ABSTRACT
The understanding of electron transfer across interfaces and nanostructures constitutes a major challenge in emerging nanodevices. Ultrafast injection of carriers through the interface at femtosecond scale has been reported in dye-TiO2 systems and in piezotronics devices based on ZnO nanotubes, followed by anomalous transport of the charges through the nanoporous network. These features which could not be expected on the basis of bulk models, motivated real-time theoretical studies of the injection dynamics and transport at the nanoscale to improving the overall efficiency. In this paper correlation functions are evaluated by the Fourier transform of the frequency-dependent conductivity of the system. Drude-Lorentz and Schmith models, fitted to experimental data, have been analyzed. We present results for TiO2 and ZnO oxides nanostructured films. It is found that the diffusion coefficient of carriers is usually very small, but can reach values comparable to the single crystal at early times after carriers are released, even in the presence of structural disorder, under conditions concerning the size of the nanoparticles, the strength of the coupling of the charges with the nanoparticles and the relaxation time. Our result for the current-current correlation function as a function of time is in agreement with the obtained result by ultrafast time THz spectroscopy.
ABSTRACT
Recently, an approach for converting nanoscale mechanical energy into electrical energy has been suggested by using piezoelectric zinc oxide (ZnO) nanowire arrays. Such devices have been shown to convert ultrasonic energy into electric energy by a deflection of the nanowires via a corrugated electrode operated up and down by the ultrasound. A typical approximately 1 pW output power for a device of a approximately 1 mm2 area and a density of approximately 10(7)/mm2 nanowires can be obtained. In order to reach the approximately 10 nW power needed to operate a nanodevice, nanogenerators of this kind need to be optimized. With the aim of fabricating low cost to efficiency ratio nanogenerators, we have considered ZnO films grown by an electrochemical technique, based on the direct precipitation of Zn hydroxide on a conducting ITO/glass substrate and subsequent heat treatment, and TiO2 films deposited from a colloidal suspension of anatase/rutile commercial powders. These methods allowed us to obtain disordered but quite uniform arrays distributed on the surface of the substrate. Preliminary results on the electrical properties are presented. Under input mechanical strain we find output powers of approximately 10(-9)/cm2 W, which are comparable to those obtained with the ZnO nanoarrays. Possible interpretations of results in terms of piezoelectricity (ZnO) and incipient ferroelectricity (TiO2) are presented and improvements of the devices are discussed.
ABSTRACT
Recently, attention has been paid to surface plasmon waves occurring at an interface as responsible for selective absorption of light and enhancement of the photoelectron current. Interest for plasmon resonance phenomena in photovoltaics is groving, for applications in very thin film-based devices. We discuss in this paper d-c open circuit voltages of nominally undoped mesoporous films of TiO2 of the type used in dye-sensitized cells, under exposure to visible and ambient electromagnetic radiation. In non-sensitized films we interpret these as arising from absorption of electromagnetic energy at plasmon resonances determined by the morphology of the films and subsequent electronic diffusion through the nanocrystals of TiO2. Typical order of magnitude of voltages V(oc) approximately equal to 1 mV indicates an equivalent temperature T approximately equal to 3 K of the radiation involved, which suggests ambient microwave background as responsabile of the effect. The order of magnitude of short-circuit current I approximately equal to 10(-9) A is consistent with experimental data of the diffusion coefficient of the oxide. We find from these data the efficiency of the process is of typical order 1% for nanocrystals of 10 nm typical size. Confirmation of these results are obtained by a Montecarlo simulation of transport taking into account the overall mechanisms of scattering, including phonons, imperfections and doping centres, and traps. We find that TiO2 is a n-type self doped material, with typical doping concentrations N (I) approximately equal to 10(18) cm(-3). The same route is applied to dye-sensitized cells under exposure to solar light for which we find a maximum efficiency of 10% in agreement with certified results on Graetzel's cells, independent of the nanocrystal size. We conclude that the enhancement of the efficiency of these electrochemical cells may be obtained by optimizing the charge current within the mesoporous films, which can be obtained by maximizing the diffusion coefficient.
