21 septembre 2009
NANOGOLF
Contact: Pierre Terech

Fig.1: Diagram of functionalized mesoporous layers before (a) and after (b) extraction of the tensio-active structuring agents. (c) Ion beam etching of the first layers to open the porosity while maintaining the chemical functions grafted in the pores.

In order to take advantage of specific properties from nano-objects, one must be able to handle them, to deposit them wherever desired, and be able to read the information they carry. This is why the elaboration of "smart" substrates to capture these nano-objects at precise locations is of great interest. We found a method for obtaining a nanostructured surface made functional: by bombarding a mesoporous and material with ions made functional, under grazing incidence conditions. Such a process enables the distribution of nanoparticles (NP) inside the pores, rather than beside them!

 

The "bottom-up" routes can generate architectures and functional nano-objects whose sizes span a range from the molecular to the nanoscopic, while in large quantities and at low costs. These nano-objects, selected for having a particularly interesting property, frequently require specific localization on a surface so as to be exploitable. For example, the surface may be coated with chemical functions to enable grafting to a target particle. But, how can one arrange the chemical functions and the nano-objects wherever one wishes to put them? Two challenges emerge: first, to draw a calibrated and structured nanoscale on the surface, onto which the objects will go. Then, organize these holes so that the nano-objects will selectively graft themselves there.

 

Fig.2: (a) Nano-structured surface exhibiting organized spherical pores and opened at the surface. (b) Naive diagram showing the use of such surfaces for localizing nano-objects.

We developed a new manufacturing process for the elaboration of surfaces structured at the nanoscale and given selective functions:  the etching of a thin mesoporous silica film by an ion beam. The process involves three steps (Fig. 1).

 

  1. Structuring (see insert): spherical or cylindrical micelles of tensio-active agents structure an inorganic volume of silica.
  2.  Extraction of the organic phase, which leads to a mesoporous material.  A thin layer (typical thickness is 100 nm) contains, at this point, pores organized in hexagonal or cubic lattices. The chemical functions are localized at the surface of these pores. Nevertheless, at this stage, the external surface of the layer is neither structured nor given selective functions.
  3. Opening the surface porosity by etching it with an ion beam. Under grazing incidence, this process preserves the functions that are selectively grafted at the surface of the pores.
 

Fig.3: (a) SEM micrographs (viewed from above) showing the NP distribution as adsorbed on a mesoporous surface made functional. (b) Statistical histogram showing the NP density number dispersed over 100 pores of a film made functional with cyanopropyl functions and over 100 pores of a bare film (not made functional).

Result: surfaces made up of spherical (Fig. 2a) or cylindrical cavities designed for hanging the nano-objects or nanoparticles (Fig. 2b), wires or tubes. One of these surfaces was tested with FePt magnetic nanoparticles, which have the advantage of maintaining magnetic orientation at room temperature, thus making them good nano-reservoirs for information storage. Particles coated with ligands were dispersed onto two surfaces, one having been given functions but not the other. Electron microscopy images (Fig. 3a), statistical analysis over more than 1500 pores), show (Fig. 3b) that the particles are mainly localized in those cavities given functions.

 

Controlled localization of these particles on a surface is crucial for applications in the field of high-density information storage. Other applications can be foreseen with this process. Diamond nanoparticles grafted along oriented cylindrical channels could allow the growth of nanowires for nanodevices for use in molecular electronics and bioelectronics. This work has been patented (BD11285).

 

Further reading: S. Dourdain, et al., Langmuir 26 (2010) 7565-7568

 

Structuring of mesoporous silica films

 

Tensio-active molecules are molecules with a hydrophilic part (red) and a hydrophobic part (green). When put in a solvent (for instance, hydrophilic), they aggregate in the form of mono-dispersified objects: spherical micelles, cylinders, lamellas. These aggregates are organized in cubic or hexagonal lattices.

We take advantage of the self-assembly property to structure an inorganic silica template. To do this, the tensio-active is mixed with the silica precursors and a volatile solvent, ethanol. The structuring of the layers is induced by a fast evaporation of the solvent at the same time the layer deposition is taking place: the rapid increase of the tensio-active concentration and of the silica precursors activates the self-assembly into micelles and, simultaneously, the silica “gel”. Thus, silica "freezes" into a solid material, the structure imposed by the micelles.

 

Maj : 20/02/2014 (976)

 

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