Band Gap Of Crystal Research Articles

Introduction of a planar defect into colloidal photonic-crystal films and their optical properties

A defect mode in the bandgap of photonic crystals is a key factor for potential applications, emission, bandpass filter, sensor, and low throughput laser. A Fabry-Perot type cavity was known as a multilayer film with a planar defect or a one-dimensional colloidal photonic crystal film with a planar defect. In this work, we have developed a simple and easy method by two colloidal crystals, i.e., face center cubic (fcc) structure, films bonded together by hot pressing to form a sandwich structure, and clear defect mode was observed in the photonic bandgap of fcc (111) direction. We have investigated the effect of the thickness of the defect layer in the sandwich structure on the optical properties. A single or double dips appeared in Bragg's diffraction peak at different planar defect thicknesses. In addition, a simulation of the reflection spectra of multilayer film calculation showed the defect mode is much influenced by the planar defect thickness.

Study on bandgap of a novel phononic crystal with low-frequency sound insulation

To solve the problem of low-frequency noise in the environment, a two-dimensional Helmholtz-type phononic crystal with a labyrinth tube was designed in the paper. First, the low-frequency band structure was calculated by the finite element method (FEM) and transfer matrix method (TMM). Second, the bandgap formation was analyzed by using an acoustic pressure field, and the “spring-oscillator” equivalent model of the structure was established. Finally, the influences of structural parameters on the first bandgap were investigated. Results show that there are four bandgaps in the frequency range of 0–300 Hz, and the lower limit of the first bandgap can be as low as 12.15 Hz, which improves the low-frequency sound insulation ability of phononic crystals of the same size. The calculation results of the two methods (FEM and TMM) are basically consistent. Research on the influencing factors of the bandgap shows that the increase in the length of the tube will reduce the upper and lower limits of the bandgap and narrow the bandgap width. With the increase of the lattice constant, the upper limit of the bandgap decreases, while the lower limit of the bandgap remains unchanged. The design provides a new method to solve the problem of low-frequency noise reduction.

Formation of van der Waals Stacked p–n Homojunction Optoelectronic Device of Multilayered ReSe2 by Cr Doping

AbstractThe formation of p‐ or n‐type material via impurity doping should be crucial and essentially prior to the establishment of junction devices in semiconductor processing. Especially in a 2D transition‐metal dichalcogenide (TMD), dopant selection for growing p‐ and n‐type TMD semiconductors may suffer much higher difficulty and complexity than conventional Si and III–V compounds owing to the complicated valences occurred in transition metals. Different amount of chromium doped in ReSe2 interestingly showing dissimilar carrier types of p‐ReSe2 with Cr 10% and 20% doping and n‐ReSe2 with Cr 0%, 1%, and 5% doping, respectively, is presented here. According to structural and optical characterization, the crystal structure and bandgap of the Cr‐doped ReSe2 remain unchanged in which a deeper donor of Cr6+ can exist in n‐ReSe2 (1% and 5%) while a shallow acceptor of Cr3+ may appear in p‐ReSe2 multilayer (10% and 20%). A p–n homojunction light‐emitting diode made by stacking multilayered n‐ReSe2 (Cr 0%) and p‐ReSe2 (Cr 20%) is first fabricated, and it emits an electroluminescence of ≈946 nm from the band edge. Another p–n homojunction solar cell is also manufactured to exhibit a maximum axial conversion efficiency of η ≈ 1.05% along with the b‐axis polarization of ReSe2.

Doping-Induced Electronic/Ionic Engineering to Optimize the Redox Kinetics for Potassium Storage: A Case Study of Ni-Doped CoSe2.

Heteroatom doping effectively tunes the electronic conductivity of transition metal selenides (TMSs) with rapid K+ accessibility in potassium ion batteries (PIBs). Although considerable efforts are dedicated to investigating the relationship between the doping strategy and the resulting electrochemistry, the doping mechanisms, especially in view of the ion and electronic diffusion kinetics upon cycling, are seldom elucidated systematically. Herein, the crystal structure stability, charge/ion state, and bandgap of the active materials are found to be precisely modulated by favorable heteroatom doping, resulting in intrinsically fast kinetics of the electrode materials. Based on the combined mechanisms of intercalation and conversion reactions, electron and K+ ion transfer in Ni‐doped CoSe2 embedded in carbon nanocomposites (Ni‐CoSe2@NC) can be significantly enhanced via electronic engineering. Benefiting from the synthetic controlled Ni grains, the heterointerface formed by the intermediate products of electrochemical reactions in Ni‐CoSe2@NC strengthens the conversion kinetics and interdiffusion process, developing a low‐barrier mesophase with optimized potassium storage. Overall, an electronic tuning strategy can offer deeper atomic insights into the conversion reaction of TMSs in PIBs.

Synthesis and characterization of biologically active novel structured cadmium barbiturate single crystal with good thermal stability

A novel structured cadmium complex based on barbiturate ligand has been synthesized at room temperature by gel diffusion technique. The prepared crystals were characterised by FTIR, PXRD, thermogravimetric and single crystal X-ray diffraction analysis. SCXRD analysis reveals that the complex belongs to orthorhombic crystal structure with space group Ibam. The optical properties were studied from UV-Visible and photoluminescence spectra. The band gap of the CdB crystals was found to be 3 eV by tauc's plot and the corresponding refractive index obtained is 1.244. The crystal shows broad emission than that of its ligand (BA) in the entire visible region. The color corresponding to maximum emission of both ligand and CdB crystals were indicated by CIE chromaticity diagram. The response of CdB crystal against applied electric field is characterised by dielectric measurements. The values of penn gap and Fermi energies were calculated. The antimicrobial activities of the complex CdB against various bacteria have been investigated and then compared it with the corresponding ligand (BA). The results showed that the complex had stronger antibacterial activities than the ligand.

Impact of arrangement, length and chemical potential on the robustness of graphene induced photonic bandgap in photonic crystals

The study of photonic crystals, artificial materials whose dielectric properties can be tailored according to the stacking pattern of their constituents, remains an attractive research area. Very recently, the propagation of light waves in periodic and quasiperiodic graphene embedded dielectric multilayers has also been considered. The presence of graphene between consecutive layers induces the emergence of the so-called graphene induced photonic bandgap. In this article, we employ a transfer-matrix treatment to study the effects of (i) random arrangement of the layers, (ii) length of the multilayer and (iii) chemical potential of graphene on the robustness of the graphene induced photonic bandgap (GIPBG) in one dimensional graphene-embedded photonic crystals. The photonic crystals considered here are composed of two dielectric materials, namely, silicon dioxide (layer A=SiO2) and titanium dioxide (layer B=TiO2). Our numerical results quantitatively show that the random arrangement and length of the multilayers cause minor changes in the bandgap edge of the low frequency GIPBG, i.e., the low frequency GIPBG is robust regarding composition and length of the structures. On the other hand, the chemical potential strongly affects the robustness of the low frequency GIPBG. In fact, the low frequency GIPBG is strongly dependent on the properties of graphene, reinforcing that it is possible to adjust its width and bandgap edge by tuning the chemical potential via a gate voltage.

Tunable photonic bandgap and reflection phase shift properties of 1D binary photonic crystal consisting of double negative and magnetic cold plasma materials

In this manuscript, by using alternate layers of double negative and magnetic cold plasma materials, we have studied the reflection and reflection phase shift properties of 1D binary photonic crystals. The simulations of the proposed work have been carried out by using the transfer matrix method with the help of MATLAB software. It has been found that the combination of the angle of incidence and external magnetic field under both right hand polarized and left handed polarized configurations can be used for right and left tuning of photonic bandgap (PBG) of the proposed structure under consideration. Under the aforementioned circumstances, the reflection phase shift corresponding to TE and TM waves can be smoothly switched between 0 to π and –π to 0, respectively, across PBGs of the proposed structure. The variation in external magnetic field values from 0 to 6 T and angle of incidence from 0° to 80° can be used for precise tuning of PBG and reflection phase shift between –π and π depending upon TM and TE polarization cases, respectively. This study may open a new gateway for designing externally tunable microwave devices like single to multichannel band-stop filters, buffers that can hold data temporarily, tunable wave plates, and tunable phase retarders.

Optical biosensors using plasmonic and photonic crystal band-gap structures for the detection of basal cell cancer

One of the most interesting topics in bio-optics is measuring the refractive index of tissues. Accordingly, two novel optical biosensor configurations for cancer cell detections have been proposed in this paper. These structures are composed of one-dimensional photonic crystal (PC) lattices coupled to two metal–insulator–metal (MIM) plasmonic waveguides. Also, the tapering method is used to improve the matching between the MIM plasmonic waveguides and PC structure in the second proposed topology. The PC lattices at the central part of the structures generate photonic bandgaps (PBGs) with sharp edges in the transmission spectra of the biosensors. These sharp edges are suitable candidates for sensing applications. On the other hand, the long distance between two PBG edges causes that when the low PBG edge is used for sensing mechanism, it does not have an overlapping with the high PBG edge by changing the refractive index of the analyte. Therefore, the proposed biosensors can be used for a wide wavelength range. The maximum obtained sensitivities and FOM values of the designed biosensors are equal to 718.6, 714.3 nm/RIU, and 156.217, 60.1 RIU−1, respectively. The metal and insulator materials which are used in the designed structures are silver, air, and GaAs, respectively. The finite-difference time-domain (FDTD) method is used for the numerical investigation of the proposed structures. Furthermore, the initial structure of the proposed biosensors is analyzed using the transmission line method to verify the FDTD simulations. The attractive and simple topologies of the proposed biosensors and their high sensitivities make them suitable candidates for biosensing applications.

Omnidirectional nonreciprocal absorber realized by the magneto-optical hypercrystal.

Photonic bandgap design is one of the most basic ways to effectively control the interaction between light and matter. However, the traditional photonic bandgap is always dispersive (blueshift with the increase of the incident angle), which is disadvantageous to the construction of wide-angle optical devices. Hypercrystal, the photonic crystal with layered hyperbolic metamaterials (HMMs), can strongly modify the bandgap properties based on the anomalous wavevector dispersion of the HMM. Here, based on phase variation competition between HMM and isotropic dielectric layers, we propose for the first time to design nonreciprocal and flexible photonic bandgaps in one-dimensional photonic crystals containing magneto-optical HMMs. Especially the zero-shift cavity mode and the blueshift cavity mode are designed for the forward and backward propagations, respectively. Our results show maximum absorption about 0.99 (0.25) in an angle range of 20-75 degrees for the forward (backward) incident light at the wavelength of 367 nm. The nonreciprocal omnidirectional cavity mode not only facilitates the design of perfect unidirectional optical absorbers working in a wide-angle range, but also possesses significant applications for all-angle reflectors and filters.

Study of crystal properties based on attention mechanism and crystal graph convolutional neural network

The prediction of crystal properties has always been limited by huge computational costs. In recent years, the rise of machine learning methods has gradually made it possible to study crystal properties on a large scale. We propose an attention mechanism-based crystal graph convolutional neural network, which builds a machine learning model by inputting crystallographic information files and target properties. In our research, the attention mechanism is introduced in the crystal graph convolutional neural network (CGCNN) to learn the local chemical environment, and node normalization is added to reduce the risk of overfitting. We collect structural information and calculation data of about 36 000 crystals and examine the prediction performance of the models for the formation energy, total energy, bandgap, and Fermi energy of crystals in our research. Compared with the CGCNN, it is found that the accuracy (ACCU) of the predicted properties can be further improved to varying degrees by the introduction of the attention mechanism. Moreover, the total magnetization and bandgap can be classified under the same neural network framework. The classification ACCU of wide bandgap semiconductor crystals with a bandgap threshold of 2.3 eV reaches 93.2%, and the classification ACCU of crystals with a total magnetization threshold of 0.5 μ B reaches 88.8%. The work is helpful to realize large-scale prediction and classification of crystal properties, accelerating the discovery of new functional crystal materials.

Effect of temperature on band gap of PbWO4 single crystals grown by Czochralski method

The structural and optical properties of PbWO4 single crystals grown by Czochralski method and investigated by x-ray diffraction (XRD) and transmission experiments. XRD pattern presented well-defined and intensive peaks related with tetragonal scheelite structure. Transmission experiments were accomplished for the first time at various temperatures between 10 and 300 K on PbWO4 single crystals to reveal variation of band gap with temperature. Derivative spectroscopy method presented the experimentally observed band gap energy as increasing from 3.20 to 3.35 eV when the temperature was decreased to 10 K from room temperature. The revealed energy was associated with transition taking place between delocalized and trap levels. Temperature-band gap energy plot was analyzed by Varshni and Bose–Einstein models. The fitting processes under the light of these models revealed optical characteristics of absolute zero experimentally observed band gap, variation rate of gap energy with temperature and Debye temperature of PbWO4 single crystal.

Thermal, structural and optical properties of magnetic BiFeO3 micron-particles synthesized by coprecipitation method: heterogeneous photocatalysis study under white LED irradiation

Bismuth ferrite, BiFeO3 (BFO) prepared via a coprecipitation method, was used as an effective photocatalyst for the degradation of methylene blue (MB) using a white light-emitting diode (LED-50 W) lamp. The as-prepared material was characterized by TGA, XRD, SEM-EDX, FTIR, and UV-vis-DRS techniques. The effects of experimental key parameters such as pH, catalyst dosage, initial dye concentration, and calcination temperature were investigated. Results showed that BFO-600 (calcined at 600 °C/3 h) was a rhombohedrally-distorted perovskite oxide with the R3c space group. The average crystal size and band gap energy for BFO-600 were found to be 24.8 nm and 2.1 eV, respectively. Under optimal operating conditions ([MB]=20 mg.L-1, BFO-600/dye mass ratio=0.5 g.L-1, pH=11, and 25 °C), MB/BFO-600/LED process exhibited an efficient photodegradation (up to 94.4%) within 120 min, much higher than the MB sorption process (MB/BFO-600/dark system, 69.4%) and the MB self-photolysis system (MB/LED, 33.9%). The photocatalytic degradation of MB was found to fit well with the pseudo-first-order kinetic model. Thus, the MB photodegradation was attributed to the generation of reactive radicals such as •OH, h+, H2O2, and MB•+ via direct heterogeneous photocatalysis and mediated by MB through a self-photosensitization process of MB over BFO-600 NPs.

Photoluminescence enhancement in monolayer MoS2 and self-assembled 3D photonic crystal heterostructures.

Self-assembled photonic crystals (PCs) have promising applications in enhancing and directional manipulation of the photoemission due to their photonic bandgaps. Here, we employed self-assembled 3D polystyrene PCs to enhance the photoluminescence (PL) of monolayer molybdenum disulfide (MoS2). Through tuning the photonic bandgap of the polystyrene crystals to overlap with the direct emission band of monolayer MoS2, the MoS2/3D-PC heterostructure showed a maximum 12-fold PL enhancement, and Rabi splitting was also observed in the reflection spectrum. The heterostructure is expected to be useful in nanophotonic emitting devices.

Construction of 1D TiO2 nanotubes integrated ultrathin 2D ZnIn2S4 nanosheets heterostructure for highly efficient and selective photocatalytic CO2 reduction

Development of low cost, highly efficient and non-noble metal photosystem is of a great significance for promoting the photoproduced carrier’s separation and acting as CO2 reduction sites. Herein, non-noble metal catalysts of TiO2 nanotubes (TNT) and hexagonal ZnIn2S4 nanosheets (ZIS) have been synthesized by simple hydrothermal methods and used for photoreduction of CO2. After optimizing the TNT-loading ratio on ZIS, 10 wt.% TNT/ZIS showed the best CO production activity (4.41 mmol g–1h−1), which was 1.5 times higher than CO production rate of pristine ZIS. Especially, TNT/ZIS can present a stable CO evolving tendency during 72 h irradiation, producing regular activity and selectivity for 4 times of recycling test under solar-light irradiation. To reveal the underlying photocatalytic mechanism, the crystal structure, nanomorphology, light absorption, energy bandgap, element component and electrochemical behaviors of those obtained composite was characterized and analyzed. The results indicate that TNT/ZIS composite with glorious CO2 reduction activity not only extends the responsive spectral range, but also pushed the transfer and separation of photoexcited electrons from ZIS to TNT and then to Co(bpy)32+ cocatalyst, in which Co(bpy)32+ can be reduced to Co(bpy)3+ and re-oxidized to its original oxidation state during CO2 conversion.

Inhibiting Ion Migration by Guanidinium Cation Doping for Efficient Perovskite Solar Cells with Enhanced Operational Stability

Perovskite solar cells (PSCs) develop great potential to make photovoltaic power generation systems more cost‐effective due to the high power conversion efficiency (PCE), low material cost, and easy fabrication. Alloyed A‐site cations surrounded by PbI6 octahedra play decisive roles in the crystal structure, bandgap, and phase stability, as well as ion migration. Herein, based on the temperature‐dependent ion conductivity measurement, the activation energy for iodide ion migration is systematically studied with different proportions of guanidinium cation (GA+) substitution. It is found that partial GA+ doping could effectively suppress iodide ion migration. The triple‐cation perovskite (MA0.8FA0.1GA0.1PbI3) PSCs achieve a PCE of 22.17% with superior operational stability maintaining 90% of their initial efficiency after 1200 h under continuous light soaking. Furthermore, it is extended to mini perovskite solar modules, 14 cm2 active area, and achieves a PCE of 19.18%.

2D FeOCl: A Highly In-Plane Anisotropic Antiferromagnetic Semiconductor Synthesized via Temperature-Oscillation Chemical Vapor Transport.

2D van der Waals (vdW) transition-metal oxyhalides with low symmetry, novel magnetism, and good stability provide a versatile platform for conducting fundamental research and developing spintronics. Antiferromagnetic FeOCl has attracted significant interest owing to its unique semiconductor properties and relatively high Néel temperature. Herein, good-quality centimeter-scale FeOCl single crystals are controllably synthesized using the universal temperature-oscillation chemical vapor transport (TO-CVT) method. The crystal structure, bandgap, and anisotropic behavior of the 2D FeOCl are explored in detail. The absorption spectrum and electrical measurements reveal that 2D FeOCl is a semiconductor with an optical bandgap of ≈2.1eV and a resistivity of ≈10-1 Ω m at 295 K, and the bandgap increases with decreasing thickness. Strong in-plane optical and electrical anisotropies are observed in 2D FeOCl flakes, and the maximum resistance anisotropic ratio reaches 2.66 at 295 K. Additionally, the lattice vibration modes are studied through temperature-dependent Raman spectra and first-principles density functional calculations. A significant decrease in the Raman frequencies below the Néel temperature is observed, which results from the strong spin-phonon coupling effect in 2D FeOCl. This study provides a high-quality low-symmetry vdW magnetic candidate for miniaturized spintronics.

Constructing thermal phonon insulators by using photonic crystals

This paper designs a thermal waveguide using thermal phonon insulators, to manipulate the loss of thermal radiation. Two-dimensional (2D) photonic crystals with dielectric background and air holes have been used in the waveguide clad to suppress thermal radiation so that thermal radiation cannot escape out from the waveguide clad. The photonic crystals may reduce the loss of thermal radiation through waveguide clad which surges the thermal energy confinement in the waveguide core and propagation along the longitudinal axis. The photonic crystals have been designed with the bandgap covering the thermal phonon frequency range and thus can be used to construct thermal phonon insulators to suppress or trap thermal emission generated through phonon wave propagation. The finite-difference-time-domain simulation results demonstrate that electromagnetic bandgap in photonic crystals has enough width in the thermal phonon frequency range to confine the thermal energy within the core of the waveguide and effective transmission of thermal radiation can be conquered within the waveguide core.

Optical properties of 87Rb atomic vapor near the 1D photonic crystal bandgap and all-optical switching of transmitted light

We investigate the steady-state and dynamic behavior of the optical properties of the laser pulse in a GaAs/AlAs 1D photonic crystal (1DPC) with an atomic vapor defect layer. We chose the atomic vapor of the 87Rb as a defect layer by driving the probe field on the D2 transition between 52P3/2 - 52S1/2 levels through the 1DPC. The effect of the photonic bandgap (PBG) on the absorption and dispersion properties of the probe field is discussed. Moreover, the transmission and reflection coefficient of the 1DPC is controlled by applying the various value of the intensity of the coupling field. By comparing these results in the vacuum and near the PBG, we find that the absorption/dispersion and transmission/reflection properties are strongly affected by the PBG. We find that all-optical properties of atomic vapor in the surrounding of 1DPC are improved due to PBG. Furthermore, the effect of the intensity of the coupling field on the all-optical switching is studded. The proposed model may provide some new possibilities for technological applications as an all-optical device based on the photonic crystal in quantum information science, quantum computing, signal processing, and quantum communications.

Improvement of the Supercapacitor Performance of Nickel Molybdenum Chalcogenides/Reduced Graphene Oxide Composites through Vanadium-Doping Induced Crystal Strain Relaxation and Band Gap Modification

Metal chalcogenide/reduced graphene oxide (RGO) composites have gained significant interest as promising electrode materials for supercapacitor application. Herein, the effect of different chalcogens (O, S, and Se) on nickel-based bimetallic composites along with the addition of a scanty amount of a heteroatom (V) is investigated. Different sizes and electronegativity of the chalcogens alter the morphology of the composites. However, V doping does not change the morphology but regulates the crystalline strain, band-gap energies, and charge transfer kinetics of the materials. All these factors are very important in controlling the performance of a supercapacitor device. Among all the doped and undoped composites, Se-based electrode materials exhibit the highest supercapacitor properties. In a three-electrode configuration, the V-doped Ni-Mo selenide/RGO (VNMSeR) composite electrode exhibits the highest specific capacitance of ∼610 C g–1 (1220 F g–1) at 2 A g–1 current density with a superior rate capability of ∼73.7%. An asymmetric supercapacitor device has been fabricated using VNMSeR as positive and thermally RGO as negative electrode. The device exhibits a maximum energy density of ∼60.5 Wh kg–1 at a power density of 1.47 kW kg–1 and shows ∼83.4% retention (50.5 Wh kg–1) in energy density when the power density increases by ∼8.25-fold (12.12 kW kg–1).

Study of IR Spectroscopy of Crystalline Materials Structure Used in Laser Shutters

The IR absorption spectra of hexagonal crystals of lithium iodate α-LiIO3, used in laser shutters for ship wiring, where the quality, transparency and power of laser radiation are of great importance, are studied. The transmission spectra allowed us to determine the width of the crystal band gap 4.37 eV along the Z-axis (C6 [0001]) and 4.46 eV along the axis perpendicular to the Z axis. The activation energies and the wavelength of the vibrational centers associated with the vibrations of the H3O+, OH-, H2O groups, heavy D2O and semi-heavy molecules HDO are determined. It is established that the band 1450-1650 cm-1 is a superposition of at least two broad absorption maxima centered at ~1550 and ~1600 cm-1. It was experimentally established that the ratio of the absorption coefficients of these bands was ~1:1 (75:74 cm-1) for crystals grown in H2O, and ~2:1 (161:84 cm-1) – for crystals grown in D2O. This allows us to assume that in a lithium iodate crystal grown in D2O, the bands 1550 cm-1 and 1600 cm-1 correspond to the vibrations of the bound HDO and H2O molecules, the band 1150 cm-1 corresponds to the vibrations of heavy water molecules. The results of the study allowed us to propose a mechanism of transport and translational diffusion of protons and to determine the presence of heavy water molecules in crystalline materials, as well as to diagnose the quality of laser crystals.