For adhesive displays, smart bandages, and inexpensive flexible plastic sensors to truly take off, they will need some method to store data long-term on plastic. Having memory options is crucial in the flexible electronics ecosystem.
However, today's versions of non-volatile memory, such as flash memory, are not quite suitable. Therefore, when researchers decided to try adapting a phase-change memory to plastic, they obtained a much better-performing memory because it is built on plastic. The energy required to reset the memory is an order of magnitude less than previous flexible versions. They reported their findings this week in the journal *Science*.
Phase-change memory (PCM) is not a clear victory for plastic electronics. It stores its bits in a resistive state. In its crystalline stage, its resistance is very low. But running enough current through the device melts the crystal, causing it to freeze in an amorphous phase, resulting in higher resistance. This process is reversible. Importantly, especially for experimental neuromorphic systems, PCM can store intermediate levels of resistance. Therefore, a single device can store more than one bit of data.
Unfortunately, the typical set of materials involved doesn't work well on flexible substrates like plastics. Researchers decided to try a material called a superlattice, a crystal composed of nanolayers of different materials. In studying these superlattices, they concluded that they must be highly thermally insulating because, in their crystalline form, there are atomic-level gaps between the layers. These "van der Waals-like voids" restrict both the flow of electric current and the flow of heat. Therefore, when current is forced through, heat doesn't flow away rapidly from the superlattice, meaning less energy is needed to switch from one stage to another.
Researchers made over 100 attempts to generate a superlattice with the correct van der Waals gaps. A black-and-white micrograph shows a superlattice structure labeled 5 nm, formed by alternating layers of antimony telluride and germanium telluride. Van der Waals-like gaps form between the layers, restricting the flow of current and heat.
Researchers have managed to confine the current within a 600-nanometer-wide porous structure surrounded by insulating alumina. The final insulating layer is the plastic itself, which offers significantly better resistance to heat flow than PCMs typically built on silicon. The completed device achieves a current density of approximately 0.1 megaamperes per square centimeter, two orders of magnitude lower than conventional silicon-based PCMs and one order of magnitude better than previous flexible devices. Furthermore, it exhibits four stable resistance states, allowing it to store multiple bits of data within a single device.
