Chemical Engineering of Cu-Sn Disordered Network Metamaterials.

The design and fabrication of large-area metamaterials is an ongoing challenge. In the present work, we propose a scalable design route and low-footprint strategy for the production of large-area, frequency-selective Cu-Sn disordered network metamaterials with quasi-perfect absorption. The nanoscale networks combine the robustness of disordered systems with the broad-band optical response known from connected wire-mesh metamaterials. Using experiments and simulations, we show how frequency-selective absorption in the networks can be designed and controlled. We observe a linear dependence of the optical response as a function of Sn content ranging from the near-infrared to the visible region. The absorbing state exhibits strong sensitivity to both changes in the global network topology and the chemistry of the network. We probe the plasmonic response of these nanometric networks by electron energy loss spectroscopy (EELS), where we resolve extremely confined gap surface-plasmon (GSP) modes.

Surface Analysis of Perovskite Oxynitride Thin Films as Photoelectrodes for Solar Water Splitting.

Perovskite oxynitride semiconductors have attracted huge interest recently as promising photoelectrode materials for photoelectrochemical (PEC) water splitting. Depicted by, the extensive studies of the PEC activity of oxynitride powder-based photoelectrodes and/or deposited thin-film electrodes. High-crystalline-quality, oxynitride thin films grown by physical vapor deposition are ideal model systems to study the fundamental physical and chemical properties of the surface of these materials, including their evolution. In this work, using a combination of high-sensitivity low-energy ion scattering (LEIS) and X-ray photoelectron spectroscopy (XPS), we monitor surface evolution of LaTiOxNy (LTON) and CaNbOxNy (CNON) thin films before and after the PEC characterizations. The as-prepared epitaxial LTON films show a preferential LaO termination at the surface layers, followed by a Ti-enriched subsurface. Whereas, the polycrystalline CNON thin films exhibit a non-uniform surface, with a mixed surface termination and a significant Ca-segregated subsurface. After the PEC characterizations, additional precipitated LaO species are found on the outer surface of the LTON epitaxial films. However, no significant surface change is observed on the polycrystalline CNON films by LEIS. The XPS analysis shows, an increase of the oxidized Ti and Nb cations (Ti4+ and Nb5+) after the PEC reaction in the LTON and CNON films, respectively. The initial drops in photocurrent for the LTON and CNON films are attributed to the changes in the surface chemical status. This work provides insight into the surface characteristics and evolution of LTON and CNON oxynitride thin films as photoelectrodes for PEC applications.

Electroless Deposition of Ni-Fe Alloys on Scaffolds for 3D Nanomagnetism.

3D magnetic nanostructures are of great interest due to the possibility to design novel properties and the benefits for both technological applications such as high-density data storage, as well as more fundamental studies. One of the main challenges facing the realization of these three-dimensional systems is their fabrication, which includes the deposition of magnetic materials on 3D surfaces. In this work, the electroless deposition of Ni-Fe on a 3D-printed, non-conductive microstructure is presented. The deposited films exhibit low coercivity, with the saturation magnetization and composition corresponding to the archetypal soft magnetic material permalloy. For fundamental studies of 3D micromagnetism, this new development in fabrication offers the possibility to combine the flexibility of 3D nanofabrication techniques such as two-photon lithography for the fabrication of 3D scaffolds with a homogeneous soft ferromagnetic thin film, and thus represents an important step toward exploring the rich physics of complex 3D magnetic architectures with tailored properties and the development of advanced applications.

Persistent Double-Layer Formation in Kesterite Solar Cells: A Critical Review.

In kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cell research, an asymmetric crystallization profile is often obtained after annealing, resulting in a bilayered - or double-layered - CZTSSe absorber. So far, only segregated pieces of research exist to characterize the appearance of this double layer, its formation dynamics, and its effect on the performances of devices. In this work, we review the existing research on double-layered kesterites and evaluate the different mechanisms proposed. Using a cosputtering-based approach, we show that the two layers can differ significantly in morphology, composition, and optoelectronic properties and complement the results with a large statistical data set of over 850 individual CZTS solar cells. By reducing the absorber thickness from above 1000 to 300 nm, we show that the double-layer segregation is alleviated. In turn, we see a progressive improvement in the device performance for lower thickness, which alone would be inconsistent with the well-known case of ultrathin CIGS solar cells. We therefore attribute the improvements to the reduced double-layer occurrence and find that the double layer limits the efficiency of our devices to below 7%. By comparing the results with CZTS grown on monocrystalline Si substrates, without a native Na supply, we show that the alkali metal supply does not determine the double-layer formation but merely reduces the threshold for its occurrence. Instead, we propose that the main formation mechanism is the early migration of Cu to the surface during annealing and formation of Cu2-xS phases in a self-regulating process akin to the Kirkendall effect. Finally, we comment on the generality of the mechanism proposed by comparing our results to other synthesis routes, including our own in-house results from solution processing and pulsed laser deposition of sulfide- and oxide-based targets. We find that although the double-layer occurrence largely depends on the kesterite synthesis route, the common factors determining the double-layer occurrence appear to be the presence of metallic Cu and/or a chalcogen deficiency in the precursor matrix. We suggest that understanding the limitations imposed by the double-layer dynamics could prove useful to pave the way for breaking the 13% efficiency barrier for this technology.

Proximity-Induced Odd-Frequency Superconductivity in a Topological Insulator.

At an interface between a topological insulator (TI) and a conventional superconductor (SC), superconductivity has been predicted to change dramatically and exhibit novel correlations. In particular, the induced superconductivity by an s-wave SC in a TI can develop an order parameter with a p-wave component. Here we present experimental evidence for an unexpected proximity-induced novel superconducting state in a thin layer of the prototypical TI, Bi_{2}Se_{3} proximity coupled to Nb. From depth-resolved magnetic field measurements below the superconducting transition temperature of Nb, we observe a local enhancement of the magnetic field in Bi_{2}Se_{3} that exceeds the externally applied field, thus supporting the existence of an intrinsic paramagnetic Meissner effect arising from an odd-frequency superconducting state. Our experimental results are complemented by theoretical calculations supporting the appearance of such a component at the interface which extends into the TI. This state is topologically distinct from the conventional Bardeen-Cooper-Schrieffer state it originates from. To the best of our knowledge, these findings present a first observation of bulk odd-frequency superconductivity in a TI. We thus reaffirm the potential of the TI-SC interface as a versatile platform to produce novel superconducting states.

Examining the surface evolution of LaTiOxNy an oxynitride solar water splitting photocatalyst.

LaTiOxNy oxynitride thin films are employed to study the surface modifications at the solid-liquid interface that occur during photoelectrocatalytic water splitting. Neutron reflectometry and grazing incidence x-ray absorption spectroscopy were utilised to distinguish between the surface and bulk signals, with a surface sensitivity of 3 nm. Here we show, contrary to what is typically assumed, that the A cations are active sites that undergo oxidation at the surface as a consequence of the water splitting process. Whereas, the B cations undergo local disordering with the valence state remaining unchanged. This surface modification reduces the overall water splitting efficiency, but is suppressed when the oxynitride thin films are decorated with a co-catalyst. With this example we present the possibilities of surface sensitive studies using techniques capable of operando measurements in water, opening up new opportunities for applications to other materials and for surface sensitive, operando studies of the water splitting process.

Revealing the role of lattice distortions in the hydrogen-induced metal-insulator transition of SmNiO3.

The discovery of hydrogen-induced electronic phase transitions in strongly correlated materials such as rare-earth nickelates has opened up a new paradigm in regulating materials' properties for both fundamental study and technological applications. However, the microscopic understanding of how protons and electrons behave in the phase transition is lacking, mainly due to the difficulty in the characterization of the hydrogen doping level. Here, we demonstrate the quantification and trajectory of hydrogen in strain-regulated SmNiO3 by using nuclear reaction analysis. Introducing 2.4% of elastic strain in SmNiO3 reduces the incorporated hydrogen concentration from ~1021 cm-3 to ~1020 cm-3. Unexpectedly, despite a lower hydrogen concentration, a more significant modification in resistivity is observed for tensile-strained SmNiO3, substantially different from the previous understanding. We argue that this transition is explained by an intermediate metastable state occurring in the transient diffusion process of hydrogen, despite the absence of hydrogen at the post-transition stage.

Anodizing of Self-Passivating W xTi1- x Precursors for W xTi1- xO n Oxide Alloys with Tailored Stability.

TiO2 and WO3 are two of the most important, industrially relevant earth-abundant oxides. Although both materials show complementary functionality and are promising candidates for similar types of applications such as catalysis, sensor technology, and energy conversion, their chemical stability in reactive environments differs remarkably. In this study, anodic barrier oxides are grown on solid-solution W xTi1- x alloy precursors covering a wide compositional range (0 ≤ x ≤ 1) with the goal of creating functional oxides with tailored stability. A strong Ti-cation enrichment in the surface region of the grown W xTi1- xO n layer is observed, which can be controlled by both the anodizing conditions and precursor composition. For Ti concentrations above 50 at. %, a continuous nanometer-thick TiO2 protective coating is achieved on top of a homogeneous W xTi1- xO n film as evidenced by X-ray photoelectron spectroscopy and transmission electron microscopy analyses. A comprehensive electrochemical assessment demonstrates a very stable passivation of the surface in both acidic and alkaline environments. This increase in chemical stability correlates directly with the presence of this protective TiO2 film. The results of this work provide insights into the oxidation behavior of W1- xTi x alloys, but more importantly demonstrate how controlled oxidation of self-passivating alloys can lead to oxide alloys with thin, protective surface layers that otherwise would require more sophisticated deposition methods.

Epitaxial Thin Films as a Model System for Li-Ion Conductivity in Li4Ti5O12.

Using an epitaxial thin-film model system deposited by pulsed laser deposition (PLD), we study the Li-ion conductivity in Li4Ti5O12, a common anode material for Li-ion batteries. Epitaxy, phase purity, and film composition across the film thickness are verified employing out-of-plane and in-plane X-ray diffraction, transmission electron microscopy, time-of-flight mass spectrometry, and elastic recoil detection analysis. We find that epitaxial Li4Ti5O12 behaves like an ideal ionic conductor that is well described by a parallel RC equivalent circuit, with an ionic conductivity of 2.5 × 10-5 S/cm at 230 °C and an activation energy of 0.79 eV in the measured temperature range of 205 to 350 °C. Differently, in a co-deposited polycrystalline Li4Ti5O12 thin film with an average in-plane grain size of <10 nm, a more complex behavior with contributions from two distinct processes is observed. Ultimately, epitaxial Li4Ti5O12 thin films can be grown by PLD and reveal suitable transport properties for further implementation as zero-strain and grain boundary free anodes in future solid-state microbattery designs.

An in-situ small angle x ray scattering analysis of nanopore formation during thermally induced chemical dealloying of brass thin foils.

The development of non-noble nano-porous metal materials is hindered by surface oxidation reactions and from the difficulty to generate long range order pore arrays. Dealloying is a promising route to generate such materials by selective chemical etching of metal alloy materials. This process can generate nano-metal materials with superior plasmonic, catalytic and adsorptive surface properties. Here, the impact of properties of the etching solution on the dealloying process to generate nano-pores across thin film alloys was investigated by in-situ SAXS dealloying experiments. Single phase CuZn alloys were used as model materials to evaluate the influence of the solution temperature on the pore formation kinetics. This novel analysis allowed to visualize the change in surface properties of the materials over time, including their surface area as well as their pore and ligament sizes. The dealloying kinetics at the very early stage of the process were found to be critical to both stable pore formation and stabilization. SAXS in-situ data were correlated to the morphological properties of the materials obtained from ex-situ samples by Rutherford back scattering and scanning electron microscopy.

Stoichiometric control of the density of states in PbS colloidal quantum dot solids.

Colloidal quantum dots, and nanostructured semiconductors in general, carry the promise of overcoming the limitations of classical materials in chemical and physical properties and in processability. However, sufficient control of electronic properties, such as carrier concentration and carrier mobility, has not been achieved until now, limiting their application. In bulk semiconductors, modifications of electronic properties are obtained by alloying or doping, an approach that is not viable for structures in which the surface is dominant. The electronic properties of PbS colloidal quantum dot films are fine-tuned by adjusting their stoichiometry, using the large surface area of the nanoscale building blocks. We achieve an improvement of more than two orders of magnitude in the hole mobility, from below 10-3 to above 0.1 cm2/V⋅s, by substituting the iodide ligands with sulfide while keeping the electron mobility stable (~1 cm2/V⋅s). This approach is not possible in bulk semiconductors, and the developed method will likely contribute to the improvement of solar cell efficiencies through better carrier extraction and to the realization of complex (opto)electronic devices.

Determination of Conduction and Valence Band Electronic Structure of LaTiOx Ny Thin Film.

The nitrogen substitution into the oxygen sites of several oxide materials leads to a reduction of the band gap to the visible-light energy range, which makes these oxynitride semiconductors potential photocatalysts for efficient solar water splitting. Oxynitrides typically show a different crystal structure compared to the pristine oxide material. As the band gap is correlated to both the chemical composition and the crystal structure, it is not trivial to distinguish which modifications of the electronic structure induced by the nitrogen substitution are related to compositional and/or structural effects. Here, X-ray emission and absorption spectroscopy are used to investigate the electronic structures of orthorhombic perovskite LaTiOx Ny thin films in comparison with films of the pristine oxide LaTiOx with similar orthorhombic structure and cationic oxidation state. Experiment and theory show the expected upward shift in energy of the valence band maximum that reduces the band gap as a consequence of the nitrogen incorporation. This study also shows that the conduction band minimum, typically considered almost unaffected by nitrogen substitution, undergoes a significant downward shift in energy. For a rational design of oxynitride photocatalysts, the observed changes of both the unoccupied and occupied electronic states have to be taken into account to justify the total band-gap narrowing induced by the nitrogen incorporation.

Three-Legged 2,2'-Bipyridine Monomer at the Air/Water Interface: Monolayer Structure and Reactions with Ni(II) Ions from the Subphase.

The behavior of compound 2 [1,3,5-tri(2,2'-bipyridin-5-yl)benzene] with three bipyridine units arranged in a star geometry is investigated in the presence and absence of Ni(ClO4)2. Its properties at the air-water interface as well as after transfer onto a solid substrate are studied by several techniques including Brewster angle microscopy, X-ray reflectivity, neutron reflectivity, X-ray photoelectron spectroscopy, Rutherford backscattering spectrometry, and atomic force microscopy combined with optical microscopy. It is found that compound 2 within the monolayers formed stays almost vertical at the interface and that at high Ni2+/2 (Ni2+/2 = 4000, 20'000) ratios two of the three bipyridine units of 2 are complexed, resulting in supramolecular sheets that are likely composed of arrays of linear metal-organic complexation polymers.

Scalable, ultra-resistant structural colors based on network metamaterials.

Structural colors have drawn wide attention for their potential as a future printing technology for various applications, ranging from biomimetic tissues to adaptive camouflage materials. However, an efficient approach to realize robust colors with a scalable fabrication technique is still lacking, hampering the realization of practical applications with this platform. Here, we develop a new approach based on large-scale network metamaterials that combine dealloyed subwavelength structures at the nanoscale with lossless, ultra-thin dielectric coatings. By using theory and experiments, we show how subwavelength dielectric coatings control a mechanism of resonant light coupling with epsilon-near-zero regions generated in the metallic network, generating the formation of saturated structural colors that cover a wide portion of the spectrum. Ellipsometry measurements support the efficient observation of these colors, even at angles of 70°. The network-like architecture of these nanomaterials allows for high mechanical resistance, which is quantified in a series of nano-scratch tests. With such remarkable properties, these metastructures represent a robust design technology for real-world, large-scale commercial applications.

Silicon etch with chromium ions generated by a filtered or non-filtered cathodic arc discharge.

The pre-treatment of substrate surfaces prior to deposition is important for the adhesion of physical vapour deposition coatings. This work investigates Si surfaces after the bombardment by energetic Cr ions which are created in cathodic arc discharges. The effect of the pre-treatment is analysed by X-ray diffraction, Rutherford backscattering spectroscopy, scanning electron microscopy and in-depth X-ray photoemission spectroscopy and compared for Cr vapour produced from a filtered and non-filtered cathodic arc discharge. Cr coverage as a function of ion energy was also predicted by TRIDYN Monte Carlo calculations. Discrepancies between measured and simulated values in the transition regime between layer growth and surface removal can be explained by the chemical reactions between Cr ions and the Si substrate or between the substrate surface and the residual gases. Simulations help to find optimum and more stable parameters for specific film and substrate combinations faster than trial-and-error procedure.

The Hidden Pathways in Dense Energy Materials - Oxygen at Defects in Nanocrystalline Metals.

Highly abundant oxygen-rich line defects (blue) can act as fast oxygen transport paths. These defects show similar chemistry and therefore similar catalytic activity to the materials surface. These results provide the opportunity to design and produce simple scalable structures as catalysts, whose functionality derives from internal defects rather than from the materials surfaces.

Hematite-NiO/α-Ni(OH)2 heterostructure photoanodes with high electrocatalytic current density and charge storage capacity.

Control of the water splitting reaction in the context of natural photosynthesis is considered as a Holy Grail of chemistry, particularly with respect to artificial photosynthesis for a sustainable energy economy. The underlying objective is to build a solar fuel generator which is economically viable and environmentally benign. Hydrogen generation by solar water splitting in photoelectrochemical cells (PEC) is currently experiencing a renaissance, and the search for high performance but low-cost photoelectrode materials is an on-going quest. We present here a photoanode heterostructure of hematite and NiO/α-Ni(OH)2, which is very efficient. We prepared the heterostructure by a "two reactor" hydrothermal modification of a pristine hematite film. The system shows promising current density of 16 mA cm(-2), several times higher than that of the pristine hematite film. In addition, the system shows charge storing capacity once exposed to AM 1.5 simulated sunlight, along with electrochromic behaviour. Interestingly, the water splitting proceeds as a dark reaction after several hours of light exposure. The abrupt increase in current density originates from the oxidized Ni(OH)2 layer which is absent in the case of pn-junction-like devices made by mere deposition of NiO on hematite by thermal annealing. Hematite alone shows no such behaviour. This kind of new PEC electrode offers a low-cost and simple way for the dual purpose applications of water splitting and charge storage.

Dealloying of platinum-aluminum thin films: dynamics of pattern formation.

The application of focused ion beam (FIB) nanotomography and Rutherford backscattering spectroscopy (RBS) to dealloyed platinum-aluminum thin films allows for an in-depth analysis of the dominating physical mechanisms of nanoporosity formation during the dealloying process. The porosity formation due to the dissolution of the less noble aluminum in the alloy is treated as result of a reaction-diffusion system. The RBS and FIB analysis yields that the porosity evolution has to be regarded as superposition of two independent processes, a linearly propagating diffusion front with a uniform speed and a slower dissolution process in regions which have already been passed by the diffusion front. The experimentally observed front evolution is captured by the Fisher-Kolmogorov-Petrovskii-Piskounov (FKPP). The slower dissolution is represented by a zero-order rate law which causes a gradual porosity in the thin film.

Ion implanted optical waveguides in nonlinear optical organic crystal.

We report for the first time to our knowledge optical waveguiding in an organic crystalline waveguide produced by ion implantation. Using H+ ions a refractive index barrier suitable for waveguiding has been realized in the highly nonlinear optical organic crystal 4-N, N-dimethylamino-4'-N'-methyl-stilbazolium tosylate (DAST). The refractive index changes in the waveguiding region as a function of the distance from the surface have been measured. Maximal refractive index changes of up to -0.2 and -0.1 at wavelengths of 633nm and 810nm have been realized, respectively. The waveguide refractive index profiles as a function of the ion fluence have been determined. Planar waveguiding has been demonstrated by polishing sharp edges and using conventional end-fire coupling. The measured losses are approximately 7 dB/cm at 1.57mum.