Band Gap Difference Research Articles

Insights into multi-scale structural evolution and dielectric response of poly(methyl acrylate) under pre-strain: A simulation study.

The structural evolution of dielectric elastomer induced by pre-strain is a complex, multi-scale process that poses a significant challenge to a deep understanding of the effect of pre-strain. Through simulation results, we identify the variation in the dielectric constant and multi-scale (electronic structure, molecular chain conformation, and aggregation structure) response of poly(methyl acrylate). As the pre-strain increases, the dielectric constant initially rises (below 200% pre-strain) and then declines (above 200% pre-strain). Analysis of the charge distribution, surface electrostatic potential, HOMO-LUMO bandgap, and electron density differences reveal that adjusting chain conformation appropriately could enhance polarity domain and electron polarization. The correlation between permittivity and segment dynamics of deformed molecules is explored, encompassing segment orientation, mean shift displacement, and diffusion coefficient. Following molecular chain orientation, the kinematic capability of the chain segment improves, which leads to an increase in the number and activity of effective dipoles and the enhancement of orientation polarization. Excessive stretching restricts the polymer molecular chain mechanically, reducing the number and activity of effective dipoles and negatively impacting electron polarization. The permittivity transitions from isotropic to anisotropic behavior when the system is subjected to strain. This study provides an interesting solution for research on multiscale responses and intrinsic mechanisms of pre-strain.

P25 and Rutile Titania in Photocatalytic Degradation of Phenol and Methylene Blue

Phenolic compounds, dyes, and their derivatives are prevalent organic pollutants in surface waters due to industrial activities, effluent discharge, and agricultural runoff. In this context, this study aimed to evaluate the photocatalytic activity of commercial P25 TiO2 (TiO2-P25) and rutile TiO2 (TiO2-R) solids for the degrading phenol and methylene blue dye in a batch reactor under UV-C irradiation at 256 nm. X-ray diffraction data confirm the predominant presence of the anatase phase in TiO2-P25 and the rutile phase in TiO2-R. Significant difference in crystallite size and specific area (SBET) between the samples were linked to their commercial synthesis method. The results demonstrated that TiO2-P25 exhibited higher photocatalytic activity than TiO2-R due to differences in band gap. The use of a 15W lamp further enhanced this efficiency. For phenol degradation, TiO2-P25 obtained 57% conversion at 9W and an 11% increase in the initial concentration at 15W, while TiO2-R showed lower efficiency. In photolysis, minimal conversion was observed. Total organic carbon (TOC) data suggest that photocatalysis generates organic metabolites rather than achieving complete mineralization, leading to the accumulation of intermediates in the effluent, particularly with the 15W lamp.

Design and performance analysis of novel perovskite/CZTSe hybrid solar cell for high efficiency

With newer developments, the solar cells in the market need to be more efficient, affordable, stable, and environment friendly. The perovskite solar cells have already shown very impressive progress in performance since their inception. It has already reached 26 % efficiency and is expected to reach more, owing to its stupendous photovoltaic nature. On the other hand, even with a very high absorption coefficient, the CZTSe solar cell has not crossed the 15 % efficiency mark. Looking at the characteristics of both the materials and considerable differences in band gap, a novel hybrid structure, HTL/Perovskite/CZTSe/CdS/ZnO, has been proposed in this work to implement Perovskite (CH3NH3Pb3−xClx) and CZTSe layer to absorb higher energy and lower energy photons respectively. In this context, choosing a proper Perovskite material becomes more crucial to have the required band alignment for the easy transportation of the excitons. The performance of the cell has been observed in varying parameters like thickness, doping, defects, and temperature. A high efficiency of 30.9 % is achieved which is much higher than the contemporary solar cells. Several other output parameters like Short Circuit Current Density (JSC), Open Circuit Voltage (VOC), Fill-Factor (FF%) characteristics have also been discussed.

Layer stacking effect on structural, vibrational, and electronic properties of Janus-Ga2SeTe crystals

The van der Waals stacking of Janus Ga2SeTe quadruple-atomic layers leads to the formation of polytype bulk crystals, where the stacking order plays a critical yet elusive role in their fundamental properties. Here, we perform first-principles density functional theory calculations to investigate the stacking-dependent structural, electronic, and vibrational properties of four Ga2SeTe polytypes. Our results indicate that the γ polytype has two sub-phases of γ [ABC] and γ [ACB]. Lattice vibration studies manifest that symmetry operations depend on the stacking orders, in which the irreducible representations of optically active modes at Γ for the β, ε, γ, and δ phases are Γ = 3A1 + 4B1 + 4E2 + 3E1, Γ = 7E+7A1, Γ = 11A1 + 11E, and Γ = 8E2 + 7E1 + 8B1 + 7A1, respectively. Moreover, taking spin–orbit coupling into account, the estimated indirect bandgaps are 0.967, 0.926, 0.879, 0.925, and 0.950 eV for β, ε, γ [ABC], γ [ACB], and δ polytypes, respectively, in which the bandgap differences between indirect and direct transitions are within 10 meV. The Bader charge analysis indicates that the stacking order modulates the charge distribution among the compositional layers. These findings provide valuable insights for the development of optoelectronic devices based on Janus structures.

Mechanical and Electronic properties of ZnIV1-xIVxN2 (IV=Si, Ge and Sn) with varied concentrations: First-principles calculations

This study employed first-principles calculations to explore the mechanical and electronic properties of ZnIV1-xIVxN2 (IV=Si, Ge, Sn) at varying concentrations. Adding Si enhances elasticity, brittleness, and hardness by reinforcing Zn/IV-N ionic bonding, while Sn addition diminishes these properties. Electronic band structures indicate that antibonding states mostly arise from IV p orbitals hybridized with N s orbitals, impacting energy bandgaps. Bader charge analysis confirms enhanced ionic bonding with Si addition and weakened bonding with Sn, consistent with IV ion sizes and charges influencing bond lengths and hybridization strengths. Our study suggests that the ductility of ZnIVN2 can be improved while maintaining its UV absorption properties by using ZnGe1-xSixN2 with x = 0.25,0.50 and ZnSi1-xSnxN2 with x = 0.25, which is dynamically stable. These compounds exhibit energy gaps of 3.29–3.83 eV and 3.52 eV, respectively. In ZnGe1-xSixN2, the energy gap ranges from 4.62 to 2.82 eV, while in ZnGe1-xSnxN2, it spans from 1.16 to 2.82 eV. For ZnSi1-xSnxN2, the energy gap extends from 4.62 to 1.16 eV. The Sn p + N s hybridization is weakest compared to the Ge p + N s and Si p + N s hybridizations, respectively, resulting in a lower energy antibonding state for ZnSnN2 > ZnGeN2 > ZnSiN2. This leads to the observed bandgap differences.

(Invited) Ultrathin Gap Nanoantenna: Crystal Phase Transitions and Luminescent Properties

We report Te-doped GaP nanowires (NWs) with positive tapering and radii measuring as low as 5 nm grown by the self-assisted vapor-liquid-solid mechanism using selective-area molecular beam epitaxy. The occurrence of the ultrathin nanoantenna showed a dependence on pattern pitch (separation between NWs) with a predominance at 600 nm pitch, and exhibited radius oscillations that correlate with polytypic zincblende/wurtzite segments. A growth model explains the positive tapering of the NW leading to an ultrathin tip from the suppression of surface diffusion of Ga adatoms on the NW sidewalls by Te dopant flux. The model also provides a relationship between the radius modulations and the oscillations of the droplet contact angle, predicting the quasi-periodic radius oscillations and corresponding crystal phase transitions. The GaP NWs showed strong low temperature micro-photoluminescence exciton-related emission, indicating a bandgap difference between the zincblende/wurtzite segments, with phonon replicas due to the transverse optical phonon mode. These results establish a link between dopants and the ability to control NW morphology, crystal phase and luminescent properties with possible applications in thermoelectrics, quantum emitters and photodetectors.

Self-Powered FTO/CdSe/Bi2Se3 Photodetector with Bipolar Photoresponse Characteristics.

Self-powered photodetectors with bipolar photoresponse characteristics are expected to play a critical role in the field of secure optical communication, artificial neuromorphic systems, and intelligent color sensors. In this work, asymmetric heterojunction devices exhibiting wavelength-dependent bipolar photoresponse with a structure of Glass/FTO/CdSe/Bi2Se3/Au were fabricated. Under a short wavelength light irradiation, the top CdSe absorber generates a high carrier concentration; the excited carriers are quickly separated by the built-in electric field induced by the FTO/CdSe diode, resulting in a negative photocurrent. For light with wavelengths beyond the CdSe absorption edge, it is absorbed by the bottom Bi2Se3 absorber, and a positive photocurrent can be observed. Therefore, based on the bandgap difference between the top CdSe absorber and the bottom Bi2Se3 absorber, combined with the photogenerated carriers separated by asymmetric back-to-back diode, a wavelength-dependent bipolar response is realized. In this work, by employing this structure, the responsivities of -33.3 and 0.3 mA/W were achieved under the illumination of 405 and 830 nm, respectively. This work provides important indications in the preparation and performance optimization for wavelength-dependent bipolar photodetectors.

Electricity generation via metal oxide-air moist interaction

Harvesting electricity from ambient moisture and water droplets through the innovative technology of hydrovoltaics has lately gained attraction as it only requires water molecules' intrinsic energy and is applicable in different weather conditions. We present a device prototype for electricity generation from air humidity. The study investigates the complex interplay of water-solid interactions, focusing on band gap differences of n-type and p-type couple metal oxides and their performance in electricity generation. The research develops Moist-Electric Generators (MEGs) using ZnO as an n-type metal oxide with various p-type metal oxides (MnOx, CuO, and CoOx) on carbon paper substrates. Among these, the ZnO/CoOx MEG exhibits superior performance, producing an open circuit voltage of nearly 0.5 V, a current of around 9.5 µA for more than 7 days, with an areal and maximum power density of 1.48 µWcm2- and 4.5 µWcm2- at 95 % RH. Detailed characterization techniques, including XRD, XPS, and SEM, confirm the successful synthesis of ZnO and CoOx electrodes. The study demonstrates the sensitivity of the ZnO/CoOx MEG to changes in relative humidity and investigates its long-term stability. The results highlight the significance of water dissociation and minor redox processes for efficient energy generation, paving the way for advanced applications in renewable energy technologies.

Construction of MoS2/NiS2 heterostructure with fast interfacial reaction kinetics for ultrafast sodium storage

Constructing heterostructure is a valid method to reinforcing sodium storage performance of transition metal chalcogenides materials. Herein, a simple, safe and controllable one step hydrothermal method is proposed to synthesize MoS2/NiS2 heterostructure. Due to the difference in band gaps and work functions of MoS2 and NiS2, the charges are redistributed at the MoS2/NiS2 heterointerfaces, thereby accelerating the migration of electrons and Na+. The heterointerfaces provide extra active sites for storing Na+, thus increasing the sodium storage capacity of the heterostructure. Furthermore, the distinct redox potentials of NiS2 and MoS2 promote the structural stability of MoS2/NiS2 heterostructure during the electrochemical reaction processes. Consequently, the obtained MoS2/NiS2 heterostructure exhibits superior rate properties (339.4 mAh g−1 at 10 A g−1) and ultra-stable cycling stability (480.5 mAh g−1 after 350 cycles at 1 A g−1). This paper presents a valid strategy for creating heterostructure anodes with excellent sodium storage properties.

Constructing ZnSe/CoSe2/N-doped carbon composite with multiple heterointerfaces using a solvent-assisted strategy to enhance lithium storage

Interface engineering, as an effective strategy for preparing high power density and long-life battery anodes, can promote the electrochemical performance of materials. Herein, metal organic frameworks (MOFs) derived N-doped carbon nanosheets loaded nanoscale ZnSe/CoSe2 particle composite with multiple heterointerfaces is fabricated by solvent assisted strategy. Wherein the content of water in the solvent is a key factor in establishing a homogeneous heterojunction containing cubic phase CoSe2 (c-CoSe2) and orthorhombic phase CoSe2 (o-CoSe2). The built-in electric field generated by the energy band gap difference between ZnSe and CoSe2 or c-CoSe2 and o-CoSe2, and a strong interaction between ZnSe/CoSe2 particles and N-doped carbon nanosheets, can reduce the migration barrier of Li+ at the interface, accelerate charge transfer, and promote the lithium storage kinetic process. In addition, nanoscale ZnSe/CoSe2 particles coupled with N-doped carbon nanosheets can also generate more active sites, reduce volume expansion, and maintain the structural integrity of the anode material. Based on the above structural advantages, the prepared ZnSe/CoSe2-3# exhibits a high reversible capacity (1188 mAh g−1 after 280 cycles at 0.5 A g−1), and excellent long-term cycling stability (788 mAh g−1 at 1 A g−1 over 600 cycles).

Data-driven strategy for bandgap database construction of perovskites and the potential segregation study

Light-induced segregation limits the practical application of mixed halide perovskites in solar cells. Herein, halide segregation is evaluated by a data-driven approach with constructing a bandgap database of 53,361 mixed ABX3 [where A = Cs, formamidinium (FA) or methylammonium (MA); B = Pb or Sn; X = Br, Cl, or I] perovskites. A transfer learning strategy was employed to fine-tune the parameters of a Graph Neural Network model using experimental and density functional theory (DFT)-calculated bandgaps. This approach accelerated the construction of a unique database, distinguishing it from others primarily focused on ABX3 perovskite element substitution. The database is characterized by continuously varying compositions and accurate bandgaps. It was utilized to calculate the free energy of 20,688 mixed iodine-bromine perovskites and generate corresponding phase diagrams for predicting their light-induced segregation behavior. It is found that the bandgap increases with decreasing ionic radii at the A-site and X-site. This composition-dependent bandgap difference drives halide segregation. Moreover, using a higher Cs content at the A-site, rather than MA, reduces this bandgap difference, enhancing photostability. The proposed data-driven strategy can facilitate the targeted design of novel perovskites with mixed compositions and the investigation of halide perovskite segregation.

Eu2+-Eu3+ Co-doping provoking polyhedron-cluster-based emission structural unit evolution triggering phases transition and blue-green-red multimode luminescence emission

A series of Ca2-xSiO4:xEu2+/Eu3+ and Ca2-2xSiO4:xEu2+/Eu3+, xM (M = Na+, K+) phosphors were prepared by co-doping Eu2+-Eu3+ in Ca2SiO4 solid solution for selectively locating Eu2+-Eu3+ ions deriving polyhedron-distorted emitting structure units’ evolution. During β, γ1,2-C2S matrix phase transitions, the structure and luminescence properties were analyzed by XRD, diffuse reflection, PL and luminescence thermal stability. The results show that Eu2+/Eu3+ occupying 6 Ca2+ sites forms 10 emission structural units in the matrix and the phase transition of γ1-Ca2-xSiO4(x = 0.0001–0.0004)→β-Ca2-xSiO4 (x = 0.0005–0.04)→β, γ2-Ca2-xSiO4 mixed phase (x = 0.08–0.32) is generated by the extrusion of structural units. Ultraviolet–visible diffuse reflectance spectra confirm the band gap difference caused by structural unit rotation. According to the bond energy method, the priority occupation order for the Eu2+/3+ ions was determined. The energy transfer of Eu2+→Eu3+ occurs when the structural units 3–10 of β, γ2-Ca2-xSiO4 mixtures coexist. After co-doped M+ as charge compensator, the luminescence intensity/thermal stability of Ca2-2xSiO4:xEu3+, xM(M = Na+, K+) were improved by 33.88 %/57.12 %, and 5.1 %/6.6 % than that of Ca2-xSiO4:xEu3+, respectively. These characteristics indicate that phosphors have broad application prospects in the field of solid-state lighting.

Facile and rapid synthesis method of dielectric nanocomposite for simultaneous enhancement of electromagnetic wave absorbing/shielding characteristics

The present study involves the fabrication of poly[4-(2-thiophene)aniline] (PTA) nanocomposite for absorption-dependent shielding applications, demonstrating enhanced absorption and small reflection properties. The Microwave absorption characteristics of the fabricated polymer were studied at different loading ratios, resulting in a minimum reflection loss (RLmini) of − 44 dB for the 8 wt % loading ratio. The macromolecular architecture of the PTA material offers higher dielectric loss resulting from higher conduction and interfacial loss. Additionally, the shielding performance is also evaluated, and the sample containing 8 wt % filler demonstrates an impressive − 36.5 dB shielding efficacy. The performance of polymers is significantly impacted by their unit periodic order arrangement. Conducting polymers with linear structure and special π-orbitals angles possess unique characteristics in comparison to their branched and the random copolymer, making them highly sought after as efficient organic conductors, magnetic substances, electrochromic materials, and high dielectric agents. Monomer units contained two periodical cyclic structures and the creation of the inductive effects of heteroatoms elongates the conjugation and reduces the energy bandgap difference between the HOMO and LUMO levels, enabling smoother electron flow and enhancing absorption capacity. The findings indicate that PTA nanocomposite is a promising radar-absorbing coating material due to its superior absorption ability and outstanding shielding performance.

Ultrafast Optical Encoder Using Photonic Crystal Ring Resonator

ABSTRACT In this paper, we design and simulate a two-dimensional photonic crystal-based high speed logical encoder for optical computing application. The encoder consists of a ring resonator, four waveguides and two reflectors arranged in a hexagonal lattice platform. The optical encoder works on the interference effect which guides the propagation of light in the desired path. The light behavior is simulated with four different logic states. The Plane Wave Expansion (PWE) method is used to analyze the band gap and Finite Difference Time Domain (FDTD) analyzes the performance characteristics of the encoder, such as bit rate, delay time, and contrast ratio. The proposed structure has a footprint area of 143.84 µm2. The small compact size structure with simple defect paths reduces the encoder delay time, which in turn increase the data bit rate. The response time of the proposed encoder is 0.119 ps and bit rate is 8.403 Tbps. The encoder is designed with very low output power in “0” logic state and high output power in “1” logic state leading to high contrast ratio of 22.57 dB. Therefore, highly efficient 4 × 2 optical encoder is suitable to build in optical integrated circuits.

An Optimized Device Structure with Improved Erase Operation within the Indium Gallium Zinc Oxide Channel in Three-Dimensional NAND Flash Applications

In this paper, we propose an optimized device structure to address issues in 3D NAND flash memory devices, which encounter difficulties when using the hole erase method due to the unfavorable hole characteristics of indium gallium zinc oxide (IGZO). The proposed structure mitigated the erase operation problem caused by the low hole mobility of IGZO by introducing a filler inside the IGZO channel. It facilitated the injection of holes into the IGZO channel through the filler, while the existing P-type doped polysilicon filler material was replaced by a P-type oxide semiconductor. In contrast to polysilicon (band gap: 1.1 eV), this P-type oxide semiconductor has a band gap similar to that of the IGZO channel (2.5 to 3.0 eV). Consequently, it was confirmed through device simulation that there was no barrier due to the difference in band gaps, enabling the seamless supply of holes to the IGZO channel. Based on these results, we conducted a simulation to determine the optimal parameters for the P-type oxide semiconductor to be used as a filler, demonstrating improved erase operation when the P-type carrier density was 1019 cm−3 or higher and the band gap was 3.0 eV or higher.

Nonvolatile Isomorphic Valence Transition in SmTe Films.

The burgeoning field of optoelectronic devices necessitates a mechanism that gives rise to a large contrast in the electrical and optical properties. A SmTe film with a NaCl-type structure demonstrates significant differences in resistivity (over 105) and band gap (approximately 1.45 eV) between as-deposited and annealed films, even in the absence of a structural transition. The change in the electronic structure and accompanying physical properties is attributed to a rigid-band shift triggered by a valence transition (VT) between Sm2+ and Sm3+. The stress field within the SmTe film appears closely tied to the mixed valence state of Sm, suggesting that stress is a driving force in this VT. By mixing the valence states, the formation energy of the low-resistive state decreases, providing nonvolatility. Moreover, the valence state of Sm can be regulated through annealing and device-operation processes, such as applying voltage and current pulses. This investigation introduces an approach to developing semiconductor materials for optoelectrical applications.

Effect of interfacial electronic structure on conductivity and space charge characteristics of core-shell quantum dots/polyethylene nanocomposite insulation

To investigate the effect of the interface electronic structure of core-shell quantum dots on the conductivity and space charge characteristics of polyethylene insulation, nanocomposite insulations, namely CdSe@ZnS/LDPE and ZnSe@ZnS/LDPE, are synthesized. The study focuses on elucidating the evolution patterns of DC conductivity and space charge in the nanocomposite insulation, and analyzing the effect of the interfacial electronic structure of core-shell quantum dots on the distribution of charge traps. Comparative analysis reveals that in contrast to LDPE insulation, ZnSe@ZnS/LDPE nanocomposite insulation demonstrates a substantial reduction in DC conductivity by 47.2% and a decrease in space charge accumulation by 40.3% under the conditions of elevated temperature and strong electric field. The increase of trap energy level means an enhanced trap effect on charger carriers. According to density functional theory, the band structure characteristics of core-shell quantum dots integrated with polyethylene are computationally assessed. The findings underscore that the band misalignment at the core-shell interface and the shell-insulation interface induces shifts in the conduction band bottom and at the valence band top, respectively. These shifts impose a confinement effect on electrons and holes, with the extent of this effect escalating with the augment of the difference in band gap between the core layer and the shell layer. Consequently, this phenomenon curtails carrier migration, thereby inhibiting space charge accumulation under the conditions of elevated temperature and strong electric fields.

Computational Modulation in Electronic Structures of Halide Perovskites via Element/Dopant/Phase.

This study employs computational chemistry to investigate the electronic properties of halide perovskite materials, focusing on structural frameworks, elemental composition, surface engineering, and defect engineering. The tetragonal phase generally exhibits higher band gaps than the cubic phase due to conduction band differences, with LiPbCl3 showing the greatest band gap difference. The ionic radius of the A element influences band gaps for both phases, with Cs having the highest impact. Surface engineering significantly affects the electronic properties, and surface direction and composition play vital roles in determining band gaps. Defect engineering induces semiconducting-to-metallic transitions, impacting band gaps. Understanding these core variables is crucial for tailoring the electronic properties of halide perovskites for photovoltaic and optoelectronic applications.

Study of Chemical Polymerization of Polypyrrole with SDS Soft Template: Physical, Chemical, and Electrical Properties.

Polypyrrole (PPy) is a conductive polymer known for its biocompatibility and ease of synthesis. Chemically polymerized PPy was synthesized in the presence of sodium dodecyl sulfate (SDS), showing correlations among chemical properties, physical morphology, and electrical properties. Focused synthesis parameters included the pyrrole (Py) concentration, SDS concentration, and ammonium persulfate (APS)/Py ratio. The addition of SDS during chemical polymerization influenced the physical morphology of PPy by altering the self-assembling process via micelle formation, yielding sheet-like morphologies. However, the phenomenon also relied heavily on other synthesis parameters. Varying SDS concentrations within the 0.01 to 0.30 M window produced PPy sheets with no significant difference in optical band gap or physical size. While using 0.10 M SDS, an increase in Py concentration from 0.10 to 0.30 M yielded a larger size of PPy as the morphology changed from sheet-like to irregular shape. The band gap dropped from 2.35 to 1.10 eV, and the conductivity rose from 6.80 × 10-1 to 9.40 × 10-1 S/m. With an increase in the APS/Py ratio, the PPy product changed from a random to a sheet-like form. The product provided a larger average size, a decreased band gap, and increased electrical conductivity. Py polymerization in the absence of SDS revealed no significant change in shape or size as the Py concentration increased from 0.10 to 0.30 M; only a sphere-like form was observed, with a large band gap and small conductivity. Results from Raman spectral analysis indicated a correlation between optical band gap, physical morphology, and bipolaron/polaron ratio, mainly at the wavelengths associated with C-C stretching and C-H deformation. The increase in average size was associated with a decrease in band gap and resistance as well as an increase in the bipolaron/polaron ratio. This work indicates a strong correlation between size, morphology, electrical properties, and the bipolaron/polaron ratio of PPy in the presence of SDS.

The effect of oxygen vacancies on the color formation of aquamarine

Oxygen vacancy is an inherent defect of aquamarine crystals. The concentration and properties of this defect have an essential influence on aquamarine crystals and optical properties. This paper reports that blue in aquamarine crystals is due to the interaction between Fe2+ and oxygen vacancy. Infrared and Raman spectra showed that more Fe2+ replaced Al3+ in dark sample A than in light sample B. Oxygen vacancies were found in all samples. Dark sample A strongly absorbed the 830 nm band associated with Fe2+ in the UV–Vis–NIR absorption spectrum. The band gap of dark sample A was smaller than that of light sample B, and the difference in band gap was related to the concentration of oxygen vacancies. X-ray Photoelectron Spectroscopy showed that the oxygen vacancy concentration in sample A was higher than that in sample B, which showed that the oxygen vacancy concentration affected the color depth of aquamarines.