Ion beam surface nanostructuring of metallic thin films on dielectric substrates and their optical properties イオンビームによる金属薄膜/誘電体表面のナノ構造化とその光学特性
Ion beam surface nanostructuring of metallic thin films on dielectric substrates and their optical properties
Noble metal nanoparticles strongly adhered on the dielectric matrices have been extensively studied because of their pronounced applications in optical devices based on tunable localized surface plasmon resonance (LSPR) absorption band. To develop optical devices by using LSPR, it is important to establish a method to disperse noble metal nanoparticles homogeneously on dielectric substrates and to tune the wavelength of LSPR. Compared with chemical synthesis methods, the noble metal nanoparticles formed by ion irradiation on the dielectric substrate draw significant interest in recent years because of the distinct optical properties due to homogeneous single layer dispersion of noble metal nanoparticles and no interlayer deposition on dielectric substrates. However, the detailed investigation of noble metal nanoparticles’ microstructure in the dielectric substrate and their relationship with the optical property have been rarely conducted. Therefore, a detailed investigation of ion irradiation induced surface nanostructuring of gold thin films on amorphous SiO2 substrate is conducted by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). In addition, the dependence of optical property on surface microstructure was studied. Also, 100 keV Ar+-ion irradiation of Ag-Au bimetallic films on amorphous SiO2 and single crystal Al2O3 have been conducted, in order to investigate the dependence of optical property tuning behavior on the gold concentration and also on substrate structure.The main objectives of this research are highlighted in chapter 1; (a) to investigate the nanoparticles microstructure dependence on ion irradiation parameters; and (b) to investigate the optical properties dependence on the nanoparticles morphology. Chapter 2 gives a description of experimental procedures. The specimen preparation, ion irradiation condition, and the characterization methods including spectrophotometry, atomic force microscopy (AFM), SEM and TEM are described.In chapter 3, ion-beam induced suface nanostructuring and the burrowing of the nanostructures by the Ar+-ion irradiation are described. 100 keV Ar+-ions were irradiated to SiO2 substrate with 30 nm gold film on the surface with various fluences. The surface morphologies were investigated by AFM and SEM. Dewetting of Au thin films due to radiation-enhanced diffusion were observed. At the same time, the burrowing of the nanostructures into dielectric matrix was observed. The burrowing of these nanostructures was verified by cross sectional microstructure observation by TEM. Finally, a single layer of spherical Au nanoparticles which formed a single layer structure deeply embedded in the SiO2 substrate was obtained. The LSPR absorption band due to the localized surface plasmon excitation of these Au nanoparticles was also confirmed by photo absorption spectra. In addition, the dependence of the optical response on ion beam energy was studied and a shift of the LSPR absorption band towards the longer wavelength (red shift) with the increase of ion beam energy was obtained.In chapter 4, the control of various parameters sensitive to the LSPR absorption band (including particle size and shape) by irradiation fluence was described. Experimentally, 100 keV Ar+-ion irradiation of 30 nm Au(50%)-Ag(50%) films deposited on the SiO2 glass substrate was conducted. By increasing the irradiation fluence, the mean size of the nanospheroids decreased, the aspect ratio approached unity and the satellite nanoparticles were formed. It resulted in a shift of the LSPR absorption band towards the shorter wavelength up to an irradiation fluence of 1.0 × 1017 cm−2. The peak was then shifted towards longer wavelength because of the multi-sphere scattering effects due to the satellite nanoclusters. Further control of LSPR absorption band over a wider range has been achieved by synthesizing bimetallic nanoparticles fabricated in the form of alloys of two metals. Experimentally, 100 keV Ar+-ion irradiation of 30 nm pure silver, pure gold, and three different bimetallic Ag-Au films on SiO2 glass substrate have been conducted, and a single layer photosensitive Ag-Au bimetallic nanoparticles embedded in a SiO2 substrate was obtained. A remarkable LSPR peaks shifted towards the longer wavelength with the increase of the Au concentration was obtained. Gans theory has been used to model the optical response of these metallic nanoparticles embedded in SiO2 substrates. This theory accounts for the main effects associated with the major behaviors of the localized surface plasmon excitation.In chapter 5, substrate dependence was investigated by use single crystal Al2O3 substrate. The process of ion irradiation induced surface nanostructuring of 30nm Au-Ag bimetallic films on single crystal Al2O3 substrate was different from that on the amorphous SiO2 substrates due to the substrate structure difference. In case of single crystal Al2O3 substrate, higher fluence of 3.8 × 1016 cm−2 (in this study) was required to make the near surface amorphous. Therefore, ion irradiation can sufficiently increase the ion-induced viscous flow to burrow Au-Ag bimetallic nanoparticles into single crystal Al2O3 substrate. This further supported the present mechanism for the ion induced burrowing of rigid nanoparticles into viscous media. In addition, dependence of the LSPR absorption band on the chemical concentration for the Au-Ag bimetallic nanostructures was also observed; however, the tendency is diverted away from the tendency on the amorphous SiO2 substrate.Chapter 6 summarized the conclusions of all these studies. Based on the aforementioned results, ion irradiation is an effective approach in surface nanostructuring and in controlling the LSPR properties of the metallic films on dielectric substrates.
Hokkaido University（北海道大学）. 博士(工学)