Insights into molecular cluster materials with adamantane-like core structures by considering dimer interactions.

A class of adamantane-like molecular materials attracts attention because they exhibit an extreme non-linear optical response and emit a broad white-light spectrum after illumination with a continuous-wave infrared laser source. According to recent studies, not only the nature of the cluster molecules, but also the macroscopic structure of the materials determines their non-linear optical properties. Here we present a systematic study of cluster dimers of the compounds AdR4 and [(RT)4 S6 ] (T = Si, Ge, Sn) with R = methyl, phenyl or 1-naphthyl to gain fundamental knowledge about the interactions in the materials. For all compounds, a similar type of dimer structures with a staggered arrangement of substituents was determined as the energetically most favorable configuration. The binding energy between the dimers, determined by including London dispersion interactions, increases with the size of the core and the substituents. The cluster interactions can be classified as substituent-substituent-dominated (small cores, large substituents) or core-core-dominated (large cores, small substituents). Among various possible dimer conformers, those with small core-core distances are energetically preferred. Trimer and tetramer clusters display similar trends regarding the minimal core-core distances and binding energies. The much lower energy barrier determined for the rotation of substituents as compared to the rotation of the cluster dimers past each other indicates that the rotation of substituents more easily leads to different conformers in the material. Thus, understanding the interaction of the cluster dimers allows an initial assessment of the interactions in the materials.

Unravelling the role of dopants in the electrocatalytic activity of ceria towards CO2 reduction in solid oxide electrolysis cells.

CO2 reduction in Solid Oxide Electrolysis Cells (SOECs) is a key-technology for the transition to a sustainable energy infrastructure and chemical industry. Ceria (CeO2) holds great promise in developing highly efficient, cost-effective and durable fuel electrodes, due to its promising electrocatalytic properties, and proven ability to suppress carbon deposition and to tolerate high concentrations of impurities. In the present work, we investigate the intrinsic electrocatalytic activity of ceria towards CO2 reduction by means of electrochemical impedance spectroscopy (EIS) on model systems with well-defined geometry, composition and surface area. Aiming at the optimization of the intrinsic catalytic properties of the material, we systematically study the effect of different dopants (Zr, Gd, Pr and Bi) on the reaction rate under varying operating conditions (temperature, gas composition and applied polarization) relevant for SOECs. The electrochemical measurements reveal the dominant role of the surface defect chemistry of the material in the reaction rate, with doping having only a mild effect on the rate and activation energy of the reaction. By analyzing the pO2 and overpotential dependence of the reaction rate with a general micro-kinetic model, we are able to identify the second electron transfer as the rate limiting step of the process, highlighting the dominant role of surface polarons in the energy landscape. These insights on the correlation between the surface defects and the electrocatalytic activity of ceria open new directions for the development of highly performing ceria-based technological electrodes.

On the thermoelectric properties of Nb-doped SrTiO3 epitaxial thin films.

The exploration for thermoelectric thin films of complex oxides such as SrTiO3-based oxides is driven by the need for miniaturized harvesting devices for powering the Internet of Things (IoT). However, there is still not a clear consensus in the literature for the underlying influence of film thickness on thermoelectric properties. Here, we report the fabrication of epitaxial thin films of 6% Nb-doped SrTiO3 on (001) (LaAlO3)0.3(Sr2AlTaO6)0.7 (LSAT) single crystal using pulsed laser deposition (PLD) where the film thickness was varied from 2 nm to 68 nm. The thickness dependence shows a subtle increase of tetragonality of the thin film lattice and a gradual drop of the electrical conductivity, the density of charge carriers, and the thermoelectric Seebeck coefficient as the film thickness decreases. DFT-based calculations show that ∼2.8% increase in tetragonality results in an increased splitting between t2g and eg orbitals to ∼42.3 meV. However, experimentally observed tetragonality for films between 68 to 13 nm is only 0.06%. Hence, the effect of thickness on tetragonality is neglected. We have discussed the decrease of conductivity and the Seebeck coefficient based on the decrease of carriers and change in the scattering mechanism, respectively.

Enforced Long-Range Order in 1D Wires by Coupling to Higher Dimensions.

One-dimensional wires are known to be inherently unstable at finite temperature. Here, we show that long-range order of atomic Au double chains adsorbed on a Si(553) surface is not only stabilized by interaction with the substrate, but spontaneous self-healing of structural defects is actually enforced by the adsorption of atomic species such as Au or H. This is true even for random adsorbate distribution. Combining atomistic models within density functional theory with low energy electron diffraction and high-resolution electron energy loss spectroscopy, we demonstrate that this apparently counterintuitive behavior is mainly caused by adsorption-induced band filling of modified surface bands, i.e., by the strong electronic correlation throughout the whole terrace. Although adsorption preferably occurs at the step edge, it enhances the dimerization and the stiffness of the Au dimers. Thus, the intertwinement of quasi-1D properties with delocalized 2D effects enforces the atomic wire order.

Electromechanically active pair dynamics in a Gd-doped ceria single crystal.

Oxygen-defective ceria, e.g. Gd-doped ceria, shows giant electromechanical properties related to a complex local rearrangement of its lattice. Although they are not entirely identified, the electroactive mechanisms arise from cation and oxygen vacancy (VO) pairs (i.e. Ce-VO), and the local structural elastic distortion in their surroundings. Here, we study the geometry and behaviour of Ce-VO pairs in a grain boundary-free bulk Ce0.9Gd0.1O1.95 single crystal under an AC electric field of ca. 11 kV cm-1. The analysis was carried out through X-ray absorption spectroscopy (XAS) techniques at the Ce L-III edge. Using Density Functional Theory (DFT) calculations, we investigated the effects of the strain on density of states and orbitals at the valence band edge. Our research indicates that electrostriction increases at low temperatures. The electromechanical strain has a structural nature and can rise by one order of magnitude, i.e., from 5 × 10-4 at room temperature to 5 × 10-3 at -193 °C, due to an increase in the population of the electrically active pairs. At a constant VO concentration, the material can thus configure heterogeneous pairs and elastic nanodomains that are either mechanically responsive or not.

Towards Understanding the Reactivity and Optical Properties of Organosilicon Sulfide Clusters.

We report the extension of the class of organotetrel sulfide clusters with further examples of the still rare silicon-based species, synthesized from RSiCl3 with R=phenyl (Ph, I), naphthyl (Np, II), and styryl (Sty, III) with Na2 S. Besides known [(PhSi)4 S6 ] (IV), new compounds [(NpSi)4 S6 ] (1) and [(StySi)4 S6 ] (2) were obtained, the first two of which underwent reactions with [AuCl(PPh3 )] to form ternary complexes. DFT studies of cluster dimers helped us understand the differences between the habit of {Si4 S6 }- and {Sn4 S6 }-based compounds. Crystalline 1 showed a pronounced nonlinear optical response, while for intrinsically amorphous 2, the chemical damage threshold seems to inhibit a corresponding observation. Calculations within the independent particle approximation served to rationalize and compare electronic and optical excitations of [(RSi)4 S6 ] clusters (R=Ph, Np). The calculations reproduced the measured data and allowed for the interpretation of the main spectroscopic features.

Atomic-scale insights into electro-steric substitutional chemistry of cerium oxide.

Cerium oxide (ceria, CeO2) is one of the most promising mixed ionic and electronic conducting materials. Previous atomistic analysis has widely covered the effects of substitution on oxygen vacancy migration. However, an in-depth analysis of the role of cation substitution beyond trivalent cations has rarely been explored. Here, we investigate soluble monovalent (Li+, Na+, K+, Rb+), divalent (Fe2+, Co2+, Mn2+, Mg2+, Ni2+, Zn2+, Cd2+, Ca2+, Sr2+, Ba2+), trivalent (Al3+, Fe3+, Sc3+, In3+, Lu3+, Yb3+, Y3+, Er3+, Gd3+, Eu3+, Nd3+, Pr3+, La3+) and tetravalent (Si4+, Ge4+, Ti4+, Sn4+, Hf4+, Zr4+) cation substituents. By combining classical simulations and quantum mechanical calculations, we provide an insight into defect association energies between substituent cations and oxygen vacancies as well as their effects on the diffusion mechanisms. Our simulations indicate that oxygen ionic diffusivity of subvalent cation-substituted systems follows the order Gd3+ > Ca2+ > Na+. With the same charge, a larger size mismatch with the Ce4+ cation yields a lower oxygen ionic diffusivity, i.e., Na+ > K+, Ca2+ > Ni2+, Gd3+ > Al3+. Based on these trends, we identify species that could tune the oxygen ionic diffusivity: we estimate that the optimum oxygen vacancy concentration for achieving fast oxygen ionic transport is ≈2.5% for GdxCe1-xO2-x/2, CaxCe1-xO2-x and NaxCe1-xO2-3x/2 at 800 K. Remarkably, such a concentration is not constant and shifts gradually to higher values as the temperature is increased. We find that co-substitutions can enhance the impact of the single substitutions beyond that expected by their simple addition. Furthermore, we identify preferential oxygen ion migration pathways, which illustrate the electro-steric effects of substituent cations in determining the energy barrier of oxygen ion migration. Such fundamental insights into the factors that govern the oxygen diffusion coefficient and migration energy would enable design criteria to be defined for tuning the ionic properties of the material, e.g., by co-substitutions.

Near interface ionic transport in oxygen vacancy stabilized cubic zirconium oxide thin films.

The cubic phase of pure zirconia (ZrO2) is stabilized in dense thin films through a controlled introduction of oxygen vacancies (O defects) by cold-plasma-based sputtering deposition. Here, we show that the cubic crystals present at the film/substrate interface near-region exhibit fast ionic transport, which is superior to what is obtained with similar yttrium-stabilized cubic zirconia thin films.

Impact of carbon-ion implantation on the nonlinear optical susceptibility of LiNbO3.

In this work we study the impact of ion implantation on the nonlinear optical properties in MgO:LiNbO3 via confocal second-harmonic microscopy. In detail, we spatially characterize the nonlinear susceptibility in carbon-ion implanted lithium niobate planar waveguides for different implantation energies and fluences, as well as the effect of annealing. In a further step, a computational simulation is used to calculate the implantation range of carbon-ions and the corresponding defect density distribution. A comparison between the simulation and the experimental data indicates that the depth profile of the second-order effective nonlinear coefficient is directly connected to the defect density that is induced by the ion irradiation. Furthermore it can be demonstrated that the annealing treatment partially recovers the second-order optical susceptibility.

Enhancement of the chemical stability in confined δ-Bi2O3.

Bismuth-oxide-based materials are the building blocks for modern ferroelectrics, multiferroics, gas sensors, light photocatalysts and fuel cells. Although the cubic fluorite δ-phase of bismuth oxide (δ-Bi2O3) exhibits the highest conductivity of known solid-state oxygen ion conductors, its instability prevents use at low temperature. Here we demonstrate the possibility of stabilizing δ-Bi2O3 using highly coherent interfaces of alternating layers of Er2O3-stabilized δ-Bi2O3 and Gd2O3-doped CeO2. Remarkably, an exceptionally high chemical stability in reducing conditions and redox cycles at high temperature, usually unattainable for Bi2O3-based materials, is achieved. Even more interestingly, at low oxygen partial pressure the layered material shows anomalous high conductivity, equal or superior to pure δ-Bi2O3 in air. This suggests a strategy to design and stabilize new materials that are comprised of intrinsically unstable but high-performing component materials.

Modeling intrinsic defects in LiNbO3 within the Slater-Janak transition state model.

Intrinsic point defects in LiNbO3, i.e., isolated Nb antisites and Li as well Nb vacancies, are investigated from first-principles within the Slater-Janak transition state model. Thereby the electronic structure of the investigated defects is calculated with hybrid exchange-correlation functionals. This approach allows for the calculation of charge transition levels without comparing the total energies of differently charged supercells. The obtained results are in agreement with previous hybrid density-functional theory calculations based on total-energy differences. Li and Nb vacancies can be formed in the V(-)(Li) and V(5-)(Nb) charge states only, as long as the host is not strongly p-type or n-type, respectively. NbLi antisites may capture one or two electrons, forming the defect states often referred to as small bound polaron and bi-polaron.

Adamantane-type clusters: compounds with a ubiquitous architecture but a wide variety of compositions and unexpected materials properties.

The research into adamantane-type compounds has gained momentum in recent years, yielding remarkable new applications for this class of materials. In particular, organic adamantane derivatives (AdR4) or inorganic adamantane-type compounds of the general formula [(RT)4E6] (R: organic substituent; T: group 14 atom C, Si, Ge, Sn; E: chalcogenide atom S, Se, Te, or CH2) were shown to exhibit strong nonlinear optical (NLO) properties, either second-harmonic generation (SHG) or an unprecedented type of highly-directed white-light generation (WLG) - depending on their respective crystalline or amorphous nature. The (missing) crystallinity, as well as the maximum wavelengths of the optical transitions, are controlled by the clusters' elemental composition and by the nature of the organic groups R. Very recently, it has been additionally shown that cluster cores with increased inhomogeneity, like the one in compounds [RSi{CH2Sn(E)R'}3], not only affect the chemical properties, such as increased robustness and reversible melting behaviour, but that such 'cluster glasses' form a conceptually new basis for their use in light conversion devices. These findings are likely only the tip of the iceberg, as beside elemental combinations including group 14 and group 16 elements, many more adamantane-type clusters (on the one hand) and related architectures representing extensions of adamantane-type clusters (on the other hand) are known, but have not yet been addressed in terms of their opto-electronic properties. In this review, we therefore present a survey of all known classes of adanmantane-type compounds and their respective synthetic access as well as their optical properties, if reported.

Small electron polarons bound to interstitial tantalum defects in lithium tantalate.

The absorption features of optically generated, short-lived small bound electron polarons are inspected in congruent lithium tantalate, LiTaO3(LT), in order to address the question whether it is possible to localize electrons at interstitial TaV:VLidefect pairs by strong, short-range electron-phonon coupling. Solid-state photoabsorption spectroscopy under light exposure and density functional theory are used for an experimental and theoretical access to the spectral features of small bound polaron states and to calculate the binding energies of the small bound TaLi4+(antisite) and TaV4+:VLi(interstitial site) electron polarons. As a result, two energetically well separated (ΔE≈0.5 eV) absorption features with a distinct dependence on the probe light polarization and peaking at 1.6 eV and 2.1 eV are discovered. We contrast our results to the interpretation of a single small bound TaLi4+electron state with strong anisotropy of the lattice distortion and discuss the optical generation of interstitial TaV4+:VLismall polarons in the framework of optical gating of TaV4+:TaTa4+bipolarons. We can conclude that the appearance of carrier localization at TaV:VLimust be considered as additional intermediate state for the 3D hopping transport mechanisms at room temperature in addition to TaLi, as well, and, thus, impacts a variety of optical, photoelectrical and electrical applications of LT in nonlinear photonics. Furthermore, it is envisaged that LT represents a promising model system for the further examination of the small-polaron based photogalvanic effect in polar oxides with the unique feature of two, energetically well separated small polaron states.

Atomistic picture of charge density wave formation at surfaces.

We used ultrafast electron diffraction and density-functional theory calculations to gain insight into the charge density wave (CDW) formation on In/Si(111). Weak excitation by a femtosecond-laser pulse results in the melting of the CDW. The immediate freezing is hindered by a barrier for the motion of atoms during the phase transition: The melted CDW constitutes a long-lived, supercooled phase and is strong evidence for a first-order transition. The freezing into the CDW is triggered by preexisting adsorbates. Starting at these condensation nuclei, the CDW expands one dimensionally on the In/Si(111) surface, with a constant velocity of more than 80 m/s.

Cluster-Glass for Low-Cost White-Light Emission.

The development of efficient and high-brilliance white-light sources is an essential contribution to innovative emission technologies. Materials exhibiting strong nonlinear optical properties, in particular second-harmonic generation (SHG) or white-light generation (WLG), have therefore been investigated with great activity in recent times. While many new approaches have been reported until now, the processability of the compounds remains a challenge. Here, a new class of materials, denoted as "cluster-glass", which do not only show superior white-light emission properties upon irradiation by an inexpensive continuous-wave infrared laser diode, but can be easily accommodated in size and shape by formation of robust glassy solids, is introduced. The cluster-glass materials are fabricated by mild heating from crystalline powders of adamantane-type clusters exhibiting a quaternary, inorganic-organic hybrid cluster core [(PhSi)(CH2 )3 (PhSn)E3 ] (E  =  S, Se, Te). The process is fully reversible and preserves the integrity of the clusters in the glass, as proven by solution spectroscopy and recrystallization. Theoretical studies corroborate the importance of the quaternary nature of the cluster cores for the observed structural and optical phenomena. Thanks to these findings, high-brilliance white-light sources can be synthesized in form of stable, robust glass of any shape, which ultimately renders them suitable for everyday's applications.

High proton conduction in grain-boundary-free yttrium-doped barium zirconate films grown by pulsed laser deposition.

Reducing the operating temperature in the 500-750 °C range is needed for widespread use of solid oxide fuel cells (SOFCs). Proton-conducting oxides are gaining wide interest as electrolyte materials for this aim. We report the fabrication of BaZr(0.8)Y(0.2)O(3-δ) (BZY) proton-conducting electrolyte thin films by pulsed laser deposition on different single-crystalline substrates. Highly textured, epitaxially oriented BZY films were obtained on (100)-oriented MgO substrates, showing the largest proton conductivity ever reported for BZY samples, being 0.11 S cm(-1) at 500 °C. The excellent crystalline quality of BZY films allowed for the first time the experimental measurement of the large BZY bulk conductivity above 300 °C, expected in the absence of blocking grain boundaries. The measured proton conductivity is also significantly larger than the conductivity values of oxygen-ion conductors in the same temperature range, opening new potential for the development of miniaturized SOFCs for portable power supply.

Evolution of Topological Surface States Following Sb Layer Adsorption on Bi2Se3.

Thin antimony layers adsorbed on bismuth selenide (Bi2Se3) present an exciting topological insulator system. Much recent effort has been made to understand the synthesis and electronic properties of the heterostructure, particularly the migration of the topological surface states under adsorption. However, the intertwinement of the topological surface states of the pristine Bi2Se3 substrate with the Sb adlayer remains unclear. In this theoretical work, we apply density functional theory (DFT) to model heterostructures of single and double atomic layers of Sb on a bismuth selenide substrate. We thereby discuss established and alternative structural models, as well as the hybridization of topological surface states with the Sb states. Concerning the geometry, we reveal the possibility of structures with inverted Sb layers which are energetically close to the established ones. The formation energy differences are below 10 meV/atom. Concerning the hybridization, we trace the band structure evolution as a function of the adlayer-substrate distance. By following changes in the connection between the Kramers pairs, we extract a series of topological phase transitions. This allows us to explain the origin of the complex band structure, and ultimately complete our knowledge about this peculiar system.

Spectroscopic Analysis of Rare-Earth Silicide Structures on the Si(111) Surface.

Two-dimensional rare-earth silicide layers deposited on silicon substrates have been intensively investigated in the last decade, as they can be exploited both as Ohmic contacts or as photodetectors, depending on the substrate doping. In this study, we characterize rare-earth silicide layers on the Si(111) surface by a spectroscopic analysis. In detail, we combine Raman and reflectance anisotropy spectroscopy (RAS) with first-principles calculations in the framework of the density functional theory. RAS suggests a weakly isotropic surface, and Raman spectroscopy reveals the presence of surface localized phonons. Atomistic calculations allow to assign the detected Raman peaks to phonon modes localized at the silicide layer. The good agreement between the calculations and the measurements provides a strong argument for the employed structural model.

Water Splitting Reaction at Polar Lithium Niobate Surfaces.

Water splitting is a highly promising, environmentally friendly approach for hydrogen production. It is often discussed in the context of carbon dioxide free combustion and storage of electrical energy after conversion to chemical energy. Since the oxidation and reduction reactions are related to significant overpotentials, the search for suitable catalysts is of particular importance. Ferroelectric materials, for example, lithium niobate, attracted considerable interest in this respect. Indeed, the presence of surfaces with different polarizations and chemistries leads to spatial separation of reduction and oxidation reactions, which are expected to be boosted by the electrons and holes available at the positive and negative surfaces, respectively. Employing the density functional theory and a simplified thermodynamic approach, we estimate the overpotentials related to the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) on both polar LiNbO3 (0001) surfaces. Our calculations performed for ideal surfaces in vacuum predict the lowest overpotential for the hydrogen evolution reaction (0.4 V) and for the oxygen evolution reaction (1.2 V) at the positive and at the negative surfaces, respectively, which are lower than (or comparable with) commonly employed catalysts. However, calculations performed to model the aqueous solution in which the reactions occur reveal that the presence of water substantially increases the required overpotential for the HER, even inverting the favorable polarization direction for oxidation and reduction reactions. In aqueous solution, we predict an overpotential of 1.2 V for the HER at the negative surface and 1.1 V for the OER at the positive surface.

Electrochemical stability of (La,Sr)CoO3-δ in (La,Sr)CoO3-δ/(Ce, Gd)O2-δ heterostructures.

A modulated coherent (La,Sr)CoO3-δ/(Ce,Gd)O2-δ heterostructure is characterized for the first time for its electronic and chemical properties. 2D-multilayer architectures are deposited on NdGaO3 (110) single crystal substrate by pulsed laser deposition, resulting in epitaxial structures with in-plane lattice rotation that, via the metal oxides' interfaces, induces mutual structural rearrangements. Our results show that (La,Sr)CoO3-d thin films of 10-100 nm are chemically unstable when exposed to air at 600 °C during electrical cyclic stress-tests. Conversely, improved stability is achieved confining LSC in the nanometric heterostructure. Remarkably, the chemical stabilization occurs without compromising substantially the electrical properties of the LSC component: the heterostructures show unexpected electrical behaviour with dominant electronic contributions, fast conductivity and mixed ionic-electronic properties, depending on the number of interfaces and the nano-scaled layers.