Band Gap Region Research Articles

Fabrication of La‐Doped MoS2 Nanosheets with Tuned Bandgap for Dye Degradation and Antimicrobial Activities, Experimental and Computational Investigations

AbstractThe development of efficient catalysts with a large number of active sites, tunable bandgap, and large surface area has been very challenging. In addition, a significant bottleneck in the application of catalysts for water treatment is their dissolution under extreme conditions, such as highly acidic or highly alkaline conditions that lead to poor application of the reported materials in real‐world applications. In this study, the lanthanum (La)‐doped molybdenum disulfide (MoS2) nanosheets are reported for efficient breakdown of toxic pollutants from wastewater under a wide pH range from strongly alkaline to strongly acidic solutions. The La‐MoS2 nanosheets (NSs) are prepared by a facile hydrothermal approach using a two‐step methodology. A redshift is observed upon La doping, indicating that the bandgap is lowered after La doping in MoS2. The changes in bandgap and electronic structure are further investigated using the density functional theory (DFT), which reveal that doping of La introduces new states within the bandgap region, allowing for further induced energy transitions. The La‐MoS2, having a doping concentration of 2%, exhibits the highest catalytic activity against methylene blue (MB) in neutral, acidic, and alkaline solutions, as well as substantial inhibitory activity for bacterial strains such as Escherichia coli (E. coli). In summary, the modified catalyst provides a pathway to design highly efficient catalysts for all pH range water treatment as well as good activity against microbes.

Self‐Tracking Solar Concentrator with Absorption of Diffuse Sunlight

AbstractThe highest solar cell efficiencies are obtained with concentrating systems. However, these systems have two major drawbacks: solar tracking is needed and diffuse sunlight is poorly absorbed. Here, a system that overcomes both limitations is presented, by exploiting light‐induced halide segregation in mixed halide perovskite films. A monolayer of silica microspheres focuses direct sunlight in a mixed halide perovskite film below. Through light‐induced phase segregation a low bandgap region forms in the focal point. Together with funneling of excited states toward the low bandgap regions, this results in a voltage increase characteristic for concentrating systems. Diffuse sunlight is still absorbed by the high bandgap material, avoiding the loss of diffuse sunlight characteristic for conventional concentrators. The formation of low bandgap regions in the perovskite is dynamic, and can follow the position of the focus as the sun moves throughout the day, making solar tracking unnecessary. With detailed optical and electrical simulations, it is shown that this concept can lead to an increase in both voltage and current. This leads to a 6.6% absolute increase in power conversion efficiency compared to the film without microspheres. Absorption and emission measurements confirm the concentration effect, showing the promising potential for a self‐optimizing concentrating system.

Insights into the Fe3+ Doping Effects on the Structure and Electron Distribution of Cr2O3 Nanoparticles.

Herein, we carefully investigated the Fe3+ doping effects on the structure and electron distribution of Cr2O3 nanoparticles using X-ray diffraction analysis (XRD), maximum entropy method (MEM), and density functional theory (DFT) calculations. We showed that increasing the Fe doping induces an enlargement in the axial ratio of c/a, which is associated with an anisotropic expansion of the unit cell. We found that as Fe3+ replaces Cr in the Cr2O3 lattice, it caused a higher interaction between the metal 3d states and the oxygen 2p states, which led to a slight increase in the Cr/Fe-O1 bond length followed by an opposite effect for the Cr/Fe-O2 bonds. Our results also suggest that the excitations characterize a well-localized bandgap region from occupied Cr d to unoccupied Fe d states. The Cr2O3 and Fe-doped Cr2O3 nanoparticles behave as Mott-Hubbard insulators due to their band gap being in the d-d gap, and Cr 3d orbitals dominate the conduction band. These findings suggest that the magnitude and the character of the electronic density near the O atom bonds in Cr2O3 nanoparticles are modulated by the Cr-Cr distances until its stabilization at the induced quasi-equilibrium of the Cr2O3 lattice when the Fe3+ doping values reaches the saturation level range.

The Relationship between Photoluminescence Emissions and Photocatalytic Activity of CeO2 Nanocrystals.

In this work, we focus on understanding the morphology and photocatalytic properties of CeO2 nanocrystals (NCs) synthesized via a microwave-assisted solvothermal method using acetone and ethanol as solvents. Wulff constructions reveal a complete map of available morphologies and a theoretical-experimental match with octahedral nanoparticles obtained through synthesis using ethanol as solvent. NCs synthesized in acetone show a greater contribution of emission peaks in the blue region (∼450 nm), which may be associated with higher Ce3+ concentration, originating shallow-level defects within the CeO2 lattice while for the samples synthesized in ethanol a strong orange-red emission (∼595 nm) suggests that oxygen vacancies may originate from deep-level defects within the optical bandgap region. The superior photocatalytic response of CeO2 synthesized in acetone compared to that of CeO2 synthesized in ethanol may be associated with an increase in long-/short-range disorder within the CeO2 structure, causing the Egap value to decrease, facilitating light absorption. Furthermore, surface (100) stabilization in samples synthesized in ethanol may be related to low photocatalytic activity. Photocatalytic degradation was facilitated by the generation of ·OH and ·O2- radicals as corroborated by the trapping experiment. The mechanism of enhanced photocatalytic activity has been proposed suggesting that samples synthesized in acetone tend to have lower e'─h· pair recombination, which is reflected in their higher photocatalytic response.

Temperature tunable Bragg transmission multichannel filters made of two quarter-wave stacks separated by defect layers

We report on simulated temperature-tunable single-channel/multichannel transmission filters with 0.37 nm/K shift in the peak wavelength is observed in the infrared region (1300 nanometers −1650 nanometers) using a one-dimensional photonic crystal structure. A single channel can be selected in the photonic bandgap region based on the thickness of the quarter wave stacks and temperature. The transmission coefficient of the transmitted defect modes is approximately the same as that required for telecommunication. For 20000 defect layers, 4000 channels were created with full width at half maximum of 0.7 picometers at the center wavelength of ∼1550 nm and channel separation of ∼0.18 nanometer between 1500 nanometers-1600 nanometers.

Defect Passivation Results in the Stability of Cesium Lead Halide Perovskite Nanocrystals

Nanoheterostructures (NHSs) based on lead halide perovskites (LHPs) and chalcogenide quantum dots have proved to be promising candidates for photovoltaic device applications. However, understanding the defect chemistry at the interfaces of LHPs and chalcogenides is essential to stabilize them and further tune their optoelectronic properties. Here, we demonstrate a route for designing CsPbBr3–PbSe NHSs and other derivatives of LHP-based NHSs using defect-rich MoSe2 nanosheets (NSs) and study the effect of the size of PbSe NPs on their optical properties. In this synthesis route, PbSe nanoparticles (NPs) are formed at an early stage of the reaction through a unique cation displacement reaction, over which CsPbBr3 nanocrystals (NCs) are epitaxially grown. Using this methodology, a nearly 3-fold enhancement in photoluminescence (PL) is achieved, whereas other selenium precursors, which form larger PbSe NPs, result in negligible PL enhancement with respect to the pure CsPbBr3 NCs. Detailed density functional theory (DFT) calculations suggest that the PbSe NPs are responsible for passivating the surface defects that consequently enhance the PL intensity. However, in the case of larger PbSe NPs, the associated valence and conduction bands lie within the band-gap region of CsPbBr3, creating a type-I heterostructure between the two materials, thereby affecting the luminescence properties. Strong passivation of surface defects in CsPbBr3–PbSe NHSs is also evidenced from low-temperature PL studies. Furthermore, the resulting CsPbBr3–PbSe NHSs demonstrate enhanced stability in the presence of water and do not degrade under ambient conditions for several months.

Microwave Propagation Characteristics in Magnetized‐Cold‐Plasma‐Based Binary Photonic Crystal with Defect of MCP Layer

AbstractIn this communication, the TMM for the analysis of transmission characteristics of MCP based PCs is employed. The periodic structure of air/right‐hand‐MCP, in asymmetric form (AB)N, is considered as RHPPC. The transmission characteristics of the same structure are analyzed with symmetrically introduced defect layer of MCP for different values of the magnetizing field. It is observed that this binary RHPPC in both cases acts as good reflectors in low and high frequency ranges for low magnetic field between 0.4T to 1.2T; and the bandgaps vanish at higher magnetic field above 1.2T, in both types of PCs. Further, it is noticed that the transmittance in defect introduced PC is low in comparison to corresponding PC without defect, for a particular value of magnetic field up to 1.2T. The defective RHPPC exhibits defect modes in the bandgap regions. Hence, an increase in magnetic field decreases the bandgaps and consequently increases the transmission in both cases; while impact of defect is to reduce the transmission. The lowest reverse transmission peak for the defective PC obtained with the value of magnetic field from 2.0T to 2.4T shows more transmission as compared to that of the PC without defect. The insights of this comparative analysis can be used in designing tunable microwave transmitters.

Operando studies of Mn oxide based electrocatalysts for the oxygen evolution reaction.

Inspired by photosystem II (PS II), Mn oxide based electrocatalysts have been repeatedly investigated as catalysts for the electrochemical oxygen evolution reaction (OER), the anodic reaction in water electrolysis. However, a comparison of the conditions in biological OER catalysed by the water splitting complex CaMn4Ox with the requirements for an electrocatalyst for industrially relevant applications reveals fundamental differences. Thus, a systematic development of artificial Mn-based OER catalysts requires both a fundamental understanding of the catalytic mechanisms as well as an evaluation of the practicality of the system for industrial scale applications. Experimentally, both aspects can be approached using in situ and operando methods including spectroscopy. This paper highlights some of the major challenges common to different operando investigation methods and recent insights gained with them. To this end, vibrational spectroscopy, especially Raman spectroscopy, absorption techniques in the bandgap region and operando X-ray spectroelectrochemistry (SEC), both in the hard and soft X-ray regime are particularly focused on here. Technical challenges specific to each method are discussed first, followed by challenges that are specific to Mn oxide based systems. Finally, recent in situ and operando studies are reviewed. This analysis shows that despite the technical and Mn specific challenges, three specific key features are common to most of the studied systems with significant OER activity: structural disorder, Mn oxidation states between III and IV, and the appearance of layered birnessite phases in the active regime.

Surface and optical properties of phase-pure silver iodobismuthate nanocrystals.

The study of surface defects is one of the forefronts of halide perovskite research. In the nanoscale regime, where the surface-to-volume ratio is high, the surface plays a key role in determining the electronic properties of perovskites. Perovskite-inspired silver iodobismuthates are promising photovoltaic absorbers. Herein, we demonstrate the colloidal synthesis of phase pure and highly crystalline AgBiI4 nanocrystals (NCs). Surface-sensitive spectroscopic techniques reveal the rich surface features of the NCs that enable their impressive long-term environmental and thermal stabilities. Notably, the surface termination and its passivation effects on the electronic properties of AgBiI4 are investigated. Our atomistic simulations suggest that a bismuth iodide-rich surface, as in the case of AgBiI4 NCs, does not introduce surface trap states within the band gap region of AgBiI4, unlike a silver iodide-rich surface. These findings may encourage the investigation of surfaces of other lead-free perovskite-inspired materials.

Bandgap prediction for a beam containing membrane-arch-mass resonators

This work aims to propose a promising locally resonating system consisting of a tensioned elastic membrane and two-arch masses attached on the membrane surface. Traditional membrane-type resonators, which usually create one obvious attenuation zone at low frequencies, might not be efficient in multi-frequency vibration suppression. The proposed structure can produce an extra clear flexural attenuation region and shift bandgap frequencies below 300 Hz. By adjusting geometric parameters (thickness, width, and location) of the arch mass, the bandgap region can be tuned. Introducing a feasible analytical model for accurately predicting the first and second initial frequencies of the bandgaps for a beam structure containing membrane-arch-mass resonators is another focus of this study. The proposed theoretical framework can be used to tune the bandgap to different target frequency ranges without knowing the actual width of the bandgap. Finite-element analysis and experiments are conducted to verify the theoretical predictions. A good agreement is seen among the theoretical, finite-element analysis, and experimental results. In addition, adjacent cells with different arch-mass distributions can generate two pairs of flexural bandgaps, increasing the practicality in engineering applications. The proposed structure might be used in low-frequency vibration isolation and filters.

Performance analysis of the salinity based on hexagonal two-dimensional photonic crystal: computational study

We have designed a unique structure for a liquid sensor based on two-dimensional PCs with a triangular lattice constant in the periodicity by drilling a hexagonal cylinder in a dielectric host material. Using the COMSOL multiphysics approach, we investigated the given structure and sensing performance based on the finite element method. We will optimize two-dimensional hexagonal photonic crystals to localize the photonic band gap region in the mid and far infra-red frequency range, as water is a good absorber for this range of frequencies. Then, we inject the central hexagonal cylinder with saline water and calculate the sensor parameters for different values of the refractive index of saline water at different frequencies related to photonic band gaps. We could reach the optimum conditions of the salinity sensor as the half diagonal of the hexagonal shape (R) = 500 nm, the perpendicular distance between the two diagonal hexagonal (D) = 250 nm, and the number of periods (N) = 5, which gives a high efficiency with sensitivity (S) = 525 nm/RIU, figure of merit (FOM) = 80.7 RIU−1, and quality factor (Q) = 375. The effects of structural characteristics on sensing performance are investigated, with new approaches for improving salinity sensors proposed. Furthermore, traditional salinity sensors may be replaced by the proposed method in the photo-sensing application, which is simple and practical for use in the thermal desalination techniques.

Identification of carbon location in p-type GaN: Synchrotron x-ray absorption spectroscopy and theory

Identifying atomic configurations of impurities in semiconductors is of fundamental interest and practical importance in designing electronic and optoelectronic devices. C impurity acting as one of the most common impurities in GaN, it is believed for a long time that it substitutes at Ga site forming CGa with +1 charge-state in p-type GaN, while it substitutes at N site forming CN with -1 charge-state in n-type GaN. However, by combining x-ray absorption spectroscopy and first-principles simulations, we observed that C is mainly occupying the N site rather than the Ga one in p-GaN. We further reveal that this is due to an H-induced EF-tuning effect. During growth, the existing H can passivate Mg dopants and upshifts the EF to the upper region of bandgap, leading to the CN formation. After the p-type activation by annealing out H, although the EF is pushed back close to the valence band maximum, whereas the extremely large kinetic barrier can prevent the migration of C from the metastable CN site to ground-state CGa site, hence stabilizing the CN configuration. Additionally, the CN with neutral charge-state (CN0) in the p-GaN is further observed. Therefore, the real C-related hole-killer in p-type GaN could be CN rather than the commonly expected CGa. Our work not only offers the unambiguous evidence for the C defect formation in p-GaN but also contributes significantly to an in-depth understanding of the C-related hole-killers and their critical role on electrical and optoelectrical properties of p-GaN and even p-AlGaN.

Double-layer stretching broadens the absorption range of the solar spectrum in XSi2N4

A new two-dimensional material MoSi2N4 and WSi2N4 has been synthesized in experiments [science 369, 670–674 (2020)]. This two-dimensional material has good band gap values and stability in optoelectronic applications. We systematically analyzed the electronic structure, and optical properties of monolayer MoSi2N4 and WSi2N4 by first-principles calculations and found that monolayer MoSi2N4 and WSi2N4 can be used as photocatalytic materials. To improve their photocatalytic properties, we used both multilayer stacking and tensile stretching methods. However, the energy band changes are not obvious. Therefore, we adopted a new approach to stretch bilayers of MoSi2N4 and WSi2N4. The results show that stretching MoSi2N4 and WSi2N4 bilayers can effectively change their bandgap regions, extend the light absorption range, and improve their photocatalytic efficiency. The prediction could provide useful information to improve the property of the materials, which deserves to be checked in further experiments.

Geometric shapes and vacancy rates for the modulation of the electronic and transport properties in MoS2 nanoflakes

The electronic and transport properties of MoS2 nanoflakes are investigated using a six-band tight-binding model. The energy band structures are modified by changing the shape, size, vacancy rate, and vacancy distribution of the flakes, resulting in additional energy states in the energy bandgap region. For large flakes, the effects of their geometric shape are found to be negligible, because shape effects on the density of states (DOS) decrease with increasing flake size. The positions and DOS of these additional in-gap states are strongly related to the vacancy rate as well as to the distribution of vacancies. The number of in-gap states and the magnitude of the density of these states are both proportional to the vacancy rate. However, if the rate is high enough for vacancy clusters to form, the transmission is somewhat degraded at certain incident energies, owing to backscattering and capture of carriers by the clusters. Since the current transmission is determined primarily by the detailed energy band structure, the in-gap states due to vacancies enable the realization of low-energy currents in devices based on low-dimensional materials.

Excitonic Effects in the Optical Properties of Scandium and Hafnium MXene Semiconductors from First‐Principles Calculations

The present study uses a full‐potential linearized augmented plane wave method and Bethe–Salpeter equation to calculate and interpret the optical properties of Sc2CT2 (T = O, F, and OH) and Hf2CO2 MXene semiconductors, including real and imaginary parts of the dielectric function along with energy loss function. Excitonic structures are observed below calculated bandgaps in these compounds. Sc2CO2 and Sc2CF2 have three excitonic features, whereas Sc2C(OH)2 and Hf2CO2 show two. As compared to other compounds, Sc2CO2 has a larger exciton binding energy. The energy loss functions display spectral features near the bandgap regions due to the excitonic structures in the imaginary part of the dielectric function.

Tunable electronic properties of diamond (100) surface via boron-nitrogen co-termination: A first-principles study

Surface terminations in diamond are indispensable to stabilize the structures and modulate the electronic properties. In this study, we investigate the structural and electronic properties of boron (B) and nitrogen (N) co-terminated diamond (100) surface with various B/N ratios by a global structure prediction method combined with the first-principles calculations. The predicted surface structures (named as BN3, B2N2, and B3N) are dynamically and thermally stable. The diamond surface characteristics change from positive electron affinity (PEA) to negative electron affinity (NEA) as the B/N ratio increases. The BN3 structure has a PEA value of 1.11 eV, and the B2N2 and B3N possess NEA values of −0.01 to −0.97 eV. Furthermore, the B and N related surface states are introduced into the bandgap region of bulk diamond. Consequently, our results will provide the theoretical guidance for designing and fabricating electronic devices based on diamond.

Tunable spectral manifestation of Tamm plasmon-polaritons in a hybrid structure with 2d black phosphorus in the terahertz range

ABSTRACT A structure consisting of the 2d black phosphorus (BP) film and a rugate filter (RF) divided by a liquid crystal (LC) layer is theoretically studied as a system in which Tamm plasmon-polaritons (TPP) can be excited. The spectral distribution of the reflection coefficient of the system in the RF band gap region is calculated for the cases when the band gap is in the regions of 1 and 10 THz. It is shown that, in both cases, the reflection coefficient dip associated with TPP excitation near the 2d BP film appears in the RF band gap region. The spectral position and magnitude of the reflection coefficient dip depend on the harmonic profile parameters of the RF refractive index, concentration and effective mass of the 2d BP charge carriers, and the LC layer thickness and refractive index. It was found that the TPP wavelength is a periodic function of the LC layer thickness and its refractive index, and with their increase, two or more TPP can appear at wavelengths within the RF band gap. The possibility of controlling the TPP spectral position using an external electric field is noted.

Unveiling photoluminescent response of Ce-doped CaCu3Ti4O12: An experimental-theoretical approach

CaCu3Ti4O12: x% Ce (x = 0.00, 0.25, 0.50, 0.75, and 1.00) ceramic composites were prepared via solid-state reaction. Theoretical atomistic simulations were combined with experimental techniques to uncover Ce effects in the (micro)structure and photoluminescence of CaCu3Ti4O12-based ceramics. Application of perovskites ceramics in optoelectronics have been limited by their specific, narrow emission range, which compromise operational efficiency, pushing for the development of novel perovskite-emissive materials. This study results confirm that Ce ions are incorporated at Ca sites within the CaCu3Ti4O12 lattice, inducing point metal and oxygen vacancies in the optical bandgap region. Shallow-level defects (VCa′′/VO⦁⦁) were associated with broadband violet-blue photoluminescent (PL) emissions. Better color rendering may be a direct consequence of crystalline field splitting/wider PL emission. Furthermore, results demonstrate that PL on CaCu3Ti4O12: Ce system intensity can be modulated by structural defects, making it promising for applications in optoelectronics.

Highly improved light harvesting and photovoltaic performance in CdTe solar cell with functional designed 1D-photonic crystal via light management engineering

Photonic-based functional designs and integrations for advanced optoelectronic devices are regarded as promising candidates considering the enhancement of efficiency and tunability. With the aim to improve photovoltaic performance by increasing photon harvesting, the study presents the prominent findings of experimental and theoretical comparison of optical and electrical evaluation integrating a functionally designed one-dimension photonic crystal (1D-PC) into CdTe solar cells. Since transparency of the CdS/CdTe heterojunction based solar cell (SC) is reduced by a photonic band gap formed by (MgF2/MoO3)N 1D-PC; namely, re-harvesting is improved by increasing absorbance. The period number at resonance wavelength of 850 nm and photocurrent density ({J}_{ph}) have remarkable influence on the investigation. For four periods, the reflectance in the region of photonic band gap is sufficient for photon harvesting and saturation occurs. The photovoltaic performances are comparatively analysed for SCs with and without 1D-PC produced at optimal values. The open-circuit voltage does not change, besides, short-circuit current density and maximum-current density vary between 15.86–17.23 mA cm−2 and, 13.08–15.41 mA cm−2. Having integrated the 1D-PC into the structure, it is concluded that the FF and power conversion efficiency increase from 55.27 to 63.35% with an improvement of 15.91% and, from 8.26 to 10.47% with an improvement of 21.10%.

Facile synthesis of ZnO–Ag nanocomposite supported by graphene oxide with stabilised band-gap and wider visible-light region for photocatalyst application

The synthesis of zinc oxide (ZnO), with the support of silver (Ag) and Graphene Oxide (GO), was carried out in several stages as a potential photocatalytic material. First, the GO was synthesized from commercial graphite using the Hummers’ method, and ZnO and Ag precursors were prepared. The second stage was the electrospinning process, followed by calcination. The solution in the electrospinning process, to produce the nanofibers, was a mixture of polymer (polyvinyl alcohol), zinc acetate, AgNO3, and GO. The fibres produced were thermally treated at 500 C for 2 h. XRD and FTIR analyses confirmed that GO was successfully synthesized from commercial graphite. ZnO and Ag had wurtzite and cubic hexagonal structures based on XRD and TEM characterization. The nanocomposites developed had increased photocatalyst characteristics: low band gap energy, such as 2.98 (ZnO), 2.76 (ZnO–Ag), 2.93 (ZnO-GO), and 2.75 (ZnO–Ag-GO). Furthermore, the nanocomposites had absorption characteristics in the visible light region. Using UV–Visible spectrophotometer, Diffuse Reflectance Spectroscopy, and Photoluminescence, it was explained that the Surface Plasmon Resonance effect possessed by Ag and GO nanoparticles had semiconductor properties that acted as electron trapping, thereby reducing or preventing the occurrence of electron–hole recombination. In conclusion, the ZnO nanocomposites with the addition of Ag and GO could improve the photocatalytic characteristics of ZnO with the potential for direct application in photodegradation of organic and textile waste in water.