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Irradiation with swift heavy ions of spherical Au nanoparticles confined within a silica matrix shapes them into prolate nanorods and nanowires whose principal axes are aligned along the beam direction. In the present paper, we investigate the role that is played by the initial nanoparticle size and concentration in this so-called ion-shaping mechanism. We have produced silica films wherein Au nanoparticles with average diameters of 15, 30, and 45 nm were embedded within a single plane and have irradiated these films at 300 K at normal incidence with 18, 25, and 54 MeV Ag ions. We demonstrate the existence of both threshold and saturation fluences for the elongation effects mentioned. The values of these critical fluences depend both on the ion energy and the initial nanoparticle size. Moreover, we show that 45 nm Au particles are not deformed when irradiated with 18 MeV Ag ions, such that this value corresponds to an energy threshold for the deformation process. As far as the influence of the nanoparticle concentration on the shaping characteristics is concerned, we have found that above the critical irradiation fluence, the deformation effect becomes very sensitive to the initial concentration of the nanoparticles.

We show that the high-energy ion irradiation of embedded metallic spherical nanoparticles ͑NPs͒ is not limited to their transformation into prolate nanorods or nanowires. Depending on their pristine size, the three following morphologies can be obtained: ͑i͒ nanorods, ͑ii͒ facettedlike, and ͑iii͒ almost spherical nanostructures. Planar silica films containing nearly monodisperse gold NPs ͑8-100 nm͒ were irradiated with swift heavy ions ͑5 GeV Pb͒ at room temperature for fluences up to 5 ϫ 10 13 cm −2 . The experimental results are accounted for by considering a liquid-solid transformation of the premelted NP surface driven by the in-plane stress within the ion-deformed host matrix. This work demonstrates the interest of using ion-engineering techniques to shape embedded nanostructures into nonconventional configurations. © 2009 American Institute of Physics. ͓DOI: 10.1063/1.3186030͔Amorphous materials subjected to high-energy ion irradiation show irreversible anisotropic plastic flow at temperatures far below the glass transition temperature. 1 They shrink in the direction of the ion beam and expand in the direction perpendicular to it. On the other hand, for crystalline materials direct irradiation-induced deformation has never been observed. To overcome this limitation, a new strategy has been recently adopted to shape metallic nanoparticles ͑NPs͒. Deformation can be induced indirectly by embedding the NPs into an ion-deformable amorphous host matrix. [2][3][4][5][6] With this technique, spherical NPs deform into prolate nanorods and nanowires, with an aspect ratio that can be tuned by varying the irradiation conditions ͑ion type, energy and fluence͒. In this work, we show that the ion-shaping mechanism is not only limited to the transformation of spherical NPs into prolate nanorods/nanowires, but that, depending on the NP size and irradiation conditions, a different class of ionshaped NPs can be obtained, namely, embedded NPs with a facettedlike morphology. This work widens the potentialities of the ion-engineering technique to shape embedded nanostructures into nonconventional configurations, allowing, simultaneously, to tune the optical features of the corresponding composite glass. 7,8 Monodisperse spherical gold NPs, with average diameters of 8, 15, 50, 80, and 100 nm ͑size dispersion 10%͒, were confined within a 350 nm thick silica film deposited onto a silicon substrate. All the NPs are in a single plane 150 nm below the sample surface, such that the energy deposited is the same for all the NPs. For more details about the sample preparation we refer the reader to the literature. 6,9 The experiments were carried out with the aid of the GANIL facilities in Caen ͑France͒. High energy ͑HE͒ was used to obtain 5 GeV Pb ions. Samples were irradiated at room temperature ͑300 K͒ and at normal incidence for fluences ranging from 1 ϫ 10 13 up to 5 ϫ 10 13 cm −2 . In order to avoid any macroscopic heating, the ion flux was limited to 3 ϫ 10 8 ions cm −2 s −1 . The electronic stopping power of the Pb ions in both Si...

Systematic investigations of the energy loss threshold above which the irradiation-induced elongation of spherical Au nanoparticles occurs are reported. Silica films containing Au nanoparticles with average diameters of 15-80 nm embedded within a single plane were irradiated with 12-54 MeV Ag and 10-45 MeV Cu ions at 300 K and at normal incidence. We demonstrate that the efficiency of the ion-induced nanoparticle elongation increases linearly with the electronic energy transferred per ion track length unit from the energetic ions to the silica film. Ion beam shaping occurs above a threshold value of the specific electronic energy transfer. Three relevant regions are identified with respect to the original size of the Au nanoparticles. For 15 and 30 nm diameter particles, elongation occurs for electronic stopping power larger than 3.5 keV nm(-1). For Au nanoparticles with 40-50 nm diameter an electronic stopping power above 5.5 keV nm(-1) is required for elongation to be observed. Elongation of Au nanoparticles with 80 nm diameter is observed for electronic stopping between ∼ 7-8 keV nm(-1). For all combinations of ions and energies, the ion track temperature profiles are calculated within the framework of the thermal spike model. The correlation between experimental results and simulated data indicates a thermal origin of the increase in the elongation rate with increasing the track diameter.

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A finite-difference time-domain method is developed for studying the plasmon enhancement of light absorption from vertically aligned GaAs nanowire arrays decorated with Au nanoparticles. Vertically aligned GaAs nanowires with a length of 1 µm, a diameter of 100 nm and a periodicity of 165–500 nm are functionalized with Au nanoparticles with a diameter between 30 and 60 nm decorated in the sidewall of the nanowires. The results show that the metal nanoparticles can improve the absorption efficiency through their plasmonic resonances, most significantly within the near-bandgap edge of GaAs. By optimizing the nanoparticle parameters, an absorption enhancement of almost 35% at 800 nm wavelength is achieved. The latter increases the chance of generating more electron–hole pairs, which leads to an increase in the overall efficiency of the solar cell. The proposed structure emerges as a promising material combination for high-efficiency solar cells.

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