Christian Lotze

Driving a macroscopic oscillator with the stochastic motion of a hydrogen molecule.

Putting Noise to Work Extracting energy from a noisy system (one that moves randomly) is possible if a mechanical system can induce periodicity on its motion, a phenomenon called stochastic resonance. Lotze et al. (p. 779; see the Perspective by Bockrath) created stochastic resonances between the cantilever attached to a scanning tunneling microscope tip and a hydrogen molecule adsorbed on a copper surface at cryogenic temperatures. The tunneling electrons at a particular bias voltage could excite the hydrogen molecules and couple their random motion to that of the cantilever, driving periodic oscillations. The random motion of an activated hydrogen molecule produces forces that drive periodic motions of a scanning probe cantilever. Energy harvesting from noise is a paradigm proposed by the theory of stochastic resonances. We demonstrate that the random switching of a hydrogen (H2) molecule can drive the oscillation of a macroscopic mechanical resonator. The H2 motion was activated by tunneling electrons and caused fluctuations of the forces sensed by the tip of a noncontact atomic force microscope. The stochastic molecular noise and the periodic oscillation of the tip were coupled in a concerted dynamic that drives the system into self-oscillation. This phenomenon could be a way for enhancing the transfer of energy from incoherent sources into coherent dynamics of a molecular engine.

Orbital redistribution in molecular nanostructures mediated by metal-organic bonds

Dicyanovinyl-quinquethiophene (DCV5T-Me2) is a prototype conjugated oligomer for highly efficient organic solar cells. This class of oligothiophenes are built up by an electron-rich donor (D) backbone and terminal electron-deficient acceptor (A) moieties. Here, we investigated its structural and electronic properties when it is adsorbed on a Au(111) surface using low temperature scanning tunneling microscopy/spectroscopy (STM/STS) and atomic force microscopy (AFM). We find that DCV5T-Me2 self-assembles in extended chains, stabilized by intercalated Au atoms. The effect of metal-ligand hybridization with Au adatoms causes an energetic downshift of the DCV5T-Me2 lowest unoccupied molecular orbital (LUMO) with respect to the uncoordinated molecules on the surface. The asymmetric coordination of a gold atom to only one molecular end group leads to an asymmetric localization of the LUMO and LUMO+1 states at opposite sides. Using model density functional theory (DFT) calculations, we explain such orbital reshaping as a consequence of linear combinations of the original LUMO and LUMO+1 orbitals, mixed by the attachment of a bridging Au adatom. Our study shows that the alignment of molecular orbitals and their distribution within individual molecules can be modified by contacting them to metal atoms in specific sites.

Electronic Structure and Luminescence of Quasi-Freestanding MoS2 Nanopatches on Au(111)

Monolayers of transition metal dichalcogenides are interesting materials for optoelectronic devices due to their direct electronic band gaps in the visible spectral range. Here, we grow single layers of MoS2 on Au(111) and find that nanometer-sized patches exhibit an electronic structure similar to their freestanding analogue. We ascribe the electronic decoupling from the Au substrate to the incorporation of vacancy islands underneath the intact MoS2 layer. Excitation of the patches by electrons from the tip of a scanning tunneling microscope leads to luminescence of the MoS2 junction and reflects the one-electron band structure of the quasi-freestanding layer.

Visualizing the Role of Molecular Orbitals in Charge Transport through Individual Diarylethene Isomers

Diarylethene molecules are prototype molecular switches with their two isomeric forms exhibiting strikingly different conductance, while maintaining similar length. We employed low-temperature scanning tunneling microscopy (STM) to resolve the energy and the spatial extend of the molecular orbitals of the open and closed isomers when lying on a Au(111) surface. We find an intriguing difference in the extension of the respective HOMOs and a peculiar energy splitting of the formerly degenerate LUMO of the open isomer. We then lift the two isomers with the tip of the STM and measure the current through the individual molecules. By a simple analytical model of the transport, we show that the previously determined orbital characteristics are essential ingredients for the complete understanding of the transport properties. We also succeeded in switching the suspended molecules by the current, while switching the ones which are in direct contact to the surface occurs nonlocally with the help of the electric field of the tip.

Charge Redistribution and Transport in Molecular Contacts

The forces between two single molecules brought into contact, and their connection with charge transport through the molecular junction, are studied here using non contact AFM, STM, and density functional theory simulations. A carbon monoxide molecule approaching an acetylene molecule (C_{2}H_{2}) initially feels weak attractive electrostatic forces, partly arising from charge reorganization in the presence of molecular . We find that the molecular contact is chemically passive, and protects the electron tunneling barrier from collapsing, even in the limit of repulsive forces. However, we find subtle conductance and force variations at different contacting sites along the C_{2}H_{2} molecule attributed to a weak overlap of their respective frontier orbitals.

Atypical charge redistribution over a charge-transfer monolayer on a metal

We report an atypical charge distribution in a highly ordered monolayer of sodium (Na) and tetracyanoquinodimethane (TCNQ) on a Au(111) surface. Na atoms incorporated in the charge-transfer layer donate their 3s electron to the lowest unoccupied orbital of the TCNQ acceptor. A fingerprint of such a TCNQ anion is observed in scanning tunneling spectroscopy as a zero-bias peak characteristic of the Kondo effect. Spatial maps of the Kondo resonance surprisingly reveal that it appears most intense on top of the Na sites. Supported by density functional theory simulations, we interpret this peculiar charge distribution pattern as originating from the extension of the singly occupied molecular orbital beyond the molecular backbone, and cloaking the Na cations. We further suggest that this deformation of molecular orbitals is a consequence of the electrostatic potential landscape of the polar Na-TCNQ layer.

Conformational adaptation and manipulation of manganese tetra(4-pyridyl)porphyrin molecules on Cu(111)

Porphyrins are highly flexible molecules and well known to adapt to their local environment via conformational changes. We studied the self-assembly of manganese meso-tetra(4-pyridyl)porphyrin (Mn-TPyP) molecules on a Cu(111) surface by low temperature scanning tunneling microscopy (STM) and atomic force microscopy (ATM). We observe molecular chains along the ⟨11¯0⟩ direction of the substrate. Within these chains, we identify two molecular conformations, which differ by the orientation of the upward bending of the macrocycle. Using density functional theory, we show that this saddle shape is a consequence of the rotation and inclination of the pyridyl groups towards Cu adatoms, which stabilize the metal-organic chains. The molecular conformations obey a strict alternation, reflecting the mutual enforcement of conformational adaptation in densely packed structures. Tunneling electrons from the STM tip can induce changes in the orientation of the pyridyl endgroups. The switching behaviour varies with the di...

Moiré structure of MoS2 on Au(111): Local structural and electronic properties

Abstract Monolayer islands of molybdenum disulfide (MoS2) on Au(111) form a characteristic moire structure, leading to locally different stacking sequences at the S-Mo-S-Au interface. Using low-temperature scanning tunneling microscopy (STM) and atomic force microscopy (AFM), we find that the moire islands exhibit a unique orientation with respect to the Au crystal structure. This indicates a clear preference of MoS2 growth in a regular stacking fashion. We further probe the influence of the local atomic structure on the electronic properties. Differential conductance spectra show pronounced features of the valence band and conduction band, some of which undergo significant shifts depending on the local atomic structure. We also determine the tunneling decay constant as a function of the bias voltage by a height-modulated spectroscopy method. This allows for an increased sensitivity of states with non-negligible parallel momentum k∥ and the identification of the origin of the states from different areas in the Brillouin zone.

Evolution of cooperativity in the spin transition of an iron(II) complex on a graphite surface

Cooperative effects determine the spin-state bistability of spin-crossover molecules (SCMs). Herein, the ultimate scale limit at which cooperative spin switching becomes effective is investigated in a complex [Fe(H2B(pz)2)2(bipy)] deposited on a highly oriented pyrolytic graphite surface, using x-ray absorption spectroscopy. This system exhibits a complete thermal- and light-induced spin transition at thicknesses ranging from submonolayers to multilayers. On increasing the coverage from 0.35(4) to 10(1) monolayers, the width of the temperature-induced spin transition curve narrows significantly, evidencing the buildup of cooperative effects. While the molecules at the submonolayers exhibit an apparent anticooperative behavior, the multilayers starting from a double-layer exhibit a distinctly cooperative spin switching, with a free-molecule-like behavior indicated at around a monolayer. These observations will serve as useful guidelines in designing SCM-based devices.Spin-crossover molecules offer a potential route towards molecular spintronics, but retaining the bistability of the spin state upon surface deposition is challenging. Here, the authors study the spin-crossover behaviours of an Fe(II) complex deposited on graphite, determining the scale limit at which cooperative spin switching becomes effective.

Control of Oxidation and Spin State in a Single-Molecule Junction

The oxidation and spin state of a metal-organic molecule determine its chemical reactivity and magnetic properties. Here, we demonstrate the reversible control of the oxidation and spin state in a single Fe porphyrin molecule in the force field of the tip of a scanning tunneling microscope. Within the regimes of half-integer and integer spin state, we can further track the evolution of the magnetocrystalline anisotropy. Our experimental results are corroborated by density functional theory and wave function theory. This combined analysis allows us to draw a complete picture of the molecular states over a large range of intramolecular deformations.