Your browser doesn't support javascript.
Mostrar: 20 | 50 | 100
Resultados 1 - 20 de 48
Filtrar
Mais filtros










Base de dados
Intervalo de ano de publicação
1.
Phys Chem Chem Phys ; 21(44): 24361-24372, 2019 Nov 28.
Artigo em Inglês | MEDLINE | ID: mdl-31657412

RESUMO

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.

2.
Rev Sci Instrum ; 90(4): 043908, 2019 Apr.
Artigo em Inglês | MEDLINE | ID: mdl-31043001

RESUMO

Multiple beam interferometry (MBI) evolved as a powerful tool for the simultaneous evaluation of thin film thicknesses and refractive indices in Surface Forces Apparatus (SFA) measurements. However, analysis has relied on simplifications for providing fast or simplified analysis of recorded interference spectra. Here, we describe the implementation of new optics and a generalized fitting approach to 4 × 4 transfer matrix method simulations for the SFA. Layers are described by dispersive complex refractive indices, thicknesses, and Euler angles that can be fitted, providing modeling for birefringent or colored layers. Normalization of data by incident light intensities is essential for the implementation of a fitting approach. Therefore, a modular optical system is described that can be retrofit to any existing SFA setup. Real-time normalization of spectra by white light is realized, alignment procedures are considerably simplified, and direct switching between transmission and reflection modes is possible. A numerical approach is introduced for constructing transfer matrices for birefringent materials. Full fitting of data to the simulation is implemented for arbitrary multilayered stacks used in SFA. This enables self-consistent fitting of mirror thicknesses, birefringence, and relative rotation of anisotropic layers (e.g., mica), evaluation of reflection and transmission mode spectra, and simultaneous fitting of thicknesses and refractive indices of media confined between two surfaces. In addition, a fast full spectral fitting method is implemented for providing a possible real-time analysis with up to 30 fps. We measure and analyze refractive indices of confined cyclohexane, the thickness of lipid bilayers, the thickness of metal layers, the relative rotation of birefringent materials, contact widths, as well as simultaneous fitting of both reflection and transmission mode spectra of typical interferometers. Our analyses suggest a number of best practices for conducting SFA and open MBI in an SFA for increasingly complex systems, including metamaterials, multilayered anisotropic layers, and chiral layers.

3.
Phys Chem Chem Phys ; 21(20): 10457-10469, 2019 May 28.
Artigo em Inglês | MEDLINE | ID: mdl-31070222

RESUMO

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.

4.
Langmuir ; 35(21): 6888-6897, 2019 May 28.
Artigo em Inglês | MEDLINE | ID: mdl-31050896

RESUMO

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.

5.
Langmuir ; 35(1): 70-77, 2019 01 08.
Artigo em Inglês | MEDLINE | ID: mdl-30525645

RESUMO

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.

6.
J Chem Phys ; 148(22): 222824, 2018 Jun 14.
Artigo em Inglês | MEDLINE | ID: mdl-29907053

RESUMO

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.

7.
Phys Chem Chem Phys ; 20(23): 16092-16101, 2018 Jun 13.
Artigo em Inglês | MEDLINE | ID: mdl-29855007

RESUMO

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.

8.
ACS Nano ; 12(3): 2998-3009, 2018 03 27.
Artigo em Inglês | MEDLINE | ID: mdl-29537818

RESUMO

A rechargeable Mg battery where the capacity mainly originates from reversible reactions occurring at the electrode/electrolyte interface efficiently avoids the challenge of sluggish Mg intercalation encountered in conventional Mg batteries. The interfacial reactions in a cell based on microwave-exfoliated graphite oxide (MEGO) as the cathode and all phenyl complex (APC) as electrolyte are identified by quantitative kinetics analysis as a combination of diffusion-controlled reactions involving ether solvents ( esols) and capacitive processes. During magnesiation, esols in APC electrolytes can significantly affect the electrochemical reactions and charge transfer resistances at the electrode/electrolyte interface and thus govern the charge storage properties of the MEGO cathode. In APC-tetrahydrofuran (THF) electrolyte, MEGO exhibits a reversible capacity of ∼220 mAh g-1 at 10 mA g-1, while a reversible capacity of ∼750 mAh g-1 at 10 mA g-1 was obtained in APC-1,2-dimethoxyethane (DME) electrolyte. The high capacity improvement not only points to the important role of the esols in the APC electrolytes but also presents a Mg battery with high interfacial charge storage capability as a very promising and viable competitor to the conventional intercalation-based batteries.

9.
Beilstein J Nanotechnol ; 9: 936-944, 2018.
Artigo em Inglês | MEDLINE | ID: mdl-29600153

RESUMO

Corrosion inhibitors are added in low concentrations to corrosive solutions for reducing the corrosion rate of a metallic material. Their mechanism of action is typically the blocking of free metal surface by adsorption, thus slowing down dissolution. This work uses electrochemical impedance spectroscopy to show the cyclic oligosaccharide ß-cyclodextrin (ß-CD) to inhibit corrosion of zinc in 0.1M chloride with an inhibition efficiency of up to 85%. Only a monomolecular adsorption layer of ß-CD is present on the surface of the oxide covered metal, with Raman spectra of the interface proving the adsorption of the intact ß-CD. Angular dependent X-ray photoelectron spectroscopy (ADXPS) and ultraviolet photoelectron spectroscopy (UPS) were used to extract a band-like diagram of the ß-CD/ZnO interface, showing a large energy level shift at the interface, closely resembling the energy level alignment in an n-p junction. The energy level shift is too large to permit further electron transfer through the layer, inhibiting corrosion. Adsorption hence changes the defect density in the protecting ZnO layer. This mechanism of corrosion inhibition shows that affecting the defect chemistry of passivating films by molecular inhibitors maybe a viable strategy to control corrosion of metals.

10.
Phys Chem Chem Phys ; 19(19): 11816-11824, 2017 May 17.
Artigo em Inglês | MEDLINE | ID: mdl-28470285

RESUMO

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.

11.
Phys Chem Chem Phys ; 19(21): 13585-13595, 2017 May 31.
Artigo em Inglês | MEDLINE | ID: mdl-28513645

RESUMO

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.

12.
ACS Appl Mater Interfaces ; 9(15): 13471-13479, 2017 Apr 19.
Artigo em Inglês | MEDLINE | ID: mdl-28355873

RESUMO

Titanium dioxide nanowire (NW) arrays are incorporated in many devices for energy conversion, energy storage, and catalysis. A common approach to fabricate these NWs is based on hydrothermal synthesis strategies. A drawback of this low-temperature method is that the NWs have a high density of defects, such as stacking faults, dislocations, and oxygen vacancies. These defects compromise the performance of devices. Here, we report a postgrowth thermal annealing procedure to remove these lattice defects and propose a mechanism to explain the underlying changes in the structure of the NWs. A detailed transmission electron microscopy study including in situ observation at elevated temperatures reveals a two-stage process. Additional spectroscopic analyses and X-ray diffraction experiments clarify the underlying mechanisms. In an early, low-temperature stage, the as-grown mesocrystalline NW converts to a single crystal by the dehydration of surface-bound OH groups. At temperatures above 500 °C, condensation of oxygen vacancies takes place, which leads to the fabrication of NWs with internal voids. These voids are faceted and covered with Ti3+-rich amorphous TiOx.

13.
Analyst ; 142(2): 273-278, 2017 01 16.
Artigo em Inglês | MEDLINE | ID: mdl-27900373

RESUMO

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.

14.
Phys Chem Chem Phys ; 18(36): 25100-25109, 2016 Sep 14.
Artigo em Inglês | MEDLINE | ID: mdl-27722623

RESUMO

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.

15.
Biointerphases ; 11(2): 021001, 2016 Jun 23.
Artigo em Inglês | MEDLINE | ID: mdl-27009436

RESUMO

Biopolymer-based systems are extensively studied as green alternatives for traditional polymer coatings, e.g., in corrosion protection. Chitosan-epoxysilane hybrid films are presented in this work as a chitosan-based protective system, which could, e.g., be applied in a pretreatment step. For the preparation of the chitosan-epoxysilane hybrid systems, a sol-gel procedure was applied. The function of the silane is to ensure adhesion to the substrate. On zinc substrates, homogeneous thin films with thickness of 50-70 nm were obtained after thermal curing. The hybrid-coated zinc substrates were characterized by infrared spectroscopy, ellipsometry, and x-ray photoelectron spectroscopy. As model corrosion experiments, linear polarization resistance was measured, and cathodic delamination of the weak polymer coating poly(vinylbutyral) (PVB) was studied using scanning Kelvin probe. Overall, chitosan-epoxysilane hybrid pretreated samples showed lower delamination rates than unmodified chitosan coatings and pure PVB. Electrochemical impedance spectroscopy confirmed a reduced ion permeability and water uptake by chitosan-epoxysilane films compared to that of a nonmodified chitosan coating. Even though the coatings are hydrophobic and contain water, they slow down cathodic delamination by limiting ion transport.


Assuntos
Biopolímeros/química , Quitosana/química , Resinas Compostas/química , Corrosão , Silanos/química , Zinco/metabolismo , Espectroscopia Fotoeletrônica , Espectrofotometria Infravermelho
17.
Phys Chem Chem Phys ; 17(47): 31670-9, 2015 Dec 21.
Artigo em Inglês | MEDLINE | ID: mdl-26555893

RESUMO

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.

18.
Chem Commun (Camb) ; 51(89): 16041-4, 2015 Nov 18.
Artigo em Inglês | MEDLINE | ID: mdl-26344142

RESUMO

After passage of a delamination front at a polymer/zinc interface, pH oscillations and oscillations in the quantity of corrosion products are observed. The reason for these oscillations is the low quantity of water in the confined reaction volume, water consumption by oxygen reduction, and water regeneration after precipitation of ZnO.

19.
Langmuir ; 31(26): 7306-16, 2015 Jul 07.
Artigo em Inglês | MEDLINE | ID: mdl-26057456

RESUMO

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.

20.
Phys Chem Chem Phys ; 17(15): 10004-13, 2015 Apr 21.
Artigo em Inglês | MEDLINE | ID: mdl-25785505

RESUMO

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.

SELEÇÃO DE REFERÊNCIAS
DETALHE DA PESQUISA