New urea-based metal-free system allows high-density data storage

As a new breakthrough in memory technology, researchers from Chiba University have developed a novel urea-based system with long-term information storage ability and high tolerance towards external thermal and electric fields. This could potentially pave the way for ultra-high-density memory devices in the future.

In today’s fast-paced digital world, an enormous amount of data is exchanged and stored on a daily basis. It’s becoming more and more apparent that the massive amounts of data we’re generating require entirely new data recording systems with higher data storage density, lighter weight, and lower environmental impact.

As a material for realizing the high-density memory storage system, axially polar-ferroelectric columnar liquid crystals (AP-FCLCs) have been attracting attention in recent years. It is a liquid crystal with a structure of parallel columns generated by molecular self-assembly, which have polarization along the column axis. The polar directions of the columns can be switched by applying an external electric field and can maintain even after the removal of the E-field. This property, along with their flexibility, metal-free composition, power-saving ability, and low environmental impact, makes APFCLCs ideal for ultra-high-density memory devices.

However, due to the fluid nature of liquid crystals, the polarity induced by an external electric field can get easily undone by external stimuli.

To solve this problem, researchers have created a polarization fixation mechanism for a urea-based AP-FCLC system, where the materials can undergo a smooth transition from the AP-FCLC phase to a crystal (Cr) phase without disturbing the induced polar structure.

“The goal was to realize a compound with three states: a writable and rewritable state, an erasure state, and a save state. Emphasis was placed on minimizing the change in molecular packing structures during the FCLC−Cr phase transition process,” explains Professor Keiki Kishikawa, who led the research team.

To realize the polarization-fixable AP-FCLC system, researchers synthesized 1,3-bis(3′,4’-di(2-butyloctyloxy)[1,1’-biphenyl]-4-yl)urea. The organic molecule consists of

• a urea at its molecular center for generating a hydrogen bonding network to form a columnar molecular aggregate in the liquid crystal state
• two biphenyl groups as substituents for generating strong intermolecular interactions in the column structure and
• four bulky alkyl groups as terminal chains to prevent tight molecular packing and enable lower-temperature FCLC-Cr phase transition.

The resulting FCLC system exhibited preservation of polarization in the Cr phase, with thermally stable polarization information storage. In addition, it was found that this ferroelectric material has a significant resistance to an external electrical field at room temperature.

Furthermore, the team found that the molecules self-sorted into nanosized helical columns, which then formed small domains and became ferroelectric in nature. The proposed framework can be used to develop stable memory materials with high tolerance towards external stimuli and low environmental impact.

“AP-FCLCs have the potential to achieve more than 10,000 times larger recording density than Blu-ray Discs, but they have not been put into practical use due to the instability issue. This work will help improve their reliability, paving the way for light-weight flexible electronic devices and incinerable confidential information-recording devices,” concludes Prof. Kishikawa.

Journal reference:

1. Hikaru Takahashi, Michinari Kohri, and Keiki Kishikawa. Axially Polar-Ferroelectric Columnar Liquid Crystalline System That Maintains Polarization upon Switching to the Crystalline Phase: Implications for Maintaining Long-Term Polarization Information. ACS Applied Nano Materials, 2023; DOI: 10.1021/acsanm.3c01508