Thermoelectric efficiency of molecular junctions.

Focus of the review is on experimental set-ups and theoretical proposals aimed to enhance thermoelectric performances of molecular junctions. In addition to charge conductance, the thermoelectric parameter commonly measured in these systems is the thermopower, which is typically rather low. We review recent experimental outcomes relative to several junction configurations used to optimize the thermopower. On the other hand, theoretical calculations provide estimations of all the thermoelectric parameters in the linear and non-linear regime, in particular of the thermoelectric figure of merit and efficiency, completing our knowledge of molecular thermoelectricity. For this reason, the review will mainly focus on theoretical studies analyzing the role of not only electronic, but also of the vibrational degrees of freedom. Theoretical results about thermoelectric phenomena in the coherent regime are reviewed focusing on interference effects which play a significant role in enhancing the figure of merit. Moreover, we review theoretical studies including the effects of molecular many-body interactions, such as electron-vibration couplings, which typically tend to reduce the efficiency. Since a fine tuning of many parameters and coupling strengths is required to optimize the thermoelectric conversion in molecular junctions, new theoretically proposed set-ups are discussed in the conclusions.

Electronic properties and Schottky barriers at ZnO-metal interfaces from first principles.

First principles calculations were performed to study the interface electronic structure and the Schottky barrier heights (SBHs) of ZnO-metal interfaces. Different kinds of metals were considered with different chemistries on the polar (0 0 0 1) and (0 0 0 1¯) ZnO surfaces. The projection of the density of states on the atomic orbitals of the interface atoms reveals that two kinds of interface electronic states appear: states due to the chemical bonding which appear at well defined energies and conventional metal-induced gap states associated with a smooth density of states in the bulk ZnO band gap region. The relative weight and distribution of the two classes of states depend on both the ZnO substrate termination and on the metal species. SBHs are found to be very sensitive to the specific interface chemical bonding. In particular, it is possible to note the occurrence of either Schottky barriers or Ohmic contacts. Our results have been compared with experiments and with available phenomenological theories, which estimate the SBH from few characteristic material parameters. Finally, the electronic and structural contributions to the SBH have been singled out and related to the different charge transfers occurring at the different interfaces.

First-principles study of silicon nanocrystals: structural and electronic properties, absorption, emission, and doping.

Total energy calculations within the Density Functional Theory have been carried out in order to investigate the structural, electronic, and optical properties of un-doped and doped silicon nanostructures of different size and different surface terminations. In particular the effects induced by the creation of an electron-hole pair on the properties of hydrogenated silicon nanoclusters as a function of dimension are discussed in detail showing the strong interplay between the structural and optical properties of the system. The distortion induced on the structure by an electronic excitation of the cluster is analyzed and considered in the evaluation of the Stokes shift between absorption and emission energies. Besides we show how many-body effects crucially modify the absorption and emission spectra of the silicon nanocrystals. Starting from the hydrogenated clusters, different Si/O bonding at the cluster surface have been considered. We found that the presence of a Si--O--Si bridge bond originates significative excitonic luminescence features in the near-visible range. Concerning the doping, we consider B and P single- and co-doped Si nanoclusters. The neutral impurities formation energies are calculated and their dependence on the impurity position within the nanocrystal is discussed. In the case of co-doping the formation energy is strongly reduced, favoring this process with respect to the single doping. Moreover the band gap and the optical threshold are clearly red-shifted with respect to that of the pure crystals showing the possibility of an impurity based engineering of the absorption and luminescence properties of Si nanocrystals.

Direct role of surface oxygen vacancies in visible light emission of tin dioxide nanowires.

Tin dioxide (SnO(2)) nanowires exhibit a strong visible photoluminescence that is not observed in bulk crystalline SnO(2). To explain such effect, oxygen vacancies are often invoked without clarifying if they represent the direct origin of luminescence or if their presence triggers other radiative processes. Here we report an investigation of the nature of the visible light emission in SnO(2) nanowires, showing that both experimental and theoretical ab initio analyses support the first hypothesis. On the basis of photoluminescence quenching analysis and of first-principles calculations we show that surface bridging oxygen vacancies in SnO(2) lead to formation of occupied and empty surface bands whose transition energies are in strong agreement with luminescence features and whose luminescence activity can be switched off by surface adsorption of oxidizing molecules. Finally, we discuss how such findings may explain the decoupling between "electrical-active" and "optical-active" states in SnO(2) gas nanosensors [G. Faglia et al., Appl. Phys. Lett. 86, 011923 (2005)].

Ab initio calculations of electron affinity and ionization potential of carbon nanotubes.

By combining ab initio all-electron localized orbital and pseudopotential plane-wave approaches we report on calculations of the electron affinity (EA) and the ionization potential (IP) of (5, 5) and (7, 0) single-wall carbon nanotubes. The role played by finite-size effects and nanotube termination has been analysed by comparing several hydrogen-passivated and not passivated nanotube segments. The dependence of the EA and IP on both the quantum confinement effect, due to the nanotube finite length, and the charge accumulation on the edges, is studied in detail. Also, the EA and IP are compared to the energies of the lowest unoccupied and highest occupied states, respectively, upon increasing the nanotube length. We report a slow convergence with respect to the number of atoms. The effect of nanotube packing in arrays on the electronic properties is eventually elucidated as a function of the intertube distance.

The rutile TiO2 (110) surface: obtaining converged structural properties from first-principles calculations.

We investigate the effects of constraining the motion of atoms in finite slabs used to simulate the rutile TiO2 (110) surface in first-principles calculations. We show that an appropriate choice of fixing atoms in a slab eliminates spurious effects due to the finite size of the slabs, leading to a considerable improvement in the simulation of the (110) surface. The method thus allows for a systematic improvement in convergence in calculating both geometrical and electronic properties. The advantages of this approach are illustrated by presenting the first theoretical results on the displacement of the surface atoms in agreement with experiment.

Skin blood flow during vasoconstrictive and vasodilative stimuli in essential hypertension patients: a laser Doppler flowmetry study.

In order to compare the extent of the elicited vasoconstrictive and vasodilative response at the microcirculatory level in essential hypertension (EH), we measured the skin blood flow by means of a laser Doppler flowmeter (LDF). Thirty-four mild-to-moderate EH patients were enrolled. Twenty-two sex- and age-matched healthy subjects were selected as a control group. The LDF measurements were carried out with the probe over the fingertip of the distal phalanx at baseline conditions (Rest flow, RF), after an ischaemic test (post-ischaemic peak flow) and during an arithmetic stress test (AT). The flow was expressed in arbitrary units. The data were processed using the Perisoft computer program. The relative flows after the ischaemic test (Rel F1) and during the AT (Rel F2) were expressed as a percentage of the previous RF values (RF1 and RF2, respectively). During the AT, the lag time was calculated (in seconds). As compared to the control subjects, RF was significantly lower in the EH group (p < 0.01). During the AT, the EH patients showed a statistically lower mean Rel F2 decrease compared to the control subjects (p < 0.01). No statistically significant difference occurred in the Rel F1 and lag time. These data suggest that the vasoconstrictive capacity of the precapillary vessels is impaired in patients with hypertension.