We investigate suspensions of 3-10 nm diameter Au, Pt, and AuPd nanoparticles as probes of thermal transport in fluids and determine approximate values for the thermal conductance G of the particle/fluid interfaces. Subpicosecond ϭ770 nm optical pulses from a Ti:sapphire mode-locked laser are used to heat the particles and interrogate the decay of their temperature through time-resolved changes in optical absorption. The thermal decay of alkanethiol-terminated Au nanoparticles in toluene is partially obscured by other effects; we set a lower limit GϾ20 MW m Ϫ2 K Ϫ1. The thermal decay of citrate-stabilized Pt nanoparticles in water gives GϷ130 MW m Ϫ2 K Ϫ1. AuPd alloy nanoparticles in toluene and stabilized by alkanethiol termination give GϷ5 MW m Ϫ2 K Ϫ1. The measured G are within a factor of 2 of theoretical estimates based on the diffuse-mismatch model.

Atomic force microscopy is used to characterize the evolution of film morphology produced by heavy-ion bombardment. Pt films, 3 and 5 nm thick, are deposited on SiO2 substrates and subsequently bombarded by 800 keV Kr+. Ion doses of >2×1014 initiate pattern formation and the dewetting of Pt films from the substrate. The film morphology becomes increasingly disconnected with increasing dose; at the highest doses, (∼2×1016 cm−2), isolated nanoparticles are formed with a uniform spacing. The results are explained by the nucleation of bare substrate patches and subsequent coarsening of the morphology by the molten zones created by individual Kr+ impacts.

Articles you may be interested inAcoustic interferometry for geoacoustic characterization in a soft-layered sediment environment J. Acoust. Soc. Am. 133, 82 (2013); 10.1121/1.4768879 Femtosecond laser-ultrasonic investigation of plasmonic fields on the metal/gallium nitride interface Appl. Phys. Lett. 97, 201102 (2010); 10.1063/1.3503633 Atomic diffusion bonding of wafers with thin nanocrystalline metal films Large excess volume in grain boundaries of stressed, nanocrystalline metallic thin films: Its effect on graingrowth kinetics Appl. Phys. Lett. 95, 163112 (2009); 10.1063/1.3248070 High-frequency surface acoustic wave propagation in nanostructures characterized by coherent extreme ultraviolet beams Appl. Phys. Lett. 94, 093103 (2009);Picosecond interferometry is used to study the acoustics waves created by heating Pt films with a subpicosecond laser pulse. Both the period of the initial oscillations in the metal film and the amplitude of the sound wave in the substrate are measured quantitatively. The platinum films are roughened by irradiation with energetic ions. The amplitude of the sound wave is doubled at those irradiation levels where the platinum coverage has been reduced by about one-half. A theory for the amplitude of the launched acoustic wave predicts that the acoustic amplitude is proportional to the mean square film thickness. Thus changes in the morphology of a partially perforated metal film can be observed using a simple, nondestructive optical technique.

Dewetting and nanopattern formation of 3–10 nm Pt thin films upon ion irradiation is studied using scanning electron microscopy (SEM). Lateral feature size and the fraction of exposed surface area are extracted from SEM images and analyzed as functions of ion dose. The dewetting phenomenon has little temperature dependence for 3 nm Pt films irradiated by 800 keV Kr+ at temperatures ranging from 80 to 823 K. At 893 K, the films dewet without irradiation, and no pattern formation is observed even after irradiation. The thickness of the Pt films, in the range 3–10 nm, influences the pattern formation, with the lateral feature size increasing approximately linearly with film thickness. The effect of different ion species and energies on the dewetting process is also investigated using 800 keV Kr+ and Ar+ irradiation and 19.5 keV He+, Ar+, Kr+, and Xe+ irradiation. The lateral feature size and exposed surface fraction scale with energy deposition density (J/cm2) for all conditions except 19.5 keV Xe+ irradiation.

Atomic force microscopy and cross-sectional transmission electron microscopy are used to characterize the evolution of nanoparticle/substrate interfaces during heavy-ion bombardment. Pt nanoparticles, prepared by annealing 3 Å Pt films on SiO2, embed into the substrates following 800 keV Kr+ irradiation. For Pt particles with diameters 5–20 nm, the depth of the embedding increases with an ion dose until the particles are fully submerged at a dose of ∼1016 cm−2. The results are explained by capillary driving forces and an ion-induced viscous flow of amorphous SiO2. The irradiation-induced viscosity of SiO2 needed to explain our results is ∼0.9×1023 Pa ion cm−2, consistent with previous measurements using stress relaxation. Similar results are obtained for 10 keV He+ irradiation, suggesting that ion-induced viscosity arises from localized defects rather than from the creation of large melt zones. The embedding of Pt particles is inhibited, however, for energetically unfavorable substrates such as alumina.