Sharpness-induced energy shifts of quantum well states in Pb islands on Cu(111).

We elucidate that the tip sharpness in scanning tunneling microscopy (STM) can be characterized through the number of field-emission (FE) resonances. A higher number of FE resonances indicates higher sharpness. We observe empty quantum well (QW) states in Pb islands on Cu(111) under different tip sharpness levels. We found that QW states observed by sharper tips always had lower energies, revealing negative energy shifts. This sharpness-induced energy shift originates from an inhomogeneous electric field in the STM gap. An increase in sharpness increases the electric field inhomogeneity, that is, enhances the electric field near the tip apex, but weakens the electric field near the sample. As a result, higher sharpness can increase the electronic phase in vacuum, causing the lowering of QW state energies. Moreover, the behaviors of negative energy shift as a function of state energy are entirely different for Pb islands with a thickness of two and nine atomic layers. This thickness-dependent behavior results from the electrostatic force in the STM gap decreasing with increasing tip sharpness. The variation of the phase contributed from the expansion deformation induced by the electrostatic force in a nine-layer Pb island is significantly greater, sufficient to effectively negate the increase of electronic phase in vacuum.

Scanning tunneling microscopy/spectroscopy of picene thin films formed on Ag(111).

Using ultrahigh-vacuum low-temperature scanning tunneling microscopy and spectroscopy combined with first principles density functional theory calculations, we have investigated structural and electronic properties of pristine and potassium (K)-deposited picene thin films formed in situ on a Ag(111) substrate. At low coverages, the molecules are uniformly distributed with the long axis aligned along the [112̄] direction of the substrate. At higher coverages, ordered structures composed of monolayer molecules are observed, one of which is a monolayer with tilted and flat-lying molecules resembling a (11̄0) plane of the bulk crystalline picene. Between the molecules and the substrate, the van der Waals interaction is dominant with negligible hybridization between their electronic states; a conclusion that contrasts with the chemisorption exhibited by pentacene molecules on the same substrate. We also observed a monolayer picene thin film in which all molecules were standing to form an intermolecular π stacking. Two-dimensional delocalized electronic states are found on the K-deposited π stacking structure.

Molecular Kondo chain.

An important development in recent synthesis strategies is the formation of electronically coupled one and two-dimensional organic systems for potential applications in nanoscale molecule-based devices. Here, we assemble one-dimensional spin chains by covalently linking basic molecular building blocks on a Au(111) surface. Their structural properties are studied by scanning tunneling microscopy and the Kondo effect of the basic molecular blocks inside the chains is probed by scanning tunneling spectroscopy. Tunneling spectroscopic images reveal the existence of separate Kondo regions within the chains while density functional theory calculations unveil antiferromagnetic coupling between the spin centers.

Reversible chiral switching of bis(phthalocyaninato) terbium(III) on a metal surface.

We demonstrate a reversible chiral switching of bis(phthalocyaninato) terbium(III) molecules on an Ir(111) surface by low temperature scanning tunneling microscopy. With an azimuthal rotation of its upper phthalocyanine ligand, the molecule can be switched between a chiral and an achiral configuration actuated by respective inelastic electron tunneling and local current heating. Moreover, the molecular chiral configuration can be interchanged between left and right handedness during the switching manipulations, thereby opening up potential nanotechnological applications.

Nanoscale redox mapping at the MoS2-liquid interface.

Layered MoS2 is considered as one of the most promising two-dimensional photocatalytic materials for hydrogen evolution and water splitting; however, the electronic structure at the MoS2-liquid interface is so far insufficiently resolved. Measuring and understanding the band offset at the surfaces of MoS2 are crucial for understanding catalytic reactions and to achieve further improvements in performance. Herein, the heterogeneous charge transfer behavior of MoS2 flakes of various layer numbers and sizes is addressed with high spatial resolution in organic solutions using the ferrocene/ferrocenium (Fc/Fc+) redox pair as a probe in near-field scanning electrochemical microscopy, i.e. in close nm probe-sample proximity. Redox mapping reveals an area and layer dependent reactivity for MoS2 with a detailed insight into the local processes as band offset and confinement of the faradaic current obtained. In combination with additional characterization methods, we deduce a band alignment occurring at the liquid-solid interface.

Design of the local spin polarization at the organic-ferromagnetic interface.

By means of ab initio calculations and spin-polarized scanning tunneling microscopy experiments the creation of a complex energy dependent magnetic structure with a tailored spin-polarized interface is demonstrated. We show this novel effect by adsorbing organic molecules containing π(p(z)) electrons onto a magnetic surface. The hybridization of the out-of-plane p(z) atomic-type orbitals with the d states of the metal leads to the inversion of the spin polarization at the organic site due to a p(z)-d Zener exchange-type mechanism. As a key result, we demonstrate the possibility to selectively and efficiently inject spin-up and spin-down electrons from a ferromagnetic-organic interface, an effect which can be exploited in future spintronic devices.

Spin- and energy-dependent tunneling through a single molecule with intramolecular spatial resolution.

We investigate the spin- and energy-dependent tunneling through a single organic molecule (CoPc) adsorbed on a ferromagnetic Fe thin film, spatially resolved by low-temperature spin-polarized scanning tunneling microscopy. Interestingly, the metal ion as well as the organic ligand show a significant spin dependence of tunneling current flow. State-of-the-art ab initio calculations including also van der Waals interactions reveal a strong hybridization of molecular orbitals and substrate 3d states. The molecule is anionic due to a transfer of one electron, resulting in a nonmagnetic (S=0) state. Nevertheless, tunneling through the molecule exhibits a pronounced spin dependence due to spin-split molecule-surface hybrid states.

Adsorption behavior of asymmetric Pd pincer complexes on a Cu(111) surface.

We address the adsorption of asymmetric Pd pincer complexes on a Cu(111) surface by scanning tunneling microscopy. The structural asymmetry is manifested in the observation of two chiral enantiomers. To enable an unambiguous identification of individual constituents, three closely related complexes with small modifications are investigated in parallel. Thereby, methyl substituents determine attractive molecule-molecule interaction. Depending on their distribution, dimerization and tetramerization can be observed.

Dynamics of molecular self-ordering in tetraphenyl porphyrin monolayers on metallic substrates.

A molecular model system of tetraphenyl porphyrins (TPP) adsorbed on metallic substrates is systematically investigated within a joint scanning tunnelling microscopy/molecular modelling approach. The molecular conformation of TPP molecules, their adsorption on a gold surface and the growth of highly ordered TPP islands are modelled with a combination of density functional theory and dynamic force field methods. The results indicate a subtle interplay between different contributions. The molecule-substrate interaction causes a bending of the porphyrin core which also determines the relative orientations of phenyl legs attached to the core. A major consequence of this is a characteristic (and energetically most favourable) arrangement of molecules within self-assembled molecular clusters; the phenyl legs of adjacent molecules are not aligned parallel to each other (often denoted as pi-pi stacking) but perpendicularly in a T-shaped arrangement. The results of the simulations are fully consistent with the scanning tunnelling microscopy observations, in terms of the symmetries of individual molecules, orientation and relative alignment of molecules in the self-assembled clusters.

Synthesis of the extended phenacene molecules, [10]phenacene and [11]phenacene, and their performance in a field-effect transistor.

The [10]phenacene and [11]phenacene molecules have been synthesized using a simple repetition of Wittig reactions followed by photocyclization. Sufficient amounts of [10]phenacene and [11]phenacene were obtained, and thin-film FETs using these molecules have been fabricated with SiO2 and ionic liquid gate dielectrics. These FETs operated in p-channel. The averaged measurements of field-effect mobility, <µ>, were 3.1(7) × 10-2 and 1.11(4) × 10-1 cm2 V-1 s-1, respectively, for [10]phenacene and [11]phenacene thin-film FETs with SiO2 gate dielectrics. Furthermore, [10]phenacene and [11]phenacene thin-film electric-double-layer (EDL) FETs with ionic liquid showed low-voltage p-channel FET properties, with <µ> values of 3(1) and 1(1) cm2 V-1 s-1, respectively. This study also discusses the future utility of the extremely extended π-network molecules [10]phenacene and [11]phenacene as the active layer of FET devices, based on the experimental results obtained.

"Naked" iron-5,10,15-triphenylcorrole on Cu(111): observation of chirality on a surface and manipulation of multiple conformational states by STM.

A new member of the metalloporphyrinoid class, the one-carbon short corrole, has been developed in the past decade to a very accessible and easily tunable compound with many potential applications in material science and catalysis. Other than for the structurally related iron porphyrins, all attempts to prepare and study the "naked" iron triphenylcorrole molecule (FeTPC) in bulk have failed. Here, we demonstrate stabilization of FeTPC as adsorbates on a surface. Local investigations by means of scanning tunneling microscopy reveal that along with the adsorption of FeTPC in a saddle conformation, surface induced chirality is the result. Using scanning tunneling microscopy as a local manipulation tool, individual molecules can be controllably switched between different orientations and conformations. Even conformations which are unfavorable during the adsorption process are feasible. The presented experiments demonstrate that metalated corroles are a highly interesting class of metalloporphyrinoids for local investigations but, in comparison with the well established class of porphyrins, add an additional degree of experimental freedom via chirality.

Spin-Dependent Molecule Symmetry at a Pentacene-Co Spinterface.

Incorporating spin-polarized scanning tunneling microscopy (SP-STM) measurements and first-principles calculations, we resolve spin-polarized states and consequent features in a pentacene(PEN)-Co hybrid system. Symmetry reduction of PEN clarifies the PEN adsorption site and the Co stacking methods. Near the Fermi energy, the molecular symmetry is spin-dependently recovered and an inversion of spin-polarization in PEN with respect to Co is observed. The experimental findings and calculation results are interpreted by a pz-d hybridization model, in which spin-dependent bonding-antibonding splitting of molecular orbitals happens at metal-organic spinterfaces.

Digitized charge transfer magnitude determined by metal-organic coordination number.

Well-ordered metal-organic nanostructures of Fe-PTCDA (perylene-3,4,9,10-tetracarboxylic-3,4,9,10-dianhydride) chains and networks are grown on a Au(111) surface. These structures are investigated by high-resolution scanning tunneling microscopy. Digitized frontier orbital shifts are followed in scanning tunneling spectroscopy. By comparing the frontier energies with the molecular coordination environments, we conclude that the specific coordination affects the magnitude of charge transfer onto each PTCDA in the Fe-PTCDA hybridization system. A basic model is derived, which captures the essential underlying physics and correlates the observed energetic shift of the frontier orbital with the charge transfer.

Real-space observation of spin-split molecular orbitals of adsorbed single-molecule magnets.

A key challenge in the field of molecular spintronics, and for the design of single-molecule magnet-based devices in particular, is the understanding and control of the molecular coupling at the electrode interfaces. It was demonstrated for the field of molecular electronics that the characterization of the molecule-metal-interface requires the precise knowledge of the atomic environment as well as the molecular orbitals being involved in electron transport. To extend the field of molecular electronics towards molecular spintronics, it is of utmost importance to resolve the spin character of molecular orbitals interacting with ferromagnetic leads. Here we present first direct real-space images of spin-split molecular orbitals of a single-molecule magnet adsorbed on a ferromagnetic nanostructure. Moreover, we are able to determine quantitatively the magnitude of the spin-splitting as well as the charge state of the adsorbed molecule.

A versatile variable-temperature scanning tunneling microscope for molecular growth.

We describe and discuss the design of a variable-temperature scanning tunneling microscope (STM) system for the study of molecules at temperatures between 18 and 300 K in ultrahigh vacuum. The STM head is a refinement of a very rigid design developed and successfully operated in Hamburg. In the current version, the head is connected to a liquid helium flow cryostat, thereby reaching a base temperature of 18 K. To minimize the heat load on the STM head, a helium back flow cooled radiation shield is installed. The dimensions and the choice of materials are based on simulations of the heat dissipation. The STM is galvanically isolated from the vacuum chamber to minimize electronic noise and mechanically decoupled by means of springs and an eddy current damping stage. Additionally, the design of the STM head allows the deposition of several molecular materials onto the same cold sample surface. The operation of the STM in imaging mode is demonstrated for TPP/Cu(111) and FePCNaClCu(111). Spectroscopic capabilities of the system are shown for electronic states on NaClCu(111) and TPP/Cu(111).

Two-electron photon emission from metallic quantum wells.

Unusual emission of visible light is observed in scanning tunneling microscopy of the quantum well system Na on Cu(111). Photons are emitted at energies exceeding the energy of the tunneling electrons. Model calculations of two-electron processes which lead to quantum well transitions reproduce the experimental fluorescence spectra, the quantum yield, and the power-law variation of the intensity with the excitation current.

Color view of atomic highs and lows in tunneling induced light emission.

Based on a detailed experimental study of light emission stimulated with a scanning tunneling microscope, we put forward a consistent picture for the atomic-scale contrasts observed to date on noble metal surfaces. Divergent contrasts near various atomic steps and conflicting interpretations of light emission from a model atomic grating, (2 x 1) reconstructed Au(110), are accounted for. The light intensity modulation results from different spatial distributions of the local density of final states in the elastic and inelastic tunneling channels.