Effect of electron-phonon interaction on the formation of one-dimensional electronic states in coupled Cl vacancies

The formation of extended electron states in one-dimensional nanostructures is of key importance for the function of molecular electronics devices. Here we study the effects of strong electron-phonon interaction on the formation of extended electronic states in intentionally created Cl vacancy pairs and chains in a NaCl bilayer on Cu(111). The interaction between the vacancies was tailored by fabricating vacancy pairs and chains of different orientation and separation with atomic precision using vertical manipulation. Small separation of divacancies led to the formation of symmetric and antisymmetric vacancy states and localized interface-states. By scanning tunneling spectroscopy (STS) we measured their energy splitting and broadening as a function of the inter-vacancy separation. Unexpectedly, the energy splitting between the vacancy states is enlarged by level repulsion resulting from phonon dressing of the electronic states, as evidenced by theory. Already for a few coupled vacancies we observe an emerging band structure of the defect band.Comment: 9 pages, 4 figure

Snell's law for surface electrons: Refraction of an electron gas imaged in real space

On NaCl(100)/Cu(111) an interface state band is observed that descends from the surface-state band of the clean copper surface. This band exhibits a Moire-pattern-induced one-dimensional band gap, which is accompanied by strong standing-wave patterns, as revealed in low-temperature scanning tunneling microscopy images. At NaCl island step edges, one can directly see the refraction of these standing waves, which obey Snell's refraction law.Comment: 4 pages, 4 figure

Charge-State-Dependent Diffusion of Individual Gold Adatoms on Ionic Thin NaCl Films

It is known that individual metal atoms on insulating ionic films can occur in several different (meta) stable charge states, which can be reversibly switched in a controlled fashion. Here we show that the diffusion of gold adatoms on NaCl thin films depends critically on their charge state. Surprisingly, the anionic species has a lower diffusion barrier than the neutral one. Furthermore, for the former we observe that the diffusion atop a bilayer of NaCl is strongly influenced by the interface between NaCl and the underlying copper substrate. This effect disappears for a trilayer of NaCl. These observations open the prospect of controlling the diffusion properties of individual metal atoms on thin insulating films

Site determination and thermally assisted tunneling in homogenous nucleation

A combined low-temperature scanning tunneling microscopy and density functional theory study on the binding and diffusion of copper monomers, dimers, and trimers adsorbed on Cu(111) is presented. Whereas atoms in trimers are found in fcc sites only, monomers as well as atoms in dimers can occupy the stable fcc as well as the metastable hcp site. In fact the dimer fcc-hcp configuration was found to be only 1.3 meV less favorable with respect to the fcc-fcc configuration. This enables a confined intra-cell dimer motion, which at temperatures below 5 K is dominated by thermally assisted tunneling.Comment: 4 pages, 4 figure

Elektronenspinresonanz an einzelnen Molekülen

Elektronenspinresonanz (ESR) ist eine weit verbreitete spektroskopische Methode zur Untersuchung von Systemen mit ungepaarten Elektronenspins, wie beispielsweise molekularen Radikalen. In der Regel sind viele Milliarden Spins erforderlich, um ein ESR-Signal zu erhalten, das dadurch einer starken Ensemblemittelung unterliegt. Das Herunterskalieren der ESR auf einzelne Moleküle ermöglicht es, die Signaturen der Moleküle einzeln zu messen – etwa bei Biomolekülen in ihrer natürlichen Umgebung. Die Einzelmolekül-ESR eröffnet mehrere neue Forschungsrichtungen, unter anderem im Bereich der Quantendetektion mit einem einzelnen Molekül. In den letzten Jahrzehnten wurden vier verschiedene Ansätze der Einzelmolekül-ESR entwickelt, die entweder auf optisch detektierter Magnetresonanz oder auf Rastersondenmikroskopie beruhen. Hier wird ein Überblick über diese vier Ansätze sowie deren Verwendung in wegweisenden Arbeiten vermittelt

Electron Spin Resonance at the Single‐Molecule Scale

Electron spin resonance (ESR) is a widely employed spectroscopic technique for studying systems with unpaired electron spins, such as molecular radicals. Typically, many billions of spins are required to get a detectable ESR signal, which is subject to extensive ensemble averaging. Downscaling ESR to a single molecule allows studying the signatures of each individual molecule separately, applicable to biomolecules in their native environment, for example. Single-molecule ESR offers several novel research avenues, such as in quantum sensing with a single molecule. Over the last decades, four different single-molecule ESR approaches have been developed, which rely on either optically detected magnetic resonance or scanning-probe microscopy. An introduction into these four approaches including their deployment in pioneering works will be provided

Charge-state lifetimes of single molecules on ultrathin insulating films

In scanning tunneling microscopy (STM) experiments of molecules on insulating films, tunneling through molecular resonances implies transiently charging the molecule. The transition back to the charge ground state by tunneling through the insulating film is crucial, for example, for understanding STM-induced electroluminescence. Here, using STM, we report on the charge-state lifetimes of individual molecules adsorbed on NaCl films of different thicknesses on Cu(111) and Au(111). To that end, we approached the tip to the molecule at resonant tunnel conditions up to a regime where charge transport was limited by tunneling through the NaCl film. The resulting saturation of tunnel current is a direct measure of the molecule's charge-state lifetime, thus providing a means to study charge and, thereby, exciton dynamics. A comparison of anion and cation lifetimes on different substrates reveals the critical role of the level alignment with the insulator's conduction and valence band, and the metal-insulator interface state

On-Surface Synthesis of Polypyridine: Strain Enforces Extended Linear Chains

Strain-induced on-surface transformations provide an appealing route to steer the selectivity towards desired products. Here, we demonstrate the selective on-surface synthesis of extended all-trans poly(2,6-pyridine) chains on Au(111). By combining high-resolution scanning tunneling and atomic force microscopy, we revealed the detailed chemical structure of the reaction products. Density functional theory calculations indicate that the synthesis of extended covalent structures is energetically favored over the formation of macrocycles, due to the minimization of internal strain. Our results consolidate the exploitation of internal strain relief as a driving force to promote selective on-surface reactions

Apparent Reversal of Molecular Orbitals Reveals Entanglement

The frontier orbital sequence of individual dicyanovinyl-substituted oligothiophene molecules is studied by means of scanning tunneling microscopy. On NaCl/Cud(111), the molecules are neutral, and the two lowest unoccupied molecular states are observed in the expected order of increasing energy. On NaCl/Cud(311), where the molecules are negatively charged, the sequence of two observed molecular orbitals is reversed, such that the one with one more nodal plane appears lower in energy. These experimental results, in open contradiction with a single-particle interpretation, are explained by a manybody theory predicting a strongly entangled doubly charged ground state