Optical Band Edge Research Articles

Influence of Substrate-Induced Charge Doping on Defect-Related Excitonic Emission in Monolayer MoS2.

Many applications of transition metal dichalcogenides (TMDs) involve transfer to functional substrates that can strongly impact their optical and electronic properties. We investigate the impact that substrate interactions have on free carrier densities and defect-related excitonic (XD) emission from MoS2 monolayers grown by metal-organic chemical vapor deposition. C-plane sapphire substrates mimic common hydroxyl-terminated substrates. We demonstrate that transferring MoS2 monolayers to pristine c-plane sapphire dramatically increases the free electron density within MoS2 layers, quenches XD emission, and accelerates exciton recombination at the optical band edge. In contrast, transferring MoS2 monolayers onto inert hexagonal boron nitride (h-BN) has no measurable influence on these properties. Our findings demonstrate the promise of utilizing substrate engineering to control charge doping interactions and to quench broad XD background emission features that can influence the purity of single photon emitters in TMDs being developed for quantum photonic applications.

Photocatalytic activity for degradation of methylene blue dye-mediated nanocomposite-copper oxide/graphene oxide preparation from blood solution via hydrothermal method with laser irradiation

Blood solution contains an abundant supply of metals, natural polysaccharides, and protein. Copper oxide/graphene oxide nanocomposite (CuO/GO NCs) was created by the hydrothermal method of mixing a blood solution with copper nitrate (Cu2NO3) salt and graphene (G) material, followed by irradiation with a pulse laser ablation (PLA) (Nd-YAG) technique at 250 Mj, 532 nm, 5 cm, and 900 pulses. Paper filters were used to investigate the methylene blue (MB) dye degradation of CuO-GO NCs produced via the PLA process. The structure and optical properties of CuO-GO-NCs were studied and analyzed using X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM) images, transmission electron microscopy (TEM) images, and UV–visible spectra. CuO/GO NCs have a cubic shape with an average crystal size range of 11–52 nm, according to XRD research. An average particle size of 40 nm is created in the surface morphology investigated with FE-SEM, indicating the form of small grains in the nanoparticle. Inside cell morphology examined by TEM, a grain particle size of 40 nm is produced, suggesting that the nanoparticle has a tiny spherical form for CuO NPs, while the morphology of GO NPs has a nanosheet-like structure. CuO/GO NCs have an optical band edge value of 2.5 eV and a wavelength of 352 nm, according to the UV–visible spectrum. Using filter paper containing CuO/GO NCs, the degradation efficiency of MB dye after 50 min in the presence of ambient light in the environment was 99 %. This is a brand-new, rapid, and safe technique that has never been used before.

Sensitive Thermochromic Behavior of InSeI, a Highly Anisotropic and Tubular 1D van der Waals Crystal.

Thermochromism, the change in color of a material with temperature, is the fundamental basis of optical thermometry. A longstanding challenge in realizing sensitive optical thermometers for widespread use is identifying materials with pronounced thermometric optical performance in the visible range. Herein, it is demonstrated that single crystals of indium selenium iodide (InSeI), a 1D van der Waals (vdW) solid consisting of weakly bound helical chains, exhibit considerable visible range thermochromism. A strong temperature-dependent optical band edge absorption shift ranging from 450 to 530 nm (2.8 to 2.3 eV) over a 380 K temperature range with an experimental (dEg/dT)max value extracted to be 1.26 × 10-3 eV K-1 is shown. This value lies appreciably above most dense conventional semiconductors in the visible range and is comparable to soft lattice solids. The authors further seek to understand the origin of this unusually sensitive thermochromic behavior and find that it arises from strong electron-phonon interactions and anharmonic phonons that significantly broaden band edges and lower the Eg with increasing temperature. The identification of structural signatures resulting in sensitive thermochromism in 1D vdW crystals opens avenues in discovering low-dimensional solids with strong temperature-dependent optical responses across broad spectral windows, dimensionalities, and size regimes.

Colloidal synthesis and optical properties of Cs2ZnBr4 and Cs3ZnBr5 nanocrystals

A framework to understand the effects of an oxide-ligand environment on the photoluminescence (PL) of rare-earth-element-doped oxynitride compounds is well established, whereas the effects of halide coordination on crystal field splitting and the associated PL emission are rarely studied. In this paper, we report a synthetic route to alkali metal halide NCs of Cs2ZnBr4 and Cs3ZnBr5 with a broad blue PL emission and large Stokes shift. In particular, the zero-dimensional Cs3ZnBr5 phase, in which the isolated [ZnBr]42− units were completely separated by surrounding Cs+ cations, exhibited a strong blue PL emission at 437 nm with a PL quantum yield of 8.92 %. The formation of additional trap states or defects in the Cs3ZnBr5 crystalline lattice can be associated with a relatively considerable bands at 345 and 400 nm of the optical band edge, resulting in singlet-to-singlet transition of localized excitons with a short lifetime of 3.74 ns. Therefore, herein we demonstrate that tunable alkali metal halide NCs have considerable potential for optoelectronic applications.

Optimization of Reaction Parameters during the Synthesis of Hexagonal Hexadecylamine Capped ZnS Nanoparticles

Zinc sulphide nanoparticles capped with hexadecylamine (ZnS-HDA) have been synthesized through thermal decomposition of (Z)-2- (pyrrolidin-2-ylidene)thiourea zinc(II) complex. The effects of temperature, time and concentration of the precursor on the properties of ZnS-HDA capped nanoparticles were investigated. The values of the optical band edges were increasing when the temperature, reaction tim and the amount of the precursor were elevated. X-ray diffraction patterns of all the synthesized nanoparticles mainly exhibited hexagonal phase structures. The morphologies of the particles were increasing with the increase in temperature, time and precursor concentration.

Structure based photocatalytic efficiency and optical properties of ZnO nanoparticles modified by annealing including Williamson-Hall microstructural investigation

Structural deficiency in annealed ZnO nanoparticles would be a critical concern for the optical properties and photocatalysis efficiency of ZnO. Therefore, the structure and microstructure of synthesized ZnO nanoparticles annealed at 500 °C, 700 °C, and 900 °C for 4 h have been analyzed by X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), and UV–visible Diffuse Reflectance Spectroscopic (UV–Vis DRS) techniques. Williamson-Hall and modified Williamson-Hall models have explored microstructural properties. The bandgap in ZnO decreases from 3.18 eV to 3.11 eV for annealing, which coincides with the assessed optical band edge approximately and the accumulation of particles observed from SEM images. Moreover, the redox potential has been calculated for annealed samples to perform the photocatalytic experiment. Additionally, the photocatalytic performance has been estimated from the degradation of UV irradiation through the studied samples. Finally, the sample ZnO annealed at 500 °C confirms maximum RhB degradation rate of 60.9% due to its lower particle size with a higher surface area. The ZnO-500 may be a potential photocatalyst material, including semiconducting properties.

Free exciton and bound excitons on Pb and I vacancies and O and I substituting defects in PbI2: Photoluminescence and DFT calculations

A complete study of structure, chemical composition, morphology and photoluminescence (PL) is performed for multilayer PbI2 flakes. The reduction of thickness results in uneven spatial mapping of PL and Raman intensities, while the positions and shapes of the spectra do not undergo notable changes. The most intense peaks in the PL spectra are the direct free exciton (FX) peak at 2.44 eV (508 nm) and the bound exciton (BX) at 2.41 eV (516 nm). The variation of above/below-bandgap excitation has allowed to find three more defect-related peaks at 2.35, 2.19 and 2.04 eV. So far, these three BXs have not been studied. Our DFT calculations combined with the experimental results allow to clarify the origins of BX peaks as the recombination from the optical conduction band edge to the defect levels located above the valence band: 2.41 eV to O substituting I (OI), 2.35 eV to Pb vacancy (VPb), and 2.19/2.04 eV to IPb with split level. We have defined that the known absorption at 1.55 eV (800 nm) is related to VI nonradiative recombination center. The temperature behaviors allow to determine the thermal dissociation energy of FX of 0.22 eV. The dependences also show a co-interaction between defect levels.

Implications of morphology on excited state absorption of α-MnO2 nanostructures

Nanoclusters, nanorods and nanowires of α-MnO2 were synthesized by reduction method at different annealing temperature (100 °C–500 °C). X-ray diffraction and Raman studies confirms the formation of pure tetragonal phase of α-MnO2. Surface morphology using SEM and HRTEM reveals the gradual transformation of α-MnO2 nanoclusters (100 °C–300 °C) to nanorods (400 °C) to well defined nanowires (500 °C). While increasing the annealing temperature, Ostwald ripening process initially withholds the agglomerated clusters and its size get reduced and fused that leads to orientation along particular direction. HRTEM clearly displays the influence of temperature on favoured growth plane where (411) of nanoclusters changed to (211) plane of nanorods and nanowires. Optical band edge absorption at 268 nm arising from d–d transitions of Mn4+ in MnO2 got blue shifted due to variations in morphology. Here with increase in temperature, aspect ratio increases which alters the number of surface atoms and consequently the cohesive energy put on effect in the widening of bandgap of the material from 4.31 eV (nanoclusters) to 4.63 eV (nanowires). Open aperture Z-scan technique using Nd: YAG laser (532 nm, 10 Hz & 9 ns) revealed all MnO2 nanostructures involve reverse saturable absorption ascribed due to excited state absorption and its strength varies with morphology. Significant absorption in the visible region (532 nm) provoked excited state absorption and intensity dependent variation of nonlinear absorption coefficient confirms the existence of sequential two photon absorption process. α-MnO2 nanowires with extended interconnected network and larger aspect ratio exhibits wider excited state cross-section (6.64 × 10−2 GM), higher two-photon absorption coefficient (2.01 × 10−10 m/W) and lower onset optical limiting threshold (2.13 × 1012 W/m2). This stronger optical limiting performance of α-MnO2 nanowires symbolizes the scope of utilizing optimal α-MnO2 nanostructure for optical limiting applications.

Heptazine-Based Ordered-Distorted Copolymers with Enhanced Visible-Light Absorption for Photocatalytic Hydrogen Production.

Poly(heptazine imide) (PHI), one of the attractive allotropes of polymeric carbon nitride, has recently received extensive attention in photocatalysis due to its extended conjugation for fast separation and transfer of the charges. However, pristine PHI bears an intrinsic optical absorption band edge at 460 nm, which largely restrains the visible light utilization. Herein, the narrow-bandgap PHI (N-PHI) with an ordered-distorted interface was fabricated from polycondensation of the mixture of NaSCN, cyanuric chloride, and LiCl. Results revealed that the enhanced optical absorption and the promoted separation and transfer of the charge carriers at the interface greatly improved the photocatalytic performance, which endowed N-PHI with an apparent quantum yield of 20 % for hydrogen production at 450 nm.

Single-layer and bilayer MoSTe for photocatalytic water splitting: Role of optical absorption correction and band edge distribution

Single-layer and bilayer Janus transition metal dichalcogenides (TMDCs) attain great attention for possible application in solar water splitting. The crucial issues for solar water splitting are the band edge positions and optical absorption of photocatalysts. However, for this large group of Janus TMDCs materials, their optical absorption correction by the effective unit cell volume and the charge distribution of band edges, were not investigated in most previous works. To clarify these issues, we herein studied the electronic and optical properties of typical single-layer and bilayer MoSTe using first-principles calculations. Remarkably, when corrected by the effective unit cell volume, the optical absorption of bilayer MoSTe is not enhanced when compared to that of single-layer MoSTe. On the other hand, single-layer MoSTe possesses no solar water-splitting ability for lack of suitable band edges, when the charge distribution of band edges is considered. Furthermore, favorable band edges for water splitting were achieved in bilayer MoSTe. In brief, this work emphasized the optical absorption correction by the effective unit cell volume and the charge distribution of band edges when searching for 2D photocatalysts stacking by Janus monolayers.

Photoassisted Charging of Li-Ion Oxygen Batteries Using g-C3N4/rGO Nanocomposite Photocatalysts.

In this work, g-C3N4/rGO nanocomposites were synthesized to use them as photocatalysts in Li-ion oxygen batteries by aiming at the reduction of the charging potential efficiently under photoassisted conditions. Fourier transform infrared (FTIR) spectra showed that novel C=C bonds formed between g-C3N4 and rGO during the decomposition of melamine and that the formation of these bonds was assumed to cause a red shift in the optical absorption band edge. The competition between the narrowing in the optical band gaps of the nanocomposites as a result of the red shift due to the presence of rGO and the degradation in the visible light utilization as a result of favorably absorbed incident light by rGO instead of g-C3N4 pointed out that the g-C3N4/3% rGO nanocomposite has the optimum light absorbance efficiency. The photoassisted charging tests indicated that the g-C3N4/3% rGO nanocomposite reduced the charging potential effectively, especially at higher current densities, and improved the cyclic discharge–charge performance of the Li-ion oxygen batteries considerably.

Hybridization of two-dimensional glass network by graphene nanopowder/PbO/CaO@P2O5 for promising telecommunication applications

In this work, the conventional lead-calcium phosphate glass, PbO/CaO@P2O5, has been doped with graphene nanopowder (G), which is considered a two-dimensional organic agent. The samples were created by adding different amounts of graphene (G) to one mole of PbO/CaO@P2O5 to create a set of phosphate graphene-based glasses named as G-PbO/CaO@P2O5.The X-ray diffraction (XRD) patterns revealed the amorphous nature of the studied samples, while the Fourier transform infrared (FTIR) spectra showed the participation of the carbon atoms in the glass matrix by forming C-O bonds. Also, FTIR showed a decrease in the number of the terminal oxygen atoms when the G-content increased. While, the UV–Vis spectra showed an increase in the value of the optical transmittance and a blue shift in the position of the optical band edge when the G-content increases. The Hydrogenic Excitonic (HEM) model has been used to fit the experimental absorption coefficient, where some valuable parameters are resulted. Also, the Drude-Lorentz approximation has been used to simulate the optical dielectric loss where two single oscillators were needed to fit the experimental data. The resulting value of the plasma frequency is enclosed in the terahertz zone. The optical characterizations suggested that the studied samples act as traps for different electromagnetic radiation and telecommunication applications.

Enhanced and stable photoelectrochemical H2 production using a engineered nano multijunction with Cu2O photocathode

Cuprous oxide (Cu2O) is one of the ideal photocathodes explored for solar water splitting applications due to its suitable optical properties and band edge positions. However, state-of-the-art Cu2O employs Au back contact for hole extraction and Pt or Ru catalyst for water reduction reactions. Moreover, photo-corrosion of Cu2O during the AM 1.5 G illumination is one of the serious challenges that limit the efficiency of water splitting reactions. In this work, a multijunction strategy in which the Cu2O is sandwiched between the stoichiometrically engineered hole extraction layer and an efficient, non-toxic MoOx catalyst layer is proposed for the enhanced charge separation and stable H2 production activity. The optimized multijunction system exhibits the highest photocurrent of 6.1 mA cm−2 at 0 V vs RHE reported for noble metal-free Cu2O photocathodes. Furthermore, a significant anodic shift in onset potential was noticed. In the multijunction, the tuned layers of NiOx, aluminum-doped zinc oxide, and MoOx act as hole scavenger, electron tunneler, and H2 catalyst, respectively. Importantly, the proposed nanolayers multijunction system demonstrates the effective utilization of noble metal and sulfide-free components for stable and enhanced H2 productions employing Cu2O photocathodes.

Photocatalytic Degradation of Phenol Red in Water on Nb(x)/TiO2 Nanocomposites

In this paper, the photocatalytic effect of Nb(x)/TiO2 nanocomposites on the degradation of phenol red (PR) was studied. Nb(x)/TiO2 nanocomposites are fabricated by a simple sol-gel route with new experimental conditions. The structural and optical properties were determined using high transmission electron microscopy (HRTEM), X-ray diffraction, Raman spectroscopy, photoluminescence, and UV-vis absorbance spectroscopy. Compared to pure anatase TiO2, the recently fabricated Nb(x)/TiO2 nanocomposite has a shift in the optical band edge to the visible wavelength. Consequently, it has high performance in adsorption capacity and photocatalytic activities. A time of 160 min has been observed to be suitable for mostly degradable 20 mgL−1 of phenol red on Nb(2.0)/TiO2 composite. The kinetic results were in good agreement with the first-order kinetic model at different concentrations. In addition, the results showed that the addition of Nb led to a significant degradation process. The decomposition of phenol red pollutants showed a synergistic effect of the Nb(2.0)/TiO2 nanocomposites on wastewater treatment.

Effect of Graphene Additives on the Structure and Optical Parameters of Pure Sodium Borate Glass

This work has been performed to understand the effect of insertion of pure graphene, a two-dimensional organic material, throughout the structural matrix of the pure sodium borate glass. Different amounts (in grams), 0 ≤ x ≤ 3, of graphene have been added to 1 mol of Na2B4O7·10H2O. Four solid samples were prepared using fast cooling from their melts at 1000 ± 10°C. The prepared solids were characterized using X-ray diffraction, which indicated the amorphous nature of the four samples with an increase in the count rate when the content of graphene increased. Fourier transform infrared (FTIR) spectral analysis confirmed that the increase in the graphene content caused an increase in BO4 concentration concerning BO3. The amount of oxidized graphene was shared as glass network formers. The measured values of bulk density and the micro-hardness slightly increased as the graphene content increased. Optical measurement showed an increase in the optical absorbance, direct band gap, indirect band gap, and optical band edge. Also, the values of optoelectronic parameters increased when the graphene content increased to be larger than that of the pure ZnO and CO-ZnO, making the studied glass good candidates for nonlinear optical applications. Both the metallization factor and Urbach's energy showed that the crystallinity degree increased when the content of graphene increased.

A Comprehensive Review of Graphitic Carbon Nitride (g-C3N4)-Metal Oxide-Based Nanocomposites: Potential for Photocatalysis and Sensing.

g-C3N4 has drawn lots of attention due to its photocatalytic activity, low-cost and facile synthesis, and interesting layered structure. However, to improve some of the properties of g-C3N4, such as photochemical stability, electrical band structure, and to decrease charge recombination rate, and towards effective light-harvesting, g-C3N4–metal oxide-based heterojunctions have been introduced. In this review, we initially discussed the preparation, modification, and physical properties of the g-C3N4 and then, we discussed the combination of g-C3N4 with various metal oxides such as TiO2, ZnO, FeO, Fe2O3, Fe3O4, WO3, SnO, SnO2, etc. We summarized some of their characteristic properties of these heterojunctions, their optical features, photocatalytic performance, and electrical band edge positions. This review covers recent advances, including applications in water splitting, CO2 reduction, and photodegradation of organic pollutants, sensors, bacterial disinfection, and supercapacitors. We show that metal oxides can improve the efficiency of the bare g-C3N4 to make the composites suitable for a wide range of applications. Finally, this review provides some perspectives, limitations, and challenges in investigation of g-C3N4–metal-oxide-based heterojunctions.

Controlled solution-based fabrication of perovskite thin films directly on conductive substrate

Organometallic perovskites are one of the most investigated materials for high-efficiency thin-film devices to convert solar energy and supply energy. In particular, methylammonium lead iodide has been used to realize thin-film perovskite solar cells, achieving an efficiency higher than 20%. Different fabrication procedures based on the spin-coating technique have been proposed, which do not ensure homogenous morphologies.In this work, we present a scalable process to fabricate methylammonium lead iodide thin films directly on conductive substrates, consisting of electrodeposition and two subsequent chemical conversions. A thorough investigation of the morphological, structural and compositional properties of the layer is performed after each fabrication step. It is demonstrated that this method allows fine control of the thickness of the layer by tuning the cell parameters during the electrodeposition step. X-ray diffraction patterns and energy-dispersive X-ray analysis indicate the achievement of high-purity methylammonium lead iodide layers. Micro-Raman analyses were used to demonstrate the formation of methylammonium lead iodide. Finally, ultraviolet-visible absorption spectra were acquired to determine the optical band edge of the layer (̴ 1.56 eV) and the absorbance of methylammonium lead iodide as a function of the film thickness. As expected, the material exploits excellent optical properties, achieving an absorption ≥ 99.9% in the entire visible range for a layer thickness of 1.3 µm. The results presented here pave the way for the application of cost-friendly solution-based processes to fabricate high-quality perovskite solar cells.

Antimicrobial activity by diffusion method using iron oxide nanoparticles prepared from (Rose plant) extract with rust iron

This research succeeded in the prepared of iron oxide NPs using rose plant extract with rust iron extract at 300°C for two hours by simple chemical method. Iron oxide NPs have been developed as an death of antimicrobial as an alternative to toxic chemical drugs to prevent negative effects on human health. Iron oxide NPs were diagnosed using X-ray diffraction (XRD) analysis, Field Emission Scanning Electron Microscopy (FE-SEM) analysis, and Photoluminescence (PL) spectrum. XRD measurements explained the small crystalline size (61 nm) with (inverse cubic) structure (wustite) for (Fe0.9nO4) NPs at 300°C using rose plant extract. FESEM showed the average grain size of Fe902O NPs (wustite) rose plant extract at 300°C was 79.59 nm. PL spectrum determined a blue shift for the optical near band edge value was 2.75 eV for Fe0.911O NPs (wustite) at 300°C. Iron oxide NPs were applied in antimicrobial for removal of toxic bacterial by diffusion method. The success of this work will open wide new horizons for us in solving the problem of spent iron and how to get rid of it and the manufacture of new nanoparticles in medical treatments.

Au integrated 2D ZnO heterostructures as robust visible light photocatalysts

Integration of semiconducting nanostructures with noble metal nanoparticles are turning highly desirable for cost efficient energy and environmental related applications. From this viewpoint, we report on a facile aqueous synthesis of polymer capped gold (Au) nanoparticles on free standing 2D layered structures of zinc oxide (ZnO) to result with ZnO/Au nanocomposites. Concentration of Au nanoparticles were observed to promote the preferential growth of ZnO along the (002) wurtzite plane. The ZnO/Au structures and their morphological dissemination was noted to be of few. This flake like structure was also noted to be greatly influenced by the concentration of Au in the colloidal blend. Optical band edge transformations noted in the absorption spectra across the lower wavelength region and the shift in surface plasmon resonance (SPR) towards the red region of the visible spectrum signify the improved absorptivity of the heterostructures along the visible spectrum. These heterostructures exhibited remarkable visible light driven photocatalytic activity (99% efficiency) on par with pristine ZnO. The findings also attest this new class of composite structures to open up new openings in diversified solar energy conversion related functions.

Preparation, Structural, Electrochemical and Photocatalytic Studies of Cadmium Sulfide Quantum Dots

We report the preparation, structural, electrochemical and photocatalytic studies of monodispersed cadmium sulphide quantum dots from didecylaminyl dithiocarbamate and 4-chloro-3-(trifluoromethyl) anilinyl dithiocarbamate cadmium(II) complexes. Powder X-ray diffraction pattern confirms hexagonal crystalline phases for the as-preapred CdS quantum dots irrespective of the precursor used with particle size of 3.39-5.51 nm. Optical absorption band edges of 515 nm were observed for the cadmium sulfide quantum dots with energy band gaps estimated from the Tauc plots of 1.97 eV for OLM-CdS1 prepared from Cd(II) didecylaminyl dithiocarbamate and 1.92 eV for OLM-CdS2 prepared from Cd(II) 4-chloro-3-(trifluoromethyl) anilinyl dithiocarbamate. These energy band gaps are blue shifted with respect to the bulk cadmium sulphide. The calculated electrochemical band gap of 2.34 V and 3.30 V are higher than band gap energy. The as-prepared CdS quantum dots were used as photocatalysts for the photocatalytic decomposition of methylene blue (MB) with efficiency of 61 % and 55 %.