Making metal–organic frameworks easier to shape for electronic devices

In 1974, chemist Richard Robson from the University of Melbourne conceived the idea of using molecules and chemical bonds instead of wooden balls and metal rods to construct structural models. This concept formed the basis for metal–organic frameworks (MOFs), porous materials created by linking metal ions with organic molecules.

MOFs boast exceptional properties due to their porous structures, enabling applications like gas storage, separation, and pollutant capture. These unique characteristics also make MOFs promising for functional devices, such as chemical sensors and electronic components.

However, MOFs' delicate structures are susceptible to damage during dry etching, a process that utilizes energetic particles like ions or electrons to create patterns. Tokyo University of Science researchers developed a method to mitigate this damage by infiltrating MOFs with ionic liquids. This approach enhances the material's hardness and maintains its structure during etching, allowing for the creation of precise patterns at the scale of hundreds of nanometers.

Assistant Professor Yumeng Zheng and Professor Kentaro Kinoshita led the study, which was published in Communications Materials. They filled the copper-based MOF Cu₃BTC₂ with four imidazolium-based ionic liquids, occupying 50%–60% of its pore volume. Using a focused gallium ion beam at room temperature, they etched various patterns, including a 5 × 5 µm² square, a pillar, and a Chinese character with lines 130–300 nm wide.

The researchers propose that ionic liquids protect MOFs by absorbing heat and dissipating electrical charge accumulated during ion beam exposure. Thermal analysis revealed that treated MOFs decomposed at higher temperatures than untreated samples, indicating improved thermal stability. Hardness tests showed that all four ionic liquid-treated samples were harder than the untreated MOF, while their stiffness remained largely unchanged.

Previously, cryogenic etching was the only known method to etch MOFs while preserving their structures. The new ionic-liquid-assisted dry etching method could potentially be applied to other MOFs, enabling room-temperature patterning without specialized equipment. This advancement could lead to the incorporation of MOFs into practical devices, such as high-sensitivity sensors, separation and analysis devices, and catalytic devices.

With over 100,000 known MOFs, this method could extend their applications by facilitating the creation of precise, complex structures.