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We report on the KrF-laser ablation synthesis, purification and photocurrent generation properties of single-wall carbon nanotubes (SWCNTs). The thermally purified SWCNTs are integrated into hybrid photovoltaic (PV) devices by spin-coating them onto n-Si substrates. These novel SWCNTs/n-Si hybrid devices are shown to generate significant photocurrent (PC) over the entire 250-1050 nm light spectrum with external quantum efficiencies (EQE) reaching up to ~23%. Our SWCNTs/n-Si hybrid devices are not only photoactive in the traditional spectral range of Si solar cells, but generate also significant PC in the UV domain (below 400 nm). This wider spectral response is believed to be the result of PC generation from both the SWCNTs themselves and the tremendous number of local p-n junctions created at the nanotubes/Si interface. To assess the prevalence of these two contributions, the EQE spectra and J-V characteristics of these hybrid devices were investigated in both planar and top-down configurations, as a function of SWCNTs' film thickness. A sizable increase in EQE in the near UV with respect to the silicon is observed in both configurations, with a more pronounced UV photoresponse in the planar mode, confirming thereby the role of SWCNTs in the photogeneration process. The PC generation is found to reach its maximum for an optimal the SWCNT film thickness, which is shown to correspond to the best trade-off between lowest electrical resistance and highest optical transparency. Finally, by analyzing the J-V characteristics of our SWCNTs/n-Si devices with an equivalent circuit model, we were able to point out the contribution of the various electrical components involved in the photogeneration process. The SWCNTs-based devices demonstrated here open up the prospect for their use in highly effective photovoltaics and/or UV-light sensors.

Densely packed epitaxial Mn-doped Si(0.3)Ge(0.7) nanodots self-assembled on Si(100) have been obtained. Their structural properties were studied using reflection high-energy electron diffraction, energy dispersive x-ray diffraction, atomic force microscopy, extended x-ray absorption fine structure measurements and high-resolution transmission electron microscopy. Mn(5)Ge(1)Si(2) crystallites embedded in Si(0.3)Ge(0.7) were found. They exhibit a ferromagnetic behaviour with a Curie temperature of about 225 K.

In this work we investigated the structural properties and photoluminescence (PL) of silicon carbide films grown in an ultra-high-vacuum (UHV) set-up by acetylene C2H2 carbonization of Si(0 0 1) surfaces. We used several UHV electron techniques to probe the local structural properties of the film and scanning tunneling microscopy (STM) to study its morphology. We followed two different growth procedures to obtain cubic 3C–SiC films: in the first case the substrate temperature is maintained at 650 °C during acetylene exposure while in the latter case room temperature exposure is followed by annealing performed at 1000 °C. Such a high temperature heating has been necessary to avoid the formation of both carbidic and graphitic environments that an annealing at lower temperatures (e.g. at 650 °C) has been evidenced. STM images showed the formation of cubic, ordered rather flat at atomic scale, nanostructures (of about 20 nm) arranged around deep, volcanoes rectangular-shaped holes. The PL spectra, recorded at 10 K, showed the presence of features typical of a Si1−xCx alloy containing around 1% or less of diluted carbon. This observation indicates that also in the case of a complete formation of cubic crystalline SiC it is impossible to avoid the coexistence with a substoichiometric alloy with carbon content of a few percent. [Copyright &y& Elsevier]

We report on the significant generation of photocurrent (PC) from planar devices built from the drop casting of UV-laser-synthesized single-wall-carbon-nanotubes (SWCNTs) onto n-Si substrate. These SWCNTs/n-Si hybrid devices are shown to generate PC with external quantum efficiencies (EQE) reaching up to ∼10%. Their EQE has been optimized by controlling the amount of deposited SWCNTs, and is shown to be significantly enhanced over all the spectral range with a pronounced boost (up to ∼25× times) around 460 nm. The extension of the photoresponse of these devices toward UV correlates well with the absorbance of SWCNTs. [ABSTRACT FROM AUTHOR]

The spontaneous formation of nanometric and highly dense (∼3×1012 cm-2) Ge droplets on thin SiO2 film on Si(001) is investigated by scanning tunneling microscopy and spectroscopy. Ge dots have been grown by depositing Ge on the clean SiO2 surface at room temperature and then annealing the sample at 500 °C. Ge dots appear to be free of germanium oxides and characterized by a flat surface with the onset of {113} faceting. I-V curves show that they have an energy gap of approximately 1.8 eV, well above that of bulk Ge. Fabrication of nanometer-sized, highly dense pure Ge droplets is very promising for nanoelectronics applications. [ABSTRACT FROM AUTHOR]

Transmission electron microscopy (TEM), electron energy loss near edge structures (EELNES) and scanning tunneling microscopy (STM) were used to distinguish silicon nanotubes (SiNT) among the reaction products of a gas phase condensation synthesis. TEM images exhibit the tubular nature with a well-defined wall. The EELNES spectra performed on each single nanotube show that they are constituted by nonoxidized silicon atoms. STM images show that they have diameter ranging from 2 to 35 nm, have an atomic arrangement compatible with a puckered structure and different chiralities. Moreover, the I-V curves showed that SiNT can be semiconducting as well as metallic in character. [ABSTRACT FROM AUTHOR]

Examines the interaction of acetylene with a clean silicon surface. Use of the ultrahigh-vacuum electronic and luminescence techniques in the study; Importance of luminescence in the evaluation of the carbon content of acetylene. Indication of the formation of a high carbon-rich layer.

Abstract: Transmission electron microscopy (TEM) and electron energy loss near-edge structure (EELNES) revealed the presence of mostly non-oxidised silicon tubular structures among the reaction products synthesized by gas phase condensation technique. Scanning tunneling microscopy (STM) showed a hexagonal atomic arrangement for straight ones. The presence of Y-, T-branched and coiled tubular structure, like in carbon nanotubes, suggests a partially sp2 hybridization. Reflection energy loss measurements confirmed the presence of thin tubular structures and gave hint of sp3 bonds. [Copyright &y& Elsevier]

Abstract: The realization of a Silicon Carbon Nanotube heterojuntion opens the door to a new generation of photodetectors (Si-CNT detector) based on the coupling between this two materials. In particular the growth of Multiwall Carbon Nanotubes on the surface of a n-doped silicon substrate results on a Schottky diode junction with precise rectifying characteristics. The obtained device presents a low dark current, high efficiency in the photoresponsivity, high linearity and a wide stability range. The junction barrier is about 3.5V in reverse polarity with a breakdown limit at more than 100V. The spectral behavior reflects the silicon spectral range with a maximum at about 880nm. [Copyright &y& Elsevier]

Abstract: A new class of radiation detectors based on carbon nanostructures as the active photosensitive element has been recently developed. In this scenario the optimization of the device, both in dark and on light irradiation, is a crucial point. Here, we report on electrical measurements performed in dark conditions on carbon nanofibers and nanotubes deposited on silicon substrates. Our experimental results were interpreted in terms of a multistep tunneling process occurring at the carbon nanostructures/silicon interface. [ABSTRACT FROM AUTHOR]

The morphology of the first stages of acetylene reaction with Si(1 1 1)7 × 7 reconstructed surface has been investigated by scanning tunneling microscopy (STM). We studied the surface modification for two substrate temperatures during reaction (650 and 700 °C) and as a function of acetylene doses. Both these parameters have been observed to play a crucial role in the carbonization reaction, the formation of nanostructures and their development. For low acetylene exposure, almost the entire surface appears √ of 3 × √ of 3-R30° reconstructed but islands, mainly decorating the step edges, are generally surrounded by 7 × 7 reconstructed areas, located deeper with respect to the plane of √ of 3 × √ of 3-R30° reconstruction. For highest acetylene dose, the surface is characterized by nanostructures and large holes (as deep as 3–5 nm) that occupy one or more terraces between the original silicon surface step edges with their inner side constituted of islands itself. For intermediate acetylene exposure, the small increase (from 650 to 700 °C) of substrate temperature during the reaction is sufficient to make the resulting carbonization mechanism switch towards the highest exposure behaviour. [Copyright &y& Elsevier]

The effect of Sb in the growth of Ge/Si systems was studied by the total reflection extended X-ray absorption fine structure technique at the Ge K-edge. (GenSi10−n)15 multilayers (n=2 and 4) were grown on Si(0 0 1) substrate kept at 550 °C with and without a predeposition of 1 ML of Sb. From the analysis of the experimental data the surfactant effect was quantified in terms of Ge–Si interdiffusion. The intermixing appeared to be appreciably reduced by the Sb action for the (Ge2Si8)15 multilayers, while for (Ge4Si6)15 samples the reduced interdiffusion effect due to Sb was not detected. In any case the Ge–Ge and the Ge–Si first neighbors bond lengths was found to be close to the sum of their costituent-element atomic radii. Finally, we found that the deposition of a submonolayer of Sb is not enough to completely prevent from interdiffusion even in case of 2 Ge monolayers buried in silicon and grown on a Si(0 0 1) substrate at 400 °C. [Copyright &y& Elsevier]

Abstract: The properties of carbon nanotubes (CNTs), the new allotropic status of carbon discovered in 1991, have been widely investigated in all possible application field. This new material in fact can be easily obtained chemically by CVD (Chemical Vapour Deposition) as a layer of nanotubes growth on a wide variety of materials. When growth on a silicon surface, CNTs create a semiconductor heterojunction with peculiar photoresponsivity properties. We studied this heterojunction with the purpose to realize a large photocathode with high quantum efficiency in a large wavelength range from UV to IR. Results obtained up to day allowed us to build a new kind of photodetector very cheap, stable and easy to manage. Recently this new device has been proposed as one of candidates for the beam monitor system of SuperB. [Copyright &y& Elsevier]

The first stages of acetylene reaction with the Si(1 1 1)7 × 7 reconstructed surface kept at 600 °C are studied by recording scanning tunneling microscopy (STM) images during substrate exposure at a C2H2 pressure of 2 × 10−4 Pa (2 × 10−2 mbar). We observed the progressive substitution of the 7 × 7 reconstruction with a carbon induced Si(1 1 1)√3×√3R30° reconstruction characterized by an atomic distance of 0.75 ± 0.02 nm, very close to that of the silicon 7 × 7 adatoms. This means that a carbon enrichment of the silicon outermost layers occurs giving rise to the formation of a Si–C phase different from the √3×√3R30° reconstruction typical of Si terminated hexagonal SiC(0 0 0 1) surface with an atomic distance of 0.53 nm. To explain STM images, we propose a reconstruction model which involves carbon atoms in T4 and/or S5 sites, as occurring for B doped Si(1 1 1) surface. Step edges and areas around the silicon surface defects are the first regions involved in the reaction process, which spreads from the upper part of the step edges throughout the terraces. Step edges therefore, progressively flakes and this mechanism leads, for the highest exposures, to the formation of large inlets which makes completely irregular the straight edge typical of the Si(1 1 1)7 × 7 terraces. These observations indicate that there occurs an atomic diffusion like that driving the meandering effect. Finally, the formation of a few crystallites is shown also at the lowest acetylene exposures. This is the first STM experiment showing the possibility to have carbon incorporation in a Si(1 1 1) matrix for higher amounts than expected, at least up to 1/6 of silicon atomic layer. [Copyright &y& Elsevier]