Journal Article 2D hafnium diselenide 2D materials 3C4Z4KG52T 81AH48963U Hafnium Molybdenum X71938L1DO artificial synapses bilayer resistive materials doping ferroelectric encoder hafnium oxide hafnium zirconium oxide image processing memristor multiply-and-accumulate operation neural network hardware rhenium disulfide spiking neural networks time-to-first-spike encoding scheme van der Waals transfer