Negative differential resistance in polymeric memory devices containing disordered block copolymers with semiconducting block

Previously, localized conducting filaments have been suggested to explain the memory effects in metal-polymer-metal stacks, and nanoparticles and fullerenes have been dispersed in polymers to promote filament electroformation under the local electric fields modified by them. Therein, however, the aggregation tendency of the dispersed additives can pose challenges for reproducibility and scalability. In this work, a triblock copolymer polystyrene-block-poly(9,9-di-n-hexyl-2,7-fluorene)-block-polystyrene (PS-b-PDHF-b-PS) with a semiconductor middle block was investigated and compared to pure insulating polystyrene (PS) as the polymer layer. The shortness of the PDHF block (2900 g/mol) in comparison to the PS blocks (17100 g/mol) leads to the disordered state, which contains PDHF chains (radius of gyration R_g^PDHF ≈ 2 nm) separated by the PS chains (radius of gyration R_g^PS ≈ 4 nm). As a layer between two Al electrodes in devices of different sizes, both polymers gave similar negative differential resistance (NDR) and scalability, indicating that the homogenously distributed nanometer-sized semiconducting chains in the insulating matrix did not promote uniformity of filament formation for NDR. An additional plasma etching suppressed the NDR, confirming the importance of electrode interface engineering to control the filament growth.