Broad Photoluminescence Research Articles

Examining the contribution of Cu and Sr codoping on luminescence properties of borate crystals

Four crystals of Ba12(BO3)6[BO3][LiF4] (LBBF) with different ratios of co-dopants, Cu and Sr, were grown from high-temperature solutions. The LBBF structure is built from a porous [Ba12(BO3)6]6+ framework with channels along the optical axis filled with anionic groups. It was found that LBBF:Cu,Sr accommodates copper ions in the channels of the structure in both mono- and divalent states with the concentration of Cu + exceeding that of Cu2+, and Sr2+ replaces Ba2+ in the framework. At a constant concentration of copper and an increase in the concentration of strontium in the initial solution, the concentration of Cu+ in LBBF does not change monotonically but has a maximum. The broad photoluminescence band at about 410–440 nm is associated with the emission of Cu+ ions, while the bands at about 310–317 nm and 600 nm are related to Cu2+. All emissions demonstrate considerable temperature quenching in the range from 77 to 300 K. The LBBF:Cu crystals exhibit intense thermoluminescence (TL) in the range from 100 to 250 K after low-temperature X-ray irradiation, and peaks at about 315 and 380 K after room-temperature irradiation. With combined doping with strontium, the TL peaks shift to the region of higher temperatures, 365 and 410 K, without weakening the high-temperature component, which intensity linearly depends on radiation dose.

Broad photoluminescence and photocatalysis from functional carbonaceous material-metallized porphyrin composites for UV-Vis light harvesting

Abstract Carbon in its many form has attracted huge attraction over the years owing to its biocompatibility, processability, stability and optical properties for its utilization in UV-Vis light harvesting in pure as well as modified schemes such as composite. This study reports green precursor synthesis as well as optical, structural, luminescence and catalysis study of functional carbonaceous material (FCM) in pure and composite form. This high-order self-assembly of FCM possess extraordinary photophysical and chemical properties with its functional groups rich surface making it suitable for π-π* transitions. Further its composite formation with a high absorption coefficient (more than 20,000 cm-1 in broad visible range 400-700 nm) and non-radiative (near IR) fluorescence quenching exhibiting metallized porphyrin is shown to be suitable for photocatalysis applications. These conclusions have been established using XRD, FESEM, EDX, FTIR, UV-Vis, FL and Raman spectroscopy measurements. Broad range photoluminescence for pure FCM (300-550 nm) as well as its porphyrin-based composite (400-550 nm) and further the application of composite in the photocatalytic ability for the degradation of standard methylene blue dye for degradation percentage of 62% in 120 mins in visible light while 30% in 180 mins in dark shows the correlation for sustainable synthesis and application of FCM-porphyrin composite.

Multifunctional nanoplatforms based on alumina-coated mesoporous silica with potential for cancer theranostics applications

Multifunctional nanoplatforms based on mesoporous silica coated with alumina were successfully and sustainably synthesized using sodium silicate extracted from rice husk ash (RHA), demonstrating the potential for cancer theranostics. The hybrid nanostructures produced (C2-600 and C4-600) from two different calcination times exhibited distinct morphologies, textural parameters, and degrees of mesoscopic organization. As expected, the Al2O3 coating on the mesoporous silica nanoparticles (MSNs) reduced the specific surface area from 720 m2/g to 122 m2/g (C2-600) and 57 m2/g (C4-600), while preserving their mesoporous structure. Additionally, both C2-600 and C4-600 showed relatively good stability across a wide pH interval, with a zeta potential of ζ = −15 mV and hydrodynamic diameter (Dh) ranging from 160 to 670 nm, depending on the pH of the medium. These peculiar characteristics resulted in high encapsulation efficiency (EE ≈ 90 %) for the doxorubicin (DOX) anticancer drug, as well as sustained drug release in simulated gastric fluid (SGF, pH 1.2) and simulated intestinal fluid (SIF, pH 7.4). Appreciably, C4-600-DOX released approximately 83 % of the drug over 96 h and demonstrated significant biodegradation in simulated biological media. Furthermore, the hybrid nanoplatforms exhibited strong optical absorption between 250 and 420 nm, along with broad and intense photoluminescence (PL) in the near-infrared (NIR) region (680–900 nm), which is highly desirable for NIR-fluorescence diagnostic imaging. Notably, the hybrid nanoplatforms without DOX were non-cytotoxic to fibroblast and Caco-2 cells, while the DOX-loaded nanoplatforms exhibited selectivity and potent anticancer activity, inhibiting approximately 80 % of Caco-2 colorectal cancer cells after 72 h. These findings demonstrate that C2-600 and C4-600 are innovative, multifunctional and biodegradable nanoplatforms with powerful potential as drug carriers and fluorescence imaging agents, making them promising candidates for cancer theranostics.

Low temperature recombination luminescence of Mg3Y2Ge3O12:Tb3+

A study was conducted to examine the recombination processes in persistent phosphor Mg3Y2Ge3O12:Tb3+ garnet at low temperatures. Photoluminescence (PL), recombination luminescence (RL), electron paramagnetic resonance (EPR), and EPR detected by PL or RL were measured.In samples with low Tb3+ concentration, a broad PL and RL band around 400–450 nm and characteristic Tb3+ lines were observed. However, in samples with high Tb3+ concentration, only Tb3+ lines were present. Both the broad-band and the line components exhibit long-lasting tunneling luminescence with hyperbolic decay. After 263 nm UV irradiation signals of intrinsic electron (F-type) and hole (V-type) trapping centres were observed in the EPR spectra. Such signals were also observed in RL-detected EPR spectra, indicating that the broad RL band at low Tb3+ concentrations originates from tunneling recombination between these intrinsic traps. At high Tb3+ concentrations, the RL-EPR spectrum was not observed, suggesting that intrinsic electron and Tb-related hole trapping centres probably participate in the tunneling recombination.

Luminescence and dielectric investigations of crystalline niobate nanoceramics prepared through aqueous chemical process

The present study reflects the synthesis of MgNb2O6 using hydrofluoric acid via a wet chemical approach, followed by characterizations involving XRD, electron microscopy, Raman spectroscopy, optical analyses, and impedance spectroscopy. The crystallite size of the synthesized material was determined to be 44 nm through XRD analysis. The lattice parameters of MgNb2O6 a, b, and c, were found to be 14.1998 Å, 5.6844 Å, and 4.9813 Å, respectively. Raman spectroscopy identified molecular bonds ranging from 253 to 1011 cm−1, mainly indicating the presence of metal oxide bonds. EDX spectra confirmed the presence of Mg, Nb, and O atoms in the prepared ceramics, indicating phase purity. FESEM analysis revealed a grain size of approximately 48 nm, with the presence of agglomerated grains. Bright spots in the SAED pattern observed by HRTEM confirmed the crystallinity of the prepared niobate materials, with the HRTEM microstructure showing a particle size near 49 nm. The crystallite size by XRD, grain size by FESEM, and particle size by HRTEM are in accordance with each other. The direct band gap was determined to be approximately 2.76 eV using UV-Visible spectroscopy. Additionally, MgNb2O6 materials exhibited a broad and strong photoluminescence emission near 445 nm with excitation at 270 nm, possibly indicating the presence of radiative defects in the crystalline nanostructure. Furthermore, impedance studies conducted between 40 and 110 MHz demonstrated a decrease in the dielectric constant at higher frequencies, reaching 21.06 at 110 MHz. A low dielectric loss was also observed at 110 MHz. The moderate band gap and strong room-temperature photoluminescence in the visible range make magnesium niobates suitable for possible applications in optical devices. This investigation shows that a dielectric constant near 21 and low dielectric loss can be achieved in the high-frequency range around 110 MHz.

Characterization of polycrystalline bulk ferroelectric ZnSnS3 synthesized by hydrothermal method for photovoltaic application

ZnSnS3, a newly theoretically predicted ferroelectric material which shows promising properties for solar cell and photovoltaic applications, was successfully grown in our laboratory by hydrothermal method. The high intensity x-ray diffraction pattern from the (211), (101̅), (200), (210), (310), (320), and (321̅) planes reveal the crystalline character of the trigonal ZnSnS3 phase. The microstructural property and elemental distribution were evaluated using SEM and TEM studies. Diffuse reflectance measurement at room temperature shows a direct bandgap of 2.62 eV along with a high energy direct transition of 3.13 eV. Two broad photoluminescence peaks at various temperatures (4–300 K) were also detected around (2.61–2.73 eV) and (3.04–3.11 eV). The emission characteristics are explained considering the shallow donor level to valence band transitions. Raman study elucidates that the synthesized ZnSnS3 possess a good number of Raman active bands. A phase transition from the ferroelectric to non-ferroelectric phase of ZnSnS3 is observed around 330 °C in DSC and dielectric measurements. The P-E measurement showed that the material is ferroelectric in nature with saturation polarization value of 21.85μC/cm2. The current-voltage characteristics showgood photovoltaic response of the fabricated Ni/ZnSnS3/Ni device in the visible range indicating its application in PV devices and photodetector.

Improvement on the onset voltage for electroluminescent devices based in a SiOx/SiOy bilayer obtained by sputtering

Abstract This work is focused on the composition, optical and electroluminescent properties of silicon rich oxide (SiOx, x < 2) films monolayers and bilayers (SiOx/SiOy) deposited by Sputtering with silicon excess between 6.2 to 10.7 at.% were deposited on p-type (100) silicon substrates. As-deposited SiOx films emit a broad photoluminescence (PL) band where the maximum peak shifts from 420 to 540 nm as the Si-excess increases from 6.2 to 10.7 at.%, respectively. The PL intensity strongly increases and the main PL peak shifts to the red region when the SiOx films are thermally annealed. The PL emission band was dependent on silicon excess and the presence of Si-O bonds defects working as emission centers. MOS-like devices were fabricated (N+ polysilicon was used as top contact and aluminum as bottom contact) to study the EL of SiOx monolayers and SiOx/SiOy bilayers. It was found that the required voltage to obtain EL was reduced when SiOx/SiOy bilayers were used in light emitting capacitors (BLECs) as compared to those with SiOx monolayers.

(Invited) Optical Excitations and Growth in Layered Organic Metal Halide Semiconductors

Hybrid organic metal halides, such as CH3NH3PbI3, have garnered attention because they are earth-abundant, solution-processable materials that can be used to form solar cells with high power conversion efficiency (>20%). There are significant questions about the unusual electronic properties of this class of materials because of the coupling between excitations and the lattice. We will present our work investigating the optoelectronic properties of layered organic metal halide systems and the relationship to structure and growth conditions. We will discuss the nature of optical excitations in layered compounds of the class R2PbX4 where R is an organic cation and X is a halide. These systems show signatures of unusually strong optical frequency magnetic dipole transitions that can be understood through self-trapped exciton formation. We will then discuss how mechanical strain during growth influences broad photoluminescence in a model layered system based on the large cation ethylammonium (EA) with the structure (EA)2(EA)n-1PbnBr3n+1. In this system the emission is strongly impacted by growth-induced strain in thin films quantified using X-ray scattering. Our results suggest that broad emission, relevant for LEDs, can be tuned in thin films. Recent work on how growth impacts the behavior of low dimensional system will also be presented.

Transiently controlled emission from ZnO surface

We report on a photon (∼3.08 eV equivalent to 402 nm) controlled optical emission from ZnO (10 1¯ 0). Under below band gap excitation (∼2.33 eV equivalent to ∼532 nm), significant photoluminescence (PL) overlapped with Raman response is observed. The broad PL consists of three bands (629 (A), 690 (B), and 751 (C) nm) attributed to the defects arising due to excess zinc and charged oxygen vacancy. By employing a second excitation source at 402 nm, we demonstrate about 50% reduction in the overall PL. We utilize the doubly positive oxygen vacancy state to control the PL emission while transiently reducing its density.

Broad photoluminescence of black phosphorus with electrochemical intercalation of organic ions

Broad photoluminescence of black phosphorus with electrochemical intercalation of organic ions

Tunable cold/warm white light emitting devices based on carbon dots with multiple emissive centers

The correlated color temperature (CCT) of lighting is closely related to human emotions. However, there is still a relative lack of white light emitting devices (W-LEDs) with tunable CCT to meet application demands in different scenarios. Here, we develop a W-LED with tunable CCT based on multiple emissive centers of carbon dots (CDots) synthesized through controlled thermal carbonization of citric acid and urea. The reaction at 160 °C for 4 h results in amorphous CDots, which exhibit excitation wavelength dependent photoluminescence (PL) containing two peaks at 446 and 520 nm. However, by increasing the reaction temperature and duration, crystalline CDots are obtained, which show broad PL peaking at 650 nm. Based on their complementary spectral ranges, a W-LED with an extremely high color rendering index of 96 is demonstrated. Moreover, a W-LED with both 365 and 405 nm electroluminescent chips is designed. The W-LED shifts from cold to warm white, when the excitation chips switch from 365 to 405 nm. This adjustable CCT can create a harmonious, enjoyable, and safe lighting environment to meet the demands of different scenes.

Infrared Photodetector Based on van der Waals MoS2/MoTe2 Hetero‐Bilayer Modulated by Photogating

AbstractPhotodetectors based on 2D hetero‐bilayers can overcome the bandgap limitations of individual 2D monolayers and operate at relatively long wavelengths. However, ultra‐low light absorptions within hetero‐bilayers result in extremely weak photoresponsivity. Here, an infrared photodetector based on the MoS2/MoTe2 type‐II hetero‐bilayer is demonstrated to reach a photoresponsivity of 0.55 A W−1 at 1550 nm, well beyond energy band cut‐offs of monolayer MoS2 and MoTe2, primarily resulted from the photogating effect. Raman and photoluminescence (PL) spectroscopy reveal strong interlayer couplings in the hetero‐bilayer, and a broad PL peak around 1550 nm is observed that is ascribed to interlayer transitions of carriers. The photodetector showcases a broadband detection capability from 1100 to 1700 nm, with a peak at 1550 nm corresponding to the interlayer absorption. Electrical characterization of the hetero‐bilayer‐based field‐effect transistor and kelvin probe force microscopy reveal efficient interlayer hole transfer. The highly responsive MoS2/MoTe2 infrared photodetector offers a large photo‐gain of ≈103 and a time constant of 130 ms. The research illuminates how interlayer transitions affect 2D hetero‐bilayer‐based photodetectors and advances the utilization of layered semiconductor heterostructures.

The Impact of the Gain Medium Properties and the Resonator Morphology on the Whispering Gallery Mode Spectrum of Polystyrene Microspheres Coated with AgInS2/ZnS Quantum Dots

The modulation of fluorophore emission by whispering gallery modes (WGMs) in active spherical resonators holds great potential for advanced photonic devices. In the present work, we fabricate and systematically study a series of active WGM resonators based on polystyrene microspheres and heavy metal-free ternary AgInS2/ZnS quantum dots with broad and tunable photoluminescence bands coated on their surface as a gain medium. The variation in the mean size and emission wavelength of the quantum dots allowed us to establish a correlation between their characteristics and the WGM properties, including the resonance peaks intensity profile and the Q-factors. Other aspects such as coupling efficiency, polarization-dependent mode damping, and peak broadening are also discussed. Furthermore, we applied statistical analysis on multiple individual resonators within the batch to calculate the standard deviation of the resonator size and ellipticity. Our study highlights that both the cavity and the gain medium influence the WGM parameters in active spherical resonators.

An EPR study of point defects in zinc oxide thin films

We studied ZnO thin films deposited on glass slides by thermal evaporation in vacuum followed by heat treatment in open air or Ar flow. The samples were characterized using x-ray diffraction, Raman scattering, photoluminescence (PL), and electron paramagnetic resonance (EPR) spectroscopy. We observed a broad PL band in the visible region at spectral positions of 504 and 560 nm and a low-intensity band in the UV region at 390 nm. The EPR spectra display a clear first derivative structure at g=1.96 at temperatures below 200 K. The PL spectrum shows red-shifted valence to conduction band emission due to electron hole recombination through shallow surface states. We found an activation energy of E a = 4.2 meV using the Arrhenius plot and estimated concentrations of paramagnetic defects and spin–spin relaxation time constants of 1016 m−3 and 10–14 s, respectively.

Investigation of structural and optoelectronic integrity of Sm3+ doped CaWO4 for LED applications

In this study we illustrate the structural, electronic and optical integrity of Ca1-xSm(2/3)xWO4 (x = 0.00, 0.02, 0.04, 0.06 and 0.08) crystals for optoelectronic applications via experimental and first principles approach. According to our experimental outcomes, CaWO4 crystallizes in scheelite type tetragonal structure devoid of impurity which depends on the A-site cationic radius and dopant-host compatibility. As a result, the dopant induced lattice distortion occur due to symmetric and asymmetric stretching of (Ca, Sm)—O and W–O bonds within [CaO8] and [WO4] cluster. According to our FT-IR and Raman spectral analysis, the non-centrosymmetric vibration within the [WO4]2- complex gives rise to internal modes while the Ca2+/Sm3+ cationic displacement and rigid molecular ionic group constitutes the external modes which equally impacts the electronic and optical characteristics of the host material. While the forbidden gap extends due to Sm3+ substitution, the delocalization of W-5d orbitals on the other hand occur due to rising distortions within the WO4 cluster. As a result, the polarization between [WO4] and [CaO8] cluster and difference in the electronic transitions controls the optical characteristics of the host material. According to our experimental and theoretical predictions, a noticeable decrease in the optical band gap (5.96–5.83 eV) occurs due to heterovalent cationic substitution (Sm3+), difference in structural order-disorder and charge carrier density. As a result, a broad photoluminescence (PL) spectrum among acceptor doped samples verifies the impact of multi-phonon or multi-level processes on the luminosity of the host material.

Biomass-derived activated carbon nanocomposites with multicolor photoluminescence

In this research work, synthesis, structural, and optical properties of activated carbon (AC) and AC/metal oxide nanocomposites (AC/ZnO and AC/Fe3O4) have been studied. Uniformly distributed metal oxide in AC (AC/ZnO and AC/Fe3O4) nanocomposites were obtained by a single-step impregnation-carbonization method, using bamboo as the biomass precursor, ZnCl2, and FeCl3 as chemical activating agents. The microstructural evolutions and optical characteristics of AC, and the composites have been investigated using X-ray diffraction (XRD), Raman, X-ray photoelectron spectroscopy (XPS), and photoluminescence spectroscopy. Both AC and composites exhibited broad photoluminescence (PL) emissions in the visible region. The PL emissions at 462 and 603 nm by AC were due to the radiative recombination of electron-hole pairs generated by sp2 sites embedded in the sp3 matrix. However, two new emission centers in addition to the above appeared at 520 and 560 nm, in both AC/ZnO and AC/Fe3O4 nanocomposites. The study suggests that metal oxide formation in composites led to a new sp2 cluster formation with a new energy band resulting in two additional emission centers in the green light region. This work provides a simple, cost-effective, and efficient way to develop broadband luminescent materials in visible regions with potential optical and sensing applications.

Fabrication and luminescence properties of (K,Rb)2CuCl3 single crystal scintillators with one-dimensional structures

(K,Rb)2CuCl3 single crystal scintillators with one-dimensional structures were successfully fabricated by the slow cooling method, and their luminescence properties were evaluated. Broad photoluminescence (PL) peaks were observed at 350–500 nm, and the highest PL quantum yields were 96.4 % for (K0.5,Rb0.5)2CuCl3 among the samples. Under X-ray irradiation, scintillation peaks attributed to the recombination of self-trapped excitons were observed within the range of 350–500 nm. Both PL and scintillation decay time constants were obtained to be 10–15 μs. The best afterglow levels at 20 ms after X-ray irradiation for 2 ms were 5 ppm for (K0.75,Rb0.25)2CuCl3. The highest scintillation light yield was 16,000 photons/MeV for (K0.75,Rb0.25)2CuCl3 under 137Cs γ-rays (662 keV) irradiation. The results indicate that (K,Rb)2CuCl3 single crystals are promising as scintillators with high LY, low afterglow, and no hygroscopicity.

Remote optical thermometry by two-dimensional LiAl5O8:Cr3+ luminescence sensor probe

Pure-phase LiAl5O8:Cr3+ leaf-like particles were synthesized by combustion synthetic route at various concentrations. Both sharp peaks and broad emission photoluminescence are apparent even at room temperature. The most optimum doping concentration of 0.75 % based on lifetime and emission intensity was chosen. Peaks of different configurations were separated by deconvolution from 120 K to 680 K, where 4T2 peak increases in temperature to the expense of the sharp 2E emissions, giving luminescence intensity ratio (LIR) relative sensitivity of 2.8 % K−1 at room temperature. Large separation between Cr3+ R lines marks their LIR as perspective at low temperatures. At high temperatures luminescence lifetime provided for 0.8 % K−1 sensitivity, while line-shift and bandwidth methods are not sensitive enough for thermometric applications. Deconvolution of peaks enabled for 10 % more precise estimation of Debye temperature (679 K).

The Role of Chemical Composition in Determining the Charge‐Carrier Dynamics in (AgI)x(BiI3)y Rudorffites

AbstractSilver‐bismuth‐based perovskite‐inspired materials (PIMs) are increasingly being explored as non‐toxic materials in photovoltaic applications. However, many of these materials exhibit an ultrafast localization of photogenerated charge carriers that is detrimental for charge‐carrier extraction. In this work, such localization processes are explored for thermally evaporated thin films of compositions lying along the (AgI)x(BiI3)y series, namely BiI3, AgBi2I7, AgBiI4, Ag2BiI5, Ag3BiI6, and AgI, to investigate the impact of changing Ag+/Bi3+ content. A persistent presence of ultrafast charge‐carrier localization in all mixed compositions and BiI3, together with unusually broad photoluminescence spectra, reveal that eliminating silver will not suppress the emergence of a localized state. A weak change in electronic bandgap and charge‐carrier mobility reveals the resilience of the electronic band structure upon modifications in the Ag+/Bi3+ composition of the mixed‐metal rudorffites. Instead, chemical composition impacts the charge‐carrier dynamics indirectly via structural alterations: Ag‐deficient compositions demonstrate stronger charge‐carrier localization most likely because a higher density of vacant sites in the cationic sublattice imparts enhanced lattice softness. Unraveling such delicate interplay between chemical composition, crystal structure, and charge‐carrier dynamics in (AgI)x(BiI3)y rudorffites provides crucial insights for developing a material‐by‐design approach in the quest for highly efficient Bi‐based PIMs.

Enhancing Single Photon Emission Purity via Design of van der Waals Heterostructures.

Quantum emitters are essential components of quantum photonic circuitry envisioned beyond the current optoelectronic state-of-the-art. Two dimensional materials are attractive hosts for such emitters. However, the high single photon purity required is rarely realized due to the presence of spectrally degenerate classical light originating from defects. Here, we show that design of a van der Waals heterostructure effectively eliminates this spurious light, resulting in purities suitable for a variety of quantum technological applications. Single photon purity from emitters in monolayer WSe2 increases from 60% to 92% by incorporating this monolayer in a simple graphite/WSe2 heterostructure. Fast interlayer charge transfer quenches a broad photoluminescence background by preventing radiative recombination through long-lived defect bound exciton states. This approach is generally applicable to other 2D emitter materials, circumvents issues of material quality, and offers a path forward to achieve the ultrahigh single photon purities ultimately required for photon-based quantum technologies.