Defect Emission Research Articles

Enhanced defect emission of TiO2-doped transparent glass-ceramics embedding ZnO quantum dots with optimized heat-treatment schedule

This work mainly concentrated on the structural and optical properties of TiO 2 -doped transparent glass-ceramics embedding ZnO quantum dots (QDs). The results show that the concentration of non-bridging oxygen and related defects in the glasses increases and thermal parameters decreases by the introduction of TiO 2 . The surface defect concentration of ZnO QDs is greatly improved and the size of ZnO QDs is reduced by heat treatment below the T g and the introduction of TiO 2 . The green emission of glasses is mainly derived from oxygen vacancy defects and the yellow emission of glass-ceramics primarily originated from oxygen interstitial defects which show a tendency of enhancing first and then reducing with the increase of TiO 2 content. Among them, the optimal content for the strongest emission intensity of TiO 2 -doped glasses and glass-ceramics was determined to be 1.0 mol%, and the relative exceeds 10 times and 5.4 times that of TiO 2 -free glasses and glass-ceramics, respectively. The average size of ZnO QDs (∼2.1 nm) remarkably close to its exciton Bohr radius is successfully prepared by the optimal heat treatment condition (630 °C/8 h, below the T g ). The results show that these glass-ceramics embedding ZnO QDs are good yellow emission materials and have application prospects in WLEDs.

Double Layer Growth of ZnO Nanorods by a Low Temperature Solution Method: Synthesis and Photoluminescence Properties

In the present work, double layer Zinc Oxide (ZnO) nanorods (NRs) were fabricated hydrothermally and their photoluminescence (PL) properties were investigated. Two different recipes and their combination were used to obtain double layer vertically well-aligned ZnO NRs. These recipes include polyethylenimine (PEI) and citrate as additives in the growth solution resulting long NRs with a broad defect emission and relatively short NRs with a near band-edge ultraviolet (UV) emission, respectively. Double layer growth of long-long (LL), long-short (LS) and short-short (SS) ZnO NRs were considered. Grown samples were annealed in a forming gas atmosphere for a better quality NR structure. LL ZnO NRs (long ZnO NRs grown on a long ZnO NR layer) with a 65 µm thickness showed a broad yellow-orange PL emission and no any near band-edge UV emission was observed. LS ZnO NRs representing short ZnO NRs grown on a long ZnO NR layer (LS1= 36 µm, LS2= 48 µm and LS3= 44 µm) showed an enhanced near band-edge UV emission when compared to that of the long ZnO NRs. The UV intensity was found to decrease with the increase in thickness of the NRs in LS samples. Finally, SS ZnO NR sample, (short ZnO NRs grown on a short ZnO NR layer) which has a thickness of 33 µm, displayed a stronger near band-edge UV emission with a negligible broad emission than that of as-grown SS ZnO NRs (UV peak intensity ratio 59). This study should be important for applications where longer NRs with enhanced PL properties are strictly required.

Reconfiguring Organic Color Centers on the sp2 Carbon Lattice of Single-Walled Carbon Nanotubes.

Organic color centers (OCCs) are atomic defects that can be synthetically created in single-walled carbon nanotube hosts to enable the emission of shortwave infrared single photons at room temperature. However, all known chemistries developed thus far to generate these quantum defects produce a variety of bonding configurations, posing a formidable challenge to the synthesis of identical, uniformly emitting color centers. Herein, we show that laser irradiation of the nanotube host can locally reconfigure the chemical bonding of aryl OCCs on (6,5) nanotubes to significantly reduce their spectral inhomogeneity. After irradiation the defect emission narrows in distribution by ∼26% to yield a single photoluminescence peak. We use hyperspectral photoluminescence imaging to follow this structural transformation on the single nanotube level. Density functional theory calculations corroborate our experimental observations, suggesting that the OCCs convert from kinetic structures to the more thermodynamically stable configuration. This approach may enable localized tuning and creation of identical OCCs for emerging applications in bioimaging, molecular sensing, and quantum information sciences.

Probing defect-originated properties, actinide-lanthanide doping, and gamma irradiation effect in Lu2Hf2O7 pyrochlore nanocrystals for phosphor and nuclear applications

Multifunctional materials are in high demand these days by virtue of their efficacy to perform more than one role and are expected to ease the high global materials crunch. This work is directed towards probing defect emission in undoped and doping of uranium (U), plutonium (Pu), curium (Cm), cerium (Ce), and samarium (Sm) Lu2Hf2O7 (LuHO) pyrochlore nanocrystals (NCs) for photoluminescence and radioactive waste immobilization. Density of state calculations shows abundant defects in the undoped LuHO NCs. Multiple visible luminescence spanning violet to red in the undoped LuHO pyrochlore NCs upon ultraviolet irradiation was ascribed to the presence of neutral and ionized oxygen vacancies. The doped LuHO NCs show typical blue (Ce3+), orange-red (Sm3+), and green (U6+) emissions endowed by 4f→5d, 4f→4f, and charge transfer transitions, respectively. Moreover, uranium is stabilized as UO66− ion (octahedral uranate) in the LuHO pyrochlore lattice. Luminescence lifetime spectroscopy and density functional theory (DFT) calculated defect formation energies suggest that Ce, Sm, Cm, and Pu ared stabilized at Lu3+ site whereas U is stabilized at Hf4+ site in the LuHO NCs. The LuHO pyrochlore NCs also demonstrate high radiation stability on gamma exposure retaining the crystallinity and undergoing structural phase transition from defect fluorite to ideal pyrochlore structure. Considering the significant health hazard posed by radioactive elements present in the spent fuel, these LuHO NCs possess high potential to immobilize U, Pu, Cm, Ce, and Sm. This work poses high significance for immobilizing radioactive nuclear waste and designing tunable luminescent solid-state NCs.

Defect emission photoluminescence peak tuning by encapsulation of Au-NPs on ZnO mesoporous nanosponges

We synthesized Au nanoparticles (Au-NPs) encapsulated zinc oxide (ZnO:Au) nanosponges by the fraternization of ZnO nanoflowers with HCl (0–30 mM) concentrated colloidal Au-NPs at room temperature. The crystallite size (D) of mesoporous ZnO:Au decreased from 44 nm to 22 nm and the strain (τ) increased from 2.5 × 10−3 to 4.7 × 10−3 as the HCl concentration of colloidal Au-NPs increased from 0 mM to 30 mM. For elemental composition of ZnO:Au nanosponges, the STM-mapping confirmed the Au-NPs encapsulation on nanostructured ZnO. The bandgap, Eg, and the Urbach energy, Eu, of ZnO:Au decreased from 3.26 to 2.96 eV, and from 0.335 to 0.318 eV, as the colloidal Au-NPs increased from 0 mM to 30 mM, respectively. The structural and microstructure analysis showed the sponge like morphology along with wurtzite hexagonal structure of ZnO, which tuned the PL emission for the sensor selectivity. The visible emission of ZnO:Au nanosponges was greatly tuned from 600 nm to 500 nm with respect to the HCl diluted colloidal Au-NPs. The excellent photoluminescence (PL) performance of nanostructured ZnO:Au was attributed to the surface-plasmon-mediated sequential transfer of defect energy from ZnO to Au and electron transfer from excited Au to ZnO.

High-quality optically defect-free 1D ZnS nanostructures by a modified thermal evaporation method

ZnS has great potential as a valuable material for nanoscale devices because of its rich morphologies and unique structure. Although much effort has been made, the growth of high-quality ZnS crystal is still a challenge. In this paper, high-quality optically defect-free 1D ZnS nanostructures, including nanorods, nanowires, and nanobelts, were successfully synthesized on a large scale by a modified thermal evaporation method. XRD patterns and HRTEM images indicated that the ZnS nanostructures are single phases with hexagonal structures. Under optical excitation, all the ZnS nanostructures show intense UV emissions and almost no defect emissions at room temperature. Sharp UV lasing-like peaks with the FWHM as narrow as 2–3 nm are achieved for the ZnS nanobelts and nanowires. The optical transition from free exciton A, free exciton B, and their associated LO phonon replicas were determined from the evolution of the NBE emissions. These high-quality nanostructures are envisaged to be highly promising for high-efficiency light-emitting devices and lasers in the UV region.

Investigation of the effect of ytterbium doping on photoluminescence and photocatalytic activity of Li4Ti5O12

AbstractThis work reports the combined effect of the morphology and crystallization to investigate the role of Yb3+ as an additive in Li4Ti5O12 crystals on its photocatalytic and photoluminescence (PL) properties. With the pure phase, the synthesized samples have a spherical morphology and spherical structure with good crystallinity and uniform particle size distribution. Li4Ti5O12 doped with ytterbium (Yb3+), were obtained by a new optimized hydrothermal approach. The crystal properties and surface morphology of the synthesized compounds were examined by X‐ray diffraction method and field emission scanning electron microscopy. Bond structures and surface areas were investigated by Fourier transform infrared spectroscopy and Brunauer‐Emmett‐Teller analyses. Finally, the excitation and emission spectra were obtained by a PL spectrophotometer. The PL results show that room‐temperature PL of Yb3+ doped Li4Ti5O12 phosphor has an emission peak at 710 nm which can be attributed to Li4Ti5O12 crystal defect emission. Moreover, it has been found that Yb doping resulted in enhancement of photocatalytic activity, evaluated by monitoring the degradation of methylene blue solution. Note that, 0.01% Yb doped lithium titania phosphor exhibited excellent photocatalytic activity.

White-light defect emission and enhanced photoluminescence efficiency in a 0D indium-based metal halide

A white-emissive metal halide (C6H8N)6InBr9with inferior luminescence due to a severe thermal quenching effect has been developed. By doping with Sb3+ions, this effect was effectively suppressed, improving the luminous efficiency up to 71.84%.

Synthesis and photocatalytic activity of Ni doped SnO2 nanoparticles for removal of toxic industrial dyes

Metal oxide semiconducting material like SnO2 has novel features like wide band gap (3.6 eV) and large exciton binding energy (130 meV) which ensure its promising optoelectronic applications. In the present work, we have carried out a detailed study on the above materials i.e. SnO2 and Sn1-xNixO2 which were synthesized using sol–gel method with stannous chloride, nickel nitrate hexahydrate, ammonium hydroxide and isopropyl alcohol as the starting materials. XRD study confirms that SnO2 and Ni-SnO2 samples are of tetragonal structure with crystal size 32 nm and 28 nm respectively as estimated from Scherrer formula. The optical band gap of pure tin oxide is found to be 3.73 eV from UV–Vis absorption measurements which shifts to 3.70 eV for Ni-SnO2. The PL spectra show several emission bands at 418 nm, 437 nm, 463 nm, 487 nm and 530 nm in the visible region. These bands are ascribed to the defect emissions arising due to oxygen vacancies. With Ni doping, the intensity of these bands increases which may be due to the increase in the density of the above defects. The photocatalytic degradation of Congo red (CR) dye was studied by irradiating UV–Vis light source and found the degradation efficiency of 83% for Ni-SnO2 sample which was 74% for the pristine sample. Ni doping helps to enhance the light absorption ability of SnO2 leading to better degradation efficiency.

ZrO2-doped transparent glass-ceramics embedding ZnO nano-crystalline with enhanced defect emission for potential yellow-light emitter applications

ZrO 2 -doped transparent silicate glasses and glass-ceramics embedding ZnO nano-crystallines were synthesized. The effects of ZrO 2 addition on the structure and optical properties of glass-ceramics were studied by DSC, XRD, SEM, IR, and PL spectra. The DSC results show that the crystallization ability of glass enhances with the addition of ZrO 2 . All the glass-ceramics after being heat-treated at 730 °C for 8 min can maintain a high transmittance in the visible range (>80%). With the increase of ZrO 2 content, the grain size of ZnO nano-crystallines shows a tendency of decline, while the bandgap and the defect-related emission (580 nm) intensity attributed to ZnO is enhanced. When the ZrO 2 -doping content is 2.5 mol%, the yellow emission (580 nm) intensity of ZnO crystals reaches the highest under excitation at 345 nm, which is 21 folds higher than that of ZrO 2 -free glass-ceramics. These results imply that the glass-ceramics embedding ZnO nano-crystallines doped with ZrO 2 would be a prospective candidate for yellow-light emitters.

Photoluminescence study of solution-deposited Cu2BaSnS4 thin films

To experimentally identify the character of radiative transitions in trigonal Cu2BaSnS4, we conduct temperature and excitation intensity dependent photoluminescence (PL) measurements in the temperature range of 15–300 K. The low-temperature near band edge PL spectrum is interpreted as the free exciton at 2.11 eV and the bound exciton at 2.08 eV, coupled with associated phonon-assisted transitions. In the low energy region, we assign the dominant defect emission at 1.96 eV to donor–acceptor-pair recombination and the weak broad emission at 1.6 eV to the free-to-bound transition. The activation energies and temperature shift for the radiative transitions are determined and discussed. Above 90 K, the free exciton recombination becomes the dominant radiative transition, with its energy shift mainly governed by the contribution of optical phonons.

ZnO – Yb2O3 composite optical ceramics: Synthesis, structure and spectral-luminescent properties

Zinc oxide optical ceramics containing 0–2 wt% ytterbium are prepared by uniaxial hot pressing of commercial oxides at 1150 and 1180 °C. The ceramics have the main crystalline phase of hexagonal wurtzite-type ZnO. Ytterbium ions do not enter the ZnO crystals but form a cubic sesquioxide phase of Yb2O3 located at the ZnO grain boundaries. Yb acts as an inhibitor for the ZnO grain growth. The ceramics exhibit transmittance up to 60 % in the visible. Their transmission in the infrared is determined by the free charge carrier absorption. The Yb3+ ions are found in C2 and C3i sites in Yb2O3 crystals. Under X-ray excitation, the ceramics exhibit intense luminescence bands in the blue (near-band-edge emission) and green (defect emission) whose positions, intensities and decay times depend on the Yb content. Yb2O3 causes a redistribution of luminescence intensity in favor of the near-band-edge emission and fastens the emission decay.

Mechanistic Investigation of Molybdenum Disulfide Defect Photoluminescence Quenching by Adsorbed Metallophthalocyanines.

Lattice defects play an important role in determining the optical and electrical properties of monolayer semiconductors such as MoS2. Although the structures of various defects in monolayer MoS2 are well studied, little is known about the nature of the fluorescent defect species and their interaction with molecular adsorbates. In this study, the quenching of the low-temperature defect photoluminescence (PL) in MoS2 is investigated following the deposition of metallophthalocyanines (MPcs). The quenching is found to significantly depend on the identity of the phthalocyanine metal, with the quenching efficiency decreasing in the order CoPc > CuPc > ZnPc, and almost no quenching by metal-free H2Pc is observed. Time-correlated single photon counting (TCSPC) measurements corroborate the observed trend, indicating a decrease in the defect PL lifetime upon MPc adsorption, and the gate voltage-dependent PL reveals the suppression of the defect emission even at large Fermi level shifts. Density functional theory modeling argues that the MPc complexes stabilize dark negatively charged defects over luminescent neutral defects through an electrostatic local gating effect. These results demonstrate the control of defect-based excited-state decay pathways via molecular electronic structure tuning, which has broad implications for the design of mixed-dimensional optoelectronic devices.

A spectral library for laser-induced fluorescence analysis as a tool for rare earth element identification

Abstract. With the recurring interest in rare earth elements (REEs), laser-induced fluorescence (LiF) may provide a powerful tool for their rapid and accurate identification at different stages along their value chain. Applications to natural materials such as minerals and rocks could complement the spectroscopy-based toolkit for innovative, non-invasive exploration technologies. However, the diagnostic assignment of detected emission lines to individual REEs remains challenging because of the complex composition of natural rocks in which they can be found. The resulting mixed spectra and the large amount of data generated demand automated approaches of data evaluation, especially in mapping applications such as drill core scanning. LiF reference data provide the solution for robust REE identification, yet they usually remain in the form of tables of published emission lines. We show that a complete reference spectra library could open manifold options for innovative automated analysis. We present a library of high-resolution LiF reference spectra using the Smithsonian rare earth phosphate standards for electron microprobe analysis. We employ three standard laser wavelengths (325, 442, 532 nm) to record representative spectra in the UV-visible to near-infrared spectral range (340–1080 nm). Excitation at all three laser wavelengths yielded characteristic spectra with distinct REE-related emission lines for EuPO4, TbPO4, DyPO4 and YbPO4. In the other samples, the high-energy excitation at 325 nm caused unspecific, broad-band defect emissions. Here, lower-energy laser excitation is shown to be successful for suppressing non-REE-related emission. At 442 nm excitation, REE reference spectra depict the diagnostic emission lines of PrPO4, SmPO4 and ErPO4. For NdPO4 and HoPO4 the most efficient excitation was achieved with 532 nm. Our results emphasise the possibility of selective REE excitation by changing the excitation wavelength according to the suitable conditions for individual REEs. Our reference spectra provide a database for the transparent and reproducible evaluation of REE-bearing rocks. The LiF spectral library is available at zenodo.org and the registered DOI https://doi.org/10.5281/zenodo.4054606 (Fuchs et al., 2020). Primarily addressing the raw material exploration sector, it aids particularly the development of advanced data processing routines for LiF analysis but can also support further research on the REE luminescence in natural rocks or artificial compounds. It gives access to traceable data for the comparison of emission line positions, emission line intensity ratios and splitting into emission line sub-levels or can be used as reference or training data for automated approaches of component assignment.

The role of sulfate ions on distinctive defect emissions in ZnO:Ce3+ nanophosphors - A study on the application in color display systems

The conventional solid-state reaction route was used to synthesize a series of SO42− (1.0 mol %) and Ce3+ (0.5, 1.0, 1.5, 2.0, and 2.5 mol%) encapsulated ZnO nanophosphors. The powder X-ray diffraction confirmed the formation of the hexagonal wurtzite structures of ZnO showing that the main crystal structure was not influenced by the low concentrations of Ce3+ and SO42− ions. The effect of sulfate ions on the defect-induced photoluminescence (PL) emission of ZnO was identified in deconvoluted PL bands and was found to correlate with the shifts observed in the band edges of the absorption spectra. Defect‐related emissions such as near‐band edge and deep-level emission were detected from undoped ZnO and ZnO–SO4 and ZnO–SO4:Ce3+ (1.0%) PL spectra. The well-known emission from Ce3+ associated with the 5D4−4Fj transitions was suppressed in the ZnO:Ce3+, while the incorporation of SO4 cations was shown to increase the defects related emission in ZnO considerably. The increase in the PL intensity was attributed to non-radiative energy transfer from SO42− to defect levels in ZnO. The proposed mechanisms of energy transfer and related electronic transitions leading to photoemission are presented. The Commission Internationale de l’Eclairage (CIE - 1931) coordinates and the correlated colour temperature and color-purity data suggest that the ZnO–SO4 and ZnO–SO4:Ce3+ phosphors hold a significant promise for applications in displays.

A prototypical near-infrared light-emitting diode from CdS/Si heterojunctions based on the defect emissions in the interface

Optimizing and tuning the interface to enhance the performance of heterojunctions is an important research topic in the optoelectronic fields. In the present work, a CdS layer of ∼5.0 nm in the thickness, which is prepared by the radio frequency magnetron sputtering method, has been incorporated in the interface of CdS/Si heterojunctions which is constructed through depositing nano CdS films on porous polysilicon by using the chemical bath deposition method. Through the annealed treatment at 500 oC in the Ar atmosphere, the CdS/Si heterojunctions show typical rectification characteristics with the improved turn-on voltage from ∼2.10 V to ∼0.83 V. At the forward voltage of ∼6.0 V, a green band can be observed. With increasing the applied voltage, the near-infrared emission band can be obtained, which is ascribed to the transition from the sulfur vacancies in the surface CdS layers to the cadmium interstitials in the interface. It is believed that understanding of near-infrared emission from CdS/Si heterojunction is crucial to the field of optoelectronic devices.

Defect Emission Energy and Particle Size Effects in Fe:ZnO Nanospheres Used in Li-Ion Batteries as Anode

Defect Emission Energy and Particle Size Effects in Fe:ZnO Nanospheres Used in Li-Ion Batteries as Anode

Correlation of Defects and Lasing Threshold for AlGaN Deep Ultraviolet Lasers Grown by Molecular Beam Epitaxy

Herein, aluminum gallium nitride (AlGaN)/AlN double‐heterostructure (DH) laser at 287 nm with a lasing threshold of 530 kW cm−2 is shown. The laser structure is grown on the AlN‐on‐sapphire template by molecular beam epitaxy. This lasing threshold is drastically reduced compared with the previously reported AlGaN DH lasers at 297 nm with a threshold of around 1 MW cm−2. The detailed studies indicate that this improvement is mainly attributed to the control of defect formation largely due to the increased growth temperature of the AlGaN active layer. This is consistent with the improved internal quantum efficiency at low excitations. Moreover, the control of Al vacancy related point defects is clearly demonstrated by the absence of defect emission in the visible band.

Solid-state thermal dewetted silver nanoparticles onto electrochemically grown self-standing vertically aligned ZnO nanorods for three-dimensional plasmonic nanostructures

Three dimensional (3D) plasmonic nanostructures composed of silver nanoparticles decorated ZnO NRs arrays, have been fabricated by a process combining the electrochemical growth of ZnO NRs and further formation of Ag nanoparticles by the solid-state thermal dewetting (SSD) process. The effect of SSD parameters on the morphological, structural and optical properties of the Ag NPs decorated ZnO NRs arrays has been investigated. It is possible to tune the bandgap of the Ag NPs@ZnO nanorods array 3D plasmonic nanostructure by tailoring the Ag nanoparticle sizes, allowing light manipulation at the nanoscale. The silver nanoparticles attached to the ZnO NRs arrays experienced surface plasmonic coupling effect, causing enhancement in the room temperature photoluminescence (PL) UV emission and quenching the corresponding visible light one. An enhancement in the near band edge emission PL intensity of ZnO to the deep level emission PL intensity ratio after Ag NPs decoration of the ZnO nanostructures corresponding to ca. 11 folds has been observed, indicating that the defect emission is obviously suppressed.

Phosphorescence quenching study of Cu(II)-ions-induced Mn-doped ZnS quantum dots revealed by intensity- and lifetime-resolved spectroscopy

Luminescence quenching of colloidal quantum dots (QDs) appeals many applications in optical sensors. Here, we have developed an intensity- and lifetime-resolved spectroscopy to reveal the phosphorescence quenching of Cu2+-induced Mn-doped ZnS QDs. Phosphorescence intensity and lifetime of QDs can be divided into three components corresponding to one by one. With increasing Cu2+ ions, the isolated Mn2+ emission is quenched by copper ions, but Mn-Mn pair and ZnS defect emissions exhibit the opposite trend. Phosphorescence quenching of QDs can be explained by the effective electron transfer from QDs to Cu2+.