No description available.

Konstanz, Univ., Diplomarb., 1999.

Colloidal semiconductor quantum dots (QDs) have gained substantial interest as adjustable, bright and spectrally tunable fluorophores in the past decades. Besides their in-depth analyses in the scientific community, first industrial applications as color conversion and color enrichment materials were implemented. However, stability and processability are essential for their successful use in these and further applications. Methods to embed QDs into oxides or polymers can only partially solve this challenge. Recently, our group introduced the embedding of QDs into ionic salts, which holds several advantages in comparison to polymer or oxide-based counterparts. Both gas permeability and environmental-related degradation processes are negligible, making these composites an almost perfect choice of material. To evaluate this new class of QD-salt mixed crystals, a thorough understanding of the formation procedure and the final composites is needed. The present work is focused on embedding both aqueous-based and oil-based metal-chalcogenide QDs into several ionic salts and the investigations of their optical and chemical properties upon incorporation into the mixed crystals. QDs with well-known, reproducible and high-quality synthetic protocols are chosen as emissive species. CdTe QDs were incorporated into NaCl as host matrix by using the straightforward "classical" method. The resulting mixed crystals of various shapes and beautiful colors preserve the strong luminescence of the incorporated QDs. Besides NaCl, also borax and other salts are used as host matrices. Mercaptopropionic acid stabilized CdTe QDs can easily be co-crystallized with NaCl, while thioglycolic acid as stabilizing agent results in only weakly emitting powder-like mixed crystals. This challenge was overcome by adjusting the pH, the amount of free stabilizer and the type of salt used, demonstrating the reproducible incorporation of highest-quality CdTe QDs capped with thioglycolic acid into NaCl and KCl salt crystals. A disadvantage of the "classical" mixed crystallization procedure was its long duration which prevents a straightforward transfer of the protocol to less stable QD colloids, e.g., initially oil-based, ligand exchanged QDs. To address this challenge, the "Liquid-liquid-diffusion-assisted-crystallization" (LLDC) method is introduced. By applying the LLDC, a substantially accelerated ionic crystallization of the QDs is shown, reducing the crystallization time needed by one order of magnitude. This fast process opens the field of incorporating ligand-exchanged Cd-free QDs into NaCl matrices. To overcome the need for a ligand exchange, the LLDC can also be extended towards a two-step approach. In this modified version, the seed-mediated LLDC provides for the first time the ability to incorporate oil-based QDs directly into ionic matrices without a prior phase transfer. The ionic salts appear to be very tight matrices, ensuring the protection of the QDs from the environment. As one of the main results, these matrices provide extraordinary high photo- and chemical stability. It is further demonstrated with absolute measurements of photoluminescence quantum yields (PL-QYs), that the PL-QYs of aqueous CdTe QDs can be considerably increased upon incorporation into a salt matrix by applying the "classical" crystallization procedure. The achievable PL enhancement factors depend strongly on the PL-QYs of the parent QDs and can be described by the change of the dielectric surrounding as well as the passivation of the QD surface. Studies on CdSe/ZnS in NaCl and CdTe in borax showed a crystal-induced PL-QY increase below the values expected for the respective change of the refractive index, supporting the derived hypothesis of surface defect curing by a CdClx formation as one main factor for PL-QY enhancement. The mixed crystals developed in this work show a high suitability as color conversion materials regarding both their stability and spectral tunability. First proof-of-concept devices provide promising results. However, a combination of the highest figures of merit at the same time is intended. This ambitious goal is reached by implementing a model-experimental feedback approach which ensures the desired high optical performance of the used emitters throughout all intermediate steps. Based on the approach, a white LED combining an incandescent-like warm white with an exceptional high color rendering index and a luminous efficacy of radiation is prepared. It is the first time that a combination of this highly related figures of merit could be reached using QD-based color converters. Furthermore, the idea of embedding QDs into ionic matrices gained considerable interest in the scientific community, resulting in various publications of other research groups based on the results presented here. In summary, the present work provides a profound understanding how this new class of QD-salt mixed crystal composites can be efficiently prepared. Applying the different crystallization methods and by changing the matrix material, mixed crystals emitting from blue to the near infrared region of the electromagnetic spectrum can be fabricated using both Cd-containing and Cd-free QDs. The resulting composites show extraordinary optical properties, combining the QDs spectral tunability with the rigid and tight ionic matrix of the salt. Finally, their utilization as a color conversion material resulted in a high-quality white LED that, for the first time, combines an incandescent-like hue with outstanding optical efficacy and color rendering properties. Besides that, the mixed crystals offer huge potential in other high-quality applications which apply photonic and optoelectronic components.

Colloidal semiconductor nanocrystals (NCs) have attracted a great deal of interest as bright and stable chromophores for a variety of applications. Their superior physicochemical properties depend on characteristics of the inorganic core, as well as on the chemical nature and structure of the stabilizing organic ligand shell. To evaluate the promising material, a thorough knowledge of structure-property relationships is still demanded. The present work addresses this challenge to three water-soluble NC systems, namely thiol-capped CdTe, thiol-capped CdHgTe, and DNA-functionalized CdTe NCs with special emphasis on the investigation of structure, modification, and influence of the ligand shell. Remarkably, CdTe NCs show bright emission in the visible spectral region and can be synthesized in high quality directly in water. It was shown that the aqueous synthesis also facilitates the preparation of strongly near-infrared (NIR) emitting CdHgTe NCs. The current work presents a detailed study on parameters, by which the emission can be tuned, such as the growth time, the initial Cd : Hg ratio, and the choice of ligand. These insights contribute to the knowledge, which is essential for the design of highly emissive and long-term stable NIR emitting NCs. Further variations of the NC/ligand system include the modification of the ligand shell of CdTe NCs with oligonucleotides based on the strong attachment of DNA molecules to the NC. The successful functionalization of NCs with single-stranded DNA molecules is very promising for the precise and programmable assembly of NCs using DNA origami structures as templates. For both, functionality and optical properties, the surface chemistry of the NCs plays a substantial role and was subject to an extensive investigation. As there is no generally applicable technique to determine the amount of stabilizers and the structure of the ligand shell, the presented study is based on a combination of various methods particularly tailored to the analysis of water-soluble CdTe NCs capped by short-chain thiols. CdTe NCs served as a model system for the described analysis of the ligand shell, since they are thoroughly studied regarding synthesis and features of the core. Aiming for the quantification of thiols, a straightforward colorimetric assay, the Ellman\'s test, is for the first time introduced for the analysis of NCs. Accompanied by elemental analysis an approximate number of thiols per NC becomes accessible. Moreover, theoretical calculations were performed to estimate the amount of ligand that would cover the NC in a monolayer of covalently bound molecules. In contrast to these results, the experimental values point to a larger amount of thiols immobilized on the NC. Attempts to remove the ligand indicate the presence of Cd in the ligand shell and thermogravimetric studies show that the ligands are not loosely assembled in the ligand shell. The outstanding conclusion of these findings involves the presence of Cd-thiol complexes in the ligand shell. Further results unambiguously show that the amount of Cd-thiol complexes present in the NC solution strongly influences the concentration-dependent emission yield of the NCs. Additional studies dedicated to the considerable influence of the ligand shell highlight a strong effect of pH, NC concentration, type and purity of the solvent, and the number of precipitation steps on the emission of water-soluble semiconductor NCs. These substantial investigations emphasize the need to carefully control the conditions applied for handling, optical measurements, and application of NCs. In order to gain a deeper insight into the complex structure of the native ligand shell, techniques deliberately chosen for the in situ analysis were applied for thioglycolic acid-capped CdTe NCs. Information from dynamic light scattering (DLS) regarding the stability and the shell thickness are consistent with previous results showing a large ligand network on the NC surface and a decreasing stability of the NCs upon dilution. Importantly, nuclear magnetic resonance (NMR) spectroscopy allows for the distinction of bound and free ligands directly in solution and proves the presence of these species for the NCs studied. In particular, the results indicate that the ligands are not strongly bound to the NC core and that both, free and bound ligand species, consist of modified thiol molecules, such as Cd-thiol complexes. These findings support previous assumptions and allow to establish a distinct picture of the ligand shell of water-soluble semiconductor NCs. Further insights were obtained from small-angle X-ray scattering (SAXS), which facilitates the identification and the determination of the composition of NC core as well as ligand shell. Element-specific SAXS yields the final proof of the presence of Cd in the ligand shell. The model developed for the optimal fitting of the experimental scattering curves additionally confirms the findings from the other methods. In conclusion, the present work contributes to the challenging goal of a comprehensive knowledge of interactions between the NC core and the ligands. The fundamental development of a structural model of water-soluble CdTe NCs including information on stoichiometries is accomplished by the combination of the techniques presented and emphasizes the challenge to assign a clear border between the ligand shell and the Cd-thiol complexes in solution. Altogether, CdTe NCs capped by thioglycolic acid are best described by a crystalline core surrounded by a water-swollen Cd-thiolate shell that considerably affects the optical properties of the system. Notably, the results of the versatile study provide the opportunity to control the overall properties and to evaluate water-soluble semiconductor NCs for particular applications in photonics and optoelectronics.

Ziel dieser Doktorarbeit war es, den Einfluss von verschiedenen Parametern auf die spektroskopischen Eigenschaften von Lanthanid-basierten Aufkonversions-Materialien zu erforschen. Ein besonderer Fokus lag dabei auf hexagonalen Natrium-Yttrium-Tetrafluorid Kristallen, die mit dreifachgeladenen Yb und Er oder Tm Ionen kodotiert wurden. Eine wesentliche Voraussetzung für mögliche Anwendungen dieser Kristalle ist ein Verständnis aller ihrer wichtigen photophysikalischen Besonderheiten. Die erste Studie dieser Doktorarbeit untersuchte daher, wieso Nanokristalle viel weniger absorbierte in ausgesendete Photonen umwandeln als mikrokristalline Teilchen. Die Ergebnisse zeigten, dass man ungeschalte Kristalle aufgrund von Oberflächen-Lösch-Effekten in zwei Teile unterteilen kann, einen strahlenden Kern und eine Schale aus stark oder vollständig gelöschten oberflächennahen Lanthanid-Ionen, welche für Kristalle abnehmender Größe einen immer größeren Volumenanteil einnimmt. Die zweite Studie untersuchte exemplarisch, ob eine kompliziertere Partikelarchitektur, bestehend aus einem einfach-dotierten Er Kern und Yb als Schalenmaterial, diesen Effizienzverlust der Lumineszenz reduzieren kann. Die Ergebnisse zeigten jedoch, dass dies nicht der Fall ist. Eine weitere Studie untersuchte den Einfluss der Konzentration der Tm Ionen in Yb, Tm kodotierten Nanokristallen auf die spektroskopischen Eigenschaften dieser Materialien und zeigte, dass für eine maximale Emission im Lichtwellenbereich über 700 nm andere Tm Konzentrationen benötigt werden als für maximale Lichtemissionen in den unteren Lichtwellenbereichen. Die letzte Studie untersuchte den Einfluss eines zuvor berichteten Zersetzungsprozesses von exemplarisch ausgewählten Yb, Tm kodotierte Nanokristallen in wässrigen Dispersionen auf deren spektroskopische Eigenschaften. Mithilfe dieser Ergebnisse war es möglich, mehrere Emissionsbanden als Parameter für das Langzeit-Stabilitäts-Monitoring dieser Materialien zu identifizieren. / This PhD thesis investigated the influence of various parameters on the spectroscopic properties of so-called upconversion nanoparticles (UCNPs). A special emphasis was dedicated to hexagonal-phase sodium yttrium tetrafluoride crystals that were codoped with trivalent Yb and either Er or Tm ions. Such UCNPs can, however, experience no breakthrough in the field of UC nanotechnology before all of their important photophysical features are understood. The first study of this PhD thesis therefore investigated, why nanocrystalline upconverters with different surface chemistries convert less absorbed to emitted photons than their microcrystalline counterparts. The results revealed that upconverting crystals apparently have to be subdivided into two parts, with one being the luminescent core and the other being a completely dark shell that is quenched by surface effects and assumes an ever increasing volumetric content for small UCNPs. The second study exemplarily investigated, if a more complex particle nanostructure that consisted of a Er doped core, surrounded by a Yb doped shell, could overcome these efficiency losses, however, it concluded that it does not. Another study explored the influence of Tm doping concentrations of Yb, Tm codoped nanocrystals on their spectroscopic properties and concluded that different Tm doping concentrations are required for a maximum upconversion luminescence in the wavelength regions above 700 nm, than for the wavelength regions below that. The last study of this PhD thesis investigated the influence of a previously reported dissolution process of UCNPs in aqueous solutions on the spectroscopic properties of exemplarily chosen Yb, Tm codoped nanocrystals. These results were then utilized to identify several upconversion emission bands that can be used as a screening parameter for the long-term stability monitoring of UCNPs.

Colloidal semiconductor nanocrystals (NCs) have attracted a great deal of interest as bright and stable chromophores for a variety of applications. Their superior physicochemical properties depend on characteristics of the inorganic core, as well as on the chemical nature and structure of the stabilizing organic ligand shell. To evaluate the promising material, a thorough knowledge of structure-property relationships is still demanded. The present work addresses this challenge to three water-soluble NC systems, namely thiol-capped CdTe, thiol-capped CdHgTe, and DNA-functionalized CdTe NCs with special emphasis on the investigation of structure, modification, and influence of the ligand shell. Remarkably, CdTe NCs show bright emission in the visible spectral region and can be synthesized in high quality directly in water. It was shown that the aqueous synthesis also facilitates the preparation of strongly near-infrared (NIR) emitting CdHgTe NCs. The current work presents a detailed study on parameters, by which the emission can be tuned, such as the growth time, the initial Cd : Hg ratio, and the choice of ligand. These insights contribute to the knowledge, which is essential for the design of highly emissive and long-term stable NIR emitting NCs. Further variations of the NC/ligand system include the modification of the ligand shell of CdTe NCs with oligonucleotides based on the strong attachment of DNA molecules to the NC. The successful functionalization of NCs with single-stranded DNA molecules is very promising for the precise and programmable assembly of NCs using DNA origami structures as templates. For both, functionality and optical properties, the surface chemistry of the NCs plays a substantial role and was subject to an extensive investigation. As there is no generally applicable technique to determine the amount of stabilizers and the structure of the ligand shell, the presented study is based on a combination of various methods particularly tailored to the analysis of water-soluble CdTe NCs capped by short-chain thiols. CdTe NCs served as a model system for the described analysis of the ligand shell, since they are thoroughly studied regarding synthesis and features of the core. Aiming for the quantification of thiols, a straightforward colorimetric assay, the Ellman\'s test, is for the first time introduced for the analysis of NCs. Accompanied by elemental analysis an approximate number of thiols per NC becomes accessible. Moreover, theoretical calculations were performed to estimate the amount of ligand that would cover the NC in a monolayer of covalently bound molecules. In contrast to these results, the experimental values point to a larger amount of thiols immobilized on the NC. Attempts to remove the ligand indicate the presence of Cd in the ligand shell and thermogravimetric studies show that the ligands are not loosely assembled in the ligand shell. The outstanding conclusion of these findings involves the presence of Cd-thiol complexes in the ligand shell. Further results unambiguously show that the amount of Cd-thiol complexes present in the NC solution strongly influences the concentration-dependent emission yield of the NCs. Additional studies dedicated to the considerable influence of the ligand shell highlight a strong effect of pH, NC concentration, type and purity of the solvent, and the number of precipitation steps on the emission of water-soluble semiconductor NCs. These substantial investigations emphasize the need to carefully control the conditions applied for handling, optical measurements, and application of NCs. In order to gain a deeper insight into the complex structure of the native ligand shell, techniques deliberately chosen for the in situ analysis were applied for thioglycolic acid-capped CdTe NCs. Information from dynamic light scattering (DLS) regarding the stability and the shell thickness are consistent with previous results showing a large ligand network on the NC surface and a decreasing stability of the NCs upon dilution. Importantly, nuclear magnetic resonance (NMR) spectroscopy allows for the distinction of bound and free ligands directly in solution and proves the presence of these species for the NCs studied. In particular, the results indicate that the ligands are not strongly bound to the NC core and that both, free and bound ligand species, consist of modified thiol molecules, such as Cd-thiol complexes. These findings support previous assumptions and allow to establish a distinct picture of the ligand shell of water-soluble semiconductor NCs. Further insights were obtained from small-angle X-ray scattering (SAXS), which facilitates the identification and the determination of the composition of NC core as well as ligand shell. Element-specific SAXS yields the final proof of the presence of Cd in the ligand shell. The model developed for the optimal fitting of the experimental scattering curves additionally confirms the findings from the other methods. In conclusion, the present work contributes to the challenging goal of a comprehensive knowledge of interactions between the NC core and the ligands. The fundamental development of a structural model of water-soluble CdTe NCs including information on stoichiometries is accomplished by the combination of the techniques presented and emphasizes the challenge to assign a clear border between the ligand shell and the Cd-thiol complexes in solution. Altogether, CdTe NCs capped by thioglycolic acid are best described by a crystalline core surrounded by a water-swollen Cd-thiolate shell that considerably affects the optical properties of the system. Notably, the results of the versatile study provide the opportunity to control the overall properties and to evaluate water-soluble semiconductor NCs for particular applications in photonics and optoelectronics.

Colloidal semiconductor quantum dots (QDs) have gained substantial interest as adjustable, bright and spectrally tunable fluorophores in the past decades. Besides their in-depth analyses in the scientific community, first industrial applications as color conversion and color enrichment materials were implemented. However, stability and processability are essential for their successful use in these and further applications. Methods to embed QDs into oxides or polymers can only partially solve this challenge. Recently, our group introduced the embedding of QDs into ionic salts, which holds several advantages in comparison to polymer or oxide-based counterparts. Both gas permeability and environmental-related degradation processes are negligible, making these composites an almost perfect choice of material. To evaluate this new class of QD-salt mixed crystals, a thorough understanding of the formation procedure and the final composites is needed. The present work is focused on embedding both aqueous-based and oil-based metal-chalcogenide QDs into several ionic salts and the investigations of their optical and chemical properties upon incorporation into the mixed crystals. QDs with well-known, reproducible and high-quality synthetic protocols are chosen as emissive species. CdTe QDs were incorporated into NaCl as host matrix by using the straightforward "classical" method. The resulting mixed crystals of various shapes and beautiful colors preserve the strong luminescence of the incorporated QDs. Besides NaCl, also borax and other salts are used as host matrices. Mercaptopropionic acid stabilized CdTe QDs can easily be co-crystallized with NaCl, while thioglycolic acid as stabilizing agent results in only weakly emitting powder-like mixed crystals. This challenge was overcome by adjusting the pH, the amount of free stabilizer and the type of salt used, demonstrating the reproducible incorporation of highest-quality CdTe QDs capped with thioglycolic acid into NaCl and KCl salt crystals. A disadvantage of the "classical" mixed crystallization procedure was its long duration which prevents a straightforward transfer of the protocol to less stable QD colloids, e.g., initially oil-based, ligand exchanged QDs. To address this challenge, the "Liquid-liquid-diffusion-assisted-crystallization" (LLDC) method is introduced. By applying the LLDC, a substantially accelerated ionic crystallization of the QDs is shown, reducing the crystallization time needed by one order of magnitude. This fast process opens the field of incorporating ligand-exchanged Cd-free QDs into NaCl matrices. To overcome the need for a ligand exchange, the LLDC can also be extended towards a two-step approach. In this modified version, the seed-mediated LLDC provides for the first time the ability to incorporate oil-based QDs directly into ionic matrices without a prior phase transfer. The ionic salts appear to be very tight matrices, ensuring the protection of the QDs from the environment. As one of the main results, these matrices provide extraordinary high photo- and chemical stability. It is further demonstrated with absolute measurements of photoluminescence quantum yields (PL-QYs), that the PL-QYs of aqueous CdTe QDs can be considerably increased upon incorporation into a salt matrix by applying the "classical" crystallization procedure. The achievable PL enhancement factors depend strongly on the PL-QYs of the parent QDs and can be described by the change of the dielectric surrounding as well as the passivation of the QD surface. Studies on CdSe/ZnS in NaCl and CdTe in borax showed a crystal-induced PL-QY increase below the values expected for the respective change of the refractive index, supporting the derived hypothesis of surface defect curing by a CdClx formation as one main factor for PL-QY enhancement. The mixed crystals developed in this work show a high suitability as color conversion materials regarding both their stability and spectral tunability. First proof-of-concept devices provide promising results. However, a combination of the highest figures of merit at the same time is intended. This ambitious goal is reached by implementing a model-experimental feedback approach which ensures the desired high optical performance of the used emitters throughout all intermediate steps. Based on the approach, a white LED combining an incandescent-like warm white with an exceptional high color rendering index and a luminous efficacy of radiation is prepared. It is the first time that a combination of this highly related figures of merit could be reached using QD-based color converters. Furthermore, the idea of embedding QDs into ionic matrices gained considerable interest in the scientific community, resulting in various publications of other research groups based on the results presented here. In summary, the present work provides a profound understanding how this new class of QD-salt mixed crystal composites can be efficiently prepared. Applying the different crystallization methods and by changing the matrix material, mixed crystals emitting from blue to the near infrared region of the electromagnetic spectrum can be fabricated using both Cd-containing and Cd-free QDs. The resulting composites show extraordinary optical properties, combining the QDs spectral tunability with the rigid and tight ionic matrix of the salt. Finally, their utilization as a color conversion material resulted in a high-quality white LED that, for the first time, combines an incandescent-like hue with outstanding optical efficacy and color rendering properties. Besides that, the mixed crystals offer huge potential in other high-quality applications which apply photonic and optoelectronic components.

Since C^C* cyclometalated Pt(II) complexes with N-heterocyclic carbene (NHC) ligands have been identified as potential emitter materials in organic light-emitting devices (OLEDs), very promising results regarding quantum yields, colour and stability have been presented. Herein, we report on four nitrile substituted complexes with a chelating NHC ligand (1-(4-cyanophenyl)-3-isopropyl-1H-benzo[d]imidazole or 4-(tert-butyl)-1-(4-cyanophenyl)-3-methyl-1H-imidazole) and a bidentate monoanionic auxiliary ligand (acetylacetone or dimesitoylmethane). The complexes have been fully characterized including extensive 2D NMR studies (COSY, HSQC, HMBC, NOESY, 195Pt NMR), three of them also by solid-state structures. Photophysical measurements in amorphous PMMA films and pure emitter films at room temperature reveal the impact of the mesityl groups in the auxiliary ligand, which led to a significant increase of the quantum yield, while the decay lifetimes decreased. The electron withdrawing nitrile groups shift the emission towards blue colour coordinates.

Halogenhaltigen atmosphärischen Spurengasen wird eine Schlüsselrolle bei lokalen, regionalen und globalen Veränderungen der Erdatmosphäre zugesprochen. Die Photolyse dieser Stoffe durch ultraviolette Strahlung der Sonne führt zum Eintrag reaktiver Atome und Radikale in die Atmosphäre. Dies betrifft vor allem den stratosphärischen Ozonabbau, der nach heutigem Kenntnisstand durch Reaktionszyklen katalysiert wird und bei dem die Photolyse halogenhaltiger Spurengase an zentraler Stelle beteiligt ist. Im Rahmen dieser Arbeit wurden primäre Photoprozesse halogenhaltiger atmosphärischer Spurengase in Laborexperimenten charakterisiert. Dabei bestand das Ziel in der Bestimmung von primären Quantenausbeuten und Verzweigungsverhältnissen konkurrierender Photolysekanäle. Hierfür wurde ein neuartiger experimenteller Ansatz genutzt, der die folgenden Komponenten beinhaltet: (a) Photolyse der Spurengase durch gepulste ultraviolette Laserstrahlung unter stoßfreien Bedingungen, (b) Ein-Photon-Ionisation der neutralen Photolyseprodukte mittels durchstimmbarer Vakuum-UV-Strahlung und (c) Identifizierung der gebildeten Photoprodukt-Ionen durch Flugzeit-Massenspektrometrie. Dieser Ansatz ermöglicht die Identifizierung aller gebildeten Photolyseprodukte, die Bestimmung des Anregungszustandes der Photoprodukte durch Ausnutzung von Autoionisationsprozessen und die Bestimmung von Verzweigungsverhältnissen und Quantenausbeuten konkurrierender Photoprozesse.Im einzelnen wurden die atmosphärischen Spurengase Chlormonoxid (ClO) und sein Dimer (Cl2O2), Nitrylchlorid (ClNO2), Brommonoxid (BrO) und Bromnitrat (BrONO2) untersucht. Die Resultate können zur Verfeinerung atmosphärischer Modelle und deren Prognosefähigkeit beitragen. Insbesondere können die Ergebnisse auch einen Beitrag zur zuverlässigen Interpretation von Ergebnissen aus Feldstudien leisten.

Nanopartikel auf Basis von NaYF4 erfreuen sich großer Beliebtheit durch ihre vielseitigen Einsatzmöglichkeiten. Durch die Dotierung mit Ytterbium und Erbium im Wirtsgitter ist es beispielsweise möglich, niedrigenergetisches Infrarotlicht in höher-ergetisches, sichtbares Licht umzuwandeln. Zudem lässt sich NaYF4 auch im Nanometermaßstab präparieren, sodass ein Einsatz in Zellen oder lebenden Organismen möglich ist, wo die zur Anregung verwendete infrarote Strahlung ohne Probleme das Gewebe durchdringen kann. Zu Beginn dieser Arbeit zeigten aufwärtskonvertierende Nanomaterialien wie NaYF4 :Yb,Er jedoch auch nach Umhüllen mit einer inaktiven Schale aus undotiertem NaYF4 nur sehr geringe Lumineszenz-Quantenausbeuten und kurze Energieniveau-Lebenszeiten. Im Rahmen dieser Arbeit wurde die Synthesemethode zur Herstellung von aufwärtskonvertierenden NaYF4 -Nanopartikeln durch den Einsatz neuer Eduktmaterialien modifiziert und die Auswirkung der Modifikationen auf die Partikeleigenschaften näher untersucht. So konnte gezeigt werden, dass durch den Einsatz einer alternativen Fluoridquelle (NaHF2) Partikel mit sehr engen Partikelgrößenverteilungen hergestellt werden können. Jedoch zeigte sich auch, dass die mit NaHF2 präparierten Partikel sich nicht mit einer Schale aus undotiertem NaYF4 umhüllen ließen. Im zweiten Teil dieser Arbeit wurde daher der Fokus auf die Verbesserung der optischen Eigenschaften gelegt. Durch die Verwendung von getrockneten Lösungsmitteln und wasserfreien Seltenerdacetaten, sowie NH4F als Fluoridquelle gelang es erstmals, aufwärtskonvertierende Kern/Schale-Nanopartikel (<50 nm) mit einer sehr hohen Lumineszenz-Quantenausbeute, ähnlich dem des makrokristallinen Referenzmaterials, herzustellen. Auch bei sehr kleinen Kern/Schale-Partikeln (≤15 nm) konnten Quantenausbeuten erzielt werden, die nur um einen Faktor 3-4 niedriger sind als beim Referenzmaterial. Dabei zeigte sich durch die Messung der Energieniveau-Lebenszeiten, dass die größten Verlustprozesse durch die Yb3+ Emission bei 940 nm auftraten und diese durch aufbringen einer Schale unterbunden werden konnten.