Forbidden Band Research Articles

Analysis of Acoustic Surface Wave Focused Unidirectional Interdigital Transducers Using Coupling-of-Mode Theory

In cell or droplet separation, high acoustic wave energy of a surface acoustic wave (SAW) device is required to generate sufficient acoustic radiation force. In this paper, the electrode width-control floating electrode focused unidirectional interdigital transducer (EWC-FEFUDT) is proposed due to its enhanced focusing properties. The performance of the EWC-FEFUDT is investigated using the Coupling-of-Mode (COM) theory, and the COM parameter is extracted using the Finite Element Method (FEM). The four different forbidden band edge frequencies account for the unidirectionality of the proposed EWC-FEFUDT. A direction angle of ϕκ−ϕζ=44.5° of the EWC-FEFUDT (Design 3) is obtained, being fairly close to the optimum value of 45°. The EWC-FEFUDT (Design 3) has a lower insertion loss (IL) of −5.1 dB and greater unidirectionality (20 × log10(D) = 13.8 dB). The SAW maximum amplitude of the EWC-FEFUDT (Design 3) is increased by about 1.5×10−4 µm compared to that of the focused interdigital transducers (FIDTs). The maximum acoustic pressure of the EWC-FEFUDT is an order of magnitude higher than that of FIDTs. The EWC-FEFUDT exhibits enhanced focusing properties. The proposed EWC-FEFUDT may provide an alternative method for cell or droplet separation in an efficient manner.

UNTANGLING THE VALLEY STRUCTURE OF STATES FOR INTRAVALLEY EXCHANGE ANISOTROPY IN LEAD CHALCOGENIDES QUANTUM DOTS

We put forward a generalized procedure which allows to restore the bulk-like electron and hole wave functions localized in certain valleys from the wave functions of quantum confined electron/hole states obtained in atomistic calculations of nanostructures. The procedure is applied to the lead chalcogenide quantum dots to accurately extract the intravalley velocity matrix elements and the constants of the effective intravalley Hamiltonian of the exchange interaction for the ground exciton state in PbS and PbSe quantum dots. Our results suggest that intravalley parameters in PbS quantum dots are much more anisotropic than the ones in PbSe. Renormalization of the velocity matrix elements, forbidden band gap, valley and exchange splittings of exciton and exciton binding energy are also calculated.

Review of: "The smaller the size of the nanoparticles, the greater the distance between the energy levels and the forbidden band, and the larger the size of the particles, the smaller the distance between the energy levels nano particles"

Review of: "The smaller the size of the nanoparticles, the greater the distance between the energy levels and the forbidden band, and the larger the size of the particles, the smaller the distance between the energy levels nano particles"

Influence of Dopant Atoms in TiO2 for Its Electron Structure and Activity of Oxygen Reduction Reaction in Acidic Environmental

Polymer electrolyte fuel cells (PEFCs) are attracting attention, but their widespread commercialization has various challenges. In particle, the Pt-based cathode catalyst has two significant problems: durability and cost. These problems will persist as long as the Pt-based catalyst is used in PEFCs. Therefore, we focused on TiO2 as the alternative Pt-based catalyst, which is highly durable and low-cost. However, TiO2 has low electron conductivity and must obtain the electron conduction path to progress the oxygen reduction reaction (ORR). In general, doping transition metal and/or introducing the oxygen vacancy into the crystal structure is conducted to enhance the electron conductivity of TiO2. It was reported that Nb-doped TiOx having oxygen vacancies showed relatively higher ORR activity. Experimentally, it is known that other metal doping and oxygen vacancies bring in high ORR activity. However, the detailed enhancement mechanism for ORR activity is still unclear.Some research groups were using theoretical calculations to investigate the mechanism of ORR, such as the reaction path and active site on the TiO2 surface [1, 2]. These reports suggest one of the potentials for the ORR mechanism but don’t completely describe the ORR on the TiO2-based catalyst because it is hard to compute the state of a realistic, accurate ORR system. Therefore, we planned the combination of experiment data and theoretical calculations, and the balance of experimental data was heavier.In this study, the 3d transition metal atom was doped to TiO2 and conducted for the reduction treatment for doped TiO2 to introduce oxygen vacancies. Their ORR activities and electron structure were evaluated experimentally. In addition, the electron structures of all samples were computed using DFT calculation and compared with experimental data.5 at% of transition metal-doped TiO2 and TiO2 were synthesized by sol-gel method. The reduction treatment in this study consisted of calcining at 800°C for 10 min under the H2+Ar mixed gas flow conditions. The electrochemical measurements used a three-electrode cell with a working electrode, a carbon plate counter electrode, an Ag/AgCl reference electrode, and 0.1 M HClO4(60°C). The potential in this study was converted from versus Ag/AgCl to versus RHE. The ORR current was obtained by subtracting currents under inert gas flow conditions from currents under O2 flow conditions. The ORR activity was judged by the potential at 5 mA g-1 of ORR current. DFT calculation was performed with Quantum ESPRESSO.All the synthesized samples had the anatase phase structure, but crystallinity was low since they formed nanoparticles or might not react with precursors completely. XPS results suggested that the dopant atom was probably introduced into the TiO2 crystal structure. The ORR activity of TiO2 was improved by doping the 3d transition metal atoms.The density of states (DOS) of modeled our doped TiO2 samples showed a new intermediate band in the forbidden band of TiO2. Now, we expect that the location, amount, density, and other circumstances of this new intermediate band may be related to ORR activity.[1] F. Zhang, et al. Applied Surface Science, 598 (2022) 153873.[2] Y. Yamamoto, et al. J.phys.Chem.C, 123 (2019) 19486-19492.

N-P Type Charge Compensatory Synergistic Effect for Schottky and Efficient Photoelectrocatalysis Applications: Doping Mechanism in (Nb/Re)@WS2/Graphene Heterojunctions.

Band gap engineering based on doped two-dimensional (2D) transition metal dichalcogenides (TMDs) has shown great potential in the design and development of new nano photoelectronic devices and their application in photoelectrocatalysis. However, there are two key issues that are difficult to take into account, namely the impurity levels induced by dopant atoms appear in the forbidden band of the doping system, which can become the recombination center of photogenerated carriers, thereby reducing the photocatalytic efficiency. Compared with the carrier mobility of the corresponding doped systems, that of intrinsic 2D TMDs is too low. Understanding the doping mechanism of heteroatoms in these systems and designing corresponding crystal structures rationally is important for solving these problems. In this study, the crystal structures of co-doped monolayer WS2 with Nb and Re atoms were designed using density functional theory, and doping systems with graphene (high carrier mobility) were assembled into a heterostructure using the concept of heterorecombination. The N-P type co-doping of Nb and Re atoms retained the continuous band characteristics of the original monolayer WS2 while also providing the high carrier mobility of graphene, yielding an excellent multipurpose material for manufacturing high-speed Schottky devices and efficient water-splitting H evolution catalysts.

Preparation and degradation efficiency of CeO2 and CdS composite photocatalysts

Cerium dioxide (CeO2) is an extremely versatile rare-earth oxide, which is chemically stable, nontoxic, and simple to prepare, making it of great interest in the field of photocatalysis. However, cerium dioxide has a low forbidden band width, its utilization of sunlight is not high, and the However, cerium dioxide has a low forbidden band width, its utilization of sunlight is not high, and the photogenerated carriers are very easy to compound. These problems lead to the low photocatalytic efficiency of cerium dioxide, which greatly limits the practical application of cerium dioxide photogeneration. These problems lead to the low photocatalytic efficiency of cerium dioxide, which greatly limits the practical application of cerium dioxide photocatalysts. morphologies was prepared by hydrothermal method using cerium nitrate as raw material, and then compounded with CdS to form a composite photocatalyst. The effect of morphology on the degradation efficiency of CeO2 and the change of the degradation efficiency of CeO2 with different morphologies after The effect of morphology on the degradation efficiency of CeO2 and the change of the degradation efficiency of CeO2 with different morphologies after compounding with CdS were investigated.

Visible Light-Driven Photocatalysis of Al-Doped SrTiO3: Experimental and DFT Study.

Environmental problems associated with water pollution caused by organic dyes have raised serious concerns. In this context, photocatalytic processes have proven to be promising and environmentally friendly methods for water purification utilising abundant solar energy. In this study, a SrTiO3-based photocatalyst was modified by doping with Al ions and the deposition of dual co-catalysts (Rh/Cr2O3 and CoOOH) to enhance the photocatalytic decomposition efficiency of methylene blue (MB). Pure perovskite SrTiO3 was synthesised by chemical precipitation followed by calcination at 1100 °C. Al-doped SrTiO3 with deposited co-catalysts showed 3.2 times higher photocatalytic activity compared to unalloyed SrTiO3 with co-catalysts in MB decomposition under visible radiation. This study highlights the effectiveness of using dual co-catalysts and low-valence metal doping to enhance the efficiency of the photocatalytic decomposition of organic pollutants. The density functional theory analysis results show that the Al doping of SrTiO3 improves charge separation and increases the lifetime of photogenerated electrons and holes while maintaining the size of the forbidden band, which confirms its effectiveness for enhancing photocatalytic activity.

Control of spin wave demultiplexing using spin current

The paper reports on the principle of signals demultiplexing in the structure ferromagnetic film/normal metal/magnonic crystal. It is discovered that in such a structure there is a formation of four band gaps — spin wave forbidden bands. It is shown that depending on the frequency, the signal comes out through different ports of the structure (ferromagnetic film or magnonic crystal), i.e. there is frequency division of channels. The influence of spin current on the signal coupling at frequencies inside the band gap significantly prevails over the effect of spin current at frequencies outside the band gap. The spin current allows effective control of the main line insertion loss of a directional coupler based on such a structure. The spin current has little effect on coupling factor and isolation.

Utilizing armchair and zigzag nanoribbons for improved detection of SO2 Toxicity with graphene biosensor

Graphene nanoribbons (GNRs), with their distinctive structural and electronic characteristics, offer significant potential for use as biosensors in the detection of sulfur dioxide (SO₂) levels. This study employs Density Functional Theory (DFT) to evaluate the sensitivity of armchair graphene nanoribbons (AGNRs) to SO₂ gas. Specifically, we investigate the influence of SO₂ molecules on the electronic and transport characteristics of both pure and Cr-doped zigzag graphene nanoribbons (ZGNRs) and Cr-doped AGNRs. Key electronic properties, including band structure, adsorption energy, and density of states (DOS), were analyzed following SO₂ adsorption. The results indicate that Cr doping leads to significant changes in the electronic properties of AGNRs upon SO₂ exposure, including alterations in the Fermi surface that result in band separation and the formation of a forbidden band. The adsorption energy of Cr-doped AGNR increased from 0.07 eV (pristine AGNR) to 0.55 eV (Cr-doped AGNR), nearly eight times higher, indicating strong chemisorption of SO₂. These findings suggest that Cr-doped AGNRs exhibit high sensitivity to SO₂, making them promising candidates for gas detection. Furthermore, this study reveals that SO₂ adsorption in the armchair direction results in a more pronounced peak-to-valley ratio compared to the zigzag direction, highlighting distinct characteristics in SO₂ adsorption behavior. These findings could facilitate the development of advanced sensors with improved sensitivity and selectivity for environmental monitoring and safety applications.

Formation of a combined electron-hole emission state in the LiRbSO4 – Eu phosphor

In the irradiated phosphor 4 LiRbSO Eu , the mechanisms of formation of the induced or combined electron-emitting state at 3.1-2.94 eV were studied using optical and thermal activation spectroscopy methods. It has been shown experimentally that the combined electron-emitting state of the phosphor is formed from the electron states of impurity and intrinsic electron and hole trapping centers of 24Eu SO   and 34 4 SO SO . Electron and hole trapping centers are created by irradiating the phosphor with photons exceeding the width of the forbidden band of the matrix, where free electrons are created in the conduction band and a hole in the valence band. The trapping center is formed by the capture of free electrons by impurities and anionic complexes according to the reaction 3 2 Eu e Eu      ,2 34 4 SO e SO    . In one process with electron centers, holes in the form of 24SO are localized. Thus, impurity and intrinsic 2 4Eu SO   and 34 4 SO SO  electron-hole trapping centers are formed. Similarly, trapping centers are formed as a result of charge transfer from the excited anion of the 24SO complex to the 3Eu  impurities and to the neighboring 2 4 SO anions according to the reaction ( 2 3 O Eu   ) and ( 2 24O SO  ), and localized holes are also formed in one act along with it. Combined electron-emitting states consisting of impurity and intrinsic electron states are excited by photons with energies of ~4.0 eV and ~4.5 eV.

Effect mechanism of Cd on band structure and photocatalytic hydrogen production performance of ZnS

ZnS is widely used in the photocatalytic decomposition of water to produce hydrogen due to its fast electron-hole pair generation and high negative potential. However, its absorption in the visible region is poor due to its wide band gap, and it has serious photogenerated carrier recombination problems. Herein, a shallow impurity energy level was introduced by doping the ZnS lattice with Cd. Due to its presence, electrons trying to return to the valence band are trapped and excited twice, suppressing the recombination of photogenerated carriers and greatly improving electron utilization. The Cd1.5-ZnS possesses a hydrogen production rate as high as 85722.20 μmol/g, which is 17 times higher than pure ZnS. Meanwhile, Cd1.5-ZnS has a narrower forbidden band and superior visible light absorption, and the serious photocorrosion problem of ZnS has been suppressed. This study provides a viable approach for the synthesis of photocatalysts with adjustable band gaps and enhanced hydrogen precipitation efficiency.

Magnetic Chitosan/ZrO2 Composites for Vanadium(V) Adsorption while Concurrently being Transformed to a Dual Functional Catalyst.

Spent adsorbents for recycling as catalysts have drawn considerable attention due to their environmentally benign chemistry properties. However, traditional thermocatalytic strategies limit their applications. Here, we developed an enhanced photocatalytic strategy to expand the range of their applications. A magnetic chitosan/ZrO2 composites (MZT) for V(V) adsorption, which were prepared using chitosan, ZrO2 and Fe3O4 by one-pot synthesis. The spent MZT as a catalyst was used to synthesize 2-phenyl-1H-benzo[d]imidazole, yielding up to 89.7 %. It also was implemented to photocatalysis reactions for recycle. The discolored rates of rhodamine B (RhB) were 72.3 % and 97.4 % by new and spent MZT, respectively. The new and spent MZT showed the forbidden bands were 251 nm and 561 nm, respectively. The result displayed spent MZT red shifted to the cyan light region. The mechanism of catalysis also has been studied in detail.

Novel CoFe2O4 / LaAlO3 nanocomposites: An impressive photocatalyst with p-n hetero-junction for improved H2 generation under visible exposure

The present work is devoted to the synthesis of perovskite LaAlO3 by Sol-Gel approach and its application for the photo-evolution of H2 in conjunction with cobalt ferrite CoFe2O4 prepared by nitrate route. The thermal analysis (TG/DSC) showed that the formation is completed at 600 °C. The X-ray diffraction (XRD) pattern is characteristic of a single phase, crystallizing in a rhombohedral symmetry with a crystallite size of 36 nm. The SEM micrographs revealed a homogeneous structure with an agglomeration of shaped grains (40–80 nm). The forbidden band (3.39 eV), calculated from diffuse reflectance data, is assigned to the ligand charge transfer O2−: 2p → Al3+: 3s orbital. As prepared, LaAlO3 is an n-type semiconductor as demonstrated from the capacitance - potential (C−2 - E) graph with an electron density of 9.4 × 1014 cm−3 due to oxygen under-stoichiometry. Attention was directed to photo-electrochemistry in connection with water reduction into hydrogen. With a negative conduction band potential (−0.11 VSCE), H2 generation can be synergized over the hetero-system by visible light. The photocatalytic capability of CoFe2O4 25%/LaAlO3 revealed a high H2 generation of 963 μmol g−1 in the alkaline electrolyte (NaOH, 0.1 M) with a catalyst dose of 1 mg mL−1, which is 1.3 times more than CoFe2O4 under similar conditions. Photoactivity is significantly improved by 60%, up to 2400 μmol g−1 in the presence of oxalate C2O42− as a hole scavenger in the hetero-junction, two hydrogen release tests were successfully recorded. The higher H2 production in the hetero-junction system is attributed to the lower recombination rate of electron/hole (e−/h+) in the material, due to the large band bending at the solid interfacial.

Formation of High-Photoresponsivity BaSi2 Films by Radio-Frequency Sputtering and Evaluation of a BaSi2/TiN/Glass Heterojunction by Transmission Electron Microscopy.

Barium disilicide (BaSi2) is a thin-film solar cell material composed of abundant elements, and its application potential is further enhanced by its formation on inexpensive substrates, such as glass. The effect of the substrate temperature on the co-sputtering of BaSi2 and Ba targets to form BaSi2 films on Si(111) and TiN/glass substrates was investigated. Contrary to expectations, the photoresponsivity reached maximum values exceeding 5 and 2 A W-1, respectively, the highest value ever reported for as-deposited samples formed at 750 °C, more than 100 °C higher than those reported previously. Because the photoresponsivity is proportional to carrier lifetime, this result indicates that high-temperature growth can bring out the high performance of BaSi2 as a light-absorbing layer. Because amorphous SiC (a-SiC) has a larger forbidden band gap and electron affinity than BaSi2, it is considered suitable as an electron transport layer (ETL) material for BaSi2 solar cells. On the basis of this, the formation of BaSi2 (absorption layer)/a-SiC (ETL)/TiN (electrode)/glass heterojunctions was also attempted, and the layered structure was examined by cross-sectional transmission electron microscopy (TEM). Polycrystalline BaSi2 films were found to be even on the amorphous layer by TEM. A high photoresponsivity of over 2 A W-1 was obtained. Therefore, the BaSi2/a-SiC/TiN structure provides a guideline for the structural design of BaSi2-based thin-film solar cells on glass.

The mechanism of adjusting the low-frequency ultra-wide band gap within lightweight spherical superstructue

Conventional resonant structures can be effective in obtaining broadband, but it is still a challenge to design small-sized and lightweight acoustic metamaterials with a low-frequency ultra-wideband. This paper proposes a new approach of designing a lightweight spherical localized resonance superstructure with adjustable stiffness ratio, and the mechanism of adjusting the low-frequency ultra-wide forbidden band is revealed. Then, the correlation between the zero value of its dynamic equivalent mass and the stiffness ratio of the system is studied. It is found that not only is the upper bound of the negative mass effectively broadened, but also the lower bound is successfully lowered only by adjusting the stiffness ratio of the sphere. Most importantly, based on the regulation mechanism with adjustable stiffness ratio, the lower boundary of the band gap is lowered from 171 Hz to 141 Hz, and the upper boundary is increased from 445 Hz to 710 Hz. Therefore, the low-frequency ultra-wideband of 141–710 Hz is obtained only by adjusting the stiffness ratio of the system and the Finite Element Method, which is highly consistent with theoretical analyses. Obviously, the mechanism of obtaining the low-frequency wideband through adjusting the stiffness ratio not only provides a novel idea for adjusting the low-frequency ultra-wideband, but also provides theoretical guidance for the developing the small-size and lightweight acoustic devices, so it would have potential application in the field of vibration and noise reduction.

Enhanced Photocatalytic Degradation of Methyl Orange by Metal‐Tio2 Nanoparticles

AbstractIn this study, a series of transition metal‐doped TiO2 was prepared by a simple sol‐solvothermal method. Mix an ethanol solution of tetrabutyl titanate and metal nitrate solution to obtain a homogeneous sol, which is then subjected to solvothermal treatment. After calcination, the metal‐TiO2 naoncomposite was obtained. Selected transition metal precursors, including Cu, Fe, Ag, Cr, Co and Zn, were doped into TiO2 nanocatalysts to assess photocatalytic degradation activities. The results show that 1 mol % Ag−TiO2 photocatalyst has a larger specific surface area, a stable anatase phase and the narrowest forbidden band energy (3.00 eV). In photocatalytic degradation of methyl orange (MO) tests, 1 mol % Ag−TiO2 could reach 100 % decolourisation after 70 min and 75.43 % mineralisation after 90 min. ESR analysis confirmed that Ag doping significantly increased the content of ⋅O2− and ⋅OH for the TiO2 catalyst, which are responsible for the oxidation reactions and consequently degradation of pollutants. The photocatalytic degradation mechanism of MO was also proposed. The TOC analyses confirmed the mineralization of MO. The excellent thermal and chemical stability of 1 mol % Ag−TiO2 has also been demonstrated by thermogravimetric analysis and cycling experiments. Electrochemical tests have also demonstrated the excellent electrochemical properties of 1 mol % Ag−TiO2.

Development of Modified TiO2 for Photocatalytic Hydrogen Production

AbstractTiO2 has advantages such as excellent photocatalytic performance, high chemical stability, low price, and not easy to produce secondary pollution. Therefore, the photocatalytic hydrogen production over TiO2 catalyst was an effective way to achieve hydrogen production from solar energy with low energy consumption and low cost. However, the TiO2 catalyst responds only to UV light due to its wide forbidden band, and the electrons and holes were prone to recombination, resulting in the decrease in photocatalytic activity and quantum efficiency. In recent years, researchers have conducted a series of modifications on TiO2 to improve its visible light response, reduce the rate of carrier recombination, and thereby enhance the efficiency of photocatalytic hydrogen production. This paper reviews several main modification methods of TiO2 including morphology regulation, ion doping, semiconductor recombination, noble metal deposition, dye sensitization and their research progress in the field of photocatalytic hydrogen production. Finally, the modification of TiO2 photocatalyst in the future was predicted.

Study on the effects caused by defect LaK in KH2PO4 crystal

Based on first-principles calculations, we have investigated the effects of different charged states of LaK on the defect formation energies (DFE), lattice distortions, electronic structures, and optical properties of paraelectric phase (PE-KDP) and ferroelectrical phase (FE-KDP) crystals. Our calculations indicate that LaK·· is readily formed in both crystal structures, and the degree of H–O bond distortion is more pronounced in PE-KDP. This could explain the formation of small growth mounds on the surface of La-doped crystals observed in the experiments, as well as the large decrease in the hardness of La-doped crystals. Furthermore, the band gap value of La-doped crystals is smaller than that of perfect crystals. This can be attributed to the fact that the 5d orbitals of La affect the 2p orbitals of O located in the conduction band minimum (CBM), resulting in a splitting of the energy levels and the formation of a defective energy level that enters the forbidden band. The optical spectra have been obtained considering the electron-phonon coupling. Large stokes redshift and nonradiative jump will lower the crystal laser damage threshold. The La-doped PE-KDP has an absorption band at 3.03 eV (409 nm) in good agreement with the experimentally observed absorption band at 390 nm.

Bilayer graphene in periodic and quasiperiodic magnetic superlattices

Starting from the effective Hamiltonian arising from the tight-binding model, we study the behaviour of low-lying excitations for bilayer graphene placed in periodic external magnetic fields by using irreducible second-order supersymmetry transformations. The coupled system of equations describing these excitations is reduced to a pair of periodic Schrödinger Hamiltonians intertwined by a second-order differential operator. The direct implementation of more general second-order supersymmetry transformations allows to create non-singular Schrödinger potentials with periodicity defects and bound states embedded in the forbidden bands, which turn out to be associated with quasiperiodic magnetic superlattices. Applications in quantum metamaterials stem from the ability to engineer and control such bound states which could lead to a fast development of the subject in the near future.

Enhancement of dye treatment efficiency of TiO2/C3N4 photocatalysts synthesized by hydrothermal method

Abstract Using sunlight to generate energy is a trend to establish a sustainable economy while protecting the environment. Photocatalytic breakdown of organic molecules on the surfaces of semiconductors is an important topic of current research. In this study, competent TiO2/g-C3N4 photocatalysts produced by the hydrothermal technique demonstrated outstanding photoactivity. To determine the best-performing photocatalyst, TiO2/g-C3N4 materials were analyzed by using Scanning Electron Microscopy (SEM), X-ray diffraction analysis (XRD), Energy Dispersive X- ray (EDX), Brunauer-Emmett-Teller (BET) analysis, and Differential Reflectance Spectroscopy (DRS). XRD results indicate that the addition of C3N4 increases the ratio of TiO2 Anatase. Optical studies unveiled a significant reduction in the forbidden band. The experiments using photocatalysis revealed that the action of TiO2/g-C3N4 on methylene blue (MB) dyes exhibited not only momentous photocatalytic performance but also excellent adsorption ability. The result of this study is anticipated to offer a solid basis for the creation and synthesis of cutting-edge materials for applications including the treatment of textile effluent.