The Investigation On The Application Of 2H Phase MoS₂ Nanosheets In Resistive Random Access Memory

Two-dimensional materials,which are composed of single or several atomic?molecular?layers separated from layered materials or prepared by other methods,have many fascinating properties different from those of bulk materials.For example,when the most popular MoS₂ is made into two-dimensional material,it will change into a direct band gap semiconductor with a band gap of¹.8 eV,showing optical,electrical,magnetic and thermal properties which are completely different from the bulk MoS₂.In the aspect of electrical properties,there have been many reports about the applications of two dimensional MoS₂ in resistive random access memory?RRAM?.However,the mechanism of it is still controversial.Especially,the research on 2H phase Mo S₂ nanosheets has not been reported.In this work,we focus on the mechanism and the turning method of RRAM based on 2H phase MoS₂ nanosheets.The following are the main results of this work:1.The initial state of Al/2H-MoS₂ nanosheets/ITO device is low-resistance state.Positive scanning range increasing to 5 V or 6 V in the first voltage scan is necessary for the initial state device to switch to high-resistance state.After the first scan,Al/2H-MoS₂ nanosheets/ITO memory device exhibits the bipolar resistive switching phenomenon.In the subsequent switching cycles,RRAM based on 2H-MoS₂ nanosheets displays low operating voltage?<1.1 V?,high ON/OFF resistance ratio?>10³?and good data retention?>1000 s?,leading to a considerable application potential in nonvolatile random access memory.2.It is confirmed that the conductive filaments exist in the device through changing the size of Al top electrode and analyzing of the RESET distribution of the 2H-MoS₂ nanosheets memory.By X-ray photoelectric spectroscopy depth profiling,it was found that oxygen content in memory layer increases,and an oxygen-rich layer is formed between the Al top electrode and the 2H-MoS₂nanosheets memory layer after the first voltage scan.The appearance of oxygen-rich layer causes the device to switch to high-resistance state during the first voltage scan.In the SET process,the oxygen ions migrate to the direction of the bottom electrode,resulting in the reduction of aluminum ions to metal aluminum in the oxygen-rich layer as well as the decrease of molybdenum ions valence in the top portion of the memory layer.During the RESET process,the migration of oxygen ions to the oxygen-rich layer leads to the oxidization of metal aluminum in the oxygen-rich layer and molybdenum ions in the top portion of the memory layer.The valence change of aluminum and molybdenum elements is caused by the oxygen ion migration,resulting in the formation and rupture of conductive filaments.The valence change mechanism is considered as the resistive switching mechanism of Al/2H-MoS₂/ITO device.3.The initial resistance of RRAMs based on 2H-MoS₂ nanosheets increases with the addition of graphene oxide?GO?in the memory layer,placing the devices in high-resistance state.With increasing the content of GO,erase-threshold voltage and low-resistance state resistance are roughly constant,while write-threshold voltage and high-resistance state resistance decrease slightly.When the GO content reaches to 50%,the RRAM based on 2H-MoS₂ nanosheets-GO with low operating voltage?<0.9 V?and high switching ratio?⁹×10²?no longer needs a high positive bias to achieve the initial RESET in the first voltage scan.