Epitaxial Growth of Thin Ferroelectric Polymer Films on Graphene Layer for Fully Transparent and Flexible Nonvolatile Memory

Abstract

Enhancing the device performance of organic memory devices while providing high optical transparency and mechanical flexibility requires an optimized combination of functional materials and smart device architecture design. However, it remains a great challenge to realize fully functional transparent and mechanically durable nonvolatile memory because of the limitations of conventional rigid, opaque metal electrodes. Here, we demonstrate ferroelectric nonvolatile memory devices that use graphene electrodes as the epitaxial growth substrate for crystalline poly(vinylidene fluoride-trifluoroethylene) (PVDF-TrFE) polymer. The strong crystallographic interaction between PVDF-TrFE and graphene results in the orientation of the crystals with distinct symmetry, which is favorable for polarization switching upon the electric field. The epitaxial growth of PVDF-TrFE on a graphene layer thus provides excellent ferroelectric performance with high remnant polarization in metal/ferroelectric polymer/metal devices. Furthermore, a fully transparent and flexible array of ferroelectric field effect transistors was successfully realized by adopting transparent poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] semiconducting polymer.

Aligning in-plane polarization multiplies piezoresponse in P(VDF-TrFE) films on graphite

“…The (100) orientation is sketched in figure 1(a), left panel; we note that graphene and the graphite substrate applied in our work have identical surface lattices. The epitaxial in-plane orientation of the lamellae, with chain axes along the 10 ̲ 10 direction of graphene, is lost in films with a thickness beyond few nm [1]. In agreement with that, our film shows some lamellae with preferred orientations resulting from epitaxy on the HOPG substrate (directions marked by yellow lines in figure 1(b)), but most lamellae are not well aligned to the hexagonal substrate lattice.…”

“…The polarization is oriented along the b direction according to [12] (figure 1(a)). Kim et al reported on an epitaxial orientation of P(VDF-TrFE) on graphene where the polymer film has a (100) out-of-plane orientation [1]. The (100) orientation is sketched in figure 1(a), left panel; we note that graphene and the graphite substrate applied in our work have identical surface lattices.…”

“…Ferroelectric polymers are attractive candidates for functional layers in electronic devices like non-volatile memories [1][2][3][4][5][6], piezoelectric and magnetoelectric sensors [7,8], energy harvesters [9,10], and capacitor-based energy storage devices [11]. They offer substantial magnitudes of piezoelectricity, dielectric permittivity, ferroelectric polarization and low leakage currents combined with mechanical flexibility, optical transparence, and inexpensive near-room-temperature processing.…”

“…Advances have been made for the often applied ferroelectric polymers like poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)) in recent years [1,14,15], but fully oriented epitaxial growth of films thicker than few nm is not yet achieved. Some applied substrates like, e.g., graphite possess high symmetry resulting in several allowed epitaxial orientations of the polymer film [1,16]. Alternatively, crystallographic alignment induced by mechanical action of a force microscopy tip in similarity to a nanoscopic drawing process has been suggested [17].…”

Totally embedded hybrid thin films of carbon nanotubes and silver nanowires as flat homogenous flexible transparent conductors

“…Transparent conductive films are important components in numerous applications such as solar cells, display technologies, and lighting1234567. Indium Tin Oxide (ITO), which has high transparency and low sheet resistance, is currently the most commonly used transparent conductor material8.…”