R. Dolbec

Sub-ppm sensitivity towards carbon monoxide by means of pulsed laser deposited SnO2:Pt based sensors

The gas-sensing properties of nanoporous SnO2, SnO2:Pt, and multilayered SnO2:Pt thin films, deposited by pulsed laser deposition, have been investigated towards low concentrations of CO, in dry air. It is shown that SnO2:Pt films presenting an optimal thickness of ∼400nm and suitably doped with Pt nanoparticle catalyst are able to detect CO concentrations (C) as low as 100ppb (parts per 109). Interestingly, the response of the sensor is found to follow a [C]0.75 law over a CO concentration range as wide as 0.1–500ppm. The multilayered approach developed here is shown to be effective in further improving the sensitivity of the sensors.

Reactive pulsed laser deposition of high-k silicon dioxide and silicon oxynitride thin films for gate-dielectric applications

We have successfully developed two reactive pulsed laser deposition (PLD) processes for the growth of high-k SiO2 and SiOxNy thin films. At a KrF laser intensity of 3×108 W/cm2, both SiO2 and SiOxNy films have been deposited by ablating a silicon target in a reactive gas atmosphere (O2 and O2/N2 mixture, respectively) on both Si (100) and Pt-coated Si substrates. Two key issues are presented here, namely (i) the effect of the deposition temperature (Td in the 20–450 °C range) and (ii) the effect of the N incorporation (in the 0.3–20 at. % concentration range) on the microstructure and electrical properties of PLD SiO2 and SiOxNy thin films, respectively. For the PLD-SiO2 films, 300 °C has been identified as the optimal deposition temperature that yields stoichiometric ([O]/[Si]≈1.9), hydrogen-free films with a low local disorder, a highly dense microstructure and a dielectric constant (k) higher than that quoted for thermally grown SiO2. On the other hand, the PLD SiOxNy films containing 20 at. % of N hav...

Electrochemical behavior of Mg–Ni–Ti thin films grown by pulsed laser deposition

Abstract Mg–Ni–Ti electrodes with various composition were successfully prepared under thin film configuration by means of pulsed laser deposition (PLD) from Mg:Ti:Ni (1:1:2) target. The film composition was found to be strongly dependent on the laser fluence. At a laser intensity of ∼4 J/cm2, a congruent deposition is obtained whereas for lower fluences, Mg-rich films are formed due to the preferential ablation of the magnesium from the target. For higher laser fluences, Mg depletion in the film is thought to be related to some preferential sputtering of the magnesium atoms from the growing film. The XRD analysis indicates that the films are amorphous regardless of their composition. However, a significant amount of MgO phase is detected in the Mg-rich films, as a consequence of the high tendency of magnesium to be oxidized. Electrochemical experiments confirm that hydrogen is absorbed into the films. However, only Mg–Ti–Ni films with a Mg content of about 14 at.% (i.e. Mg16Ti26Ni58 and Mgi3Ti28Ni59 compositions) maintain their integrity during extended electrochemical tests, while those with higher Mg contents (i.e. Mg24Ti22Ni54 and Mg73Ti5N22 film compositions) show a rapid degradation of their electrochemical activity due to film delamination related to their poor initial adherence on the substrate and/or to the strain generated into the film by hydrogen absorption. Electrochemical experiments focused on Mg16Ti26Ni58 film indicate that its maximum discharge capacity is low (∼6.4 μAh/cm2.μm) that is not surprising taking into consideration its low Mg proportion. The electrode capacity reached its maximum after about 30 charge/discharge cycles, remained constant for ca. 45 cycles and then decreased due to film degradation. This study confirms the possibility, through an adequate choice of the PLD parameters, to obtain a congruent deposition despite the large different in the thermophysical properties of the constitutive elements but on the other hand, it illustrates the difficulty to obtain Mg–Ni–Ti thin films with a large electrochemical capacity and good capacity retention in contrast to that observed with Mg–Ni–Ti powder electrodes.

Comments on “Synthesis and structural characterization of rutile SnO2 nanocrystals” by Z. Chen, J.K.L. Lai, C.H. Shek, and H. Chen [J. Mater. Res. 18, 1289 (2003)]

The nanostructure, stoichiometry and sensing performance of SnO 2 films grown by pulsed laser deposition (PLD) have been shown to be highly sensitive to the deposition conditions. In particular, PLD deposition under vacuum is known to produce films that are composed of both a polycrystalline SnO 2 phase and an amorphous SnO phase, for deposition temperatures in the 20-600 °C range. The presence of such an amorphous SnO phase in the films greatly limits their practical use as gas-sensing 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.

Structural investigations of Inductively Coupled Plasma ultra-thin silicon nanowires

We demonstrated the high throughput production of ultra-thin SiNWs by the innovative Inductively Coupled Plasma (ICP) approach. Our investigations revealed that the vast majority (∼95%) of the ICP produced SiNWs grew according to the Oxide Assisted Growth mechanism, and the 5% through the Vapor-Liquid-Solid mechanism. These SiNWs present an intriguing internal nanostructure, that provides a new kind of nanocomposite, where quantum confinement effects are expected. Indeed, an intense photoluminescence emission in the near infra-red was observed. These results prove the ICP as a genuinely bulk process, which can be exploited for large scale production of thin SiNWs to be integrated into attractive large-area and flexible optoelectronic devices.

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.