Nitrogen-Doped Partially Reduced Graphene Oxide Rewritable Nonvolatile Memory

As memory materials, two-dimensional (2D) carbon materials such as graphene oxide (GO)-based materials have attracted attention due to a variety of advantageous attributes, including their solution-processability and their potential for highly scalable device fabrication for transistor-based memory and cross-bar memory arrays. In spite of this, the use of GO-based materials has been limited, primarily due to uncontrollable oxygen functional groups. To induce the stable memory effect by ionic charges of a negatively charged carboxylic acid group of partially reduced graphene oxide (PrGO), a positively charged pyridinium N that served as a counterion to the negatively charged carboxylic acid was carefully introduced on the PrGO framework. Partially reduced N-doped graphene oxide (PrGODMF) in dimethylformamide (DMF) behaved as a semiconducting nonvolatile memory material. Its optical energy band gap was 1.7–2.1 eV and contained a sp2 C═C framework with 45–50% oxygen-functionalized carbon density and 3% doped nitrogen atoms. In particular, rewritable nonvolatile memory characteristics were dependent on the proportion of pyridinum N, and as the proportion of pyridinium N atom decreased, the PrGODMF film lost memory behavior. Polarization of charged PrGODMF containing pyridinium N and carboxylic acid under an electric field produced N-doped PrGODMF memory effects that followed voltage-driven rewrite-read-erase-read processes.

chemically synthesized N-doped semiconducting PrGOs were produced using a mild reducing agent, the polar aprotic solvent DMF, and yielded different band gap values. Reaction time-dependent GO reduction with DMF resulted in a variety of optical band gaps from 2.55 eV (GO) to 1.35 eV (120 min PrGODMF). In two-terminal memory devices with the N-doped semiconducting PrGOs sandwiched between top and bottom metal electrodes, the current hysteresis exhibited two conducting states in current–voltage characteristics. However, for fully reduced GO with a low oxygen-functionalized carbon density (small amount of a negatively charged oxygen) and highly oxidized GO with a high oxygen-functionalized carbon density (no positively charged pyridinium), the current hysteresis was negligible. As carefully designed, the semiconducting PrGODMF devices containing sp2 C═C frameworks with 2.7–3.2% nitrogen atoms and an oxygen-functionalized carbon density of 45–50% (especially 1.5% pyridinium N atoms) clearly displayed rewritable nonvolatile memory behaviors, suggesting that novel N-doped PrGODMF could form a charge transporting path by associating with oxygen/nitrogen-functional groups such as Hδ+N-PrGO–COOδ−/Hδ+N-PrGO–COOδ− in interlayers of PrGODMF sheets. As a whole, the voltage-induced polarization of the film allowed the switching of an OFF state to an ON state, producing a rewritable nonvolatile memory.

Reference

Nitrogen-Doped Partially Reduced Graphene Oxide Rewritable Nonvolatile Memory
Sohyeon Seo, Yeoheung Yoon, Junghyun Lee, Younghun Park, and Hyoyoung Lee
ACS Nano Article ASAP