Computational Study of RRAM memory materials and metal-organic frameworks for catalyst
1.A (resistance random access memory) RRAM memory cell has a capacitor-like structure consisting of insulating or semiconducting materials sandwiched between two metallic electrodes. The working principle of this kind of memory cell is based on change of the resistance across the insulating or semiconducting materials under an electrical plus. The SrTiO3 (STO) based memory cell shows a bipolar switching behavior i.e. the resistance change not only depends on the strength of the bias voltage but also on the polarity. The driving mechanism of the resistive switching in STO is still not well understood. In transition metal oxides, the positively charged oxygen vacancies is much more mobile than cations and can migrate to cathode which blocks the ion exchange reactions during the electroforming process. Therefore, an oxygen-deficient region starts to growth and expand towards the anode as the transition metal cations can capture electron from cathode reducing their valence state to accommodate this deficiency. This is also known as valence change mechanism (VCM). However, this kind ion-migration-based switching is still not well understood now and open questions remain about microscopic details of transport properties of the ions, the electronic charge transport of the conductive filament, detailed defect structure and the electrochemical redox reactions involved, and so forth. In this study, we will employ ab initio simulation methods e.g. density functional theory (DFT) and ab initio molecular dynamics (AIMD) to study driving mechanism of resistive switching in STO. A deep understanding on the resistive switching phenomenon in STO can be achieved in the end. Key factors that influence the resistive switching behavior will be proposed, which can be employed for designing and improving the performance of RRAM devices. 2. Last decade, several promising metal-organic frameworks (MOFs) were experimentally synthesized. Despite tremendous experimental efforts, it is difficult to unravel these materials only via experimental techniques. However, it is undoubtedly recognized by the scientific community that these materials have a large potential for valorization. Importantly to note is that these materials are highly tunable, as both the metal-(hydroxy)-oxide and the organic linkers can be modified. Furthermore, this feature makes it possible to find the right material for a particular application. Therefore, with a proper computational screening for a particular application, we could save a lot of experimental efforts. Focus within this part of the research project will be to study the electronic properties of modified UiO-66-type materials (initially synthesized as Zr-terephthalates) for water oxidation. Within these materials, it is well known, that defects (missing linkers and missing inorganic bricks) create other oxide terminations within the inorganic part. As such terminations are totally different compared to oxide materials, the behavior of this nano-oxide clusters for water oxidation can be altered at will by metal doping and linker exchange.