Magnetic Field Alignment Of Graphene Flakes:Properties And Applications

Graphene has attracted enormous interest in the last decade because of its excellent thermal,optical,electrical,mechanical and magnetic properties for a wide range of potential applications,such as gas sensors,integrated circuits,graphene transistors,thermal conductor,transparent electrode and energy storage materials.Currently,the graphene produced enormously in the industry is graphene nanosheets fabricated by solution exfoliation of graphite.Because the intrinsic properties of graphene are anisotropic,only when graphene flakes are assembled in the same direction,the unique thermal,optical,mechanical and electrical properties of a single graphene sheet can be fully expended to the whole device.The previous research indicated that the electric and magnetic field can provide flexible and non-contact control techniques for lowdimensional materials.Electric field alignment of graphene has been demonstrated not very effective.Although magnetic field alignment of graphene oxide sheets has been realized by coating with iron oxide nanoparticles,the magnetic alignment of graphene flakes with its intrinsic property remains a challenge.In order to overcome this challenge,we made theory analysis and simulation about the orientation control of diamagnetic graphene flakes with a magnetic field.The graphene flakes were prepared by solution exfoliation of graphite and then aligned by the magnetic field in the experiment.The anisotropic optical properties of aligned graphene were studied and some optical devices were fabricated as the application examples.(1)We theoretically investigated the stable levitation and orientation control of graphene with a magnetic field.The stable levitation conditions of different shapes graphene laminate above four interleaved magnets were studied by simulation.It was found that only regular polygon shapes with even numbers of sides and rounded shape graphene can be stably levitated.In addition,the orientation control of micro-sized graphene flakes above a permanent magnet is simulated in both air and liquid environments.Graphene flakes experience magnetic torques and turn to a tilted angle in the magnetic field.The magnetic torque strength and direction can be tuned by controlling the distance and the relative position between the magnets and graphene flakes.The tilted angle is influenced by magnetic torque,sample gravity and resistance of liquid.The understanding of orientation control of graphene flake is essential to align graphene sheets in experiment.(2)We demonstrated the orientation control of graphene flakes by a static magnetic field of common commercial magnets.The graphene dispersion exhibited macroscopic anisotropy in optical properties.Subsequently,a magnetic field sensor and graphene display were fabricated as two examples of related device applications.The magnetic alignment of graphene relies on its intrinsic strong diamagnetic susceptibility.Because of the magnetic response and anisotropic optical reflection,a liquid suspension of graphene flakes can be used as a magnetic field sensor with sensitivity and spatial resolution higher than traditional iron filings or particles.The graphene suspension is then packaged in a glass cell as a writing and/or display board which can be controlled by magnets or magnetic field.What's more,when it works as a reflective display,no external back illumination and optical polarizers are needed.(3)By using a rotating magnetic field produced by a pair of small NdFeB magnets,we realized planar alignment of graphene dispersion,namely all graphene flakes oriented along one direction and parallel to each other.The graphene dispersion aligned planarly exhibited further improved optical anisotropy and potential novel device applications.Compared with partially aligned sheets by a static magnetic field,planarly aligned graphene suspensions exhibit nearly perfect alignment order parameter,a much higher birefringence,order parameter,and anisotropic absorption/transmission.By mixing graphene and Ultra-Violet(UV)cured resin,the alignment of graphene can be fixed after UV illumination.With the assistance of a mask,we can align and fix graphene in UV polymer with a special pattern and prepare an all-graphene permanent display film.This display film shows complimentary transmission/reflection patterns with a very wide viewing angle and high bright-dark contrast.The ability to align and pattern graphene orientation in three dimensions opens up further exploration and wider device applications of graphene.Magnetic alignment of graphene by intrinsic diamagnetism promotes fundamental properties research of aligned graphene flakes.The macroscopic control and alignment of graphene transfer unique properties of graphene from microscopic to the macroscopic scale,making it possible to fabricate graphene in large size devices.What's more,our demonstration opens door to enormous graphene device applications such as transparent electrode,electromagnetic interference shielding,thermal conductor,mechanical reinforcement et al.