States In Band Gap Research Articles

Hybrid Bandgaps in Mass-coupled Bragg Atomic Chains: Generation and Switching

In this work, without introducing mass-in-mass units or inertial amplification mechanisms, we show that two Bragg atomic chains can form an acoustic metamaterial that possesses different types of bandgaps other than Bragg ones, including local resonance and inertial amplification-like bandgaps. Specifically, by coupling masses of one monatomic chain to the same masses of a diatomic or triatomic chain, hybrid bandgaps can be generated and further be switched through the adjustment of the structural parameters. To provide a tuning guidance for the hybrid bandgaps, we derived an analytical transition parameter (p-value) for the mass-coupled monatomic/diatomic chain and analytical discriminants for the mass-coupled monatomic/triatomic chain. In our proposed mass-coupled monatomic/triatomic chain system, each set of analytical discriminants determines a hybrid bandgap state and a detailed examination reveals 14 different bandgap states. In addition to bandgap switching, the analytical p-value and discriminants can also be used as a guide for designing the coupled-chain acoustic metamaterials. The relations between the mass-coupled monatomic/triatomic chain system and a three-degree-of-freedom (DOF) inertial amplification system further indicate that the band structure of the former is equivalent to that of the latter through coupling masses by negative dynamic stiffness springs.

Deep-subwavelength interface states in mechanical systems

Topological interface states in mechanical systems are the analogues of the edge modes in topological insulators from the field of condensed matter physics. The interface states produced and reported in the existing literature are located in the Bragg Scattering band gaps of phononic crystals. The corresponding frequencies of the interface states are thus high. This paper proposes a novel mechanical system that can produce an ultra-low frequency interface state in the deep-subwavelength region. Due to the periodic spring connections to the ground, a quasi-static band gap with a starting frequency of 0 Hz can be produced in the proposed system. The spring connections can be tuned to alter the polarization direction of the mode shape. Through a mass-spring model, the formation of an interface state in the quasi-static band gap is demonstrated. Moreover, it is found that by carefully configuring the spring connections to the ground, the interface state in the quasi-static band gap can be tuned to an arbitrarily low frequency. In addition, practical systems based on rod and beam structures are constructed following a similar design methodology. Theoretical analyses demonstrate that for the longitudinal/transverse wave mode in the topological rod/beam structure, a deep-subwavelength interface state can always form in the quasi-static band gap. The possibility of producing the deep-subwavelength interface state at an arbitrarily low frequency is also confirmed in a rod and a beam lattice system.

Highly efficient and durable electron transport layer for QDSSC: An integrated approach to address recombination losses

Quantum dot sensitized solar cells (QDSSC) are up-and-coming low cost third-generation solar cells technology. The distinctive properties of quantum dots (QDs) feature QDSSC as an optimistic future of photovoltaics. The real concern with these devices is the wide band gap semiconductors used as the electron transport layer. Electron transport layers in quantum dot sensitized solar cells suffer from performance losses arising from recombination of electrons. This work tries to entail an integrated approach of tailoring the nanostructure as an exclusive electron transport layer to overcome the recombination losses. We adopted doping of Neodymium into TiO2 (Nd-TiO2) to create intermittent band gap states that can extend the charge carrier lifetime and rendered the defect material (Nd-TiO2) as a hybrid composite with 1D Graphene nanoribbons for quick electron extraction. This approach carefully alters ETL's functionality, and its utility has emerged into a potential electrode for QDSSC. Our tailored photoanode reports a higher efficiency of ~ 6% for cadmium sulfide QDs deposited via successive ionic layer adsorption and reaction (SILAR) and stands as highly efficient photoanode for QDSSC. This opens a clear channel in the realm of applications like photocatalysis, photovoltaics, and energy devices that involve metal oxide nanostructures for efficient charge separation.

Insight into the effects of oxygen vacancy on the toluene oxidation over α-MnO2 catalyst

In order to clarify the role of oxygen vacancy (OV), five α-MnO2 catalysts with abundant OVs are fabricated via a novel and facile redox-precipitation approach and employed to the toluene oxidation in air. The concentration of OVs in α-MnO2 catalysts is regulated via the alkyl chain length of alcohols, and its correlation with catalytic performances is scientifically investigated based on various characterization technologies and density functional theory (DFT) calculation. The α-MnO2-C2 catalyst exhibits excellent catalytic activity (T90 = 217 °C), stability, and water resistance for toluene oxidation in air. The OVs can induce the new bandgap states (BGS), which upshift the antibonding orbitals relative to the Fermi level (Ef), eventually favoring the formation of adsorbed active oxygen species. Furthermore, the OVs cause an increase in the amount of Mn3+, resulting in the elongated Mn–O bonds due to the strong Jahn-Teller effect of Mn3+. Therefore, the synergistic effects of OVs benefit toluene oxidation through L-H and MvK mechanisms over the prepared α-MnO2-Cx catalysts. This work reveals the important role of OVs in the promotion of toluene catalytic oxidation activity and also may provide new insights for the design of high-performance VOCs oxidation elimination catalyst.

First-Principles-Based Quantum Transport Simulations of Interfacial Point Defect Effects on InAs Nanowire Tunnel FETs

The effects of a single point defect on InAs gate-all-around nanowire tunnel FETs (NW TFETs) are investigated. We considered two kinds of interfacial defects, the arsenic dangling bond (As <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">DB</sub> ) and the arsenic antisite (As <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">In</sub> ). The critical physics related to the point defects, which are the charge trapping (CT), the trap-assisted tunneling, and their interplay, are rigorously captured using the full quantum transport model with the physical defect Hamiltonian obtained from the density functional theory calculations. We found that both defects on the NW cause the bandgap states, which crucially affects the TFETs performances. Through nonequilibrium Green’s function method self-consistently coupled with Poisson’s equation, the characteristics of the point defects under finite bias conditions are analyzed. We found that the CT is strong in As <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">In</sub> , and thus the defect level, referenced to the valence band edge, is significantly changed by the gate bias. The strong CT leads to the screening of the gate field, which makes the defect level of As <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">In</sub> effectively pinned near the drain Fermi level. The radial distributions of the density of states (DOS) and the phase relaxation time are analyzed. As <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">In</sub> shows the strongly localized DOS at the defect center, while the DOS of As <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">DB</sub> is relatively delocalized due to the coupling with the valence bands. The impact of the defects on the TFET performances is examined as the gate length is scaled down. It is shown that As <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">In</sub> acts as a limiting factor for scaling down of the TFETs.

Anisotropic d-d Transition in Rutile TiO2.

The band gap state of TiO2, which is dominated by Ti3+ 3d character, is of great relevance to light absorption, electron trapping, charge recombination, and conduction band structure. Despite the importance, the explanation of the excitation from this state is controversial. To this end, the electronic structures of TiO2(110) and TiO2(011)-(2 × 1) have been systematically measured with two-photon photoemission spectroscopy. The results reveal the anisotropic nature of the electronic structure in rutile TiO2 at seemingly equivalent directions of [110] and [11̅0], the long axes of the TiO6 blocking unit. Although the resonant energy of these two d-d transitions is identical, the energy levels are systematically shifted by 0.1 eV. We propose this anisotropy originates from the broken symmetry of the rutile TiO2 crystals caused by the surface. The proposed asymmetry-caused electronic structure anisotropy could be generalized to other similar materials and may affect associated catalytic properties. This work provides an important benchmark for related calculations.

New Simulation Method for Dependency of Device Degradation on Bending Direction and Channel Length.

The dependency of device degradation on bending direction and channel length is analyzed in terms of bandgap states in amorphous indium-gallium-zinc-oxide (a-IGZO) films. The strain distribution in an a-IGZO film under perpendicular and parallel bending of a device with various channel lengths is investigated by conducting a three-dimensional mechanical simulation. Based on the obtained strain distribution, new device simulation structures are suggested in which the active layer is defined as consisting of multiple regions. The different arrangements of a highly strained region and density of states is proportional to the strain account for the measurement tendency. The analysis performed using the proposed structures reveals the causes underlying the effects of different bending directions and channel lengths, which cannot be explained using the existing simulation methods in which the active layer is defined as a single region.

Structural, Raman and optical investigations of barium titanate nanoparticles

Barium titanate nanoparticles were synthesized via a low temperature sol-gel technique using acetylacetone as a chelating agent. The X-ray diffraction pattern confirmed the formation of tetragonal perovskite structure of the material. The average crystallite size was found to be 49 nm using the Debye-Scherrer method. The Williamson–Hall plot was drawn for evaluating the micro-strain and crystallite size. Raman spectroscopic features typical for the tetragonal phase were identified for Barium titanate powder. In addition to these, an absorption band edge and an optical bandgap were found to be of 406 nm and 3.14 eV respectively. Barium titanate displays polychromatic luminescent peaks in UV -visible regions and it was attributed to the near-band-edge transition and defect states in the optical band gap of the perovskite structure.

Second‐Harmonic Generation Based on the Dual‐Band Second‐Order Topological Corner States

Coherent frequency conversions using multiband topological edge states in photonic structures made of nonlinear optical materials have been extensively investigated. In practice, however, coherent sources localized at the subwavelength scale are of great importance. Herein, efficient second‐harmonic generation (SHG) using dual‐band second‐order topological corner states is reported. By properly adjusting the structural parameters, dual‐band corner states can be obtained in the first and second bandgaps, when the eigenfrequency of the corner state in the higher bandgap is twice that of the corner one in the lower gap. In a square lattice made of tellurium as an example, high SHG efficiency in the order of 10−1% can be obtained for a peak pump intensity of around 20 GW cm−2. The results presented provide the realizability of subwavelength‐scale coherent sources, which are robust against the structural disorder or defects.

Photocatalytic degradation of methylene blue using polyaniline-based silver-doped zinc sulfide (PANI-Ag/ZnS) composites.

This study set out to determine the photocatalytic degradation potential of polyaniline-based silver-doped zinc sulfide composite (PANI-Ag/ZnS) for effective degradation of methylene blue. The heterogeneous photocatalytic experiments were carried out by irradiating aqueous dye solutions with ultraviolet light (UV-254 nm). The catalysts (ZnS, Ag/ZnS, PANI-ZnS, and PANI-Ag/ZnS) were prepared successfully and characterized by Fourier Transforms Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM), and Energy-dispersive X-ray diffraction (EDX). Combined with density functional theory calculations, a set of calculations has been performed for optimization, computation, and accuracy of the structure. After the optimization, the equilibrium lattice were a=b= (0.54447 nm), in good agreement with experimental results (a=b=c=0.54093 nm). Fermi energy levels Ef, indicating Ag-doped in ZnS as the impurity acceptor and for better visible-light photo-catalysis, narrow bandgap, and acceptor states are beneficial. The optimization of effective parameters like pH, catalyst dose, oxidant dose, dye concentration, and reaction time was carried out. The best degradation efficiency (> 95%) of PANI-Ag/ZnS composite against methylene was achieved within 60 min of reaction time under optimized conditions. The optimized conditions were recoded as follows: pH = 7, catalysts dose = 30 mg/L, oxidant dose = 3 mM, and irradiation time = 60 min under UV-254 nm for all catalysts. The central composite design (CCD) under the Response Surface Methodology (RSM) was chosen as a statistical tool to obtain the correlation of influential parameters. Five successive reusability trials were carried out to check the stability of catalysts.

Co-doped magnetic N-TiO2-x/rGO heterojunction@cellulose nanofibrous flakelet for enhanced photocatalytic oxidation and facile separation: Efficient charge separation and self-floatability

Visible-light-absorbing N-TiO2-x/rGO@cellulose nanohybrid photocatalyst with magnetism and self-floatability was fabricated via a facile in-situ nanocomposite strategy. The co-doping levels of nitrogen and oxygen vacancies formed bandgap states highly broadening the visible-light harvesting. The N-TiO2-x/rGO heterojunction remarkably enhanced the interfacial charge separation for excellent photocatalytic activity. Bisphenol A (10 mg·L−1) was photodegraded practically (96.2%) by this nanohybrid within 60 min through the synergistic oxidation via persulfate activation. Radical trapping experiments and EPR measurement revealed that the photo-induced superoxide radical (·O2–) and hydroxyl radical (·OH) were the predominant reactive oxidizing species. The nanohybrids were stably floating on aqueous solutions for an unprecedented long period of time (up to 5 months) and can be reused without difficult separation or photocatalytic attenuation. Contact angle measurement and thermodynamic model were combined to reveal the floatable nature. This work is therefore of interest for inexpensive and scalable manufacturing of multi-featured visible-light photocatalyst towards environmental remediation applications.

Impact of Cation Substitution in (AgxCu1−x)2ZnSnSe4 Absorber‐Based Solar Cells toward 10% Efficiency: Experimental and Theoretical Analyses

Solar cells based on kesterite Cu2ZnSnSe4 (CZTSe) compounds with earth‐abundant elements are highly desirable for the low‐cost and high‐efficiency production of renewable energy. However, the occurrence of intrinsic defects substantially impairs the photovoltaic properties of CZTSe. Herein, a cation substitution method to control and passivate the defect states in bandgap of kesterite CZTSe by incorporating Ag ions is introduced. Intensity‐dependent low‐temperature photoluminescence measurements show that Ag incorporation can reduce the density and depth of intrinsic defects in CZTSe. The results reveal that 10% Ag‐alloyed CZTSe provides the shallowest defect states and less nonradiative recombination. It is also confirmed by first‐principles calculations that Ag incorporation enables the formation and suppresses the beneficial and detrimental defects, respectively. Based on the theoretical results, the observed subband photoluminescence peaks can be assigned to the intrinsic point and cluster defects. The best power conversion efficiency of 10.2% is achieved for the 10% Ag‐alloyed CZTSe cell, along with an enhanced open‐circuit voltage. These results open up a new avenue for further improving the performances of CZTSe‐based device via defect engineering.

Substitutional Lithium Doping on Germanene: A First-principles Study

The density functional theory (DFT) based first-principles calculations have been adopted for the study of structural, electronic and magnetic properties of pure and single Lithium (Li) atom doped germanene monolayer. Due to the higher intrinsic carrier mobilities and large spin orbit gap, germanene has great possibility of being integrated into the silicon based semiconductor industry. Different studies have been done to change the band gap value from its’ pristine zero band gap state. We have doped the single Li atom into the germanene system with the intention of tuning the band gap and other electronic and magnetic properties. Band structure calculations show pristine germanene is semi-metallic in nature whereas the Li doped system is fully metallic with the overlapping of the conduction and valance bands in the Fermi level. Under density of states (DOS) calculations, it is evident that both pristine and doped system are non-magnetic in nature with symmetric DOS plot.

Effect of embedding of CrSi2 and β-FeSi2 nanocrystals into n-type conductivity silicon on the transport and thermal generation of carriers

The possibility of ordered embedding of CrSi2 and β-FeSi2 nanocrystals (NCs) into a Si(111) substrate of n-type conductivity has been demonstrated. The types of NCs and silicon matching were determined, and the types of the crystal lattice deformations of CrSi2 and β-FeSi2 NCs were established. For six-layer heterostructures with embedded CrSi2 and CrSi2 + β-FeSi2 NCs, a change in the Seebeck coefficient sign from negative to positive was observed for the first time in a 250–400 K temperature range, which indicates a nanocrystals to silicon hole injection. An estimate was made of the minimum number of holes, injected from a single nanocrystal, which are necessary to compensate for the thermo-emf from an n-type substrate. Band diagrams were constructed for the β-FeSi2-p NC/Si-n and CrSi2-p NC/Si-n heterojunctions, and the discontinuities in the valence and conduction bands were determined. The minimum barrier (0.065 eV) for holes in the Si-p/NC β-FeSi2-p/Si-n system indicates their thermal generation as the main mechanism. The main mechanism of hole injection from Si-p/NC CrSi2-p/Si-n system is tunneling through the surface states in the Si band gap due to the large discontinuity (1.075 eV) in the Si valence band.

Structural investigation and optical properties of Fe, Al, Si, and Cu–ZnTiO3 nanocrystals

Synthesis, morphology control, and diffuse reflectance of Fe/Al/Si/Cu–ZnTiO3 prepared using the sol-gel method and calcined at 750 °C have been inspected. The Fe, Al, Si, and Cu can interchange the Ti ions and form sols in the ZnTiO3 matrix. The microstructural and optical properties of the ZnTiO3 were affected by the dopant. X-ray diffraction patterns and FullProf software exhibited the forming of the ZnTiO3 rhombohedral phase. The introduction of Fe, Al, Si, and Cu in the rhombohedral ZnTiO3 structure results in trigonal and triclinic phases for Al and Si and monoclinic phases for Fe and Cu. The various dopants convince the topical ZnTiO3 surface morphology modification observed by SEM. The total TGA weight loss of about<3% through the heating from ambient to 700 °C for all tested samples reflects its high stability.The prepared nanocrystalline powders undergo both direct and indirect energy band transitions with a direct type, which is more probable where it needs lower energy. The bandgap value of the nanocrystalline powder is slightly increased by different doping unless Fe, which leads to its decrease. The doped samples show a significant shift in the bandgap due to the introduction of electronic level, which forms the lowest unoccupied molecular orbital within the bandgap states of ZnTiO3.

Topological electronic states and thermoelectric transport at phase boundaries in single-layer WSe2: an effective Hamiltonian theory

Monolayer transition metal dichalcogenides in the distorted octahedral 1T′ phase exhibit a large bulk bandgap and gapless boundary states, which is an asset in the ongoing quest for topological electronics. In single-layer tungsten diselenide (WSe2), the boundary states have been observed at well ordered interfaces between 1T′ and semiconducting (1H) phases. This paper proposes an effective 4-band theory for the boundary states in single-layer WSe2, describing a Kramers pair of in-gap states as well as the behaviour at the spectrum termination points on the conduction and valence bands of the 1T′ phase. The spectrum termination points determine the temperature and chemical potential dependences of the ballistic conductance and thermopower at the phase boundary. Notably, the thermopower shows an ambipolar behaviour, changing the sign in the bandgap of the 1T′–WSe2 and reflecting its particle-hole asymmetry. The theory establishes a link between the bulk band structure and ballistic boundary transport in single-layer WSe2 and is applicable to a range of related topological materials.

Investigation of structural, spectral, optical and nonlinear optical properties of nanocrystal CdS: Electrodeposition and quantum mechanical studies

Nanocrystalline CdS semiconductor was synthesized by electrodeposition technique, and characterized by X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscope (SEM), and FTIR spectroscopic methods. XRD analysis result showed that it is a hexagonal crystal structure. SEM micrographs showed that the CdS nanocrystal is homogeneously distributed on the surface. The roughness of the CdS thin films was measured by AFM. The presence of band in the FTIR spectrum at 558 cm−1 corresponds to the stretching of CdS. CdS nanocrystals were modeled separately and calculated in the Gaussian program. Structural, spectroscopic, and nonlinear optical (NLO) properties were investigated using the DFT/Lanl2dz and semi-empirical/pm6 level. Besides, by making energy calculations, Homo, Lumo, band gap, and total state density (TDOS) were calculated. The experimental band gap was measured at 2.40 eV, while this calculated for hexagonal and cubic structure as 2.21 and 2.01 eV with semiemprical/pm6 model, 2.44 and 2.25 eV with DFT/lanl2dz level.These results support that CdS nanostructures are crystallized in hexagonal structure. With this study, the obtaining of nano structured CdS films by electrodeposition method, their structural, optical and surface properties were experimentally examined, supported by theoretical calculations, and additionally NLO properties were investigated. It has been determined that the nanocrystals obtained are suitable material for optoelectronic applications.

Optical absorption and linear/nonlinear parameters of polyvinyl alcohol films doped by fullerene

New polymeric films were developed for optoelectronic devices by an interaction between Fullerene (C60) and polyvinyl alcohol (PVALC) via the casting method. The structure analysis was studied by Gaussian fitting of XRD patterns and SAED (Selected area electron diffraction). Using HRTEM and SEM, the particle shape and the nanocomposite surface on PVALC are investigated. The change in the functional group of the composites was observed via FT-IR and Raman spectroscopy. In this work, we calculated the bandgap and localized state's width, in the forbidden band, via the absorption coefficient obtained from the Beer-Lambert relation. The bandgap of nanocomposite films was reduced to 4.05 eV. Moreover, due to the surface plasmon absorption in nanocomposite films, k (Extinction index) increases with the doping concentration. The bandgap and refractive index (n) relations were studied via various empirical formulas to calculate the average value of n. The linear optical parameters, such as 1st-order susceptibility (χ(1)), high-frequency dielectric (e∞), and static dielectric (eo) constants, were calculated. The nonlinear susceptibility χ(3) raised from 0.928 × 10−12 esu to 1.779 × 10−12 esu. Also, the refractive index (n2) enhanced from 1.665 × 10−11 esu to 2.993 × 10−11 esu. The optical limiting performance and the ability of nanocomposite to absorb laser beams were tested. The result suggests that C60 has a considerable effect on the PVALC matrix, and the samples have the necessary guidance for laser CUT-OFF and photonic applications.

Morphology Control in 2D Carbon Nitrides: Impact of Particle Size on Optoelectronic Properties and Photocatalysis

AbstractThe carbon nitride poly(heptazine imide), PHI, has recently emerged as a powerful 2D carbon nitride photocatalyst with intriguing charge storing ability. Yet, insights into how morphology, particle size, and defects influence its photophysical properties are virtually absent. Here, ultrasonication is used to systematically tune the particle size as well as concentration of surface functional groups and study their impact. Enhanced photocatalytic activity correlates with an optimal amount of those defects that create shallow trap states in the optical band gap, promoting charge percolation, as evidenced by time‐resolved photoluminescence spectroscopy, charge transport studies, and quantum‐chemical calculations. Excessive amounts of terminal defects can act as recombination centers and hence, decrease the photocatalytic activity for hydrogen evolution. Re‐agglomeration of small particles can, however, partially restore the photocatalytic activity. The type and amount of trap states at the surface can also influence the deposition of the co‐catalyst Pt, which is used in hydrogen evolution experiments. Optimized conditions entail improved Pt distribution, as well as enhanced wettability and colloidal stability. A description of the interplay between these effects is provided to obtain a holistic picture of the size–property–activity relationship in nanoparticulate PHI‐type carbon nitrides that can likely be generalized to related photocatalytic systems.

Promoted charge separation from nickel intervening in [Bi2O2]2+ layers of Bi2O2S crystals for enhanced photocatalytic CO2 conversion

Improving the separation efficiency of charge carriers and abilities for CO2 adsorption and activation of photocatalysts is still a challenge. Herein, we successfully introduced Ni2+ into [Bi2O2]2+ layers in Bi2O2S crystals via a facile hydrothermal reaction, which markedly enhanced their photocatalytic CO2 reduction performance. With full-spectrum light irradiation, the photocatalytic CO2 reduction activity of the optimized sample NBOS-0.24 is ca. 13 times and 8 times higher than that of pristine Bi2O2S under air and 99 % CO2 atmosphere, respectively. Both experimental results and theoretical calculations verify that implanting of Ni2+ into [Bi2O2]2+ layers generates new band gap states and increases the concentration of oxygen vacancies in Bi2O2S photocatalyst, which boosts the photogenerated charge separation efficiency and greatly improves the CO2 adsorption and activation capacities over the photocatalyst. This work shows a promising [Bi2O2]2+-containing photocatalyst and reveals the mechanism of introducing metal ions to boost its photocatalytic CO2 reduction performance.