Large Direct Band Gap Research Articles

Continuous-Flow Synthesis of Zn1-xMnxS Nanoparticles at Ambient Conditions.

With its large direct band gap and good chemical stability, ZnS is suitable for many applications, including light-emitting diodes, panel displays, and photodetection. Here, nanoparticles of ZnS are synthesized phase pure under ambient conditions by precipitation in a simple and scalable continuous-flow reactor. Furthermore, different degrees of Zn substitution with Mn have been investigated, Zn1-xMnxS, with x = 0.05, 0.19, and 0.25 according to X-ray fluorescence measurements. The products are analyzed with multitemperature synchrotron powder X-ray diffraction (PXRD) and X-ray total scattering. The analysis reveals phase-pure synthesis products with the sphalerite structure and crystallite sizes in the range of 3.8-4.7 nm in agreement with scanning transmission electron microscopy. Only Zn0.75Mn0.25S shows traces of Mn3O4, indicating that x = 0.25 is above the substitution limit as the impurity appears. Substitution of Zn with Mn in the nanoparticles is confirmed by energy-dispersive X-ray spectroscopy, as well as an observed decrease in the band gap, decrease in the sphalerite-to-wurtzite phase transition temperature, and increase in the unit cell dimensions with increasing Mn content. Based on the modeling of the PXRD Rietveld refined atomic displacement parameters, the Debye temperature for ZnS and Zn0.95Mn0.05S is determined to be 322 ± 13 and 394 ± 22 K, respectively.

Advancements in Optoelectronics: Harnessing the Potential of 2D Violet Phosphorus

AbstractRecently, 2D violet phosphorus (VP), a new kind of allotrope of phosphorus has attained significant attention owing to its remarkable electronic, optical, and magnetic characteristics. Tunable, large direct bandgap, high charge carrier mobility, and large optical absorption make it a potential candidate for realizing photoelectronic applications. VP demonstrates unique electronic structure, chemical stability, strong light‐matter interaction, and thermal stability that can be utilized for assembling intelligent photoelectronic devices. This review article provides comprehensive investigations of the latest synthesis techniques employed to develop the VP including its structural characterizations, stability, and degradation mechanisms. Furthermore, this review demonstrates the diverse applications of VP in optoelectronics, including photodetectors, polarization detection, imaging systems, neuromorphic optoelectronic devices, and many others. Finally, challenges and future research directions associated with the VP in terms of optoelectronics are discussed. Overall, as presented, the review offers an extensive study of VP, covering its synthesis, structural insights mechanisms, and applications in optoelectronics with the aim of stimulating further research and development in this growing field.

Hidden Direct Bandgap of Bi2O2Se by Se Vacancy and Enhanced Direct Bandgap of Bismuth Oxide Overlayer.

The Bi2O2Se surfaces are well-known to possess 50% Se vacancies, yet they have shown no in-gap states within the indirect bandgap (∼0.8 eV). We have found that the hidden in-gap states arising from the Se vacancies in a 2 × 1 pattern induce a reduced direct bandgap (∼0.5 eV). Such a reduced direct bandgap is responsible for the high electron mobility of Bi2O2Se. Moreover, the Bi oxide overlayers of the Bi thin films, formed through air exposure and annealing, unexpectedly exhibit a large direct bandgap (∼2.1 eV). The simplified fabrication of Bi oxide overlayers provides promise for improving Bi2O2Se electronic devices and enhancing photocatalytic activity.

NiO as Hole Transporting Layer for Inverted Perovskite Solar Cells: A Study of X‐Ray Photoelectron Spectroscopy

AbstractHygroscopic and acidic nature of organic hole transport layers (HTLs) insisted to replace it with metal oxide semiconductors due to their favorable charge carrier transport with long chemical stability. Apart from large direct bandgap and high optical transmittance, ionization energy in the range of −5.0 to −5.4 eV leads to use NiO as HTL due to good energetic matching with lead halide perovskites. Analyzing X‐ray photoelectron spectroscopic (XPS) data of NiO, it is speculated that p‐type conductivity is related to the NiOOH or Ni2O3 states in the structure and the electrical conductivity can be modified by altering the concentration of nickel or oxygen vacancies. However, it is difficult to separate the contribution from nonlocal screening, surface effect and the presence of vacancy induced Ni3+ ion due to very strong satellite structure in the Ni 2p XPS spectrum of NiO. Thus, an effective approach to analyze the NiO XPS spectrum is presented and the way to correlate the presence of Ni3+ with the conductivity results which will help to avoid overestimation in finding the oxygen‐rich/deficient conditions in NiO.

Computationally-driven discovery of second harmonic generation in EuBa3(B3O6)3 through inversion symmetry breaking

Nonlinear optical (NLO) crystals with superior properties are significant for advancing laser technologies and applications. Introducing rare earth metals to borates is a promising and effective way to modify the electronic structure of a crystal to improve its optical properties in the visible and ultraviolet range. In this work, we computationally discover inversion symmetry breaking in EuBa3(B3O6)3, which was previously identified as centric, and demonstrate noncentrosymmetry via synthesizing single crystals for the first time by the floating zone method. We determine the correct space group to be P6¯. The material has a large direct bandgap of 5.56 eV and is transparent down to 250 nm. The complete anisotropic linear and nonlinear optical properties were also investigated with a d11 of ∼0.52 pm/V for optical second harmonic generation. Further, it is Type I and Type II phase matchable. This work suggests that rare earth metal borates are an excellent crystal family for exploring future deep ultraviolet (DUV) NLO crystals. It also highlights how first principles computations combined with experiments can be used to identify noncentrosymmetric materials that have been wrongly assigned to be centrosymmetric.

Free-standing 2D gallium nitride for electronic, excitonic, spintronic, piezoelectric, thermoplastic, and 6G wireless communication applications

Two-dimensional gallium nitride (2D GaN) with a large direct bandgap of ~5.3 eV, a high melting temperature of ~2500 °C, and a large Young’s modulus ~20 GPa developed for miniaturized interactive electronic gadgets can function at high thermal and mechanical loading conditions. Having various electronic, optoelectronic, spintronic, energy storage devices and sensors in perspective and the robust nature of 2D GaN, it is highly imperative to explore new pathways for its synthesis. Moreover, free-standing sheets will be desirable for large-area applications. We report our discovery of the synthesis of free-standing 2D GaN atomic sheets employing sonochemical exfoliation and the modified Hummers method. Exfoliated 2D GaN atomic sheets exhibit hexagonal and striped phases with microscale lateral dimensions and excellent chemical phase purity, confirmed by Raman and X-ray photoelectron spectroscopy. 2D GaN is highly stable, as confirmed by TGA measurements. While photodiode, FET, spintronics, and SERS-based molecular sensing, IRS element in 6G wireless communication applications of 2D GaN have been demonstrated, its nanocomposite with PVDF exhibits an excellent thermoplastic and piezoelectric behavior.

First-principles investigations of the crystal and electronic structures, dynamical and mechanical stabilities, Born effective charges and dielectric permittivities of a novel tetragonal zirconia

In this paper, a novel tetragonal ZrO2 phase has been predicted from first-principles calculations. The detailed crystal structural analysis shows that the tetragonal ZrO2 has a space group I41/amd (D4h19, No. 141) with 4 formula units per unit cell and 16 symmetry operations as well as six-fold cation coordination. The Zr atoms occupy 4a Wyckoff positions (0, 1/2, 1/4) and (1/2, 1/2, 1/2) and the O atoms occupy 8e Wyckoff positions (0, 0, z+1/8), (0, 1/2, z+3/8), (1/2, 0, -z+5/8) and (1/2, 1/2, -z+3/8) with z = 0.0761. The electronic structures, dynamical and mechanical stabilities investigations show the tetragonal ZrO2 is dynamical and mechanical stable as well as with a large direct band gap of 3.9015eV at Γ point. The phonon dispersion is divided into two regions: the low frequency part (0–8.6240 THz) and the high frequency part (10.3453–18.4034 THz) with a separation gap of 1.7213 THz. The magnitudes of Born effective charges are greater than the nominal chemical valences of +4 and −2 for Zr and O ions, respectively, reflecting a strong dynamic charge transfer from Zr to O atoms and thus a p-d hybridized Zr–O ionic bond. A diagonal and anisotropic static dielectric permittivity εij0 is obtained between ab-plane ε110=ε220=17.80 and along c axis ε330=14.77.

Bottom-Up Growth of Monolayer Honeycomb SiC.

The long theorized two-dimensional allotrope of SiC has remained elusive amid the exploration of graphenelike honeycomb structured monolayers. It is anticipated to possess a large direct band gap (2.5eV), ambient stability, and chemical versatility. While sp^{2} bonding between silicon and carbon is energetically favorable, only disordered nanoflakes have been reported to date. Here we demonstrate large-area, bottom-up synthesis of monocrystalline, epitaxial monolayer honeycomb SiC atop ultrathin transition metal carbide films on SiC substrates. We find the 2D phase of SiC to be almost planar and stable at high temperatures, up to 1200 °C in vacuum. Interactions between the 2D-SiC and the transition metal carbide surface result in a Dirac-like feature in the electronic band structure, which in the case of a TaC substrate is strongly spin-split. Our findings represent the first step towards routine and tailored synthesis of 2D-SiC monolayers, and this novel heteroepitaxial system may find diverse applications ranging from photovoltaics to topological superconductivity.

Synergetic optimization of thermoelectric properties in SnSe film via manipulating Se vacancies

Thin film thermoelectric materials have attracted much attention due to the application prospect in nanoscale devices. However, the commonly used optimization strategy in bulk materials, such as defect engineering, is hard to be realized in films. In this work, we synthesized SnSe films by the magnetron sputtering method and manipulated the Se vacancy concentration simply by controlling the sputtering time. The carrier transport properties were systematically investigated by combining theoretical simulation and experimental measurement. In the 130 nm-thick ultrathin film, Se vacancies were formed due to the difference in atomic mass between Sn and Se. The electrical conductivity and carrier mobility are improved by the weak electron-phonon coupling strength and small indirect band gap. Meanwhile, the thermal conductivity of the vacancy structure has an average reduction of 33.8% compared with the pristine SnSe films due to the strong vacancy scattering. The Seebeck coefficient maintains above 300 μV K-1 due to the relatively large direct band gap. A high ZT of 0.6 was finally achieved at 700 K in the film sample with Sn:Se ratio of 1.08, 50% improvement over the pristine SnSe film. However, a transition to the metallic characteristic occurs when Se vacancy concentration further increases, resulting in deterioration of ZT at high temperature. The present work demonstrates a guiding principle of manipulating vacancy concentration for high thermoelectric performance and reveals how vacancies influence energy transfer.

First-principles study of SrTe and BaTe: Promising wide-band-gap semiconductors with ambipolar doping

Transparent wide-band-gap (WBG) semiconductors are crucial components in diverse (opto)electronic and energy devices. Using first-principles calculations, we demonstrate that alkaline earth tellurides MTe (M = Sr or Ba) are promising WBG semiconductors that can be ambipolarly doped and transparent to visible light. Because of their large direct band gaps (3.74 eV for SrTe and 3.09 eV for BaTe), 100 nm thick MTe films exhibit significant transmittance (over 80%) for visible light. The effective mass of electrons and holes in MTe is predicted to be small (<1 m0) enough to show high carrier mobilities. From the analysis of the defect properties, we show that the major carrier type (electrons vs. holes) and its concentration can be controlled by adjusting the synthetic conditions. We also find that the valence band maximum of MTe is relatively shallow. Thus, MTe can be utilized as an electron-blocking (hole-transport) layer in emerging perovskite solar cells.

Ground state and phase stability of GaP-ZnS and GaP-ZnSe quaternary systems from first-principles

Quaternary alloys systems with large direct bandgap are promising for electronic devices and energy applications. Here we performed a crystal structure search for GaP-ZnS and GaP-ZnSe quaternary alloys using an ab initio evolutionary algorithm. The analysis of structures reveals the existence of trigonal (R3m) phases for GaPZnS and GaPZnSe alloys, which are thermodynamically slightly unstable, but dynamically and mechanically stable. State-of-the-art local density approximation half-occupation (LDA-1/2) approach divulges that both materials exhibit wide band-gap character (EG(x)∼2.78eV, and 2.48eV for GaPZnS and GaPZnSe respectively). Moreover, GaPZnS is found to be indirect; whereas GaPZnSe evinces a quasi-direct band-gap character. Besides, we predict a relatively weak band-gap bowing of 0.84eV for GaPZnS and 0.96eV for GaPZnSe quaternary alloys.

Full characterization of spontaneous parametric down conversion in non-ideal quarter-wavelength semiconductor Bragg reflection waveguide

Entangled photons are important for testing foundations of quantum physics and are at the heart of quantum technology. Integrated photonics has overwhelming dominance in terms of density and performance, making it a promise route for scalable quantum information processing. AlGaAs-based materials having large second-order non-linearities, direct bandgap and strong electro-optical effect can offer distinct advantages in quantum light source. Here we report a non-ideal quarter-wavelength Bragg reflection waveguide for generating three types of spontaneous parametric down-conversion processes. A general solution to the dispersion equation is derived and employed for designing high efficiency devices by taking into account the influence of core layer aluminium concentration. We further design and fabricate a Bragg reflection waveguide sample based on the analysis, and experimentally characterize its phase matching types and spectral brightness. Our work paves the path for the development of portable quantum light sources.

A simple and robust machine learning assisted process flow for the layer number identification of TMDs using optical contrast spectroscopy

Layered transition metal dichalcogenides (TMDs) like tungsten disulphide (WS2) possess a large direct electronic band gap (∼2 eV) in the monolayer limit, making them ideal candidates for opto-electronic applications. The size and nature of the bandgap is strongly dependent on the number of layers. However, different TMDs require different experimental tools under specific conditions to accurately determine the number of layers. Here, we identify the number of layers of WS2 exfoliated on top of SiO2/Si wafer from optical images using the variation of optical contrast with thickness. Optical contrast is a universal feature that can be easily extracted from digital images. But fine variations in the optical images due to different capturing conditions often lead to inaccurate layer number determination. In this paper, we have implemented a simple Machine Learning assisted image processing workflow that uses image segmentation to eliminate this difficulty. The workflow developed for WS2 is also demonstrated on MoS2, graphene and h–BN, showing its applicability across various classes of 2D materials. A graphical user interface is provided to enhance the adoption of this technique in the 2D materials research community.

Facile synthesis and photodetection characteristics of new pyrrolo[2,3-b]pyrrole-based metal-free organic dyes containing phenols as the potential candidates towards energy conversion

A new characteristic with inexpensive preparation tools and best application performance is needed. These new properties have been achieved by using novel heterocyclic dyes by attaching different electron-rich aromatics to the pyrrolo[2,3-b]pyrrole system. A new derivatives of diethyl 3,4-diamino-1,6-diphenyl-1,6-dihydropyrrolo [2,3-b]pyrrole-2,5-dicarboxylate (PPy) (compound 1) was easily and efficiently synthesized using catalyzed condensation reaction. The final push-pull conjugated dyes were yielded by azo coupling with resorcinol, and 2-naphthol, denoted as compounds 2 and 3, respectively. Very good adhesive and homogenous thin films of these compounds are successfully prepared by thermal evaporation technique. On the basis of its corrected analytical and spectral data, the structure of these compounds was determined. The relation between dye structures, photophysical, and molecular structures had been discussed. The unique simulation characteristics of the prepared compounds 1–3 were achieved by density functional theory, DFT/B3LYP, using 6–311 ++ G (d,p) basis set. Optical spectra have revealed a large direct band gap at 3.99, 3.15, and 2.85 eV for compounds 1, 2, and 3 respectively, corresponding to π−π* transitions. Finally, an experimental approach was used to determine the exciton binding energy to depict Frenkel exciton binding energy. The properties of these compounds proved that it is more suitable as a potential application for energy conversion.

Impact of strain on the electronic, phonon, and optical properties of monolayer transition metal dichalcogenides XTe2 (X = Mo and W)

Here, we provide a systematic assessment of biaxial strain effects on the electronic, phonon, and optical properties of monolayer transition metal dichalcogenides (TMDs) XTe2 (X = Mo and W) using density functional theory calculations. We observed a large direct bandgap of 1.163 eV and 0.974 eV for MoTe2 and WTe2, which reduced to 1.042 eV and 0.824 eV in the spin–orbit coupling ambient. The XTe2 structures show a tunable bandgap with the variation of the applied biaxial strains. Due to the breaking of inversion symmetry, a large spin-valley coupling emerged at the valance band edges for both MoTe2 and WTe2 monolayers under applied biaxial strain. The phonon properties with different biaxial strains reveal that monolayer MoTe2 is more stable than the WTe2 structure. The calculated optical properties demonstrate that the dielectric constant and absorption coefficient of MoTe2 and WTe2 move to higher photon frequencies when the compressive strain is increased. On the other hand, with the increase in tensile strain, a red-shift behavior is found in the calculated optical properties, indicating the suitability of the XTe2 monolayer for different infrared and visible light optical applications.

Enhanced strong coupling of WSe2 monolayer by Bound State in the continuum

Due to the large binding energy and direct bandgap, transition-metal dichalcogenides (TMDCs) monolayers have been considered a perfect platform for realising strong coupling at room temperature. It is well established that the quality factor (Q-factor) plays a crucial role in enhancing strong coupling. In this work, we demonstrate the improved strong coupling between the exciton of the WSe2 monolayer and the high Q cavity resonance based on symmetry protected magnetic dipole (MD) bound state in the continuum (BIC). We have found that the Rabi-splitting of the strongly coupled system could be largely enhanced by adjusting the location of the TMDC monolayer, increasing the Q-factor, and reducing the grating thickness. After carefully adjusting the three critical parameters, a Rabi-splitting as high as 38 meV was achieved limited by the oscillator strength of the WSe2 monolayer. Our system could be considered an excellent platform to realise ultra-thin polaritonic devices.

Photo- and exchange-field controlled line-type resonant peaks and enhanced spin and valley polarizations in a magnetic WSe2 junction

Existing resonant tunneling modes in the shape of line-type resonances can improve the transport properties of the junction. Motivated by the unique structural properties of monolayer WSe2 e.g. significant spin–orbitcoupling and large direct band gap, the transport properties of a normal/ferromagnetic/normal WSe2 junction with large incident angles in the presence of exchange field (h), off-resonance light () and gate voltage (U) is studied. In a certain interval of U, the transmission shows a gap with optically controllable width, while outside it, the spin and valley resolved transmissions have an oscillatory behavior with respect to U. By applying (h), an optically (electrically) switchable perfect spin and valley polarizations at all angles of incidence have been found. For large incident angles, the transmission resonances change to spin-valley-dependent separated ideal line-type resonant peaks with respect to U, results in switchable perfect spin and valley polarizations, simultaneously. Furthermore, even in the absence of U, applying h or at large incident angles can give some spin-valley dependent ideal transmission peaks, making h or a transmission valve capable of giving a switchable fully spin-valley filtering effect. These findings suggest some alternate methods for providing high efficiency spin and valley filtering devices based on WSe2.

Synthesis and characterization of Mg-doped ZnO thin-films for photovoltaic applications

Zinc oxide (ZnO) has gained significant interest in the last few years due to its attractive properties for the transparent electronics industry, particularly due to its large direct band gap (Eg) ∼ 3.3 eV and exciton binding energy ∼60 meV. In this study, we report the elaboration and characterization of ZnO and Mg-doped ZnO (MZO) thin films with different Mg doping concentrations (0%, 2%, 3%, 4%, 5%, respectively).The Mg-doped ZnO (MZO) thin films were synthesized by the sol–gel immersive method on glass substrates, employing dehydrated zinc acetate, magnesium, ethanol as solvent, and MEA as stabilizer. The impact of Mg doping concentration on the structural and optical properties of the prepared films was examined using X-ray diffraction and UV visible spectroscopy, respectively. The (002) peak of the Mg-doped ZnO thin films is observed to be shifted to a higher angle as a consequence of Mg doping. The transmittance spectra reveal that Mg doping may increase the optical band gap of ZnO thin films. The band gap is adjusted from 2.95 eV to 3.10 eV by varying the Mg doping concentration between 2% and 5%.

First-principles investigation of structural, electronic and optical properties of quasi-one-dimensional barium cadmium chalcogenides Ba2CdX3 (X = S, Se, Te) using HSE06 and GGA-PBE functionals

Structural, electronic and optical properties of quasi-one-dimensional barium cadmium chalcogenides Ba2CdS3, Ba2CdSe3 and Ba2CdTe3 are studied using density functional theory. Structural properties of these materials are investigated by incorporating Van der Waals correction with PBE exchange correlation functional. Results of structural analysis are in good agreement with the previously reported experimental observations and confirm the quasi-one-dimensional structure of the compounds. Ba2CdX3 chalcogenide structures crystallize in orthorhombic phase with space group Pnma-D2h16. Band structure calculations with HSE06 show that Ba2CdSe3 and Ba2CdTe3 are wide bandgap materials with direct bandgap of 2.56 eV for Ba2CdSe3 and 2.16 eV for Ba2CdTe3. Ba2CdS3 is an insulator with direct bandgap 3.16 eV. Optical properties are investigated by calculating the refractive index, birefringence, dichroism, dielectric function and absorption coefficient using the HSE06 method. Ba2CdX3 materials exhibit large optical anisotropy with considerable birefringence and dichroism. The comparatively high values of absorption coefficients, extinction coefficients, birefringence and dichroism make Ba2CdX3 materials suitable for applications as polarizers and sensing devices, while the large direct bandgaps indicate that they are promising candidates in photovoltaic applications.

Crystal structure and photo-physical properties in a perovskite–type semiconducting hybrid phase: {(m-C6H7N2O2)2CdCl4}n

A new intercalation crystalline polymer compound of bis m-nitroanilinium tetrachlorocadmate (II) {(m-C6H7N2O2)2CdCl4}n was synthesized and analyzed using single crystal SXRD, differential scanning calorimetry (DSC), DFT analysis, thermal gravimetric analysis (TGA) and FT-IR, Raman, UV–Vis, fluorescence spectroscopy techniques. X-ray diffraction analyses (SXRD, PXRD) show a layered structure consisting of alternating organic bilayers and two-dimensional inorganic sheets in which each CdCl6 octahedron shares four corners with adjacent octahedra. The crystal packing is consolidated by means of classic and non-classic hydrogen bonds and π-π interactions. At room temperature photoluminescence spectra of {(m-C6H7N2O2)2CdCl4}n yield broad peaks in the 469–770 nm range with full width at half maximum (FWHM) values up to 153 nm. Besides, this compound exhibits a semiconducting behavior with bright red-light under 360 nm ultraviolet photoexcitation and possesses a large Stokes shift and direct band gap of 2.69 eV which overlaps well with solar spectrum. The CIE chromaticity coordinates of {(m-C6H7N2O2)2CdCl4}n are (x = 0.4704 and y = 0.4523). The color rendering index CRI and the low correlated color temperature CCT are 84 and 2861 K, respectively. Electronic structure (BS, DOS and PDOS), and optical properties (dielectric constant ε(ω), refractive index n(ω), reflectivity R(ω), absorption coefficient α(ω), optical conductivity σ(ω) and energy loss function L(ω) with the incident photon energy) were determined using (DFT) calculations by CASTEP code.