Atomic scale nanoelectronics for quantum neuromorphic devices: comparing different materials

I review the advancements of atomic scale nanoelectronics towards quantum neuromorphics. First, I summarize the key properties of elementary combinations of few neurons, namely long-- and short--term plasticity, spike-timing dependent plasticity (associative plasticity), quantumness and stochastic effects, and their potential computational employment. Next, I review several atomic scale device technologies developed to control electron transport at the atomic level, including single atom implantation for atomic arrays and CMOS quantum dots, single atom memories, Ag$_2$S and Cu$_2$S atomic switches, hafnium based RRAMs, organic material based transistors, Ge$_2$Sb$_2$Te$_5$ synapses. Each material/method proved successful in achieving some of the properties observed in real neurons. I compare the different methods towards the creation of a new generation of naturally inspired and biophysically meaningful artificial neurons, in order to replace the rigid CMOS based neuromorphic hardware. The most challenging aspect to address appears to obtain both the stochastic/quantum behavior and the associative plasticity, which are currently observed only below and above 20 nm length scale respectively, by employing the same material.