Direct Electronic Transitions Research Articles

Study of the optical and photovoltaic properties in nanoparticles of BiFeO3

Bismuth ferrite ceramic powders (BiFeO3) were synthesized by a combustion reaction in solution, using the mixture of urea and glycine as fuels, whose objectives were to improve their optical, electrical and photovoltaic response properties. X-ray diffraction analysis indicated the formation of the single phase BiFeO3. Transmission electron microscopy (TEM) shows images of uniform nanoparticles of 70 nm on average. Through diffuse reflectance we obtain a band gap with a direct electronic transition of 2.14 eV, this value presents an improvement compared to that reported by other authors in BiFeO3 ceramic powders. The value of the resistivity obtained by the 4-pointed method was ρ = 0.486 × 106 Ω.cm, checking that our material is a semiconductor. Finally, by building a prototype of Ag/BiFeO3/Ag, with visible light (λ = 405 nm), a photovoltaic response of 0.2 nA at room temperature was achieved.

Solvents mediated-synthesis of 3D-BiOX (X = Cl, Br, I) microspheres for photocatalytic removal of gaseous Hg0 from the zinc smelting flue gas

Three-dimensional (3D-BiOX (X = Cl, Br, I)) microspheres were fabricated by a facile solvothermal method with ethylene glycol (EG) as the solvent. The as-prepared microspheres compounds BiOX were characterized by XRD, SEM, BET, UV–vis DRS, FT-IR and XPS techniques and employed as a photocatalyst to photocatalytic oxidize elemental mercury (Hg0) from a smelting flue gas. Results showed that the band gaps values of 3.12, 2.69 and 1.77 eV were obtained for the direct electronic transition of BiOCl, BiOBr and BiOI samples, respectively. The specific surface area and surface hydroxyl groups are the two crucial factors affecting the photocatalytic oxidation of Hg0 BiOI microsphere possesses the maximum of the surface hydroxyl groups, largest BET specific surface area (35.8964 m2/g), strongest optical absorption properties, narrowest band gap width (1.77 eV) and the highest photocatalytic activity (ηoxi = 96.83%) under UV–vis irradiation. The photogenerated holes (h+) produced in the Valence Band (VB) can react with surface-bonded H2O (ad) or OH−, I− to produce hydroxyl radicals (OH) and I2, respectively. The photogenerated electrons (e−) produced in the Conduction Bands (CB) can react with adsorbed dissolved oxygen to form super-oxide radicals (O2−). In addition, for the BiOBr reaction system, the OH, h+ and O2− could play key roles in the formation of HgO, while in BiOI photocatalytic system, except for OH, O2− and h+, I2 could also be an important species for the generation of HgO and HgI2 species.

New coating synthesis comprising CuO:NiO/C to obtain highly selective surface for enhancing solar energy absorption

A novel nano coating was synthesized by spin and casting methods to obtain high-performance selective surfaces to enhance solar energy absorption. Nanomaterials (CuO:NiO) and carbon (fly ash) were used to provide a cost-efficient coating with high absorption efficiency. The materials were prepared with different carbon doping content, and the coating of the nanocomposite film was deposited by these techniques on pre-cleaned copper and glass substrates. Energy dispersive X-ray analysis was used to determine the component element for carbon (fly ash), and its diameter was measured using scanning electron microscopy. The optical properties were investigated by UV spectrometry and reflectivity tests in a range of 250–1300 nm at room temperature. The absorbance coefficient, transmittance, reflectance, skin depth, optical density, optical energy gap and Urbach’s parameters of the nanocomposite thin films were also determined. The data were analyzed and interpreted in terms of the theory of phonon-assisted direct electronic transitions. The Eg of the doped carbon was measured in different composition ratios of CuO:NiO (A = 0.5:2.5, B = 1:2, C = 1.5:1.5, D = 2:1, E = 2.5:0.5) wt.%, and fixed carbon content at 7 wt.%. The results of the doped samples revealed an energy gap value of 2.5–3.9 eV. When the ratio of the CuO content ranged from 0.5 to 2.5, composition B was found to have three regions in its figure that were dependent on the CuO content in the nanocomposite mixture. The optical band gap values were highly dependent on the CuO content in the nanocomposite films.

Understanding how the mixed alkaline-earth effect tunes transition metal oxides-tellurite glasses properties

Mixed alkaline-earth effect (MAEE) on the transition metal oxides (TMO)-tellurite glasses [TMO = MoO3, Nb2O5 and WO3] modified with BaO/MgO mix is analyzed to understand how feasible it is to tune structural and optical properties of a tellurite glass. Changes induced by the three transition metal oxides on the tellurite polyhedral produce some modifications on the cut-off wavelength, optical band gap, Urbach's energy, refractive index and permittivity values where the MAEE manifest itself in different ways. The changes in the glass structural properties are dominated by the TMO type while optical and electrical properties depend mostly on the combination between the TMO and the [BaO/(BaO + MgO)] ratio. Glass transition temperature is discussed in terms of some parameters like number of bonds per unit volume, average stretching force constant, molar volume, oxygen packing density values (OPD) and FTIR spectra. In this work we provide evidence that the behavior attributed to the relationship between structure and electrical or optical properties in pure oxide and even binary systems is not reproduced by multi-component glassy systems. A comparison between traditional and new optical methods confirm allowed direct inter-band electronic transitions in the studied glasses evidencing that the a priori parameter election is not always an accurate procedure.

Structural characterization and optical properties of zeolitic imidazolate frameworks (ZIF-8) for solid-state electronics applications

Zeolitic imidazolate frameworks (ZIF-8) thin films were fabricated via layer by a layer deposition method. The investigation of X-ray diffraction (XRD) of the ZIF-8 powder form was seen to be polycrystalline with a single cubic-phase structure. The lattice parameters and Miller index were calculated using Rietveld refinement by Fullprof software. XRD of ZIF-8 film demonstrates an amorphous structure. The morphology of the ZIF-8 films has therefore been studied with the scanning laser microscope method and found that the homogeneity and surface enhance with increasing the film thickness. On the other hand, it is found that ZIF-8 films have a direct electronic transition with two energy bandgaps (3.62 and 4.58 eV) and their values independent of the thickness of the films. The optical constants were calculated, and the refractive index esteems increment with expanding of the film thickness. This outcome might be credited to the interlude of the film in the preliminary phase of the deposition. The nonlinear optical constants for ZIF-8 film were calculated, and the third-order optical nonlinearity was expanded with expanding the film thickness.

Optical and Structural Characterization of Honeycomb-Like Ag2S Nanoparticles by a Simplified and Stable Wet Chemical Synthesis Method

The goal of this paper is to present a stable and new formulation route for the synthesis of silver sulfide nanoparticles with stoichiometric Ag2S composition in aqueous solution. The structural, optical, and morphological properties of Ag2S nanoparticles were studied by using characterization techniques such as ultraviolet–visible spectroscopy (UV–Vis), transmission electron microscopy and x-ray photoelectron spectroscopy (XPS). From the UV–Vis spectrum, the direct and indirect energy gap values of 3.56 and 1.89 eV were calculated, related to direct and indirect transitions of electrons, as an estimation of bandgaps. Also, an optical band gap shift with respect to bulk bandgap is observed reported in the literature and is related to nanoparticle size decrease. Furthermore, from the high-resolution transmission electron microscopy micrograph an orthorhombic crystallographic structure was determined with lattice parameters 4.77 × 6.92 × 6.88 A and the nanoparticle surface showed a honeycomb-like interference pattern. Finally, the expected chemical composition was proved by the low- and high-resolution XPS spectra.

Birefringence of SnSe single crystals in excitonic and electronic transitions region

Absorption spectra in temperature range 300–10 K were studied. The minimal band gap A1 (1.091 eV at 300 K) is formed by direct allowed in E∣∣c polarization and forbidden in E∣∣a polarizations transitions. The next interval B1 (1.316 eV) is formed by direct transitions allowed in E∣∣a polarization and forbidden in E∣∣c polarization. Angular dependences of the electron transitions in the band gap minimum were investigated. Spectral dependences of refractive index (n) were calculated from wavelength modulation transmission (ΔT/Δλ) and reflection (ΔR/Δλ) spectra in the region of direct electron transitions. The absorption edge shifts to higher energies with temperature decreasing, and temperature coefficient of edge absorption shift (β) is 3.4 × 10−3 eV K−1.

Focus on scavengers’ effects and GC-MASS analysis of photodegradation intermediates of sulfasalazine by Cu2O/CdS nanocomposite

Coupling of Cu2O and CdS nanoparticles (NPs) showed a boosted photocatalytic activity in the degradation of Sulfasalazine (SSZ) in aqueous solution. Some characterization techniques including x-ray diffraction (XRD), scanning electron microscope equipped with an energy dispersive X-ray (EDX) analyzer, Transmission electron microscope (TEM), x-ray mapping, photoluminescence spectroscopy (PL) and diffuse reflectance spectroscopy (DRS) was used for characterizing of samples. A blue shift in band gap energy was obtained for the composite concerning the single semiconductors and values of 2.42, 2.43 and 2.52 eV were obtained for the direct electronic transition of Cu2O, CdS and CdS/Cu2O samples, respectively. PL results confirmed an elongate lifetime of e/h pairs in the composite. The optimal conditions were CSSZ: 10 mg L−1, pH 5, 25 min irradiation time and 0.5 g L−1 of the Cu2O/CdS catalyst. The chemical oxygen demand (COD) showed about 82% of SSZ molecules were mineralized during the photodegradation process. The effects of scavengers showed that the photogenerated holes have a major role in SSZ degradation with respect to other reactive species. GC-Mass analysis was done for detecting the degradation intermediates formed during the photodegradation process. The initial pH of the solution was changed from 5 to about 7.5 at the end of the photodegradation process.

Influence of Doping on Optical Properties of (CdS:Fe+3)Thin Films Prepared by Chemical Spray pyrolysis

In the present work, undoped CdS and doped with iron CdS:Fe+3 thin films have been prepared by chemical spray pyrolysis method on glass substrate with different temperature from cadmium nitrate solution with constant thickness(450 ± 5 nm), and study the effect of the percentage of doping with iron on optical properties of prepared films.
 The optical properties have been studied from transmittance and absorbance spectral within wavelengths range (380-900 nm). The results showed that all the prepared films has direct electronic transitions and optical energy gap between (2.31-2.44 eV). They also showed that the transmittance and optical energy gap of films prepared from nitrate solution increase with increasing of substrate temperature, then transmittance start downward with the continued increase in temperature (400, 450 oC), and a decrease in the optical energy gap with increasing doping percentage with iron.

Effect of Fluorine doping on the Optical properties of NiO thin Films deposited by Spray Pyrolysis

Thin films of undoped Nickel oxide and fluorine doped Nickel oxide have been deposited utilizing easy and cheapest technique, which is chemical spray pyrolysis. Absorbance and transmittance spectra were achieved by double beam spectrophotometer in the wavelength range (300-900) nm. Direct electronic transition for the deposited films, the optical energy gap was evaluated to be increase as the doping ratio increase. All the optical constants such as reflectance and refractive index, are affected by increase their value with the increase in F content. The calculated skin depth was found to decrease upon the increase of fluorine.

Novel synthesis and optical properties of CaSb2O6 nanoparticles via thermal decomposition of thioacetate precursor

A new molecular precursor Ca[SbO(SCH2COO)Cl H2O] was synthesized from the reaction of calcium thioacetate with SbCl3 in (1:1) ratio. The precursor was characterized by elemental analyses, infrared spectra (IR), and thermal analyses (TGA). IR spectra indicated that thioacetate chelates Sb in a binegative bidentate manner via thiol sulfur and carboxylate oxygen after deprotonation. TGA results indicated that the precursor decomposes in three steps.Calcium antimonate (CaSb2O6) was synthesized from the thermal decomposition of the molecular precursor at 250 °C in air. The calcination product was characterized by X-ray diffractions (XRD), scanning electron microscope (SEM), and transmittance electron microscope (TEM). The results of X-ray diffractions indicated the formation of calcium antimonate nanoparticles which crystallizes in hexagonal system. The crystallite size was found to be 32 nm. Electron microscopes (SEM and TEM) indicated the formation of aggregates of spherical nanoparticles that forms rod like shape. The optical band gap (Eg) measurements showed allowed direct electronic transition. The determined Eg value is 3.9 eV, which is larger than the bulk.

Composition dependence of structural and linear and non-linear optical properties of CdS1−xMnx semiconducting thin films

This work reported the structure and optical investigations of CdS1−xMnx (x = 0.00, 0.02, 0.04, 0.06, 0.08, 0.10) thin films. The thin films of CdS1−xMnx with different compositions were synthesized using electron beam evaporation method under high vacuum conditions at room temperature. X-ray diffraction (XRD) analyses revealed the polycrystalline nature of the deposited films. The observed diffraction peaks were well fitted with a hexagonal crystal structure. In addition, the intensity of XRD peaks decreased by increasing the Mn concentration indicating a monotonical deterioration in the crystalline quality by the doping. The (101) plane is the preferential orientation for the crystal growth of the studied films. The film thickness as well as a refractive index were calculated by Swanepoel’s method. The direct and indirect electronic transitions were found to be responsible for the high absorption process in the studied films. Furthermore, both direct and indirect bandgaps decreased with increasing the Mn content. Other optical parameters such as optical conductivity, real and imaginary parts of dielectric constant, dispersion energy, and dissipation factor were determined and showed a strong dependence on the Mn content. These findings were discussed in terms of the localized states.

Synthesis, characterization, molecular docking and cytotoxicity studies on N-benzyl-2-isonicotinoylhydrazine-1-carbothioamide and its metal complexes

N-benzyl-2-isonicotinoylhydrazine-1-carbothioamide (H3L) and its complexes with Co(II), Ni(II), Cu(II) and Zn(II) have been synthesized. The metal complexes have been characterized by elemental and thermal analyses, spectral studies of FT-IR, 1H and 13C NMR, ESR, MS, and UV-Visible, conductance and magnetic moments measurements. The free ligand exists in the keto-thione form, but it coordinates to the metal ion in the keto-thiol form. The 1:1 coordinated species, M(II)-H3L, are formed. The optical band gap (Eg) measurements indicated direct electronic transition and semi-conducting nature of the compounds. The cytotoxicity of H3L and its Zn(II) complex were tested against both HCT116 and HePG2. The ligand showed strong cytotoxicity against the two investigated cell lines, while Zn(II) complex exhibited moderate cytotoxic effect. The molecular docking on both colon and liver cancer proteins were studied and the data support the experimental results.

Influence of Cr3+ substitution on structural, morphological, optical, and magnetic properties of nickel ferrite thin films

The NiFe2−xCrxO4 (0.0 ≤ x ≤ 1.0 in the step of x = 0.2) thin films have been synthesized using chemical spray deposition technique. The effects of Cr3+ content on the structural, morphological, optical and magnetic properties of these thin films have been investigated. The X-ray diffraction patterns confirmed single phase cubic spinel structure with space group of Fd-3m. The lattice constant decreases with increases Cr3+ concentration obeying Vegard’s law. The functional studied of Cr3+ substitution NiFe2O4 thin films were formed two major transmission bands, high-frequency band (ν1) around at 665 cm−1 is due to intrinsic vibrations of (A) site and the low frequency band (ν2) around 428 cm−1 is due to [B] site, respectively. Surface morphology was investigated in terms of root mean square roughness, average roughness (Ra), surface kurtosis and surface skewness of Ni–Cr ferrite thin films. The energy band gap for allowed direct electronic transition found to be in the range of 2.75–3.02 eV with Cr3+ content. According to VSM results show that, the saturation magnetizations of the thin film samples were found to be in the range of 234.12–46.65 emu/cc. The Cr3+ concentration increases with a reduction in hysteresis losses as well as slight reduction in the saturation magnetization.

Electronic and optical properties of HgIn2S4 thiospinels

Photoluminescence (PL), transmission (T), reflection (R), wavelength modulated transmission (ΔT/Δλ) and reflection (ΔR/Δλ) spectra of thiospinel HgIn2S4 crystals were investigated in temperature interval form 10 to 300 K. The band gap 1.64 eV (300 K) and 1.666 eV (10 K) are formed by electron transitions from L to Γ points of Brillouin zone. The edge temperature shift coefficient β (ΔEg/ΔT) is equal to 4.3 10–3 eV K−1. The direct energy gap in Γ point (Egdir) corresponds to 1.748 eV. The top valence bands in k = 0 are split by the crystal field on 25 meV. Direct electron transitions a1–a7, observed in energy range 1–6 eV, were interpreted conform the theoretically calculated band structure. The optical functions (n, k, ε1 and ε2) were determined by Kramers-Kronig analysis.

Synthesized pure cobalt oxide nanostructure and doped with yttrium by hydrothermal method for photodetector applications

In this study, pure Co3O4 nano structure and doping with 4 %, and6 % of Yttrium is successfully synthesized by hydrothermal method.The XRD examination, optical, electrical and photo sensingproperties have been studied for pure and doped Co3O4 thin films.The X-ray diffraction (XRD) analysis shows that all films arepolycrystalline in nature, having cubic structure.The optical properties indication that the optical energy gap followsallowed direct electronic transition calculated using Tauc equationand it increases for doped Co3O4. The photo sensing properties ofthin films are studied as a function of time at different wavelengths tofind the sensitivity for these lights.High photo sensitivity doped Co3O4 with 6% of Yttrium, is a118.774% at wavelength 620 nm, while for pure Co3O4 films nosensitivity at the same wavelength. So, higher sensitivity is found fordoping Co3O4 with fast rise and fall times less than 1s.

Study Influence of Substrate Temperature on Optical Properties of CdS Thin Films Prepared by Chemical Spray pyrolysis

This study aims to prepare Cadmium Sulphide (CdS) thin films using thermal Chemical Spray Pyrolysis (CSP) on glass of different temperatures substrate from cadmium nitrate solution. Constant thickness was (430 ± 20 nm) and the effect of substrate temperature on the optical properties of prepared thin films.
 Optical properties have been studied from transmittance and absorbance spectral within wavelengths range (360 - 900 nm). The results show that all the prepared films have a direct electron transitions and optical energy gap between (2.31-2.44 eV). They also show that the transmittance and optical energy gap of films prepared from nitrate solution increase with increasing of substrate temperature, then transmittance start downward with the continued increase in temperature (400, 450) oC.

Band-fluctuations model for the fundamental absorption of crystalline and amorphous semiconductors: a dimensionless joint density of states analysis

We develop a band-fluctuations model which describes the absorption coefficient in the fundamental absorption region for direct and indirect electronic transitions in disordered semiconductor materials. The model accurately describes both the Urbach tail and absorption edge regions observed in such materials near the mobility edge in a single equation with only three fitting parameters. An asymptotic analysis leads to the universally observed exponential tail below the bandgap energy and to the absorption edge model at zero Kelvin above it, for either direct or indirect electronic transitions. The latter feature allows the discrimination between the absorption edge and absorption tails, thus yielding more accurate bandgap values when fitting optical absorption data. We examine the general character of the model using a dimensionless joint density of states formalism with a quantitative analysis of a large amount of optical absorption data. Both heavily doped p-type GaAs and nano-crystalline Ga1-xMnxN, as examples for direct bandgap materials, as well as amorphous Si:Hx, SiC:Hx and SiNx, are modeled successfully with this approach. We contrast our model with previously reported empirical models, showing in our case a suitable absorption coefficient shape capable of describing various distinct materials while also maintaining the universality of the exponential absorption tail and absorption edge.

Unearthing the factors governing site specific rates of electronic excitations in multicomponent plasmonic systems and catalysts.

We use experimental and computational studies of core-shell metal-semiconductor and metal-molecule systems to investigate the mechanism of energy flow and energetic charge carrier generation in multicomponent plasmonic systems. We demonstrate that the rates of plasmon decay through the formation of energetic charge carriers are governed by two factors: (1) the intensity of the local plasmon induced electric fields at a specific location in the multicomponent nanostructure, and (2) the availability of direct, momentum conserved electronic excitations in the material located in that specific location. We propose a unifying physical framework that describes the flow of energy in all multicomponent plasmonic systems and leads us towards molecular control of the energy flow and excited charge carrier generation in these systems.

Novel photocatalyst nitrogen-doped simonkolleite Zn5(OH)8Cl2·H2O with vis-up-conversion photoluminescence and effective visible-light photocatalysis

As photocatalysts exhibit selectivity toward various pollutants, it is necessary to develop different and novel photocatalysts. In this work, a novel photocatalyst-nitrogen-doped simonkolleite Zn5(OH)8Cl2·H2O (DSM) is prepared through a new facile method: calcinating the mixture of zinc hydroxide, urea, and guanidine hydrochloride at 575 °C for 1 h in a furnace with an air atmosphere. The as-prepared sample was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL) spectra, UV–visble near-infrared diffuse reflection spectra (UV–Vis–NIR DRS), Brunauer–Emmett–Teller (BET) method, Ramen spectra and Zeta potential measurement, photocatalytic properties, as well as active species trapping experiments. XRD and XPS show the as-prepared powder is nitrogen-doped simonkolleite Zn5(OH)8Cl2H2O (DSM) with a small ZnCl2 fraction. SEM investigation indicates that the as-prepared powder possesses a flower-like layered shape. The UV–Vis–NIR exhibits that after doping, the DSM possesses strong light absorption in the ranges of 300–500 and 1400–2500 nm, a direct electronic transition with a band gap energy of 2.469 eV. PL measurement reveals a strong photoluminescence and an up-conversion from lower to higher-energy visible light in as-prepared samples. Zeta potential investigations show that during photocatalysis, the charges on as-prepared photocatalyst are positive. The photocatalytic experiments show a good dark adsorption, a high photodegradation (99.4% at 60 min), a high pseudo-first-order constant (k) of 0.0261 min−1. Meanwhile, the active species trapping experiments suggest that hole (h+) is the dominant active species during photocatalysis. It is concluded that the doping favors in enhancing vis-light-photocatalysis. This work makes a significant contribution to the literature.