The optimization of conductance change behavior in synaptic devices based on analog resistive memory is studied for the use in neuromorphic systems. Resistive memory based on Pr 1−x Ca x MnO 3 (PCMO) is applied to a neural network application (classification of Modified National Institute of Standards and Technology handwritten digits using a multilayer perceptron trained with backpropagation) under a wide variety of simulated conductance change behaviors. Linear and symmetric conductance changes (e.g., self-similar response during both increasing and decreasing device conductance) are shown to offer the highest classification accuracies. Further improvements can be obtained using nonidentical training pulses, at the cost of requiring measurement of individual conductance during training. Such a system can be expected to achieve, with our existing PCMO-based synaptic devices, a generalization accuracy on a previously-unseen test set of 90.55%. These results are promising for hardware demonstration of high neuromorphic accuracies using existing synaptic devices.Index Terms-Resistive random-access memory (ReRAM), memristor, long-term potentiation (LTP), long-term depression (LTD), hardware neural network (HNN), bio-inspired system.

Since the advent of dye-sensitized solar cells (DSCs), which have achieved $11% of power conversion efficiency (PCE) in TiO 2 -based photoelectrodes, a lot of efforts have been devoted to make low-cost, light-weight, high-performance photovoltaic devices. [1][2][3] Nanostructured metal oxides are one of key factors in determining the PCE of DSCs, because the nanostructured networks provide a huge surface area to accommodate a large quantity of dye molecules that relate to the light harvesting of a photoelectrode in DSCs.ZnO is a good alternative of TiO 2 because it has a similar band gap but higher electron mobility than TiO 2 . [4][5][6][7] The mobility of ZnO is about 115-155 cm 2 V À1 s À1, much higher than that of TiO 2 , $10 À5 cm 2 V À1 s À1. Recently, DSCs with photoelectrodes made of submicrometer-sized aggregates of ZnO nanocrystallites demonstrated a PCE of 5.4% due to much enhanced light scattering without compromising the surface area for dye molecule adsorption. [8][9][10] A porous structured ZnO aggregates of nanocrystallites were thought to be helpful to retain their high surface area. Although this PCE is still lower than that of TiO 2 DSCs, it doubled the PCE of ZnO nanocrystallite DSCs.Atomic layer deposition (ALD) has been used to introduce extremely thin and conformal coating due to its unique self-limiting nature and low growth temperature; lots of semiconductor materials like TiO 2 , ZnO, SnO, and Al 2 O 3 can be grown by ALD. [11][12][13] In this study, we utilized ALD to deposit ultrathin TiO 2 layer on the porous structure of ZnO aggregates and demonstrated much enhanced PCE of ZnO DSC with photoelectrodes made of submicrometer-sized aggregates of ZnO nanocrystallites.As illustrated schematically in Figure 1a-c, TiO 2 ultrathin layer deposited by ALD would form a complete and conformal coverage on the surface and even inside pores of ZnO that would otherwise be exposed to dye electrolyte during the dye loading. Consequently, all the dye molecules would adsorb onto the surface of TiO 2 coating. Such an ultrathin and conformal ALD coating would not change the morphology the underline ZnO structures as shown in Figure 1e and 1f. The coating of TiO 2 layer on the surface of ZnO by ALD is presumably so thin that would not affect any detectable change in the morphology by means of scanning electron microscopy (SEM). Brunauer Emmett Teller (BET) results demonstrate that micropores inside each aggregate still remain after ALD, indicating that the porous structure of ZnO is preserved. As shown in Table 1, the slight decrease in the size and volume of the micropore was observed due to the introduction of ALD-TiO 2 layer. In addition, the connections between adjacent ZnO nanocrystallites would retain to ensure a favorable electron motion through ZnO (as suggested in Fig. 1d). Such structure would improve the surface stability with enhanced dye loading on the ZnO surface, while retains the advantage of high electron mobility in ZnO.It is reported that the growth rate of TiO 2 at the substrate tempera...

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V2O5 xerogel films were fabricated by casting V2O5 sols onto fluorine-doped tin oxide (FTO) glass substrates and annealing at 300 °C for 3 h in different annealing atmospheres of air and nitrogen. Films prepared in different annealing conditions possess different grain sizes and crystallinity, while the vanadium ion oxidation state also varies, as identified by X-ray absorption spectroscopy. A nitrogen annealing atmosphere induces the presence of defects, such as V4+ ions, and associated oxygen vacancies. Thus, the presence of defects, whether on the film surface or in the bulk, can be controlled by using air and nitrogen annealing atmospheres in the proper order. Electrochemical impedance analyses reveal enhanced charge-transfer conductivity in films with more V4+ and oxygen vacancies on the film surface, that is, a film annealed, first, for 0.5 h in air and then for 2.5 h in nitrogen. Lithium-ion intercalation measurements show that, at a charge/discharge current density of 600 mA g−1, this film possesses a noticeably better lithium-ion storage capability than films without surface defects. This sample starts with an initial discharge capacity of 139 mA h g−1, and the capacity increases slowly to a maximum value of 156 mA h g−1 in the 15th cycle, followed by a mild capacity degradation in later cycles. After 50 cycles, the discharge capacity is still as high as 136 mA h g−1. A much improved lithium-ion intercalation capacity and cyclic stability are attributed to V4+ surface defects and associated oxygen vacancies introduced by N2 annealing.