Mark A. MacDonald

Enhancing multiphoton upconversion through energy clustering at sublattice level.

The applications of lanthanide-doped upconversion nanocrystals in biological imaging, photonics, photovoltaics and therapeutics have fuelled a growing demand for rational control over the emission profiles of the nanocrystals. A common strategy for tuning upconversion luminescence is to control the doping concentration of lanthanide ions. However, the phenomenon of concentration quenching of the excited state at high doping levels poses a significant constraint. Thus, the lanthanide ions have to be stringently kept at relatively low concentrations to minimize luminescence quenching. Here we describe a new class of upconversion nanocrystals adopting an orthorhombic crystallographic structure in which the lanthanide ions are distributed in arrays of tetrad clusters. Importantly, this unique arrangement enables the preservation of excitation energy within the sublattice domain and effectively minimizes the migration of excitation energy to defects, even in stoichiometric compounds with a high Yb(3+) content (calculated as 98 mol%). This allows us to generate an unusual four-photon-promoted violet upconversion emission from Er(3+) with an intensity that is more than eight times higher than previously reported. Our results highlight that the approach to enhancing upconversion through energy clustering at the sublattice level may provide new opportunities for light-triggered biological reactions and photodynamic therapy.

Kinetic control and thermodynamic selection in the synthesis of atomically precise gold nanoclusters.

This work presents a combined approach of kinetic control and thermodynamic selection for the synthesis of monodisperse 19 gold atom nanoclusters protected by thiolate groups. The step of kinetic control allows the formation of a proper size distribution of initial size-mixed Au(n)(SR)(m) nanoclusters following the reduction of a gold precursor. Unlike the synthesis of Au(25)(SR)(18) nanoclusters, which involves rapid reduction of the gold precursor by NaBH(4) followed by size focusing, the synthesis of 19-atom nanoclusters requires slow reduction effected by a weaker reducing agent, borane-tert-butylamine complex. The initially formed mixture of nanoclusters then undergoes size convergence into a monodisperse product by means of a prolonged aging process. The nanocluster formula was determined to be Au(19)(SC(2)H(4)Ph)(13). This work demonstrates the importance of both kinetic control of the initial size distribution of nanoclusters prior to size focusing and subsequent thermodynamic selection of stable nanoclusters as the final product.

Ultrathin Bi2S3 nanowires: surface and core structure at the cluster-nanocrystal transition.

Herein, we present the structural characterization of the core and surface of colloidally stable ultrathin bismuth sulfide (Bi(2)S(3)) nanowires using X-ray Absorption Spectroscopy (EXAFS and XANES), X-ray Photoelectron Spectroscopy (XPS), and Nuclear Magnetic Resonance (NMR). These three techniques allowed the conclusive structural characterization of the inorganic core as well as the coordination chemistry of the surface ligands of these structures, despite the absence of significant translational periodicity dictated by their ultrathin diameter (1.6 nm) and their polycrystallinity. The atomic structure of the inorganic core is analogous to bulk bismuthinite, but Bi atoms display a remarkably higher coordination number than in the bulk. This can be only explained by a model in which each bismuth atom at the surface (or in close proximity to it) is bound to at least one ligand at any time.

X-ray spectroscopy studies on the surface structural characteristics and electronic properties of platinum nanoparticles

The surface structural characteristics and electronic behavior of three platinum nanoparticle (NP) samples prepared with tertiary amine (Pt-TA), primary amine (Pt-PA), and thiol (Pt-SR) molecules were studied using Pt 4f, 5d, and S 2p x-ray photoelectron spectroscopy (XPS), Pt L(3)-edge x-ray absorption spectroscopy (XAS), and theoretical projected local density of states (l-DOS) calculations. Transmission electron microscopy and XPS composition analysis indicated that the three NPs were all very small (1-2 nm), the NP size decreasing in the order of Pt-TA>Pt-PA approximately Pt-SR. All the three samples showed a positive Pt 4f binding energy (BE) shift relative to that of the bulk, in the order of bulk<Pt-TA<Pt-PA<Pt-SR. The origin of the BE shift was elucidated by XAS and deconvolution of the Pt 4f XPS peak, indicating that the observed BE shifts were largely associated with the initial state effect (i.e., nanosize and surface structure). The surface and size effects on the electronic behavior of Pt were further studied by valence band XPS and the results were interpreted with calculated d-DOS of three Pt(55) model clusters with varied surface structures. Finally, the implication of these results on tuning the electronic properties of Pt NPs with size, surface, and alloying effects was discussed.