High Upconversion Research Articles

Rare-earth doped nano-glass-ceramics for extending spectral response of water-splitting semiconductor electrodes by high intense UV-blue up-conversion: Turning the sun into blue

The use of hydrogen to feed Fuel Cells emerges as an attractive renewable solution to meet the terawatt carbon-neutral energy demand challenge. Rare-earth doped nano-glass-ceramics, presenting outstanding high intense UV-blue up-conversion processes, appear as an interesting approach to assist long wavelength light harvesting of solar irradiation, extending the photo-catalytic response to the red and near-infrared region to overcome the drawback of large band-gap materials used in the splitting of water into oxygen and hydrogen in a photo-electrochemical cell.

High upconversion quantum yield at low pump threshold in Er3+/Yb3+ doped La2O2S phosphor

Abstract In this letter, we report the absolute upconversion quantum yields (QY) for 9% Yb3+ and 1% Er3+ co-doped La2O2S and its comparison with NaYF4 doped with 20% Yb3+, 2% Er3+ for all the upconversion emissions centered at 410, 550, 667, and 822 nm at various excitation power densities at 977 nm excitation. The maximum upconversion QY for 9% Yb3+ and 1% Er3+ co-doped La2O2S was measured to be 5.83±0.87% at the excitation power density of 13 W/cm2. An excitation power dependent study revealed that although absolute upconversion QY measured for NaYF4: 20% Yb, 2% Er is higher at higher excitation power densities, La2O2S: 9% Yb, 1% Er possesses higher QYs at the lower excitation power densities.

Comparative Study of Upconverting Nanoparticles with Various Crystal Structures, Core/Shell Structures, and Surface Characteristics

Upconverting nanoparticles (UCNPs) have been studied as novel bioimaging probes owing to the absence of autofluorescence and excellent photostability. For practical applications, biocompatible UCNPs with high upconversion efficiency, bright luminescence, and good colloidal stability are desirable. Herein, we report a quantitative and systematic study on the upconversion luminescence from a set of NaYF4:Yb3+,Er3+-based nanoparticles by varying crystal structures, core/shell structures, and surface ligands. Upconversion luminescent properties in colloidal solution and at the single-particle level were examined. Hexagonal-phase core/shell UCNPs exhibited the most intense luminescence among various structures, while the excellent photostability was observed in all different types of UCNPs. To optimize the biomedical imaging capability of UCNPs, various surface coating strategies were tested. By quantitative spectroscopic measurements of surface-modified UCNPs in water, it was suggested that encapsulation with polyethylene glycol (PEG)-phospholipid was found to be effective in retaining both upconversion luminescence intensity and dispersibility in aqueous environment. Finally, UCNPs with different crystal structures were applied and compared in live cells.

Microwave-assisted synthesis and up–down conversion luminescent properties of multicolor hydrophilic LaF3:Ln3+nanocrystals

Monodisperse water-soluble LaF(3):Ln(3+) nanocrystals (NCs) have been successfully fabricated via a fast, facile and environmentally-friendly microwave-assisted modified polyol process with polyvinylpyrrolidone (PVP) as an amphiphilic surfactant. The obtained NCs can be well dispersed in hydrophilic solutions with small sizes in the range of 9-12 nm. The LaF(3):Ln(3+) NCs (Ln = Eu, Nd, Ce, Tb, Yb, Er, Yb, Ho and Yb, Tm) have the unique feature of up-down conversion from visible to NIR emission owing to the ladder-like arranged energy levels of Ln(3+) and in particular, the high efficiency upconversion of the two-photon, obtained from excitation by a continuous 980 nm laser. This investigation focuses on both the up and down conversion fluorescence properties of water-soluble monodisperse crystalline LaF(3):Ln(3+) NCs in such a small size. Furthermore, the three-dimensional PDMS rod-like fluorescence displays and a silica surface modification by a core/shell structure on the obtained NCs can improve the biocompatibility, indicating potential applications in optical 3D devices and as bio-probes.

Synthesis, Tunable Multicolor Output, and High Pure Red Upconversion Emission of Lanthanide-Doped Lu2O3Nanosheets

Yb3+and Ln3+(Ln = Er, Ho) codoped Lu2O3square nanocubic sheets were successfully synthesized via a facile hydrothermal method followed by a subsequent dehydration process. The crystal phase, morphology, and composition of hydroxide precursors and target oxides were characterized by X-ray diffraction (XRD), field emission scanning electron microscope (FE-SEM), and energy-dispersive X-ray spectroscope (EDS). Results present the as-prepared Lu2O3crystallized in cubic phase, and the monodispersed square nanosheets were maintained both in hydroxide and oxides. Moreover, under 980 nm laser diode (LD) excitation, multicolor output from red to yellow was realized by codoped different lanthanide ions in Lu2O3. It is noteworthy that high pure strong red upconversion emission with red to green ratio of 443.3 of Er-containing nanocrystals was obtained, which is beneficial forin vivooptical bioimaging.

Aqueous phase synthesis of upconversion nanocrystals through layer-by-layer epitaxial growth for in vivo X-ray computed tomography

Lanthanide-doped core-shell upconversion nanocrystals (UCNCs) have tremendous potential for applications in many fields, especially in bio-imaging and medical therapy. As core-shell UCNCs are mostly synthesized in organic solvents, tedious organic-aqueous phase transfer processes are usually needed for their use in bio-applications. Herein, we demonstrate the first example of one-step synthesis of highly luminescent core-shell UCNCs in the "aqueous" phase under mild conditions using innocuous reagents. A microwave-assisted approach allowed for layer-by-layer epitaxial growth of a hydrophilic NaGdF4 shell on NaYF4:Yb, Er cores. During this process, surface defects of the nanocrystals could be gradually passivated by the homogeneous shell deposition, resulting in obvious enhancement in the overall upconversion emission efficiency. In addition, the up-down conversion dual-mode luminescent NaYF4:Yb, Er@NaGdF4:Ce, Ln (Eu, Tb, Sm, Dy) nanocrystals were also synthesized to further validate the successful formation of the core-shell structure. More significantly, based on their superior solubility and stability in water solution, high upconversion efficiency and Gd-doped predominant X-ray absorption, the as-prepared NaYF4:Yb, Er@NaGdF4 core-shell UCNCs exhibited high contrast in in vitro cell imaging and in vivo X-ray computed tomography (CT) imaging, demonstrating great potential as multiplexed luminescent biolabels and CT contrast agents.

Intense Red Upconversion Emission and Shape Controlled Synthesis of Gd2O3:Yb/Er Nanocrystals

Yb/Er codoped Gd(OH)3was synthesized firstly via a simple hydrothermal treatment of the corresponding nitrate in the presence of alkali by tuning the pH values. The Gd2O3:Yb/Er nanocrystals were obtained via sintering the corresponding hydroxides precursors. The as-prepared samples were characterized by the typical X-ray diffraction, energy-dispersive X-ray spectroscopy, transmission electron microscopy (TEM), high-resolution TEM, and spectrophotometer. The results revealed that two shapes of the as-prepared Gd2O3:Yb/Er nanocrystals can be readily tuned from lemon-like particle to rod-like structure via tuning pH values from 7 to 14. Moreover, compared with the samples prepared at pH 7, the Gd2O3:Yb/Er nanocrystals prepared at pH 14 exhibit enhanced red upconversion emission and higher upconversion luminescence intensity under the excitation of 980 nm laser.

Visible green upconversion luminescence of Er3+/Yb3+/Li+ co-doped CaWO4 particles

The upconversion (UC) luminescence of Li+/Er3+/Yb3+ co-doped CaWO4 phosphors is investigated in detail. Single crystallized CaWO4:Li+/Er3+/Yb3+ phosphor can be obtained, co-doped up to 25.0/5.0/20.0 mol% (Li+/Er3+/Yb3+) by solid-state reaction. Under 980 nm excitation, CaWO4:Li+/Er3+/Yb3+ phosphor exhibited strong green UC emissions visible to the naked eye at 530 and 550 nm induced by the intra-4f transitions of Er3+ (2H11/2,4S3/2→4I15/2). The optimum doping concentrations of Yb3+/Li+ for the highest UC luminescence were verified to be 10/15 mol%, and a possible UC mechanism that depends on the pumping power is discussed in detail.

The development of triplet-triplet annihilation upconversion

Triplet-triplet annihilation (TTA) has attracted much attention due to its requirement of low excitation power density (solar light is sufficient), high upconversion quantum yield, readily tunable excitation/emission wavelength and potential applications in photovoltaics and photocatalysis, etc. This review introduced the mechanism of TTA upconversion and method for calculation of upconversion quantum yield, as well as the design rationales of the reported sensitizers were reviewed in detail. Photo upconversions in rubbery polymeric materials and polymer nanoparticles and applications in fields like photoelectrochemistry, solar cells, bioimaging <italic>in vivo</italic> were also reviewed.

Bright dual-mode green emission from selective set of dopant ions in β-Na(Y,Gd)F4:Yb,Er/β-NaGdF4:Ce,Tb core/shell nanocrystals

Bright dual-mode green-emitting core/shell nanoparticles (NPs) were synthesized by doping selective set of lanthanide ions. Up-conversion (UC) green-emitting β-NaY0.2Gd0.6F4:Yb0.18,Er0.02 NPs (8.3 nm) were used as core material. Bright down-conversion (DC) green-emitting β-NaGd0.8F4:Ce0.15,Tb0.05 NPs showed ca. 31 times higher photoluminescence (PL) intensity than β-NaGdF4:Tb NPs and they were served as shell material with their excellent PL properties. The UC/DC core/shell NPs showed bright green light under excitations of 980 nm near infrared (NIR) light and 254 nm ultraviolet (UV) light, respectively. The UC/DC core/shell NPs showed ca. 11 times higher UC PL intensity than core UCNPs. Consequently, the core/shell NPs doped with selective set of lanthanide ions showed bright dual-mode green emission under excitations of NIR light and UV light, indicating that they are promising for application to optical imaging.

Synergetic Effect in Triplet–Triplet Annihilation Upconversion: Highly Efficient Multi‐Chromophore Emitter

The synergetic effect on the efficiency of triplet–triplet-annihilation-assisted photon upconversion, as a result of a synthetic procedure for simultaneous and independent optimization of the intrinsic efficiencies of triplet–triplet transfer and triplet–triplet annihilation, is demonstrated (see picture). The new system shows high upconversion quantum yields of order of 0.11. Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

A novel contrast agent with rare earth-doped up-conversion luminescence and Gd-DTPA magnetic resonance properties

The magnetic-luminescent multifunctional nanoparticles based on Gd-DTPA and NaYF4:Yb, Er were successfully synthesized by the conjugation of activated DTPA and silica-coated/surface-aminolated NaYF4:Yb, Er nanoparticles through EDC/NHS coupling chemistry. The as-prepared products were characterized by powder X-ray diffraction, transmission electron microscopy, dynamic light scattering, energy dispersive X-ray analysis, and fourier transform infrared spectrometry. The room-temperature upconversion luminescent spectra and T1-weighted maps of the obtained nanoparticles were carried out by 980nm NIR light excitation and a 3T MR imaging scanner, respectively. The results indicated that the as-synthesized multifunctional nanoparticles with small size, highly solubility in water, and both high MR relaxivities and upconversion luminescence may have potential usage for MR imaging in future.

Efficient Upconverting Nanophosphors for Imaging and Photodynamic Therapy

ABSTRACTErbium-Ytterbium codoped nanophosphor systems are explored for high efficiency upconversion. The NIR to visible upconversion from 1550 nm and 980 nm excitation are of particular interest to us for biomedical applications such as imaging, sensing, and photodynamic therapy. Variations in synthesis method and rare earth concentration are carried out in sodium, potassium, and transition metal based phosphor materials. The spectroscopic properties of the material dry and in biologically appropriate solution are taken. After bioconjugation, these particles will be used in a mouse model to demonstrate that cancer imaging with a near-infrared excitation source is possible.

Absolute quantum yields in NaYF4:Er,Yb upconverters – synthesis temperature and power dependence

We investigate the temperature and power dependence of the emission quantum yield of NaYF4:Er,Yb upconverters. We show that the quantum yields may vary by almost three orders of magnitude. We demonstrate that formation of the hexagonal phase of this material is not in itself sufficient to guarantee high upconversion efficiency.

High upconversion gain optoelectronic mixer using uni-travelling carrier phototransistors

The optoelectronic mixing performance of a uni-travelling carrier heterojunction phototransistor (UTC-HPT) was compared with its performance as a photodiode (PD). The upconverted output power in HPT mode was 24 dBm higher than in PD mode.

Large scale electrochemical synthesis of high quality carbon nanodots and their photocatalytic property

High quality carbon nanodots (C-dots) with high purity were synthesized through a mild, one-step electrochemical approach, without the assistance of any chemicals but only pure water. This high productivity method makes the synthetic process of C-dots synthesis both economical as well as environment-friendly. The as prepared C-dots are predominantly multi-layer graphene oxide, with luminescence and high up-conversion photoluminescence (emission of light at shorter wavelengths than the excitation wavelength). Meanwhile, C-dots showed peroxidise mimetic function and visible-light-sensitive photocatalytic activity for methyl orange degradation. In addition, a novel photocatalyst (TiO(2)/C-dots) was obtained by combining C-dots with TiO(2) through an easy hydrothermal method. Remarkably, TiO(2)/C-dots exhibited an excellent visible-light photocatalytic activity.

Tuning the photophysical properties of N^N Pt(ii) bisacetylide complexes with fluorene moiety and its applications for triplet–triplet-annihilation based upconversion

Fluorene-containing aryl acetylide ligands were used to prepare N^NPt(II) bisacetylide complexes, where aryl substituents on the fluorene are phenyl (Pt-1), naphthal (Pt-2), anthranyl (Pt-3), pyrenyl (Pt-4), 4-diphenylaminophenyl (Pt-5) and 9,9-di-n-octylfluorene (Pt-6) (where N^N ligand = 4,4′-di-tert-butyl-2,2′-bipyridine, dbbpy). All the complexes show room temperature (RT) phosphorescence. The emissive T1 excited states of Pt-1, Pt-5 and Pt-6 were assigned as metal-to-ligand-charge-transfer state (3MLCT), whereas for Pt-2, Pt-3 and Pt-4, the emissive T1 excited states were identified as the intraligand state (3IL), based on steady state emission spectra, the lifetime of the T1 state, emission spectra at 77 K, spin density analysis and the time-resolved transient absorption spectroscopy. Exceptionally long lived T1 excited state was observed for Pt-3 (τ = 66.7 μs) and Pt-4 (τ = 54.7 μs), compared to a model complex dbbpy Pt(II) Bisphenylacetylide (τ = 1.25 μs). RT phosphorescence of anthracene was observed at 780 nm with Pt-3. The critical role of the fluorene is to move the absorption of the complexes to the red-end of the spectra, but at the same time, without compromising the energy level of the T1 state of the complexes. The advantage of this unique spectral tuning effect and the long-lived T1 excited states of Pt-4 was demonstrated by the enhanced performance of the complexes as triplet sensitizers for triplet–triplet annihilation (TTA) based upconversion; an upconversion quantum yield (ΦUC) up to 22.4% was observed with Pt-4 as the sensitizer. Other complexes described herein show negligible upconversion. The high upconversion quantum yield of Pt-4 is attributed to its intense absorption of visible light and long-lived T1 excited state. Based on the result of Pt-4, we propose that weakly phosphorescent, or non-phosphorescent transition metal complexes can be used as triplet sensitizers for TTA upconversion, compared to the phosphorescent triplet sensitizers currently used for TTA upconversion. Our results will be useful for the design of transition metal complexes to enhance the light-absorption and thereafter the cascade photophysical processes, without decreasing the T1 excited state energy levels, which are important for the application of the complexes as triplet sensitizers in various photophysical processes.

Organic Triplet Sensitizer Library Derived from a Single Chromophore (BODIPY) with Long-Lived Triplet Excited State for Triplet–Triplet Annihilation Based Upconversion

Triplet-triplet annihilation (TTA) based upconversions are attractive as a result of their readily tunable excitation/emission wavelength, low excitation power density, and high upconversion quantum yield. For TTA upconversion, triplet sensitizers and acceptors are combined to harvest the irradiation energy and to acquire emission at higher energy through triplet-triplet energy transfer (TTET) and TTA processes. Currently the triplet sensitizers are limited to the phosphorescent transition metal complexes, for which the tuning of UV-vis absorption and T(1) excited state energy level is difficult. Herein for the first time we proposed a library of organic triplet sensitizers based on a single chromophore of boron-dipyrromethene (BODIPY). The organic sensitizers show intense UV-vis absorptions at 510-629 nm (ε up to 180,000 M(-1) cm(-1)). Long-lived triplet excited state (τ(T) up to 66.3 μs) is populated upon excitation of the sensitizers, proved by nanosecond time-resolved transient difference absorption spectra and DFT calculations. With perylene or 1-chloro-9,10-bis(phenylethynyl)anthracene (1CBPEA) as the triplet acceptors, significant upconversion (Φ(UC) up to 6.1%) was observed for solution samples and polymer films, and the anti-Stokes shift was up to 0.56 eV. Our results pave the way for the design of organic triplet sensitizers and their applications in photovoltaics and upconversions, etc.

An 80 GHz High Gain Double-Balanced Active Up-Conversion Mixer Using 0.18 $\mu{\rm m}$ SiGe BiCMOS Technology

This letter reports the performance of a W-band (75-110 GHz) high gain double-balanced active up-conversion mixer fabricated in a low-cost 200 GHz f T and f max 0.18 μm SiGe BiCMOS technology. Integrated on-chip baluns are used at RF and LO ports to facilitate on-wafer characterization. The mixer achieves a measured single sideband (SSB) power conversion gain of 6.6 dB and 3.2 dB at 75 GHz and 80 GHz, respectively. The measured output-referred 1 dB compression point (OP1dB) is -4.8 dBm and -7.3 dBm at 75 GHz and 80 GHz, respectively. The overall chip area is 830 μm×690 μm. The mixer draws a total of 31.5 mA from a nominal 3.3 V supply. To the best of authors' knowledge, this is the W-band active up-conversion mixer with the highest conversion gain reported among all prior published millimeter-wave silicon-based up-conversion mixers.

Intense ultraviolet and blue upconversion emissions in Yb 3+–Tm 3+ codoped stoichiometric Y 7O 6F 9 powder

Stoichiometric Y 7O 6F 9 powder codoped with Yb 3+–Tm 3+ was synthesized via co-precipitation and subsequent calcining route. The results of X-ray diffraction and transmission electron microscopy reveal that when the calcining temperature is beyond 800 °C, orthorhombic YF 3 nanoparticles can be completely oxidized into orthorhombic Y 7O 6F 9 powder. Under the excitation of a 980 nm laser, Y 7O 6F 9 powder exhibits multicolor UC emission in regions spanning the UV to the NIR. In addition, the upconversion emission intensities of YF 3, Y 7O 6F 9 and Y 2O 3 powders were compared under the same dopant condition (Yb/Tm=5/0.5 mol%). The low phonon energy revealed by Raman spectra helped to understand the high efficient upconversion emission of Y 7O 6F 9 and the main phonon vibration of Y 7O 6F 9 lies at 472 cm −1, which is far lower that of Y 2O 3 (at 708 cm −1). Our results indicate that orthorhombic rare earth ions doped Y 7O 6F 9 is an efficient matrix for UV and blue UC emission, and has potential applications in color displays, anti-counterfeiting and multicolor fluorescent labels.