NaYF4 Nanocrystals Research Articles

Thermometric Analysis of Nanoaperture-Trapped Erbium-Containing Nanocrystals.

Temperature changes in plasmonic traps can affect biomolecules and quantum emitters; therefore, several works have sought out the capability of measuring the local temperature. Those works used ionic nanopore currents, fluorescence emission variations, and fluorescence-based diffusion tracking to measure the temperature dependence of shaped nanoapertures in metal films. Here, we make use of a stable erbium-containing NaYF4 nanocrystal that gives local temperature dependence while trapped in the nanoaperture hot spot. Ratiometric analysis of the emission at different wavelengths gives local temperature variation. Since the gold film dominates the thermal characteristic, we find that films of thickness 70, 100, and 130 nm give 0.64, 0.37, and 0.25 K/mW temperature change with laser power. Therefore, using thicker films can be effective in reducing the heating when it is not desired.

Effect of microwave heat-treatment on up-conversion luminescence of Er3+/Yb3+ co-doped NaYF4 oxy-fluoride glass-ceramics

The microscopic properties of fluoride grains are known to play critical roles in the up-conversion luminescence properties of phosphors carrying rare-earth-based luminescence centers. To study the effect of different heating methods during preparation on the microstructure and up-conversion luminescence performance of oxy-fluoride glass-ceramics, we synthesized Er3+/Yb3+ co-doped NaYF4 nanocrystals containing transparent glass-ceramics by microwave heat-treatment and the normally heating methods. We characterized the internal structure of glass-ceramics by X-ray diffraction, transmission electron microscopy, mid-infrared spectroscopy, and Raman spectroscopy. While glass-ceramics samples prepared by both methods have extremely high optical transmittance similar to that of a glass, the microwave heat-treatment promoted faster grain growth rate, the glass-ceramics sample had better crystallinity and much improved up-conversion luminescence intensity compared to the sample obtained by the normally heating method.

Synthesis and characterization of fluorinated polyimide doped with NaYF4 nanocrystals for optical waveguide applications

Fluorinated polyimide materials were selected for their exceptional flexibility, high thermal stability, and minimal absorption loss. These materials were subsequently employed in the production of single-mode polymer optical waveguide devices using photolithography. In this study, a fluorinated polyimide material doped with NaYF4 nanocrystals was prepared and confirmed to exhibit low absorption loss near 1550 nm, which is within the common communication window. The refractive index of the fluorinated polyimide films was precisely adjusted by the NaYF4 doping, the adjustable ranges for the refractive index were 1.95% and 1.72% for doped and undoped, respectively. Secondly, utilizing the NaYF4 nanocrystals doped-fluorinated polyimide materials, a three-layer single-mode polymer optical waveguide device for C-band (near 1550 nm) optical interconnection was fabricated by etching the underlying cladding and employing silicon nitride as a mask. The transmission performance of the device was excellent, with a total waveguide coupling and transmission loss of 1.502 dBm.

The influence of Al2O3 concentration on the NaYF4 crystallization in oxyfluoride glass–ceramics

AbstractThe influence of Al2O3 addition on the precipitation of NaYF4 crystals in oxyfluoride glasses has been investigated through the thermal, structural, and optical characterization of the parent glasses and corresponding glass–ceramics (GCs). The high‐resolution transmission electron microscopy analysis of the GC with 5 mol% of Al2O3 shows the presence of phase‐separated droplets about 69 nm in size containing several NaYF4 nanocrystals with the diameter of about 10–15 nm. Raman and magic angle spinning nuclear magnetic resonance (MAS‐NMR) spectroscopy were used to examine the structural changes attributed to the addition of Al2O3. The 19F MAS‐NMR analysis indicated that fluorine atoms are present in different chemical environments (Na–F, Na–Si–F, Na–Al–F, and NaYF4). The increasing amount of Al2O3 reduces the crystallization of NaYF4 phase due to the increased fraction of fluorine bound in Na–Al–F environments. The visible luminescence investigation of the glasses and GCs demonstrated that the intensity of Er3+ ions emission transitions in the GCs was higher than that of the parent glass. This difference was attributed to the presence of Er3+ ions bound in NaYF4 crystalline phase. Further evidence that the Er3+ ions were present in NaYF4 phase was provided by the fact that the excited level Er3+:4S3/2 lifetime was increased in GCs as compared to parent glass.

The Formation of NaYF4 : Er3+, Yb3+ Nanocrystals Studied by In Situ X‐ray Scattering: Phase Transition and Size Focusing

Abstractβ‐NaYF4 nanocrystals are a popular class of optical materials. They can be doped with optically active lanthanide ions and shaped into core‐multi‐shell geometries with controlled dopant distributions. Here, we follow the synthesis of β‐NaYF4 nanocrystals from α‐NaYF4 precursor particles using in situ small‐angle and wide‐angle X‐ray scattering and ex situ electron microscopy. We observe an evolution from a unimodal particle size distribution to bimodal, and eventually back to unimodal. The final size distribution is narrower in absolute numbers than the initial distribution. These peculiar growth dynamics happen in large part before the α‐to‐β phase transformation. We propose that the splitting of the size distribution is caused by variations in the reactivity of α‐NaYF4 precursor particles, potentially due to inter‐particle differences in stoichiometry. Rate equation modeling confirms that a continuous distribution of reactivities can result in the observed particle growth dynamics.

Laser Pyrolysis Synthesis of Upconverting Lanthanide-Doped NaYF4 Nanocrystals for Anticounterfeiting Applications

Laser pyrolysis, an aerosol nanosynthesis technique, uses a 10.6 μm CO2 laser to rapidly heat precursors to induce decomposition and particle nucleation. A photosensitizing agent, usually SF6, absorbs the laser beam and transfers energy to the precursors. Here, we exploit the presence of SF6 in the system and use it as a safe source of fluorine to synthesize NaYF4 nanocrystals in a single-step process. NaYF4 is an excellent host for rare-earth ions to create upconverting nanocrystals (UCNCs) that absorb infrared photons and emit at shorter wavelength. We thus dope the NaYF4 nanocrystals during synthesis with ytterbium as a sensitizer and erbium, thulium, or holmium as an upconversion emitter. Each emitter produces upconversion emission at specific wavelengths in the visible spectral range after near-infrared light (∼980 nm peaked emission from laser diode) is absorbed by ytterbium followed by energy transfer to emitter. We then show that calcining these materials, in a simple second step, dramatically increases upconversion emission intensity. Finally, we optimize the laser pyrolysis process and postsynthesis calcination to produce the most intense emission at each emission color. We also show that these laser-synthesized UCNCs can be dispersed in a broad range of solvents and retain their visible upconversion emission. This dispersibility could be particularly valuable in creating inks for printing in anticounterfeiting applications. Proof-of-concept use of the synthesized UCNC for anticounterfeiting applications was demonstrated by drop-casting a dispersion of UCNC in stencils on paper, to create various luminescent patterns that were visible under irradiation by a 980 nm laser diode. Overall, this study demonstrates a promising vapor-phase route to rapid, single-step, continuous synthesis of lanthanide-doped fluoride nanomaterials for anticounterfeiting and related applications.

Effect of glass-ceramics network intermediate Al2O3 content on up-conversion luminescence in Er3+/Yb3+ co-doped NaYF4 oxy-fluoride glass-ceramics

Effect of alumina as a glass network intermediate on the up-conversion luminescence (UCL) in NaYF4:Er3+/Yb3+ co-doped oxy-fluoride glass-ceramics (GCs) was investigated. Combinations of smaller NaYF4 nanocrystals (10 and 13 nm) and lower Al2O3 contents (5% and 10%), as well as larger NaYF4 nanocrystals (26 and 40 nm) and higher lower Al2O3 contents (15% and 20%) were prepared after heat-treatment, respectively. The glass network of intermediate partial replacement of SiO2 with Al2O3 was investigated, and the consequence on the response to the up-conversion of the lanthanide ions was also studied. The UCL properties of Er3+ ions were changed in accordance with the addition of Al2O3, the red UCL intensity decreased with an increased Al2O3 concentration, while the green emission intensity showed opposite tendency. Our results showed that adding Al2O3 to 20 mol% is an effective strategy to simultaneous control of the magnitude and luminescence properties of lanthanide ion doped GCs.

Quantitatively identifying the upconversion luminescence mechanisms of Er3+:NaYF4 nanocrystals in aluminosilicate glass ceramics under femtosecond laser field

Upconversion luminescence of lanthanide ion doped nanocrystals under ultrashort laser pulse excitation can find significant applications in areas like super-resolution nonlinear microscopic imaging and attracts increasing attention. Though major excitation mechanisms, such as excited state absorption (ESA), energy transfer upconversion (ETU) and multi-photon absorption (MPA), have been analyzed and discussed intensively, quantitative differentiation of their respective contributions with convincing experimental proof has seldom been reported however. Here we report experimental demonstration of a simple but effective way to quantitatively identify simultaneous MPA from other mechanisms like ESA and ETU involved in the Er3+ doped NaYF4 nanocrystals in aluminosilicate glass ceramics under femtosecond laser field excitation. Comparative studies using laser pulses of the same spectral bandwidth but different time duration or repetition rate are performed and convincing experimental evidences for their respective contribution weights are obtained. The dependence of the MPA excitation efficiency on the spectral bandwidth of the femtosecond laser pulse is also explored for maximizing the upconversion luminescence yield. And predictions from a theoretical model agree well with experimental observations.

Rose Bengal Decorated NaYF4:Tb Nanoparticles for Low Dose X-ray-Induced Photodynamic Therapy in Cancer Cells.

The use of scintillating nanoparticles (ScNPs) in X-ray-induced photodynamic therapy (X-PDT) is a technique for deep tissue-localized tumor therapy with few side effects. ScNPs transfer X-ray-induced energy to photosensitizers, which generate massive amounts of reactive oxygen species (ROS) and kill cancer cells. Here we fabricated rose bengal (RB)-installed, Tb3+-rich NaYF4 nanocrystals (NaYF4:Tb@RB), in which optically inert Y3+ enables highly efficient energy transfer via high amounts of Tb3+ doping. NaYF4:Tb was prepared via solvothermal synthesis to have an average size of 7.6 nm, followed by coating with poly(maleic anhydride-alt-1-octedecene)-poly(ethylene glycol) with a molecular weight of 2000 (C18PMH-PEG2k). Further, RB was covalently conjugated to carboxyl groups generated from PMH on NaYF4:Tb using an ethylenediamine linker. NaYF4:Tb@RB exhibited a hydrodynamic diameter of ∼75 nm with a ζ-potential of -12 mV. NaYF4:Tb@RB efficiently generated ROS in cultured luciferase-expressing murine epithelial breast cancer (4T1-luc) cells under low dose X-ray irradiation (0.5 Gy). The ROS generation amounts of NaYF4:Tb@RB were 1.5-2-fold higher than those of NaGdF4:Tb@RB, in which host nanocrystals were prepared with optically active Gd3+. Flow cytometric and confocal microscopic analyses showed higher intracellular ROS production of NaYF4:Tb@RB, compared to NaYF4:Tb and RB, resulting in higher X-ray-induced DNA damage in cultured 4T1-luc cells. Ultimately, NaYF4:Tb@RB elicited significant cytotoxicity after X-ray irradiation (0.5 Gy), while inducing marginal cytotoxicity without X-ray irradiation. Altogether, this research proposes a promising ScNP design for efficient X-PDT agents that make the better use of incident X-ray energy while causing the fewest side effects.

Near-infrared quantum cutting in NaYF4:Ce3+,Yb3+ nanocrystals for luminescent solar concentrators

A series of Ce3+-Yb3+ ions codoped NaYF4 nanocrystals were prepared by hydrothermal method. The structure, morphologies, photoluminescence properties and fluorescence dynamics were studied systemically. In Ce3+-Yb3+ ions codoped NaYF4 nanocrystals, strong near-infrared (NIR) emission at 900–1100 nm was observed under the excitation of 5d level of Ce3+ ions. The lifetime of Ce3+ ions decreased increases with increasing Yb3+ ions concentration. The back energy transfer from Yb3+ ions to Ce3+ ions in NaYF4 nanocrystals does not occur. As a near-infrared luminescent convertor, the kinds of materials have potential to be applied in luminescent solar concentrators (LSCs) and efficient NIR emitters.

Modulating the thermally coupled status of energy levels in rare earth ions for sensitive optical temperature sensing

Luminescence thermometry technique, which utilizes the rare earth ions activated luminescent nanomaterials as sensing media, has presented great potential in various fields owing to its high spatial, temporal, and thermal resolutions. Nevertheless, the development of sensitive luminescent nanothermometers working in the biological window remains a challenge. Here, Tm3+-Yb3+ codoped NaYF4 nanocrystals, the excitation and emission wavelengths of which are both located within the near-infrared region, are investigated for optical temperature measurement via the fluorescence intensity ratio (FIR) approach. The thermal population and decay rates involved in the thermally coupled levels (3F2, 3 and 3H4) are verified to play an important role in regulating the thermal coupling status. The measurement sensitivity of as-prepared nanothermometers is optimized through modulating the Tm3+ surrounding environment and a maximum sensitivity of about 2.13% K−1 is achieved in the physiological temperature range. Besides, a parameter named as thermally coupled degree is defined in this work to quantify the coupling between the thermally coupled levels. According to the conclusion of our work, special attention has to be taken for rationally constructing FIR-based optical temperature sensors.

Intense continuous‐wave laser and mode‐locked pulse operation from Yb3+‐doped oxyfluoride glass–ceramic fibers

AbstractUltrafast fiber lasers, due to their short pulse duration, excellent beam quality, and high brightness, are extensively used in precision processing, biomedicine, nonlinear optics, and spectroscopy. However, great challenges still exist in improving the optical conversion efficiency in glass‐based gain media because of the high non‐radiative transition probability. Here, we demonstrate an oxyfluoride glass–ceramic (GC) fiber containing NaYF4:Yb3+ nanocrystal that enables enhanced 1064‐nm continuous‐wave laser output with an optical signal‐to‐noise ratio of 60 dB. Compared with the as‐prepared glass fiber, the optical conversion efficiency of GC fiber is improved from 24.2% to 30.0%. The improvement of laser action is mainly caused by the preferential incorporation of Yb3+ into the NaYF4 nanocrystal with low phonon energy. Using this well‐developed GC fiber, we successfully built a passively mode‐locked pulsed fiber laser that deliveries laser pulses with a pulse duration of 8.1 ps and a repetition frequency of 56.92 MHz. These results highlight that the GC strategy may provide a roadmap for the development of ultrafast fiber laser and the application of GC fibers in various optoelectronic fields.

Oxyfluoride glass-ceramics for upconversion all-optical combinational logic gate operations

All-optical logic gates operations are highly desired for optical communications and data processing because of the high computing speed, low power consumption, and parallel processing. Here, we develop a combinational upconversion (UC) all-optical logic system based on glass-ceramic (GC) material, which uses two external near-infrared excitation sources as inputs signal and UC emission as optical outputs. The prepared transparent oxyfluoride GC containing Er:NaYF4 nanocrystals could well respond to dual-wavelength excitation, realizing the enhancement of UC luminescence intensity and the adjustment of emission color based on the synergistic effect of photons. This special UC system can be used to design a variety of all-optical logic gates and combinational logic operations through observing the two luminescence emission intensities and the ratio of these emission intensities under the dual-wavelength excitation input conditions. The results provide a path for the development of novel photonic logic devices for future optical computing technologies.

Intense and broadband mid-infrared emission by nano-crystallization of rare‐earth doped oxyfluoride glass-ceramic

Transparent oxyfluoride glass ceramics (GCs) comprised of a uniform oxide glass phase and a well-distributed fluoride crystal phase are regarded as suitable optical gain materials because of the combination of facile fabricability of glass and low phonon energy of nanocrystals. Here, oxyfluoride GCs containing NaYF4: Er3+/Ho3+/Dy3+ nanocrystals were controllably prepared. Due to the preferential incorporation of rare-earth ions into NaYF4 nanocrystals with low phonon energy and the efficient energy transfer between active ions, enhanced and broadband mid-infrared (MIR) emission at 2.5–3.3 µm was obtained from the GCs, which was nearly unobserved in precursor glass (PG) because of the high phonon energy of glass matrix. Compared with Er3+ singly-doped sample, the full width at half maximum (FWHM) is more than 3 times broader in Er3+/Ho3+/Dy3+ triply-doped sample. Furthermore, by the match of the refractive index of glass with NaYF4 nanocrystal, transparent GCs with average crystal size of 80 nm and high transmittance over 80% were achieved simultaneously. The desirable optical properties endow the GCs with potential applications in MIR broadband tunable lasers.

Volume and surface effects on two-photonic and three-photonic processes in dry co-doped upconversion nanocrystals

Despite considerable advances in synthesizing high-quality core/shell upconversion (UC) nanocrystals (NC; UCNC) and UCNC photophysics, the application of near-infrared (NIR)-excitable lanthanide-doped UCNC in the life and material sciences is still hampered by the relatively low upconversion luminescence (UCL) of UCNC of small size or thin protecting shell. To obtain deeper insights into energy transfer and surface quenching processes involving Yb3+ and Er3+ ions, we examined energy loss processes in differently sized solid core NaYF4 nanocrystals doped with either Yb3+ (YbNC; 20% Yb3+) or Er3+ (ErNC; 2% Er3+) and co-doped with Yb3+ and Er3+ (YbErNC; 20% Yb3+ and 2% Er3+) without a surface protection shell and coated with a thin and a thick NaYF4 shell in comparison to single and co-doped bulk materials. Luminescence studies at 375 nm excitation demonstrate back-energy transfer (BET) from the 4G11/2 state of Er3+ to the 2F5/2 state of Yb3+, through which the red Er3+4F9/2 state is efficiently populated. Excitation power density (P)-dependent steady state and time-resolved photoluminescence measurements at different excitation and emission wavelengths enable to separate surface-related and volume-related effects for two-photonic and three-photonic processes involved in UCL and indicate a different influence of surface passivation on the green and red Er3+ emission. The intensity and lifetime of the latter respond particularly to an increase in volume of the active UCNC core. We provide a three-dimensional random walk model to describe these effects that can be used in the future to predict the UCL behavior of UCNC.

Internal OH\u2212 induced cascade quenching of upconversion luminescence in NaYF4:Yb/Er nanocrystals

Internal hydroxyl impurity is known as one of the main detrimental factors affecting the upconversion (UC) efficiency of upconversion luminescence (UCL) nanomaterials. Different from surface/ligand-related emission quenching which can be effectively diminished by, e.g., core/shell structure, internal hydroxyl is easy to be introduced in synthesis but difficult to be quantified and controlled. Therefore, it becomes an obstacle to fully understand the relevant UC mechanism and improve UC efficiency of nanomaterials. Here we report a progress in quantifying and large-range adjustment of the internal hydroxyl impurity in NaYF4 nanocrystals. By combining the spectroscopy study and model simulation, we have quantitatively unraveled the microscopic interactions underlying UCL quenching between internal hydroxyl and the sensitizers and activators, respectively. Furthermore, the internal hydroxyl-involved UC dynamical process is interpreted with a vivid concept of “Survivor effect,” i.e., the shorter the migration path of an excited state, the larger the possibility of its surviving from hydroxyl-induced quenching. Apart from the consistent experimental results, this concept can be further evidenced by Monte Carlo simulation, which monitors the variation of energy migration step distribution before and after the hydroxyl introduction. The new quantitative insights shall promote the construction of highly efficient UC materials.

Structural evolution, crystallization behaviour and mid-infrared emission properties in Yb/Ho codoped oxyfluoride germanosilicate glass ceramics with varied Si/Ge ratio

Changes in glass structure and crystallization behaviour from oxyfluoride silicate glass to oxyfluoride germanate glass were studied by modifying the ratio of Si/Ge in oxyfluoride germanosilicate glass ceramics, and transparent glass ceramic with low light scattering and efficient mid-infrared emissions were obtained. The NaYF4 nanocrystals crystallization ability of oxyfluoride glass was decreased with the increase in Ge content, which can be attributed to the weakened oxide and fluoride phase separation in GeO2-rich glass. Upconversion and down-conversion emission spectra together with Ho3+: 5I6 energy level fluorescent decay curves have been discussed for SiO2-rich glass ceramics and GeO2-rich glass ceramics, in which the fluoride crystallization induced the decrease of multi-phonon non-radiative decay rates plays the main role in the energy transfer processes. The influence of Si/Ge ratio on 2 μm and 3 μm fluorescent emissions in glasses and glass ceramics, and its link with glass network structural changes were discussed. The optimized Yb/Ho codoped oxyfluoride germanosilicate glass ceramic with low light scattering, high peak emission cross section(11.18 × 10−21cm2) and maximum gain coefficient (1.387 cm−1) at 2888 nm provides a potential application in the development of new 3 μm laser devices based on transparent oxyfluoride glass ceramic materials.

Powder Nano-Beam Diffraction in Scanning Electron Microscope: Fast and Simple Method for Analysis of Nanoparticle Crystal Structure.

We introduce a novel scanning electron microscopy (SEM) method which yields powder electron diffraction patterns. The only requirement is that the SEM microscope must be equipped with a pixelated detector of transmitted electrons. The pixelated detectors for SEM have been commercialized recently. They can be used routinely to collect a high number of electron diffraction patterns from individual nanocrystals and/or locations (this is called four-dimensional scanning transmission electron microscopy (4D-STEM), as we obtain two-dimensional (2D) information for each pixel of the 2D scanning array). Nevertheless, the individual 4D-STEM diffractograms are difficult to analyze due to the random orientation of nanocrystalline material. In our method, all individual diffractograms (showing randomly oriented diffraction spots from a few nanocrystals) are combined into one composite diffraction pattern (showing diffraction rings typical of polycrystalline/powder materials). The final powder diffraction pattern can be analyzed by means of standard programs for TEM/SAED (Selected-Area Electron Diffraction). We called our new method 4D-STEM/PNBD (Powder NanoBeam Diffraction) and applied it to three different systems: Au nano-islands (well diffracting nanocrystals with size ~20 nm), small TbF3 nanocrystals (size < 5 nm), and large NaYF4 nanocrystals (size > 100 nm). In all three cases, the STEM/PNBD results were comparable to those obtained from TEM/SAED. Therefore, the 4D-STEM/PNBD method enables fast and simple analysis of nanocrystalline materials, which opens quite new possibilities in the field of SEM.

Measuring 3D orientation of nanocrystals via polarized luminescence of rare-earth dopants

Orientation of nanoscale objects can be measured by examining the polarized emission of optical probes. To retrieve a three-dimensional (3D) orientation, it has been essential to observe the probe (a dipole) along multiple viewing angles and scan with a rotating analyzer. However, this method requires a sophisticated optical setup and is subject to various external sources of error. Here, we present a fundamentally different approach employing coupled multiple emission dipoles that are inherent in lanthanide-doped phosphors. Simultaneous observation of different dipoles and comparison of their relative intensities allow to determine the 3D orientation from a single viewing angle. Moreover, the distinct natures of electric and magnetic dipoles originating in lanthanide luminescence enable an instant orientation analysis with a single-shot emission spectrum. We demonstrate a straightforward orientation analysis of Eu3+-doped NaYF4 nanocrystals using a conventional fluorescence microscope. Direct imaging of the rod-shaped nanocrystals proved the high accuracy of the measurement. This methodology would provide insights into the mechanical behaviors of various nano- and biomolecular systems.

Optical tweezers beyond refractive index mismatch using highly doped upconversion nanoparticles.

Optical tweezers are widely used in materials assembly1, characterization2, biomechanical force sensing3,4 and the in vivo manipulation of cells5 and organs6. The trapping force has primarily been generated through the refractive index mismatch between a trapped object and its surrounding medium. This poses a fundamental challenge for the optical trapping of low-refractive-index nanoscale objects, including nanoparticles and intracellular organelles. Here, we report a technology that employs a resonance effect to enhance the permittivity and polarizability of nanocrystals, leading to enhanced optical trapping forces by orders of magnitude. This effectively bypasses the requirement of refractive index mismatch at the nanoscale. We show that under resonance conditions, highly doping lanthanide ions in NaYF4 nanocrystals makes the real part of the Clausius-Mossotti factor approach its asymptotic limit, thereby achieving a maximum optical trap stiffness of 0.086 pN μm-1 mW-1 for 23.3-nm-radius low-refractive-index (1.46) nanoparticles, that is, more than 30 times stronger than the reported value for gold nanoparticles of the same size. Our results suggest a new potential of lanthanide doping for the optical control of the refractive index of nanomaterials, developing the optical force tag for the intracellular manipulation of organelles and integrating optical tweezers with temperature sensing and laser cooling7 capabilities.