Indirect Optical Gap Research Articles

Unveiling the Structural Properties, Optical Behavior, and Thermoelectric Performance of 2D CsSn2Br5 Halide Obtained by Mechanochemistry.

Metal halide perovskites with a two-dimensional structure are utilized in photovoltaics and optoelectronics. High-crystallinity CsSn2Br5 specimens have been synthesized via ball milling. Differential scanning calorimetry curves show melting at 553 K (endothermic) and recrystallization at 516 K (exothermic). Structural analysis using synchrotron X-ray diffraction data, collected from 100 to 373 K, allows for the determination of Debye model parameters. This analysis provides insights into the relative Cs-Br and Sn-Br chemical bonds within the tetragonal structure (space group: I4/mcm), which remains stable throughout the temperature range studied. Combined with neutron data, X-N techniques permit the identification of the Sn2+ lone electron pair (5s2) in the two-dimensional framework, occupying empty space opposite to the four Sn-Br bonds of the pyramidal [SnBr4] coordination polyhedra. Additionally, diffuse reflectance UV-vis spectroscopy unveils an indirect optical gap of approximately ∼3.3 eV, aligning with the calculated value from the B3LYP-DFT method (∼3.2 eV). The material exhibits a positive Seebeck coefficient as high as 6.5 × 104 μV K-1 at 350 K, which evolves down to negative values of -3.0 × 103 μV K-1 at 550 K, surpassing values reported for other halide perovskites. Notably, the thermal conductivity remains exceptionally low, between 0.32 and 0.25 W m-1 K-1.

The effect of Zn2+ ion substitution on nanoparticles of ZnxMn(1-X) Fe2O4 (X = 0.2, 0.4, 0.6, 0.8) towards signal surface plasmon resonance (SPR)

ABSTRACT This study examined the effect of Zn2+ ion substitution on ZnxMn(1-x) Fe2O4 nanoparticles (x = 0.2, 0.4, 0.6, 0.8) towards signal Surface Plasmon Resonance (SPR). The co-precipitation method synthesised the nanoparticles at a temperature of 90°C. The X-Ray Diffraction (XRD) results showed a polycrystalline structure with a crystallite size varied from 19.05 nm to 19.77 nm. In addition, as the Zn element increased in the ZnxMn(1-x)Fe2O4 nanoparticles, the degree of crystallinity increased by 21.72%, respectively. The optical properties of the nanoparticles ZnxMn(1-x) Fe2O4 indicate absorbance at wavelengths of 330.36–331.74 nm. The absorbance level increased as the Zn element decreased, resulting a direct optical gap of 3.47 eV and an indirect optical gap of 3.23 eV. The Attenuated Total Reflection (ATR) curve in the SPR biosensor indicated a shift in the reflective angle of the Prism/Au/Air configuration, which is 47.78° on Prisma/Au/nanoparticle of Zn0.8Mn0.2Fe2O4/air by 50.75°, and the prism/Au/nanoparticle of Zn0.2Mn0.8Fe2 O4/air by 51.04°. The substitution of Zn ions in the compound Zn x Mn(1-x)Fe2O4 affects the resulting ATR curve. The shift in the SPR angle is caused by changes in the refractive index in each layer which is influenced by the concentration of each material which has an impact on the absorbance ability and photon reflectance. This research has the potential to continue with the testing of dielectric and magnetic properties, as well as special testing for biological analytes compatible with ZnxMn(1-x)Fe2O4 nanoparticles.

Preparation of lead hexaferrite filled PVA- PVP nanocomposite and a study of its structural features, optical and dielectric properties

Polymer blend films based on PVA-PVP filled with PbFe12O19 hexa-ferrites nanoparticles were created by solution casting technique. XRD, HRTEM, FESEM, FTIR, UV–vis spectroscopy and Broad-band dielectric spectroscopy were used to examine the properties of the prepared polymer composite films. X-ray diffraction pattern of the powder confirmed the formation of PbFe12O19 with hexagonal phase and that for the films confirmed the incorporation of the PbFe12O19 nanoparticles in the PVA-PVP blends. HRTEM confirms the formation of hexagonal nanoparticles with average size ranged from 7 nm to 21 nm. Field emission scanning electron microscopy (FESEM) studied the morphology of the nanocomposite films. There are noticeable variations in FT-IR spectra that confirm the incorporation of PbFe12O19, NPs in PVA-PVP blend. The absorbance was found to increase with red shift in the ultraviolet-visible region while the transmittance decreases with increasing PbFe12O19, nanoparticles. By incorporating, PbFe12O19 the direct optical gap and indirect optical gap gradually decreased from 5.20 eV to 4.44 eV and 4.97 eV to 4.34 eV, respectively while the Urbach energy increases from 0.274 eV to 0.678 eV. According to the Wemple-Didomenico single oscillator model, the optical dispersion parameters have been examined and are found to be strongly affected by PbFe12O19, loading. Additionally, the nanocomposite film containing 7.5% PbFe12O19, demonstrated outstanding optical shielding. The complex dielectric constant (ε*), complex electric modulus (M*) and electrical conductivity, of NCPs materials have been studied with the frequency changing from 0.1 Hz to 10 MHz. The real permittivity () of PVA-PVP blend polymer film increased while the real electric modulus (M′) is found to decrease by the incorporation of PbFe12O19, nanoparticles up to 2.5 wt.% and the imaginary component of permittivity follows the same pattern of The examined nanocomposite films show promise for flexible optoelectronics, photosensors, optical shielding and nano-dielectric materials.

Influence of KI salt concentration on the hydroxypropyl methylcellulose films: Optical study

Herein the optical properties of Hydroxypropyl methylcellulose/Potassium iodide (HPMC/KI) composite films were determined. The casting technique was introduced to make HPMC/KI films with different KI salt concentrations (0.1–1) wt%. The absorbance model was used to determine parameters like absorption edge, refractive index, real and imaginary sections of the optical dielectric function, extinction coefficient, Urbach energy, band gap, and optical conductivity in the spectral range 200–800 nm. As shown by the study, KI salt doping affects the optical properties of HPMC. The absorption edge (Ee) was widely displaced towards a region of lower photonic energy. The direct and indirect optical bandwidth gaps were lowered from 5.6 to 2.56 eV and 5.86 to 2.5 eV for the 1 wt % HPMC/KI film, respectively. The optical dielectric loss method was effectively employed as an alternate method for estimating the optical band gap. In addition, Tauc's extrapolation method identified the kind of electronic transition. The variation of the optical energy band gap and optical dielectric constant (ε1) based on KI salt concentration was used to investigate the credibility of Penn's model. In KI salt-composite films, an increase in Urbach energy and optical conductivity was observed which may be evidence of a large shift from tail-to-tail and band-to-tail transitions. Meanwhile, X-ray diffraction (XRD) examination revealed that the KI salts damaged the HPMC polymer's crystalline phase. Lastly, the films were subjected to Fourier Transform Infrared Spectroscopy (FTIR). The considerable variation in transmittance and band change was observed for doped films in FTIR spectra.

Zero-Dimensional Lead-Free Halide with Indirect Optical Gap and Enhanced Photoluminescence by Sb Doping.

Three new lead-free organic-inorganic metal halides (OIMHs) (C7H8N3)3InX6·H2O (X = Cl, Br) and (C7H8N3)2SbBr5 were synthesized. First-principles calculations indicate that the highest occupied molecular orbitals (HOMOs) of the two In-based OIMHs are constituted of π orbitals from [C7H8N3]+ spacers. (C7H8N3)3InX6·H2O (X = Cl, Br) shows an indirect optical gap, which may result from this organic-contributed band edge. Despite the indirect-gap nature with extra phonon process during absorption, the photoluminescence of (C7H8N3)3InBr6·H2O can still be significantly enhanced through Sb doping, with the internal photoluminescence quantum yields (PLQY) increased 10-fold from 5% to 52%. A white light-emitting diode (WLED) was fabricated based on (C7H8N3)3InBr6·H2O:Sb3+, exhibiting a high color-rendering index of 90. Our work provides new systems to deeply understand the principles for organic spacer choice to obtain the 0D metal OIMHs with specific band structure and also the significant enhancement of luminescence performance by chemical doping.

Rashba Splitting in Two Dimensional Hybrid Perovskite Materials for High Efficient Solar and Heat Energy Harvesting.

The physical properties of two-dimensional (2D) lead halide based hybrid perovskites are quite exciting and challenging. Further, the role of organic cations in 2D perovskites is still in a debate. We investigated layered (CH3(CH2)3NH3)2(CH3NH3)Pb2I7 2D Ruddlesden-Popper (2DRP) phase (M1) and 2D derivative of CH3NH3PbI3 (M2) using density functional theory. The spin orbit coupling mediates the significantly large Rashba splitting energy of 328.5 meV for M2, which is higher than earlier 2D hybrid perovskites. At the picosecond time scale, the dynamical Rashba effect was observed due to organic and inorganic cation dynamics. Two step absorption suggests an indirect optical gap of 2.38 and 2.15 eV for M1 and M2, respectively and solar performance depicts excellent power conversion efficiency of 14.92% and 19.75% for M1 and M2, respectively. For the first time, we explored the thermoelectric properties of 2D hybrid perovskites and perceived high power factor for p-type doping in M2. Our findings suggest that these novel 2D perovskites have the potential to be used in solar and heat energy harvesting.

Influence of phase transformation on structure–property relationship in quaternary In10Sb10Ag10Se70 chalcogenide films

This work reports the effect of thermal annealing (353–413 K) on structure and physical properties of thermally evaporated In10Ag10Sb10Se70 chalcogenide films. The samples were characterized by X-ray diffraction, transmission electron microscopy, TEM, Raman spectroscopy, optical absorption and photoconductivity measurements. Thermal annealing favours amorphous to polycrystalline structure transition with the growth of Se, AgInSe2 and AgSbSe2 phases. A significant change in position/shape of Raman bands upon annealing has been observed. The indirect optical gap decreases from 1.44 to 0.68 eV while tailing parameter increases with annealing. Photosensitivity decreases from 29.3 to 14.2 with annealing temperature. The decay of photocurrents is well fitted to stretched exponential function; the value of decay time constant increases while local minimum value of dispersion parameter at 353 K has been observed. These results are explained on the basis of amorphous crystalline phase transformation in chalcogenides and are important for developing new materials for emerging technologies.

SYNTHESIS, CHARACTERIZATION, AND APPLICATION OF Fe3O4 NANOPARTICLES AS A SIGNAL AMPLIFIER ELEMENT IN SURFACE PLASMON RESONANCE BIOSENSING

The Fe3O4 nanoparticle has potential applications to be active materials for biosensor. This research was aimed at synthesizing and applying Fe3O4 nanoparticles for signal amplification of Surface Plasmon Resonance (SPR)-based biosensor. The synthesis was carried out by using the coprecipitation method at a temperature of 80oC for 60 minutes. NaOH of 5 molars was used as a precipitating agent. The Fe3O4 nanoparticles were characterized for their microstructure by using X-Ray Diffraction (XRD) and Transmission Electron Microscopy (TEM) techniques. The crystallinity of nanoparticles is 87% and the crystallite size is 15.70 nm. The shape of Fe3O4 is nearly spherical. The optical properties were characterized by using Fourier Transform Infrared Spectroscopy (FTIR) and Specular Reflection UV-Vis Spectroscopy (SR UV-Vis). There were vibrations of Fe3+-O and Fe2+-O bonds at characteristic frequencies of 586.86 cm-1 and 422.64 cm-1. The direct and indirect optical gap energies were 3.43 eV and 3.10 eV. The magnetic properties measured by Vibrating Sample Magnetometer (VSM) confirmed that Fe3O4 was ferrimagnetic. The Fe3O4 was successful to be applied as a signal amplifier for SPR-based biosensors due to their optical and magnetic properties. The SPR angle shifted to a higher value and the change was 3.57o.

Optical and structural properties of drop-cast PVA/PEG polyblends

In the present study, the drop cast films of polyvinyl alcohol (PVA), polyethylene glycol (PEG) and optimized PVA:PEG (50:50) polyblend were studied. The pure polymer and polyblends samples were characterised by using Scanning Electron Microscopy (SEM), Fourier Transform Infrared Spectroscopy (FTIR) and Optical Absorption Spectroscopic Techniques. Changes in the morphology of optimized PVA:PEG polyblend has been observed in SEM photographs. FTIR results have been observed to form H-bond between PVA and PEG which confirms the plasticizing effect of PEG on PVA. The absorption maxima is observed at 325 nm and 300 nm for the pure PEG and PVA samples, while it is found to shift to 320 nm for the polyblends. The shift in peak related to absorption edge may attribute to the change in polymeric chain mobility as well as crystallization behaviour during the blending process. The value of indirect optical gap is found to be 1.75 eV, which very less as compared to the pure components. An increase in the value of B−1 has been observed indicating an enhancement of disorder with the formation of polyblends. These results are very important for realizing their applications as optical components.

Optical and Thermoelectric Properties of Surface-Oxidation Sensitive Layered Zirconium Dichalcogenides ZrS2−xSex (x = 0, 1, 2) Crystals Grown by Chemical Vapor Transport

In this work, structure, optical, and thermoelectric properties of layered ZrS2−xSex single crystals with selenium composition of x = 0, 1, and 2 were examined. Single crystals of zirconium dichalcogenides layer compounds were grown by chemical vapor transport method using I2 as the transport agent. X-ray diffraction (XRD) and high-resolution transmission electron microscope (HRTEM) results indicated that ZrS2−xSex (x = 0, 1, and 2) were crystalized in hexagonal CdI2 structure with one-layer trigonal (1T) stacking type. X-ray photoelectron and energy dispersive X-ray measurements revealed oxidation sensitive behavior of the chalcogenides series. Transmittance and optical absorption showed an indirect optical gap of about 1.78 eV, 1.32 eV, and 1.12 eV for the ZrS2−xSex with x = 0, 1, and 2, respectively. From the result of thermoelectric experiment, ZrSe2 owns the highest figure-of merit (ZT) of ~0.085 among the surface-oxidized ZrS2−xSex series layer crystals at 300 K. The ZT values of the ZrS2−xSex (x = 0, 1, and 2) series also reveal increase with the increase of Se content owing to the increase of carrier concentration and mobility in the highly Se-incorporated zirconium dichalcogenides with surface states.

A COMPREHENSIVE STUDY ON THE MICROSTRUCTURE AND OPTICAL PROPERTIES OF Fe3O4 NANOPARTICLES BY VARIATION OF TEMPERATURE AND NaOH CONCENTRATION

This work was aimed at optimizing the microstructure and optical properties of Fe3O4 by variation of synthesis temperature and NaOH concentration. The nanoparticles have been synthesized from iron sand by using the coprecipitation method. The temperature was varied of 60oC, 80oC, and 100oC, while NaOH concentration was 3 molar, 5 molar, and 7 molars. The microstructural parameters were characterized by using X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) techniques. The optical properties were characterized by using Fourier Transform Infrared Spectroscopy (FTIR) and UV-Vis Spectroscopy, consecutively. The results showed that the crystallite size of Fe3O4 increase with an increase of synthesis temperature due to higher thermal energy driving the nucleation process. The crystallite sizes are in the range of 3.77 nm – 20.37 nm. Increasing NaOH concentration also affected the increase in crystallite sizes. On the other hand, an excess of NaOH concentration influences the formation of smaller crystallite sizes of Fe3O4 with smaller crystal density and larger microstrain. The existence of Fe2+-O and Fe3+-O vibration in FTIR spectra confirmed the formation of Fe3O4 nanoparticles. The direct and indirect optical gap energies of Fe3O4 were in the range of 3.15 eV – 3.39 eV and 2.39 eV – 2.91eV. The synthesis temperature and NaOH concentration have a significant role in controlling the properties of Fe3O4.

Narrow optical gap ferroelectric Bi2ZnTiO6 thin films deposited by RF sputtering

Thriving ferroelectric oxide Bi2ZnTiO6 thin films with a 1.48 eV optical gap.

Relative energies and electronic structures of CoO polymorphs through ab initio diffusion quantum Monte Carlo

We present a many-body diffusion quantum Monte Carlo (DMC) study on the ground- and excited state properties of crystalline CoO polymorphs. To our knowledge, DMC is the only electronic structure method available to provide correct energetic ordering within experimental error bars between the three CoO polymorphs: rocksalt, wurtzite, and zinc blende. We compare these results to density functional theory (DFT) using state-of-the-art functionals such as SCAN. For the structural properties, such as the lattice parameters and bulk moduli, our results are comparable to HSE and SCAN. Using DMC, we calculated the indirect and direct optical gaps as 3.8(2) and 5.2(2) eV. Our indirect optical gap compares well with the conductivity measurements of 3.6(5) eV and $GW$ calculations with 3.4 eV. Similarly, we obtained the DMC indirect and direct quasiparticle gaps as 3.9(2) and 5.5(2) eV. DMC direct quasiparticle gaps compare well with the direct band gap of 5.53 eV obtained from ellipsometry studies.

Structural, optical and photoelectrical properties of thermally annealed amorphous In15Sb15Se70 chalcogenide films

In this work, the effect of annealing on the structure and photoelectrical properties of In15Sb15Se70 films has been investigated. The annealed films (333–453 K) were characterized by XRD, SEM, TEM, optical absorption, Raman spectroscopy and photoconductivity measurements. The amorphous/crystalline structure of as-prepared and annealed samples was confirmed by XRD and SEM/TEM studies. The crystallization of Se, Sb2Se3 and In4Se3 phases has been observed upon annealing of the film samples. The position and shape of Raman bands have been significantly altered with annealing temperature. The indirect optical gap varies from 1.53 to 1.08 eV which is very useful for solar cell applications. DC activation energy and photosensitivity were found to decrease with annealing temperature. The decay of photocurrents was well fitted to stretched exponential function and the minimum value for fit parameters for the sample annealed at 373 K has revealed least defect states in mobility gap for In15Sb15Se70 films.

Temperature dependence of direct and indirect band gaps of Bi13I2S18 hexagonal rod crystals

Single crystals of Bi13I2S18 have been grown by chemical vapor transport method. The band-edge properties of Bi13I2S18 are evaluated using temperature-dependent optical absorption and thermal-modulated reflectance (TR) measurements in the temperature range between 30 and 300 K. Single-crystal X-ray diffraction of different orientations showed structural anisotropy and identified the crystalline phase of the as-grown Bi13I2S18 hexagonal micro pillars. According to transmission and TR measurements, the Bi13I2S18 hexagonal rod is confirmed to be an indirect semiconductor that simultaneously possesses indirect (Egind at ∼0.73 eV) and direct (Egd at ∼1.06 eV) optical gaps. The averaged value of direct and indirect gaps is close to 1 eV that may efficiently absorb the sunlight spectrum for solar energy application. Temperature-energy variation and line-width broadening behavior (TR) of the band-edge transition of Bi13I2S18 are also analyzed and discussed herein.

Illumination effect on the structural and optical properties of nano meso nickel (II) tetraphenyl-21H, 23H-porphyrin films induces new two hours photo bleached optical sensor

250nm thickness of meso nickel (II) tetraphenyl-21H, 23H-porphyrin, NiTPP films were prepared by thermal evaporation technique on a quartz substrate. The prepared films were illuminated for 10, 20, 30, 60, 120, 240, 360 and 420min. Films of 120 and 240min of illumination time had been affected while others had no response. The as prepared meso NiTPP films are amorphous and the illuminated one are crystalline with amorphous background included within nano scale. The illuminated films were accompanied by photo bleaching effect. Two gaps could be distinguished; indirect optical gap,Egiopt and the fundamental energy gap, Eg. Optical parameters; refractive index, absorption coefficient, single-oscillator energy, dispersion energy, Lattice dielectric constant, the ratio of the carrier concentration to the effective mass, optical conductivity and energy loss function were studied. All mentioned parameters were affected by the illumination time. The response of nano meso NiTPP films to the mention illumination time has pulse shape, so it is recommended to be used as optical sensor.

Evidence of indirect gap in monolayer WSe2

Monolayer transition metal dichalcogenides, such as MoS2 and WSe2, have been known as direct gap semiconductors and emerged as new optically active materials for novel device applications. Here we reexamine their direct gap properties by investigating the strain effects on the photoluminescence of monolayer MoS2 and WSe2. Instead of applying stress, we investigate the strain effects by imaging the direct exciton populations in monolayer WSe2–MoS2 and MoSe2–WSe2 lateral heterojunctions with inherent strain inhomogeneity. We find that unstrained monolayer WSe2 is actually an indirect gap material, as manifested in the observed photoluminescence intensity–energy correlation, from which the difference between the direct and indirect optical gaps can be extracted by analyzing the exciton thermal populations. Our findings combined with the estimated exciton binding energy further indicate that monolayer WSe2 exhibits an indirect quasiparticle gap, which has to be reconsidered in further studies for its fundamental properties and device applications.

Effect of excess oxygen for CuFeO2.06 delafossite on thermoelectric and optical properties

This work presents the role of excess oxygen in CuFeO2.06 compounds on thermoelectric and optical properties. The CuFeO2.06 specimens were synthesized by solid state reaction method. X-ray diffraction technique has confirmed the CuFeO2 structure for the specimens. In particularly, CuFeO2.06 specimen revealed the structural extension of lattice parameter: a and c. Also, the specimen found increasing excess oxygen of approximately 3% as a resulted enhancement of mixed valence state of Cu+ and Cu2+ ions. XPS showed mixed valence state of the Cu+/Cu2+ ions, and Fe3+ and Fe2+ ions was also found in the CuFeO2.06 specimen. Mixed valence states contributed the co-existence of hole and electron carriers for conduction. Consequently, electrical conductivity of the CuFeO2.06 specimen increased up to 23S/cm at 873K. Also, increasing Seebeck coefficient was shown to be approximately 302μV/K at 873K. The CuFeO2.06 specimen was found power factor to be approximately 2.1 × 10−4W/m∙K2 at 873K. The indirect optical gap of CuFeO2.06 (2.40 eV) was lower than that of the CuFeO2 (2.60 eV). Thus, thermoelectric and optical properties were governed by an existence of excess oxygen.

Effect of composition on steady state and transient photoconductivity in isocoordinated InxSb30−xSe70 (0 ≤ x ≤ 25) chalcogenide films

The study of steady state and transient photocurrent measurement provide important information about carrier generation and recombination phenomena in various semiconducting systems for photo-sensor device applications. In the present work, the composition dependent analysis of photocurrents was studied for thermally evaporated Se-rich InxSb30−xSe70 films of average thickness 800 nm. The indirect optical gap has been calculated from the transmission and reflection data and the variation of molecular units was studied from the Raman spectroscopy. The initial rise of photocurrent sharply to approach a steady state value during illumination and fast decay to a constant persistent current after stopping the illumination has been observed. The intensity dependence of photocurrent obeys the power law IPh = Fγ, where the value of exponent tells about the recombination process. The decay of photocurrent has been fitted with stretched exponential function for different compositions and at different light intensities. These results are important for the development of low cost photo absorbers for solar cell applications and visible region responsive photo sensor devices.

Local Strain Induced Band Gap Modulation and Photoluminescence Enhancement of Multilayer Transition Metal Dichalcogenides

The photocarrier relaxation between direct and indirect band gaps along the high symmetry K−Γ line in the Brillion zone reveals interesting electronic properties of the transition metal dichalcogenides (TMDs) multilayer films. In this study, we reported on the local strain engineering and tuning of an electronic band structure of TMDs multilayer films along the K−Γ line by artificially creating one-dimensional wrinkle structures. Significant photoluminescence (PL) intensity enhancement in conjunction with continuously tuned optical energy gaps was recorded at the high strain regions. A direct optical band gap along K–K points and an indirect optical gap along Γ–K points measured from the PL spectra of multilayer samples monotonically decreased as the strain increased, while the indirect band gap along Λ–Γ was unaffected owing to the same level of local strain in the range of 0%–2%. The experimental results of band gap tuning were in agreement with the density functional theory calculation results. Local s...