Remotely controlled isomer selective molecular switching

Nonlocal addressing—the “remote control”—of molecular switches promises more efficient processing for information technology, where fast speed of switching is essential. The surface state of the (111) facets of noble metals, a confined two-dimensional electron gas, provides a medium that enables transport of signals over large distances and hence can be used to address an entire ensemble of molecules simultaneously with a single stimulus. In this study we employ this characteristic to trigger a conformational switch in anthradithiophene (ADT) molecules by injection of hot carriers from a scanning tunneling microscope (STM) tip into the surface state of Cu(111). The carriers propagate laterally and trigger the switch in molecules at distances as far as 100 nm from the tip location. The switching process is shown to be long-ranged, fully reversible, and isomer selective, discriminating between cis and trans diastereomers, enabling maximum control.PostprintPeer reviewe

Bipolar conductance switching of single anthradithiophene molecules

The authors acknowledge funding by the Emmy-Noether-Program of the Deutsche Forschungsgemeinschaft, the SFB 767, and the Baden-Württemberg Stiftung. R.P. and A.A. thank the Basque Departamento de Universidades e Investigacion (grant no. IT-756-13) and the Spanish Ministerio de Economia y Competitividad (grant no. FIS2013-48286-C2-8752-P) for financial support.Single molecular switches are basic device elements in organic electronics. The pentacene analogue anthradithiophene (ADT) shows a fully reversible binary switching between different adsorption conformations on a metallic surface accompanied by a charge transfer. These transitions are activated locally in single molecules in a low-temperature scanning tunneling microscope . The switching induces changes between bistable orbital structures and energy level alignment at the interface. The most stable geometry, the “off” state, which all molecules adopt upon evaporation, corresponds to a short adsorption distance at which the electronic interactions of the acene rings bend the central part of the molecule toward the surface accompanied by a significant charge transfer from the metallic surface to the ADT molecules. This leads to a shift of the lowest unoccupied molecular orbital down to the Fermi level (EF). In the “on” state the molecule has a flat geometry at a larger distance from the surface; consequently the interaction is weaker, resulting in a negligible charge transfer with an orbital structure resembling the highest occupied molecular orbital when imaged close to EF. The potential barrier between these two states can be overcome reversibly by injecting charge carriers locally into individual molecules. Voltage-controlled current traces show a hysteresis characteristic of a bipolar switching behavior. The interpretation is supported by first-principles calculations.PostprintPeer reviewe

Electric-field-driven direct desulfurization

The ability to elucidate the elementary steps of a chemical reaction at the atomic scale is important for the detailed understanding of the processes involved, which is key to uncover avenues for improved reaction paths. Here, we track the chemical pathway of an irreversible direct desulfurization reaction of tetracenothiophene adsorbed on the Cu(111) closed-packed surface at the submolecular level. Using the precise control of the tip position in a scanning tunneling microscope and the electric field applied across the tunnel junction, the two carbon–sulfur bonds of a thiophene unit are successively cleaved. Comparison of spatially mapped molecular states close to the Fermi level of the metallic substrate acquired at each reaction step with density functional theory calculations reveals the two elementary steps of this reaction mechanism. The first reaction step is activated by an electric field larger than 2 V nm–1, practically in absence of tunneling electrons, opening the thiophene ring and leading to a transient intermediate. Subsequently, at the same threshold electric field and with simultaneous injection of electrons into the molecule, the exergonic detachment of the sulfur atom is triggered. Thus, a stable molecule with a bifurcated end is obtained, which is covalently bound to the metallic surface. The sulfur atom is expelled from the vicinity of the molecule.PostprintPeer reviewe

Chiral and catalytic effects of site-specific molecular adsorption

Open access funded by Max Planck Society. The authors acknowledge the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany’s Excellence Strategy-EXC-2123 Quantum Frontiers - 390837967; Core program PC2-PN23080202 and the PN-III-P2-2.1-PED-2021-0378 (contract no. 575PED/2022) granted projects, financed by the Romanian Ministry of Research, Innovation and Digitalization/UEFISCDI; and the generous allocation of computer time at the computing center of Donostia International Physics Center and at the Red Española de Supercomputación (project QHS-2021-2-0019). A.A. acknowledges support from Project No. PID2019-103910GB-I00, funded by MCIN/AEI/10.13039/501100011033/ and FEDER Una manera de hacer Europa, and Project No. IT-1527-22 funded by the Basque Government.The changes of properties and preferential interactions based on subtle energetic differences are important characteristics of organic molecules, particularly for their functionalities in biological systems. Only slightly energetically favored interactions are important for the molecular adsorption and bonding to surfaces, which define their properties for further technological applications. Here, prochiral tetracenothiophene molecules are adsorbed on the Cu(111) surface. The chiral adsorption configurations are determined by Scanning Tunneling Microscopy studies and confirmed by first-principles calculations. Remarkably, the selection of the adsorption sites by chemically different moieties of the molecules is dictated by the arrangement of the atoms in the first and second surface layers. Furthermore, we have investigated the thermal effects on the direct desulfurization reaction that occurs under the catalytic activity of the Cu substrate. This reaction leads to a product that is covalently bound to the surface in chiral configurations.Publisher PDFPeer reviewe

Controlling single molecule conductance by a locally induced chemical reaction on individual thiophene units

The authors acknowledge the Emmy-Noether-Program of the Deutsche Forschungsgemeinschaft, the SFB 767, Core Program PN19-03 (contract number 21 N/08.02.2019) founded by the Romanian Ministry of Research and Innovation, Basque Departamento de Universidades e Investigación (grant no. IT-756-13), the Spanish Ministerio de Economía y Competitividad (grant no. FIS2013-48286-C2-8752-P and FIS2016-75862-P) andthe Operational Programme Research, Development and Education financed by European Structural and Investment Funds and the Czech Ministry of Education, Youth and Sports (Project No. SOLID21 CZ.02.1.01/0.0/0.0/16_019/0000760).Among the prerequisites for the progress of single‐molecule‐based electronic devices are a better understanding of the electronic properties at the individual molecular level and the development of methods to tune the charge transport through molecular junctions. Scanning tunneling microscopy (STM) is an ideal tool not only for the characterization, but also for the manipulation of single atoms and molecules on surfaces. The conductance through a single molecule can be measured by contacting the molecule with atomic precision and forming a molecular bridge between the metallic STM tip electrode and the metallic surface electrode. The parameters affecting the conductance are mainly related to their electronic structure and to the coupling to the metallic electrodes. Here, the experimental and theoretical analyses are focused on single tetracenothiophene molecules and demonstrate that an in situ‐induced direct desulfurization reaction of the thiophene moiety strongly improves the molecular anchoring by forming covalent bonds between molecular carbon and copper surface atoms. This bond formation leads to an increase of the conductance by about 50 % compared to the initial state.Publisher PDFPeer reviewe

Electronic and magnetic interactions in single molecular and atomic contacts

This thesis encompasses an investigation of organic molecules on metallic substrates as well as a study of electronic and magnetic effects in atomic-sized palladium contacts. For this, a scanning tunneling microscope, operating at 6 K and ultra-high vacuum is used. The clean environment allows characterization of single molecules and atoms under well-defined conditions. Moreover, by examining at low-temperatures it is possible to study electronic and magnetic effects spectroscopically, which would otherwise be thermally obscured. The first part of this thesis focuses on studying organic molecules absorbed on metallic substrates. The molecules under investigation are thiophene derivatives of the archetypical organic semiconductor pentacene. The thiophene derivatives, tetracenothiophene (TCT) and anthradithiophene (ADT), contain either one or two thiophene groups at the terminal benzene rings. The dependence of the deposition temperature as well as the impact of different metallic substrates on their adsorption behavior is studied. When depositing the molecules on a Au substrate at room temperature, the molecules stay fully intact. In contrast, when deposited onto a Cu substrate, a breaking of the thiophene entity is observed. The breaking can be prevented by cooling the substrate to 200 K during deposition. Conductance measurements on the TCT molecules with an intact and broken thiophene entity reveal differences in their transport characteristics. The intact ones exhibit lower conductances than the broken ones. This can essentially be attributed to the fact that the latter ones bind covalently to the substrate, which facilitates transport. ADT deposited on Cu(111) can be reversibly and repeatedly interconverted between two distinct adsorption congurations associated with different conductances, and thus has been investigated as a molecular switch. The switching mechanism is related to a change between two different bonding configurations of the molecule to the substrate. The non-switched state ("off") can be ascribed to a strongly bound state and the switched state ("on") to a weaker bound configuration. The switching process is not only restricted to the location beneath the tip but also can be initiated within an extended area of about 100 nm in radius. This long-ranged addressing is enabled by means of surface state carriers of the Cu(111) surface. The non-local switching process is fully reversible, isomer selective and efficient over radial distances of about 100 nm. The second part of this work examines atomic-sized palladium (Pd) contacts. Pd exhibits outstanding properties owing to its high density of states (DOS) at the Fermi level. This renders Pd a "nearly ferromagnetic" material. Upon size confinement the DOS can be pushed to higher values and Pd can develop a net magnetization. Transport measurements were carried out for two distinct contact geometries; a tip-adatom and and a tip-surface contact geometry. Whereas for the tip-adatom contacts no magnetic state can be detected, measurements conducted for the tip-surface geometry might indicate the presence of a magnetic moment. The differences are attributed to the adsorption configuration of the single bridging atom in the junction. Spectra taken in point-contact with an adatom reveal a distinct inverted V-shaped feature. Presently we attribute this feature to paramagnons

On surface electric field driven chemical reaction of individual molecular subunits

Resumen del trabajo presentado al International workshop On-Surface Synthesis (OSS), celebrado en Sant Feliu de Guíxol (España) del 23 al 28 de septiembre de 2018.Understanding and controlling elementary steps in chemical reactions will help to improve their efficiency significantly. Here we present on surface chemistry studies of a direct desulfurization process of a single thiophene unit connected to a molecular backbone on a Cu(111) surface. We can trigger the chemical reaction by the electric field confined in the tunnel junction of a scanning tunneling microscope. The precise control of the external stimulus allows resolving the elementary reaction steps of this direct desulfurization process. High resolution scanning tunneling microscopy (STM) and atomic force microscopy images clearly reveal the breaking of the thiophene ring after the chemical reaction. An atomically precise reconstruction of the initial and final product was possible with the help of density functional theory calculations. The breaking of the thiophene ring results in the formation of strong C-Cu bonds. Measuring the conductance through single molecules with the STM locally we find that this strong anchoring between the C-atoms and the Cu-surface atoms leads to an increase of the conductance by 50% compared to before the reaction.Peer reviewe