Ion-Beam-Assisted Deposition of Metal Nanocluster thin Films with Nonlinear Optical Properties
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oxidation of TaIr metallic glasses. [4] The second method involves in-situ oxidation of the active metal by means of oxygen ion beam bombardment during deposition. EXPERIMENTAL PROCEDURES The thin films were deposited by evaporation or IBAD in a multihearth IBAD vacuum chamber with a base pressure of lxl0-8 mm Hg. The evaporants consisted of Nb and Au. Typical evaporation rates were 1-1.5 As-1 for Nb and 0.2-0.4 As-1 for Au. Films were deposited simultaneously on Si and Si0 2 substrates. Optical properties were measured using the films deposited on Si0 2 substrates while the films on Si substrates were used for microstructural characterization by TEM due to the relative ease of sample preparation using Si substrates. The substrate temperatures ranged from room temperature to 730'C. Room temperature refers to a condition in which no heat was applied to the substrate; however, during deposition the substrate surface temperature rose to -90'C. Ion beams of 02+ (300-500 eV and 40-200 jiAcm-2) were used for IBAD. The highest 02+ current densities yielded -2.5 0 atoms for every Nb atom deposited. Post-deposition oxidizing anneals were conducted in flowing 02 at temperatures ranging from 200-800'C for times between 10 and 120 min. Nanocluster sizes and distributions were determined from TEM photomicrographs of planview or cross-sectional samples on Si substrates. Sample preparation for TEM was by conventional means. In the case of plan-view samples, the substrate was etched away from the back side leaving an electron-transparent film. Cross-sectional samples were thinned to electron transparency by ion-milling on a liquid nitrogen-cooled stage. The NLO properties of the nanoclusters in oxidized niobium were probed experimentally using DFWM and NLT according to procedures described in detail elsewhere.[5] The output of the laser system had a pulse width of less than 1.2 ps FWHM with pulse energies up to 1 mJ and was operated at 590.5 nm for these experiments. The spot size at the sample was 2.4 mm. Data analysis was based on the equations found in reference [6]. RESULTS Microstructure and Linear Optical Properties of Evaporated Au Nanocluster Films Figure 1 shows a bright field transmission electron micrograph (TEM) and electron diffraction pattern of a film consisting of Au nanoclusters in a matrix of randomly-oriented polycrystalline Nb 0 5 fabricated as follows. Niobium and gold were co-evaporated at rates of 1.2 As-] and 0.2 As-1, respectively, onto substrates of (100) Si and Si0 2 , held at room temperature.. After deposition, the films were annealed at 600'C for 30 min. The oxidizing anneal resulted in a change in the appearance of the film from opaque and metallic to transparent and blue in color. Energy dispersive X-ray analysis of the TEM sample from the Si substrate showed that the dark circular features in the film were rich in Au. The size of these Au nanoclusters ranged from 50-250A and examination of transverse TEM cross sections showed that their shape was spherical. Figure 2 is a plot of the optical absorpt
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