Vacuum-Evaporable Spin-Crossover Complexes in Direct Contact With a Solid Surface: Bismuth vs. Gold
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)

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.

  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}
Publication Date (Web): December 21, 2016
Distance- and spin-resolved spectroscopy of iridium atoms on an iron bilayer
J. Schöneberg, N.M. Caffrey, P. Ferriani, S. Heinze, R. Berndt, Phys. Rev. B 94, 115418 (2016)

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.

  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},
  url = {http://link.aps.org/doi/10.1103/PhysRevB.94.115418},
  doi = {http://doi.org/10.1103/PhysRevB.94.115418}
Vacuum synthesis of magnetic aluminum phthalocyanine on Au(111)
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)

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.

  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}
Manipulation of Cyclohexene-Based Organic Molecules on Various Metallic Substrates
M. Gruber, R. Berndt, J. Phys. Chem. C 120, 18642 (2016)

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.

  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}
Surface cis Effect: Influence of an Axial Ligand on Molecular Self-Assembly
T. Knaak, T.G. Gopakumar, B. Schwager, F. Tuczek, R. Robles, N. Lorente, R. Berndt, J. Am. Chem. Soc. 138, 7544 (2016)

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.

  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}
Switching of an Azobenzene-Tripod Molecule on Ag(111)
K. Scheil, T.G. Gopakumar, J. Bahrenburg, F. Temps, R.J. Maurer, K. Reuter, R. Berndt, J. Phys. Chem. Lett. 7, 2080 (2016)

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.

  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}
AFM Imaging of Mercaptobenzoic Acid on Au(110): Submolecular Contrast with Metal Tips
N. Hauptmann, R. Robles, P. Abufager, N. Lorente, R. Berndt, J. Phys. Chem. Lett. 7, 1984 (2016)

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.

  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}
Isolated supramolecules on surfaces studied with scanning tunneling microscopy
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 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.

  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}
Generation of spin in single cholesterol molecules on gold
S. Karan, R. Berndt, Phys. Chem. Chem. Phys. 18, 9334 (2016)

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.

  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}
Ballistic Anisotropic Magnetoresistance of Single-Atom Contacts
J. Schöneberg, F. Otte, N. Néel, A. Weismann, Y. Mokrousov, J. Kröger, R. Berndt, S. Heinze, Nano Lett. 16, 1450 (2016)

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.

  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}
Spin Manipulation by Creation of Single-Molecule Radical Cations
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)

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.

  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},
  url = {http://link.aps.org/doi/10.1103/PhysRevLett.116.027201},
  doi = {http://doi.org/10.1103/PhysRevLett.116.027201}
Shot noise from single atom contacts in a scanning tunneling microscope
A. Burtzlaff, N.L. Schneider, A. Weismann, R. Berndt, Surf. Sci. 643, 10 (2016)

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.

  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}