16 research outputs found
First-principles calculation of the thermoelectric figure of merit for [2,2]paracyclophane-based single-molecule junctions
Here we present a theoretical study of the thermoelectric transport through
{[}2,2{]}para\-cyclo\-phane-based single-molecule junctions. Combining
electronic and vibrational structures, obtained from density functional theory
(DFT), with nonequilibrium Green's function techniques, allows us to treat both
electronic and phononic transport properties at a first-principles level. For
the electronic part, we include an approximate self-energy correction, based on
the DFT+ approach. This enables us to make a reliable prediction of all
linear response transport coefficients entering the thermoelectric figure of
merit . Paracyclophane derivatives offer a great flexibility in tuning
their chemical properties by attaching different functional groups. We show
that, for the specific molecule, the functional groups mainly influence the
thermopower, allowing to tune its sign and absolute value. We predict that the
functionalization of the bare paracyclophane leads to a largely enhanced
electronic contribution to the figure of merit.
Nevertheless, the high phononic contribution to the thermal conductance
strongly suppresses . Our work demonstrates the importance to include the
phonon thermal conductance for any realistic estimate of the for
off-resonant molecular transport junctions. In addition, it shows the
possibility of a chemical tuning of the thermoelectric properties for a series
of available molecules, leading to equally performing hole- and
electron-conducting junctions based on the same molecular framework.Comment: 8 pages, 7 figure
Heat dissipation and its relation to thermopower in single-molecule junctions
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.Comment: 26 pages, 9 figures, submitted to New Journal of Physic
Tuning the Bandgap Character of Quantum-Confined SiâSn Alloyed Nanocrystals
Nanocrystals in the regime between molecules and bulk give rise to unique electronic properties. Here, a thorough study focusing on quantum-confined nanocrystals (NCs) is provided. At the level of density functional theory an approximate (quasi) band structure which addresses both the molecular and bulk aspects of finite-sized NCs is calculated. In particular, how band-like features emerge with increasing particle diameter is shown. The quasiband structure is used to discuss technological-relevant direct bandgap NCs. It is found that ultrasmall Sn NCs have a direct bandgap in their at-nanoscale-stable α-phase and for high enough Sn concentration (â41%) alloyed SiâSn NCs transition from indirect to direct bandgap semiconductors. The calculations strongly support recent experiments suggesting a direct bandgap for these systems. For a quantitative comparison many-body GW + BetheâSalpeter equation (BSE) calculations are performed. The predicted optical gaps are close to the experimental data and the calculated absorbance spectra compare well with the corresponding measurements
Stability of siliconâtin alloyed nanocrystals with high tin concentration synthesized by femtosecond laser plasma in liquid media
Nanocrystals have a great potential for future materials with tunable bandgap, due to their optical properties that are related with the material used, their sizes and their surface termination. Here, we concentrate on the siliconâtin alloy for photovoltaic applications due to their bandgap, lower than bulk Si, and also the possibility to activate direct band to band transition for high tin concentration. We synthesized siliconâtin alloy nanocrystals (SiSn-NCs) with diameter of about 2â3 nm by confined plasma technique employing a femtosecond laser irradiation on amorphous siliconâtin substrate submerged in liquid media. The tin concentration is estimated to be ⌠17 % , being the highest Sn concentration for SiSn-NCs reported so far. Our SiSn-NCs have a well-defined zinc-blend structure and, contrary to pure tin NCs, also an excellent thermal stability comparable to highly stable silicon NCs. We demonstrate by means of high resolution synchrotron XRD analysis (SPring 8) that the SiSn-NCs remain stable from room temperature up to 400âC, with a relatively small expansion of the crystal lattice. The high thermal stability observed experimentally is rationalized by means of first-principle calculations
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
Length dependence of the thermal conductance of alkane-based single-molecule junctions : an ab-initio study
Motivated by recent experiments, we present here a systematic ab-initio study of the length dependence of the thermal conductance of single-molecule junctions. We make use of a combination of density functional theory with non-equilibrium Green's function techniques to investigate the length dependence of the phonon transport in single alkane chains, contacted with gold electrodes via both thiol and amine anchoring groups. Additionally, we study the effect of the substitution of the hydrogen atoms in the alkane chains by heavier fluorine atoms to form polytetrafluoroethylenes. Our results demonstrate that (i) the room-temperature thermal conductance is fairly length-independent for chains with more than 5 methylene units and (ii) the efficiency of the thermal transport is strongly influenced by the strength of the phononic metal-molecule coupling. Our study sheds new light onto the phonon transport in molecular junctions, and it provides clear guidelines for the design of molecular junctions for thermal management.publishe
Characteristics of amine-ended and thiol-ended alkane single-molecule junctions revealed by inelastic electron tunneling spectroscopy
A combined experimental and theoretical analysis of the charge transport through single-molecule junctions is performed to define the influence of molecular end groups for increasing electrode separation. For both amine-ended and thiol-ended octanes contacted to gold electrodes, we study signatures of chain formation by analyzing kinks in conductance traces, the junction length, and inelastic electron tunneling spectroscopy. The results show that for amine-ended molecular junctions no atomic chains are pulled under stretching, whereas the Au electrodes strongly deform for thiol-ended molecular junctions. This advanced approach hence provides unambiguous evidence that the amine anchors bind only weakly to Au