Upconversion Photoluminescence Intensity Research Articles

Biaxial strain tuned upconversion photoluminescence of monolayer WS2

Monolayer tungsten disulfide (1L-WS2) is a direct bandgap atomic-layered semiconductor material with strain tunable optical and optoelectronic properties among the monolayer transition metal dichalcogenides (1L-TMDs). Here, we demonstrate biaxial strain tuned upconversion photoluminescence (UPL) from exfoliated 1L-WS2 flakes transferred on a flexible polycarbonate cruciform substrate. When the biaxial strain applied to 1L-WS2 increases from 0 to 0.51%, it is observed that the UPL peak position is redshifted by up to 60 nm/% strain, while the UPL intensity exhibits exponential growth with the upconversion energy difference varying from − 303 to − 120 meV. The measured power dependence of UPL from 1L-WS2 under biaxial strain reveals the one photon involved multiphonon-mediated upconversion mechanism. The demonstrated results provide new opportunities in advancing TMD-based optical upconversion devices for future flexible photonics and optoelectronics.

Upconversion photoluminescence of monolayer WSe2 with biaxial strain tuning.

Mechanical strain can be used to tune the optical properties of monolayer transition metal dichalcogenides (1L-TMDs). Here, upconversion photoluminescence (UPL) from 1L-WSe2 flakes is tuned with biaxial strain induced by cruciform bending and indentation method. It is found that the peak position of UPL is redshifted by around 24 nm as the applied biaxial strain increases from 0% to 0.51%. At the same time, the UPL intensity increases exponentially for the upconversion energy difference that lies within a broad range between -157 meV to -37 meV. The observed linear and sublinear power dependence of UPL emission in 1L-WSe2 with and without biaxial strain at three different excitation wavelengths of 784 nm, 800 nm, and 820 nm indicates the multiphonon-assisted one-photon upconversion emission process. The results of strain-dependent UPL emission from 1L-TMDs pave a unique path to the advances in photon upconversion applications and optoelectronic devices.

Stability of Y2O3:Tm,Yb phosphor thin films during cathodoluminescence and upconversion

An Y2O3:Tm,Yb thin film was synthesized by the sol-gel method using spin coating. X-ray powder diffraction revealed the formation of a Y2O3 cubic phase. The cathodoluminescence and upconversion photoluminescence of the film were investigated, including assessment of emission stability. Auger electron spectroscopy was used to monitor its chemical state concurrently with the cathodoluminescence intensity using the same electron beam. Auger peaks associated with carbon, yttrium and oxygen were detected and their Auger peak-to-peak heights indicated that carbon was removed from the surface during the initial stage of degradation, whereas yttrium and oxygen remained almost unchanged. The cathodoluminescence degradation of the Tm emissions at 457 nm and 975 nm exhibited a continuous decline until the electron dose was about 100 C/cm2 and then stabilized. The upconversion photoluminescence intensity of the Tm emission at 488 nm remained stable for a period of 28 h and did not exhibit degradation.

Phonon-assisted upconversion photoluminescence of monolayer MoS2 at elevated temperatures.

Upconversion photoluminescence (UPL) lies at the heart of optical refrigeration and energy harvesting. Monolayer transition metal dichalcogenides (TMDCs) have been identified as an excellent platform with robust phonon-exciton coupling for studying the phonon-assisted UPL process. Herein, we investigate the multiphonon-assisted UPL emission in monolayer MoS2 at elevated temperatures and the temperature-dependent phonon contributions in the UPL process. When temperature goes up from 295 K to 460 K, the enhancement of the integrated UPL intensity is demonstrated due to the increased phonon population and the reduced phonon numbers involved in the UPL process. Our findings reveal the underlying mechanism of phonon-assisted UPL at high temperatures, and pave the way for the applications of photon upconversion in display, nanoscale thermometry, anti-Stokes energy harvesting, and optical refrigeration.

Photochemical Upconversion in Solution: The Role of Oxygen and Magnetic Field Response

Upconversion processes effectively convert two or more low energy photons into one higher energy photon, and they have diverse prospective applications in photovoltaics and biomedicine. We focus on two specific mechanisms for photochemical upconversion in solution: triplet-triplet annihilation (TTA) and singlet oxygen mediated energy transfer (SOMET). TTA is spin-selective, whereas SOMET is not, so the interplay between these two upconversion mechanisms can be examined via their different magnetic field responses. A kinetic model is developed and applied to explain the different photoluminescence profiles of oxygenated versus deoxygenated systems. From the magnetic field response, the triplet-triplet annihilation rate constant is estimated. The conditions required to maximize upconversion photoluminescence intensity in oxygenated solution are determined, providing a set of design principles to guide molecule choices for robust and air-stable upconversion systems in the future.

Unveiling room temperature upconversion photoluminescence in monolayer WSe2.

Upconversion photoluminescence (UPL) is a phenomenon describing an anti-Stokes process where the emitted photons have higher energy than the absorbed incident photons. Transition metal dichalcogenides (TMDCs) with strong photon-exciton interactions represent a fascinating platform for studying the anti-Stokes UPL process down to the monolayer thickness limit. Herein, we demonstrate room-temperature UPL emission in monolayer WSe2 with broadband near-infrared excitation. The measured excitation power dependence of UPL intensity at various upconversion energy gains unveils two distinguished upconversion mechanisms, including the one-photon involved multiphonon-assisted UPL process and the two-photon absorption (TPA) induced UPL process. In the phonon-assisted UPL regime, the observed exponential decay of UPL intensity with the increased energy gain is attributed to the decreased phonon population. Furthermore, valley polarization properties of UPL emission with circular polarization excitation is investigated. The demonstrated results will advance future photon upconversion applications based on monolayer TMDCs such as night vision, semiconductor laser cooling, and bioimaging.

Enhanced Upconversion Photoluminescence of LiYF4: Yb3+/Ho3+ Crystals by Introducing Mg2+ Ions for Anti-Counterfeiting Recognition

By doping appropriate lanthanide ions, LiYF4 as a host luminescent material can simultaneously exhibit bright visible-light emission. A series of LiYF4:Yb3+/Ho3+ microparticles with different Mg2+ doping concentrations were synthesized and investigated. The crystal structure of the synthesized microparticles was tested by X-ray diffraction (XRD). Notably, a significant increase in the upconversion photoluminescence intensity of upconversion microparticles (UCMPs) was obtained by introducing Mg2+ ions under 980 nm laser excitation, and achieved a maximum level when the concentration of Mg2+ ions was 8 mol%. Additionally, the practicality of the resultant UCMPs used as the raw material of anti-counterfeiting ink was systematically investigated. These results prove that the Mg2+-doped LiYF4:Yb3+/Ho3+ are very promising as screen-printing materials for anti-counterfeiting recognition labels.

High electrical properties and good upconversion luminescence in SrBi1.94-xEr0.06YbxNb2O9 lead-free piezoelectric ceramics

SrBi1.94-xEr0.06YbxNb2O9 (x = 0.000, 0.002, 0.004 and 0.006, noted as SBEN-xYb) composites were prepared using traditional solid-phase sintering technology. The influences of (Er, Yb)3+ co-doping on the electrical and luminous properties as well as energy band structure of SrBi2Nb2O9 (SBN) ceramics were systematically investigated. The outstanding comprehensive characteristics with low dielectric loss (tanδ ~ 0.073%), high remnant polarization (2Pr ~ 10.73 μC/cm2), large piezoelectric responses (d33 ~ 15 pC/N) and excellent upconversion photoluminescence (UC-PL) are simultaneously observed by Yb doping with a concentration of x = 0.004. The UC-PL spectrum excited by 980 nm laser shows double bands at 533 nm and 554 nm in the green region, and single band at 672 nm in the red region. A density functional theory (DFT) calculation was performed to reveal the physical origin of electrical conductivity and UC-PL. It is seen that the band gap consistently decreases, while the contribution from f state of (Er, Yb)3+ is intensified with increasing Yb3+, elaborating the variations of electrical conductivity and UC-PL intensity. Related mechanisms for the improvement of UC-PL and electrical properties were discussed.

Upconversion photoluminescence of perovskite nanoparticles encapsulated in porous sub-micron spheres supporting Mie resonances.

Currently, halide perovskites are very perspective materials not only for photovoltaics but also for nanophotonic and especially nonlinear optics. These materials have already demonstrated high two-, three- and many- photon absorption coefficients, strong Kerr-nonlinearity, and high-efficient second harmonic generation. Easy and cheap fabrication gives halide perovskites a wide area for scientific research and engineering applications. However, to achieve the stability of perovskites is still a challenging task, which scientific community is working on. In this work, we study a new form of encapsulation of perovskite nanoparticles in sub-micron porous dielectric nanospheres. Due to small pores in such spheres, perovskites are not only protected from external factors, but also are confined in size, which brings several features in the photoluminescence emission. We also show resonant properties of spherical sub-micron particles, which can be used for enhancing upconversion photoluminescence intensity.

Non-radiative relaxation and nonlinear properties of YVO4:Yb3+, Er3+ upconversion nanoparticles

• Analysis of actual energy transfer channels in YVO 4 :Yb 3+ ,Er 3+ UCNP is presented. • Simple (reduced) model for energy transfer processes is proposed. • Pump power dependence of upconversion photoluminescence is yielded. • It is established that multiphonon transitions mainly effect on upconversion nonlinearity. • Theoretical results could be useful in UCNPs characterization by spectroscopic data analysis. We analyzed nonlinear emission properties of oxide nanocrystals YVO 4 doped with Yb 3+ and Er 3+ ions which are one of the most interesting types of upconverting nanoparticles (UCNPs). As is widely believed the deviation of the power-dependent upconversion phosphorescence intensity from a quadratic character is attributed to a saturation effect occurring at the relatively high pump power. In this paper, we focused on studying upconversion nonlinearity at the low pump power when excited states populations are considerably less then ground states populations. The analysis of actual energy transfer channels allowed us to propose a reduced model having simple solutions. The obtained analytic expressions for the dependences of upconversion photoluminescence intensity and its nonlinearity on the pump power revealed that multiphonon transitions are a main factor responsible for the variation of upconversion nonlinearity in a range between 1 and 2. This result could be used in spectroscopic experiments for simple and direct identification of the non-radiative relaxation efficiency in YVO 4 :Yb, Er UCNPs.

Upconversion photoluminescence analysis of fluoroquinolones.

The characteristics of the upconversion (UC) photoluminescence (PL) of fluoroquinolones (FQs) are reported here for the first time. Using UC PL, a simple, sensitive, and cost-effective method of determining FQs was developed that eliminated interference arising from biological background fluorescence and did not require UV light to excite the FQ (in contrast to downconversion PL), which is an advantage given that UV is potentially damaging to organisms. Ciprofloxacin (CPFX) and levofloxacin (LVFX), two important FQs, were studied. The effects of acidity, temperature, the solvent used, ionic strength, and stable time on the UC PL from the two FQs were also investigated. The UC PL intensity was found to be proportional to the concentration of CPFX over the range 0.05-100μmol/L with a correlation coefficient of 0.9992, and proportional to the concentration of LVFX over the range 0.05-100μmol/L with a correlation coefficient of 0.9991. The limit of detection (LOD) was 6.05nmol/L for CPFX and 5.64nmol/L for LVFX. The proposed method was successfully used to determine FQs in human serum and pharmaceutical samples. The recoveries of the two FQs ranged from 96.0% to 103.2% and the RSD was < 2.62%. Graphical abstract.

Erbium–Ytterbium Upconversion in Polyvinyl Chloride Films

We have prepared a series of polyvinyl chloride–yttrium oxysulfide composites doped with ytterbium and erbium ions and measured their upconversion photoluminescence spectra and volume resistivity. The results demonstrate that the green photoluminescence intensity in the composites is considerably higher than that in yttrium oxysulfide doped with erbium and ytterbium. The resistivity of the composites drops by eight orders of magnitude on heating. Besides, with increasing erbium–ytterbium ion pair concentration, the upconversion photoluminescence intensity in the red and green spectral regions rises, and additional photoluminescence peaks emerge, two in the green and one in the red, owing to the Stark splitting of the erbium terms under the effect of the chlorine present in the polyvinyl chloride. On heating, the PVC–Y2O2S:Yb3+/Er3+ composite passes into a semiconducting state.

1.2 µm and 1.5 µm near-infrared photoluminescence and visible upconversion photoluminescence in GeGaS:Er3+/Ho3+ glasses under 980 nm excitation

We report on the Er3+ ↔ Ho3+ optical interaction on the 1.2 and 1.5 µm near-infrared and visible upconversion photoluminescence in Er3+/Ho3+-co-doped GeGaS glasses at pumping wavelength of 980 nm. Since the 980 nm pumping wavelength is off resonance for Ho3+ intra-4f electronic transitions, we have utilized the Er3+ sensitization with observed efficient Er3+ → Ho3+ energy transfer. As a result, the Ho3+ 1.2 µm emission in Er3+/Ho3+-co-doped GeGaS glass has been notably improved while the intense Er3+ 1.5 µm emission has been maintained. The visible upconversion photoluminescence has been observed at 530, 550 and 660 nm and was assigned mainly to intra-4f electronic transitions within Er3+ ions. The overall upconversion photoluminescence intensity significantly increases by addition of Ho3+ ions into GeGaS:Er glass which may be explained by energy transfer processes between Er3+ and Ho3+ ions leading to population of the Er3+ upper energy levels. The Ho3+ 1.2 µm and Er3+ 1.5 µm emission has much potential in telecommunication while the intense visible upconversion photoluminescence in sensing.

Comparison of the up-conversion photoluminescence for GAP, GAG and GAM phosphors

GdAlO3:Er3+/Yb3+, Gd3Al5O12:Er3+/Yb3+ and Gd4Al2O9:Er3+/Yb3+ phosphors were prepared by co-precipitation. The effects for Gd2O3-Al2O3 composite oxides as the host materials with different crystal structures such as GdAlO3, Gd3Al5O12 and Gd4Al2O9 were investigated. It was found that the perovskite structured GdAlO3:Er3+/Yb3+ (GAP phosphor) could be obtained from the precursor when the calcination temperature was 1000 °C, while the garnet structured Gd3Al5O12:Er3+/Yb3+ (GAG phosphor) could be formed when the calcination temperature was 1300 °C, but the monoclinic-structured Gd4Al2O9:Er3+/Yb3+ (GAM phosphor) could be formed only when the calcination temperature was raised up to 1500 °C. The difference of the up-conversion photoluminescence (UCPL) spectra under 980 nm between the GAP, GAG and GAM phosphors was studied. The result showed that the UCPL intensity of the GAP phosphor was close to that of the GAM phosphor with much higher red-to-green intensity ratio than that of GAP phosphor. The UCPL intensity of GAG phosphor was the weakest among them. Finally, the factors which influenced on the UCPL of the GAP, GAG and GAM phosphors were discussed.

An effective method to detect the Curie transition of Er3+/ Yb3+ co-doped BaTiO3 ceramics by up-conversion photoluminescence intensity ratio

In this work, BaTi1−x−yO3:xEr/yYb (BTO:xEr/yYb, x = 0, y = 0; x = 0.005, y = 0.005; x = 0.01, y = 0.005; x = 0.01, y = 0.01; x = 0.015, y = 0.01) ceramics were prepared using a conventional solid-reaction method to investigate the relation between the Curie transition and up-conversion photoluminescence (UC PL) properties. We did not find that the green or red UC PL absolute intensity of Er3+ ions had any direct correlation with the Curie phase transition. However, the variation of the ratio Ig/Ir of green and red UC emission intensities with temperature has similar behavior as that of dielectric constant. An evident peak of Ig/Ir ratio was also obtained at TC in all the BTO:xEr/yYb ceramics with different Er3+/Yb3+ co-doping contents. The present study demonstrates that UC PL intensity ratio, rather than the absolute PL intensity, is an effective optical non-contact method to detect the Curie phase transition of ferroelectrics.

Effect of phase structure changes on the lead-free Er3+-doped (K0.52Na0.48)1−xLixNbO3 piezoelectric ceramics

Er3+ doped (K0.52Na0.48)1−xLixNbO3 ceramics had been prepared through a conventional solid state reaction routine and their phase structure, ferroelectric, piezoelectric, and photoluminescence properties were systematically investigated. X-ray diffraction results clearly illustrated a phase transition from the orthorhombic to coexistence and further to tetragonal phase with the increase of Li content. Via measuring the ferroelectric and piezoelectric properties of the ceramics, it found that the polymorphic phase transition (PPT) effect contributed the obvious enhancement in electrical properties of the Er3+ doped (K0.52Na0.48)0.94Li0.06NbO3 ceramic which lay in the composition region of coexistence phase structure. The up-conversion photoluminescence intensity of green emission and red emission first increased as x increase from 0 to 0.06 and then decreased for x = 0.08. The most obvious enhancement in the up-conversion photoluminescence intensity was also achieved in the composition lying in the vicinity of PPT region. And the above circumstance caused the consideration that here the essence of the coexistence phase may be a phase structure with lower crystal symmetry instead of just physical combination of orthorhombic and tetragonal phase.

Optically Monitoring Mineralization and Demineralization on Photoluminescent Bioactive Nanofibers.

Bone regeneration and scaffold degradation do not usually follow the same rate, representing a daunting challenge in bone repair. Toward this end, we propose to use an external field such as light (in particular, a tissue-penetrating near-infrared light) to precisely monitor the degradation of the mineralized scaffold (demineralization) and the formation of apatite mineral (mineralization). Herein, CaTiO3:Yb(3+),Er(3+)@bioactive glass (CaTiO3:Yb(3+),Er(3+)@BG) nanofibers with upconversion (UC) photoluminescence (PL) were synthesized. Such nanofibers are biocompatible and can emit green and red light under 980 nm excitation. The UC PL intensity is quenched during the bone-like apatite formation on the surface of the nanofibers in simulated body fluid; more mineral formation on the nanofibers induces more rapid optical quenching of the UC PL. Furthermore, the quenched UC PL can recover back to its original magnitude when the apatite on the nanofibers is degraded. Our work suggests that it is possible to optically monitor the apatite mineralization and demineralization on the surface of nanofibers used in bone repair.

Modifying optical, electric and magnetic properties of multifunctional Gd3+-doped Ba4La0.95Er0.05TiNb9O30 ceramics

Tetragonal tungsten bronze (TTB) structure Ba4La0.95−xEr0.05GdxTiNb9O30 (BLETN:Gd3+) with different Gd3+ concentrations have been synthesized, and their structural, up-conversion photoluminescence (UC-PL), dielectric, ferroelectric (FE), and magnetic properties have been investigated in this work. With the incorporation of Gd3+, it is found that a BLETN:Gd3+ system exhibits multifold responds to the external electric, magnetic, and optical stimuli. Specially, enhanced UC-PL intensity, obvious relaxor-like FE state, and paramagnetic (PM) feature have been simultaneously realized in this single phase TTB-type compound. It is believed that the BLETN:Gd3+ system involving bright UC emission and promising electromagnetic properties may act as a potential material for future multifunctional device applications.

Quenching of the upconversion luminescence of NaYF4:Yb3+,Er3+and NaYF4:Yb3+,Tm3+nanophosphors by water: the role of the sensitizer Yb3+in non-radiative relaxation

We have studied the mechanisms of water-based quenching of the upconversion photoluminescence of upconverting nanophosphors (UCNPs) via luminescence decay measurements for a better understanding of the non-radiative deactivation pathways responsible for the relatively low upconversion luminescence efficiency in aqueous solutions. This included both upconversion luminescence measurements and the direct excitation of emissive energy states of Er(3+) and Yb(3+) dopants in NaYF4:Yb(3+),Er(3+) UCNPs by measuring the decays at 550 and 655 nm upon 380 nm excitation and at 980 nm upon 930 nm excitation, respectively. The luminescence intensities and decays were measured from both bare and silanized NaYF4:Yb(3+),Er(3+) and NaYF4:Yb(3+),Tm(3+) UCNPs in H2O and D2O. The measurements revealed up to 99.9% quenching of the upconversion photoluminescence intensity of both Er(3+) and Tm(3+) doped bare nanophosphors by water. Instead of the multiphonon relaxation of excited energy levels of the activators, the main mechanism of quenching was found to be the multiphonon deactivation of the Yb(3+) sensitizer ion caused by OH-vibrations on the surface of the nanophosphor. Due to the nonlinear nature of upconversion, the quenching of Yb(3+) has a higher order effect on the upconversion emission intensity with the efficient Yb-Yb energy migration in the ∼35 nm nanocrystals making the whole nanophosphor volume susceptible to surface quenching effects. The study underlines the need of efficient surface passivation for the use of UCNPs as labels in bioanalytical applications performed in aqueous solutions.

Effect of exciton oscillator strength on upconversion photoluminescence in GaAs/AlAs multiple quantum wells

We report upconversion photoluminescence (UCPL) in GaAs/AlAs multiple quantum wells. UCPL from the AlAs barrier is caused by the resonant excitation of the excitons in the GaAs well. When the quantum well has sufficient miniband width, UCPL is hardly observed because of the small exciton oscillator strength. The excitation-energy and excitation-density dependences of UCPL intensity show the exciton resonant profile and a linear increase, respectively. These results demonstrate that the observed UCPL caused by the saturated two-step excitation process requires a large number of excitons.