Capillary condensation is a naturally occurring phenomenon and involves uptake of vapour by the pores in nanomaterials. Michael Rubner and Robert Cohen's team of the Massachusetts Institute of Technology have now exploited the effect to make inorganic composite coatings with 50% or more nanoparticle loading. The researchers did this by simply exposing nanoparticles to organic vapours in a sealed container and heating the lot up to 100 °C. The organic vapours spontaneously condense in between the nanoparticles and no co-solvents or other chemicals are required in the process.

Condensation occurs in nanometre-scale pores thanks to interfacial curvature-induced pressure effects. "It is not exactly the particle that is functionalized, but rather the void in between particles that is filled in a tunable fashion with an arbitrary chemical condensate," explained team member Zekeriyya Gemici.

Anti-reflection coating
The resulting two-stack organic-inorganic composite structure is an excellent anti-reflection coating and is equivalent to those made by much more complicated methods, such as vacuum deposition that produces seven-stack fully inorganic antireflection coatings. But the new composite is only half as thick and does not crack when exposed to large temperature swings on plastic substrates, which could make it useful for next-generation cell phone camera microlenses, says Gemici. These devices must undergo temperature increases from room temperature to around 260 °C when fabricated by soldering, and traditional glassy coatings crack during the process.

Beyond such a proven application, capillary condensation might also be employed to make transparent materials with very high particle loading – for example, high refractive index and highly transparent thin films that incorporate optically non-linear polymers desirable for holographic memory storage devices, adds Gemici. "Using our technique, we might create a porous matrix of titania nanoparticles (which have a high refractive index) to condense monomers of optically non-linear polymers into the porous and transparent scaffold," he told nanotechweb.org.

According to the team, the biggest, and perhaps only challenge, in bringing this technology to bear on applications like mobile phone cameras is the current financial crisis. "As soon as consumer electronics manufacturers are comfortable investing in new technologies once again, I am confident that our technology will find its way into the market," said Gemici.

The researchers say they would now like to combine capillary condensation with the layer-by-layer technique that is a speciality in the Cohen and Rubner labs at MIT to explore many more novel coating systems. According to Gemici, such hybrid technology might be used to develop a perfect water-based coating methodology that could one day replace many solvent-based or vacuum techniques. "Water-based coating techniques are of course both environmentally friendly and highly cost-effective compared with these methods," he added.

The work, supported by Essilor International SA and the MIT MRSEC program, was published in Nano Letters. Gemici also acknowledges KAUST for fellowship support.