Sharp Absorption Edge Research Articles

Highly luminescent CdS nanoparticles synthesized using microwave irradiation of Dithiocarbazate ligand as a single molecular precursor source

In this paper, we have reported microwave-assisted synthesis of highly luminescent cadmium sulfide nanocrystals using dithiocarbazate Cd (II) chloride complex single-molecule precursor route. The precursors were heated using microwave irradiation to obtain CdS nanocrystals. The irradiation of precursor in DMSO environment played an important role in reducing reaction time, minimizing possibility of side reactions, and resulted in obtaining good quality particles. The obtained samples were analyzed using structural, morphological, and optical characterizations. X-ray diffraction studies confirm the hexagonal crystalline phase. High-resolution electron microscopy and transmission electron microscopy exhibit rod-shape structures with an average particle size of 80 nm. Sharp absorption edge at 320 nm indicates good quality of the particles. Strong luminescence peaks in the visible wavelength range 450–490 nm is observed in PL spectra. Another luminescence peak at 680 nm for excitation at 320 nm ensured the involvement of deep donor and acceptor levels in the crystals. The luminescence spectra show strong dependence on excitation wavelength which makes these particles suitable for various optoelectronic applications. The method reported here is a good candidate for obtaining large-scale high-quality CdS nanoparticles for optoelectronic applications.

Spontaneous anion-exchange synthesis of optically active mixed-valence Cs2Au2I6 perovskites from layered CsAuCl4 perovskites.

Cs2Au2I6, a lead-free photovoltaic material, has been synthesized via controlled and systematic addition of hydroiodic acid (HI) to CsAuCl4. X-ray diffraction studies suggest the formation of Cs2Au2I6 when a threshold concentration of HI is added to CsAuCl4. The final compound shows good stability against light, oxygen and moisture and at temperatures up to 140 °C without any phase degradation. The stability of Cs2Au2I6 is also confirmed by its high negative formation energy and the convex hull diagram constructed using Density Functional Theory (DFT). Absorption studies suggest an abrupt band shift from 2.31 eV to 1.06 eV when HI concentration reaches the threshold limit (∼100 μl). A sharp absorption edge was found for Cs2Au2I6 with an Urbach energy of 59 meV, indicating lower structural disorder and higher crystallinity.

Inverse Perovskite Oxysilicides and Oxygermanides as Candidates for Nontoxic Infrared Semiconductor and Their Chemical Bonding Nature.

We have synthesized inverse-perovskite-type oxysilicides and oxygermanides represented by R3SiO and R3GeO (R = Ca and Sr) and studied their characteristics in the search for nontoxic narrow band gap semiconductors. These compounds exhibit a sharp absorption edge around 0.9 eV and a luminescence peak in the same energy range. These results indicate that the obtained materials have a direct-band electronic structure, which was confirmed by hybrid DFT calculations. These materials, made from earth abundant and nontoxic elements and with a relatively light electron/hole effective mass, represent strong candidates for nontoxic optoelectronic devices in the infrared range.

TEMPERATURE DEPENDENCE OF HOMOGENEOUS ANATASE-PHASED TiO2FILMS CHARACTERIZATION AND GAS-SENSING BEHAVIORS

Anatase-phased TiO2films were prepared at different temperatures (350, 400, 450 and 500∘C) using automated nebulizer spray pyrolysis (ANSP) method. The structural study (XRD) revealed the amorphous nature at 350∘C and remaining samples (400, 450 and 500∘C) show the tetragonal structure with 2[Formula: see text], 38.43, 48.49 and 55.54 corresponding to (101), (004), (200) and (105) reflected planes and it is well fitted with standard data. The compositional XPS analysis confirmed the core level primary element of Ti 2p, O 1s and valance band (VB) of Ti 3p, Ti 3s, O 2s peaks in the prepared samples. The 3D optical profilometer has shown that the thickness of the prepared films was decreased by increase in temperature. The AFM study exhibited average roughnesses (Ra) of the prepared films such as 0.058, 0.147, 0.176 and 0.194[Formula: see text]nm, respectively. The surface morphological study of FESEM has shown the cracked uneven distributed nature (350∘C) turn into evenly distributed closed packed agglomerated particles by the influence of temperature. The oscillating nature of transmittance (%) with redshift of the sharp absorption edge was observed in UV–Vis–NIR spectrophotometer and found the bandgap value about 3.58[Formula: see text]eV to 3.33[Formula: see text]eV through Tauc’s relation. The gas-sensing behavior has shown better response to C2H6O reducing gas at 300∘C operating temperature with 150 ppm gas concentration.

In situ NMR Investigation of the Photoresponse of Perovskite Crystal

Summary Hybrid perovskites are outstanding candidates for use in optoelectronic devices. Nonetheless, hybrid perovskites present numerous fundamental issues that are not yet fully understood. This study seeks to address this problem by reporting on the occurrence of ultraslow dynamic orientation changes in the methylammonium (MA) cations and the corresponding changes in the inorganic lattice of MAPbI3 perovskite crystals occurring over a time scale of 100 s under continuous illumination based on in situ single-crystal 2H nuclear magnetic resonance and in situ X-ray diffraction results. The demonstrated ultraslow dynamic changes in MAPbI3 are far slower than elementary molecular motions typically having time scales of femtoseconds and picoseconds and have never been reported previously. The ultraslow dynamic changes have been correlated with the reduction of band gap and the increase of absorption coefficient of MAPbI3, providing the molecular origins of the stabilization process in the photoresponse of the MAPbI3 device.

Physicochemical and magnetic properties of pure and Fe doped TiO2 nanoparticles synthesized by sol-gel method

Pure and iron-doped TiO2 nanoparticles (NPs) for different moles were synthesized by the sol–gel method. The synthesized products were characterized by sophisticated instrumental techniques such as powder X-ray diffraction (XRD), Scanning Electron Microscope (SEM), ultraviolet–visible spectroscopy (UV–Vis) and vibrational sample magnetometer (VSM). XRD pattern confirmed the Rutile structure of Pure TiO2 and Fe doped TiO2 NPs. The doping of Fe into TiO2 proved a great decrease in the size of nanoparticles as compared to undoped TiO2. The various functional groups were identified using FTIR analysis. The surface morphology of pure and Fe doped TiO2 nanoparticles were revealed by scanning electron microscopic analysis. The SEM micrographs revealed flakes like structure of pure and Fe doped TiO2 NPs. UV–Vis Spectrometer showed a sharp absorption edge for pure TiO2 in the ultraviolet region at a wavelength of about 375 nm and the absorption edge for Fe doped TiO2 NPs at a wavelength of about 339 nm. Vibrating Sample Magnetometer (VSM) revealed the ferromagnetic property. The value of magnetic moment showed an increase due to the suitable moles of Fe in TiO2 NPs.

Ganymede's Far‐Ultraviolet Reflectance: Constraining Impurities in the Surface Ice

AbstractWe present reflectance spectra of Ganymede's leading and trailing hemispheres in the wavelength range 138–215 nm, obtained by the Hubble Space Telescope Cosmic Origins Spectrograph (HST/COS) in 2014. The most notable feature of both spectra is the absence of a sharp water absorption edge at ~165 nm, seen in laboratory measurements of ice reflectivity and in previous observations of Saturn's icy moons and rings. Rather than displaying a sharp change in the reflectivity at the wavelength of the water ice absorption edge, Ganymede's reflectance gradually increases with wavelength at λ > 165 nm. We show that the observed shape of Ganymede's UV reflectance is inconsistent with intimate mixture models of pure ice with UV‐dark materials including tholins, amorphous carbon, graphite, and silicates. However, we find that intraparticle models, in which a small proportion of a UV‐absorbing contaminant is trapped as inclusions within the ice matrix, are able to suppress the 165 nm feature at contaminant concentrations of <1%. We show that models of ice with inclusions of silicates, Triton‐type tholin, or H2O2 are able to produce the observed gradual increase in reflectivity at λ > 165 nm, but additional absorbing surface materials are required to produce a good fit to Ganymede's previously observed near‐UV and visible reflectance.

A Family of Small Molecular Materials Enabling Consistently Lower Recombination Losses in Organic Photovoltaic Devices

Hybridization between the charge transfer (CT) state of a donor–acceptor pair and lowest exciton state of the donor or the acceptor is reported to be effective for reducing recombination loss in organic photovoltaic (OPV) devices. Although this approach shows great success in maximizing open circuit voltage (Voc), it is typically accompanied by low device performance. Here, “complete boron sub‐(na)phthalocyanine devices” with strong hybridization resulting in lower recombination loss (≈0.47 eV) while not penalizing charge separation dynamics (internal quantum efficiency (IQE) > 80% and fill factor (FF) > 70%) are reported. Interestingly, when boron sub‐(na)phthalocyanine is paired with any other active material used in this study (“partial boron sub‐(na)phthalocyanine device”), recombination losses are still consistently maintained at lower levels (<0.53 eV). These observations denote the capability of boron sub‐(na)phthalocyanine to result in lower recombination loss devices while pairing with other materials. Special intrinsic characteristics of these materials (high dielectric constant, sharp absorption edge, unusually high absorption coefficient) and hybridization collectively result in reduced recombination loss and efficient charge generation in these systems.

Single‐Source, Solvent‐Free, Room Temperature Deposition of Black γ‐CsSnI3 Films

AbstractThe presence of a nonoptically active polymorph (yellow‐phase) competing with the optically active polymorph (black γ‐phase) at room temperature in cesium tin iodide (CsSnI3) and the susceptibility of Sn to oxidation represent two of the biggest obstacles for the exploitation of CsSnI3 in optoelectronic devices. Here room‐temperature single‐source in vacuum deposition of smooth black γ −CsSnI3 thin films is reported. This is done by fabricating a solid target by completely solvent‐free mixing of CsI and SnI2 powders and isostatic pressing. By controlled laser ablation of the solid target on an arbitrary substrate at room temperature, the formation of CsSnI3 thin films with optimal optical properties is demonstrated. The films present a bandgap of 1.32 eV, a sharp absorption edge, and near‐infrared photoluminescence emission. These properties and X‐ray diffraction of the thin films confirm the formation of the orthorhombic (B‐γ ) perovskite phase. The thermal stability of the phase is ensured by applying in situ an Al2O3 capping layer. This work demonstrates the potential of pulsed laser deposition as a volatility‐insensitive single‐source growth technique of halide perovskites and represents a critical step forward in the development and future scalability of inorganic lead‐free halide perovskites.

Development and characterization of TlGaSe2 thin film-based photodetector for visible-light photodetector applications

In current work a thermal evaporation technique has been employed to fabricate the thin films from the grown TlGaSe2 single crystal and investigated. X-ray diffraction study revealed the formation of polycrystalline films of monoclinic crystal system. The crystallite size was estimated to be 11 nm. Atomic force and scanning electron microscopy studies shows the nanocrystalline film fabrication and the size of grain was estimated. Optical study shows that the grown film is about 55% transparent in 800–1500 nm region and possess a sharp absorption edge. The direct energy gap of TlGaSe2 films was estimated around 2.31 eV. The photo current of the fabricated TlGaSe2 photodetector increases about 6 times compare to dark when exposed under 5 mW/cm2 illumination. The fabricated detector possesses high external quantum efficiency of 158% and also the detectivity reached to 5.16 × 1010 at V = 10 V. The on/off behaviour of the device was also studied and found that the response time for growth and decay is 88 ms and 90 ms, respectively.

Water-Stable 1D Hybrid Tin(II) Iodide Emits Broad Light with 36% Photoluminescence Quantum Efficiency.

The optical and light emission properties of tin and lead halide perovskites are remarkable because of the robust room-temperature (RT) performance, broad wavelength tunability, high efficiency, and good quenching resistance to defects. These highly desirable attributes promise to transform current light-emitting devices, phosphors, and lasers. One disadvantage in most of these materials is the sensitivity to moisture. Here, we report a new air-stable one-dimensional (1D) hybrid lead-free halide material (DAO)Sn2I6 (DAO, 1,8-octyldiammonium) that is resistant to water for more than 15 h. The material exhibits a sharp optical absorption edge at 2.70 eV and a strong broad orange light emission centered at 634 nm, with a full width at half-maximum (fwhm) of 142 nm (0.44 eV). The emission has a long photoluminescence (PL) lifetime of 582 ns, while the intensity is constant over a very broad temperature range (145-415 K) with a photoluminescence quantum yield (PLQY) of at least 20.3% at RT. Above 415 K the material undergoes a structural phase transition from monoclinic (C2/c) to orthorhombic (Ibam) accompanied by a red shift in the band gap and a quench in the photoluminescence emission. Density functional theory calculations support the trend in the optical properties and the 1D electronic nature of the structure, where the calculated carrier effective masses along the inorganic chain are significantly lower than those perpendicular to the chain. Thin films of the compound readily fabricated from solutions exhibit the same optical properties, but with improved PLQY of 36%, for a 60 nm thick film, among the highest reported for lead-free low-dimensional 2D and 1D perovskites and metal halides.

Ultraviolet‐Protective Transparent Photovoltaics Based on Lead‐Free Double Perovskites

Perovskite solar cells have attracted great research interest as a promising candidate for silicon solar cells. Plenty of work has been reported to use perovskites to semitransparent windows and transparent photovoltaic (TPV) devices to obtain multifunctional systems. However, the narrow bandgap and sharp absorption edge of the typical perovskites prevent them from achieving the highest transparency to satisfy the requirements of aesthetic and integration, and the poor stability and toxic Pb compositions hinder their practical application. Herein, lead‐free halide double perovskites with a wide bandgap and indirect bandgap characteristics is introduced to fabricate long‐term stable transparent photovoltaic devices exhibiting high visible transmittance (73%) and considerable energy conversion efficiency (1.56%). Through further theoretical calculation and evaluation, a new strategy using indirect bandgap material on TPV devices is proposed to combine the enhancement of these two parameters. This approach will be a significant compliment to near‐infrared‐absorbing solar cells to selectively harvest light in the invisible region to obtain highly performing multi‐junction smart windows on buildings, vehicles and mobile electronics, providing a new reasonable idea to realize TPVs with high efficiency and transparency simultaneously.

Growth properties of gallium oxide on sapphire substrate by plasma-assisted pulsed laser deposition

Gallium oxide was deposited on a c-plane sapphire substrate by oxygen plasma-assisted pulsed laser deposition (PLD). An oxygen radical was generated by an inductive coupled plasma source and the effect of radio frequency (RF) power on growth rate was investigated. A film grown with plasma assistance showed 2.7 times faster growth rate. X-ray diffraction and Raman spectroscopy analysis showed β-Ga2O3 films grown with plasma assistance at 500 °C. The roughness of the films decreased when the RF power of plasma treatment increased. Transmittance of these films was at least 80% and showed sharp absorption edge at 250 nm which was consistent with data previously reported.

Synthesis of nanostructured Nickel compounds on conductive metallic substrates

Abstract Nickel hydroxide, oxide, and sulfide films were synthesized on conductive Ni metallic substrates by solid-vapor reactions, calcination and hydrothermal methods. The synthesized films were characterized by XRD, XPS, FE-SEM, UV–Vis Spectroscopy and Cyclic Voltammetry (CV). The morphological analyses of Ni(OH)2 and NiO films showed flower-like nanostructures with thicknesses from 20 to 40 nm; meanwhile the NiS films exhibits nano-granular morphology. All films showed high optical reflectance in the visible region and a sharp absorption edge below 400 nm. The specific capacitance of the Nickel compound ranges from 134 to 311 mF cm−2 (at 20 mV s−1) suggesting potential as conductive electrodes for energy storage and electrochemical conversion cells.

Filterless narrowband photodetectors employing perovskite/polymer synergetic layers with tunable spectral response

Abstract Spectrally selective light detection, especially the detection within desired spectral range, is crucial for many fields, such as image sensing, optical communication, climate monitoring and national defense. Conventional narrowband photodetectors with optical filters are face to the architectural complexity and the increased device cost. Here, a smart strategy for achieving the narrowband light detection with photodetectors is proposed by employing perovskite/polymer synergetic layers, in which the perovskite layer acts as an electronically-quenching optically-active (EQOA) layer and the polymer layer acts as the active layer. The full width at half maximum of this narrowband photodetectors can reach minimum 50 nm due to the sharp absorption edges of perovskite materials, and the optimized photodetectors exhibit suppressed dark current of 1.85 × 10 − 5 mA cm − 2 , high specific detectivity of 1.37 × 10 12 Jones (cm Hz1/2 W−1). Furthermore, the spectral response can be tuned via managing the bandgap of the perovskite material, and a series of narrowband photodetectors with response spectral range between 680 nm and 710 nm were fabricated. This provides a new design paradigm for a narrowband photodetector with many imaging applications.

Maximum Efficiencies and Performance-Limiting Factors of Inorganic and Hybrid Perovskite Solar Cells

The Shockley and Queisser limit, a well-known efficiency limit for a solar cell, is based on unrealistic physical assumptions and its maximum limit is seriously overestimated. To understand the power loss mechanisms of record-efficiency cells, a more rigorous approach is necessary. Here, we have established a new formalism that can accurately predict absolute performance limits of solar cells in conventional thin film form. In particular, we have estimated the maximum efficiencies of 13 well-studied solar cell materials [GaAs, InP, CdTe, a-Si:H, CuInSe2, CuGaSe2, CuInGaSe2, Cu2ZnSnSe4, Cu2ZnSnS4, Cu2ZnSn(S,Se)4, Cu2ZnGeSe4, CH3NH3PbI3, HC(NH2)2PbI3] in a 1-um-thick physical limit. Our calculation shows that over 30% efficiencies can be achieved for absorber layers with sharp absorption edges (GaAs, InP, CdTe, CuInGaSe2, Cu2ZnGeSe4). Nevertheless, many record-efficiency polycrystalline solar cells, including hybrid perovskites, are limited by open-circuit voltage and fill-factor losses. We show that the maximum conversion efficiencies described here present new alternative limits that can predict the power generation of real-world solar cells.

Properties and performance of photocatalytic CeO2, TiO2, and CeO2–TiO2 layered thin films

CeO2, TiO2, and CeO2–TiO2 layered thin films of different compositions were deposited on polished fused silica glass substrates using spin coating and then annealed at 550 °C for 1 h. Poorly crystalline fluorite-type CeO2 and anatase-type TiO2 were the only phases detected. The valence effects for Ce are attributed to intervalence charge transfer (IVCT) while those for TiO2 are attributed to simultaneous Ti4+ → Ti3+ redox and IVCT. Contamination of the grain boundaries by Si from the substrate is likely to have contributed to blockage of surface-active sites. While the roughnesses of the former thin films exhibited the expected direct correlation with the grain size, those of the latter two types of thin films showed a converse relation, suggesting that the grain boundaries of the layered TiO2 thin films included a recrystallized CeO2–TiO2 invariant point liquid and/or a CeO2–TiO2–SiO2 glass. These would have derived from the other sources of grain boundary contamination, which are Si contamination from the substrate and/or exsolved CeO2 beyond the solubility limit of TiO2. The UV–Vis spectra demonstrated two principal types of curves, with the CeO2 and higher TiO2 concentration giving typical spectra with sharp absorption edges while those of the lower TiO2 concentration indicated highly defective nanostructures with diffuse absorption edges. These data support the conclusion that CeO2 initially decreases the Eg by enhancing nucleation and recrystallization through defect formation but subsequently increases the Eg through supersaturation and consequent lattice distortion. The CeO2 thin films exhibited much worse photocatalytic performances than those of the TiO2 thin films, although no significant differences between the TiO2 thin films and the CeO2–TiO2 layered thin films were apparent, which probably resulted from the competing effects of blockage of the active sites on the grain boundaries and the extent of recrystallization of the thin film phases, the latter of which is more significant.

Dynamic shortening of disorder potentials in anharmonic halide perovskites

Halide perovskites are semiconductors that exhibit sharp optical absorption edges and small Urbach energies allowing for efficient collection of sunlight in thin-film photovoltaic devices. However, halide perovskites also exhibit large nuclear anharmonic effects and disorder, which is unusual for efficient optoelectronic materials and difficult to rationalize in view of the small Urbach energies that indicate a low amount of disorder. To address this important issue, the disorder potential induced for electronic states by the nuclear dynamics in various paradigmatic halide perovskites is studied with molecular dynamics and density functional theory. We find that the disorder potential is dynamically shortened due to the nuclear motions in the perovskite, such that it is short-range correlated, which is shown to lead to favorable distributions of band edge energies. This dynamic mechanism allows for sharp optical absorption edges and small Urbach energies, which are highly desired properties of any solar absorber material.

Mid-infrared (3–8 μm) Ge1−ySny alloys (0.15 < y < 0.30): Synthesis, structural, and optical properties

Ge1−ySny alloys with compositions in the 0.15 < y < 0.30 range have been grown directly on Si substrates using a chemical vapor deposition approach that allows for growth temperatures as high as 290 °C. The films show structural properties that are consistent with results from earlier materials with much lower Sn concentrations. These include the lattice parameter and the Ge-Ge Raman frequency, which are found to depend linearly on composition. The simplicity of the structures, directly grown on Si, makes it possible to carry out detailed optical studies. Sharp absorption edges are found, reaching 8 μm near y = 0.3. The compositional dependence of edge energies shows a cubic deviation from the standard quadratic alloy expression. The cubic term may dramatically impact the ability of the alloys to cover the long-wavelength (8–12 μm) mid-IR atmospheric window.

Influence of silica nanoparticles incorporated with chitosan/polyacrylamide polymer nanocomposites

Nanocomposite films based on chitosan (Cs) and polyacrylamide (PAM) embedded with silica nanoparticles (SiO2) were prepared. The films were studied and characterized using different techniques. The X-ray diffraction revealed the presence of the semi-crystalline nature of Cs/PAM blend. No peaks characterizing pure SiO2 were observed due to the masking effect of the Cs/PAM blend matrix resulting of the low amount of SiO2. The main characteristic IR-bands to the vibrational groups for Cs/PAM were observed. No changes in the position of IR bands were seen after incorporation of SiO2 nanoparticles. The UV–vis spectra showed an absorption band at 253nm with a sharp absorption edge which indicates the semi-crystalline nature of Cs/PAM matrices. The spectra optical parameters were measured and characterized as a function of photon energy. The value of optical energy gap (Eg) was estimated using indirect transition model and explained due to local cross-linking in the amorphous regions of Cs/PAM. The plot of dielectric loss and dielectric constant ε′ and ε″ with the frequency was gradually decreased with the increase of the frequency and reaches to constant values at higher frequencies due to polarization effects. The plot of real impedance (Z′) with imaginary impedance Z″ gives a perfect semicircle correlated to Debye behavior.