Surface Space Charge Layer Research Articles

Competing descriptions of diffusion profiles with two features: Surface space-charge layer versus fast grain-boundary diffusion

Two different physical processes, (i) fast grain-boundary diffusion (FGBD) of oxygen and (ii) hindered oxygen diffusion in a surface space-charge layer, yield oxygen isotope diffusion profiles in a similar form. Two features are observed, with the short, sharp profile close to the surface being followed by a longer, shallower profile. In this study, we develop a procedure for deciding which of the two descriptions applies to experimentally measured profiles. Specifically, we solve Fick's second law, using finite-element simulations, to obtain oxygen isotope diffusion profiles for the two cases. Each set of profiles is then analysed in terms of the competing description. In this manner, we derive falsifiable conditions that allow physical processes to be assigned unambiguously to the two features of such isotope profiles. Applying these conditions to experimental profiles for SrTiO3 single crystals published in the literature, we find that FGBD is an invalid model for describing the diffusion processes.

Enhancing ethanol detection by heterostructural silver nanoparticles decorated polycrystalline zinc oxide nanosheets

Noble metals decorated two-dimensional nanostructures with a large surface-to-volume ratio raise substantial promise for many technological applications particularly for high-performance chemical sensing. Here, we design and synthesize a hierarchical heterojunction comprising ultrathin two-dimensional polycrystalline ZnO nanosheets on a large scale, which serve as reaction positions for reduced gases, and uniformly dispersed Ag nanoparticles to enhance reaction activity. We find that the Ag nanoparticles in the heterostructural system act as strong electron acceptors, inducing remarkable surface space charge layers. As a result, the heterostructural system shows significantly improved gas-sensing performance to ethanol as compared to pure ZnO. Such metal nanoparticles decorated two-dimensional polycrystalline ZnO nanosheets can be applied in not only as gas-sensing but also catalytic and photocatalytic fields.

Defect Chemistry of Perovskite Titanates: From Bulk Materials to Nanostructured Materials

To describe the electrical conductivities of perovskite titanates (e.g. SrTiO3 and BaTiO3) and the relation to parameters like temperature, oxygen partial pressure or acceptor- or donor-type dopants, the defect chemistry has been extensively studied since the late 1960s. The studies finally lead to the formulation of the point defect chemistry on the basis of defect chemical reactions, exchange processes with the atmosphere and changes of the valence state regarding their individual thermal activation and thermo­dynamic equilibriums. Comparing with microstructured counterparts, nanostructured materials have significantly higher interface (surface and/or grain boundary) to bulk fraction, which leads to some unique effects in nanostructured perovskite titanates. One effect is that the space charge layer becomes increasingly important. Owing to the high surface-to-volume ratio, the surface space charge layer plays an important role in thin films. For example, the p-type conductivity of the epitaxial SrTiO3 thin films decreases, and the n-type conductivity increases with decreasing film thickness. This phenomenon is attributed to the charge carrier redistribution in the space charge layer. Moreover, the defect thermodynamics at nanometer-scale is different. E.g. the oxidation enthalpy of nanocrystalline BaTiO3 ceramics is found to be about one-third of that of microcrystalline BaTiO3. As a result, the p-type electrical conductivity of nanocrystalline BaTiO3 is about 2 orders of magnitude higher than that of microcrystalline BaTiO3.

Detection of NOx down to ppb levels at room temperature based on highly mesoporous hierarchical Ni(OH)2–In(OH)3 double hydroxide composites

The 3D flower-like mesoporous hierarchical architecture materials were fabricated via a facile one-pot reflux method. The composites were carefully characterized by XRD, SEM, BET, XPS, EIS and MS plot measurements. The results indicated that the morphology of the obtained products could be controlled by adjusting the reaction time, and the sample with the mass ratio of 30 % In(NO3)2·6H2O/Ni(NO3)2·6H2O (labeled as NIDH-30) formed 3D flower-like mesoporous hierarchical architecture, which was composed of nanopetals with porous thin nanosheets and the size of In(OH)3 nanoparticles around 8–13 nm. As a result, NIDH-30 exhibited excellent sensing properties at ultra low detection limit of 9.7 ppb for detection of NOx. The response toward 97 ppm NOx could reach to 60 % and the response time was 1.2 s. Moreover, it showed a great selectivity and stability at room temperature. The excellent gas sensing performance could be ascribed to higher surface area and space-charge layer which induce the highly effective surface interaction between the target gas molecules and the surface active sites. Synergetic effect and electronic effect also favoring the effective adsorption of NOx on the surface. All the factors were obviously beneficial for the mesoporous hierarchical Ni(OH)2–In(OH)3 double hydroxide composites as gas sensor for monitoring air pollution.

Finite-size versus interface-proximity effects in thin-film epitaxialSrTiO3

The equilibrium electrical conductivity of epitaxial ${\text{SrTiO}}_{3}$ (STO) thin films was investigated as a function of temperature, $950\ensuremath{\le}$ $T$/K $\ensuremath{\le}1100$, and oxygen partial pressure, ${10}^{\ensuremath{-}23}\ensuremath{\le}$ $p$${\text{O}}_{2}$/bar $\ensuremath{\le}1$. Compared with single-crystal STO, nanoscale thin-film STO exhibited with decreasing film thickness an increasingly enhanced electronic conductivity under highly reducing conditions, with a corresponding decrease in the activation enthalpy of conduction. This implies substantial modification of STO's point-defect thermodynamics for nanoscale film thicknesses. We argue, however, against such a finite-size effect and for an interface-proximity effect. Indeed, assuming trapping of oxygen vacancies at the STO surface and concomitant depletion of oxygen vacancies---and accumulation of electrons---in an equilibrium surface space-charge layer, we are able to predict quantitatively the conductivity as a function of temperature, oxygen partial pressure, and film thickness. Particularly complex behavior is predicted for ultrathin films that are consumed entirely by space charge.

Revisiting SrTiO3as a photoanode for water splitting: development of thin films with enhanced charge separation under standard solar irradiation

Strontium titanate (SrTiO3) is an n-type semiconductor with high chemical and photochemical stability. This wide band gap oxide has a band gap energy of about 3.2 eV as well as a favorable energy for photocatalysis. In this study, we demonstrate an alternative and superior method to produce Nb-doped and undoped SrTiO3 photoanode thin films based on a colloidal deposition process which possess good activity under standard solar illumination conditions. Methanol was used as “hole scavenger,” and the results showed that the semiconductor–liquid junction (SCLJ) charge accumulation is not an important mechanism to control the photocurrent density and overpotential. In addition, experimental results suggest that the dominance of photocurrent density is controlled by the potential at the surface space charge layer for the Nb-doped SrTiO3 and by recombination at the depletion layer for the undoped oxide.

Surface states and space charge layer electronic parameters specification for long term air-exposed copper phthalocyanine thin films

The photoemission yield spectroscopy (PYS) study of the 1-year air-exposed thin 16-nm copper phthalocyanine (CuPc) layers deposited on n- and p-type Si (111) native substrates was presented. The occupied electronic states in the band gap and in the upper part of the valence band have been identified from PYS spectra and their possible origin was discussed. Furthermore, space charge layer electronic parameters of organic thin films and their variations upon the exposure were determined. Different values of work function were received as equal 4.64eV and 4.80eV for copper phthalocyanine on n-Si substrate and on p-Si one respectively while ionization energies were estimated accordingly as 5.18eV and 5.21eV. It was found that after the long term air exposure the work function and surface band bending of CuPc increased by 0.94eV and 0.56eV respectively for the layers on n-Si and by 1.05eV and 0.59eV for those on p-Si indicated their higher gas sensitivity. Additionally, we detected the significant surface dipole effect on both phthalocyanine samples manifested by strong shift in electron affinity of 0.38eV and 0.46eV for layers on n-Si and p-Si substrate respectively. To explain these changes we developed the model of CuPc surface-ambient gas electronic interaction.

Electrostatic drift effects on near-surface defect distribution in TiO2

The present work employs a combination of isotopic self-diffusion measurements and diffusion-drift modeling to identify a unique mechanism for defect accumulation in surface space-charge layers of TiO2. During oxygen gas-exchange experiments at elevated temperatures, rutile (110) surfaces inject charged oxygen interstitials into the underlying bulk. Yet near-surface electric fields attract the injected defects back toward the surface, retarding their diffusional migration and leading to longer residence times within the space-charge layers. The extended residence time enhances kick-in reactions, resulting in measureable pile-up of the isotope. Related effects probably generalize to other related semiconductors.

Migration of Ion Vacancy in Hydroxylated Oxide Film Formed on Cr: A Density Functional Theory Investigation

A theoretical study of ion vacancy migration properties in hydroxylated oxide film (passive film) formed on Cr surface has been carried out based on density functional theory (DFT). The ion vacancy migration involves a rather complex regime, which includes inner-Cr2O3, outer-Cr(OH)3 layers, and also the Cr/Cr2O3, Cr2O3/Cr(OH)3 and Cr(OH)3/solution interfaces. An atomic model has been proposed to simulate this system, where the diffusion coefficient of ion vacancy has been achieved as well as considering the influence of electric fields within n-p junction and surface space-charge layer. The results show that in the inner-Cr2O3 layer only O2– vacancy could migrate with a self-diffusion coefficient of 3.54 × 10–21 cm2/s at 25 °C; in the outer-Cr(OH)3 layer, only Cr3+ vacancy could migrate with a self-diffusion coefficient of 1.71 × 10–24 cm2/s at 25 °C, which indicates that the outer-Cr(OH)3 layer also contributes to the protection of passive film. At the Cr2O3/Cr(OH)3 interface, the self-diffusion coeffici...

Behavior of oxygen vacancies in single-crystal SrTiO3: Equilibrium distribution and diffusion kinetics

${}^{18}$O/${}^{16}$O exchange and subsequent time-of-flight secondary ion mass spectrometry (ToF-SIMS) analysis was employed to investigate the transport of oxygen, and thus the behavior of oxygen vacancies, in [nominally undoped, (100) oriented] single-crystal SrTiO${}_{3}$ substrates. Isotope exchange anneals were performed as a function of temperature, 948 $l$ $T$/K $l$ 1123, at an oxygen activity $a$O${}_{2}$ $=$ 0.50 and as a function of oxygen activity, 0.01 $l$ $a$O${}_{2}$ $l$ 0.70, at $T$ $=$ 1073 K. All isotope profiles show the same characteristic form: an initial drop over tens of nanometers close to the surface, which is attributed to an equilibrium space-charge layer depleted of oxygen vacancies, followed by a profile extending several microns into the solid, which is attributed to diffusion in a homogeneous bulk phase. The entire isotope profile can be described quantitatively by a numerical solution to the diffusion equation with a position-dependent diffusion coefficient; the description yields the tracer diffusion coefficient in the bulk ${D}^{*}$(\ensuremath{\infty}), the surface exchange coefficient ${k}_{\mathrm{s}}^{*}$, and the space-charge potential ${\ensuremath{\Phi}}_{0}$. All ${D}^{*}$(\ensuremath{\infty}) data are consistent with nominally undoped SrTiO${}_{3}$ substrates being weakly acceptor doped; the activation enthalpy for the migration of oxygen vacancies in bulk SrTiO${}_{3}$ is found to be $\ensuremath{\Delta}{H}_{\mathrm{mig},\mathrm{V}}$ \ensuremath{\approx} 0.6 eV. The surface termination of the SrTiO${}_{3}$ substrates was seen to affect significantly the surface exchange coefficient ${k}_{\mathrm{s}}^{*}$. Values of ${\ensuremath{\Phi}}_{0}$ obtained as a function of $T$ and $a$O${}_{2}$ are approximately 0.5 V, indicating strong depletion of oxygen vacancies within the equilibrium surface space-charge layers. Thermodynamic modeling indicates that space-charge formation at the TiO${}_{2}$-terminated (100) surface is driven by the Gibbs formation energy of oxygen vacancies at the interface being lower than in the bulk.

Surface photovoltage effect at thep-WSe2:Rb surface: Photoemission experiment and numerical model

It is shown that a combined experimental and theoretical study of the surface photovoltage (SPV) effect can be utilized for the quantification of a number of material parameters in a semiconductor-adsorbate model system. At the Rb-adsorbed surface of the semiconducting layered transition metal dichalcogenide WSe${}_{2}$, a large SPV effect of $\ensuremath{\approx}$600 meV is observed already at a moderate photon flux. Using valence band PES and an auxiliary tunable light source, the surface potential and the SPV effect are traced as a function of the adsorbate density and absorbed light intensity. We find that a major part of the stationary band back bending as a function of photon flux is constrained to a highly sensitive region, that is, a large fraction (50%--80%) of the total SPV magnitude rises within less than 1 order of magnitude. The effect is reproduced by a numerical model involving semiclassical charge carrier dynamics in the surface space charge layer. Among the parameters determined from the simulation, we find an approximate bulk acceptor density between $0.9\ifmmode\times\else\texttimes\fi{}{10}^{17}$ and $2\ifmmode\times\else\texttimes\fi{}{10}^{17}$ cm${}^{\ensuremath{-}3}$, a broad distribution of adsorbate donor levels at $\ensuremath{\approx}$0.2 eV above the conduction band minimum, and electron and hole carrier mobilities ${\ensuremath{\mu}}_{e}=0.7$ cm${}^{2}$/(V s), ${\ensuremath{\mu}}_{h}=1.5$ cm${}^{2}$/(V s) in the crystallographic $c$ direction.

Band-bending effects on scanning tunneling microscope images of subsurface dopants: First-principles calculations

Tip-induced band-bending (TIBB) effects on scanning tunneling microscope (STM) images have been investigated. The TIBB has an extremal value where the surface space charge layer turns from the inversion region to the depletion one with increasing dopant concentration at a fixed sample bias. Unignorable TIBB remains even for the usual degenerate semiconductor with a dopant concentration such as 1018 cm−3 for Si. The STM images for H-terminated Si(111) surfaces with dopants substituted at the subsurface have been simulated using first-principles calculations within the density functional theory. The subsurface dopants on the STM images become distinguishable more obviously in consideration of the TIBB effect, specifically for acceptors in the occupied state images and for donors in the empty state. As a result, the TIBB effect improves agreement between the experimental and the theoretical sample biases at which the dopant’s feature can be observed clearly.

Theoretical and experimental depth-resolved cathodoluminescence microanalysis of excitonic emission from ZnO epilayers

In this letter, the excitonic emissions from ZnO epilayers were studied by depth-resolved cathodoluminescence (CL) microanalysis. An excellent agreement between the redshift of the experimental depth-resolved CL emission and theoretically simulated values was observed, which clearly identified its origin to be the strong internal absorption in ZnO epilayer. Moreover, the rigorous Monte Carlo simulated CL generation profiles with the correction of self-absorption exhibit the reasonable correspondence with the measured CL intensities. The intensity discrepancies for low and high excitation cases have been interpreted by the occurrence of the excess carrier nonradiative recombination on the surface space charge layer and the exciton nonradiative quenching processing by defects or structural disorders near the interface of ZnO and sapphire.

Using 18O/16O exchange to probe an equilibrium space-charge layer at the surface of a crystalline oxide: method and application

The use of an (18)O/(16)O exchange experiment as a means for probing surface space-charge layers in oxides is examined theoretically and experimentally. On the basis of a theoretical treatment, isotope penetration profiles are calculated for (18)O/(16)O exchange across a gas-solid interface and subsequent diffusion of the labelled isotope through an equilibrium space-charge layer depleted of mobile oxygen vacancies and into a homogeneous bulk phase. Profiles calculated for a range of conditions all have a characteristic shape: a sharp drop in isotope fraction close to the surface followed by a normal bulk diffusion profile. Experimental (18)O profiles in an exchanged (001) oriented single crystal of Fe-doped SrTiO(3) were measured by time-of-flight secondary ion mass spectrometry (ToF-SIMS). By extracting the space-charge potential from such profiles, we demonstrate that this method allows the spatially resolved characterization of space-charge layers at the surfaces of crystalline oxides under thermodynamically well-defined conditions.

Surface waves and space charge layers in a spatially inhomogeneous plasma

A theory of surface waves in a layer of a spatially inhomogeneous cold electron plasma is presented. Four types of surface waves are revealed, and the conditions under which they can exist are determined. Complex frequency spectra are obtained, and the mechanisms for wave damping by plasma inhomogeneity are discussed.

Laudatio for Gerhard Abstreiter

Laudatio for Gerhard Abstreiter

Laudatio for Gerhard Abstreiter

Laudatio for Gerhard Abstreiter

Electrical Conductivity of Epitaxial SrTiO3 Thin Films as a Function of Oxygen Partial Pressure and Temperature

SrTiO3 (100) epitaxial films with thicknesses of 3, 1 μm, and 250 nm were prepared on MgO (100) substrates by pulsed‐laser deposition. The electrical conductivities of the thin films were systematically investigated as a function of temperature and ambient oxygen partial pressure. This was made possible by using a specially designed measurement setup, allowing the reliable determination of resistances of up to 25 GΩ in the temperature range of 600°–1000°C under continuously adjustable oxygen partial pressures ranging from 10−20 to 1 bar. The capabilities of the measurement setup were tested thoroughly by measuring a SrTiO3 single crystal. The well‐known characteristics, e.g., the decline of the conductivity with a slope of –1/4 under reducing conditions and the opposite +1/4 behavior in oxidizing atmospheres, are found in the log(σ)–log(pO2) profiles of the epitaxial films. However, the p‐type conductivity decreases, and the n‐type conductivity increases with decreasing film thickness. This phenomenon is attributed to the charge carrier redistribution in the surface space charge layers. Owing to the high surface‐to‐volume ratio, the space charge layers play an important role in thin films.

Growth, structure and electrical properties of tungsten oxide nanorods

Tungsten trioxide has shown good sensing properties towards various gases. Recently thin nanostructured WO 3 films have been tested. Due to their large surface area to volume ratio they exhibit good sensitivity depending on the grain size. However in conventional WO 3 thin films the average grain size exceeds the thickness of the surface space charge layer, so the electrical conduction is mainly controlled by the carriers transport across the grain boundaries. An alternative way seems to be in a monocrystalline material with nanometric dimensions. Our objective is to fabricate nanosized tungsten oxide rods and to test their sensing properties under gas adsorption. In this work, we focus on the growth, the structure and the electrical properties of tungsten nanorods. The tungsten oxide nanorods were grown by vapour transport from a WO 3 layer onto a substrate (Mica). The nanorods growth was controlled by the temperature gradient between the WO 3 layer and the substrate. Their morphology was investigated by AFM and their structure by TED and TEM. We have investigated the conductivity of the WO 3 nanorods with a technique derived from Atomic Force Microscopy operating in contact mode with a conductive tip (Resiscope).

Monolithic t‐ZrO2 Nanopowder through a ZrO(OH)2·xH2O Polymer Precursor

Monolithic t‐ZrO2 (tetragonal structure) nanopowder is synthesized with an amorphous ZrO(OH)2·xH2O polymer precursor. The H2O molecules impart the structure and promote reconstructive thermal decomposition of the structure to t‐ZrO2 nanoparticles on heating at temperatures as low as 200°C. A prevalent endothermic heat output in the dissociation process controls the local temperature in exothermic nucleation and growth of various groups of the reaction species so that it is self‐controlled in high‐energy nanoparticles. Crystallites are, on average, d= 8 nm diameter, and they have a high value of Gibbs free energy or lattice volume Vo= 0.06770 nm3. The excess volume decreases to Vo= 0.06705 nm3 if the reaction temperature is increased to >200°C, i.e., approximately the bulk value of 0.06681 nm3; there is a minor increase to d= 12 nm at 600°C. Many oxygen vacancies in the thin surface space‐charge layers seem to support the stability of small particles in this particular polymorph. A pure m‐ZrO2 (monoclinic structure) appears with d= 22 nm at temperature as low as 800°C. The results are analyzed using X‐ray diffractometry, microstructure, infrared spectroscopy, and thermal studies of the polymer precursor and derivative t‐ZrO2 nanoparticles.