Growth Mechanisms of Inductively-Coupled Plasma Torch Synthesized Silicon Nanowires and their associated photoluminescence properties.

Ultra-thin Silicon Nanowires (SiNWs) were produced by means of an industrial inductively-coupled plasma (ICP) based process. Two families of SiNWs have been identified, namely long SiNWs (up to 2-3 micron in length) and shorter ones (~100 nm). SiNWs were found to consist of a Si core (with diameter as thin as 2 nm) and a silica shell, of which the thickness varies from 5 to 20 nm. By combining advanced transmission electron microscopy (TEM) techniques, we demonstrate that the growth of the long SiNWs occurred via the Oxide Assisted Growth (OAG) mechanism, while the Vapor Liquid Solid (VLS) mechanism is responsible for the growth of shorter ones. Energy filtered TEM analyses revealed, in some cases, the existence of chapelet-like Si nanocrystals embedded in an otherwise silica nanowire. Such nanostructures are believed to result from the exposure of some OAG SiNWs to high temperatures prevailing inside the reactor. Finally, the intense photoluminescence (PL) of these ICP-grown SiNWs in the 620-950 nm spectral range is a clear indication of the occurrence of quantum confinement. Such a PL emission is in accordance with the TEM results which revealed that the size of nanostructures are indeed below the exciton Bohr radius of silicon.

Field electron emission enhancement of graphenated MWCNTs emitters following their decoration with Au nanoparticles by a pulsed laser ablation process.

A plasma-enhanced chemical vapor deposition (PECVD) process was adapted to alter the growth of multiwall carbon nanotubes (MWCNTs) so that graphene sheets grow out of their tips. Gold nanoparticle (Au-NP) decoration of graphenated MWCNTs (g-MWCNTs) was obtained by subsequent decoration by a pulsed laser deposition (PLD) process. By varying the number of laser ablation pulses (N(Lp)) in the PLD process, we were able to control the size of the gold nanoparticles and the surface coverage of the decorated g-MWCNTs. The presence of Au-NPs, preferentially located at the tip of the g-MWCNTs emitters, is shown to significantly improve the field electron emission (FEE) properties of the global g-MWCNT/Au-NP nanohybrid films. Indeed, the electric field needed to extract a current density of 0.1 µA cm(-)(2) from the g-MWCNT/Au-NP films was decreased from 2.68 V µm(-1) to a value as low as 0.96 V µm(-1). On the other hand, UV photoelectron spectroscopy (UPS) characterization revealed a decrease in the global work function of the Au-decorated g-MWCNT nanohybrids compared to that of bare g-MWCNT emitters. Surprisingly, the work function of g-MWCNT was found to decrease from 4.9 to 4.7 eV with the addition of Au-NPs-a value lower than the work function of both materials worth 5.2 and 4.9 eV for gold and g-MWCNT, respectively. Our results show that the N(Lp) dependence of the FEE characteristics of the g-MWCNT/Au-NP emitters correlates well with their work function changes. Fowler-Nordheim-theory-based calculations suggest that the significant FEE enhancement of the emitters is also caused by the Au-NPs acting as nanoscale electric field enhancers.

Pulsed laser ablation based direct synthesis of single-wall carbon nanotube/PbS quantum dot nanohybrids exhibiting strong, spectrally wide and fast photoresponse.

Pulsed laser ablation for the direct synthesis of single-wall carbon nanotube/PbS-quantumdot(SWCNT/PbS-QD) nanohybrids is demonstrated. The latitude of the developed pulsed laser deposition process permits not only the control of the size of the PbS-QDs but also the straightforward integration of these novel SWCNT/PbS-QD nanohybrids into photoconductive (PC) devices. Thus, by optimizing the nanohybrid characteristics, PC devices exhibiting not only fast but also strong photoresponse (as high as 1350% at 405 nm) are achieved.

Enhanced UV photoresponse of KrF-laser-synthesized single-wall carbon nanotubes/n-silicon hybrid photovoltaic devices.

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.