Luminescence Spectroscopy and Visible Upconversion Properties of Er3+ in ZnO Nanocrystals

Solution Processed Zn1-x-ySmxCuyO Nanorod Arrays for Dye Sensitized Solar Cells

Cu- and Sm-doped ZnO nanorod arrays were grown with 1 wt% of Sm and different weight percents (0.0, 0.5, 1.0 and 1.5 wt%) of Cu by two-step hydrothermal method. The influence of Cu concentration and precursor of Sm on the structural, optical and photovoltaic properties of ZnO nanorod arrays was investigated. An X-ray diffraction study showed that the nanorod arrays grown along the (002) plane, i.e., c-axis, had hexagonal wurtzite crystal structure. The lattice strain is present in all samples and shows an increasing trend with Cu/Sm concentration. Field emission scanning electron microscopy was used to investigate the morphology and the nanorod arrays grown vertically on the FTO substrates. The diameter of nanorod arrays ranged from 68 nm to 137 nm and was found highly dependent on Cu concentration and Sm precursor while the density of nanorod arrays almost remains the same. The grown nanorod arrays served as photoelectrodes for fabricating dye-sensitized solar cells (DSSCs). The overall light to electricity conversion efficiency ranged from 1.74% (sample S₁, doped with 1 wt% of Sm and 0.0 wt% of Cu) to more than 4.14% (sample S₄, doped with 1 wt% of Sm and 1.5 wt% of Cu), which is 60% higher than former sample S1. The increment in DSSCs efficiency is attributed either because of the doping of Sm³⁺ ions which increase the absorption region of light spectrum by up/down conversion or the doping of Cu ions which decrease the recombination and backward transfer of photo-generated electrons and increase the electron transport mobility. This work indicates that the coupled use of Cu and Sm in ZnO nanorod array films have the potential to enhance the performance of dye-sensitized solar cells.

Characteristics and Photovoltaic Applications of Au-Doped ZnO-Sm Nanoparticle Films

Au-doped ZnO-samarium nitrate (Sm) nanoparticles with fixed concentrations of Sm (1 wt %) and various concentrations of Au (0.0, 0.5, 1.0 and 1.5 wt %) were prepared and used as photoelectrodes to enhance the photovoltaic efficiency of dye-sensitized solar cells (DSSCs). The cell fabricated with 1.5 wt % of Au-doped ZnO-Sm nanoparticles film achieved an optimal efficiency of 4.35%, which is about 76% higher than that of 0.0 wt % of Au-doped ZnO-Sm-based cell (2.47%). This increase might be due to the formation of a blocking layer at the ZnO-Sm/Au interface, which inhibits the recombination of electrons. This increase may also be attributed to the addition of rare-earth ions in ZnO to enhance the non-absorbable wavelength region of light via up/down-conversion of near-infrared and ultraviolet radiations to visible emission and reduce the recombination loss of electron in the cell. The efficiency of cells may be increased by the blocking layer and up/down-conversion process and thus promote the overall performance of the cells. This work indicates that Au-doped ZnO-Sm nanoparticle films have the potential to increase the performance of DSSCs.

Investigation of 4f-Related Electronic Transitions of Rare-Earth Doped ZnO Luminescent Materials: Insights from First-Principles Calculations

Rare-earth (RE) doped zinc oxides (ZnO) are regarded as promising materials for application in versatile color-tuned devices. However, the understanding of underlying luminescence mechanism and the rule of 4 f-related electronic transition is still limited, which is full of significance for the exploration of advanced RE-based ZnO phosphors. Thus, a series of ZnO : RE (RE=Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb) phosphors have been investigated by means of first-principles calculations. Meanwhile, we also consider the effect of native defects (V_O , V_Zn ) on the luminescence of ZnO : RE phosphors. Accordingly, four types of electric-dipole allowed transition processes are figured out in ZnO : RE family. Additionally, we manifest that the V_O can further improve the luminescent performance of ZnO : RE phosphors, and give insightful guidance to design desired RE-based ZnO materials with excellent luminescence.

Comparative study on the photoluminescence properties of monoclinic and cubic erbium oxide

As a heavy rare earth oxide, erbium oxide (Er₂O₃) has many attractive properties. Monoclinic Er₂O₃ has useful properties not found in stable cubic Er₂O₃, such as unique optical properties and high radiation damage tolerance. In this study, pure cubic and mixed phase of cubic and monoclinic Er₂O₃ coatings were prepared. Photoluminescence properties of these coatings were characterized by a confocal micro-Raman spectrometer equipped with 325, 473, 514, 532, 633 nm lasers, and the influence of microstructure on the fluorescence properties was analyzed in detail. The room temperature fluorescence peaks of cubic Er₂O₃ were assigned. Furthermore, a novel method for rapid phase identification of Er³⁺ doped cubic and monoclinic rare earth sesquioxides at room temperature was proposed.

Achieving Thermo-Mechano-Opto-Responsive Bitemporal Colorful Luminescence via Multiplexing of Dual Lanthanides in Piezoelectric Particles and its Multidimensional Anticounterfeiting

Optical characteristics of luminescent materials, including emission color (wavelength), lifetime, and excitation mode, play crucial roles in data communication and information security. Conventional luminescent materials generally display unicolor, unitemporal, and unimodal (occasionally bimodal) emission, resulting in low-level readout and decoding. The development of multicolor, multitemporal, and multimodal luminescence in a single material has long been considered to be a significant challenge. In this study, for the first time, the superior integration of colorful (red-orange-yellow-green), bitemporal (fluorescent and delayed), and four-modal (thermo-/mechano-motivated and upconverted/downshifted) emissions in a particular piezoelectric particle via optical multiplexing of dual-lanthanide dopants is demonstrated. The as-prepared versatile NaNbO₃ :Pr³⁺ ,Er³⁺ luminescent microparticles shown are particularly suitable for embedding into polymer films to achieve waterproof, flexible/wearable and highly stretchable features, and synchronously to provide multidimensional codes that can be visually read-out using simple and commonly available tools (including the LED of a smartphone, pen writing, cooling-heating stimuli, and ultraviolet/near-infrared lamps). These findings offer unique insight for designing highly integrated stimuli-responsive luminophors and smart devices toward a wide variety of applications, particularly advanced anticounterfeiting technology.

Designing Dual Emissions via Co-doping or Physical Mixing of Individually Doped ZnO and Their Implications in Optical Thermometry

Here, we report on the novel design of dual emission via defect state engineering in codoped oxide microstructures and its implication in fluorescence intensity ratio (FIR) based optical temperature sensing. Eu- and Er-co-doped ZnO (EuEr:ZnO) microrods prepared by hydrothermal method. The emission peaks corresponding to Eu³⁺ and Er³⁺ are observed suggesting dual emission from codoped ZnO. Interestingly, Er³⁺ peak intensity decreases and that of Eu³⁺ increases with increase of temperature as is the case of individual doped cases and dual emission is also achieved via phyical mixing of the individual doped ZnO. The opposite trend is due to the electron transfer from the defect levels of host ZnO to Eu³⁺ and not to Er³⁺. Overall, our results pave the way in designing dual emission that can be exploited in FIR based temperature sensing. As an example, we probe temperature dependency of congo-red and polyvinyle alcohol (PVA) composite using EuEr:ZnO as optical probe for temperature sensing.

Structural investigations of (Mn, Dy) co-doped ZnO nanocrystals using X-ray absorption studies

EXAFS measurements on sol gel derived (Mn, Dy) co-doped ZnO nanocrystals show that oxygen vacancies are created near the Dy sites into ZnO lattice. Thus, oxygen vacancy assisted bound magnetic polarons contribute to the RTFM in the Dy doped samples.

Enhanced upconversion emission in ZrO₂-Al₂O₃ composite oxide

Aqueous solutions of zirconium oxychloride and aluminum nitrate were coprecipitated and crystallized to form a ZrO2-Al2O3 solid solution. The upconversion (UC) emission from different Er(3+)-doped samples was studied. An enhancement of the green UC emission by as much as 22 times was achieved by co-doping with Yb(3+) and Mo(6+) ions due to an energy transfer at a higher excited-state energy, which partly avoided the non-radiative decay processes at the lower energy levels of Er(3+). The UC emission of the ZrO2-Al2O3 composite system series doped with different agents was enhanced. Excess oxygen vacancies are generated by forming ZrO2-Al2O3 solid solutions, which have an energy level close to the (4)F7/2 level of the Er(3+) ions. The defect state promoted the energy transfer process resulting in an eight-fold increased green UC emission in ZrO2-Al2O3 solid solutions. The solid solutions have a superior color chromaticity of x = 0.25 and y = 0.71 due to the evident enhancement in the green to red emission ratio in the 8ZrO2-2Al2O3 sample.

Spectral-converting study of Ba((9-3)(m+n)/2)Er(m)Yb(n)Y2Si6O24 (m = 0.005 - 0.2, n = 0 - 0.3) orthosilicate phosphors

Optical materials composed of Ba((9-3)(m+n)/2)Er(m)Yb(n)Y2Si6O24 (m = 0.005-0.2, n = 0-0.3) were prepared using a solid-state reaction. The X-ray diffraction patterns of the obtained phosphors were examined to index the peak positions. The photoluminescence (PL) excitation and emission spectra of the Er(3+)-activated phosphors and the critical emission quenching as a function of Er(3+) content in the Ba(9-3m/2)Er(m)Y2Si6O24 structure were monitored. The spectral conversion properties of Er(3+) and Er(3+)-Yb(3+) ions doped in Ba9Y2Si6O24 phosphors were elucidated under diode-laser irradiation at 980 nm. Up-conversion emission spectra and the dependence of the emission intensity on pump power for the Ba8.55Er0.1Yb0.2Y2Si6O24 phosphor were investigated. The desired up-conversion of the emitted light, which passed through the green, yellow, orange and red regions of the spectrum, was achieved through the use of appropriate Er(3+) and/or Yb(3+) concentrations in the host structure and 980 nm excitation light. The up-conversion mechanism in the phosphors is described by an energy-level schematic.

Photoluminescent ZnO Nanoparticles and Their Biological Applications

During the past decades, numerous achievements concerning luminescent zinc oxide nanoparticles (ZnO NPs) have been reported due to their improved luminescence and good biocompatibility. The photoluminescence of ZnO NPs usually contains two parts, the exciton-related ultraviolet (UV) emission and the defect-related visible emission. With respect to the visible emission, many routes have been developed to synthesize and functionalize ZnO NPs for the applications in detecting metal ions and biomolecules, biological fluorescence imaging, nonlinear multiphoton imaging, and fluorescence lifetime imaging. As the biological applications of ZnO NPs develop rapidly, the toxicity of ZnO NPs has attracted more and more attention because ZnO can produce the reactive oxygen species (ROS) and release Zn²⁺ ions. Just as a coin has two sides, both the drug delivery and the antibacterial effects of ZnO NPs become attractive at the same time. Hence, in this review, we will focus on the progress in the synthetic methods, luminescent properties, and biological applications of ZnO NPs.

Infrared to infrared upconversion emission in Pr(3+)/Yb(3+) co-doped La2O3 and La(OH)3 nano-phosphors: a comparative study

The Pr(3+)/Yb(3+) co-doped La2O3 and La(OH)3 nano-phosphors have been synthesized through solution combustion method. The structure and morphology of the samples have been studied using X-ray diffraction (XRD) and transmission electron microscopy (TEM). The physical and optical properties of the samples have been measured and compared. A broad intense infrared emission centered at 850nm due to (1)I6→(1)G4 transition along with sharp green emission centerd at 513nm due to (3)P0→(3)H4 transition are observed on excitation with 976nm laser. The emission intensity of Pr(3+) is optimized with concentration and it is maximum at 0.08mol%. The annealed samples are found to be more crystalline and emit larger photoluminescence due to removal of quenching centers. The power dependent study of green upconversion emission indicates the involvement of two photons. The phosphor in La(OH)3 phase is more stable though the photoluminescence emission is slightly weak. La(OH)3 is less toxic compared to La2O3 and is biocompatible. It generates more heat and can be used in biothermal treatment.

Structural characterizations and intense green upconversion emission in Yb3+, Pr3+ co-doped Y2O3 nano-phosphor

We report the structural and optical properties of Yb(3+), Pr(3+) co-doped Y2O3 nano-phosphor synthesized through solution combustion method. The structural studies reveal the nano-crystalline structure of the sample. The energy dispersive spectroscopy (EDS) measurements confirm the presence of Y, O, Pr and Yb elements in the sample. Fourier transform infrared studies show the vibrational features of the samples. The fluorescence spectra of the samples have been monitored on excitation with 976 nm and the intense green upconversion emission observed at 552 nm is due to (3)P0→(3)H5 electronic transition. The concentration of Pr(3+) ion in the sample is optimized and the fluorescence intensity is maximum at 0.08 mol% of Pr(3+). The power dependence studies reveal the involvement of two photons in the emission process. The possible mechanism of upconversion has been discussed on the basis of schematic energy level diagram. The sample annealed at higher temperature enhances the fluorescence intensity up to 8 times and this enhancement is discussed in terms of the removal of optical quenching centers. The nano-phosphor can be applicable in the field of display devices and green laser.

Upconversion luminescence of cerium-stabilized high temperature phase zirconia phosphors with a high Er3+ doping concentration

As the Er³⁺ concentration increases, the luminescence can be tuned from green to red and the red emission intensity reaches its maximum with an Er³⁺ concentration as high as 10 mol%.

Nanoparticulate coatings with efficient up-conversion properties

Nanoparticulate films with high up-conversion emission (UC) properties were prepared by spray-deposition of nanometer-sized YVO4:Yb,Er particles. The optical properties of YVO4:Yb,Er were optimized upon annealing before the film deposition in order to get the highest possible UC signal in the considered type of system. Thanks to a simple model and some time-resolved spectroscopic investigations, the contribution of the scattering to the UC signal could be separated from the intrinsic properties (crystallinity, surface defects) of the material. The films obtained by this technique present the advantages of having both high UC and good transparency.

Synthesis and up/down conversion luminescence properties of Na0.5R0.5MoO4:Ln3+ (R3+ = La, Gd), (Ln3+ = Eu, Tb, Dy, Yb/Er) thin phosphor films grown by pulsed laser deposition technique

(Na_0.5R_0.5)MoO₄:Ln³⁺ thin phosphor films can serve as an excellent material for electro/cathodo-luminescence and display applications.

Triple excitation with dual emission in paramagnetic ZnO:Er3+ nanocrystals

Confocal fluorescence images and emission spectra of ZnO:Er^3± nanocrystals showing triple excitation.

Introducing dual excitation and tunable dual emission in ZnO through selective lanthanide (Er3+/Ho3+) doping

Dual emission in Ho³⁺ doped ZnO showing red and green emission600 dpi in TIF format)??> under UV (DC) and IR (UC) excitation.

Photofunctional hybrids of rare earth complexes covalently bonded to ZnO core-shell nanoparticle substrate through polymer linkage

A novel series of multi-component hybrids are assembled based on rare earth coordinated to rare earth ion (Eu(3+), Tb(3+), Sm(3+), Dy(3+)) complex systems and ZnO nanocomposites through three different ester units (ethyl methacrylate (EMA), 2-hydroxyethyl methacrylate (HEMA) and 2,2,3,4,4,4-hexafluorobutyl methacrylate (HFMA)) as functional polymer linkages. Methacrylic-group-modified ZnO nanoparticles (designated ZnO-MAA) are synthesized based on the reaction between zinc methacrylate and LiOH with the molar ratio 1 : 3.5 via sol-gel process. The final hybrid materials are prepared by introducing rare earth complexes into ZnO-MAA matrix via addition polymerization reaction in the presence of benzoyl peroxide (BPO) as the initiator. The detailed characterization and luminescence of these hybrid materials are discussed. It is found that ZnO-MAA-HEMA/EMA/HFBMA-RE-phen hybrid systems have effective intramolecular energy transfer process and exhibit longer lifetime and higher quantum efficiency.

Intense near-infrared emission from ZnO-LiYbO(2) hybrid phosphors through efficient energy transfer from ZnO to Yb(3+)

The ZnO-LiYbO(2) hybrid phosphors were sintered by the solid-state reaction method, in which the intense near-infrared emission around 1000 nm due to Yb(3+ 2)F(5/2)-->(2)F(7/2) transition was obtained due to the efficient energy transfer from ZnO to Yb(3+) ions. The growth of the LiYbO(2) crystal and the formation of the diffusion layer between LiYbO(2) and ZnO were confirmed by XRD, SEM and EDX studies. The high efficient energy transfer is benefited from the inter-diffusion of Li(+), Yb(3+) and Zn(2+) in the diffusion region. The spectroscopy results clearly indicated that the ZnO-LiYbO(2) hybrid phosphors can harvest the energy from near-UV photons in a broad wavelength region and effectively convert them into Yb(3+) near-infrared emission.

Near-infrared luminescence of Nd3+ and Tm3+ ions doped ZnO nanocrystals

Intense near-infrared luminescence of Nd(3+) and Tm(3+) ions in the region of 860-1550 nm were achieved in 10-15 nm wurtzite ZnO nanocrystals fabricated by a facile sol-gel process. The optical properties of Nd(3+) and Tm(3+) ions were investigated by using the steady-state and time-resolved laser spectroscopy. Due to the well-ordered crystal-field surroundings experienced by Nd(3+) and Tm(3+) ions, sharp and well resolved emission lines of Nd(3+) and Tm(3+) ions were identified at 4-300 K. Time-resolved luminescence and decay behaviors of the (4)F(3/2)-->(4)I(11/2) transition of Nd(3+) ions reveal the existence of multiple Nd(3+) sites in ZnO nanocrystals.

Autofluorescence-free in vivo multicolor imaging using upconversion fluoride nanocrystals

Non-invasive fluorescence imaging is an important technique in biology. However, detection of traditional biomarker emissions is accompanied by a high background signal. In this study we examined whether upconversion sodium yttrium fluoride (NaYF(4)) nanocrystals were suitable for autofluorescence-free multicolor fluorescence imaging in a living animal. Tissue autofluorescence was induced with a 405 nm light source, then rats were subjected to injection of fluorescein isothiocyanate (FITC), cadmium selenide/zinc sulfide (CdSe/ZnS) quantum dots (QDs), or NaYF(4):ytterbium/thulium (Yb(3+)/Tm(3+)), NaYF(4):Yb(3+)/holmium (Ho(3+)), and NaYF(4):Yb(3+)/Ho(3+)/cerium (Ce(3+)) nanocrystals. Imaging with NaYF(4) nanocrystals (974 nm laser) completely removed the high tissue autofluorescence, in marked contrast to imaging with FITC and QDs (405 nm light). Optical imaging experiments demonstrated that multiple biological targets and organs could be imaged at the same time using multicolor NaYF(4) upconversion nanocrystals under a single excitation wavelength (974 nm). These data demonstrated the proof-of-principle that autofluorescence-free multicolor imaging using near-infrared to visible upconversion of NaYF(4) nanocrystals excited by laser can be performed in a living animal.

Spectroscopic evidence of the multiple- site structure of Eu(3+) ions incorporated in ZnO nanocrystals

Hexagonal Eu(3+):ZnO nanocrystals were synthesized by a modified solgel method. By means of the site-selective spectroscopy at 10 K, two kinds of luminescence sites of Eu(3+) are identified. One site exhibits a long lifetime of (5)D(0) and sharp emission and excitation peaks, which are ascribed to the inner lattice site with an ordered crystalline environment. The other site associated with the distorted lattice sites near the surface shows a relatively short lifetime of (5)D(0) and significantly broadened fluorescence lines. The energy transfer from the nanocrystal host to Eu(3+) confirms that Eu(3+) ions can, to some extent, be incorporated into the ZnO nanocrystal.

Synthesis, structure and optical properties of rare-earth benzene carboxylates

Two series of rare-earth isophthalates of the general formula, [M(2)(H(2)O)][{C(6)H(4)(COO)(2)}(2){C(6)H(4)(COOH)(COO)}(2)].H(2)O, M=La (I), Pr (Ia), and Nd (Ib) and [M(2)(H(2)O)(2)][{C(6)H(4)(COO)(2)}(3)].H(2)O, M=Y (II), Gd (IIa), and Dy (IIb) have been prepared by the reaction of the corresponding trivalent lanthanide salts and isophthalic acid under mild hydrothermal conditions. The La (I), Pr (Ia) and Nd (Ib) have MO(9) polyhedra connected to the isophthalate anions forming a two-dimensional structure, whereas Y (II), Gd (IIa) and Dy (IIb) have MO(7) and MO(8) polyhedral units connected to the isophthalate anions forming a different, but related two-dimensional structure. Both the structures are stabilized by hydrogen bonding and pi...pi/CH...pi interactions. Partial substitution of Eu and Tb (2 and 4%) at the La (I) and Y (II) sites give rise to characteristic red/pink or green luminescence, indicating a ligand-sensitized metal-centered emission. The Nd (Ib) compound shows interesting UV and blue emission through an up-conversion process.

Near-Infrared (NIR) to Red and Green Up-Conversion Emission from Silica Sol−Gel Thin Films Made with La0.45Yb0.50Er0.05F3 Nanoparticles, Hetero-Looping-Enhanced Energy Transfer (Hetero-LEET): A New Up-Conversion Process

Bright green and red luminescence has been generated with a 980 nm diode laser from silica sol-gel thin films made with La0.45Yb0.50Er0.05F3 nanoparticles through a newly described hetero-looping-enhanced energy-transfer (hetero-LEET) up-conversion process, which exhibits a power dependence similar to that of a photon avalanche (PA). The hetero-LEET mechanism is potentially more efficient than PA, ground-state absorption/excited-state absorption (GSA/ESA), and energy-transfer (ETU) mechanisms because it combines resonant ground-state absorption with a looping or feedback process.

Structures, Photoluminescence, Up-Conversion, and Magnetism of 2D and 3D Rare-Earth Coordination Polymers with Multicarboxylate Linkages

Four new rare-earth compounds, [Eu(NDC)1.5(DMF)2] (1), [Nd2(NDC)3(DMF)4].H2O (2), [La2(NDC)3(DMF)4].0.5H2O (3), and [Eu(BTC)(H2O)] (4), where NDC = 1,4-naphthalenedicarboxylate, BTC = 1,3,5-benzenetricarboxylate, and DMF = N,N-dimethylformamide, have been synthesized through preheating and cooling-down crystallization. Compounds 1-3 possess similar 2D structures, in which the NDC ligands link M(III) (M = La, Nd, and Eu) ions of two adjacent double chains constructed by NDC ligands and dinuclear M(III) building units. In compound 4, the Eu(III) ion is seven-coordinated by O atoms from six BTC ligands and one terminal water molecule in a distorted pentagonal-bipyramidal coordination environment. If the BTC ligand and the Eu(III) ion are regarded as six-connected nodes, respectively, the structure of compound 4 can be well described as a 3D six-connected net. Furthermore, compounds 1 and 4 exhibit strong red luminescence upon 355-nm excitation. Compound 2 displays interesting emissions in the near-IR region, and yellow (580 nm) pumping of this compound results in UV and intense blue emissions through an up-conversion process. The magnetic properties of compounds 1, 2, and 4 have been studied through measurement of their magnetic susceptibilities over the temperature range of 4-300 K.