Search

Lead-free ceramics based on Na1/2Bi1/2TiO3 (NBT) have been shown to exhibit excellent ferroelectric properties putting them amongst the best materials to replace lead-based piezoelectrics. The defect chemistry of NBT is, however, very complex. High oxygen ionic conductivity can be induced by acceptor doping or bismuth evaporation, which is quite detrimental to the ferroelectric properties. Nevertheless, this conductivity is non-linearly dependent on the acceptor concentration, which allows for tuning of NBT-based material from highly ionically conducting to highly resistive. This unique behavior raises the question of whether NBT ceramics also need to be treated differently with respect to high electric fields as they are used for ferroelectric and dielectric applications. High field electrodegradation experiments have been performed on NBT and a solid solution with BaTiO3 (BT) to elucidate this. It could be shown that reversible electroformation can be induced, which is non-linearly dependent on acceptor concentration. Additionally, a fast surface degradation process could be identified, which could be attributed to a reversible field-induced change in composition at the anode due to sodium becoming mobile. These results will be of high importance for reliability investigations of NBT based material and also hint towards possible applications in resistive switching memory applications.

Antiferroelectric materials exhibit a unique electric-field-induced phase transition, which enables their use in energy storage, electrocaloric cooling, and nonvolatile memory applications. However, in many prototype antiferroelectrics this transition is irreversible, which prevents their implementation. In this work, we demonstrate a general approach to promote the reversibility of this phase transition by targeted modification of the material's local structure. A new NaNbO3-based composition, namely (1− x)NaNbO3−xSrSnO3, was designed with a combination of first-principles calculations and experimental characterization. Our theoretical study predicts stabilization of the antiferroelectric state over the ferroelectric state with an energy difference of 1.4 meV/f.u. when 6.25 mol % of SrSnO3 is incorporated into NaNbO3. A series of samples was prepared using solid-state reactions, and the structural changes upon SrSnO3 incorporation were investigated using X-ray diffraction and 23Na solid-state nuclear magnetic resonance spectroscopy. The results revealed an increase in the unit cell volume and a more disordered, yet less distorted local Na environment, which were related to the stabilization of the antiferroelectric order. The SrSnO3-modified compositions exhibited well-defined double polarization loops and an eight times higher energy storage density as compared to unmodified NaNbO3. Our results indicate that this first-principles calculations based approach is of great potential for the design of new antiferroelectric compositions.

β-CuGaO2 possesses a direct band gap of 1.5 eV and is therefore expected as a suitable light absorber layer for thin-film solar cells. To evaluate the potential of β-CuGaO2 as a light absorber, the...

The exchange of 18O between gaseous molecular oxygen and thin-film samples of Ce0.99Gd0.01O1.995 with two different, nominal surface orientations [(111) and (110)] was studied. Oxygen isotope exchange experiments were conducted in the temperature range of 573 ≤ T/K ≤ 673 at an oxygen activity of aO2 = 0.2. Subsequently, secondary ion mass spectrometry (SIMS) measurements were performed on the thin-film samples to obtain 18O isotope depth profiles. All 18O diffusion profiles showed two features, suggesting spatially nonuniform oxygen tracer diffusion coefficients in the samples. A numerical solution to the diffusion equation was used to describe the experimental profiles and yielded oxygen tracer diffusion coefficients D* and oxygen surface exchange coefficients k*. Values of D* obtained were found, surprisingly, to be different for the two orientations and also orders of magnitude lower than values for ceramic samples in this temperature range. As possible explanations, we examine quantitatively the effect of halide anion impurities and the effect of ultrasmall columnar grains on oxygen tracer diffusion. Surface exchange coefficients for the (111) oriented surface were found to be roughly 1 order of magnitude higher than those for (110). We discuss two possible explanations for the observed behavior: the enrichment of anion impurities at the surface and the interaction between the surface and vapor water in the gas phase.

Different magnetron sputtering‐based deposition methods of nickel oxide SiO₂‐passivated Si surfaces are compared. Results highlight that the presence of oxygen in the deposition chamber during reactive sputtering drastically affects the Si/SiO₂ interface. An alternative method for the preparation of NiO is the sputtering of metallic nickel in oxygen‐free atmosphere followed by a post oxidation of the deposited layer in an oxygen atmosphere without plasma exposition is proposed. This method is introduced as metal layer oxidation (MLO). Using this technique, the barrier height on n‐type silicon increases from ≈0.4 eV for reactively sputtered NiO to more than 0.6 eV for the MLO method. In situ photoelectron spectroscopy evidences the formation of an extra electronic state when NiO is reactively sputtered, which is assigned to the intense oxygen ion bombardment of the Si/SiO₂ surface during the process. This extra‐electronic state pins the silicon energy bands in an undesirable position. The extra‐electronic state is associated with oxygen interstitial in the SiO₂ implanted during reactive sputtering.

In order to answer the question of whether Schottky barriers on polar ZnO surfaces are different at Zn- and O-terminated surfaces, the interface formation of $n$-type ZnO and different high work function metals and metal oxides (Pt, ${\mathrm{PtO}}_{x}$, and ${\mathrm{RuO}}_{2}$) with Schottky barrier heights of up to 1.5 eV has been studied using photoelectron spectroscopy with in situ sample preparation. The experiments are designed to exclude the effects of substrate reduction and consequent Fermi level pinning by high concentrations of oxygen vacancies. Moreover, by including the Zn LMM Auger emission in the analysis, it is demonstrated that an accurate extraction of barrier heights needs to take into account that the screening of the photoelectron core hole can change in the course of contact formation. The polarization dependence of Schottky barriers, which is important for piezotronic applications, is in most cases dominated by the influence of defects. Reducing the influence of defects, up to $\ensuremath{\sim}240$ meV higher Schottky barriers are revealed on oxygen-terminated surfaces. This is opposite to what has been reported in the literature but agrees with the dependence of barrier heights expected for an incomplete screening of the polarization of ZnO by the electrode as for ferroelectric materials.

Due to its simplicity, flexibility and conformity, electroless plating presents itself as an attractive route towards functional metal nanostructures. Despite the importance for creating multimetallic materials with enhanced properties, the complex interactions between the components in electroless plating baths make alloy formations a challenging objective. In this work, we outline an electroless plating strategy fabricating Pd−Pt alloy nanomaterials, which is based on arbitrarily miscible plating baths for the individual metals. To demonstrate the excellent nanoscale conformity and homogeneity of our plating system, we apply it to ion track‐etched polymer templates with large inner surfaces as ambitious substrates, resulting in the formation of 3D free‐standing PdₓPt₁₀₀₋ₓ‐nanotube‐networks (NTNWs). Based on the electro‐oxidation of methanol as a model reaction, we utilize the compositional freedom provided by our syntheses for optimizing the catalytic performance of our metal NTNWs, which heavily depends on the Pd−Pt ratio. Within our system, the highest surface normalized activity was found for the Pd₂₀Pt₈₀ NTNW, reaching more than a two‐fold increase of the peak current density in comparison to pure Pt. Overall, our reaction system provides a versatile toolkit for fabricating intricate Pd−Pt nanostructures of arbitrary elemental composition, and constitutes a starting point for designing new electroless alloy plating baths.

The (photo)electrochemical water-splitting mechanism has not yet been completely resolved on an atomic level, partly because the interaction between water and the (photo)electrocatalytic surface has not yet been completely elucidated. Because of its ability to oxidize water upon light exposure, BiVO4 was chosen in this work as a prototype photocatalytic material to investigate the effects of water on a pristine metal oxide surface. In particular, polycrystalline surface-cleaned BiVO4 samples were exposed to water in controlled experiments at room temperature and at 77 K, analyzing any electronic and chemical changes at the surface by applying photoelectron spectroscopy. The results show that upon water exposure, hydroxide species form, the Fermi level position rises, the amount of oxygen at the surface decreases, and bismuth and vanadium partially reduces. Accordingly, the interaction of water with bismuth vanadate seemingly follows a reductive water dissociation process.

We studied the behavior of oxygen interstitials in donor-doped ceria using equilibrium conductivity measurements, conductivity relaxation experiments, and (18O2/16O2) isotope exchange experiments. ...

Barium Titanate and Ni-based multilayer ceramic capacitors have wide commercial applicability, and interfaces are critical to the overall device behavior as they can help control unwanted leakage currents. Here we make use of photoemission methods to investigate the electrostatic barriers formed at BaTiO3/Ni(O) interfaces to understand the implications for electron injection. We find the interface Fermi level in BaTiO3 to evolve smoothly during Ni deposition with a Schottky barrier height for electrons of 0.68 eV, whereas with NiO the Fermi level evolves rapidly with an electron injection barrier of 1.49 eV. In-situ poling shows the Schottky barrier at the BaTiO3/Ni interface is not significantly altered by ferroelectric polarization, consistent with the good screening of the Ni electrode. This study presents a direct quantitative measurement of the interface barrier heights and highlights the significance of the oxidation state of the electrode.

A transition to couple an antenna to spoof plasmon-polariton waveguides at terahertz frequencies is presented. The optimization of the transition is discussed. Examples of how antenna coupling can enable THz on-chip applications are given in a biosensor and a phase shifter. The sensing of deeply subwavelength biological cells demonstrates the high sensitivity to even small concentrations of bacteria. And while the phase shifter only exhibits a small angle range, its high transmission, and easy integration makes it suitable for applications such as an on-chip phased array.

The electronic properties and growth morphology of Si(111)/ZnS heterostructure modified by a GaSe van der Waals termination layer, which provides a chemical and an electronic passivation of the Si(111)���surface is investigated by surface sensitive methods (STM, LEED and SXPS). The van der Waals termination layer suppresses the Si���S interface reaction, which is observed for non terminated substrates. The interface electronic properties are nearly unchanged, but the morphology of the growing ZnS���film is dramatically changed. With the passivation layer ZnS grows as orientated pyramides with a (111) base area surface orientaion and (100)���facets. The sticking coefficient is reduced by a factor of ���10 compared to bare Si(111), where ZnS grows as an untextured polycrystalline layer.

In this investigation, we studied electronic properties of the CdS/Zn1���xMgxO (x=0,0.15) interface using photoelectron spectroscopy. ZnO and (Zn,Mg)O films were deposited by magnetron sputtering from ceramic targets on thermally evaporated CdS. Valence-band offsets of ��EV=1.2��0.1 eV are determined for both interfaces. The gap difference of 0.3 eV between ZnO and Zn0.85Mg0.15O is therefore fully accommodated by a different conduction-band energy, which should be well suited for modulation doping in ZnO/(Zn,Mg)O heterostructures.

Under electrical bias, mixed ionic conductors such as SrTiO��� are characterized by oxygen vacancy migration which leads to resistance degradation. The defect chemistry to describe the relationship between conductivity and oxygen vacancies is usually obtained by high temperature conductivity data or quenching experiments. These techniques can investigate the equilibrated state only. Here, we introduce a new approach using in-situ impedance studies with applied dc voltage to analyze the temperature dependent electrical properties of degraded SrTiO��� single crystals. This procedure is most beneficial since it includes electric field driven effects. The benefits of the approach are highlighted by comparing acceptor doped and undoped SrTiO���. This approach allows the determination of the temperature activation of both anodic and cathodic conductivity of Fe-doped SrTiO��� in the degraded state. The anodic activation energy matches well with the published results, while the activation energy of the degraded cathode region reported here is not in agreement with earlier assumptions. The specific discrepancies of the experimental data and the published defect chemistry are discussed, and a defect chemistry model that includes the strong temperature dependence of the electron conductivity in the cathode region is proposed.

Doping limits, band gaps, work functions and energy band alignments of undoped and donor-doped transparent conducting oxides Zn���, In���O���, and SnO��� as accessed by X-ray and ultraviolet photoelectron spectroscopy (XPS/UPS) are summarized and compared. The presented collection provides an extensive data set of technologically relevant electronic properties of photovoltaic transparent electrode materials and illustrates how these relate to the underlying defect chemistry, the dependence of surface dipoles on crystallographic orientation and/or surface termination, and Fermi level pinning.

Valence-band photoelectron spectroscopy of CuInSe���, CuInS���, and CuGaSe��� surfaces and interfaces give evidence for the formation of Cu vacancies when the Fermi level moves upwards in the band gap due to contact formation. The effect might be a key issue in understanding basic properties of solar cell devices based on these materials.

International audience; Facet-engineered monoclinic scheelite BiVO4 particles decorated with various cocatalysts were successfully synthesized by selective sunlight photodeposition of metal or metal oxy(hydroxide) nanoparticles onto the facets of truncated bipyramidal BiVO4 monoclinic crystals coexposing {010} and {110} facets. X-ray photoelectron spectroscopy, scanning electron microscopy, and scanning Auger microscopy revealed that metallic silver (Ag) and cobalt (oxy)hydroxide (CoOx(OH)y) particles were selectively deposited onto the {010} and {110} facets, respectively, regardless of the cocatalyst amount. By contrast, the nickel (oxy)hydroxide (NiOx(OH)y) photodeposition depends on the nickel precursor amount with an unprecedented selectivity for 0.1 wt % NiOx(OH)y/BiVO4 with a preferential deposition onto the {010} facets and the edges between the {110} facets. Moreover, these noble metal-free heterostructures led to remarkable photocatalytic properties for rhodamine B photodecomposition and sacrificial water oxidation reactions. For instance, 0.2 wt % CoOx(OH)y/BiVO4 led to one of the highest oxygen evolution rates, i.e., 1538 μmol h–1 g–1, ever described which is ten times higher than that found for bare BiVO4. The selective deposition of cobalt (oxy)hydroxide species onto the more electron-deficient facet of truncated bipyramidal monoclinic BiVO4 particles favors photogenerated charge carrier separation and therefore plays a key role for efficient photochemical oxygen evolution.

Bead foams serve in a wide variety of applications, from insulation and packaging to midsoles in shoes. However, the currently used materials are limited to somewhat low temperature or exhibit significant changes in modulus in the temperature range of many applications due to their glass transition. By comparison, polycarbonate (PC) exhibits almost constant mechanics for temperatures up to 130 &deg, C. Therefore, it appears as an advantageous base material for bead foams. The aim of the publication is to provide comprehensive data on the properties of expanded PC (EPC) in comparison to already commercially available expanded polypropylene, EPP, and expanded polyethylene-terephthalate, EPET. A special focus is set on the thermo-mechanical properties as these are the most lacking features in current materials. In this frame, dynamic mechanical analysis, and tensile, bending, compression and impact tests at room temperature (RT), 80 &deg, C, and 110 &deg, C are conducted for the three materials of the same density. Already at RT, EPC exhibits superior mechanics compared to its peers, which becomes more pronounced toward higher temperature. This comes from the low sensitivity of properties to temperature as EPC is used below its glass transition. In summary, EPC proves to be an outstanding foam material over a broad range of temperatures for structural applications.

More than the sum of its parts! Moving from binary to ternary blends of active materials in cathode materials for lithium ion batteries provides a versatile design beyond the properties of the individual components. Synergistic effects improve the rate capability, power, thermal, and chemical stability of the blend. Overall advantageous performance is achieved while economizing 25 % cobalt and nickel of a NMC622 based electrode. The combination of two active materials into one positive electrode of a lithium-ion battery is an uncomplicated and cost-effective way to combine the advantages of different active materials while reducing the disadvantages of each material. In this work, the concept of binary blends is extended to ternary compositions. The combination of three different active materials provides high versatility in designing the properties of an electrode. Therefore, the unique properties of a layered oxide, phospho-olivine, and spinel type material are mixed to design a high-energy cathode with improved environmental friendliness. Four different compositions of blend electrodes are investigated, each with individual benefits. Synergistic effects improved the rate capability, power density, thermal and chemical stability simultaneously. The blend electrode consisting of 75 % NMC, 12.5 % LMFP and LMO provides similar energy and power density as a pure NMC electrode while economizing 25 % cobalt and nickel., publishedVersion

An approach to determine the defect energy levels of the Fe impurities in BaTiO3 and SrTiO3 single crystals using electrical conductance measurements is presented. The defect levels are obtained from the dependence of the activation energy of electrical transport on the oxygen vacancy concentration, which is varied by stepwise re-oxidation of a reduced sample. An energy level at 0.7–0.8 eV below the conduction band minimum ECB is identified for BaTiO3, which can be assigned to the Fe2+/3+-transition in good agreement with literature. In contrast, the conductivity of Fe-doped SrTiO3 does not show a defect energy level in the upper half of the band gap, indicating that the Fe2+/3+-transition in SrTiO3 is near the conduction band minimum. The often reported alignment of defect energy levels, which is fulfilled for the Fe3+/4+-transition in BaTiO3 and SrTiO3, does not hold for the Fe2+/3+-transition in these compounds. This limits the applicability of Fe-doped SrTiO3 as a model system for studying resistance degradation in acceptor-doped high-permittivity dielectrics.

Oriented NiO films ((111), (100), and (110)), reactively direct current-sputtered on oriented platinum films on top of MgO substrates, were studied electrochemically on a rotating disk electrode for their performance toward the oxygen evolution reaction (OER). From the results, two electrochemical domains can be defined. At low overpotential ( 1.55 V), the ranking is NiO(110) > NiO(111) > NiO(100). The observed differences were attributed to the fact that NiO(111) and NiO(100) produce an hydroxide phase with the same electrocatalytic properties, but the NiO(100) sample is covered less by the hydroxide and thus produces less active sites. Moreover, NiO(110) forms the more stable and catalytically most active hydroxide phase. We attribute the observed differences to the formation of the β-NiOOH phase on the NiO(110) orientation, being more active than γ-NiOOH, which is su...

Polycrystalline anatase thin films, (001)- and (101)-oriented anatase TiO 2 single crystals and (001)- and (110)-oriented rutile TiO 2 single crystals with various surface treatments were studied by photoelectron spectroscopy to obtain their surface potentials. Regardless of orientations and polymorph, a huge variation of the Fermi level and work function was achieved by varying the surface condition. The most strongly oxidized surfaces are obtained after oxygen plasma treatment with a Fermi level ∼2.6 eV above the valence band maximum and ionization potentials of up to 9.5 eV (work function 7.9 eV). All other treated anatase surfaces exhibit an ionization potential independent of surface condition of 7.96 ± 0.15 eV. The Fermi level positions and the work functions vary by up to 1 eV. The ionization potential of rutile is ∼0.56 eV lower than that of anatase in good agreement with recent band alignment studies.

Acoustoelastic (AE) effect of a mechanically pre-stressed layered piezoelectric structure of BST is investigated. Two samples of BST (Ba0.8Sr0.2TiO3) thin films were grown by rf-magnetron sputtering deposition techniques on a Pt(100)/MgO(100) and Pt(110)/MgO(110) substrates. The crystallographic orientation of BST thin films is analyzed by X-ray diffraction (XRD). A laser acoustic waves (LA-waves) technique is used to generate surface acoustic waves (SAW) propagating in both samples for epitaxial relationship examination. We perform an extended model to determine the residual stress and strain in BST films using the (XRD) X-ray diffraction measurement and the piezoelectric constitutive equations. Theoretical analysis of (SAW) propagation in a pre-stressed layered piezoelectric structure is achieved to discuss the effect of the measured residual stresses of BST films on the propagation behavior of Rayleigh waves and on the coupled electromechanical factor.

The long‐term stability of electronic devices at high temperatures and electric fields might be strongly influenced by the electronic properties of interfaces. A modification of Schottky barrier heights at electrode interfaces of functional oxides upon changes of the external oxygen partial pressure is well documented in literature. In this work, an experimental approach using X‐ray photoelectron spectroscopy is presented, which enables to study transient changes in the Schottky barrier height induced by electrical degradation. A rise of the Fermi level at the RuO2 cathode interface of Fe‐doped SrTiO3 single crystals by 0.6 eV is observed in the course of resistance degradation. The change of the effective barrier height is associated to the migration of oxygen vacancies towards the cathode and accompanied by the observed reduction of Ti. Different scenarios are discussed to explain the origin of barrier modification and the localization of the reduced Ti.

The energy band alignment of antiferroelectric (Pb,La)(Zr,Sn,Ti)O 3 is studied with photoelectron spectroscopy using interfaces with high work function RuO 2 and low work function Sn-doped In 2 O 3 (ITO). It is demonstrated how spectral deconvolution can be used to determine absolute Schottky barrier heights for insulating materials with a high accuracy. Using this approach it is found that the valence band maximum energy of (Pb,La)(Zr,Sn,Ti)O 3 is found to be comparable to that of Pb- and Bi-containing ferroelectric materials, which is ∼1 eV higher than that of BaTiO 3 . The results provide additional evidence for the occupation of the 6s orbitals as origin of the higher valence band maximum, which is directly related to the electrical properties of such compounds. The results also verify that the energy band alignment determined by photoelectron spectroscopy of as-deposited electrodes is not influenced by polarisation. The electronic structure of (Pb,La)(Zr,Sn,Ti)O 3 should enable doping of the material without strongly modifying its insulating properties, which is crucial for high energy density capacitors. Moreover, the position of the energy bands should result in a great freedom of selecting electrode materials in terms of avoiding charge injection.

Lead-free ceramics based on Na1/2Bi1/2TiO3 (NBT) have been shown to exhibit excellent ferroelectric properties putting them amongst the best materials to replace lead-based piezoelectrics. The defect chemistry of NBT is, however, very complex. High oxygen ionic conductivity can be induced by acceptor doping or bismuth evaporation, which is quite detrimental to the ferroelectric properties. Nevertheless, this conductivity is non-linearly dependent on the acceptor concentration, which allows for tuning of NBT-based material from highly ionically conducting to highly resistive. This unique behavior raises the question of whether NBT ceramics also need to be treated differently with respect to high electric fields as they are used for ferroelectric and dielectric applications. To elucidate this, high field electrodegradation experiments have been performed on NBT and a solid solution with BaTiO3 (BT). It could be shown that reversible electroformation can be induced, which is non-linearly dependent on acceptor concentration. Additionally, a fast surface degradation process could be identified, which could be attributed to a reversible field-induced change in composition at the anode due to sodium becoming mobile. These results will be of high importance for reliability investigations of NBT based material and also hint towards possible applications in resistive switching memory applications.

The interaction of BiFeO3 and Co-doped BiFeO3 thin-film surfaces with water vapor is examined using photoelectron spectroscopy. Water exposure results in an upward shift of the Fermi energy, which is limited by the reduction of Bi and Fe in undoped BiFeO3 and by the reduction of Co in oxidized Co-doped BiFeO3. The results highlight the importance of surface potential changes induced by the interaction of solid surfaces with water and the ability of photoelectron spectroscopy to quantitatively determine electrochemical reduction potentials and defect energy levels., This work was supported by the LOEWE program of the State of Hesse, Germany, within the project FLAME (Fermi Level Engineering of Antiferroelectric Materials for Energy Storage and Insulation Systems). M.C. and P.M. acknowledge the financial support from the Spanish MINECO (“Severo Ochoa” Programme for Centres of Excellence in R& D SEV-2015-0496 and MAT 2017-83169-R AEI/FEDER). T.R. and M.M. acknowledge the Slovenian Research Agency for funding (Program P2-0105 and Ph.D. Project PR-08350).

Hematite Fe$_2$O$_3$ seed layers are shown to constitute a pathway to prepare highly conductive transparent tin-doped indium oxide (ITO) thin films by room temperature magnetron sputtering. Conductivities of up to $\sigma = 3300\,{\rm S/cm}$ are observed. The improved conductivity is not restricted to the interface but related to an enhanced crystallization of the films, which proceeds in the rhombohedral phase., Comment: 5 pages, 3 figures

Low-temperature-processed ITO thin films offer the potential of overcoming the doping limit by suppressing the equilibrium of compensating oxygen interstitial defects. To elucidate this potential, electrical properties of Sn-doped In 2 O 3 (ITO) thin films are studied in dependence on film thickness. In-operando conductivity and Hall effect measurements during annealing of room-temperature-deposited films, together with different film thickness in different environments, allow to discriminate between the effects of crystallization, grain growth, donor activation and oxygen diffusion on carrier concentrations and mobilities. At 200 ∘ C , a control of carrier concentration by oxygen incorporation or extraction is only dominant for very thin films. The electrical properties of thicker films deposited at room temperature are mostly affected by the grain size. The remaining diffusivity of compensating oxygen defects at 200 ∘ C is sufficient to screen the high Fermi level induced by deposition of Al 2 O 3 using atomic layer deposition (ALD), which disables the use of defect modulation doping at this temperature. The results indicate that achieving higher carrier concentrations in ITO thin films requires a control of the oxygen pressure during deposition in combination with seed layers to enhance crystallinity or the use of near room temperature ALD.

The modification of the work function of Sn-doped In2O3 (ITO) by vacuum adsorption of 4-(Dimethylamino)benzoic acid (4-DMABA) has been studied using in situ photoelectron spectroscopy. Adsorption of 4-DMABA is self-limited with an approximate thickness of a single monolayer. The lowest work function obtained is 2.82 ± 0.1 eV, enabling electron injection into many organic materials. In order to identify a potential influence of the ITO substrate surface on the final work function, different ITO surface orientations and treatments have been applied. Despite the expected differences in substrate work function and chemical bonding of 4-DMABA to the substrate, no influence of substrate surface orientation is identified. The resulting work function of ITO/4-DMABA substrates can be described by a constant ionization potential of the adsorbed 4-DMABA of 5.00 ± 0.08 eV, a constant band alignment between ITO and 4-DMABA and a varying Fermi energy in the ITO substrate. This corresponds to the behaviour of a conventional semiconductor heterostructure and deviates from the vacuum level alignment of interfaces between organic compounds. The difference is likely related to a stronger chemical bonding at the ITO/4-DMABA interface compared to the van der Waals bonding at interfaces between organic compounds.

The influence of dopant species and concentration on the grain boundary scattering of differently doped In 2 O 3 thin films is studied by means of room temperature and temperature dependent Hall effect measurements. Barrier heights at grain boundaries E B are evaluated from temperature dependent carrier mobility taking the theoretically calculated temperature dependence of intragrain mobility into account. It is thereby shown that also samples with a negative temperature coefficient of mobility exhibit significant grain boundary barrier heights and that E B is usually underestimated when evaluated based on Seto's model. It is also shown that the most commonly used Sn doping of In 2 O 3 with a dopant concentration > 2 wt . % SnO 2 leads to significantly enhanced grain boundary scattering compared to nominally undoped, Zr-doped and H-doped films. An effect of grain boundary scattering is even observed for carrier concentrations ~ 10 21 cm − 3 if the films exhibit a pronounced (100) texture. The poor grain boundary properties of highly Sn-doped In 2 O 3 are attributed to segregation of the Sn dopants, which is also indicated by measurements of surface Sn concentration.

Energy band alignments at heterointerfaces play a crucial role in defining the functionality of semiconductor devices, yet the search for material combinations with suitable band alignments remains a challenge for numerous applications. In this work, we demonstrate how changes in deposition conditions can dramatically influence the functional properties of an interface, even within the same material system. The energy band alignment at the heterointerface between Cu2O and ZnO was studied using photoelectron spectroscopy with stepwise deposition of ZnO onto Cu2O and vice versa. A large variation of energy band alignment depending on the deposition conditions of the substrate and the film is observed, with valence band offsets in the range ΔEVB = 1.45-2.7 eV. The variation of band alignment is accompanied by the occurrence or absence of band bending in either material. It can therefore be ascribed to a pinning of the Fermi level in ZnO and Cu2O, which can be traced back to oxygen vacancies in ZnO and to metallic precipitates in Cu2O. The intrinsic valence band offset for the interface, which is not modified by Fermi level pinning, is derived as ΔEVB ≈ 1.5 eV, being favorable for solar cell applications.

Differently oriented CeO2 thin films were prepared by radio frequency magnetron sputter deposition from a nominally undoped CeO2 target. (111), (110) and (100) oriented films were achieved by deposition onto Al2O3(0001)/Pt(111), MgO(110)/Pt(110) and SrTiO3:Nb(100) substrates, respectively. Epitaxial growth is verified using X-ray diffraction analysis. The films were analyzed by in situ photoelectron spectroscopy to determine the ionization potential, work function, Fermi level position and Ce3+ concentration at the surface in dependence of crystal orientation, deposition conditions and post-deposition treatment in reducing and oxidizing atmosphere. We observed a very high variation of the work function and ionization potential of more than 2 eV for all surface orientations, while the Fermi level varies by only 0.3 eV within the energy gap. The work function generally decreases with increasing Ce3+ surface concentration but comparatively high Ce3+ concentrations remain even after strongly oxidizing treatments. This is related to the presence of subsurface oxygen vacancies.

In this paper a promising solution for the notorious problem of manufacturing a stable low ohmic back contact of a CdTe thin film superstrate solar cell is presented without using elemental copper. Instead we have used a Cu2O layer inserted between the CdTe absorber and metal contact (Au). In contrast to the barrier free band alignment gained by using the transitivity rules, XPS measurements show a barrier in the valence band of the Cu2O layers directly after deposition, which results in a low performing JV curve. The contact can be improved by a short thermal treatment resulting in efficiencies superior to copper based contacts for standard CdS/CdTe hetero junction solar cells prepared on commercial glass/FTO substrates. By replacing the CdS window layer with a CdS:O buffer layer efficiencies of >15% could be achieved.

Defect chemistry and transport in Fe-doped SrTiO3 single crystal are studied to understand its resistance degradation mechanism. The temporal evolution of electric conductivity under a voltage stress was obtained computationally by solving the transport equations for ionic and electronic defects coupled with the defect reaction equilibrium equations. The computational results are compared to the corresponding experimental measurement under similar conditions. It is shown that the local electron and hole concentrations are controlled by the local electronic defect equilibria rather than by their quasi-steady state diffusional transport. It is the electric field-induced migration of oxygen vacancies and the subsequent instantaneous reestablishment of the local defect equilibria that lead to the resistance degradation. The resistance degradation behavior and the defect distributions under a long-term voltage stress are strongly influenced by the sample-annealing oxygen partial pressure, degrading electric field, and temperature. The present study contributes to the understanding of resistance degradation mechanism and provides guidance to improve the lifetime and reliability of wide band-gap semiconducting capacitors.

CuO thin films were grown by reactive magnetron sputtering and analyzed with respect to their structural, chemical, optical, and electrical properties. The films are strongly p-type with carrier concentrations of ∼ 1020 cm−3. Carrier accumulation at grain boundaries is evident from temperature and gas phase dependent conductivity measurements. The films show high absorption coefficients with a band gap of approximately 1.5 eV. Interface formation with TiO2 and Au was studied by photoelectron spectroscopy. Regarding the TiO2/CuO interface, a strong band bending in the TiO2 substrate and a valence band offset of ∼ 2.4 eV were observed. The TiO2 conduction band is therefore approximately midgap of CuO at the interface. CuO is chemically reduced upon Au deposition but an ideal ohmic contact is formed with a Fermi level at the valence band maximum of CuO. Solar cell device structures were prepared of fluorine doped SnO2/TiO2 substrates and Au back contacts, revealing open circuit voltages of

International audience; Facet-engineered anatase TiO2 with NiO nanoparticles heterocontacts were successfully prepared by selective photodeposition of NiO nanoparticles onto the {101} facet of the top-truncated bipyramidal TiO2 anatase nanocrystals coexposed with {001} and {101} facets. The morphology and electronic properties of the resulting 0.1–10 wt % NiO-decorated TiO2 were investigated by X-ray diffraction, high-resolution electron microscopy, N2 sorption analysis, and UV–vis spectroscopy. Furthermore, a careful determination of the energy band alignment diagram was conducted by a model experiment using XPS and UPS to verify charge separation at the interface of the NiO−TiO2 heterostructure. The model experiment was performed by stepwise deposition of NiO onto oriented TiO2 substrates and in-situ photoelectron spectroscopy measurements without breaking vacuum. Core levels showed shifts of 0.58 eV toward lower binding energies, meaning an upward band bending in TiO2 at the NiO–TiO2 interface. Furthermore, 0.1 wt % NiO–TiO2 exhibited 50% higher activities than the pure TiO2 for methylene blue (MB) photodecomposition under UV irradiation. This enhanced photocatalytic activity of NiO–TiO2 nanocomposites was related to the internal electric field developed at the p–n NiO−TiO2 heterojunction, leading to vectorial charge separation. Finally, mechanistic studies conducted in the presence of carrier or radical scavengers revealed that holes dominantly contributed to the photocatalytic reactions in the case of NiO–TiO2 photocatalysts while electrons played the main role in photocatalysis for the pure TiO2 materials.

The optical band gap is a major selection criterion for an absorber in photocatalytic water splitting. Due to its ideal value hematite has been intensively investigated without reaching the expectation, yet. In this work, the Fermi level positions in hematite due to doping and contact formation are investigated. An upper boundary for the Fermi level position at 1.8 eV above the valence band maximum due to the formation of polarons is identified. This results in a different concept of the effective band gap for hematite which we believe is transferable to any material with competing polaron formation after optical excitation: the optical band gap of 2.2 eV deviates from an effective electronic band gap of 1.75 eV. The polaron state acts as a limit in (quasi-)Fermi level shift, restricting the potential of charge transfer reactions. Additionally, it has led to an incorrect determination of the band edge positions of hematite in electrochemical contacts.

The photocatalytic (PC) performance of titanium dioxide (TiO2) nanoparticles strongly depends on their specific surface, the presence of crystal defects, their crystal phase, and the exposed crystal facets. In order to understand which of these factors contributes most significantly to the PC activity of TiO2 colloids, all of them have to be individually analyzed. This study entails the synthesis of five anatase nanocrystal samples. By maintaining the same reactant ratios as well as hydrothermal sol–gel synthesis route and only varying the autoclaving time or temperature, different crystallite sizes are obtained under comparable experimental conditions. A decrease in PC performance with increase in specific surface area is found. Such an unexpected counterintuitive result establishes the basis for a better understanding of the crucial factors that ultimately determine the PC activity. These are investigated by studying nanocrystals bulk and surface structure and morphology using a selection of complementary analysis methods (X-ray photoelectron spectroscopy (XPS), X-ray absorption fine structure (XAFS), X-ray diffraction (XRD)…). It is found that a change in the nanocrystal morphology from an equilibrium state truncated tetragonal bipyramid to a more elongated rod-like structure accompanied by an increase in oxygen vacancies is responsible for an augmented PC activity of the TiO2 nanocrystals.

SnO2 films doped with antimony or tantalum were sputterdeposited for comparison, using an identical set of parameters. The influence of dopant concentration and choice of deposition parameters such as substrate temperature on the optoelectronic properties, especially film resistivity, were determined. Comparative analysis shows that tantalum doping yields lower film resistivity, probably due to an increased inhibiting influence of grain boundary scattering in the case of antimony doping. Sputter-deposited tantalum-doped films with lower than previously achieved resistivity 5.4 × 10−4 Wcm, carrier density 4.5 × 1020 cm−3, and mobility 25.7 cm2 Vs−1 are reported, while maintaining optical transmittance above 85% at a film thickness 400 nm. Ta/Sb co-doped thin films were synthesized for the first time, achieving similar results.

Chemical and electronic properties of dielectric oxide interfaces as obtained using photoelectron spectroscopy are presented and discussed. Interface preparation includes the deposition of metals onto dielectrics and vice versa as well as the effect of postdeposition treatments. Most interfaces are not abrupt as either reduction in the oxide surface occurs during metal deposition or oxidation of the metal substrate is induced by oxide deposition. The Schottky barrier heights at these interfaces are strongly affected by the interface chemistry. Reactive interfaces exhibit a strong Fermi level pinning due to defect formation. Nonreactive interfaces, which are obtained by depositing metallic oxide electrodes, exhibit an unpinned Schottky–Mott-like barrier formation. Barrier heights can therefore be modified by more than 1 eV with suitable electrode material and processing. Postdeposition oxidation and reduction treatments and ferroelectric polarization can lead to comparable changes of barrier height. Interface studies between dielectric oxides reveal the dependence of valence band maximum and conduction band minimum energies. Due to transitivity of band alignment, these can be arranged on an absolute energy scale. The range of Fermi level positions in dielectric oxides, which can also be obtained from photoelectron spectroscopy and which is limited by intrinsic defect formation, is comparable when the oxides aligned on the energy scale determined by the photoemission experiments. The band alignment therefore indicates if a material can be made n-type or p-type by donor or acceptor doping. The range of Fermi levels in the oxides corresponds also with the range of the Fermi levels at oxide/metal interfaces.

Cu- and V-doped BNKT10-based piezoelectric ceramics with up to 0.5 at.% dopant concentration were synthesized and displayed more homogeneous grain growth compared to undoped BNKT10 ceramics. The defect chemistry and defect structure, studied by X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR), indicate a slightly rhombic electronic environment with major unidirectional octahedral distortion of the local environment of Cu. The solubility limit of Cu2+ in this material system is lower than 0.05 at.% Cu; above this limit, a Cu segregation at the grain boundaries is prevalent, unlike in PZT and KNN. Here, V was shown to be incorporated into the perovskite lattice and possess oxidation states of +4 and +5, acting both as isovalent and donor dopant, predominantly compensated by A-site vacancies. A trend toward higher ceramic densities, higher maximum polarization, and higher remanent polarization with increasing Cu concentration was observed. A maximum mechanical coupling factor could be obtained in the case of doping with 0.4 at.% V and 0.1 at.% Cu with a planar coupling of 0.19 and a thickness coupling factor of 0.56.

An electrochemically stable and cobalt-free cathode material is prepared by impregnating Li2MnO3 with nickel nitrate and subsequent calcination at 650 °C. X-ray diffraction and scanning electron microscopy reveal a phase pure material with a homogenous distribution of nickel over the particle surface. Structural changes related to the treatment before and after cycling as well as electrochemical performance are investigated. The surface modified samples exhibit improved cycling stability and rate capability. In particular, the material with a nickel content of 5.7 wt% shows 173 mAh g−1 at rate C/10 and 104 mAh g−1 at rate 3C. Furthermore, a constant capacity of 155 mAh g−1 is obtained at rate C/5. Derivative plots of galvanostatic curves in combination with Raman spectroscopy depict a reduced transition from a layered to spinel-like structure compared with pristine Li2MnO3, while ICP measurements reveal a drastically decreased dissolution of manganese.

In this work the current knowledge on the electrical degradation of polymer-based light-emitting diodes is reviewed focusing especially on derivatives of poly(p-phenylene-vinylene) (PPV). The electrical degradation will be referred to as electrical fatigue and is understood as mechanisms, phenomena and material properties that change during continuous operation of the device at constant current. The focus of this review lies especially on the effect of chemical synthesis on the transport properties of the organic semiconductor and the device lifetimes. In addition, the prominent transparent conductive oxide indium tin oxide as well as In���O��� will be reviewed and how their properties can be altered by the processing conditions. The experiments are accompanied by theoretical modeling shining light on how the change of injection barriers, charge carrier mobility or trap density influence the current���voltage characteristics of the diodes and on how and which defects form in transparent conductive oxides used as anode.

International audience; The interface formation and energy-band alignment at interfaces between polycrystalline BiVO4 and high-work-function RuO2 and low-work-function Sn-doped In2O3 (indium tin oxide) have been studied using photoelectron spectroscopy with in situ thin-film deposition of the contact materials. The Schottky barrier heights for both contact films differ by 0.85 eV, which is smaller than the difference in work function and the differences observed for other semiconducting oxides, indicating a partial Fermi-level pinning. On the basis of the present results and the comparison with other photoelectrochemically active oxides, the differences of band alignment obtained from solid/electrolyte and from solid/solid interfaces, which can exhibit substantial differences, are discussed.