Formation of Hybrid Silicon Nanostructures via Capillary Instability Triggered in Inductively-Coupled-Plasma Torch Synthesized Ultra-Thin Silicon Nanowires

This paper provides a report on the formation of two main classes of hybrid silicon nanostructures via the capillary instability induced in ultra‐thin silicon nanowires (SiNWs) when subject to high temperature annealing. The first class of hybrid Si nanostructures shows a high‐level nanostructural order, and regroups (i) periodic strings of almond‐shaped Si nanocrystals (SiNCs) having average dimension of 3 nm and connected by ultra‐thin (≈2 nm) SiNWs and (ii) spherical SiNC chains (mean diameter of 6 nm, spaced averagely by 16 nm), both embedded into silica NWs. In the second class of hybrid Si nanostructures, the SiNCs have different dimensions (in the 3–14 nm size range) and shapes, or a modulated Si core inside a SiO2 shell, thus exhibiting much higher nanostructural complexity. The self‐assembly of such nanostructures is related to the gas ambient under which the thermal treatment is performed. However, by increasing the annealing temperature, the SiNWs’ cores preferentially evolve toward the spherical SiNC chain morphology. The ultra‐thin diameter (2–3 nm) of the initial SiNWs is a key feature to induce the hybrid Si nanostructures formation. This study opens up the prospects of in situ controlling the formation of Si nanocrystals inside silica NWs, which will lead to the tailoring of their optoelectronic properties

Packing-induced electronic structure changes in bundled single-wall carbon nanotubes

The electronic structure of free-standing parallel and braided bundles of single-wall carbon nanotubes (similar to 1.2 nm diameter) was probed by transmission electron microscopy and electron energy loss spectroscopy. The observed dramatic changes in the carbon K(1s) near-edge structures are attributed to the tubes' structural packing in bundles which consequently alters their electronic structure. The pi(*)- and the sigma(*)-states are shown to be strongly affected by the way the tubes are packed in the bundles (i.e., parallel, braided, turned, or twisted). (c) 2005 American Institute of Physics

Effect of coiling on the electronic properties along single-wall carbon nanotubes

Straight and coiled single-wall carbon nanotubes (SWCNTs) synthesized by laser vaporization were dispersed on highly oriented pyrolitic graphite. Their morphology and electrical properties were investigated by scanning tunneling microscopy (STM). STM images revealed that the SWCNTs (either straight or coiled) often self-organize into bundles of two or more tubes and are rarely found alone. The conductance measured along a periodically coiled CNT was found to increase at locations where the CNT is squeezed, while it decreases significantly in unsqueezed regions characterized by an unperturbed hexagonal network. This provides experimental evidence of significant conductance modulation along a one-dimensional system on the nanometer scale. © 2004 American Institute of Physics

Structural and photoluminescence properties of silicon nanowires extracted by means of a centrifugation process from plasma torch synthesized silicon nanopowder

We report on a method for the extraction of silicon nanowires (SiNWs) from the by-product of a plasma torch based spheroidization process of silicon. This by-product is a nanopowder which consists of a mixture of SiNWs and silicon particles. By optimizing a centrifugation based process, we were able to extract substantial amounts of highly pure Si nanomaterials (mainly SiNWs and Si nanospheres (SiNSs)). While the purified SiNWs were found to have typical outer diameters in the 10-15 nm range and lengths of up to several μm, the SiNSs have external diameters in the 10-100 nm range. Interestingly, the SiNWs are found to have a thinner Si core (2-5 nm diam.) and an outer silicon oxide shell (with a typical thickness of ∼5-10 nm). High resolution transmission electron microscopy (HRTEM) observations revealed that many SiNWs have a continuous cylindrical core, whereas others feature a discontinuous core consisting of a chain of Si nanocrystals forming a sort of 'chaplet-like' structures. These plasma-torch-produced SiNWs are highly pure with no trace of any metal catalyst, suggesting that they mostly form through SiO-catalyzed growth scheme rather than from metal-catalyzed path. The extracted Si nanostructures are shown to exhibit a strong photoluminescence (PL) which is found to blue-shift from 950 to 680 nm as the core size of the Si nanostructures decreases from ∼5 to ∼3 nm. This near IR-visible PL is shown to originate from quantum confinement (QC) in Si nanostructures. Consistently, the sizes of the Si nanocrystals directly determined from HRTEM images corroborate well with those expected by QC theory

Tuning the Charge-Transfer Property of PbS-Quantum Dot/TiO₂-Nanobelt Nanohybrids via Quantum Confinement

A newly designed photoactive nanohybrid structure based on the combination of near-infrared PbS quantum dots (QDs) as light harvester and one-dimensional TiO₂ nanobelts (NBs) to guide the flow of photogenerated charge carriers is reported. Efficient electron transfer from photoexcited PbS QDs to TiO₂ NBs has been demonstrated to occur in the developed PbS-QD/TiO₂-NB nanohybrids, and the charge-transfer property can be tuned through the size quantization effect of PbS QDs. Moreover, the use of TiO₂ NBs instead of TiO₂ NPs permits a larger critical size of PbS QDs capable of injecting electrons into TiO₂ NBs, which, in turn, markedly extends the “effective” absorption of the PbS-QD/TiO₂-NB nanohybrids to a longer wavelength region up to 1400 nm. Such an extension of the “effective” absorption is a major asset for improving the overall photoconversion efficiency of PbS-QD/TiO₂-NB nanohybrids-based photovoltaic devices

Self-assembly of silicon nanowires studied by advanced transmission electron microscopy

Scanning transmission electron microscopy (STEM) was successfully applied to the analysis of silicon nanowires (SiNWs) that were self-assembled during an inductively coupled plasma (ICP) process. The ICP-synthesized SiNWs were found to present a Si–SiO2 core–shell structure and length varying from ≈100 nm to 2–3 μm. The shorter SiNWs (maximum length ≈300 nm) were generally found to possess a nanoparticle at their tip. STEM energy dispersive X-ray (EDX) spectroscopy combined with electron tomography performed on these nanostructures revealed that they contain iron, clearly demonstrating that the short ICP-synthesized SiNWs grew via an iron-catalyzed vapor–liquid–solid (VLS) mechanism within the plasma reactor. Both the STEM tomography and STEM-EDX analysis contributed to gain further insight into the self-assembly process. In the long-term, this approach might be used to optimize the synthesis of VLS-grown SiNWs via ICP as a competitive technique to the well-established bottom-up approaches used for the production of thin SiNWs

Anharmonicity in single-wall carbon nanotubes as evidenced by means of extended energy loss fine structure spectroscopy analysis

A comparative study of the structure of free-standing parallel bundles of single-wall carbon nanotubes (SWCNTs), multiwalled carbon nanotubes (MWCNTs), and highly oriented pyrolytic graphite (HOPG) was achieved by means of transmission electron microscopy and electron energy loss spectroscopy analyses. In particular, the carbon K (1s) extended fine structure of SWCNTs is found to be characterized by an apparent contraction of the nearest neighbors distance. This contraction is interpreted here to originate from an asymmetric pair distribution function, mostly due to the high out-of-plane vibrational motion of the C atoms, as for the case of chemisorbed atoms on clean surfaces. In contrast, the MWCNTs did not exhibit any signature of such an anharmonic effect because of their more rigid structure. This indicates that the SWCNTs pair potential is significantly broader and its effect is much weaker than that experienced by the same C-C pair embedded in a multiwall nanotube

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