The carrier capture and relaxation of Type II ZnTe/ZnSe quantum dots(QDs) were investigated with ultrafast photoluminescence upconversion. We found that carrier relaxation of QDs under Volmer-Weber(VW) growth mode exhibits faster decay and rise than that of QDs under Stranski-Krastanow(SK) growth mode due to the wetting layer in SK growth mode provides as a pathway for carriers to diffuse and migrate from large(small) to small (larger) QDs. The wetting layer level was found by analyze the decay time of PL with different wavelength and temperature. The PL of VW mode and SK mode by using 532nm Nd-YAG laser also prove the existence of wetting layer. We interpret our results of VW mode in terms of Auger process with large carrier density.

The hot carrier dynamics of Zn0.91Cd0.09Se/ZnSe multi-quantum wells were studied using the femtosecond time-resolved photoluminescence upconversion technique. The carrier cooling behavior was investigated for different compositions at various lattice temperatures. The hot carriers generated photoexcitation by 405nm Ti:sapphire laser pulses release their excess energy primarily through carrier-LO-phonon interaction. As the excess energy reduce to the amount that lower than the energy of LO phonon, the excess energy was released by carrier-TA-phonon scattering before radiative recombination occurs. We have determined the scattering times of carrier-LO-phonon scattering at different lattice temperatures. No hot phonon effects was found at low photoexcited carrier density. The dependence of photoluminescence lifetime on wavelength was also discussed.

Cancer currently represents one of the greatest burdens on human health in the world, claiming in excess of 7 million lives a year worldwide. Advances in both our understanding of the disease as well as our ability to diagnose it before it has had a chance to metastasize will lead to a reduction in its burden on society. To these ends, optical imaging techniques are particularly attractive. The ability to resolve cellular details noninvasively is paramount to improved cancer detection and to research on diseased tissue and cells. Lanthanoid nanoparticles, a group of photoluminescent contrast agents developed within the last two to three decades, have numerous unique optical properties that enable their use in improved and novel optical techniques. They possess large Stokes and anti-Stokes shifts, sharp electronic transitions, long luminescence lifetimes, and exceptional photostability. For these reasons, they are a good choice for biomedical applications that benefit from low background fluorescence or long illumination times. The major goal of the research presented in this dissertation was to synthesize functional lanthanoid nanoparticles for optical imaging modalities, and to explore their potential uses in a variety of biomedical applications. To this end, the research can be broken up into three specific aims. The first aim was to successfully and reproducibly synthesize downconverting and upconverting lanthanoid nanoparticles, and to functionalize these nanoparticles for use in optical techniques that would aid in the research and diagnosis of cancer. The second aim was to conduct a thorough investigation of the optical properties of these nanoparticles, and the third aim was to explore the utility of these nanoparticles in a variety of biomedical applications. First, both downconverting and upconverting lanthanoid nanoparticles were synthesized using several different methods, resulting in nanoparticles of varying size and surface functionality. Novel methods were employed to improve the utility of these nanoparticles for specific applications, including the incorporation of a mixed surface ligand population in downconverting lanthanoid nanoparticles and the use of a biomimetic surface coating to render upconverting nanoparticles water dispersible. These coated particles were further functionalized by the addition of folic acid and an antibody for epidermal growth factor receptor, both of which bind to cell surface receptors overexpressed in a number of cancers. Second, the spectral properties of lanthanoid nanoparticles were explored in detail, with special attention paid to many of the unique optical properties of upconverting lanthanoid nanoparticles. This included the discovery of one such unique property, the excitation frequency dependent emission of NaYF₄ nanocrystals codoped with Yb³⁺ and Er³⁺. Third, lanthanoid nanoparticles were used as contrast agents in a number of biomedical applications, including the development of a homogenous assay based on diffusion enhanced luminescence resonance energy transfer, a wide-field luminescence lifetime microscope, and a super resolution microscope based on the aforementioned excitation frequency dependent emission of NaYF₄:Yb³⁺,Er³⁺ nanoparticles. Specific binding of functionalized upconverting lanthanoid nanoparticles was investigated with laser scanning multiphoton microscopy, and an image processing technique was developed to overcome the challenge of working with long lived luminescent contrast agents using this imaging modality.

The present work reports the study of noncoherent photon upconversion (NCPU) via triplet-triplet annihilation (TTA) in polymer systems. This upconversion mechanism has application in photovoltaic devices through the utilization of sub-band gap photons for potentially enhanced power conversion efficiencies. First, homomolecular TTA was studied in zinc tetraphenylporphyrin (ZnTPP) in polymer matrices. Here, ZnTPP acts as both the sensitizer and upconverting emitter as TTA yields an S2 excited porphyrin. Use of poly(methyl methacrylate) (PMMA) as the host polymer demonstrates aggregation-driven upconverted fluorescence (UC) by TTA (TTA-UC). The dye-loading ratio of the precursor solution was varied, controlling the degree of pre-aggregation. Power-dependence studies of the champion film demonstrated that TTA-UC is occurring toward the strong annihilation kinetic limit. A sub-linear dependence of upconverted fluorescence on film thickness was observed in this system. The ZnTPP study was extended to polymers possessing low glass transition temperatures, representing molecular diffusion-driven TTA-UC. Upconverted fluorescence was not observed in ZnTPP in a polyurethane (PU) matrix, likely due to coordination of the PU to the axial position of the Zn2+ ion. Low intensity NCPU via homomolecular TTA was observed in ZnTPP in a poly(ethylene glycol) (PEG) matrix, but the kinetic limit was not determined due to film photodegradation. Dye-loading studies revealed that porphyrin self-quenching was evident at low dye concentrations. Likely reasons for the low upconverted fluorescence intensities realized are this self-quenching and the possibility of PEG coordination to the Zn2+ ion, though it is believed self-quenching is the dominant parasitic effect. Strategies to determine the effect and extent of polymer coordination to the Zn2+ ion are discussed. The study of polymer-based NCPU is extended to a pair of macromolecules, each containing a single ruthenium tris(bipyridine) (Ru(bpy)3) core and multiple pendant arms, which in turn, each contain eight 9,10-diphenylanthracene (DPA) moieties. A power-dependence study of NCPU in this system is reported, and TTA-UC in the weak annihilation kinetic limit was observed. Upconverted fluorescence quantum yields vary linearly with excitation power in both polymers, consistent with the observed kinetic limit. Stern-Volmer experiments have compared the quenching of Ru(bpy)3 phosphorescence (Ph) by monomeric and polymeric DPA. These data show an enhancement in quenching rate constant for the DPA polymer (pDPA). Kinetic analysis of the Ru-DPA polymers has revealed that the energy scheme realized in this system is intrachain TTET from Ru(bpy)3 core to DPA emitter followed by interchain TTA between excited DPA moieties. Low intensity upconverted fluorescence is observed in Ru-DPA containing thin films. Based on the results presented, the requirements of future photophysically-active polymers are discussed with regards to meaningful application in photovoltaics.

The widely available charge coupled device (CCD) and lately CMOS imaging devices have created many applications on a mass scale. However these devices are limited to wavelengths shorter than about 1 μm. Hybrid photon upconversion devices have been developed recently. The end goal is to achieve an alternative technology for imaging in the 1.5-μm region. The hybrid upconversion idea relies on the integration of a photodetector and an organic light emitting diode (OLED). Under a forward bias for the OLED, the detected signal in the Photodetector is sent to the OLED, resulting in an increase in emission at a shorter wavelength and therefore achieving optical up conversion. An OLED device can simply consists of a stack of anode, a hole transport layer (HTL), a light-emitting layer, an electron transport layer (ETL), a cathode layer, and it typically emits visible light. As each organic molecule is a topologically perfect structure, the growth of each organic layer does not require “lattice matching”, which has been the fundamental limit for inorganic semiconductor monolithic devices. Thus, integration of an OLED with a III–V compound semiconductor is a highly feasible and desirable approach for making low-cost, large-area, potentially high efficiency devices. This thesis addresses the physics, fabrication and characterization of hybrid near infrared optical upconverters and their imaging application. Firstly, one novel hybrid optical upconverter structure is presented, which substantially improves the upconversion efficiency by embedding a metal mirror. Efficient carrier injection from the inorganic photodetector to the OLED is achieved by the insertion of a thin Au metal embedded mirror at the inorganic-organic interface. The upconversion efficiency was improved by more than 100%. Secondly, the overall upconversion efficiency can be increased significantly, by introducing a gain mechanism into the Photodetector section of the upconverter. A promising option to implement gain is a heterojunction phototransistor (HPT). An InGaAs-InP HPT was integrated with an OLED, which converts 1.5-μm Infrared light to visible light with a built-in electrical gain (~94). The overall upconversion efficiency was improved to be 1.55 W/W. Thirdly, this upconversion approach can also be used to realize a pixelless imaging device. A pixelless hybrid upconversion device consists of a large-area single-mesa device, where the OLED output is spatially correlated with the input 1.5-µm scene. Only the parts receiving incoming photons will emit output photons. To achieve this functionality, photon-generated carriers must flow mainly along the layer-growth direction when injected from the InGaAs light absorption layer into OLED light emission layer. A prototype of pixelless imaging device based on an i-In0.53Ga0.47As/C60 heterojunction was demonstrated, which minimized lateral current spreading. This thesis presents experimental results of the first organic/inorganic hybrid optical amplifer and the first hybrid near infrared imaging device.

This work is focused on examining photoluminescent properties of InAs quantum dots (QDs) on GaAs substrate covered by GaAs1-xSbx strain reducing capping layer (SRL) prepared by Stranski-Krastanow method. We measured luminescence decay time of two samples with different concentration of Sb in this layer. We investigated the influence of temperature, intensity and wavelength of the excitation pulse on the luminescent decay time. We also compared the properties of the samples after excitation by 760 nm pulse and 850 nm pulse - the former one is energetically above the substrate band gap; in the second case we excited only the QDs and the wetting layer (WL). We consequently derived recombination and relaxation processes occurring inside InAs QDs and also the transport of charge carriers from the substrate and the WL into QDs. One part of this diploma thesis was to learn about the methods of measuring ultrafast photoluminescence and build the experimental set-up.

In dieser Doktorarbeit wurden mit unterschiedlichen Ionen dotierte Nanokristalle untersucht: Zum einen lanthanoid-dotierte LiYF4-Nanokristalle sowie Nanokristalle weiterer LiSEF4-Materialien (SE = Gd-Lu) und zum anderen Nanopartikel aus antimondotiertem Zinndioxid, einem n-leitenden Oxid. Ein zentrales Ergebnis des ersten Teils der Dissertation war die Synthese von LiYF4:Yb,(Er)/LiYF4-Kern/Schale-Nanokristallen, die eine Zerfallszeit der Yb3+-Emission aufweisen, die Czochralski-Laserkristallen von LiYF4:Yb sehr ähnlich ist. Diese Ähnlichkeit bestätigt die hohe Qualität des neu entwickelten Kern/Schale-Syntheseverfahrens. Die Co-Dotierung der LiYF4-Kern-Partikel mit Yb3+ und Er3+ führte zu einer Upconversion-Emission der Partikel, die für ein System dieser Größe und ohne zusätzliche Dotierungsionen die bisher höchste Quantenausbeute von 1,25 % zeigt. Eine hohe Qualität der LiYF4-Nanopartikel hinsichtlich ihrer schmalen Partikelgrößenverteilung und hohen Form-Homogenität konnte über die Bildung von kolloidalen Kristallen bestätigt werden. Die Herstellung weiterer LiSEF4 Nanokristalle mit den Seltenen Erden (SE) = Lu-Gd führte zu einer systematischen Zunahme der Größe der Partikel von LiLuF4 bis LiTbF4. Die Synthese von LiSEF4-Partikeln mit SE = Gd oder mit Seltenenerd-Ionen mit noch höheren Ionenradien war mit diesem Syntheseverfahren allerdings nicht möglich. Daher wurde das eigenständig entwickelte Kern/Schale-Verfahren zur Umhüllung von LiYF4-Kern-Partikeln mit LiYF4 auf LiGdF4 als Schale übertragen und die Bildung einer LiGdF4-Schale auf LiYF4-Kern-Partikeln spektroskopisch bestätigt. Im zweiten Teil dieser Arbeit wurden im Rahmen eines BMBF-Projektes: „Aktiv steuerbare elektrochrome Gradientenfilter für die Anwendung in optischen Bildaufnahmesystemen“ antimon-dotierte Zinndioxid-Nanopartikel-Suspensionen und -Pasten hergestellt, um n-Leitfähigkeit des SnO2 zu erreichen. Für die Verwendung im ektrochromen Gradientenfilter wurden mittels Rakel- oder Druckprozess nanoporöse Dünnschichtelektroden generiert. Über die Kopplung verschiedener Farbstoffmoleküle konnte der Kontrast des Gradientenfilters in Abhängigkeit des elektro-chemischen Potentials beliebig variiert werden.

The synthesis and surface modification of upconverting nanoparticles (UCNPs) composed of a host lattice NaYF4 doped with sensitizers (Yb3+, Nd3+) and luminescent emitters (Er3+) were investigated for potential integration in biological applications.The fascination of NaYF4: Nd3+, Yb3+, Er3+ upconverting nanoparticles derives from their capacity to be excited in the biologically transparent window (650-950 nm) enabling deep tissue penetration. In particular, the ability to convert near infrared radiation into visible light (upconversion), which prevents autofluorescence and over-heating effect of biological tissues.In biological applications especially in vivo, morphology and size of the nanoparticles plays a crucial role in determination of cellular responses and fate in living organism. Heterogeneously sized nanoparticles, in contrast to uniform ones, might be distributed unevenly in the organism causing undesirable toxic side effects. Therefore, precise control of the nanoparticle size, distribution, and reproducibility were main tasks in the first part of this work. Colloidal upconverting nanoparticles were synthesized using coprecipitation method. Synthetic parameters such as reaction temperature (280-320 °C), and time (5-30 min) were used to tailor the nanoparticle morphology, crystal phase (cubic or hexagonal) and particle size (sub-10 - 20 nm). Integration of these nanoparticles in biological applications requires dispersibility in aqueous media. Hence hydrophobic UCNPs were surface-modified with low molecular weight ligands including O-phospho-L-threonine, alendronic acid, and PEG-phosphate ligands to generate water-dispersible UCNPs. Furthermore, in this work, photocrosslinking of diacetylenes is presented as an effective way to create robust UCNPs with a crosslinked shell. Finally, the protein corona formation on UCNPs coated with charged, zwitterionic and nearly neutral ligands was investigated. The composition of protein binding to UCNP is notably influenced by the surface charges of the UCNPs. Overall, the results obtained in the frame of this work show that the NaYF4: Nd3+, Yb3+, Er3+ UCNPS have the potential to replace conventional fluorophores in bioimaging applications due to their remarkable optical properties, as well as the derivatization flexibility of their surface

The increasing prevalence of glass windows in modern buildings has raised the demand for solar control windows that possess climate-appropriate properties. Glass windows made of abundant and low-cost materials which can both decrease the heating energy consumption as well as enhance the light climate indoors would sufficiently meet the goals of economical yet uplifting buildings. The main objective of this thesis was to examine whether a plasmonic hybrid interface, comprising three layers of thin films (gold nanoparticles of approximately 10 nm, ZrO2 with a thickness range of 20-35 nm, and ZnO with a thickness of approximately 20 nm), could achieve the upconversion of infrared light to visible light through a multiphoton absorption process in the ZnO layer. If successful, this configuration, in conjunction with an established layer capable of downconverting ultraviolet light to visible light, would be applied to commercially available glass windows to enhance the solar utilization and improve indoor lighting conditions. ZnO was selected as the upconversion material due to its wide emission range in the visible spectrum, indicative of intermediate electron states between the valence and conduction bands suitable for excitation. The objective of the plasmonic material, the gold nanoparticles, was to increase the probability of the upconversion process by utilizing the enhanced electric field resulting from plasmons localized at the surface of the gold nanoparticles. ZrO2 served as a separator layer between the plasmonic material and the ZnO, to effectively preventing charge transfer and ensuring that any upconversion or other photoluminescence processes were purely photonic. Various optical experimental techniques were employed in this study to assess any upconversion, plasmon enhancement, and map the intermediate electron states of the ZnO. The ZrO2 layer successfully prevented charge transfer between the layers. However, the influence ofthe gold’s surface plasmons and it’s enhanced electric field on ZnO emission varied among the samples, likely due to the synthesis processes. Ultimately, the plasmonic hybrid interface investigated in this thesis did not exhibit detectable upconversion when illuminated with either 600 or 750 nm light. Further research is necessary to increase the density of intermediate electron states in ZnO, along with optimization of the thin film synthesis to enhance plasmon effects. These advancements would augment the probability of detectable upconversion.

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