Silver Cluster Formation in Implanted A1 2 O 3 Single Crystals
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Here, we report on the results of a multitechnique study, including RBS, optical absorption and Raman spectroscopy, on the Ag-cluster formation in implanted sapphire crystals. RBS experiments were used to determine the depth-profile of the implanted ions. The formation of nanoclusters of Ag in A12 0 3 gives rise to strong optical absorption at the surface plasmon energy and this is responsible for the coloration of samples. Optical absorption spectroscopy in the UV-Vis region was exploited to study the effects of the ion implantation on the surface plasmon resonance of Ag colloids. The broad absorption band observed at about 440 nm in heavily implanted samples shifts to shorter wavelengths in samples which have been implanted with low fluences of silver ions. Low-frequency Raman spectroscopy measurements were also performed under nearlyresonant excitation with the Ag surface plasmon energy. A strong scattering from acoustic vibrations localized at the surface of the silver clusters is observed. Both optical absorption and Raman spectroscopy results indicate an increasing size of Ag colloids with the ion fluence, and, from low-frequency Raman spectra, an estimation of the mean size of metals particles is obtained. EXPERIMENTAL PROCEDURES Commercial A12 0 3 crystals were cut in form of small pallets with the typical dimensions of 5x5xl mm and then implanted at room temperature with 300 keV-Ag÷. The ion fluences ranged from 0.8 to 13.6x1016 ions/cm2 and the current density was 1 gA/cm2 . The implanted samples were stored in air. Under white light, the samples implanted at low doses show a pale yellowish color, which tends to became more and more reddish at higher ion fluences. RBS measurements were carried out using 2.0 MeV He+ beams and the scattering angle was 1650. The optical absorption spectra in the range 200-800 nm (or 1.55-6.2 eV) were measured at room-temperature with a Carry 14 dual beam spectrophotometer. Virgin A120 3 crystals was used as a reference sample. Low-frequency Raman scattering spectra were collected at room temperature using standard equipment consisting of a 1 meter focal length, Jobin-Yvon double-pass monochromator equipped with holographic gratings (2000 lines/mm) and a photon counting system. For the spectra, excitation with either 488.0 nm or 514.5 nm lines of an Ar ion laser were employed. During the measurements the samples were mounted on a home-made sample holder to form an angle near to Brewster angle with the incident beam. Two significant polarization settings were used in order to probe the symmetry character of the vibrational modes responsible for the observed scattering, with the electric field of the incident beam parallel (V) or perpendicular (H) to the scattering plane, the scattered light being filtered by a polarizer with vertical axis (V). These settings allowed us to
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obtain VV and HV spectra, respectively. The laser operates so that both spectra, and, therefore, the two spectral intensities, can be used to determine the depolarization factor of the low energy scat
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