Xiaoning Zhao

Oxidized carbon quantum dot–graphene oxide nanocomposites for improving data retention of resistive switching memory

Data retention of nano-sized conducting filaments is a critical reliability issue in the pursuit of low-power graphene oxide-based resistive switching (RS) memory devices. Herein, an improvement in the low resistance state retention is demonstrated in fabricated oxidized carbon quantum dot (OCQD)–graphene oxide nanocomposites. Reliable RS characteristics with good retention properties were achieved instead of volatile switching, even with a relatively low compliance current of 100 μA. More epoxy groups were introduced as the concentration of embedded OCQDs was increased, resulting in a larger high resistance state, a higher set voltage, and deeper trapping levels. The dependence of the set switching time on the temperature acts as experimental verification that the oxygen migration energy barrier Ea was improved from 0.37 to 0.78 eV after embedding the OCQDs, which explains the enhancement of the low resistance state retention based on a filamentary model.

Multilevel Resistive Switching in P–N Heterostructure Memory

High storage density is an important requirement for resistive random access memory (RRAM) devices. Multilevel resistive switching (RS) in RRAMs does not require much change to current technologies compared with device size reduction and 3D integration. Herein, five stable resistance states can be obtained in a Pt/p-NiO/n+-Si memory device by controlling the current compliance (CC). The RS mechanism can be attributed to the formation and rupture of localized conducting filaments (CFs) in an NiO film. The conductivity of the low resistance states (LRS) is determined through the combined action of the P-N junction and localized CFs. The CC can be used to effectively modulate the formation of localized CFs and junction resistance. Importantly, different LRS have large differences in resistance values, resulting in multilevel memory. A model is suggested and discussed to account for the observed multilevel memory operation.

Photocatalytic Reduction of Graphene Oxide–TiO2 Nanocomposites for Improving Resistive‐Switching Memory Behaviors

Graphene oxide (GO)-based resistive-switching (RS) memories offer the promise of low-temperature solution-processability and high mechanical flexibility, making them ideally suited for future flexible electronic devices. The RS of GO can be recognized as electric-field-induced connection/disconnection of nanoscale reduced graphene oxide (RGO) conducting filaments (CFs). Instead of operating an electrical FORMING process, which generally results in high randomness of RGO CFs due to current overshoot, a TiO2 -assisted photocatalytic reduction method is used to generate RGO-domains locally through controlling the UV irradiation time and TiO2 concentration. The elimination of the FORMING process successfully suppresses the RGO overgrowth and improved RS memory characteristics are achieved in graphene oxide-TiO2 (Go-TiO2 ) nanocomposites, including reduced SET voltage, improved switching variability, and increased switching speed. Furthermore, the room-temperature process of this method is compatible with flexible plastic substrates and the memory cells exhibit excellent flexibility. Experimental results evidence that the combined advantages of reducing the oxygen-migration barrier and enhancing the local-electric-field with RGO-manipulation are responsible for the improved RS behaviors. These results offer valuable insight into the role of RGO-domains in GO memory devices, and also, this mild photoreduction method can be extended to the development of carbon-based flexible electronics.

Reversible alternation between bipolar and unipolar resistive switching in Ag/MoS2/Au structure for multilevel flexible memory

Reversible alternation between bipolar and unipolar resistive switching (RS) modes was observed by controlling voltage-polarity in Ag/MoS2/Au memory on polyethylene terephthalate (PET) substrate. The temperature-dependent low-resistance-state and Raman measurement indicated that Ag and sulphur vacancy (Vs)-based conductive filaments (CFs) were responsible for these two RS modes. The two kinds of CFs had different responses under positive read voltages. Thus, a new operation scheme of multilevel memory was demonstrated in which multiple states were distinguished by CF composition rather than resistance values. The memory capacity of the cell could be further extended by adjustments in CFs’ size in each mode. The RS performance of the device did not degrade under bending conditions even over 104 bending cycles, which indicated good mechanical flexibility. The present Ag/MoS2/Au memory has promise for future high-density flexible information storage.