Surface Band Research Articles

3D-Micro-Flower like [formula omitted] Nanoplate Solid Solution: A facile and rapid room-temperature synthesis with excellent photocatalytic decomposition of antibiotics in water

This research addresses the environmental threat posed by residual antibiotics in water and explores the potential of the photocatalytic process as an eco-friendly solution for antibiotic wastewater treatment. Here in, a series of BiOBr1−xClx nanoplate solid solutions with varied Br:Cl molar ratios were synthesized through a simple co-precipitation method. These solid solutions exhibited superior visible-light-driven photocatalytic activity compared to pristine BiOCl and BiOBr. Notably, the BiOBr0.25Cl0.75 sample demonstrated exceptional performance, achieving 89% and 99% degradation efficiency for ciprofloxacin (CIP) and tetracycline hydrochloride (TCH), respectively, within 20minutes under optimum conditions. The outstanding performance is attributed to factors such as a large specific surface area, suitable morphology and band gap, effective separation of photo-generated electron-hole pairs, and the presence of meso-size pores in the structure. Thermodynamic studies confirmed the exothermic and spontaneous nature of the photocatalytic reactions. The proposed degradation pathway of CIP and TCH, along with the photocatalytic mechanism, is elucidated. The solid solution, BiOBr1−xClx, exhibits facile recyclability, robust stability, and excellent adaptability to actual aquatic environments, establishing its potential as a promising eco-friendly photocatalyst for antibiotic pollution control.

Scanning electron microscopy imaging of multilayer-doped GaN: Effects of surface band bending, surface roughness, and contamination layers on doping contrast

Dopant profiling by a scanning electron microscope possesses great potential in the semiconductor industry due to its rapid, contactless, non-destructive, low cost, high spatial resolution, and high accuracy characteristics. Here, the influence of plasma and wet chemical treatments on doping contrast was investigated for a multilayered p-n GaN specimen, which is one of the most promising third-generation wide bandgap semiconductors. Angle-resolved x-ray photoelectron spectroscopy and atomic force microscope were employed to characterize the degree of surface band bending, surface roughness, gallium oxides, and hydrocarbons on the surface of GaN. N2 and air plasmas were unable to remove the surface contamination layers, although the degree of surface band bending was suppressed. In contrast, wet chemical methods offer superior capability in removing contamination layers; however, the surface roughness was increased to varying degrees. Notably, NH4F solution is capable of improving the doping contrast. The underlying mechanism was elucidated from the perspective of surface band bending, surface roughness, and contamination. The findings reported here will provide a feasible solution for effective characterization of semiconductor materials and devices.

Improvement of charge trapping memory performance by modulating band alignment with oxygen plasma

Metal-oxide charge trapping memory (CTM) integration into amorphous and organic flexible devices encounters challenges due to high-temperature treatment. Our approach enhances memory performance via room-temperature oxygen plasma treatment, subtly adjusting surface band alignment without changing the original material structure and surface roughness. Infiltration of oxygen plasma induces band alignment bending, creating a barrier for charge trapping. The device with oxygen plasma treatment exhibits an impressive 19.06 V memory window and a charge trapping density of 3.58 × 1013/cm2. In comparison, the memory window of untreated device only has 5.56 V, demonstrating that oxygen plasma treatment significantly improves memory characteristics. The charge retention rate exhibits outstanding stability, potentially reaching 94% after a decade. It should be noted that careful control during plasma treatment is crucial to maintaining optimal memory effects. This facile, efficient technique, applicable to various oxide layers, offers a way for future advancements in metal-oxide CTM technology.

Enhanced Carrier Selectivity and Superior Passivation in Cost‐Effective Heterogeneous Hybrid Transport Layers for Next Generation Silicon Solar Cells

AbstractHybrid hole transport layers (HHTLs) consisting of‐a low‐cost, low‐temperature, dopant‐free, organic, hole‐selective poly(3,4‐ethylenedioxythiophene):polystyrene sulphonate (PEDOT:PSS) and an inorganic homogenous tunnel oxides (HoTOs), such as SiOx, TiOx, and AlOx have allowed significant improvements in carrier selectivity as well as in power and cost efficiency. These key parameters can be further improved by introducing heterogeneous tunnel oxides (HeTOs), formed of two different inorganic oxides. This work examines the potential of heterogeneous HHTLs and their impact on passivation as well as on the interfacial properties in silicon solar cells. HeTOs of SiOx/TiOx and SiOx/AlOx with negative fixed charge are studied and optimized. A heterogeneous HHTL with AlOx (>1.5 nm) resulted in a high lifetime (1.2 ms), low surface recombination velocity, and improved surface band bending at the interface due to the presence of higher negative fixed charge (1012 cm−2) within these layers. The passivation properties further improved on light soaking and annealing due to oxidation of PSS and AlOx/SiOx. Overall, these HHTLs demonstrated a hole selectivity (S10) of 12.5 and very high efficiencies up to of 26.1%, surpassing homogenous‐SiOx/PEDOT:PSS by 1.4%.

Transport and electronic structure properties of MBE grown Sn doped Ga2O3 homo-epitaxial films

In this work, we report the transport, defect state and electronic structure properties of unintentionally doped (UID) and Sn doped β-Ga2O3 homo-epitaxial thin films grown by molecular beam epitaxy (MBE) with electron density ranging from 2.1 × 1016 to 2.8 × 1019 cm−3. The UID film with an electron density of 2.1 × 1016 cm−3 exhibits a notable RT mobility of 129 cm2/Vs and a peak mobility of 900 cm2/Vs at 80 K, achieving the state-of-the-art level for MBE-grown Ga2O3 films. Temperature dependent Hall measurement reveal that Sn dopants have an activation energy of 56.7 meV. Synchrotron-based photoemission spectroscopy were further used to study insights into the evolution of electronic properties induced by Sn doping. An in-gap defect state was observed at the 1.5 eV above the valence band maximum for the Sn-doped Ga2O3 film. The in-gap state acts as self-compensating centers affecting the overall doping efficiency and mobility. Furthermore, photoemission spectroscopic study also reveals an upward surface band bending existing at the surface region of Sn doped Ga2O3 films. The identification of the in-gap state and surface upward band bending have significant implications for understanding the doping mechanisms in Ga2O3 and its electronic device applications.

Extended Surface Bands Enabled Lasing Emission and Wavelength Switch from Sulfur Quantum Dots.

The development of a lasing wavelength switch, particularly from a single inorganic gain material, is challenging but highly demanded for advanced photonics. Nonetheless, all current lasing emission of inorganic gain materials arises from band-edge states, and the inherent fixed bandgap limitation of the band-edge system leads to the inaccessibility of lasing wavelength switching from a single inorganic gain material. Here the realization of a single inorganic gain material-based lasing wavelength switch is reported by proposing an alternative lasing emission strategy, that is, lasing emission from surface gain. Previous efforts to achieve surface-gain-enabled lasing emission have been hindered by the limited gain volume provided by surface states due to the broad emission bandwidth and/or low emission efficiency. This challenge is overcome by introducing extended surface bands onto the surface of sulfur quantum dots. The extended surface bands contribute to a high photoluminescence quantum yield and narrow emission bandwidth, thereby providing sufficient gain volume and facilitating stimulated emission. When combined with whispering gallery mode microcavity, surface gain enabled lasing emission manifests an ultralow threshold of 8.3 µJcm-2. Remarkably, the reconfigurable perturbation to surface gain, facilitated by molecular affinity, allows for the realization of the lasing wavelength switch from a single inorganic gain material.

The de Haas–van Alphen quantum oscillations in the kagome metal RbTi3Bi5

Abstract The kagome system has attracted great interest in condensed matter physics due to its unique structure that can host various exotic states such as superconductivity (SC), charge density waves (CDWs) and nontrivial topological states. The topological semimetal RbTi3Bi5 consisting of a Ti kagome layer shares a similar crystal structure to the topological correlated materials AV3Sb5 (A = K, Rb, Cs) but without the absence of CDW and SC. Systematic de Haas–van Alphen oscillation measurements are performed on single crystals of RbTi3Bi5 to pursue nontrivial topological physics and exotic states. Combining this with theoretical calculations, the detailed Fermi surface topology and band structure are investigated. A two-dimensional Fermi pocket β is revealed with a light effective mass, consistent with the semimetal predictions. The Landau fan diagram of RbTi3Bi5 reveals a zero Berry phase for the β oscillation in contrast to that of CsTi3Bi5. These results suggest that kagome RbTi3Bi5 is a good candidate for exploring nontrivial topological exotic states and topological correlated physics.

Sol Gel obtained Ti/Co/Mn oxides doped with 5wt% CuO for the photocatalytic removal of organic azo dyes from wastewater

Various organic pollutants continually poison natural water resources due to rapid modernization driven by industrial growth. As a result, communities throughout the globe are paying close attention to the health and environmental issues raised by these harmful organic pollutants. Here, we prepared TiO2-CuO(5wt%), TiO2-CuO(5wt%)@Co3O4, and TiO2-CuO(5wt%)@Mn2O3 photocatalysts by employing the sol-gel method for the photocatalytic removal of azo dyes. These composites' surface morphology, composition, band gap, and purity were analyzed using SEM, EDS, TEM/HRTEM, XRD, FTIR, Raman, XPS, DRS, and BET measurements. The 5 % CuO doping caused a significant change in the electronic structure of the composites and contributed to the superior photocatalytic performance of the composites against azo dyes at neutral pH and under visible light irradiation. The function of reactive oxygen species (ROS) was also studied using different scavengers and enhancers; •OH was found to be the main ROS for the photodegradation of azo dyes.

Moiré superlattices of antimonene on a Bi(111) substrate with van Hove singularity and Rashba-type spin polarization

Moiré superlattices consisting of two-dimensional materials have attracted immense attention because of emergent phenomena such as flat band-induced Mott insulating states and unconventional superconductivity. However, the effects of spin-orbit coupling on these materials have not yet been fully explored. Here, we show that single- and double-bilayer antimony honeycomb lattices, referred to as antimonene, form moiré superlattices on a Bi(111) substrate due to lattice mismatch. Scanning tunnelling microscopy (STM) measurements reveal the presence of spectral peaks near the Fermi level, which are spatially modulated with the moiré period. Angle-resolved photoemission spectroscopy (ARPES) combined with density functional theory calculations clarify the surface band structure with saddle points near the Fermi level, which allows us to attribute the observed STM spectral peaks to the van Hove singularity. Moreover, spin-resolved ARPES measurements reveal that the observed surface states are Rashba-type spin-polarized. The present work has significant implications in that Fermi surface instability and symmetry breaking may emerge at low temperatures, where the spin degree of freedom and electron correlation also play important roles.

Chemical and Electronic Structure of the i‐ZnO/InxSy:Na Front Contact Interface in Cu(In,Ga)(S,Se)2 Thin‐Film Solar Cells

AbstractThe chemical and electronic structure of the front contact i‐ZnO/InxSy:Na interface for Cu(In,Ga)(S,Se)2‐based thin‐film solar cells is investigated using a combination of x‐ray and electron spectroscopies. Upon i‐ZnO sputter deposition on the InxSy:Na buffer layer, we find an intermixed heterojunction and the formation of InOx and Na2SO4. The window layer is shown to consist of a mixture of Zn(OH)2 and ZnO, with decreasing relative Zn(OH)2 content for thicker window layers. Moreover, we observe diffusion of sodium to the surface of the window layer. We derive electronic surface band gaps of the i‐ZnO and InxSy:Na layers of 3.86 ± 0.18 eV and 2.60 ± 0.18 eV, respectively, and find a largely flat conduction band alignment at the i‐ZnO/InxSy:Na interface.

First-principles calculations of cubic boron arsenide surfaces

The properties of cubic boron arsenide (c-BAs) (100), (110), and (111) surfaces are investigated by performing first-principles calculations using the slab and Green's function surface models with different terminals. The (111) surface with As-termination is found to be the most stable structure among the studied surfaces, with its lowest surface energy (1.70–1.92 J m−2) and largest surface density (20.24 nm−2). The electronic affinity of these surfaces lie in the range 4.62–6.17 eV, which is higher than that of common semiconductor materials, such as silicon (4.05 eV) and germanium (4.13 eV), implying that the electrons at the bottom of the conduction band require more energy to escape. The surface states of the structures with As-termination in the surface band structures are generally more numerous and extended than those with B-termination. The absorption peak of the bulk c-BAs is located in the ultraviolet region, and the light absorption ranges of the surfaces are significantly extended compared with the bulk c-BAs, due to the surface states inside the bandgap.

Quantum-Confined Lifshitz Transition on Weyl Semimetal Td-MoTe2.

Adsorption of alkali atoms onto material surfaces is widely utilized for controlling electronic properties and is particularly effective for two-dimensional materials. While tuning the chemical potential and band gap and creating quantum-confined states are well established for alkali adsorption on semiconductors, the effects on semimetallic systems remain largely elusive. Here, utilizing angle-resolved photoemission spectroscopy measurements and density functional theory calculations, we disclose the creation of two-dimensional electron gas and the quantum-confined Lifshitz transition at the surface of a Weyl semimetal Td-MoTe2 by potassium adsorption. Electrons from potassium adatoms are shown to be transferred mainly to the lowest unoccupied band within the gapped part of the Brillouin zone, which, in turn, induces strong surface band bending and quantum confinement in the topmost layer. The quantum-confined topmost layer evolves from a semimetal to a strong metal with a Lifshitz transition departing substantially from the bulk band. The present finding and its underlying mechanism can be exploited for the creation of electronic heterojunctions in van der Waals semimetals.

(Invited) Semiconductor Facet Effects Toward Photocatalysis

Photo-induced charge transfer has been found to strongly depend on the exposed semiconductor surfaces. For instance, {110}-bound Cu2O rhombic dodecahedra exhibit a high photocatalytic activity toward dye degradation, but {100}-terminated Cu2O cubes are inert. Photocatalytic behaviors of Cu2O-based semiconductor heterostructures, such as Cu2O-ZnS and Cu2O-ZnO composites, can both enhance and suppress photocatalytic activity depending on the contacting planes at the heterojunction. Significantly, Cu2O photocatalytic activity can be greatly enhanced by functionalizing with conjugated molecules such as 4-nitrophenylacetylene (4-NA) to tune the surface band structure. This can turn the inactive cubes to possess a superior photocatalytic activity. Recently, Cu2O rhombic dodecahedra with and without 4-NA modification have been employed in photocatalytic hydroxylation of boronic acids, aryl sulfide oxidation, and amine coupling. SrTiO3 truncated rhombic dodecahedra were also found to be effective in amine coupling. Oxidative cyclization of thioamides is the latest example of surface control to photocatalytic organic transformations.

Density Functional Tight-Binding Models for Band Structures of Transition-Metal Alloys and Surfaces across the d-Block.

First-principles electronic structure simulations are an invaluable tool for understanding chemical bonding and reactions. While machine-learning models such as interatomic potentials significantly accelerate the exploration of potential energy surfaces, electronic structure information is generally lost. Particularly in the field of heterogeneous catalysis, simulated electron band structures provide fundamental insights into catalytic reactivity. This ab initio knowledge is preserved in semiempirical methods such as density functional tight binding (DFTB), which extend the accessible computational length and time scales beyond first-principles approaches. In this paper we present Shell-Optimized Atomic Confinement (SOAC) DFTB electronic-part-only parametrizations for bulk and surface band structures of all d-block transition metals that enable efficient predictions of electronic descriptors for large structures or high-throughput studies on complex systems outside the computational reach of density functional theory.

Spectroscopic analysis of color origins in titanium-based thin films deposited by cathodic arc deposition

The distinctive colors observed in titanium-based (Ti-based) decorative coatings fabricated by cathodic arc deposition originate from the unique electronic, morphological, and chemical properties of these films. Most Ti-based film colors arise from their surface band gaps, as measured by electron energy loss spectroscopy, with the exception of the lowest brightness (TLB) and green hues. The green color in titanium dioxide films typically results from interference between light reflected from the film and substrate interfaces. In contrast, the black color in Ti-based coatings stems from the chemical structure, specifically the presence of Ti-C and C=C(sp2) bonds within the film. Moreover, the large band gap of the TLB film surface is attributed to surface oxidation, as detected by soft X-ray spectroscopy. Significantly, the film thickness and surface roughness are characterized by field emission secondary electron microscopy and atomic force microscopy, respectively.

Oxygen Switchable Photo-Hydrovoltaic Effect along the Silicon-Water Interface.

Moving boundaries of electrical double layers have shown promising capability in driving directional electron flows in solids, leading to a range of hydrovoltaic effects. The recent discovery of a photohydrovoltaic phenomenon utilizes a moving illumination zone to generate moving boundaries with different properties at the solid-water interface, referred to as the kinetic photovoltaic effect. Here, oxygen was found to act as a chemical switch to turn on and off the kinetic photovoltaic effect. Introducing oxygen would rapidly diminish the kinetic photovoltage in p-Si. On the contrary, degassing oxygen leads to a gradual recovery, whose rate can be facilely speeded up by more than one order through electrostatic gating. Mechanistic investigations of the oxygen switch behavior uncovered a dependence of surface band bending intensity of silicon on oxygen adsorption, which highlights the role of gas molecules, often overlooked, in applications based on semiconductor-liquid interfaces, such as photoelectrochemistry.

Exploring innovative antibacterial properties of porous ALT (Al2O3/TiO2) composite

Among all metallic materials, titanium is mostly employed in medical applications. In this study, single TiO2 layers on Ti as well as composite ALT(Al2O3/TiO2) layers on Ti for medical applications were synthesized using titanium and aluminum cathodic cages. SEM, EDX, XRD, PL, UV–Vis analyses of untreated and plasma-processed samples were used to determine the surface morphology, chemical content, microstructure, band gap, and photocatalytic behavior, respectively. E. coli and S. aureus immersion bacterial solutions were used to test the antibacterial performance of untreated and plasma-processed specimens. Comparing the composite to a single TiO2 and an untreated sample in PL spectra revealed a smaller band gap. When compared to untreated specimens, coatings made of TiO2 and composite ALT demonstrated significant antibacterial properties. The results showed that ALT-60min displayed unique antibacterial behavior because the band gap was reduced and antibacterial effectiveness was increased. This work explains the distinctive antibacterial activity of composite nanostructures made for medicinal use.

Ag-doped SrTiO3: Enhanced water splitting for hydrogen production

This study investigates the effect of Ag-doping on the (001) surface of SrTiO3 (STO) to enhance hydrogen production via water-splitting employing density functional theory (DFT) simulations. It was found that the heterolytic water dissociation on Ag-doped (001)-STO occurs via a first-order reaction with an energy barrier of 5.74 kJ/mol, representing a reduction of 94% compared to undoped (001)-STO. The Gibbs free energy of H2 formation catalyzed by Ag-doped (001)-STO is approximately −302 kJ/mol, indicating the spontaneity of this process. Additionally, the surface band gap energy decreases by ∼2 eV after water-splitting, suggesting charge transfer from the Ag-doped STO to the water and leading to new electron-hole recombination states. This research underscores the potential of Ag-doping in enhancing the efficiency of hydrogen production through water-splitting, contributing to the development of greener and more efficient energy technologies.

L -gap surface resonance at Pt(111): Influence of atomic structure, d bands, and spin-orbit interaction

Pt(111) hosts a surface resonance with peculiar properties concerning energy vs momentum dispersion and spin texture. At variance with the free-electron-like behavior of the L-gap Shockley-type surface states on the fcc(111) surfaces of Au, Ag, and Cu, it splits into several branches with distinct spin polarization around the center of the surface Brillouin zone Γ¯. Theoretical predictions based on density-functional theory vary depending on the particular functionals used. To clarify this issue, we investigate the atomic structure of Pt(111) by low-energy electron diffraction and the unoccupied electronic structure by spin- and angle-resolved inverse photoemission. The experimental results are backed by theoretical studies using different functionals, which show that the characteristics of the surface band depend critically on the lattice constant. From the analysis of the energy-dependent low-energy electron diffraction intensities, we derive structural parameters of the Pt(111) surface relaxation with high accuracy. In addition, we give an unambiguous definition of the nonequivalent mirror-plane directions Γ¯M¯ and Γ¯M¯′ at fcc(111) surfaces, which is consistent with band-structure calculations and inverse-photoemission data. Concerning the surface resonance at the bottom of the L gap, we identified a delicate interplay of several contributions. Lattice constant, hybridization with d bands, and the influence of spin-orbit interaction are critical ingredients for understanding the peculiar energy dispersion and spin character of the unoccupied surface resonance. Published by the American Physical Society 2024

Crystal field anisotropy correlated charge disproportionation in Ruddlesden-Popper Sr3-xCaxFe2O7-δ perovskites

A series of Ca2+-doped Ruddlesden-Popper phase perovskites Sr3-xCaxFe2O7-δ (x = 0∼0.4) were synthesized. Strong asymmetric FeO6 octahedral distortion was confirmed at x = 0.2 owing to the anisotropic chemical pressure applied from preferable occupation of Ca2+ ion at Sr2 site. Unusual high-charged oxidation state of iron ion with enhanced p→d charge transfer was collectively evidenced by X-ray photoelectron spectroscopy and Mössbauer spectroscopy. Combined electron emission spectroscopy further revealed a narrowing of the surface band gap accompanied by an apparent lowering of the conduction band edge at x = 0.2. These observations can be explained by a nontrivial partial charge disproportionated (CD) regime: d5L + d5L → d5L1−n + d5L1+n, which reflects a significant correlation between anisotropic chemical pressure and CD ground state. The d5L1−n ground state, which dominates the conduction band, is largely stabilized by enhancing the d(Fe)-p(O) hybridization along the c-axis. In addition, abrupt changes in both magnetism and dc electrical properties were also observed at x = 0.2. It is believed that the anisotropic crystal field correlated electronic state fluctuation of iron may facilitate magnetic phase competition and carrier transport. This work provides a comprehensive understanding of the crystallographic anisotropy and correlated valence electron behavior in Fe-base Ruddlesden-Popper perovskites, as well as quasi two-dimensional perovskite materials.