Cerium oxide (CeO₂) has been studied for its potential in resistive switching applications due to its unique properties, such as its ability to exist in multiple oxidation states (Ce³⁺ and Ce⁴⁺), which can be leveraged for non-volatile memory devices. For CeO₂ to be suitable for resistive switching, its morphology plays an essential role in enhancing the switching behaviour, including parameters like switching speed, endurance, and retention.
Suitable Morphology of Cerium Oxide for Resistive Switching:
Nanostructures (Nanoparticles, Nanowires, Nanorods):
Nanoparticles: CeO₂ nanoparticles are often preferred because their small size can lead to a large surface area and high interface density, facilitating the formation of oxygen vacancies which are key to the resistive switching mechanism.
Nanowires/Nanorods: These are particularly attractive due to their one-dimensional structure, which can provide better electronic conduction paths and improve the efficiency of resistive switching. The high aspect ratio and surface-to-volume ratio allow for more uniform switching behaviour.
Thin Films:
Thin films of CeO₂ (often in the nanometer range) are widely used, as they can be easily integrated into existing memory devices. The thinness helps in reducing the required voltage for switching, while the film thickness also influences the characteristics of the switching, such as the ON/OFF ratio and switching endurance.
Polycrystalline or Amorphous Structures:
Polycrystalline CeO₂ has been shown to exhibit a favourable resistive switching behaviour due to grain boundaries, which can act as traps for oxygen vacancies and influence the conduction mechanism.
Amorphous CeO₂ may also be beneficial for resistive switching, as amorphous materials tend to have a higher density of defects and oxygen vacancies, which can improve switching behaviour.
Porous Structures:
Porous CeO₂ can enhance resistive switching performance because the pores can increase the surface area, leading to a higher number of oxygen vacancies. The increased surface area and defects contribute to better charge storage and switching efficiency.
Core-Shell Structures:
Core-shell CeO₂ structures, where CeO₂ forms the shell around another material or phase, have been studied to enhance the stability and control of resistive switching. The shell can serve to control the migration of oxygen vacancies and improve the device’s retention properties.
High Oxygen Vacancy Density:
The resistive switching properties of CeO₂ heavily rely on oxygen vacancy dynamics. The morphology that can introduce a high density of oxygen vacancies (such as smaller particles or high surface-to-volume ratio) tends to have enhanced switching characteristics. Materials with a high number of defects (e.g., CeO₂ with a reduced oxidation state or partial non-stoichiometry) are more likely to show resistive switching behaviour.
In summary, nanostructured CeO₂ (especially nanowires, nanoparticles, and thin films) with a high surface area, high oxygen vacancy density, and controlled defects typically exhibit the most favourable properties for resistive switching. The morphology that maximizes the formation and migration of oxygen vacancies will lead to better memory performance, such as lower voltage switching, higher ON/OFF ratios, and longer cycling endurance.
