Width Of Space Charge Layer Research Articles

Exploring the role of Ti3C2TxMXene in photoelectrochemical water splitting over solution-processed BiVO4 photoanode

Transition metal carbides, nitrides, or carbonitrides known as MXenes commonly improve the efficiency of photo(electro)catalysts, but their role is still not fully explored. Herein, we developed a novel and facile solution-processing method to fabricate BiVO4 and Ti3C2Tx-modified BiVO4 (Ti3C2Tx/BiVO4) photoanodes to explore the role of Ti3C2Tx MXene in photoelectrochemical water splitting. The charge transfer and recombination kinetics were studied by transient open-circuit potential, transient photocurrent, photoelectrochemical impedance spectroscopy, and intensity-modulated photocurrent spectroscopy. It was demonstrated that Ti3C2Tx forms a Schottky-like junction at the BiVO4/Ti3C2Tx interface and that it acts as a hole transport/storage layer. The Ti3C2Tx layer creates a built-in electric field at the surface that increases the band bending and space charge layer width, eventually enhancing the charge separation efficiency. Modification of Ti3C2Tx/BiVO4 with a water oxidation catalyst such as cobalt phosphate (Co-Pi) was essential to deliver the maximum photocurrent. For instance, the Co-Pi/Ti3C2Tx/BiVO4 photoanode delivered 4.72 mA/cm−2 at 1.23 VRHE, which is 2- and 11-fold higher than those of Ti3C2Tx/BiVO4 and pristine BiVO4 photoanodes, respectively. This study provides a practical protocol for fabricating Ti3C2Tx/BiVO4 and reveals the role of MXene in photoelectrochemical water splitting.

Enhanced grain boundary conductivity of Gd and Sc co-doping BaZrO3 proton conductor for proton ceramic fuel cell

Applicability of BaZrO3 proton conductor as electrolytes in fuel cells and electrolysis cells is limited due to lower grain boundary conductivity and serious sintering activity. Herein, we report the enhancement of grain boundary conductivity and sintering performance of BaZrO3 proton conductor by Sc and Gd co-doping at B site. Combining experimental characterization and theoretical calculations, we found that the incorporation of Sc can effectively promote the generation of proton defects and lower the proton migration energy barrier. Meanwhile, the clustering of positively charged oxygen vacancy to acceptors can suppress proton trapping, thus positively affecting the long-range proton transport and effectively improving bulk conductivity. In addition, the space charge quantification by Mott-Schottky approximation confirms that Sc and Gd co-doping is significantly reduce the Schottky barrier height and the width of space charge layer, resulting in lower proton depletion and higher grain boundary conductivity. Benefiting from these two positive effects, these electrolytes show the highest proton conductivity of 2.99 × 10-3 ∼ 3.63 × 10-2 S cm−1 in the temperature range of 250 ∼ 750 ℃. This indicates that co-doping of Gd and Sc is one of the effective and feasible strategy to improve the sinterability and conductivity of BaZrO3-based proton conductors.

Fully-depleted dual P–N heterojunction with type-II band alignment and matched build-in electric field for high-efficient photocatalytic hydrogen production

In this work, a dual p-n heterojunction of Cu2O/Ni(OH)2/TiO2 with type-II band alignment and matched build-in electric field was fabricated. This dual p-n heterojunction promoted the separation and transfer of charge carriers, which is much more efficient than individual p-n heterojunction. Photocatalytic hydrogen evolution under the simulated sunlight shows a high rate of 6145 μmol g−1 h−1 for Cu2O/Ni(OH)2/TiO2, which is 1.9 and 2.7 times that of Ni(OH)2/TiO2 and Cu2O/TiO2, respectively. The apparent quantum yield of Cu2O/Ni(OH)2/TiO2 is about 20.2% under the irradiation of monochromatic light (λ = 420 nm). The recycling test of hydrogen evolution also verified a high stability for this dual heterojunction. The type-II band alignment and matched build-in electric field was confirmed, which accelerate the migration of charge carriers. The width of space-charge layer was calculated and proved that the p-n junctions in Cu2O/Ni(OH)2/TiO2 are fully-depleted, which largely reduced the bulk recombination of charge carriers. The synergistic effect of the improved visible-light response, type-II band alignment, matched build-in electric field, and fully-depleted space-charge layer contributes to the enhanced photocatalytic hydrogen evolution.

Langmuir-Blodgett-assembled monolayer zinc ferrite nanoparticle film with unique photogenerated charge carrier separation efficiency and charge transfer behavior

For figuring out the separation performance and transfer behavior of photogenerated charge carriers in porous spinel structured zinc ferrite (ZnFe2O4, ZFO) ultrathin films, novel ZFO nanoparticle films (LB-ZFO) were prepared on the fluorine-doped tin oxide conductive glass by the Langmuir-Blodgett (LB) method in this study. As illuminated by the light with the wavelength ranging from 320 nm to 440 nm, monolayer LB-ZFO film (LB-ZFO-1) showed relatively higher surface photovoltage than the 5-layered and 9-layered LB-ZFO films, which suggested the film thickness reduction facilitated the photogenerated charge carrier separation. Moreover, the LB-ZFO-1 film possessed lower flat band potential and larger space charge layer width in comparison with the 5-layered and 9-layered LB-ZFO films, further proving that stronger driving force was available for separating the photogenerated electrons and holes by reducing the film thickness. All the as-prepared LB-ZFO films exhibited the capacitor behavior because of their porous structure. The photoelectrocatalytic (PEC) activity of LB-ZFO-1 film was still low even though the electron transfer resistance at the LB-ZFO-1 film/electrolyte interface greatly decreased from 518.60 Ω to 92.56 Ω. Therefore, it suggests that the capacitor behavior existing in porous films is probably one of key factors that resulted in poor PEC activities of LB-ZFO films, which should be induced considerable attentions in future.

High-resolution Scanning Transmission EBIC Analysis of Misfit Dislocations at Perovskite pn-Heterojunctions

Fundamental losses of photovoltaic energy conversion are transmission of sub band gap photons and thermalisation which are the underlying physics of the Shockley-Queisser limit defining maximum conversion efficiency of single-junction solar cells. Strongly correlated materials such as perovskites are promising candidates to exceed this limit by exploiting (i) long wavelength light absorption and (ii) the existence of long-living intraband excitations indicating that harvesting hot excess carriers might be feasible in such systems. In this work, we study pn-heterojunctions produced from Pr1-xCaxMnO3 on SrTi1-yNbyO3 by means of microscopic techniques. Such systems exhibit relevant quantities such as space charge layer width, screening lengths and excess carrier diffusion lengths in the 1-10 nm range which makes the use of standard methods such as electron beam induced current a challenging task. We report scanning transmission electron beam induced current experiments of misfit dislocations at the heterojunction. The dislocation-induced reduction of the charge collection is studied with nanometer spatial resolution. Effects of surface recombination and the heterojunction electric field are discussed.

Defect Engineering in Semiconductors: Manipulating Nonstoichiometric Defects and Understanding Their Impact in Oxynitrides for Solar Energy Conversion

AbstractNonstoichiometric defects, as manifested by slight deviation of elemental compositions from chemical formulas, are common yet highly important in solid materials. Oxynitrides with a relatively large O/N ratio variation are theoretically predicted to change their electronic structures and charge transport behaviors with these nonstoichiometric defects. However, little experimental effort is devoted to understanding the impact of such nonstoichiometric defects regarding varied O/N ratios in oxynitrides for solar water splitting. The main reason is the lack of suitable oxynitride research models for aforementioned nonstoichiometric defect study without interference from other factors. Using TaON as a prototypical material, finely tuned O/N ratios can dramatically influence its photoresponse. In‐depth analysis further reveals a significant impact of nonstoichiometric defects (O/N ratios) on TaON's charge carrier densities, charge separation, and transport. Finally, manipulating nonstoichiometric defects of O/N ratios demonstrates its ability to control the space charge layer width and film conductivity of TaON photoanodes for high efficiency water splitting. Therefore, a fine understanding and control of nonstoichiometric defects would be highly important for future development of high efficiency oxynitrides for water splitting.

Analysis of electronic parameters and frequency-dependent properties of Au/NiO/n-GaN heterojunctions

The electrical and frequency-dependent properties of ten Au/NiO/n-GaN heterojunctions fabricated with similar conditions are assessed by I–V, C–V, and G–V measurement methods. In addition, C–f and G–f measurements are conducted in the frequency range of 1 kHz–1 MHz. The electronic parameters are changed from junction to junction even if they are fabricated in the similar way. The calculated barrier height and ideality factor values are fitted by the Gaussian distribution function. Statistical analysis of the data provides the mean barrier height and ideality factor values of 0.84 eV and 2.70 for the heterojunction. Besides, the mean barrier height (Ф b), donor concentration (N d), space charge layer width (W D), and Fermi level (E F) are determined from the C–V data and the corresponding values are 1.30 eV, 2.00 × 1017 cm−3, 8.222 × 10−6 cm, and 0.018 eV, respectively. The interface state density (N SS) and relaxation time (τ) are assessed from C–f and G–f measurements. Moreover, the dielectric constant (e′), dielectric loss (e″), tangent loss (tan δ), and electrical conductivity (σ ac) are determined from C–f and G–f data in the frequency range of 1 kHz–1 MHz with various biases (0.1–0.6 V). e′ and e″ are decreased with increasing frequency.

Effect of strain on space charge layer in GaN nanowires investigated by in-situ off-axis electron holography

Effect of strain on space charge (SC) layer in nanowires (NWs) has been examined by in situ off-axis electron holography, where GaN NWs attach to an Au electrode inside a transmission electron microscope (TEM). Based on the phase image reconstructed from the complex hologram, the width of SC layer in a strained GaN NW is significantly reduced to about 60nm, comparing to the 85nm of the unstrained NW. About 29% reduction of the SC layer in the strained GaN NW resulted from significant decrease of electrons flowed from the GaN into Au. First principle calculations show that the strain reduced bandgap of GaN, narrowing the difference between GaN NW and Au electrode in Fermi level.

Use of Surface Photovoltage Spectroscopy to Measure Built-in Voltage, Space Charge Layer Width, and Effective Band Gap in CdSe Quantum Dot Films.

Surface photovoltage spectroscopy (SPS) was used to study the photochemistry of mercaptoethanol-ligated CdSe quantum dot (2.0-4.2 nm diameter) films on indium doped tin oxide (ITO) in the absence of an external bias or electrolyte. The n-type films generate negative voltages under super band gap illumination (0.1-0.5 mW cm(-2)) by majority carrier injection into the ITO substrate. The photovoltage onset energies track the optical band gaps of the samples and are assigned as effective band gaps of the films. The photovoltage values (-125 to -750 mV) vary with quantum dot sizes and are modulated by the built-in potential of the CdSe-ITO Schottky type contacts. Deviations from the ideal Schottky model are attributed to Fermi level pinning in states approximately 1.1 V negative of the ITO conduction band edge. Positive photovoltage signals of +80 to +125 mV in films of >4.0 nm nanocrystals and in thin (70 nm) nanocrystal films are attributed to electron-hole (polaron) pairs that are polarized by a space charge layer at the CdSe-ITO boundary. The space charge layer is 70-150 nm wide, based on thickness-dependent photovoltage measurements. The ability of SPS to directly measure built-in voltages, space charge layer thickness, sub-band gap states, and effective band gaps in drop-cast quantum dot films aids the understanding of photochemical charge transport in quantum dot solar cells.

Laterally Inhomogeneous Barrier Analysis Using Capacitance–Voltage Characteristics of Identically Fabricated Schottky Diodes

Au Schottky contacts (50 dots) on n-Si (100) were fabricated by thermal evaporation under the same conditions. The mean of the electrical parameters of the diodes were investigated by means of capacitance–voltage (C–V) measurements at 1 MHz. Even if the diodes were all equally fabricated, there was a diode-to-diode change. The values of barrier height (ΦB) were determined from the C −2–V characteristics, which ranged from 0.812 eV to 0.837 eV. The Gaussian fit of the barrier height distributions gave a mean of barrier height value of 0.822 eV and a standard value of 0.005 eV. Furthermore, the mean values of other parameters such as the carrier donor concentration (N D), the diffusion potential at zero bias (V 0), the Fermi level (E F), the image force lowering (ΔΦb) and the space charge layer width (W D) were investigated and determined to be 1.311 × 1015 cm−3, 0.575 V, 0.257 eV, 1.363 × 10−2 eV, and 7.573 × 10−5 cm, respectively.

Low-Temperature Atomic Layer Deposition of Crystalline and Photoactive Ultrathin Hematite Films for Solar Water Splitting.

We developed a low-temperature atomic layer deposition route to deposit phase pure and crystalline hematite (α-Fe2O3) films at 230 °C without the need for postannealing. Homogenous and conformal deposition with good aspect ratio coverage was demonstrated on a nanostructured substrate and analyzed by transmission electron microscopy. These as-deposited α-Fe2O3 films were investigated as photoanodes for photoelectrochemical water oxidation and found to be highly photoactive. Combined with a TiO2 underlayer and a low-cost Ni(OH)2 catalyst, hematite films of less than 10 nm in thickness reached photocurrent densities of 0.3 mA cm(-2) at 1.23 V vs RHE and a photocurrent onset potential of less than 0.9 V vs RHE, previously unseen for films this thin and without high temperature annealing. In a thickness-dependent photoelectrochemical analysis, we identified a hematite thickness of only 10 nm to yield the highest internal quantum efficiency when using a suitable underlayer such as TiO2 that induces doping of the hematite film and reduces electron/hole recombination at the back contact. We find that, at high bias potentials, photocurrent density and quantum efficiency proportionally increase with light absorption in films thinner than 10 nm and are limited by the space charge layer width in thicker films. Thus, we propose to apply hematite films of 10 nm in thickness for future developments on suitable nanostructured conductive scaffolds that can now be extended to organic scaffolds due to our low-temperature process.

Investigation of diode parameters using I–V and C–V characteristics of Al/maleic anhydride (MA)/p-Si structure

Al/maleic anhydride (MA)/p-Si metal–polymer–semiconductor (MPS) structures were prepared on p-Si substrate by spin coating. Device parameters of Al/MA/p-Si structure have been determined by means of capacitance–voltage (C–V) and conductance–voltage (G–V) measurements between 700 kHz and 1⋅5 MHz and current–voltage (I–V) measurements at 300 K. The parameters of diode such as the ideality factor, series resistance, barrier height (BH) and flat band barrier height were calculated from the forward bias I–V characteristics. The investigation of interface states that density and series resistance from C–V and G–V characteristics in Al/MA/p-Si device has been reported. The frequency dependence of the capacitance could be attributed to trapping states. Several important device parameters such as the BH ϕb, fermi energy (EF), diffusion voltage (VD), donor carrier concentration (ND) and space charge layer width (WD) for the device have been obtained between 700 kHz and 1⋅5 MHz. The I–V, C–V-f and G–V-f characteristics confirm that the parameters like the BH, interface state density (Dit) and series resistance (Rs) of the diode are strongly dependent on the electrical parameters in the MPS structures.

Analysis of device parameters for Au/tin oxide/n-Si(1 0 0) metal–oxide–semiconductor (MOS) diodes

In present paper, the device parameters of tin oxide/n-Si(100) structure have been determined by means of capacitance–voltage (C–V) and conductance–voltage (G–V) measurements between 500Hz and 1MHz and current–voltage (I–V) measurements between −2 and +3V at 300K. This device has denoted good rectifying behavior and the I–V data could be described by thermionic emission (TE) technique. The values of ideality factor (n) and barrier height (ΦB) for the sample have been determined to be 3.724 and 0.624eV, respectively. The measured values of capacitance and conductance for the series resistance under all the biases have been corrected influence to calculate the real values of capacitance and conductance. The frequency dependence of the capacitance may be attributed to trapping states. Interface trap states of the MOS device increased by decreasing the frequency and were calculated as 1.12×1011 and 6.62×1011eV−1cm−2 for 1MHz and 100kHz, respectively. Several important device parameters such as barrier height (ΦB), fermi energy (EF), diffusion voltage (VD), donor carrier concentration (ND) and space charge layer width (WD) for the device have been obtained between 100kHz and 1MHz.

Photoelectrocatalytic performance of TiO2 nanoparticles incorporated TiO2 nanotube arrays

Highly ordered TiO2 nanotube arrays decorated by TiO2 nanoparticles were prepared via electrochemical anodization method, followed by evaporation induced self-assembly (EISA) strategy. Their photocatalytic (PC) and photoelectrocatalytic (PEC) degradation performance on different dyes were investigated. The dyes used were methylene blue (MB), methyl orange (MeO), rhodamine B (RhB) and reactive brilliant red (K-2G). The structure and compositions of photocatalysts were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). The optical properties of the samples were evaluated by UV–vis spectra. Effects of the incorporation of TiO2 nanoparticles on the charge transfer rate and charge carrier concentration of the TiO2 nanotube arrays during the photocatalytic and photoelectrocatalytic reactions were investigated with electrochemical impedance spectroscopy (EIS) and Mott–Schottky analysis. In spite of the negative effects such as the decreased absorption in UV region and increased charge transfer resistance, incorporation of TiO2 nanoparticles into TiO2 nanotube arrays was found to not only increase the surface area of TiO2 nanotube arrays, but also enhance the width of space charge layer which promotes the separation of the photo-generated hole/electron pairs efficiently under external bias. Moreover, anodic bias can improve the degradation rates of anionic dyes more than cationic dyes.

Conductivity study of dense BaZr0.9Y0.1O(3 − δ) obtained by spark plasma sintering

10% yttrium doped barium zirconate (BZY10) was synthesized by solid state reaction and a 99.8% dense and transparent sample was prepared by spark plasma sintering (SPS) at 1700°C for 5 minutes. A single phase compound was obtained, with no evaporation of barium. High-Resolution Transmission Electron Microscopy (HRTEM) images revealed a distinct grain boundary across two grains, confirming no secondary phase formation along grain boundaries. The conductivity was determined as a function of temperature, at two different water vapor pressures, as well as in the D2O vapor exchanged state. The activation energy corresponding to protonic conduction has been determined, and compared to values from literature. The decrease in activation energy with increasing lattice parameters is confirmed. Furthermore, the effective space charge layer width has been estimated at 1.2–1.3nm.

Impact of Zr/Ti ratio in the PZT on the photoreduction of silver nanoparticles

Silver nanoparticle deposition from an aqueous solution of silver nitrate onto the surface of PZT thin films of stoichiometric compositions PbZr 0.3Ti 0.7O 3 and PbZr 0.52Ti 0.48O 3 has been investigated. The impact of Zr/Ti ratio on the photochemical properties of PZT is shown by the preferential growth of silver nanoparticles onto the surface. Photoreduction of silver occurs on both c + and c − domains on PbZr 0.52Ti 0.48O 3 whereas it occurs only on c + domains on a PbZr 0.3Ti 0.7O 3 surface. The difference in deposition pattern is attributed to difference in magnitude of spontaneous polarization, effective hole concentration and band gap of the two samples which impacts shape and width of space charge layer in the two samples resulting in a change in band bending at the surface.

Direct Observation of the Valence Band Edge by in Situ ECSTM-ECTS in p-Type Cu2O Layers Prepared by Copper Anodization

Polycrystalline Cu2O layers have been selectively grown by electrochemical anodization of polycrystalline Cu electrodes in an alkaline medium (pH 12.85). Uniform layers with thicknesses around 100 nm have been obtained. Using electrochemical impedance spectroscopy, it was concluded that the Cu2O films behave as a p-type semiconductor. The Mott−Schottky plot gives a value for the flat band potential of UFB = −255 mV vs silver/silver chloride electrode (SSC), an estimated carrier density NA = 6.1 × 1017 cm−3, and the space charge layer width was calculated to be WSCL = 9 nm at a band bending of 120 mV. The electronic structure of the Cu|Cu2O|electrolyte interface was for the first time probed by in situ electrochemical tunneling spectroscopy. The use of in situ electrochemical scanning tunneling microscopy allows us to directly observed the valence band edge and determine its position against the absolute energy scale to be EVB = −4.9 eV. Finally, we constructed a quantitative electronic diagram of the Cu|Cu2O|electrolyte interface, where the positions of the valence and conduction band edges are depicted, as well as the edge of the previously reported electronic subband.

Quantitative analysis of doping profile on beveled p-type GaAs structures by micro-Raman spectroscopy

Micro-Raman spectra taken from beveled p-GaAs samples covering a range of concentrations (1×1018–2×1019cm−3) have been studied. This study concentrated on the evaluation of the changes of Raman intensities measured in frequency positions of transversal optical and longitudinal optical phonons along the bevel. These changes are dependent on doping concentration and due to this fact we have extracted a calibration function, which will be used to determine the doping concentration in nanometer-thin layers. There are two physical values, which was derived from this study: width of surface-depletion layer and width of space-charge layer. Good correspondence between experimental results and theoretical calculations is the proof of correctness of the model of explanation of the changes of micro-Raman and their analysis. The chemical procedure for preparation of homogeneous oxide-free bevel-shaped p-type GaAs structures with small angle was also developed.

Space charge layer dynamics at oxide-semiconductor interfaces under optical modulation: Theory and experimental studies by non-contact photocarrier radiometry

The dynamic theory of the optically modulated space charge layer (SCL) resulting from band-bending at a Si - SiO 2 interface was developed in terms of the density of interface charges occupying bandgap energy states. Expressions were derived for these interface charge densities, interacting with the free carrier-density-wave generated in the SCL and in the quasi-neutral region (hulk) by an intensity-modulated super-bandgap laser. The PCR theory incorporating these effects was further developed. It was found to involve the dc, fundamental, and entire harmonic spectrum of the excitation frequency as a result of the optical modulation of the curvature of the energy bands and the SCL width at the interface. PCR was used experimentally with a harmonically modulated low-power laser pump and a superposed dc super-bandgap optical bias (a secondary laser beam) to control and monitor the space-charge-layer width in oxidized p-Si - SiO 2 and n-Si - SiO 2 interfaces (wafers) exhibiting charged interface-state related band-bending. The application of the theory to the experiments yielded the various transport parameters of the samples as well as depth profiles of the SCL exhibiting complete (p-type Si) or partial (n-type Si) band-flattening, to a degree controlled by the widely different minority carrier capture cross-section at each interface. The uncompensated charge density at the interface was also calculated from the theory.

Theory of space-charge layer dynamics at oxide-semiconductor interfaces under optical modulation and detection by laser photocarrier radiometry

The dynamic theory of the optically modulated space-charge layer (SCL) resulting from band bending at a Si–SiO2 interface was developed in terms of the density of interface charges occupying band-gap energy states. Expressions were derived for these interface charge densities, interacting with the free-carrier density wave generated in the SCL and in the quasineutral region (bulk) by an intensity-modulated super-band-gap laser. The residual and modulated interface charge coverage affects the band-edge-to-impurity state recombination and the concomitant near-infrared photon emission comprising the photocarrier radiometry (PCR) signal. The PCR theory incorporating these effects was further developed. It was found to involve the dc, fundamental, and entire harmonic spectrum of the excitation frequency as a result of the optical modulation of the curvature of the energy bands and the SCL width at the interface.