(color online) CNT-based ultrahigh density RRAM.^[5,45] (a), (b) Fabrication process of CNT-based RRAM with a metal/a-C/CNT structure (a-C: amorphous carbon) and the corresponding switching characteristics. (c), (d) Fabrication process of CNT-based RRAM with a metal/a-C/CNT/a-C/metal structure and the corresponding switching characteristics. (e), (f) Fabrication process of CNT-based RRAM with a CNT/a-C/CNT structure and the corresponding switching characteristics.