Band Gap Size Research Articles

Energy gap and aromatic molecular rings.

The manuscript combines rational density functional theory simulations and experimental data to investigate the electrical properties of eight polycyclic aromatic hydrocarbons (PAHs). The optimized geometries reveal a preference for one-row, two-row and three-row ring distributions. Band structure plots demonstrate an inverse correlation between the number of aromatic rings and band gap size, with a specific order observed across the PAHs. Gas phase simulations support these findings, though differences in values are noted compared to the literature. Introducing a two-row ring distribution concept resolves discrepancies, particularly in azulene. The B3LYP function successfully bridges theoretical and experimental gaps, particularly in large PAHs. The manuscript highlights the potential for designing electronic devices based on different-sized PAHs, emphasizing a multi-ring distribution approach and opening new avenues for practical applications.

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.

Influence of shear strain on the electronic properties of monolayers MoS2, WS2, and MoS2/WS2 vdW heterostructure.

This study investigates the dynamic stability of monolayers MoS2, WS2, and MoS2/WS2 van der Waals heterostructures (vdWHs) and the influence of shear strain on their electronic properties. The computational results of the binding energy and phonon dispersion demonstrate the excellent dynamic stability of MoS2/WS2 vdWHs. The MoS2/WS2 vdWH, with a type-II band alignment and an indirect bandgap, reduces electron-hole recombination, enhancing the efficiency and performance of optoelectronic devices. Under shear strain, the bandgap size and type of monolayers MoS2, WS2, and MoS2/WS2 vdWHs were effectively modulated, along with the interlayer charge redistribution in the MoS2/WS2 vdWHs. This work reveals the tunability of the electronic properties of monolayers MoS2, WS2, and MoS2/WS2 vdWHs under shear strain, offering new possibilities and solutions for developing optoelectronic devices, sensors, and related fields. This work employed the CASTEP module within the Materials Studio software package for first-principles calculations. Ultrasoft pseudopotentials were employed during geometry optimizations to account for ion-electron interactions using the GGA-PBE functional for exchange-correlation potentials. The electronic configurations of the S, Mo, and W atoms were chosen as their typical arrangements: (3s2p4), (4s2p6d55s1), and (5s2p6d46s2), respectively. A vacuum layer of 20Å was added to avoid interactions between the atomic layers. A cutoff energy of 500eV was set for structural optimization and self-consistent calculations, with k-point grids of 6 × 6 × 1 and 9 × 9 × 1. During the structural optimization process, the energy convergence criterion was set to 1 × 10-5eV, and the thresholds for interatomic forces and stresses were set to 0.01eV/Å and 0.01 GPa, respectively. Grimmer's DFT-D2 correction accounted for the interlayer vdW interactions in the MoS2/WS2 vdWH, while the phonon dispersion was calculated using the linear response method.

Synthesis and Exploration of Barium Stannate-Zirconate BaSn1-xZrxO3 (0 ≤ X ≤ 1) Solid Solutions as Photocatalysts.

Employing ionic liquid-assisted microwave synthesis and moderate heat treatment allows for the preparation of otherwise difficult-to-obtain perovskite-type BaSn1-xZrxO3 solid solutions (0 ≤ x ≤ 1). The impact of substituting Sn for the crystal structure, crystallinity, morphology, and photocatalytic performance was investigated. The obtained materials are characterized by X-ray diffraction, scanning electron microscopy, Brunauer-Emmett-Teller (BET) surface area analysis, X-ray photoelectron spectroscopy, UV-Vis diffuse reflectance spectroscopy, photoluminescence spectroscopy, and Raman and IR spectroscopy. SEM images show that the morphology of the samples varies from rods for x = 0, 0.2 to spherical for x = 0.5, 0.8, 1. Upon Zr for Sn substitution, the band gap changes from 3.1 to 5.0 eV as the valence and conduction bands move to lower and higher energies. The photocatalytic activities of the BaSn1-xZrxO3 samples in the hydroxylation of terephthalic acid (TA) follow the order BaSn0.5Zr0.5O3> BaSn0.8Zr0.2O3> BaSnO3> BaSn0.2Zr0.8O3> BaZrO3. The superior photocatalytic activity of BaSn0.5Zr0.5O3 can be attributed to the synergistically favorable combination of a suitable band structure, band gap size, and increased surface area-to-volume ratio, resulting in a diminished crystalline particle size unattainable from samples prepared via traditional synthetic routes or without ionic liquid.

Superpentagraphene C34: A two-dimensional network structure of C20 fullerene

The well-known smallest icosahedral fullerene C20 is comprising five-membered carbon-rings in all sp2 bonding states. Here we identify by ab initio calculations a new quasi two-dimensional (2D) carbon allotrope in sp2-sp3 bonding states, comprising the unusual five-membered C20 fullerene as the basic structural units, with a 34-atom hexagonal cell in P6/mmm(D6h1) symmetry, and term it as spg-C34 superpentagraphene. Total energy calculations show that it is energetically more stable than the previously reported 2D carbon forms such as pentagraphene and ph-graphene. Its structural stability has been verified by phonon mode analysis and ab initio molecular dynamics simulations. Electronic band calculations reveal that this 2D spg-C34 carbon is a semiconductor with a sizable indirect band gap of 3.31 eV. Moreover, we show that such 2D spg-C34 carbon structure has a larger layer modulus (281 N/m), a larger in-plane stiffness (524 N/m), and a smaller Poisson's ratio (0.069) than the values for graphene. These results establish a new exotic quasi-2D carbon phase and offer insights into its outstanding electronic and mechanical properties of two-dimensional carbon structure.

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.

Multi-Layer Palladium Diselenide as a Contact Material for Two-Dimensional Tungsten Diselenide Field-Effect Transistors.

Tungsten diselenide (WSe2) has emerged as a promising ambipolar semiconductor material for field-effect transistors (FETs) due to its unique electronic properties, including a sizeable band gap, high carrier mobility, and remarkable on-off ratio. However, engineering the contacts to WSe2 remains an issue, and high contact barriers prevent the utilization of the full performance in electronic applications. Furthermore, it could be possible to tune the contacts to WSe2 for effective electron or hole injection and consequently pin the threshold voltage to either conduction or valence band. This would be the way to achieve complementary metal-oxide-semiconductor devices without doping of the channel material.This study investigates the behaviour of two-dimensional WSe2 field-effect transistors with multi-layer palladium diselenide (PdSe2) as a contact material. We demonstrate that PdSe2 contacts favour hole injection while preserving the ambipolar nature of the channel material. This consequently yields high-performance p-type WSe2 devices with PdSe2 van der Waals contacts. Further, we explore the tunability of the contact interface by selective laser alteration of the WSe2 under the contacts, enabling pinning of the threshold voltage to the valence band of WSe2, yielding pure p-type operation of the devices.

Magnetization direction-controlled topological band structure in TlTiX (X = Si, Ge) monolayers

The quantum anomalous Hall (QAH) insulator is a vital material for the investigation of emerging topological quantum effects, but its extremely low working temperature limits experiments. Apart from the temperature challenge, effective regulation of the topological state of QAH insulators is another crucial concern. Here, by first-principles calculations, we find a family of stable two-dimensional materials TlTiX (X = Si, Ge) are large-gap QAH insulators. Their extremely robust ferromagnetic (FM) ground states are determined by both the direct- and super-exchange FM coupling. In the absence of spin–orbit coupling (SOC), there exist a spin-polarized crossing point located at each K and Kʹ points, respectively. The SOC effect results in the spontaneous breaking of C 2 symmetry and introduces a mass term, giving rise to a QAH state with sizable band gap. The tiny magnetocrystalline anisotropic energy (MAE) implies that an external magnetic field can be easily used to align magnetization deviating from z direction to the x–y plane, thereby leading to a transformation of the electronic state from the QAH state to the Weyl half semimetals state, which indicate monolayers TlTiX (X = Si, Ge) exhibit a giant magneto topological band effect. Finally, we examined the impact of stress on the band gap and MAE, which underlies the reasons for the giant magneto topological band effect attributed to the crystal field. These findings present novel prospects for the realization of large-gap QAH states with the characteristic of easily modifiable topological states.

Progress in Electronic, Energy, Biomedical and Environmental Applications of Boron Nitride and MoS2 Nanostructures.

In this review, we examine recent progress using boron nitride (BN) and molybdenum disulfide (MoS2) nanostructures for electronic, energy, biomedical, and environmental applications. The scope of coverage includes zero-, one-, and two-dimensional nanostructures such as BN nanosheets, BN nanotubes, BN quantum dots, MoS2 nanosheets, and MoS2 quantum dots. These materials have sizable bandgaps, differentiating them from other metallic nanostructures or small-bandgap materials. We observed two interesting trends: (1) an increase in applications that use heterogeneous materials by combining BN and MoS2 nanostructures with other nanomaterials, and (2) strong research interest in environmental applications. Last, we encourage researchers to study how to remove nanomaterials from air, soil, and water contaminated with nanomaterials. As nanotechnology proceeds into various applications, environmental contamination is inevitable and must be addressed. Otherwise, nanomaterials will go into our food chain much like microplastics.

Band gap engineering and photocatalytic degradation of lead-free halide chalcogenide Cs(Ag1-xCux)2I3

Lead halide chalcogenides are becoming more and more widely used in photocatalysis, but the toxicity of elemental lead has forced the study of environmentally friendly lead-free halide chalcogenides. In this paper, the band gap modulation of Cs(Ag1-xCux)2I3 and its application in photocatalytic degradation were investigated. Cs(Ag1-xCux)2I3 single crystals were prepared by an anti-solvent method. Experimental analyses showed that copper substitution could modulate the band gap size and improve the photocatalytic performance, in which Cs(Ag0.8Cu0.2)2I3 degraded 99.0 % of methylene blue pollutants in 4 h with good reusability. This study demonstrated that Cs(Ag1-xCux)2I3 has good application prospects in photocatalytic degradation.

Effect of Codoping Zinc Oxide Nanoparticles with Sulfur and Nitrogen on Its Energy Bandgap, Antioxidant Properties, and Antibacterial Activity

Zinc oxide nanoparticles (ZnO-NPs) are used in various fields such as industrial, environmental remediation, catalytic, and antibacterial applications. However, their ability to absorb visible light is limited due to their high-energy bandgap and fast electron-hole recombination, which restricts their use. To enhance the efficiency of ZnO-NPs in medical and other applications, surface functionality can be modified through doping. Here, we investigated the effects of S and N doping on the energy bandgap of ZnO-NP and their antimicrobial and antioxidant activities. The results showed that the optical bandgap energy of pure ZnO-NPs was 2.98 eV while that of 6% N-ZnO, 4% S-ZnO, and S4-N6-ZnO was 2.78, 2.69, and 2.63 eV, respectively. The energy bandgap reduction is attributed to the changes in the electronic level of zinc oxide as the result of doping. The crystal size of pure ZnO-NPs, 6% N-ZnO, 4% S-ZnO, and S4-N6-ZnO was 29.06, 27.05, 29.02, and 25.06 nm, respectively, as calculated from XRD data using FWHM. Following the bandgap and particle size reduction, the antimicrobial activities of the dual-doped ZnO-NPs surpassed that of the pure ZnO-NPs. Moreover, dual doping improved the antioxidant activity of ZnO-NPs from 52.45% to 88.89% for the optimized concentration. Therefore, incorporating S and N as dual dopants can enhance the functionality and efficiency of ZnO-NPs in various fields.

Exploring electronic characteristics of bilayer HfS2 under mechanical strain and external electric field: A first-principles approach

This study employed the density functional theory (DFT) to explore the dynamic stability of different stacking configurations of bilayer HfS2 (2L-HfS2) and the influence of external conditions on its electronic properties. These results indicate that the AA configuration is the most stable among the various stacking configurations. Calculations of the carrier effective mass revealed that the artificially stacked 2L-HfS2 maintained a small effective mass of electrons and holes and exhibited high carrier mobility. Further investigation revealed diverse trends in the electronic properties of 2L-HfS2 under vertical strain, in-plane strain, shear strain, and an applied external electric field. These variations encompass bandgap size alterations, band structure evolution, and transitions in the metallic behavior. This systematic study elucidates the regulatory effects of different strains and external electric field conditions on the electronic properties of 2L-HfS2, thereby providing a vital theoretical foundation and experimental guidance for its application in electronics and optoelectronics.

Search on stable binary and ternary compounds of two-dimensional transition metal halides

Ab initio driven density functional theory-based high throughput simulations have been conducted to search for stable two-dimensional (2D) structures based on transition metal halides. Binary MeX2 and MeXY (Me—transition element, X and Y–Cr, Br, I, where X ≠ Y) 2D structures in two structural polymorphic modifications, which are 1T-phase and 1H-phase, have been studied. The main structural stability criteria, such as heat formation energy, elasticity constants, and phonon spectra and the following ab initio molecular dynamics simulations have been used to determine the stability of studied compounds. It has been shown that 35 MeX2 and 32 MeXY 2D structures comply with given stability criteria. Photocatalytic properties of these stable 2D MeX2 and 2D MeXY have been investigated. Based on the calculated band gap size E g, work function Ф and electron affinity χ, it has been found that among all stable compounds 13 MeX2 and 16 MeXY 2D structures are promising photocatalysts for water splitting. However, only 7 compounds have solar-to-hydrogen (STH) efficiency overcome the 10% threshold, which is a critical parameter for solar hydrogen generation to be an economically viable resource. Among MeX2 2D structures 1T-CdI2 and 1H-VBr2 possess a STH efficiency of 11.58% and 17.23%. In the case of 2D MeXY, STH efficiencies are 22.79% (1T-ZnClI), 15.20% (1T-CdClI), 22.13% (1T-ZnBrI), 12.11% (1T-CdBrI) and 19.76% (1H-VClBr). Moreover, as a result of this work, a comprehensive publicly available database, containing detailed calculation parameters and fundamental properties of the discovered 2D transition metal halides, has been created.

Synthesis and characterization of novel banana-graphene nanofibrous membrane from viscous liquid for bandgap formation

Organic semiconductors with wide bandgaps have potential applications in organic light-emitting diode displays, organic photovoltaic devices, organic field-effect transistors and solar cells. However, organic semiconductors are not good conductors of electricity. This study synthesizes the banana-graphene nanofibrous membrane using an electric spin process to form a tunable bandgap to enhance conductivity. The chemical process was followed to prepare liquid viscous from banana stalks. Different percentages of graphene (1.2 wt.%, 2.4 wt.% and 3.6 wt.%) were used with the liquid viscous to form sizeable and tunable bandgaps in the composites and enhance the conductivity of the materials. The results showed 5.67 %, 24.5 %, and 27.8 % lower bandgap for 1.2 wt.%, 2.4 %, and 3.6 wt.% graphene-contained samples, respectively, as compared to the polyvinyl alcohol (PVA) viscous fibre (3.35 eV). With the addition of graphene with PVA viscous fibre, the properties of electrospun nanofibrous graphene were changed from insulator to semiconductor. The band gap property was controlled by incorporating graphene with various percentages. The morphology of the surfaces showed that nanofibers are bonded with each other, and graphene nanoparticles are uniformly distributed. The results of this work can be considered to further progress in manufacturing nanocomposites for organic semiconductor applications.

Quantum octets in high mobility pentagonal two-dimensional PdSe2

Two-dimensional (2D) materials have drawn immense interests in scientific and technological communities, owing to their extraordinary properties and their tunability by gating, proximity, strain and external fields. For electronic applications, an ideal 2D material would have high mobility, air stability, sizable band gap, and be compatible with large scale synthesis. Here we demonstrate air stable field effect transistors using atomically thin few-layer PdSe2 sheets that are sandwiched between hexagonal BN (hBN), with large saturation current > 350 μA/μm, and high field effect mobilities of ~ 700 and 10,000 cm2/Vs at 300 K and 2 K, respectively. At low temperatures, magnetotransport studies reveal unique octets in quantum oscillations that persist at all densities, arising from 2-fold spin and 4-fold valley degeneracies, which can be broken by in-plane and out-of-plane magnetic fields toward quantum Hall spin and orbital ferromagnetism.

Intrinsic edge states and strain-tunable spin textures in the Janus 1T-VTeCl monolayer.

Using first-principles calculations and micro-magnetic simulations, we investigate the electronic structures, the effect of biaxial strain on the topological characteristics, magnetic anisotropy energy (MAE), Dzyaloshinskii-Moriya interaction (DMI) and spin textures in the Janus 1T phase VTeCl (1T-VTeCl) monolayer. Our results show that 1T-VTeCl has an intrinsic edge state, and a topological phase transition with a sizeable band gap is achieved by applying biaxial strain. Interestingly, the MAE can be switched from the in-plane to the off-plane with a compressive strain of -5%. Microscopically, the origin of MAE is mainly associated with the large spin-orbit coupling (SOC) from the heavy nonmagnetic Te atoms rather than that from the V atoms. Furthermore, the induced DMI (0.09 meV) can occur stabilizing magnetic merons without applying temperatures and magnetic fields. Then, the skyrmions, frustrated antiferromagnetism and vortex are induced after applying a suitable compressive strain. Our study provides compelling evidence that the 1T-VTeCl monolayer with topological properties holds great potential for application in spintronic devices, as well as information storage devices based on different magnetic phases achievable through strain engineering.

Carrier transport in bulk and two-dimensional Zn2(V,Nb,Ta)N3 ternary nitrides.

Density functional theory-based simulations are applied to study the electronic structures, carrier masses, carrier mobility and carrier relaxation times in bulk and two-dimensional (2D) Zn2(V,Nb,Ta)N3 ternary nitrides. Bulk Zn2(V,Nb,Ta)N3 possess moderate band gap sizes of 2.17 eV, 3.11 eV, and 3.40 eV, respectively. Two-dimensional Zn2(V,Nb,Ta)N3 have slightly higher band gap sizes of 2.77 eV, 3.33 eV, and 3.23 eV, respectively. Carrier mass, carrier mobility and carrier relaxation time are found to be anisotropic in all the studied structures. Bulk and 2D samples show an order of magnitude higher electron mobility compared to hole mobility. The highest electron mobility in bulk Zn2NbN3 and Zn2TaN3 is about ∼103 cm2 V-1 s-1. Importantly, for 2D Zn2NbN3, an abnormally high electron mobility of 1.67 × 104 cm2 V-1 s-1 is observed, which is not inferior to the highest known electron mobility values in 2D materials. Such a high electron mobility in 2D Zn2NbN3 can be attributed to a strong delocalization of the conduction band minimum, which is responsible for electron transport. Therefore, this work opens up new materials for high performance nanodevices, such as tandem solar cells and field-effect transistors. This study also provides deep physical insights into the nature of carrier transport mechanisms in bulk and 2D Zn2(V,Nb,Ta)N3 ternary nitrides.

Ideal two-dimensional quantum spin Hall insulators MgA2Te4 (A = Ga, In) with Rashba spin splitting and tunable properties.

For decades, topological insulators have played a pivotal role in fundamental condensed-matter physics owing to their distinctive edge states and electronic properties. Here, based on in-depth first-principles calculations, we investigate the MgA2Te4 (A = Ga, In) structures belonging to the MA2Z4 2D material family. Among them, the topological insulator MgGaInTe4 exhibits band inversion and a sizeable bandgap of up to 60.8 meV which satisfies the requirement for room-temperature realization. Under the spin-orbit coupling effect, MgGaInTe4 with inversion asymmetry undergoes Rashba spin splitting. The Rashba-like and Dirac-type edge states emerge from different terminals along (010) for MgGaInTe4. The external vertical electric field is verified to modulate the inverted bandgap and topological state of MgGaInTe4 by converting a nontrivial state to a trivial state and MgIn2Te4 with the original trivial state to a nontrivial one. Accordingly, MgGaInTe4 and MgIn2Te4 have significant potential for application in topological quantum field-effect transistors. Our research identifies that the MgA2Te4 (A = Ga, In) structures have huge potential to be candidate 2D materials for spintronics and topological quantum devices.

Electronic properties of MoSe2 nanowrinkles.

Mechanical deformations, either spontaneously occurring during sample preparation or purposely induced in their nanoscale manipulation, drastically affect the electronic and optical properties of transition metal dichalcogenide monolayers. In this first-principles work based on density-functional theory, we shed light on the interplay among strain, curvature, and electronic structure of MoSe2 nanowrinkles. We analyze their structural properties highlighting the effects of coexisting local domains of tensile and compressive strain in the same system. By contrasting the band structures of the nanowrinkles against counterparts obtained for flat monolayers subject to the same amount of strain, we clarify that the specific features of the former, such as the moderate variation of the band-gap size and its persisting direct nature, are ruled by curvature rather than strain. The analysis of the wave-function distribution indicates strain-dependent localization of the frontier states in the conduction region while in the valence, the sensitivity to strain is much less pronounced. The discussion about transport properties, based on inspection of the effective masses, reveals excellent perspectives for these systems as active components for (opto)electronic devices.

Theoretical investigation of spin Hall conductivity in graphene/MoS2 van der Waals heterostructure under external electric field

Generation and controlling of spin current in two-dimensional (2D) materials such as graphene are challenging due to small intrinsic spin-orbit coupling (SOC) of the 2D materials. We demonstrate from first-principles calculations that the finite spin Hall conductivity (SHC) of 25Ω−1cm−1 is obtained in van der Waals heterostructure of graphene/MoS2, and the SHC is maximized up to 170Ω−1cm−1 by applying positive electric field, while application of negative electric field hardly affects the SHC. The underlying mechanism of large SHC is discussed in terms of spin Berry curvature in Dirac-cone bands of graphene in the heterostructure, where a size of band gap is modulated sensitively by a magnitude of the external electric field.