In a previous report, we showed that by confining the filament formation within a thin Talayer in a Ta/TaObilayer structure, endurance was improved and random distributions were suppressed. (22) Meanwhile, others have reported on the activity of negative oxygen ions (O) modifying deposited microstructures through affecting adatom surface mobility during dc magnetron-sputtering processes. (28) Here, we incorporated oxygen gas during deposition of noble metal top electrodes (TEs) to reduce the roughness of the TE/oxide interface to further control resistance switching. A clear transitioning of the Talayer from a resistive to conducting state is thoroughly investigated through a transmission electron microscopy (TEM) study, showing the suppression of any ECM phenomenon and demonstrating nearly pure VCM resistance switching mechanisms. We were able to achieve a high endurance multistate device by using an Ir electrode, chosen for its redox potential, which is known to play an important role during VCM switching. (1,2,20)

Recently, breakthroughs into the neural biologic mechanisms have brought us closer to a realizable model for the human brain. (6) However, even some of the most basic biological components have been impossible or unwieldy to mimic using silicon chips. (5,23,24) The drive toward neuromorphic computing has led to the importance of multivalued conduction (MVC) states in memristor materials, (8,18,25) the final goal being an almost continuous number of stable, distinguishable, and addressable states. However, the filamentary nature of conduction channels in memristors makes it extremely challenging to fabricate structures which show stable MVC while also having sufficient endurance ( Figure S1 ). The major failure mechanism during endurance cycling is related to the formation of a thick filament path which can no longer be reversibly ruptured during normal operation. Meanwhile, MVC states should occur through either the control of filament thickness or number. Because of the continuously changing conduction paths during nanoscale switching, (26,27) it has been challenging to achieve both high endurance and MVC states.

Resistance-change transition metal oxides (TMOs) have been actively investigated for technological application in traditional devices such as memory (1,2) and logic (3,4) while also being intensively studied for novel concepts such as neuromorphic devices. (5−8) TMOs demonstrate resistance switching across a wide variety of oxide materials, (9,10) electrode materials, (11,12) deposition conditions, film thickness, (13) bilayered structures, (14,15) buffering layers, (16) and programming conditions. (17−19) Generally, resistance-changing TMOs are found to be either an electrochemical metallization memory (ECM) or valence change memory (VCM) (1,2,20) depending on the nanoscale mechanism. For some oxide materials such as tantalum oxide (TaO) investigated here, the actual mechanism is some mixture of both ECM and VCM. (21) Despite complications from two competing resistance switching mechanisms in TaO, it has previously been shown to be a robust candidate for semiconductor memory applications. (17,22)

Results and Discussion ARTICLE SECTIONS Jump To

2 O 5–x (5 nm)/TaO 2–x (15 nm) bilayer memristors, we fabricated three samples. All samples were deposited on Pt bottom electrodes (BEs) with a thin 2 nm Al 2 O 3 buffer layer deposited by atomic layer deposition placed in between the BE and TaO x layers to act as a diffusion barrier, as described by other researchers. 2 O 3 layer acted to stabilize cycle to cycle variation and otherwise did not contribute significantly to the overall device resistance or switching properties as also reported. 2 O 5–x and the respective TEs. The sample in 2 O 5–x layer. An amorphous Pt region bounded by the red-dotted line and indicated by the blue arrow was found for the sample deposited at 100% argon. Diffraction spectra showing that metallic Ir as opposed to IrO 2 were deposited despite the oxygen present during sputtering is included in the To investigate the effects of electrode material and deposition conditions on resistance switching in Ta(5 nm)/TaO(15 nm) bilayer memristors, we fabricated three samples. All samples were deposited on Pt bottom electrodes (BEs) with a thin 2 nm Albuffer layer deposited by atomic layer deposition placed in between the BE and TaOlayers to act as a diffusion barrier, as described by other researchers. (29) The Allayer acted to stabilize cycle to cycle variation and otherwise did not contribute significantly to the overall device resistance or switching properties as also reported. (30) Cross-sectional TEM images of each respective sample are shown in Figure 1 a–c. The TEM images show the interface between Taand the respective TEs. The sample in Figure 1 a used a sputtered Pt electrode (Pt) using 100% argon gas, the sample in Figure 1 b used a Pt electrode sputtered with 2% oxygen and 98% argon gas mixture for a reduced interface roughness (RIR-Pt), and the sample in Figure 1 c was an Ir electrode sputtered with 2% oxygen and 98% argon gas (RIR-Ir). We calculated peak-to-valley differences by measuring the average maximum observed peak to valley heights across four positions in two separate samples. Pt has a difference of 3.75 ± 1 nm, RIR-Pt with a difference of 2.19 ± 1 nm, and RIR-Ir with a difference of 2.06 ± 1 nm, generally, demonstrate the RIR through the incorporation of 2% oxygen during TE sputter deposition. The larger magnified images in each respective figure show the boundary between the crystalline noble metal electrodes and Talayer. An amorphous Pt region bounded by the red-dotted line and indicated by the blue arrow was found for the sample deposited at 100% argon. Diffraction spectra showing that metallic Ir as opposed to IrOwere deposited despite the oxygen present during sputtering is included in the Figure S2

Figure 1 Figure 1. Electrode effect on resistance switching. TEM cross-section images of samples with (a) Pt, (b) RIR-Pt, and (c) RIR-Ir electrodes. The interface is magnified to detail in the changing interface roughness. Electrical resistance switching dc I–V sweeps for samples with (d) Pt, (e) RIR-Pt, and (f) RIR-Ir electrodes. The inset (red line) shows the marked differences between switching behavior depending on electrode type and deposition condition. Schematic drawing describing the onset from (g) ECM to (h) ECM + VCM to (i) mainly VCM-type switching depending on the electrode type.

I–V) sweep measurements were performed to demonstrate electrical resistance switching across all three samples, as shown in 2 , x memristors were enhanced.I–V curve for RIR-Ir in Electrical dc current–voltage () sweep measurements were performed to demonstrate electrical resistance switching across all three samples, as shown in Figure 1 d–f. After an initial electroforming (31) step shown by the red curves in each respective graph, subsequent application of an appropriate external voltage bias could switch each sample from a high resistance state (HRS) to a low resistance state (LRS) at approximately −1 V and vice versa at approximately +2 V. Details on the dependence of the switching process depending on electroforming conditions are reported elsewhere. (22,27,31) We note that there is no abrupt increase in the current during electroforming of RIR-Ir in contrast to both Pt and RIR-Pt. We believe that this is related to suppression of ECM mechanisms, however, further study is needed. Comparison of the reset switching behavior (from LRS to HRS) also indicated a transition of the switching mechanism. For Pt samples in Figure 1 d, an abrupt drop in current is observed as usually indicative of filamentary switching mechanisms. (32,33) For RIR-Pt in Figure 1 e, a similar drop occurs, however, an extended gradual reduction also appears past 2 V. In RIR-Pt, the smoother interface reduces local electrical field concentrations allowing us to observe behavior associated with nonfilamentary switching mechanisms. It has recently been reported that because of the higher catalytic activity of Ir and IrO (34) nonfilamentary mechanisms in TaOmemristors were enhanced. (20,21) This is shown in thecurve for RIR-Ir in Figure 1 f, where no abrupt drop in current can be observed at all.

x memories with Ir electrodes A simple model describing the observed changes by the electrode type are described in Figure 1 g–i. For Pt with the largest roughness, local-field concentrations lead to the evolution of percolating filament paths. (22) Upon improvement of the interface roughness for RIR-Pt, we believe that the average number of local filament paths is reduced because of the slight reduction in the reset current (the maximum current during the reset process). In addition, the gradual decrease in current after the abrupt drop for RIR-Pt indicates the presence of VCM-related switching mechanisms. Finally, for RIR-Ir, in contrast to previous studies on TaOmemories with Ir electrodes (35) Figure S3 ), the VCM mechanism completely dominates switching. The interface roughness played a much more significant role than expected in determining which switching mechanism was observed even for the same electrode material. We confirmed further evidence for the suppression of filamentary switching mechanisms by comparing samples at different cell sizes for Pt device and RIR-Ir device. The Pt devices showed almost no area dependence in the LRS, as expected for filamentary conduction mechanisms, whereas RIR-Ir samples showed changes with cell area ( Figure S4 ).

2 O 5–x . Wedig et al., 2 O 5–x and Pt and that carbon can act to decrease interface dynamics, suppressing oxygen reactions. Second, the presence of oxygen during sputtering of noble metal electrodes may help to maintain any surface stoichiometry of Ta 2 O 5–x , which can be damaged during sputtering because of highly energetic metal or gas ions. The incorporation of oxygen during noble metal sputtering may also affect switching behavior in two additional ways. First is that oxygen plasma can clean any carbon contamination found on the surface of the Ta. Wedig et al., (20) reported that ECM-type switching did not occur for structures where carbon was deposited between Taand Pt and that carbon can act to decrease interface dynamics, suppressing oxygen reactions. Second, the presence of oxygen during sputtering of noble metal electrodes may help to maintain any surface stoichiometry of Ta, which can be damaged during sputtering because of highly energetic metal or gas ions.

2 O 5–x layer as its resistance increases. Intermediate-state switching was possible from any one state to another. Further electrical measurements and TEM analysis were performed using the RIR-Ir sample. Cross-point metal–insulator–metal structures were fabricated using traditional semiconductor processes ( Figure S5 ). The most important difference for RIR-Ir samples compared to Pt or RIR-Pt ones was that multiple stable resistance states were easily observed for both switching to the LRS and to the HRS. Figure 2 a shows a single sample that was consecutively switched from the highest resistance state in red to three lower resistance states (in green, blue, and black, respectively). Each state was observed at gradually increasing voltages by carefully reversing the voltage sweep as soon as any change in conductivity was measured. Figure 2 b shows the opposite process for the same sample, switching from the lowest resistance states in red to three higher resistance states (in green, blue, and black, respectively). Here, although the respective voltages gradually increased as well, the transition between the first (red) and second (green) sweeps ends up at a similar voltage likely because of the voltage-drop changes across the Talayer as its resistance increases. Intermediate-state switching was possible from any one state to another.

Figure 2 Figure 2. MVC state characterization of RIR-Ir (TE)/TaO x devices. (a) I–V curves showing the gradual changes from HRS to LRS (set) through two intermediate resistance states. (b) I–V curves showing the gradual changes from LRS to HRS (reset) through two intermediate resistance states. (c) Comparison between varying Ta 2 O 5–x /TaO 2–x bilayer structure relative thickness, showing the HRS resistance decided by the Ta 2 O 5–x thickness and reset voltage condition. (d) Plot of HRS resistance and LRS resistance vs sample thickness (plasma oxidation cycle). Thicker Ta 2 O 5–x thickness leads to a greater on/off ratio for identical reset switching conditions because of the larger voltage drop occurring across the Ta 2 O 5–x layer, whereas it transitions from the LRS to the HRS.

2–x layer (base layer) and darker contrast in the Ta 2 O 5–x layer (oxygen-exchange layer) because of a relatively higher concentration of oxygen (I–V sweep measurement data. The first sample consisted of an RIR-Ir TE with 5 nm thick Ta 2 O 5–x layer and 15 nm thick TaO 2–x layer. The second sample had a 10 nm thick Ta 2 O 5–x layer and a 10 nm thick TaO 2–x layer. All samples had a total thickness of 20 nm, a 2 nm thick buffer Al 2 O 3 layer, and W BE. The role of Al 2 O 3 was to act as an oxygen diffusion barrier to prevent oxidation of the BE. Next, we fabricated four different bilayer samples for further electrical measurement ( Figure S6 ). Two of the samples are shown as a schematic and with cross-sectional high-angle annular dark-field (HAADF) scanning TEM (STEM) images, showing brighter contrast in the TaOlayer (base layer) and darker contrast in the Talayer (oxygen-exchange layer) because of a relatively higher concentration of oxygen ( Figures S7 and S8 ), in Figure 2 c with correspondingsweep measurement data. The first sample consisted of an RIR-Ir TE with 5 nm thick Talayer and 15 nm thick TaOlayer. The second sample had a 10 nm thick Talayer and a 10 nm thick TaOlayer. All samples had a total thickness of 20 nm, a 2 nm thick buffer Allayer, and W BE. The role of Alwas to act as an oxygen diffusion barrier to prevent oxidation of the BE. (29,30)

I–V sweep measurement data show that the HRS level is determined by Ta 2 O 5–x thickness, as seen from the comparison of the red (5 nm/15 nm) and blue (10 nm/10 nm) sweep data, as well as reset voltage as seen from the comparison of the red (+3 V) and green (+2.5 V) sweep data. As seen earlier, both samples showed a gradual change during the reset process. The LRS levels were similar across all samples; differences in the HRS could be explained from the Ta 2 O 5–x layer thickness. We believe that significant changes to the TaO 2–x layer resistance do not occur during resistance switching. Meanwhile, the Ta 2 O 5–x layer undergoes around a 40 times resistance change and is responsible for resistance switching. We plotted the resistance versus thickness and plasma oxidation cycles for all four samples with the same programming conditions (set to LRS at −2 V, and reset to HRS at +3 V) in 2 O 5–x layer will become more completely reset. Thesweep measurement data show that the HRS level is determined by Tathickness, as seen from the comparison of the red (5 nm/15 nm) and blue (10 nm/10 nm) sweep data, as well as reset voltage as seen from the comparison of the red (+3 V) and green (+2.5 V) sweep data. As seen earlier, both samples showed a gradual change during the reset process. The LRS levels were similar across all samples; differences in the HRS could be explained from the Talayer thickness. We believe that significant changes to the TaOlayer resistance do not occur during resistance switching. Meanwhile, the Talayer undergoes around a 40 times resistance change and is responsible for resistance switching. We plotted the resistance versus thickness and plasma oxidation cycles for all four samples with the same programming conditions (set to LRS at −2 V, and reset to HRS at +3 V) in Figure 2 d showing the relatively small changes for the LRS, and a much larger 2 orders of magnitude difference for the HRS. The approximate four times difference in thickness alone does not account for the 40 times difference in resistance. The observation of the intermediate states in our devices in Figure 2 a,b, as well as sweep data in Figure 2 c, shows that even for the same sample (red vs green) a higher reset voltage (+3 V vs +2.5 V) leads to a higher HRS resistance. Therefore, when using identical reset voltage conditions, devices with a relatively thicker Talayer will become more completely reset.

However, if the Ta 2 O 5–x layer became too thin, dielectric breakdown could occur in the TaO 2–x layer from higher voltages needed to maintain larger on/off ratios.

x memristor at 1 μm × 1 μm cell size. To verify the robustness of MVC states, the 5 nm Ta 2 O 5–x /15 nm TaO 2–x sample was cycled across four intermediate conductivity states from ∼6 × 10–6 to ∼4 × 10–8 S (read at +0.5 V), as shown in 12 cycles, as demonstrated in 2 O 5–x /TaO 2–x bilayer memristors retained their conductance states beyond an extrapolated 10-year time period for all programmed conductance levels. We measured the actual data with a read voltage of 0.5 V up to 2 × 104 s, whereas the sample was heated to 85 °C over the entire period. 3 Figure shows MVC state cycling characterization and endurance reliability of RIR-Ir TaOmemristor at 1 μm × 1 μm cell size. To verify the robustness of MVC states, the 5 nm Ta/15 nm TaOsample was cycled across four intermediate conductivity states from ∼6 × 10to ∼4 × 10S (read at +0.5 V), as shown in Figure 3 a,b. We were able to switch between states using both voltage pulse height ( Figure 3 a) and voltage pulse width ( Figure 3 b). The total operating cycles for two states were tested to 10cycles, as demonstrated in Figure 3 c, using voltage pulses to reset and set the device while using two smaller reading pulses in between as shown in Figure S9 for measurement scheme. Meanwhile, Figure 3 a shows stability of up to 50 000 cycles for resistance switching between all intermediate states (see Methods/Experimental Section ). The retention reliability of individual MVC states was also expectedly high, as shown in Figure 3 d. Ta/TaObilayer memristors retained their conductance states beyond an extrapolated 10-year time period for all programmed conductance levels. We measured the actual data with a read voltage of 0.5 V up to 2 × 10s, whereas the sample was heated to 85 °C over the entire period.

Figure 3 Figure 3. MVC state cycling characterization and endurance reliability of RIR-Ir TaO x memristor at 1 × 1 μm2 cell size. (a,b) Multivalued cycling data for TaO x memristor devices with RIR-Ir TE sample. Switching was performed by −3 V, 100 ns for the set process. The reset process was performed using a 100 ns pulse width, and a pulse height ranging from +3.4 to +4.2 V for (a) and +4.2 V pulse height, with pulse widths ranging from 10 to 100 ns for (b). (c) Electrical repeatable bistable conductance change endurance data. The bilayer memristor device with RIR-Ir electrode did not fail even after 1012 cycles. Actual state reading for the data shown was performed separately by periodic dc measurement 5 times per order of magnitude. (d) MVC retention measurements for the four states shown in (a). Accelerated failure measurements performed at 85 °C show no degradation.

2 O 5–x layers with RIR-Ir electrode, switched in situ to be in the HRS. 2 O 5–x thickness upon applying increasing reset stress voltages in situ from +0.5 to +1.2 V at a constant 100 ms stress time. The reverse process of reduction in Ta 2 O 5–x thickness also occurred upon reversing the bias direction to −1.2 V. Similarly, TEM with specially constructed electrodes (see Methods ) to apply bias voltages in situ was performed to study the switching behavior of Talayers with RIR-Ir electrode, switched in situ to be in the HRS. Figure 4 a shows the increase from 7.4 to 8.3 nm in Tathickness upon applying increasing reset stress voltages in situ from +0.5 to +1.2 V at a constant 100 ms stress time. The reverse process of reduction in Tathickness also occurred upon reversing the bias direction to −1.2 V. Similarly, Figure 4 b shows an increase from 7.1 to 7.9 nm upon increasing stress voltage times in situ from 0 to 100 ms at a constant +1 V.

Figure 4 Figure 4. In situ STEM analysis and direct band gap measurements at the Ta 2 O 5–x and TaO 2–x layers with RIR-Ir electrode. (a,b) In situ HAADF-STEM observations while external bias was applied with (a) voltage height and polarity or (b) varying stress time. (c,d) Aberration-corrected HAADF-STEM images of the same position (c) before (HRS) and (d) after (LRS) in situ voltage-induced structural changes in the Ta 2 O 5–x and TaO 2–x layers occurs using a bias of −1 V at 100 ms stress times. (e) VEELS of Ta 2 O 5–x layer measured at the local region-labeled A reset and A set in (c,d), respectively using a monochromated STEM/EELS technique. (f) VEELS of TaO 2–x layer measured at the local region-labeled B reset and B set in (c,d) using a monochromated STEM/EELS technique. (g) In situ resistance measurement of the HRS (c) and LRS (d).

2 O 5–x and the TaO 2–x layers after each set and reset operation was compared quantitatively. We found that the intensity ratio of the Ta 2 O 5–x area (darker region), which was in the insulating state, to the TaO 2–x area (brighter region) under the HRS (I reset-Ta 2 O 5–x /I reset-TaO 2–x = ∼0.35) ( 2 O 5–x area, which was in the conductive state, to the TaO 2–x area under the LRS (I set-Ta 2 O 5–x /I set-TaO 2–x = ∼0.17) (2–) migration from the Ta 2 O 5–x to the TaO 2–x region in the LRS ( We programmed the TEM samples into the LRS in situ by applying a bias of −1 V at 100 ms stress times. Figure 4 c shows the sample (HRS) before application of in situ set bias, and Figure 4 d shows the same sample (LRS) after set bias. The intensity of HAADF-STEM images of the Taand the TaOlayers after each set and reset operation was compared quantitatively. We found that the intensity ratio of the Taarea (darker region), which was in the insulating state, to the TaOarea (brighter region) under the HRS (= ∼0.35) ( Figure 4 c) was higher than that of the Taarea, which was in the conductive state, to the TaOarea under the LRS (= ∼0.17) ( Figure 4 d) because of the formation of Ta-rich phases via oxygen-ion (O) migration from the Tato the TaOregion in the LRS ( Figures S10, S11 and Table S1 ).

2 O 5–x and TaO 2–x layers measured at the local regions labeled A set (or A reset ) and B set (or B reset ), as indicated in 2 O 5–x layer in the HRS at approximately 4.5 ± 0.1 eV. This value was consistent with the band gap measurement result using Auger electron spectroscopy-reflective electron energy loss spectroscopy (EELS) ( 2–x layer regardless of the set and reset operations ( 2–x region remains in a relatively unchanged conducting metallic state. These results support the conclusion that the resistive switching mechanism here is associated with transformation of the insulating Ta 2 O 5–x layer into the conducting Ta-rich oxide layer ( The direct observation of the electrochemical redox reactions in our sample was carried out by spectroscopic analysis. Figure 4 e,f shows the comparative valence electron energy-loss spectra (VEELS) (36) of the Taand TaOlayers measured at the local regions labeled A(or A) and B(or B), as indicated in Figure 4 c (HRS) and 4 d (LRS). In Figure 4 e, we observe the band gap onset of the insulating Talayer in the HRS at approximately 4.5 ± 0.1 eV. This value was consistent with the band gap measurement result usingelectron spectroscopy-reflective electron energy loss spectroscopy (EELS) ( Figure S12 ) and the literature. (37) In contrast, the bandgap onset is not found in the TaOlayer regardless of the set and reset operations ( Figure 4 f), which indicates that the TaOregion remains in a relatively unchanged conducting metallic state. These results support the conclusion that the resistive switching mechanism here is associated with transformation of the insulating Talayer into the conducting Ta-rich oxide layer ( Figure S13 ). Figure 4 g measures the resistance in situ of the states represented in Figure 4 c (HRS) and 4 d (LRS).

2 O 5–x /TaO 2–x interface was not distorted by filament paths, or interface roughness increases during programming from the HRS to the LRS, 2 O 5–x interface when compared to Pt/Ta 2 O 5–x interface. 2 O 5–x may be suppressed when large interface roughness leads to localized field concentrations. This type of continuous structure change contrasts with previous observations in TMO memories with filamentary mechanisms (38) and likely leads to observed MVC states. The Ta/TaOinterface was not distorted by filament paths, or interface roughness increases during programming from the HRS to the LRS, (32) and rather the interface remained relatively flat. The more uniform interface may be related to the RIR-Ir/Tainterface when compared to Pt/Tainterface. (22,32) The observation of VCM switching in Tamay be suppressed when large interface roughness leads to localized field concentrations.

Recently, memristors have been actively investigated for artificial synapse applications (6,25) in neuromorphic computing (5,18,39) (details in Supporting Information section S1 and Figure S14). A typical biological neuron with several synapses is shown in Figure 5 a. A simple circuit inset shows synapse at the portion of the brain that memristors attempt to mimic. The conductivity of synapse connections is believed to be affected by learning through external stimuli. (40) A well-known description of the synaptic conductance changes based on the spike-timing-dependent plasticity (STDP) model, which is our most current understanding of synapses function, is depicted in Figure 5 b. (40) When the postsynaptic neuron fires closely following the presynaptic neuron, there is a large increase in conductance of the synapse. As the delay between neuron firing decreases, the conductivity change becomes larger.

Figure 5 Figure 5. Comparison of MVC between RIR-Ir and Pt electrodes for synaptic applications. (a) Representation of neurons and synapses in the human brain. The magnified synapse represents the portion which we mimic using solid-state devices. (b) Conductance changes which occur in biological synapses depending on the time delay between the firing of connected neurons. (c,d) Compared conductance changes in RIR-Ir (c) and Pt (d) electrodes for TaO x memristors depending on a simulated time delay. The RIR-Ir electrode shows gradual changes, where the Pt electrode shows an abrupt change at Δt = 350 ns. (e,f) MVC cycling operation of RIR-Pt (e) and RIR-Ir (f) electrode device. Intermediate states are apparent and stable in RIR-Ir electrode devices. (g) Plot of on/off ratio for intermediate states depending on reset voltage. A linear relationship increasing 1 order per 1 V between reset voltage and on/off ratio is demonstrated in RIR-Ir electrodes.

2 O 5–x /TaO 2–x bilayer memristor cross-point structures with RIR-Ir electrodes (t. In our case, we defined Δt as the time delay between incoming programming pulses and a periodic “postsynaptic” pulse. The exact methods are more fully described in t = 350 ns. For artificial synapse applications especially, the MVC property is essential. Ta/TaObilayer memristor cross-point structures with RIR-Ir electrodes ( Figure S15 ) showed a similar behavior to synapses, as demonstrated in Figure 5 c, showing the changes in conductance versus Δ. In our case, we defined Δas the time delay between incoming programming pulses and a periodic “postsynaptic” pulse. The exact methods are more fully described in Supporting Information section S2 and Figure S16. Also, analogous properties to STDP (dependent on time correlation) for both long-term potentiation and long-term depression was demonstrated ( Supporting Information Figure S17). In contrast to the MVC changes that were observed for RIR-Ir electrode samples, Pt electrode samples in Figure 5 d showed a clear threshold between almost full conductance change and no conductance change at Δ= 350 ns. For artificial synapse applications especially, the MVC property is essential. (8,18,25)