# Solution-Processed Memristor Devices Using a Colloidal Quantum Dot-Polymer Composite
## 1 Introduction  
Recently, memristors have received significant interest for next-generation electronics such as neuromorphic computing systems, owing to their potential advantages such as high energy efficiency, good scalability, and compatibility with conventional complementary metal-oxide-semiconductor fabrication processes [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]. Particularly, memristors are considered as the fourth fundamental non-linear circuit element linking charge and magnetic flux [11]. In the memristor device, the electrical resistance state can be changed according to the history of the external stimulation, such as the voltage pulses, allowing the emulation of learning behavior and various neuromorphic functions. To realize the memristor device, various material systems have been investigated including ferroelectrics [12], ferromagnetics [13], and phase-changing materials [4, 14]. In addition, organic/inorganic hybrid materials have received much attention recently due to their wide tunability in electrical properties, good mechanical flexibility, low-temperature process, and large-area scalability [15, 16]. Moreover, since the organic/inorganic hybrids can be deposited by a simple solution process, the use of sophisticated vacuum deposition processes can be omitted, enabling the realization of cost-effective and area-scalable neuromorphic systems.  
In the case of organic/inorganic hybrid-based memristors, it is claimed that the inorganic components such as the metallic or semiconducting nanoparticles play an important role in regulating the resistance state by acting as charge storage or charge trapping centers [17, 18]. For the inorganic component, colloidal quantum dots (CQDs) can be a good candidate since they are solution processable and their bandgap can be tuned by adjustingthe size of the QDs [19; 20]. Particularly, when the QDs are embedded in an insulating polymer with a large bandgap over 3 eV [21], effective charge trapping can be expected at the QDs due to the relatively small bandgap of the QDs. Moreover, the QD-polymer memristors can provide potential advantages such as good mechanical flexibility [22], high optical transparency, and low voltage operation for efficient energy consumption [20]. To date, although some research has been carried on QD-polymer-based memristors [23], there is still a need for optimizing the QD concentration since the trapping and de-trapping of charge carriers at the QDs largely affect the memristive characteristics. Furthermore, a study on the conduction mechanism is required to understand the dominant conduction mechanism of the QD-polymer-based memristors and the possible origin of the degradation mechanisms under repeated operations.  
In this study, we demonstrate solution-processed memristor devices using a CdSe/ZnS QD-poly(methyl methacrylate) (PMMA) composite, and their electrical characteristics were investigated. Particularly, to obtain stable memristive characteristics, we optimized the concentration of CdSe/ZnS QDs in the PMMA matrix. The CdSe/ZnS QD:PMMA memristor with an optimized QD ratio exhibited decent memristive characteristics with a high current switching ratio up to \(\sim\)10\({}^{4}\) and a relatively long retention time over 10\({}^{4}\) s. In addition, the operational stability of the device was investigated by performing the cyclic endurance test and it was found that the memristor devices showed stable switching behavior up to 400 cycles. Furthermore, by investigating the conduction behavior of the memristors, we have deduced the possible mechanisms for the degradation of the switching characteristics over long switching cycles. Particularly, by analyzing the I-V characteristics, we show that the dominant charge conduction mechanism changed from trap-free space charge-limited current (SCLC) conduction to trap charge-limited current (TCLC) conduction, indicating the creation of additional trap states during the repeated operation, disturbing the memristive operation.