Broad Photoluminescence Band Research Articles

Detailed photoluminescence study of Cu2Ge(SSe)3 microcrystals

We present a detailed temperature and laser power dependent photoluminescence (PL) study of Cu2Ge(S0.4Se0.6)3 microcrystals. At T = 20 K, two relatively narrow PL peaks were detected at about 1.16 eV (peak No. 1) and 1.12 eV (peak No. 2) and a weak, broad PL band was detected at about 0.82 eV (peak No. 3). The temperature and laser power dependencies indicate that at T = 20 K, the properties of PL peak Nos. 1 and 2 can be explained by the distant donor–acceptor (DA) pair model where a donor defect has a depth of ED ≈ 20 meV and ED ≈ 60 meV for peak Nos. 1 and 2, respectively. The depth of acceptor defects was 57 and 76 meV for peak Nos. 1 and 2, respectively. At around T = 90 K, the DA pair recombination of peak No. 1 gradually starts to transform into the conduction band–acceptor recombination, but peak No. 2 shows a DA pair recombination even at room temperature. The estimated bandgap energy of this compound at room temperature was Eg = 1.225 eV.

Link between Structural and Optical Properties of CoxFe3–xO4 Nanoparticles and Thin Films with Different Co/Fe Ratios

CoxFe3–xO4 nanoparticles (x = 0.4 to x = 2.5) and thin films (x = 0.9 to x = 2.2) are analyzed by Raman, absorption, and photoluminescence spectroscopy to link structural and optical properties to different cobalt to iron (Co/Fe) ratios. Raman spectroscopy shows that with decreasing Co content, the crystal structure changes from a predominantly normal cubic spinel phase to a mixed inverse spinel phase. This finding is supported by absorption spectroscopy that points out that inter valence charge transfer (IVCT) processes between octahedrally coordinated Co2+ and Fe3+ cations become more prominent with increasing Fe content. Independent of the Co/Fe ratio, CoxFe3–xO4 nanoparticles show a broad photoluminescence (PL) band with a maximum at around 510 nm. Time-resolved photoluminescence spectroscopy shows subnanosecond lifetimes and temperature-resolved photoluminescence experiments reveal that the green PL increases with decreasing temperature (300 to 10 K) while showing no temperature-dependent shift in energy. It is proposed that this green PL originates from OH-groups on the particles’ surface.

Core-clad terbium doped chalcogenide glass fiber with laser action at 5.38 μm

The core-clad Tb3+-doped Ga-Ge-As-Se/Ge-As-S chalcogenide glass fiber is fabricated and tested in laser experiment. To prepare the high-purity terbium doped Ga-Ge-As-Se core glass, the special multi-stage purification technique is developed. The 1050 ppmw Tb3+-doped core Ga3.2Ge24.9As15.3Se56.6 glass is characterized by an extra-low content of limiting hydrogen impurity, high stability against crystallization and strong broadband photoluminescence in the middle infrared range. The doped chalcogenide fiber has a total diameter of 400 μm, a core diameter of 18 μm, and minimum optical losses of 3 ± 0.3 dB/m at a wavelength of 7.1 μm. Using a Tm3+ 1.98 μm fiber laser as a pump source, the threshold of 5.38 μm lasing with spike structure was reached in chalcogenide fiber for the first time.

Single-Mode Lasing in Polymer Circular Gratings.

In recent years, conjugated polymers have become the materials of choice to fabricate optoelectronic devices, owing to their properties of high absorbance, high quantum efficiency, and wide luminescence tuning ranges. The efficient feedback mechanism in the concentric ring resonator and its circularly symmetric periodic geometry combined with the broadband photoluminescence spectrum of the conjugated polymer can generate a highly coherent output beam. Here, the detailed design of the ultralow-threshold single-mode circular distributed feedback polymer laser is presented with combined fabrication processes such as electron beam lithography and the spin-coating technique. We observe from the extinction spectra of the circular gratings that the transverse electric mode shows no change with the increase of incident beam angle. The strong enhancement of the conjugated polymer photoluminescence spectra with the circular periodic resonator can reduce the lasing threshold about 19 µJ/cm2. A very thin polymer film of about 110 nm is achieved with the spin-coating technique. The thickness of the gain medium can support only the zero-order transverse electric lasing mode. We expect that such a low threshold lasing device can find application in optoelectronic devices.

Femtosecond Laser Fabrication of Hybrid Metal-Dielectric Structures with Nonlinear Photoluminescence

Fabrication of hybrid micro- and nanostructures with a strong nonlinear response is challenging and represents a great interest due to a wide range of photonic applications. Usually, such structures are produced by quite complicated and time-consuming techniques. This work demonstrates laser-induced hybrid metal-dielectric structures with strong nonlinear properties obtained by a single-step fabrication process. We determine the influence of several incident femtosecond pulses on the Au/Si bi-layer film on produced structure morphology. The created hybrid systems represent isolated nanoparticles with a height of 250–500 nm exceeding the total thickness of the Au-Si bi-layer. It is shown that fabricated hybrid nanostructures demonstrate enhancement of the SHG signal (up to two orders of magnitude) compared to the initial planar sample and a broadband photoluminescence signal (more than 200 nm in width) in the visible spectral region. We establish the correlation between nonlinear signal and phase composition provided by Raman scattering measurements. Such laser-induced structures have significant potential in optical sensing applications and can be used as components for different nanophotonic devices.

Tailoring the photoluminescence of BaMoO4 and BaWO4 hierarchical architectures via precipitation induced by a fast precursor injection

We investigate the effects of temperature and reaction time on structural, morphological, and optical properties of BaMoO4 and BaWO4 synthesized via precipitation induced by a fast precursor injection. Oriented aggregation and matter transfer influenced the particle sizes and shapes. The morphological changes were responsible for manipulating the maximum photoluminescence (PL) emissions from blue to red, exclusively in BaWO4. The broadband PL profiles and band gap energies (from 4.08 to 4.98 eV) suggested the participation of direct electronic transitions among intermediary energy levels in the forbidden region. The proposed template/surfactant-free approach has the potential to growth hierarchical structures of tungstates/molybdates with improved optical properties.

Photoinduced Dynamics of (CH3NH3)4Cu2Br6 Thin Films Indicating Efficient Triplet Photoluminescence.

Hybrid organic-inorganic halogenidocuprates based on copper(I) represent materials with rich structural diversity and high photoluminescence (PL) quantum yield, yet the mechanism responsible for their efficient, strongly Stokes-shifted emission is still unclear. Here we report the successful preparation of (CH3NH3)4Cu2Br6 thin films with a zero-dimensional molecular salt structure featuring "isolated" [Cu2Br6]4- ions. Time-resolved broadband PL measurements provide an excited-state lifetime of 114 μs at 298 K. Results from femto- to microsecond UV-vis-NIR transient absorption experiments combined with DFT/TDDFT calculations suggest the formation of a long-lived structurally relaxed triplet species through intersystem crossing (61 ps), which almost exclusively decays by phosphorescence. In addition, time scales for structural relaxation and cooling processes are extracted from a global kinetic analysis of the transient spectra. Calculations for the isolated [Cu2Br6]4- anion and the (CH3NH3)4Cu2Br6 crystal suggest a strong impact of the crystal environment on the structure of the anion.

Broad band photoluminescence of g-C3N4/ZnO/ZnS composite towards white light source

We have found one step synthesized g–C3N4/ZnO/ZnS composite to be promising for engineering of white-light sources. The composite was fabricated by simultaneous pyrolytic decomposition of thiourea and zinc acetate with a consequent insitu polymerization of the products at 550–625 °C and characterized by SEM, XRD, EDX and low temperature PL spectroscopy. The composite synthesized at 550 °C demonstrates blue PL with a peak at 2.74 eV while higher temperatures of the synthesis result in a broad spectra with a maximum at 2.22 eV and comprise of the bands of g–C3N4 and mixed ZnO/ZnS as they have been resolved by low temperature PL. The proper selection of the synthesis temperature allows an extension of light emitting spectra of g–C3N4/ZnO/ZnS composites from blue to white light range. The results obtained show ternary systems containing g-C3N4 and wide band-gap semiconductors are suitable for white-light emitting and optoelectronic devices.

Assessing the atomic structure of the defect complex in a solid electrolyte by photoluminescence measurements

We demonstrate that photoluminescence in solid electrolytes is sensitive to the atomic structure of the defect complex that is decisive to the ionic conduction. We systematically measure the photoluminescence spectra of a typical solid oxide electrolyte, stabilized zirconia sintered pellets. Based on the comparison with the photoluminescence spectrum of a single crystal, we assign the broad long-lived photoluminescence band in the visible region near 2.4 eV to that related to the defect complex. Because the electronic state of the oxygen vacancy is sensitive to the surrounding ions, which has been indicated in previous investigations of the local structure around the dopants and vacancies, we are able to assign each sample’s photoluminescence characteristics to a certain atomic arrangement that is considered plausible based on previous investigations. Photoluminescence spectroscopy is applicable to various solid electrolytes and can become a powerful tool for their characterization.

Understanding photoluminescence in semiconductor Bragg-reflection waveguides

Compared to traditional non-linear optical crystals, like BaB2O4, KTiOPO4 or LiNbO3, semiconductor integrated sources of photon pairs may operate at pump wavelengths much closer to the bandgap of the materials. This is also the case for Bragg-reflection waveguides (BRWs) targeting parametric down-conversion (PDC) to the telecom C-band. The large non-linear coefficient of the AlGaAs alloy and the strong confinement of the light enable extremely bright integrated photon pair sources. However, under certain circumstances, a significant amount of detrimental broadband photoluminescence has been observed in BRWs. We show that this is mainly a result of linear absorption near the core and subsequent radiative recombination of electron–hole pairs at deep impurity levels in the semiconductor. For PDC with BRWs, we conclude that devices operating near the long wavelength end of the S-band or the short C-band require temporal filtering shorter than 1 ns. We predict that shifting the operating wavelengths to the L-band reduces the amount of photoluminescence by 70% and making small adjustments in the material composition results in its total reduction of 90%. Such measures enable us to increase the average pump power and/or the repetition rate, which makes integrated photon pair sources with on-chip multi-gigahertz pair rates feasible for future devices.

Triple‐Site Dopant–Defect Complexes in Mg–H‐Codoped GaN: First‐Principles Identification

There are dozens of double‐site and triple‐site dopant–defect complexes in Mg–H‐codoped GaN that can compensate the MgGa acceptor and thus limit p‐type conductivity; however, their properties have not been systematically studied. Using first‐principles calculations, herein, it is found that the well‐known double‐site complexes MgGa–VN and MgGa–HN can still act as donors, although they are already donor–acceptor‐compensated complexes. Therefore, they can compensate another MgGa acceptor and form the triple‐site complexes MgGa–MgGa–VN and MgGa–MgGa–HN, which have never been reported. The formation energies of the two triple‐site complexes can be low, especially in Ga‐rich and n‐type GaN, so they have non‐negligible concentration and thus play an important role in limiting the p‐type conductivity. The MgGa–MgGa–VN complex produces a defect level that can not only induce nonradiative electron–hole recombination, but also give rise to a red and broad photoluminescence band at 1.76 eV, providing another explanation for the red band observed in heavily Mg‐doped GaN. These triple‐site dopant–defect complexes should be taken into account in future study of defects in GaN and related semiconductors.

Фотолюминесценция композитов полиметилметакрилат / [(Zn,Cd)S : Mn,Cu,Eu

Nanoscale structures based on zinc and cadmium sulfides doped with Mn, Cu, and Eu ions were synthesized by the method of emerging reagents in a polymerizing medium of methyl methacrylate. The broadband photoluminescence of the compositions is associated with recombination processes at the defect levels of semiconductor structures. Narrow photoluminescence bands occur during electron transitions between the energy levels of Eu3+ ions. The excitation of photoluminescence occurs as a result of the interband transition and electron transitions to the levels of structural defects, as well as during self-absorption and energy transfer to the levels of Eu3+ ions.

Broad-band photoluminescence of donor–acceptor pairs in tetrahedrite Cu10Cd2Sb4S13 microcrystals

We present temperature and laser power dependent photoluminescence (PL) study of Cd substituted tetrahedrite Cu10Cd2Sb4S13 microcrystals. At T = 10 K a single broad, asymmetric and structureless PL band was detected at about 1.08 eV. The temperature and laser power dependencies indicate that the properties of PL emission can be explained by the distant donor–acceptor (DA) pair model, where a donor defect has a depth of E D ≈ 30 meV and an acceptor defect E A = 88 ± 6 meV. It was shown that the shape of the DA pair band could be effectively described using statistical distribution of donor–acceptor defects, recombination probability of DA pairs with different spatial separation, relatively strong electron–phonon coupling and occupation probabilities of donor and acceptor defects. At T = 200 K the DA pair recombination gradually starts to transform into conduction band-acceptor recombination.

Defect-related photoluminescence and photoluminescence excitation as a method to study the excitonic bandgap of AlN epitaxial layers: Experimental and ab initio analysis

We report defect-related photoluminescence (PL) and its vacuum ultraviolet photoluminescence excitation (PLE) spectra of aluminum nitride layers with various layer thicknesses and dislocation densities grown on two different substrates: sapphire and silicon. The defect-related transitions have been distinguished and examined in the emission and excitation spectra investigated under synchrotron radiation. The broad PL bands of two defect levels in the AlN were detected at around 3 eV and 4 eV. In the PLE spectra of these bands, a sharp excitonic peak originating most probably from the A-exciton of AlN was clearly visible. Taking into account the exciton binding energy, the measurements allow determination of the bandgaps of the investigated AlN samples and their temperature dependencies. Next, they are compared with the literature data obtained by other experimental techniques for bulk AlN crystals and layers grown on different substrates. The obtained results revealed that the AlN bandgap depends on the substrate. The theoretical analysis using density functional theory calculations showed that the effect is induced by the tetragonal strain related to the lattice mismatch between the substrate and the AlN layer, which has a strong influence on the spectral positions of the intrinsic excitons, and consequently on the bandgap of AlN layers.

Insight into the enhanced photocatalytic properties of AgBr/Ag4P2O7 composites synthesized via in situ ion exchange reaction

Photocatalysis is a promising advanced oxidation technology aimed to assist in the degradation of toxic contaminants found in wastewaters into harmless compounds. Here, we report a simple and versatile synthetic route based on in situ ion exchange reaction to produce AgBr/Ag4P2O7 composites with improved photocatalytic performance. All AgBr/Ag4P2O7 composites prepared with different Br/P ratios exhibited superior photocatalytic activities compared to their respective single counterparts. The optimized condition was found at 50 mol%, whose photocatalyst was able to degrade the rhodamine B (RhB) up to 4 min under ultraviolet irradiation. This high photocatalytic performance was maintained even after five successive cycling runs. In the photocatalytic reactions by using AgBr/Ag4P2O7 composites, OH•, Br•, O2•− radicals and photogenerated holes were the main active species responsible for the oxidation of RhB molecules. Our samples presented indirect transitions with band gap energies from 3.20 eV to 3.40 eV, resulting in maximum photoluminescence (PL) emissions within the ultraviolet energy range. Thus, the broadband PL profiles were predominantly controlled by shallow-level defect states in the band gap. These results hold great promise for generating new silver-based materials with enhanced photocatalytic and optical properties.

Low thermal expansion and broad band photoluminescence of Zr0.1Al1.9Mo2.9V0.1O12 * *Project supported by the National Natural Science Foundation of China (Grant Nos. 11874328, U1731121, and 41401384) and the Shandong Province Higher Educational Science and Technology Program, China (Grant No. J17KB127).

A new material of Zr0.1Al1.9Mo2.9V0.1O12 is synthesized by the traditional solid state synthesis method. The phase transition, coefficient of thermal expansion, and luminescence properties of Zr0.1Al1.9Mo2.9V0.1O12 are explored with Raman spectrometer, dilatometer, and x-ray diffraction (XRD) diffractometer. The results show that the Zr0.1Al1.9Mo2.9V0.1O12 possesses the strong broad-band luminescence characteristics almost in the whole visible region. The sample is crystallized in a monoclinic structure group of P21/a (No. 14) crystallized at room temperature (RT). The crystal is changed from monoclinic to orthorhombic structure when the temperature increases to 463 K. The material has very low thermal expansion performance in a wide temperature range. Its excellent low thermal expansion and strong pale green light properties in a wide temperature range suggest its potential applications in light-emitting diode (LED) and other optoelectronic devices.

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.

Time-resolved and Temperature-dependent Broadband Emission of Plasmon-coupled Quantum Dots

The broadband photoluminescence (PL) emissions from CdSe QDs and plasmon-coupled QDs were characterized with time-resolved and temperature-dependent spectroscopy for the application of solid-state white light. The origin of broad spectral emission includes the transitions from bandedge and surface-trapped states. The emission intensity enhancement of plasmon-coupled QDs with respect to that of bare QDs is attributable to the reduction of nonradiative decay and the local field enhancement with plasmon-exciton coupling through the Coulomb interaction. The temperature-dependent and time-resolved PL spectroscopy revealed the existence of selective contribution strength of both the local field enhancement and the reduction of nonradiative decay with plasmon-exciton coupling at different spectral regions.

Solution-Based Synthesis of Layered Two-Dimensional Oxides as Broadband Emitters.

Preparing transition-metal oxides in their two-dimensional (2D) form is the key to exploring their unrevealed low-dimensional properties, such as the p-type transparent superconductivity, topological Mott insulator state, existence of the condensed 2D electron/hole gas, and strain-tunable catalysis. However, existing approaches suffer from the specific constraint techniques and precursors that limit their product types. Here, we report a solution-based method to directly synthesize KNbO2 in 2D by an out-of-the-pot growth process at low temperature, which is observed directly in real time. The developed method can also be applied to other 2D ternary oxide syntheses, including CsNbO2 and composited NaxK1-xNbO2, and it can be extended to the preparation of self-assembled nanofilms. In addition, We demonstrate the emission of broadband photoluminescence (PL, λ ∼ 350-800 nm) from as-synthesized single-crystal 2D KNbO2 sheets down to a single unit cell thickness. The ultra-broadband emission is ascribed to the self-trapped excitation state (STEs) from the in-phase distortion of the NbO6 octahedrons in 2D NbO2- layers. Beyond the broader luminescent range and the robust material thermal stability of niobates, the absence of sample size restrictions and the large aspect ratio of the 2D oxide sheets will provide opportunities in miniaturizing and advancing 2D-materials integrated optoelectronic devices.

Formation and optical properties of IR photoluminescence centres in bismuth-containing aluminophosphate glass

We have prepared aluminophosphate glass samples differing in bismuth oxide content and demonstrating broadband near-IR photoluminescence. Analysis of their photoluminescence spectra leads us to conclude that they contain two main types of emission centres, one of which seems to be a bismuth monocation and the other is a cluster ion. In addition to these luminescence centres, the aluminophosphate glasses contain a nonluminescent bismuth-containing centre responsible for the broad, strong optical absorption band peaking at 450 nm.