eV Direct Band Gap Research Articles

Type‐II C2N/ZnTe Van Der Waals Heterostructure: A Promising Photocatalyst for Water Splitting

AbstractBased on density functional theory calculations under the framework of first‐principles, the electronic and optical characteristics of C2N/ZnTe van der Waals (vdW) heterostructure are investigated in detail. An inherent type‐II band alignment arises in the heterostructure, which possesses a 1.94 eV direct band gap. With light irradiation, the photogenerated electron‐hole pairs of heterostructure are separated on different monolayers. This helps to extending the lifetime of carriers. Additionally, the lower effective carrier mass is owned for the heterostructure. The heterostructure has suitable band gap and band edge positions, as well as, enhanced visible light absorption capacity, which can facilitate photocatalytic water splitting in acidic conditions. More interestingly, the redox reaction of photocatalytic water splitting can still proceed on the heterostructure after exerting a biaxial tensile strain of 2–6%. These results suggest that C2N/ZnTe vdW heterostructure will be a promising water splitting photocatalyst.

Phase Transformation and Superstructure Formation in (Ti0.5, Mg0.5)N Thin Films through High-Temperature Annealing

(Ti0.5, Mg0.5)N thin films were synthesized by reactive dc magnetron sputtering from elemental targets onto c-cut sapphire substrates. Characterization by θ–2θ X-ray diffraction and pole figure measurements shows a rock-salt cubic structure with (111)-oriented growth and a twin-domain structure. The films exhibit an electrical resistivity of 150 mΩ·cm, as measured by four-point-probe, and a Seebeck coefficient of −25 µV/K. It is shown that high temperature (~800 °C) annealing in a nitrogen atmosphere leads to the formation of a cubic LiTiO2-type superstructure as seen by high-resolution scanning transmission electron microscopy. The corresponding phase formation is possibly influenced by oxygen contamination present in the as-deposited films resulting in a cubic superstructure. Density functional theory calculations utilizing the generalized gradient approximation (GGA) functionals show that the LiTiO2-type TiMgN2 structure has a 0.07 eV direct bandgap.

Assessment of GaPSb/Si tandem material association properties for photoelectrochemical cells

Here, the structural, electronic and optical properties of the GaP1-xSbx/Si tandem materials association are determined in view of its use for solar water splitting applications. The GaPSb crystalline layer is grown on Si by Molecular Beam Epitaxy with different Sb contents. The bandgap value and bandgap type of GaPSb alloy are determined on the whole Sb range, by combining experimental absorption measurements with tight binding (TB) theoretical calculations. The indirect (X-band) to direct (Γ-band) cross-over is found to occur at 30% Sb content. Especially, at a Sb content of 32%, the GaP1-xSbx alloy reaches the desired 1.7eV direct bandgap, enabling efficient sunlight absorption, that can be ideally combined with the Si 1.1 eV bandgap. Moreover, the band alignment of GaP1-xSbx alloys and Si with respect to water redox potential levels has been analyzed, which shows the GaPSb/Si association is an interesting combination both for the hydrogen evolution and oxygen evolution reactions. These results open new routes for the development of III-V/Si low-cost high-efficiency photoelectrochemical cells.

Exploring the potential of MnX (S, Sb) monolayers for antiferromagnetic spintronics: A theoretical investigation

In this study, we predicted new two-dimensional tetragonal structures of t-Mn2X2 (X = S, Sb) sheets on the basis of first-principles plane wave calculations within density functional theory with Hubbard U model. Stability tests such as phonon spectrum calculation and molecular dynamic simulations reveal that the 2D t-Mn2X2 structures are dynamically and thermally stable at least in room temperature. Our theoretical calculations have shown that t-Mn2X2 structures have two Raman active and seven infrared active modes. The t-Mn2Sb2 sheet exhibits metallic property, whereas t-Mn2S2 shows semiconducting property with a 0.68 eV indirect bandgap. Exploring of the favorable magnetic orientation calculations revealed that both 2D t-Mn2X2 structures prefer antiferromagnetic spin configuration. Estimated critical temperatures for the phase transition from antiferromagnetic spin order to paramagnetic case are 720 K and 545 K for t-Mn2S2 and t-Mn2Sb2, respectively. These relatively high Néel temperatures and their suitable electronic properties for many applications clearly qualify that the 2D t-Mn2X2 sheets can be a good candidate for room temperature antiferromagnetic device applications.

Boron-rich boron carbide from soot: a low-temperature green synthesis approach

Boron carbide is a promising super-hard semiconducting material for refractory applications ranging from the nuclear industry to spacecraft. The present work is the first report of not only turning futile soot, containing carbon allotropes in varying composition, into boron-rich boron carbide (BC), but also developing it by a low-cost, low-temperature, and green synthesis method. The BC synthesised from gingelly oil soot is subjected to structural, morphological, and optical characterisations. The field emission scanning electron microscope shows beautiful flower-like morphology, and the thermogravimetric analysis reveals the high-temperature stability of the sample synthesised. The Tauc plot of the sample indicates a 2.38 eV direct bandgap. The formation of BC and boron-rich carbide evidenced by X-ray diffraction studies is confirmed through Raman and Fourier transform infrared spectroscopic signatures of B–C and C–B–C bonds. The fluorescence, power spectrum, and CIE analyses carried out suggest the blue light emission for excitation at 350 nm.

Boron phosphide as a p -type transparent conductor: Optical absorption and transport through electron-phonon coupling

Boron phosphide has recently been identified as a potential high hole mobility transparent conducting material. This promise arises from its low hole effective masses. However, BP has a relatively small 2 eV indirect band gap which will affect its transparency. In this work, we computationally study both optical absorption across the indirect gap and phonon-limited electronic transport to quantify the potential of boron phosphide as a \emph{p}-type transparent conductor. We find that phonon-mediated indirect optical absorption is weak in the visible spectrum and that the phonon-limited hole mobility is very high (around 900 cm$^2$/Vs) at room temperature. This exceptional mobility comes from a combination of low hole effective mass and very weak scattering by polar phonon modes. We rationalize the weak scattering by the less ionic bonding in boron phosphide compared to oxides. We suggest this could be a general advantage of non-oxides for \emph{p}-type transparent conducting applications. Using our computed properties, we assess the transparent conductor figure of merit of boron phosphide and shows that it exceeds by one order of magnitude that of established \emph{p}-type transparent conductors, confirming the potential of this material.

Electronic and optical properties of PdSe2 from monolayer to trilayer

Based on density functional theory (DFT), the electronic and optical properties of PdSe2 from monolayer to trilayer are systematically studied. The 1.37eV indirect band gap of monolayer PdSe2 reflects its semiconductor behavior, and the band gap tends to decrease with the increase of the number of layers. The band gap of PdSe2 from monolayer to trilayer are basically consistent with the experimental values. The conduction band minimum (CBM) and valence band maximum (VBM) mostly come from the Pd-4d orbitals and Se-4p orbitals. Interestingly, two structures in the trilayer PdSe2 of ABA and ABB stackings show prominent light absorption, and their maximum light absorption values occur near the energy of 10.6eV in xx direction. The positions of absorption edge appear red shift with the increase of the number of layers in all directions. Additionally, the trilayers have high refractive index and reflectivity. From monolayer to trilayer, the static refractive index n (0) increases gradually. Therefore, they may have promising applications in optical coatings and tunable optoelectronic devices.

First Principles Research on the Magnetic Properties of Na and Cl doped ZnO

ZnO is considered as a wide bandgap material because it has a 3.4 eV direct bandgap. This wide bandgap characteristic causes good transparency, high electron mobility and luminescence at room temperature.The unique and tuneable properties of nanostructured ZnO shows excellent stability in chemically as well as thermally stable n-type semiconducting material with wide applications such as in luminescent material, supercapacitors, battery and solar cells. To be applied to a variety of needs, price control bandgap is needed. Likewise, control over the magnetic nature. Therefore we need a study related to bandgap modification, one of them is by giving impurity atoms. Atom Na and Cl were chosen as representatives of donors and acceptors. Atomistic calculations use the Functional Density Theory method which is implemented in ABINIT software. Relaxation and convergence research results are used to find the most stable energy value of ZnO.. The results showed Magnetic Properties in ZnO doping Na obtained magnetization values of 1.4802 𝜇𝐵 greater than pure ZnO that is 0.9394 𝜇𝐵 while ZnO doping Cl obtained magnetization values of 0.8593 𝜇𝐵 smaller than pure ZnO. In conclusion the ZnO doping magnetic properties of Na increase magnetization and Cl doping also change the magnetic properties by decreasing ZnO magnetization.

Synthesis of undoped and Fe doped nanoparticles of TiO2 via co-precipitation technique and their characterizations

TiO2 nanoparticles attracted much attention due to photocatalytic activity and solar energy applications. The present work demonstrates the preparation of undoped and Fe doped nanoparticles of TiO2 by co-precipitation procedure at 450 °C. From XRD measurement the mean size of crystallite was estimated by Debye Scherer and W-H plot technique. The morphological study of spherical sized nanoparticles was carried out by Scanning electron microscopy. EDS results show signs of no impurity. The optical band gap values of undoped and doped samples were determined to be 3.35 and 3.25 eV for direct bandgap correspondingly.SEM and EDS are in good agreement with XRD results.

Electronic structure of exfoliated millimeter-sized monolayer WSe2 on silicon wafer

The monolayer WSe2 is interesting and important for future application in nanoelectronics, spintronics and valleytronics devices, because it has the largest spin splitting and longest valley coherence time among all the known monolayer transition-metal dichalcogenides (TMDs). To obtain the large-area monolayer TMDs' crystal is the first step to manufacture scalable and high-performance electronic devices. In this letter, we have successfully fabricated millimeter-sized monolayer WSe2 single crystals with very high quality, based on our improved mechanical exfoliation method. With such superior samples, using standard high resolution angle-resolved photoemission spectroscopy, we did comprehensive electronic band structure measurements on our monolayer WSe2. The overall band features point it to be a 1.2eV direct band gap semiconductor. Its spin-splitting of the valence band at K point is found as 460 meV, which is 30 meV less than the corresponding band splitting in its bulk counterpart. The effective hole masses of valence bands are determined as 2.344 me at Gamma, and 0.529 me as well as 0.532 me at K for the upper and lower branch of splitting bands, respectively. And screening effect from substrate is shown to substantially impact on the electronic properties. Our results provide important insights into band structure engineering in monolayer TMDs. Our monolayer WSe2 crystals may constitute a valuable device platform.

Diboron-porphyrin monolayer: A new 2D semiconductor

We investigate a new 2D semiconductor material with BC10N2 stoichiometry based on the molecular structure of diboron porphyrin. Ab-initio molecular dynamics simulations indicate mechanical stability for temperatures up to T=4000 K. Electronic structure calculations based on density functional theory predict a 0.6 eV direct band gap. We employ the Boltzmann transport equation within the relaxation time approximation to analyze electronic transport properties as a function of charge carrier density at room temperature. The lattice thermal conductivity at room temperature was obtained from non-equilibrium molecular dynamics simulations, and we found anisotropic conductivities given by 160 and 115 W/m·K along perpendicular in-plane directions. The thermoelectric figure of merit ZT calculated at 300 K has a maximum value of 0.008. This result indicates that, in spite of its interesting anisotropic transport properties, BC10N2 is not a suitable candidate for thermoelectric applications.

Electronic and optical properties of SnX2(X = S, Se)—InSe van der Waal’s heterostructures from first-principle calculations

In this work from first-principles simulations we investigate bilayer van der Waal’s heterostructures (vdWh) of emerging 2-dimensional optical materials SnS2 and SnSe2 with monolayer InSe. With density functional theory calculations, we study the structural, electronic, optical and carrier transport properties of the SnX2 (X = S, Se)-InSe vdWh. Calculations show SnX2-InSe in its most stable stacking form (named AB-1) to be a material with a small (0.6–0.7 eV) indirect band-gap. The bilayer vdWh shows broad spectrum optical response, with number of peaks in the infra-red to visible region. In terms of carrier transport properties, asymmetry in conductance was observed with respect to the transport direction and electron and hole transmission. The findings are promising from the viewpoint of nanoelectronics and photonics.

Band Alignments, Band Gap, Core Levels, and Valence Band States in Cu3BiS3 for Photovoltaics.

The earth-abundant semiconductor Cu3BiS3 (CBS) exhibits promising photovoltaic properties and is often considered analogous to the solar absorbers copper indium gallium diselenide (CIGS) and copper zinc tin sulfide (CZTS) despite few device reports. The extent to which this is justifiable is explored via a thorough X-ray photoemission spectroscopy (XPS) analysis: spanning core levels, ionization potential, work function, surface contamination, cleaning, band alignment, and valence-band density of states. The XPS analysis overcomes and addresses the shortcomings of prior XPS studies of this material. Temperature-dependent absorption spectra determine a 1.2 eV direct band gap at room temperature; the widely reported 1.4-1.5 eV band gap is attributed to weak transitions from the low density of states of the topmost valence band previously being undetected. Density functional theory HSE06 + SOC calculations determine the band structure, optical transitions, and well-fitted absorption and Raman spectra. Valence band XPS spectra and model calculations find the CBS bonding to be superficially similar to CIGS and CZTS, but the Bi3+ cations (and formally occupied Bi 6s orbital) have fundamental impacts: giving a low ionization potential (4.98 eV), suggesting that the CdS window layer favored for CIGS and CZTS gives detrimental band alignment and should be rejected in favor of a better aligned material in order for CBS devices to progress.

Thick hydride vapor phase epitaxial growth of ZnSe on GaAs (1 0 0) vicinal and orientation patterned substrates

Zinc selenide is attractive for numerous optical applications in the mid and longwave infrared from solid state lasers to non-linear optical frequency conversion. Its use has been limited by the availability of single crystal and epitaxial materials. To meet these challenges we have developed a low-pressure hydride vapor phase epitaxy (HVPE) process for growth of high quality thick ZnSe layers on GaAs (1 0 0) vicinal and orientation-patterned GaAs (OP-GaAs) substrates. We show growth of >300 µm thick layers at growth rates >50 µm/hr. With only 0.26% lattice mismatch between ZnSe and GaAs, epitaxial layers with crystal quality comparable to bulk ZnSe substrates are achieved. X-ray reciprocal space maps show the ZnSe to be fully relaxed. Cross-sectional TEM imaging analysis shows stacking faults at the interface that propagate in to the growing film consistent with a full relaxed film. Surface morphology shows the presence of pits that have their long axis perpendicular to [1–10] direction. Low temperature photoluminescence reveals band to band emission that consisted of the 2.797 eV direct band gap along with the strong exciton emission at 2.802 eV. Growth on OP templates revealed low fidelity domains of thickness >50 µm. This work presents a process to produce high quality thick layers potentially suitable for laser and frequency conversion for defense and civilian applications.

Ab Initio Simulation of Ta2O5: A High Symmetry Ground State Phase with Application to Interface Calculation

AbstractIt is reported that the recently predicted triclinic γ‐phase ground state Ta2O5 by Yang and Kawazoe can be assigned a much more symmetric I41/amd space group, and is isomorphic to P‐Nb2O5. Interestingly, the well‐known high temperature α‐phase Ta2O5 also has the I41/amd symmetry, but is unstable at zero temperature according to our phonon dispersion calculation. A thorough energy comparison of the βAL, δ, λ, Β, LSR, βR, Pm, Cmmm, γ, and α phases of Ta2O5 is carried out using density functional theory under the generalized gradient approximation (GGA). The GGA‐1/2 method is applied in calculating the electronic structure of various phases, where the tetragonal γ‐phase demonstrates a 4.24 eV indirect band gap, close to experimental value. The high symmetry tetragonal phase together with computationally efficient GGA‐1/2 method greatly facilitates the ab initio simulation of Ta2O5‐based devices. As an example, the Ohmic contact nature between metal Ta and Ta2O5 by calculating an interface model of b.c.c. Ta and tetragonal γ‐Ta2O5, using GGA‐1/2 has been explicitly shown.

Tunable electronic and optical properties of arsenene/MoTe2 van der Waals heterostructures

The effects of biaxial strain and interlayer separation on the electronic structures and optical properties of arsenene/MoTe2 van der Waals heterostructures are investigated based on first principles density functional theory calculations. The six possible high-symmetry stacking methods are considered for arsenene/MoTe2 heterostructures. The type I band alignment with a 1.504 eV direct band gap is obtained in the most stable arsenene/MoTe2 heterostructure. Both biaxial strain and interlayer separation induce direct-indirect band gap transition and semiconductor–metal transition. The arsenene/MoTe2 heterostructures have appreciable adsorption of visible light and ultraviolet light. We also consider the effects of different biaxial strains and interlayer distances on the optical properties of arsenene/MoTe2 heterostructure, and find the optical properties can be effectively modulated by biaxial strains.

The electric field modulation of electronic properties in a type-II phosphorene/PbI2 van der Waals heterojunction.

Lead iodide (PbI2), a high-quality, single-layer, large-area material, has recently been experimentally acquired in a relatively simple manner. As a layered semiconductor material with an ideal band gap, it is also an important precursor of lead halide perovskites, making it an ideal material for manufacturing the next-generation optoelectronic devices. However, at present, there have been few theoretical studies reported on PbI2. Moreover, by constructing a vertical van der Waals (vdW) heterojunction, the excellent properties of various materials can be well utilized. Therefore, the study of a two-dimensional (2D) vdW heterojunction based on phosphorene/PbI2 (P/PbI2) will be very useful. In this study, a P/PbI2 vdW heterojunction was constructed, and its electronic properties were studied using the first-principles calculations based on the density functional theory (DFT) method. The calculation result shows that the P/PbI2 vdW heterojunction has a distinct type-II band alignment, whose direct band gap value is 0.52/0.83 eV in DFT/HSE06. Moreover, the band gap of the heterojunction can be effectively modulated under the control of an electric field, and the value of the band gap can vary from 0 to 0.90/1.54 eV in DFT/HSE06. Collectively, these findings provide an effective approach for designing new PbI2-based vdW heterojunctions and adjusting the electronic properties in solar energy and optoelectronic devices.

Phononic Stability Analysis of Two-Dimensional Carbon Nitride Monolayers

Abstract:In this study we examined the structural, dynamical stability and electronical properties of carbon nitrides monolayers as C6N6 and C6N8. We found that buckled form of C6N8 monolayer is dynamically stable instead of planar C6N8, which is many times studied in the literature. While planar C6N8 has negative optical phonon modes, with proper created buckling in the structure can dissappear these imaginarities and makes the system dynamically stable. This buckled C6N8 has 2.05 eV direct band gap, which falls in the visible region. Other investigated carbon nitride is C6N6 and as is known in the literature planar C6N6 monolayer is stable, while created buckling results instability for the structure. We believe that with this study, confliction on the stability of carbon nitride structures will annihilate and investigation can focus on the planar C6N6 and buckled C6N8 monolayers.

KAgSe: A New Two-Dimensional Efficient Photovoltaic Material with Layer-Independent Behaviors.

Recent advances in the development of two-dimensional (2D) materials have stimulated people's interest and enthusiasm to discover new kinds of 2D functional materials. In this paper, we propose a novel 2D layered semiconductor KAgSe using the first-principles calculation method, which displays excellent photovoltaic properties with proper direct band gap and significant carrier mobility. By evaluating the cohesive energy, vibrational phonon spectrum, and temporal evolution of the total energy at a high temperature of 500 K, the KAgSe monolayer is proved to be stable. Finite cleavage energy comparable to that of black phosphorus implies the feasibility of mechanical exfoliation of a KAgSe monolayer from the bulk. Layered KAgSe shows a ∼1.5 eV direct band gap, which is roughly independent of the number of layers. Remarkable optical absorption coefficients in the visible light region and significant carrier mobilities reveal a favorable application prospect of layered KAgSe in photovoltaic devices. Especially, the layer-independent optical absorption provides enormous convenience and less difficulty in experimental fabrication of photoelectronic devices which are based on finite layer KAgSe. To further explore the photovoltaic behaviors, the polarization angle-related photocurrent is evaluated for the KAgSe monolayer-based nanodevice by irradiating a beam of linearly polarized light to the scattering region. Moreover, large photon responsivity and external quantum efficiency are also obtained for the KAgSe monolayer.

Comparative Study of ∼2.05 eV Lattice-Matched and Metamorphic (Al)GaInP Solar Cells Grown by MOCVD

This work investigates two conceptually different approaches, both based on the AlGaInP alloy family, to produce metal–organic chemical vapor deposition grown 2.05 eV direct bandgap materials for use in the top cell of AM0 multijunction solar cells. The first approach achieves the target bandgap via (Al0.32Ga0.68) 0.52In0.48P lattice-matched to GaAs. The second approach achieves the target bandgap via relaxed, lattice-mismatched Ga0.66In0.34P. Solar cell characterization and analytical device modeling reveal merits and challenges for both approaches. Both devices are found to possess comparable minority carrier diffusion lengths within the p-type base and an equal bandgap-voltage offset of 0.54 V, suggesting effectively similar base material quality. However, the metamorphic Ga0.66In0.34P cell exhibits a superior quantum efficiency across all wavelengths, particularly at short wavelengths. Modeling results attribute the enhanced short wavelength response to the wider direct bandgap of the internally lattice-matched Al0.69In0.31P window and a longer emitter diffusion length. As such, despite the non-negligible lattice mismatch, the enhanced short wavelength response for the metamorphic Ga0.66In0.34P approach proved significantly advantageous, at least with respect to the growth conditions used in this study. Assuming the use of a perfect antireflection coating, the lower parasitic optical absorption from the wider direct bandgap window alone provides as much as ∼0.72 mA/cm2 additional photocurrent. Combined with the higher quality emitter layer observed in this study, ∼1.1 mA/cm2 additional photocurrent was achieved for the metamorphic Ga0.66In 0.34P approach. These values correspond to ∼3.9% and ∼6.0% of the total available photocurrent for a 2.05 eV bandgap solar cell.