Electro-active properties of nanostructured films of cytosine and guanine nucleobases.

The discovery of multifunctional properties related to electro-activity of organic systems of biomolecules is important for a variety of applications, especially for devices in the realm of biocompatible sensors and/or bioactuators. A further step towards such applications is to prepare thin films with the required properties. Here, the investigation is focused on the characterization of films of guanine and cytosine nucleobases, prepared by thermal evaporation-an industrial accessible deposition technique. The cytosine films have an orthorhombic non-centrosymmetric structure and grow in two interconnected nanostructured fractal patterns, of nearly equal proportion. Piezoresponse force microscopy images acquired at room temperature on the cytosine films display large zones with antiparallel alignment of the vertical components of the polarization vector. Guanine films have a dense nano-grained morphology. Our studies reveal electrical polarization switching effects which can be related to ferroelectricity in the films of guanine molecules. Characteristic ferroelectric polarization-electric-field hysteresis loops showing large electrical polarization are observed at low temperatures up to 200 K. Above this temperature, the guanine films have a preponderant paraelectric phase containing residual or locally induced nano-scopic ferroelectric domains, as observed by piezoresponse force microscopy at room temperature.

Controlling Single Molecule Conductance by a Locally Induced Chemical Reaction on Individual Thiophene Units.

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.

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.

Chiral and Catalytic Effects of Site-Specific Molecular Adsorption.

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.

Molecular sensitised probe for amino acid recognition within peptide sequences.

The combination of low-temperature scanning tunnelling microscopy with a mass-selective electro-spray ion-beam deposition established the investigation of large biomolecules at nanometer and sub-nanometer scale. Due to complex architecture and conformational freedom, however, the chemical identification of building blocks of these biopolymers often relies on the presence of markers, extensive simulations, or is not possible at all. Here, we present a molecular probe-sensitisation approach addressing the identification of a specific amino acid within different peptides. A selective intermolecular interaction between the sensitiser attached at the tip-apex and the target amino acid on the surface induces an enhanced tunnelling conductance of one specific spectral feature, which can be mapped in spectroscopic imaging. Density functional theory calculations suggest a mechanism that relies on conformational changes of the sensitiser that are accompanied by local charge redistributions in the tunnelling junction, which, in turn, lower the tunnelling barrier at that specific part of the peptide.

Magnetocaloric and Giant Magnetoresistance Effects in La-Ba-Mn-Ti-O Epitaxial Thin Films: Influence of Phase Transition and Magnetic Anisotropy.

Magnetic perovskite films have promising properties for use in energy-efficient spintronic devices and magnetic refrigeration. Here, an epitaxial ferromagnetic La0.67Ba0.33Mn0.95Ti0.05O3 (LBMTO-5) thin film was grown on SrTiO3(001) single crystal substrate by pulsed laser deposition. High-resolution X-ray diffraction proved the high crystallinity of the film with tetragonal symmetry. The magnetic, magnetocaloric and magnetoresistance properties at different directions of the applied magnetic field with respect to the ab plane of the film were investigated. An in-plane uni-axial magnetic anisotropy was evidenced. The LBMTO-5 epilayer exhibits a second-order ferromagnetic-paramagnetic phase transition around 234 K together with a metal-semiconductor transition close to this Curie temperature (TC). The magnetic entropy variation under 5 T induction of a magnetic field applied parallel to the film surface reaches a maximum of 17.27 mJ/cm3 K. The relative cooling power is 1400 mJ/cm3 K (53% of the reference value reported for bulk Gd) for the same applied magnetic field. Giant magnetoresistance of about 82% under 5 T is obtained at a temperature close to TC. Defined as the difference between specific resistivity obtained under 5 T with the current flowing along the magnetic easy axis and the magnetic field oriented transversally to the current, parallel and perpendicular to the sample plane, respectively, the in-plane magneto-resistance anisotropy in 5 T is about 9% near the TC.

Cyclophane with eclipsed pyrene units enables construction of spin interfaces with chemical accuracy.

Advanced functionality in molecular electronics and spintronics is orchestrated by exact molecular arrangements at metal surfaces, but the strategies for constructing such arrangements remain limited. Here, we report the synthesis and surface hybridization of a cyclophane that comprises two pyrene groups fastened together by two ferrocene pillars. Crystallographic structure analysis revealed pyrene planes separated by ∼352 pm and stacked in an eclipsed geometry that approximates the rare configuration of AA-stacked bilayer graphene. We deposited this cyclophane onto surfaces of Cu(111) and Co(111) at submonolayer coverage and studied the resulting hybrid entities with scanning tunnelling microscopy (STM). We found distinct characteristics of this cyclophane on each metal surface: on non-magnetic Cu(111), physisorption occurred and the two pyrene groups remained electronically coupled to each other; on ferromagnetic Co(111) nanoislands, chemisorption occurred and the two pyrene groups became electronically decoupled. Spin-polarized STM measurements revealed that the ferrocene groups had spin polarization opposite to that of the surrounding Co metal, while the pyrene stack had no spin polarization. Comparisons to the non-stacked analogue comprising only one pyrene group bolster our interpretation of the cyclophane's STM features. The design strategy presented herein can be extended to realize versatile, three-dimensional platforms in single-molecule electronics and spintronics.

Thin Films Based on Cobalt Phthalocyanine:C60 Fullerene:ZnO Hybrid Nanocomposite Obtained by Laser Evaporation.

Matrix-assisted pulsed laser evaporation (MAPLE) was used to deposit hybrid nanocomposite thin films based on cobalt phthalocyanine (CoPc), C60 fullerene and ZnO nanoparticles. The inorganic nanoparticles, with a size of about 20 nm, having the structural and optical properties characteristic of ZnO, were chemically synthesized by a simple precipitation method. Furthermore, ZnO nanoparticles were dispersed in a dimethyl sulfoxide solution in which CoPc and C60 had been dissolved, ready for the freezing MAPLE target. The effect of the concentration of ZnO nanoparticles on the structural, morphological, optical and electrical properties of the CoPc:C60:ZnO hybrid nanocomposite layers deposited by MAPLE was evaluated. The infrared spectra of the hybrid nanocomposite films confirm that the CoPc and C60 preserve their chemical structure during the laser deposition process. The CoPc optical signature is recognized in the ultraviolet-visible (UV-Vis) spectra of the obtained layers, these being dominated by the absorption bands associated to this organic compound while the ZnO optical fingerprint is identified in the photoluminescence spectra of the prepared layers, these disclosing the emission bands linked to this inorganic semiconductor. The hybrid nanocomposite layers exhibit globular morphology, which is typical for the thin films deposited by MAPLE. Current-voltage (J-V) characteristics of the structures developed on CoPc:C60:ZnO layers reveal that the addition of an appropriate amount of ZnO nanoparticles in the CoPc:C60 mixture leads to a more efficient charge transfer between the organic and inorganic components. Due to their photovoltaic effect, structures featuring such hybrid nanocomposite thin films deposited by MAPLE can have potential applications in the field of photovoltaic devices.

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.

Bipolar Conductance Switching of Single Anthradithiophene Molecules.

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.

Ordered arrays of metal-organic magnets at surfaces.

We report here on the molecular self-assembly of 7, 7', 8, 8' tetracyanoquinodimethane (TCNQ) (schematic representation of the molecule on figure 1(b)) on a single-crystal Cu(111) surface, followed by deposition of Fe and Mn, and on the formation of an ordered metal­organic coordination network of Fe (or Mn)­TCNQ units, which contains magnetic ions separated by 0.9 nm.