Upconversion Materials Research Articles

Effect of Mn4+ ions on the structure and luminescence properties of NaY(MoO4)2: Yb3+/Er3+ phosphor

Abstract In this work, we prepare a series of phosphor of NaY(MoO4)2: Yb3+/Er3+/Mn4+ using high temperature solid state method, the results of X-ray diffraction (XRD) shows that the doped Yb3+, Er3+ and Mn4+ ions will make lattice be smaller size. The reduction of the binding energy of O-1s XPS is due to the vacancy of O2− ions in the matrix lattice which is brought by the substitution of Mn4+ and Mo6+ ions. Under the laser excitation of 980 nm, the Er3+ ions exhibit strong green up-conversion emission and weak red up-conversion emission at 532, 554 and 658 nm, corresponding to the transitions of 2H11/2/4S3/2 → 4I15/2, 4F9/2 → 4I15/2 of Er3+ ions. The reasons for the high green emission intensity are discussed in detail by analyzing the Raman spectroscopy of NaY (MoO4)2: Yb3+/Er3+/Mn4+ phosphor. It can be inferred that the emissions at 554 nm and 658 nm are a two-photon process by the relationship between pump power and up-conversion luminescence intensity, and the possible up-conversion emission mechanism has been explored. Meanwhile, the luminescence life of green and red light decreases with the increasing doping concentration of Mn4+ ions. After doping Mn4+ ions, the symmetry of the matrix lattice decreases, and the emission intensity of up-conversion will be greatly improved. The study of this work expands the research scope of traditional conversion materials, and offers reference for the exploration of up-conversion luminescent materials based on transition metals.

Unraveling the Important Role of High-Lying Triplet-Lowest Excited Singlet Transitions in Achieving Highly Efficient Deep-Blue AIE-Based OLEDs.

Aggregation-induced emission (AIE) materials are attractive for achieving highly efficient nondoped organic light-emitting diodes (OLEDs) owing to their strong luminescence in the solid state. However, the electroluminescence efficiency of most AIE-based OLEDs remains low owing to the waste of triplet excitons. Here, using theoretical calculations, photophysical dynamics, and magnetoluminescence measurements, the spin conversion process is demonstrated between the high-lying triplet state (Tn ) and the lowest excited singlet state (S1 ) in AIE materials. Moreover, the relative positions of Tn (n<4) and S1 are shown to have a significant impact on the spin-conversion efficiency, thus influencing the harvesting of triplet excitons and the device efficiency. Finally, by selecting an upconversion material with an appropriate energy level for further utilizing the triplet excitons, a deep-blue fluorescent OLED with CIE coordinates of (0.15, 0.08), amaximum external quantum efficiency of 10.2%, low efficiency roll-off, and a high brightness of 16817cdm-2 is developed. This is one of the most efficient deep-blue OLEDs based on AIE materials reported so far. These findings also provide new insights into the design of more efficient AIE molecules and corresponding OLEDs by managing high-lying triplet excitons.

First-Principles Calculation of Photoelectric Property in Upconversion Materials through In3+ Doping.

Multicolor turning holds great promise in optical intelligent recognition and optical imaging. Here, Er3+, Yb3+, and In codoped ZnO (Er/Yb/IZO) with a uniform block strucuture is obtained. The doping of In3+ ions enhances the multicolor upconversion luminescence (UCL) intensity of Er/Yb/IZO. Particularlly, the UCL of Er/Yb/I2ZO turns from red through yellow to dominant green emission via increasing density power from 2.54 to 10.19 W/cm2, thus realizing the power sensitiviy. First-principles theory is used to design a In3+, Yb3+, and Er3+ codoped ZnO. The band structure, total density of state and optical coefficient of Er/Yb/IZO have been studied via a generalized gradient approximation within density functional theory (DFT). The potential electron density and total electron density of the O atom increase with In3+ and Er3+/Yb3+ doping, which indicate that substitution of Zn2+ by In3+ and Er3+/Yb3+ generate positive vacancies on the surface. The band gap of Er/Yb/IZO decreases compare with that of pure ZnO. Furthermore, the optical coefficient of In3+ doping is enhanced compare with that of pure ZnO via using DFT calculations.

Upconversion properties in lanthanide doped layered-perovskite, CsBiNb2O7

Despite advances of lanthanide-doped upconversion (UC) materials, the applications such as light-emitting diode and biological imaging are limited by low quantum efficiency. For this context, the understanding of unique interactions between the doped-lanthanides and the host crystals has attracted a huge amount of the researcher's interest. In particular, it was revealed that doping lanthanide ions in a non-centrosymmetric site of host lattice is the cause of relaxation of the Laporte selection rule in the 4f-4f transition of lanthanide ions. One of the layered perovskites CsBiNb2O7 is known to have non-centrosymmetric sites, which would lead to highly bright UC emission. Nevertheless, to our knowledge, there has been no research on the UC comparison between host materials of CsBiNb2O7 with other hosts. In this article, we present the UC intensity comparison of Yb3+-Er3+ ion doped CsBiNb2O7, NaYF4, BaTiO3, and SrTiO3 hosts (the UC in CsBiNb2O7:Er3+,Yb3+ was 2.4 times that of NaYF4:Er3+,Yb3+ and ∼70 times that of SrTiO3:Er3+,Yb3+). After that, we dig into UC, downshifting, and double beam system UC properties. The activator concentration was optimized by varying the doping ratio of Yb3+ and Er3+, and we found out the main reason for the concentration quenching behavior in Er3+ ion doped CsBiNb2O7 is dipole-dipole interaction. In addition, the double excitation experiment indicates that the absorption (4I15/2 → 4I13/2) factor is stronger than the stimulated emission (4I13/2 → 4I15/2) factor in CsBiNb2O7 under 1540 nm laser irradiation.

Improvement of luminescence properties of NaYF4 :Yb3+ /Er3+ upconversion materials by a cross-relaxation mechanism based on co-doped Ho3+ ion concentrations.

NaYF4 :Yb3+ /Er3+ /Ho3+ nanophosphors were successfully synthesized using a solvothermal method and with various concentrations of Ho3+ ions. The crystal structure, grain size, morphology, and luminescence properties were analyzed by X-ray diffraction, field-emission scanning electron microscopy, and photoluminescence measurements. All samples were crystallized as a cubic structure; it was confirmed that all samples exhibited strong green emission and weak red emission generated at a particular level of the activated ions. The strongest upconversion fluorescence intensity was observed in the Ho3+ and Er3+ ions co-doped NaYF4 materials with a Ho3+ ion concentration of 0.005 mol. Only the green fluorescence intensity at the 542 nm centre increased strongly due to the 4 S3/2 →4 I15/2 energy transfer. This increase in upconversion fluorescence intensity at a selected wavelength was described as a cross-relaxation mechanism due to the addition of Ho3+ ions.

Controllable crystal form transformation and luminescence properties of up-conversion luminescent material K3Sc0.5Lu0.5F6: Er3+, Yb3+ with cryolite structure.

In this paper, a novel cryolite-type up-conversion luminescent material K3Sc0.5Lu0.5F6: Er3+, Yb3+ with controllable crystal form was synthesized by a high temperature solid state method. K3Sc0.5Lu0.5F6: Er3+, Yb3+ can crystallize in monoclinic or cubic form at different temperatures. The composition, structure and up-conversion luminescence (UCL) properties of K3Sc0.5Lu0.5F6: Er3+, Yb3+ samples with different crystal form were investigated in detail. It is impressive that both monoclinic and cubic forms of K3Sc0.5Lu0.5F6: Er3+, Yb3+ show green emission (2H11/2/4S3/2→4I15/2). The luminescence intensity of cubic K3Sc0.5Lu0.5F6 is much higher than that of the monoclinic form, and the reasons are also discussed in detail. The results show that the luminescence intensity of up-conversion materials can be effectively tuned by controlling the crystal form. According to the power dependent UCL intensity, the UCL mechanism and electronic transition process were discussed. In addition, the fluorescence decay curves were characterized and the thermal coupling levels (TCLs) of Er3+ (2H11/2/4S3/2 → 4I15/2) in the range of 304–574 k were used to study the optical temperature sensing characteristics. All the results show that K3Sc0.5Lu0.5F6: Er3+, Yb3+ can be used in electronic components and have potential application value in temperature sensing fields.

Expanding the synthetic toolbox to access pristine and rare-earth-doped BaFBr nanocrystals.

A new synthetic route to access pristine and rare-earth-doped BaFBr nanocrystals is described. Central to this route is an organic-inorganic hybrid precursor of formula Ba5(CF2BrCOO)10(H2O)7 that serves as a dual-halogen source. Thermolysis of this precursor in a mixture of high-boiling point organic solvents yields spherical BaFBr nanocrystals (≈20 nm in diameter). Yb:Er:BaFBr nanocuboids (≈26 nm in length) are obtained following the same route. Rare-earth-doped nanocrystals display NIR-to-visible photon upconversion under 980 nm excitation. The temperature-dependence of the green emission from Er3+ may be exploited for optical temperature sensing between 150 and 450 K, achieving a sensitivity of 1.1 × 10-2 K-1 and a mean calculated temperature of 300.9 ± 1.5 K at 300 K. The synthetic route presented herein not only enables access to unexplored upconverting materials but also, and more importantly, creates the opportunity to develop solution-processable photostimulable phosphors based on BaFBr.

Green Synthesis of Upconverting NaYF4 and NaGdF4 Materials and Energy Levels Determination

An alternative green route using a domestic microwave oven to synthesize NaGdF4:Yb,Tm and NaYF4:Yb,Er was successfully developed. Decreasing dopants concentration increased the β-NaREF4/α-NaREF4 ratio in NaGdF4. The scanning electron microscopy (SEM) images of NaGdF4 showed particles ranging from 500 nm to 1 μm with formation of aggregates up to 100 μm, with two different surface morphologies. The NaGdF4:Yb,Tm emission spectrum showed white-like emission due to Er3+ and Eu3+ contamination. The bandgaps of undoped NaGdF4 and NaYF4 were measured using synchrotron radiation vacuum-UV (VUV) diffuse reflectance, with values of 10.20 eV for the cubic phase and 10.80 eV for the hexagonal phase for both materials. The determination of the RE2+/3+ energy level positions in these fluorides sheds new light on the design of efficient upconversion materials.

Nanostructure Design, Multi-Color Manipulation and Application of Orthogonal Upconversion Materials

Nanostructure Design, Multi-Color Manipulation and Application of Orthogonal Upconversion Materials

Enhanced up-conversion luminescence and temperature-sensing of GdVO4:Ln3+ with dual-wavelength excitation.

There exists a tendency in the research of up-conversion materials to shift the excitation from 980 nm to multiple excitation wavelengths. A series of GdVO4:Ln3+ with similar sizes and irregular prism morphology were successfully prepared by a co-precipitation technique. A wide multi-color emission corresponding to different Ln3+ doping was also obtained under single excitation. It is worth pointing out that among all the studied samples, only the emission intensity of GdVO4:Yb3+/Er3+ excited at two-wavelengths (980 nm + 1550 nm) simultaneously was enhanced by a factor of 1.87, compared to the sum of emission intensities excited at two single-wavelengths separately. Moreover, to further enhance the up-conversion luminescence intensity, cation ions (Lu3+/Y3+) and anion ions (PO43-) were also doped into the host, and the luminous intensity was also improved to a certain extent. A possible mechanism for energy transfer and possible transitions were also suggested and discussed in detail using an energy level diagram. In addition, not only a high record value of Sa (0.0069 K-1) but also a high Sr (1.13% K-1) is achieved for GdVO4:Yb3+/Er3+ in the physiological temperature range (273-453 K). Combining a much intensified dual-wavelength up-conversion signal and good temperature-sensing properties, this work can be extended to the surges of other lanthanide ion doped systems pumped by using multiple-wavelength lasers, and can also open new possibilities for up-conversion color displays and anti-counterfeiting applications.

Solvothermal Synthesis of Lanthanide‐doped NaYF4 Upconversion Crystals with Size and Shape Control: Particle Properties and Growth Mechanism

AbstractLanthanide‐doped NaYF4 upconversion nano‐ and microcrystals were synthesized via a facile solvothermal approach. Thereby, the influence of volume ratios of ethylene glycol (EG)/H2O, molar ratios of NH4F/RE3+ (RE3+ represents the total amount of Y3+ and rare‐earth dopant ions), Gd3+ ion contents, types of activator dopant ions, and different organic co‐solvents on the crystal phase, size, and morphology of the resulting particles were studied systematically. A possible formation mechanism for the growth of crystals of different morphology is discussed. Our results show that the transition from the α‐ to the β‐phase mainly depends on the volume ratio of EG/H2O and the molar ratio of NH4F/RE3+, while the morphology and size could be controlled by the type of organic co‐solvent and Gd3+ dopant ions. Furthermore, the reaction time has to be long enough to convert α‐NaYF4 into β‐NaYF4 during the growth process to optimize the upconversion luminescence. The formation of larger β‐NaYF4 crystals, which possess a higher upconversion luminescence than smaller particles, proceeds via intermediates of smaller crystals of cubic structure. In summary, our synthetic approach presents a facile route to tailor the size, crystal phase, morphology, and luminescence features of upconversion materials.

Dual-color upconversion photoluminescence induced by disordered Yb3+-Ti4+ pairs and defects in highly stable langbeinite-type Rb2Ti0.80Yb1.20(PO4)3 single crystal

Inorganic upconversion materials show significant importance in both basic research and applied area, and searching for suitable host material and understanding of fundamental photophysical process are particularly worth studying in upconversion application. Herein, a heavily Yb3+-substituted langbeinite-type Rb2Ti0.80Yb1.20(PO4)3 (RTYP) millimeter-sized single crystal, which has been successfully grown by high temperature solution method, exhibits efficient dual-color upconversion photoluminescence under 980 nm excitation. The lifetimes of two-photon blue emission and one-photon red emission are 4.1 μs and 17.1 μs, respectively. X-ray structural analyses reveal that the dual-color emissions are ascribed to disordered Yb3+-Ti4+ pairs and crystal defect, and the higher intensity of red emission is attributed to Yb3+-defect energy transfer activated by Ti4+. Notably, the blue emission originates from cooperative luminescence of Yb3+ dimer, and the red emission originates dominantly from the cooperative sensitization of the intrinsic defect caused by excited Yb3+ dimer. Moreover, RTYP with disordered structure shows high thermal stability up to 1,150 °C. This work not only demonstrates the promising potential of langbeinite-type materials in multicolor upconversion application, but also sheds light on the photophysical process associated with disordered crystal structure.

Luminescent Er3+ centers in CaSc2O4:Er3+:Yb3+ upconversion phosphor

The structure of luminescent centers in the upconversion material Er:Yb:CaSc2O4 is investigated using high-resolution optical spectroscopy methods at low temperature (10K). Two luminescent centers were identified in Er(2 at%):CaSc2O4: one corresponding to Er3+ on Ca2+ positions (center I) and one corresponding to Er3+ on Sc3+ positions (center II). Both X-ray diffraction and optical spectroscopy indicate an increase in the population of center II with the increase of doping concentrations. The Stark energy levels corresponding to multiplets 4I15/2, 4I13/2, 4F9/2 and 4S3/2 were obtained for the two luminescent centers, using selectively-excited luminescence. Upconversion measurements performed at low temperatures (10K) showed that the contribution of center I is dominant. The temperature dependence of the upconversion intensity and of the red to green ratio is discussed.

Bright constant color upconversion based on dual 980 and 1550 nm excitation of SrF2:Yb3+, Er3+ and β-NaYF4:Yb3+, Er3+ micropowders― considerations for persistence of vision displays

Abstract Upconversion phosphors are of interest for 2D head-up and 3D volumetric displays. These exploit persistence of vision; fast scanning of the near-infrared excitation means that the material at each emission point is only excited for a short fraction of the time it takes to trace the whole image. To achieve an average luminance on the order of 100 cd m−2 (necessary for the display to be visible in ambient indoor lighting), the luminance during excitation must be several orders of magnitude higher than its time-averaged value. For this purpose, efficient energy-transfer upconversion materials such as the benchmark β-NaYF4:Yb3+,Er3+ are of obvious interest. However, under 980 nm excitation the perceived color of their emission varies with their luminance, limiting their applicability for a grayscale display. We demonstrate that under dual 980 nm and 1550 nm excitation, a constant green color can be maintained at CIE coordinates (0.31, 0.66), as the luminance is varied. At moderate power densities of 100 W cm-2 at each excitation wavelength, a luminance of 8 × 106 cd m−2 can be achieved. Conservatively considering that the luminance will be reduced by 5 orders of magnitude in a laser scanning display, this material will still be bright enough to be viewed in indoor ambient light. We also investigate SrF2:Yb3+,Er3+, and find color tuning from the red to green is possible under dual wavelength excitation, but only at limited luminance, so color tuning in this manner would only be appropriate for displays viewed in a dark environment.

Green, room-temperature, fast route for NH4Yb2F7:Tm3+ nanoparticles and their blue upconversion luminescence properties

Abstract A pure ammonium fluorolanthanate NH4Yb2F7 matrix and its derivative doped Tm3+ were successfully synthesized by an ultra-fast solution route at room temperature. The as-prepared nanoparticles (NPs) were about 70 nm in diameter with the uniform size. In addition, NH4Yb2F7:Tm NPs demonstrated a bright blue upconversion luminescence feature, which can be observed with the naked eye. There are five emission bands in the range of 400–800 nm and the primary blue and red bands derived from the transitions of 1D2→3F4 and 1G4→3F4, respectively. The blue emission was populated via a three-photon process based on the relationship between emission intensity and excitation pump power. Our ultra-fast synthesis route presents a green and one-step strategy for the large-scale production. Moreover, the excellent blue upconversion will arouse extensive interest in upconversion studies in ammonium fluorolanthanate based hosts and pave the way for further development of upconversion material family.

A three-mode self-referenced optical thermometry based on up-conversion luminescence of Ca2MgWO6:Er3+,Yb3+ phosphors

To develop new up-conversion luminescent materials for optical thermometry, a sequence of Ca2MgWO6:xEr3+,yYb3+ phosphor samples were synthesized via high temperature solid-state reaction method. Their structure, morphology and luminescent performances were completely explored via X-ray diffraction, scanning electron microscopy and photoluminescence. Excited by 980 nm laser, all phosphor samples emit intense up-conversion luminescence of Er3+ at 531, 549 and 661 nm. Besides, these emissions were greatly enhanced with addition of Yb3+ ions. Benefiting from the different temperature dependence of three emission peaks, a dual-mode temperature sensor based on fluorescence intensity ratio (2H11/2/4S3/2 and 2H11/2/4F9/2) is realized by using single luminescent center. Meanwhile, the fluorescence lifetime of 4S3/2 state of Er3+ ions was used as the third temperature detection signal. More importantly, the phosphors exhibit superior thermal stability that will be helpful to obtain high precision temperature measurement. All results show that Ca2MgWO6:Er3+, Yb3+ samples could have potential applications for self-referenced optical thermometry.

Determination of the refractive index of BaY2F8:Er3+ (0.5 mol% to 30 mol%) in the 300 nm to 1800 nm range by ellipsometry; a record-breaking upconversion material

This work reports the ellipsometric study of trivalent erbium (Er3+) doped monocrystalline barium yttrium fluoride (BaY2F8), which has recently been shown to be one of the best photon upconversion (UC) materials available. This spans the BaY2F8 applications in a large range of wavelengths, from ultraviolet (UV) to near-infrared (NIR). We detail the optical properties of BaY2F8: Er3+ (0.5 mol%, 10 mol%, and 30 mol%), measured via variable angle spectroscopic ellipsometry over a spectral range from 300 nm to 1800 nm, reporting for the first time the indices of refraction for BaY2F8:Er3+. The upconversion external photoluminescence quantum yield (ePLQY) of the BaY2F8:Er3+ samples have also been studied by exciting at λ=1493 nm. The highest ePLQY of BaY2F8 was found for the largest dopant concentration 30 mol% Er3+ reaching the value of 3.62% ± 0.01%, at an irradiance of (6.23 ± 0.45) ×10−2 W/cm2. The refractive index (λ= 589.3 nm) was determined to be 1.4808 ± 0.014 for 0.5 mol%, 1.4980 ± 0.003 for 10 mol%, and 1.5022 ± 0.006 for 30 mol%. Increasing Er3+ doping concentration increased the refractive index. All samples decreased monotonically with increasing wavelength. The Brewster angle of BaY2F8:Er3+ is observed to be ≈56∘, whilst the Abbe number of the samples was found to be as high as 124.62. These findings provide valuable insight into the optical properties of BaY2F8:Er3+ in the wide range of frequencies that is has proven useful.

Highly Effective Near-Infrared Activating Triplet-Triplet Annihilation Upconversion for Photoredox Catalysis.

Organic triplet-triplet annihilation upconversion (TTA-UC) materials have considerable promise in areas as broad as biology, solar energy harvesting, and photocatalysis. However, the development of highly efficient near-infrared (NIR) light activatable TTA-UC systems remains extremely challenging. In this work, we report on a method of systematically tailoring an annihilator to attain such outstanding systems. By chemical modifications of a commonly used perylene annihilator, we constructed a family of perylene derivatives that have simultaneously tailored triplet excited state energy (T1) and singlet excited state energy (S1), two key annihilator factors to determine TTA-UC performance. Via this method, we were able to tune the TTA-UC system from an endothermic type to an exothermic one, thus significantly elevating the upconversion performance of NIR light activatable TTA upconversion systems. In conjunction with the photosensitizer PdTNP (10 μM), the upconversion efficiency using the optimal annihilator (100 μM) identified in this study was measured to be 14.1% under the low-power density of NIR light (100 mW/cm2, 720 nm). Furthermore, using such a low concentration of perylene derivative, we demonstrated that the optimal TTA-UC pair developed in our study can act as a highly effective light wavelength up-shifter to enable NIR light to drive a photoredox catalysis that otherwise requires visible light. We found that such an NIR driven method is highly effective and can even surpass directly visible light driven photoredox catalysis. This method is important for photoredox catalysis as NIR light can penetrate much deeper in colored photoredox catalysis reaction solutions, especially when done in a large-scale manner. Furthermore, this TTA-UC mediated photoredox catalysis reaction is found to be outdoor sunlight operable. Thus, our study provides a solution to enhance NIR activatable organic upconversion and set the stage for a wide array of applications that have previously been limited by the suboptimal efficiency of the existing TTA upconversion materials.

980 nm-excited multiphoton photocarrier separation process of Yb3+ ions under internal electric field and its upconverting modification on Eu3+ ions

In this work, we report a new optical properties and modification methods for the rare-earth (RE) ion doped upconversion (UC) materials. Here, The Yb3+- Eu3+ co-doped BiOCl layered micrometer crystals were synthesized via the traditional solid state reaction. The experiment results show that due to efficient photocarrier space separation role of IEF, a band-to-band multiphoton excitation process of Yb3+ ions and the subsequent photopolarization role have been triggered inside BiOCl crystals via a commercial 980 nm laser. Then, it combines the photopolarization electric field (PPF) of the laser beam to generate a multiplexed tunable photopolarization on the Eu3+ ions. Consequently, the uponverting Stark splitting and emission intensity ratio of electrical dipole transitions (EDTs) of Eu3+ ions could be tailored simultaneously, either by pumping power or Yb3+ ion doping level, but showing an opposite trend. This research provides new insights into the interaction between the light and materials as well as the UC tailored method of lanthanide ions, which may inspire the design of opto-electric multifunctional devices.

Two-step light conversion with quantum dots inside non-linear crystals.

Up-conversion of infrared (IR) to visible light is demonstrated via an unconventional two-step process combining second harmonic generation with photoluminescence. This is achieved in a composite material formed by single crystals of KH2PO4 (KDP), in which CdTe/CdS quantum dots (QDs) are embedded during a crystal growth. The second harmonic generation from the IR laser frequency in KDP takes place, and then, generated light is absorbed by QDs and luminescence is emitted. Compared to the most common up-converting materials based on lanthanide ions, our novel composites use only abundant elements and have several other properties with application potential: the excitation and emission can cover a broader spectral range, and the response is much faster (ns compared to ms) and is strongly dependent on the direction of the incident beam.