Optically Transparent Nanofiber Paper

M Optically Transparent Nanofiber Paper M U N IC By Masaya Nogi,* Shinichiro Iwamoto, Antonio Norio Nakagaito, and Hiroyuki Yano A T IO N Glass has well-suited low thermal expansion for use in electronic devices, but it is fragile, and the search for a stronger, more flexible optically clear medium has gone on for many years. Plastics have been widely studied; however, most of them have large coefficients of thermal expansion (CTE, approx. 50 ppm K ), and foldable plastics in particular exhibit extremely large CTEs, in excess of 200 ppm K . Further, the functional materials deposited on plastic substrates are prone to be damaged by the temperatures involved in the assembly and mounting processes due to themismatch between CTEs from differentmaterials. This article reports on what might be best described as optically transparent paper. It is a foldable nanofiber material with low thermal expansion (CTE <8.5 ppm K ) prepared using 15 nm cellulose nanofibers with the same chemical constituents as conventional paper and a production process also similar to that of conventional paper. The only difference is in the fiber width and the size of the interstitial cavities (Fig. 1). The foldable, low-CTE, and optically transparent nanofiber paper is the perfect candidate for substrates for continuous roll-to-roll processing in the future production of electronic devices, such as flexible displays, solar cells, e-papers, and a myriad of new flexible circuit technologies, and could replace the costly conventional batch processes based on glass substrates currently used. We project that it will also replace conventional paper as an advanced information medium, which can still be produced using traditional paper-making equipment that is used in production today. Cellulose nanofibers are the main component of plant and wood pulp fibers. These tiny elements with diameters of 15–20 nm are composed of bundles of cellulose microfibrils smaller than 4 nm in width, which, in turn, are composed of long cellulose molecules laterally stabilized by hydrogen bonds forming highly crystalline domains. As such, cellulose nanofibers have a CTE of 0.1 ppm K , which is as low as that of quartz glass, and an estimated strength of 2–3 GPa, rendering it five times stronger than mild steel. The nanofibers also exhibit good heat-transfer properties comparable to glass. Another significant property of the nanofibers is that light scattering can be suppressed. If the cellulose nanofibers are densely packed, and the interstices between the fibers are small enough to avoid