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ACS Appl Mater Interfaces ; 9(19): 16627-16634, 2017 May 17.
Artigo em Inglês | MEDLINE | ID: mdl-28425281


Despite the astonishing values of the power conversion efficiency reached, in just less than a decade, by the carbon nanotube/silicon (CNT/Si) solar cells, many doubts remain on the underlying transport mechanisms across the CNT/Si heterojunction. Here, by combining transient optical spectroscopy in the femtosecond timescale, X-ray photoemission, and a systematic tracking of I-V curves across all phases of the interlayer SiOx growth at the interface, we grasp the mechanism that adequately preserves charge separation at the junction, hindering the photoexcited carrier recombination. Moreover, supported by ab initio calculations aimed to model the complex CNT-Si heterointerface, we show that oxygen-related states at the interface act as entrapping centers for the photoexcited electrons, thus preventing recombination with holes that can flow from Si to CNT across the SiOx layer.

Nanotechnology ; 27(14): 145605, 2016 Apr 08.
Artigo em Inglês | MEDLINE | ID: mdl-26916977


A novel carbon-based nanostructured material, which includes carbon nanotubes (CNTs), porous carbon, nanostructured ZnO and Fe nanoparticles, has been synthetized using catalytic chemical vapour deposition (CVD) of acetylene on vertically aligned ZnO nanorods (NRs). The deposition of Fe before the CVD process induces the presence of dense CNTs in addition to the variety of nanostructures already observed on the process done on the bare NRs, which range from amorphous graphitic carbon up to nanostructured dendritic carbon films, where the NRs are partially or completely etched. The combination of scanning electron microscopy and in situ photoemission spectroscopy indicate that Fe enhances the ZnO etching, and that the CNT synthesis is favoured by the reduced Fe mobility due to the strong interaction between Fe and the NRs, and to the presence of many defects, formed during the CVD process. Our results demonstrate that the resulting new hybrid shows a higher sensitivity to ammonia gas at ambient conditions (∼60 ppb) than the carbon nanostructures obtained without the aid of Fe, the bare ZnO NRs, or other one-dimensional carbon nanostructures, making this system of potential interest for environmental ammonia monitoring. Finally, in view of the possible application in nanoscale optoelectronics, the photoexcited carrier behaviour in these hybrid systems has been characterized by time-resolved reflectivity measurements.

J Phys Chem Lett ; 6(18): 3632-8, 2015 Sep 17.
Artigo em Inglês | MEDLINE | ID: mdl-26722734


Photochemically activated reactions, despite being a powerful tool to covalently stabilize self-organized molecular structures on metallic surfaces, have struggled to take off due to several not yet well understood light-driven processes that can affect the final result. A thorough understanding of the photoinduced charge transfer mechanisms at the organic/metal interface would pave the way to controlling these processes and to developing on-surface photochemistry. Here, by time-resolved two-photon photoemission measurements, we track the relaxation processes of the first two excited molecular states at the interface between porphyrin, the essential chromophore in chlorophyll, and two different orientations of the silver surface. Due to the energy alignment of the porphyrin first excited state with the unoccupied sp-bands, an indirect charge transfer path, from the substrate to the molecule, opens in porphyrin/Ag(100) 250 fs after the laser pump excitation. The same time-resolved measurements carried out on porphyrin/Ag(111) show that in the latter case such an indirect path is not viable.