Antiphase Boundaries Constitute Fast Cation Diffusion Paths in SrTiO3 Memristive Devices

AbstractResistive switching in transition metal oxide‐based metal‐insulator‐metal structures relies on the reversible drift of ions under an applied electric field on the nanoscale. In such structures, the formation of conductive filaments is believed to be induced by the electric‐field driven migration of oxygen anions, while the cation sublattice is often considered to be inactive. This simple mechanistic picture of the switching process is incomplete as both oxygen anions and metal cations have been previously identified as mobile species under device operation. Here, spectromicroscopic techniques combined with atomistic simulations to elucidate the diffusion and drift processes that take place in the resistive switching model material SrTiO3 are used. It is demonstrated that the conductive filament in epitaxial SrTiO3 devices is not homogenous but exhibits a complex microstructure. Specifically, the filament consists of a conductive Ti3+‐rich region and insulating Sr‐rich islands. Transmission electron microscopy shows that the Sr‐rich islands emerge above Ruddlesden–Popper type antiphase boundaries. The role of these extended defects is clarified by molecular static and molecular dynamic simulations, which reveal that the Ruddlesden–Popper antiphase boundaries constitute diffusion fast‐paths for Sr cations in the perovskites structure

Conductive Self-Assembled Monolayers of Paramagnetic {CoIICo4III} and {Co4IICo2III} Coordination Clusters on Gold Surfaces

Two polynuclear cobalt(II,III) complexes, [Co5(N3)4(N-n-bda)4(bza·SMe)2] (1) and [Co6(N3)4(N-n-bda)2(bza·SMe)5(MeOH)4]Cl (2), where Hbza·SMe = 4-(methylthio)benzoic acid and N-n-H2bda = N-n-butyldiethanolamine, were synthesized and fully characterized by various techniques. Compound 1 exhibits an unusual, approximately C2-symmetric {CoII Co4II } core of two isosceles Co3 triangles with perpendicularly oriented planes, sharing a central, high-spin CoII ion residing in a distorted tetrahedral coordination environment. This central CoII ion is connected to four outer, octahedrally coordinated low-spin CoIII ions via oxo bridges. Compound 2 comprises a semi-circular {Co4IICo2III } motif of four non-interacting high-spin CoII and two low-spin CoIII centers in octahedral coordination environments. Self-assembled monolayers (SAMs) of 1 and 2 were physisorbed on template-stripped gold surfaces contacted by an eutectic gallium-indium (EGaIn) tip. The acquired current density-voltage (I-V) data revealed that the cobalt-based SAMs are more electrically robust than those of the previously reported dinuclear {CuIILnIII} complexes with Ln = Gd, Tb, Dy, or Y (Schmitz et al., 2018a). In addition, between 170 and 220°C, the neutral, mixed-valence compound 1 undergoes a redox modification, yielding a {Co5}-based coordination cluster (1-A) with five non-interacting, high-spin octahedral CoII centers as indicated by SQUID magnetometry analysis in combination with X-ray photoelectron spectroscopy and infrared spectroscopy. Solvothermal treatment of 1 results in a high-nuclearity coordination cluster, [Co10(N3)2(N-n-bda)6(bza·SMe)6] (3), containing 10 virtually non-interacting high-spin CoII centers. © Copyright © 2019 Schmitz, Qiu, Glöß, van Leusen, Izarova, Nadeem, Griebel, Chiechi, Kögerler and Monakhov

Conductive Self-Assembled Monolayers of Paramagnetic {CoII Co 4 III } and { Co 4 II Co 2 III } Coordination Clusters on Gold Surfaces

Two polynuclear cobalt(II,III) complexes, [Co5(N3)4(N-n-bda)4(bza·SMe)2] (1) and [Co6(N3)4(N-n-bda)2(bza·SMe)5(MeOH)4]Cl (2), where Hbza·SMe = 4-(methylthio)benzoic acid and N-n-H2bda = N-n-butyldiethanolamine, were synthesized and fully characterized by various techniques. Compound 1 exhibits an unusual, approximately C2-symmetric {CoIICoIII4} core of two isosceles Co3 triangles with perpendicularly oriented planes, sharing a central, high-spin CoII ion residing in a distorted tetrahedral coordination environment. This central CoII ion is connected to four outer, octahedrally coordinated low-spin CoIII ions via oxo bridges. Compound 2 comprises a semi-circular {CoII4CoIII2} motif of four non-interacting high-spin CoII and two low-spin CoIII centers in octahedral coordination environments. Self-assembled monolayers (SAMs) of 1 and 2 were physisorbed on template-stripped gold surfaces contacted by an eutectic gallium-indium (EGaIn) tip. The acquired current density-voltage (I-V) data revealed that the cobalt-based SAMs are more electrically robust than those of the previously reported dinuclear {CuIILnIII} complexes with Ln = Gd, Tb, Dy, or Y (Schmitz et al., 2018a). In addition, between 170 and 220°C, the neutral, mixed-valence compound 1 undergoes a redox modification, yielding a {Co5}-based coordination cluster (1-A) with five non-interacting, high-spin octahedral CoII centers as indicated by SQUID magnetometry analysis in combination with X-ray photoelectron spectroscopy and infrared spectroscopy. Solvothermal treatment of 1 results in a high-nuclearity coordination cluster, [Co10(N3)2(N-n-bda)6(bza·SMe)6] (3), containing 10 virtually non-interacting high-spin CoII centers

Exploring the Ligand Functionality, Electronic Band Gaps, and Switching Characteristics of Single Wells–Dawson‐Type Polyoxometalates on Gold

The miniaturization, high performance, energy efficiency, and new added functionalities are the essential drivers of modern information data storage and processing technologies. Polyoxometalates (POMs) characterized by atomically well-defined structures with discrete energy levels and the ability to undergo redox transformations are viewed as promising active components for the integration into the next-generation (beyond-CMOS) hybrid nanoelectronics. Herein, new fundamental insights into the application of organically augmented POMs on conducting surfaces are offered. Three key findings resulting from scanning probe investigations combined with integral spectroscopic methods used to explore tris(alkoxo)-ligated, vanadium-containing Wells-Dawson-type POM structures on Au(111) are reported on. First, it is shown how the (OCH2)3C–R ligands, depending on the structurally exposed R group (R = CH2SMe and NHCOC6H4SMe), influence the self-assembly behavior of the synthesized POMs on gold. Second, the impact of the employed (OCH2)3C–R ligands and the determined assembly characteristics on the relative position of POM's electronic band structure against the Fermi level of the gold surface are explained. Third, the on-surface conductance switching of single POM structures due to external electrical stimuli is demonstrated. The author's experimental efforts enable to discover highly sought-after multi-level resistive switching orchestrated by electrically accessible V(3d) states in the POM single-molecules at room temperature in a narrow voltage range

Antiphase Boundaries Constitute Fast Cation Diffusion Paths in SrTiO3 Memristive Devices

Resistive switching in transition metal oxide-based metal-insulator-metal structures relies on the reversible drift of ions under an applied electric field on the nanoscale. In such structures, the formation of conductive filaments is believed to be induced by the electric-field driven migration of oxygen anions, while the cation sublattice is often considered to be inactive. This simple mechanistic picture of the switching process is incomplete as both oxygen anions and metal cations have been previously identified as mobile species under device operation. Here, spectromicroscopic techniques combined with atomistic simulations to elucidate the diffusion and drift processes that take place in the resistive switching model material SrTiO3 are used. It is demonstrated that the conductive filament in epitaxial SrTiO3 devices is not homogenous but exhibits a complex microstructure. Specifically, the filament consists of a conductive Ti3+-rich region and insulating Sr-rich islands. Transmission electron microscopy shows that the Sr-rich islands emerge above Ruddlesden–Popper type antiphase boundaries. The role of these extended defects is clarified by molecular static and molecular dynamic simulations, which reveal that the Ruddlesden–Popper antiphase boundaries constitute diffusion fast-paths for Sr cations in the perovskites structure. © 2020 The Authors. Published by Wiley-VCH Gmb

Antiphase Boundaries Constitute Fast Cation Diffusion Paths in SrTiO 3 Memristive Devices

Resistive switching in transition metal oxide‐based metal‐insulator‐metal structures relies on the reversible drift of ions under an applied electric field on the nanoscale. In such structures, the formation of conductive filaments is believed to be induced by the electric‐field driven migration of oxygen anions, while the cation sublattice is often considered to be inactive. This simple mechanistic picture of the switching process is incomplete as both oxygen anions and metal cations have been previously identified as mobile species under device operation. Here, spectromicroscopic techniques combined with atomistic simulations to elucidate the diffusion and drift processes that take place in the resistive switching model material SrTiO3 are used. It is demonstrated that the conductive filament in epitaxial SrTiO3 devices is not homogenous but exhibits a complex microstructure. Specifically, the filament consists of a conductive Ti3+‐rich region and insulating Sr‐rich islands. Transmission electron microscopy shows that the Sr‐rich islands emerge above Ruddlesden–Popper type antiphase boundaries. The role of these extended defects is clarified by molecular static and molecular dynamic simulations, which reveal that the Ruddlesden–Popper antiphase boundaries constitute diffusion fast‐paths for Sr cations in the perovskites structure

CCDC 1936118: Experimental Crystal Structure Determination

Related Article: Sebastian Schmitz, Xinkai Qiu, Maria Glöß, Jan van Leusen, Natalya V. Izarova, Muhammad Arif Nadeem, Ryan C. Chiechi, Paul Kögerler, Kirill Yu. Monakhov|2019|Frontiers in Chemistry|7|681|doi:10.3389/fchem.2019.0068