jacob, David and Lorente, Nicolas and Berndt, Richard},
title = {Spin Control Induced by Molecular Charging in a Transport Junction},
journal = {Nano Letters},
volume = {18},
number = {1},
pages = {88-93},
year = {2018},
note = {PMID: 29232947},
url = {https://doi.org/10.1021/acs.nanolett.7b03411},
doi = {http://doi.org/10.1021/acs.nanolett.7b03411}
}

##### T. Knaak, M. Gruber, S. Puhl, F. Benner, A. Escribano, J. Heck, R. Berndt, J. Phys. Chem. C 121, 26777 (2017)

Abstract:
Interconnected molecular magnetic centers on metallic surfaces are of interest for molecular spintronics. Complexes composed of two or three cobaltocene units linked by naphthalene or benzene groups are successfully deposited on Au(111) and Cu(111) by sublimation and electrospray deposition. Low-temperature scanning tunneling microscopy and spectroscopy are employed to investigate the deposited compounds and their spin state. Although all molecules are composed of the same magnetic cobaltocene unit, only one compound shows a zero-bias feature compatible with a Kondo resonance, whose amplitude varies from molecule to molecule. The amplitude variation and its absence for the other investigated complexes are attributed to different moleculeâ€“substrate coupling, which is strongly influenced by the linker. Parameters influencing the moleculeâ€“substrate coupling and molecular properties are extracted from the experimental data. These key parameters should be considered for future strategies of interconnected magnetic centers on metallic substrates.

###### [BibTeX]
@article{TKN_JPCC17_2,
author = {Knaak, Thomas and Gruber, Manuel and Puhl, Sarah and Benner, Florian and Escribano, Alejandra and Heck, J\"{u}rgen and Berndt, Richard},
title = {Interconnected Cobaltocene Complexes on Metal Surfaces},
journal = {J. Phys. Chem. C},
volume = {121},
number = {48},
pages = {26777-26784},
year = {2017},
url = { http://dx.doi.org/10.1021/acs.jpcc.7b07302},
doi = {http://doi.org/10.1021/acs.jpcc.7b07302}
}

##### K. Scheil, N. Lorente, M.-L. Bocquet, P.C. Hess, M. Mayor, R. Berndt, J. Phys. Chem. C 121, 25303 (2017)

Abstract:
The structure of molecular adlayers is of great interest for surface functionalization. As molecular complexity increases, the subtle interplay of the relevant interactions becomes more difficult to unravel. Here, we present a scanning tunneling microscope (STM) and atomic force microscope study along with free-energy calculations using density functional theory on two closely related NDI-cyclophane molecules. These three-dimensional double-decker molecules are designed to attach to the substrate with one subunit while the other functional moiety is exposed to the environment. The molecular arrangements obtained on Ag(111) are rationalized by the inclusion of adatoms from the substrate into the structure. The presence of adatoms is identified by a drastic change in corrugation of the STM images that takes place at moderate bias voltages. Our calculations using density functional theory of the systemâ€™s free-energy yield that two adatoms favorably coadsorb with the molecules.

###### [BibTeX]
@article{KSC_JPCC17,
author = {Scheil, Katharina and Lorente, Nicol\'{a}s and Bocquet, Marie-Laure and Hess, Pascal Christian and Mayor, Marcel and Berndt, Richard},
journal = {J. Phys. Chem. C},
volume = {121},
number = {45},
pages = {25303-25308},
year = {2017},
url = {http://dx.doi.org/10.1021/acs.jpcc.7b08953},
doi = {http://doi.org/10.1021/acs.jpcc.7b08953}
}

##### T. Knaak, M. Gruber, C. Lindström, M.-L. Bocquet, J. Heck, R. Berndt, Nano Lett. 17, 7146 (2017)

Abstract:
Magnetic sandwich complexes are of particular interest for molecular spintronics. Using scanning tunneling microscopy, we evidence the successful deposition of 1,3,5-tris(η6-borabenzene-η5-cyclopentadienylcobalt) benzene, a molecule composed of three connected magnetic sandwich units, on Cu(111). Scanning tunneling spectra reveal two distinct spatial-dependent narrow resonances close to the Fermi level for the trimer molecules as well as for molecular fragments composed of one and two magnetic units. With the help of density functional theory, these resonances are interpreted as two Kondo resonances originating from two distinct nondegenerate d-like orbitals. These Kondo resonances are found to have defined spatial extents dictated by the hybridization of the involved orbitals with that of the ligands. These results opens promising perspectives for investigating complex Kondo systems composed of several â€œKondoâ€ orbitals.

###### [BibTeX]
@article{TKN_NLE17,
author = {Knaak, Thomas and Gruber, Manuel and Lindstr\"{o}m, Christoph and Bocquet, Marie-Laure and Heck, J\"{u}rgen and Berndt, Richard},
title = {Ligand-Induced Energy Shift and Localization of Kondo Resonances in Cobalt-Based Complexes on Cu(111)},
journal = {Nano Letters},
volume = {17},
number = {11},
pages = {7146-7151},
year = {2017},
note = {PMID: 29045149},
url = { http://dx.doi.org/10.1021/acs.nanolett.7b04181},
doi = {http://doi.org/10.1021/acs.nanolett.7b04181}
}

##### T. Jasper-Toennies, M. Gruber, S. Karan, H. Jacob, F. Tuczek, R. Berndt, Nano Lett. 17, 6613 (2017)

Abstract:
The switching between two spin states makes spin-crossover molecules on surfaces very attractive for potential applications in molecular spintronics. Using scanning tunneling microscopy, the successful deposition of [Fe(pap)2]+ (pap = N-2-pyridylmethylidene-2-hydroxyphenylaminato) molecules on Cu2N/Cu(100) surface is evidenced. The deposited FeIII spin-crossover compound is controllably switched between three different states, each of them exhibiting a characteristic tunneling conductance. The conductance is therefore employed to readily read the state of the molecules. A comparison of the experimental data with the results of density functional theory calculations reveals that all Fe(pap)2 molecules are initially in their high-spin state. The two other states are compatible with the low-spin state of the molecule but differ with respect to their coupling to the substrate. As a proof of concept, the reversible and selective nature of the switching is used to build a two-molecule memory.

###### [BibTeX]
@article{TJT_NLE17,
author = {Jasper-Toennies, Torben and Gruber, Manuel and Karan, Sujoy and Jacob, Hanne and Tuczek, Felix and Berndt, Richard},
title = {Robust and Selective Switching of an FeIII Spin-Crossover Compound on Cu2N/Cu(100) with Memristance Behavior},
journal = {Nano Letters},
volume = {17},
number = {11},
pages = {6613-6619},
year = {2017},
note = {PMID: 29023129},
url = {http://dx.doi.org/10.1021/acs.nanolett.7b02481

},
doi = {http://doi.org/10.1021/acs.nanolett.7b02481}
}

##### H. Zheng, J. Zhang, R. Berndt, Scientific Reports 7, 10764 (2017)

Abstract:
Double quantum dots (DQDs) are a versatile platform for solid-state physics, quantum computation and nanotechnology. The micro-fabrication techniques commonly used to fabricate DQDs are difficult to extend to the atomic scale. Using an alternative approach, which relies on scanning tunneling microscopy and spectroscopy, we prepared a minimal DQD in a wide band-gap semiconductor matrix. It is comprised of a pair of strongly coupled donor atoms that can each be doubly charged. The donor excitation diagram of this system mimicks the charge stability diagram observed in transport measurements of DQDs. We furthermore illustrate how the charge and spin degrees of freedom of the minimal DQD may be used to obtain a single quantum bit and to prepare a Bell state. The results open an intriguing perspective for quantum electronics with atomic-scale structures.

###### [BibTeX]
@article{HZH_SIR17,
author = {Hao Zheng and Junyi Zhang and Richard Berndt},
title = {A minimal double quantum dot},
journal = {Scientific Reports},
volume = {7},
pages = {10764},
year = {2017},
doi = {http://doi.org/10.1038/s41598-017-10814-z}
}

##### Y. Zhang, Y. Wang, J.-T. Lü, M. Brandbyge, R. Berndt, Angew. Chem. 129, 11931 (2017)

Abstract:
Atomic-scale mechanochemistry is realized from force exerted by a C60-functionalized scanning tunneling microscope tip. Two conformers of tin phthalocyanine can be prepared on coinage-metal surfaces. A transition between these conformers is induced on Cu(111) and Ag(100). Density-functional calculations reveal details of this reaction. Because of the large energy barrier of the reaction and the strong interaction of SnPc with Cu(111), the process cannot be achieved by electrical means.

###### [BibTeX]
@article{YZH_ACH17,
author = {Yajie Zhang and Yongfeng Wang and Jing-Tao Lü and Mads Brandbyge and Richard Berndt},
title = {Mechanochemistry Induced Using Force Exerted by a Functionalized Microscope Tip},
journal = {Angew. Chem.},
volume = {129},
number = {39},
pages = {11931--11935},
publisher = {Wiley},
year = {2017},
doi = {https://doi.org/10.1002/ange.201704940}
}

##### K. Buchmann, N. Hauptmann, A.S. Foster, R. Berndt, J. Phys.: Condens. Matter 29, 394004 (2017)

Abstract:
Single Sn-phthalocyanine (SnPc) molecules adsorb on Cu(100) with the Sn ion above (Sn-up) or below (Sn-down) the molecular plane. Here we use a combination of atomic force microscopy (AFM), scanning tunnelling microscopy (STM) and first principles calculations to understand the adsorption configuration and origin of observed contrast of molecules in the Sn-down state. AFM with metallic tips images the pyrrole nitrogen atoms in these molecules as attractive features while STM reveals a chirality of the electronic structure of the molecules close to the Fermi level EF, which is not observed in AFM. Using density functional theory calculations, the origin of the submolecular contrast is analysed and, while the electrostatic forces turn out to be negligible, the van der Waals interaction between the phenyl rings of SnPc and the substrate deform the molecule, push the pyrrole nitrogen atoms away from the substrate and thus induce the observed submolecular contrast. Simulated STM images reproduce the chirality of the electronic structure near EF.

###### [BibTeX]
@article{KBU_JPCM17,
author = {Kristof Buchmann and Nadine Hauptmann and Adam S Foster and Richard Berndt},
title = {Submolecular resolution in scanning probe images of Sn-phthalocyanines on Cu(100) using metal tips},
journal = {Journal of Physics: Condensed Matter},
volume = {29},
number = {39},
pages = {394004},
year = {2017},
url = {http://stacks.iop.org/0953-8984/29/i=39/a=394004},
doi = {http://doi.org/10.1088/1361-648X/aa7dbd}
}

##### P.-J. Peters, F. Xu, K. Kaasbjerg, G. Rastelli, W. Belzig, R. Berndt, Phys. Rev. Lett. 119, 066803 (2017)

Abstract:
The light emission from a scanning tunneling microscope operated on a Ag(111) surface at 6 K is analyzed from low conductances to values approaching the conductance quantum. Optical spectra recorded at sample voltages V reveal emission with photon energies hν > 2eV. A model of electrons interacting coherently via a localized plasmon-polariton mode reproduces the experimental data, in particular, the kinks in the spectra at eV and 2eV as well as the scaling of the intensity at low and intermediate conductances.

###### [BibTeX]
@article{PPE_PRL17,
author = {Peters, Peter-Jan and Xu, Fei and Kaasbjerg, Kristen and Rastelli, Gianluca and Belzig, Wolfgang and Berndt, Richard},
title = {Quantum Coherent Multielectron Processes in an Atomic Scale Contact},
journal = {Phys. Rev. Lett.},
volume = {119},
pages = {066803},
publisher = {American Physical Society},
year = {2017},
doi = {http://doi.org/10.1103/PhysRevLett.119.066803}
}

##### M. Zugermeier, M. Gruber, M. Schmid, B.P. Klein, L. Ruppenthal, P. Muller, R. Einholz, W. Hieringer, R. Berndt, H.F. Bettinger, J.M. Gottfried, Nanoscale 9, 12461 (2017)

Abstract:
Heptacene was generated by surface-assisted didecarbonylation of an α-diketone precursor on a Ag(111) surface. Monitoring of the surface reaction and characterization of the adsorbed heptacene was performed with scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, and density functional theory (DFT) calculations. The surface-assisted formation of heptacene occurs around 460 K. Both the heptacene and the precursor molecules are oriented along the high-symmetry directions of the (111) surface and their molecular π systems face towards the substrate. The interaction with the Ag(111) substrate is not laterally uniform, but appears to be strongest on the central part of the molecule, in line with the expectations from Clar's rule. In the STM images, heptacene shows a dumbbell shape, which may correspond to the substantial out-of-plane deformations of heptacene on Ag(111). As revealed by DFT, the center of the molecule is closer to the surface than the outer parts. In addition, the inner rings are most affected by charge redistribution between surface and molecule. Heptacene acts as an acceptor and receives a negative charge of -0.6e from the Ag(111) surface. Since vacuum-sublimable α-diketone precursors for even larger acenes are available, the approach is promising for the on-surface synthesis of higher acene homologues such as octacene and nonacene.

###### [BibTeX]
@article{MZU_NSC17,
author = {Zugermeier, Malte and Gruber, Manuel and Schmid, Martin and Klein, Benedikt P. and Ruppenthal, Lukas and Muller, Philipp and Einholz, Ralf and Hieringer, Wolfgang and Berndt, Richard and Bettinger, Holger F. and Gottfried, J. Michael},
title = {On-surface synthesis of heptacene and its interaction with a metal surface},
journal = {Nanoscale},
volume = {9},
pages = {12461-12469},
publisher = {The Royal Society of Chemistry},
year = {2017},
url = {http://dx.doi.org/10.1039/C7NR04157H},
doi = {http://doi.org/10.1039/C7NR04157H}
}

##### X. Zhang, N. Li, Y. Zhang, R. Berndt, Y. Wang, Phys. Chem. Chem. Phys. 19, 14919 (2017)

Abstract:
Hierarchical self-assembly of 13-cis-retinoic acid on Au(111) and Ag(111) was investigated using low-temperature scanning tunnelling microscopy. On both surfaces, molecules form dimers by hydrogen bonds and the dimers arrange into ordered two-dimensional arrays through van der Waals forces. Three packing modes are observed on Au(111) and only one on Ag(111). We tentatively attribute the different patterns on the two surfaces to a stronger molecule-substrate interaction on Ag(111) and site-dependent molecular adsorption on different atomic lattices. In addition, 13-cis-ReA on Au(111) can be made to carry a localized spin.

###### [BibTeX]
@article{XZH_PCCP17,
author = {Zhang, Xue and Li, Na and Zhang, Yajie and Berndt, Richard and Wang, Yongfeng},
title = {13-cis-Retinoic acid on coinage metals: hierarchical self-assembly and spin generation},
journal = {Phys. Chem. Chem. Phys.},
volume = {19},
pages = {14919-14923},
publisher = {The Royal Society of Chemistry},
year = {2017},
url = {http://dx.doi.org/10.1039/C7CP01568B},
doi = {http://doi.org/10.1039/C7CP01568B}
}

##### T. Jasper-Tönnies, A. Garcia-Lekue, T. Frederiksen, S. Ulrich, R. Herges, R. Berndt, Phys. Rev. Lett. 119, 066801 (2017)

Abstract:
A freestanding molecular wire is placed vertically on Au(111) using a platform molecule and contacted by a scanning tunneling microscope. Despite the simplicity of the single-molecule junction, its conductance G reproducibly varies in a complex manner with the electrode separation. Transport calculations show that G is controlled by a deformation of the molecule, a symmetry mismatch between the tip and molecule orbitals, and the breaking of a C≡C triple in favor of a Au–C–C bond. This tip-controlled reversible bond formation or rupture alters the electronic spectrum of the junction and the states accessible for transport, resulting in an order of magnitude variation of the conductance.

###### [BibTeX]
@article{TJT_PRL17,
author = {Jasper-T\"onnies, Torben and Garcia-Lekue, Aran and Frederiksen, Thomas and Ulrich, Sandra and Herges, Rainer and Berndt, Richard},
title = {Conductance of a Freestanding Conjugated Molecular Wire},
journal = {Phys. Rev. Lett.},
volume = {119},
pages = {066801},
publisher = {American Physical Society},
year = {2017},
doi = {http://doi.org/10.1103/PhysRevLett.119.066801}
}

##### T. Jasper-Tönnies, M. Gruber, S. Karan, H. Jacob, F. Tuczek, R. Berndt, J. Phys. Chem. Lett. 8, 1569 (2017)

Abstract:
Spin-crossover molecules on metallic substrates have recently attracted considerable interest for their potential applications in molecular spintronics. Using scanning tunneling microscopy, we evidence the first successful deposition of a charged FeIII spin-crossover complex, [Fe(pap)2]+ (pap = N-2-pyridylmethylidene-2-hydroxyphenylaminato), on Au(111). Furthermore, the bulk form of the molecules is stabilized by a perchlorate counterion, which depending on the deposition technique may affect the quality of the deposition and the measurements. Finally, we evidence switching of the molecules on Au(111).

###### [BibTeX]
@article{TJT_JPCL17,
author = {Jasper-Tönnies, Torben and Gruber, Manuel and Karan, Sujoy and Jacob, Hanne and Tuczek, Felix and Berndt, Richard},
title = {Deposition of a Cationic FeIII Spin-Crossover Complex on Au(111): Impact of the Counter Ion},
journal = {J. Phys. Chem. Lett.},
volume = {8},
number = {7},
pages = {1569-1573},
year = {2017},
note = {PMID: 28319403},
url = { http://dx.doi.org/10.1021/acs.jpclett.7b00457},
doi = {http://doi.org/10.1021/acs.jpclett.7b00457}
}

##### S. Ossinger, H. Naggert, L. Kipgen, T. Jasper-Toennies, A. Rai, J. Rudnik, F. Nickel, L.M. Arruda, M. Bernien, W. Kuch, R. Berndt, F. Tuczek, J. Phys. Chem. C 121, 1210 (2017)

Abstract:
To investigate the ability for spin-state switching of spin-crossover (SCO) complexes adsorbed to solid substrates, the SCO complex [Fe(H2B(pz)2)2(phenme4)] (pz = pyrazole, phenme4 = 3,4,7,8-tetramethyl-1,10-phenanthroline) is prepared. The new complex is investigated by magnetic susceptibility measurements and Mössbauer spectroscopy in the solid state and by temperature-dependent UV/Vis-spectroscopy in a thin film deposited by physical vapour deposition (PVD) on quartz glass. Thermal and light-induced SCO are observed in the bulk and the film on glass. Submonolayers of this complex obtained by PVD are studied by temperature-dependent near-edge x-ray absorption fine structure (NEXAFS) on Au(111) as well as Bi(111) and by scanning tunneling microscopy (STM) on Au(111). NEXAFS shows thermal and light-induced spin-state switching of the complex on Bi(111), however, with a large temperature-independent high-spin fraction (≈ 50%). On the other hand, combined evidence from NEXAFS and STM indicates that on Au(111) the complex dissociates into [Fe(H2B(pz)2)2] and phenme4. Similar observations are made with the parent complex [Fe(H2B(pz)2)2(phen)], which on Bi(111) stays intact and exhibits thermal as well as light-induced SCO, but on Au(111) dissociates into [Fe(H2B(pz)2)2] and phen.

###### [BibTeX]
@article{SOS_JPCC16,
author = {Ossinger, Sascha and Naggert, Holger and Kipgen, Lalminthang and Jasper-Toennies, Torben and Rai, Abhishek and Rudnik, Julian and Nickel, Fabian and Arruda, Lucas M. and Bernien, Matthias and Kuch, Wolfgang and Berndt, Richard and Tuczek, Felix},
title = {Vacuum-Evaporable Spin-Crossover Complexes in Direct Contact With a Solid Surface: Bismuth vs. Gold},
journal = {J. Phys. Chem. C},
volume = {121},
number = {2},
pages = {1210-1219},
year = {2017},
url = {http://dx.doi.org/10.1021/acs.jpcc.6b10888},
doi = {http://doi.org/10.1021/acs.jpcc.6b10888}
}

##### N. Li, H. Wang, D.-L. Song, C. Li, R. Li, S.-M. Hou, Y.-F. Wang, R. Berndt, Dalton Trans. 45, 16566 (2016)

Abstract:
Single Co adatoms adsorbed on a double-layer NaCl film supported by Cu(111) were negatively charged after applying a positive voltage pulse to the sample in a scanning tunnelling microscope. Density functional calculations showed that the magnetic moment of Co changed from 3μB to 2.2μB after charge state manipulation.

###### [BibTeX]
@article{NLI_DTR16,
author = {Li, Na and Wang, Hao and Song, Dao-Liang and Li, Chao and Li, Ruoning and Hou, Shi-Min and Wang, Yong-Feng and Berndt, Richard},
title = {Charging single Co atoms on ultrathin NaCl films},
journal = {Dalton Trans.},
volume = {45},
pages = {16566-16569},
publisher = {The Royal Society of Chemistry},
year = {2016},
url = {http://dx.doi.org/10.1039/C6DT01963C},
doi = {http://doi.org/10.1039/C6DT01963C}
}

##### J. Schöneberg, N.M. Caffrey, P. Ferriani, S. Heinze, R. Berndt, Phys. Rev. B 94, 115418 (2016)

Abstract:
The induced spin-polarization of Ir atoms on a ferromagnetic Fe double layer on W(110) has been investigated with spin-polarized scanning tunneling microscopy. An unoccupied state is observed with a spin-polarization exceeding 60% that is inverted with respect to the Fe layer. This inversion is due to the tunneling gap acting as an orbital and spin filter. Distance dependent measurements show that the spin-polarization remains approximately constant over the entire experimentally accessible range, from far in the tunneling regime to 1Å from the point of contact formation. This is corroborated by density functional theory calculations which show that the inversion of spin-polarization occurs within 0.5Å of the adatom.

###### [BibTeX]
@article{JSC_PRB16,
author = {Schöneberg, Johannes and Caffrey, Nuala Mai and Ferriani, Paolo and Heinze, Stefan and Berndt, Richard},
title = {Distance- and spin-resolved spectroscopy of iridium atoms on an iron bilayer},
journal = {Phys. Rev. B},
volume = {94},
pages = {115418},
year = {2016},
doi = {http://doi.org/10.1103/PhysRevB.94.115418}
}

##### I.-P. Hong, N. Li, Y.-J. Zhang, H. Wang, H.-J. Song, M.-L. Bai, X. Zhou, J.-L. Li, G.-C. Gu, X. Zhang, M. Chen, J.M. Gottfried, D. Wang, J.-T. Lu, L.-M. Peng, S.-M. Hou, R. Berndt, K. Wu, Y.-F. Wang, Chem. Commun. 52, 10338 (2016)

Abstract:
Air-unstable magnetic aluminum phthalocyanine (AlPc) molecules are prepared by an on-surface metalation reaction of phthalocyanine with aluminum (Al) atoms on Au(111) in ultrahigh vacuum. Experiments and density functional theory calculations show that an unpaired spin is located on the conjugated isoindole lobes of the molecule rather than at the Al position.

###### [BibTeX]
@article{IHO_CCO16,
author = {Hong, I-Po and Li, Na and Zhang, Ya-Jie and Wang, Hao and Song, Huan-Jun and Bai, Mei-Lin and Zhou, Xiong and Li, Jian-Long and Gu, Gao-Chen and Zhang, Xue and Chen, Min and Gottfried, J. Michael and Wang, Dong and Lu, Jing-Tao and Peng, Lian-Mao and Hou, Shi-Min and Berndt, Richard and Wu, Kai and Wang, Yong-Feng},
title = {Vacuum synthesis of magnetic aluminum phthalocyanine on Au(111)},
journal = {Chem. Commun.},
volume = {52},
pages = {10338-10341},
publisher = {The Royal Society of Chemistry},
year = {2016},
url = {http://dx.doi.org/10.1039/C6CC03359H},
doi = {http://doi.org/10.1039/C6CC03359H}
}

##### M. Gruber, R. Berndt, J. Phys. Chem. C 120, 18642 (2016)

Abstract:
Shikimic-acid and beta-carotene molecules were investigated on various metallic substrates using scanning tunneling microscopy at ~5 K. We studied the potential ability of these metal-free organic molecules to be switched into a magnetic state as it was recently evidenced for retinoic acid and cholesterol on Au(111). While our attempts to generate spin within shikimic-acid and beta-carotene molecules proved unsuccessful, switching attempts for different molecule/substrate pairs were thoroughly analyzed so as to tentatively elucidate the underlying causes of the vain attempts. We speculate that, by avoiding the identified causes, other organic molecule/substrate pairs can be found for spin manipulation.

###### [BibTeX]
@article{GRU_JPCC16,
author = {Manuel Gruber and Richard Berndt},
title = {Manipulation of Cyclohexene-Based Organic Molecules on Various Metallic Substrates},
journal = {J. Phys. Chem. C},
volume = {120},
number = {33},
pages = {18642-18650},
year = {2016},
url = {http://dx.doi.org/10.1021/acs.jpcc.6b06089},
doi = {http://doi.org/10.1021/acs.jpcc.6b06089}
}

##### T. Knaak, T.G. Gopakumar, B. Schwager, F. Tuczek, R. Robles, N. Lorente, R. Berndt, J. Am. Chem. Soc. 138, 7544 (2016)

Abstract:
Adding ligands to molecules can have drastic and unforeseen consequences in the final products of a reaction. Recently a surface trans effect due to the weakening of a molecule-surface bond was reported. Here, we show a surface cis effect where an axial ligand at adsorbed transition-metal complexes enables lateral bonding among the molecules. In the absence of this ligand, the intermolecular interaction is repulsive and supramolecular patterns are not observed. Fe-tetramethyl-tetraazaannulene on Au(111) was investigated using low-temperature scanning tunneling microscopy and spectroscopy along with density functional theory calculations. At low coverages, the molecules remain isolated. Exposure to CO leads to axial CO bonding and induces reordering into extended clusters of chiral molecular trimers. The changed self-assembly pattern is due to a CO-induced modification of the molecular structure and the corresponding charge transfer between the molecule and the substrate, which in turn changes the lateral intermolecular forces.

###### [BibTeX]
@article{TKN_JACS16,
author = {Thomas Knaak and Thiruvancheril G. Gopakumar and Bettina Schwager and Felix Tuczek and Roberto Robles and Nicolás Lorente and Richard Berndt},
title = {Surface cis Effect: Influence of an Axial Ligand on Molecular Self-Assembly},
journal = {J. Am. Chem. Soc.},
volume = {138},
number = {24},
pages = {7544 -- 7550},
year = {2016},
url = {http://dx.doi.org/10.1021/jacs.6b03710},
doi = {http://doi.org/10.1021/jacs.6b03710}
}

##### K. Scheil, T.G. Gopakumar, J. Bahrenburg, F. Temps, R.J. Maurer, K. Reuter, R. Berndt, J. Phys. Chem. Lett. 7, 2080 (2016)

Abstract:
The trans-cis isomerization makes azobenzene (AB) a robust molecular switch. Once adsorbed to a metal, however, the switching is inefficient or absent due to rapid excited-state quenching or loss of the trans-cis bistability. We find that tris-[4-(phenylazo)-phenyl]-amine is a rather efficient switch on Ag(111). Using scanning tunneling and atomic force microscopy at submolecular resolution along with density functional theory calculations, we show that the switching process is no trans-cis isomerization but rather a reorientation of the N-N bond of an AB unit. It proceeds through a twisting motion of the azo-bridge that leads to a lateral shift of a phenyl ring. Thus, the role of the Ag substrate is ambivalent. While it suppresses the original bistability of the azobenzene units, it creates a new one by inducing a barrier for the rotation of the N-N bond.

###### [BibTeX]
@article{KSC_JPCL16,
author = {Katharina Scheil and Thiruvancheril G. Gopakumar and Julia Bahrenburg and Friedrich Temps and Reinhard Johann Maurer and Karsten Reuter and Richard Berndt},
title = {Switching of an Azobenzene-Tripod Molecule on Ag(111)},
journal = {J. Phys. Chem. Lett.},
volume = {7},
number = {11},
pages = {2080--2084},
year = {2016},
note = {PMID: 27193044},
url = {http://dx.doi.org/10.1021/acs.jpclett.6b01011},
doi = {http://doi.org/10.1021/acs.jpclett.6b01011}
}

##### N. Hauptmann, R. Robles, P. Abufager, N. Lorente, R. Berndt, J. Phys. Chem. Lett. 7, 1984 (2016)

Abstract:
A self-assembled monolayer of mercaptobenzoic acid (MBA) on Au(110) is investigated with scanning tunneling and atomic force microscopy (STM and AFM) and density functional calculations. High-resolution AFM images obtained with metallic tips show clear contrasts between oxygen atoms and phenyl moieties. The contrast above the oxygen atoms is due to attractive covalent interactions, which is different than previously reported high-resolution images, where Pauli repulsion dominated the image contrast. We show that the bonding of MBA to the substrate occurs mainly through dispersion interactions, whereas the thiol-Au bond contributes only a quarter of the adsorption energy. No indication of Au adatoms mediating the thiol-Au interaction was found in contrast to other thiol-bonded systems. However, MBA lifts the Au(110)-(2x1) reconstruction.

###### [BibTeX]
@article{NHA_JPCL16,
author = {Nadine Hauptmann and Roberto Robles and Paula Abufager and Nicolas Lorente and Richard Berndt},
title = {AFM Imaging of Mercaptobenzoic Acid on Au(110): Submolecular Contrast with Metal Tips},
journal = {J. Phys. Chem. Lett.},
volume = {7},
number = {11},
pages = {1984-1990},
year = {2016},
note = {PMID: 27183144},
url = {http://dx.doi.org/10.1021/acs.jpclett.6b00684},
doi = {http://doi.org/10.1021/acs.jpclett.6b00684}
}

##### C. Xie, Q.-M. Wu, R.-N. Li, G.-C. Gu, X. Zhang, N. Li, R. Berndt, J. Kröger, Z.-Y. Shen, S.-M. Hou, Y.-F. Wang, Chin. Chem. Lett. 27, 807 (2016)

Abstract:
Abstract To date, supramolecular chemistry is an ever growing research field owing to its crucial role in molecular catalysis, recognition, medicine, data storage and processing as well as artificial photosynthetic devices. Different isolated supramolecules were prepared by molecular self-assembly on surfaces. This review mainly focuses on supramolecular aggregations on noble metal surfaces studied by scanning tunneling microscopy, including dimers, trimers, tetramers, pentamers, wire-like assemblies and Sierpinski triangular fractals. The variety of self-assembled structures reflects the subtle balance between intermolecular and molecule-substrate interactions, which to some extent may be controlled by molecules, substrates and the molecular coverage. The comparative study of different architectures helps identifying the operative mechanisms that lead to the structural motifs. The application of these mechanisms may lead to novel assemblies with tailored physicochemical properties.

###### [BibTeX]
@article{CXI_CCL16,
author = {Chao Xie and Qi-Meng Wu and Ruo-Ning Li and Gao-Chen Gu and Xue Zhang and Na Li and Richard Berndt and Jörg Kröger and Zi-Yong Shen and Shi-Min Hou and Yong-Feng Wang},
title = {Isolated supramolecules on surfaces studied with scanning tunneling microscopy},
journal = {Chin. Chem. Lett.},
volume = {27},
number = {6},
pages = {807 - 812},
year = {2016},
url = {http://www.sciencedirect.com/science/article/pii/S1001841716300511},
doi = {http://doi.org/10.1016/j.cclet.2016.03.022}
}

##### S. Karan, R. Berndt, Phys. Chem. Chem. Phys. 18, 9334 (2016)

Abstract:
Compact islands of cholesterol on Au(111) were investigated with scanning tunneling microscopy at ∼ 5 K. Single molecules have been switched among several states, three of which exhibit a sharp spectroscopic feature at the Fermi level. This feature signals the presence of a localized spin and suggests that the molecule may be controllably switched between paramagnetic and diamagnetic states.

###### [BibTeX]
@article{SKA_PCCP16,
author = {Karan, Sujoy and Berndt, Richard},
title = {Generation of spin in single cholesterol molecules on gold},
journal = {Phys. Chem. Chem. Phys.},
volume = {18},
pages = {9334--9337},
publisher = {The Royal Society of Chemistry},
year = {2016},
url = {http://dx.doi.org/10.1039/C5CP07410J},
doi = {http://doi.org/10.1039/C5CP07410J}
}

##### J. Schöneberg, F. Otte, N. Néel, A. Weismann, Y. Mokrousov, J. Kröger, R. Berndt, S. Heinze, Nano Lett. 16, 1450 (2016)

Abstract:
Anisotropic magnetoresistance, that is, the sensitivity of the electrical resistance of magnetic materials on the magnetization direction, is expected to be strongly enhanced in ballistic transport through nanoscale junctions. However, unambiguous experimental evidence of this effect is difficult to achieve. We utilize single-atom junctions to measure this ballistic anisotropic magnetoresistance (AMR). Single Co and Ir atoms are deposited on domains and domain walls of ferromagnetic Fe layers on W(110) to control their magnetization directions. They are contacted with nonmagnetic tips in a low-temperature scanning tunneling microscope to measure the junction conductances. Large changes of the magnetoresistance occur from the tunneling to the ballistic regime due to the competition of localized and delocalized d-orbitals, which are differently affected by spin-orbit coupling. This work shows that engineering the AMR at the single atom level is feasible.

###### [BibTeX]
@article{JSC_NLE16,
author = {J. Schöneberg and F. Otte and N. Néel and A. Weismann and Y. Mokrousov and J. Kröger and R. Berndt and S. Heinze},
title = {Ballistic Anisotropic Magnetoresistance of Single-Atom Contacts},
journal = {Nano Letters},
volume = {16},
number = {2},
pages = {1450-1454},
year = {2016},
note = {PMID: 26783634},
url = {http://dx.doi.org/10.1021/acs.nanolett.5b05071},
doi = {http://doi.org/10.1021/acs.nanolett.5b05071}
}

##### S. Karan, N. Li, Y. Zhang, Y. He, I.-P. Hong, H. Song, J.-T. Lü, Y. Wang, L. Peng, K. Wu, G.S. Michelitsch, R.J. Maurer, K. Diller, K. Reuter, A. Weismann, R. Berndt, Phys. Rev. Lett. 116, 027201 (2016)

Abstract:
All-trans-retinoic acid (ReA), a closed-shell organic molecule comprising only C, H, and O atoms, is investigated on a Au(111) substrate using scanning tunneling microscopy and spectroscopy. In dense arrays single ReA molecules are switched to a number of states, three of which carry a localized spin as evidenced by conductance spectroscopy in high magnetic fields. The spin of a single molecule may be reversibly switched on and off without affecting its neighbors. We suggest that ReA on Au is readily converted to a radical by the abstraction of an electron.

###### [BibTeX]
@article{SKA_PRL16,
author = {Karan, Sujoy and Li, Na and Zhang, Yajie and He, Yang and Hong, I-Po and Song, Huanjun and Lü, Jing-Tao and Wang, Yongfeng and Peng, Lianmao and Wu, Kai and Michelitsch, Georg S. and Maurer, Reinhard J. and Diller, Katharina and Reuter, Karsten and Weismann, Alexander and Berndt, Richard},
title = {Spin Manipulation by Creation of Single-Molecule Radical Cations},
journal = {Phys. Rev. Lett.},
volume = {116},
pages = {027201},
publisher = {American Physical Society},
year = {2016},
doi = {http://doi.org/10.1103/PhysRevLett.116.027201}
}

##### A. Burtzlaff, N.L. Schneider, A. Weismann, R. Berndt, Surf. Sci. 643, 10 (2016)

Abstract:
Abstract The shot noise of the current through single-atom contacts has been measured in a scanning tunneling microscope at a temperature of 5 K. Electrical measurements at frequencies up to 120 kHz were performed on Au, Fe, and Co atoms on Au(111) using Au tips. The data from Fe and Co indicate spin polarized transmission through a single conductance channel. Optical measurements at sub-petahertz frequencies were carried out on Cu adatoms on Cu(111) using Cu tips. The data are consistent with previous observations from coinage metal contacts.

###### [BibTeX]
@article{ABU_SSI16,
author = {Andreas Burtzlaff and Natalia L. Schneider and Alexander Weismann and Richard Berndt},
title = {Shot noise from single atom contacts in a scanning tunneling microscope},
journal = {Surf. Sci.},
volume = {643},
pages = {10 -- 12},
year = {2016},
note = {Present challenges in surface science, a special issue in honour of Dietrich Menzel},
url = {http://www.sciencedirect.com/science/article/pii/S0039602815001983},
doi = {http://doi.org/10.1016/j.susc.2015.07.006}
}