In this work we analyze the coupled piezoelectric and semiconductive behavior of vertically aligned ZnO nanowires under uniform compression. The screening effect on the piezoelectric field caused by the free carriers in vertically compressed zinc oxide nanowires (NWs) has been computed by means of both analytical considerations and finite element calculations. We predict that, for typical geometries and donor concentrations, the length of the NW does not significantly influence the maximum output piezopotential because the potential mainly drops across the tip, so that relatively short NWs can be sufficient for high-efficiency nanogenerators, which is an important result for wet-chemistry fabrication of low-cost, CMOS- or MEMS-compatible nanogenerators. Furthermore, simulations reveal that the dielectric surrounding the NW influences the output piezopotential, especially for low donor concentrations. Other parameters such as the applied force, the sectional area and the donor concentration have been varied in order to understand their effects on the output voltage of the nanogenerator.

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The equilibrium piezoelectric potential distribution in a deformed ZnO semiconductive nanowire has been systematically investigated in order to reveal its dependence on the donor concentration, applied force, and geometric parameters. In particular, the donor concentration markedly affects the magnitude and distribution of the electric potential. At a donor concentration of N D > 10 18 cm -3 , the piezopotential is almost entirely screened. Among the other parameters, a variation in the length of the nanowire does not signifi cantly affect the potential distribution. KEYWORDSNanogenerator, ZnO, nanowire Energy harvesting is gaining more and more importance in the development of self-powered devices for different applications, such as implanted biomedical devices, environmental monitoring, wireless sensor networks, and personal electronics. Piezoelectricity is one of the most convenient methods for harvesting energy from the environment. Mechanical energy can be harvested from a variety of sources such as body movement and muscle stretching, acoustic waves, and blood fl ow, and can be converted into electricity by the piezoelectric effect.Piezoelectric nanogenerators using nanowires (NWs) have been recently reported for converting mechanical energy into electricity on the nanoscale [1,2]. NWs made of wurtzite structured materials, in particular of ZnO, may take advantage of both the semiconducting and piezoelectric properties of these materials to scavenge energy from the environment,. The principles of a nanogenerator have already been established [3 6]. A piezoelectric potential is created in the NW as a result of elastic deformations produced by an external force; this potential can drive the fl ow of charge carriers through an external load. A Schottky barrier formed at the junction of the NW with the electrode controls the flow direction of the charge carriers [3]. This principle has been used in different systems, such as a single nanowire bent by the tip of an atomic force microscope (AFM) [3] and an array of aligned NWs with a zigzag top electrode and with ultrasonic excitation [4]. Furthermore, based on the coupled piezoelectric and semiconducting properties of NWs, a new fi eld of nanopiezotronics has been introduced [7], which utilizes the piezoelectric potential to modulate the Nano Research 625 Nano Res (2009) 2: 624 629 carrier transport process in the NW itself for building various electronic devices and components, such as transistors and diode.The theoretical basis of the nanogenerator and nanopiezotronics arises from the voltage drop created across the cross section of the NW when it is laterally deflected. The surface of the compressed side of the NW will exhibit a negative potential, while the surface of the stretched side will present a positive potential. The piezoelectric potential is created by the polarization of anions and cations inside the NW and will remain as long as the NW is maintained in a deformed configuration, because the polarization charges cannot freely move. This co...

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Background/purpose: The relationship between diseases and alterations of the airborne chemicals emitted from the body has been found in many different pathologies and in particular for various forms of cancer. Metabolism of cancer cells is greatly altered during their lifetime; then, modification of chemicals is supposed to be large around cancer tissues. Positive hints in this direction were provided, as an example, on studying the breath composition of lung cancer-affected subjects. Besides the conventional analytical approaches, in recent years sensor arrays were also applied to these researches considering the chemical composition changes as those occurring in other applications such as for instance, those dealing with food quality measurements. Methods: In this paper, the first application of sensor arrays to study the differentiation between melanomas and nevi, namely malignant and benign affection of melanocytary cells, respectively, is presented and discussed. The localization of lesions on the skin surface made possible the utilization of differential measurements aimed at capturing the differences between two adjacent skin regions. This approach strongly reduces the influence of skin headspace variability due to the peculiar subjective odour background and the skin odour variability. The measurement campaign involved 40 cases; 10 of these were diagnosed melanomas referred to surgical intervention. Nine of these diagnoses were further confirmed by histological examinations of the removed tissue and one was a false positive. Results: The differences in the chemical composition of headspace were verified with a gas-chromatographic investigation, and the classification of electronic nose data provided an estimated cross-validated accuracy of the same order of magnitude as the currently used diagnostic instruments. Conclusion: Electronic nose sensors have been shown to have good sensitivity towards volatile organic compounds emitted by skin lesions, and the method seems to be effective for malign lesions identification. The results presented in this paper encourage a second experimental campaign with a larger number of participants and a systematic use of gas chromatography mass spectrometer technology in order to identify some possible melanoma biomarkers

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