Small Band Gap Research Articles

Structural, optical and electrical studies of non-toxic oxide- RMnO3 [R=Y, Er, Yb] hexagonal manganite thin-films

Recently, ferroelectric-photovoltaics have come under the spotlight as a potential class of materials for application in photovoltaic devices. However, the broad bandgap of these ferroelectric-photovoltaic materials causes them to have modest photocurrents[1]. To overcome this challenge hexamanganites can be used to achieve high photovoltaic efficiency as it has a small band gap. Here we present a stable, non-toxic, and cost-efficient hexamanganite RMnO3 [R=Y, Er, Yb] thin films prepared by spray pyrolysis method. The formation of the single phase is confirmed by X-ray diffraction analysis. Morphological studies show grains are uniform and closely packed and the grain size increases with the decrease in ionic radii of rare earth ions. A narrow optical band gap of nearly 1.5 eV is observed for the films. The results of this investigation show that Ultraviolet ray irradiation can be used to tune the bandgap. With a carrier concentration of around 10+14 cm−3, Hall measurements proved that the films are p-type semiconductors. This research illuminates the exploration of stable oxide semiconductors with a small band gap for applications in futuristic solar cells.

Theoretical exploration of band gap error dependency on band gap size in density functional theory Calculations: CdTe and GeTe as representative cases of two band structure semiconductor types

This study addresses the prevalent band gap error problem in density functional theory (DFT) with local density or generalized gradient approximation (LDA/GGA), widely considered as the “standard model” for investigating the band structure of semiconductors. Despite previous DFT efforts focusing on improving the exchange correlation energy to mitigate the underestimation of the band gap, the correlation between the band gap error and the intrinsic electronic structure of semiconductors has been overlooked. From two principal observations: (i) traditional semiconductors typically have a p-s type band structure, and (ii) a smaller band gap is linked to less underestimation, we derive two crucial questions: (i) How is band gap error manifested in s-p type semiconductors? (ii) Does the error vanish as the band gap approaches zero, akin to metals? Through the examination of CdTe and GeTe, representing p–s and s–p type semiconductors with strain-engineered band gaps, our calculations indicate that for CdTe, the band gap is underestimated, whereas for GeTe, it is overestimated. Significantly, these errors persist even as the band gap of semiconductors tends to zero, unlike in metals. This persistent discrepancy stems from the discontinuity in wavefunctions between the semiconductors' VBM and CBM.

Facile green preparation of Y2O3 decorated with ZnFe2O4 using citrus limon (L.) osbeck leaf extract for photocatalytic degradation of malachite green

The Y2O3 decorated with ZnFe2O4 was successfully synthesized using Citrus limon (L.) Osbeck leaf extract for the first time. A modification of Y2O3 with ZnFe2O4 nanoparticles was carried out because Y2O3 is stable but has a wide band gap, making it less active in visible light. On the other hand, ZnFe2O4 has a small band gap and is cheap. The synthesized ZnFe2O4/Y2O3 nanocomposites, ZnFe2O4, and Y2O3 nanoparticles were characterized using FTIR, XRD, TEM, SEM, and DRS UV–Vis. TEM and DRS UV–Vis results show that the ZnFe2O4/Y2O3 nanocomposite particle has a size of 49,61 nm with a unique shape and an optical band gap of 2.08 eV. The photocatalytic activity of Y2O3, ZnFe2O4, and ZnFe2O4/Y2O3 was observed based on the photodegradation of malachite green (MG) dye under visible light. The results demonstrate that ZnFe2O4/Y2O3 can degrade MG with a photodegradation percentage of 95 % within 120 min, which is better than pure ZnFe2O4 and Y2O3. After the fourth cycle, the photodegradation percentage of MG by ZnFe2O4/Y2O3 remained at 88 %, demonstrating good reusability of ZnFe2O4/Y2O3 for photocatalytic degradation. These findings demonstrate that the use of ZnFe2O4 as a modifier may increase the photocatalytic performance of Y2O3.

DFT-1/2 for ionic insulators: Impact of self-energy potential on band gap correction

DFT-1/2 is an efficient band gap correction method for density functional theory under local density approximation or generalized gradient approximation. Nevertheless, it still predicts lower band gaps than experimental in highly ionic insulators, such as LiF and CsCl. In this study, we emphasize that the problem is more severe for halides. For example, the DFT-1/2 band gap for Li2O is much more satisfactory than that for LiF. Notwithstanding the traditional belief that high ionicity is the key, we ascribe the main reason to the fact that the self-energy potentials (SEPs) used for anions were derived based on neutral atoms. Extensive tests have shown that an SEP derived from an atom/ion with fewer electrons is stronger than that derived with more electrons. The degree of enhancement in the O SEP, based on O rather than O2–, could generally compensate for the loss of SEP in the trimming process. In the case of halides, such compensation is insufficient, resulting in small band gaps. For ionic nitrides involving N3–, the SEP is undesirably large if it is derived from a neutral N atom. Equivalent scaling factors are prescribed for the SEPs of F and N, with the former being greater and the latter being smaller than unity. With proper SEP scaling factors according to the charge carried by the anion, the DFT-1/2 band gaps are greatly improved for highly ionic insulators.

Exploration of key electronic properties and optical nonlinearity of diamine based Schiff bases: Spectroscopic characterization and DFT study

The study focuses on the synthesis and theoretical investigation of a series of newly synthesized diamine-based Schiff bases. Structural confirmation of the synthesized Schiff bases was achieved through various spectroscopic techniques. DFT calculations, utilizing the M06/6311G (d, p) functional, were conducted to assess NLO characteristics. The broader absorption spectrum (444.0 nm) and a smaller bandgap (4.4 eV) were observed for compound:1E,1′E)-N,N'-(1,4-phenylene)bis(1-(4-nitrophenyl)methanimine compared to the other synthesized compounds. The NBOs results provided insight into the hyperconjugative phenomenon that contributes to the stability of entitled compounds. Among all the examined compounds, 1E,1′E)-N,N'-(1,4-phenylene)bis(1-(4-nitrophenyl)methanimine exhibited the highest value (3.7 × 10−29esu) for first hyperpolarizability (βtot). Moreover, the calculated first hyperpolarizability for the newly synthesized diamine-based compounds is greater than standard para-nitroaniline. Above findings ascertained the potential of synthesized compounds for advanced applications in high-tech NLO technologies.

Perovskite half tandem v-shaped grating nanostructure solar cells: Improvement by light trapping and carrier transfer towards efficiency enhancement

In this paper, the enhancement of the perovskite solar cells (PSCs) efficiency through the implementation of a half-tandem structure featuring two absorbent layers of MAPbI3 and MoTe2 with a V-shaped nanostructured grating was investigated. MoTe2 was utilized as the second lower absorbent with a smaller bandgap than the first and upper absorbent, MAPbI3, to cover the incident light absorption in the infrared wavelength range. Additionally, the structure was also optimized by introducing a nanostructured V-shaped grating to increase the light propagation path and improve the absorption in the active layers. Also, the critical role of the electron transfer layer (ETL) and hole transfer layer (HTL) in carrier transfer and prevention of charge leakage was examined for materials including TiO2, TiO2-Gr (5%) nanocomposite, and Al-doped ZnO (AZO) as ETLs, and Cu2FeSnS4 (CFTS) and reduced Graphene Oxide (rGO) as HTLs in four distinct cell configurations. In this study, the optical and electrical models were solved to determine the optimal solar cell structure and materials through a comparative analysis of structures. Our findings revealed substantial improvements in short-circuit current density (Jsc), open-circuit voltage (Voc), and power conversion efficiency (PCE), where they increased from 15.27 to 31.55 mA/cm2, 0.91 to 1.30 V, and 11.78 to 27.63%, respectively. This resulted in a 2.35 times efficiency enhancement for the nanostructured V-shaped half-tandem solar cell with MAPbI3 (250 nm) and MoTe2 (150 nm) as absorbent layers, AZO (40 nm) as ETL and rGo (40 nm) as HTL compared to the planar non-tandem solar cell with MAPbI3 (250 nm) as absorbent layer, TiO2 (40 nm) as ETL and CFTS (40 nm) as HTL.

Synthesis, studies on properties of Eu3+/Dy3+ co–doped BaMoO4 nanophosphors via hydrothermal method

AbstractThe article shows the outstanding properties and luminescence ability of Eu3+/Dy3+ co–doped BaMoO4 (BMED) materials, which have been successfully fabricated by the hydrothermal method. The crystalline structure analysis of BMED nanophosphor by X–ray diffraction (XRD) shows that it has a structure single–phase structure, with tetragonal BaMoO4 (JCPDS 029–0193), consist of spherical nanoparticles with a size diameter of 31.79–52.99 nm. The characteristic vibrational modes of molybdate lattice in the Raman spectroscopy confirmed the successful Eu3+/Dy3+ co–doped into the host matrix. The field emission scanning electron microscopy (FE–SEM) image shows that the BMED consists of quite uniform nanoparticle with sizes less than 50 nm. The energy dispersive spectra (EDS) confirm the purity and uniform distribution of elements Eu, Dy, Mo, O, Ba throughout in the solid sample. The BMED nanophosphor has is small bandgap from 2.83 to 3.00 eV. The photoluminescence spectra show that BMED have strong red emission at 616 nm of the transition (5D0 → 7F2) with luminescence lifetime from 0.330 to 1.429 ms. The BMED nanophosphor have potential application for the production of warm wLEDs, agricultural lighting and biomedical marking.

Chiral Viologen-Derived Water-Stable Small Band Gap Lead Halides: Synthesis, Characterization, and Optical Properties.

Chiral organic-inorganic metal halides (OIMHs) are attractive for their potential applications in chiral optoelectronics and spintronics, such as circular polarized light emitters, detectors, and chiral-induced spin selectivity. Here, we report three pairs of chiral OIMHs with great water stability constructed from chiral viologens. These OIMHs contain either 1D or 0D structures, however, with small band gaps around 2 eV. Circular dichroism (CD) spectroscopy on transparent thin films of two OIMH pairs showed a wide CD response covering most of the visible light range. Although the chiral center is not directly attached to the pyridinium in these chiral viologens, the chirality is still successfully transferred into both the band gap and the exciton absorption ranges. Liquid and solid CD studies of the chiral viologens further indicate that the chiral induction inside these OIMHs is possibly through chiral crystallization. This work demonstrated the design strategy of water-stable, small band gap chiral OIMHs through chiral viologens. These low-dimensional chiral materials may provide an interesting system to investigate chiral induction, and their broad CD response may enable their potential application as circular photodetectors with a wide detection range.

Stability of 2D Crystals from Tessellation

AbstractReticular chemistry has been a cornerstone in the design of novel 2D materials. Despite numerous possibilities for topological arrangements, only a few with high symmetry can form stable networks. Here, starting from 2D carbons, four types of highly stable tessellations are discovered, which consist of chains of non‐hexagonal rings separated by hexagonal ribbons. A modified Read–Shockley model is established to perfectly describe the stability of these highly stable frameworks, which is based on the interaction between non‐hexagonal rings. Moreover, these four types of tessellations and the modified Read–Shockley model are found to be of general validity in designing highly stable 2D materials, which is verified by the calculations on polymorphs of boron nitride and molybdenum disulfide. Besides, among the studied 2D carbon allotropes, two semi‐metallic structures with highly anisotropic Dirac cones and one semimetal with a Dirac nodal line at the Fermi level are discovered, as protected by their D2h symmetry. Spin‐orbital coupling is further found to open small bandgaps for these three Dirac structures, making them nontrivial topological insulators. The in‐depth understanding of the stability of 2D crystals in this study provides a new way for rational design of 2D crystals that may show peculiar electronic structures.

An Account on BiVO4 as Photocatalytic Active Matter.

Photocatalytic materials are gaining popularity and research investment for developing light-driven micromotors. While most of the early work used highly stable TiO2 as a material to construct micromotors, mostly in combination with noble metals, other semiconductors offer a wider range of properties, including independence from high-energy UV light. This review focuses on our work with BiVO4 which has shown promise due to its small band gap and resulting ability to absorb blue light. Additionally, this salt's well-defined crystal structures lead to exploitable charge separation on different crystal facets, providing sufficient asymmetry to cause active propulsion. These properties have given rise to fascinating physical and chemical behaviors that show how rich and variable active matter can become. Here, we present the synthesis of different BiVO4 microparticles and their material properties that make them excellent candidates as active micromotors. A critical factor in understanding inherently asymmetric micromotors is knowledge of their flow fields. However, due to their small size and the need to use even smaller tracer particles to avoid perturbing the flow field, measuring flow fields at the microscale is a difficult task. We also present these first results, which allow us to demonstrate the correlation between chemical reactivity and the flow generated, leading to active motion. Due to the nontoxic nature of BiVO4, these visible-light-responsive microswimmers have been used to study the first steps toward applications, even in sensitive areas such as food technology. Although these initial tests are far from being realized, we have to face the fact that a single microswimmer will not be able to perform macroscale tasks. Therefore, we present the reader with the first simple studies of collective motion, hoping for many new contributions to the field. The one-step synthesis of BiVO4 clearly paves the way for studies requiring large numbers of particles. We predict that the combination of promising applications for a nontoxic material which is readily synthesized in large quantities will contribute pivotally to advance the field of active matter beyond the proof-of-concept stage.

Bio-Inspired Photosensory Artificial Synapse Based on Functionalized Tellurium Multiropes for Neuromorphic Computing.

Nanomaterials like graphene and transition metal dichalcogenides are being explored for developing artificial photosensory synapses with low-power optical plasticity and high retention time for practical nervous system implementation. However, few studies are conducted on Tellurium (Te)-based nanomaterials due to their direct and small bandgaps. This paper reports the superior photo-synaptic properties of covalently bonded Tellurium sulfur oxide (TeSOx) andTellurium selenium oxide (TeSeOx)nanomaterials, which are fabricated by incorporating S and Se atoms on the surface of Te multiropes using vapor deposition. Unlike pure Te multiropes, the TeSOx and TeSeOx multiropes exhibit controllable temporal dynamics under optical stimulation. For example, the TeSOx multirope-based transistor displays a photosensory synaptic response to UV light (λ = 365nm). Furthermore, the TeSeOx multirope-based transistor exhibits photosensory synaptic responses to UV-vis light (λ = 365, 565, and 660nm), reliable electrical performance, and a combination of both photodetector and optical artificial synaptic properties with a maximum responsivity of 1500 AW-1 to 365nm UV light. This result is among the highest reported for Te-heterostructure-based devices, enabling optical artificial synaptic applications with low voltage spikes (1V) and low light intensity (21 µW cm-2), potentially useful for optical neuromorphic computing.

Investigating the adsorption performance of CO, NO, NO2, NH3, and H2S molecules on C9N4 through DFT for potential gas sensor applications

In this work, the feasibility of using transition metal doped C9N4 as an efficient gas sensor is comprehensively investigated by density functional theory methods. Firstly, the calculated Ef and Udiss values show that most of M-C9N4 and M2-C9N4 have good thermodynamic and electrochemical stability. Secondly, for CO, NO, NO2, NH3, and H2S on Cr-C9N4, the Eads values are −2.07, −3.05, −2.53, −1.76, and −2.14 eV, respectively. For these gases on Fe2-C9N4, the Eads values are −2.60, −3.07, −2.45, −2.39, and −2.21 eV, respectively. The results show that Cr-C9N4 and Fe2-C9N4 have good adsorption strengths for all gases, which can provide good theoretical guidance for the design of excellent sensing materials. Finally, by calculating band structures of all catalysts, they all have small band gap values, such as, Fe2-C9N4 with the 0.10 eV, which all shows stable catalysts possess good conductive properties. The DOS results show that there is an obvious orbital hybridization between metal, N, C, and O atoms, indicating that there is a certain interaction between them, which is an important factor leading to the good adsorption of the catalyst to the molecular gas. This work can provide theoretical guidance for searching efficient sensor materials.

Modeling the Zigzag Curves of Transition Metal Ions' Charge Transition Levels in Crystals.

Predicting the defect levels of transition metal (TM) dopants in the band gap of crystals is critical in determining the charge states of TM dopants and explaining their electronic and optical properties. By analyzing the calculated charge transition levels and the crystal-field strengths of all the 3d-TM ions in several insulators, we demonstrate that the variation trend of the 3d-TM dopants in a crystal is a scaling of the variation of 3d-electron binding energies (ionization potential) of the free TM ions corrected by adding the contribution of the 3d-orbital's crystal-field splitting. We therefore develop a model to predict the relative location of TM ions' defect levels in the band gap from the defect level and crystal-field splitting of a reference TM ion in the host of concern. The model is applied to predict the defect levels of the series of TM ions in β-Ga2O3 and ZnO, which have moderate to small band gaps, making some of the levels fall into the conduction or valence bands. These results show that the model may serve as a quick reference for related material design and optimization.

Vacancy and strain effects on electronic structures of monolayer hexagonal 1T-phase pdSe2

Two-dimensional transition metal dichalcogenides (TMDs) such as MoS2 and WS2 have traditionally been used, but Group-10 TMDs have recently attracted attention due to their small and tunable band gap and high mobility. This paper investigates the impact of vacancy and strain on the structure of hexagonal single-layer PdSe2 using density functional theory (DFT) calculations. The sublayers can introduce strain into the structure when building these 2D surfaces. Structures with chalcogen vacancies are more stable than those with metallic vacancies. When Se atoms are removed, the band gap disappears, and the semiconductor phase changes from semiconductor to conductor. The band gap can be recreated by applying a suitable percentage of strain using the vacancy mode. The work function (WF) was also calculated. WF decreases with compressive strain but increases with Se removal and tensile strain.

22% Record Efficiency in Nanorod Light-Emitting Diodes Achieved by Gradient Shells.

The external quantum efficiency (EQE) in light-emitting diodes (LEDs) based on isotropic quantum dots has approached the theoretical limit of close to 20%. Anisotropic nanorods can break this limit by taking advantage of their directional emission. However, the progress towards higher EQE by using CdSe/CdS nanorods (NRs) faces several challenges, primarily involving the low quantum yield and unbalanced charge injection in devices. Herein, the seeded growth method is modified and anisotropic nanorods are obtained with photoluminescence quantum yield up to 98% by coating a gradient alloyed CdZnSe shell around conventional spherical CdSe seeds. This intermediate alloyed CdZnSe shell combined with a subsequent rod-shaped CdZnS/ZnS shell can effectively suppress the electron delocalization in the typical CdSe/CdS nanorods due to their small conduction bandgap offset. Additionally, this alloyed shell can reduce the hole-injection barrier and create a larger barrier for electron injection, both effects promoting a balanced injection of electrons and holes in LEDs. Hence, LEDs are reached with high brightness (160341cd m-2) and high efficiency (EQE = 22%, current efficiency = 23.19cd A-1), which are the highest values to date for nanorod LEDs.

Exceptional Thermoelectric Performance of Cu2(Zn,Fe,Cd)SnS4 Thin Films.

High-quality Cu2(Zn,Fe,Cd)SnS4 (CZFCTS) thin films based on the parent CZTS were prepared by aerosol-assisted chemical vapor deposition (AACVD). Substitution of Zn by Fe and Cd significantly improved the electrical transport properties, and monophasic CZFCTS thin films exhibited a maximum power factor (PF) of ∼0.22 μW cm-1 K-2 at 575 K. The quality and performance of the CZFCTS thin films were further improved by postdeposition annealing. CZFCTS thin films annealed for 24 h showed a significantly enhanced maximum PF of ∼2.4 μW cm-1 K-2 at 575 K. This is higher than all reported values for single-phase quaternary sulfide (Cu2BSnS4, B = Mn, Fe, Co, Ni) thin films and even exceeds the PF for most polycrystalline bulk materials of these sulfides. Density functional theory (DFT) calculations were performed to understand the impact of Cd and Fe substitution on the electronic properties of CZTS. It was predicted that CZFCTS would have a smaller band gap than CZTS and a higher density of states (DoS) near the Fermi level. The thermal conductivity and thermoelectric figure of merit (zT) of the CZFCTS thin films have been evaluated, yielding an estimated maximum zT range of 0.18-0.69 at 550 K. The simple processing route and improved thermoelectric performance make CZFCTS thin films extremely promising for thermoelectric energy generation.

Induced out-of-plane piezoelectricity and giant Rashba spin splitting in Janus WSiZ3H (Z = N, P, As) monolayers toward next-generation electronic devices

Two-dimensional (2D) piezoelectric nanomaterials have widely been studied recently due to their promise for various applications in technology. Investigation of vertical piezoelectricity will contribute to a deeper understanding of the intrinsic mechanism of piezoelectric effects in the 2D structures. In this paper, we report a first-principle study for the structural, electronic, piezoelectric, and transport properties of new-designed Janus WSiZ3H (Z= N, P, and As) monolayers. The structural stability of WSiZ3H is theoretically confirmed based on the energetic, phonon dispersion, and also elastic analyses. At the ground state, while WSiN3H is an indirect semiconductor, both WSiP3H and WSiAs3H are predicted to be direct semiconductors with smaller bandgaps. When the spin-orbit coupling effects are taken into account, a large valley spin splitting is observed at the K point of WSiZ3H materials. Interestingly, a giant Rashba spin splitting is found in WSiP3H and WSiAs3H with Rashba constant αR up to 770.91 meV Å. Additionally, our first-principles study indicates that Janus WSiZ3H monolayers are piezoelectric semiconductors with high out-of-plane piezoelectric coefficient |d31|, up to 0.15 pm/V, due to the broken mirror symmetry. Besides, with high electron mobilities and also possessing direct band gaps, WSiP3H and WSiAs3H monolayers are favorable for applications in optoelectronics.

Accurate and rapid predictions with explainable graph neural networks for small high-fidelity bandgap datasets

Accurate and rapid bandgap prediction is a fundamental task in materials science. We propose graph neural networks with transfer learning to overcome the scarcity of training data for high-fidelity bandgap predictions. We also add a perturbation-based component to our framework to improve explainability. The experimental results show that a framework consisting of graph-level pre-training and standard fine-tuning achieves superior performance on all high-fidelity bandgap prediction tasks and training-set sizes. Furthermore, the framework provides a reliable explanation that considers node features together with the graph structure. We also used the framework to screen 105 potential photovoltaic absorber materials.

Strain-Induced Topological Phase Transitions Covering the Indicator in Orthorhombic Li2AuBi.

The symmetry indicator is widely used to classify topological materials hosting inversion symmetry. We find orthorhombic Li2AuBi in space group Cmcm is a topological insulator with under no strain via first-principles calculations. Due to small band gaps in the kz = 0 plane, the band inversions can be selectively induced by moderate external strains to realize phases covering all values of = 1, 2, 3, and 0. Detailed phase diagrams are plotted under various moderate strains. The (001) surface states and their associated Fermi surfaces and spin textures are calculated. The topological surface states have different connectivities and different spin textures for the four different phases. The tunability of topological surface states via moderate strain suggests Li2AuBi as an attractive topological material for device applications.

Geometry optimization and energy parameter calculations using density-functional theory method and molecular docking of anticonvulsant therapeutic metal complexes of gabapentin

This work aims to give computational studies of Mn(II), Co(II), Ni(II) and Cu(II) complexes of gabapentin (Gpn), formulized as [M(Gpn)(H2O)3(Cl)].nH2O complexes (where n = 2-6), using DFT method. They were previously synthesized and characterized. DFT calculations are in good agreement with practical studies. Bond lengths of metal complexes reduced or increased rather than that of ligand due to complexation. Bond angles of complexes predict the octahedral environment around the central metal ions predicting sp3d2 or d2sp3hybridization. The calculated energy parameters are negative indicating stability of metal complexes. The small energy band gap of compounds predicts the higher biological activity and high tendency of electron transfer. The comparable frequencies of theoretical and experimental IR may be attributed to different phases of measurement. The induced fit docking SP G-score of the molecular interactions of drug (Gpn) and its metal(II) complexes show that all investigated compounds have a good interaction towards sertonine receptor 5-HT2C and D2 dopamine receptor proteins. Co(II)-Gpn interacts with active site residues of sertonine receptor 5-HT2C with an excellent dock score of -7.370 kcal/mol and RMSD = 1.581 Å. On the other hand, Ni(II)-Gpn has the best dock score of -6.638 kcal/mol and RMSD = 1.995 Å with D2 dopamine receptor.
 KEY WORDS: Gabapentin, Transition metals, DFT-method, molecular docking
 Bull. Chem. Soc. Ethiop. 2024, 38(2), 511-526. 
 DOI: https://dx.doi.org/10.4314/bcse.v38i2.18