Properties Of Semiconductor Surfaces Research Articles

Tuning the Surface Properties of CuO Films Using the Precursor Aging Approach for Enhanced Photoelectrocatalytic Reactions

AbstractThe surface properties of semiconductors have a significant influence on their photoelectrocatalytic efficiency. This research presents a precursor aging method for tuning the surface properties of CuO films, for enhanced catalytic response. A precursor solution made using copper acetate, polyethylene glycol (PEG) 400, and diethanolamine, and aged for 1, 40, 80, 120, 150, 180, and 250 days is used in each case to fabricate CuO photocathodes via the dip‐coating method. The films fabricated using the 1‐day‐old precursor reveal compact and homogeneous nanoparticles. The films eventually get tuned to yield highly porous and rougher surfaces after aging the precursors for 180–250 days. The film's bandgap decreases by 9% after 180–250 days of precursor aging. Photocathodes prepared using the 180‐day‐old precursor produce the optimum photocurrent density of 1.6 mA cm−2 at 0.35 V versus reversible hydrogen electrode (RHE), representing a 196% increase relative to the films fabricated using the 1‐day‐old solution. They also produce an anodic onset potential shift of 260 mV. This improved photoelectrocatalytic response is due to the porous morphology of the films, which produces a larger surface area that enhances light absorption, increases active sites for catalytic reactions, and reduces charge transfer resistance at the photocathode‐electrolyte interface.

Photoelectrochemical Modelling of Semiconducting Electrodes for Neural Interfacing

Semiconducting electrodes are increasingly utilised for neural interfacing applications, such as neural recording, stimulation, and photomodulation. To characterize the performance of these electrodes, photoelectrochemical analysis is often undertaken in biologically relevant electrolytes. These include electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and for photomodulation applications, photocurrent (PC) measurements. From such measurements, it is possible to deduce key properties of semiconductor surfaces, such as electrochemical impedance and capacitance, as well as mechanisms of charge transfer. To extract these parameters from the experimental data, equivalent electrical circuit modelling is often employed, but usually only for a single technique at a time which often misses key insights about the processes occurring at the electrode-electrolyte interface. Here we present an equivalent circuit model that simultaneously describes the results from CV, EIS, and PC transient measurements. Using semiconducting nitrogen-doped ultrananocrystalline diamond (N-UNCD) electrodes in saline solution, we show that the model describes physical mechanisms that occur at the interface with electrolyte, encompassing the space charge region, the electrical double layer, and the electrolyte. Using the model we are able to optimize parameters relevant for neural interfacing and suggest that this framework may assist in the characterization of other semiconducting electrodes.

Cr2O3 layer inhibits agglomeration of phosphine-protected Au9 clusters on TiO2 films.

The properties of semiconductor surfaces can be modified by the deposition of metal clusters consisting of a few atoms. The properties of metal clusters and of cluster-modified surfaces depend on the number of atoms forming the clusters. Deposition of clusters with a monodisperse size distribution thus allows tailoring of the surface properties for technical applications. However, it is a challenge to retain the size of the clusters after their deposition due to the tendency of the clusters to agglomerate. The agglomeration can be inhibited by covering the metal cluster modified surface with a thin metal oxide overlayer. In the present work, phosphine-protected Au clusters, Au9(PPh3)8(NO3)3, were deposited onto RF-sputter deposited TiO2 films and subsequently covered with a Cr2O3 film only a few monolayers thick. The samples were then heated to 200°C to remove the phosphine ligands, which is a lower temperature than that required to remove thiolate ligands from Au clusters. It was found that the Cr2O3 covering layer inhibited cluster agglomeration at an Au cluster coverage of 0.6% of a monolayer. When no protecting Cr2O3 layer was present, the clusters were found to agglomerate to a large degree on the TiO2 surface.

Nanometric and surface properties of semiconductors correlated to photocatalysis and photoelectrocatalysis applied to organic pollutants – A review

This review study summarizes relevant topics about the photoelectrocatalytic semiconductors for the degradation of organic pollutants. Among the topics are A) the basic concepts of photocatalysis and photoelectrocatalysis for the advanced oxidation of organics in water. B) The band theory about heterojunctions, Schottky barriers, and doping of these semiconductors being photocatalysts. C) Proposing the outlook for photocatalysts, as those systems arranging doped semiconductors with heterojunctions and having Schottky junctions, a set of tables and sections show reported cases that complement the semiconductor characteristics providing the percentage of degradation attained, such as photoanodes of Boron-doped TiO2 nanotubes, TiO2 - Sb2S3 composites, and so forth. D) The novel characteristics of graphene-based materials for improving photoelectrocatalytic systems. E) A systematic way of classifying and understanding photoelectrocatalytic semiconductors based on their relevant characteristics and photoelectrochemical mechanisms. Moreover, this work shows the photoelectrocatalytic properties of the recently reported graphene oxide - reduced graphene oxide (GO-rGO) nanohybrids, which can also be used for the degradation of organic pollutants. Finally, based on all the previously gathered and summarized information, some inferences are made about the photoelectrocatalytic properties of some GO-rGO based heterojunctions for the degradation of organic pollutants.

Photoelectrochemical performance enhancement of low-energy Ar+ irradiation modified TiO2

To evaluate the effects of surface properties of semiconductor and enhance semiconductor’s activity for photocatalytic water oxidation, low-energy Ar+ irradiation was employed to modify the rutile TiO2 (110) surface and control the formation of oxygen vacancy density in TiO2, which was subsequently used for the photoelectrochemical (PEC) oxygen evolution reaction (OER). Specifically, by controlling the substrate temperature during Ar+ irradiation, the surface morphology and oxygen vacancy density of rutile TiO2 (110) could be modified, such that the effects of surface crystallinity, optical absorption and electrical properties of TiO2 could be compared for OER. We found that the intrinsic activity of TiO2 for PEC OER does not exhibit a scaling relationship with respect to the density of oxygen vacancy, optical adsorption or charge carrier concentration. Rather, a linear relationship could be observed between the photocurrent density and the interfacial charge-transport conductivity. Furthermore, ambient pressure X-ray photoelectron spectroscopy (APXPS) was used to investigate the effect of low-energy Ar+ irradiation on the interaction between TiO2 and water. It was demonstrated that using single crystal model systems with controlled properties, this study has thus provided a clear yet detailed analysis on key factors in tuning the photoelectrochemical performance of TiO2.

Surface-engineering of decahedron shaped bismuth vanadate for improved photoelectrochemical water oxidation by indium doping coupled with graphitic carbon nitride quantum dots

Surface of a photocatalyst critically determines the efficiency of water oxidation by providing suitable reaction sites and energetics. Manipulating the properties of semiconductor surface for better charge extraction and efficiency forms the basis of present study. We chose monoclinic BiVO4 as a model system, due to its suitable band structure and stability with a theoretical current density approaching 7 mA/cm2 is a promising material for photoelectrochemical water oxidation. An in-situ growth procedure yields, indium doped BiVO4 (BiVO4:In) photoanode with distinctive morphology. This photoanode forms a type II heterojunction (BiVO4:In)-CNQD) with g-C3N4 quantum dots (CNQD) loading to address the shortcomings. A photocurrent density of ~2.42 mA/cm2 with a solar to hydrogen conversion of 2.66% with an average Faradaic yield of 90% is yielded, which is ~4-folds higher than its bare counterpart. (BiVO4:In)-CNQD photoanode shows a significant cathodic shift of 136 mv than bare BiVO4 photoanode. Indium in BiVO4 favors the creation of oxygen vacancies, which improves charge separation and charge carrier density in BiVO4:In photoanode. It is also observed that the BiVO4:In photoanode shows higher surface hydrophilic characteristics, results in more polar surface and water adsorption on the surface, leading to more active water at the reaction site.

Phenethylammonium Functionalization Enhances Near-Surface Carrier Diffusion in Hybrid Perovskites

Understanding semiconductor surface properties and manipulating them chemically are critical for improving their performance in optoelectronic devices. Hybrid halide perovskites have emerged as an exciting class of highly efficient solar materials; however, their device performance could be limited by undesirable surface properties that impede carrier transport and induce recombination. Here we show that surface functionalization of methylammonium lead iodide (MAPbI3) perovskite with phenethylammonium iodide (PEAI), a commonly employed spacer cation in two-dimensional halide perovskites, can enhance carrier diffusion in the near-surface regions and reduce defect density by more than 1 order of magnitude. Using transient transmission and reflection microscopy, we selectively imaged the transport of the carriers near the (001) surface and in the bulk for single-crystal MAPbI3 microplates. The surface functionalization increases the diffusion coefficient of the carriers in the 40 nm subsurface region from ∼0.6 cm2 s-1 to ∼1.0 cm2 s-1, similar to the value for bulk carriers. These results suggest the PEA ligands are effective in reducing surface defect and phonon scattering and shed light on the mechanisms for enhancing photophysical properties and improving solar cell efficiency.

Behavior of E. coli with Variable Surface Morphology Changes on Charged Semiconductor Interfaces.

Bacterial behavior is often controlled by structural and composition elements of their cell wall. Using genetic mutant strains that change specific aspects of their surface structure, we modified bacterial behavior in response to semiconductor surfaces. We monitored the adhesion, membrane potential, and catalase activity of the Gram-negative bacterium Escherichia coli (E. coli) that were mutant for genes encoding components of their surface architecture, specifically flagella, fimbriae, curli, and components of the lipopolysaccharide membrane, while on gallium nitride (GaN) surfaces with different surface potentials. The bacteria and the semiconductor surface properties were recorded prior to the biofilm studies. The data from the materials and bioassays characterization supports the notion that alteration of the surface structure of the E. coli bacterium resulted in changes to bacterium behavior on the GaN medium. Loss of specific surface structure on the E. coli bacterium reduced its sensitivity to the semiconductor interfaces, while other mutations increase bacterial adhesion when compared to the wild-type control E. coli bacteria. These results demonstrate that bacterial behavior and responses to GaN semiconductor materials can be controlled genetically and can be utilized to tune the fate of living bacteria on GaN surfaces.

Silicon surface functionalization based on cavitation processing

Functionalization of semiconductor substrates, specifically silicon, has focused attention toward such technological applications as photovoltaic and biosensing. Several common chemical approaches have been employed for this purpose. In this paper, we focus on the alternative approach dealing with ultrasound-assisted modification of silicon. The aim was to identify the chemical functional groups and to evaluate the biocompatibility of the ultrasonically processed semiconductor. The properties of the Boron-doped p-type (100) silicon wafers subjected to cavitation impacts have been studied. It was shown that high-intensity (15W/cm2) and high-frequency (1÷6MHz) sonication of silicon wafers in the liquid nitrogen in focused mode of acoustic action induces changes of the optical, chemical, and structural properties of semiconductor surface. The cavitation processing of the Si samples besides a structuring of semiconductor surface and possible phase transformation of bulk material, results in the formation of functional oxide layers consisted of such materials as SiO2, CaSiO3 and Ca2SiO4. The formation of a new phase on the silicon surface is confirmed by the results of X-ray diffraction and μ-Raman spectroscopy investigations. The obtained composite structure has demonstrated photoluminescence in the spectral range of 500–800nm. Biocompatibility of the ultrasonically processed wafers was confirmed by the hydroxyapatite formation on the Si surface after storage in simulated body fluid solution.

Role of the dithiolate backbone on the passivation of p-GaAs(111)B surface

Effects of the self-assembled layers of biphenyl 4,4′ –dithiol (BPDT) and 1,8 –octanedithiol (ODT) on the chemical and electronic properties of p-doped GaAs(111)-As terminated substrate, (p-GaAs(111)B), were investigated in order to evidence whether the hydrocarbon moiety plays a role in tailoring the semiconductor surface properties. The electrochemical properties of the BPDT and ODT coated p-GaAs(111)B substrates were studied by Electrochemical Impedance Spectroscopy (EIS). The structural and chemical changes caused by the dithiolate layers formed on p-GaAs(111)B substrates were monitored by Atomic Force Microscopy (AFM) and X-ray Photoelectron Spectroscopy (XPS) investigations, respectively. The XPS data showed that BPDT and ODT bind to p-GaAs(111)B via the thiol group. The ODT layer provides better protection against the further oxidation in air of p-GaAs(111)B substrate compared to the BPDT layer. The EIS investigations are in good agreement with XPS results, pointing to better insulating properties of the ODT layer compared to the BPDT layer in the electron transfer at the electrode/solution interface. The results evidenced that backbone plays an important role in tailoring the properties of p-GaAs(111)B substrate.

Dye molecule bonded titanium alkoxide: a possible new type of dye for sensitized solar cells.

An organic dye coordinated titanium iso-propoxide compound is designed and synthesized. Taking advantage of the hydrolysis of the titanium alkoxide moiety on the surface of TiO2 electrode, the dye-semiconductor surface properties, including anchoring and dispersivity, are improved, which opens a new perspective to explore dyes for DSSCs.

Sonosynthesis of microstructures array for semiconductor photovoltaics

The properties of the silicon samples subjected to cavitation impacts have been studied. It was shown that high-intensity (15W/cm2) and high-frequency (1–6MHz) sonication of silicon samples in the liquid nitrogen induces changes of the physical, chemical, and structural properties of semiconductor surface. Optical, atomic force and scanning electron microscopy techniques as well as energy dispersive X-ray spectroscopy, X-ray diffraction and photoresponse spectroscopy were used. The experimental study demonstrates the microstructure formation as well as a change of the chemical composition at the silicon surface. It was found that a significant rise in value and expansion of the spectral range of photosensitivity take place after cavitation treatment. The photoresponse of about 2.0eV can be connected with the formation of Si-rich SiNx compound inside the ultrasonically structured region of Si. The obtained value of the refractive index confirms this assumption.

Foundations of ab initio simulations of electric charges and fields at semiconductor surfaces within slab models

Semiconductor surfaces were divided into charge categories, i.e., surface acceptor, donor, and neutral ones that are suitable for simulations of their properties within a slab model. The potential profiles, close to the charged surface states, accounting for explicit dependence of the point defects energy, were obtained. A termination charge slab model was formulated and analyzed proving that two control parameters of slab simulations exist: the slope and curvature of electric potential profiles which can be translated into a surface and volumetric charge density. The procedures of slab model parameter control are described and presented using examples of the DFT simulations of GaN and SiC surfaces showing the potential profiles, linear or curved, depending on the band charge within the slab. It was also demonstrated that the field at the surface may affect some surface properties in a considerable degree proving that verification of this dependence is obligatory for a precise simulation of the properties of semiconductor surfaces.

Modulating Semiconductor Surface Electronic Properties by Inorganic Peptide–Binders Sequence Design

The use of proteins and peptides as part of biosensors and electronic devices has been the focus of intense research in recent years. However, despite the fact that the interface between the bioorganic molecules and the inorganic matter plays a significant role in determining the properties of such devices, information on the electronic properties of such interfaces is sparse. In this work, we demonstrate that the identity and position of single amino acid in short inorganic binding protein-segments can significantly modulate the electronic properties of semiconductor surfaces on which they are bound. Specifically, we show that the introduction of tyrosine or tryptophan, both possessing an aromatic side chain which higher occupied molecular orbitals are positioned in proximity to the edge of GaAs valence band, to the sequence of a peptide that binds to GaAs (100) results in changes of both the electron affinity and surface potential of the semiconductor. These effects were found to be more pronounced than the effects induced by the same amino acids once bound on the surface in a head-tail configuration. Furthermore, the relative magnitude of each effect was found to depend on the position of the modification in the sequence. This sequence dependent behavior is induced both indirectly by changes in the peptide surface coverage, and directly, probably, due to changes in the orientation and proximity of the tyrosine/tryptophan side group with respect to the surface due to the preferred conformation the peptide adopts on the surface. These studies reveal that despite the use of short protein oligomers and aiming at a non-natural-electronic task, the well-known relations between the proteins' structure and function is preserved. Combining the ability to tune the electronic properties at the interface with the ability to direct the growth of inorganic materials makes peptides promising building blocks for the construction of novel hybrid electronic devices and biosensors.

Quantitative Determination of Nanoscale Electronic Properties of Semiconductor Surfaces by Scanning Tunnelling Spectroscopy

Simulation of tunnelling spectra obtained from semiconductor surfaces permits quantitative evaluation of nanoscale electronic properties of the surface. Band offsets associated with quantum wells or quantum dots can thus be evaluated, as can be electronic properties associated with particular point defects within the material. An overview of the methods employed for the analysis is given, emphasizing the critical requirements of both the experiment and theory that must be fulfilled for a realistic determination of electronic properties.

Correction of systematic errors in scanning tunneling spectra on semiconductor surfaces: The energy gap of Si(111)-7×7at 0.3 K

The investigation of the electronic properties of semiconductor surfaces using scanning tunnelling spectroscopy (STS) is often hindered by non-equilibrium transport of the injected charge carriers. We propose a correction method for the resulting systematic errors in STS data, which is demonstrated for the well known Si(111)-(7x7) surface. The surface has an odd number of electrons per surface unit cell and is metallic above 20 K. We observe an energy gap in the ground state of this surface by STS at 0.3 K. After correction, the measured width of the gap is (70 +- 15) meV which is compatible with previous less precise estimates. No sharp peak of the density of states at the Fermi level is observed, in contrast to proposed models for the Si(111)-(7x7) surface.

Electrostatic condition for the termination of the opposite face of the slab in density functional theory simulations of semiconductor surfaces

It is proved that in slab simulations of uniform semiconductor surfaces the electric field in the vacuum space should vanish. In standard approach this condition was achieved by introduction of the dipole correction [J. Neugebauer and M. Scheffler, Phys. Rev. B 46, 16067 (1992)]. An effective and stable method of exact solution of Poisson equation, based on Laplace correction, which attains the zero field condition in the vacuum, is described. The dipole correction to the slab energy is removed. Additionally, a method of the control of electric field within the slab is introduced, applicable in direct simulations of Fermi level influence on the properties of semiconductor surfaces.

On Topographic Properties of Semiconductor Surfaces and Thin Film Systems

The microroughness of the semiconductor/oxide interface substantially influence properties of the whole structure. In our work Si/SiO2/SiOx structures were prepared by using low temperature nitric acid oxidation technique and by the electron gun technique and then the whole structure was passivated by the HCN technique. In the present study we investigate the surface morphology evolution during the creation of the SiO2/SiOx double-layer and after the passivation steps. Surface roughness properties are studied by the fractal geometry methods. The complexity of analysed surface is sensitive to the oxidation and passivation steps and the proposed fractal complexity measure values enable quantifying of the fine surface changes.

Valence Surface Electronic States on Ge(001)

The optical, electrical, and chemical properties of semiconductor surfaces are largely determined by their electronic states close to the Fermi level (E{F}). We use scanning tunneling microscopy and density functional theory to clarify the fundamental nature of the ground state Ge(001) electronic structure near E{F}, and resolve previously contradictory photoemission and tunneling spectroscopy data. The highest energy occupied surface states were found to be exclusively back bond states, in contrast to the Si(001) surface, where dangling bond states also lie at the top of the valence band.

Investigation of electron properties of semiconductor surface by modulation spectroscopy of electroreflection

The relation of the Franz-Keldysh oscillations to electronic parameters of semiconductor materials is analyzed using the high-field measurement mode. The potential of using modulation spectroscopy of electroreflection for investigating electronic properties of a semiconductor surface is shown by the example of electroreflection spectra of n-GaAs (100) homoepitaxial films with the electron concentration of 1017–1018 cm−3. The spectra are measured by the Schottky-barrier method at a room temperature using unpolarized light in the spectral range of 1.3–1.65 eV in the vicinity of the transition E 0 (Γ8V -Γ6C ). From the quantitative analysis of electroreflection spectra, electronic parameters of films are obtained: the electron-transition energy E 0, the electro-optical energy ħϑ, the surface electric field F s, the energy-relaxation time τ of charge carriers, the oscillation length λKF of the wave function of a quantum-mechanical particle with a reduced effective mass μ for a given surface electric field F s, and the electron mobility μe.