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A fascinating aspect of nanoscale ferroelectric materials is the emergence of topological polar textures, which include various complex and stable polarization configurations. The manipulation of such topological textures through external stimuli like electric fields holds promise for advanced nanoelectronics applications. There are, however, several challenges to reach potential applications, among which reliably creating and controlling these textures at the nanoscale on silicon, and with lead-free compounds. We report the realization of epitaxial BaTiO3 nanoislands on silicon, with a lateral size as small as 30-60 nm, and demonstrate stable center down-convergent polarization domains that can be reversibly switched by an electric field to center up-divergent domains. Piezoresponse force microscopy data reconstruction and phase field modeling give insight into the 3D patterns. The trapezoidal-shape nanoislands give rise to center down-convergent lateral swirling polarization component with respect to the nanoisland axis, which prevents the formation of bound charges on the side walls, therefore minimizing depolarization fields. The texture resembles a swirling vortex of liquid flowing into a narrowing funnel. Chirality emerges from the whirling polarization configurations. The ability to create and electrically manipulate chiral whirling polar textures in BaTiO3 nanostructures grown monolithically on silicon holds promise for applications in future topological nanoelectronics. Epitaxial BaTiO3 ultra scaled nanoislands (30−60 nm) on Si exhibit an electrically switchable topological center down-convergent polar pattern. The peculiar island shape induces a lateral swirling component, which confers chirality to the polar domains. [ABSTRACT FROM AUTHOR]

Amorphous gallium oxide thin films were grown by plasma-enhanced atomic layer deposition on (100) silicon substrates from trimethylgallium Ga(CH3)3 precursor and oxygen plasma. At 200 °C, the growth per cycle is in the range of 0.65–0.70 Å for O2 plasma exposure times ranging from 3 up to 30 s during each cycle. The effect of O2 plasma exposure times on the interfacial SiOx regrowth and the electrical properties was investigated. In situ spectroscopic ellipsometry shows that the SiOx regrowth occurs during the first three cycles and is limited to 0.27 nm for plasma times as long as 30 s. Increasing the O2 plasma exposure during each ALD cycle leads to a drastic decrease in the leakage current density (more than 5 orders of magnitude for 30 nm films), which is linked to the suppression of oxygen vacancy states as evidenced by spectroscopic ellipsometry. Interestingly, an increase in the dielectric constant with increasing O2 plasma exposure time is observed, reaching a value of ε r ∼ 14.2 , larger than that of single crystalline β-Ga2O3. This study highlights the crucial role of oxygen plasma exposure time in the control and tuning of the electrical properties of amorphous gallium oxide films. [ABSTRACT FROM AUTHOR]

Tetranuclear, intensely blue-coloured CuI complexes were synthesised in which two Cu2X3− units (X=Br or I) are bridged by a dicationic GFA (guanidino-functionalised aromatic) ligand. The UV/Vis spectra show a large metal-to-ligand charge-transfer (MLCT) band around 638 nm. The tetranuclear 'low-temperature' complexes are in a temperature-dependent equilibrium with dinuclear CuI 'high-temperature' complexes, which result from the reversible elimination of two CuX groups. A massive thermochromism effect results from the extinction of the strong MLCT band upon CuX elimination with increasing temperature. For all complexes, quantum chemical calculations predict a small and method-dependent energy difference between the possible electronic structures, namely CuI and dicationic GFA ligand (closed-shell singlet) versus CuII and neutral GFA ligand (triplet or broken-symmetry state). The closed-shell singlet state is disfavoured by hybrid-DFT functionals, which mix in exact Hartree-Fock exchange, and is favoured by larger basis sets and consideration of a polar medium. [ABSTRACT FROM AUTHOR]

We report the synthesis and properties of the new redox-active ligands 4,4′,5,5′-tetrakis(tetramethylguanidinyl)binaphthyl and 4,4′,5,5′-tetrakis( N, N′-dimethylethyleneguanidinyl)binaphthyl. The first dinuclear zinc complexes were prepared. Oxidative C-C coupling of these compounds leads to perylene derivatives. [ABSTRACT FROM AUTHOR]

The limited long-term stability of metal halide perovskite-based solar cells is a bottleneck in their drive toward widespread commercial adaptation. The organic hole-transport materials (HTMs) have been implicated in the degradation, and metal oxide layers are proposed as alternatives. One of the most prominent metal oxide HTM in organic photovoltaics is MoO 3 . However, the use of MoO 3 as HTM in metal halide perovskite-based devices causes a severe solar cell deterioration. Thus, the formation of the MoO 3 /CH 3 NH 3 PbI 3- x Cl x (MAPbI 3- x Cl x ) heterojunction is systematically studied by synchrotron-based hard X-ray photoelectron spectroscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and Raman spectroscopy. Upon MoO 3 deposition, significant chemical interaction is induced at the MoO 3 /MAPbI 3- x Cl x interface: substoichiometric molybdenum oxide is present, and the perovskite decomposes in the proximity of the interface, leading to accumulation of PbI 2 on the MoO 3 cover layer. Furthermore, we find evidence for the formation of new compounds such as PbMoO 4 , PbN 2 O 2 , and PbO as a result of the MAPbI 3- x Cl x decomposition and suggest chemical reaction pathways to describe the underlying mechanism. These findings suggest that the (direct) MoO 3 /MAPbI 3- x Cl x interface may be inherently unstable. It provides an explanation for the low power conversion efficiencies of metal halide perovskite solar cells that use MoO 3 as a hole-transport material and in which there is a direct contact between MoO 3 and perovskite.

The chemical and electronic structure of MoO 3 thin films is monitored by synchrotron-based hard X-ray photoelectron spectroscopy while annealing from room temperature to 310 °C. Color-coded 2D intensity maps of the Mo 3d and O 1s and valence band maximum (VBM) spectra show the evolution of the annealing-induced changes. Broadening of the Mo 3d and O 1s spectra indicate the reduction of MoO 3 . At moderate temperatures (120-200 °C), we find spectral evidence for the formation of Mo 5+ and at higher temperatures (>165 °C) also of Mo 4+ states. These states can be related to the spectral intensity above the VBM attributed to O vacancy induced gap states caused by partial filling of initially unoccupied Mo 4d-derived states. A clear relation between annealing temperature and the induced changes in the chemical and electronic structure suggests this approach as a route for deliberate tuning of MoO 3 thin-film properties., Competing Interests: The authors declare no competing financial interest.

We report on the first valence tautomeric dinuclear copper complex, featuring 2,3,5,6-tetrakis(tetramethylguanidino)pyridine as a bridging redox-active GFA (guanidino-functionalized aromatic) ligand. The preferred electronic structure of the complex is massively influenced by the environment. In the solid state and in nonpolar solvents a paramagnetic, dinuclear Cu(II) complex with a neutral GFA ligand is present. In polar solvents, the electronic structure changes to a diamagnetic, dinuclear Cu(I) complex with a twofold-oxidized GFA ligand. Using acetone as a solvent, both electronic structures are accessible due to a temperature-dependent equilibrium between the two valence tautomeric complexes. Our results pave the way for a broader use of valence tautomeric transition-metal complexes in catalytic reactions since anionic coligands can now be tolerated owing to the neutral/positively charged GFA ligand., (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Herein the synthesis, structures and properties of hydrogen-bonded aggregates involving redox-active guanidine superbases are reported. Reversible hydrogen bonding is switched on by oxidation of the hydrogen-donor unit, and leads to formation of aggregates in which the hydrogen-bond donor unit is sandwiched by two hydrogen-bond acceptor units. Further oxidation (of the acceptor units) leads again to deaggregation. Aggregate formation is associated with a distinct color change, and the electronic situation could be described as a frozen stage on the way to hydrogen transfer. A further increase in the basicity of the hydrogen-bond acceptor leads to deprotonation reactions., (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)