Probing Interaction Distance of Surface Quenchers in Lanthanide-Doped Upconversion Core–Shell Nanoparticles

Core–shell structures, which employ an optically inert shell to physically separate the emitting core from the surface quenchers, are often designed to optimize the emission efficiency of nanoscale emitters. However, it remains unclear that at what distance the effects of different surface quenchers, such as defects and adsorbed moieties, can be completely screened by the shell. Here, in a model upconversion system, we examine the interaction distance of surface quenchers in core–shell nanoparticles by using upconversion spectroscopy. Steady-state as well as time-resolved spectra show that the quenching effect of surface-adsorbed hydroxyl (OH) group diminishes at a distance (shell thickness) of 3.5 nm in diameter and 8.0 nm in length, which is larger than that for oleate-capped counterparts. With the increase of pumping density, the interaction distance of the surface quenchers does not apparently change, whereas saturation of the surface-related states notably reduces the optimal shell thickness for surface passivation

Understanding Enhanced Upconversion Luminescence in Oxyfluoride Glass-Ceramics Based on Local Structure Characterizations and Molecular Dynamics Simulations

In this Article, large enhancement in upconversion (UC) luminescence was verified in a transparent aluminosilicate glass-ceramics (GCs) containing CaF₂ nanocrystals (NCs) codoped with Er³⁺ and Yb³⁺ ions. On the basis of the joint spectroscopic and structural characterizations, we suggest that the precipitation of fluoride NCs is correlated with the pre-existence of the fluoride-rich domains in the as-melt glass, which is supported by scanning transmission electron microscopy (STEM) and reproduced by molecular dynamics (MD) simulation. The precipitation of the fluoride NCs starts from a phase-separated as-melt glass consisting of fluorine-rich and oxygen-rich domains, while the spatial distribution of rare earth (RE) ions and the vibration energies of the bonds connecting RE ions remain almost unchanged after crystallization. In the GCs, both the fluoride domain and the oxygen-containing polyhedrons surrounding RE ions experience significant ordering, which may affect the UC emission for both glasses and GCs. We therefore attribute the enhanced UC emissions of the GCs to the long-range structural ordering and the change of site symmetry surrounding RE ions, rather than the preference of RE ions in migrating from fluoride-rich phase to the fluoride NCs. Our results may have strong implications for a better understanding of the enhanced UC emission in similar oxyfluoride GCs

Integrated Strategy for High Luminescence Intensity of Upconversion Nanocrystals

The growing applications of upconversion nanocrystals in bioimaging, therapeutics, and photonics have given rise to a demand of high quality nanocrystals with desirable luminescence intensity. Although the design of optimal nanocrystals such as core–shell nanostructures has improved the intensity, the internal links between dopant concentration balance, epitaxial growth protection, and shell thickness effect encounter a compromised situation that lacks of integrated consideration and comprehensive assessment. Here we propose an integrated strategy based on a core–shell design for the enhancement of upconversion luminescence intensity. Epitaxial protection can enable higher activator accommodation capacity in limited spatial scale, which leads to an Er³⁺ concentration threshold improvement in β-NaYF₄ core–shell nanocrystals from 2 to 6 mol %. We further perform a comprehensive assessment of the nanocrystals with convincing performance improvement in ensemble spectroscopic intensity, upconversion quantum yield, and single nanocrystal intensity. Our findings provide improved understanding of electronic behaviors in multiphoton upconversion and opportunities for diverse applications requiring high quality upconversion nanocrystals

Supplementary document for Magnetically driven micro-optical choppers fabricated by two-photon polymerization - 6173146.pdf

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Magnetic Tuning of Optical Hysteresis Behavior in Lanthanide-Doped Nanoparticles

Magnetic-optical bifunctional materials have attracted tremendous interest due to their potential applications in biomedicine as well as multifunctional sensors. However, much attention has been paid on the bifunctional materials rendering magnetic and optical behavior individually, rather than the interaction between magnetic field and optical process. In this paper, we examine the coupling of magnetic field with photoluminescence in Eu³⁺-doped NaGdF₄ nanoparticles. The Zeeman effect induced by magnetic field is clearly observed from the shift of luminescence bands and the splitting of the emission peaks. Furthermore, the luminescence intensity of different transitions of Eu³⁺ in paramagnetic NaGdF₄ exhibits a hysteresis behavior when the magnetic field is scanned between 0 and 40 T. Compared with the optical behavior of Eu³⁺ in the nonmagnetic NaYF₄, this optical hysteresis behavior of luminescence intensity is tentatively ascribed to the magnetic response of the paramagnetic dopant ions in both hosts. Due to the high magnetic field sensitivity, the Eu³⁺-doped bifunctional nanoparticles could be used as optical probes in sensor and biomedical areas

Anisotropically Enhanced Nonlinear Optical Properties of Ensembles of Gold Nanorods Electrospun in Polymer Nanofiber Film

Polymeric nanofibers containing gold nanorods (GNRs) are aligned in a uniform orientation through electrospinning. The dispersive and absorptive parts of the third-order optical nonlinear optical refractive index of the composite film measured by polarization dependent <i>z</i>-scan method are demonstrated to be anisotropically enhanced. Anisotropic optical response of the aligned GNRs and its connection with the ultrafast electron dynamics are discussed in light of the results of resonant femtosecond pump–probe experiments. The significant appearance of anisotropic nonlinear optical properties of ensembles of GNRs is attributed to the sensitive excitation of longitudinal surface plasmon resonance (LSPR) of highly aligned GNRs. For the macroscopic applications of ensembles of GNRs, such as passive mode-locking and all-optical switching, the experimental results demonstrate that the alignment of GNRs through electrospinning should be very high efficient, and economic

Two-Dimensional GeSe as an Isostructural and Isoelectronic Analogue of Phosphorene: Sonication-Assisted Synthesis, Chemical Stability, and Optical Properties

Monochalcogenides of germanium (or tin) are considered as isoelectronic and isostructural analogues of black phosphorus. Here, we demonstrate the synthesis of atomically thin GeSe by direct sonication-assisted liquid phase exfoliation (LPE) of bulk microcrystalline powders in organic solvents. The thickness of the GeSe sheets is dependent on the exfoliation conditions, and highly crystalline few-layer GeSe sheets of 4–10 layer stacks with lateral sizes over 200 nm were obtained. In ambient atmosphere, the LPE sheets deposited on the substrate demonstrate strong resistance against degradation, while decomposition into elemental Ge and Se nanostructures occurs at a moderate rate for ethanol dispersions. Density functional theory calculation together with optical characterizations confirm the blue-shifted bandgap for the GeSe sheets as a result of strong quantum confinement effect. In addition, we show that the few-layer GeSe sheets with favorable optical bandgap allow for efficient solar light harvesting for photocurrent generation based on a photoelectrochemical cell. Our joint theoretical and experimental results suggest that GeSe sheets of atomic thickness could be a new two-dimensional semiconductor that can be exploited for potential applications in optoelectronics and photonics

Near-Infrared Emission and Photon Energy Upconversion of Two-Dimensional Copper Silicates

BaCuSi₄O₁₀ (Han blue), CaCuSi₄O₁₀ (Egyptian blue), and SrCuSi₄O₁₀ are pigments found in many ancient artifacts all over the world. Behind their brilliant color, we demonstrate here that these ancient pigments are strong candidates for photonic materials due to their bright Stokes and anti-Stokes emissions. These pigments give near-infrared emissions (NIR) from Cu²⁺ centered at around 930 nm under excitation of 440–800 nm light. This NIR emission can also be produced by pumping using a NIR laser diode. With the rise of pumping density, the emission bandwidth increases notably and stretches to the visible region, giving rise to bright and broadband photon upconversion (UC). This photon UC process is interpreted in terms of laser-driven blackbody radiation from the ancient pigments

Ultrasensitive Polarized Up-Conversion of Tm³⁺–Yb³⁺ Doped β‑NaYF₄ Single Nanorod

Up-conversion luminescence in rare earth ions (REs) doped nanoparticles has attracted considerable research attention for the promising applications in solid-state lasers, three-dimensional displays, solar cells, biological imaging, and so forth. However, there have been no reports on REs doped nanoparticles to investigate their polarized energy transfer up-conversion, especially for single particle. Herein, the polarized energy transfer up-conversion from REs doped fluoride nanorods is demonstrated in a single particle spectroscopy mode for the first time. Unique luminescent phenomena, for example<i>,</i> sharp energy level split and singlet-to-triplet transitions at room temperature, multiple discrete luminescence intensity periodic variation with polarization direction, are observed upon excitation with 980 nm linearly polarized laser. Furthermore, nanorods with the controllable aspect ratio and symmetry are fabricated for analysis of the mechanism of polarization anisotropy. The comparative experiments suggest that intraions transition properties and crystal local symmetry dominate the polarization anisotropy, which is also confirmed by density functional theory calculations. Taking advantage of the REs based up-conversion, potential application in polarized microscopic multi-information transportation is suggested for the polarization anisotropy from REs doped fluoride single nanorod or nanorod array

Tip-Enhanced Upconversion Luminescence in Yb³⁺–Er³⁺ Codoped NaYF₄ Nanocrystals

Lanthanide doped upconversion nanoparticles reveal enormous potential for biomedical applications. However, they are limited by low upconversion efficiency. In this paper, we demonstrate tip-enhanced upconversion luminescence (UCL) from a single Yb³⁺–Er³⁺-codoped NaYF₄ nanoparticle with a maximum enhancement factor of 11 by the plasmonic effect, which is a solution for improving the conversion efficiency and will become a potential technique for the applications in sensitive imaging and detection via speeding up both the absorption and the emission processes of lanthanide doped upconversion nanoparticles. By investigating the optical properties of the tip-enhanced UCL at 550 and 660 nm separately in Yb³⁺–Er³⁺-codoped NaYF₄ nanoparticles, we observe that the gold-coated tip influences the upconversion process in Yb³⁺–Er³⁺-codoped NaYF₄ particles by improving the reception and the transmission of incident electromagnetic fields