A detailed understanding of the resistive switching mechanisms that operate in redox-based resistive random-access memories (ReRAM) is key to controlling these memristive devices and formulating appropriate design rules. Based on distinct fundamental switching mechanisms, two types of ReRAM have emerged: electrochemical metallization memories, in which the mobile species is thought to be metal cations, and valence change memories, in which the mobile species is thought to be oxygen anions (or positively charged oxygen vacancies). Here we show, using scanning tunnelling microscopy and supported by potentiodynamic current-voltage measurements, that in three typical valence change memory materials (TaO(x), HfO(x) and TiO(x)) the host metal cations are mobile in films of 2 nm thickness. The cations can form metallic filaments and participate in the resistive switching process, illustrating that there is a bridge between the electrochemical metallization mechanism and the valence change mechanism. Reset/Set operations are, we suggest, driven by oxidation (passivation) and reduction reactions. For the Ta/Ta2O5 system, a rutile-type TaO2 film is believed to mediate switching, and we show that devices can be switched from a valence change mode to an electrochemical metallization mode by introducing an intermediate layer of amorphous carbon.

Transition-metal dichalcogenides (TMDs) have emerged in recent years as a special group of two-dimensional materials and have attracted tremendous attention. Among these TMD materials, molybdenum disulfide (MoS) has shown promising applications in electronics, photonics, energy, and electrochemistry. In particular, the defects in MoS play an essential role in altering the electronic, magnetic, optical, and catalytic properties of MoS, presenting a useful way to engineer the performance of MoS. The mechanisms by which lattice defects affect the MoS properties are unsettled. In this work, we reveal systematically how lattice defects and substrate interface affect MoS electronic structure. We fabricated single-layer MoS by chemical vapor deposition and then transferred onto Au, single-layer graphene, hexagonal boron nitride, and CeO as substrates and created defects in MoS by ion irradiation. We assessed how these defects and substrates affect the electronic structure of MoS by performing X-ray photoelectron spectroscopy, Raman and photoluminescence spectroscopies, and scanning tunneling microscopy/spectroscopy measurements. Molecular dynamics and first-principles based simulations allowed us to conclude the predominant lattice defects upon ion irradiation and associate those with the experimentally obtained electronic structure. We found that the substrates can tune the electronic energy levels in MoS due to charge transfer at the interface. Furthermore, the reduction state of CeO as an oxide substrate affects the interface charge transfer with MoS. The irradiated MoS had a faster hydrogen evolution kinetics compared to the as-prepared MoS, demonstrating the concept of defect controlled reactivity in this phase. Our findings provide effective probes for energy band and defects in MoS and show the importance of defect engineering in tuning the functionalities of MoS and other TMDs in electronics, optoelectronics, and electrochemistry.

The local electronic properties of tantalum oxide (TaO x , 2 ≤ x ≤ 2.5) and strontium ruthenate (SrRuO 3 ) thin-film surfaces were studied under the influence of electric fields induced by a scanning tunneling microscope (STM) tip. The switching between different redox states in both oxides is achieved without the need for physical electrical contact by controlling the magnitude and polarity of the applied voltage between the STM tip and the sample surface. We demonstrate for TaO x films that two switching mechanisms operate. Reduced tantalum oxide shows resistive switching due to the formation of metallic Ta, but partial oxidation of the samples changes the switching mechanism to one mediated mainly by oxygen vacancies. For SrRuO 3 , we found that the switching mechanism depends on the polarity of the applied voltage and involves formation, annihilation, and migration of oxygen vacancies. Although TaO x and SrRuO 3 differ significantly in their electronic and structural properties, the resistive switching mechanisms could be elaborated based on STM measurements, proving the general capability of this method for studying resistive switching phenomena in different classes of transition metal oxides. KEYWORDS: resistive switching, strontium ruthenate, tantalum oxide, scanning tunneling microscopy, electric field effect R edox-based resistance switching random access memories (ReRAMs) are considered as the next-generation memory devices to replace the present flash-based technology.1,2 ReRAMs have a simple metal−solid electrolyte− metal architecture, storing binary code information using the change in the resistance induced by filament formation and rupture, defining the low-resistive ON state (also denoted as LRS) and the high-resistive OFF state (or HRS), respectively. High scalability, CMOS compatibility, switching times in the subnanosecond range, excellent endurance and retention, and low power consumption are key but otherwise difficult-toduplicate features of ReRAM devices.

Plastic strain engineering was applied to induce controllable changes in electronic and oxygen ion conductivity in oxides by orders of magnitude, without changing their nominal composition. By using SrTiO 3 as a model system of technological importance, and by combining electrical and chemical tracer diffusion experiments with computational modeling, it is revealed that dislocations alter the equilibrium concentration and distribution of electronic and ionic defects. The easier reducibility of the dislocation cores increases the n-type conductivity by 50 times at oxygen pressures below 10 −5 atm at 650 °C. At higher oxygen pressures the p-type conductivity decreases by 50 times and the oxygen diffusion coefficient reduces by three orders of magnitude. The strongly altered electrical and oxygen diffusion properties in SrTiO 3 arise because of the existence of overlapping electrostatic fields around the positively charged dislocation cores. The findings and the approach are broadly important and have the potential for significantly impacting the functionalities of electrochemical and/or electronic applications such as thin film oxide electronics, memristive systems, sensors, micro-solid oxide fuel cells, and catalysts, whose functionalities rely on the concentration and distribution of charged point defects.

One-dimensional defects are created in [001] and [110] oriented TiO 2 single crystals by uniaxial pressure. Transmission electron microscopy (TEM) characterization shows them to preferably lie on {110} planes. Electrical properties studied as a function of oxygen partial pressure reveal their infl uence on ionic and electronic charge carriers. At high oxygen partial pressures (1 bar-10 − 5 bar) the conductivity due to positive charge carriers is strongly enhanced, e.g., the ionic conductivity is increased by more than two orders of magnitude, when the electrical measurement axis lies on the slip plane. In contrary, no changes are observed when the measurement axis does not lie on the slip planes. At low oxygen partial pressures ( < 10 − 15 bar), irrespective of orientation and presence of dislocation, there is no change in the n-type conductivity. The observed phenomena can be well explained within the space charge model, assuming the dislocation cores to exhibit an excess negative charge (increased titanium vacancy concentration). The present study gives a clear correlation between line defects and point defect concentrations in such an oxide for the fi rst time.