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Posted: Sep 26, 2012

Just say no to cracks

(Nanowerk Spotlight) One way to use engineered nanoparticles in the real world is in thin films. Such nanoparticulate thin films are thin layers, sometimes only a few nanometers thick, of composite materials that contain nanoparticles. These new materials have a wide range of applications in drug delivery, nanoelectronics, magnetic storage devices, sensors, or optical coating. However, most processes used to fabricate thin nanocomposite films with high nanoparticle fillings suffer from random nanoparticle agglomeration causing formation of irregularly shaped nanostructured features within the composite. Another complication arises from cracks that develop during the fabrication of the films. When a film is made from a suspension of nanoparticles, cracks form if the film thickness exceeds a critical value – these cracks are sometime called "mud cracks" because they resemble the cracking pattern that is observed in a dry lake bed.
"Conventionally, to enable the formation of crack-free nanoparticle films, the film deposition process or the nanoparticle suspension had to be changed quite drastically," Daeyeon Lee, Assistant Professor of Chemical and Biomolecular Engineering at the University of Pennsylvania, explains to Nanowerk. "For example, a chemical called a 'binder' is added to the nanoparticle suspension to prevent cracking. While this method works relatively well, the addition of the binder usually changes the composition and properties of the resulting nanoparticle films. Other conventional methods include using supercritical CO2 drying or changing the substrate for the film formation. These again require drastic changes to the film fabrication process and may require expensive equipment."
In a paper in the September 17, 2012 online edition of Nano Letters ("Avoiding Cracks in Nanoparticle Films"), Lee and his team have now described a simple method for fabricating thick, crack-free silica nanoparticle films by subsequent deposition of thin, crack-free silica nanoparticle multilayers.
silica nanoparticle films
Optical microscope images of 22 nm silica nanoparticle films formed at a rotational rate of 2000 rpm and deposited on glass substrates: (a) 480 nm single-coated film generated from a nominal suspension concentration of 22.5 wt % silica. This image displays a fully interconnected crack network in the final film; (b) 494 nm multi-coated film consisting of 14 total coatings deposited consecutively from a 2 wt % silica suspension. Each coating has a nominal thickness of 35 nm. Subsequent coatings were performed with about a 20 min time duration between each deposition. The multi-coated film was observed to remain crack-free throughout its fabrication which is confirmed by this image. (Reprinted with permission from American Chemical Society)
"Our method does not require any added chemicals nor does it require new substrates or new deposition technique/equipment," says Lee. "Thus, this new technique is practical and can be used by anyone who is interested in making thick crack-free nanoparticle films."
The essence of these new results is that nanoparticle films can be generated without crack formation. The key to this technique is the depositing of several thin crack-free layers sequentially on top of one another, instead of generating a thick film in one deposition step.
The researchers believe that the nanoparticles in the film covalently bond to one another – even though they are at room temperature – after the deposition of each crack-free layer, giving them the strength to resist crack formation during subsequent layer depositions.
Lee and his team found this method somewhat by accident. "We were interested in learning how to control the thickness of nanoparticle films by controlling the nanoparticle concentration and spin coating condition" he recounts. "We kept getting cracked films and learned that this is a fundamental problem that has intrigued people in a variety of fields including physics, chemistry and engineering. Then we accidentally found that thin crack-free films do not 'dissolve' when they are immersed in the solvent that was used to prepare the nanoparticle suspension. This rather trivial finding motivated us to try the multicoating method, and we quickly realized how powerful this method is."
One of the biggest advantages of this method is that crack-free films with complex structures can be generated without relying on heat treatment, which is often used to prevent crack formations.
One specific application of this method, which is also discussed in the Nano Letters paper, is the fabrication of Bragg reflectors that exhibit structural color. These are one-dimensional photonic crystals that has iridescence like the butterfly wing. These structures are generally made by processes that require high temperature treatment.
"Our multicoating method completely eliminates the need for such thermal treatment and enables the formation of highly reflective Bragg reflector with structural color," Lee says. "This is just one illustrative example of how our method can drastically change the way nanoparticle-film based structures can be generated without the concern for crack formation."
He notes that this new work also presents some interesting physics that researchers may not have explored before or be aware of concerning how nanoparticle films assemble from suspension. Additionally, it may be possible to use these multicoated films to study crack propagation or even to design novel materials with well controlled crack structures.
The team believes that one of the most important applications of this crack-free film fabrication will be in the area of energy conversion and storage. Many energy devices are made of nanoparticle films. For example, the electrodes in lithium ion batteries and fuel cells are essentially films made of multiple types of nanoparticles. The method developed by Lee's team could potentially be used to generated thick crack-free films – even on thermally labile surfaces to make flexible devices – that will optimize the properties of these electrodes, which of course, will impact the battery and fuel cell performance.
According to Lee, future directions in this research field include 1) understanding the fundamental mechanism behind the crack suppression process; 2) further optimizing the film processing; for example, improving (shortening) the processing time required to generate thick crack-free films; 3) improving the mechanical durability and reliability of assembled films, and understanding their mechanical failure mechanisms; 4) and fabricating functional films from novel nanomaterials that have unique properties.
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