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We report on the chemical design of chiral molecular junctions with stress-dependent conductance, whose helicity is maintained during the stretching of a single molecule junction due to the stapling of both ends of the inner helix. In the reported compounds, different conductive pathways are observed, with clearly different conductance values and plateau-length distributions, attributed to different conformations of the helical structures. The large chiro-optical responses and the potential use of these molecules as unimolecular spin filters have been theoretically proved using state-of-the-art Density Functional Theory (DFT) calculations, including a fully ab-initio estimation of the CISS-originating spin polarization which is done, for the first time, for a realistic molecular system, We thank the Spanish MICIN for the “María de Maeztu” Programme for Units of Excellence in R&D (grant No. CEX2018-000805-M). Financial support from MCIN/AEI/ 10.13039/501100011033 is acknowledged by L.A.Z. (grant PID2021-125604NB-I00), E. L. (PID2021-127964NB-C21), D. M. and J. M. C. (PID2020-113059GB-C21), A. M. (PID2021-127964NB-C22) and A. O. G. for her FPU contract (FPU16/02597). We also thank the Universidad Autónoma de Madrid and the Comunidad de Madrid (grants No. SI3/PJI/2021-00191, S2018/NMT-4321 through the Nanomag COST-CM Program and E.L. for Atracción de Talento grant 2019-T1/IND-16384). J. J. P. acknowledges financial support from Spanish MICINN (PID2019- 109539GB-C43) and the Generalitat Valenciana through Programa Prometeo/2021/01. This research was also funded by FEDER/Junta de Andalucía-Consejería de Economía y Conocimiento/P20 00162 and the Visiting Scholar Program in the University of Granada. IMDEA Nanociencia acknowleges support from the ’Severo Ochoa’ Programme for Center of Excelence in R&D (CEX2020-001039-S).

Spin-orbit coupling gives rise to a range of spin-charge interconversion phenomena in non-magnetic systems where certain spatial symmetries are reduced or absent. Chirality-induced spin selectivity (CISS), a term that generically refers to a spin-dependent electron transfer in non-magnetic chiral systems, is one such case, appearing in a variety of seemingly unrelated situations ranging from inorganic materials to molecular devices. In particular, the origin of CISS in molecular junctions is a matter of an intense current debate. Here we derive a set of geometrical conditions for this effect to appear, hinting at the fundamental role of symmetries beyond otherwise relevant quantitative issues. Our approach, which draws on the use of point-group symmetries within the scattering formalism for transport, shows that electrode symmetries are as important as those of the molecule when it comes to the emergence of a spin-polarization and, by extension, to the possible appearance of CISS. It turns out that standalone metallic nanocontacts can exhibit spin-polarization when relative rotations which reduce the symmetry are introduced. As a corollary, molecular junctions with $\textbf{achiral}$ molecules can also exhibit spin-polarization along the direction of transport, provided that the whole junction is chiral in a specific way. This formalism also allows the prediction of qualitative changes of the spin-polarization upon substitution of a chiral molecule in the junction with its enantiomeric partner. Quantum transport calculations based on density functional theory corroborate all of our predictions and provide further quantitative insight within the single-particle framework., 34 pages, 4 figures, 1 table

Multiple experiments on the electron transport through solid-state junctions based on different proteins have suggested that the dominant transport mechanism is quantum tunneling (or coherent trans...

The effects of antiaromaticity and destructive quantum interference (DQI) are investigated on the charge transport through dibenzo-[a,e]pentalene (DBP). 5,10-Connectivity gives high single-molecule conductance whereas 2,7 gives low conductance due to DQI. Comparison of the 5,10-DBP with phenyl and anthracene analogues yields the trend GDBP ≈ GAnth > GPh, despite the aromatic anthracene having a larger HOMO–LUMO gap than 5,10-DBP. This is explained by unfavourable level alignment for 5,10-DBP.

We have explored the use of constrained density functional theory (cDFT) for molecular junctions based on benzenediamine. By elongating the junction, we observe that the energy gap between the ionization potential and the electronic affinity increases with the stretching distance. This is consistent with the trend expected from the electrostatic screening. A more detailed analysis shows how this influences the charge distribution of both the individual metal layers and the molecular atoms. Overall, our work shows that constrained DFT is a powerful tool for studying screening effects in molecular junctions, We thank the Spanish MINECO for the “María de Maeztu” Programme for Units of Excellence in R&D (grant No. CEX2018-000805-M). L.A.Z. thanks financial support from the Universidad Autónoma de Madrid and the Comunidad de Madrid (grant No. SI3/PJI/2021-00191). J.J.P. acknowledges financial support from Spanish MICINN through Grant No. PID2019- 109539GBC43, the Comunidad Autónoma de Madrid through the Nanomag COST-CM Program (Grant No. S2018/NMT-4321) and the Generalitat Valenciana through Programa Prometeo/2021/01

Understanding charge transport in DNA molecules is a long-standing problem of fundamental importance across disciplines1,2. It is also of great technological interest due to DNA’s ability to form versatile and complex programmable structures. Charge transport in DNA-based junctions has been reported using a wide variety of set-ups2–4, but experiments so far have yielded seemingly contradictory results that range from insulating5–8 or semiconducting9,10 to metallic-like behaviour11. As a result, the intrinsic charge transport mechanism in molecular junction set-ups is not well understood, which is mainly due to the lack of techniques to form reproducible and stable contacts with individual long DNA molecules. Here we report charge-transport measurements through single 30-nm-long double-stranded DNA (dsDNA) molecules with an experimental set-up that enables us to address individual molecules repeatedly and to measure the current–voltage characteristics from 5 K up to room temperature. Strikingly, we observed very high currents of tens of nanoamperes, which flowed through both homogeneous and non-homogeneous base-pair sequences. The currents are fairly temperature independent in the range 5–60 K and show a power-law decrease with temperature above 60 K, which is reminiscent of charge transport in organic crystals. Moreover, we show that the presence of even a single discontinuity (‘nick’) in both strands that compose the dsDNA leads to complete suppression of the current, which suggests that the backbones mediate the long-distance conduction in dsDNA, contrary to the common wisdom in DNA electronics2–4. Measurements of electrical conductance in double-stranded DNA suggest that the backbones mediate the long-distance conduction in dsDNA, contrary to the common wisdom in DNA electronics.

A sample-type protein monolayer, that can be a stepping stone to practical devices, can behave as an electrically driven switch. This feat is achieved using a redox protein, cytochrome C (CytC), with its heme shielded from direct contact with the solid-state electrodes. Ab initio DFT calculations, carried out on the CytC-Au structure, show that the coupling of the heme, the origin of the protein frontier orbitals, to the electrodes is sufficiently weak to prevent Fermi level pinning. Thus, external bias can bring these orbitals in and out of resonance with the electrode. Using a cytochrome C mutant for direct S-Au bonding, approximately 80 % of the Au-CytC-Au junctions show at greater than 0.5 V bias a clear conductance peak, consistent with resonant tunneling. The on-off change persists up to room temperature, demonstrating reversible, bias-controlled switching of a protein ensemble, which, with its built-in redundancy, provides a realistic path to protein-based bioelectronics.

We carried out first-principles density-functional theory calculations to study the work of separation for five different metal–metal interfaces, each of them comprising thin layers of selected metals (Cr, W, Ta, Al or Ti) lying on top of Au surfaces. We found that the highest work of separation is obtained for one-atom-thick layers. Increasing the number of atomic layers leads the work of separation to oscillate with the thickness, and ultimately tend to a limiting value for a large number of layers. Interestingly, for most cases the lowest work of separation is obtained for two-atom layers. We find that this behaviour is mirrored by the quantity of charge transferred between the two metals on the one hand, and their spatial distance on the other.

We have joined two fundamental concepts of organic chemistry to provide a deep, yet intuitive, understanding of how side groups influence destructive quantum interference (DQI) in the transport through conjugated molecules. Using density functional theory combined with Green's function techniques, and employing tight-binding models in which all the π-systems are considered, we elucidate the separate roles of bond-resonance and induction in tuning DQI. We show that the position of the anti-resonances produced by DQI is sensitive to the number of side groups, but not in a simple additive way. Instead, addition of multiple groups results in a weaker overall contribution per group, and this can be understood using a straight forward graphical analysis. Furthermore, we show that additional fine tuning of DQI is possible via attachment of a chain of atoms to a second site around the ring. DQI is controlled by modifying the length of the chain, thus providing exquisite control over the anti-resonance position. This insight provides chemists with a large number of options to tune DQI for unprecedented device optimization.

Herein, we present, for the first time, a 2D-MOF based on copper and 4-hydroxypyrimidine-5-carbonitrile as the linker. Each MOF layer is perfectly flat and neutral, as is the case for graphene. High pressure X-ray diffraction measurements reveal that this layered structure can be modulated between 3.01 to 2.78 Å interlayer separation, with an evident piezochromism and varying conductive properties. An analysis of the band structure indicates that this material is conductive along different directions depending on the application of pressure or H doping. These results pave the way for the development of novel layered materials with tunable and efficient properties for pressure-based sensors., European Union (EU) 604391, Swiss National Science Foundation (SNSF) 200020_162861, Spanish Government CEX2018-000805-M CTQ2015-69163-CO2R1 PGC2018-102052-B-C21 PGC2018-102052-A-C22 PGC2018102047-B-I00 FIS2016-80434-P PID2019-109539GB-C43, Junta de Andalucia FQM-394, Comunidad Autonoma de Madrid through the Nanomag COST-CM Program S2018/NMT-4321, University of Seville, National Council for Scientific and Technological Development (CNPq) 405107/2017-0

Motivated by recent experiments, we performed a theoretical study of electron transport through single-molecule junctions incorporating four kinds of homopeptides (based on alanine, glutamic acid, lysine, and tryptophan). Our results suggest that these molecules are rather insulating and operate in off-resonance tunneling as their main transport mechanism. We ascribe their poor performance as conductors to the high localization of their frontier orbitals. We found that binding scenarios in which side chains lie on the side of gold protuberances could give rise to an increase in conductance with respect to end-to-end binding configurations. These findings provide an insight into the conductance mechanism of the building blocks of proteins and identify key issues that need to be further investigated., L.A.Z. was funded by the Spanish MINECO through the grant MAT2014-58982-JIN. J.C.C. acknowledges financial support from the Spanish MINECO (FIS2014-53488-P and FIS2017-84057-P) as well as DFG and SFB767 for sponsoringhis stay at the University of Konstanz as Mercator Fellow

Using the break junction (BJ) technique we show that Au(RS)2 units play a significant role in thiol-terminated molecular junctions formed on gold. We have studied a range of thiol-terminated compounds, either with the sulfur atoms in direct conjugation with a phenyl core, or bonded to saturated methylene groups. For all molecules we observe at least two distinct groups of conductance plateaus. By a careful analysis of the length behavior of these plateaus, comparing the behavior across the different cores and with methyl sulfide anchor groups, we demonstrate that the lower conductance groups correspond to the incorporation of Au(RS)2 oligomeric units at the contacts. These structural motifs are found on the surface of gold nanoparticles but they have not before been shown to exist in molecular-break junctions. The results, while exemplifying the complex nature of thiol chemistry on gold, moreover clarify the conductance of 1,4-benzenedithiol on gold. We show that true Au-S-Ph-S-Au junctions have a relatively narrow conductance distribution., Comment: 9 pages, 7 figures

We present a theoretical study of the blue-copper azurin extracted from Pseudomonas aeruginosa and several of its single amino acid mutants. For the first time, we consider the whole structure of this kind of protein rather than limiting our analysis to the copper complex only. This is accomplished by combining fully ab initio calculations based on density functional theory with atomic-scale molecular dynamics simulations. Beyond the main features arising from the copper complex, our study reveals the role played by the peripheral parts of the proteins. In particular, we find that oxygen atoms belonging to carboxyl groups which are distributed all over the protein contribute to electronic states near the HOMO. The contribution of the outer regions to the electronic structure of azurins had so far been overlooked. Our results highlight the need to investigate them thoroughly; this is especially important in prospect of understanding complex processes such as the electronic transport through metal-metalloprotein-metal junctions, We thank the financial support from the Spanish MINECO (MAT2014-58982-JIN, FIS2017-84057-P, MDM-2014-0377 and MAT2017-83273-R). J. G. V. acknowledges funding from a Marie Sklodowska-Curie Fellowship within the Horizons 2020 framework (grant number DLV-795286)

The study of thermoelectricity in molecular junctions is of fundamental interest for the development of various technologies including cooling (refrigeration) and heat-to-electricity conversion 1-4 . Recent experimental progress in probing the thermopower (Seebeck effect) of molecular junctions 5-9 has enabled studies of the relationship between thermoelectricity and molecular structure 10,11 . However, observations of Peltier cooling in molecular junctions-a critical step for establishing molecular-based refrigeration-have remained inaccessible. Here, we report direct experimental observations of Peltier cooling in molecular junctions. By integrating conducting-probe atomic force microscopy 12,13 with custom-fabricated picowatt-resolution calorimetric microdevices, we created an experimental platform that enables the unified characterization of electrical, thermoelectric and energy dissipation characteristics of molecular junctions. Using this platform, we studied gold junctions with prototypical molecules (Au-biphenyl-4,4'-dithiol-Au, Au-terphenyl-4,4''-dithiol-Au and Au-4,4'-bipyridine-Au) and revealed the relationship between heating or cooling and charge transmission characteristics. Our experimental conclusions are supported by self-energy-corrected density functional theory calculations. We expect these advances to stimulate studies of both thermal and thermoelectric transport in molecular junctions where the possibility of extraordinarily efficient energy conversion has been theoretically predicted 2-4,14 .

Licensee MDPI, Basel, Switzerland. Protein-based electronics is an emerging field which has attracted considerable attention over the past decade. Here, we present a theoretical study of the formation and electronic structure of a metal-protein-metal junction based on the blue-copper azurin from pseudomonas aeruginosa. We focus on the case in which the protein is adsorbed on a gold surface and is contacted, at the opposite side, to an STM (Scanning Tunneling Microscopy) tip by spontaneous attachment. This has been simulated through a combination of molecular dynamics and density functional theory. We find that the attachment to the tip induces structural changes in the protein which, however, do not affect the overall electronic properties of the protein. Indeed, only changes in certain residues are observed, whereas the electronic structure of the Cu-centered complex remains unaltered, as does the total density of states of the whole protein, This research was funded by the Spanish MINECO (MAT2014-58982-JIN, FIS2017-84057-P, MDM-2014-0377 and MAT2017-83273-R). J.G.V. acknowledges funding from a Marie Sklodowska-Curie Fellowship within the Horizons 2020 framework (grant number DLV 795286). I.D.-P. thanks the ERC project Fields4CAT (ref. 772391) for financial support

In the growing field of biomolecular electronics, blue-copper Azurin stands out as one of the most widely studied protein in single-molecule contacts. Interestingly, despite the paramount importance of the structure/dynamics of molecular contacts in their transport properties, these factors remain largely unexplored from the theoretical point of view in the context of single Azurin junctions. Here we address this issue using all-atom Molecular Dynamics (MD) of Pseudomonas Aeruginosa Azurin adsorbed to a Au(111) substrate. In particular, we focus on the structure and dynamics of the free/adsorbed protein and how these properties are altered upon single-point mutations. The results revealed that wild-type Azurin adsorbs on Au(111) along two well defined configurations: one tethered via cysteine groups and the other via the hydrophobic pocket surrounding the Cu 2 + . Surprisingly, our simulations revealed that single amino-acid mutations gave rise to a quenching of protein vibrations ultimately resulting in its overall stiffening. Given the role of amino-acid vibrations and reorientation in the dehydration process at the protein-water-substrate interface, we suggest that this might have an effect on the adsorption process of the mutant, giving rise to new adsorption configurations.

Motivated by a recent experiment [C. Guo et al., Proc. Natl. Acad. Sci. U. S. A. 113, 10785 (2016)], we carry out a theoretical study of electron transport through peptide-based single-molecule junctions. We analyze the pristine hepta-alanine and its functionalizations with a single tryptophan unit, which is placed in three different locations along the backbone. Contrary to expectations from the experiment on self-assembled monolayers, we find that insertion of tryptophan does not raise the electrical conductance and that the resulting peptides instead remain insulating in the framework of a coherent transport picture. The poor performance of these molecules as conductors can be ascribed to the strongly off-resonant transport and low electrode-molecule coupling of the frontier orbitals. Although the introduction of tryptophan increases the energy of the highest occupied molecular orbital (HOMO) of the peptides in the gas phase, the new HOMO states are localized on the tryptophan unit and therefore essentially do not contribute to coherent charge transport.

We have developed a new and user-friendly interface energy calculation method that avoids problems deriving from numerical differences between bulk and slab calculations, such as the number of k points along the direction perpendicular to the interface. We have applied this to 36 bcc-fcc metal interfaces in the (100) orientation and found a clear dependence of the interface energy on the difference between the work functions of the two metals, on the one hand, and the total number of d electrons on the other. Greater mechanical deformations were observed in fcc crystals than in their bcc counterparts. For each bcc metal, the interface energy was found to follow the position of its d band, whereas the same was not observed for fcc., Comment: 10 pages, 7 figures, and 3 tables, with supporting information provided as an ancillary file. As accepted on 10th January 2018 for publication in Materials and Design, for which see https://doi.org/10.1016/j.matdes.2018.01.019

Bioelectronics moves toward designing nanoscale electronic platforms that allow in vivo determinations. Such devices require interfacing complex biomolecular moieties as the sensing units to an electronic platform for signal transduction. Inevitably, a systematic design goes through a bottom-up understanding of the structurally related electrical signatures of the biomolecular circuit, which will ultimately lead us to tailor its electrical properties. Toward this aim, we show here the first example of bioengineered charge transport in a single-protein electrical contact. The results reveal that a single point-site mutation at the docking hydrophobic patch of a Cu-azurin causes minor structural distortion of the protein blue Cu site and a dramatic change in the charge transport regime of the single-protein contact, which goes from the classical Cu-mediated two-step transport in this system to a direct coherent tunneling. Our extensive spectroscopic studies and molecular-dynamics simulations show that the proteins' folding structures are preserved in the single-protein junction. The DFT-computed frontier orbital of the relevant protein segments suggests that the Cu center participation in each protein variant accounts for the different observed charge transport behavior. This work is a direct evidence of charge transport control in a protein backbone through external mutagenesis and a unique nanoscale platform to study structurally related biological electron transfer.

We present a theoretical study of the electronic transport through Pt nanocontacts. We show that the analysis of the tunnelling regime requires a very careful treatment of the technical details. For instance, an insufficient size of the system can cause unphysical charge oscillations to arise along the transport direction; moreover, the use of an inappropriate basis set can deviate the distance dependence of the conductance from the expected exponential trend. While the conductance decay can be either corrected by employing ghost atoms or a large-cutoff-radius basis set, the same does not apply to the corrugation, for which only the second option is recommended. Interestingly, these details were not found to have a remarkable impact in the contact regime. These findings are important for theoretical studies of distance-dependent phenomena in scanning-probe and break-junction experiments., Comment: 7 pages, 8 figures

We studied the electron-transport properties of ten different amino acids and one dimer (di-methionine) using the mechanically controlled break-junction (MCBJ) technique. For methionine and cysteine, additional measurements were performed with the scanning tunneling microscope break-junction (STM-BJ) technique. By means of a statistical clustering technique, we identified several conductance groups for each of the molecules considered. Ab initio calculations revealed that the observed broad conductance distribution stems from the possibility of various binding geometries which can be formed during stretching combined with a multitude of possible conformational changes. The results suggest that it would be helpful to explore different experimental techniques such as recognition tunneling and conditions to help identify the nature of amino-acid-based junctions even further, for example, with the goal to establish a firm platform for their unambiguous recognition by tunneling break-junction experiments.

One-dimensional nanostructures at silicon surfaces have potential applications in nanoscale devices. Here we propose a mechanism of dipole-directed assembly for the growth of lines of physisorbed dipolar molecules. The adsorbate chosen was a halide, in preparation for the patterned imprinting of halogen atoms. Using scanning tunnelling microscopy, physisorbed 1,5-dichloropentane on Si(100)-2x1 was shown to self-assemble at room temperature into molecular lines that grew predominantly perpendicular to the Si-dimer rows. Line formation was triggered by the displacement of surface charge by the dipolar adsorbate. Experimental and simulated scanning tunnelling microscopy images were in agreement for a range of positive and negative bias voltages. The geometry of the physisorbed molecules and nature of their binding were evident from the scanning tunnelling microscopy images, as interpreted by scanning tunnelling microscopy simulation.

We modelled the adsorption of tetra-pyridyl-porphyrin (TPyP) and Fe(II)-tetra-pyridyl- porphyrin (Fe-TPyP) molecules on Ag(1 1 1). TPyP adsorbs in a flat geometry at a distance of 5.6 A from the surface. The dihedral angle of the pyridyl rings is found to be 70°. The molecular structure remains largely unaffected by the adsorption. The structure of the molecular adlayer should thus be determined mainly by lateral intermolecular interactions. These conclusions are confirmed by scanning tunnelling microscopy observations. Upon metalation spin-polarized states with strong Fe d-character appear in Fe-TPyP near the centre of the gap between the highest occupied and the lowest unoccupied states of a TPyP molecule. We find also that the gap is widened in the presence of Fe. This implies a shift of 0.3 eV of unoccupied states as identified by scanning tunnelling spectroscopy simulations and confirmed by experimental data.

In this contribution we demonstrate structural control over a transport resonance in HS(CH2)n[1,4 −C6H4](CH2)nSH (n = 1, 3, 4, 6) metal-molecule-metal junctions, fabricated and tested using the scanning tunnelingmicroscopy-based I (z)method. The Breit-Wigner resonance originates from one of the arene π-bonding orbitals, which sharpens and moves closer to the contact Fermi energy as n increases. Varying the number of methylene groups thus leads to a very shallow decay of the conductance with the length of the molecule. We demonstrate that the electrical behavior observed here can be straightforwardly rationalized by analyzing the effects caused by the electrostatic balance created at the metal-molecule interface. Such resonances offer future prospects in molecular electronics in terms of controlling charge transport over longer distances, and also in single-molecule conductance switching if the resonances can be externally gated, This research was supported by the EPSRC (Grant No. EP/H035184/1), by MINECO under Grant No. FIS2013-47328, by the European Union structural funds and the Comunidad de Madrid MAD2D-CM Program under Grant. P2013/MIT-2850, and by Generalitat Valenciana under Grant PROMETEO/2012/011.

We describe the synthesis and single-molecule electrical transport properties of a molecular wire containing a π-extended tetrathiafulvalene (exTTF) group and its charge-transfer complex with F4TCNQ. We form single-molecule junctions using the in situ break junction technique using a homebuilt scanning tunneling microscope with a range of conductance between 10 G0 down to 10-7 G0. Within this range we do not observe a clear conductance signature of the neutral parent molecule, suggesting either that its conductance is too low or that it does not form a stable junction. Conversely, we do find a clear conductance signature in the experiments carried out on the charge-transfer complex. Due to the fact we expected this species to have a higher conductance than the neutral molecule, we believe this supports the idea that the conductance of the neutral molecule is very low, below our measurement sensitivity. This idea is further supported by theoretical calculations. To the best of our knowledge, these are the first reported single-molecule conductance measurements on a molecular charge-transfer species, Financial support by the European Commission (EC) FP7 ITN “MOLESCO” Project No. 606728, the European Research Council (ERC-436 2012 ADG_20120216-Chirallcarbon), the CAM (PHOTOCARBON project S2013/MIT-2841, NANOFRONTMAG-CM project S2013/MIT-2850 and MAD2D project S2013/MIT-3007), FP7- ENERGY-2012-1-2STAGE-number 309223 (PHOCS) and the Spanish MICINN/MINECO through the programs MAT2011- 25046, MAT2014-57915-R and PRI-PIBUS-2011-1067 is acknowledged. MB was partly supported by a FY2012 (P12501) Postdoctoral Fellowship for Foreign Researchers from the Japan Society for Promotion of Science (JSPS) and by a JSPS KAKENHI, “Grant-in-Aid for JSPS Fellows”, grant no. 24·02501. YA is also thankful to another KAKENHI, “Grantin-Aid for Scientific Research on Innovation Areas, Molecular Architectonics: Orchestration of Single Molecules for Novel Functions” (#25110009). LAZ was supported by the Spanish MICINN under Grant MAT2011-23627. FP acknowledges support by the Carl-Zeiss foundation and the Collaborative Research Center 767 “Controlled Nanosystems: Interaction and Interfacing to the Macroscale”

In this paper, we have systematically studied how the replacement of a benzene ring by a heterocyclic compound in oligo(phenyleneethynylene) (OPE) derivatives affects the conductance of a molecular wire using the scanning tunneling microscope-based break junction technique. We describe for the first time how OPE derivatives with a central pyrimidine ring can efficiently link to the gold electrode by two pathways presenting two different conductance G values. We have demonstrated that this effect is associated with the presence of two efficient conductive pathways of different length: the conventional end-to-end configuration, and another with one of the electrodes linked directly to the central ring. This represents one of the few examples in which two defined conductive states can be set up in a single molecule without the aid of an external stimulus. Moreover, we have observed that the conductance through the full length of the heterocycle-based OPEs is basically unaffected by the presence of the heterocycle. All these results and the simplicity of the proposed molecules push forward the development of compounds with multiple conductance pathways, which would be a breakthrough in the field of molecular electronics.

Poly(phenyleneethynylene) (PPE) conjugated polymers bearing glucose units are used as enantioselective active layers in vapor-sensing devices. As a novel approach, a conjugated polymer bearing chiral pendant groups is used to detect different enantiomers, natural and synthetic menthol molecules in the present case. The surface analytical characterization of the organic layer reveals a compact and smooth morphology. By means of a quartz crystal microbalance revealing system, the polymer bearing glucose chiral sites is demonstrated to interact more favorably with the natural menthol. Promising perspectives are seen for the use of such polymers in chiral, chemically sensitive resistors or even transistors.

Motivated by recent experiments [Lee et al. Nature 498, 209 (2013)], we present here a detailed theoretical analysis of the Joule heating in current-carrying single-molecule junctions. By combining the Landauer approach for quantum transport with ab initio calculations, we show how the heating in the electrodes of a molecular junction is determined by its electronic structure. In particular, we show that in general the heat is not equally dissipated in both electrodes of the junction and it depends on the bias polarity (or equivalently on the current direction). These heating asymmetries are intimately related to the thermopower of the junction as both these quantities are governed by very similar principles. We illustrate these ideas by analyzing single-molecule junctions based on benzene derivatives with different anchoring groups. The close relation between heat dissipation and thermopower provides general strategies for exploring fundamental phenomena such as the Peltier effect or the impact of quantum interference effects on the Joule heating of molecular transport junctions., 26 pages, 9 figures, submitted to New Journal of Physics

This document is the Accepted Manuscript version of a Published Work that appeared in final form in Journal of the American Chemical Society, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see http://dx.doi.org/10.1021/ja3100116, We present a theoretical study of electron transport through single-molecule junctions incorporating a Pt6 metal cluster bound within an organic framework. The insertion of this molecule between a pair of electrodes leads to a fully atomically engineered nanometallic device with high conductance at the Fermi level and two sequential high on/off switching states. The origin of this property can be traced back to the existence of a degenerate HOMO consisting of two asymmetric orbitals with energies close to the Fermi level of the metal leads. The degeneracy is broken when the molecule is contacted to the leads, giving rise to two resonances that become pinned to the Fermi level and display destructive interference, This research was supported by the Comunidad de Madrid through the project NANOBIOMAGNET S2009/MAT1726, by the Generalitat Valenciana through the project PROMETEO2012/011 and by the Spanish MICINN under the Grant Nos. FIS2010-21883 and CONSOLIDER CSD2007-0010. EL was funded by the EU through the ELFOS Network (FP7-ICT2009-6)

Atomic and single-molecule junctions represent the ultimate limit to the miniaturization of electrical circuits. They are also ideal platforms to test quantum transport theories that are required to describe charge and energy transfer in novel functional nanodevices. Recent work has successfully probed electric and thermoelectric phenomena in atomic-scale junctions. However, heat dissipation and transport in atomic-scale devices remain poorly characterized due to experimental challenges. Here, using custom-fabricated scanning probes with integrated nanoscale thermocouples, we show that heat dissipation in the electrodes of molecular junctions, whose transmission characteristics are strongly dependent on energy, is asymmetric, i.e. unequal and dependent on both the bias polarity and the identity of majority charge carriers (electrons vs. holes). In contrast, atomic junctions whose transmission characteristics show weak energy dependence do not exhibit appreciable asymmetry. Our results unambiguously relate the electronic transmission characteristics of atomic-scale junctions to their heat dissipation properties establishing a framework for understanding heat dissipation in a range of mesoscopic systems where transport is elastic. We anticipate that the techniques established here will enable the study of Peltier effects at the atomic scale, a field that has been barely explored experimentally despite interesting theoretical predictions. Furthermore, the experimental advances described here are also expected to enable the study of heat transport in atomic and molecular junctions, which is an important and challenging scientific and technological goal that has remained elusive., Comment: supporting information available in the journal web site or upon request

Employing ab-initio electronic structure calculations combined with the non-equilibrium Green's function technique, we study the dependence of the thermopower Q on the conformation in biphenyl-based single-molecule junctions. For the series of experimentally available biphenyl molecules, alkyl side chains allow us to gradually adjust the torsion angle \phi\ between the two phenyl rings from 0 to 90{\deg} and to control in this way the degree of \pi-electron conjugation. Studying different anchoring groups and binding positions, our theory predicts that the absolute values of the thermopower decrease slightly towards larger torsion angles, following an a+b*cos^{2}\phi\ dependence. The anchoring group determines the sign of Q and a,b, simultaneously. Sulfur and amine groups give rise to Q,a,b>0, while for cyano Q,a,b, Comment: 8 pages, 4 figues, 3 tables

We present a theoretical study of the conductance and thermopower of single molecule junctions based on C60 and C60-terminated molecules.We first analyze the transport properties of gold-C60-gold junctions and show that these junctions can be highly conductive (with conductances above 0.1G0, where G0 = 2e2/h is the quantum of conductance). Moreover, we find that the thermopower in these junctions is negative due to the fact that the lowest unoccupied molecular orbital dominates the charge transport, and its magnitude can reach several tens of microvolts per kelvin, depending on the contact geometry. On the other hand, we study the suitability of C60 as an anchoring group in single-molecule junctions. For this purpose, we analyze the transport through several dumbbell derivatives using C60 as anchors, and we compare the results with those obtained with thiol and amine groups. Our results show that the conductance of C60-terminated molecules is rather sensitive to the binding geometry. Moreover, the conductance of the molecules is typically reduced by the presence of the C60 anchors, which in turn makes the junctions more sensitive to the functionalization of the molecular core with appropriate side groups, We thank T. Frederiksen, G. Foti, E. Leary, and E. Scheer for fruitful discussions. S.B, L.A.Z., and J.C.C. were funded by the EU through the network BIMORE (Grant No. MRTN-CT-2006-035859) and by the Comunidad de Madrid through the program NANOBIOMAGNET S2009/MAT1726. F.P. acknowledges funding through a Young Investigator Group and the DFG Center for Functional Nanostructures (Project C3.6)

We fabricate and characterize carbon-fiber tips for their use in combined scanning tunneling and force microscopy based on piezoelectric quartz tuning fork force sensors. An electrochemical fabrication procedure to etch the tips is used to yield reproducible sub-100-nm apex. We also study electron transport through single-molecule junctions formed by a single octanethiol molecule bonded by the thiol anchoring group to a gold electrode and linked to a carbon tip by the methyl group. We observe the presence of conductance plateaus during the stretching of the molecular bridge, which is the signature of the formation of a molecular junction., Conference Proceeding (Trends in NanoTechnology 2011, Tenerife SPAIN); Nanoscale Research Letters, (2012) 7:254

We present a theoretical study of the conductance of atomic junctions comprising single noble gas atoms (He, Ne, Ar, Kr, and Xe) coupled to gold electrodes. The aim is to elucidate how the presence of noble gas atoms affects the electronic transport through metallic atomic-size contacts. Our analysis, based on density functional theory and including van der Waals interactions, shows that for the lightest elements (He and Ne) no significant current flows through the noble gas atoms and their effect is to reduce the conductance of the junctions by screening the interaction between the gold electrodes. This explains the observations reported in metallic atomic-size contacts with adsorbed He atoms. Conversely, the heaviest atoms (Kr and Xe) increase the conductance because of the additional current path provided by their valence p states, L.A.Z. and J.C.C. were funded by the EU through BIMORE (MRTN-CT-2006-035859) and by the Comunidad de Madrid through the program NANOBIOMAGNET S2009/MAT1726. M.B acknowledges funding through the Center for Functional Nanostructures and the DFG priority program 1243, and F.P. through the Young Investigator Group

We present a combined experimental and theoretical study of the electronic transport through single-molecule junctions based on nitrile-terminated biphenyl derivatives. Using a scanning tunneling microscope-based break-junction technique, we show that the nitrile-terminated compounds give rise to well-defined peaks in the conductance histograms resulting from the high selectivity of the N-Au binding. Ab initio calculations have revealed that the transport takes place through the tail of the LUMO. Furthermore, we have found both theoretically and experimentally that the conductance of the molecular junctions is roughly proportional to the square of the cosine of the torsion angle between the two benzene rings of the biphenyl core, which demonstrates the robustness of this structure-conductance relationship.

A combined experimental and theoretical study is presented revealing the influence of metal molecule coupling on electronic transport through single-molecule junctions. Transport experiments through tolane molecules attached to gold electrodes via thiol, nitro, and cyano anchoring groups are performed. By fitting the experimental current voltage characteristics to a single-level tunneling model, we extract both the position of the molecular orbital closest to the Fermi energy and the strength of the metal molecule coupling. The values found for these parameters are rationalized with the help of density-functional-theory-based transport calculations. In particular, these calculations show that the anchoring groups determine the junction conductance by controlling not only the strength of the coupling to the metal but also the position of the relevant molecular energy levels.

It has been shown that electron transitions, as measured in a scanning tunnelling microscope, are related to chemical interactions in a tunnelling barrier. Here, we show that the shape and apparent height of subatomic features in both, measurements of the attractive forces in an atomic force microscope, and measurements of the tunneling current between the Si(1 1 1) surface and an oscillating cantilever, depend directly on the available electron states of the silicon surface and the silicon tip. Simulations and experiments confirm that forces and currents show similar subatomic variations for tip-sample distances approaching the bulk bonding length.

The following article appeared in Applied Physics Letters vol 99,12 (2011): 123105 and may be found at http://scitation.aip.org/content/aip/journal/apl/99/12/10.1063/1.3643031?showFTTab=true&containerItemId=content/aip/journal/apl, We study electron transport through single-molecule junctions formed by an octanethiol molecule bonded with the thiol anchoring group to a goldelectrode and the opposing methyl endgroup to a carbon tip. Using the scanning tunneling microscope based break junction technique, we measure the electrical conductance of such molecular junctions. We observe the presence of well-defined conductance plateaus during the stretching of the molecular bridge, which is the signature of the formation of a molecular junction., Acknowledges fellowship support from the Comunidad de Madrid (Spain). This work was supported by MICINN (Spain) through the programs MAT2008-01735 and CONSOLIDER-INGENIO-2010 CSD-2007-00010, by Comunidad de Madrid through the program NANOBIOMAGNET S2009/MAT1726, by the EU through the networks BIMORE (MRTN-CT-2006-035859),and by the EC through the network FP7 ITN “FUNMOLS”Project Number 212942.

The adsorption of semifluorinated alkanethiols on Au/mica was studied by scanning tunneling microscopy (STM). The adlayer structure produced is based on a p(2 x 2) structure though lines of molecules displayed extensive kinks and bends. In addition, a considerable variation in the contrast of molecular features is found. Molecular modeling calculations confirm that, for the fluorinated thiols, inequivalently adsorbed molecules within a p(2 x 2) registry are present, an aspect that endows the local structure of the adlayer with a higher flexibility in comparison to nonfluorinated thiols, where one adsorption site is strongly favored in a (radical 3 x radical 3) R30 degrees structure. Simulated STM imaging on the optimized systems successfully recovered the effects on the molecular feature contrast induced by the flexibility of the fluorinated thiol adlayer.