Anisotropic, Transparent Films with Aligned Cellulose Nanofibers.

Transparent films or substrates are ubiquitously used in photonics and optoelectronics, with glass and plastics as traditional choice of materials. Transparent films made of cellulose nanofibers are reported recently. However, all these films are isotropic in nature. This work, for the first time, reports a remarkably facile and effective approach to fabricating anisotropic transparent films directly from wood. The resulting films exhibit an array of exceptional optical and mechanical properties. The well-aligned cellulose nanofibers in natural wood are maintained during delignification, leading to an anisotropic film with high transparency (≈90% transmittance) and huge intensity ratio of transmitted light up to 350%. The anisotropic film with well-aligned cellulose nanofibers has a mechanical tensile strength of up to 350 MPa, nearly three times of that of a film with randomly distributed cellulose nanofibers. Atomistic mechanics modeling further reveals the dependence of the film mechanical properties on the alignment of cellulose nanofibers through the film thickness direction. This study also demonstrates guided liquid transport in a mesoporous, anisotropic wood film and its possible application in enabling new nanoelectronic devices. These unique and highly desirable properties of the anisotropic transparent film can potentially open up a range of green electronics and nanofluidics.