NIR-Vis Up-Conversion Luminescence in the Yb3+,Er3+ Doped Y2O2S, ZrO2, and NaYF4 Nanomaterials

Since the discovery of the up-conversion phenomenon, there has been an ever increasing interest in up-converting phosphors in which the absorption of two or more low energy photons is followed by emission of a higher energy photon. Most up-conversion luminescence materials operate by using a combination of a trivalent rare earth (lanthanide) sensitizer (e.g. Yb or Er) and an activator (e.g. Er, Ho, Tm or Pr) ion in a crystal lattice. Up-converting phosphors have a variety of potential applications as lasers and displays as well as inks for security printing (e.g. bank notes and bonds). One of the most sophisticated applications of lanthanide up-conversion luminescence is probably in medical diagnostics. However, there are some major problems in the use of photoluminescence based on the direct UV excitation in immunoassays. Human blood absorbs strongly UV radiation as well as the emission of the phosphor in the visible. A promising way to overcome the problems arising from the blood absorption is to use a long wavelength excitation and benefit from the up-conversion luminescence. Since there is practically no absorption by the whole-blood in the near IR region, it has no capability for up-conversion in the excitation wavelength region of the conventional up-converting phosphor based on the Yb3+ (sensitizer) and Er3+ (activator) combination. The aim of this work was to prepare nanocrystalline materials with high red (and green) up-conversion luminescence efficiency for use in quantitative whole-blood immunoassays. For coupling to biological compounds, nanometer-sized (crystallite size below 50 nm) up-converting phosphor particles are required. The nanocrystalline ZrO2:Yb3+,Er3+, Y2O2S:Yb3+,Er3+, NaYF4:Yb3+,Er3+ and NaRF4-NaR’F4 (R: Y, Yb, Er) materials, prepared with the combustion, sol-gel, flux, co-precipitation and solvothermal synthesis, were studied using the thermal analysis, FT-IR spectroscopy, transmission electron microscopy, EDX spectroscopy, XANES/EXAFS measurements, absorption spectroscopy, X-ray powder diffraction, as well as up-conversion and thermoluminescence spectroscopies. The effect of the impurities of the phosphors, crystallite size, as well as the crystal structure on the up-conversion luminescence intensity was analyzed. Finally, a new phenomenon, persistent up-conversion luminescence was introduced and discussed. For efficient use in bioassays, more work is needed to yield nanomaterials with smaller and more uniform crystallite sizes. Surface modifications need to be studied to improve the dispersion in water. On the other hand, further work must be carried out to optimize the persistent up-conversion luminescence of the nanomaterials to allow for their use as efficient immunoassay nanomaterials combining the advantages of both up-conversion and persistent luminescence.Siirretty Doriast

Multiphoton-absorption-excited up-conversion luminescence in optical fibers

We experimentally demonstrate a previously unforeseen nonlinear effect in optical fibers: up-conversion luminescence generation excited by multiphoton absorption of femtosecond infrared pulses. We directly estimate the average number of photons involved in the up-conversion process, by varying the wavelength of the pump source. We highlight the role of nonbridging oxygen hole centers and oxygen-deficient center defects and directly compare the intensity of side-scattered luminescence with numerical simulations of pulse propagation

Lanthanide doped NaYF4 up-conversion luminescence materials

The thesis includes literature and experimental parts. The first part introduces the reader to the topic while the later part contains synthesis and characterization details, results and publications. Up-conversion (UPC) luminescence materials are very interesting and they have plenty of potential applications of which bioanalytical ones are probably the most promising. A lot of research has been done to study and develop UPC luminescence materials and lanthanide (Ln) doped NaYF4 is maybe the most popular - especially Yb3+,Er3+ doped NaYF4. The trivalent lanthanides are very good for UPC luminescence due to their ladder-like energy level structures and long excited state lifetimes. Er3+, Tm3+ and Ho3+ activators are the best ones and often they are used together with an Yb3+ sensitizer which enhances the UPC efficiency. Moreover, NaYF4 is considered as the best host. The aim of this work was to prepare, study and improve UPC luminescence materials based on NaYF4. Yb3+ was used as the sensitizer and Pr3+, Nd3+, Sm3+, Eu3+, Tb3+, Dy3+, Ho3+, Er3+ and Tm3+ were used as activators. The materials were prepared with co-precipitation synthesis and studied with TG-DSC, XPD, TEM, TOF-SIMS, XPS and EXAFS. UPC luminescence was studied under 976 nm excitation. The synthesis was optimized so that the obtainable luminescence intensity is now even 2 orders of magnitude stronger than before. Several factors contributing to differences between the old and the new improved material were found: The cubic-to-hexagonal phase transition temperature is ca. 100 °C lower. The crystal structure is always hexagonal. The particles are covered with sodium. There are less lattice strains. The Na site is partly occupied with Yb3+ ions which enhances the Yb3+-Ln3+ energy transfer. In the end, UPC luminescence is obtained from all the lanthanides except Sm3+ and Dy3+Lantanideilla seostetut NaYF4-pohjaiset up-konversioluminesenssimateriaalit Väitöskirja sisältää kirjallisen ja kokeellisen osan. Kirjallinen osa johdattelee lukijan aiheeseen ja kokeellinen osa sisältää tiedot valmistus- ja tutkimusmenetelmistä sekä tulokset ja julkaisut. Up-konversioluminesenssimateriaalit ovat erittäin mielenkiintoisia ja niillä on useita potentiaalisia sovelluskohteita joista bioanalyyttiset sovellukset vaikuttaisivat olevan lupaavimpia. Up-konversioluminesenssimateriaaleja on tutkittu ja kehitetty paljon ja lantanideilla (Ln) seostetut NaYF4-materiaalit ovat ehkä kaikkein suo¬situimpia - erityisesti Yb3+,Er3+ seostettu NaYF4. Kolmivalenssiset lanta¬nidit sopivat erittäin hyvin up-konversioluminesenssiin, koska niillä on tikapuumaiset energiatilarakenteet ja virittyneiden tilojen eliniät ovat riittävän pitkiä. Er3+-, Tm3+- ja Ho3+-aktivaattorit ovat parhaimpia ja usein niiden kanssa käytetään Yb3+-herkistintä mikä parantaa up-konversion tehokkuutta. NaYF4:a pidetään parhaimpana pohja-aineena. Työn tarkoitus oli valmistaa, tutkia ja parantaa NaYF4:iin pohjautuvia up-konversioluminesenssimateriaaleja. Yb3+-herkistintä ja seuraavia aktivaattoreita käytettiin: Pr3+, Nd3+, Sm3+, Eu3+, Tb3+, Dy3+, Ho3+, Er3+ ja Tm3+. Materiaalit valmistettiin kerasaostusmenetelmällä ja niitä tutkittiin TG-DSC, XPD, TEM, TOF-SIMS, XPS ja EXAFS menetelmillä. Up-konversioluminesenssia tutkittiin 976 nm virityksellä. Synteesiä optimoitiin, minkä seurauksena luminesenssin intensiteetti on nyt jopa kaksi kertalukua voimakkaampi kuin ennen. Vanhan ja uuden parannellun materiaalin ominaisuuksissa havaittiin useita eroavuuksia: Kuutiollinen-heksagoninen rakennemuutoslämpötila on nyt n. 100 °C matalampi. Kiderakenne on aina heksagoninen. Partikkelit ovat natriumin peitossa. Hilajännityksiä on vähemmän. Na-paikka on osittain täytetty Yb3+-ioneilla mikä parantaa Yb3+-Ln3+-ener¬giansiirtoa. Nyt up-konversioluminesenssia voidaan havaita kaikista muista lantanideista paitsi Sm3+:sta ja Dy3+:staSiirretty Doriast

Room-temperature exciton storage in elongated semiconductor nanocrystals

Journal ArticleThe excited state of colloidal nanoheterostructures consisting of a spherical CdSe nanocrystal with an epitaxially attached CdS rod can be perturbed effectively by electric fields. Field-induced fluorescence quenching coincides with a conversion of the excited state species from the bright exciton to a metastable trapped state (dark exciton) characterized by a power-law luminescence decay. The conversion is reversible so that up to 10% of quenched excitons recombine radiatively post turn-off of a 1 us field pulse, increasing the delayed luminescence by a factor of 80. Excitons can be stored for up to 105 times the natural lifetime, opening up applications in optical memory elements

Photoluminescence upconversion at GaAs/InGaP2 interfaces driven by a sequential two-photon absorption mechanism

This paper reports on the results of an investigation into the nature of photoluminescence upconversion at GaAs/InGaP2 interfaces. Using a dual-beam excitation experiment, we demonstrate that the upconversion in our sample proceeds via a sequential two-photon optical absorption mechanism. Measurements of photoluminescence and upconversion photoluminescence revealed evidence of the spatial localization of carriers in the InGaP2 material, arising from partial ordering of the InGaP2. We also observed the excitation of a two-dimensional electron gas at the GaAs/InGaP2 heterojunction that manifests as a high-energy shoulder in the GaAs photoluminescence spectrum. Furthermore, the results of upconversion photoluminescence excitation spectroscopy demonstrate that the photon energy onset of upconversion luminescence coincides with the energy of the two-dimensional electron gas at the GaAs/InGaP2 interface, suggesting that charge accumulation at the interface can play a crucial role in the upconversion process

Replacement of glass-former B2O3 by GeO2 in an amorphous host evidenced by optical methods

Two completely different glass-host matrices containing lead, i.e. borate and germanate glasses doped with erbium were studied. The replacement of glass-former B2O3 by GeO2 in an amorphous host was evidenced by optical methods. The luminescence decay from the⁴I13/2 upper laser state of Er³⁺ ions is relatively short, whereas the up-converted emission signal is reduced definitely in borate glass containing lead due to its high B-O stretching vibrations. The results indicate that germanate glasses containing lead are promising for near-infrared luminescence and up-conversion applications

Stress Dependence of Exciton Relaxation Processes in Cu2O

A comprehensive study of the exciton relaxation processes in Cu2O has led to some surprises. We find that the ortho-para conversion rate becomes slower at high stress, and that the Auger nonradiative recombination rate increases with stress, with apparently no Auger recombination at zero stress. These results have important consequences for the pursuit of Bose-Einstein condensation of excitons in a harmonic potential.Comment: 10 figures, 1 tabl

Mutual energy transfer luminescent properties in novel CsGd(MoO4)2:Yb3+,Er3+/Ho3+ phosphors for solid-state lighting and solar cells

In this work, we prepared a novel kind of Yb3+, Er3+/Ho3+ co-doped CsGd(MoO4)(2) phosphors with a different structure from the reported ALn(MoO4)(2) (A = Li, Na or K; Ln = La, Gd or Y) compounds using a high-temperature solid-state reaction method. X-ray diffraction showed that the as-prepared samples had a pure phase. Based on the efficient energy transfer from Yb3+ to Er3+/Ho3+, the up-conversion (UC) luminescence of the optimal CsGd(MoO4)(2): 0.30Yb(3+), 0.02Er(3+) sample showed intensely green light with dominant emission peaks at 528 and 550 nm corresponding to Er3+ transitions H-2(11/2)-I-4(15/2) and S-4(3/2)-> I-4(15/2), respectively, as well as a weak emission peak originating from F-4(9/2)-I-4(15/2) at 671 nm, under 975 nm laser excitation. The CsGd(MoO4)(2): Yb3+, Ho3+ samples mainly displayed two emission bands around 540 and 660 nm together with a negligible one at 755 nm, which corresponded to Ho3+ transitions F-4(4),F-5(2)-> I-5(8), F-5(5)-> I-5(8) and F-4(4),F-5(2)-> I-5(7), respectively, under 975 nm laser excitation. With increasing Yb3+ concentration in CsGd(MoO4)(2): Yb3+, Ho3+ phosphors, the emission color could be tuned from orange red to light yellow due to the large energy gap between levels F-4(4),F-5(2) and F-5(5). In addition, the CsGd(MoO4)(2): Yb3+, Er3+ showed green light under 376 nm UV irradiation similar to that upon 975 nm laser excitation. However, the emissions for CsGd(MoO4)(2): Yb3+, Ho3+ samples under 358 nm UV or 449 nm blue excitation showed dominant emission peaks at 540 nm and weak 660 nm and 752 nm peaks, which were a bit different from those under 975 nm excitation. Interestingly, we observed efficient energy transfer phenomena (possible quantum cutting) from Er3+/Ho3+ to Yb3+ and a Yb3+-O2- charge transfer (CT) transition in the molybdates, which was deduced from the visible and near-infrared emission spectra and the decrease of the Er3+/Ho3+ luminescent lifetimes with increasing Yb3+ concentration in the CsGd(MoO (4))(2): Yb3+, Er3+/Ho3+ samples. The luminescence properties of these phosphors suggest their potential possibility for applications in solid-state lighting and displays as well as in c-Si solar energy conversion systems

Characterisation of periodically poled materials using nonlinear microscopy

Periodically poled crystalline materials are extremely attractive for processes such as second harmonic generation and optical parametric generation due to their very high conversion efficiency. For optimal performance, fabrication of poled regions with sub-micron tolerance is required. In this paper we introduce multi-photon laser scanning luminescence microscopy as a powerful minimally-invasive measurement technique which provides information about internal device structure with high spatial resolution that cannot be easily obtained with existing methods. A comparative study of confocal and multi-photon imaging of periodically poled crystalline materials is also performed