Indirect Transitions Research Articles

Tuning the band gap and effective mass of black arsenic phosphide monolayer by in-plane strain

Based on first-principles calculations, the electronic properties of a black AsP monolayer were investigated by applying an external strain. It was found that the electronic property and effective mass of black AsP monolayer exhibits strong anisotropy. Comparing with the armchair direction, the zigzag direction was more resistant to elastic deformation. When the strain was applied along the zigzag direction, a direct-indirect-direct transition of the band gap occurred with increasing tensile strain and compressive strain. The direct to indirect transition occurred at a −2% compressive strain when the strain was applied along the armchair direction. The direct-indirect transition occurred at biaxial compressive and tensile stains of −3% and 3%, respectively. In addition, the effective mass of electrons and holes along the armchair direction is an order of magnitude smaller than that associated with the zigzag direction, suggesting that electron and hole transport will occur preferentially along the armchair direction. Moreover, we found that the effective mass of the black AsP monolayer can be significantly tuned by strain.

Structural and optical characteristics of poly(vinyl alcohol)/carboxymethyl cellulose/ curcumin nanocomposites

ABSTRACT. Curcumin nanoparticles (CurNP’s) were successfully synthesized, characterized, and used as a cross-linking dopant of polyvinyl alcohol/ sodium carboxymethyl cellulose (PVA/CMC) semi-natural polymer blend. Synthesized nanocomposite films of (PVA/CMC/CurNP’s) were characterized using Fourier transforms infrared FTIR spectroscopy and tested for their resistance of different bacterial grams. Obtained data shows that studied PVA polymerized and cross-linked with CMC as a result for hydrogen bonding between the carboxylic groups and with the non-substituted hydroxyl groups of the cellulose molecule. The optical energy gap was found to be sensitive for the CurNP’s doping level, and the indirect transition was dominant in the studied samples. The addition of CurNP’s appears to increase the activity index of all samples against both gram-negative and positive bacteria, and their activity increases with increasing dopant level until a specific optimal concentration.
 
 KEY WORDS: Curcumin nanoparticles, Semi-natural polymer blend, Antibacterial, FTIR, Optical energy gap
 
 Bull. Chem. Soc. Ethiop. 2021, 35(3), 615-624.
 DOI: https://dx.doi.org/10.4314/bcse.v35i3.12

Effect of Zinc Oxide on the Optical Properties of Polyvinyl Alcohol/Graphene Oxide Nanocomposite

In this work, the conventional casting method was utilized to synthesize nanocomposites based on polyvinyl alcohol (PVA) and both zinc oxide nanoparticles (ZnO-NPs) and graphene oxide (GO). ZnO-NPs have been synthesized by the co-precipitation method while GO with Hummer's method. The synthesized nanocomposites were characterized by UV-vis spectroscopy. Films have been abbreviated to contain different amounts of ZnO as follows: (2, 4, 6, and 8 wt%) and constant amount of GO (2 wt.%) within the PVA matrix using an ultrasonic system to disperse ZnO and GO in the PVA matrix completely. The synthesized nanocomposites were characterized optically by utilizing a UV–Vis-NIR spectrophotometer. The addition of nanofiller to PVA increases the absorption of PVA. At the same time, the optical band gap (for both direct and indirect transitions) was diminished as they depend on the ZnO and GO concentration. PVA/ZnO and PVA/GO nanocomposites present good optical properties. This confirmed that the synthesized ZnO and GO- substituted PAV can be utilized as a solid polymer electrolyte in optoelectronic applications.

Energy gaps, refractive index and photon emission from point defects in copper-doped Gd2O3 nanocrystalline films

Thin nanocrystalline films of gadolinium oxide (undoped and doped with copper ions) were synthesized by means of reactive anodic evaporation technique. Chemical composition inspection and structural-phase analysis of the samples obtained were performed by XPS and XRD methods. Optical characteristics (energy gaps, refractive index, emission parameters) of thin films were determined by means of absorption, transmission and photoluminescence spectroscopies. It was found that injection of copper leads to an insignificant decrease in the transparency gaps for direct and indirect transitions of Gd2O3 film. On the basis of interference effects, a decrease in refractive index and an increase in its dispersion in Gd2O3:Cu film were established. The excited by energy transfer from "defective" Gd3+ ions emission bands at 2.9 eV and 3.2 eV were associated with oxygen vacancies (F+- centers) localized near C2 and S6 cationic positions in Gd2O3 host. The injection of copper ions into Gd2O3 host stimulates the formation of lattice defects and leads to an increase in the luminescence yield. An analytical expression was obtained for describing luminescence flare-up and quenching. The corresponding thermal activation barriers were determined. The transfer of excitation energy between intrinsic defects of Gd2O3 film provides the conversion of hard UV radiation into soft UV and visible radiation.

Characterisation of the transition type in optical band gap analysis of in-flame soot

Semiconductor materials are characterised by a band gap between conduction and valence band, which can either be of direct or indirect nature depending on the crystal momentum. In case of applying the so-called Tauc method to infer the band gap value, both transition types are described by different values for the transition type exponent. In the case of soot particles, the optical band gap is an important quantity during the evolution process to describe the optical characteristics mainly linked to the underlying structural properties. In this work, an approach to infer the optical band gap value together with the transition type exponent is used to shed light on the electronic nature and optical properties of soot during its formation process. In situ UV–Vis-NIR extinction measurements as well as ex situ Raman spectroscopy are performed in a non-premixed ethylene/air flame at two different positions in the flame reflecting different stages of the soot formation process. The results reveal two important tendencies: first, an increase in the band gap energy of young soot is observed, possibly caused by the curvature of initially planar polycyclic aromatic hydrocarbon monomers due to the presence of odd-numbered rings in the hexagonal aromatic lattice. Second, a decrease of the transition type exponent value can be observed, indicating a shift from an indirect transition to a more direct band structure. Further, the indirect band gap assumption for mature soot and its band gap value close to 0.2 eV can be confirmed experimentally. The Raman results provide additional insight in the atomic fine structure of the soot particles regarding the defect density and the loss of amorphous character with increasing maturity.

Preparation and improved photochemical properties of V-doped Nb2Te4O13

Mixed oxides combining d0-and p-cations with stereoactive lone pair electrons exhibit excellent polarization abilities for potential photoelectric applications. This work reports the photochemical properties of V-doped Te–Nb mixed oxide Nb2Te4O13, which has the typical framework constructed by NbO6 (d0-) octahedra, TeO4, and TeO3 (p-) polyhedra. The samples were prepared using the sol-gel method, which resulted in the final nanoparticles. The luminescence spectra, lifetimes of photoinduced electrons and holes, and conductivity properties of the samples were characterized. Nb2Te4O13 has an indirect transition with a band energy of 2.98 eV V5+-doping shows a narrowing effect on the band gap, enhancing the harvest of the optical absorption in the visible light region. Photocatalysis tests were conducted via the photodegradation of RhB solutions under visible-light irradiation. V5+-doping depresses the intrinsic luminescence efficiency, while it significantly enhances photodegradation. It is suggested that the induced redox couples V5+/4+ and defects are responsible for the improvements of photocatalysis of Nb2Te4O13. This work offers an effective method to modify photochemical properties via redox couples and defects induced by impurity ion doping.

Pronounced effect of PbI2 nanoparticles doping on optoelectronic properties of PVA films for photo-electronic applications

Composite films of polyvinyl alcohol/lead iodide (PVA/PbI2) were prepared with different concentrations of PbI2 using low-cost casting process. The characteristics of the prepared films were analyzed by XRD and SEM. The XRD results showed increased crystalline nature with PbI2 content and the SEM micrographs revealed homogeneous particles distribution of PbI2 within the polymer. The optical parameters of PVA/PbI2 films were measured through UV–Vis absorption-transmittance spectroscopy. The indirect energy gap for PVA was found to be 4.75 eV, while the PVA/PbI2 film with 10 wt% PbI2 showed direct transition of 2.47 eV and indirect transition of 2.15 eV. The PVA film resistivity was found to be 1.79 × 109 Ω cm in the dark and decreased to1.85 × 108 Ω cm at 7000 lux. The film resistivity was found to decrease as the PbI2 content ratio increased up to 10 wt%. The results also showed gradual increase in photo sensitivity with increasing PbI2 content ratio in the composite films. The optical properties revealed that the composite films (PVA/PbI2) could be used in different optoelectronic applications.

Structural and optical characteristics of charge transfer complex of plasma deposited 2-(diethylamino)ethyl methacrylate thin films and iodine

The effect of iodine doping on the optical and structural characteristics of plasma polymerized 2-(diethylamino)ethyl methacrylate (PPDEAEMA) thin films are described in this paper. Scanning electron micrographs show that both as-deposited and iodine doped PPDEAEMA thin films are homogeneous. Thermogravimetric analysis demonstrates the stability temperature as 560 K for iodine doped PPDEAEMA. The degradation temperature of iodine doped PPDEAEMA is slightly greater than that of as deposited PPDEAEMA. Fourier transform infrared spectroscopy shows shifts in absorption bands due to the incorporation of iodine in PPDEAEMA. The absorption coefficient and allowed direct and indirect transition energy gaps were calculated from the ultraviolet–visible absorbance spectra. The energy band gap value decreases due to iodine doping. Urbach energy, steepness parameter and extinction coefficient were calculated as well for the PPDEAEMA thin films of thicknesses 100, 200 and 300 nm which were iodine doped for 45, 360 and 720 min.

Far-UV spectroscopy of mono- and multilayer hexagonal boron nitrides

Hexagonal boron nitrides (hBNs) have a very high luminescence efficiency and are promising materials for deep-UV emitters. Although intense deep-UV emissions have been recorded in various forms of hBN excited by photons or energetic electrons, information on the electronic structure of the conduction band has been derived mainly from theoretical works. Therefore, there is a lack of high-resolution absorption data in the far-UV region. In this study, the far-UV absorption spectra of chemical-vapor-deposition-grown mono- and multilayer hBNs were recorded at 10 and 298 K. In addition to the previously reported band at 6.10 eV, two absorption bands at 6.82 and 8.86 eV were observed for the first time in thin-film hBN. Furthermore, excitation of the hBN thin film samples with 6.89-eV photons revealed intense emission peaks at 6.10 (mono) and 5.98 (multi) eV with a bandwidth of ∼0.7 eV. Comparing the absorption and photoluminescence data, we believe that both direct and indirect transitions occur in the radiative processes.

STRUCTURAL, OPTICAL, AND PARAMAGNETIC INVESTIGATIONS OF (Mn2+) IONS DOPED ZINCALUMINOBOROPHOSPHATE (ZnAlBP) GLASS SYSTEM

Concentration variation of Mn2+ ions doped zincaluminoborophosphate (ZnAlBP) glasses of 40 ZnO + 25 H3BO3 + (30-x) P2O5 + 5 Al2O3 + x MnO with 0  x  0.8 (x=0, 0.2, 0.4, 0.6 and 0.8) have been prepared and investigated by structural and spectroscopic techniques. The XRD diffractograms confirm the amorphous nature of the prepared samples. The FTIR spectra reveal the existence of BO3 and BO4 units along with fundamental P-O-H and B-O-H structural units in the present glass system. The Optical absorption spectra exhibit two low intensity bands centered at 407 nm (24,570 cm-1) and 689 (14,514 cm-1). These bands were assigned to 6A1g(S) → 4A1g (G) + 4Eg (G) and6A1g(S) → 4T1g (G) characteristic transitions of Mn2+ ions in octahedral symmetry. Tauc’s plots were drawn to measure the optical band gaps for the direct and indirect transitions. The energy gaps show a decreasing trend for the selected ZnAlBP: Mn2+ glass system. The well-resolved hyperfine splitting EPR spectra of prepared glass samples exhibit resonance at g ≈ 1.98 indicating the Mn2+ ions are in octahedral site symmetry. The hyperfine splitting factor (hfs), the number of spin levels (N), hyperfine constant (A), zero-field splitting parameter (D), and paramagnetic susceptibility (χ) have been determined. The magnitude of g indicates the predominant ionicity in the glasses.

Synthesis, physical, optical, structural and radiation shielding characterization of borate glasses: A focus on the role of SrO/Al2O3 substitution

In a bid to expand the amount of information available on glass systems and their potential applications for radiation shielding design, glass samples with the compositions (30-x)SrO-xAl 2 O 3 –68B 2 O 3 –2V 2 O 5 (x = 5, 7.5, 10, 12.5&15 mol %) coded as SABV0 - 4 were prepared by the melt-quenching technique and analyzed for their optical, structural, physical, and radiation shielding features. The glassy (amorphous) nature of the SABV glass samples was affirmed by broad peaks of X-ray diffraction spectra. Calculated values of density and molar volume shown opposite behavior and the variation of these values were discussed as structural modifications in the glass matrix. From recorded optical absorption spectra optical band gap energy (Eg)-indirect transition, Urbach energy and optical basicity were estimated. FTIR spectra were recorded for all the samples in the range 400 cm −1 to 4000 cm −1 . The FTIR absorbance spectra unveiled the SABV network structure mainly incorporating of BO 3 and BO 4 units. Raman spectroscopy is achieved to detect the structural changes and at higher wavenumber, B–O stretching modes in [BO 3 ] observed with one or two NBO's. The results of ESR spectra of glasses have indicated the highly covalent environment of vanadium ions. Analysis of the photon shielding parameters of the glasses which were obtained primarily from FLUKA Monte Carlo simulations and XCOM computations revealed photon energy and glass chemical composition dependence. The mass attenuation coefficient and effective atomic number ranged from 0.2668 to 0.3385 cm 2 g -1 and 12.98–15.93 accordingly as the weight fraction of Sr increased from 16.06 to 26.72% in the glasses. Generally, photon shielding ability of the SABV glasses follows the trend: SABV0 > SABV1 > SABV2 > SABV3 > SABV4. The thermal neutron total cross section follows the same trend with values fluctuating between 71.9553 and 80.6268 cm −1 . However, SABV1 showed superior fast neutron moderating capacity among the glasses. The present SABV glasses showed outstanding photon shielding ability compared to common shields. The prepared glasses are thus suitable candidates for radiation protection applications.

A study on the optical and dielectric behavior of Au-PVA nanocomposite films

In the current work, gold nanoparticles (AuNPs) embedded poly(vinyl alcohol) (PVA) composites were fabricated via solution casting technique. The AuNPs were synthesised in situ by the reduction of the metal precursor in the PVA matrix, acts as reducing and stabilising agent. The Plasmon resonance bands registered around 562–564 nm wavelength range of UV–Visible (UV–Vis) absorption spectra evidenced AuNPs’ generation in PVA. X-ray diffraction (XRD) studies revealed the FCC crystalline structure of the formed AuNPs. The generation of AuNPs in PVA has significantly reduced the optical energy gap for both direct and indirect transitions. Dielectric properties of the nanocomposites were investigated in the frequency range of 50 Hz-50 MHz revealed that dielectric constant and dielectric loss are enhanced in nanocomposites. Cole-Cole plot shows semi-circular arc originating from single point implying broad relaxation exists in nanocomposites. DC conductivity was enhanced with increase in gold salt concentration in Au-PVA nanocomposite, which is two orders of magnitude higher than the pristine PVA films.

Band gap tuning and p to n-type transition in Mn-doped CuO nanostructured thin films

Here we discuss the synthesis of copper (II) oxide (CuO) and manganese (Mn)-doped CuO thin films varying with 0 to 8 at% Mn using the spray pyrolysis technique. As-deposited film surfaces comprised of agglomerated spherical nanoparticles and a semi-spongy porous structure for 4 at% Mn doping. Energy dispersive analysis of X-rays confirmed the chemical composition of the films. X-ray diffraction spectra showed a polycrystalline monoclinic structure with the predominance of the ( 11) peak. Optical band gap energy for direct and indirect transitions was estimated in the ranges from 2.67–2.90 eV and 0.11–1.73 eV, respectively. Refractive index and static dielectric constants were computed from the optical spectra. Electrical resistivity of CuO and Mn-doped CuO (Mn:CuO) thin films was found in the range from 10.5 to 28.6 Ω·cm. The tiniest electron effective mass was calculated for 4 at% Mn:CuO thin films. P to n-type transition was observed for 4 at% Mn doping in CuO films. Carrier concentration and mobility were found in the orders of 1017 cm–3 and 10–1 cm2/(V·s), respectively. The Hall coefficient was found to be between 9.9 and 29.8 cm3/C. The above results suggest the suitability of Mn:CuO thin films in optoelectronic applications.

Heterovalent substitution of Nb5+, Ta5+ on Te4+ sites and induced modification on luminescence and photocatalysis of Te-V mixed oxide Te2V2O9

Materials combining d0 and p cations with stereo-active lone pair electrons have excellent polarization for photoelectric applications. This work reports the modifications of photochemical properties via Nb5+-, Ta5+-doping in Te-V mixed oxide Te2V2O9 with d0-(V5+) and p-(Te4+) cations. Nb5+-, Ta5+-doped Te2V2O9 nanobelts were prepared using the sol-gel method. The structure, morphologies, and optical absorption properties were reported. The calculation and experimental results indicate that the Te-V mixed oxide Te2V2O9 has an indirect transition with band energy of 2.19 eV. The photodegradation of RhB dye solutions was conducted under the irradiation of visible-light irradiation (λ ≥ 420 nm). The conductivity properties, lifetime of charge carriers were characterized via impedance spectra and decay curves, respectively. The multivalent cations were confirmed via XPS measurements. The experiments concluded that both Nb5+ and Ta5+ ions can depressed the luminescence, and enhanced photocatalysis to some extent. While, compared with Ta5+, the Nb5+ doping shows much more significant improvement of photodegradation. It was concluded that the redox couples V5+/4+ induced by Nb5+ doping attribute to the photocatalysis. This work offers an effective method to modify the photochemical properties via the redox couples and defects induced by heterovalent substitution on the cations.

Исследование влияния оптического излучения на резистивное переключение МДП-структур на основе ZrO-=SUB=-2-=/SUB=-(Y) на подложках Si(001) с наноостровками Ge

The effect of optical radiation in the visible and near-infrared bands on resistive switching of a MOS stack based on ZrO2(Y) film on an n-Si(001) substrate with self-assembled Ge nanoislands on its surface has been studied. An increase in the resistive switching logical gap was observed upon the photoexcitation, in particular, when the photon energies were smaller than the Si band gap. The effect was associated with the impact of the photovoltage at the Si/Ge/ZrO2(Y) interface. In the latter case, the effect is associated with spatially indirect interband optical transitions in Ge nanoislands.

Investigation of the effect of optical radiation on resistive switching of MIS-structures based on ZrO-=SUB=-2-=/SUB=-(Y) on Si(001) substrates with Ge nanoislands

The effect of optical radiation in the visible and near-infrared bands on resistive switching of a MOS stack based on ZrO2(Y) film on an n-Si(001) substrate with self-assembled Ge nanoislands on its surface has been studied. An increase in the resistive switching logical gap was observed upon the photoexcitation, in particular, when the photon energies were smaller than the Si band gap. The effect was associated with the impact of the photo-emf at the Si/Ge/ZrO2(Y) interface. In the latter case, the effect is associated with spatially indirect interband optical transitions in Ge nanoislands. Keywords:: memristor, photo-induced resistive switching, yttria-stabilized zirconia, MIS-structure, Ge/Si nanoislands.

Impact of indirect transitions on valley polarization in WS2 and WSe2.

Controlling the momentum of carriers in semiconductors, known as valley polarization, is a new resource for optoelectronics and information technologies. Materials exhibiting high polarization are needed for valley-based devices. Few-layer WS2 shows a remarkable spin-valley polarization above 90%, even at room temperature. In stark contrast, polarization is absent for few-layer WSe2 despite the expected material similarities. Here, we explain the origin of valley polarization in both materials based on the interplay between two indirect optical transitions. We show that the relative energy minima at the Λ- and K-valleys in the conduction band determine the spin-valley polarization of the direct K-K transition. Polarization appears as the energy of the K-valley rises above the Λ-valley as a function of temperature and number of layers. Our results advance the understanding of the high spin-valley polarization in WS2. This insight will impact the design of both passive and tunable valleytronic devices operating at room temperature.

Structural, Opto-Physical, Photoluminescence, and Optical Limiting Properties of Polyvinyl (Pyrrolidone and Alcohol) Blend Film Doped with Co-Metal

The optical properties of polyvinyl (pyrrolidine and alcohol) and PV(P/A) blend polymer have been improved using Co-metal as a filler for optoelectronic and optical shielding applications. The casting technique of solutions was used in order to prepare composite films with different Co-ratios (x: 0-18.5). XRD, FT-IR, SEM, and UV-visible spectrophotometry were operated to study the structure, morphology, and optical features of the flexible plain blend and composite films. The semi-crystalline nature of the films was influenced by the filler ratio of nanoparticles. Homogenous dispersion with some agglomeration of Co-particles has been observed in the host blend matrix. Then again, a successful interaction between the host matrix and particles was ensured by the FT-IR spectroscopic and XRD measurements. The cut-off absorbance edge of composite films is red-shifted from 200 nm (host blend) to 228 nm. The indirect transition optical band gap was confirmed from the Tauc's and optical dielectric loss calculations. Its value goes back from 5.15 eV (plain blend) to 4.43 eV for 18.5 wt% Co-blend composite film. An improvement in the extinction coefficient, optical conductivity, and refractive index of the composite films was achieved compared with the plain blend. The non-linear parameters of the composites were also enhanced. Photoluminescence (PL) emission spectra of PV(P/A) blend films doped with various weight percentages of Co-metal were examined at a wavelength of 750 nm. The optical shielding performance of the prepared composites is recommended for laser cut-off. Furthermore, the ability to tailor the optical properties of blend film makes it more effective for various applications including optical devices, non-linear optoelectronics, and reflective coating. Keywords: extinction coefficient, PV(P/A) polymer blend, transition band gap, photoluminescence, NL optical parameters, optical shielding.

Lattice composition dictated photocatalytic activity of bismuth chromium niobate pyrochlore under visible light

Bismuth-based pyrochlore oxides are known for excellent physicochemical properties due to their insignificant structural tolerance corresponding to vacancy and misplacement. Herein, Bi1.34CrNbO6 (BCN), a rare phase that stabilizes as a defect pyrochlore structure is reported for its photocatalytic activity. The co-precipitation method of synthesis was adopted and the phase was corroborated by the X-ray diffraction technique. The surface topography done by scanning and imaging using high-resolution microscopes (FESEM and HRTEM) revealed that the material possessed irregular morphology with particle size around ∼ 15 nm. From the optical property analysis, the possible low-energy direct and indirect transitions of 2.95 and 1.65 eV can be understood. Butler – Shinley calculation of redox potentials demonstrates the obtained phase is suitable for photodecomposition of water. The photocatalytic efficiency was systemically investigated on the degradation of Rhodamine B (RhB) dye on a lab-scale using a 300 W tungsten lamp. The process of band formation and the band offset positions of BCN concerning Cr incorporation results in superior photocatalytic efficiency.

Deposition of copper sulfide films on polyamide surface

In this paper, we present a novel and low - cost method for preparing copper sulfide films on polyamide. Non-treated as well as pre-treated PA6 films by 3 different methods (in boiled water; in NaOH solution; in boiled water and then in NaOH solution) were used for the formation of Cu2S layers by the sorption-diffusion method. Molten sulfur has been used as a sulfurization agent. The XRD, FTIR, and UV-VIS methods were used to characterize the structural, optical, and electrical properties of samples and to track changes in samples after each treatment stage. The sheet resistance of Cu2S layers depends on the pre-treatment method and varied from 7 k?/sq to 6 M?/sq. The optical band gaps (Eg) for direct and indirect transitions are determined to be 2.61-2.67 eV and 1.40-1.44 eV, respectively. Furthermore, the optical constants n, k, and ? are determined from UV-VIS measurements.