Eu:YPO 4 •xH 2 O (x = 0.5−1) nanocrystals were synthesized by a liquid−solid−solution (LSS) solvothermal method and dispersed in chloroform. In order to sensitize the emission from the Eu 3+ ions, 2-thenoyltrifluoroacetone (HTTFA) was used to replace a significant fraction of the oleate capping ligand on the as-prepared Eu:YPO 4 •xH 2 O (x = 0.5−1) nanocrystals. During the ligand exchange, HTTFA reacts with oleate, forming oleic acid and 2-thenoyltrifluoroacetonate, TTFA. The negatively charged TTFA then displaces the neutral oleic acid ligand from the surface of the nanoparticles. The resulting surface-modified samples were less dispersible in chloroform than were the as-prepared, oleate-capped nanoparticles but were easily dispersed in pyridine, forming very clear mixtures. The resulting surface-modified nanoparticles were characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, transmission electron microscopy (TEM), UV−vis absorption spectroscopy, photoluminescence (PL) spectroscopy, and time-resolved luminescence spectroscopy. XRD analysis indicates that the samples are crystalline with a hexagonal phase. The oleate-capped Eu:YPO 4 •xH 2 O (x = 0.5−1) nanocrystals have a zeolite structure with a porous surface. The morphology and quality of the nanoparticles remained unchanged upon ligand exchange. The FTIR spectrum of the surfacemodified (TTFA-sensitized) Eu:YPO 4 •xH 2 O (x = 0.5−1) nanocrystals shows signals for both 2-thenoyltrifluoroacetate and oleate. Using UV−vis absorbance and elemental analysis, it is estimated that approximately half of the native oleate capping ligands are replaced with TTFA. Colloidal dispersions in pyridine show characteristic emission of Eu 3+ 5 D 0 → 7 F J (J = 0−4) when excited at the TTFA absorbance band at 350 nm. Ligand excitation at 350 nm results in an enhancement of external quantum efficiency of Eu 3+ emission of up to 4700× relative to direct Eu 3+ excitation at 464 nm. The ability to sensitize emission from these nanocrystals greatly increases their potential for application in display and lighting fields.

Electron−phonon coupling controls nonradiative relaxation dynamics of the photoexcited electron−hole pair in semiconductor nanostructures. Here the optoelectronic properties for Al-and P-codoped silicon quantum dots (QDs) are calculated by combining time-dependent density matrix methodology and ab initio electronic structure methods. The energy-band landscape of the codoped Si QD is elucidated via time evolution of population density distributions in energy and in coordinate space. Multiple nonradiative relaxation pathways result in a specific charge-separated state, where a hole and an electron are localized on Al and P dopants, respectively. Analysis of the simulated nonradiative decay shows that high-energy photoexcitation relaxes to the band gap edge within 10 ps, forming the final charge-transfer state. We also simulate time-resolved emission spectra of the codoped Si QD that reveals optical and IR emissions below the optical band gap. These emission features are attributed to the intraband transitions introduced by doping.

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Unimolecular gas-phase laser-photodissociation reaction mechanisms of open-shell lanthanide cyclopentadienyl complexes, Ln(Cp)3 and Ln(TMCp)3, are analyzed from experimental and computational perspectives. The most probable pathways for the photoreactions are inferred from photoionization time-of-flight mass spectrometry (PI-TOF-MS), which provides the sequence of reaction intermediates and the distribution of final products. Time-dependent excited-state molecular dynamics (TDESMD) calculations provide insight into the electronic mechanisms for the individual steps of the laser-driven photoreactions for Ln(Cp)3. Computational analysis correctly predicts several key reaction products as well as the observed branching between two reaction pathways: (1) ligand ejection and (2) ligand cracking. Simulations support our previous assertion that both reaction pathways are initiated via a ligand-to-metal charge-transfer (LMCT) process. For the more complex chemistry of the tetramethylcyclopentadienyl complexes Ln(TMCp)3, TMESMD is less tractable, but computational geometry optimization reveals the structures of intermediates deduced from PI-TOF-MS, including several classic “tuck-in” structures and products of Cp ring expansion. The results have important implications for metal–organic catalysis and laser-assisted metal–organic chemical vapor deposition (LCVD) of insulators with high dielectric constants.