NaYF4:Yb,Er Upconversion Nanocrystals: Investigating Energy Loss Processes for the Systematic Enhancement of the Luminescence Efficiency

Aufkonvertierende (upconverting; UC) Nanomaterialien bilden eine neue Klasse nichtlinearer lumineszenter Reporter, die nah-infrarotes (NIR) Anregungslicht in Photonen von höherer Energie umwandeln. Das effizienteste bekannte UC-System bildet hierbei β-NaYF4: 20%Yb(III), 2%Er(III) mikrokristallines Bulkmaterial, für welches UC-Quantenausbeuten (ΦUC) von 10 % berichtet werden, während ΦUC von Nanokristallen (nanocrystals; NC) um mehrere Größenordnungen niedriger sein können. Um die Effizienz von UC-Nanomaterialien zu erhöhen, werden NC üblicherweise mit inerten Schalen versehen. In dieser Arbeit werden mehrere verschiedene Bulkmaterialien spektroskopisch untersucht, um ein Vergleichsmaterial auszuwählen, das als Maßstab für alle folgenden, vergleichbaren Messungen an NC dient. Die Oberfläche von ultrakleinen (3.7±0.5) nm NC wird mit Schalen von bis zu 10 nm Dicke versehen, um die optimale Schalendicke für vollständige Oberflächenpassivierung zu identifizieren, allerdings weisen die Ergebnisse auf eine mögliche Kern-Schale-Durchmischung hin. In einer zweiten Studie werden die unterschiedlichen Dotanden, Er(III) und Yb(III), auf ihre optischen Eigenschaften sowie die Einflüsse von Energietransfer (ET) und von ihrer Umgebung spektroskopisch untersucht. Dabei kann klar zwischen Oberflächeneffekten und oberflächenunabhängigen Volumeneffekten unterschieden werden. Die Ergebnisse werden durch ein einfaches Monte-Carlo-Modell gestützt, durch das die größen- und leistungsdichte-(P-)abhängigen Populierungsdynamiken der strahlenden Banden von Er(III) vorhergesagt werden können. Zuletzt werden durch eine verbesserte Synthesemethode UCNC mit stark verbesserten Lumineszenzeigenschaften hergestellt, mit denen bei vergleichsweise niedrigen P die gleichen ΦUC wie beim Bulkmaterial erreicht werden. Dies liefert einen Einblick in vielfältige Anwendungsmöglichkeiten für UCNC.Upconversion (UC) nanomaterials are an emerging new class of non-linear luminescent reporters which convert near-infrared (NIR) excitation light into higher-energy photons. The most efficient known UC material is the β-NaYF4: 20%Yb(III), 2%Er(III) bulk (microcrystalline) phosphor with reported UC quantum yields (ΦUC) of 10 %, while ΦUC of nanocrystals (NC) can be several orders of magnitude lower. Strategies to improve the efficiency of UC nanomaterials include surface passivation with inert shells. In this work, several different bulk materials are compared to select one benchmark material for comparisons with NC analyzed with the same measurement techniques. The surface of ultrasmall (3.7 ± 0.5) nm NC is coated with inert shells of up to 10 nm thickness to identify an optimal shell thickness for complete surface passivation, but the results suggest core-shell intermixing. To distinguish between the different dopant ions, Er(III) and Yb(III), and the effect of energy transfer (ET) in a second study, single- and co-doped UCNC are investigated spectroscopically and the influence of their environment is determined thoroughly. Herein, a clear distinction between surface-related and surface-independent, volume-related effects is achieved and the results are emphasized by the use of a simple random walk model which accurately predicts size- and power density (P)-dependent population dynamics of the emissive bands of Er(III). Finally, utilizing an improved synthesis technique, UCNC with enhanced luminescence properties are produced, reaching the same ΦUC as the benchmarked bulk material at reasonably low P, providing an insight into numerous possible applications of UCNC

Coordination mechanism of cyanine dyes on the surface of core@active shell β-NaGdF4_{4}:Yb3+^{3+},Er3+^{3+} nanocrystals and its role in enhancing upconversion luminescence

The sensitization of lanthanide-doped upconversion nanocrystals (UCNCs) using organic dyes with a broad and intense optical absorption is an interesting approach for efficient excitation-energy harvesting and enhancing the upconversion luminescence of such UCNCs. In this work, an ultrasmall (∼6.5 nm in diameter) β-NaGdF$_{4}$:Yb$^{3+}$,Er$^{3+}$ core and related core@shell UCNCs were sensitized using six NIR-excitable cyanine dyes with a wide range of functional groups and optical properties. The greatest UC enhancement of 680-times was observed for the conjugate between the Cy 754 dye and β-NaGdF$_{4}$:Yb$^{3+}$,Er$^{3+}$@NaGdF$_{4}$:10%Yb$^{3+},30$^{3+} core@shell UCNCs excited using a 754 nm laser. The enhancement was estimated relative to NaGdF$_{4}$:Yb$^{3+}$,Er$^{3+}$@NaGdF$_{4}$:10%Yb$^{3+},30$^{3+} core@shell UCNCs capped with oleic acid and excited using a similar intensity (75 W cm$^{-2}$) of a 980 nm laser. UC intensity measurements for identical dye-sensitized UCNCs carried out in methanol and in deuterated methanol under argon, as well as in air, allowed us to reveal the connection of the dye triplet states with UCNC sensitization as well as of the hydroxyl groups with quenching of the excited states of lanthanide ions. For UCNCs dispersed in methanol, the strong quenching UC luminescence was always observed, including core@shell UCNCs (with a shell of ∼2 nm). A strong influence of the triplet states of the dyes was observed for the two dyes Cy 754 and Cy 792 that bind firmly to UCNCs and allow the distances between the dye and the UCNC to be reduced, whereas the contribution of this sensitization pathway is very insignificant for Cy 740 and Cy 784 dyes that bind weakly to UCNCs

Pattern overlap implies runaway growth in hierarchical tile systems

We show that in the hierarchical tile assembly model, if there is a producible assembly that overlaps a nontrivial translation of itself consistently (i.e., the pattern of tile types in the overlap region is identical in both translations), then arbitrarily large assemblies are producible. The significance of this result is that tile systems intended to controllably produce finite structures must avoid pattern repetition in their producible assemblies that would lead to such overlap. This answers an open question of Chen and Doty (SODA 2012), who showed that so-called "partial-order" systems producing a unique finite assembly *and" avoiding such overlaps must require time linear in the assembly diameter. An application of our main result is that any system producing a unique finite assembly is automatically guaranteed to avoid such overlaps, simplifying the hypothesis of Chen and Doty's main theorem

The Leptoquark Hunter's Guide: Pair Production

Leptoquarks occur in many new physics scenarios and could be the next big discovery at the LHC. The purpose of this paper is to point out that a model-independent search strategy covering all possible leptoquarks is possible and has not yet been fully exploited. To be systematic we organize the possible leptoquark final states according to a leptoquark matrix with entries corresponding to nine experimentally distinguishable leptoquark decays: any of {light-jet, b-jet, top} with any of {neutrino, $e/\mu$, $\tau$}. The 9 possibilities can be explored in a largely model-independent fashion with pair-production of leptoquarks at the LHC. We review the status of experimental searches for the 9 components of the leptoquark matrix, pointing out which 3 have not been adequately covered. We plead that experimenters publish bounds on leptoquark cross sections as functions of mass for as wide a range of leptoquark masses as possible. Such bounds are essential for reliable recasts to general leptoquark models. To demonstrate the utility of the leptoquark matrix approach we collect and summarize searches with the same final states as leptoquark pair production and use them to derive bounds on a complete set of Minimal Leptoquark models which span all possible flavor and gauge representations for scalar and vector leptoquarks.Comment: 19 pages + references and appendices, 18 figures, 15 tables. Added references, fixed typo