Simulated annealing, spin-coating, and poling: in silico fabrication of ferroelectric polyvinylidene fluoride polymers on graphene as a model of a low-energy-consumption switching device

Abstract

Ferroelectric polyvinylidene fluoride (PVDF), particularly its β-phase crystals with well-aligned all-trans polymer chains and prominent polarization in a composite with other organic or inorganic materials, has attracted great attention in various areas of energy harvesting, storage, and saving. However, since the β phase is not the most stable polymorph of PVDF, a simple solution casting at a low temperature produces a PVDF film with a limited β-phase content, a low crystallinity, and/or a high porosity. Additional thermal, mechanical, and electrical controls such as annealing, stretching, spin-coating, and poling are required to maximize both the β-phase content and crystallinity of the PVDF film. Herein, the amorphous-to-β-phase crystallization achieved by such processes are mimicked in silico at the molecular level, revealing the effect of each process on the quality of the processed film. The content of β-phase crystal, which is negligible after the simulated annealing beyond the melting temperature (300 K to 500 K and back to 300 K), increases to 80% after the SLLOD simulations at a shear velocity of 5.5 m/s (i.e., by the simulated spin coating of approximately 3000-5000 rpm) and increases further to 100 % when combined with a high electric field of 0.18 GV/m (i.e., by the simulated electric poling). The perfectly polarized dipole moments of such β-phase PVDF thin films, when deposited on graphene, can induce electrostatic doping (i.e., create charge carriers) in the underlying graphene, even at zero electric field, resolving the zero-bandgap (i.e., no-OFF-state) issues of graphene while maintaining its high carrier mobility and low-power operation. Indeed, the current-voltage (I-V) curves mimicked by non-equilibrium Green’s function calculations on a model device of field-effect transistor show a modulation of the doping level and in turn the conductance of graphene, virtually achieving an I_ON/I_OFF ratio of up to 20, when the orientation of the PVDF polarization was flipped by a bias gate voltage sweep. We exvision that such devices can eventually lead to low-power-consumption high-ON/OFF-ratio graphene-channel field-effect transistors and non-volatile memories.