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A systematic study of the luminescence properties of monodisperse β-NaYF4: 20% Yb3+, 2% Er3+ upconversion nanoparticles (UCNPs) with sizes ranging from 12–43 nm is presented utilizing steady-state and time-resolved fluorometry.
Special emphasis was dedicated to the absolute quantification of size- and environment-induced quenching of upconversion luminescence (UCL) by highenergy O–H and C–H vibrations from solvent and ligand molecules at different excitation power densities (P). In this context, the still-debated Population pathways of the 4F9/2 energy level of Er3+ were examined. Our results Highlight the potential of particle size and P value for color tuning based on the pronounced near-infrared emission of 12 nm UCNPs, which outweighs the red Er3+ emission under “strongly quenched” conditions and accounts for over 50% of total UCL in water. Because current rate equation models do not include such emissions, the suitability of these models for accurately simulating all (de)population pathways of small UCNPs must be critically assessed. Furthermore, we postulate population pathways for the 4F9/2 energy level of Er3+, which correlate with the size-, environment-, and P-dependent quenching states of the higher Er3+ energy levels.
Colloidal semiconductor nanoparticles with a spherical core and an elongated shell form bright emitters with a high absorption cross section. They show great potential for a multitude of optoelectronic applications such as LEDs or photovoltaic cells and can be used as gain material or as markers for bio imaging. For most of these applications high fluorescence quantum yields are a figure of merit for the emitter quality. In this work we investigate how the ensemble quantum yield is affected by the properties of the individual particles. In particular, we prove the role of non-emitting particles as well as the role of blinking. Using a combination of AFM and spatially resolved photoluminescence spectroscopy we measured hundreds of individual CdSe/CdS dot/rod particles of different shell lengths exciting with two different excitation wavelengths for shell or core excitation, respectively.
The rational design of brighter upconversion nanoparticles (UCNPs) requires a better understanding of the radiationless deactivation pathways in these materials. Here, we demonstrate the potential of excitation power density (P)-dependent studies of upconversion (UC) luminescence intensities, slope factors, and absolute quantum yields (ΦUC) of popular β-NaYF4:20% Yb3+,2% Er3+ UCNPs of different surface chemistries in organic solvents, D2O, and water as a tool to gain deeper insight into the UC mechanism including population and deactivation pathways particularly of the red emission. Our measurements, covering a P regime of three orders of magnitude, reveal a strong difference of the P-dependence of the ratio of the green and red luminescence bands (Ig/r) in water and organic solvents and P-dependent population pathways of the different emissive energy levels of Er3+. In summary, we provide experimental evidence for three photon processes in UCNPs, particularly for the red emission. Moreover, we demonstrate changes in the excited population dynamics via bi- and triphotonic processes dependent on the environment, surface chemistry, and P, and validate our findings theoretically
Here, we report on the recent advancements in the synthesis and optical properties of novel core-shell CdSe semiconductor nanocrystals (SCNC) with either a thick monolithic shell of CdS or thin multi-shells of succeeding CdS and ZnS monolayers, characterized by steady state and time-resolved fluorescence spectroscopy at the ensemble level. SCNC samples, chosen upon the size of their photoluminescence quantum yield (PL QY), were subsequently investigated at the single particle level using a custom build confocal laser scanning microscope to obtain information on brightness, blinking behavior, and decay kinetics of single SCNCs and correlate these properties with their surface morphology. The results of this study can help to identify new synthetic routes and surface modifications to colloidally and photochemically stable SCNCs with a PL QY of close to unity.
The design of bright of upconversion nanoparticles requires an improved understanding of the radiationless deactivation pathways in UCNP, that are affected by their size1,2, surface chemistry,2 and microenvironment 3,4. In this respect, we discuss experimental requirements of an integrating sphere setup for absolute measurements of the excitation power density dependent quantum yield5,6 (UC(P)) in the VIS to IR spectral region. We report systematic studies of the influence of the UCNP size, surface ligands, and solvent on the UC(P), the up- and downconversion luminescence decay behavior, the power dependent intensities of the individual emission bands, and their slope factors.
Hexagonal beta-NaYF₄ doped with 20 % Yb and 2 % Er is an efficient upconversion (UC) phosphor for the conversion of 976 nm to 845 nm, 655 nm and 540 nm light. The emission behavior of this material is strongly influenced by the particle size, surface chemistry, and microenvironment. Especially the design of nm-sized UC particles for bioanalytical applications requires reliable spectroscopic tools for the characterization of the optical properties of these materials like the UC quantum yield (QYUC) in aqueous media. The UC emission originates from multiphotonic absorption processes, rendering the QYUC excitation power density (P) dependent. The P-dependent absolute measurement of QYUC in aqueous media with an excitation wavelength of 976 nm presents a considerable challenge due to the low absorption coefficients of the UC materials and the absorption of water at this wavelength.
Here, we present the P-dependent QYUC of 21 nm, 26 nm and 31 nm-sized NaYF₄ UC particles dispersed in cyclohexane and aqueous solutions, in view of bioanalytical applications. To gain a better insight of the photonic nature of the upconversion mechanism and the quenching processes involved, the luminescence decay behavior as well as the P-dependent red-to-green luminescence intensity ratios and slope factors of the different emission bands are studied in both solvents. These results present an important step to the rational design of brighter upconversion nanoparticles.
In order to assess the anisotropy-related uncertainties of relatively determined photoluminescence quantum yields (ΦPL) of molecular emitters and luminescent nanomaterials, we compared ΦPL values measured without and with polarizers using magic angle conditions and studied systematically the dependence of the detected emission intensity on the polarizer settings for samples of varying anisotropy. This includes a dispersion of a spherical quantum dot (QD) with an ideally isotropic emission, a solution of a common small organic dye in a fluid solvent as well as dispersions of elongated quantum dot rods (QDR) with an anisotropic luminescence and a small organic dye in a rigid polymeric matrix, as ideally anisotropic emitter. Our results show that for instruments lacking polarizers, anisotropy-related measurement uncertainties of relative photoluminescence quantum yields can amount to more than 40%, with the size of these systematic errors depending on the difference in emission anisotropy between the sample and the standard.