Role of Annealing with Electric Field Toward Improvement of Ferroelectric and Electroactive Properties of PVDF Copolymer and Terpolymer Thin Films.

The present study elucidates the role of annealing with electric field on lamellar crystalline structure and molecular orientation of polymer chains in ferroelectric copolymer (P(VDF-TrFE)) and ferroelectric terpolymer (P(VDF-TrFE-CFE)) spin-coated thin films. The ferroelectric polymer thin films annealed under an electric field support the growth of nanostructure with an "edge-on" lamellar crystalline structure having in-plane molecular chain orientation. The poled P(VDF-TrFE) thin films have higher remnant polarization (Pr) ≈6.2 µC cm-2 and saturation polarization (Ps) ≈8.2 µC cm-2 at an applied electric field of 250 MV/m compared to unpoled thin films having Pr ≈4.7 and Ps ≈6.2 µC cm-2. Also, poled P(VDF-TrFE) thin films show lower coercive field (Ec) ≈94 MV/m compared to an unpoled thin film having Ec ≈105 MV/m. Similarly, poled PVDF-TrFE-CFE thin film shows better ferroelectric properties having Pr ≈0.4 and Ps ≈5.7 µC cm-2 at an applied electric field of 200 MV m-1 compared to unpoled thin films having Pr ≈0.4 and Ps ≈4.1 µC cm-2. The storage energy efficiency of unpoled and poled P(VDF-TrFE-CFE) thin films is measured to be ≈75% and 80%. Annealing of ferroelectric P(VDF-TrFE) polymer thin films under an electric field demonstrates improved ferroelectric and electroactive properties.

Interfacial Interaction of Absorbate Copper Phthalocyanine with PVDF Based Ferroelectric Polymer Substrates: A Spectroscopic Study.

Studies of CuPc thin films on underlying ferroelectric copolymeric and terpolymeric substrates have been performed by ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. Work function (WF) and highest occupied molecular orbital (HOMO) energy level shift observed from UPS spectroscopy for successive deposition of CuPc molecules on ferroelectric polymer surfaces confirm the formation of interface dipole at the CuPc-ferroelectric polymer interface owing to charge transfer from the tailing region of the CuPc HOMO density of states (DOS) to the ferroelectric polymer layer. According to our thickness dependent XPS data, CuPc molecules are coupled to the organic ferroelectric surfaces through the central metal atom of the CuPc molecules, i.e., copper atom, and the halogens of underlying ferroelectric polymer surfaces, and hence support the charge transfer phenomenon from CuPc molecules to the ferroelectric polymer substrate. Polarization dependent NEXAFS results reveal that CuPc molecules retain their tilted geometrical configuration even at submonolayer thickness of the molecular films on both ferroelectric surfaces and confirms the electronic structural disturbance associated with structural modification of CuPc molecules due to interfacial charge transfer. Therefore, the energy level alignment with increment in the thickness of CuPc films at both the organic semiconductor-ferroelectric polymer interface is controlled by the charge transfer phenomenon from deposited CuPc molecules to the organic ferroelectric substrates. Our results provide a clear understanding about chemical interactions, molecular configurations, energy level alignment, and their correlation at CuPc/polymeric ferroelectric interfaces that can be important for organic nonvolatile memory and synaptic based thin-film transistor devices.