Lanthanide-doped upconversion nanoparticles (UCNPs) have rich photophysics exhibiting complex luminescence kinetics. In this work, we thoroughly investigated the luminescence response of UCNPs to excitation pulse durations. Analyzing this response opens new opportunities in optical encoding/decoding and the assignment of transitions to emission peaks and provides advantages in applications of UCNPs, e.g., for better optical sectioning and improved luminescence nanothermometry. Our work shows that monitoring the UCNP luminescence response to excitation pulse durations (while keeping the duty cycle constant) by recording the average luminescence intensity using a low-time resolution detector such as a spectrometer offers a powerful approach for significantly extending the utility of UCNPs.

Since selective detection of multiple narrow spectral bands in the near-infrared (NIR) region still poses a fundamental challenge, we have, in this work, developed NIR photodetectors (PDs) using photon upconversion nanocrystals (UCNCs) combined with perovskite films. To conquer the relatively high pumping threshold of UCNCs, we designed a novel cascade optical field modulation strategy to boost upconversion luminescence (UCL) by cascading the superlensing effect of dielectric microlens arrays and the plasmonic effect of gold nanorods, which readily leads to a UCL enhancement by more than four orders of magnitude under weak light irradiation. By accommodating multiple optically active lanthanide ions in a core-shell-shell hierarchical architecture, developed PDs on top of this structure can detect three well-separated narrow bands in the NIR region, i.e., those centered at 808, 980, and 1540 nm. Due to the large UCL enhancement, the obtained PDs demonstrate extremely high responsivities of 30.73, 23.15, and 12.20 A W-1 and detectivities of 5.36, 3.45, and 1.91 x 10(11) Jones for 808, 980, and 1540 nm light detection, respectively, together with short response times in the range of 80-120 ms. Moreover, we demonstrate for the first time that the response to the excitation modulation frequency of a PD can be employed to discriminate the incident light wavelength. We believe that our work provides novel insight for developing NIR PDs and that it can spur the development of other applications using upconversion nanotechnology. Cascade amplified upconversion luminescence: Applied in narrow band NIR photodetection Selective detection of multiple narrow spectral bands in the near-infrared (NIR) region is still a challenge. Recently, Hongwei Song and Wen Xu at Jilin University/China, Haichun Liu at KTH Royal Institute of Technology/Sweden, and their co-workers have successfully fabricated a novel multiple NIR bands photo-detectors (PDs) by combining multiple-excitation-bands core-shell upconversion nanocrystals (UCNCs) with MAPbI(3) perovskite photoelectric conversion layer. Through a cascade optical field modulation strategy, a combination of microlenses and gold plasmon nanocrystals, the emission intensity of the UCNCs and the photoelectric signal of the PDs can be enhanced four orders of magnitude. Moreover, the excitation frequency of the PD has been employed to discriminate the wavelength of incident light for the first time. This work provides a novel insight for developing multiple bands NIR PDs, and for applications of upconversion nanotechnology.

Colloidal InP and ZnS-coated InP quantum dots (QDs) were synthesized by hot injection method and successfully employed as sensitizer for the first time in quantum dot sensitized solar cells (QDSSCs). Colloidal InP QDs has not been used in QDSSCs due to low stability and high sensitivity to the moisture. In this work ZnS-coating were applied as a strategy to increase the stability and protect InP against moisture and interaction with electrolyte. The nature of low toxicity of such QDs compared to high toxic Cd-based QDs was the main idea to employ “green” and heavy metal-free InP-ZnS QDs for future solar cell application. It is found that ZnS shell-coating caused the absorption onset to shift toward longer wavelength and broader absorption. In the solar cell device, ZnS shell not only acts as protection agent and increases the life time for InP QDs but also enhances the power conversion efficiency by more than 2 times.

YTTERBY, a small village very close to Stockholm where I live, is the place in the world which has lent its name to the largest number of elements in the periodic table, namely four - YTTRIUM, YTTERBIUM, ERBIUM and TERBIUM. Three more lanthanide elements were discovered from the now empty quarry located in this village. By the time of their discoveries in the 19th century little could be known about their fantastic properties, the versatility of their use and functionality in what we now call nanotechnology. This is a circumstance that motivated me to rather recently enter lanthanide research, in particular studies of their outstanding optical properties for the purpose of information technology and energy harvesting.

So far, upconversion nanoparticles (UCNPs) have been much explored as unique spectral converters for various applications, like biotechnology, information technology and photovoltaic devices due to properties like sharp emission profiles, low autofluorescence and large anti-Stoke shifts. Still, there is much to explore and to understand in order to fully utilize the very unique properties of UCNPs. The kinetic dynamics of the upconversion process is one such aspect that is not well understood, and a deeper understanding of the kinetic dynamics of lanthanide upconversion systems could thus broaden their applications. Therefore, the work of this thesis is focused on investigating the kinetic dynamics of upconversion processes mainly based on systems with NaYF4 as host material, and Yb3+/Er3+ or Yb3+/Tm3+ embedded as sensitizer/activator. Through rate equation models, the kinetic dynamics of upconversion are comparatively investigated with numerical simulations and analytical derivation. The temporal response regarding upconverted luminescence and quantum yield power density dependence, excitation duration response and excitation frequency response of the upconversion systems are investigated and the corresponding applications for multicolor imaging, optical encoding, photovoltaics, IR photodetectors are explored and analyzed in the thesis, taking advantage of the kinetic properties.

YTTERBY, en liten by nära Stockholm där jag bor, är den plats i världen som har lånat sitt namn till det högsta antalet element i det periodiska systemet, nämligen fyra - YTTRIUM, YTTERBIUM, ERBIUM och TERBIUM. Ytterligare tre lantanidelement upptäcktes från det nu tomma stenbrottet som ligger i denna by. Vid deras upptäckter på 1800-talet kunde man inte ana deras fantastiska egenskaper, mångsidigheten i deras användning och deras funktionalitet i det vi nu kallar nanoteknologi. Detta är en omständighet som motiverade mig ganska nyligen att intressera mig för lantanidforskning, i synnerhet studier av deras enastående optiska egenskaper och deras energitillämpningar och användning inom informationsteknik.

Hittills har uppkonverterande nanopartiklar (UCNPs) utforskats mycket som unika spektralkonverterare för olika applikationer, som bioteknik, informationsteknologi och fotovoltaiska enheter på grund deras egenskaper som skarpa emissions profiler, låg autofluorescens och stora anti-Stoke skift. Det finns fortfarande mycket att utforska och förstå för att utnyttja de mycket unika egenskaperna hos dessa partiklar. Den kinetiska dynamiken i upkonverteringsprocessen är en sådan aspekt som inte är väl undersökt ännu, och en djupare förståelse av den kinetiska dynamiken i uppkonverterande lantanid system kan bredda deras tillämpningar. Därför har jag fokuserat arbetet med den här avhandlingen på att undersöka den kinetiska dynamiken i upkonverterings processen huvudsakligen baserat på system med NaYF4 som värdmaterial och Yb3+/Er3+ eller Yb3+/Tm3+ inbäddat som sensibilisator/aktivator. Genom simuleringar av ekvationsmodeller har jag undersökt den kinetiska dynamiken i uppkonversionen jämförande numerisk simulering och analytisk härledning. Det temporära svaret med avseende på uppkonverterad luminescens, det s.k. täthetsberoendet av kvantutbytet och excitation frekvens responsen för olika upkonversionssystem har studerats. Motsvarande tillämpningar för flerfärgs avbildning, optisk kodning, fotovoltaik och IR fotodetektorer undersöks och analyseras i avhandlingen, med speciell fokus på de kinetiska egenskaperna.

Nd3+-sensitized upconversion nanoparticles (UCNPs) can be excited by both 980 and 808 nm light, which is regarded as a particularly advantageous property of these particles. In this work, we demonstrate that the nanoparticles can exhibit significantly different response when excited at these two excitation wavelengths, showing dependence on the intensity of the excitation light and the way it is distributed in time. Specifically, with 808 nm excitation saturation in the emitted luminescence is more readily reached with increasing excitation intensities than upon 980 nm excitation. This is accompanied by delayed upconversion luminescence (UCL) kinetics and weaker UCL intensities. The different luminescence response at 808 and 980 nm excitation reported in this work is relevant in a manifold of applications using UCNPs as labels and sensors. This could also open new possibilities for multi-wavelength excitable UCNPs for upconversion color display and in laser-scanning microscopy providing selective readouts and sub-sectioning of samples.

The dearth of high upconversion luminescence (UCL) intensity at low excitation irradiance hinders the prevalent application of lanthanide-doped upconversion nanoparticles (UCNPs) in many fields ranging from optical bioimaging to photovoltaics. In this work, we propose to use microlens arrays (MLAs) as spatial light modulators to manipulate the distribution of excitation light fields in order to increase UCL, taking advantage of its nonlinear response to the excitation irradiance. We show that multicolored UCL from NaYF4:Yb3+,Er3+@NaYF4:Yb3+,Nd3+ and NaYF4:Yb3+,Tm3+@NaYF4:Yb3+,Nd3+ core/shell UCNPs can be increased by more than one order of magnitude under either 980 or 808 nm excitation, by simply placing a polymeric MLA onto the top of these samples. The observed typical green (525/540 nm) and red (654 nm) UCL bands from Er3+ and a blue (450/475 nm) UCL band from Tm3+ exhibit distinct enhancement factors due to their different multi-photon processes. Importantly, our ray tracing simulation reveals that the MLA is able to spatially confine the excitation light (980 and 808 nm) by orders of magnitude, thus amplifying UCL by more than 225-fold (the 450 nm UCL band of Tm3+) at low excitation irradiance. The proposed MLA method has immediate ramifications for the improved performance of all types of UCNP-based devices, such as UCNP-enhanced dye sensitized solar cells demonstrated here.

In this study, we systematically investigate the decay characteristics of upconversion luminescence (UCL) under anti-Stokes excitation through numerical simulations based on rate-equation models. We find that a UCL decay profile generally involves contributions from the sensitizer's excited-state lifetime, energy transfer and cross-relaxation processes. It should thus be regarded as the overall temporal response of the whole upconversion system to the excitation function rather than the intrinsic lifetime of the luminescence emitting state. Only under certain conditions, such as when the effective lifetime of the sensitizer's excited state is significantly shorter than that of the UCL emitting state and of the absence of cross-relaxation processes involving the emitting energy level, the UCL decay time approaches the intrinsic lifetime of the emitting state. Subsequently, Stokes excitation is generally preferred in order to accurately quantify the intrinsic lifetime of the emitting state. However, possible cross-relaxation between doped ions at high doping levels can complicate the decay characteristics of the luminescence and even make the Stokes-excitation approach fail. A strong cross-relaxation process can also account for the power dependence of the decay characteristics of UCL.