Direct Band Gap Character Research Articles

First-principles study on the electronic structure and photocatalytic properties of novel two-dimensional Janus CrXCN4 (X = Si, Ge)

Abundant studies demonstrate the significant role of Janus-structured two dimensional semiconductors as photocatalytic materials, highlighting their substantial advantages and importance in photocatalysis. In this work, CrXCN4 (X = Si, Ge) Janus monolayers were constructed based on CrC2N4, and the thermal stability, thermodynamic stability, mechanical stability, electronic properties, and optical properties of the monolayers were systematically investigated. Furthermore, an investigation was conducted to examine the impact of biaxial strain on their electronic and light absorption properties. The findings reveal that both monolayers exhibit direct band gap characteristics, with high absorption coefficients for visible light owing to their appropriate band gaps (1.44 eV for CrSiCN4 and 1.15 eV for CrGeCN4, respectively). At compressive strains exceeding 3%, the CrSiCN4 monolayer demonstrates an optimal band edge position, suggesting its potential as a photocatalyst for overall water splitting. Furthermore, as the compressive strain increases, the absorption spectra have blue-shifted and the absorption coefficient becomes higher, exceeding 2 × 105 cm−1 under a −3% compressive strain. Our study highlights the potential applications of CrXCN4 monolayers in the field of optoelectronic device, particularly emphasizing the promising candidacy of CrSiCN4 as an efficient photocatalyst.

Structural and electronic properties of GaN: Ab initio study within LDA and LDA+U methods

Structural and electronic properties of the GaN were simulated based on Density Functional Theory implementing Local Density Approximation methods. Hubbard U correction gives us an opportunity to find the correct energy gap for GaN in agreement with known experimental results. Choosing more accurate investigation methods leads to calculating accurate electronic band structure and in the future predicting some physical properties of related material. The bottom of the conduction band and the top of the valence band are formed mainly by p-orbitals of host Ga and N atoms. The present study shows the direct band gap character of GaN with a wurtzite structure

Reconfiguring the Optical Selection Rule in Ultramicrotome‐Crafted Vertically Aligned InSe Ribbons

AbstractDue to the high carrier mobility and direct bandgap character, indium selenide (InSe) exhibits great advantages for advanced optoelectronic applications. However, the unique optical section rule in InSe weakens the light‐matter interaction when the electric field of illuminated light is perpendicular to the c‐axis of planar InSe flake. To overcome this constraint, ultramicrotome technique is introduced to achieve vertically aligned InSe ribbons, aiming to reconfigure the optical paths for improved optical absorption and device performance. The well‐designed InSe ribbons are acquired with uniform morphology, showing an integrated structure free of cracks or fragments. First, the structural symmetry, crystal orientation, and optical anisotropy of InSe ribbon are carefully revealed by polarization‐dependent Raman spectrum, second harmonic generation (SHG), and photoluminescence measurements. Then, the back focal plane (BFP) imaging of photoluminescence distinguishes the in‐plane and out‐of‐plane dipole emission of InSe ribbons and planar InSe flakes, respectively. Notably, the ribbon exhibits new PL peaks with energy lower than the exciton peak of InSe at low temperature, which is attributed to Se vacancies. The work by introducing vertically aligned InSe nanoribbons provides a new strategy to manipulate the light‐matter interactions and defect engineering for novel optoelectronics, which can be also extended to other 2D semiconductors.

Gap type effect on photocatalytic degradation using newly hydrotalcite nanoposite precursor synthesized by hydrothermal method

<p>A novel technique involving hydrothermal synthesis was employed to prepare nanocomposite precursors of lamellar double hydroxide based basic mixed oxides. This method extended ongoing research on iron-impregnated hydrotalcite-supported nanomaterials. The iron-based Fe/MgAl-h nanocomposite was prepared via dry impregnation, introducing a known % by weight of Fe (NO3)3.9H2O metal salt into the hydrothermally synthesized MgAl-h structure. For comparative analysis, the photocatalytic performance of Fe/MgAl-h was compared with the coprecipitation-synthesized Fe/MgAl-c after calcination at 400°C, using various characterization techniques. Structural examination revealed a double-layered hydroxyl structure and the presence of iron oxide phase Fe2O3 in Fe/MgAl-h400. Optical assessments indicated an indirect band gap energy of 1.89 eV for solid Fe/MgAl-h400, which is suitable for visible light absorption. Photocatalytic experiments for methylene blue (MB) dye degradation in aqueous solution were performed under artificial irradiation with a tungsten lamp as the visible light source, mirroring the conditions of Fe/MgAl-c400. Using 50 mg/L MB concentration and 0.5 g/L catalyst quantity, increased MB removal efficiency with time was observed. A comparative study of hydrothermal and coprecipitation synthesis methods showed decreased photodegradation for the hydrothermally prepared solid. Fe/MgAl-h400 and Fe/MgAl-c400 exhibited reaction efficiencies of approximately 46% and 67%, respectively, possibly attributed to their distinct indirect and direct band gap characteristics. Moreover, a wider band gap in a semiconductor material offers several advantages for the degradation of dyes.</p>

Theoretical discovery of the phase stability, mechanical, optoelectronic, and photovoltaic properties of AZn3N3 (A = Sc, Y, La)

In this study, the detailed first-principles calculations are employed to evaluate the structural stability, mechanical, optoelectronic, and photovoltaic properties of AZn3N3 (A = Sc, Y, La). Our calculations show that the thermodynamic, dynamical, and mechanical stability are firstly verified for the hexagonal phase with space group P63/mmc of AZn3N3. The direct band gap feature with the value of 1.7–1.8 eV is revealed for these novel compounds, and the energy gap variation is less than 100 meV. They have relatively high effective masses and exciton binding energies. Their similar absorption spectra are observed in the visible light region. Based on the spectroscopic limited maximum efficiency (SLME) model, their maximum efficiencies are about 18–20 % in the thickness of 0.5 μm. Given their optoelectronic and photovoltaic properties, ternary zinc nitrides AZn3N3 (A = Sc, Y, La) may be potential candidates for tandem solar cells.

Tunable photoelectric properties of monolayer Mo1-x W x Te2 alloys: a first-principles study.

Monolayer MoTe2 and WTe2 within the two-dimensional transition metal dichalcogenides (TMDCs) material family exhibit broad potential for application in optoelectronic devices owing to their direct band gap characteristics. In this work, upon alloying these materials into a monolayer system denoted as Mo1-x W x Te2, intriguing alterations are observed in the electronic and optoelectronic properties. The photoelectric attributes of these alloys can be tailored by manipulating the respective ratios of molybdenum to tungsten (Mo/W). This investigation employs first-principles calculations based on density functional theory (DFT) to assess physical traits of two-dimensional monolayered structures composed from varying compositions of Mo1-x W x Te2. Our findings reveal that while maintaining a direct band gap characteristic across all compositions studied, there is also a reduction observed in electron effective mass near the Fermi level. Moreover, changing in the Mo/W ratio allows gradual adjustments in electronic properties such as density of states (DOS), work function, dielectric function, absorptivity, and reflectivity. Phonon dispersion curves further demonstrate the stability of Mo1-x W x Te2 systems. Notably, Mo0.5W0.5Te2 exhibits lower polarizability and reduced band gap when compared against MoTe2 and WTe2 counterparts. This research underscores how alloying processes enable customizable modifications in the electronic and optoelectronic properties of Mo1-x W x Te2 monolayer materials which is essential for enhancing nanoscale electronic and optoelectronic device design.

Computational Screening of 2D All‐Inorganic Lead‐Free Halide Perovskites A3B2X9 for Photovoltaic and Photocatalytic Applications

AbstractExploration of low‐dimensional Pb‐free halide perovskites with high stability and outstanding properties is still a pursuing target for developing integrated optoelectronic devices. Herein, comprehensive computational screening of a new class of two‐dimensional (2D) all‐inorganic Pb‐free A3B2X9 perovskites is performed based on the first‐principles calculations. The results indicate that the structural and electronic properties of 2D A3B2X9 structures strongly depend on the B‐X bonding interactions, which makes that their thermodynamic stability follows the trend of A3Bi2X9≈A3Sb2X9>A3In2X9>A3Ga2X9 and their interlayer interactions show a reversal trend. Owing to the lack of lone‐pair electron effect, A3In2X9 indicate direct bandgap characteristics and present the relatively smaller bandgaps and higher electron mobilities than A3Sb2X9 and A3Bi2X9. Benefit from optimal bandgaps (0.8–2.1 eV) and large absorption coefficients (104–105 cm−1) in the visible region, A3B2I9 (B = In, Sb, Bi) exhibit high power conversion efficiency up to 18.2%. Moreover, A3B2I9 (B = Sb, Bi) is verified as efficient photocatalysts for overall water splitting. The theoretical solar‐to‐hydrogen efficiency of Rb3Bi2I9 and Cs3Bi2I9 are >16%. This work suggests huge potential of 2D A3B2X9 perovskites for photovoltaic and photocatalytic applications.

Electroluminescence from Megasonically Solution-Processed MoS2 Nanosheet Films.

Due to their superior optoelectronic properties, monolayer two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted significant attention for electroluminescent devices. However, challenges in isolating optoelectronically active TMD monolayers using scalable liquid phase exfoliation have precluded electroluminescence in large-area, solution-processed TMD films. Here, we overcome these limitations and demonstrate electroluminescence from molybdenum disulfide (MoS2) nanosheet films by employing a monolayer-rich MoS2 ink produced by electrochemical intercalation and megasonic exfoliation. Characteristic monolayer MoS2 photoluminescence and electroluminescence spectral peaks at 1.88-1.90 eV are observed in megasonicated MoS2 films, with the emission intensity increasing with film thickness over the range 10-70 nm. Furthermore, employing a vertical light-emitting capacitor architecture enables uniform electroluminescence in large-area devices. These results indicate that megasonically exfoliated MoS2 monolayers retain their direct bandgap character in electrically percolating thin films even following multistep solution processing. Overall, this work establishes megasonicated MoS2 inks as an additive manufacturing platform for flexible, patterned, and miniaturized light sources that can likely be expanded to other TMD semiconductors.

Electronic, transport and optical properties of ZnO/GaN heterostructures: first-principles study

The electronic, transport and optical properties of the trilayer of ZnO and GaN heterostructures are investigated using density functional study to understand its role in optoelectronic devices. For layered systems, Zn over N and Ga over O stacking arrangement of ZnO over GaN is most favorable. The calculated formation energies reflect the energetically favorable condition of ZnO/GaN heterostructures. The GaN/ZnO/GaN is a more energetically favorable stacking arrangement as compared to ZnO/GaN/ZnO. The band gap of trilayer systems decreases as compared to that of bilayer and monolayer. The ZnO/GaN bilayer and ZnO/GaN/ZnO trilayer show direct band gap characteristics with the value of 1.71 and 1.61 eV, respectively. The GaN/ZnO/GaN shows an indirect band gap of 1.47 eV. The higher recombination rate of ZnO/GaN/ZnO is useful to develop a base for optical emission devices. The transport calculations show that, the magnitude of current flowing through the system increases with the layers of heterosystems and specifically higher for GaN/ZnO/GaN heterostructure. The enhanced channel conductance and higher mobility of GaN/ZnO/GaN heterostructure are crucial for the development of high mobility transistors. The improved absorption energy and dielectric properties are observed for trilayer systems as compared to that of the bilayer and monolayer and may be useful for optical devices. The higher optical efficiency is observed for GaN/ZnO/GaN as compared to ZnO/GaN/ZnO heterostructure system and opens up a way toward optical waveguides and reflectors.

Prediction of a monolayer spin-spiral semiconductor: CoO with a honeycomb lattice

The recent successful fabrication of two-dimensional (2D) CoO with nanometer-thickness motivates us to investigate monolayer CoO due to possible magnetic properties induced by Co atoms. Here, we employ first-principles calculations to show that monolayer CoO is a 2D spin-spiral semiconductor with a honeycomb lattice. The calculated phonon dispersion reveals the monolayer's dynamical stability. Monolayer CoO exhibits a type-I spin-spiral magnetic ground state. The spin-spiral state and the direct bandgap character are both robust under biaxial compressive strain (−5%) to tensile strain (5%). The bandgap varies only slightly under either compressive or tensile strain up to 5%. These results suggest a potential for applications in spintronic devices and offer a new platform to explore magnetism in the 2D limit.

Enhanced free-exciton luminescence in Cs2SnBr6−xIx: A first-principles study

A2BX6-type tin halide perovskites have received extensive attention as environment-friendly lead-free luminescent materials. However, the limited knowledge of the partial anion substitution effects on their luminescent properties hinders their further development in this field. Therefore, we investigated such effects using Cs2SnBr6 (CSB) double perovskite as an example in this work. We first found that CSB remains stable upon partial anion substitution of bromine by iodine. Next, we explored the electronic origin of the tunable band gaps upon the presence of both Jahn-Teller-like or non-Jahn-Teller-like distortions in [SnBr6−xIx]2− (x = 1–3) octahedrons. As a further stage, we performed hybrid HSE06 functional calculations considering spin–orbit coupling (SOC) to obtain more accurate band gaps. Importantly, all substituted materials except Cs2SnBr4I2 (structure 2a) show direct band gap characteristics and enhanced light emissions. Finally, we identified the free exciton (FE) emission mechanism is more favorable than self-trapped exciton (STE) emission in these materials owning to their high electronic dimensionality, small effective mass, non-Jahn-Teller-like distortion, small exciton binding energy, small energy separation, small self-trapping depth, low detrapping barrier, and non-local deformation in the presence of heavy halogen. We believe our findings of Cs2SnBr6−xIx would promote the design of tin halide perovskites with better luminescent properties.

Tuning the Electronic and Mechanical Properties of Kagome Graphene via Hydrogenation

Surface passivation is proved to be an effective way to adjust material properties or to explore new two-dimensional (2D) materials. Herein, we proposed three hydrocarbons with high stability for the first time via hydrogenation on the Kagome graphene, namely, C6H4, C6H6-I, and C6H6-II. Unlike the Kagome graphene, which is metallic, all these 2D monolayers are wide-bandgap semiconductors (4.06–4.81 eV). Among them, C6H4 is an indirect bandgap semiconductor, but both C6H6-I and C6H6-II possess the direct bandgap feature. Considerable carrier mobilities (102 to 103 cm2 V–1 s–1) have been further confirmed in the three hydrocarbons on the basis of modified deformation potential theory. Specifically, for C6H4, the hole mobilities are as high as 104 to 105 cm2 V–1 s–1, comparable to those of graphene and black phosphorus. The intrinsic vertical electric field induced by the asymmetric crystal structures in C6H4 and C6H6-I will be beneficial to the spatial separation of electrons and holes in semiconductors, promising in the field of optoelectronics. In addition, hydrogenation has a great influence on the mechanical properties of Kagome graphene, no matter whether it is Young’s modulus, Poisson’s ratio, or ideal tensile strength. Particularly, in-plane axial negative Poisson’s ratios (−0.011/–0.018 along the a-/b-direction) were found in C6H6-I, mainly originated from the interaction of carbon pentagons and octagons. These interesting findings in our work may pave the way for the application of hydrogenated Kagome graphene in the future.

Band and luminescence regulation of SiGeSn ternary alloy: A first-principles investigation

Current experiments show that the ternary alloy SiyGezSnx has larger crystal lattice and bandgap control range than group IV binary alloy, and it is a popular material for the preparation of silicon-based high-efficiency light sources. However, the physical laws of the bandgap and luminescence properties of SiyGezSnx with concentration changes have not been fully elucidated because of high experimental costs and time-consuming research. Therefore, the band structures and optical gains of SiyGezSnx (x∈0–0.35, y∈0–1) were investigated in this study using a first-principles method based on density functional theory combined with special quasi-random structures and Heyd-Scuseria-Ernzerhof hybrid functional correction. The quantitative relationship between the bandgap and concentration change of SiyGezSnx was theoretically clarified. In addition, the optical gain coefficient of the SiyGezSnx with direct bandgap characteristics was studied. Compared with the structure of GeSn with the same Sn concentration, SiGeSn can maintain the direct bandgap when Si is introduced. The results show that the bandgap increases with increasing Si concentration whereas it decreases when more Sn is introduced. Thus, the best way to maintain a direct bandgap is to simultaneously increase the Si and Sn concentrations. To facilitate experimental research, a formula to determine the direct bandgap region of SiyGezSnx was realized. Regarding optical gains, the positive gain regions are consistent with the direct bandgap regions, and the optical gain values are related to fluctuations in electron and hole numbers in the different valleys. For Siy(y=0–0.21)GezSnx(x=0.1–0.31), there are two maximum optical gain regions around Ge0.9Sn0.1 and Si0.17Ge0.56Sn0.27. These results can directly provide theoretical guidance for experimental research, as well as accelerate the research and development of group IV alloy high-efficiency light sources.

Structural, Mechanical, Electronic and Optical Properties of KNaY (Y = S, Se and Te) as Ferroelectric Compounds

The purpose of this work is an exploration of new materials which show ferroelectric behavior. We have analyzed the structural, mechanical, electronic, and optical properties of ternary inter-alkali chalcogenide compounds KNaY (Y = S, Se and Te) using the first principal calculations. The atomic positions and structural parameters are in agreement with other studies. The elastic parameters designate that these compounds have a fragile behavior on the other hand. The electronic band structure calculations suggest that these compounds are semiconductors with a direct energy band gap and anionic character, in agreement with literature data. The optical properties show that these compounds can be a good candidate for optoelectronic devices.

Robust type-II BP/AlN van der Waals heterostructure: A first-principles study

To achieve photoelectric appliance, effective carrier separation is a vital question. Here, we construct van der Waals (vdW) heterostructure by superimposing two different two-dimensional (2D) layer materials BP and AlN via vdW force. The results show that 2D BP/AlN heterostructure exhibits direct band gap characteristics and has a type-II band alignment, which accelerates the effective separation of electron-hole pairs. In addition, under the modulation of applied external electric field and biaxial strain, this heterostructure still keeps a type-II band alignment. This result reveals that 2D BP/AlN heterostructure shows a wide application prospect in the future photoelectric nanodevices.

Siloxene: An advanced metal-free catalyst for efficient photocatalytic reduction of aqueous Cr(VI) under visible light

In this work, we reported the synthesis of siloxene catalysts from calcium silicide at various topotactic reaction conditions and demonstrated their photocatalytic activity for Cr(VI) reduction under visible light irradiations. The topotactic reaction time influenced the surface area, structure, and optical characteristics of the siloxene, which altered the Cr(VI) photocatalytic elimination efficiency. A large specific surface area (47.04 m2 g−1) and a narrow bandgap (2.45 eV) with hydroxyl-terminated layered silicon of the S-12 h catalyst could enable more active sites for photocatalytic reaction, which results in the improved catalytic performance for Cr(VI) removal. The Density Functional Theory calculations proved the siloxene’s direct bandgap feature and narrow bandgap. The initial Cr(VI) concentration, solution pH, temperature, inert gas, and different scavenger's effects on the photocatalytic performance of the S-12 h catalyst were also investigated. The S-12 h photocatalyst exhibited the Cr(VI) reduction efficiency of 98.6% for the electroplating wastewater that demonstrates the potential application in industrial for treatment of heavy metal ions removal.

Electronic, thermoelectric, transport and optical properties of MoSe2/BAs van der Waals heterostructures

The density functional theory (DFT) calculations were performed to systematically study the geometrical, electronic, thermoelectric, transport and optical properties of MoSe2/BAs van der Waals heterostructures (vdWHs). The different MoSe2 and BAs stacking configurations effect hardly on the band structure. The MoSe2/BAs vdWHs possesses excellent dynamical, thermal and mechanical stability, with a direct bandgap of 1.04 eV and type-I band alignment. The Seebeck coefficient and ZT suggest the possibility of MoSe2/BAs vdWHs for thermoelectric applications. The in-plane strains and external electric field can modulate the band structure to achieve the transition from type-I to type-II band alignment, and the direct band gap feature remains preserved. Under the in-plane strains and external electric field, one can find a remarkably high optical absorption coefficient (~105 cm−1) in the visible-ultraviolet region and the red shift in the optical absorption spectrum. Its high energy conversion efficiency of 20.08% making the heterostructure extremely potential in solar energy harvesting of low-dimensional excitonic solar cells. These properties of MoSe2/BAs vdWHs show their promising applications in optoelectronic, nanoelectronic and thermoelectronic fields.

Lead‐Free Double Perovskite Cs2AgInCl6

AbstractLead‐free halide perovskites have drawn wide attention as alternatives to their toxic and poorly stable lead‐based counterparts. Among them, double perovskites with Cs2AgInCl6 composition, often doped with various elements, have been in the spotlight owing to their intriguing optical properties, namely, self‐trapped exciton (STEs) emission and dopant‐induced photoluminescence. This interest has sparked different synthesis approaches towards both crystals and nanocrystals, and the exploration of many alloy compositions with mono‐ and trivalent cations other than Ag+ and In3+. In this Minireview we describe the recent developments on Cs2AgInCl6 bulk crystals and nanocrystals, their synthesis strategies, intrinsic optical properties, and tunable photoluminescence originating from different alloying and doping effects. We also discuss progress on computational studies aimed at understanding the thermodynamic stability, the role of defects, and the origin of photoluminescence in relation to the STEs and the direct band gap character.

Lead-Free Double Perovskite Cs2 AgInCl6.

Lead-free halide perovskites have drawn wide attention as alternatives to their toxic and poorly stable lead-based counterparts. Among them, double perovskites with Cs2 AgInCl6 composition, often doped with various elements, have been in the spotlight owing to their intriguing optical properties, namely, self-trapped exciton (STEs) emission and dopant-induced photoluminescence. This interest has sparked different synthesis approaches towards both crystals and nanocrystals, and the exploration of many alloy compositions with mono- and trivalent cations other than Ag+ and In3+ . In this Minireview we describe the recent developments on Cs2 AgInCl6 bulk crystals and nanocrystals, their synthesis strategies, intrinsic optical properties, and tunable photoluminescence originating from different alloying and doping effects. We also discuss progress on computational studies aimed at understanding the thermodynamic stability, the role of defects, and the origin of photoluminescence in relation to the STEs and the direct band gap character.

First-principles investigation of the valley and electrical properties of carbon-doped α-graphyne-like BN sheet* *Project supported by the Special Foundation for Theoretical Physics Research Program of China (Grant No. 11847065) and the Natural Science Foundation of Shanxi Province, China (Grant No. 201901D211115).

Based on ab initio density functional theory calculations, we demonstrate that two carbon-doped boron nitride analog of α-graphyne structures, B3C2N3 and BC6N monolayers, are two-dimensional direct wide band gap semiconductors, and there are two inequivalent valleys in the vicinities of the vertices of their hexagonal Brillouin zones. Besides, B3C2N3 and BC6N monolayers exhibit relatively high carrier mobilities, and their direct band gap feature is robust against the biaxial strain. More importantly, the energetically most favorable B3C2N3 and BC6N bilayers also have direct wide band gaps, and valley polarization could be achieved by optical helicity. Finally, we show that BC6N monolayer might have high efficiency in photo-splitting reactions of water, and a vertical van der Waals heterostructure with a type-II energy band alignment could be designed using B3C2N3 and BC6N monolayers. All the above-mentioned characteristics make B3C2N3 and BC6N monolayers, bilayers, and their heterostructures recommendable candidates for applications in valleytronic devices, metal-free photocatalysts, and photovoltaic cells.