Bistable Organic Memory Device with Gold Nanoparticles Embedded in a Conducting Poly(<i>N</i>-vinylcarbazole) Colloids Hybrid

Abstract

We report on the nonvolatile memory characteristics of a bistable organic memory (BOM) device with Au nanoparticles (NPs) embedded in a conducting poly(N-vinylcarbazole) (PVK) colloids hybrid layer deposited on flexible poly(ethyleneterephthalate) (PET) substrates. Transmission electron microscopy (TEM) images show the Au nanoparticles distributed isotropically around the surface of a PVK colloid. The average induced charge on Au nanoparticles, estimated using the C−V hysteresis curve, was large, as much as 5 holes/NP at a sweeping voltage of ±3 V. The maximum ON/OFF ratio of the current bistability in the BOM devices was as large as 1 × 105. The cycling endurance tests of the ON/OFF switching exhibited a high endurance of above 1.5 × 105 cycles, and a high ON/OFF ratio of ∼105 could be achieved consistently even after quite a long retention time of more than 1 × 106 s. To clarify the memory mechanism of the hole-mediated bistable organic memory device, the interactions between Au nanoparticles and poly(N-vinylcarbazole) colloids was studied by estimating the density of states and projected density of state calculations using density functional theory. Au atom interactions with a PVK unit decreased the band gap by 2.96 eV with the new induced gap states at 5.11 eV (HOMO, E0) and LUMO 4.30 eV and relaxed the HOMO level by 0.5 eV (E1). E1 at ∼6.2 eV is very close to the pristine HOMO, and thus the trapped hole in E1 could move to the HOMO of pristine PVK. From the experimental data and theoretical calculation, it was revealed that a low-conductivity state resulted from a hole trapping at Eo and E1 states and subsequent hole transportation through Fowler−Nordheim tunneling from E1 state to Au NPs and/or interface trap states leads to a high conductivity state.