Correlating Carrier Density and Emergent Plasmonic Features in Cu_2–<i>x</i>Se Nanoparticles

Recently, a wide variety of new nanoparticle compositions have been identified as potential plasmonic materials including earth-abundant metals such as aluminum, highly doped semiconductors, as well as metal pnictides. For semiconductor compositions, plasmonic properties may be tuned not only by nanoparticle size and shape, but also by charge carrier density which can be controlled via a variety of intrinsic and extrinsic doping strategies. Current methods to quantitatively determine charge carrier density primarily rely on interpretation of the nanoparticle extinction spectrum. However, interpretation of nanoparticle extinction spectra can be convoluted by factors such as particle ligands, size distribution and/or aggregation state which may impact the charge carrier information extracted. Therefore, alternative methods to quantify charge carrier density may be transformational in the development of these new materials and would facilitate previously inaccessible correlations between particle synthetic routes, crystallographic features, and emergent optoelectronic properties. Here, we report the use of ⁷⁷Se solid state nuclear magnetic resonance (NMR) spectroscopy to quantitatively determine charge carrier density in a variety of Cu_2–<i>x</i>Se nanoparticle compositions and correlate this charge carrier density with particle crystallinity and extinction features. Importantly, we show that significant charge carrier populations are present even in nanoparticles without spectroscopically discernible plasmonic features and with crystal structures indistinguishable from fully reduced Cu₂Se. These results highlight the potential impact of the NMR-based carrier density measurement, especially in the study of plasmon emergence in these systems (i.e., at low dopant concentrations)

Efficient Energy Transfer from Near-Infrared Emitting Gold Nanoparticles to Pendant Ytterbium(III)

Here, we demonstrate efficient energy transfer from near-infrared-emitting <i>ortho</i>-mercaptobenzoic acid-capped gold nanoparticles (AuNPs) to pendant ytterbium­(III) cations. These functional materials combine the high molar absorptivity (1.21 × 10⁶ M^–1 cm^–1) and broad excitation features (throughout the UV and visible regions) of AuNPs with the narrow emissive properties of lanthanides. Interaction between the AuNP ligand shell and ytterbium is determined using both nuclear magnetic resonance and electron microscopy measurements. In order to identify the mechanism of this energy transfer process, the distance of the ytterbium­(III) from the surface of the AuNPs is systematically modulated by changing the size of the ligand appended to the AuNP. By studying the energy transfer efficiency from the various AuNP conjugates to pendant ytterbium­(III) cations, a Dexter-type energy transfer mechanism is suggested, which is an important consideration for applications ranging from catalysis to energy harvesting. Taken together, these experiments lay a foundation for the incorporation of emissive AuNPs in energy transfer systems

Clathrate Ba₈Au₁₆P₃₀: The “Gold Standard” for Lattice Thermal Conductivity

A novel clathrate phase, Ba₈Au₁₆P₃₀, was synthesized from its elements. High-resolution powder X-ray diffraction and transmission electron microscopy were used to establish the crystal structure of the new compound. Ba₈Au₁₆P₃₀ crystallizes in an orthorhombic superstructure of clathrate-I featuring a complete separation of gold and phosphorus atoms over different crystallographic positions, similar to the Cu-containing analogue, Ba₈Cu₁₆P₃₀. Barium cations are trapped inside the large polyhedral cages of the gold–phosphorus tetrahedral framework. X-ray diffraction indicated that one out of 15 crystallographically independent phosphorus atoms appears to be three-coordinate. Probing the local structure and chemical bonding of phosphorus atoms with ³¹P solid-state NMR spectroscopy confirmed the three-coordinate nature of one of the phosphorus atomic positions. High-resolution high-angle annular dark-field scanning transmission electron microscopy indicated that the clathrate Ba₈Au₁₆P₃₀ is well-ordered on the atomic scale, although numerous twinning and intergrowth defects as well as antiphase boundaries were detected. The presence of such defects results in the pseudo-body-centered-cubic diffraction patterns observed in single-crystal X-ray diffraction experiments. NMR and resistivity characterization of Ba₈Au₁₆P₃₀ indicated paramagnetic metallic properties with a room-temperature resistivity of 1.7 mΩ cm. Ba₈Au₁₆P₃₀ exhibits a low total thermal conductivity (0.62 W m^–1 K^–1) and an unprecedentedly low lattice thermal conductivity (0.18 W m^–1 K^–1) at room temperature. The values of the thermal conductivity for Ba₈Au₁₆P₃₀ are significantly lower than the typical values reported for solid crystalline compounds. We attribute such low thermal conductivity values to the presence of a large number of heavy atoms (Au) in the framework and the formation of multiple twinning interfaces and antiphase defects, which are effective scatterers of heat-carrying phonons