34 research outputs found
Resonant transport and electrostatic effects in single-molecule electrical junctions
In this contribution we demonstrate structural control over a transport resonance in HS(CH2)n[1,4 âC6H4](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 Ï-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 gatedThis 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
A group-theoretic approach to the origin of chirality-induced spin selectivity in non-magnetic molecular junctions
Spin-orbit coupling gives rise to a range of spin-charge interconversion
phenomena in non-magnetic systems where 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
contend that the necessary conditions for the CISS effect to appear can be
generally and fully understood on the basis of a complete symmetry analysis of
the molecular junction, and not only of the molecule. 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, therefore,
a possible appearance of CISS. It turns out that standalone metallic
nanocontacts can exhibit spin-polarization when relative rotations are
introduced which reduce the symmetry. As a corollary, molecular junctions with
molecules can also exhibit spin polarization along the
direction of transport, provided that the whole junction is chiral. This
formalism also allows to predict the qualitative changes on 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.Comment: 19 pages, 4 figures, 1 tabl
Electron scattering in atomic force microscopy experiments
It has been shown that electron transitions, as measured in a scanning
tunnelling microscope (STM), are related to chemical interactions in a
tunnelling barrier. Here, we show that the shape and apparent height of
subatomic features in an atomic force microscopy (AFM) experiment on Si(111)
depend directly on the available electron states of the silicon surface and the
silicon AFM tip. Simulations and experiments confirm that forces and currents
show similar subatomic variations for tip-sample distances approaching the bulk
bonding length.Comment: 5 pages and 4 figure
Carbon-fiber tips for scanning probe microscopes and molecular electronics experiments
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.Comment: Conference Proceeding (Trends in NanoTechnology 2011, Tenerife
SPAIN); Nanoscale Research Letters, (2012) 7:25
Single-molecule conductance of a chemically modified, {\pi}-extended tetrathiafulvalene and its charge-transfer complex with F4TCNQ
We describe the synthesis and single molecule electrical transport properties
of a molecular wire containing a -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 home-built
scanning tunneling microscope with a range of conductance between 10 G
down to 10 G. 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 stable junctions.
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 is further supported by our theoretical
calculations. To the best of our knowledge, these are the first reported single
molecule conductance measurements on a molecular charge-transfer species
Chiral Single-Molecule Potentiometers Based on Stapled ortho- Oligo(phenylene)ethynylenes
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
A Molecular Platinum Cluster Junction: A Single-Molecule Switch
We present a theoretical study of the electronic transport through
single-molecule junctions incorporating a Pt6 metal cluster bound within an
organic framework. We show that the insertion of this molecule between a pair
of electrodes leads to a fully atomically engineered nano-metallic 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
HOMO which consists of two degenerate and asymmetric orbitals, lying close in
energy to the Fermi level of the metallic leads. Their degeneracy is broken
when the molecule is contacted to the leads, giving rise to two resonances
which become pinned close to the Fermi level and display destructive
interference.Comment: 4 pages, 4 figures. Reprinted (adapted) with permission from J. Am.
Chem. Soc., 2013, 135 (6), 2052. Copyright 2013 American Chemical Societ
Exploring Cosmic Origins with CORE: Cosmological Parameters
We forecast the main cosmological parameter constraints achievable with theCORE space mission which is dedicated to mapping the polarisation of the CosmicMicrowave Background (CMB). CORE was recently submitted in response to ESA'sfifth call for medium-sized mission proposals (M5). Here we report the resultsfrom our pre-submission study of the impact of various instrumental options, inparticular the telescope size and sensitivity level, and review the great,transformative potential of the mission as proposed. Specifically, we assessthe impact on a broad range of fundamental parameters of our Universe as afunction of the expected CMB characteristics, with other papers in the seriesfocusing on controlling astrophysical and instrumental residual systematics. Inthis paper, we assume that only a few central CORE frequency channels areusable for our purpose, all others being devoted to the cleaning ofastrophysical contaminants. On the theoretical side, we assume LCDM as ourgeneral framework and quantify the improvement provided by CORE over thecurrent constraints from the Planck 2015 release. We also study the jointsensitivity of CORE and of future Baryon Acoustic Oscillation and Large ScaleStructure experiments like DESI and Euclid. Specific constraints on the physicsof inflation are presented in another paper of the series. In addition to thesix parameters of the base LCDM, which describe the matter content of aspatially flat universe with adiabatic and scalar primordial fluctuations frominflation, we derive the precision achievable on parameters like thosedescribing curvature, neutrino physics, extra light relics, primordial heliumabundance, dark matter annihilation, recombination physics, variation offundamental constants, dark energy, modified gravity, reionization and cosmicbirefringence. (ABRIDGED