Synthesis of InN@SiO₂ Nanostructures and Fabrication of Blue LED Devices

We synthesized InN@SiO₂ nanostructures (i.e., nanoparticles and nanowires) by varying the annealing temperature and nitridation conditions of In₂O₃@SiO₂ nanoparticles in the presence of ammonia. The In₂O₃@SiO₂ nanoparticles were synthesized using a urea-based homogeneous precipitation of indium hydroxide on the surface of the SiO₂ (15 nm) nanoparticles, followed by annealing at 600 °C in air. Subsequently, nitridation of In₂O₃@SiO₂ nanoparticles in ammonia at 600 °C for 2 h resulted in InN@SiO₂ nanoparticles. The sizes of InN nanoparticles are ∼5 nm on the silica surface. Nitridation at the same temperature for 3–5 h gave InN nanoparticles of size ∼20 nm. Furthermore, on annealing above 650 °C the InN nanoparticles grew in the form of nanowires. The nanowires are 4–5 μm in length and have a diameter of 100 nm. The photoluminescence peak of both InN@SiO₂ nanoparticles and nanowires is centered at 442 nm (λ_exi = 325 nm). Subsequently, the surface of InN@SiO₂ nanoparticles was modified by reacting with dodecyltriethoxysilane at 80 °C, which enabled them to be dispersible in toluene. The surface-modified InN@SiO₂ nanoparticles were used to fabricate blue electroluminescence devices which showed blue electroluminescence peak centered at 442 nm. The Commission Internationale de I’Eclairage (CIE) coordinates of InN@SiO₂ nanoparticles are <i>X</i> = 0.15 and <i>Y</i> = 0.13, which is well within the blue region and commercially appropriate

Blue Electroluminescence from Eu²⁺-Doped GaN@SiO₂ Nanostructures Tuned to Industrial Standards

We have tuned the blue electroluminescence of surface-modified Eu²⁺-doped GaN@SiO₂ nanoparticle to industrial standards with a peak at 450 nm and Commission Internationale de I’Eclairage (CIE–1931) coordinates of <i>X</i> = 0.15 and <i>Y</i> = 0.15. The blue electroluminescence was observed on applying a 14 V forward bias to the devices. The Eu²⁺-doped GaN@SiO₂ nanoparticles were obtained on nitridation of Eu³⁺-doped Ga₂O₃@SiO₂ nanoparticles at 900 °C. Subsequently, the surface of the Eu²⁺-doped GaN@SiO₂ nanostructure was modified by reacting with dodecyletriethoxysilane at 80 °C, which enabled it to be dispersible in toluene, xylene, and benzene. Tuning of the CIE coordinates is dependent on the ratio of nitrogen to oxygen in the coordinating sphere of Eu²⁺ ions which is the origin of the blue electroluminescence

Lanthanide-Based Heteroepitaxial Core–Shell Nanostructures: Compressive <i>versus</i> Tensile Strain Asymmetry

Heteroepitaxial core–shell nanostructures have been proven advantageous in a wide variety of applications, ranging from luminescence enhancement, band gap engineering, multimodal theranostics, to catalysis. However, precisely tailoring the epitaxial growth is challenging, and a general understanding of the parameters that impact epitaxial growth remains unclear. Here we demonstrate the critical role of the sign of the lattice mismatch of the shell relative to the core (compressed/tensile) in generating lanthanide-based core–shell structures, a parameter conventionally not considered in heteroepitaxial design. We took advantage of the very gradual contraction of lanthanide ions along the series to control precisely both the magnitude and the sign of lattice mismatch and investigated multiple sodium lanthanide fluoride (NaLnF₄) core–shell heterostructures of variable composition and size. We discovered that the tensile strained shells adapt to the core crystallite shape (<i>i.e.</i>, conformal) and lattice structure (<i>i.e.</i>, coherent), while under identical magnitude of mismatch, the compressively strained shells are neither conformal nor coherent to the core. This striking asymmetry between the tensile and compressively strained epitaxial growth arises from the fundamental anharmonicity of the interatomic interactions between the attractive and repulsive pairs. From a broader perspective, our findings redefine the <i>a priori</i> design consideration and provide a fundamental insight on the necessity to include the sign of lattice mismatch and not just its magnitude in designing heteroepitaxial core–shell nanostructures

Validation of Inner, Second, and Outer Sphere Contributions to T₁ and T₂ Relaxation in Gd³⁺-Based Nanoparticles Using Eu³⁺ Lifetime Decay as a Probe

Paramagnetic lanthanide-based NPs are currently designed as magnetic resonance imaging (MRI) contrast agents to obtain optimal relaxivities at high magnetic fields of 7, 9.4, and 11.7 T where human imaging has been possible yielding high contrast to noise ratio in the MR images compared to the clinical field of 3 T. However, the underlying longitudinal (T₁) and transverse (T₂) relaxation mechanisms of the NP-based contrast agents based on the spatial motion and proximity of water protons with respect to the paramagnetic ions on the surface of NPs are still not well understood, specifically, in terms of contributions from inner, second, and outer spheres of coordination of water molecules to the NPs. Gd³⁺-based NPs, e.g., NaGdF₄, are promising T₁ contrast agents owing to the paramagnetic Gd³⁺ possessing a symmetric ⁸S_7/2-state and slow electronic relaxation relevant to its efficiency to produce a positive (T₁) contrast. Here, water-dispersed NaGdF₄:Eu³⁺ (3 nm diameter, TEM) and NaYF₄–NaGdF₄:Eu³⁺ core–shell NPs (18.3 nm core diameter with 0.5 nm thick shell, TEM) were studied for their <i>r</i>₁ and <i>r</i>₂ relaxivities at 9.4 T. Excited state lifetime decays of Eu³⁺ dopants, which are highly sensitive to proximate water molecules, were analyzed, demonstrating a dominance of inner and second sphere contribution over outer sphere to the T₁ and T₂ relaxations in smaller NaGdF₄:Eu³⁺ NPs while exclusively outer sphere in NaYF₄–NaGdF₄:Eu³⁺ core–shell NPs

Four-Fold Enhancement of the Activation Energy for Nonradiative Decay of Excitons in PbSe/CdSe Core/Shell versus PbSe Colloidal Quantum Dots

PbSe/CdSe core/shell quantum dots (QDs) were prepared and investigated as thick films using temperature-dependent photoluminescence. In addition to increased photostability, the CdSe shell leads to a four-fold increase of the activation energy for nonradiative exciton decay for the core/shell QDs compared to that for the bare PbSe QDs. The onset for exponential decay of luminescence is ∼240 K in the core/shell samples. From further analysis of the temperature-dependent photoluminescence shift and emission line width, we find that the cation exchange reaction broadens the QD size distribution and increases the temperature-independent state broadening. However, the temperature-dependent contribution to the line shape of the core/shell QDs is similar to that in the cores

Long-Term Colloidal Stability and Photoluminescence Retention of Lead-Based Quantum Dots in Saline Buffers and Biological Media through Surface Modification

Lead-based quantum dots (QDs) can be tuned to emit in the transparent region of the biological tissue (700 to 1100 nm) which make them a potential candidate for optical bioimaging. However, to employ these QDs as biolabels they have to retain their luminescence and maintain their colloidal stability in water, physiological saline buffers, different pH values, and biological media. To achieve this, four different surface modification strategies were tried: (1) silica coating; (2) ligand exchange with polyvinylpyrrolidone; (3) polyethyleneglycol-oleate (PEG-oleate) intercalation into the oleate ligands on the surface of the QDs; and (4) intercalation of poly­(maleicanhydride-<i>alt</i>-1-octadecene) (PMAO) into the oleate ligands on the surface of the QDs and further cross-linking of the PMAO. The first two methods exhibited excellent dispersion stability in water, but did not retain their photoluminescence. On the other hand, the intercalation strategy with PEG-oleate helped the QDs retain their luminescence but with poor colloidal stability in water. The fourth and final strategy involving intercalation and cross-linking of the amphiphilic polymer PMAO provided the QDs with colloidal stability in water but also resulted in the QDs retaining their luminescence as well. This process involved two steps; (1) the intercalation between octadecene chains of PMAO with the oleates on the surface of the QDs with some of the anhydride rings opened with PEG-amine; (2) the anhydride rings were cross-linked with bis­(hexamethylene)­triamine (BHMT) to avoid detachment of the polymer from the surface of QDs because of the polymer’s dynamic nature in solvents. The presence of PEG molecules potentially improves the biocompatibility of the QDs and the presence of carboxylic acids after reaction with BHMT makes them suitable for further surface functionalization with antibodies, proteins, and so forth. The surface-modified QDs have excellent dispersibility in water, saline buffers, and in various pH conditions for more than 7 months and more than 20 days in serum-supplemented growth media. In addition to the colloidal stability, the QDs retained their photoluminescence even after 7 months in the aforementioned aqueous media. The intercalation and cross-linking process have also made the QDs resistant to oxidation when exposed to ambient atmosphere and aqueous media

Analysis of the Shell Thickness Distribution on NaYF₄/NaGdF₄ Core/Shell Nanocrystals by EELS and EDS

The structure and chemical composition of the shell distribution on NaYF₄/NaGdF₄ core/shell nanocrystals have been investigated with scanning transmission electron microscopy (STEM), electron energy loss spectroscopy (EELS), and energy-dispersive X-ray spectroscopy (EDS). The core and shell contrast in the high-angle annular dark-field (HAADF) images combined with the EELS and EDS signals indicate that Gd is indeed on the surface, but for many of the particles, the shell growth was anisotropic

Local Structure of Rare-Earth Fluorides in Bulk and Core/Shell Nanocrystalline Materials

NaREF₄ (rare-earth, RE = Y, Lu) nanocrystals (NCs) containing Ln³⁺-dopants are of great interest due to their unique ability to downshift, downconvert, and upconvert light. While the luminescent properties and microscale structures of such NCs are well understood, relatively little is known about their molecular-level structures, the nature of the NC cores and shells, and the interactions of the stabilizing ligands at the NC surface. To address these issues, we present a comprehensive characterization of bulk β-NaYF₄, one of the most commonly used hosts for Ln³⁺-dopants in luminescent NCs. Using a combination of powder X-ray diffraction (pXRD) and multinuclear (⁸⁹Y, ²³Na, ¹⁹F) solid-state nuclear magnetic resonance (SSNMR) spectroscopy, we provide conclusive evidence that this compound crystallizes in a hexagonal <i>P</i>6₃/<i>m</i> structure, resolving a longstanding debate. The structure of this bulk form is related to the structure of NaYF₄/NaLuF₄ core/shell NCs. From the similarities between the ¹⁹F and ²³Na SSNMR spectra of the bulk and NC materials, it is concluded that the NCs have the same β-NaYF₄ and β-NaLuF₄ phases as the bulk compounds. A series of cross-polarization NMR experiments confirm the presence of oleates on the surface of the particle via their proximity to surface Na⁺ sites

Cation Exchange: A Facile Method To Make NaYF₄:Yb,Tm-NaGdF₄ Core–Shell Nanoparticles with a Thin, Tunable, and Uniform Shell

Cation exchange was performed on up-conversion NaYF₄:Yb,Tm nanoparticles, resulting in NaYF₄:Yb,Tm-NaGdF₄ core–shell nanoparticles as indicated by electron energy-loss spectroscopy 2D mapping. Results show that core–shell nanoparticles with a thin, tunable, and uniform shell of subnanometer thickness can be made via this cation exchange process. The resulting NaYF₄:Yb,Tm-NaGdF₄ core–shell nanoparticles have an enhanced up-conversion intensity relative to the initial core nanoparticles. As potential magnetic resonance imaging (MRI) contrast agents, they were tested for their proton relaxivities. The r₁ relaxivity per Gd³⁺ ion of the nanoparticles with a thin NaGdF₄ shell (ca. 0.6 nm thick) measured at 9.4 T was found to be 2.33 mM^–1·s^–1. This r₁ relaxivity is among the highest in all the reported NaYF₄–NaGdF₄ core–shell nanoparticles. The r₁ relaxivity per nanoparticle is 1.56 × 10⁴ mM^–1·s^–1, which is over 4000 times higher than commercial Gd³⁺-complexes. The very high proton relaxivity per nanoparticle is critical for targeted MRI as such nanoparticles provide strong contrast even in low concentrations. The presented cation exchange method is a promising way to manufacture core–shell nanoparticles with up-conversion NaYF₄:Yb,Tm core and paramagnetic NaGdF₄ shell for bimodal imaging, i.e. MR and optical imaging

An Effective Polymer Cross-Linking Strategy To Obtain Stable Dispersions of Upconverting NaYF₄ Nanoparticles in Buffers and Biological Growth Media for Biolabeling Applications

Ligands on the nanoparticle surface provide steric stabilization, resulting in good dispersion stability. However, because of their highly dynamic nature, they can be replaced irreversibly in buffers and biological medium, leading to poor colloidal stability. To overcome this, we report a simple and effective cross-linking methodology to transfer oleate-stabilized upconverting NaYF₄ core/shell nanoparticles (UCNPs) from hydrophobic to aqueous phase, with long-term dispersion stability in buffers and biological medium. Amphiphilic poly­(maleic anhydride-<i>alt</i>-1-octadecene) (PMAO) modified with and without poly­(ethylene glycol) (PEG) was used to intercalate with the surface oleates, enabling the transfer of the UCNPs to water. The PMAO units on the phase transferred UCNPs were then successfully cross-linked using bis­(hexamethylene)­triamine (BHMT). The primary advantage of cross-linking of PMAO by BHMT is that it improves the stability of the UCNPs in water, physiological saline buffers, and biological growth media and in a wide range of pH values when compared to un-cross-linked PMAO. The cross-linked PMAO–BHMT coated UCNPs were found to be stable in water for more than 2 months and in physiological saline buffers for weeks, substantiating the effectiveness of cross-linking in providing high dispersion stability. The PMAO–BHMT cross-linked UCNPs were extensively characterized using various techniques providing supporting evidence for the cross-linking process. These UCNPs were found to be stable in serum supplemented growth medium (37 °C) for more than 2 days. Utilizing this, we demonstrate the uptake of cross-linked UCNPs by LNCaP cells (human prostate cancer cell line), showing their utility as biolabels