Indirect Band Gap Transition Research Articles

Abnormal Response of Indirect Excitons to Non‐Uniform Elastic Strain in MoS2 Flakes

AbstractRegulating the electronic structures and carrier dynamics in 2D materials via elastic strain is a fascinating avenue for tailoring their optoelectronic properties at atomic scale. Here, the abnormal response of indirect excitons to non‐uniform elastic strain in MoS2 flakes is reported. The non‐uniform elastic strain in the MoS2 flakes is introduced by transferring them to pre‐prepared convex cross structures on a SiO2/Si substrate, where the topography and strain distribution are determined by atomic force microscopy and Raman spectroscopy, respectively. It is observed that the emission energy of the indirect excitons shows an unexpected blue‐shift followed by a red‐shift with increasing the local tensile strain, which is in sharp contrast to the linear red‐shift of the direct excitons. Density functional theory calculations reveal that the abnormal energy shift of the indirect excitons in the MoS2 flakes arises from the strain‐induced competition between two indirect bandgap transitions and the spatial exciton funnel effect in the non‐uniform strain field. This work provides new insights into the elastic strain effect on the indirect excitons in 2D semiconductors, which possesses potential applications in flexible optoelectronic devices.

Modulation of photoluminescence properties of transition metal dichalcogenides through temperature and Au/Ag nanoparticles

Abstract Transition metal dichalcogenides (TMDCs) attract significant attention because of their remarkable optical properties. Nowadays, the light emission efficiency of TMDCs is still inferior. Temperature and plasma resonance effects can be important approaches in the modulation of their luminescence performance. Here, multilayer WSe2 and monolayer MoSe2 were synthesized. Under the temperature control system, two photoluminescence (PL) peaks are observed from multilayer WSe2. The higher-energy PL peak results from K-Γ indirect band gap transition, which is then demonstrated by the first principle calculation. Otherwise, PL enhancement is realized on monolayer MoSe2 decorated with Au nanoparticles. The PL of WS2 is inhibited by hybridization with Ag nanoparticles.

A novel and promising Penta-Octa-Based silicon carbide semiconductor

Penta-octa-graphene (POG) consists of pentagonal and octagonal carbon rings, hosting type-I and type-II Dirac line nodes due to its sp2 and sp3 mixed bonds. Inorganic analogs of 2D carbon lattices have increased the potential applications and changed the main properties of carbon-based structures. Therefore, this work proposes penta-octa-graphene based on silicon carbide using DFT simulations. With a cohesive energy of −5.22 eV/atom, POG-Si5C4 is energetically viable in comparison with other silicon carbide-based monolayers. Phonon dispersion analysis confirms the POG-Si5C4 dynamical stability. MD simulations demonstrate that this new monolayer can withstand temperatures up to 1020 K. Electronic analysis indicates it is a semiconductor with an indirect band gap transition of 2.02 eV. The mechanical properties exhibit anisotropy, with Young’s modulus ranging from 38.65 to 99.47 N/m and an unusual negative Poisson’s ratio of −0.09. The band edge alignment suggests that POG-Si5C4 holds potential for hydrogen generation through photocatalytic water splitting. This research opens possibilities for designing inorganic penta-octa-based structures and provides insights for future experimental and theoretical studies focused on exploring and optimizing advanced silicon-carbide 2D materials.

Van der Waals heterostructures of AlAs and InSe: Stacking-dependent Raman spectra and electric field dependence of electronic properties

In the present work, the electronic and vibrational properties of a van der Waals type heterostructure, composed of single layers of AlAs and InSe, are investigated using density functional theory (DFT)-based first-principles calculations. Vibrational analyses reveal that dynamically stable single layers of AlAs and InSe form van der Waals type heterostructure which is shown to exhibit stacking-dependent Raman spectra by means of the frequency shifts. According to our findings, a type-II band alignment with a direct band gap of 1.84 eV is found in the ground state stacking of AlAs/InSe vertical heterostructure, in contrast to the indirect band gap behaviors of each individual layer. Moreover, the application of an external vertical electric field shows that the both band alignment type and the electronic behavior of the heterostructure can be tuned. The heterostructure is found to exhibit direct to indirect band gap transition under negative electric field as well as a transition from type-II to type-I heterojunction under negative fields up to 0.3 V/Å. The stronger fields along the same direction results in overlapping of valence states of each layer and lead to a non-linear change of the energy band gap. Overall, the predicted van der Waals type heterobilayer of InSe and AlAs with stacking-dependent vibrational features and well-controlled electronic properties under external field is shown to be potential candidate for optical and optoelectronic applications.

Fundamental properties of the MgCl2 monolayer from first-principles calculations

The band gap of 2D MgCl2 can be modulated flexibly in a large range by applied strain, layer control and cutting into nanoribbons and a direct–indirect band gap transition can be obtained.

Thickness-dependent optical bandgap of WS2/glass thin films deposited PLD method and their optical limitations and nonlinear optical properties by using the CW laser beam(532 nm)

WS2 thin films were deposited on glass substrates (WS2/glass) using the pulsed laser deposition (PLD) method with thicknesses of 30 and 190 nm. The crystalline structure and elemental composition of the WS2 thin films were analyzed through X-ray diffraction (XRD) and energy-dispersive X-ray spectroscopy (EDX). Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were employed to measure uniform, hexagonally cross-sectioned cones with smooth and even surfaces. Raman spectroscopy was utilized to assess the crystallinity and layer count in the WS2 thin films. Optical absorption, Tauc, and Urbach plots were employed to determine the optical bandgap energy (Eg) and defect energy Urbach (ED) of WS2 thin films with thicknesses of 30 nm and 190 nm. The 190 nm thick film exhibited an indirect bandgap transition with an energy value of 1.41 eV, whereas the 30 nm thin film showed a direct bandgap transition with an energy value of 1.98 eV. The defect energy Urbach was measured at 1.28 eV for the 30 nm WS2 film and 0.83 eV for the 190 nm film. Open aperture (OA) Z-scan analysis was conducted using a continuous-wave (CW) laser at 532 nm. The nonlinear absorption coefficient (β) was calculated for two different intensities, yielding values of 20.62 and 17.4 cmW for the 30 nm WS2 thin film and 13.75 and 13.35 cmW for the 190 nm film. The values of Imχ(3) and the figure of merit increased as the thin film size decreased for different sizes of WS2 films (30 nm and 190 nm). The optical limiting threshold and clamping point for the 30 nm WS2 thin film were found to be between 30–110 mW. It is evident that the higher β value for the WS2 thin film (30 nm) indicates a more effective limiting behavior.

Novel buckled graphenylene-like InN and its strain engineering effects

This paper reveals the structural, electronic and mechanical properties of a novel inorganic graphenylene based on indium nitride (IGP-InN). The IGP-InN was characterized via density functional theory (DFT) simulations. The phonon dispersion shows the dynamic stability of IGP-InN, and molecular dynamics simulations confirm its thermal stability up to 700 K. Besides the electronic properties, the indirect band gap transition with energy (Egap) of 2.49 eV makes IGP-InN suitable for optoelectronic applications under UV–visible. Also, the Egap tunability with mechanical strain was analyzed, with a decrease of 1.19 eV in the Egap for tensile strains. The structural buckling plays an important role, with a transition to a planar structure for tensile strains. This work reveals a new class of 2D materials, buckled inorganic graphenylenes, and provides valuable insights into designing and optimizing graphenylene-like materials.

Plasma defect-engineering of bulk oxygen-deficient zirconia

Oxygen-deficient zirconia (ZrO2-x) has recently emerged as a promising material for light absorption and photocatalytic applications. However, the economic and environmentally friendly manufacture of bulk ZrO2-x remains challenging and has limited widespread adoption. In this study, we present a novel low-pressure (300 Pa) plasma treatment (H2 gas at 500 °C for 5 h) capable of producing fully-dense bulk ZrO2-x without significant structural modifications. EPR (electron paramagnetic resonance) and XPS (X-ray photoelectron spectroscopy) characterisation of the plasma treated zirconia indicate the formation of Zr3+ ions and F2+ (VO··) centres. The increase of oxygen vacancies is also supported by the greater exothermic heat flow and relative mass gain observed through TGA (thermogravimetric analysis) and DSC (differential scanning calorimetry) analyses. Diffuse reflectance spectroscopy (DRS) reveals a substantial enhancement in light absorption, with an average increase of 66.2 % and >65 % absolute absorption across the entire spectrum (200–3000 nm). XPS and DRS measurements suggest significant reduction in both direct (from 4.84 to 2.61 eV) and indirect (from 3.19 to 1.45 eV) bandgap transition. By effectively enhancing the light absorption capability, reducing bandgap transitions, and maintaining the structural integrity of zirconia, low-pressure plasma treatments offer a promising and scalable approach for the environmentally friendly production of next-generation ZrO2-x materials.

Tunable Schottky barrier of in-plane MoSSe/Borophene heterojunctions under electric field and strain

The electronic properties of Janus MoSSe/Borophene in-plane heterojunctions are investigated through the first principle calculations. The constructed MoSSe/Borophene heterojunctions with different structures of Borophene exhibit different electronic properties. All the heterojunctions present the p-type Schottky contact. Furthermore, the electronic properties of MoSSe/Borophene heterojunctions can be modulated by electric field and mechanical strain. Specifically, as the applied electric field changes, the band gap also changes and Ohmic contact is achieved. Additionally, the strain induces a direct to indirect band gap transition of Janus MoSSe and changed contact type. The results demonstrate that the tunable electronic properties of Janus MoSSe/Borophene heterojunction make it become a promising candidate for the electronic devices.

Optical properties of multilayer MoS2 modified with thermal and shape transformation by laser irradiation

In this study, the optical properties of multilayer MoS2 at various temperatures and incident powers were studied. The multilayer MoS2 exhibits two excitonic bands and an indirect band gap transition. We found from the incident power dependence of photoluminescence that the bands exhibit a redshift with increasing incident power. For high-power excitation, a new band emerges near the indirect band and intensity ratio between the excitonic band and indirect band shows peculiar incident power dependence. We conclude that the emergence of a new band and incident power-dependent intensity ratio originate from the elevation of temperature and the morphological change, respectively.

The Acoustophotoelectric Effect: Efficient Phonon-Photon-Electron Coupling in Zero-Voltage-Biased 2D SnS2 for Broad-Band Photodetection.

Two-dimensional (2D) layered metal dichalcogenides constitute a promising class of materials for photodetector applications due to their excellent optoelectronic properties. The most common photodetectors, which work on the principle of photoconductive or photovoltaic effects, however, require either the application of external voltage biases or built-in electric fields, which makes it challenging to simultaneously achieve high responsivities across broad-band wavelength excitation─especially beyond the material's nominal band gap─while producing low dark currents. In this work, we report the discovery of an intricate phonon-photon-electron coupling─which we term the acoustophotoelectric effect─in SnS2 that facilitates efficient photodetection through the application of 100 MHz order propagating surface acoustic waves (SAWs). This effect not only reduces the band gap of SnS2 but also provides the requisite momentum for indirect band gap transition of the photoexcited charge carriers, to enable broad-band photodetection beyond the visible light range, while maintaining pA-order dark currents─ without the need for any external voltage bias. More specifically, we show in the infrared excitation range that it is possible to achieve up to 8 orders of magnitude improvement in the material's photoresponsivity compared to that previously reported for SnS2-based photodetectors, in addition to exhibiting superior performance compared to most other 2D materials reported to date for photodetection.

Layer thickness-dependent optical properties of GaTe

Gallium Telluride (GaTe) has emerged as a potential candidate for optoelectronic applications due to its outstanding optical properties. However, a comprehensive understanding of the thickness-dependent optical properties, particularly the bandgap and vibration modes, is still lacking. In this study, we investigated the dependence of bandgap and Raman-active modes on layer thickness by measuring micro-photoluminescence (PL) and micro-Raman spectroscopy of GaTe at 77 K. Our results revealed a significant dependence of bandgap and Raman-active modes on layer thickness, with the bandgap of GaTe flakes increasing from 1.736 to 2.09 eV as the thickness decreases from bulk to 1.5 nm. Furthermore, the quantum confinement effect led to a direct to indirect bandgap transition in few-layered GaTe. We also interpret interlayer coupling and long-range Coulomb interactions between layers resulted in the appearance of an extra Raman peak associated with the shear or breathing modes and structural change for GaTe-flakes with decreasing their thickness down to a few layers. Additionally, we observed a softening of Ag3, Ag4, and Ag5 modes and stiffening of others with reducing thickness. Our results provide a comprehensive understanding of the thickness dependence of optical properties in GaTe material.

Strain engineering on structural, optoelectronic and photocatalytic properties of BP, BAs and BSb monolayers

Recently, hydrogen energy gains a lot of attention for researcher due to its clean resource and easily production in photocatalytic process. Two dimension materials have superior chemical and physical properties compared to their bulk counterparts, having promising advantages in photocatalytic process. We perform density functional theory calculation and investigated the structural, optoelectronic and photocatalytic properties of BP, BAs and BSb monolayer. The calculated band structure shows that all these monolayers have direct band nature. A direct to indirect band gap transition is observed for 8% and 10% compressive strain for BSb monolayer. The band gap values show increase for compressive strain while decrease monotonically under tensile strain. The partial density of states confirms the different atoms contribution in valence and conduction bands. Effective masses and work function are also calculated and found to be vary under compressive and tensile strain. A blue shift in the excitonic peaks for compressive strain, while a red shift is observed for tensile strain. Photocatalytic response shows that BP (for unstrained and strain) and BAs (for 4%, 6% and 10% compressive strain) are good candidates for full water splitting. Furthermore, the thermal stability confirms that, these systems are stable under both tensile and compressive strain.

Enhancing of 4F3/2 → 4I9/2 transition of Nd3+ doped BaO-Ga2O3– Al2O3–B2O3 glasses for near-infrared laser applications

Neodymium, Barium Gallium Aluminium Borate glasses having the composition of [35% B2O3 + 35% BaO + 15% Ga2O3 + (15-x)% Al2O3 + x% Nd2O3, (x = 0, 0.5, 1 and 2%)] were prepared using traditional melt quenching method. Physical properties such as refractive index (n), molar volume (VM), density (ρ), band-gap (Eg), and Urbach energy (ΔE) were measured and calculated. The results revealed that our glass has allowed indirect band-gap transitions and the addition of Nd3+ reduced the Eg energies from 3.24 to 3.01 eV. The Judd-Ofelt parameters were obtained using the least-square technique and they were used to compute the total radiative probability (At), radiative lifetime (τr), fluorescence branching ratio (Br), and quantum efficiency (η) for the meta-stable level 4F3/2. The glass system showed a high Spectroscopic factor (χ) ranging from 1.61 to 1.98. The Br showed that the 4F3/2 → 4I9/2 transition (883 nm) was enhanced at the expense of the 4F3/2 → 4I11/2 transition (1064 nm) and (4F3/2 → 4I13/2) transition (1342 nm). A relatively high quantum efficiency (η = 0.78) was noticed for our glass. Generally the spectroscopic properties and gain factor (γ) showed that the present glass system is a promising candidate for near-infrared laser applications (883 nm).

Novel octa-graphene-like structures based on GaP and GaAs.

The discovery of graphene gave way to the search for new two-dimensional structures. In this regard, octa-graphene is a carbon allotrope consisting of 4- and 8-membered rings in a single planar sheet, drawing the research community's attention to study their inorganic analogs. Considering the promising properties of octa-graphene-like structures and the role of GaAs and GaP in semiconductor physics, this study aims to propose, for the first time, two novel inorganics buckled nanosheets based on the octa-graphene structure, the octa-GaAs and octa-GaP. This work investigated the structural, electronic, and vibrational properties of these novel octa-graphene-based materials. The octa-GaP and octa-GaAs have an indirect band gap transition with a valence band maximum between M and Г points and a conduction band minimum at Г point with energy of 3.05 eV and 2.56 eV, respectively. The QTAIMC analysis indicates that both structures have incipient covalent in their bonds. The vibrational analysis demonstrates the occurrence of ΓRaman = 6Ag + 6Bg and ΓRaman = 12A' + 12B″ for octa-GaP and octa-GaAs, respectively. The symmetry reduction of octa-GaAs leads to activating inactive modes observed in the octa-GaP structure. The frontier crystalline orbitals are composed by Ga(px) and P(py and pz) orbitals for octa-GaP and Ga(px and py) and As(s, py, and pz) for octa-GaAs in the valence bands while in the conduction bands by Ga(py, pz, and s) for both compounds and P(px and pz) and As(py). The phonon bands demonstrate the absence of the negative frequency modes and the structural stability of these new nanosheets. This report aims to reveal the fundamental properties of both newfound materials for stimulating experimental research groups in the search for synthesis routes to obtain this structure. This work used the DFT/B3LYP approach implemented in the CRYSTAL17 computational package. Ga, As, and P atomic centers were described by triple-zeta valence with polarization (TZVP) basis set. The vibrational analysis was carried out via coupled-perturbed Hartree-Fock/Kohn Sham (CPHF/KS) method, and the chemical bonds were evaluated via the quantum theory of atoms in molecules and crystals (QTAIMC).

Extracting Optical Parameters of Cu–Mn–Fe Spinel Oxide Nanoparticles for Optimizing Air-Stable, High-Efficiency Solar Selective Coatings

High-temperature Cu–Mn–Fe spinel-oxide nanoparticle solar selective absorber coatings are investigated experimentally and theoretically. A general realistic approach to evaluate absorption coefficient spectra from the optical measurements of the nanoparticle-pigmented coatings is developed by solving the inverse problem of the four-flux-radiative transfer model. The derived absorption properties are utilized to elucidate the direct and indirect bandgap transitions in the visible spectral regime as well as the sub-bandgap absorption in the infrared range. Furthermore, the optical properties of NP materials can be directly applied to optimize the solar selective coating design and analyze thermal degradation mechanisms. The analysis reveals that the Cu–Mn–Fe spinel oxides are semiconductors with an indirect bandgap ranging from 1.7 to 2.1 eV, while iron-free CuMn2O4 is a direct bandgap material with Eg = 1.84 eV. With the same coating thickness and nanoparticle load, the solar absorptance ranks in the order of Mn2O3 < MnFe2O4 < CuFe2O4 < CuFeMnO4 < CuMn2O4. The optimized spray-coated iron-free CuMn2O4 NP-pigmented coating demonstrates a high solar absorptance of 97%, a low emittance of 55%, a high optical-to-thermal energy conversion efficiency of ∼93.5% under 1000× solar concentration at 750 °C, and long-term endurance upon thermal cycling between 750 °C and room temperature in air. The optical parameter analysis approach can be easily extended to other material systems to facilitate the search and optimization of high-temperature nanoparticle-pigmented solar selective coatings.

Halide Double Perovskite Nanocrystals Doped with Rare-Earth Ions for Multifunctional Applications.

Most lead-free halide double perovskite materials display low photoluminescence quantum yield (PLQY) due to the indirect bandgap or forbidden transition. Doping is an effective strategy to tailor the optical properties of materials. Herein, efficient blue-emitting Sb3+ -doped Cs2 NaInCl6 nanocrystals (NCs) are selected as host, rare-earth (RE) ions (Sm3+ , Eu3+ , Tb3+ , and Dy3+ ) are incorporated into the host, and excellent PLQY of 80.1% is obtained. Femtosecond transient absorption measurement found that RE ions not only served as the activator ions but also filled the deep vacancy defects. Anti-counterfeiting, optical thermometry, and white-light-emitting diodes (WLEDs) are exhibited using these RE ions-doped halide double perovskite NCs. For the optical thermometry based on Sm3+ -doped Cs2 NaInCl6 :Sb3+ NCs, the maximum relative sensitivity is 0.753% K-1 , which is higher than those of most temperature-sensing materials. Moreover, the WLED fabricated by Sm3+ -doped Cs2 NaInCl6 :Sb3+ NCs@PMMA displays CIE color coordinates of (0.30, 0.28), a luminous efficiency of 37.5lm W-1 , a CCT of 8035 K, and a CRI over 80, which indicate that Sm3+ -doped Cs2 NaInCl6 :Sb3+ NCs are promising single-component white-light-emitting phosphors for next-generation lighting and display technologies.

Strain induced effects on the electronic and phononic properties of 2H and 1T[formula omitted] monolayer MoS2

First-principles calculations have been performed on the 2H and the 1T′ phase of single-layer MoS2. We have addressed the strain-induced tunability of the electronic and the phononic properties of both phases, and compared their stability. For the 2H phase, a direct to indirect band gap transition, followed by the band gap lowering leading to a semiconductor to metal transition, is observed. The applied strain destroys the semimetallic nature of the 1T′ phase. Significant strain-induced changes in the frequency and the dispersion of the phonon branches are observed. With increasing strain, the out-of-plane acoustic mode (ZA) turns imaginary, suggesting a possibility of phase transition or instability of the crystal structure. The 2H phase appears to withstand a larger amount of strain indicating better stability compared to the 1T′ phase. We also highlight the significance of anisotropic strain-engineering in tuning the electronic and phononic properties and the stability limit of monolayer 1T′-MoS2.

Resistive switching characteristics of W/TiO2/ITO devices

Materials with resistance-switching properties are proving to be promising for the novel non-volatile memories-resistive random-access memory (ReRAM). Despite extensive research on electric field-induced resistance switching, there are diverse opinions about its reliability and working mechanism. In this work, we analysed the impact of SET and RESET voltages on the switching characteristics and the possible current conduction mechanism of W/TiO2/ITO devices. By simple solution processing and spin coating, titanium dioxide thin films were grown on ITO, glass, silicon, and quartz substrates. The precursor and solvent used for the synthesis of the active layer were titanium isopropoxide and ethanol, respectively. Polycrystalline films with an anatase structure were formed after annealing at 400 °C with a 20 nm average crystallite size, as evidenced by X-ray diffraction patterns. The crystalline nature of the fabricated film has been found to improve on annealing. Using Raman spectroscopy, the chemical bonding states of the film were studied. A UV–visible spectrometer study confirms that anatase TiO2 possesses an indirect band gap transition with a 3.42 eV band gap. The device shows reversible, reproducible, stable, and bipolar resistance-switching behaviour as indicated by current–voltage (I-V) measurements, and it was found that there exists a minimum voltage (∼-4 V) to set the device into an ON state. Compared to crystalline TiO2, theamorphous active layer offers much better switching characteristics, such as good cyclic endurance with a Roff-Ronratio ∼ 10.The current–voltage characteristics were fitted to various carrier transport mechanismssuch as Ohmic conduction, Schottky emission (SE), Space-charge-limited conduction (SCLC), and Poole-Frenkel emission (PF). Among these, the ohmic and SE mechanisms were detected at low and high fields, respectively. Schottky barrier modulation at the metal-oxide interface has been identified as the motive for switching.

First principles study of BAs/MoSi2N4 van der Waals heterostructure: tunable electronic and optical properties via vertical strain.

Constructing van der Waals heterostructures from layered materials can form new optoelectronic devices with superior performance to the individual monolayers. Here, we use first-principles calculations to explore the modulation of a two-dimensional BAs/MoSi2N4 van der Waals heterostructure via strain, including the structure stabilities, electronic properties, charge transfer, and optical properties. Our calculated results reveal that the BAs/MoSi2N4 heterostructure has a direct bandgap of 0.72 eV and type-I band alignment. In addition, the BAs/MoSi2N4 heterostructure exhibits enhanced light absorption in the visible light region. The electronic properties of the BAs/MoSi2N4 heterostructure are tunable by vertical strain, exhibiting a direct to indirect bandgap transition as well as a type-I to type-II band alignment transition when the vertical distance is reduced. Our research provides a comprehensive understanding of the electronic and optical properties of the BAs/MoSi2N4 heterostructure and could be helpful for their potential applications in optoelectronic devices.