Unique Coexistence of Two Resistive Switching Modes in a Memristor Device Enables Multifunctional Neuromorphic Computing Properties

We report on hybrid memristor devices consisting of germanium dioxide nanoparticles (GeO2 NP) embedded within a poly(methyl methacrylate) (PMMA) thin film. Besides exhibiting forming-free resistive switching and an uncommon “ON” state in pristine conditions, the hybrid (nanocomposite) devices demonstrate a unique form of mixed-mode switching. The observed stopping voltage-dependent switching enables state-of-the-art bifunctional synaptic behavior with short-term (volatile/temporal) and long-term (nonvolatile/nontemporal) modes that are switchable depending on the stopping voltage applied. The short-term memory mode device is demonstrated to further emulate important synaptic functions such as short-term potentiation (STP), short-term depression (STD), paired-pulse facilitation (PPF), post-tetanic potentiation (PTP), spike-voltage-dependent plasticity (SVDP), spike-duration-dependent plasticity (SDDP), and, more importantly, the “learning–forgetting–rehearsal” behavior. The long-term memory mode gives additional long-term potentiation (LTP) and long-term depression (LTD) characteristics for long-term plasticity applications. The work shows a unique coexistence of the two resistive switching modes, providing greater flexibility in device design for future adaptive and reconfigurable neuromorphic computing systems at the hardware level.

materials, we infer that the switching mechanism in the material is because of the GeO 2 rather than the GeS 2 .

Figure S1.
Raman spectra of the GeO 2 NPs prepared by a low temperature solvothermal process and showing peaks at 445 cm -1 (attributable to α-quartz GeO 2 ) and 344 cm -1 (attributable to GeS 2 ).].Here  () is the memory (current) level at a given time ,  0 is the memory level at  = 0,  is the characteristic relaxation time, which can be used to evaluate the forgetting rate. is the stretch  showing good reproducible switching performance for 100 sweeps in the non-volatile mode.

Mixed Hybrid
In Figure S7, the effect of NP concentration on the I-V characteristics, the OFF/ON ratio and the conduction mechanism was studied.As can be seen from Figure S7a, the device with R m = 0.05% (blue curve) shows a typical I-V bipolar resistive switching behavior where the switching between HRS and LRS and vice versa occurs at different voltage polarities.It is well known that the PMMA is a good insulator and has no RS properties, 5 (see Figure S7a, red curve) and hence, the resistive switching properties of the devices are dominated by GeO 2 NPs.Importantly, most of the pristine devices were originally in LRS.During the sweep from 0 V to 1.5 V, the current decreased with a particularly large transition at about 1 V, denoting the switch from the LRS to the HRS.In contrast, upon sweeping from 0 to -1.5 V, the current increased with a sharp transition at -0.8 V, denoting the switch back to the LRS.The device has a large resistance OFF/ON ratio of larger than two orders of magnitude.The asymmetric shape of the I-V curve can be ascribed to the different work functions of the ITO and Al electrodes, as has been shown by others 6,7 Note, a few devices were initially in the HRS, which can be attributed to the inhomogeneous distribution of GeO 2 NPs in the PMMA across the substrate.Upon increasing the NP concentration to Rm = 0.15%, the devices exhibited smooth bipolar transitions between HRS and LRS with a very small OFF/ON resistance ratio and had the highest conductance (see Figure S7a, yellow curve).The effect of GeO 2 NPs on the RS properties can be clearly seen in Figure S7b, which plots the ON and OFF currents and the OFF/ON resistance ratio as a function of the NP concentration.
To investigate the conduction mechanisms in the mixed hybrid device, ITO/GeO 2 :PMMA/Al, the I-V characteristics of devices at R m = 0.05% and 0.15% were replotted on a log I -log V scale, as shown in Figure S7c and S7d, respectively.For the devices with R m = 0.05%, ohmic and SCLC transport dominate, whereas for the higher concentration, R m = 0.15%, only ohmic conduction is observed.Cumulative probability of the c) HRS and LRS and d) V SET and V RESET .Although the device is initially in the HRS, the current level for the HRS is still higher (by more than two orders) than that of devices that are LRS in their pristine state.(i.e. the former devices are more conducting because of the presence of conducting filaments).This results in a small window between HRS and LRS of about 4 in comparison with devices that have an LRS in their pristine condition, which instead typically exhibit a larger window of more two orders of magnitude.

Single Layer and Bi-layer GeO 2 NP Devices
Besides mixed hybrid memristor devices, another two device architectures were studied.The first device structure, called a single layer device, consists of only GeO 2 NPs, and has the configuration of ITO/GeO 2 /Al.The second device structure, called a bi-layer device, consists of two distinct layers of the GeO 2 NPs and PMMA, and has the configuration of ITO/PMMA/GeO 2 /Al.These device architectures can be seen from the insets in Figure S10a.Both the ITO/GeO 2 /Al and ITO/PMMA/GeO 2 /Al based devices were fabricated by spin-coating a mixed GeO 2 NPs/toluene solution (800 rpm, 10 s) onto the substrates, followed by an annealing process at 100 C for an hour to form a thin-film of approximate thickness ~150 -200 nm.
However, in the case of the ITO/PMMA/GeO 2 /Al devices, prior to depositing the GeO 2 NPs film, a 40 nm thick of PMMA was deposited by spin-coating a PMMA solution (3000 rpm, 30 s) on the ITO substrate with annealing at 40 C for 10 minutes.For both device types, a final Al films (200 nm) were thermally deposited onto the substrates using a shadow mask with 400 m diameter circular dots to act as the top electrodes.
Figure S10a shows a comparison of the I-V sweeps of three different device types, the single layer GeO 2 NP device, bi-layer GeO 2 NP/PMMA device and the control device containing only PMMA.The GeO 2 NP containing devices were more conductive than the other devices and were swept at low voltages between -1.3 V and 1.3 V, see Figure S10a (yellow curve).As observed previously in other nanoparticle based resistive switching memory devices 8,9 no forming step was needed to initiate the switching process.The single layer device, ITO/GeO 2 /Al, had the lowest resistivity with an initial LRS.Devices of this nature exhibited a bipolar resistive switching behavior (negative SET and positive RESET) with a low OFF/ON resistance ratio of ~ 2.
Although the resistance OFF/ON ratio was small, these devices, having very smooth I-V sweeps and no sudden jumps in the current, exhibited the most reliable switching properties of all of the GeO 2 device types.The I-V sweeps for a device showing a reliable and reproducible switching are presented in Figure S10b.
Figure S10a also shows an I-V sweep of the control device containing only an insulating layer of PMMA (200 nm), ITO/PMMA/Al, (red curve).This device, as expected, had the highest resistivity because of the highly insulating nature of the PMMA material. 10The device also showed no resistive switching between HRS and LRS, even though the device was swept to higher voltages, ±2 V.The bi-layer device, ITO/PMMA/GeO 2 /Al, also exhibited resistive switching effects but had a vastly improved resistance OFF/ON ratio of three orders of magnitude, see Figure S10a (blue curve).Successive I-V curves for a device showing a reliable and reproducible switching are presented in Figure S10c.In contrast to the ITO/GeO 2 /Al device, which was in the LRS state in its pristine state, the ITO/PMMA/GeO 2 /Al device existed first in the HRS.There are also other notable differences between the two device types.The single layer device typically switched at much higher potentials, ~ 1.2 V, in contrast to ~ 0.47 V for the bi-layer device.The type of switching was also very different.In the single layer device the switch between the high and low resistance states occurred smoothly whereas the bi-layer device exhibited very sharp, first-order like transitions, which is typically indicative of a filamentary switching process, resulting in a larger OFF/ON resistance ratio of about 3 orders of magnitude, see Figure S10d.Additionally, adding the PMMA layer to the GeO 2 NPs decreased the power consumption from 171 µW for the single layer device to 1.43 µW for the bi-layer device, as shown in Figure S10d.The power consumption (  =   ×   ) was calculated at a read voltage of 300 mV and the OFF currents of the devices.These comparisons indicate the switching characteristics of inorganic GeO 2 NPs can be improved by the adding of organic PMMA, making the bi-layer device a promising candidate for fabrication low power operation and large OFF/ON resistance ratio non-volatile memories. 11 explore the origin of the resistive switching characteristics in the single layer (GeO 2 ) and bilayer (PMMA/GeO 2 ) devices, the conduction mechanisms of the HRS and LRS for both devices were investigated.Figure S10e shows the plot of log I vs. log V for the single layer GeO 2 NP device in the HRS and LRS.Since the device is initially in the LRS it is likely that conductive pathways of some type are already present within the switching material after device fabrication.
The straight lines with a slope of approximately 1 for both the HRS and LRS indicate ohmic conduction, which is usually attributed to free carriers being thermally generated. 12The switch to the HRS at approximately 1.0 V is fast and not much relevant information can be gained from fits in this region.On the other hand, the bi-layer device, whilst exhibiting ohmic conduction for the LRS and for the HRS state at low applied potential, Figure S10f, undergoes a very large jump from high resistance to low resistance at 0.47 V.The shape of this transition has the hallmark of a mechanism involving space charge limited conduction (SCLC), which is a well-known effect that occurs in insulating materials that contain trap states.In this scenario ohmic conduction occurs at low potential, but as the voltage is increased there is a transition to a trap-controlled SCLC conduction regime (0.23 ≤ V ≤ 0.41), having a characteristic slope of 2.0 on a log-log graph and described by the Mott-Gurney law, 13 as given by: where J is the current density, ε is the dielectric constant, μ is the free carrier mobility, V is the applied voltage, and d is the insulator thickness.
Further increment in the applied voltage up to 0.44 V causes a sharp, first-order transition with a slope of 10.Typically, large values of slope in the trap-filling regime indicates the presence of a large number of traps with an exponential distribution over energy, most likely this injection occurs at the GeO 2 NPs/PMMA interface with the trap states being in the GeO 2 NPs, 12,14,15 since the control PMMA device did not show a RS effect and/or trapping effect.It is also worth mentioning that the trapping effect may occur at the GeO 2 NPs/PMMA interface due to the dangling bonds on the NP surface.After the filling of all traps there should be a change in the conduction mechanism to a trap-free space charge limited conduction (TF-SCLC).However, in this case the slope of the I-V curve is already 1.0, in contrast to the normal slope value of 2.0, which indicates that the TF-SCLC state might be short-lived or more likely, the switch to the LRS state has already occurred as evidenced by the value of the slope being the same as the return path from the maximum voltage (1.3 V) back to 0 V.The current transport in the return path is again governed by ohmic conduction. 16,17lthough both the single layer and bi-layer GeO 2 NPs devices exhibit clear bipolar RS properties at low applied bias, there are significant differences in their I-V characteristics and switching properties.Namely, 1. the pristine devices have different initial states (HRS, LRS); 2. the shape of the I-V curves and the abrupt HRS/LRS transition; 3. the value of the resistance OFF/ON ratio.We discuss the different switching properties and conduction mechanisms in the following.
The initial resistance state of pristine single layer devices is the LRS, whereas for the bi-layer devices, it is HRS.The LRS of the single layer devices is easier to understand since it likely indicates the GeO 2 NPs contain a high amount of oxygen vacancy defects, (V O ), 18 which makes the thin-film more conducting.The GeO 2 NP thin-film is also in direct contact with the Al and ITO electrodes and from the shape and symmetry of the I-V curve, it appears that there is a good ohmic contact with the electrodes, facilitating easy charge transport in both the LRS and HRS case.The formation of ohmic contact in both the HRS and LRS in the single layer device is evidenced by the slope of the graph (≈1), symmetric profile of the I-V curve and the V SET and V RESET , having the same absolute magnitude i.e. −1.26 V and +1.22 V, respectively.The ohmic contacts appear to be largely unaffected by the bias, but it could be minor changes in this which cause the observed changes in resistance between HRS and LRS.
In the bi-layer devices case, the initial HRS can be attributed to the presence of the 40 nm thick PMMA, which acts as an insulating barrier between the GeO 2 NPs film and the ITO bottom electrode.However, the mechanism of switching is unclear since either SCLC with trap-filling occurs at the GeO 2 NPs/PMMA interface, as shown by the modelling in Figure S10f, or, with the application of a sufficient potential, oxygen vacancies could migrate from the GeO 2 NPs into the PMMA layer to form a conducting filament, 19 which can produce similar switching characteristics in the I-V curve.The thickness of the PMMA layer is quite thin, ~40 nm, and it is quite likely that a few spots exist in the device where it may be substantially thinner.In these places the electric field is larger and filament formation would be easier.This would lead to current hot spots in the devices, which are commonly seen in the field. 20,21We exclude the migration of aluminum ions into the PMMA in this case since the control device ITO/PMMA/Al exhibited no evidence of switching.
A noticeable difference between the bi-layer device and the single layer device is that the transition from the HRS to the LRS (and vice versa) is smooth for the single layer device.The I-V sweeps do not contain abrupt transitions and the shape of sweeps do not significantly change after repeated cycles, indicating the devices are much more stable.This type of resistive switching, often termed homogeneous switching 22 or non-filamentary, 8,23 is typically due to gradual changes in the materials properties or electronic structure of the device, such as changes in the Schottky-barrier heights at interfaces between the functional material and electrode.In contrast, the bi-layer device exhibits sharp, 1 st order like transitions between the ON and OFF states with large changes in the conductance, often more than two orders of magnitude.This is typical of the trapping and de-trapping effect-based switching mechanism. 24,25e switching mechanism of the bi-layer device is expected to be based on trapping and detrapping effect that can occur at the PMMA/GeO 2 interface.Figure S11 shows a schematic diagram of the electronic structure for the ITO/PMMA/GeO 2 /Al under a positive voltage polarity, where the ITO is positive with respect to Al.The GeO 2 NPs can act as charge trapping centers at the interface.When a positive voltage is applied, the injected charges are trapped by GeO 2 NP trapping sites and once a sufficient positive voltage is reached, all the trap sites are filled, and as a result switching the device to the LRS.When a negative voltage is applied, the trapped charges are removed, and consequently switching the device to the HRS.This is consistent with the fitting I-V curve in Figure S10f, which shows the trap-mediated SCLC effect.The LUMO and HOMO represent the energy levels of the lowest unoccupied molecular orbital and the highest occupied molecular orbital of the PMMA film, respectively.The energy levels for GeO 2 NPs were taken from ref. 26 and those for PMMA from ref. 13 Upon application of a positive Regardless of the precise switching mechanism, our research confirms that the addition of a 40 nm thick PMMA layer improves the RS properties of devices by increasing the resistance OFF/ON ratio of devices and decreasing the power consumption.Additionally, the devices are also forming-free and device yield is improved, most likely because the PMMA layer prevents possible short-circuits between the two electrodes during fabrication and device operation.

Figure S2 .
Figure S2.Atomic Force microscopy (AFM) of a GeO2 NP:PMMA hybrid device showing the

Figure S3 .Figure S4 .Figure
Figure S3.Stopping voltage-dependent switching modes under negative polarity conditions.a) Consecutive I-V curves for a device consisting of GeO 2 NPs embedded within PMMA at R m =0.05% showing a short-term memory at a stopping voltage of −1.5 V. b) The switching between LRS and intermediate state upon repeated cycling between 0 V and −1.5 V. c) I-V curve at the stopping voltage of 0 V (in a voltage direction of 0 V → −1.0 V → 0 V).d) Consecutive I-V curves upon sweeping the device between ±1.0 V, showing the non-volatile bipolar memory fitted to be 0.2 in this work.The fitted relaxation time was found to be 200 ms.f) long-term potentiation and long-term depression under 50 positive pulses (0.5 V, 3 ms) and 50 negative pulses (−0.5 V, 3 ms), respectively.

Figure S7 .
Figure S7.a) I-V sweep for a device consists of GeO 2 NPs embedded within PMMA at R m =0.15% (yellow curve), R m =0.05% (blue curve) and PMMA only (red curve).The inset shows schematic of the resistive switching device architecture.b) Plot of the OFF/ON resistance ratio (right axis)

Figure
Figure S8.a) Successive I-V curves of a typical hybrid device, ITO/GeO 2 :PMMA/Al (with Rm = 0.05%), exhibiting the HRS state in its pristine condition.b) DC endurance for the HRS and LRS.

Figure
Figure S9.a) Successive I-V curves of a typical hybrid device, ITO/GeO 2 :PMMA/Al (with Rm = 0.15%), showing reproducible switching performance.b) DC endurance for the HRS and LRS.c)

Figure S10 .
Figure S10.a) I-V curves of resistive switching devices based on ITO/GeO 2 /Al (yellow curve), ITO/PMMA/GeO 2 /Al (blue curve) and ITO/PMMA/Al (red curve).The inset show schematics of the three memristor device architectures.b) Successive I-V curves of a single layer ITO/GeO 2 /Al device, showing a reproducible switching performance.c) Successive I-V curves of a bi-layer

Figure S11 .
Figure S11.Schematic diagram of the energy levels for the bilayer ITO/PMMA/GeO 2 /Al device.
voltage on the ITO electrode, electrons are initially injected from the Al electrode into the GeO 2 NPs via thermally generated carriers.Increasing the applied voltage gradually increases the via the SCLC, but in this case, injected electrons begin to be trapped by the GeO 2 NP trapping sites.Below the SET voltage, the device is still in the OFF state since some of the trapped electrons are transferred from the GeO 2 NPs to the ITO electrode via the LUMO level of PMMA, leaving the GeO 2 NPs partially occupied.Once the GeO 2 NP trap sites are fully occupied, as shown by the sharp increase in current, the device switches to the ON state at a SET voltage of 0.5 V.However, when the voltage is switched to negative bias conditions, the trapped electrons are gradually released, resulting in switching the device to the OFF state at a RESET voltage of ∼−1 V.