Band Gap Properties Research Articles

Band gap characteristics of multi-functional lattice metamaterials with adjustable thermal expansion and Poisson's ratio

The current development of science and technology has put increased demands on the versatility of metamaterials. In this paper, the band gap characteristics of a lattice metamaterial that can simultaneously modulate thermal expansion and Poisson’s ratio are studied, the dispersion characteristics of elastic waves propagating in periodic lattice metamaterials are analyzed, and the effects of different structures and parameters on the band gap are discussed. The results indicate that the configuration has excellent band gap properties while satisfying the tunable coefficient thermal expansion (CTE)and Poisson’s ratio functions. Through reasonable material selection and shape design, it is expected to achieve a multi-objective win-win for specific thermal expansion properties, Poisson’s ratio, and band gap design, resulting in better tunability and versatility of the metamaterial.

Investigations on physical, structural, elastic, optical and radiation shielding properties of boro-tellurite glasses for radiation waste management applications

The boro-tellurite glasses with the chemical composition 29B2O3+20TeO2+50MO +1Dy2O3 containing different modifiers (where M = Ba, Li2, Sr, Ca, Mg and Zn) have been produced via conventional melt quenching method. The elastic, structural, physical, optical bandgap properties and the high energy photon shielding behavior of the produced glasses were extensively inspected through appropriate characterization techniques. The X-ray diffraction (XRD) pattern confirms amorphous character of the as-quenched samples. The Fourier-Transform Infrared (FTIR) spectral analysis reveals the presence of functional groups by the way of their stretching and bending vibrational modes. The estimation of parameters like Boron–Boron separation (dB−B), Bond density (n0), oxygen molar volume (V0), and Oxygen Packing Density (OPD) were facilitated to understand the detailed structural modifications and the moduli of elasticity (E, K, G, σ) parameters were determined for the fabricated the boro-tellurite glasses. The barium incorporated present boro-tellurite glass possess lower OPD and higher average molecular weight and it might make the glass network densely packed. To ensure the radiation protection ability, defending parameters such as equivalent atomic number, mass attenuation coefficient (MAC), Effective electron density, half-value layer (HVL), mean free path (MFP) and build-up factors of the titled glasses were determined employing the user friendly Phy-X software. Barium cations added boro-tellurite glass found to have the shortest MFP and HVL, as well as the lowest EBF in the low and intermediate energy zones indicating the potential for using the barium cations incorporated boro-tellurite glass as alternatives for conventional concretes in shielding and waste management applications.

Light intensity and temperature effect on the DC parameters of optimized nc-3C-Sic:H Based solar cell

The Hydrogenated nanocrystalline cubic silicon carbide (nc-3C-Si:H) based solar cell with interesting efficiency, find her place promoted in photovoltaic applications. To improve the sunlight absorption and its electrical performances, the structure of the solar cell can be optimized, combining new materials to form multi-layers solar cells. In this context, the nc-3C-SiC:H, having a wide band gap, excellent optical, electrical and structural properties is used as wide bandgap absorber layer. In this work, we report a theoretical study of new optimized nc-3C-SiC:H/ i-a-Si:H/ a-Si:H/ µc-Si solar cell using the SCAPS one dimension program. We investigate the temperature and incident light intensity effect on the DC parameters of the solar cell. For the proposed structure, optimal power conversion efficiency (PCE) of 16.89% is registered under AM1.5G spectrum at room temperature.

S–CdIn2S4: A novel near-infrared emitter triggered by low potential for constructing a dual-mode immunosensing platform

CdIn2S4 is an interesting ternary metal sulfide whose narrow band gap and tunable optical properties offer new opportunities for the development of novel ECL emitters. Here, we use a simple hydrothermal synthesis to obtain hollow spindle CdIn2S4 (S–CIS), which exhibits strong near-infrared electrochemiluminescence (ECL) emission with K2S2O8 as a coreactant at a low excitation potential (−1.3 V), which is encouraging. The lower excitation potential of S–CIS is probably due to the low band gap energy, which makes the excitation potential positively shift. This lower excitation potential reduces the side-reactions caused by high voltages, effectively avoiding irreversible damage to biomolecules, and protecting the biological activity of antigens and antibodides. In this work, new features of S–CIS in ECL studies are also presented, demonstrating that the ECL emission mechanism of S–CIS is generated by surface state transitions and that S–CIS exhibits excellent near-infrared (NIR) characteristics. Importantly, we introduced S–CIS into electrochemical impedance spectroscopy (EIS) and ECL to the construct a dual-mode sensing platform to achieve AFP detection. The two models with intrinsic reference calibration and high accuracy showed outstanding analytical performance in AFP detection. The detection limits were 0.862 pg mL−1 and 16.8 fg mL−1, respectively. This study demonstrates the key role and great application potential of S–CIS as a novel NIR emitter with easy preparation, low cost and great performance in the development of a simple, efficient and ultrasensitive dual-mode response sensing platform for early clinical use.

Preparation of ZnMoO4 for ultraviolet detection by laser ablation in liquid combined with hydrothermal method

Ternary metal oxide（TONs）has attracted extensive attention in the field of ultraviolet detection due to its adjustable band gap and excellent photoelectric properties. Here, ternary metal oxide ZnMoO4 is prepared by laser ablation in liquid combined with hydrothermal method. During the laser ablation, highly active solutions containing Mo6+, Zn2+, MoO3, ZnO, MoO42- and Zn(OH)2 are obtained, which was put into the reactor for hydrothermal reaction. Both hydrothermal time and hydrothermal temperature has effects on the morphology and size of the products. ZnMoO4 nanowires can be prepared with hydrothermal condition of 200 °C and 15 h. The products have high absorption in ultraviolet and low absorption in the visible region, namely good selectivity to the spectral region. A UV photodetector based on ZnMoO4 material with 4.70 eV band gap is constructed and its photoelectric performances are studied. It is found that the dark current of ZnMoO4 UV detector is very small, about 0.01 nA. The current increases rapidly as the voltage increases. And with the increase of light intensity, the current increases due to the increase of photo-generated carriers. The current in the I-T curve presents a good periodic rise and fall. The results indicate that ZnMoO4 is a suitable material to be used in ultraviolet detectors.

Materials property prediction with uncertainty quantification: A benchmark study

Uncertainty quantification (UQ) has increasing importance in the building of robust high-performance and generalizable materials property prediction models. It can also be used in active learning to train better models by focusing on gathering new training data from uncertain regions. There are several categories of UQ methods, each considering different types of uncertainty sources. Here, we conduct a comprehensive evaluation on the UQ methods for graph neural network-based materials property prediction and evaluate how they truly reflect the uncertainty that we want in error bound estimation or active learning. Our experimental results over four crystal materials datasets (including formation energy, adsorption energy, total energy, and bandgap properties) show that the popular ensemble methods for uncertainty estimation are NOT always the best choice for UQ in materials property prediction. For the convenience of the community, all the source code and datasets can be accessed freely at https://github.com/usccolumbia/materialsUQ.

Growth of wafer-scale chromium sulphide and selenide semiconductor films

Two-dimensional (2D) transition metal chalcogenides have attracted enormous attention due to their stunning properties and great prospects for applications. Most of the reported 2D materials have layered structure, and non-layered transition metal chalcogenides are rare. Particularly, chromium chalcogenides are highly complexed in terms of structural phases. Researches on their representative chalcogenides, Cr2S3 and Cr2Se3, are insufficient and most of them focus on individual crystal grains. In this study, large-scale Cr2S3 and Cr2Se3 films with controllable thickness are successfully grown, and their crystalline qualities are confirmed by multiple characterizations. Moreover, the thickness-dependent Raman vibrations are investigated systematically, presenting slight redshift with increasing thickness. The fundamental physical properties of grown Cr2S3 and Cr2Se3 films, including optical bandgap, activation energy and electrical properties, are measured with different thicknesses. The 1.9 nm thick Cr2S3 and Cr2Se3 films show narrow optical bandgap of 0.732 and 0.672 eV, respectively. The electrical properties of Cr2S3 films demonstrate p-type semiconductor behaviours, while the Cr2Se3 films exhibit no gate response. This work can provide a feasible method for growing large-scale Cr2S3 and Cr2Se3 films, and reveal fundamental information of their physical properties, which is helpful for future applications.

Enhanced NIR fluorescence quantum yield of graphene quantum dots using dopants

In the present work, several efforts have been made theoretically to achieve an excellent non-toxic fluorescent graphene quantum dot (GQD) in the near-infrared region for the application of bio-imaging and sensing. Although the QY of GQDs is a maximum of 98.5% in the visible region, it is still very low, and it is as low as 7% in NIR. Sulfur and its group elements have been used for doping because they are pretty cheap and nontoxic and hence suitable for this application. The surface-doped position is considered for studying their effect on the energy band gap, absorption and fluorescence properties. The HOMO and LUMO isosurfaces have been analyzed in order to comprehend the nature of the dominant transition taking place in absorption spectra. Additionally, the quantitative indices, transition density matrix contour maps, and charge difference density have all been examined in order to determine whether this particular transition is locally excited or involves charge transfer. Following this, the QY of each GQD has been determined by considering the fluorescence spectra. The wavelength of fluorescence of doped GQDs is found to be in the region of 800–1400 nm, i.e. in NIR, which is strongly desirable for bio-imaging and bio-sensing applications. With a fluorescence of ∼850 nm, sulfur-doped GQDs (S-GQD: C52S2H18) have the greatest QY, 26%, which is larger than the 7% achieved earlier in NIR and such a high QY in NIR is being reported for the first time.

Defect-enriched red phosphorus nanosheets as efficient and stable photothermal absorber material for interfacial solar desalination

The development of efficient photothermal materials has gained increasing attention for solar thermal-energy conversion. A good photothermal material has a strong light-absorption property, low thermal conductivity, high wettability, and high solar-to-thermal conversion efficiency. In this study, we prepared uniform, sub-micron, and defect-rich red phosphorus (RP) nanosheets via a simple ball-milling method to demonstrate their efficient and durable solar steam generation properties. Three RPs of different sizes were prepared by controlling the milling time (0 h: RP0, 30 h: RP30, and 60 h: RP60). Notably, RP60 exhibited the smallest particle (lateral) size, nanosheet morphology, and defect-rich surface. Furthermore, RP60 exhibited the distinctive properties of a small band gap (1.44 eV), low thermal conductivity (0.07 W/m·K), and low heat capacity (0.66 J/g·K) with exceptional wettability. With simulated sunlight illumination (100 mW/cm2, 1 sun), the RP60 photothermal absorber demonstrated a high water evaporation rate of 1.34 kg/m2·h with a stable solar steam generation efficiency of 74.1 % for over 10 h. A one-dimensional water path and porous polyurethane support facilitated the water supply, large contact area, heat localization, and steam escape. By employing plasmonic resonance-heating silver nanoparticles on the RP60, we achieved a significantly improved solar steam generation efficiency of over 96.0 % and a water evaporation rate of 1.75 kg/m2·h. This study highlights the critical role of morphology, particle size, and defects control in improving the photothermal properties for efficient and durable interfacial seawater desalination.

Bandgap formation and low-frequency structural vibration suppression for stiffened plate-type metastructure with general boundary conditions

Metastructures with unique mechanical properties have shown attractive potential application in vibration and noise reduction. Typically, most of the metastructures deal with the vibration bandgap properties of infinite structures without considering specific boundary condition and dynamic behaviors, which cannot be directly applied to the engineering structures. In this research, we design a Stiffened Plate-type Metastructure (SPM) composed of a plate with periodic stiffeners and cantilever beam-type resonators subjected to general boundary conditions for low-frequency vibration suppression. The effects of boundary conditions and the number and orientation of the stiffeners on Locally Resonant (LR) type bandgap properties in SPM are further investigated. An analytical modeling framework is developed to predict the bandgap formations and vibration behaviors of SPMs in finite-size configuration. The governing equations of the SPM reinforced by various arrangements of stiffeners are derived based on the first-order shear deformation theory and Hamilton’s principle, and a Fourier series combined with auxiliary functions is employed to satisfy the arbitrary boundary conditions. Finite element analysis and experimental investigations of vibration behaviors for the SPM are carried out to validate the accuracy and reliability of the present analytical model. For practical designs of the SPMs with specific boundary conditions, it is found that there exist optimal numbers of stiffeners and resonators which can produce the significant LR-type bandgap behaviors. Furthermore, various arrangements of stiffeners and resonators are explored for different boundary conditions by breaking the requirement of spatially periodicity. It is shown that for the designed SPM, the vibration modes of its host structure should be considered to widen the frequency range in which the resonators transfer and store energy, and hence improve the performance of low-frequency vibration suppression. The present work can provide a significant theoretical guidance for the engineering application of metamaterial stiffened structures.

Synthesis and characterization (optical and antibacterial) of silver doped zinc oxide nanoparticles

In the present work, Zn1-xAgxO (x = 0, 0.03, 0.05, and 0.07) nanoparticles were synthesized by the sol-gel synthesis method. Microstructural, morphological, optical, and antibacterial characterizations were performed. The wurtzite crystalline structure was confirmed through XRD analysis. The maximum crystallite size was found for Zn0.93Ag0.07O (∼22.36 nm). Rietveld refinement was performed to understand better crystal symmetry. The optical band gap property exhibited a decremental pattern with the increasing Ag content. The lowest band gap was achieved by Zn0.93Ag0.07O (∼3.26 eV). The Fourier transform infrared spectroscopy (FTIR) analysis ensured the bond stretching of Zn–O and Ag–O near the absorbance band of 490 cm−1. Further, the Thermogravimetric analysis (TGA) was performed to determine the samples' weight loss, and the minimum weight loss was observed for Zn0.93Ag0.07O (∼9–11%). Finally, the antibacterial characterization was performed using Escherichia coli (gram-negative bacteria), and again Zn0.93Ag0.07O exhibited better performances than other synthesized nanoparticles.

AcesExperimental and theoretical study on photocatalytic nitrogen reduction catalyzed by Fe doped MoO2

Transition metal oxides are suitable for photocatalytic nitrogen reduction (pNRR) due to their properties of narrow band gap and high energy density. Still, it is also limited by the defects of high photogenerated carrier recombination and low adsorption capacity of nitrogen molecules, which hinder the further improvement of the activity. For high carrier recombination and low activation rate of the nitrogen molecule, herein, Fe-doped MoO2 catalyst (Fe−MoO2) was synthesized for pNRR. Combined with the experimental characterization and theoretical calculation, it shows that the doping of Fe atoms, one hand, improves the conduction band position of MoO2 and enhances the reduction ability, which is favorable to nitrogen reduction. On the other hand, it not only enhances the adsorption of Mo sites and N2 molecules, but also changes the surface electronic structure and promotes the electron transfer from MoO2 to Fe sites, and the two act synergistically to adsorb and activate nitrogen more effectively. The activity of Fe−MoO2 for pNRR is as high as 96.62 μmol g−1h−1, which is about 2.5 times that of pristine MoO2, and the activity decreases by only 10% after three cycles.

Effect of Al Incorporation on the Structural and Optical Properties of Sol-Gel AZO Thin Films.

ZnO and Al-doped ZnO (AZO) thin films were prepared using the sol-gel method and deposited on a Silicon (Si(100)) substrate using the dipping technique. The structure, morphology, thickness, optical constants in the spectral range 300-1700 nm, bandgap (Eg) and photoluminescence (PL) properties of the films were analyzed using X-ray diffractometry (XRD), X-ray fluorescence (XRF), atomic force microscopy (AFM), scanning electron microscopy (SEM), spectroscopic ellipsometry (SE), Raman analysis and PL spectroscopy. The results of the structure and morphology analyses showed that the thin films are polycrystalline with a hexagonal wurtzite structure, as well as continuous and homogeneous. The PL background and broader peaks observable in the Raman spectra of the AZO film and the slight increase in the optical band gap of the AZO thin film, compared to undoped ZnO, highlight the effect of defects introduced into the ZnO lattice and an increase in the charge carrier density in the AZO film. The PL emission spectra of the AZO thin film showed a strong UV line corresponding to near-band-edge ZnO emission along with weak green and red emission bands due to deep-level defects, attributed to the oxygen-occupied zinc vacancies (OZn lattice defects).

The Formation Mechanism of (001) Facet Dominated α-FAPbI3 Film by Pseudohalide Ions for High-Performance Perovskite Solar Cells.

Formamidinium lead triiodide (α-FAPbI3 ) has been widely used in high-efficiency perovskite solar cells due to its small band gap and excellent charge-transport properties. Recently, some additives show facet selectivity to generate a (001) facet-dominant film during crystallization. However, the mechanism to realize such (001) facet selectivity is not fully understood. Here, the authors attempted to use three ammonia salts NH4 X (X are pseudohalide anions) to achieve better (001) facet selectivity in perovskite crystallization and improved crystallinity. After addition, the (001) facet dominance is generally increased with the best effect from SCN- anions. The theoretical calculation revealed three mechanisms of such improvements. First, pseudohalide anions have larger binding energy than the iodine ion to bind the facets including (110), (210), and (111), slowing down the growth of these facets. The large binding energy also reduces nucleation density and improves crystallinity. Second, pseudohalide ions improve phase purity by increasing the formation energies of the δ-phase and other hexagonal polytypes, retarding the α- to δ-phase transition. Third, the strong binding of these anions can also effectively passivate the iodine vacancies and suppress nonradiative recombination. As a result, the devices show a power conversion efficiency of 24.11% with a Voc of 1.181V.

Highly strained and heavily doped germanium thin films by non-equilibrium high-speed CW laser annealing for optoelectronic applications

The integration of light transmitters into electronic circuits is essential for the development of optical communication systems. A direct bandgap semiconductor with excellent electrical properties is preferable for a cost-effective and high-performance device. Germanium is considered a promising candidate for CMOS-compatible optoelectronics, even though the light emission is inefficient due to its indirect bandgap property. It has been reported that applying tensile strain modifies the energy band structure thus making quasi-direct bandgap emission possible. In addition, n-type doping has been expected to improve this emission. We have realized high tensile strain in polycrystalline germanium grown on a quartz substrate by employing continuous wave laser annealing (CWLA). In this study, we investigated the implementation of a similar annealing technique with antimony doped samples to combine both tensile strain and n-type doping in a Ge-on-insulator (GeOI) structure. We investigated various samples with different interfaces to achieve large-grained polycrystalline films on insulator. Characterizations of the CWLA processed samples revealed the successful introduction of both tensile strain and n-type doping in GeOI. The properties were confirmed by the enhancement of direct Γ-LH emission that observed even for room-temperature optical measurements. These results provide an important insight that will assist the feasible and large-scale development of germanium-based optoelectronic devices.

Effect of strain on the spin-polarization, mechanical and magnetic properties of Co[formula omitted]TiGe heusler alloy: A First Principle Study

In our work, we studied the effect of isotropic strain from −6% to 8% (compressive to tensile) on the Band gap, Elastic, Spin-polarization and Magnetic properties of Co2TiGe heusler alloy. We found that the half-metallicity (100% Polarization) is maintained under uniform strain from −2% to 4% but for the strain −4% to −6% the Spin-polarization drops quickly from 94% to 5.363%. From the study of electronic band structure we found that the energy band gap decreases with the increase of tensile strain and increase with the increase of compressive strain. For strain free compound the calculated magnetic moment of the material is 2.00 μB. We have found that under −4% to 8% uniform strain the total magnetic moment of the compound remains constant. But for strain −6% it decreases to 1.91 μB. We have calculated the mechanical properties of the material under different uniform strain. So, from the computed values we found the Bulk modulus, Young modulus, Shear modulus, Pugh’s ratio and Poisson ratio decreases with the tensile strain and increases with the compressive strain. With the increase of tensile strain (up to 8%) brittle nature of the material increases but with the increase of compressive strain (up to −6%) ductile of the material increases.

Optical Properties of PT‐Based Relaxor Ferroelectric Crystals

AbstractDuring the last two decades, PT‐based relaxor ferroelectric crystals such as (1−x)Pb(Zn1/3Nb2/3)O3–xPbTiO3 (PZN–PT), (1−x)Pb(Mg1/3Nb2/3)O3–xPbTiO3 (PMN–PT), and (1−x−y)Pb(In1/2Nb1/2)O3–yPb(Mg1/3Nb2/3)O3–xPbTiO3 (PIN–PMN–PT or PIMNT) crystals, are widely studied due to their huge piezoelectric properties and super‐high electro‐mechanical coupling factors. They have excellent properties with compositions around the morphotropic phase boundary (MPB). It is gratifying that the PT‐based relaxor crystals are replacing traditional PZT piezo‐ceramics in many application fields. Most ferroelectrics with oxygen‐octahedral structure, which show outstanding electro‐mechanical properties, also have excellent optical performances. Ferroelectric single crystals with large electro‐optic (EO) modulation are widely applied in laser communication devices. For the practical applications, the knowledge of detailed optical parameters is desirable. This paper tries to show a global review on the optical properties of PT‐based relaxor ferroelectric crystals. In the present review, optical properties of the crystals are systematically summarized, including refractive index dispersion, transmittance, band gap, EO, acousto‐optic, and photorefractive properties. These properties change with the crystal composition, orientation, and poling condition. The purpose of the review is to provide a resource for the researchers who are concerned with basic physical investigation or optical device applications of the PT‐based relaxor ferroelectric crystals.

Review on III–V Semiconductor Nanowire Array Infrared Photodetectors

AbstractIn recent years, III–V semiconductor nanowires have been widely investigated for infrared photodetector applications due to their direct and suitable bandgap, unique optical and electrical properties, flexibility in device design and to create heterostructures, and/or grow on a foreign substrate such as Si with more effective strain relaxation compared with planar structures. In particular, vertically aligned and ordered nanowire arrays have emerged as a promising photodetector platform, since their geometry‐related light absorption and carrier transport properties can be tailored to achieve high photodetector performance and new functionalities. In this article, the state‐of‐the‐art progress in the development of various types of infrared photodetectors based on III–V semiconductor nanowire arrays is reviewed. The nanowire synthesis/fabrication methods are introduced briefly at first, followed by discussions on the working principle and device performance of various types of nanowire array‐based photodetectors and their emerging applications. Finally, we analyze the challenges and present the perspectives for the development of future low‐cost, large‐scale, high‐performance nanowire array infrared photodetectors for practical applications.

Advances in Polymethine Dyes for Near‐Infrared Organic Photodiodes†

Comprehensive SummaryNear‐infrared organic photodiodes (NIR OPDs) have tremendous potential in industrial, military, and scientific applications, due to their unique features of lightweight, low toxicity, high structural flexibility, cooling‐system‐free, etc. However, the overall performance of currently available NIR OPDs still lags behind the commercial inorganic photodetectors, ascribed to the critical challenge of realizing organic semiconductors with sufficiently low optical bandgap and excellent optoelectronic properties simultaneously. Among various types of NIR‐absorbing organic semiconductors, polymethine dyes not only possess advantages of simple synthesis and structural diversity, but also show fascinating optical and aggregation features in the solid state, making them attractive material candidates for NIR OPDs. In this review, after a brief introduction of NIR OPDs and polymethine dyes, we comprehensively summarize the advances of polymethine dyes for broadband and narrowband NIR OPDs, and further introduce their applications in all‐organic optical upconversion devices and photoplethysmography sensors. In particular, the relationship between the chemical structure and the aggregation behaviors of polymethine dyes and the device performance is carefully discussed, providing some important molecular insights for developing high performance NIR OPDs.

Design of Lead Hybrid Halide Perovskite for Solar Cells

Hybrid organic-inorganic halide perovskites solar cells have attracted extensive interest because of their outstanding properties, including an optimal band gap, high carrier mobility, and excellent optoelectronic merits. We study the electronic and crystal structural properties of hybrid organic-inorganic halide APbX3 (A = Cs, methylammonium (MA), formamidinium (FA), X = I, Br) perovskites using first-principles calculations based on density functional theory. We find that halide atoms and A-site cations strongly affect their structural and electronic properties. The radius of a halide atom and the size of an organic molecule determine their lattice parameters and bond length. A relatively large halide atom can increase the value of the lattice parameters (a and b). Meanwhile, the electronic properties (band gap & carrier effective mass) of the Pb-based hybrid halide APbX3 can be effectively modified by adopting appropriate A- and X-site atoms or organic sections. We predict that HOIPs may have outstanding potential in solar light harvesting with promoted power conversion efficiency due to a tunable band gap and excellent electronic properties.