Electronics printed on paper promise to be cheap, flexible, and recyclable, and could lead to applications such as smart labels on foods and pharmaceuticals or as wearable medical sensors. Many engineers have managed to print transistors and solar cells on paper, but one key component of a smart device has been missing—memory. Now a group of researchers from Taiwan has developed a method that uses ink-jet technology to print working memory on an ordinary piece of paper (ACS Nano 2014, DOI: 10.1021/nn501231z).
One challenge of using paper as a base for electronic memory is that, because it’s made of fibers, it is very rough and porous on a microscopic level, making it difficult to lay down the thin, uniform layers of materials that typical memory technologies, such as dynamic random access memory (DRAM), require. To get around this problem, the developers of the new printed memory decided to build resistive random access memory (RRAM), a relatively new type of memory with a structure simple enough to cope with such surface variations. In an RRAM device, an insulator can be set to different levels of electrical resistance by applying a voltage across it; one level of resistance corresponds to the 1s of digital logic, the other to the 0s. So each bit in RRAM consists of an insulator sandwiched by two electrodes.
The team, led by Ying-Chih Liao, Si-Chen Lee, and Jr-Hau He of National Taiwan University, built the device with silver, titanium dioxide, and carbon, although other combinations of a metal, an insulator, and a conductor could be used. They started by using a screen-printing process to coat a carbon paste onto the paper to form the bottom electrode. The process was repeated 10 times to reduce surface roughness, then the coated paper was cured at 100 °C for 10 minutes in a vacuum. They then made an ink by mixing TiO2 nanoparticles in acetyl acetone and used an ink-jet printer to deposit a layer of the particles on top of the carbon, where it would act as the insulator. Once that dried, the researchers used a solution of ethylene glycol and water containing silver nanoparticles, and they printed silver dots on top of the TiO2 layer to serve as top electrodes.
The researchers could print memory bits that were 100 µm on a side. At that size, an 8.3- by 11.7-inch piece of paper can hold 1 MB of memory. Der-Hsien Lien, the paper’s lead author, says existing ultrafine ink-jet technology can produce smaller dots that would allow the same piece of paper to hold 1 gigabyte. Reading and writing the bits takes 100 to 200 microseconds, not fast enough for high-performance computing but adequate for the type of applications the team envisions, Lien says. The memory also proved to be robust; it can be bent at least 1,000 times with no degradation in performance.
Rodrigo Martins, a materials scientist at the New University of Lisbon who also works on paper-based electronics, says paper memory is an important step for the development of applications such as smart sensors printed on stickers or labels. Researchers are also developing flexible electronics based on polymeric materials, but he thinks paper-based devices may be cheaper and easier to recycle. Such devices bring the field closer to powerful, wearable electronics, he says.