We have designed and operated a device consisting of three nanoscale tunnel junctions biased below the Coulomb gap. Phase shifted r.f. voltages of frequency f applied to two gates “pump” one electron per cycle through the device. This is shown experimentally by plateaus in the current-voltage characteristic at I = ± ef, the sign of the current depending on the relative phase of the r.f. voltages and not on the sign of the bias voltage.

In this work, we bridge the gap between short-range tunneling in molecular junctions and activated hopping in bulk organic films, and greatly extend the distance range of charge transport in molecular electronic devices. Three distinct transport mechanisms were observed for 4.5-22-nm-thick oligo(thiophene) layers between carbon contacts, with tunneling operative when d < 8 nm, activated hopping when d > 16 nm for high temperatures and low bias, and a third mechanism consistent with field-induced ionization of highest occupied molecular orbitals or interface states to generate charge carriers when d = 8-22 nm. Transport in the 8-22-nm range is weakly temperature dependent, with a field-dependent activation barrier that becomes negligible at moderate bias. We thus report here a unique, activationless transport mechanism, operative over 8-22-nm distances without involving hopping, which severely limits carrier mobility and device lifetime in organic semiconductors. Charge transport in molecular electronic junctions can thus be effective for transport distances significantly greater than the 1-5 nm associated with quantum-mechanical tunneling.all-carbon molecular junction | attenuation coefficient | field ionization | strong electronic coupling C harge transport mechanisms in organic and molecular electronics underlie the ultimate functionality of a new generation of electronic devices. Understanding, controlling, and designing molecular devices for use as practical components requires an intimate knowledge of the system energy levels and operative transport mechanisms, and how key variables such as molecule length, identity, temperature, etc., affect device performance parameters. Especially interesting in this context is the relationship between organic electronic devices, which typically have active layer thicknesses of tens to hundreds of nanometers, and molecular electronic devices reported to date, in which at least one dimension for charge propagation is below 10 nm. Indeed, many types of functional organic electronic devices have been demonstrated, including thinfilm transistors, organic light-emitting diodes, and memory cells (1, 2). Bridging the gap between organic and molecular devices may therefore reveal pathways for improving the performance of such devices, or even lead to new types of devices based on alternative transport mechanisms.The great majority of molecular electronic devices investigated to date have transport distances of <5 nm between the contacts, where the prevalent transport mechanism is quantum-mechanical tunneling. For this distance range, there is general agreement that the conductance scales exponentially with length, with an attenuation coefficient (β), defined as the slope of ln J vs. thickness (d), equal to 8 to 9 nm −1 for aliphatic molecules (3-6) and 2-3 nm −1 for aromatic molecules (7)(8)(9)(10)(11)(12)(13)(14). A few molecular electronic systems have been investigated beyond 5 nm (15, 16), some of which exhibit a decrease in β to less than 1 nm −1 . Such small values of β ar...

We have measured the difference between the free energies of an isolated superconducting electrode with odd and even number of electrons using a Coulomb blockade electrometer. The decrease of this energy difference with increasing temperature is in good agreement with theoretical predictions assuming a BCS density of quasiparticle states, except at the lowest temperatures where the results indicate the presence of an extra energy level inside the gap.PACS numbers: 74.50.+r, 73.40.Rw, 74.25.Bt The key concept of the Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity [1] is the pairing of electrons. A surprising feature of the theory appears when one considers a macroscopic piece of superconducting metal with a fixed number of electrons TV. If TV is even, all the electrons can condense in the ground state. If TV is odd, however, one electron should remain as a quasiparticle excitation. In principle, if one would measure the energy required to add one electron to the superconductor, there should be a difference between the cases of even and odd TV. This fundamental even-odd asymmetry, which might vanish due to sample imperfections [2], does not manifest itself in conventional experiments on superconductors because these experiments are only sensitive to a finite fraction of quasiparticles. In this Letter, we report a new experiment based on singleelectron tunneling [3] with which we measured the evenodd free energy difference introduced by Tuominen et al. [4].Consider a superconducting-normal (SN) tunnel junction in series with a voltage source U and a capacitor C s (see Fig. 1), a basic Coulomb blockade circuit whose normal-normal junction version has been nicknamed the electron "box" [5,6]. The superconducting electrode which is common to both the junction and the capacitor is surrounded everywhere by insulating material. When the junction tunnel resistance R t is such that R t^> R[(=h/e 2 J the number n of excess electrons on this "island" is a good quantum number [3,7L The w-dependent part of the ground-state energy of the circuit, including the work done by the source £/, is given by En^Ecin -C s U/e) 2 + <£", where E c = : e 2 /2Cz is the electrostatic energy of one excess electron on the island, Cz the total capacitance of the island, and &" is the nonelectrostatic part of the energy of the island. For a normal island G n = 0 [ Fig. 2(a)], whereas for a superconducting island, one has S n -DoPn where Do is the energy difference between the odd-n and even-/? island ground states, and p n =nmod2[ Fig. 2(c)]. The BCS theory yields D 0 =A where A is the superconducting gap of the island. In equilibrium at zero temperature, n will be determined by the lowest E n and is therefore given by a staircase function of U [Figs. 2(b) and 2(d)]. In the normal case, the steps are of equal size, whereas in the superconducting case even-AZ steps are longer than odd-Ai steps. For Do> E c , the odd-n steps disappear, while for Do

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Molecular junctions were fabricated with the combined use of electrochemistry and conventional CMOS tools. They consist of a 5-10 nm thick layer of oligo(1-(2-bisthienyl)benzene) between two gold electrodes. The layer was grafted onto the bottom electrode using diazonium electroreduction, which yields a stable and robust gold-oligomer interface. The top contact was obtained by direct electron-beam evaporation on the molecular layers through masks defined by electron-beam lithography. Transport mechanisms across such easily p-dopable layers were investigated by analysis of current density-voltage (J-V) curves. Application of a tunneling model led to a transport parameter (thickness of ~2.4 nm) that was not consistent with the molecular thickness measured using AFM (~7 nm). Furthermore, for these layers with thicknesses of 5-10 nm, asymmetric J-V curves were observed, with current flowing more easily when the grafted electrode was positively polarized. In addition, J-V experiments at two temperatures (4 and 300 K) showed that thermal activation occurs for such polarization but is not observed when the bias is reversed. These results indicate that simple tunneling cannot describe the charge transport in these junctions. Finally, analysis of the experimental results in term of "organic electrode" and redox chemistry in the material is discussed.

Quantum interference in cross-conjugated molecules embedded in solid-state devices was investigated by direct current-voltage and differential conductance transport measurements of anthraquinone (AQ)-based large area planar junctions. A thin film of AQ was grafted covalently on the junction base electrode by diazonium electroreduction, while the counter electrode was directly evaporated on top of the molecular layer. Our technique provides direct evidence of a large quantum interference effect in multiple CMOS compatible planar junctions. The quantum interference is manifested by a pronounced dip in the differential conductance close to zero voltage bias. The experimental signature is well developed at low temperature (4 K), showing a large amplitude dip with a minimum >2 orders of magnitude lower than the conductance at higher bias and is still clearly evident at room temperature. A temperature analysis of the conductance curves revealed that electron-phonon coupling is the principal decoherence mechanism causing large conductance oscillations at low temperature.