Methyl Methacrylate Copolymer with Pendant Thioxanthenone Groups as Active Layer for Resistive Memory Devices

An electro-active copolymer of methyl methacrylate and 2-((4-acroylpiperazine-1-yl)methyl)-9H-thioxanthene-9-one (poly(MMA-co-ThS)) was synthesized by radical polymerization. The copolymer has good solubility in most organic solvents, thermal stability up to 282 °C and excellent ability to form thin films on silicon wafers. Poly(MMA-co-ThS) films exhibited an electrochemical and electrochromic activity resulting in the formation of long-lived radical anion states of pendant thioxanthone groups inside the film. These states exhibit optical transitions in the visible region as a broad optical absorption band, 500<λ<900 nm (1.38opt(ThS)<2.48 eV) with a maximum at λ_max=675 nm (1.84 eV). Using temperature measurements of the current-voltage characteristics of p-Si(100)/poly(MMA-co-ThS)/Al devices, it was shown that the charge transport in the film occurs by a multiphonon mechanism, which is quantitatively described by the Nasyrov-Gritsenko model of phonon-assisted tunneling between traps. The value of the optical transition energy of the trap, determined by the Nasyrov-Gritsenko model, W_opt=1.8 eV, is in a good agreement with W_opt(ThS), confirming the nature of the traps as 9H-thioxanthen-9-one structures. The n⁺⁺Si(100)/poly(MMA-co-ThS)/Al memory device exhibited a memristive effect (reversible ON/OFF switching of the device) with an initial "forming" cycle followed by repetitive memory cycles characterized by bipolar switching.