Broad Photoluminescence Emission Band Research Articles

A luminescent binuclear Cu(I) complex constructed by bipyridine, TD-DFT calculation, and its fluorescent sensing properties for VOCs

The design and research of materials with strong luminescence and fluorescence sensing for the detection of volatile organic compounds (VOCs) are of great significance and challenge. Herein, we report a binuclear Cu(I) complex, [Cu2(POP)2(MeCN)2(4,4′-bipy)](PF6)2 (2), which is synthesized rationally through a substitution reaction between complex [Cu(POP)(4-PBO)(MeCN)](PF6) (1) and 4,4′-bipy lignand (POP is bis[2-(diphenylphosphino)phenyl]ether, 4,4′-bipy is bipyridine, 4-PBO is 2-(4′-pyridyl)-benzoxazole). The complex 2 was characterized by single-crystal X-ray diffraction as an isolated binuclear structure constructed by a bridged 4,4′-bipy ligand. Under UV irradiation, both the powder and doped thin film of 1 and 2 exhibit broadband photoluminescence emission. The TD-DFT calculation reveals the details of the electronic transitions corresponding to the light absorption and emission of the two complexes, and also illustrates that the improved luminescence of 2 compared to 1 is due to the T1 excited state transforms from a localized transition to a charge transfer transition similar to the S1 excited state and the extremely small energy gap ΔE(S1-T1) value of 0.09 eV (2). Based on these complexes, fluorescent test strips were developed. It is gratifying to find that the test strip based 2 shows a distinguishable fluorescence response behavior to a variety of VOCs. Especially, the test strip showed a rapid quenching fluorescence response to acetophenone, and a rapid quenching followed by rapid enhancement fluorescence response to acetonitrile. Therefore, it can be used for rapid and highly selective fluorescence sensing detection of acetonitrile and acetophenone.

Revealing photoluminescence and nonlinear optical absorption characteristics of PbMo0.75W0.25O4 single crystal for optical limiting applications

Nonlinear absorption properties of PbMo0.75W0.25O4 single crystal fabricated by the Czochralski method were studied. The band gap energy of the crystal was determined as 3.12 eV. Urbach energy which represents the defect states inside the band gap was found to be 0.106 eV. PbMo0.75W0.25O4 single crystal has a broad photoluminescence emission band between 376 and 700 nm, with the highest emission intensity occurring at 486 nm and the lowest intensity peak at 547 nm, depending on the defect states. Femtosecond transient absorption measurements reveal that the lifetime of localized defect states is found to be higher than the 4 ns pulse duration. Open aperture (OA) Z-scan results demonstrate that the PbMo0.75W0.25O4 single crystal exhibits nonlinear absorption (NA) that includes two-photon absorption (TPA) as the dominant mechanism at the 532 nm excitations corresponding to 2.32 eV energy. NA coefficient (β eff ) increased from 7.24 × 10−10 m W−1 to 8.81 × 10−10 m W−1 with increasing pump intensity. At higher intensities β eff tends to decrease with intensity increase. This decrease is an indication that saturable absorption (SA) occurred along with the TPA, called saturation of TPA. The lifetime of the defect states was measured by femtosecond transient absorption spectroscopy. Saturable absorption behavior was observed due to the long lifetime of the localized defect states. Closed aperture (CA) Z-scan trace shows the sign of a nonlinear refractive index. The optical limiting threshold of PbMo0.75W0.25O4 single crystal at the lowest intensity was determined as 3.45 mJ/cm2. Results show that the PbMo0.75W0.25O4 single crystal can be a suitable semiconductor material for optical limiting applications in the visible region.

Pressure Coupled Lanthanide Ion Doping to Enhance Optical Properties in BaTiO3.

Self-trapped excitons (STEs) typically give broadband photoluminescence emission with a large Stokes shift, which is important for the enhancement of the optical properties of materials. Here, low-dimensional La-doped BaTiO3 nanocrystals with defects are prepared using supercritical CO2 (SC CO2). The generation of the STEs is facilitated by doping La3+ ions and introducing CO2 pressure, which effectively enhance the luminescence intensity of BaTiO3. This discovery shows that the La ion doping concentration can modulate the photoluminescence of BaTiO3 nanocrystals under pressure. This work deepens the understanding of the influence of rare-earth-doped luminescent materials under pressure and provides insight to improve the capabilities of optical devices.

Synthesis and photoluminescence characterizations of Zn2+ and Tb3+ codoped CeO2 phosphors for temperature sensing applications

Cyan light-emitting Ce0.985-xZnxO2:0.015 Tb3+ (x = 0 to 0.2) phosphors were synthesized using the ethylenediaminetetraacetic acid-assisted hydrothermal method. The X-ray diffraction and refinement analyses of the prepared phosphors indicated that the formed face-centered cubic structure remained intact even after the doping of large quantities of Zn2+ ions. However, the incorporation of Zn2+ ions increased the Ce3+/Ce4+ ratio, resulting in the enhancement of oxygen vacancies in the prepared phosphors. The generation of oxygen vacancies caused the evolution of a broad photoluminescence emission band ranging from 400 to 525 nm with a characteristic Tb3+ emission of approximately 543 nm. Two-emission regions in Ce0.885Zn0.1O2:0.015 Tb3+ phosphors were utilized for measuring the fluorescence intensity ratio (FIR) as a function of temperature ranging from 303 to 523 K. At 523 K, the FIR values dropped to approximately 40% of the starting temperature value. The variation of FIR values followed the Boltzmann behavior. The Boltzmann fitting demonstrated the feasibility of the present phosphors for temperature sensor applications. The optimum absolute sensor sensitivity of Ce0.885Zn0.1O2:0.015 Tb3+ phosphors was measured to be 0.0043 K−1 at 398 K with a resolution of approximately 1 K−1. Moderate temperature sensitivity, negligible hysteresis loop, and excellent reversibility revealed the suitability of Ce0.885Zn0.1O2:0.015 Tb3+ phosphors for sensing the temperature in various electronic devices.

A three-coordinate cuprous complex exhibiting both photochromic and fluorescent sensing properties, and some TD-DFT calculations

Photo-functional materials with photochromic and fluorescent properties are very attractive because they have a wide range of applications. The design and research of fluorescent sensing materials for VOCs (volatile organic compounds) are also of great significance and challenge. Herein, we report a three-coordinate cuprous complex, XantphosCuI (1) (Xantphos is 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene), and its reversible photochromism, photoluminescence, and fluorescent sensing for VOCs. The complex 1 was characterized by single-crystal X-ray diffraction as an isolated molecule with a triangle CuIP2 coordination geometry. In solution, complex 1 undergoes a reversible photochromic transition by alternating non-irradiation and irradiation with 365 nm UV light. Under UV light excitation, both the powder and doped thin film exhibit broadband photoluminescence emission. TD-DFT computational studies have been conducted to further understand the electronic transition properties of this compound. Based on 1, a fluorescent test strip was developed, and its fast and selective fluorescent sensing of pyridine vapor through enhanced mechanism was successfully realized. When exposed to vapor, the fluorescence of the test strip quickly becomes extremely bright. The fluorescent enhanced mechanism of the material was studied with the assistance of TD-DFT calculations, and it was proposed that the enhancement should be attributed to the charge transfer mechanism transition of the excited state after the coordination increase.

Theory and experiments of pressure-tunable broadband light emission from self-trapped excitons in metal halide crystals

Hydrostatic pressure has been commonly applied to tune broadband light emissions from self-trapped excitons (STE) in perovskites for producing white light and study of basic electron-phonon interactions. However, a general theory is still lacking to understand pressure-driven evolution of STE emissions. In this work we first identify a theoretical model that predicts the effect of hydrostatic pressure on STE emission spectrum, we then report the observation of extremely broadband photoluminescence emission and its wide pressure spectral tuning in 2D indirect bandgap CsPb2Br5 crystals. An excellent agreement is found between the theory and experiment on the peculiar experimental observation of STE emission with a nearly constant spectral bandwidth but linearly increasing energy with pressure below 2 GPa. Further analysis by the theory and experiment under higher pressure reveals that two types of STE are involved and respond differently to external pressure. We subsequently survey published STE emissions and discovered that most of them show a spectral blue-shift under pressure, as predicted by the theory. The identification of an appropriate theoretical model and its application to STE emission through the coordinate configuration diagram paves the way for engineering the STE emission and basic understanding of electron-phonon interaction.

Wide‐Bandgap Double Perovskites with Multiple Longitudinal‐Optical Phonon Scattering

AbstractAlloyed lead‐free double perovskites display intense photoluminescence, are environmentally friendly, and their devices show long‐term operation. Thanks to these properties, which make them excellent warm white‐emitting materials, they have recently received great attention in lighting applications. An important factor to tune the optical properties of alloyed lead‐free double perovskites is the presence of self‐trapped excitons. Here, it is demonstrated that in lead‐free double perovskites, the strong electron–phonon coupling plays a crucial role in the generation of self‐trapped excitons. The strong electron–phonon coupling is confirmed by a large Huang–Rhys factor and by the presence of multiphonon transitions. In particular, sharp emission lines superimposed on the broad photoluminescence emission band of one of these samples (Cs2Ag0.6Na0.4InCl6 0.5%Bi) are observed; these are due to the strong coupling of longitudinal‐optical phonons with excited electronic states caused by the tetragonally distorted AgCl6 octahedrons. Such a strong coupling of longitudinal‐optical phonons to electrons can effectively modulate the photophysical properties of alloyed double perovskites, and its understanding is, thus, of paramount importance for the design of future optoelectronic devices.

Tuning intrinsic photoluminescence from light-emitting multispectral nanoporous anodic alumina photonic crystals

To control and harness the intrinsic photoluminescence of solid-state, light-emitting materials produced by self-organization approaches remain challenging. This study demonstrates how the intrinsic broadband photoluminescence emission from nanoporous anodic alumina (NAA) produced by anodization of aluminum in oxalic acid electrolyte can be precisely tuned by engineering its structure in the form of photonic crystals (PCs). A combination of pulse and constant anodization in distinct acid electrolytes makes it possible to engineer a novel heterogeneous optical structure consisting of two layers: (i) a non-emitting, light-filtering layer in the form of multi-spectral nanoporous anodic alumina photonic crystals (MS–NAA–PCs) on its top (i.e., 58 µm thick and average pore diameter of 17 nm) and (ii) an intrinsically light-emitting layer of NAA at its bottom (i.e., 50 µm thick an average pore diameter of 40 nm). MS–NAA–PCs are engineered to feature three intense, well-resolved photonic stopbands (PSBs), the positions of which are spaced at specific regions of the visible spectrum from ∼380 to 560 nm. It is demonstrated that the PSBs of the non-emitting MS–NAA–PCs on top of the heterogeneous optical structure act as a light-filtering component, which makes it possible to narrow and tune the characteristically broad, Gaussian-like photoluminescence emission from the underlying light-emitting NAA layer. This structural design makes it possible to narrow the width of photoluminescence emission up to ∼50 nm and blue shift its position for ∼15 nm. Our advances pave the way for novel designs of intrinsic, light-emitting NAA-based PC structures, which could find broad applicability across light technologies, such as sensing and biosensing, photodetection, and solar light harvesting.

Strong visible-light emission in annealed poly(acrylic acid)

An intense and broadband photoluminescence emission is demonstrated in a bio-compatible film produced by thermal annealing of poly(acrylic acid) (PAA). The annealing process enhances the emission intensity from weakly emitting sub-luminophores of C–O and C=O functional groups by a factor of more than two orders of magnitude. The emission property can be sensitively tuned by the annealing temperature, duration, the PAA solution preparation conditions such as solvent type and solvent:PAA ratio. The strong luminescence is attributed to the formation of rigid molecular structure due to aggregation and crosslinking. FTIR measurements show that the aggregation and crosslinking processes subdue nonradiative recombination pathways.

Erbium-Doped Nanoparticle-Polymer Composite Thin Films for Photonic Applications: Structural and Optical Properties.

Erbium-doped nanocrystal (NC)-dispersed polymer thin films are attractive core materials for use in optical waveguides as they can provide high optical gain and enable the formation of compact waveguide amplifiers. Nonetheless, there are significant challenges associated with obtaining good dispersibility of NCs into a polymer matrix and favorable optical properties. Therefore, in this paper, we report the fabrication of Er3+-doped ceria (EGC) NCs employing the Leeds alginate process (LAP) and their incorporation into a siloxane polymer matrix. The surface morphology and compositional, structural, and optical properties of the fabricated films are evaluated to assess the NC dispersion and their suitability for the waveguide amplifier. The photoluminescence (PL) and lifetime measurements of the NCs–polymer nanocomposite thin film samples show intense, broadband PL emission of the Er3+ ions at 1534 nm (4I13/2 → 4I15/3 transition) with a full width at half-maximum (fwhm) of ∼64 nm and lifetime in the range of 2.6–3.0 ms. The inhomogeneously broadened PL spectra and improvement in lifetime of NCs in the polymer are important results that we report. The EGC NCs–polymer nanocomposite thin films also exhibit excellent transparency in the NIR wavelength range and a refractive index in the range of 1.53–1.58 in the visible wavelength. The work presented here clearly demonstrates the potential of using high-quality Er-doped nanocomposite polymer thin films for interesting applications such as compact low-cost waveguide amplifiers and lasers.

Gallium Oxide Nanowire with Twinning Structure and Its Photoluminescence Property.

In this paper, β-Ga₂O₃ nanowires were synthesized by vapor transport method at different temperatures. The as-prepared samples were analyzed for crystal structure by X-ray diffraction (XRD), transmission electron microscopy (TEM) and selected area electron diffraction (SAED), and for morphology using scanning electron microscopy (SEM). The results show that the Ga₂O₃ nanowires present a monoclinic structure, the length and diameter of the Ga₂O₃ nanowires increased with the growth temperature. A majority of the Ga₂O₃ nanowires present longitudinal twinning structures. A broad photoluminescence emission band was observed from the Ga₂O₃ nanowires at room temperature, which is caused by different kinds of vacancy defects. Our study shows an unusual twinning structure of β-Ga₂O₃ nanowires, which may be helpful to understand the growth mechanism of nanowires.

Controlled synthesis, mechanism and optical properties of triangular prism-like CeO2 nanostructures with corrugated surface

The large-scale uniform triangular prism-like CeO2 nanostructures with rough surface have been purposefully fabricated by a simple and efficient hydrothermal route. X-ray diffractometer (XRD), scanning electron microscope (SEM), X-ray photoelectron spectrum, Raman spectra and photoluminescence (PL) spectra were employed to research the obtained CeO2 samples. It is found that many Ce3+ ions and oxygen vacancies in the surface of cubic fluorite structure CeO2 samples. The possible mechanism involved the dissolution of rod-like structures and followed by the recrystallization of triangular prism-like structures was discovered. Ce3+ ions and oxygen vacancies may account for the similar broad photoluminescence emission bands for all samples, while their different emission intensity is attributed to the effects of morphology and size of the samples.

Diimidazolium Halobismuthates [Dim]2[Bi2X10] (X = Cl-, Br-, or I-): A New Class of Thermochromic and Photoluminescent Materials.

We present a novel family of polyhalide salts of Bi(III) with the general formula [Dim]2[Bi2X10], where Dim2+ is the diimidazolium cation (C9H14N4)2+ and X is Cl-, Br-, or I-. Single-phase materials are easily obtained by means of a mild solution chemistry method performed at room temperature. This [Dim]2[Bi2X10] family exhibits a crystal structure based on halobismuthate [Bi2X10]4- dimers, built by distorted {BiX6} octahedra interconnected by edge sharing, and sandwiched between two diimidazolium cations. The optical band gaps displayed by these materials (1.9-3.2 eV) allow their classification as semiconductors. Additionally, the three halides display photoluminescence with emission in the visible range. The behavior of [Dim]2[Bi2I10] is particularly interesting, as it shows an optical band gap of 1.9 eV, a broad band photoluminescence emission, and a relatively long emission lifetime of 190 ns. Moreover, the iodide and bromide compounds also exhibit a reversible solid state thermochromism, being the first example of a bromobismuthate with this property. The diimidazolium cations play an important structural role by stabilizing the crystal structure and balancing the charges of the [Bi2X10]4- dimers. Furthermore, density functional theory calculations suggest that they play a key role in the thermochromic behavior. Therefore, compounds [Dim]2[Bi2X10] (X = Cl-, Br-, or I-) represent a very versatile family in which the optical band gap can be tuned by changing the halide or temperature. This makes them promising new materials for different optoelectronic applications, in particular for obtaining new solar absorbers.

Cu(II) ion based 1D coordination polymer: Its thermal, fluorescence properties, Hirshfeld surface analysis and theoretical calculations

ABSTRACTIn this investigation, the reaction of [Cu(2,2′-bpy)(H2O)4][ClO4]2 with thiosemicarbazide (tsc) yielded [Cu(2,2′-bpy)(NCS)2]n frameworks, which has crystallized in noncentrosymmetric space group (Cc). The complex has been characterized by elemental analyses, FT-IR spectrum, powder X-ray diffraction analysis and single-crystal diffraction analysis. This complex is 1D polymer with high thermal stability and showing positive cotton effect. These results have provided some interesting insights into its nonlinear optical applications. Solid state fluorescence spectrum shows broad photoluminescence emission bands, which are assigned as the bands of intra-ligand charge transfer and MLCT (metal to ligand charge transfer) transitions. During investigation of various intermolecular interactions, both single crystal X-ray diffraction and Hirshfeld surface analysis give comparable results.

Thermoluminescence of the persistent-luminescence phosphor, BaAl2O4; A stuffed tridymite

BaAl2O4 is a stuffed tridymite used as a long-lasting phosphor. The thermoluminescence of BaAl2O4 prepared by solution-combustion is reported. Analysis of the sample using X-ray diffraction shows that it formed as a single phase compound with a hexagonal structure following annealing at 1200 °C. A broad photoluminescence emission band between 300 and 650 nm was detected due to excitation at 248 nm. The phosphor showed a natural TL peak at 102 °C for measurement at 1 oCs−1 and, when beta irradiated to 100 Gy, two broad peaks at 123 and 318 °C also for heating at 1 oCs−1. The analysis of the main glow peak at 123 °C suggests that it is a combination of several collocated peaks, that is, peaks embedded within each other. We resolved four such components labelled peaks 1 to 4. The thermoluminescence decreases with heating rate in a way consistent with thermal quenching whose activation energy was determined as ∼0.65eV using peak 3. Interestingly, this value of the activation energy for thermal quenching for BaAl2O4, a stuffed derivative of silica, is similar to literature values for quartz (a silica), suggesting that the recombination centre in the two cases may be similar.

Strong Electron–Phonon Coupling and Self-Trapped Excitons in the Defect Halide Perovskites A3M2I9 (A = Cs, Rb; M = Bi, Sb)

The optical and electronic properties of Bridgman grown single crystals of the wide-bandgap semiconducting defect halide perovskites A3M2I9 (A = Cs, Rb; M = Bi, Sb) have been investigated. Intense Raman scattering was observed at room temperature for each compound, indicating high polarizability and strong electron–phonon coupling. Both low-temperature and room-temperature photoluminescence (PL) were measured for each compound. Cs3Sb2I9 and Rb3Sb2I9 have broad PL emission bands between 1.75 and 2.05 eV with peaks at 1.96 and 1.92 eV, respectively. The Cs3Bi2I9 PL spectra showed broad emission consisting of several overlapping bands in the 1.65–2.2 eV range. Evidence of strong electron–phonon coupling comparable to that of the alkali halides was observed in phonon broadening of the PL emission. Effective phonon energies obtained from temperature-dependent PL measurements were in agreement with the Raman peak energies. A model is proposed whereby electron–phonon interactions in Cs3Sb2I9, Rb3Sb2I9, and Cs3Bi...

Color Change Effect in an Organic–Inorganic Hybrid Material Based on a Porphyrin Diacid

Porphyrinic materials show a range of interesting and useful optical and electrical properties. The less well-known subclass of porphyrin diacids has been used in this work to construct an ionic hybrid organic–inorganic material in combination with a halogenidometalate anion. The resulting compound, [H6TPyP][BiCl6]2 (1) (TPyP = tetra(4-pyridyl)porphyrin), has been obtained via a facile solution-based synthesis in single crystalline form. The material exhibits a broad photoluminescence emission band between 650 and 850 nm at room temperature. Single crystals of [H6TPyP][BiCl6]2 show a photocurrent in the fA and a much higher dark current in the nA range. They also display an unexpected reversible color change upon wetting with different liquids. This phenomenon has been investigated with optical spectroscopy, SEM, XPS, and NEXAFS techniques, showing that a surface-based structural coloration effect is the source of the color change. This stands in contrast to other materials where structural coloration typ...

Benzimidazolium Lead Halide Perovskites: Effects of Anion Substitution and Dimensionality on the Bandgap

We present the synthesis and structural characterization of a series of benzimidazolium‐based lead halide perovskites including (C7H7N2)2PbCl4, (C7H7N2)2PbBr4, (C7H7N2)2PbI4, and (C7H7N2)PbI3, which serves as a platform to investigate the change in optical properties as a function of the halide and the dimensionality of the inorganic sublattice. The structural similarity of the layered systems with A2MX4 stoichiometry was verified by single‐crystal X‐ray diffraction and solid‐state NMR spectroscopy. The optical properties were analyzed by absorption and photoluminescence (PL) measurements, confirming the decrease in bandgap when exchanging the chloride with its higher homologues Br and I. In addition, comparison of the layered compound (C7H7N2)2PbI4 with the known compound (C7H7N2)PbI3 reveals an increase in bandgap on going from a 2D to a dimensionally reduced 1D topology. Besides, significant differences in the PL spectra of the halides are observed, which likely are due to recombination of self‐trapped excitons stabilized through lattice distortions or permanent lattice defects in the compounds featuring broad PL emission bands.

A new zinc(II) coordination compound of meta-aminobenzoate and 4,4′-bipyridine with broad-band photoluminescence emission

A new coordination compound with a 1-D chain-like structure, [ZnL2(4,4′-bipy)]n (L=meta-aminobenzoate; 4,4′-bipy=4,4′-bipyridine), has been hydrothermally prepared and characterized by single-crystal X-ray diffraction analysis, elemental analysis, and photoluminescence determination. The obtained yellow prismatic crystal belongs to triclinic space group P1¯ with a=7.9053(16), b=11.664(2), c=12.404(3)Å, α=98.25(3), β=95.54(3), γ=103.08(3)°, Z=2, R1=0.0439 and wR2=0.0757. The 1-D chain contains rarely both μ-N,O-bridging bidentate and non-bridging monodentate coordination modes of L ligands, and they are consolidated by N–H···O, N–H···N, and C–H⋯π hydrogen bonds as well as π⋯π stacking interactions to yield a 3-D supramolecular network. The title compound exhibits a very broad emissive band (λem,max=520nm) when illuminated by 370nm light, where the high-energy and low-energy components are tentatively ascribed to ligand-centered and interligand transition, respectively, on the basis of experimental and theoretical data. This work may shed light on the design and synthesis of white-light-emitting coordination compounds.

Photoluminescence optimization of BCNO phosphors synthesized using citric acid as a carbon source

Citric acid was used as carbon source for the optimization of the photoluminescence (PL) performance of boron carbon oxynitride (BCNO) phosphor. Citric acid was chosen as an alternative carbon source because of its simple molecular structure, low decomposition temperature, relative inexpensiveness, and environmental friendliness. The prepared sample exhibited a single, homogeneous, and broad photoluminescence emission band whose peak varied from near-UV (400nm) to yellow-visible (500nm) upon excitation at 365nm. The effects of varying the synthesis temperature, molar ratio of the carbon/boron and nitrogen/boron sources, and addition of SiO2 nanoparticles on the PL properties were also studied. The optimized BCNO phosphors may find potential use in white LED applications.