Wide Optical Band Gap Research Articles

High-Temperature and Large-Polarization Ferroelectric with Second Harmonic Generation Response in a Novel Crown Ether Clathrate.

Molecular ferroelectrics of high-temperature reversible phase transitions are very rare and have attracted increasing attention in recent years. In this paper is described the successful synthesis of a novel high-temperature host-guest inclusion ferroelectric: [(C6 H5 NF3 )(18-crown-6)][BF4 ] (1) that shows a pair of reversible peaks at 348 K (heating) and 331 K (cooling) with a heat hysteresis about 17 K by differential scanning calorimetry measurements, thus indicating that 1 undergoes a reversible structural phase transition. Variable-temperature PXRD and temperature-dependent dielectric measurements further prove the phase-transition behavior of 1. The second harmonic response demonstrates that 1 belongs to a non-centrosymmetric space group at room temperature and is a good nonlinear optical material. In its semiconducting properties, 1 shows a wide optical band gap of about 4.43 eV that comes chiefly from the C, H and O atoms of the crystals. In particular, the ferroelectric measurements of 1 exhibit a typical polarization-electric hysteresis loop with a large spontaneous polarization (Ps ) of about 4.06 μC/cm2 . This finding offers an alternative pathway for designing new ferroelectric-dielectric and nonlinear optical materials and related physical properties in organic-inorganic and other hybrid crystals.

Covalent Triazine Framework as an Efficient Photocatalyst for Regeneration of NAD(P)H and Selective Oxidation of Organic Sulfide.

Covalent triazine frameworks (CTFs), belonging to the super-family of covalent organic frameworks, have attracted significant attention as a new type of photosensitizer due to the superb light-harvesting ability and efficient charge transfer originating from the large surface area. However, the wide optical band gap in CTFs, which is larger than 3.0 eV, hinders the efficient light harvesting in the visible range. To overcome this limitation, we developed the new type CTFs photocatalyst based on the donor-acceptor conjugation scheme by using melamine (M) and 2,6-diaminoanthraquinone (AQ) as monomeric units. The melamine-2,6-diaminoanthraquinone-based covalent triazine frameworks (M-AQ-CTFs) photocatalyst shows the excellent light-harvesting capacity with high molar extinction coefficient, and the suitable optical band gap involving the internal charge transfer character. Combination of M-AQ-CTFs and artificial photosynthetic system including the organometallic rhodium complex, acting as an electron mediator, exhibited the excellent photocatalytic efficiency for the regeneration of the nicotinamide cofactors such as NAD(P)H. In addition, this photocatalyst showed the high photocatalytic efficiency for the metal-free aerobic oxidation of sulfide. This study demonstrates the high potential of CTFs photocatalyst with the donor-acceptor conjugated scheme can be actively used for artificial photosynthesis.

Frequency and temperature dependent electrical characteristics of P-doped nc-SiOX:H thin films

Phosphorus-doped nc-SiOX:H films with elevated dark-conductivity and wide optical band gap are prepared via one-step low-temperature (~ 250 °C) processing, by optimizing H2-dilution [RH = H2/(He + H2)] of the (SiH4 + CO2 + PH3) precursor gasses in RF plasma-CVD. The band tail defect dynamics in covalently bonded disordered oxide-semiconductor system has been extensively studied via frequency and temperature dependent conductivity-response, impedance-characteristics, and dielectric-properties, in doped-silicon-oxygen and amorphous-nanocrystalline complex system. The temperature dependent ac-conductivity obeys the Universal Jonscher power-law and the behaviour of extracted parameters from nonlinear-fitting identifies non-overlapping small-polaron-tunnelling (NSPT) model to prevail in ac-conduction mechanism. Modelling of the impedance spectra over wide frequency range via equivalent electric circuit consisting of two parallel R||CPE connected serially, confirms individual contribution of separate nc-Si and a-SiOX:H moieties. The charge carrier density-of-states, N(EF), participating in ac-conduction at ~ 373 K and ~ 104 Hz increases from ~ 7.6 × 1015 eV−1cm−3 to ~ 1.4 ×1017 eV−1cm−3 with RH elevated from 0.1 to 0.4, corresponding to increased O-content simultaneous to reduced H-content in the Si-network possessing sequentially higher-crystallinity and wider optical band gap.

A Donor Polymer with a Good Compromise between Efficiency and Sustainability for Organic Solar Cells

Impressive advancements in solution‐processed bulk heterojunction (BHJ) solar cells have been driven to a large extent by the rational design of conjugated polymers as photoactive donors. These achievements have been obtained without paying much attention to green metrics parameters (such as E factor) and to the increasing synthetic complexity (SC), which is a bottleneck for industrial scalability. In this context, a novel donor copolymer (PBDT3T) based on benzodithiophene and terthiophene building blocks with ester functionalities is synthesized. The 26‐fold‐reduced E factor for the benzodithiophene monomer synthesis and the SC index of 24 for PBDT3T, much lower compared with benchmark donor polymers, provide a hint for good scalability, sustainability, and low costs. PBDT3T features a relatively wide optical bandgap and a deep highest occupied molecular orbital (HOMO), meeting the requirements for suitable donor material in both fullerene and nonfullerene‐based solar cells. PBDT3T is studied in binary and ternary blend, using PC71BM and ITIC as acceptors. The best performances are obtained in the ternary blend devices, reaching power conversion efficiency (PCE) values of 7.14%. Trade‐off considerations between PCE and SC make PBDT3T promising on an industrial perspective.

Influence of Cu2+ and Sn2+ ions on optical properties of MgS Nanoparticles: A comparative Analysis

Cu2+ and Sn2+ ions doped MgS nanoparticles were synthesized by chemical route without the aid of any capping agent. The role of Cu2+ and Sn2+ ions on the structural, optical, photoluminescence properties of MgS nanoparticles have been investigated through various analyses like XRD, TEM, EDX UV- vis absorption, transmittance studies, PL spectra and FT-IR. The average particle size was calculated as ∼ 10 nm and it was varied based on the ionic radius of the dopant. From this comparative study, it was observed that Cu2+ (2%) doped MgS showed a blue shift in Absorbance spectra, wide optical band gap and also exhibited better photoluminescence property. A strong PL emission was received in UV region and in the Red region. This combination of materials can be utilized in solar panel manufacturing as buffer material, optoelectronic devices and in red-emitting luminescent displays.

Self-Assembled Organic Cations-Assisted Band-Edge Tailoring in Bismuth-Based Perovskites for Enhanced Visible Light Absorption and Photoconductivity.

Bismuth-based zero-dimensional perovskites garner high research interest because of their advantages, such as excellent moisture stability and lower toxicity in comparison to lead-based congeners. However, the wide optical bandgap (>2 eV) and poor photoconductivity of these materials are the bottlenecks for their optoelectronic applications. Herein, we report a combined experimental and theoretical study of the structural features and optoelectronic properties of two novel and stable zero-dimensional bismuth perovskites: (biphenyl bis(methylammonium))1.5BiI6·2H2O (BPBI) and (naphthalene diimide bis(ethylammonium))1.5BiI6·2H2O (NDBI). NDBI features a remarkably narrower bandgap (1.82 eV) than BPBI (2.06 eV) because of the significant orbital contribution of self-assembled naphthalene diimide cations at the band edges of NDBI. Further, the FP-TRMC analysis revealed that the photoconductivity of NDBI is about 3.7-fold greater than that of BPBI. DFT calculations showed that the enhanced photoconductivity in NDBI arises from its type-IIa band alignment, whereas type-Ib alignment was seen in BPBI.

Role of electron carrier selective contact layer of lithium fluoride films with wide bandgap and low work function for silicon heterojunction solar cells

Wide bandgap with low work function-based materials are widely used as an electron transport layer to achieve high efficiency in silicon heterojunction (SHJ) solar cells. We presented transparent Lithium fluoride (LiFx) films with a wide optical bandgap and various thicknesses for the applications of SHJ solar cells. The high-resolution transmission electron microscopic (HR-TEM) analysis was used to confirm the thickness of LiFx layers. The aluminium electrode's work-function was reduced because of the high dipole moment by the LiFx and lowers an energy barrier at back contact. Enhancement of quantum efficiency (QE) at longer wavelength regions and operating voltage was due to lower energy barrier height can improve the short-circuit current density (Jsc), open-circuit voltage (Voc), and hence solar cell efficiency. Thermally evaporated-based LiFx films showed an optical transmittance of 88 ~ 90% in the visible-NIR (380–1100) nm wavelength region. LiFx/a-Si:H(i)/c-Si/a-Si:H(i)/LiFx structure for 20 nm thick LiFx showed lifetime of 1129.73 μs and implied iVoc of 731.7 mV. SHJ solar cells were fabricated using various thicknesses 1, 10 and 20 nm, respectively. The SHJ solar cells fabricated on 20 nm thick LiFx film yield the maximum efficiency of 17.16% and was used for the baseline condition of simulations. The simulated performance of the SHJ solar cell was improved from 0.098 to 22.58% as a function of variable LiFx work function from 3.2 to 3.9 eV.

Electrical and Optical Studies of Reactively Sputtered Indium Oxide Thin Films

Electrical and optical properties of transparent conducting Indium Oxide (In2O3) thin films were studied. These films were deposited by reactive RF sputtering process using Indium target in the presence of oxygen as reactive gas with argon. The pressure and the oxygen gas flow ratios during sputtering were varied to investigate their effect on the resistivity, optical transmission, refractive index, and optical bandgap of In2O3 thin films. The lowest reported resistivity in this study is 2 × 10−3 Ω-cm. The optical transmission was more than 80% in the wavelength range of 400 nm to 800 nm. The optical study revealed the wide optical bandgap of 3.712 eV and 3.875 eV of the deposited In2O3 films. The films showed refractive indices in the range of 1.99 to 2.22. The elemental composition of In2O3 films has been studied through X-ray Photoelectron Spectroscopy (XPS).

Investigation of Electrical and Optical Properties of Low Resistivity Indium Oxide Thin Films

Indium oxide (In2O3) films were deposited by reactive sputtering using an indium target in the presence of oxygen and argon as sputtering gases. The electrical and optical properties of In2O3 films specifically sheet resistance, resistivity, optical transparency, refractive index, and optical bandgap were investigated as a function of oxygen gas flow ratio, keeping the substrate at room temperature. The lowest reported resistivity in this study is 2 x 10-3 Ω-cm. The optical transmission was more than 80% in the wavelengths of 400 nm to 800 nm. The optical study revealed the wide optical bandgap of 3.875 eV of the deposited In2O3 films. The films showed refractive indices in the range of 1.99 to 2.22. The elemental composition of In2O3 films has been studied through X-ray Photoelectron Spectroscopy (XPS).

Alloy Engineering of a Polar (Si,Ge)2N2O System for Controllable Second Harmonic Performance.

Although silicon oxynitrides are important semiconductors for many practical applications, their second-order nonlinear optical (NLO) properties have never been systemically explored. Using the first-principles calculations, in this article, we discover that the sinoite (e.g., silicon oxynitride Si2N2O) can simultaneously exhibit wide optical band gap, strong second-harmonic generation (SHG) effect, and large birefringence, which are further confirmed by our preliminary experimental measurement. Importantly, we propose that alloying engineering can be further applied to control the balanced NLO properties in the Si2N2O system. Based on first-principles cluster expansion theory, we demonstrate that alloying Ge into Si2N2O can form low formation-energy Si2(1-x)Ge2xN2O alloys, which can in turn achieve controllable phase-matching harmonic output with high SHG efficiency at different energy ranges. Therefore, alloy engineering could provide a unique approach to effectively control the NLO performance of Si2(1-x)Ge2xN2O, making this polar alloy system hold potential applications in tunable laser conversion and controllable optical devices.

Effect of Ca2+/Hf4+ modification at A/B sites on energy-storage density of Bi0.47Na0.47Ba0.06TiO3 ceramics

• By incorporating CaHfO 3 , the E b is greatly enhanced from 120 kV/cm for BNBT to 280 kV/cm for BNBT-12CH. • BNBT-12CH ceramic achieves a large recoverable energy-storage density of 4.2 J/cm 3 . • The η and W rec of BNBT-12CH ceramic gradually improve with increasing temperature from 25 °C to 140 °C. Pb-free relaxor ferroelectric materials are attractive because of their fast charge–discharge capability and environmental safety. Here, a series of new-type Pb-free ferroelectric ceramics are designed and fabricated: (1 - x )Bi 0.47 Na 0.47 Ba 0.06 TiO 3 - x CaHfO 3 (abbreviated as BNBT- 100x CH). Through the modification of Ca 2+ /Hf 4+ at A/B sites, a wider optical band gap and submicron grains are obtained, and the polar nano regions are induced in the structure. Therefore, an optimal W rec of 4.2 J/cm 3 is obtained for the BNBT-12CH ceramic under 280 kV/cm. In addition, the BNBT-12CH ceramic displays better energy-storage characteristics at high temperatures within the test range of 25–140 °C. All of these features indicate that the modification of BNT-based ceramics by Ca 2+ /Hf 4+ has a significant effect on energy-storage density. This work also offers a novel guidance for the development and application of Hf element.

Photoluminescent and semiconductive properties of a novel praseodymium compound

A praseodymium complex, [Pr(IA)2(H2O)4] nnCl (1) (HIA = isonicotinic acid), has been prepared via a solvothermal reaction and characterized by single crystal X-ray diffraction. The praseodymium ion is coordinated by eight oxygen atoms to form a slightly distorted square antiprismatic geometry. Complex 1 is characteristic of a one-dimensional (1 D) chain-like structure. Solid-state photoluminescence spectra reveal that complex 1 has an emission at 660 nm of the red region. This emission is probably ascribed to the characteristic emission 3 P 0→3 F 2 transition of the praseodymium ions. Compound 1 has CIE (Commission Internationale de I'Éclairage) chromaticity coordinates of (0.5328, 0.4653). The CCT (Correlated Color Temperature) is 2180 K for compound 1. Solid-state UV/vis diffuse reflectance spectrum reveals that complex 1 possesses a wide optical band gap of 5.08 eV.

In situ preparation and photophysical properties of a novel europium complex

Under hydrothermal conditions, a novel europium complex [Eu(H2O)2 (Me-IN)3]n·2nH2O·3nClO4 (1; Me ​= ​methyl; IN ​= ​isonicotinic acid) was synthesized in situ and characterized. Complex 1 crystallizes in the monoclinic space group P21/c with four formula units in one cell. Complex 1 is characterized by a one-dimensional (1-D) chain-like structure. The solid state photoluminescence emission is in the red region and the emission peaks should come from the 5D0 → 7F1, 5D0 → 7F2 and 5D0 → 7F4 characteristic emission of the Eu(III) ions. It has a CIE (Commission Internationale de L’Eclairage) chromaticity coordinates of (0.5442, 0.4444). A solid-state UV–visible diffuse reflectance spectrum unveiled that it possesses a wide optical band gap of 4.98 ​eV.

Polymer Thermal Treatment Production of Cerium Doped Willemite Nanoparticles: An Analysis of Structure, Energy Band Gap and Luminescence Properties.

The contemporary market needs for enhanced solid–state lighting devices has led to an increased demand for the production of willemite based phosphors using low-cost techniques. In this study, Ce3+ doped willemite nanoparticles were fabricated using polymer thermal treatment method. The special effects of the calcination temperatures and the dopant concentration on the structural and optical properties of the material were thoroughly studied. The XRD analysis of the samples treated at 900 °C revealed the development and or materialization of the willemite phase. The increase in the dopant concentration causes an expansion of the lattice owing to the replacement of larger Ce3+ ions for smaller Zn2+ ions. Based on the FESEM and TEM micrographs, the nanoparticles size increases with the increase in the cerium ions. The mean particles sizes were estimated to be 23.61 nm at 1 mol% to 34.02 nm at 5 mol% of the cerium dopant. The optical band gap energy of the doped samples formed at 900 °C decreased precisely by 0.21 eV (i.e., 5.21 to 5.00 eV). The PL analysis of the doped samples exhibits a strong emission at 400 nm which is ascribed to the transition of an electron from localized Ce2f state to the valence band of O2p. The energy level of the Ce3+ ions affects the willemite crystal lattice, thus causing a decrease in the intensity of the green emission at 530 nm and the blue emission at 485 nm. The wide optical band gap energy of the willemite produced is expected to pave the way for exciting innovations in solid–state lighting applications.

Dominance of c-axis orientation on the carrier transport properties of Sn doped ZnO thin films

Sol-gel derived pure and Tin (Sn) doped ZnO thin films were deposited on the glass substrate by cost-effective spin-coating method. The influence of Sn incorporation on the structural, morphological, optical, and electrical properties of ZnO films was investigated by varying Sn concentration. X-ray diffraction reveals the hexagonal crystal structure with a preferential orientation of the crystallites along c-axis in ZnO film after doping with Sn. The transparency of ZnO thin film in the visible region is increased significantly from 72% to 93% after Sn incorporation into the ZnO matrix. The grain size of the film decreased from 27 nm to 20 nm after doping with Sn. The widening of optical band gap from 3.23 to 3.29 eV i.e., a blue shift of absorption edge with Sn doping is attributed to a combination of the Burstein-Moss effect and electron-impurity scattering. Urbach energy values have shown that Sn dopant decreases the width of the band tail of localized states. The electrical properties of the films revealed the decrement of the conductivity in the doped film. The decrement in the conductivity upon Sn doping is explained considering the grain-boundary conduction model and piezoelectric scattering mechanism. The Figure of Merit was calculated to assess the efficiency of the prepared film and was found that the film Sn: ZnO (3 at.%) shows the highest value. The obtained results confirm that Sn dopant has considerable effects on the properties of ZnO thin films and can be used as promising transparent conducting oxide films for optoelectronic applications.

Synthesis and photoluminescence of organotin-dithiothreitol clusters

Three novel organotin oxysulfide hybrid clusters (BuSn)4(DTT)4M (M ​= ​Sn4+ (1), Ti4+ (2), Hf4+ (3); DTT ​= ​1,4-dithiothreitol) were prepared by hydrothermal methods and their crystal structures were determined by single-crystal X-ray diffraction. These clusters are isostructural, pentanuclear complexes containing four peripheral butyltin fragments, and a central metal site of either tin, titanium or hafnium. Solid state variable temperature fluorescence spectra showed that the position and the intensity of key emission peaks of the three isomorphic compounds are significantly different. Compound 1 gave bimodal emission at 420 and 445 ​nm upon excitation at 360 ​nm, and its peak positions remained unchanged. Compounds 2 and 3 were excited by 365 and 460 ​nm radiation, respectively, and their maximum emission peaks were red-shifted in different degrees with decreasing temperature. All their peak intensity increased with decreasing temperature. Solid-state UV/visible diffuse reflectance spectra displayed that these three isostructural compounds have wide optical band gaps of 3.72, 2.68, and 3.19 ​eV, respectively.

Bifunctional CO2 Plasma Treatment at the i/p Interface Enhancing the Performance of Planar Silicon Heterojunction Solar Cells

Hydrogenated nanocrystalline silicon (nc‐Si:H) or hydrogenated nanocrystalline silicon oxide (nc‐SiOx:H) as window layers have ample potential to improve the short‐circuit current density (JSC) of silicon heterojunction (SHJ) solar cells due to their wide optical bandgap. However, the growth of their nanocrystals within a 20 nm‐thickness on intrinsic hydrogenated amorphous silicon (i‐a‐Si:H) poses a challenge. Plasma treatment (PT) after i‐a‐Si:H deposition is usually used to improve passivation performance. Herein, the i/p interface is subjected to CO2 PT: a 20 s post CO2 PT is proven to be beneficial in obtaining improved passivation performance. This can be achieved by activated hydrogen which can be diffused to the wafer surface and can saturate the dangling bonds on it. Extending the CO2 PT time from 20 to 40 s, the oxygen (O) insertion into i‐a‐Si:H starts to increase slowly, which leads to marginal change of microstructure in the p layer. Strained SiSi bonds by oxygen incorporation into the a‐Si:H network generate nucleation sites and consequently accelerate the nucleation of p‐nc‐Si:H on i‐a‐Si:H. Finally, it is demonstrated that the power conversion efficiency (PCE) of planar SHJ solar cells is enhanced from 12.65% to 19.06% by a 20 s CO2 PT at the i/p interface.

Optimization of transparent amorphous oxide-metal-amorphous oxide multilayer with high figure of merit for high transmittance and low resistivity

Highly transparent amorphous oxide-metal-amorphous oxide (OMO) multilayers were fabricated at room temperature using amorphous semiconducting Si–Zn–Sn–O (SZTO) metal oxide and metallic silver. Amorphous SZTO (a-SZTO) showed high refractive index of 2.04 which is comparable to crystalline ITO, and wide optical bandgap (E g ) of 3.26 eV, allowing transmitting of wavelength above 380 nm. SZTO based OMO showed low resistivity at the level of 10 −5 Ω cm by inserting an Ag layer. After optimizing the optical and electrical properties by changing Ag and SZTO sublayer thickness, a transparent conductive electrode with high average transmittance of 96.5% in visible region and low resistivity of 7.158 × 10 −5 Ω cm was fabricated at room temperature. The results show that SZTO is a promising sublayer for high performance transparent conducting oxides applications. • SZTO OMO multilayer is a transparent conductive electrode having high transmittance and low resistivity. • It was calculated that the optical bandgap has a wide bandgap. • The optical and electrical properties were optimized by changing the Ag and SZTO sub-layer thickness.

Preparation and characterization of a gadolinium compound with high thermal stability

A new lanthanide compound [Gd(2,5-HPA)(2,5-PA)]n (1; 2,5-H2PA = 2,5-pyridinedicarboxylic acid) was obtained through hydrothermal reactions and structurally characterized by single-crystal X-ray diffraction. It possesses a three-dimensional (3-D) framework structure. Solid-state photoluminescence experiment revealed that it shows dark blue emission band, which can be assigned to the characteristic emission of the 4f electron intrashell transition of 6P7/2 → 8S7/2 (Gd3+). The energy transfer mechanism was explained by an energy level diagram of the gadolinium ion and 2,5-pyridinedicarboxylic acid ligand. It displayed remarkable CIE chromaticity coordinates of 0.1346 and 0.0678. The solid-state UV/Vis diffuse reflectance spectra unveiled that it possesses a wide optical band gap of 3.52 eV. Thermogravimetry (TG) measurements revealed that this compound is highly thermal stable up to around 500 °C.
 
 KEY WORDS: CIE, Energy transfer, Lanthanide, Photoluminescence, Thermal stability
 
 Bull. Chem. Soc. Ethiop. 2020, 34(3), 479-488.
 DOI: https://dx.doi.org/10.4314/bcse.v34i3.5

Mg(OH)2 nanostructures using impure brine: optimization of synthesis parameters by Taguchi robust design and study of optical properties

In this research, high purity magnesium hydroxide (Mg(OH)2) nanoparticles were successfully synthesized by the wet chemical precipitation method from an impure precursor. The effective parameters of the synthesis to achieve magnesium hydroxide nanoparticles with smaller crystallite sizes such as the precipitator agent concentration, the reaction temperature, the amount of surfactant, and the suspension aging time were studied. Optimum conditions were determined by using the Taguchi design method. Each of these factors was investigated in three levels. The analysis of variance (ANOVA) was exploited to determine the contribution of various process parameters on the response. Statistical analysis results indicate that among investigated parameters, the reaction temperature, as well as the suspension aging time, has the most effect on the magnesium hydroxide nanoparticles’ crystallite size. Additionally, there is reasonable accordance between the predicted crystallite average size (17.14 ± 3.1 nm) and the obtained crystallite average size in optimum conditions (19.22 nm). Structural and phase investigations, the chemical composition, as well as optical and morphological properties of synthesized nanoparticles, were precisely investigated through XRD, EDS, FESEM, TEM, FTIR, and UV–Vis. The synthesized sample in optimum conditions had a wide optical band gap due to the smallest crystallite size.