Fig. 1. (Color online) The structure and characterization of the Au/WO_x/Ti memristive device. (a) Schematic illustration of the biological synapse connecting pre-synaptic and post-synaptic neurons. (b) Schematic illustration of the device, including Au top electrodes, WO_x films and Ti bottom electrodes. (c) An overhead view of the device obtained by an optical microscope. (d) A cross-sectional SEM image of the device.

Fig. 2. (Color online) Demonstration of spike-intensity-dependent synaptic plasticity in a Au/WO_x/Ti memristive device. (a, b) I–V characteristics of the device obtained under positive/negative bias; the voltage sweep range was 0 to 2 V (−2 V) then back to 0 V. (c) Schematic diagram of PPF measurement. (d) The variation of PPF according to relative spike timing.

Fig. 3. (Color online) The transition from STP to LTP by adjusting spike intensity. (a) The device received input stimuli with different features, including a spike train with an amplitude of 0.2 V, weak stimuli with an amplitude of 1.5 V, and strong stimuli with an amplitude of 3 V. (b) Memorization of the image “T” to demonstrate the transition from STM to LTM. Case 1: the conductance of the memristive array before stimulation, after 1.5-V stimulation, and after stimulation for 60 s; Case 2: the conductance of the memristive array before stimulation, after 3-V stimulation, and after stimulation for 60 s. The different color levels represent different magnitude conductance values.

Fig. 4. (Color online) Demonstration of image Boolean intersection operation in the memristive synapse array. (a) The stimulation condition for inputting the images “X” and “Y”. (b) The conductance states of the devices under different inputting conditions.

Fig. 5. (Color online) Demonstration of the image. (a) Boolean subtraction and (b) Boolean union operations in the memristive synapse array.