Symposium MD7 : Advances in Lanthanide Materials for Imaging, Sensing, Optoelectronics and Recovery/Recycling

In this talk, our research directed toward developing new paradigms of lanthanide coordination chemistry and producing nanoscopic lanthanide-containing clusters with interesting materials properties will be presented. Inspired by the potential applications of polynuclear lanthanide hydroxide complexes, we developed the rational synthesis of these otherwise synthetically elusive species by the approach of ligand-controlled hydrolysis of the lanthanide ions. Nanoscopic and structurally pleasing lanthanide-hydroxo cluster motifs have been uncovered. Extension of this approach to mixed d-f systems affords giant heterometallic clusters. Properties of these cluster compounds pertinent to their potential applications in fuel technology and magnetic cooling will be discussed.

Lanthanide-activated upconversion (UC) materials have the remarkable ability to generate visible or UV light upon NIR excitation. However, the poor efficiency of UC, particularly at low excitation power densities, is the major limitation for many potential applications of these materials. Because upconversion is a multi-photon process, UC intensity has a greater-than-linear dependence on excitation power density. It is common practice, therefore, to focus the excitation beam within a small sample volume to achieve high excitation power densities, resulting in brighter, more efficient upconversion. This often precludes large-area UC imaging in the absence of either powerful unfocused excitation or tightly-focused, rastered excitation. Plasmonic enhancement is a major strategy being pursued to improve the efficiency of upconversion. Properly designed plasmonic surfaces can generate patterned regions of highly-focused electric fields upon large-area illumination with excitation light. However, significant fundamental questions still remain with regard to the interaction of UC nanocrystals with plasmonic particles and surfaces. One difficulty in characterizing this interaction is that the observed enhancement is very sensitive to the position of the UC nanocrystal relative to the plasmonic architecture. Enhancements observed from ensembles of UC nanocrystals can be dramatically affected by the inhomogeneity of the particle distribution relative to the field ‘hot spots.’ In the study presented here, however, the geometry of the UC nanocrystals relative to the plasmonic surfaces is carefully controlled and the spectral data acquired with high spatial resolution, extending even to the single particle level. A model plasmonic architecture is used consisting of Au micro-cavity arrays. Single crystals of NaYF₄: Yb, Ln (Ln=Er, Tm) of uniform size are imaged in the hemispherical cavity wells. UC images of the crystals is captured with a hyperspectral microscope, exciting the arrays with 980 nm light, using full field illumination. Single crystal orientation is confirmed by comparing the optical and hyperspectral images with the scanning electron microscope images. The project introduces a new approach for evaluating the effects of a plasmonic surface on UC efficiency.

Metal phosphonates are widely studied since the last decades due to their potential applications in several domains such as ion exchangers, proton conductors or opto-electronics and non-linear optical materials. Lanthanide aryl and alkyl phosphonates LnH(O₃PR)₂ (Ln = La, Y, Eu, Tb; R = CH₃, C₂H₅, C₃H₇, C₆H₅) are particularly interesting for several reasons : (i) they exhibit an intense luminescence comparable to commercial phosphors like Y₂O₃:Eu^{3+ 1,2,3} ; (ii) they are easy to produce from cheap organic precursors by low temperatures processes and (iii) they are stable up to 400°C – 500°C. All these reasons make them competitive candidates for phosphors materials in LED applications.

Recently the R = C₆H₅ members of the LnH(O₃PR)₂ series have been successfully nanostructured to enhance luminescence^4,5,6. We propose here another route to improve and control the luminescent properties of these materials, by selecting the most efficient host matrices and lanthanide doping rates. Based on this approach, the La_1-xLn_xH(O₃PR)₂ (0 ≤ x ≤ 1; Ln = Eu, Tb) series have been synthesized by hydrothermal methods at 180°C and studied.

The crystal structures have been analyzed by X ray diffraction, and the microstructures by scanning electron microscopy. The LaH(O₃PR)₂ structures are composed of alternating organic and inorganic layers. Within the inorganic layers, the lanthanum ions are disposed in chains of LaO₈ polyhedra linked by opposite edges. Crystal structures and microstructures of La_1-xLn_xH(O₃PR)₂ compounds will be compared and presented.

The photophysical properties were investigated by photoluminescence (PL), PL excitation and time resolved PL spectroscopies. The La_1-xEu_xH(O₃PR)₂ phosphonates exhibit typical Eu³⁺ red luminescence, due to 4f-4f transitions. The La_1-xTb_xH(O₃PR)₂ phosphonates exhibit, under 4f-4f and 4f-5d excitations, 4f-4f blue to green emissions, which can be tuned by the lanthanide doping rates and the excitation wavelengths. Weakly Eu³⁺ / Tb³⁺ codoped materials exhibit a white emission under l_exc = 378 nm ; giving alternative candidates for white phosphors applications. Whereas heavily Eu³⁺ or Tb³⁺ doped materials exhibit red and green emissions which can be optimized by selection of the R groups of the LaH(O₃PR)₂ host matrices and of the Eu³⁺ and Tb³⁺ contents. Effects of host matrices, lanthanide doping rates and excitation wavelengths upon Tb³⁺ cross relaxation and concentration quenching phenomenon in La_1-xLn_xH(O₃PR)₂ materials will be presented in details and linked with the evolution of decay times and photoluminescence intensities.

¹ Rosa, I. L. V et al. J. of Alloys and Compounds, 275-277, 315-317, (1998)
² Rosa, I. L. V. et al. J. Fluoresc. 16, 455–459 (2006).
³ Mutelet, B. et al. Dalton Trans 44, 1186–1192 (2015).
⁴ S.Y. Song et al. Inorg. Chem. 45, 1201-1207, (2006)
⁵ W. Di et al. J. Phys. Chem. C, 114, 6290-6297, (2010)
⁶ W. Di et al. CrystEngComm, 13, 5226-5233, (2011)

The shapes of absorption and emission spectra that are observed in luminescence spectroscopy give essential information about the type of electronic transition that is studied. Electronic transitions in luminescent molecules or luminescent centers in crystals couple to vibrations. This results in broadening of absorption and emission bands, as well as in the occurence of a Stokes shift. In principle, one can derive from the Stokes shift the Huang-Rhys parameter S, which describes the microscopic details of the vibrational coupling and can be related to the equilibrium position offset between the ground state and excited state potential energy surfaces of the transitions. A relation between E_Stokes and S allows for a direct link between the Stokes shift and the molecular properties of the system which can also be addressed through quantum mechanical calculations.

Suprisingly, the two commonly found textbook relations E_Stokes=(2S-1)hω/2π and E_Stokes=2Shω/2π are contradictory. We have attempted to resolve this ambiguity. We investigate how E_Stokes is related to S by modelling the emission spectra as a function of the Huang-Rhys parameter. To this end we use the simple Franck-Condon overlap integrals and take into account the effects of a finite temperature through line broadening and thermal occupation of excited vibrational states. We show that in different ranges of temperature, different approximate relations between E_Stokes and S are appropriate. The two textbook relation are valid in limiting situations and set boundaries to the values that the Stokes shift can take.

Moreover, we demonstrate a new method for relating the absorption and emission spectra to the microscopic properties of the system: instead of using the maxima of the bands, the difference between the barycenters of absorption and emission bands can be used to determine S in an unambiguous way. The position of the barycenter is, contrary to the Stokes shift, unaffected by temperature.

This work settles the debate about the validity of the two relations that are commonly used to relate the Stokes shift to the Huang-Rhys parameter in the field of luminescence spectroscopy. It finds immediate application in all the fields in which this technique is used to determine the Huang-Rhys parameter from experimentally observed spectra.

Near-infrared (NIR) luminescent materials containing lanthanide(III) ions are used increasingly in telecommunications engineering, laser technology and biomedical science due to their extraordinary optical properties. However, lanthanides are constrained intrinsically by the Laporte-forbidden 4f – 4f transitions that render direct excitation of them rather inefficient; their extreme susceptibility towards non-radiative quenching by water’s vibrational O-H harmonic oscillations cannot be overstated. These constraints make lanthanide luminescence studies challenging. To sensitize and protect the lanthanide ion, porphyrin-based macrocycles, in this regard, are one of the promising organic antenna ligands to be utilized. With high absorption cross sections for intense UV absorption and low energy triplet states well matched to the lanthanides’ NIR emitting states, porphyrins can absorb the excitation light effectively, transfer the energy to the lanthanide ion efficiently, and coordinate with them to form robust complexes. In the 1^st part of lecture, we will highlight some recent advances in the development of NIR emissive porphyrin-based lanthanide complexes, porphyrin-coated lanthanide nanoparticles, and their potential bio-applications in terms of being NIR probes and antitumor agents.

In the second half, a new gadolinium complex (Gd-N) as a tumor-specific photodynamic therapy (PDT) agent will be discussed. Gd-N has great potential of serving as an anti-cancer torpedo boat which is equipped with visible-to-NIR emission for tumor imaging, discriminating radar for tumor targeting, and powerful ¹O₂ torpedo generation for tumor killing. This new gadolinium complex here can be promising and applicable for the development of long-term live cancer cell tracking and imaging, as well as modern practical PDT.

References:
1. D. Qi, Q. Wang, H. Li, T. Zhang, R. Lan, W.-K. Wong, K.-L. Wong, S. Li, F. Lu, Molecular Biosystems, (2015) In Press. DOI: 10.1039/C5MB00497G.
2. T. Zhang, R. Lan, L. Gong, B. Wu, Y. Wang, D. W. J. Kwong, W.-K. Wong, K.-L. Wong, D. Xing, ChemBioChem, (2015) In Press 10.1002/cbic.201500349.
3. T. Zhang, R. Lan, C.-F. Chan, G.-L. Law, W.-K. Wong, K.-L. Wong, Proceedings of National Academy of Science USA, 111 (2014) E5492-E5497.
4. T. Zhang, C.-F. Chan, R. Lan, W.-K. Wong, K.-L. Wong, Chemistry-A European Journal, (2014), 971-973.
5. T. Zhang, C.-F. Chan, J. Hao, G.-L. Law, W.-K. Wong, K.-L. Wong, RSC Advances, 3 (2013) 382-385.
6. T. Zhang, C.-F. Chan, R. Lan, H. Li, N.-K. Mak, W.-K. Wong, K.-L. Wong, Chemical Communications, 49 (2013) 7258-7260.
7. J. Zhang, H. Li, C.-F. Chan, R. Lan, W.-L. Chan, G.-L. Law, W.-K. Wong, K.-L. Wong, Chemical Communications, 48 (2012) 9646-9648.
8. T. Zhang, X. Zhu, W.-M. Kwok, C. T.-L. Chan, H.-L. Tam, W.-K. Wong, K.-L. Wong, Journal of the American Chemistry Society, 50, (2011) 20120-20122.

To generate a nanoscale optical probe for interrogating dynamics in biological systems, we have developed a new approach called cathodoluminescence-activated imaging by resonance energy transfer (CLAIRE) that combines the nanoscale focusing of scanning electron microscopy and the chemical specificity and non-invasiveness of fluorescence microscopy. Our super-resolution optical imaging platform consists of a high-brightness, rapidly scannable, 20-nm optical spot in a cathodoluminescent cerium-doped yttrium aluminum perovskite (YAlO₃:Ce, YAP:Ce) thin film. The nanoscale excitation volume is generated by a low energy, focused electron beam from a scanning electron microscope. Optical excitations in the ultrathin (10-20 nm) YAP:Ce thin film, activated by the electron beam, interact in the near-field with the sample. Interactions between the excited cerium dopants in the YAP:Ce film and the sample can provide contrast by quenching or enhancing the emission of the YAP:Ce film. Additionally, excitations in the film can be non-radiatively transferred to adjacent luminescent samples via Förster resonance energy transfer (FRET). By correlating the position of the electron beam with the cathodoluminescence intensity of the film or with fluorescence from the sample, we generate images with nanoscale resolution and high optical contrast. Thus far, we have imaged metal nanostructures based on cathodoluminescence-activated enhancement of the YAP:Ce film emission. We have also used FRET to image soft samples, such as organic light-emitting polymer blends, which would otherwise be damaged by direct electron excitation. Recently, we have begun imaging dried biological samples and tracking the movement of samples in fluid environments. By encapsulating aqueous biological samples adjacent to the YAP:Ce film, we anticipate imaging biological dynamics, such as diffusion on lipid membranes, on the nanoscale.

Metal boride ceramics are of great interest as high performance materials due to their high melting points, thermal stability, chemical inertness and extreme hardness. In particular, lanthanum hexaboride also possesses other useful properties such as a very low work function and one of the highest electron emissivities known. However, even with these exceptional properties, its industrial use is limited due to its difficulty in being processed into thin films.

To that end, we have developed a technique that utilizes a lithium ion incorporation-exfoliation reaction to convert lathanum hexaboride into its constituent nanosheets, making it solution-processable. Using this synthetic methodology, we obtained LaB₆ nanosheets with lateral dimensions on the micron scale and thickness less than 5 nm. The effect of using different starting material morphologies, lithium ion sources and reaction conditions on the size, shape and morphology of the nanosheets were also studied. We are currently investigating the Li ion incorporation into the lanthanum hexaboride lattice and further evaluating the near infra-red absorption properties of LaB₆ nanosheets.

One of our main research thrusts in the field of colloidal nanomaterials and their applications is based on the trivalent lanthanide ions, which have unique optical and magnetic properties. We cover the range from synthesis and characterisation to (in vivo) applications, with a recent focus on MRI (Magnetic resonance Imaging) for cancer diagnostics. The colloidal stability is typically obtained by a “soft” layer of organic molecules. To this purpose, we have developed synthesis techniques to make core-shell architectures which allows the optical and magnetic properties to be optimized separately. Additionally, we do surface modifications by organic chemistry to impart compatibility with biological media and to impart binding specificity (e.g. through antibodies).
The synthesis and basic characterisation of the nanoparticles will only briefly be discussed, but the core-shell synthesis will be discussed in some detail. Some emphasis will be on the challenges to prove the actual formation of core-shell structures and the Ln³⁺ dopant distribution. The main focus of the talk is on their surface modification and use as optical and magnetic probes for bioimaging. Soem recent in vivo MRI studies will be included showing their potential as MRI contrast agent at high magnetic fields in brain, breast and prostate cancer.

Imaging complex materials at the single-molecule level reveals spatial and temporal heterogeneities that are lost in ensemble imaging experiments. An ongoing challenge is the development of probes with the photostability, brightness, and continuous emission necessary at higher single-molecule excitation intensities. We have found that lanthanide-doped upconverting nanoparticles (UCNPs), which absorb multiple photons in the NIR and emit at higher energies in the NIR or visible, have unusual properties that make them valuable luminescent reporters, including: no measurable photobleaching, even under prolonged single-particle excitation; no overlap with autofluorescence in biological systems; an absence of on-off blinking; and efficiencies 5-6 orders of magnitude higher than the most efficient 2-photon fluorescence processes. But the brightness of UCNPs has been limited by a poor understanding energy transfer and relaxation within individual nanocrystals and unavoidable trade-offs between brightness and size. We have recently engineered UCNPs under 10 nm in diameter that are over an order of magnitude brighter under single-particle imaging conditions than the brightest ensemble compositions, allowing us to visualize single upconverting nanoparticles as small as fluorescent proteins. We have used a combination of advanced characterization (single-nanocrystal lifetimes and full emission spectra) and theoretical modeling to find that surface effects become critical at diameters under 20 nm, and that the higher fluences used in single-molecule imaging fundamentally change the factors that determine nanocrystal brightness. We find that factors known to increase brightness in bulk experiments are unimportant at higher excitation powers, and that, paradoxically, the brightest probes under single-molecule excitation are barely luminescent at the ensemble level. We have also applied this modeling strategy to develop efficient hybrid UCNP-dye biosensors that may be deployed as local sensors of cell signaling.

For more than a decade, we have been pushing the boundaries of special material class that emits light when it is stressed, strained, and/or fractured. These materials have been proposed for the active element of impact sensors, earthquake detectors, and stress sensors in a variety of applications. While there are a number of techniques currently being used for damage detection and monitoring of civil, aerospace, and military structures and aircraft, they do not provide in-situ and distributed sensing. Wiedemann and Schmidt defined triboluminescence (TL) as the emission of light produced by the fracturing of materials. In recent years, triboluminescent materials have been proposed for use as the active element in smart vehicular or static structural sensors. To sense damage, these materials would be embedded into the object. If damage occurs, the embedded triboluminescent material would give off visible light. This light could be transmitted by lightweight fiber optics or wireless detector to a computer-based detection system to warn occupants in real time that a significant impact event has occurred. In addition, the triboluminescent-based sensor could allow for real-time monitoring of both the magnitude and location of damage with influence to the host object. For this type of sensor to be adopted, it must operate in the harshest of environments, like space. However, in order for this concept to go from the laboratory to use in real world applications, the light emission from these functional materials must be quite bright so that inexpensive light detectors can be used. One of the brightest materials found thus far is europium tetrakis dibenzoylmethide triethylammonium (EuD₄TEA). This material has a luminescent emission yield that is so bright that it can be observed in daylight. Other lanthanides can be used to synthesize tetrakis compounds, each with different light emission characteristics. Tetrakis compounds can also be doped with other compounds to make them brighter. This talk will provide an overview into our research to synthesize and characterize tetrakis-based luminescent materials

CeO₂ has been intensively studied due to their notable physical and chemical properties. Applications in fields like catalysis, membranes, photocatalysis, fuel cells, gas sensors and medicine have been reported for this material. In catalysis, the ability of cerium ions to change their oxidation state, from Ce⁺³ to Ce^+4, explained its efficiency to store and transport oxygen. In this work, monodisperse CeO₂ microspheres were prepared by a microwave-assisted coprecipitation method, using ceric nitrate and formic acid. The reaction between the latter produced the precipitation of a white powder, with simultaneous emission of NO₂. X-ray diffraction results revealed the formation of cerium formate, at room temperature. Calcination of the precipiate from 300^oC produced single-phase CeO_2, which has cubic crystal structure. The morphology of the samples corresponds to monodisperse microspheres, with a size of approximately 1 μm. Observation by transmission electron microscopy of samples calcined at 500^oC show the formation of nanograins and abundant nanoporosity. In order to evaluate the synthesis parameters, the effect of the calcination temperature and the concentration of ceric nitrate were investigated. The results show that the increase on the calcination temperature caused the thermal decomposition of organic matter, which produced nanoporosity and microspheres with rough surface. On the other hand, the increase on the amount of ceric nitrate in solution, yielded larger particles with uneven size. Measurements of the gas sesning properties were performed on thick films made by depositing a suspension of the CeO₂ powder, on alumina substrates. Alternating current (AC) and direct current (DC) were used in this characterization. The films exhibited a reliable and quantitative detection of carbon monoxide from 250^oC. According to its electrical response, CeO₂ has the n-type semiconductor behavior; which is described by a decrease of the impedance in CO flows. Moreover, the increase on the applied frequency produced reproducible and more stable gas response partterns. The operating temperature played also a key role in the sensor performace, by decreasing the response time to about 10 s, at 300^oC. A comparison of these results with those previously reported for this material was done.

We have prepared many large families of 4f/3d materials in the search for compounds with unusual optical and magnetic properties. While long-range magnetic coupling has remained elusive, a number of unexpected structures and structure-property relationships have been observed. In particular, this talk will focus on origin of atypical optical properties such as thermochromism and the Alexandrite Effect in these families of compounds.

Bulk glass and crystalline materials are usually used for upconversion, but bulk materials are often not suitable for certain applications such as biological imaging or biolabelling. Upconversion materials on nanometer scales are desirable. However, synthesis of small nanocrystals that exhibit high monodispersibility and excellent upconversion properties remains a challenge.
In this work, nanoscale monodisperse rare-earth-metal-doped oxysulfate RE₂O₂SO₄ (RE = Y, Gd) hollow spheres were prepared for upconversion via a biomolecule-assisted hydrothermal method followed by calcination. The size and crystallinity of nanospheres, as well as the upconversion properties could be tuned by changing the experimental conditions such as surfactants, hydrothermal/annealing temperature, and chemical compositions. Upconversion properties revealed that the energy transfer between the sensitizer and the activator was strongly depended on the concentration of activator, the ratio of sensitizer to activator, chemical compositions and crystallinity of the hollow spheres.

Rare earth (RE) doped semiconductors have found applications as efficient luminescent materials, optical amplifiers, lasers, photon down- and up-conversion coatings among others. The interest on RE-doped materials is attributed to the narrow light emission spectral lines and long fluorescence lifetimes they present at wavelengths that are practically unaffected by the host environment. This is owing to the fact that the RE luminescence arises from the partially filled intra-4f shell electronic transitions. The 4f electrons are shielded from the 6s bonding electrons by the 5s and 5p shells. Consequently, the mixing with the 4f electrons is very low, and thus the effect on the 4f energy levels due to the RE bonding with the host matrix atoms is weak. However, the excitation mechanisms and subsequent recombination in RE ions are highly susceptible to host matrix electronic properties, such as defect states and the optical bandgap.

It is well established that the light emission intensity of RE doped materials can be enhanced after annealing treatments [1]. The study of the annealing behavior can help elucidate the underlying mechanism for producing optically active RE centers in a host [2,3]. However, the variation of the light emission intensity with thermal annealing treatments not only obeys the increase of luminesce centers. The annealing treatments also induce the variation of non-radiative recombination paths, thus modifying the light emission intensity by energy migration between luminescence centers and the host matrix. In the present work, Tb ions were incorporated into a-SiC:H thin films by RF-sputtering. After deposition, we performed thermal annealing treatments to induce the RE activation. We assessed the latter activation by the systematical analysis of the Urbach energy and by recording sub-bandgap excitation photoluminescence spectra for different Tb concentrations and annealing temperatures. Finally, in order to discriminate the effect on the luminescence of the Tb³⁺ activation from the variation of host-related recombination paths, we report the variation of the light emission intensity versus de Tb³⁺ concentration at different annealing temperatures. We describe the dependence of the intensity with the RE concentration by a rate equation model [4]. Three fitting parameters were extracted for each annealing temperature from our comprehensive dataset. These parameters are related to the RE activation degree, the interaction probability between RE ions, and the probability of energy loss through a host-related non-radiative recombination channel, respectively. Surprisingly, the results show a suppression of the self quenching by a thermally-induced reduction of the RE interaction probability.

References
[1] A. R. Zanatta, J. Phys. D: Appl. Phys. 42 (2009) 025109.
[2] A. Janotta et al, Phys. Rev. B 68 (2003) 165207.
[3] S. Abedrabbo et al, J. of Phys. D: Appl. Phys. 44 (2011) 315401.
[4] F. Benz et al, J. Lumin 137 (2013) 73.

Nanothermometry has indeed experienced an unprecedented growth over the past five years following, in most examples, the technological trends of sub-micron miniaturization, namely in the temperature mapping of microcircuits and microfluidic devices.¹ There is also a high demand of nanothermometry for biomedical uses, like the development of intracellular thermometers capable of accurate temperature determination in living cells.²
Despite promising progresses on the temperature mapping of microfluidic devices and living cells, precision control of fluid temperature by accounting for local temperature gradients, heat propagation and accurate temperature distributions have not yet been satisfactorily addressed. The major obstacle for this has been the unavailability of a thermometer with the following requirements: i) high temperature resolution (ca. 0.1 degree); ii) ratiometric temperature output; iii) high spatial resolution (