We perform a set of experiments on photo-heating in a water droplet containing gold nanoparticles (NPs). Using photo-calorimetric methods, we determine efficiency of light-to-heat conversion (η) which turns out to be remarkable close to 1, (0.97< η <1.03). Detailed studies reveal a complex character of heat transfer in an optically-stimulated droplet. The main mechanism of equilibration is due to convectional flow. Theoretical modeling is performed to describe thermal effects at both nano- and millimeter-scales. Theory shows that the collective photo-heating is the main mechanism. For a large concentration of NPs and small laser intensity, an averaged temperature increase (at the millimeter-scale) is significant (~ 7 °C) whereas, on the nanometer scale, the temperature increase at the surface of a single NP is small (0.02 °C). In the opposite regime, a small NP concentration and intense laser irradiation, we find an opposite pictures: a temperature increase at the millimeter-scale is small (0.1 °C) but a local, nanoscale temperature has strong local spikes at the surfaces of NPs (3 °C). These studies are crucial for the understanding of photo-thermal effects in NPs and for their potential and current applications in nano-and bio -technologies.

No abstract

A thin film of Al(0.94)Ga(0.06)N embedded with Er(3+) ions is used as an optical temperature sensor to image the temperature profile around optically excited gold nanostructures of 40 nm gold nanoparticles and lithographically prepared gold nanodots. The sensor is calibrated to give the local temperature of a hot nanostructure by comparing the measured temperature change of a spherical 40 nm gold NP to the theoretical temperature change calculated from the absorption cross section. The calibration allows us to measure the temperature where a lithographically prepared gold nanodot melts, in agreement with the bulk melting point of gold, and the size of the nanodot, in agreement with SEM and AFM results. Also, we measure an enhancement in the Er(3+) photoluminescence due to an interaction of the NP and Er(3+). We use this enhancement to determine the laser intensity that melts the NP and find that there is a positive discontinuous temperature of 833 K. We use this discontinuous temperature to obtain an interface conductance of ∼10 MW/m(2)-K for the gold NP on our thermal sensor surface.

A temperature-dependent photoluminescent thin film of Al(0.94)Ga(0.06)N doped with Er(3+) is used to measure the temperature of lithographically prepared gold nanodots. The gold nanodots and thin film are excited simultaneously with a continuous wave (CW) Nd:YAG 532 nm laser. The gold nanodot is submersed under water, and the dot is subsequently heated. The water immediately surrounding the nanodot is superheated beyond the boiling point up to the spinodal decomposition temperature at 594 ± 17 K. The spinodal decomposition has been confirmed with the observation of critical opalescence. We characterize the laser scattering that occurs in unison with spinodal decomposition due to an increased coherence length associated with the liquid-liquid transition.

Cadmium chalcogenide nanocrystals combined with co-catalyst nanoparticles hold promise for efficient solar to hydrogen conversion. Despite the progress, achieving high efficiency is hampered by high charge recombination rates and sample degradation. Here, we vary the decoration of platinum nanoparticles on CdS nanorods to demonstrate the important role of pathways for the photoelectrons to the co-catalyst. Contrary to expectations, the shortening of the path, by increasing the number of co-catalyst particles, increases the transfer rate but decreases the photocatalytic performance. This is because subsequent electron transfer to the acceptor is much slower; therefore, the recombination rate with the nearby holes increases. We show that with tip-decorated nanorods, the quantum yield of H2 production can reach and sustain nearly 90%, provided an efficient mechanism of mediated hole extraction is employed. The approach demonstrates that highly efficient photocatalysts may be prepared with only a minimal amount of co-catalyst and thereby suggests future pathways for solar to H2 conversion with semiconductor nanocrystals.

Abstract.Highly porous poly (ε-caprolactone) microfiber scaffolds can be fabricated using electrospinning for tissue engineering applications. Melt electrospinning produces such scaffolds by direct deposition of a polymer melt instead of dissolving the polymer in a solvent as performed during solution electrospinning. The objective of this study was to investigate the significant parameters associated with the melt electrospinning process that influence fiber diameter and scaffold morphology, including processing temperature, collection distance, applied voltage and nozzle size. The mechanical properties of these microfiber scaffolds varied with microfiber diameter. Additionally, the porosity of scaffolds was determined by combining experimental data with mathematical modeling. To test the cytocompatability of these fibrous scaffolds, we seeded neural progenitors derived from murine R1 embryonic stem cell lines onto these scaffolds where they could survive, migrate, and differentiate into neurons, demonstrating the potential of these melt electrospun scaffolds for tissue engineering applications.

No abstract

The thermo-optical properties of gold nanoparticles (NPs) embedded in an ice matrix were investigated using photoluminescence and Raman spectroscopy. An intense laser beam alone will not melt ice, but the addition of embedded Au NPs allows for melting with resonant laser light of relatively weak intensity. This is due to the strong absorption of Au NPs in the plasmon resonance regimen. We can determine the threshold melting power, P melting (T), where T is the background temperature by recording time-resolved Raman scattering signals of the system. A resultant loss of ice signal indicates melting and an absence of conversion to water implicates an irreversible loss of water molecules to the gas phase due to the location of the Au NP agglomerate at or near the ice/vapor surface. For fully embedded NP agglomerates, the ice/water phase transition can be monitored through Raman spectroscopy and the number of NPs in an agglomerate and their interactions can have a greater effect on localized heat generation. The local temperature inside and around the NP agglomerate depends strongly on its geometry and leads to a large scatter in the measured P melting as a function of the reduced temperature for different agglomerates. Immobilized Au NP agglomerates can also be characterized using single-particle spectroscopy, and results show that the plasmon emission of Au NPs scales with the number of NPs in an agglomerate.