# Solution-Processed Memristor Devices Using a Colloidal Quantum Dot-Polymer Composite
## 3 Results
### Degradation Mechanisms  
To closely investigate the possible origin for the instability occurring at high-number cycles, the conduction mechanism change during the switching operation was analyzed in detail. Figure 5a,b show the ln \(I\) vs. In \(V\) plots for the CdSe/ZnS QD-PMMA memristor before (fresh device) and after the degradation (device underwent 500 endurance cycles), respectively. The fresh device showed a clear difference in the conduction mechanism at HRS and LRS regions. Initially in the HRS region, the I-V curve followed I \(\propto\) V\({}^{0.2}\). Then, at around the set voltage (+2.3 V), the slope (\(\alpha\)) becomes larger than 2, indicating a transition to the trap filling regime or the TCLC [30]. The presence of the TCLC regime is an indication that the traps are exponentially distributed in the forbidden gap [30], and at this stage,  
Figure 3: **(a)** I-V characteristics of CdSe/ZnS QD-PMMA memristors during a cyclic test (50 cycles) (QD: 1 wt%). **(b)** The variation of LRS and HRS currents during the cyclic test. **(c)** The distribution of set and reset voltages during the cyclic test.  
the injected electrons fill the trap levels. Followingly, after the set process, the dominant conduction mechanism changes to trap-free SCLC, showing the relation, I \(\propto\) V1.[89]. Ideally, for the trap-free SCLC, the I-V is related by the following equation [17; 31].  
\[I\propto\frac{9\varepsilon_{i}\mu V^{2}}{8d^{3}} \tag{1}\]  
where \(\varepsilon_{i}\) is the insulator permittivity, \(\mu\) is the carrier mobility and \(d\) is the insulator thickness. As displayed in Figure 5a, the \(\alpha\) value in the LRS region (1.89) is similar to that of the ideal case, suggesting that most of the traps are filled with the electrons during the set process. These transitions in conduction mechanism are in a good agreement with the operation mechanism described in Figure 2. After the degradation, however, the dominant conduction mechanisms and their transitions are significantly different from the fresh device. Figure 5b shows the ln \(I\) vs. ln \(V\) plot, which is measured after the degradation. Interestingly, the I-V characteristics were similar in both forward and reverse sweeps. For the forward and reverse sweeps, the corresponding \(\alpha\) values were 3.38 and 3.58, respectively. Thus, the dominant conduction mechanism is considered as the TCLC, suggesting that during the repeated operation, additional traps are created in the CdSe/ZnS QD-PMMA layer, preventing the occurrence of trap-free SCLC behavior. Previously, it has been reported that electronic trap states can be created and eliminated in the CdSe CQDs [31]. Particularly, by doping and charging, atomic dimmers at the QD surface can be formed or broken, creating trap states within the bandgap [32]. Moreover, the oxidation of reactive metallic components in QD particles can be another possible reason for the creation of additional trap states. Nevertheless, from our observation, it is clear that the dominant conduction mechanism is changed after the degradation, possibly originating from the creation of additional trap states in the CdSe/ZnS QD-PMMA layer. Therefore, to obtain highly reliable QD-polymer-based memristors, surface-stabilized QDs and the prevention of oxidative reactions would be necessary.  
Figure 4: (**a**) The data retention characteristics of the CdSe/ZnS QD-PMMA memristor (QD: 1 wt%). The LRS and HRS states were programmed with biases of +3 V and \(-\)3 V, respectively. Additioanally, the V\({}_{\text{read}}\) was +1 V. (**b**) The variation of current switching ratio during the data retention test up to 10\({}^{4}\) s. (**c**) The endurance cyclic test data of the CdSe/ZnS QD-PMMA memristor (500 cycles). The LRS and HRS states were programmed with biases of +3 V and \(-\)3 V, respectively. Additionally, the V\({}_{\text{read}}\) was +1 V. (**d**) The variation of current switching ratio during the endurance test.