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Oxides nanowires by CVD

Updated on November 30, 2009
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Oxides nanowires by CVD Students,: Y. Lai, Objectives : The bottom-up growth of IV/IV or III/V semiconductor nanowires by the VLS method has been extensively studied. As far as oxides are concerned, similar approaches were applied mainly to the growth of ZnO nanowires. However, nanowires of binary metal oxides or complex metal oxides (strongly correlated systems) - and their integration on silicon - would be of high interest owing to their large variety of properties. Also, from a fundamental point of view, there is a limited knowledge about their physical properties (except for ZnO). Before envisaging complex oxides, we focused on the synthesis of MgO nanowires, which can be used as a template and which has also many other interests (tunneling barrier, catalyst applications). So far, the CVD of MgO nanowires had been realized at high temperature (typically 900°C) starting from powders such as MgB2 or Mg3N2. Our goal was to achieve the growth by liquid injection CVD at moderate temperatures and, importantly, starting from conventional precursors used for the growth of oxide films. Indeed, this would then open the route to the synthesis of more complex wires, for example with modulated composition or structure, by using several injectors and their extremely rapid response time. Main results : We demonstrated the growth of MgO nanowires by liquid injection MOCVD at a temperature as low as 600°C and starting from the b-diketonate    Mg(tmhd)2 The synthesis was carried out on MgO (001) and Si (001) substrate (see Fig. 1.20). Gold was used as a catalyst.  
Figure 1.20 - SEM images (tilted) of MgO nanowires synthesized by MOCVD at 600°C on Au-coated substrates (~ 2 nm) : (a) on MgO (001) - (b) on Si (001).   A thin gold layer of few nanometers was sputtered onto the substrates prior to deposition. Both on MgO and Si substrates, TEM showed that the growth direction is along the [001]. On MgO, the wires are grown epitaxial, vertically aligned perpendicular to the substrate plane, while on Si (001), various orientations are observed. 
Figure 1.21 - TEM images of MgO nanowires epitaxially grown at 600°C by MOCVD on Au-coated (~ 2 nm) MgO (001) substrates : (a) HRTEM image showing Au particle at the tip of the wire, (b) : nanowire with Au nanoparticles distributed along its radial axis, ( c) : HRTEM image showing facets along sides of the wire. The wires exhibit a square-rod shape with facets of {001}-type on the sides. They have a tapered shape, which was also reported for PLD-grown wires. The bottom diameter is typically of 15 - 20 nm while the top diameter is of 4 - 5 nm. The observation of a roundshape gold nanoparticle at the tip of the wires (Fig. 1.21), pointed out to a vapor-liquid-solid (VLS) growth mechanism, while the contribution from a solid/vapor mechanism is not precluded. The length was typically of 700 nm for 85 min deposition using an injection period of 3s. The dimension of the wires depend on several parameters, such as the starting gold catalyst, the temperature, the total pressure and the oxygen partial pressure. We demonstrated that the growth on MgO substrates can be switch from vertical to horizontal by  decreasing the time period at which the reactants are injected into the deposition chamber. Finally, in most of the wires, discrete gold nanoparticles with oblong shape were observed along the central axis and with a decreasing diameter from bottom to top, as shown in Fig. 1.21. Embedded gold particles in a dielectric matrix are highly interesting for optical applications. The reason for such growth is under investigation. Collaborations CMTC Grenoble INP, EMAT, Anvers.
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Date of update November 30, 2009

Univ. Grenoble Alpes