Shift In Band Gap Research Articles

Inkless Rewritable Photonic Crystals Paper Enabled by a Light-Driven Azobenzene Mesogen Switch.

Rewritable paper, as an environment-friendly approach of information transmission, has potential possibility to conserve energy and promote a sustainable development of our society. Recently, photonic crystals (PCs) have become a research hotspot in the development of rewritable paper. However, there are still many shortcomings that limit the further application of PC paper, such as slow response sensitivity, short-cycle lifetime, poor storage stability, and so on. Herein, we constructed an optically rewritable azobenzene inverse opals (AZOIOs) with a thin film (ca. 1 μm) plated on an inverse opal structure based on the UV/vis switchable structure color of the sample. The top thin film acts as a protective layer to avoid the large deformation of the pore structure and the bottom inverse opal structure with refractive index/pore structure change that provides reversible structure color. Large, reversible, and rapid bandgap shift (ca. 60 nm, 2 s) of AZOIOs can be repeated more than 100 times under alternating UV/vis irradiation based on isomerization of high content of the azobenzene group. On-demand long-time preservation pattern can be obtained by the appearance of azobenzene's intrinsic color. The proof of concept for rewritable PC paper is demonstrated herein. Such inkless rewritable colorful paper paves a way for developing novel display technology.

Fluorescent Detection of Carbon Disulfide by a Highly Emissive and Robust Isoreticular Series of Zr-Based Luminescent Metal Organic Frameworks (LMOFs)

Carbon disulfide (CS2) is a highly volatile neurotoxic species. It is known to cause atherosclerosis and coronary artery disease and contributes significantly to sulfur-based pollutants. Therefore, effective detection and capture of carbon disulfide represents an important aspect of research efforts for the protection of human and environmental health. In this study, we report the synthesis and characterization of two strongly luminescent and robust isoreticular metal organic frameworks (MOFs) Zr6(µ3-O)4(OH)8(tcbpe)2(H2O)4 (here termed 1) and Zr6(µ3-O)4(OH)8(tcbpe-f)2(H2O)4 (here termed 2) and their use as fluorescent sensors for the detection of carbon disulfide. Both MOFs demonstrate a calorimetric bathochromic shift in the optical bandgap and strong luminescence quenching upon exposure to carbon disulfide. The interactions between carbon disulfide and the frameworks are analyzed by in-situ infrared spectroscopy and computational modelling by density functional theory. These results reveal that both the Zr metal node and organic ligand act as the preferential binding sites and interact strongly with carbon disulfide.

Structural and optical modifications of CdS properties in CdS-Au thin films prepared by CBD

In this work highlights the modifications of the crystalline structure and properties of the nanostructured cadmium sulphide (CdS) thin films, including gold quantum dots grown to form a hybrid metal-semiconductor system. By means of chemical bath deposition (CBD) good quality thin films are obtained, with the advantage of doping in-synthesis with no required additional steps. Film thicknesses around 100 nm were determined by ellipsometry. The binding energies of elements in the CdS and CdS-Au samples were measured by X-ray photoelectronic spectroscopy (XPS). The materials crystalline structure was studied by X-ray diffraction (XRD). A change from disperse like-single-crystalline (highly oriented) CdS NP’s films to monodisperse CdS NP’s–Au QD’s like-single-crystalline samples was observed. This behaviour was confirmed by TEM micrographs. The samples properties pass from an intermediate quantum confinement regime (IQCR) in CdS to a strong one (SQCR) in CdS-Au. The Raman vibrational spectra allowed the analysis of the phonon emissions in CdS, where the Raman shift gives structural information and confirms the effects of IQCR in CdS and the SQCR in CdS-Au. The optical UV–Vis characterization describes the effect of the structural modifications, with an optical band gap shift at higher energy in the CdS-Au sample related to the SQCR. Photoluminescence (PL) measured at room-temperature shows a decrease of PL intensity in CdS-Au film due to via defects-recombination, with respect to that in CdS. This fact is interpreted as an effect of a decreasing of surface defects due to passivation by the Au presence.

Enhanced second-order Stark effect in twisted bilayer graphene quantum dots

External electric field and interlayer twist introduce diverse changes in their confined electronic states of bilayer graphene quantum dots. Using a quantum-dot model, the band gaps of twisted bilayer graphene in finite sizes of about 1.4–2.4 nm with varying twist angles are studied in the presence of an electrostatic field perpendicular to the flakes by means of first-principles calculations. The size-dependent gaps are widened by the interlayer twist, but narrowed by the applied field. Their coupling, however, results in an enhanced Stark response in the twisted structures of which the field-induced band-gap variations are about 3–4 times as large as that of the corresponding untwisted structures under the same field strength. The exceptional Stark shifts come from the field-induced asynchronous shifts in their occupied and virtual energy levels, which are further enhanced by strong interlayer coupling at specific twist angles. Moreover, the shift of band gaps with the field strength follows the quadratic Stark response with large second-order shifting coefficients. The enhanced and tunable Stark shift suggests a gateway to the band engineering of bilayer graphene quantum dots by tuning their sizes, twist angles and their coupling with applied field.

Extrinsic Influences on Photoluminescence Spectral Lineshape in Thin Films

AbstractAccurate determination of the photoluminescence (PL) spectrum of molecules is crucial in many areas including molecular diagnostics, genome analysis, and organic electronics. However, interpretation of experimental spectra from thin films can be complicated by phenomena extrinsic to the site of photon emission such as optical interference and self‐absorption. In this study, the alteration of fluorescence spectral lineshape is modeled from a thin film due to extrinsic factors as function of film thickness, incident angle, bandgap, and Stokes shift. The method is experimentally verified for thin (<400 nm) layers of molecular semiconductors on quartz substrates. Finally, a chart is provided for estimating the magnitude of the error due to extrinsic factors in the bandshape of fluorescence spectra from thin films. This demonstrates that the 0‐0/0‐1 intensity ratio, an indicator of the nature of excitonic coupling, is especially sensitive to possibly unconsidered influence from extrinsic effects.

Exploring the AgSb1−xBixI4 phase diagram: Thermochromism in layered CdCl2-type semiconductors

AgSbI4 and several AgSb1–xBixI4 compositions have been synthesized via solid state reactions. Both end members and all intermediate compositions crystallize with the CdCl2 structure type (space group ​= ​R3¯m) and a random distribution of the Ag+ and Sb3+/Bi3+ cations. Hypothetical cation ordered structures have been generated and their stabilities evaluated with density functional theory. This analysis suggests that while charge balanced layers are strongly favored, the long-range interactions that favor ordered stacking of layers are too weak to produce three-dimensional cation ordering. The colors of these materials change noticeably when heated, signaling a large thermochromic effect. For example, the band gap of AgSb0·25Bi0·75I4 changes from 1.68 ​eV ​at room temperature to 1.55 ​eV ​at 150 ​°C. This thermochromic shift in band gap is larger than comparable diamond-like semiconductors, such as CdTe. VASP calculations and molecular dynamics simulations indicate the large thermochromic effect is primarily caused by significant electron-phonon coupling.

Band structure engineering in gallium sulfide nanostructures

The gallium sulfide monolayer is demonstrated as a promising two-dimensional semiconductor material with considerable properties. The present work, investigates the bandgap modulation of GaS monolayer under biaxial or uniaxial strain, using density functional theory calculation. We find that GaS monolayer shows an indirect bandgap that limits its optical applications. The uniaxial strain shifts bandgap from indirect to direct in gallium sulfide monolayer at the strain of \(\varepsilon \)= − 10 percent along y direction. Also, by analyzing Poisson’s ratio, we figure out that Gallium Sulfide monolayer has isotropic mechanical nature under pressure. The detailed reasons for the bandgap modulation are discussed by analyzing the projected density of states. Besides, we investigate indirect to direct bandgap shifts on Gallium Sulfide nanoribbons through varying nanoribbon width as well as edge hydrogen passivations. The direct bandgap accompanied by the isotropic mechanical feature in the Gallium Sulfide monolayer, facilitates and moves up its practical application.

Perovskite/Silicon Tandem Solar Cells: Effect of Luminescent Coupling and Bifaciality

The power conversion efficiency of the market‐dominating silicon photovoltaics approaches its theoretical limit. Bifacial solar operation with harvesting additional light impinging on the module back and the perovskite/silicon tandem device architecture are among the most promising approaches for further increasing the energy yield from a limited area. Herein, the energy output of perovskite/silicon tandem solar cells in monofacial and bifacial operation is calculated, for the first time considering luminescent coupling (LC) between two sub‐cells. For energy yield calculations, idealized solar cells are studied at both standard testing as well as realistic weather conditions in combination with a detailed illumination model for periodic solar panel arrays. Typical experimental photoluminescent quantum yield values reveal that more than 50% of excess electron–hole pairs in the perovskite top cell can be utilized by the silicon bottom cell by means of LC. As a result, LC strongly relaxes the constraints on the top‐cell bandgap in monolithic tandem devices. In combination with bifacial operation, the optimum perovskite bandgap shifts from 1.71 eV to the range 1.60–1.65 eV, where already high‐quality perovskite materials exist. The results are very important for developing optimal perovskite materials for tandem solar cells.

Designing spinel NiCr2O4 loaded Bi2O3 semiconductor hybrid for mitigating the charge recombination and tuned band gap for enhanced white light photocatalysis and antibacterial applications

This study investigates the enhancement of Bi2O3 activity as photocatalyst by loading NiCr2O4 as an effective catalyst. High-resolution transmission electron microscopy (HRTEM) analysis revealed the attachment of NiCr2O4 nanospheres on surface of sheet-like Bi2O3 nanoparticle (NPs). X-ray powder diffraction (XRD) revealed the successful fabrication of crystalline nanocomposites (NCs) with coexistence of NiCr2O4 and Bi2O3. X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray spectroscopy (EDAX) confirmed the elemental composition (Ni, Cr, Bi and O) with no trace of impurity. Peaks related to tetrahedral and octahedral oxygen-metal bonds were observed in fourier-transform infrared spectroscopy (FTIR). BET analyzer indicates presence of more reactive sites in NCs than individual NPs. UV–vis diffuse reflectance spectroscopy (DRS) showed shift in band gap of NiCr2O4 (2.1 eV) and Bi2O3 (2.8 eV) to 2.3 eV for NiCr2O4-Bi2O3 NCs with visible light sensitization. The life time of charge carriers in NCs was greatly improved and it was revealed by photoluminescence (PL) study. The photocatalytic performance of NiCr2O4-Bi2O3 NCs was higher with apparent kinetic constant (kapp) of 0.012 min−1 which was 4–6 times boosted than its pure individual parts (Bi2O3 = 0.003 min−1; NiCr2O4 = 0.002 min−1). The study identified that ·OH play major role in dye degradation. The NiCr2O4-Bi2O3 NCs exhibited a stable photocatalysis performance with good photostability after six cycles. In addition, NCs exhibited excellent antimicrobial activity against Escherichia coli and Bacillus subtilis. The potential usage of visible-light energy and operation simplicity prompted NiCr2O4-Bi2O3 NCs as a promising magnetically removable candidate for photodegradation of toxic organic contaminants in the aqueous solutions and for antimicrobial applications.

Tunable emission of electro-spun ceramic ZnS:Cu:Co nanofibers for photonic applications: structure and optical properties

Lab-made electrospinning method together with thermal treatment can be considered simple, cost-effective, and non-toxic approach. ZnS ceramic nanofibers were fabricated by thermally treated ZnO nanofibers in H2S atmosphere. The cubic phase of fabricated ZnS nanofibers is verified by XRD measurements. The nano-nature with an average fiber diameter of 110 nm was confirmed by electron microscopy (SEM). While the EDS spectrum showed the compositional information. The optical properties were investigated to study the effect of doped elements by measuring the absorption spectra. The blue shift of the optical bandgap of synthesized ZnS:Cu:Co fibers with respect to the value of bulk was investigated by the UV–Visible spectroscopy. The bandgap values and its nature of deviation was studied extensively. A photoluminescence (PL) study of the ZnS and ZnS:Cu:Co fibers at room temperature indicated a strong emission band over the visible region centered at 520- 556 nm. This tuned luminescence band was due to the presence of doping of Cu and Co. These results revealed that, such low cost synthesized materials are appropriate for wide use in optoelectronic devices by tuning the optical bad-gap of ZnS nanofibers using these dopants.

Studies of magnetic behavior of chemically synthesized interacting superparamagnetic copper ferrite nanoparticles

The magnetic property of nanocrystalline copper ferrite (CuFe2O4, CF) with narrow size distribution synthesized by co-precipitation technique is investigated. The occupancy of cations among the tetrahedral and octahedral sites is determined from the Rietveld analysis of the X-ray diffraction (XRD) pattern. Microstructural, optical, and magnetic properties of the ferrite nanoparticles (NPs) are analyzed in detail using high-resolution transmission electron microscopy (HRTEM), UV–Vis absorption spectroscopy, Fourier-transform infrared (FTIR) spectroscopy, Raman spectroscopy, and superconducting quantum interference device (SQUID) magnetometer. The blue shift in optical band gap compared to bulk one is observed due to the smaller particle size. The particle size distribution inferred by mathematical analysis from the variation of difference between field cooled (FC) and zero field cooled (ZFC) magnetization with temperature supports the HRTEM observation. M–T curve initially proposes the presence of superparamagnetic (SPM) NPs with non-interacting core and interacting surface spin. Therefore magnetic behavior of the sample is explained by core–shell model. This fact is substantiated from the good fitting of M–H curves at different temperatures using a combined equation generated from ‘Law of approach’ to saturation magnetization and Langevin function. With the lowering of temperature magnetic blocking and random spin freezing process occurs. The ratio of $$M_{{\text{R}}} /M_{{\text{s}}}$$ (remanent/saturation magnetization) less than 0.5 for all temperatures implies the presence of uniaxial anisotropy of the single-domain NPs. The unsaturated nature of the M–H curves may be due to the combined contribution from SPM core and spin canting in the mixed spinel CuFe2O4.

Structural and optical study of high concentrate Co-doped ZnO nanostructures

High concentrate Co doped ZnO nanostructures, were prepared using solution method. Prepared samples were characterized by different characterization techniques. Structural analysis was done by X ray-diffraction (XRD) and found the formation of the wurtzite hexagonal structure of ZnO without any essence presence of the Co related clusters. Field emission scanning electron microscope (FESEM) images indicated change in surface morphology of ZnO due to introducing of Co ions into ZnO lattice. UV visible spectroscopy showed red shift in the optical band gap of ZnO because of Co doping. Fourier transform infrared spectroscopy (FTIR) showed stretching vibration mode of Zn-O at 527 cm-1which indicated the formation of the wurtzite hexagonal structure of ZnO.

Energy bandgap variation in semiconductor compound nanomaterials

In the present work, we have used phenomological models for analyzing the impact of shape and size on energy band gap in semiconducting nanomaterial compounds. The models used presently are Qi model, Bond energy model and Guisbiers model. The extension of melting temperature expression for nanomaterials of the models considered is done and shape and size dependent expression of energy band gap is obtained. In this paper, we have taken group III-V semiconductor compound nanomaterials i.e., AlN, GaN, InN, GaAs and InAs. It is clear from the results obtained that decrease in the size of the semiconductor compound nanomaterials led to band gap expansion and this increase is significant for particle size below 5 nm. Comparison of the results predicted using different models with the available experimental and simulated results is done. Guisbiers model is found best out of the models considered to study the band gap expansion in semiconducting nanomaterial compounds. The energy band gap shift in valence and conduction band with size is determined in nanosemiconductors.

Enhanced photocatalytic activity of Ho3+ doped ZnO NPs synthesized by modified sol-gel method: An experimental and theoretical investigation

All nanocrystalline ZnO nanoparticles (NPs) were synthesized via modified sol-gel method and origin of the enhanced photocatalytic activity of the Zn1−xHoxO (x = 0.01, 0.03 and 0.05) NPs were investigated. Structural properties and morphological evolution were characterized using X-ray diffraction (XRD) and field emission scanning electron microscope (FE-SEM), respectively. Ultraviolet visible (UV-Vis) spectroscopy showed red shift of optical band gap on doping with minimum band gap of 3.10 eV for Zn0.97Ho0.03O. Photoluminescence (PL) spectroscopy showed decrease in electron-hole recombination rate with increasing doping up to 3 mol% doping. Photocatalytic activity was measured from degradation of rhodamine B (RhB) and reaction kinetics were analyzed. Theoretical study showed good agreement with experimental band gap results. Decreased electron-hole recombination rate with doping due to electron trapping by partially filled 4f orbitals of Ho3+ ions, change in NPs morphology and the red shift of the optical band gap resulted the best photocatalytic performance of Zn0.97Ho0.03O NPs.

Response of structural and optical properties against proton irradiation in AlN:Tm thin films

Thulium (Tm) doped aluminum nitride (AlN) thin films are deposited by radio frequency (RF) magnetron sputtering in a pure nitrogen atmosphere. As-deposited thin films are irradiated at room temperature with 335 keV protons and a fluence of 1 × 1014 ions/cm2. The effects of irradiation on the structural and optical properties of the deposited thin films are investigated. Rutherford backscattering (RBS) is performed to determine the thickness and stoichiometric properties of the films while the structural properties of the material before and after irradiation are investigated with x-ray diffraction (XRD). Non-ionizing energy loss (NIEL) study demonstrates that scattering events increase as the ions penetrate deep into the matrix and result in higher damage in the material. Fourier transform infrared spectroscopy (FTIR) is used to see the absorption spectra of thin films which reveals that the implanted films have great capabilities of solar energy storage. Bandgap of the film is determined using diffused reflection spectroscopy (DRS) technique, where a slight upwards shift in bandgap is observed due to irradiation induced material damages.

Enhanced Optical and Electrical Properties of Ti Doped In<sub>2</sub>O<sub>3</sub> thin Films Treated by Post-deposition Electron Beam Irradiation

Transparent and conductive Ti doped In<sub>2</sub>O<sub>3</sub> (TIO) films were prepared on slide glass substrate using a radio frequency (RF) magnetron sputter and then subjected to Transparent and conductive Ti doped In<sub>2</sub>O<sub>3</sub> (TIO) films were prepared on a glass slide substrate using radio frequency (RF) magnetron sputter. The film surface was then subjected to intense electron beam irradiation, to study the influence of incident energy on the visible transmittance and electrical resistivity of the films. All x-ray diffraction plots exhibited some diffraction peaks of the cubic bixbyite In<sub>2</sub>O<sub>3</sub> (222), (400), (332), (431), (440), and (444) planes regardless of the electron irradiation energy, while the characteristic diffraction peak for crystalline TiO<sub>2</sub> did not appear even when irradiated at 1500 eV. In atomic force microscope analysis, the surface roughness of the as deposited TIO films was found to be 0.63 nm. As the electron irradiation energy was increased up to 1500 eV, the root mean square roughness decreased down to 0.36 nm. The films electron irradiated at 1500 eV showed higher visible transmittance of 83.2% and the lower resistivity of 6.4 × 10<sup>-4</sup> Ωcm compared to the other films. From the electrical properties and optical band gap observation, it is supposed that the band gap shift is related to the carrier density. The band gap enlarged from 4.013 to 4.108 eV, along with an increase in carrier density from 9.82 × 10<sup>19</sup> to 3.22 × 10<sup>20</sup> cm<sup>-3</sup>.

Effect of annealing temperature on structure and optical properties of CdO nanocrystaline thin film prepare by chemical bath deposition method

CdO thin film has been deposited by chemical bath deposition method (CBD) on the glass substrate. Effect of annealing temperature( 573,623 and 673 K) on the structural and optical properties of the films has been investigated. The crystal structure investigated by X-ray diffraction method. Annealed CdO films are polycrystalline in nature with cubic structure having a preferential orientation along (1 1 1) plane. Analysis of XRD indicates that the intensities of peaks of the crystalline phase have increased with the increasing of annealing temperature. The structural parameters for the CdO thin films as the grain size, strain, dislocation density, and texture coefficient were calculated. Particle size for the preferential orientation at different annealing temperature is (16.95, 18.24 nm) for annealing temperature 623 and 673 K respectively. Optical properties study of CdO thin films using UV-vis spectrophotometer shows blue shift in the energy band gap as an indication of quantum confinement. It is observed that the band gap energy decreases with increasing of annealing temperature. The direct and indirect band gap energy values were found to be (2.95 2.87) and (1.945,1.914) eV respectively.

Carbazole-based π-conjugated 2,2′-Bipyridines, a new class of organic chromophores: Photophysical, ultrafast nonlinear optical and computational studies

The developmen of donor-acceptor (D-A) architecture based organic chromophores with large two-photon absorption (2 PA) cross-sections are essential for myriad of applications, ranging from nonlinear microscopy to biomedical imaging. Here, we present the results from a comprehensive study of 2 PA cross-sections of a new series of carbazole mono substituted-π-conjugated-2,2′-bipyridine derivatives with D-π-A architecture, possessing carbazole as the donor moiety and bipyridine core as an acceptor. Further, we have extended the π-conjugation by introducing the phenyl and butoxy substituted phenyl linkers. The fluorescence properties of these D-A chromophores are highly sensitive to solvent polarity and alteration of electron donor functionalities. The relevant computation studies support our experimental results, e.g., a shift in emission maxima and band gaps of the molecules. The third-order nonlinear optical (NLO) properties of the title donor-acceptor (D-A) chromophores demonstrated that 2 PA cross-section values are in the range of 6–39 GM, obtained using ~50 fs (fs) laser pulses at a wavelength of 800 nm. Further, the nonlinear refractive indices (n2) of these chromophores were found to be ~10−14 cm2/W, rendering them potential optical switching candidates since the coefficients were obtained with kHz, fs pulses extracting pure electronic nonlinearities. Based on our experimental findings and theoretical calculations presented in this study, we believe that carbazole-based π-conjugated 2,2′-bipyridines, demonstrated in this work, would be potential ligands to obtain a series of transition metal coordination complexes of interesting physical properties (for example, superior NLO behaviour).

Synthesis, characterization and anti-bacterial activity of Mg and Ba-doped ZnO Nanoparticles

Mg and Ba-doped zinc oxide nanoparticles were prepared by chemical precipitation method. The prepared nanoparticles were annealed at 700 °C. The Wurtzite geometry was confirmed in the X-ray diffraction. The crystallite size was found from the Debye-Scherer’s formula. The dopant Ba highly influences crystalline size than the dopant Mg. From Tauc’s plot, the band gap of the prepared samples was calculated and analyzed. The red shifted band gap was observed due to the metal dopant. From the charge carrier concentration, it was observed that the prepared nanoparticles were p-type in nature. The refractive index decreases as the metal dopant concentration increases. High metal doping concentration inhibits the growth of E. coli. Ba-doped zinc oxide nanoparticles have high anti-bacterial activity than Mg-doped nanoparticles.

Investigation of Electrodeposited Nano ZnO as a Photoanode for Dye-Sensitized Solar Cell (DSSC)

This paper reports the deposition and characterization of nanoporous zinc oxide (ZnO) by using a simple and low-cost electrodeposition method. SEM, XRD and UV-Vis are used to study the structural, morphology, and optical properties of ZnO thin films. ZnO has a nanosheet-like structure that is 1 m in size. The optical energy band gap shifts from 3.44 eV to 3.46 eV after the dying process. The organic dye of Clitoria ternatea was extracted and used as a synthesizer. An anthocyanin peak is observed in the absorption spectrum. The zinc oxide films are employed as photoanodes on dye-sensitized solar cells (DSSCs). The I-V characteristic curve parameters show a PCE of 0.027% and a fill factor of 38%. Keywords: Nano porous zinc oxide, Dye-Sensitized, Electrodeposition DOI: 10.7176/JETP/10-6-03 Publication date: October 31 st 2020