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.

The influence of non-equilibrium and equilibrium processes during growth of LaAlO3/SrTiO3 (LAO/STO) heterostructures is analyzed. We investigate the electronic properties of LAO/STO heterostructures obtained at constant growth conditions after annealing in different oxygen atmospheres within the typical growth window (1 × 10−4 mbar –1 × 10−2 mbar). The variation of annealing conditions is found to cause a similar change of electronic properties as observed for samples grown in different oxygen pressure. The results indicate that equilibrium defect formation is the dominant process for establishing the properties of the two-dimensional electron gas (2DEG), while growth dynamics play a minor role in the typical LAO/STO growth regime. Furthermore, the effects of non-equilibrium processes occurring during growth are investigated in detail by quenching just-grown LAO/STO heterostructures directly after growth. We show that during growth the sample is pushed into a non-equilibrium state. After growth, the sample then relaxes towards equilibrium, while the relaxation rate strongly depends on the ambient pressure. The observed relaxation behavior is mainly associated with a reoxidation of the STO bulk, while the 2DEG is formed immediately after the growth.

Metal–semiconductor Schottky interfaces are of high interest in many fields of semiconductor physics. One type of electronic devices based on Schottky contacts are resistive switching cells. The mostly applied analytical models are insufficient to describe all Schottky contact systems, which further impedes finding the correct conduction mechanism and may lead to physical misunderstandings. In this work, the electron transport properties of the resistively switching SrTiO3/Pt interface model system are investigated using a combination of experimental and theoretical methods. Temperature‐dependent I–V curves are measured and analyzed using an analytical approach, an atomistic approach based on density functional theory and the nonequilibrium Green's function formalism, and a continuum modeling approach. The findings suggest two different conduction mechanisms. Instead of a current transport over the barrier, as in the case of Schottky emission theory, the simulations show that tunneling through the Schottky barrier dominates. In the low voltage range, only thermally excited electrons can tunnel into the conduction band. For higher voltages, the SrTiO3 conduction band and the Fermi level at the injecting Pt‐electrode are aligned, allowing also electrons at the Fermi‐edge to tunnel. Consequently, the temperature dependence changes, leading to a crossing of the I–V curves at different temperatures.

The understanding of the molecular state of vanadium-oxo clusters (polyoxovanadates, POVs) in solution and on surface is a key to their target application in catalysis as well as molecular electronics and spintronics. We here report the results of a combined experimental and computational study of the behavior of nucleophilic polyoxoanions [V IV 10 V V 8 O 42 (I)] 5− charged balanced by Et 4 N + in water, in a one-phase organic solution of N,N-dimethylformamid (DMF) or acetonitrile (MeCN), in a mixed solution of MeCN−water, and at the hybrid liquid−surface interface. The molecular characteristics of the compound (NEt 4 ) 5 [V 18 O 42 (I)] (1) in the given environments were studied by microspectroscopic, electrochemical, scattering, and molecular mechanics methods. Contrary to the situation in pure water, where we observe great agglomeration with a number of intercalated H 2 O molecules between POVs that are surrounded by the Et 4 N + ions, no or only minor agglomeration of redox-active POVs in an unprecedented cation-mediated fashion was detected in pure DMF and MeCN, respectively. An inclusion of 1% water in the MeCN solution does not have an effect significant enough to reinforce agglomeration; however, this leads to the POV•••POV interface characterized by the presence of the Et 4 N + ions and a small number of H 2 O molecules. Water amounts of ≥5% trigger the formation of higher oligomers. The deposition of compound 1 from MeCN onto an Au(111) surface affords nearly roundshaped particles (∼10 nm). The use of DMF instead of MeCN results in bigger, irregularly shaped particles (∼30 nm). This change of solvent gives rise to more extensive intermolecular interactions between polyoxoanions and their countercations as well as weaker binding of ion-pairing induced agglomerates to the metallic substrate. Lower concentration of adsorbed molecules leads to a submonolayer coverage and an accompanied change of the POV's redox state, whereas their higher concentration results in a multilayer coverage that offers the pristine mixed-valence structure of the polyoxoanion. Our study provides first important insights into the reactivity peculiarities of this redox-responsive material class on a solid support.

Oxide-based metal-insulator-metal structures are of special interest for future resistive random-access memories. In such cells, redox processes on the nanoscale occur during resistive switching, which are initiated by the reversible movement of native donors, such as oxygen vacancies. The formation of these fi laments is mainly attributed to an enhanced oxygen diffusion due to Joule heating in an electric fi eld or due to electrical breakdown. Here, the development of a dendrite-like structure, which is induced by an avalanche discharge between the top electrode and the Ta 2 O 5-x layer, is presented, which occurs instead of a local breakdown between top and bottom electrode. The dendrite-like structure evolves primarily at structures with a pronounced interface adsorbate layer. Furthermore, local conductive atomic force microscopy reveals that the entire dendrite region becomes conductive. Via spectromicroscopy it is demonstrated that the subsequent switching is caused by a valence change between Ta 4+ and Ta 5+ , which takes place over the entire former Pt/Ta 2 O 5-x interface of the dendrite-like structure.