Optical Absorption Band Research Articles

Layer‐Dependent Electronic and Optical Properties of 2D Black Phosphorus: Fundamentals and Engineering

AbstractIn 2D materials, the quantum confinement and van der Waals‐type interlayer interactions largely govern the fundamental electronic and optical properties, and the dielectric screening plays a dominant role in the excitonic properties. This suggests strongly layer‐dependent properties and a central topic is to characterize and control the interlayer interactions in 2D materials and heterostructures. Black phosphorus is an emerging 2D semiconductor with unusually strong interlayer interactions and widely tunable direct bandgaps from the monolayer to the bulk, offering an ideal platform to probe the layer‐dependent properties and the crossover from 2D to 3D (i.e., the scaling effects). In this review, a comprehensive and thorough summary of the fundamental physical properties of black phosphorus is presented, with a special focus on the layer‐dependence character, including the electronic band structures, optical absorption and photoluminescence, and excitonic properties, as well as the band structure engineering by means of electrical gating, strain, and electrochemical intercalation. Finally, an outlook is given for the future research.

Organic cation rotation in HC(NH2)2PbI3 perovskite solar cells: DFT & DOE approach

In the past few years, hybrid organic–inorganic perovskites have become one of the most interesting and efficient materials for photovoltaics and optoelectronics applications. Here, we report the key role of organic cation rotation in the formamidinium lead iodide HC(NH2)2PbI3 perovskites (FAPbI3) in different rotational modes and angles. Using density functional theory (DFT) calculations, we show the significant effect of rotation on overall electronic and optical properties of FAPbI3 such as band structure, DOS, PDOS, electron density, and optical absorption spectra. Moreover, design of experiment (DOE) methodology was applied to generate a program to simultaneously optimize five distinct factors of solar cell with minimum number of calculations and high accuracy. This approach yields a detailed and remarkable insight from the interactions of mutual effective factors and how they affect the final Voc, Jsc, FF, and PCE% of the FAPbI3 solar cell. Solar cells based on this architecture exhibit power-conversion efficiency more than 20% when measured under the illumination of AM1.5G, which makes it some of the high-performing perovskite-based solar cells.

Effect of alkylthiolated hetero-aromatic rings on the photovoltaic performance of benzodithiophene-based polymer/fullerene solar cells

Three new donor–acceptor (D–A) copolymers based on benzodithiophene (BDT) with alkylthiolated aromatic rings furan, thiophene, and benzene and 5,6-difluoro-4,7-bis(4-(2-octyldodecyl)thiophen-2-yl)benzo[c][1,2,5]thiadiazole (DTBT) units, namely PBDTFS-DTBT, PBDTTS-DTBT, and PBDTPS-DTBT, respectively, were synthesized and applied in solution-processed fullerene type organic solar cells (OSCs). The target polymers were systematically investigated by thermogravimetric analysis (TGA), ultraviolet-visible (UV–vis) absorption spectra, cyclic voltammetry (CV), density functional theory (DFT) and photovoltaic measurements. The experimental results imply that the electron push-pull effect and torsion angles between the conjugated side hetero-aromatic rings and BDT core play a key role in tuning the optical absorption bands, molecular energy levels, hole mobilities, and morphology of the corresponding polymers. The OSCs based on PBDTTS-DTBT and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) show the highest power conversion efficiency (PCE) of 7.80% among the three polymeric analogues, with an open-circuit voltage (Voc) of 0.92 V, a short-circuit current density (Jsc) of 11.95 mA cm−2 and fill factor (FF) of 70.76%. Hence, the results in this work indicate that the photovoltaic properties of the BDT-BT-based polymers can be effectively modulated by introducing conjugated alkylthiolated hetero-aromatic rings with the varied electron donating ability and steric hindrance.

Photovoltaic Performance of Thin-Film CdTe for Solar Cell Applications

In the current investigation, we report a theoretical study to acquire the highest feasible efficiency of cadmium telluride (CdTe) thin-films. It is well recognized that CdTe crystallizes in cubic zinc-blende structure and its direct band gap of 1.5 eV turned it out as a potential candidate for photovoltaic (PV) applications. Our calculations are founded on Shockley-Queisser (SQ) limit to simulate the open-circuit voltage, current density, and filling factor versus the variation of photon energy up to 4.0 eV. These key parameters of SQ change with the variation of energy between 0.3 to 3.5 eV. This is owing to the strong optical absorption (> 104 cm−1) and direct band gap of 1.5 eV, which make CdTe thin-film suitable for single junction solar cell and ideal for PV applications. It is observed that the optical absorption enhances as the thickness of the absorbed layer increases. This will effectively provide a theoretical support to the industry of global solar energy that is anticipated to be sustainable in the future.

Transformations Among Colloidal Semiconductor Magic-Size Clusters.

A knowledge of colloidal semiconductor magic-size clusters (MSCs) is essential for understanding how fundamental properties evolve during transformations from individual molecules to semiconductor quantum dots (QDs). Compared to QDs, MSCs display much narrower optical absorption bands; the higher cluster stability gives rise to a narrower size distribution. During the production of binary QDs such as II-VI metal (M) chalcogenide (E) ones, binary ME MSCs observed were interpreted as side products and/or the nuclei of QDs. Prior to the current development of our two-step approach followed by our two-pathway model, it had been extremely challenging to synthesize MSCs as a unique product without the nucleation and growth of QDs. With the two-step approach, we have demonstrated that MSCs can be readily engineered as a sole product at room temperature from a prenucleation stage sample, also called an induction period (IP) sample. It is important that we were able to discover that the evolution of the MSCs follows first-order reaction kinetics behavior. Accordingly, we proposed that a new type of compound, termed as "precursor compounds" (PCs) of MSCs, was produced in an IP sample. Such PCs are optically transparent at the absorption peak positions of their MSC counterparts as well as to longer wavelengths. It is thought that quasi isomerization of a single PC results in the development of one MSC.In this Account, we provide an overview of our latest advances regarding the transformations among binary CdE MSCs as well as from binary CdTe to ternary CdTeSe MSCs. Optical absorption spectroscopy has been employed to study these transformations, all of which display well-defined isosbestic points. We have proposed that these MSC to MSC transformations occur via their corresponding PCs, also called immediate PCs. It is reasonable that the as-synthesized PC (in an IP sample) and the immediate PC (in an incubated and/or diluted sample) probably have different configurations. A transformation between two PCs may involve an intermolecular reaction, with either first-order reaction kinetics or a more complicated time profile. A transformation between one immediate PC and its counterpart MSC may contain an intramolecular reaction. The present Account, which addresses the PC-enabled MSC transformations with isosbestic points probed by optical absorption spectroscopy, calls for more experimental and theoretical attention to understand these magic species and their transformation processes more precisely.

Preparation, Structural, Electrochemical and Photocatalytic Studies of Cadmium Sulfide Quantum Dots

We report the preparation, structural, electrochemical and photocatalytic studies of monodispersed cadmium sulphide quantum dots from didecylaminyl dithiocarbamate and 4-chloro-3-(trifluoromethyl) anilinyl dithiocarbamate cadmium(II) complexes. Powder X-ray diffraction pattern confirms hexagonal crystalline phases for the as-preapred CdS quantum dots irrespective of the precursor used with particle size of 3.39-5.51 nm. Optical absorption band edges of 515 nm were observed for the cadmium sulfide quantum dots with energy band gaps estimated from the Tauc plots of 1.97 eV for OLM-CdS1 prepared from Cd(II) didecylaminyl dithiocarbamate and 1.92 eV for OLM-CdS2 prepared from Cd(II) 4-chloro-3-(trifluoromethyl) anilinyl dithiocarbamate. These energy band gaps are blue shifted with respect to the bulk cadmium sulphide. The calculated electrochemical band gap of 2.34 V and 3.30 V are higher than band gap energy. The as-prepared CdS quantum dots were used as photocatalysts for the photocatalytic decomposition of methylene blue (MB) with efficiency of 61 % and 55 %.

Microwave and Induction Heating-Assisted Biosynthesis of 12CaO 7Al2O3 Electride from Aloe Vera Leaf Extract

The white powders used as precursor powders for the synthesis of 12CaO×7Al2O3 electride (C12A7:e-) were prepared by biosynthesis method using Aloe vera extract and microwave assisted synthesis. The C12A7:e- crystals were synthesized by induction heating process under a reducing atmosphere at different times of 3, 4 and 5 min. The structure of C12A7:e- powders was characterized by X-ray diffraction. The XRD analysis revealed that pure C12A7:e- powders were obtained from white precursor powders with an induction heating process time of 5 min. To confirm that the white precursor powders were transformed into C12A7:e- after induction heating process for 3, 4 and 5 min, the optical absorption spectra of powders were investigated by an UV-Vis diffuse reflectance spectrometer in the wavelength range of 200 – 800 nm. The results show the optical absorption bands at 2.8 eV for the white precursor powders with induction heating time of 3, 4 and 5 min. This is due to the C12A7 was transformed into electride (C12A7:e-). Therefore, the results on the optical absorption spectra are in good agreement with the XRD results.

Effect of electrodeposition time on morphology and photoelecrochemical performance of bismuth vanadate films

In this work, facile and low cost electrodeposition method has been used to prepare bismuth vanadate (BiVO4) thin films on fluorine-doped tin oxide glass in order to study photoelectrochemical water splitting performance. The electrochemical deposition was performed at different times (5, 10, 15, 20, 25, 30, 35 and 40 min) using vanadyl sulfate and bismuth (III) nitrate solutions. The phase and microstructure analyses have been performed using X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM). The morphological properties of the prepared films are dependent on the electrodeposition time, based on the FE-SEM results. In addition, UV–Vis diffuse reflection spectroscopy was used to confirm the optical absorption properties and band gap of the samples prepared. Linear sweepvoltammetry, chronoamperometryandopen circuit potential under xenon light irradiation were used to study the photoelectrochemical properties of the films prepared. Variations of the electrodeposition time remarkably changed the photoelectrochemical properties of the films. A high photocurrent density of 0.068 mA cm−2 at 1 V vs. Ag/AgCl was shown by the film deposited for 30 min. In addition, all the samples showed good photo responses and photo stabilities.

Quantum–classical mechanics: On the problem of a two-photon resonance band shape in polymethine dyes

In the framework of a new physical theory — quantum–classical mechanics, on the basis of a dynamic electron-nuclear-reorganization resonance and the law of conservation of energy, an explanation is given for the strong detuning of the resonance and the associated significant parasitic transformation of the shape of the resonant optical absorption band as a result of the transition from linear to nonlinear, two-photon absorption in polymethine dyes in solutions. Based on this explanation and model band shapes of the theoretical fit to the experimental optical bands (Hales et al. (2010)), the conditions for reconstructing the resonance shape of the band for two-photon absorption and its redshift are predicted.

Strain-mediated unusual bandgap bowing in continuous composition tuned monolayer Mo1−xWxS2 ternary alloys

Band gap engineering via 2D alloying is a vital strategy for three-atom-thick transition metal dichalcogenides based optoelectronics, valleytronics and nanophotonics. Here we demonstrate the growth of Mo1−xWxS2 ternary alloy monolayers and precise compositional tuning for the entire range of x from 0 to 1 using the gas-phase precursor approach. By means of Raman spectroscopy we show that W alloying in MoS2 lattice can lead to a tensile strain of ∼0.8%. The alloying-induced tensile strain plays a key role in observing redshift in optical absorption and photoluminescence (PL) bands and resulted an unusual bandgap bowing. The coupling of tensile strain and alloying effect allowed us to tune the overall PL emission energy to as large as 185 meV. Our optical spectroscopy results indicate three different phase-regions for the Mo1−xWxS2 alloy system. For x < 0.37, the alloys exhibit MoS2-like nature, whereas, WS2-like behavior is observed for x > 0.64, and a mixed behavior for 0.37 ≤ x ≤ 0.64.

Enhanced removal of organic dye by activated carbon decorated TiO2 nanoparticles from Mentha Aquatica leaves via ultrasonic approach

The development and investigation of environmental-friendly photocatalysts remain a challenge for eco-system remediation concerns. Herein, an ultrasonically-driven, biogenic, green synthesis of titanium dioxide (TiO2) nanoparticles (NPs) utilizing the extract of Mentha Aquatica leaves is reported. The TiO2 nanoparticles were further functionalized with carbon (TiO2@Carbon). The product composition and structure were examined through Raman, BET, and XPS analysis. The XRD studies of the TiO2 NPs confirmed the anatase phase of nanostructures. The TiO2@Carbon NPs displayed enhanced photocatalytic activity(doubled) compared to pristine TiO2 due to their increased surface area, a bathochromic shift in the optical absorption band, and effective charge separation. Significantly, greater than 90% RhB dye was decayed during the photocatalytic oxidation method, which is validated via trapping active species experiment. Carbon is a powerful substrate for the TiO2 hybrid and can be reused over successive five catalytic cycles. The experimental results demonstrated the TiO2@Carbon as a potential candidate for highly-efficient, eco-friendly, reusable, cost-effective photocatalyst.

Effect of substrate temperature on thermally evaporated CdS/CZTS for solar cell fabrication

The metal chalcogenides has a potential application in energy harvesting systems due to the good optical absorption, much abundance and suitable band potentials. In view of this, n-type Cadmium sulphide (CdS) thin films were thermally evaporated on conducting glass substrate (patterned Indium doped Tin Oxide (ITO) substrates, Ossila S211) followed by the p-type Copper Zinc Tin Sulphide (CZTS) films using nanocrystalline CZTS which in turn was obtained by mechanosynthesis at different milling rates. The mechanosynthesized CZTS nanoparticles and its corresponding films obtained by thermal evaporation were subjected to various characterization techniques such as XRD, UV–Vis-NIR spectroscopy and Hall Effect Measurement techniques in order to understand its physical properties, to access the rate of absorbance and to determine the type of conduction. Moreover, n-CdS as well as p-CZTS films were deposited at different substrate temperatures and studied the effect of temperature on the structural and optoelectronic behaviour of the respective films toward the fabrication of heterojunction solar cells. Finally, the p-n heterojunction was formed using n-CdS/p-CZTS films on patterned ITO substrates.

Unexpected three-state hydrochromism of a donor-acceptor self-complex with fluctuations in relative humidity.

Water in our environment is ever present, particularly in our atmosphere, from which it may be adsorbed by materials hygroscopically. At the molecular level, the binding of water molecules to various materials is driven by weak interactions but can have profound effects on physical properties, including the donor-acceptor interactions in charge transfer (CT) salts. Herein we present the unexpected three-state hydrochromatic switching of a bipyridinium-based donor-acceptor self-complex with changes in relative humidity (RH) and subsequent stable hydrate formation. RH is typically an overlooked variable that can vary greatly. These findings suggest that care should be taken to consider fluctuations in RH when characterizing the solid state optical band gap and CT absorption bands for organic donor-acceptor CT salt complexes.

The optical properties and carrier mobility of MH3 (M = Co, Rh and Ir) monolayers.

A new series of two-dimensional transition metal hydrides MH3 (M = Co, Rh, Ir) are investigated using first principles calculations. Their electronic structures, phonon dispersion, optical absorptions, and carrier mobilities are obtained and discussed. Our results on the basis of the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional reveal that CoH3, RhH3 and IrH3 are indirect semiconductors with band gaps of 2.54 eV, 1.80 eV and 1.82 eV, respectively. Moreover, MH3 monolayers show strong optical absorption in the visible and near-ultraviolet light regions. Under tensile strain, the band gaps decrease and the optical absorption is enhanced in the visible region. The obtained carrier mobilities are found to be anisotropic along the armchair and zigzag directions. The holes along the armchair are more easily transferred with high mobility. The strong optical absorption intensity and the relatively high carrier mobilities make MH3 monolayers (especially RhH3 and IrH3) potential candidates for applications in photovoltaics.

Tunable electronic and optical properties of a BAs/As heterostructure by vertical strain and external electric field.

Based on the first-principles method, we investigated the electronic properties of a BAs/arsenene (As) van der Waals (vdW) heterostructure and found that it has an intrinsic type-II band alignment with a direct band gap of 0.25 eV, which favors the separation of photogenerated electrons and holes. The band gap can be effectively modulated by applying vertical strain and external electric field, displaying a large alteration in the band gap via the strain and experiencing an indirect-to-direct band gap transition. Moreover, the band gap of the heterostructure varies almost linearly with external electric field, and the semiconductor-to-metal transition can be realized in the presence of a strong electric field. The calculated band alignment and optical absorption reveal that the BAs/As heterostructure could present an excellent light-harvesting performance. The absorption strength can be tuned mainly by interlayer coupling, while external electric field shows clear regulating effects on the absorption strength and absorption edge.

Effect of Divalent Metal Ions on Thermal Evolution and Band Gap Energy of Cadmium Oxide (CdO) Nanocrystallites: Comparetive Study

In this study, pure and divalent metal ions doped cadmium oxide samples are synthesized by cost effective co-precipitation method at the nanoscale dimensions. Here, we have represented a comprehensive study which conclude that how structural and optical behavior of undoped cadmium oxide has been modified with single doping of Cu, Mn and their co-doping (Cu-Mn) also. The prepared nanopowder is being investigated using different complementary techniques like TG-DTA and UV-Vis spectroscopy to explore their bandgap energy and thermal behavior. A strong and broad optical absorption band is obtained at 262, 250, (312 and 475), (250-350) nm for respectively samples. The optical band gap values are calculated using Tauc’s plot as 4.45, 3.86, 2.39 and 2.44 eV for undoped CdO, Cu doped CdO, Mn doped CdO and Cu-Mn co-doped CdO respectively. UV-Vis spectra revealed that the prepared samples are having high absorbance and extended sufficiently into visible region. Therefore, it is suggested that the co-doping efficiently alter the band gap of the CdO and make it suitable for optoelectronics and other applications like gas sensing, photo catalyst etc.

Pb5Sb8Se17-δ nanocrystals: A new solar absorber material with an optimal bandgap and an efficiency near 6 % under 0.05 sun

Ternary semiconductor Pb5Sb8S17 has been shown to be a candidate solar absorber. This work presents a new solar absorber of the same group – Pb5Sb8Se17. Pb5Sb8S17 precursor nanocrystals (NCs) are first synthesized using solution processing. A subsequent S to Se ion exchange transforms the Pb5Sb8S17 precursor into the monoclinic Pb5Sb8Se17-δ phase. The substitution of S by Se reduces the band gap Eg from 1.75 (Pb5Sb8S17) to 1.41 eV (Pb5Sb8Se17-δ), increasing the optical absorption band from 300 to 750 (Pb5Sb8S17) to 300–900 nm (Pb5Sb8Se17-δ). The Eg of 1.4 eV equals to the ideal Eg for the Shockley-Queisser-limit. Liquid-junction quantum dot-sensitized solar cells (QDSSCs) are fabricated from the Pb5Sb8Se17-δ NCs with three types of counter electrode (CE): Pt, Au and NiSe. The cell using NiSe CE yields the best PCE of 2.73% (1 sun). Under reduced sun intensities, the PCE increases to 5.47% (0.1 sun) and further to 5.91% (0.05 sun). The external quantum efficiency (EQE) spectrum yields an EQE-integrated photocurrent Jph of 26.8 mA/cm2. The PCE of Pb5Sb8Se17 is 50% higher than that of the host Pb5Sb8S17 (3.95%). The optimal Eg, a high Jph and a high PCE suggest that Pb5Sb8Se17-δ has potential to be an efficient solar absorber.

Thermally stimulated infrared shift of cadmium oxide optical absorption band edge

The effect of annealing temperature on the optical properties of nanosized cadmium oxide with rock-salt structure synthesized by thermal treatment of cadmium formate Cd(HCOO)2·2H2O was studied using X-ray diffraction and UV–Vis–NIR spectroscopy methods. The samples were obtained by heating of as-prepared cadmium oxide in air at a temperature successively increasing from 500 to 900 °C with an interval of 50 °C. The exposure time at each temperature was 1 h. It was established that as a result of annealing in air in this thermal regime the CdO band gap decreased from 1.95 to 1.50 eV with the corresponding change of the powder color from reddish-brown to black. The particle size estimated by the Williamson-Hall method and the density decreased from 85.5 to 79.8 nm and from 8.24 to 8.20 g/cm3, respectively. With the use of the first-principle calculations of the electronic band structure, optical absorption and vacancy formation energy it was shown that the reason of the band gap narrowing is the appearance of vacancies in the CdO oxygen sublattice, the concentration of which grows with increasing annealing temperature.

Thermoluminescence of fluorapatite mineral

Fluorapatite [Ca10(PO4)6F2] is a mineral of the apatite group of the highest occurrence in the world among phosphates. The material is known for its diverse luminescent properties, as fluorescent lamps and lasers. The thermoluminescence (TL) signal of apatite from various origins has been investigated in the past years, mainly of the Mexican origin. In this work, the objective was to investigate the TL of samples of the Brazilian natural fluorapatites submitted to beta-ionizing radiation. The chemical composition of the samples was obtained by the technique of X-ray fluorescence and the electronic transitions were verified by optical absorption in the visible region. The mineral presented a wide optical absorption band between 500 nm and 750 nm, typical of fluorapatite, and SEM/EDS showed a high percentage of purity. The registered Raman spectrum exhibits the characteristics modes from the PO43− tetrahedra, observed in fluorapatite, and a band at 1103 cm−1 characteristic of the (CaO3)2- carbonate group. The TL measurements were carried out using a commercial Risø TL/OSL reader, model DA-20, equipped with a built in 90Sr/90Y beta source, with different radiation doses (1–5 Gy). The TL glow curve showed three peaks located at about 350 K, 470 K and 570 K (heating rate of 1 K/s). The activation energy and the frequency factor associated with each peak were determined by different methods. In addition, the TL response (area under the peaks) has shown linear behavior in relation to the doses.

Regulating electronic properties of BiOBr to enhance visible light response via 3d transition metals doping: DFT + U calculations

AbstractIn our work, the formation energies, band structures, densities of states, effective masses and optical absorption properties of pure BiOBr and 3d transition metals‐doped BiOBr have been calculated using DFT + U method. Ti, V, Fe, Cr, Co, Ni and Cu doping can induce impurity energy levels, originating from spin‐up or ‐down orbits of 3d transition metals (TMs), within the forbidden band of BiOBr, but Sc, Mn and Zn atoms only change the electronic delocalization in the valence band or conduction band region of BiOBr. Furthermore, with introduction of 3d TMs atoms, there exist the redshift phenomena for optical absorption band edge of BiOBr to different extents. The photo response priority order, structural stability and recombination probability of photoinduced carriers for 3d TMs‐doped BiOBr are summarized. Our theoretical findings should well explain the experimental observations in the previous literatures, and provide promising prediction and significant guidance for the well‐construction of BiOBr‐based photocatalyst systems.