PL Centers Research Articles

Multi-functional ratiometric detection based on dual-emitting N-doped carbon dots

Ratiometric fluorescence probes based on multi-emission carbon dots improve accuracy and sensitivity on detecting various environment issues. Herein, a novel dual-emitting N-doped carbon dots (N-CDs) was synthesized from citric acid and urea via a solvothermal method in N,N-dimethylformamide (DMF). The blue and orange emissions of N-CDs in water were modulated, and pure white light-emitting with Commission Internationale de L'Eclairage (CIE) coordinates of (0.33, 0.33) was achieved. The two PL centers behaved differently for Fe3+, Cu2+ and Ag+ ions, with the limit of detection (LOD) of ppm as fluorescence probes. Additionally, N-CDs displayed unique solvatochromism phenomenon. A new green emission appeared in organic solvents and gradually quenched with the increase of solvent polarity. The ratiometric PL displayed an excellent linear response for detecting water, and the LOD was between 0.003 % and 0.3 % in DMF, ethanol, isopropanol and N-methylpyrrolidone. Furthermore, N-CDs exhibited pH-sensitive response in the range of 4.0–7.0 and temperature-dependent response during heating–cooling cycles between 15 and 70 °C. A simple, efficient and reliable multi-functional ratiometric probe for detecting metal ions, water content, pH and temperature simultaneously was realized. However, there is a need for future application research to overcome the limitation imposed by the excitation wavelength of 330 nm.

DLTS analysis of interface and near-interface bulk defects induced by TCO-plasma deposition in carrier-selective contact solar cells

We investigate the electrical characteristics of defects at the SiO2/Si interface, within the adjacent Si crystal, and through the depth profile of the bulk defect using three-dimensional deep-level transient spectroscopy (3D-DLTS). These defects are introduced by the reactive plasma deposition technique employed for depositing transparent conductive oxides in the fabrication of carrier-selective contact-type solar cells. To control the surface potential near the Si surface, we apply a varying voltage to obtain DLTS signals as functions of both temperature and Fermi level at the SiO2/Si interface. Using machine learning for 3D-DLTS spectral analysis, we estimate the capture cross sections, energy levels, densities, and depth profiles of these process-induced defects. The experimental results indicate the existence of three types of electron traps within the bulk defects, ranging from the interface to a depth of ∼70 nm. The electrical properties of these bulk defects suggest the presence of oxygen-related defects within Si. On the other hand, regarding the properties of interface defects, the capture cross sections and the defect densities are estimated as a function of their energy levels. They suggest that the defects at the SiO2/Si interface are likely oxygen-related PL centers.

White Light Emission from Single-Component Cs7Cd3Br13:Pb2+,Mn2+ Crystals with High Quantum Efficiency and Enhanced Thermodynamic Stability

Single-component white light emitters are particularly attractive for fabricating economical solid-state devices for display and lighting. Herein, an efficient white light with photoluminescence quantum yields approaching 98% has been obtained in Mn2+ and Pb2+ co-doped Cs7Cd3Br13 crystals. The quantified bonding and nonbonding characters calculated by the crystal orbital Hamilton population analysis demonstrate that the Mn2+ and Pb2+ occupy preferentially in the [CdBr4]2– tetrahedrons rather than the [CdBr6]4– octahedrons in Cs7Cd3Br13. According to the PL spectral characteristic and the theoretical results, the broadband white light emission from Cs7Cd3Br13:Pb2+,Mn2+ crystals is ascribed to the multiple PL origins, i.e., the first self-trapped exciton (STE1) from the intrinsic trapping states of host lattice Cs7Cd3Br13, the d–d transition of Mn2+, and the second self-trapped exciton (STE2) induced by the introduction of Pb2+, respectively. The results calculated by density functional theory reveal that the incorporation of Mn2+ and Pb2+ has significantly improved the thermodynamic stability of the Cs7Cd3Br13 structure with lower defect formation energies, which is verified by the experimental observation in the temperature-dependent PL spectra of the emitting crystals. The findings here provide a new perspective for a single-component white light via creating multiple PL centers in a single matrix as well as help in the elucidation of the preferential site occupancy mechanism of the dopants in different symmetrical units.

Highly efficient solid-state luminescence of carbonized polymer dots without matrix

Development of high-performance solid state luminescent carbon-based nanomaterials remains challenging. Here, strong blue-green fluorescent carbonized polymer dots (CPDs) from o-aminobenzenethiol and thiosalicylic acid (oABT-TSA-CPDs) with an absolute photoluminescence quantum yield (PLQY) of 76% in solid state without matrix were synthesized. Through adjusting the reaction temperature and time, the PL centers were proved to be carbon core state and surface state associated to carbonyl group which was the source of strong fluorescence emission in solid state. The mechanism of the unique phenomenon of enhanced emission from ethanol solution (PLQY = 7%) to powder (PLQY = 76%) was investigated by analyzing the chemical properties and structures of oABT-TSA-CPDs at different temperatures and oABT-TSA-CPDs/PVC composites, and was confirmed as fixation of PL centers.

Synthesis of highly efficient green emissive carbon dots towards UV encryption fluorescent ink

Blue and red emitting carbon dots (CDs) have attracted extensive research interests, while the research on green emissive CDs (G-CDs) are often ignored. In this work, G-CDs with high photonluminescence quantum yield (PLQY) were synthesized through a microwave assisted solvothermal method, where 1-amino-2-naphthol-4-sulfonic acid (ANSA) and ethylenediamine were used as precursors, and ethanol as solvent. The chemical structure, surface composition and optical properties of G-CDs suggest that the G-CDs have a structure of carbon core coated by functional groups such as hydroxyl, sulfinyl, and amino groups. The mass fractions of C, N, O and S elements are 75.83%, 14.26%, 9.31% and 0.8%, respectively. The excitation and emission wavelengths are 439 nm and 528 nm, respectively. The G-CDs have considerable photonluminescence efficiency with QY up to 33.8%, which exceeds those of many other G-CDs. The G-CDs exhibit excitation wavelength-independent emission behavior, indicating that they possess a unique PL center. Furthermore, the possible synthesis mechanism and a simple application of the G-CDs are briefly introduced. This work is of great value for filling the gap of G-CDs with high PLQY and promoting the practical application of CDs.

Stable Multimode Luminescence Performances of SrGa12O19: Sm3+, Tb3+ Phosphor for Anti-Counterfeiting Application

Fluorescence materials have been widely employed for anti-counterfeiting techniques owing to their high-throughput, facile identification, and simplicity of production. However, the stability of the materials is a prerequisite for their subsequent application. Here, a series of SrGa12O19: Sm3+, Tb3+ phosphors with multi-color luminescence are obtained successfully by the traditional solid-state method. These Sm3+/Tb3+ co-doped phosphors emit green, orange and yellow-green under the excitation of 254 nm, 365 nm, and 254 nm+365 nm UV lamps, respectively. After removal of the UV lamp, the green long persistent luminescence (LPL) phenomenon is exhibited and then vanished 15 s later. The dynamic PL and LPL are associated with the interaction between PL and trapping centers. Notably, as-obtained phosphors show excellent stability against both air water resistance, and high temperature, which features the as-obtained phosphors a great application potential in high-level anti-counterfeiting with high stability.

Multi-site occupation of Cr3+ toward developing broadband near-infrared phosphors

The near-infrared (NIR) light source has a huge potential for foodstuff analysis, biometric identification, physiological monitoring, and night vision technology with real-time and non-destructive advantages. Demand-led growth impels the exploration of novel NIR phosphors with broadband emission. Here we report a new kind of blue-light excitable NIR phosphor Mg7Ga2GeO12: xCr3+, in which the multi-site occupation of Cr3+ induces broadband NIR emission in the range of 600–1200 nm with a large full width at half maximum (FWHM) of ~190 nm and a NIR photoluminescence quantum yield (PLQY) of 12.06% at x = 0.02. The FWHM can be effectively adjusted from 101 to 226 nm via multisite occupancy preference of Cr3+ ions. The PL intensity remains ~50% and nearly 100% of the initial level after heated up to 400 K and immersed in hot water (100 °C) for 24 h, respectively, demonstrating good thermal stability and robust chemical stability. The broadband emission originated from multi-site PL centers of Cr3+ is discussed in detail by the temperature-dependent PL behaviors and PL decay lifetime. Finally, a broadband emission NIR-LED is fabricated, based on which a proof-of-concept application for night vision is demonstrated. This work not only provides a strategy for the design and developing of novel broadband NIR phosphors, but also indicates the potential for various advanced spectroscopy applications.

Characterization of order-disorder transition in MgAl2O4:Cr3+ spinel using photoluminescence

Photoluminescence spectroscopy of Cr3+ shows considerable potential for probing local cation arrangement in MgAl2O4 spinel while undergoing order-disorder phase transition. The PL spectrum consists of R peaks (caused by ordered PL centers) and N peaks (caused by disordered PL centers). The peak dominating in the PL spectra dramatically switched from R peaks to N peaks around 850 °C and showed rather few changes above 900 °C. We defined the percent ratio of the integrated area of N peaks to the total integrated area under all peaks as a parameter to monitor the degree of disordering. This parameter can quantitatively indicate the variation of neighboring atom arrangement around Cr3+ during the order-disorder transition. A thermodynamic model based on this spectroscopic parameter can be built, and we can model the transition with an activation energy of 254 ± 49 kJ/mol. This study also attempted to determine the ability to restore the ordering of heated spinels by annealing them at 650 °C up to 8 days. Our results clearly indicated that the spectra cannot be fully restored back to the ordered state within our experimental time span, thus makes PL spectroscopy a reliable method to distinguish natural spinel from heated ones.

Temperature Sensing Using Upconversion in Rare Earth Doped Thermographic Phosphors

Thermometry based on the use of thermographic phosphors utilises the physical properties of the phosphor particles for assessing temperature. The phosphor particles used in thermometry are usually inorganic materials having a powder form. Fluorescence intensity ratio (FIR) technique is widely used for the temperature sensors based on phosphors doped with RE3+, providing high detection spatial resolution, precision and excellent sensitivity. Such a phosphor consists of a host material and a doping agent from which the light is emitted. Low temperature sensing using and NIR to visible upconversion (UC) emission could be very useful for biological applications. One of the important conditions an upconversion material should satisfy in order to be used as temperature sensor is that the radiative transitions from a fluorescent level should dominate its non-radiative transitions. This could be achieved only when the host has considerably low maximum phonon energy. This condition is also relevant to achieve UC, since a material which has low maximum phonon energy can be effectively used for RE3+ UC. These thermographic phosphors cover a wide range of temperatures, from cryogenic temperatures up to 1700 ºC or higher, making them suitable for many different applications. Each phosphor that is selected is highly sensitive within a specific range of temperatures, exhibiting accuracies in the order of 1-5 ºC. After excitation of the thermographic phosphor, the subsequent emission is imaged onto a detector. The temperature can then be deduced from the spectral or temporal properties of the recorded signal. This technique provides a two dimensional measurements and remote thermometry, as well as a high degree of accuracy. These advantages have allowed thermographic phosphors to be used in a wide variety of applications and in harsh environments. Many of the trivalent lanthanides such as Er3+, Tm3+, Ho3+, Nd3+, Dy3+ and Eu3+ have been used as PL centers of optical thermometry. But the Er3+ ion has proved itself to be an efficient UC centre among RE3+ ions and tremendous increase in its efficiency when combined with Yb3+ ions, acting as sensitizer, makes it the ultimate choice among UC ions. Apart of Er3+, Yb3+ combination, Tm3+ ions can also find their way for a better sensitivity depending on the host characteristics. In view of the above advantages of thermographic phosphors, structural, physical, morphological and temperature sensing characteristics of visible upconversion emission of different hosts namely SrLaAlO3, BiNbO4, LaNbO4 and YNbO4 doped with rare earth ions at low temperatures and excited by NIR excitation using FIR technique would be presented in detail.

Phase and optical properties of solvothermal prepared Sm2O3 doped ZrO2 nanoparticles: The effect of oxygen vacancy

This work reports the synthesis and characterization of samaria doped zirconia (SDZ) nanoparticles by solvothermal method. The samples were characterized by XRD, SEM, UV–vis and PL spectra. Effect of oxygen vacancies on the phase and optical properties were carefully discussed. The as-prepared products showed cubic structured nanoparticles with average size of 1.1 nm. Fully stabilized cubic ZrO2 could be obtained via doping 8mol % of Sm2O3 into its lattice. UV-blue absorption spectra of ZrO2 host showed obscure absorption edge, while those doped by Sm3+ showed distinguished spectra. No luminescence from Sm3+ center could be observed on PL spectra of all of the samples. Nine different Gaussian-Lorentz amplitude bands should be used to fully deconvolve the acquired PL spectra, and all of them were proven to be originated from oxygen vacancies. The oxygen vacancies in SDZ played as the roles of phase stabilizer, quencher of Sm3+ fluorescence, and PL center of UV-blue emission.

Insights on the atomistic origin of X and W photoluminescence lines in c-Si from ab initio simulations

We have used atomistic simulations to identify and characterize interstitial defect cluster configurations candidate for W and X photoluminescence centers in crystalline Si. The configurational landscape of small self-interstitial defect clusters has been explored through nanosecond annealing and implantation recoil simulations using classical molecular dynamics. Among the large collection of defect configurations obtained, we have selected those defects with the trigonal symmetry of the W center, and the tetrahedral and tetragonal symmetry of the X center. These defect configurations have been characterized using ab initio simulations in terms of their donor levels, their local vibrational modes, the defect induced modifications of the electronic band structure, and the transition amplitudes at band edges. We have found that the so-called I3-V is the most likely candidate for the W PL center. It has a donor level and local vibrational modes in better agreement with experiments, a lower formation energy, and stronger transition amplitudes than the so-called I3-I, which was previously proposed as the W center. With respect to defect candidates for the X PL center, our calculations have shown that none of the analyzed defect candidates match all of the experimental characteristics of the X center. Although the Arai tetra-interstitial configuration previously proposed as the X center cannot be excluded, the other defect candidates for the X center found, I3-C and I3-X, cannot be discarded either.

Insight into excitation-related luminescence properties of carbon dots: synergistic effect from photoluminescence centers in the carbon core and on the surface

EDE and EIE properties of CDs attribute to the synergistic effect from PL centers in the carbon core and on the surface.

Investigating the surface state of graphene quantum dots.

A universal route to GQDs is developed based on "solution phase-based scissor" methods. The PL centers of the GQDs are systematically studied and are proved to be the surface state. This is related to the hybridization structure of the edge groups and the connected partial graphene core. Through experiment and analysis, we have preliminarily proved that the efficient edge groups for green emission are mainly carboxyl, carbonyl and amide. This is indicated by the following three factors: firstly, the PL of GQDs is enhanced by UV exposure, during which partial -OH groups are converted into carboxyl groups; secondly, the PL properties of GQDs can be further improved by one-step solvothermal treatment, in which partial carboxyl groups are converted to amide groups and the surface state of the GQDs is enhanced; thirdly, reduced m-GQDs possess more -OH groups compared with reduced GQDs, resulting in more blue PL centers (the carboxyl, carbonyl and amide-based green centers are converted to -OH-based blue centers). The present work highlights a very important direction for the understanding of the PL mechanism of GQDs and other related carbon-based materials.

Luminescence centers in proton irradiated single crystal CVD diamond

Diamond displays a large variety of luminescence centers which define its optical properties and can be either created or modified by irradiation. The main purpose of the present work is to study the radiation hardness of several of such centers in homoepitaxial single-crystal CVD diamond by following the evolution of photoluminescence and ionoluminescence upon 2 MeV proton irradiation. Luminescence decays were observed with values of the fluence at half of the starting luminescence ( F 1/2) of the order of 10 14 cm − 2 . The 3H center displayed a non-monotonic behavior, with a growing behavior and a subsequent decay with a rather high F 1/2 value (in the order of few a 10 16 cm − 2 ), maintaining at the highest fluences an intensity significantly higher than the blue A-band. A simple model based on a double-exponential trend was defined to fit with satisfactory accuracy the evolution of the 3H center. Several PL centers (namely: 3H, TR12, 491 nm and 494 nm) exhibited clear correlations and anti-correlations in their fluence dependences, which were considered in the attempt to acquire some insight into their possible alternative attributions.

Photoluminescence of Si or Ge nanocrystallites embedded in silicon oxide

This paper presents the results of photoluminescence, its temperature dependence and Raman scattering investigations on magnetron co-sputtered silicon oxide films with (or without) embedded Si (or Ge) nanocrystallites. It is shown the oxide related defect origin of the visible PL centers peaked at 1.7, 2.06 and 2.30eV. The infrared PL band centered at 1.44–1.58eV in Si–SiOx, system has been analyzed within a quantum confinement PL model. Comparative PL investigation of Ge–SiOx system has confirmed that high energy visible PL bands (1.60–1.70 and 2.30eV) are connected with oxide related defects in SiOx. The PL band in the spectral range of 0.75–0.85eV in Ge–SiOx system is attributed to exciton recombination inside of Ge NCs.

Capture cross sections of defect states at the Si/SiO2 interface

Modulation capacitance voltage spectroscopy is applied to study the interaction between interface defect states and the conduction band of thermally oxidized n-type silicon wafers, which were prepared using a broad spectrum of preparation conditions. The modulation frequency response of metal oxide semiconductor samples is measured in depletion and accumulation as a function of temperature and of the position of the Fermi level at the interface. The data reveal two different sets of states, the capture cross sections of which differ by as much as 1–2 orders of magnitude. We assign these states to two types of defects which originate from silicon dangling bonds with three backbonded silicon atoms (Pb center) and silicon dangling bonds where one backbond is substituted by oxygen (PL center). Both capture cross sections decrease monotonically with decreasing energetic separation from the conduction band.

Photoluminescence Characterization of Defects in Thermal Oxide

ABSTRACTDefects in thermal oxides were investigated by a photoluminescence technique. Thermal oxides with a thickness of 100 nm grown either by dry or wet oxidation were studied. A broad PL band at 2-4 eV was observed for both dry and wet oxides. Effects of annealing under vacuum or in atmospheres of Ar or N2 on the PL were also examined. The PL intensity was enhanced for the case of wet oxide by vacuum annealing at 700 °C. High-temperature anneal above 750 °C without O2 further generated PL centers for both dry and wet oxides. The formation mechanism of the PL centers will be discussed in terms of the decomposition of oxide at Si/SiO2 interface.

Effect of high-temperature annealing on electrical and optical properties of undoped semi-insulating GaAs

A comprehensive characterization, including room temperature Hall effect, near infrared absorption, temperature dependent dark current and photocurrent (using 1.13 eV light), normalized thermally stimulated current (NTSC), photoluminescence at 4.2 K in both near band edge and deep level regions, and selective pair photoluminescence (SPL) at 2 K, has been carried out on undoped semi-insulating GaAs samples, cut from four wafers which were grown by the low pressure liquid encapsulated Czochralski technique and annealed by three different schedules: a 1100 °C anneal with either fast or slow cooling, or a 1000 °C standard anneal. The 1100 °C anneal clearly introduces higher concentrations of NTSC traps near 0.3 and 0.5 eV, a PL center at 0.8 eV, and acceptor centers, which are mainly due to the point defects and increase the resistivity. Slow cooling to some extent reduces all of these additional centers. The SPL measurements show changes in the relative intensities of C, Zn, and Si related emissions with changes in annealing conditions.

ESR and PL centers induced by gamma rays in silica

We have studied the point defects created by γ irradiation in various types of commercial silica glasses, including both natural and synthetic samples, with different OH content, in the low dose regime (0.05–100 Mrad). We found that the growth rate of E′ centers depends strongly on the silica type, ranging from 2 × 10 15 cm −3 Mrad −1 to 6 × 10 17 cm −3 Mrad −1. Samples of natural silica are rather susceptible to γ ray exposure, as E′ concentration saturates (typically 5 × 10 17 cm −3) for doses as low as a few Mrads. For both synthetic and natural samples, the radiation hardness is higher in wet than in dry silica. Moreover, we found a strict correlation between the concentration of E′ centers and the γ-induced absorption band at 5.8 eV. Finally, exposure to γ rays generates in all the samples a photoluminescence band at 4.4 eV, whose excitation spectrum has a maximum at 4.95 eV. This band exhibits a sublinear growth kinetics in all the investigated samples.

Localized Nature of Photoluminescence from Anodically Oxidized Porous Silicon

ABSTRACTPhotoluminescence (PL) properties from anodically oxidized porous silicon (PS) have been investigated. Large temperature coefficients (-0.5 meV/K) of PL peak energy are observed for relatively strongly oxidized PS specimens which show saturation of PL peak blue shift upon further anodic oxidation. Time-resolved PL data show that the PL decay is characteristic of thermal activation process with an energy of 9-27 meV at low temperatures between 80 and 180 K. Those results can be explained using a luminescence model which assumes several PL centers in the energy gap and considers thermally activated and tunneling processes.