Single NaYF4 Research Articles

Energy transfer and upconversion emission properties of single NaYF4: Yb3+/Ln3+@NaYF4: Yb3+/Ln3+ core-shell microcrystal

In this work, the energy transfer process and upconversion luminescence properties of NaYF4: Ln3+@NaYF4: Ln3+ (Ln = Yb3+, Er3+, Tm3+) core-shell microcrystals were studied through introducing different Yb3+ ions in core and shell. Compared with Yb3+/Tm3+/Er3+ co-doped NaYF4 microcrystals, the NaYF4:Yb3+/Tm3+@NaYF4: Yb3+/Er3+ core-shell microcrystals displays greater stability spectral characteristics under NIR 980 nm excitation. This is because the core-shell structures can control the excitation energy acquisition channel and avoid interactions between Er3+ and Tm3+ ions. Meanwhile, the luminescence properties of NaYF4@NaYF4 core-shell microcrystals can also be precisely controlled by changing concentrations of Yb3+ ions in the core and shell, respectively. The experimental results further indicate that upconversion emission of microcrystals can also be precisely controlled by constructing core-shell structures. The multicolor emission core-shell microcrystals may have great potential in display, anti-counterfeiting and micro-photoelectric technology applications.

Single up-conversion nanocrystal as a local temperature probe of electrically heated silver nanowire.

Luminescence thermometry is a powerful technique for monitoring temperature in a sensitive, remote (through light), and minimally invasive manner. Up to now, many macroscopic and microscopic luminescence temperature probes exploiting different temperature sensing schemes have been investigated, with the majority of the studies using aggregates of nanothermometers. This work presents isolated single up-converting NaYF4:Er3+/Yb3+ nanocrystals as functional temperature indicators operating in a standard confocal microscopy configuration. More specifically, the nanocrystals were used to monitor the temperature of a single silver nanowire, whose temperature was controlled electrically via the Joule process. We demonstrate that individual nanocrystals placed near the nanowire can precisely determine the temperature distribution in its surroundings. These results, which combine nanoscopic heat generation with temperature readout using isolated nanocrystals, represent an essential step for the application of isolated single nanoprobes for luminescence thermometry at the nanoscale.

Polarized Luminescence of Sm3+-Doped Single NaYF4 and BiPO4 Microcrystals

Polarized Luminescence of Sm3+-Doped Single NaYF4 and BiPO4 Microcrystals

Vectorial probing of electric and magnetic transitions in variable optical environments and vice-versa

We use europium doped single crystalline NaYF4 nanorods for probing the electric and magnetic contributions to the local density of optical states (LDOS). Reciprocically, we determine intrinsic properties of the emitters (oscillator strength, quantum yield) by comparing their measured and simulated optical responses in front of a mirror. We first experimentally determine the specifications of the nanoprobe (orientation and oscillator strength of the electric and magnetic dipoles moments) and show significant orientation sensitivity of the branching ratios associated with electric and magnetic transitions. In a second part, we measure the modification of the LDOS in front of a gold mirror in a Drexhage’s experiment. We discuss the role of the electric and magnetic LDOS on the basis of numerical simulations, taking into account the orientation of the dipolar emitters. We demonstrate that they behave like degenerated dipoles sensitive to polarized partial LDOS.

Supersensitive sensing based on upconversion nanoparticles through cascade photon amplification at single-particle level

Single-particle sensing strategy is expected to achieve super-sensitivity with extremely low detection limit by improving the contact surface with targeted object and excluding the statistical effect of nanoparticle ensembles. However, single-particle sensing based on upconversion nanoparticles (UCNPs) is rarely reported, limited by the detection difficulty of single particles. Herein, we designed a cascade photon amplification structure of surface plasmonic photonic crystals consisting of polymethyl methacrylate (PMMA) opal photonic crystals (OPCs) with plasmonic CsxWO3 NPs onto its surface voids. Finite-difference time-domain simulations and atomic force microscopy (AFM) characterization evidence that UCNPs at single-particle level (abbreviated as UCNP) are located at the maximum amplification of optical field. Combined with the advanced fluorescence microscope equipped with AFM, the upconversion emissions of single NaYF4:Yb3+, Er3+ NP (~45 nm) are greatly boosted ~1600 folds along with lower pumping thresholds by cascade photon amplification effect for the first time. As a proof of concept, such enhanced UCNP is employed as a fluorescent probe to develop a novel UCNP senor for supersensitive dithiothreitol detection with a detection limit of 0.25 nM, two orders of magnitude lower than the previous reports. This work provides a new attempt to develop high performance of UCNP for constructing supersensitive sensors.

Enhancing upconversion emission of Er3+ in single β-NaYF4 microrod through constructing different inert and active shells with doping Yb3+ ions

The construction of core-shell structure helps in enhancing upconversion luminescence in lanthanide-doped micro/nanomaterials. This study aimed to synthesize a series of NaYF4 and NaErF4 core-shell microrods based on epitaxial growth technology so as to enhance the upconversion emission. The upconversion luminescence properties of single NaYF4:Yb3+/Er3+ and NaErF4 microrods with different core-shell structures were carefully studied using a confocal microscope spectroscopy test system under 980-nm near-infrared excitation. The upconversion emission intensity and red-to-green emission intensity ratio of Er3+ ions were remarkably enhanced in different single microrods by building different inert and active core-shell structures. The higher red-to-green ratio was mainly due to the cross-relaxation between Er3+ ions and the energy back-transfer process from Er3+ to Yb3+ ions, which were explained on the basis of spectral characteristics and rate equations. These tunable NaYF4 and NaErF4 core-shell microrods had the potential to be applied to displays, micro-lasers, and anti-counterfeiting.

Multicolor upconversion emission of Ho3+ in single NaYF4 microrod

Lanthanide-doped upconversion nano/micro-materials exhibit superior and fascinating optical properties due to their unique 4f–4f transitions. They have been extensively investigated in various fields. Thus, studying the upconversion optical characteristic, especially for Er3+, Tm3+, and Ho3+ ions, is extremely important and useful. In this work, a strong upconversion emission with a candy-like fluorescent pattern has been discovered in a single NaYF4: Yb3+/Ho3+ microrod via confocal microscopy under NIR 980 nm excitation. When the Yb3+ ion-doped concentrations and the excitation power are increased, the Ho3+ ions show an increased blue emission intensity and high R/G ratio in single NaYF4 microrod, which is due to the cross-relaxation between Ho3+ ions. The corresponding upconversion emission behaviors and physical mechanisms have been studied on the basis of the spectral characteristics of Ho3+ ions and rate equations. Meanwhile, the candy-like emission pattern of single NaYF4 microrod is explained by establishing optical models, which because of the occurrence of fluorescence total reflection. The present study is of great importance in NaYF4 micro-materials doped with rare-earth ions, thereby providing new applications in optical anti-counterfeiting, 3D-displays, and wave-guides.

Upconversion luminescence quenching mechanism of single Au nanoparticles decorated NaYF4: Yb3+, Er3+ hexagonal disk

Different amounts of Au nanoparticles decorated NaYF4: Yb3+, Er3+ hexagonal disks were synthesized by using a hydrothermal strategy. In comparsion with single NaYF4: Yb3+, Er3+ hexagonal disk, the average upconversion quenching factors for the green and red upconversion emission bands of single Au nanoparticles decorated hexagonal disk increased obviously with the increasing of the amounts of uploaded Au nanoparticles. The local field distribution accompanied with the enhancement factors were obtained by finite element method. Based on the cavity quantum optics theory, a quantitative analysis for the Au nanoparticles induced quenching was addressed. The results show that the presence of the higher mode Purcell factors induced nonradiative energy transfer from the upconverter to the Au nanoparticles, which contributed to the quenching significantly. The modest weakening of local fields only played a minor role. It could offer a novel theory guidance for the seeking of the novel plasmonic nanostructure-upconverter hybrid system.

Tunable upconversion emission of Ho3+/Yb3+-doped single β-NaYF4 microrod

In this work, a series of hexagonal phase NaYF4: Yb3+/Ho3+ microrods have been successfully synthesized through hydrothermal methods, and their optical properties studied by a confocal microscope equipment with a 980 nm laser. An intense green and red upconversion emissions with candy-like emission pattern are observed from a single NaYF4: Yb3+/Ho3+ microrod. As the codoping Ce3+ ion concentrations increases from 0% to 12% in the microrods, the interesting phenomenon that fluorescence emission of Ho3+ ion is tuned green to red emission has been obtained. The red-to-green ratio emission of Ho3+ ions is enhanced about 11-fold, which is due to the two efficient cross-relaxation processes between Ho3+ and Ce3+ ions, which leading to the populations increase of the 5F5 state. The possible upconversion emission mechanisms are discussed based on the emission spectra of Ho3+ ions. The current study suggests that the NaYF4: Yb3+/Ho3+ microrods with the strong red upconversion emission may have great potential applications in micro-fiber laser, multicolor display, micro-optoelectronic device.

Luminescence regulation mechanism of single layer NaYF4: Yb3+, Er3+@NaYF4 nanocrystals on Au nanolayer supported by anodic aluminum oxide template

A kind of novel hybrid nanostructure of anodic aluminum oxide (AAO) template or Au sputtered AAO template covered by a single layer NaYF4: Yb3+, Er3+@NaYF4 nanocrystals (NCs) was prepared by a facile strategy. Two kinds of available models that include with and without Au nanolayer were suggested for simulations. Significant downconversion luminescence (DCL) enhancement was observed when the supported template changed from single AAO to Au nanolayer covered AAO. A formula for luminescence enhancement factor based on the cavity quantum optics theory was constructed. With luminescence dynamics analyses, the DCL enhancement was ascribe to the main positive influence of excitation enhancement, besides the minor negative impact of the competition among the nonradiative loss of resonant cavity, the higher order mode loss of Purcell factor, and the increase of radiative transition rate. It proposed a new idea for surface plasmon (SP) induced luminescence regulation mechanism exploration.

Tunable flower-like upconversion emission and directional red radiation in a single NaYF4:Yb3+/Tm3+ microcrystal particle

Rear earth doped upconversion (UC) luminescence materials have attracted considerable attentions in color display and biological labeling due to their tunable emission colors property. In this report, we designed and fabricated NaYF4: Yb3+/Tm3+ microcrystals with novel morphology, which exhibit some unique UC luminescence phenomena. The experimental findings show that a flower-like blue UC emission pattern and the directional red radiation are readily manipulated by controlling the focal point position of the excitation laser on the single microcrystal particle (MP). The unique phenomena are attributed to optical waveguide effect with total internal reflection of the UC luminescence at the microcrystal-air interface. The findings not only provide a new strategy for designing UC nano-/microcrystals with particular and tunable luminescence properties, but also open a promising route for the fabrication of new optoelectronic devices and the construction of three-dimensional color display systems.

Atomistic evolution during the phase transition on a metastable single NaYF4:Yb,Er upconversion nanoparticle

The phase evolution of as-prepared NaYF4:Yb,Er upconversion nanoparticles (UCNPs) with a metastable cubic structure is studied based on in situ heating experiments via transmission electron microscopy (TEM). The atomistic behavior on the single NaYF4:Yb,Er UCNP is observed during the phase transition. The formation and evolution of voids on the NaYF4:Yb,Er UCNP appear at a temperature below 420 °C. Small circular voids are transformed at the initial stage to a large, hexagonal-pillar shaped single void. Two different routes to reach the stable α-phase from the metastable cubic structure are identified on a single NaYF4:Yb,Er UCNP. The first is via a stable β-phase and the second is a direct change via a liquid-like phase. The specific orientation relationships, [110]cubic//[11bar{2}0]hexagonal and {002}cubic//{2bar{2}00}hexagonal, between the cubic and hexagonal structures are confirmed. Additionally, a few extra-half planes terminated in the cubic structures are also observed at the cubic/hexagonal interface.

Spectroscopic exploration of upconversion luminescence behavior of rare earth-doped single-particle micro/nanocrystals

In recent years, rare earth-doped upconversion (UC) micro/nanocrystals are useful for many applications, especially in biology because of their unique luminescent properties and specific geometry. The luminescence efficiency of lanthanide-doped UC nanoparticles is of particular importance for their applications. However, the unsatisfactory UC efficiency is still one of the main hurdles. In the present article, a series of Yb3+/Er3+ doped NaYF4 micro/nanoparticles with different ratios of length to diameter are successfully synthesized by a facile hydrothermal route. X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy (EDX) analyses, photoluminescence spectra, and the dynamic process of the luminescence are used to characterize the samples. The intrinsic structural feature of fluoride, the solution pH value, and organic additive Cit3- account for the ultimate shape evolution of the final products. The ratio of length to diameter of NaYF4 microrod can be tuned only by varying the value of pH or the amount of an organic additive (Cit3-). The UC characteristics of a single NaYF4:Yb3+/Er3+ microrod obtained by tuning the value of pH or the amount of Cit3- are investigated by laser confocal microscopy with a 980 nm laser. The two series of codoped fluoride crystals both exhibit the characteristic UC luminescence from Er3+ ions and display the rich luminescence patterns in space. The UC luminescence from a single NaYF4:Yb3+/Er3+ microrod obtained by tuning the value of pH is brighter than that from a single NaYF4:Yb3+/Er3+ microrod with the same size obtained by tuning the amount of Cit3-. The EDX analysis indicates that the number of Na+ defects depends on the specific synthesis conditions of the sample. The Na+ defects of samples obtained by tuning the values of pH are lower than those of samples with the same size obtained by tuning the amount of Cit3-. It conduces to reducing Na+ defects at lower pH value. The parameters of the luminescence kinetics are found to be unambiguously dependent on the size of sample, which relates to higher energy phonon of surface and Na+ defects. The mechanism of luminescence enhancement by pH controlling is explored, and a mechanism based on the reduced intrinsic defects of Na+ is proposed. The investigation not only enriches the controllable synthesis approach of fluoride micro/nanomaterials, but also indicates the potential applications of rare earth materials with a rich luminescence pattern in the photonic devices and anti-counterfeiting devices.

Ultrafast Synthesis and Coating of High-Quality β-NaYF4:Yb3+,Ln3+ Short Nanorods.

An ultrafast route to prepare up-converting single β-phase NaYF4:Yb3+,Ln3+ (Ln: Er, Tm, or Tb) short nanorods (UCNRs) of high quality was developed. This new procedure affords reactive-surface nanorods that are easily coated by direct injection of suitable capping ligands. Thus highly crystalline nanorods with excellent UC fluorescence and good solvent-selective dispersion are obtained, which represents a significant advance in the field and enlarges their use for biomedical and other technological applications. Unlike other methodologies, the short reaction time provides a kinetic control over crystallization processes, and the β-phase and rod morphology is preserved regardless of the optically active Ln3+ ion. The UC emission was finely tuned by using the most popular Yb3+/Tm3+ and Yb3+/Er3+ pairs. More importantly, UCNRs doped with the unusual Yb3+/Tb3+ pair, with no ladder-like energy levels, provided a nice emission upon near-infrared excitation, which constitutes the first example of phonon-assisted cooperative sensitization to date in pure β-NaYF4 nanocrystals.

Tuning Plasmonic Enhancement of Single Nanocrystal Upconversion Luminescence by Varying Gold Nanorod Diameter.

Plasmonic enhancement induced by metallic nanostructures is an effective strategy to improve the upconversion efficiency of lanthanide-doped nanocrystals. It is demonstrated that plasmonic enhancement of the upconversion luminescence (UCL) of single NaYF4 :Yb3+ /Er3+ /Mn2+ nanocrystal can be tuned by tailoring scattering and absorption cross sections of gold nanorods, which is synthesized wet chemically. The assembly of the single gold nanorod and single upconversion nanocrystal is achieved by the atomic force microscope probe manipulation. By selecting two kinds of gold nanorods with similar longitudinal surface plasmon resonance wavelength but different diameters (27.3 and 46.7 nm), which extinction spectra are separately dominant by the absorption and scattering, the maximum UCL enhancement by a factor of 110 is achieved with the 46.7 nm-diameter gold nanorod, while it is 19 for the nanorod with the diameter of 27.3 nm. Such strong enhancement with the larger gold nanorod is due to stronger scattering ability and greater extent of the near-field enhancement. The enhanced UCL shows a strong dependence on the excitation polarization relative to the nanorod long axis. Time-resolved measurements and finite-difference time-domain simulations unveil that both excitation and emission processes of UCL are accelerated by the nanorod plasmonic effect.

Spectral and spatial characterization of upconversion luminescent nanocrystals as nanowaveguides.

Lanthanide upconversion (UC) luminescent nanocrystals exhibit a uniquely sharp multiband emission over a broad spectral bandwidth covering the ultraviolet region to the near-infrared (NIR) region when subjected to NIR excitation, which is vital for multichannel optical communication using wavelength-division multiplexing to achieve high transmission rates. In this study, we experimentally and theoretically investigated the spectral and spatial characterization of a single NaYF4:Yb3+,Tm3+(Yb3+,Er3+) UC nanocrystal as a nanowaveguide. We suggest that a UC nanocrystal can be used as a nanowaveguide because it produces a range of output colors simultaneously and provides unaltered emission bands during propagation. Via the observation of single NaYF4:Yb3+,Tm3+(Yb3+,Er3+) UC nanocrystals, we found, for the first time, that a single UC nanocrystal exhibited wavelength- and position-dependent UC emissions. In addition, by adding Ag coating to the UC nanocrystal to act as a plasmonic waveguide and introducing a photonic crystal, the scattering loss of the UC emissions was significantly suppressed in the middle of the NaYF4 nanocrystal, indicating efficient light guiding through the UC nanocrystal. Our discovery provides a basic understanding of the use of UC nanocrystals as nanowaveguides at the single-nanoparticle level, expanding our knowledge of the performance optimization of UC nanomaterials.

Simultaneous quasi-one-dimensional propagation and tuning of upconversion luminescence through waveguide effect.

Luminescence-based waveguide is widely investigated as a promising alternative to conquer the difficulties of efficiently coupling light into a waveguide. But applications have been still limited due to employing blue or ultraviolet light as excitation source with the lower penetration depth leading to a weak guided light. Here, we show a quasi-one-dimensional propagation of luminescence and then resulting in a strong luminescence output from the top end of a single NaYF4:Yb3+/Er3+ microtube under near infrared light excitation. The mechanism of upconversion propagation, based on the optical waveguide effect accompanied with energy migration, is proposed. The efficiency of luminescence output is highly dependent on the concentration of dopant ions, excitation power, morphology, and crystallinity of tube as an indirect evidence of the existence of the optical actived waveguide effect. These findings provide the possibility for the construction of upconversion fiber laser.

Determining the 3D orientation of optically trapped upconverting nanorods by in situ single-particle polarized spectroscopy.

An approach to unequivocally determine the three-dimensional orientation of optically manipulated NaYF4:Er(3+),Yb(3+) upconverting nanorods (UCNRs) is demonstrated. Long-term immobilization of individual UCNRs inside single and multiple resonant optical traps allow for stable single UCNR spectroscopy studies. Based on the strong polarization dependent upconverted luminescence of UCNRs it is possible to unequivocally determine, in real time, their three-dimensional orientation when optically trapped. In single-beam traps, polarized single particle spectroscopy has concluded that UCNRs orientate parallel to the propagation axis of the trapping beam. On the other hand, when multiple-beam optical tweezers are used, single particle polarization spectroscopy demonstrated how full spatial control over UCNR orientation can be achieved by changing the trap-to-trap distance as well as the relative orientation between optical traps. All these results show the possibility of real time three-dimensional manipulation and tracking of anisotropic nanoparticles with wide potential application in modern nanobiophotonics.

Mechanistic investigation of upconversion luminescence in Er3+-doped BaCl2, BaF2 and NaYF4 phosphors

Near infrared to visible up-conversion luminescence in Er3+ single doped BaCl2, BaF2 and NaYF4 phosphors were compared. Intense blue emission at 410 nm due to 2H9/2 → 4I15/2 transition and greenish-blue emission at 496 nm due to 4F7/2 → 4I15/2 transition were only observed in BaCl2:Er3+ when excited by 808 nm and 976 nm laser diodes, respectively. The phenomena can be ascribed to the direct two-step excitation processes which occur in the chlorides where multiphonon relaxation is rapidly suppressed. In addition, the intensity of upconversion emission in BaCl2:Er3+ is greatly enhanced upon broadband near-infrared excitation.

Tailoring Plasmonic Enhanced Upconversion in Single NaYF4:Yb(3+)/Er(3+) Nanocrystals.

By using silver nanoplatelets with a widely tunable localized surface plasmon resonance (LSPR), and their corresponding local field enhancement, here we show large manipulation of plasmonic enhanced upconversion in NaYF4:Yb3+/Er3+ nanocrystals at the single particle level. In particular, we show that when the plasmonic resonance of silver nanolplatelets is tuned to 656 nm, matching the emission wavelength, an upconversion enhancement factor ~5 is obtained. However, when the plasmonic resonance is tuned to 980 nm, matching the nanocrystal absorption wavelength, we achieve an enhancement factor of ~22 folds. The precise geometric arrangement between fluorescent nanoparticles and silver nanoplatelets allows us to make, for the first time, a comparative analysis between experimental results and numerical simulations, yielding a quantitative agreement at the single particle level. Such a comparison lays the foundations for a rational design of hybrid metal-fluorescent nanocrystals to harness the upconversion enhancement for biosensing and light harvesting applications.