CVD for MOF fab

Bioscience engineers from KU Leuven in Belgium and colleagues at CSIRO, Australia and MBI, Singapore, have devised a new approach to the synthesis of so-called metal-organic frameworks (MOFs) with nanoscale pores that allows them to fabricate very thin films having numerous potential high-tech applications. MOFs are essentially a three-dimensional network of an organic compound interwoven with metal ions wherein the organic component keeps the metal ions apart opening up a regular pattern of nanoscopic pores within the 3D lattice and lending the materials to applications in catalysis and separation science and other areas where a large internal surface area relative to volume is critical, such as gas storage or in gas sensors. MOF surface areas can range from 1000 to 5000 square meters per gram of material.

"Researchers are already looking into applications," explains KU Leuven chemist Rob Ameloot. "They are examining the use of MOFs as catalysts and for gas storage." However, MOFs are difficult to produce without lab-scale wet chemistry that simply results in a powder. For integrated, nanoscale applications, engineers would, however, need these materials not in powder form but as a thin film deposited on a suitable substrate.

Team member and lead author in the research paper published in Nature Materials, Ivo Stassen explains that the team hoped to develop an alternative to wet chemistry that would allow them to make highly pure, thin MOF films. "Vapor-phase deposition is already a common method to produce high-tech devices," he explains. "We are the first to use this method for the production of these highly porous materials," namely zeolitic imidazolate MOFs, ZIF-8.

The team firstly deposits layers of zinc and let them react with the vapor of the organic material. The organic material permeates the zinc, the volume of the whole expands, and it is fully converted into a material with a regular structure and nanopores," he adds. The team is able to fine tune the outcome by tweaking the precise conditions. More importantly it sidesteps the liquid phases that are entirely incompatible with microelectronic circuit fabrication and so opens up the possibility of integrating MOF films into devices without the corrosion or contamination risks of liquid processing. Stassen is collaborating with the Leuven-based research centre "imec", which specializes in nanoelectronics, the two centers have submitted a joint patent application for the technology.

"This alternative production method opens up new possibilities for MOFs in terms of applications and industries," Stassen adds. "Chemical vapor deposition (CVD) is a common technique in nanofabrication. Therefore, new MOF applications can be developed relatively quickly: gas sensors, nanochip components, and improved batteries," he says.

David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the bestselling science book "Deceived Wisdom".