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.

["Initiative" in primary care: survey on hypertension prevalence and definition of cardiovascular risk profile in ASL 3 of Lagonegro (PZ)]. / "L'iniziativa" per la medicina generale: survey sulla prevalenza dei casi d'ipertensione arteriosa e definizione del profilo di rischio cardiovascolare nell'ASL 3 di Lagonegro (PZ).

The objective of our study was to estimate the prevalence of not-diagnosed hypertension and to determine 10-year risk of coronary heart disease in a random sample of individuals in ASL 3 of Lagonegro (PZ). The sample was based on the whole population assisted by 21 General Practitioners. All General Practitioners filled a form for each patient containing social-demographic, anthropometric and clinical data. The study included only patients aged 35 to 74 years, without a previous diagnosis of myocardial infarction, congestive heart failure and that were not taking antihypertensive drugs (N=335). The directly standardized prevalence of prehypertension was 0.54 (IC95%: 0.47-0.59) and that of hypertension was 0.33 (IC95%: 0.28-0.38). The prevalence of both, prehypertension and hypertension was higher in males than in females, as well as in old compared to young adults. Cardiovascular risk factors as Body Mass Index (BMI), diabetes, a family history of dyslipidemia and diabetes were more frequent among prehypertensive and hypertensive individuals, than in normotensive ones. We also estimated the risk of an acute coronary event in patients whit blood pressure higher than 140/90 mmHg. The calculation, carried out through Framingham algorithm, showed that 1 out of 10 hypertensive subject had a 10-years risk of developing coronary heart disease and that males had a higher risk compared to females. Our study highlights the fundamental role of General Practitioners may play in earlier diagnosis prehypertension and hypertension in the general population, thus adopting practices oriented to healthy promotion and prevention.

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.

A C13-norisoprenoid gentiobioside from quince fruit.

The beta-D-gentiobioside [beta-D-glucopyranosyl(1----6)-beta-D-glucopyranoside] of 3-hydroxy-beta-ionol has been isolated and characterized in quince (Cydonia oblonga) fruit through spectral and chemical studies. Model experiments carried out with this new natural compound revealed its important role as precursor of a number of C13-norisoprenoid flavour compounds of quince essential oil.