Tunable Band Gap Research Articles

Tunable band gap energy of l-Cysteine-assisted formation of Mn-doped ZnS interconnected nanoparticles for electro-optic applications

Interconnected pure and Mn2+-doped ZnS (ZnS: Mn) nanostructures were obtained via l-Cysteine-assisted synthetic strategy. The obtained interconnected nanostructures have been investigated systematically for their respective structural, morphological, optical and photoelectric properties. The attained XRD results reveal the cubic zinc blended structure of ZnS for all the synthesized specimens. The average grain size is found to be about ∼4.5 nm for the pure ZnS nanoparticles and ∼4.2 nm for the 3% Mn2+-doped ZnS nanoparticles which have been computed using the Debye‐Scherrer equation. The recorded TEM results indicate the interconnected nanoparticles of 4–7 nm forming the flakes which are about 200 nm in size for both the pure and Mn2+-doped ZnS NPs. Optical absorbance spectrum indicates the incorporation of Mn which induces a red shift whereby the maximum absorption of the material is witnessed. The band gap energy of the pure, 1%, 2% and 3% Mn2+-doped ZnS NPs are found to be 4.09, 3.75, 3.38 and 3.23 eV, respectively. PL peak intensity is linearly decreased with respect to the doping percentage. Enhanced electrical conductivity for ZnS nanomaterials doped with Mn2+ has been authenticated experimentally. Based on the optical and electrical properties, the Mn-doped ZnS NPs can be a potential candidate for electro-optic applications.

Recent Progress on the Surface and Bulk Modification of Carbon Nitrides for Photocatalytic Carbon Dioxide Reduction

Photocatalytic CO2 reduction has been considered as a green and sustainable approach to solve energy and environmental problems. However, the CO2 reduction efficiency of various types of semiconductor photocatalysts studied at this stage is far from realizing practical applications. The challenge of photocatalytic CO2 reduction is to design photocatalysts with excellent catalytic performance. This article summarizes the carbon nitride materials for efficient CO2 reduction and obtaining high value‐added products. The physicochemical properties of carbon nitride materials are discussed in terms of the photocatalytic mechanisms (bandgap tuning, high specific surface area, shorter electron–hole transmission paths, increasing the number of active sites, etc.). The present review provides the latest research progress of carbon nitride modification strategies. The prospects and challenges for the application of carbon nitride materials for CO2 reduction are presented. This review article will provide useful guidance for further research on carbon nitride materials for efficient photocatalytic CO2 reduction.

Theoretical Study of Electric, Dielectric, and Optical Properties in Ion Doped Multiferroic SrFe12O19 Nanoparticles.

Electric, dielectric, and optical (band gap) properties of pure multiferroic as well as La- and Ni-doped SrFe12O19 (SFO) (at different sites) are investigated using a microscopic model and Green's function technique. The concentration dependence of the polarization P is considered for substitution of rare earths ions on the Sr sites. For a small La ion doping concentration, x = 0.1, La-doped SFO is ferroelectric, whereas for a larger doping concentration, for example x = 0.5, it is antiferroelectric. The real part of the dielectric constant ϵ increases with an increasing magnetic field h. ϵ decreases with an increasing frequency and La dopants. Therefore, La-doped SFO is suitable for microwave application with a low dielectric constant. The magnetic properties of pure SFO NPs are also studied. Ni doping at the Fe site of SFO leads to enhanced ferroelectric polarization and dielectric constant. The band gap decreases or increases by substitution of Ni or In ions on the Fe site, respectively. The results reveal that the tuned band gap of Ni-doped SFO makes it a crucial candidate for optoelectronic and solid oxide fuel cell applications.

Narrowing of band gap and decrease in dielectric loss in La1-xSrxMnO3 for x = 0.0, 0.1, and 0.2 manganite nanoparticles

Polycrystalline compounds of lanthanum strontium manganite (La1-xSrxMnO3, LSMO) are widely used in electronic storage devices due to their minimal losses and electronic charge transport properties. In this study, we investigated how varying substitutions of Sr2+ for values of x = 0.0, 0.1, and 0.2 affected the tuning of the optical band gap and dielectric losses in La1-xSrxMnO3 nanoparticles. Synthesized samples were structurally analyzed via X-ray diffraction. A rhombohedral R3¯c crystal structure was confirmed for all prepared samples; crystallite size ranging from 15 nm to 20 nm was estimated along with other lattice parameters. Polygonal or hexagonal-like morphology was revealed by field emission scanning electron microscopy, with a moderate size distribution of nanoparticles affected by thinner grain boundaries in doped LSMO. Energy dispersive spectroscopy was employed to confirm the elemental composition of each compound, and the infrared spectrum indicated bonding in the fingerprint region It was observed that there was a significant reduction in the optical band gap, which was measured using ultraviolet spectrometer absorption data. The band gap decreased from 4.34 eV to 4.11 eV. This reduction was found to be related to the difference in refractive index, which was calculated using both Moss and Herve–Vandamme relations. In parallel, frequency-dependent dielectric analysis revealed that frequency was proportional to the increase in Sr content, inversely affecting dielectric loss. Moreover, the AC conductivity of the prepared samples increased with the rise in Sr content, as described by Johnscher’s universal power law in the region of high frequency.

On a Nonlinear Locally Resonant Metamaterial With Resistance-Inductance Shunt

Abstract Numerous recent works have established the potential of various types of metamaterials for simultaneous vibration control and energy harvesting. In this paper, we investigate a weakly nonlinear metamaterial with electromechanical (EM) local resonators coupled to a resistance-inductance shunt circuit, a system with no previous examination in the literature. An analytical solution is developed for the system, using the perturbation method of multiple scales, and validated through direct numerical integration. The resulting linear and nonlinear band structures are used for parametric analysis of the system, focusing on the effect of resonator and shunt circuit parameters on band gap formation and vibration attenuation. This band structure analysis informs further study of the system through wavepacket excitation as well as spectro-spatial analysis. The voltage response of the system is studied through spatial profiles and spectrograms to observe the effects of shunt inductance, nonlinearity, and their interactions. Results describe the impact of adding a shunted inductor, including significant changes to the band structure; multiple methods of tuning band gaps and pass bands of the system; and changes to wave propagation and voltage response. The results demonstrate the flexibility of the proposed metamaterial and its potential for both vibration control and energy harvesting, specifically compared to a previously studied system with resistance-only shunt.

A DFT Study of Bandgap Engineering and Tuning of Structural, Electronic, Optical, Mechanical and Transport Properties of Novel [Ba4Sb4Se11]: Sr3+ Selenoantimonate for Optoelectronic and Energy Exploitations

A DFT Study of Bandgap Engineering and Tuning of Structural, Electronic, Optical, Mechanical and Transport Properties of Novel [Ba4Sb4Se11]: Sr3+ Selenoantimonate for Optoelectronic and Energy Exploitations

External electric field assisted energy band gap modulation and optical properties of SiGe/AsSb heterobilayers

In this work, first-principle calculations within Density Functional Theory (DFT) were performed to study the structural, electronic, and optical properties of eight different bilayer van der Waals (vdW) heterostructures composed of SiGe and AsSb monolayers. These heterobilayers exhibit large energy bandgap (90 meV–459 meV) with linear energy–momentum relationship, implying high charge carrier mobility. Application of external vertical electric field can effectively modulate the energy bandgap of these heterobilayers from 64 meV to 459 meV, depending on stacking orders. Our study shows that a moderate amount of electric field can almost linearly tune the energy bandgap, however, semiconductor to metal transition occurs beyond a certain limit. The mechanism of energy bandgap tuning by the external electric field was examined by studying differential charge density. The optical properties such as complex dielectric function, electron energy loss function, complex refractive index, reflectivity, absorption coefficient, and optical conductivity of these SiGe/AsSb heterobilayers are studied for parallelly and perpendicularly polarized incident light. Our research demonstrates that these heterostructures exhibit variable plasma frequency depending on the stacking order of the heterostructures, anisotropic reflectivity i.e., birefringence properties in response to the incident light's polarization. Also, they exhibit high refractive indices (∼3) and absorption coefficients (∼105 cm−1) from visible to ultraviolet incident light frequencies. Our study proposes several SiGe/AsSb heterobilayers with unique electronic and optical properties which may have potential applications in nanoscale electronic and optoelectronic applications.

Using perovskite solar cells with tunable bandgaps for beam-splitting photovoltaic-thermal System

ABSTRACT The beam-splitting PV-T system has great potential for solar energy production. However, few studies have been conducted on hybrid systems featuring the newly emerging perovskite solar cells. This paper proposes a novel beam-splitting PV-T system with perovskite solar cells (PSCs). PSCs with various bandgaps are fabricated for the system by replacing Iodine ion (I−) in FA1-xMAxPbI3 with different proportions of Bromine ion (Br−). The trends of the photovoltaic properties of PSC are also analyzed and validated through the characterization of perovskite films. Systems of various spectral response ranges are then simulated and optimized using the experimental data. The obtained results show that the beam-splitting PV-T system can achieve a theoretical solar-to-electric efficiency of 25.74%. This value exceeds that of the single CSP and single PSC, which are 23.97% and 17.27%, respectively. When the ratio of Br− increases, the efficiency tends to decrease. Although the introduction of Br− fails to increase the overall efficiency, the changed spectral response range brings a shift in optimum cutoff wavelength, and the efficiency only slightly decreases with a lower ratio of Br− mixing. The results of this study provide a reference for beam-splitting PV-T utilization.

Effect of in contents on the electronic and optical properties of 2D GaN/InxGa1-xN heterostructures

Two-dimensional gallium nitride (2D GaN) has recently received considerable attention due to its excellent physical properties, including high stability, wide and tunable bandgap, and magnetism. 2D GaN offers an accessible way to apply in photocatalytic hydrogen production as composing to vdW heterostructures. Here, we report a first principles study based on the HSE06 functional for the 2D van der Waals (vdW) GaN/InxGa1-xN heterostructures with a tunable bandgap achieved through modulation of indium (In) contents. Such heterostructure is a bilayer vertically stacked by a monolayer of GaN and a monolayer of InxGa1-xN, both of which are graphite-like structures. In particular, the heterostructure bandgap decreases from 3.37 eV to 2.23 eV as In content increases from 0 to 0.5, accompanying by a significant transition from indirect to direct band gap. All of the studied heterostructures have suitable band edge positions for the redox potential of water decomposition at pH 0. Furthermore, increasing In content improves the visible light absorption, carrier mobility and establishes a more effective built-in electric field in the heterostructures. Our results prove that 2D GaN/InxGa1-xN heterostructure is highly suitable as a catalyst for photocatalytic water splitting and provides theoretical guidance for applying 2D GaN in photocatalytic hydrogen production.

Advancing bandgap tuning: Novel nitrogen doping in KLaTiO4 with uncompromised crystallinity

KLaTiO4 is a promising photocatalyst, producing 9.54(11) µmol of hydrogen gas per hour. Its high bandgap, of 4.09 eV, makes it unsuitable for direct photocatalysis under sunlight irradiation. In this study two novel methods of doping KLaTiO4 with nitrogen were studied, in an attempt to reduce the bandgap of KLaTiO4 without inducing structural degradation of the photocatalyst, which adversely affects the hydrogen evolution rate. The first method involves post-synthesis calcination of KLaTiO4 with urea under a nitrogen atmosphere, and the second method attempts to introduce nitrogen during synthesis, by replacing TiO2 with TiN as a starting reagent. The samples were structurally characterised using powder X-ray diffraction and their bandgap and photocatalytic performance determined by Tauc plot and hydrogen evolution testing. Density Function Theory calculations have been used to probe the likely site of nitrogen doping.

Prediction of Operational Lifetime of Perovskite Light Emitting Diodes by Machine Learning

Perovskite light‐emitting diodes (LEDs) with advantages of high electroluminescence efficiency at high brightness, good color purity, and tunable bandgap, are believed to have potential applications in the next generation display and lighting technologies. Due to the complex degradation process, mathematic models to describe the degradation process of perovskite LEDs are absent. In this work, it is found that the mathematical fitting methods which have been widely used to describe the decay trend of organic LEDs and quantum‐dot LEDs, are unable to accurately predict the lifetime of perovskite LEDs. Then an ensemble machine learning model is developed, which utilizes data augmentation technique to predict T50 of perovskite LEDs based on features before T80, achieving an accuracy of 0.995. Furthermore, the model can also accurately predict the T90 lifetime of quantum‐dot LEDs (QLEDs) using features before T98, suggesting it is a useful tool to efficiently evaluate LED lifetimes.

Preparation and optoelectronic performance of two-dimensional MoSe2/WSe2 lateral and vertical heterostructures

Two-dimensional materials Transition Metal Dichalcogenides (TMDs) are a novel type of semiconductor material with tunable bandgap and atomic-level thickness. Different TMDs can be flexibly combined to form heterostructures with distinct properties. In this paper, we have prepared two-dimensional MoSe2/WSe2 vertical and lateral heterostructures by using CVD and a combination of PVD and CVD, respectively. Subsequently, the structure, electronic and optoelectronic properties of these two types of heterostructures were characterized. The results indicate that field-effect transistors based on single-layer MoSe2/WSe2 lateral heterostructures, and single-layer WSe2/multi-layer MoSe2 and single-layer MoSe2/multi-layer WSe2 vertical heterostructures exhibit significant rectification effects. The in-plane monolayer MoSe2/WSe2 heterostructure devices exhibit the on/ff ratio of up to 104, and the monolayer-multilayer vertical heterostructure reaching 106, significantly outperforming most known devices of the same type. Moreover, the optical response of both in-plane and vertical heterostructures field-effect transistors reach 10 ms. Our research findings offer pathways for the flexible selective growth of in-plane and vertical heterostructures based on TMDs, and provide insights into achieving diverse performance in logic devices.

Aperiodic Bragg Reflectors for Tunable High-Purity Structural Color Based on Phase Change Material.

Tunable thin-film coating-based reflective color displays have versatile applications including image sensors, camouflage devices, spatial light modulators, and intelligent windows. However, generating high-purity colors using such coatings have posed a challenge. Here, we reveal high-purity color generation using an ultralow-loss phase change material (Sb2S3)-based tunable aperiodic distributed Bragg reflector (A-DBR). By strategically adjusting the periodicity of the adjacent layers of A-DBRs, we realize a narrow photonic bandgap with high reflectivity to generate high-purity orange and yellow colors. In particular, we demonstrate an A-DBR with a large photonic bandgap tunability by changing the structural phase of Sb2S3 layers from amorphous to crystalline. Moreover, we experimentally tailor multistate tunable colors through external optical stimuli. Unlike conventional nano thin-film coatings, our proposed approach offers an irradiance-free, narrowband, and highly reflective color band, achieving exceptional color purity by effectively suppressing reflections in off-color bands.

2D Nanostructured MXene-Based Silver Nanoparticles for Photocatalytic Degradation of Safranin Dye

Due to their unique chemical structure, MXenes have been recognized as a potential material, having a high surface area, high thermal and electrical conductivity, and a tunable band gap, showing great hydrophilicity and stability. The adsorption and reducing properties of MXene-based 2D nanomaterials make them efficient photocatalysts for degrading organic pollutants. Silver nanoparticles were synthesized over the exfoliated MXene sheets (1:50 and 1:20 by weight to silver salt) using polyvinyl pyrrolidone as a dispersant. The elemental composition and morphology of the nanocatalysts Ag20@Ti3C2Tx and Ag50@Ti3C2Tx were analyzed by X-ray diffraction (XRD) analysis, X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray (EDX) spectroscopy, and field-emission scanning electron microscopy (FESEM). FESEM micrographs reveal porous exfoliated Ti3C2Tx sheets obtained by continuously stirring Ti3AlC2 for 44 h at 60 °C, providing a platform for the growth of Ag nanoparticles. Diffused reflectance spectroscopy (DRS) indicates that the bare silver nanoparticles show a decrease in the band gap value from 2.4 to 1.35 and 1.41 eV in Ag50@Ti3C2Tx and Ag20@Ti3C2Tx, respectively, which enables the nanocomposites to show excellent catalytic performance and degrade around 99% of safranin dye within 15 min at a concentration of 5 mg Ag50@Ti3C2Tx.

Digital Alloy-Grown InAs/GaAs Short-Period Superlattices with Tunable Band Gaps for Short-Wavelength Infrared Photodetection.

The InGaAs lattice-matched to InP has been widely deployed as the absorption material in short-wavelength infrared photodetection applications such as imaging and optical communications. Here, a series of digital alloy (DA)-grown InAs/GaAs short-period superlattices were investigated to extend the absorption spectral range. The scanning transmission electron microscopy, high-resolution X-ray diffraction, and atomic force microscopy measurements exhibit good material quality, while the photoluminescence (PL) spectra demonstrate a wide band gap tunability for the InGaAs obtained via the DA growth technique. The photoluminescence peak can be effectively shifted from 1690 nm (0.734 eV) for conventional random alloy (RA) InGaAs to 1950 nm (0.636 eV) for 8 monolayer (ML) DA InGaAs at room temperature. The complete set of optical constants of DA InGaAs has been extracted via the ellipsometry technique, showing the absorption coefficients of 398, 831, and 1230 cm-1 at 2 μm for 6, 8, and 10 ML DA InGaAs, respectively. As the period thickness increases for DA InGaAs, a red shift at the absorption edge can be observed. Furthermore, the simulated band structures of DA InGaAs via an environment-dependent tight binding model agree well with the measured photoluminescence peaks, which is advantageous for a physical understanding of band structure engineering via the DA growth technique. These investigations and results pave the way for the future utilization of the DA-grown InAs/GaAs short-period superlattices as a promising absorption material choice to extend the photodetector response beyond the cutoff wavelength of random alloy InGaAs.

Flexible and transparent polylactic acid/hydrophobically modified nanobagasse cellulose/tannic acid/MXene films with highly efficient ultraviolet shielding

Ultraviolet (UV) radiation is potentially harmful to human health, while traditional petroleum-based conventional UV shielding materials can damage the environment, which necessitates the development of environmentally friendly transparent UV shielding materials. Ti3C2 (MXene) nanosheets were dispersed in a polylactic acid (PLA) matrix via the synergistic effect of hydrophobically modified nanobagasse cellulose (HC-NBC) and tannic acid (TA) to prepare solution-cast transparent UV shielding PLA/HC-NBC/TA/MXene (PLA/NTM) composite films with tunable bandgaps. The bandgap and UV shielding performance of the PLA/NTM composite films were regulated by varying the MXene addition amount. The bandgap of the PLA/NTM composite film with an MXene content of 1 wt% decreased to 2.37 eV, and the UVC, UVB, and UVA shielding ratios were 97.3, 96.9, and 90.8%, respectively. The visible light transmissibility of the PLA/NTM composite film was 41.8%, and its tensile strength increased by 58.4%. Furthermore, molecular dynamics simulations of the PLA/NTM composite system demonstrated that the hydrogen bonds between TA and HC–NBC synergistically promoted MXene dispersion in the PLA matrix, improving the UV shielding and mechanical properties of the PLA/NTM composite films. The findings of this study provide a new technical reference for the preparation of high-performance transparent UV shielding films.

Tunable band gaps and high carrier mobilities in germanene by Si doping in the presence of an external electric field: Field effect transistors

We have systematically investigated the electronic properties of Si doping germanene under external electric fields by the first-principle calculations. The results show that a wide range of linearly tunable and sizable direct band gaps for the Ge31Si1 and Ge28Si4 systems (3–1048 meV and 6–716 meV) are merely determined by the strength of the electric field. More importantly, the high carrier mobilities of the stable Ge31Si1 and Ge28Si4 systems are largely retained under external electric field. The mechanism of charge transfer confirms the p-type semiconductor properties in the Si doping germanene systems. These results indicate that the electronic properties of Si doping germanene can be effectively modulated under the external electric field, providing a potential application in novel electronic devices, such as Field effect transistors.

Optical Analysis of Perovskite III-V Nanowires Interpenetrated Tandem Solar Cells.

Multi-junction photovoltaics approaches are being explored to mitigate thermalization losses that occur in the absorption of high-energy photons. However, the design of tandem cells faces challenges such as light reflection and parasitic absorption. Nanostructures have emerged as promising solutions due to their anti-reflection properties, which enhances light absorption. III-V nanowires (NWs) solar cells can achieve strong power conversion efficiencies, offering the advantage of potentially integrating tunnel diodes within the same fabrication process. Metal halide perovskites (MHPs) have gained attention for their optoelectronic attributes and cost-effectiveness. Notably, both material classes allow for tunable bandgaps. This study explores the integration of MHPs with III-V NWs solar cells in both two-terminal and three-terminal configurations. Our primary focus lies in the optical analysis of a tandem design using III-V semiconductor nanowire arrays in combination with perovskites, highlighting their potential for tandem applications. The space offered by the compact footprint of NW arrays is used in an interpenetrated tandem structure. We systematically optimize the bottom cell, addressing reflectivity and parasitic absorption, and extend to a full tandem structure, considering experimentally feasible thicknesses. Simulation of a three-terminal structure highlights a potential increase in efficiency, decoupling the operating points of the subcells. The two-terminal analysis underscores the benefits of nanowires in reducing reflection and achieving a higher matched current between the top and the bottom cells. This research provides significant insights into NW tandem solar cell optics, enhancing our understanding of their potential to improve photovoltaic performance.

Nonlinear optical response of strain-mediated gallium arsenide microwire in the near-infrared region

Abstract Gallium arsenide (GaAs) semiconductor wires have emerged as potent candidates for nonlinear optical devices, necessitating bandgap engineering for an expanded operational wavelength range. We report the successful growth of strain-mediated GaAs microwires (MWs) with an average diameter of 1.1 μm. The axial tensile strain in these wires, as measured by X-ray diffraction and Raman scattering, ranges from 1.61 % to 1.95 % and from 1.44 % to 2.03 %, respectively. This strain condition significantly reduces the bandgap of GaAs MWs compared to bulk GaAs, enabling a response wavelength extension up to 1.1 μm. Open aperture Z-scan measurements reveal a nonlinear absorption coefficient of −15.9 cm/MW and a third-order magnetic susceptibility of −2.8 × 10−8 esu at 800 nm for these MWs. I-scan measurements further show that the GaAs saturable absorber has a modulation depth of 7.9 % and a nonsaturation loss of 3.3 % at 1050 nm. In laser applications, GaAs MWs have been effectively used as saturable absorbers for achieving Q-switched and dual-wavelength synchronous mode-locking operations in Yb-bulk lasers. These results not only offer new insights into the use of large diameter semiconductor wires but also expand the potential for applications requiring bandgap tuning.

Effect of biaxial strain on electronic and optical properties of vertically stacked HfS2/HfSe2 heterostructures

The structural, electronic, and optical properties of HfS2/HfSe2 heterostructures (HSs) have been investigated using Density Functional Theory. The HfS2 and HfSe2 monolayers with H and T phases with different stacking configurations were considered for the formation of HSs. HfS2(1T)/HfSe2(2H) is found to be the most stable HS as compared to other phases. All phases of HSs show indirect band-gaps HfS2(1T) (2.07 eV), HfSe2(2H) (1.63 eV), and HfS2(1T)/HfSe2(2H) (1.44 eV) with valence band (VB) of HS dominated by HfS2(1T) and the conduction band (CB) dominated by HfSe2(2H) leading to type-II configuration. The optical properties of HS show ab-sorption peaks in the visible region of 420–430 nm with an absorption coefficient of 1.82–1.89 × 105 cm−1. Further, the stability of the HS increases with tensile strain and decreases with compressive strain. Similarly, the band gap increases from 1.44 eV to 1.51 eV with the tensile strain and decreases to 1.30 eV with compressive strain. The tunability of electronic band gap and band alignment of Hf-based HSs by external strain makes them useful for possible use in futuristic optoelectronic devices.