New non-volatile memory platform built with covalent organic frameworks

Researchers at Institute of Science Tokyo have created a new material platform for non-volatile memories using covalent organic frameworks (COFs), which are crystalline solids with high thermal stability. The researchers successfully installed electric-field-responsive dipolar rotors into COFs.

Due to the unique structure of the COFs, the dipolar rotors can flip in response to an electric field without being hampered by a steric hindrance from the surroundings, and their orientation can be held at ambient temperature for a long time, which are necessary conditions for non-volatile memories. The study is published in the Journal of the American Chemical Society.

Humans have made great efforts to record information by inventing recording media such as clay, paper, compact disks, and semiconductor memories. As the physical entity that holds information—such as indentations, characters, pits, or transistors—becomes smaller and its areal density becomes higher, the information is stored with higher density. In rewritable memories, the class called “non-volatile memories” are suitable for storing data for a long time, such as for days and years.

Recently, molecular technology has evolved. One class of molecular technology consists of molecules that exhibit mechanical motions. They are called “molecular machines” or “nanomachines.” If a mechanical entity rotates or flips around a chemical bond, which serves as an axis, the material class is particularly called “molecular rotors.”

Use of molecular rotors to store information may cause a breakthrough. This is because the size of molecules is a few orders of magnitude smaller than the sizes of pits in a compact disk and transistors in semiconductor memories, and organic molecules are inherently highly designable. Although applications using molecular machines have been explored extensively, the attempts to develop non-volatile memories have been scarce, mainly because the simultaneous satisfaction of the following three requisites has been so challenging.

1. To control the orientation of molecular rotors with an electric field, the rotors have to have a dipole—a spatial displacement of a positive charge and a negative charge necessary to gain a force from the applied electric field.
2. The rotors must not flip at ambient temperatures so that their orientations are held for a long period.
3. There must be adequate spaces around the rotors so that they can flip without being hampered by the steric hindrance that may be caused by the tight packing of the molecules in the solid phase. Additionally, the substance has to be heat durable up to the temperatures current computational components ordinarily undergo, which is often up to 150°C.

New materials developed by the researchers of Institute of Science Tokyo have achieved these three requisites simultaneously, with very high thermal durability up to near 400°C. By demonstrating these novelties for the first time, the researchers have created a material foundation for molecular-machine-based non-volatile memories that potentially store information at higher density than current technologies.

The researchers selected covalent organic frameworks (COFs) as a platform for the aim. COFs are an emerging class of crystalline solids formed by periodically connecting two kinds of building block molecules by covalent bonds. For one building block, they chose a tetrahedral, four-handed molecule. For the other building block, they newly developed a flat, three-handed molecule in which three dipolar rotors (1,2-difluorophenyl, DFP) and three aryl groups are alternately positioned around the central benzene ring.

Previously, these aryl groups were shown to suppress the flip of the DFP rotors at ambient temperatures in a toluene solution, which satisfied requisites 1 and 2 above, but the high density of the molecular solid sterically hindered the flip of the rotors in the solid phase, which could not satisfy requisite 3.

Interestingly, the COFs they developed exhibited an unprecedented shape dimorphism, in which the COFs grew to a hexagonal prism shape or a membrane shape, depending on the solvent composition used for the growth. Furthermore, from X-ray structural analyses, these new COFs turned out to have an unprecedented sln topology, which has a low density inherently and has not been reported for COFs.

“Due to the substantially low density of about 0.2 g/cm³ caused by the unique sln topology possessed by the COFs, the dipole rotors incorporated into the periodic network constituting the COFs have adequate spaces around them, allowing them to flip without suffering from the steric hindrance from their surroundings.

“This is a breakthrough, because our COFs are a rare solid in which dipolar rotors can flip when they are brought to elevated temperatures above 200°C or undergo sufficiently strong electric fields, but their orientations can be held for a long time at ambient temperatures. These uniquenesses have been realized by our careful selection of the building block molecules to create the COFs for this aim,” says Professor Yoichi Murakami, the leader of this project.

Additionally, Murakami pointed out the significance of the work also exists in the extension of the diversity of COFs by their discoveries of sln topology and shape dimorphism, both of which were unknown for COFs previously.

These COF-based solids may be a new platform for storing information with further higher density after proper scale-up and device demonstration are made subsequently.