Optical Properties of Nonspherical Silver Nanoparticles Embedded in a Plasma Polymer Thin Film Matrix
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1 2 an a.c. discharge with a power density of 0.15 W cm- at a monomer flow rate of 0.1 Pa 1 s- . During the plasma polymerization, silver particles were embedded by simultaneous evaporation. The deposition reactor was described in more detail in [2]. The nanoparticle-containing plasma polymer films were deposited as multilayer systems with metal particles only in one plane between two plasma polymer layers. Electron micrographs of these multilayer systems showed well-separated single particles. For the experiments presented in this paper, potassium bromide coated glass slides and uncoated quartz slides were placed in the depostion chamber. For the TEM-investigations, the films were lifted off in distilled water and picked up with usual copper grids. The quartz slides were used 100 nm of the multilayer system was determined for the optical measurements. The thickness d by interferometry. Electron microscopy was carried out using a Philips electron microscope CM 20 FEG at 200 kV acceleration voltage. The optical extinction spectrum was measured with a commonly used 1 spectrophotometer in the spectral region between 10000 cm-1 and 35000 cm- .
RESULTS Particle size and shape Size and shape of the embedded particles which are fixed during film deposition mainly depend on the amount of the evaporated metal, the vacuum pressure during the deposition and the distance of the sample from the metal evaporation source. As shown formerly [3], the particles have a broad size distribution which can be described with a log-normal distribution. Furthermore, for samples with mean particle sizes larger than z 5 nm deviations from the spherical shape cannot be neglected. Fig. 1 shows a TEM-micrograph of a sample prepared as described above and giving an impression of the sizes and shapes of the embedded silver nanoparticles. In order to get quantitative results, the TEM-micrograph was analysed with an optical image processing system (Kontron KS 400). The TEM-micrograph from Fig. 1 was scanned with a CCD camera and transfered into a digital image with a size of 784 x 600 pixels, using several image improvement routines. In this paper, also with respect to the results of the cross-sectional transmission electron microscopy (XTEM) examination [11], we assume the particles to be elongated spheroids (rotational symmetric ellipsoids), which are described by the mean half axis A (major axis) and the lower half axis B (minor axis). The values for A and B were determined from image processing based on the pixel coordinates of the digital image. In total, 1049 particles were analyzed. The results are given in a three-dimensional histogram (Fig. 2). The x-direction gives the minor axis B, the y-direction the major axis A and the z-direction the number of particles found in the A-B-interval (count). The mean particle size is 2R = 22.6 ± 9.4nm. There is a substantial number of large particles with an eccentricity e = 1 - (B/A) 2 > 0. Especially, these particles have a considerable influence on the half width of the plasma resonance absorption.
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