Electric Field Induced Birefringence in Isotropic Suspensions of Nanoplates

Abstract

Isotropic suspensions of colloidal nanoplates are of potential use in electronic displays. Electric field induced-birefringence and the response time of platelets are the two most relevant important properties for display applications. The Kerr coefficient quantifies how high is the field induced-birefringence for a given strength of electric field. The higher the Kerr coefficient, the lower is the required voltage for switching the pixels ON. Therefore, the battery consumption would be lower for high Kerr coefficient platelets. While graphene oxide platelets have shown a potential to be used in displays due to very high Kerr coefficient and very high diameter-to-thickness ratio, a systematic study on the variation of this ratio to study its effect on Kerr coefficient has not yet been done. Very dilute aqueous suspensions of 2D nanoplates of ?-zirconium phosphate (?-Zr(HPOv4) abbreviated as ?-ZrP) in isotropic phases were tested for electric field induced-birefringence. Different reaction conditions of synthesizing pristine ?-ZrP disks in hydrothermal reactor allowed controlled variation of disk sizes. The ?-ZrP disks were exfoliated in aqueous medium using tetra-(n)-butyl hydroxide (Buv4NOH). After exfoliation, nanoplates with uniform thickness (~3 nm) and diameter-to-thickness ratios of 300 to 900 were obtained and then tested for electric field induced-birefringence. The systematic variation in aspect ratio allowed us to study the dependence of Kerr coefficient on nanoplate diameter-to-thickness ratio. The obtained Kerr coefficient of ZrP nanoplates is higher than other platelet systems such as gibbsite, beidellite, but lower than graphene oxide. Due to the shape anisotropy of 2D materials, the electrical polarizability in the plane of this 2D material (??v?) and in the direction perpendicular to the plane (???) are different. The anisotropy in electric polarizability, defined as ???=??v????v?, induces birefringence in isotropic samples. The ?? value was measured for two different sizes of nanoplates and compared with those predicted by Maxwell-Wagner-O’Konski. By comparing the field interaction parameter of nanoplates for electric and magnetic fields, it turned out that a Tesla of magnetic field and a volt per millimeter of electric field have the same effect in terms of induced-birefringence. Hence, it can be concluded that electric field can easily induce-birefringence in isotropic suspensions of nanoplates as compared to magnetic field.