Catalysis, hydrogen storage and gas sensing and purification are just a few of the promising technologies for materials called metal–organic frameworks, which are mostly…empty space. Metal–organic frameworks (MOFs) are the molecular equivalent of a building scaffold: metal ion ‘joints’ are connected by organic ‘linkers’ to form a low-density porous material with exceptionally high capacity for the storage of gases such as hydrogen and methane. The gases travel easily through the porous architecture and can be either selectively adsorbed by the organic rungs or flow past them depending on chemical interactions between the two parts of the structure.

Now, with an eye toward exploiting these properties in nanoscale devices, a team of scientists in Japan1 has succeeded in controlling the growth of MOFs, which has enabled them to prepare fully crystalline thin MOF films.

“We expect our crystalline MOF films to perform ideally in applications requiring highly selective gas separation and the preferential transportation of adsorbed molecules or ions,” says Rie Makiura, a scientist from Kyushu University who co-led the study.

MOFs perform optimally when their scaffold-like structure is highly crystalline and free of breaks or broken channels, which obstruct the flow of gas. However, it has previously only been possible to produce partially crystalline MOF films.

Fig. 1: Schematic illustration of the scaffold-like architecture of a metal–organic framework consisting of pyridine rings (gray/purple), and larger porphyrin rings encapsulating cobalt atoms (pink).From Ref. 1 © 2010 R. Makiura, H. Kitagawa

Makiura and her colleagues developed a two-step fabrication process that allowed them to build up a fully crystalline MOF film of arbitrary thickness. First, they deposited a monolayer of pyridine — a ring-shaped organic compound — and cobalt-bearing porphyrin molecules on the surface of a liquid bath containing copper ions. They then compressed the floating layer from all sides, causing the three components to assemble into a two-dimensional, crystalline metal–organic sheet that subsequently slipped off the surface onto a silicon wafer. Repeating this process, the researchers could construct a multilayer film while ensuring that each layer was itself perfectly crystalline (Fig. 1).

Makiura believes the structure of these thin MOF films could be useful in ‘gate-opening’ selective gas absorption, by which the layers slide with respect to one another in response to a particular gas squeezing its way between the framework’s rungs. According to Makiura, these MOF films would be the first example of gate-opening devices in thin-film form.