# Solution-Processed Memristor Devices Using a Colloidal Quantum Dot-Polymer Composite
## 3 Results  
### Device Structure and Electrical Characteristics  
Figure 1a shows the device structure of the CdSe/ZnS QD-PMMA memristor device. The device has an asymmetric electrode structure with a high work-function ITO (\(\Phi\) \(\sim\) 4.8 eV) used as a bottom electrode and a low work-function Al (\(\Phi\)\(\sim\) 4.3 eV) used as a top electrode [24]. The CdSe/ZnS QD-PMMA layer, which is positioned between the ITO and Al electrodes, had an average thickness of 204.8 nm. As shown in Figure 1b, the CdSe/ZnS QDs were well dispersed in the PMMA matrix without noticeable agglomeration of the QD particles (QD concentration: 1 wt%). Figure 1c shows the light absorbance and normalized PL spectra of the CdSe/ZnS QDs, which were measured in the toluene solution. The CdSe/ZnS QDs showed an emission peak centered at around 530 nm, which corresponds to a bandgap of \(\sim\)2.34 eV. Additionally, the full-width-half-maximum of the PL peak was \(\sim\)34.5 nm, showing a narrow distribution of the QD size. In the QD-polymer based memristors, electron trapping and de-trapping occurring at the QDs play the dominant role, affecting memory characteristics such as the set/reset voltages, memory margin, and the current switching ratio [25]. In particular, the large band offset between the polymer and the QDs allows effective electron trapping at the QDs. Since the charge trapping characteristics are mainly governed by the embedded CdSe/ZnS QDs, it is necessary to optimize the CdSe/ZnS QD concentration in the polymer matrix. To optimize the QD concentration, we varied the concentration of CdSe/ZnS QDs as 1, 3, and 5 wt%, which correspond to volume concentrations of 0.21, 0.63, and 1.05 v/v%, respectively. Typical I-V characteristics of CdSe/ZnS QD-PMMA memristor devices are shown in Figure 1d when the CdSe/ZnS QD concentration was 1 and 3 wt%. Clearly, in both devices, set and reset operations accompanying abrupt changes of resistance states were observed. Initially, upon sweeping the bias to the positive bias direction, the device was in a high-resistance state (HRS). Then, at around 2.5 to 3 V, a transition to a low-resistance state (LRS) was observed, which is regarded as the set process. Particularly, at around the set bias, the current variation was approximately \(\sim\)10\({}^{4}\) and 10\({}^{2}\)\(\sim\)10\({}^{3}\) for 1 wt% and 3 wt% devices, respectively, which are comparable to those of TiO\({}_{2}\)-based memristors [26]. The LRS maintained after the set process until a sufficient negative  
Figure 1: (**a**) Device structure and an optical image of solution-processed CdSe/ZnS QD-PMMA memristors. (**b**) Cross-sectional TEM images of the CdSe/ZnS QD-PMMA memristor. The inset shows the structure of CdSe/ZnS QDs with oleic acid as the ligand. (**c**) Light absorption and photoluminescence (PL) characteristics of CdSe/ZnS QDs. (**d**) Current-voltage (I-V) characteristics of CdSe/ZnS QD-PMMA memristors with QD concentrations of 1 wt% and 3 wt%, and (**e**) 5 wt%. bias was applied (\(\text{---}\)2 V); then, a transition from LRS to HRS was induced. Increasing the CdSe/ZnS QD concentration to 5 wt%, however, resulted in abnormal I-V behavior showing mostly either insulating or conducting characteristics. In particular, as shown in Figure 1e, the device with conducting behavior followed the Ohmic conduction mechanism (linear I-V) with a relatively large current level (\(>\)10\({}^{-2}\) A), while the device with insulating behavior exhibited relatively low current levels (\(<\)10\({}^{-7}\) A). Since the PMMA is an insulating polymer and only a weak electric field is applied to the device during the operation (\(<\)0.20 MV/cm), it is likely that the occurrence of conducting or insulating characteristics is due to the agglomeration of CdSe/ZnS QDs in PMMA. For instance, when the number of QD particles is low at a specific region, the device could operate similar to a metal-insulator-metal diode exhibiting insulating behavior.