Role of aluminium hydrides in localised corrosion of aluminium revealed by operando Raman spectroscopy.

Filiform corrosion (FFC) is characteristic of metals such as aluminium and magnesium, usually takes place on coated metals, and spreads from coating defects in the form of filaments with a width on the order of 100 µm. In this work, in situ and operando Raman spectroscopy and optical microscopy were used to characterize the composition and distribution of corrosion products inside growing filaments. The filament head contains water (OH stretching modes, 3000-3600 cm-1), and corrosion products based on aluminium oxide with both tetrahedrally (840 cm-1) and octahedrally (600 cm-1) coordinated Al3+, and with some hydroxyl group content (3075, 1420, 1164 cm-1). Remarkable is the prominent presence of structural motifs as in γ-AlH3 (1045, 1495 cm-1). The tail contains predominantly aluminium oxide with octahedrally coordinated Al3+ and in addition carbonate (1100 cm-1) and aluminium chloride (347 cm-1). Video recordings of the active filigree show hydrogen evolution inside the active head and a very fast precipitation of corrosion products. Re-dissolution, transport and re-formation of the corrosion products is also observed, accompanying start-stop-cycles of the propagation of FFC; this mechanism leads to wavy surface morphologies by lifting of certain coating areas after the passage of the corrosion front as evidenced by 3D optical profilometer analysis. When exposed to the acidic head conditions for a sufficient time, the initiation of other forms of localised corrosion, such as pitting, is possible, which in turn facilitates further propagation of the filament. The in situ detection of hydride which transforms into the typical aluminium corrosion products in due course points to a prominent role of hydride as intermediate in the aqueous corrosion of aluminium.

Operando studies of Mn oxide based electrocatalysts for the oxygen evolution reaction.

Inspired by photosystem II (PS II), Mn oxide based electrocatalysts have been repeatedly investigated as catalysts for the electrochemical oxygen evolution reaction (OER), the anodic reaction in water electrolysis. However, a comparison of the conditions in biological OER catalysed by the water splitting complex CaMn4Ox with the requirements for an electrocatalyst for industrially relevant applications reveals fundamental differences. Thus, a systematic development of artificial Mn-based OER catalysts requires both a fundamental understanding of the catalytic mechanisms as well as an evaluation of the practicality of the system for industrial scale applications. Experimentally, both aspects can be approached using in situ and operando methods including spectroscopy. This paper highlights some of the major challenges common to different operando investigation methods and recent insights gained with them. To this end, vibrational spectroscopy, especially Raman spectroscopy, absorption techniques in the bandgap region and operando X-ray spectroelectrochemistry (SEC), both in the hard and soft X-ray regime are particularly focused on here. Technical challenges specific to each method are discussed first, followed by challenges that are specific to Mn oxide based systems. Finally, recent in situ and operando studies are reviewed. This analysis shows that despite the technical and Mn specific challenges, three specific key features are common to most of the studied systems with significant OER activity: structural disorder, Mn oxidation states between III and IV, and the appearance of layered birnessite phases in the active regime.

Controlling amphipathic peptide adsorption by smart switchable germanium interfaces.

The in situ control of reversible protein adsorption to a surface is a critical step towards biofouling prevention and finds utilisation in bioanalytical applications. In this work, adsorption of peptides is controlled by employing the electrode potential induced, reversible change of germanium (100) surface termination between a hydrophobic, hydrogen terminated and a hydrophilic, hydroxyl terminated surface. This simple but effective 'smart' interface is used to direct adsorption of two peptides models, representing the naturally highly abundant structural motifs of amphipathic helices and coiled-coils. Their structural similarity coincides with their opposite overall charge and hence allows the examination of the influence of charge and hydrophobicity on adsorption. Polarized attenuated total reflection infrared (ATR-IR) spectroscopy at controlled electrode potential has been used to follow the adsorption process at physiological pH in deuterated buffer. The delicate balance of hydrophobic and electrostatic peptide/surface interactions leads to two different processes upon switching that are both observed in situ: reversible adsorption and reversible reorientation. Negatively charged peptide adsorption can be fully controlled by switching to the hydrophobic interface, while the same switch causes the positively charged, helical peptide to tilt down. This principle can be used for 'smart' adsorption control of a wider variety of proteins and peptides and hence find application, as e.g. a bioanalytical tool or functional biosensor.

Superoxide intermediate in the oxygen reduction on a zinc hydroxide model corrosion product.

The inhibition of the electrochemical oxygen reduction reaction (ORR) by zinc corrosion products plays an important role in the corrosion protection of galvanized steel. Hence, the electrocatalytic mechanism of the ORR on electrodeposited zinc hydroxide-based model corrosion products was investigated by in situ and operando attenuated total reflection infrared (ATR-IR) spectroscopy, supplemented by density functional theory (DFT) calculations. Model corrosion products containing flake-like crystalline Zn5(NO3)2(OH)8 were cathodically electrodeposited on germanium(100) electrodes from a zinc nitrate precursor electrolyte. Substantial amounts of the films are non-crystalline, and their surfaces predominantly consist of zinc oxide and hydroxide species, as evidenced by x-ray photoelectron spectroscopy. ATR-IR spectra show a peak at 1180 cm-1 during cathodic currents in O2-saturated NaClO4 solution. This peak is assigned to a surface-bound superoxide, the only ORR intermediate detected. Absorbance from the intermediate increases with increasing cathodic current, indicating an increase in surface concentration of superoxide intermediates at larger ORR current densities. The zinc hydroxide ages in the experiments, most likely by a transformation into zinc oxide, consistent with the observed decrease in absorbance over time of the OH bending mode of zinc hydroxide at 1380 cm-1. This aging is a time-dependent chemical process, implying that pure chemical aging is important in actual corrosion products as well. DFT calculations of adsorbed superoxide yield a Zn-O bond length similar to the bond length in Zn-O, thus enhancing superoxide interaction with undercoordinated tetrahedral Zn2+ sites on the surface. Thus, such active sites catalyze the first reduction step in the ORR.

Time-Evolved SERS Signatures of DEP-Trapped Aß and Zn2+Aß Peptides Revealed by a Sub-10 nm Electrode Nanogap.

Alzheimer's disease (AD) has become highly relevant in aging societies, yet the fundamental molecular basis for AD is still poorly understood. New tools to study the undergoing structural conformation changes of amyloid beta (Aß) peptides, the pathogenic hallmark of AD, could play a crucial role in the understanding of the underlying mechanisms of misfolding and cytotoxicity of this peptide. It has been recently reported that Zn2+ interacts with Aß and changes its aggregation pathway away from less harmful fibrillar forms to more toxic species. Here, we present a versatile platform based on a set of sub-10 nm nanogap electrodes for the manipulation and sensing of biomolecules in the physiological condition at a low copy number, where molecules are trapped via dielectrophoresis (DEP) across the nanogap, which also serves as a surface-enhanced Raman spectroscopy hotspot. In this study, we demonstrate that our electrode nanogap platform can be used to study the structural difference between Aß40 and ZnAß40 peptides at different aggregation stages in the physiologically relevant concentration and in solution phase. The Raman spectroscopic signatures of the DEP-captured neuropeptides prove the device to be attractive as a label-free bioanalytical tool.

What is the trigger for the hydrogen evolution reaction? - towards electrocatalysis beyond the Sabatier principle.

The mechanism of the hydrogen evolution reaction, although intensively studied for more than a century, remains a fundamental scientific challenge. Many important questions are still open, making it elusive to establish rational principles for electrocatalyst design. In this work, a comprehensive investigation was conducted to identify which dynamic phenomena at the electrified interface are prerequisite for the formation of molecular hydrogen. In fact, what we observe as an onset of the macroscopic faradaic current originates from dynamic structural changes in the double layer, which are entropic in nature. Based on careful analysis of the activation process, an electrocatalytic descriptor is introduced, evaluated and experimentally confirmed. The catalytic activity descriptor is named as the potential of minimum entropy. The experimentally verified catalytic descriptor reveals significant potential to yield innovative insights for the design of catalytically active materials and interfaces.

Carbon-Sulfur Bond Cleavage During Adsorption of Octadecane Thiol to Copper in Ethanol.

The effect of the solvent on the formation of thiol self-assembled monolayers (SAMs) on oxide-covered, reactive metals is more involved than in the well-studied gold-thiol system. In this work, copper covered with a native oxide was modified with 1-octadecanethiol (ODT) in either tetrahydrofuran or ethanol. Infrared spectroscopy indicated the formation of crystalline chain packing of alkyl chains from both solvents. Surface coverage was approximately equal in both systems, with differences in tilt angles of the chains. A detailed analysis by X-ray photoelectron spectroscopy showed the formation of Cu2S and copper-bound carbon when the adsorption was carried out in ethanol. This observation can be explained by the cleavage of the C-S bond in ODT during adsorption. Based on the analogy of preparations, we reason that the solvation of ODT in ethanol must be such that it weakens the C-S bond in ODT, thus enabling the cleavage of this bond. Based on the evidence presented here, it is not possible to distinguish between surface solvation and bulk solvation. Electrochemical linear sweep voltammetry shows that SAMs from both solvents have an enhancing protective effect compared to the native oxide layer. The results from this work show interesting possibilities for the preparation of adsorbed monolayers with chemical interaction to reactive metals, with some similarities to carbene-based SAMs.

Pretreatment with a ß-Cyclodextrin-Corrosion Inhibitor Complex Stops an Initiated Corrosion Process on Zinc.

Metal pretreatment is typically the first step in a reliable corrosion protection system. This work explores the incorporation of complexes between the cyclic oligosaccharide ß-cyclodextrin (ß-CD) and the molecular organic corrosion inhibitor 2-mercaptobenzothiazole (MBT) into an oxide-based pretreatment layer on metallic zinc. The layers were produced by a precorrosion step in the presence of ß-CD. The resulting films have a morphology dominated by spherical particles. X-ray photoelectron spectroscopy investigations of the surfaces show the sulfur atoms of MBT to be partially oxidized but mostly intact. Samples pretreated with such a layer were subsequently coated with a model polymer coating, and the delamination of this model coating from an artificial defect was monitored by a scanning Kelvin probe (SKP). The SKP results show a slow down of delamination after several hours of the ongoing corrosion process for surfaces pretreated with the complexes. Finally, an increase in the electrode potential in the defect was observed, with a subsequent complete stop in delamination and repassivation of the defect after ≈10 h. This repassivation is attributed to the release of MBT after the initiation of the corrosion process. Most likely, the increase of pH, combined with the availability of aqueous solution, facilitates the MBT release after the initiation of a corrosion process. Consequently, complexes formed from ß-CD and corrosion inhibitors can be effectively incorporated into inorganic pretreatments, and the inhibitor component can be released upon start of the corrosion process.

The reassigned pseudo Wigner-Ville transform in electrochemical noise analysis.

Several different time-frequency transforms from signal processing were used to analyze electrochemical noise data to determine frequency components contained within the noise record and their time evolution. Bilinear time-frequency representations (TFR) based on the Wigner-Ville distribution (WVD) were compared with a special focus on the reassigned smoothed pseudo WVD (RSPWVD). Spectra obtained with WVD were compared with traditional linear time-frequency representations, such as short time Fourier transform and wavelet transform. Comparison to other TFRs showed that the RSPWVD suppressed artifacts, provided better resolution of the time-frequency analysis in both time and frequency domains, and improved the overall readability of a representation. The obtained spectra from RSPWVD were consistent with the results from DWT, but permitted a more comprehensive analysis of transients. Consequently, RSPWVD is suitable for electrochemical noise analysis. In the presence of Cl-, RSPWVD showed that the passivity of Al was compromised, as evidenced by the presence of various current transients in the frequency range from 10-2 to 1 Hz.

Alkaline manganese electrochemistry studied by in situ and operando spectroscopic methods - metal dissolution, oxide formation and oxygen evolution.

Manganese-based systems are considered as candidate electrocatalysts for the electrochemical oxygen evolution reaction (OER), because of their abundance in biochemical oxygen producing catalyst systems. In this work, the surface of metallic manganese was investigated in situ and operando in potentiodynamic cyclic voltammetry (CV) experiments and potentiostatic chronoamperometry (CA) experiments in NaOH. In both cases, the surfaces were initially reduced. At corresponding potentials, no oxide species can be detected by Raman spectroscopy, though electrochemical data and the absence of dissolution above the reversible potential for reactions of type Mn → MnII indicate that the material is passive. The CV shows anodic peaks at potentials in line with expectations on the basis of thermodynamic data for the oxidation to Mn3O4 and Mn2O3; the thickness of the surface layer increases by a few nm during these peaks, as evidenced by spectroscopic ellipsometry. Dissolution of Mn as evidenced by downstream electrolyte analysis by inductively coupled plasma mass spectrometry in a scanning flow cell (SFC-ICP-MS) of the electrolyte is negligible in the range of electrode potential vs. Ag|AgCl|3 M KCl, EAg|AgCl, up to 0.3 V. Remarkably, Raman spectra already show the occurrence of α-MnO2 at EAg|AgCl > -0.25 V, which is ca. 0.5 V below the potential at which oxidation to MnO2 is expected. This observation is attributed to disproportionation above a certain level of MnIII. For EAg|AgCl > 0.4 V, dissolution sets in, at a constant layer thickness. Above the onset potential of the OER, at EAg|AgCl≈ 0.6 V, SFC-ICP-MS analysis shows fast dissolution, and the oxide layer thickness is constant or increases. CA experiments during the OER show strong dissolution, and the re-formation of a strongly disordered, ß-MnO2-like oxide, which exists in a quasi-stationary state at the interface. Several CV cycles increase the dissolution per cycle and the fraction of α-MnO2 on the surface which cannot be reduced. The high dissolution currents show that metallic Mn is hardly suitable as an OER catalyst, however, at least the MnIV oxides remain stationarily present in the system.

Oxide-organic heterostructures: a case study of charge transfer disturbance at a SnO2-copper phthalocyanine buried interface.

Reduced tin dioxide/copper phthalocyanine (SnOx/CuPc) heterojunctions recently gained much attention in hybrid electronics due to their defect structure, allowing tuning of the electronic properties at the interface towards particular needs. In this work, we focus on the creation and analysis of the interface between the oxide and organic layer. The inorganic/organic heterojunction was created by depositing CuPc on SnOx layers prepared with the rheotaxial growth and vacuum oxidation (RGVO) method. Exploiting surface sensitive photoelectron spectroscopy techniques, angle dependent X-ray and UV photoelectron spectroscopy (ADXPS and UPS, respectively), supported by semi-empirical simulations, the role of carbon from adventitious organic adsorbates directly at the SnOx/CuPc interface was investigated. The adventitious organic adsorbates were blocking electronic interactions between the environment and surface, hence pinning energy levels. A significant interface dipole of 0.4 eV was detected, compensating for the difference in work functions of the materials in contact, however, without full alignment of the energy levels. From the ADXPS and UPS results, a detailed diagram of the interfacial electronic structure was constructed, giving insight into how to tailor SnOx/CuPc heterojunctions towards specific applications. On the one hand, parasitic surface contamination could be utilized in technology for passivation-like processes. On the other hand, if one needs to keep the oxide's surficial interactions fully accessible, like in the case of stacked electronic systems or gas sensor applications, carbon contamination must be carefully avoided at each processing step.

Vibrational spectroscopic study of pH dependent solvation at a Ge(100)-water interface during an electrode potential triggered surface termination transition.

The charge-dependent structure of interfacial water at the n-Ge(100)-aqueous perchlorate interface was studied by controlling the electrode potential. Specifically, a joint attenuated total reflection infrared spectroscopy and electrochemical experiment was used in 0.1M NaClO4 at pH ≈ 1-10. The germanium surface transformation to an H-terminated surface followed the thermodynamic Nernstian pH dependence and was observed throughout the entire pH range. A singular value decomposition-based spectra deconvolution technique coupled to a sigmoidal transition model for the potential dependence of the main components in the spectra shows the surface transformation to be a two-stage process. The first stage was observed together with the first appearance of Ge-H stretching modes in the spectra and is attributed to the formation of a mixed surface termination. This transition was reversible. The second stage occurs at potentials ≈0.1-0.3 V negative of the first one, shows a hysteresis in potential, and is attributed to the formation of a surface with maximum Ge-H coverage. During the surface transformation, the surface becomes hydrophobic, and an effective desolvation layer, a "hydrophobic gap," developed with a thickness ≈1-3 Å. The largest thickness was observed near neutral pH. Interfacial water IR spectra show a loss of strongly hydrogen-bound water molecules compared to bulk water after the surface transformation, and the appearance of "free," non-hydrogen bound OH groups, throughout the entire pH range. Near neutral pH at negative electrode potentials, large changes at wavenumbers below 1000 cm-1 were observed. Librational modes of water contribute to the observed changes, indicating large changes in the water structure.

10 nm deep, sub-nanoliter fluidic nanochannels on germanium for attenuated total reflection infrared (ATR-IR) spectroscopy.

The fabrication of sub-nanoliter fluidic channels is demonstrated, with merely 10 nm depth on germanium, using conventional semiconductor device fabrication methods and a polymer assisted room-temperature sealing method. As a first application, an ultralow volume (650 pL) was studied by ATR-IR spectroscopy. A detection limit of ∼7.9 × 1010 molecules of human serum albumin (HSA) (∼0.2 mM) in D2O was achieved with highly specific ATR-IR spectroscopy.

Electrode potential dependent desolvation and resolvation of germanium(100) in contact with aqueous perchlorate electrolytes.

The electrode potential dependence of the hydration layer on an n-Ge(100) surface was studied by a combination of in situ and operando electrochemical attenuated total reflection infrared (ATR-IR) spectroscopy and real space density functional theory (DFT) calculations. Constant-potential DFT calculations were coupled to a modified generalised Poisson-Boltzmann ion distribution model and applied within an ab initio molecular dynamics (AIMD) scheme. As a result, potential-dependent vibrational spectra of surface species and surface water were obtained, both experimentally and by simulations. The experimental spectra show increasing absorbance from the Ge-H stretching modes at negative potentials, which is associated with an increased negative difference absorbance of water-related OH modes. When the termination transition of germanium from OH to H termination occurs, the surface switches from hydrophilic to hydrophobic. This transition is fully reversible. During the switching, the interface water molecules are displaced from the surface forming a "hydrophobic gap". The gap thickness was experimentally estimated by a continuum electrodynamic model to be ≈2 Å. The calculations showed a shift in the centre of mass of the interface water by ≈0.9 Å due to the surface transformation. The resulting IR spectra of the interfacial water in contact with the hydrophobic Ge-H show an increased absorbance of free OH groups, and a decreased absorbance of strongly hydrogen bound water. Consequently, the surface transformation to a Ge-H terminated surface leads to a surface which is weakening the H-bond network of the interfacial water in contact.

Charge transfer quantification in a SnOx/CuPc semiconductor heterostructure: investigation of buried interface energy structure by photoelectron spectroscopies.

A tin oxide/copper phthalocyanine (CuPc) layer stack was investigated with two complementary photoemission methods. Non-destructive analysis of the electronic properties at the SnOx/CuPc interface was performed applying angle-dependent measurements with X-ray photoelectron spectroscopy (ADXPS) and energy-resolved photoemission yield spectroscopy (PYS). The different components (related to oxide layer and organic overlayer as well as to contamination features) observed in the spectra were assigned to a particular layer by relative depth plot analysis. ADXPS allowed determination of the chemical and electronic structure of the investigated samples. The addition of the organic ultra-thin film to the oxide layer caused a significant increase of the structure's photoemission yield. The combination of ADXPS and PYS allowed determination of the work function of constituent layers, and charge transfer phenomena at the SnOx/CuPc buried interface. An interface dipole of 0.23 eV was detected, assigned to charge transfer across the interface from the oxide layer towards the organic film. The energy level alignment at the SnOx/CuPc interface was determined, and presented in a band-like diagram, together with depth-dependent changes of the core energy levels of the structure's constituents. Finally the role of the oxide's defect-related energy levels in the charge transfer was discussed. The results obtained exhibit significance ranging from investigation, basic understanding and application of such hybrid films. Application of these results in hybrid electronic devices can help understanding and furthering this technology.

Mechanism of the potential-triggered surface transformation of germanium in acidic medium studied by ATR-IR spectroscopy.

In acidic solution, germanium surfaces undergo a transformation to hydrogen-terminated surfaces at sufficiently negative electrode potentials. Herein, we used in situ and operando attenuated total reflection infrared (ATR-IR) spectroscopy coupled to electrochemical experiments to study the details of this surface transformation on Ge(111) and Ge(100) in 0.1 M HClO4. The ATR-IR data gathered during the surface transformation are consistent with an interpretation according to which an intermediate state exists of a surface with mixed termination. In the mixed termination, both H and OH are bound to the surface, which showed a Ge-H stretching mode at ∼2025-2030 cm-1. At sufficiently negative potentials, the surfaces became fully hydrogen terminated. ATR-IR spectra can be understood by assigning the peak at ∼1977-1990 cm-1 to the stretching mode of GeH1 species on Ge(111), and the peak at ∼2000-2015 cm-1 to a stretching mode of GeH2 species on Ge(100). Measurements of the linear dichroism showed the GeH1 species to be oriented predominantly upright. The transition dipole moment of the GeH2 species was oriented parallel to the surface, as expected for an antisymmetric stretching mode.

Surface Functionalization of Oxide-Covered Zinc and Iron with Phosphonated Phenylethynyl Phenothiazine.

Phenothiazines are redox-active, fluorescent molecules with potential applications in molecular electronics. Phosphonated phenylethynyl phenothiazine can be easily obtained in a four-step synthesis, yielding a molecule with a headgroup permitting surface linkage. Upon modifying hydroxylated polycrystalline zinc and iron, both covered with their respective native oxides, ultrathin organic layers were formed and investigated by use of infrared (IR) reflection spectroscopy, X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (ToF-SIMS), contact angle measurement, and ellipsometry. While stable monolayers with upright oriented organic molecules were formed on oxide-covered iron, multilayer formation is observed on oxide-covered zinc. ToF-SIMS measurements reveal a bridging bidentate bonding state of the organic compound on oxide-covered iron, whereas monodentate complexes were observed on oxide-covered zinc. Both organically modified and unmodified surfaces exhibit reactive wetting, but organic modification makes the surfaces initially more hydrophobic. Cyclic voltammetry (CV) indicates redox activity of the multilayers formed on oxide-covered zinc. On the other hand, the monolayers on oxide-covered iron desorb after electrochemical modifications in the state of the oxide, but are stable at open circuit conditions. Exploiting an electronic coupling of phenothiazines to oxides may thus assist in corrosion protection.

A new look at oxide formation at the copper/electrolyte interface by in situ spectroscopies.

The widely used engineering material copper is a prototype of an electrochemically passive metal. In this work, the passive films on evaporated copper in 0.1 M NaOH are investigated in situ and operando by spectroscopic ellipsometry and Raman spectroscopy, both conducted during oxidation in potentiostatic step experiments. Oxide growth is initiated by jumping from a potential at which the surface is oxide-free to -0.1 V vs. Ag|AgCl|3 M KCl (+0.11 V vs. standard hydrogen electrode, SHE). At subsequent electrode potential jumps, no corresponding jumps in the thickness are observed; instead, oxide growth proceeds steadily. Above +0.3 V vs. Ag|AgCl|3 M KCl (+0.51 V vs. SHE), the oxide layer thickness remains constant at ≈7 nm. Raman spectra show a peak at 530 cm(-1), which agrees with the dominant peak in spectra of copper mixed oxide, Cu4O3 (Cu2(I)Cu2(II)O3). Crystalline Cu4O3 nucleates from a precursor state showing strong photoluminescence (PL), which hints at the involvement of Cu2O. Overall, the PL spectra of the growing oxide and absorption spectra indicate the presence of Cu2O in the thin films. Absorption spectra cannot be understood as a superposition of the spectra from different well-described copper oxides, which points to defect-rich oxides that show rather different spectra. Raman spectra also point to an involvement of both crystalline and amorphous oxides that coexist. The results show that the passive layers on copper are more complex than the duplex layers described in the literature; they do contain an oxide with a mixed valency of copper.

Zinc oxide as a defect-dominated material in thin films for photovoltaic applications--experimental determination of defect levels, quantification of composition, and construction of band diagram.

In the present work, thin ZnO layers were synthesized by the sol-gel method with subsequent spin-coating on Si(100). We show that the detailed analysis of lab-recorded photoemission spectra in combination with Kelvin probe data yielded the work function, ionization energy, and valence band - Fermi level separation - and hence enabled the construction of band diagrams of the examined layers. With small modifications in preparation, very different films can be obtained. One set shows a homogeneous depth-dependent n carrier distribution, and another a significant carrier concentration gradient from n-type conductivity to almost metal-like n(+) character. Likewise, the surface morphology can be tuned from a uniform, compact surface with spherical single-nm sized grain-like features to a structured surface with 5-10 nm tall crystallites with (002) dominating crystal orientation. Based on the band-bending and the energy levels observed, defects of contradictory nature, i.e. acceptor-donor-trap (ADT) properties, were identified. These defects may be groups of point defects, with opposite character. The ADT states affect the energy levels of the oxide layers and due to their nature cannot be considered in the photoemission experiment as mutually independent. The versatile nature of the synthesis provides us with the opportunity to tune the properties with a high degree of freedom, at low processing costs, yielding layers with an exotic electronic structure. Such layers are interesting candidates for applications in photovoltaic and nanoelectronic devices.

Low-copy number protein detection by electrode nanogap-enabled dielectrophoretic trapping for surface-enhanced Raman spectroscopy and electronic measurements.

We report a versatile analysis platform, based on a set of nanogap electrodes, for the manipulation and sensing of biomolecules, as demonstrated here for low-copy number protein detection. An array of Ti nanogap electrode with sub-10 nm gap size function as templates for alternating current dielectrophoresis-based molecular trapping, hot spots for surface-enhanced Raman spectroscopy as well as electronic measurements, and fluorescence imaging. During molecular trapping, recorded Raman spectra, conductance measurements across the nanogaps, and fluorescence imaging show unambiguously the presence and characteristics of the trapped proteins. Our platform opens up a simple way for multifunctional low-concentration heterogeneous sample analysis without the need for target preconcentration.