Nanomaterials


COMMON TECHNOLOGICAL PLATFORM FOR PREPARATION OF NANOSTRUCTURED AND MONODIMENSIONAL OXIDES

Responsible: Prof. Elisabetta Comini

This activity will deal with the task of preparing oxides as quasi 1D structures and thin and thick films and will feed all the other downstream activities

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Preparation of oxide quasi 1D structures

SENSOR, since it was established, addressed its activity in the field of material research for functional applications trough the bottom-up fabrication of highly ordered, crystalline, quasi 1-dimensional (1-D) nanostructures of ionic metal oxides. The increasing scientific interest in 1-D systems such as nanowires and nanorods stimulates their functional exploitation, and single-crystalline 1-D nanostructures are nowadays emerging as building blocks for a new generation of electronic, and optoelectronic nanometer-scaled devices with superior performances. The potential of the bottom-up approach for the preparation of a new generation of size- and shape- controlled nanostructured materials has been demonstrated for a number of pure oxides. The fabrication techniques of homogeneous 1-D nanostructures have pursued the control over shape, aspect-ratio, and the crystalline arrangement to a considerable degree, and the improvement of the synthesis methods has recently achieved the fabrication of chemically non-homogeneous 1-D nanowires, heterogeneous structures, and eventually the direct integration of functional nanostructures into nano-devices.
At SENSOR we synthesize and characterize quasi monodimensional single crystals of Zn, Sn, Cu and In oxide nanowires (NWs) by Physical and Chemical Vapour Deposition (PVD and CVD) techniques. Thermal evaporation of oxide or precursors powders under controlled conditions is obtained in tubular furnaces at low pressure. Hierarchical structures of metal oxides were obtained by several deposition steps, using catalyst assisted process.


Figure 1. SEM images of copper oxide nanowires prepared by thermal oxidation.


Figure 2. Hierarchical nanostructures synthesized by two steps growth PVD process using A. Au nanoparticles and B. Zinc acetate seeds as catalyst for the second growth.


Figure 3. SEM image of ZnO nanorods obtained by MOCVD.

Furthermore liquid phase growth techniques are have been studied for the preparation of Zn and Cu oxide nanostructures. We have obtained Cu2O bi-pyramids by reduction of Cu(OH)2 using hydrazine as reducing agent.


Figure 4. SEM Image of Cu2O bi-pyramids.

The last technique that has been developed in the last year is electrochemical anodization. The surface of metal can be easily and cheaply oxidize and modified by electrochemical anodization method.  Anodization of titanium foils and thin films on solid and polymeric flexible substrates were performed using a two-electrode electrochemical cell. The morphology and shape of resulting anodized titanium is strongly dependent on the experimental conditions such as the type and concentration of electrolytes, the applied voltage and current, the temperature, the anodization time.


Figure 5. Left: SEM top-view of TiO2 nanotubular arrays obtained on flexible polymeric substrate. Right: SEM image of the cross-section of TiO2 nanotube arrays formed on flexible polymeric substrate.

The prepared nanostructrure are then fully characterised in terms of morphological, structural, electrical and optical properties in form of bundles, vertically aligned NWs and single NW transistors. Nanowire applications at SENSOR will cover in the period 2009-2011 the following applications: Chemical Sensing, Dye and Quantum Dots based solar cells, Single Nanowire Transistors for biosensing, Electron sources for X ray emitters, Innovative electrodes for Electrochemical Biosensing, Innovative Electrodes for Li batteries, Thermolectrics, ZnO nanowires for optoelectronics (diodes, uv detector).

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Preparation of oxides by thin film technology

Magnetron sputtering is a technique to prepare thin films that is easily scalable on the industrial scale; in Sensor Lab sputtering is used for deposition of metal contacts, catalyzer layers, sensing layers (metal oxide semiconductors), buffer layer for solar cell applications and Transparent Conducting Oxides (TCO). The thin films can be deposited on different substrates (alumina, silicon and glass are the most common used in our lab).

We prepare by RF and pulsed DC sputtering different oxides for sensing applications (ZnO, SnO2, In2O3, WO3, MoO3, TiO2, Ga2O3, and several mixed oxides like SnO2-In2O3, TiO2-Fe2O3, WO3-MoO3 and TiO2-WO3). Deposition can be conducted in inert or reactive atmosphere at a substrate temperature in the range 20°C-  400°C.

TCO
can be prepared by RF sputtering starting from ternary compound target (4” and 2”): Indium Tin Oxide, Cadmium Stannate, Copper delafossite and Zinc Tin Oxide. In case of deposition from ternary target the influence of the type of substrate and of oxygen pressure, gas pressure and sputtering power is strong; thus the preparation and annealing procedure must be carefully selected.