A memristor is a two-terminal 'memory resistor' electronic device, in which a metal oxide switching layer is sandwiched between two metal electrodes [1][2][3][4]. In general, memristors offer non-linear switching characteristics, and materials and process compatibility with advanced silicon manufacturing. These attributes have spurred the exploration of memristors as synaptic devices for realizing spike-based hardware learning systems that are capable of processing unstructured, temporal data [5][6][7][8][9][10]. However, for memristor-based technologies to be viable, the device should exhibit several key characteristics. It should have a compact nanoscale footprint, operate at a voltage close to 1V that is compatible with complementary metal oxide semiconductor (CMOS) technology, have reproducible electrical characteristics, and possess high switching speed to minimize the energy consumption [11]. Furthermore, the hardware integration of synaptic connections in advanced neural networks requires memristors with multiple resistive states [12,13]. These are challenging requirements and are difficult to implement without significant innovations.The phenomenological principle of memristor device operation is based on the change in the physical properties of a conductive filament (associated with the presence of oxygen vacancies) by applying an electric field across the metal oxide switching layer [14][15][16]. The resulting motion of the oxygen vacancies alters the device resistance between low (Set) and high (Reset) states, depending on the direction and the amplitude of the electric field. So far, a variety of structures from a large set of materials (various metal oxide switching layers and metal electrodes) have been studied in the literature [4,17,18]. Several key findings can be drawn from those studies regarding the performance, energy and scalability of this type of devices. The most important finding reveals the trade-off between the switching energy and the data retention time-that is often referred to as voltage-time dilemma [19]. This trade-off is associated with the energy barrier of the device structure. For example, devices made of metal oxides with small energy bandgap (E g ), such as titanium oxide (TiO x , E g~3 .4eV), generally exhibit low operating voltage and compromised data retention [20], while those with large bandgap, such as hafnium oxide (HfO x , E g~5 .4eV) demonstrate the opposite [21]. However, the fabrication of devices with bilayer switching stacks has shown to be effective in mitigating this trade-off. In particular, the improvement in data retention was obtained by the incorporation of an ultra-thin wide bandgap metal oxide capping layer (for example aluminum oxide) [22]. On the other hand, the addition of a reactive capping metal (for example titanium, hafnium, etc.) as an oxygen scavenging layer provided a pathway for reducing the operating voltage of the devices [23,24]. Despite significant advances, a sub-1V memristive device that simultaneously affords built-in analog behavior...