Conjugated Polymer-Functionalized 2D MXene Nanosheets for Nonvolatile Memory Devices with High Environmental Stability

The major disadvantages of MXene with abundant low-valence metal species are its poor environmental stability and poor solution processability. The best way to address these problems is to covalently functionalize MXene with highly soluble conjugated polymers, which can not only significantly enhance the environmental stability and solubility of MXene but also further expand its application scope in optoelectronic and electronic devices. By using 4-bromobenzoyl-functionalized MXene (Ti3C2TX) as a key two-dimensional template, poly[(9,9-dihexyl-9H-fluorene)-alt-(1,4-diethynylbenzene)] (PDFD) polymer chains were grown directly from the MXene surface via surface-directing Sonogashira–Hagihara polymerization. The achieved PDFD–MXene is highly soluble in common organic solvents. As expected, the as-fabricated Al/PDFD–MXene/ITO device showed bistable electrical switching and nonvolatile resistive random access memory (RRAM) performance. After annealing at 150 °C in N2, the observed ON/OFF current ratio and switch-on voltage changed from 9.4 × 103 and 0.90 V before annealing to 7.16 × 104 and 0.46 V after annealing, respectively. In contrast to PDFD–MXene, their blends showed a poor RRAM performance. The achieved ON/OFF current ratio and switch-on voltage were 4.4 × 102 and 1.12 V, respectively. After exposed to damp air, the PDFD–MXene-based device displayed outstanding environmental stability and could work well for at least 60 days at a humidity of 50% and a temperature of 30 °C. Unlike PDFD–MXene, their blends-based memory device showed a very unstable performance with increasing relative humidity.