Upconversion Measurements Research Articles

Novel protocol for rapid evaluation of plasmonic enhancement for up-converting phosphors applied to Y2O3 doped with Er3+ and Er3+/Yb3+

A novel technique was created to quickly evaluate the impact of various ratios on the efficiency of up-conversion emission using a combination of up-conversion nanophosphors (UCNPs) and gold nanoparticles (Au NPs). Four different types of Y2O3 UCNPs with different Er3+ or Er3+/Yb3+ doping, which affects the emission colour as well as the up-conversion mechanism, were prepared via the microwave-assisted hydrothermal method. Au NPs were produced independently using the reduction technique. Different ratios of UCNPs were mixed with the Au NPs in water. The mixed powder composites were extracted for analysis. X-ray powder diffraction was utilized to study the structure of UCNPs with and without Au NPs, while transmission electron microscopy was used to study the morphology and size of Au NPs. Absorption and up-conversion measurements were also made. The novel protocol allowed rapid evaluation of four different Y2O3 UCNPs of Er3+ and Er3+/Yb3+ doped with concentrations optimized for red and green emission over a range of Au/UCNP concentrations, which might usually be done in four separate studies. Despite the wide range of variables, only a decrease in UC emission was measured in the presence of Au NPs, suggesting that the conditions for plasmonic enhancement are limited and not easy to attain. In spite of this, in many phosphor/metal NP systems, the innovative prototyping process can enable quick assessment of possible plasmonic enhancement.

Yb3+ concentration influence on NIR and upconversion emission and temperature sensing properties of Er3+/Yb3+ co-doped Ta2O5 particles

We synthesized Er3+ and Yb3+ co-doped tantalum oxide (Ta2O5) spherical sub-micrometric particles by a facile method involving formation of tantalum glycolates after pouring in acetone. Then, we investigated how the annealing temperature and Yb3+ concentration (0–30 mol %) affect their structural and luminescence properties. Particle sizes ranged from 146 to 258 nm for the samples annealed at 800 °C. XRD analysis revealed the orthorhombic L-Ta2O5 crystalline structure. The different symmetry sites that the rare earth ions can occupy within the Ta2O5 structure led to intense and broad emission bands centered around 1530 nm. Up-conversion measurements and the calculated number of photons for intense green and red emissions obtained with 980 and 1550 nm excitations helped us to elucidate the mechanisms involved at both excitation wavelengths. We investigated how the 0.5 mol % Er3+/1.5 mol % Yb3+ co-doped Ta2O5 sample performs as a primary thermometer by using the Boltzmann distribution law to predict the absolute temperature from the ratio between the Er3+ 2H11/2 → 4I15/2 and 4S3/2 → 4I15/2 transitions. We obtained a maximum relative thermal sensitivity of 0.97 ± 0.04 % K−1 and a minimum δT of 2.41 K. The calculated repeatability was above 95%. These results show that promising nanothermometer based on Ta2O5 with controlled size and morphology can be designed.

Comparing Pr3+ and Nd3+ for deactivating the Er3+: 4I13/2 level in lanthanum titanate glass

Erbium lanthanum titanate glasses were prepared by levitation melting for the spectroscopic study of ways to promote the mid-infrared fluorescence. Two series of heavily erbium doped glasses (15 wt%) were prepared with the addition of either Pr3+ or Nd3+ in amounts relative to Er3+ of 0.05, 0.1, and 0.2. Both ions quench the lower Er3+ laser level with the Pr3+ doing so more rapidly. Although high co-dopant concentrations result in higher energy transfer, as clearly evidenced in upconversion and downconversion fluorescence measurements, the mid-infrared lifetime also suffers a reduction and, therefore, a balance must be struck in the co-dopant concentration. Lifetime and spectral measurements indicate that, at a fixed relative co-dopant amount, Pr3+ is more effective than Nd3+ at removing the bottleneck of the Er3+ 4I13/2 level. Moreover, consideration of the lifetimes alongside the absorption data of the individual ions indicates that despite the large absorption cross-section of Nd3+ at 808 nm, the concentration needed to yield more absorbed power than utilizing direct 976 nm excitation of Er3+ results in unfavorable lifetimes of the mid-infrared transition. In the end, Pr3+ prevails as the superior co-dopant in terms of the effects on fluorescence lifetimes as well as potential laser system design considerations. In a unique self-doping approach, a reducing melt atmosphere of Ar instead of O2 creates a small fraction of Ti3+. In 5Er2O3-12La2O3-83TiO2 glass, the presence of Ti3+ quenches the 4I13/2 emission about 2.6 times more than the 4I11/2 when lifetimes are compared to an O2 melt environment. As an additional means of increasing the mid-infrared emission, the effect of temperature on the mid- and near- infrared lifetimes of a lightly doped lanthanum titanate composition is investigated between 77-300 K. The mid-infrared lifetime increases by ∼30% while the near-infrared lifetime increases by ∼10%, which suggests in addition to co-doping, active cooling of the gain media will further enhance performance.

Multimode Luminescent Properties from Tb3+-Yb3+ Codoped CaMoO4 for Solar Cell Spectral Conversion

NIR and visible emitting CaMoO4:Tb3+/Tb3+-Yb3+ phosphors were synthesized to harness the energy losses attribute to wavelength mismatch between response spectrum of incident solar photons and solar cells. The X-ray diffraction (XRD), field emission scanning microscopy (FE-SEM), fourier transform infrared spectroscopy (FTIR), UV–vis diffuse reflectance spectroscopy, photoluminescence (PL), up-conversion and life time measurements were used to characterized the prepared phosphors. XRD study followed by Rietveld refinement confirms the tetragonal crystal structure with space group I41a(88). The phosphors exhibit strong green emission (5D4→7F4) in visible region after an excitation wavelengths 378 nm and 980 nm, respectively through down-shifting (DS) and up-conversion (UC) processes. The UV photons (378 nm) are also split it into two NIR photons (2F7/2→2F5/2) through down-conversion process. Fluorescence quenching was observed by increasing concentration of Tb3+ and Yb3+ ions. Lifetime measurements clearly illustrates the energy transfer from Tb3+ to Yb3+ ions and mechanism involved is co-operative energy transfer. The estimated values of energy transfer efficiency (ETE) and corresponding theoretical internal quantum efficiency are achieved as 69% and 169% respectively.

Excited-State Dynamics in 4-[4'(Dimethylamino)styryl]pyridine, a Photobase: Role of Photoinitiated Proton-Coupled Electron Transfer.

The photoinitiated proton-coupled electron transfer (PCET) process in photoacid-based adducts is predominantly governed by the evolution of the electron-proton transfer state. However, such a process is underexplored in the case of photobases as the excited states evolve through multiple competitive channels. Here, we elucidate the excited-state dynamics of a photobase, 4-[4'-(dimethylamino)styryl]pyridine (DMASP), in the presence of hexafluoroisopropanol (HFIP) that enables PCET. Transient absorption measurements show the evolution of a protonated species in the excited state with a time constant of ∼2.5 ps. Fluorescence upconversion measurements reveal the signatures of an emissive intramolecular charge transfer state and a protonated state. The role of such states is further confirmed by time-resolved measurements in the presence of trifluoroacetic acid and computational analysis. Furthermore, the proton-abstraction dynamics of DMASP is analyzed in bulk methanol and butanol solvents. The extent of proton abstraction by DMASP is found to be higher in the presence of HFIP when compared with the normal alcohols over a time period of a few picoseconds.

Polymorphism and up-conversion photoluminescence properties of Y2WO6 doped with Yb3+/Er3+ under excitation at 975 nm

This article reports the synthesis and luminescent properties (up-conversion and down-shifting) of the tetragonal and orthorhombic polymorphs of Y2WO6 doped with Erbium (Er3+) and Ytterbium (Yb3+) cations. The syntheses were carried out by a high temperature solid state reaction. The orthorhombic phase was prepared by pressing the parent compounds into pellets of 13 mm diameter. The samples were characterized by powder X-ray diffraction (PXRD) and scanning electron microscopy (SEM). Luminescence up-conversion measurements were analyzed using a tunable continuous wave (CW) Ti:sapphire laser system tuned at 975 nm. The results show that the green up-conversion emission is more intense than the red one in both polymorphs. To gain further clarity on this matter, we have also conducted a study on down-shifting emission spectra measured at 378 nm, where only the strong green emission is observed. The pump power dependence and the decay curves of the (4S3/2 → 4I15/2) transition were also evaluated.

Spectroscopic properties of Er3+-doped barium phosphate glasses for optical gain media

Barium metaphosphate glasses (PBaLa) doped with erbium (Er3+) ions have been prepared by melt quenching method and studied their optical, structural characteristics by using FT-IR, absorption, radiative de-excitation spectra, decay curves and up-conversion measurements. Judd-Ofelt (Ωλ, λ = 2,4 and 6) parameters have been determined using the absorption transitions of the Er3+(1.0 mol %)-doped glasses which are in turn taken to predict the radiative characteristics that include branching ratio (βR), radiative emission probability (AR) and lifetime (τR) of some important fluorescent levels of Er3+:glasses. Visible emission spectra measured under 378 nm excitation reveals three emission transitions at 663 (4F9/2 → 4I15/2), 547 (4S3/2→ 4I15/2) and 524 nm (2H11/2 → 4I15/2) corresponding to Er3+ ions. Among these, the green emission at 547 nm is relatively intense. Near-Infrared emissions were measured by exciting at 980 nm which exhibits broad emission at 4I13/2→ 4I15/2 (1.53 µm). The stimulated emission cross-section (σemi), FWHM and gain bandwidth (σemi× FWHM) were predicted to be 13.58 × 10-21 cm−2, 59 nm and 801.2 cm3, respectively, for PBaLaEr1.0 glass. The lifetime of 4I13/2 level has been shortened from 2.53 to 0.8 ms with an increase in Er3+ ions concentration from 0.1 to 2 mol %. NIR to visible up-conversion has also been measured for various concentrations of Er3+-doped PBaLaEr glasses with 980 nm excitation. These findings indicate that the Er3+-doped PBaLaEr glasses can be considered for waveguide laser and optical amplifier applications at 1.53 µm.

Mn Doping in Low-Dimensional Perovskites: A Switch for Controlling Dopant and Host Emission

The growth of solid-state photonics relies on the design of highly efficient devices with tunable photoluminescence (PL) based on thermally stable phosphors. In this study, a room-temperature, all-ambient synthesis of Mn-doped hybrid lead halide perovskite phosphors is presented. By applying temperature-dependent (4.2–300 K) steady-state and transient PL measurements, the photophysical pathways of the hybrid system were identified and the carrier recombination processes at play were defined. At higher Mn-doping concentrations (>4%), the excitonic white light host emission nearly disappears, and a bright Mn transition appears at ∼650 nm with a record PL quantum yield of ∼87.4%. The intense sensitized Mn luminescence results from rapid and efficient exciton-to-dopant energy transfer, as evidenced by time-gated and femtosecond fluorescence upconversion measurements. A millisecond PL decay time attributed to the Mn dopant and a microsecond PL decay time associated with the excitonic emission of the host were found, indicating energy and reverse energy transfer in the system. These findings provide unique insights into the chemical tailoring of the Mn-dopant emission by investigating the coupling between the host semiconductor and the dopant ion, a stepping stone toward the development of high-performance solid-state light-emitting diodes and scintillators.

Visible-light communication using thermally evaporated CsPbBr3 perovskite thin films

Lead halide perovskites are widely used in optoelectronic applications owing to their promising photophysical properties, but developing components of the order of nanoscale remains challenging. In this work, the authors fabricated cesium lead bromide (CsPbBr3) thin films of varying thicknesses and investigated their visible-light communication (VLC) performance. The thickness of the CsPbBr3 thin films was precisely controlled by using a single-beam thermal evaporation technique, and their morphology was analyzed through scanning electron microscopy and x-ray diffraction. Thicker films were found to have a homogeneous surface, with gain boundaries of increasing size and fewer surface trap states than the thinner films. Furthermore, we identified the thickness-dependent photoluminescence (PL) property of the CsPbBr3 thin films based on steady-state PL measurements and verified it by using time-correlated single-photon counting as well as femtosecond upconversion measurements. Films with thicknesses of 5, 10, and 20 nm, with enhanced surface homogeneity and purity, were used in a VLC link as color-converting fluorescent components. The 20-nm-thick CsPbBr3 film delivered the best performance because it had the highest PL intensity and the most suitable morphology, with a –3-dB bandwidth of 30.7 MHz and a net data rate of 330 Mb/s. These results reflect a facile and well-controlled approach to fabricating such films that can be used for high-power, high-speed, and large-area transmission and detection of visible-light signals.

Photoisomerization of linear merocyanines in salt solutions

AbstractThe trans‐cis isomerization in the excited state of linear merocyanine L‐Mero4 and phenyl substituted linear merocyanine P‐L‐Mero4 in salt solution and in ionic liquid was investigated using frequency upconversion measurements. Strontium chloride and cesium iodide were added to solvent dimethyl sulfoxide (DMSO) and dimethylformamide (DMF) to vary the ionic strength. The time‐resolved fluorescence curves of merocyanines displayed multiple exponential decay behavior. The second temporal component with time constant τ2 ≈ 2.8 (11) ps of L‐Mero4 (P‐L‐Mero4) in DMSO was assigned to the duration to reach the isomerization equilibrium between the trans and the twisted conformers. The τ2 increased at higher salt concentrations and was explained by the attachment of salt ions on the polar excited merocyanines decelerating the isomerization rate. The rotational correlation time constants obtained from the anisotropy decay of fluorescence were 360 and 240 ps in neat DMSO for L‐Mero4 and P‐L‐Mero4, respectively, and they increased to 790 and 450 ps in the most concentrated SrCl2. Using Perrin relation, we estimated the increase in the rotating volume at [SrCl2] = 536 mM, revealing ≈15 SrCl2 molecules surrounding L‐Mero4 and 7 SrCl2 on P‐L‐Mero4. The experimental data indicated that the ion–molecule interaction was stronger with SrCl2 and on L‐Mero4 than on P‐L‐Mero4.

Comparative study of upconversion and photoacoustic measurements of Er3+/Yb3+ doped La2O3 phosphor under 980 nm

The Er3+/Yb3+doped La2O3 phosphor samples were synthesized by the combustion method and then photoluminescence and photoacoustic spectroscopic studies were done. Prepared samples were annealed at 800 °C, 1000 °C and 1300 °C and all samples were found in pure hexagonal phase as confirmed by XRD analysis. From FE-SEM images it is found that particle size increases with increase in annealing temperature. The frequency upconversion emission spectra of samples were recorded by exciting the sample with 980 nm diode laser and maximum emission intensity is obtained for the sample annealed at 1000 °C for 2 h. A photoacoustic cell was designed and wavelength dependent photoacoustic spectra were measured. The effect of sample storage time on radiative and non-radiative emission properties of sample was checked by measuring upconversion emission and photoacoustic spectra, simultaneously. It is observed that the emission intensity and photoacoustic signal both decreases with time. The maximum photoacoustic signal is obtained around 974 nm wavelength and it indicates its potential for photo-thermal therapy using infrared excitation.

Charge-Transfer-Mediated Mechanism Dominates Oxygen Quenching of Ligand-Protected Noble-Metal Cluster Photoluminescence.

Photoluminescence (PL) quenching of ligand-protected noble-metal clusters (NMCs) by molecular oxygen is often used to define whether the PL of NMC is fluorescent or phosphorescent, and only energy transfer has been always considered as the quenching mechanism. Herein, we performed the Rehm-Weller analysis of the O2-induced PL quenching of 13 different NMCs and found that the charge-transfer (CT)-mediated mechanism dominates the quenching process. The quenching rate constant showed a clear dependence on the CT driving force, varied markedly from 106 to 109 M-1s-1. Transient absorption spectroscopy and photon upconversion measurements confirmed the triplet sensitization of aromatic molecules by NMCs regardless of the quenching degree by O2, establishing that the PL of NMCs under investigation originates from the excited triplet state (i.e., phosphorescence). The results herein provide an essential indicator for correctly determining whether the PL of an NMC is fluorescent or phosphorescent.

Modulating white light emission temperature in Ho3+/Yb3+/Tm3+ triply doped nanostructured GeO2-Nb2O5 materials for WLEDs applications

Materials based on Ho3+/Yb3+/Tm3+ triply doped with GeO2-Nb2O5 were prepared by the sol-gel methodology followed by thermal annealing at temperatures ranging from 700 to 1100 °C, which affected the crystalline structure and the white light generation of the solids. Trigonal GeO2 was detected in all the samples, whereas a tetragonal GeO2.9Nb2O5 mixed oxide was obtained between 900 and 1100 °C. The spectroscopic properties of the samples depended on the final crystalline structure. When upconversion measurements were performed by using a CW 980 nm laser as excitation source, a cool white light generation was detected for the sample annealed at 900 °C, more specifically; a cool white light has been acquired. In addition, the sample calcined at 1100 °C presented color purity higher than 96% in the yellow-green region where the anti-Stokes process was also being carried out. Excitation with ultraviolet light (245–280 nm) also generated white light. The emitted white light modulation was remarkable. The Cold white light was emitted under shorter wavelengths excitation, whereas warm white light was emitted under longer wavelengths excitation. The outstanding results show the potential of these materials for photonic applications, especially regarding white light emission in solid-state lighting for indoor and outdoor environments.

Enhancement of dual-mode emission and temperature sensing performance in Y2Zr2O7: Er3+ nano phosphor by incorporation of lithium ions

The present work has been planned with the primary objective to study the effect of Li+ doping on photoluminescence (PL) emission intensity and temperature sensing performance of Y2Zr2O7(YZO): Er3+ phosphors. The hydrothermal method was employed to synthesize the YZO: 4Er3+, xLi+ (x = 0, 3, 5, 10 mol%) phosphors. The formation of the phase and the crystallinity of the prepared samples were examined from the XRD results. The cell parameters were estimated from Rietveld refinement. The surface morphology and elemental analysis were studied using the FESEM and EDX techniques. UV–Vis–NIR diffuse reflectance spectroscopy was utilized to find the optical band gap of the prepared samples. The Li+ doped sample exhibits better optical absorption than the sample without Li+ ions. The FTIR spectroscopy confirms the presence of the desired functional groups within the samples. XPS measurements were performed to find the bonding state of the compositions. Photoluminescence down-conversion and up-conversion measurements were carried out under 378 nm and 976 nm excitation, respectively. The optical thermometry of the prepared phosphors was investigated within the temperature range 303K–630K. The reported phosphor shows a significant amount of intensity enhancement after Li+ doping in both the down conversion and up conversion processes. In general, the charge compensation effect is used to explain this type of result. As the phosphor is already charge balanced, the phenomenon mentioned above cannot be considered. We have explained the various contributing factors responsible for the changes in intensity and correlated them with the different experimental results collected by characterizing the prepared samples. Overall, the obtained results suggest that the reported phosphor may act as multifunctional material.

Defect-Interceded Cascading Energy Transfer and Underlying Charge Transfer in Europium-Doped CsPbCl3 Nanocrystals.

Rare-earth ion (RE3+) doping in cesium lead chloride (CsPbCl3) has unlocked novel prospects to explore changes in optical, magnetic, and charge carrier transport properties. This leads to a huge advancement in optoelectronic applications, yet deep understanding of the photophysics governing the energy transfer processes is lacking and demands vital attention. Herein, we probe into the mechanistic transfer processes from the band edge of the host (CsPbCl3) to the dopant europium ion (Eu3+) with the aid of femtosecond fluorescence upconversion and transient absorption (TA) spectroscopy. The upconversion measurement portrays a defect-mediated cascading energy transfer from CsPbCl3 to Eu3+ and further cross-relaxation among Eu3+ states. Moreover, TA studies reveal that there is charge transfer from the band edge of CsPbCl3 to doping-induced shallow defect states. Furthermore, two-photon absorption study establishes no compromise in the transfer mechanism even upon bandgap excitation. This work validates that Eu-CsPbCl3 is an apt entrant for optoelectronic applications.

Multiphoton upconversion and non-resonant optical nonlinearity in perovskite quantum dot doped glasses.

By incorporating the CsPbBr3 quantum dots (QDs) into a glass host, we report for the first time, to our knowledge, the measurement of non-resonant optical nonlinearity and multiphoton upconversion (UC) processes for this QD-in-glass composite. We observe up to four-photon stable UC photoluminescence under excitation by infrared femtosecond pulses, low optical limiting thresholds, and high nonlinear optical absorption coefficients close to those of colloid processed metal halide perovskite (MHP) QDs. Combined with high robustness against air and moisture, the monolithic inorganic glass with incorporated MHP QDs could be a better platform for exploiting strong light-matter interaction for MHPs.

Hole injection from P1 dye hot-excited states in p-type dye-sensitized films: a fluorescence study.

We present a study of the excited state relaxation dynamics of the photosensitizer P1 used in p-type dye-sensitized solar cells. Comparative femtosecond fluorescence upconversion measurements in solution and in films show that the dye undergoes a picosecond electronic relaxation from the bright Franck-Condon (FC) state to a low-emitting charge-transfer (CT) state in polar environment. The fluorescence is moderately quenched in solution and on the mesoporous Al2O3 isolator but dramatically more on NiO semiconductor. We assign this sub-picosecond process to the hole injection thus confirming that the electron transfer is from the FC state directly into the NiO valence band.

Some aspects about time broadening in fluorescence up-conversion measurements.

Several aspects contributing to the temporal broadening in the measurement of ultrafast fluorescence by means of up-conversion wave mixing are presented: the characteristics of the sample, those of the collection optics, and the wave mixing with the gate pulse in a non-linear crystal. It is concluded that these contributions are emission wavelength dependent and can be as important as the pulse durations in determining the instrument response function in this technique.

Concurrent Energy- and Electron-Transfer Dynamics in Photoexcited Mn-Doped CsPbBr3 Perovskite Nanoplatelet Architecture.

Mn-doped perovskites have already been widely explored in the context of interesting optical, electronic, and magnetic properties. Such fascinating traits showcased by them explain the huge augmentation in the device efficiency, directing their widespread application in the field of solar cells, energy- harvesting sectors, and light-emitting diodes. However, the underlying photophysics governing the overall charge carrier dynamics in Mn-doped CsPbBr3 nanoplatelets (NPLs) has never been discussed and therefore demands an in-depth investigation. Herein, fluorescence up-conversion and femtosecond transient absorption (TA) spectroscopy are employed for gaining a comprehensive understanding of the excited-state dynamics and the fundamental energy/charge-transfer processes for two-dimensional CsPbBr3 nanoplatelets (NPLs) and their Mn-doped counterparts. The up-conversion measurement clearly suggests the possibility of energy-transfer pathways in the Mn-doped CsPbBr3 NPLs. Interestingly, strong indication of charge transfer (CT) in Mn-doped CsPbBr3 NPLs was unambiguously established by an ultrafast TA approach. Our investigation clearly suggests that both the probable processes viz. the ultrafast energy and electron transfers noticeable in the Mn2+-doped CsPbBr3 NPLs are utterly competitive and rapid owing to the highly confined nature of the two-dimensional NPLs. This extensive probing of concurrent charge/energy-transfer processes may pave help clarify unresolved anomalies in Mn-doped perovskites, which may prove advantageous for a wide range of practical applicability.

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.