Band Structure Of Semiconductors Research Articles

Enhanced piezoelectricity induced by transition metal atoms adsorption on monolayer and bilayer MoS2

Piezoelectricity in MoS2 has attracted extensive attention because of potential applications in energy harvesting and sensors. However, the piezoelectricity of MoS2 monolayer is weaker than those of traditional piezoelectric materials. Here, based on first principles calculations, we report the large work function transition metal atoms (TMs = Ni, Pd, Pt and Ir) adsorbed on monolayer and bilayer MoS2 with large out-of-plane piezoelectric polarization. For TMs adsorbed on monolayer MoS2, the Ir and Ni adsorption exhibit stronger adsorption energy and larger migration barrier compared with Pd and Pt adsorption. All structures maintain dynamical stability at 300 K and exhibit p-type semiconducting band structures. The larger out-of-plane piezoelectric coefficients induced by adsorption increase with increasing the adsorption concentration, accompanied with slightly decreased in-plane piezoelectric coefficients, which is attributed to more and more electrons participating in redistribution along the out-of-plane direction. For TMs adsorbed bilayer MoS2, the energetically favorable configuration has same polarization orientation between two monolayers, which results in increased in-plane piezoelectric coefficients. The out-of-plane piezoelectric coefficients further increase due to the coupling of interlayer vertical polarization and TMs adsorption induced vertical polarization. Our results provide a possible way to increase the piezoelectricity of MoS2.

Interaction-Induced Crystalline Topology of Excitons.

We apply the topological theory of symmetry indicators to interaction-induced exciton band structures in centrosymmetric semiconductors. Crucially, we distinguish between the topological invariants inherited from the underlying electron and hole bands and those that are intrinsic to the exciton wave function itself. Focusing on the latter, we show that there exists a class of exciton bands for which the maximally localized exciton Wannier states are shifted with respect to the electronic Wannier states by a quantized amount; we call these excitons shift excitons. Our analysis explains how the exciton spectrum can be topologically nontrivial and sustain exciton edge states in open boundary conditions even when the underlying noninteracting bands have a trivial atomic limit. We demonstrate the presence of shift excitons as the lowest energy neutral excitations of the Su-Schrieffer-Heeger model in its trivial phase when supplemented by local two-body interactions.

2D g-C3N5 p-Doping of Donor Material for High-Efficiency Organic Solar Cells.

Molecular doping of organic semiconductor is a great strategy for significantly regulating the electronic band structure of organic semiconductor while increasing charge mobility and carrier concentration. Here, a facile strategy is presented by introducing 2D g-C3N5 as a p-dopant into PM6, improving the charge mobility and hole carrier concentration of PM6. Moreover, the electron transfer between PM6 and g-C3N5 can effectively downshift the Fermi energy level and highest occupied molecular orbital (HOMO) energy level of PM6, which leads to the increase the built-in electric field of organic solar cells (OSCs). The addition of g-C3N5 also effectively enhances the crystallization of active layer, thereby improving the stability of OSCs. As a result, a champion bulk-heterojunction (BHJ) and layer-by-layer (LbL) structure OSCs are successfully achieved featuring a high-power conversion efficiency of 18.10%/18.25%, simultaneously having excellent device stability. This work shows that introducing a low concentration dopant into organic donor is an effective method for improving the electrical performance of organic donor and the efficiency of OSCs.

Kinetics-controlled epitaxial growth and bipolar transport properties of semimetal Bi4Se3

(Bi2)m(Bi2Se3)n (m, n: integers) compounds with an infinitely adaptive superlattice structure exhibit several fascinating topological phases. Here, we study kinetics-controlled epitaxial growth of Bi4Se3 on mica by co-evaporating Bi and Se using molecular beam epitaxy technique, as well as the transport properties of Bi4Se3. By precisely controlling the beam fluxes of Bi and Se and growth temperature, we can tune the growth modes from van der Waals' condensation to spiral growth, thus achieving single-crystalline Bi4Se3 of dislocation-free microplate or mounded thin-film morphologies. This reflects a transition from near-thermodynamic-equilibrium to non-thermodynamic-equilibrium growth processes of single-crystalline Bi4Se3. Thin-film Bi2+xSe3 (1.7 < x < 2) solid-solution phases consisting of randomly stacked Bi2 and Bi2Se3 units are also prepared as comparative samples. Hall and thermopower properties suggest that the as-grown Bi4Se3 films exhibit semimetallic bipolar conduction behaviors while the Bi2+xSe3 films present typical semiconducting transport characteristics with a n-type polarity. Due to semiconductor band structures, the Bi2+xSe3 films show superior thermopower to that of semimetal Bi4Se3.

Photocatalytic Activity and Antibacterial Properties of ZnO/CNTs Composites

Photocatalytic technology is one of the promising technologies for wastewater treatment. Herein, zinc oxide/multi-walled carbon nanotubes (ZnO/CNTs) photocatalyst was successfully prepared by hydrothermal method with combining in-situ synthesis technology. The micro-morphology, crystalline structure, surface chemical elements, and optical properties were characterized by SEM, TEM, XRD, FTIR, UV-Vis, and DRS technologies. The ZnO/CNTs photo-catalyst exhibited enhancement photo activity for degradation of organic pollutants under simulated light irradiation. Specifically, the photo-catalytic activity of the ZnO/CNTs catalysts improved with the rise of CNTs content in the composites. Investigation on the photo-degradation mechanism verified that the presence of CNTs in the catalyst not only optimized the band structure of ZnO semiconductor but also contributed to the transfer of photo-generated electrons and reducing the recombination of electron-hole pairs due to its excellent conductivity. Moreover, the active radical groups such as superoxide radical (O-2), hole (h+), and hydroxyl radical (·OH) played the dominated role for the pollutants degradation under the simulated sunlight irradiation. In addition, ZCT20 catalysts and light irradiation had synergistic effects on antibacterial activity, whose antibacterial rates against E. coli and S. aureus were up to 99.96% and 99.94%, respectively. Investigation on antibacterial mechanisms revealed that the existence of ROS and the continuous release of Zn2+ played an important role for improving the antibacterial activity of the ZCT20 catalyst under the simulated sunlight irradiation.

Theoretical exploration of band gap error dependency on band gap size in density functional theory Calculations: CdTe and GeTe as representative cases of two band structure semiconductor types

This study addresses the prevalent band gap error problem in density functional theory (DFT) with local density or generalized gradient approximation (LDA/GGA), widely considered as the “standard model” for investigating the band structure of semiconductors. Despite previous DFT efforts focusing on improving the exchange correlation energy to mitigate the underestimation of the band gap, the correlation between the band gap error and the intrinsic electronic structure of semiconductors has been overlooked. From two principal observations: (i) traditional semiconductors typically have a p-s type band structure, and (ii) a smaller band gap is linked to less underestimation, we derive two crucial questions: (i) How is band gap error manifested in s-p type semiconductors? (ii) Does the error vanish as the band gap approaches zero, akin to metals? Through the examination of CdTe and GeTe, representing p–s and s–p type semiconductors with strain-engineered band gaps, our calculations indicate that for CdTe, the band gap is underestimated, whereas for GeTe, it is overestimated. Significantly, these errors persist even as the band gap of semiconductors tends to zero, unlike in metals. This persistent discrepancy stems from the discontinuity in wavefunctions between the semiconductors' VBM and CBM.

Topological Structure Realized in Cove-Edged Graphene Nanoribbons via Incorporation of Periodic Pentagon Rings.

Cove-edged zigzag graphene nanoribbons are predicted to show metallic, topological, or trivial semiconducting band structures, which are precisely determined by their cove offset positions at both edges as well as the ribbon width. However, due to the challenge of introducing coves into zigzag-edged graphene nanoribbons, only a few cove-edged graphene nanoribbons with trivial semiconducting bandgaps have been realized experimentally. Here, we report that the topological band structure can be realized in cove-edged graphene nanoribbons by embedding periodic pentagon rings on the cove edges through on-surface synthesis. Upon noncontact atomic force microscopy and scanning tunneling spectroscopy measurements, the chemical and electronic structures of cove-edged graphene nanoribbons with periodic pentagon rings have been characterized for different lengths. Combined with theoretical calculations, we find that upon inducing periodic pentagon rings the cove-edged graphene nanoribbons exhibit nontrivial topological structures. Our results provide insights for the design and understanding of the topological character in cove-edged graphene nanoribbons.

Semiconductor Band Structure, Symmetry, and Molecular Interface Hybridization for the Chemist.

Merging molecular bonding concepts with semiconductor- and materials-based concepts of band structure is challenging due to the mutually exclusive historical development and notations used in those respective fields: symmetry adapted linear combinations (SALCs) and Mulliken terms for molecules, versus k space and Bloch sums for materials. This lack of commonality brings the issue of hybridization (aka electronic coupling) between molecules and materials to the forefront in many aspects of modern chemical research─including nanocrystal properties, solar energy conversion, and molecular computing. It is thus critical to establish a holistic approach to hybridizing orbital (molecule) and plane-wave (semiconductor/material) systems to better describe symmetry-based molecule|material bonding and the corresponding symmetry-adapted molecular orbital (MO) diagrams. Such a unified approach would enable the construction of testable hypotheses about the role of symmetry and electronic structure in determining the extent of electronic coupling between molecular orbitals and semiconductor band structure. This Perspective provides an analysis and compendium of "translations" between the physics and chemistry language of group theory. In this vein, this approach describes the symmetries─and corresponding point groups─that occur in k space along the available descent in symmetry pathways (k space vectors). As a result, chemists may arrive at a more intuitive understanding of the band symmetries of semiconductors, as well as insights into the corresponding algebraic formulations. This analysis can ultimately generate MO diagrams for hybrid molecule|material systems. Lastly, an Outlook provides some context to the application of this analysis to modern problems at the interface of molecular and materials chemistry.

Interaction of an Electromagnetic H-Wave with an “Insulator–Semiconductor–Insulator” Nanostructure in the View of Semiconductor Band Structure Anisotropy

Interaction of an Electromagnetic H-Wave with an “Insulator–Semiconductor–Insulator” Nanostructure in the View of Semiconductor Band Structure Anisotropy

Extrinsic n-type semiconductor transition in ZrSe2 with the metallic character through hafnium substitution

Two-dimensional (2D) layered materials exhibit versatile electronic properties in their different phases. The intrinsic electronic properties of these materials can be modulated through doping or intercalation. In this study, we investigated the electronic properties of Hf-doped ZrSe2 single crystals using angle-resolved photoemission spectroscopy (ARPES) combined with first-principles density functional theory (DFT) calculations. It is observed that the valence band maxima of ZrSe2, located below the Fermi level, undergo a significant change with the introduction of Hf substitution. Hf can introduce extra charges into the conduction band, rather than making a mixed structure of HfSe2 and ZrSe2 band structure, which can cross the Fermi level. Compared to the semiconducting band structure of ZrSe2, we observed that the conduction band crosses the Fermi level at the high symmetry M point in Hf-doped ZrSe2. This suggests an increase of electron-type carriers around the Fermi level, resulting in an extrinsic charge carrier density in the conduction band, which can form a metallic behaviour. It can be noticed that the Hf cations can create disorder in the form of excess atoms of Zr, which yields more carriers in the conduction band in the shape of “smeared bands”. The tails of the “smeared band” occupied the d-orbitals extended into the Fermi level and left the d-band below. Similarly, the electrical resistance measurements further confirm the metallic-like character of Hf-doped ZrSe2 compared to the semiconductor ZrSe2, indicating increased carriers. This metallic-like behavior is suggested to be predisposed by the extrinsic electrons induced by the substitutional disorder. This study further demonstrates the possibility of band gap engineering through heavy metal doping in 2D materials.

Profiling Electronic and Phononic Band Structures of Semiconductors at Finite Temperatures: Methods and Applications

Semiconductor devices are often operated at elevated temperatures that are well above zero Kelvin, which is the temperature in most first-principles density functional calculations. Computational approaches to computing and understanding the properties of semiconductors at finite temperatures are thus in critical demand. In this review, we discuss the recent progress in computationally assessing the electronic and phononic band structures of semiconductors at finite temperatures. As an emerging semiconductor with particularly strong temperature-induced renormalization of the electronic and phononic band structures, halide perovskites are used as a representative example to demonstrate how computational advances may help to understand the band structures at elevated temperatures. Finally, we briefly illustrate the remaining computational challenges and outlook promising research directions that may help to guide future research in this field.

Analyzing k · p modeling in highly mismatched alloys and other III–V semiconductors

This Tutorial provides a comprehensive overview of various k⋅p models used to describe the electronic band structures of semiconductors with cubic diamond and zinc blende symmetries. Our primary focus is on III–V semiconductors, with a particular emphasis on highly mismatched alloys. We begin our exploration with the six-band k⋅p model, which effectively captures interactions within the highest valence bands. Following that, we delve into the intricacies of the eight-band k⋅p model, which takes into account strain effects and modifications to energy dispersion. The Tutorial also introduces the band anticrossing model and its corresponding ten-band k⋅p models, specifically tailored for dilute nitride semiconductors. Furthermore, we extend our discussion to the valence band anticrossing model and its application to the 14-band k⋅p model in the context of dilute bismide materials. Additionally, we emphasize the significance of more comprehensive models, exemplified by the 30-band k⋅p model, for faithfully representing the entire Brillouin zone.

Enhanced azo dye reduction at semiconductor-microbe interface: The key role of semiconductor band structure

Low-energy environmental remediation could be achieved by biocatalysis with assistance of light-excited semiconductor, in which the energy band structure of semiconductor has a significant influence on the metabolic process and electron transfer of microbes. In this study, direct Z-scheme and type II heterojunction semiconductor with different energy band structure were successfully synthesized for constructing semiconductor-microbe interface with Shewanella oneidensis MR-1 to achieve acid orange7 (AO7) biodegradation. UV–vis diffuse reflection spectroscopy, photoluminescence spectra and photoelectrochemical analysis revealed that the direct Z-scheme heterojunction semiconductor had stronger reduction power and faster separation of photoelectron-hole, which was beneficial for the AO7 biodegradation at semiconductor-microbe interface. Riboflavin was also involved in electron transfer between the semiconductor and microbes during AO7 reduction. Transcriptome results illustrated that functional gene expression of Shewanella oneidensis MR-1 was upregulated significantly with photo-stimulation of direct Z-scheme semiconductor, and Mtr pathway and conductive pili played the important roles in the photoelectron utilization by Shewanella oneidensis MR-1. This work is expected to provide alternative ideas for designing semiconductor-microbial interface with efficient electron transfer and broadening their applications in bioremediation.

Electrochemical and microscopic characterization of titanium dioxide electrodes modified with platinum nanoparticles

This study investigates glass/Ti/Pt/TiO2 surfaces, wherein Pt nanoparticles (NPs) were potentiostatically deposited with an amount of Pt that varies based on deposition time. The size and distribution of NPs were analyzed by scanning electron microscopy (SEM). Subsequently, a thicker titanium dioxide film was grown via anodization. Topography and roughness were examined by atomic force microscopy (AFM). Remarkably, TiO2 grows independently of Pt NPs and remains stable under working conditions, including acid, neutral, and alkaline media. Under steady-state conditions, the open circuit potentials (OCPs) of the modified semiconductor/electrolyte interfaces corresponding to 1, 5, and 10 s of electrodeposited Pt, showed a shift of 167 mV, 42 mV, and 24 mV toward more positive values, respectively. Notably, these surfaces exhibit the activity of a Pt quasi-electrode and the band structure of a titanium dioxide semiconductor, making them ideal for use as photoanodes. In addition, it can be highlighted that the methodology employed in the preparation of the surfaces allows for reproducibility.

Spin-dependent thermoelectric transport properties of Cr-doped blue phosphorene

We systematically investigate the thermoelectric (TE) properties of the Cr-doped blue phosphorene (blue-P) along the armchair and zigzag directions. First, we find the semiconducting band structure of the blue-P will become spin-polarized due to the Cr-doping, and can be seriously changed by the doping concentration. Then we show the Seebeck coefficient, the electronic conductance, the thermal conductance, and the figures of merit ZTs are all dependent on the transport directions and doping concentration. However, two pairs of the peaks of the charge and spin ZTs can be always observed with the low-height (high-height) pair on the side of the negative (positive) Fermi energy. In addition, at temperature 300 K the extrema of the charge (spin) ZTs of the blue-P along the two directions are kept to be larger than 22 (90) for the different doping concentrations and will be further enhanced at lower temperature. Therefore, we believe the Cr-doped blue-P should be a versatile high-performance TE material which may be used in the fields of the thermorelectrics and spin caloritronics.

A Review on Oxygen-Deficient Titanium Oxide for Photocatalytic Hydrogen Production

Photocatalytic technology based on the specific band structure of semiconductors offers a promising way to solve the urgent energy and environmental issues in modern society. In particular, hydrogen production from water splitting over semiconductor photocatalysts attracts great attention owing to the clean source and application of energy, which highly depends on the performance of photocatalysts. Among the various photocatalysts, TiO2 has been intensively investigated and used extensively due to its outstanding photocatalytic activity, high chemical stability, non-toxicity, and low cost. However, pure TiO2 has a wide band gap of approximately 3.2 eV, which limits its photocatalytic activity for water splitting to generate hydrogen only under ultraviolet light, excluding most of the inexhaustible sunlight for human beings. Fortunately, the band gap of semiconductors can be manipulated, in which introducing oxygen defects is one of the most effective measures to narrow the band gap of titanium oxides. This review considers the fundamentals of photocatalytic water splitting for hydrogen production over TiO2, discusses the latest progress in this field, and summarizes the various methods and strategies to induce oxygen defects in TiO2 crystals. Then, the next section outlines the modification approaches of oxygen-deficient titanium oxide (TiO2−δ) to further improve its photocatalytic performance. Finally, a brief summary and outlook of the studies on TiO2−δ photocatalysts for water splitting to produce hydrogen are presented.

In situ fabrication of type-II BiOIO3/Bi-MOF heterostructures with enhanced photocatalytic activity

BiOIO3/Bi-MOF (BF-x) composites with a porous hierarchical rod-like structure are prepared by in situ growth of BiOIO3 on Bi-MOF rod-like template. BiOIO3/Bi-MOF exhibit enhanced adsorption and photodegradation efficiency of RhB and TC. Within 60 min irradiation of Xe light, the optimal BF-40 composite can degrade 99% of RhB and 78% of TC, and the apparent rate constant k for photodegrading RhB (TC) is approximately 8.1 (3.9) and 29.6 (12.8) times higher than that of BiOIO3 and Bi-MOF, respectively. The BiOIO3/Bi-MOF heterojunction has larger specific surface area, improved light-harvesting ability and boosted separation and transfer efficiency of photocarrier, which benefit to enhance the photocatalytic activity of BiOIO3/Bi-MOF heterojunction. According to the semiconductor band structure theory and the results of radicals quenching experiment, the photocarriers at the interface of BiOIO3/Bi-MOF heterojunction follow the traditional type-II migration path, and a possible type-II photocatalytic degradation mechanism is proposed. This work provides more perspective in the development of in situ growth of 2D materials on MOF with the same metal element for water treatment and other potential applications.

Fermi level induced band edge alignment and band bending in Ag3PO4/Cu2O p-n heterojunction for proficient photocatalytic applications

Fabrication of binary p-n heterojunction and the band alignment at the interface of the individual semiconductor photocatalyst has been extensively studied to verify superior charge separation and migration capability in photocatalytic applications. Here, a simple wet chemical followed by a hydrothermal fabrication strategy was introduced for the development of the binary p-n Ag3PO4/Cu2O heterostructures. The tunable band structure of individual semiconductors with the work function (φ), witnessed a band bending at the space charge region. The bending at the interface induces a carrier concentration gradient and manifests a rectifying current transport diode. The fabricated p-n heterostructures and carrier migration between n-type Ag3PO4 and p-type Cu2O were verified by different morphological and physicochemical methods. The band banding at the interface, leading to a narrow depletion region, favors the tunneling of electron-hole pairs through a Z-scheme carrier transport mechanism. The electron-hole pair movement has further been confirmed by considering the band edge position after contact, photocatalytic scavengers, and the radical trapping experiment. The Ag3PO4/Cu2O p-n heterojunction photocatalyst manifested a 737.4 μmol g−1h−1 of H2 generation and 91% endosulfan degradation efficiency, these are 12 and 9 times higher than that of pure Ag3PO4. The p-n heterojunction photocatalyst displayed a higher current density with electron-hole migration efficiency, synergistically enhancing the catalytic activity through the interfacial space charge junction.

Hybrid Cathodic/Anodic Electrosynthesis of Phase Pure Ag4V2O7 Thin Films

Here, we demonstrate a two-step electrosynthesis approach for the preparation of silver pyrovanadate, Ag4V2O7 in thin-film form. In the first, cathodic step, polycrystalline Ag was deposited on fluorine doped tin oxide (FTO) substrate from a non-aqueous bath. Aqueous pyrovanadate species were then generated by aging of a CO2-infused sodium orthovanadate (Na3VO4) solution for three weeks. Silver ions were subsequently generated in situ in this medium using anodic stripping of the Ag/ITO films from the first step. Interfacial precipitation of the Ag+ ions with the pyrovanadate species afforded the targeted product in phase pure form. The various stages of the electrosynthesis were monitored in situ via the combined use of voltammetry, electrochemical quartz crystal nanogravimetry (EQCN), and coulometry. The Ag4V2O7 thin films were characterized by a variety of experimental techniques, including X-ray diffraction, laser Raman spectroscopy, diffuse reflectance spectroscopy, scanning electron microscopy, and high-resolution transmission electron microscopy. Surface photovoltage spectroscopy, ambient-pressure photoemission spectroscopy, and Kelvin probe contact potential difference (work function) measurements afforded information on the energy band structure of the p-type Ag4V2O7 semiconductor. Finally, the electrochemical and photoelectrochemical properties of the electrosynthesized Ag4V2O7 thin films were studied in both aqueous and non-aqueous electrolytes.

Optimizing the band structure of sponge-like S-doped poly(heptazine imide) with quantum confinement effect towards boosting visible-light photocatalytic H2 generation

Simultaneously manipulating the nanostructure and band structure of semiconductors for boosting the photocatalytic performance of photocatalyts is highly desirable. Herein, a series of hierarchical sponge-like S-doped poly(heptazine imide) (HS-SPHI) assembled by ultrathin nanosheets were successfully fabricated via a facile bottom-up supramolecular preassembly approach using melamine (MA) and trithiocyanuric acid (TTCA) as precursors. Benefiting from the synergistic effect of the S-doping and their unique hierarchical porous structure coupled with quantum confinement effect, the as-obtained HS-SPHIs are endowed with extended visible-light response, improved charge separation efficiency, enlarged specific surface area, and enhanced thermodynamic driving force for water reduction. As a result, all the HS-SPHIs exhibit remarkable boosting visible-light (>420 nm) photocatalytic H2evolution (PHE). The maximum PHE rate achieved by HS-SPHI-650 can be up to 3584.2 μmol g−1h−1, with an apparent quantum efficiency (AQE) of 14.67 % at 420 nm, which is about 22.4 times than that of pristine bulk g-C3N4 (B-GCN). We believe that this work will provide a significant strategy for optimizing the band structure of PCN in order to improve its photocatalytic performance.