20 research outputs found
Quantum properties of atomic-sized conductors
Using remarkably simple experimental techniques it is possible to gently
break a metallic contact and thus form conducting nanowires. During the last
stages of the pulling a neck-shaped wire connects the two electrodes, the
diameter of which is reduced to single atom upon further stretching. For some
metals it is even possible to form a chain of individual atoms in this fashion.
Although the atomic structure of contacts can be quite complicated, as soon as
the weakest point is reduced to just a single atom the complexity is removed.
The properties of the contact are then dominantly determined by the nature of
this atom. This has allowed for quantitative comparison of theory and
experiment for many properties, and atomic contacts have proven to form a rich
test-bed for concepts from mesoscopic physics. Properties investigated include
multiple Andreev reflection, shot noise, conductance quantization, conductance
fluctuations, and dynamical Coulomb blockade. In addition, pronounced quantum
effects show up in the mechanical properties of the contacts, as seen in the
force and cohesion energy of the nanowires. We review this reseach, which has
been performed mainly during the past decade, and we discuss the results in the
context of related developments.Comment: Review, 120 pages, 98 figures. In view of the file size figures have
been compressed. A higher-resolution version can be found at:
http://lions1.leidenuniv.nl/wwwhome/ruitenbe/review/QPASC-hr-ps-v2.zip (5.6MB
zip PostScript
Current rectification in a single molecule diode: the role of electrode coupling
We demonstrate large rectification ratios (> 100) in single-molecule
junctions based on a metal-oxide cluster (polyoxometalate), using a scanning
tunneling microscope (STM) both at ambient conditions and at low temperature.
These rectification ratios are the largest ever observed in a single-molecule
junction, and in addition these junctions sustain current densities larger than
10^5 A/cm^2. By following the variation of the I-V characteristics with
tip-molecule separation we demonstrate unambiguously that rectification is due
to asymmetric coupling to the electrodes of a molecule with an asymmetric level
structure. This mechanism can be implemented in other type of molecular
junctions using both organic and inorganic molecules and provides a simple
strategy for the rational design of molecular diodes
Characterization of single-molecule pentanedithiol junctions by inelastic electron tunneling spectroscopy and first-principles calculations
We study pentanedithiol molecular junctions formed by means of the
break-junction technique with a scanning tunneling microscope at low
temperatures. Using inelastic electron tunneling spectroscopy and
first-principles calculations, the response of the junction to elastic
deformation is examined. We show that this procedure makes a detailed
characterization of the molecular junction possible. In particular, our results
indicate that tunneling takes place through just a single molecule.Comment: 5 pages, 4 figures (accepted in Phys. Rev. B
Atomically thin mica flakes and their application as ultrathin insulating substrates for graphene
We show that it is possible to deposit, by mechanical exfoliation on SiO2/Si
wafers, atomically thin mica flakes down to a single monolayer thickness. The
optical contrast of these mica flakes on top of a SiO2/Si substrate, which
depends on their thickness, the illumination wavelength and the SiO2 substrate
thickness, can be quantitatively accounted for by a Fresnel law based model.
The preparation of atomically thin insulating crystalline sheets will enable
the fabrication of ultrathin defect-free insulating substrates, dielectric
barriers or planar electron tunneling junctions. Additionally, we show that
few-layer graphene flakes can be deposited on top of a previously transferred
mica flake. Our transfer method relies on viscoelastic stamps, as those used
for soft lithography. A Raman spectroscopy study shows that such an all-dry
deposition technique yields cleaner and higher quality flakes than conventional
wet-transfer procedures based on lithographic resists.Comment: 11 pages, 5 figures, 1 graphical abstrac
Exploring the Impact of the HOMO–LUMO Gap on Molecular Thermoelectric Properties: A Comparative Study of Conjugated Aromatic, Quinoidal, and Donor–Acceptor Core Systems
Thermoelectric materials have garnered significant interest for their potential to efficiently convert waste heat into electrical energy at room temperature without moving parts or harmful emissions. This study investigated the impact of the HOMO–LUMO (H-L) gap on the thermoelectric properties of three distinct classes of organic compounds: conjugated aromatics (isoindigos (IIGs)), quinoidal molecules (benzodipyrrolidones (BDPs)), and donor–acceptor systems (bisÂ(pyrrol-2-yl)Âsquaraines (BPSs)). These compounds were chosen for their structural simplicity and linear Ď€-conjugated conductance paths, which promote high electrical conductance and minimize complications from quantum interference. Single-molecule thermoelectric measurements revealed that despite their low H-L gaps, the Seebeck coefficients of these compounds remain low. The alignment of the frontier orbitals relative to the Fermi energy was found to play a crucial role in determining the Seebeck coefficients, as exemplified by the BDP compounds. Theoretical calculations support these findings and suggest that anchor group selection could further enhance the thermoelectric behavior of these types of molecules
Long-lived charged states of single porphyrin-tape junctions under ambient conditions
The ability to control the charge state of individual molecules wired in two-terminal single-molecule junctions is a key challenge in molecular electronics, particularly in relation to the development of molecular memory and other computational componentry. Here we demonstrate that single porphyrin molecular junctions can be reversibly charged and discharged at elevated biases under ambient conditions due to the presence of a localised molecular eigenstate close to the Fermi edge of the electrodes. In particular, we can observe long-lived charge-states with lifetimes upwards of 1–10 seconds after returning to low bias and large changes in conductance, in excess of 100-fold at low bias. Our theoretical analysis finds charge-state lifetimes within the same time range as the experiments. The ambient operation demonstrates that special conditions such as low temperatures or ultra-high vacuum are not essential to observe hysteresis and stable charged molecular junctions
Molecular Structure-(Thermo)electric Property Relationships in Single-Molecule Junctions and Comparisons with Single- and Multiple-Parameter Models
The most probable single-molecule conductance of each member of a series of 12 conjugated molecular wires, 6 of which contain either a ruthenium or platinum center centrally placed within the backbone, has been determined. The measurement of a small, positive Seebeck coefficient has established that transmission through these molecules takes place by tunneling through the tail of the HOMO resonance near the middle of the HOMO–LUMO gap in each case. Despite the general similarities in the molecular lengths and frontier-orbital compositions, experimental and computationally determined trends in molecular conductance values across this series cannot be satisfactorily explained in terms of commonly discussed “single-parameter” models of junction conductance. Rather, the trends in molecular conductance are better rationalized from consideration of the complete molecular junction, with conductance values well described by transport calculations carried out at the DFT level of theory, on the basis of the Landauer–Büttiker model
Monarchy and military practice during the reign of Alfonso XI of Castile (1312–1350)
My dissertation argues that Alfonso, while not the radical innovator that Salvador de MoxĂł depicted, was an accomplished commander who cleverly responded to the challenges he faced. He introduced reforms culminating developments begun in the thirteenth century. He modified the caballerĂa popular, specifically making the upkeep of a horse and service mandatory for anyone with a specific amount of wealth. He also increased the central role of the Crown in recruitment both by augmenting royal armies\u27 size and disbursing higher monetary sums to others to recruit increasingly larger contingents. Heavier fiscal demands notwithstanding, the Castilian monarchy achieved a greater degree of control over the recruitment process. Alfonso and his commanders were constantly concerned with logistics. The Castilian frontier defense network continued to function with fortifications drawing on their possessions, but invariably depending on resources provided local towns, lords, or the Crown. Castilian commanders paid ample attention to the acquisition of the supplies necessary for proficient military activities. Once a host was on the field, its needs were met through the aggregate of the goods brought with them, forage, and long-distance supply routes. How Alfonso dealt with the logistical challenges goes a long way towards explaining his military successes. Despite being plagued by fiscal problems, Alfonso adeptly marshalled the necessary financial resources for his campaigns. While traditional revenue sources remained important, Alfonso relied on extraordinary collections like the Cortes-approved servicios [levies] and the alcabala, a kingdom-wide 5% sales tax, to underwrite the increasingly larger outlays required by his campaigns. With his Ordenamientos of 1338 and 1348, he implemented measures that tapped income sources that otherwise would have remained in private hands and out of reach. Finally, warfare was intricately linked to naval activity. Royal navies were instrumental in transporting troops and supplies to land armies, participating in joint military operations, patrolling the Strait of Gibraltar to protect Castile\u27s interests, and engaging enemy vessels to gather intelligence, draw them into battle, or cut them off through blockades. These activities prepared Castile for its development as the naval power it became in the fifteenth and sixteenth centuries
Thermopower measurements in molecular junctions
The measurement of thermopower in molecular junctions offers complementary information to conductance measurements and is becoming essential for the understanding of transport processes at the nanoscale. In this review, we discuss the recent advances in the study of the thermoelectric properties of molecular junctions. After presenting the theoretical background for thermoelectricity at the nanoscale, we review the experimental techniques for measuring the thermopower in these systems and discuss the main results. Finally, we consider the challenges in the application of molecular junctions in viable thermoelectric devices
Engineering the thermopower of C60 molecular junctions
We report the measurement of conductance and thermopower of C60 molecular junctions using a scanning tunneling microscope (STM). In contrast to previous measurements, we use the imaging capability of the STM to determine precisely the number of molecules in the junction and measure thermopower and conductance continuously and simultaneously during formation and breaking of the molecular junction, achieving a complete characterization at the single-molecule level. We find that the thermopower of C60 dimers formed by trapping a C60 on the tip and contacting an isolated C60 almost doubles with respect to that of a single C60 and is among the highest values measured to date for organic materials. Density functional theory calculations show that the thermopower and the figure of merit continue increasing with the number of C60 molecules, demonstrating the enhancement of thermoelectric preformance by manipulation of intermolecular interactions