Silver Negative-ion Implantation Research Articles

Silver Negative-Ion Implantation into Thermally Grown Thin SiO2 Film on Si Substrate and Heat Treatment for Formation of Silver Nanoparticles

Silver negative ion implantation into 50- and 25-nm-thick SiO2 film on Si and subsequent heat treatment were conducted to form nanoparticles. Optical properties showed formation of Ag nanoparticles in the films. Transmission electron microscope (TEM) images of the 50-nm-thick SiO2 sample implanted at 30 keV showed nanoparticles with diameter of several nm. The location of nanoparticles corresponded to the calculated profile. On the other hand, the 25-nm-thick SiO2 sample implanted at 10 keV with dose of 5×1015 ions/cm2 showed monolayered crystallized nanoparticles along the interface of SiO2/Si. The Rutherford backscattering spectrometry (RBS) spectra showed Ag atoms distributed at the surface and at a depth corresponded to the calculated profile after annealing at 500 °C. The surface accumulation seems to be resulted from thermal diffusion of implanted atoms during implantation. A small peak in concentration was also observed at 22 nm in depth. This suggests existence of a diffusion barrier for Ag atoms. At 700 °C, a main peak appeared at 20 nm in depth.

Photocatalytic properties of sol–gel titania film under fluorescent-light irradiation improved by silver negative-ion implantation

We have investigated on improvement of photocatalytic properties under fluorescent light of anatase-type titanium oxide film by using silver negative-ion implantation. The anatase films were prepared in a sol–gel method on a glass plate with a commercial sol material by spin-coating and baking at 500°C for 30min. Typical thickness of films was about 500nm. Silver negative ions were implanted to these films at 30keV with a dose in 1–5×1016ions/cm2. For forming Ag nanoparticles and recovering ion-induced defects, the implanted samples were annealed at temperature in a range of 400–600°C. All the samples showed a strong optical absorption at photon energy of about 2.2eV after annealing at 300–500°C. This means the existence of Ag nanoparticles in the films. In reduction test of methylene blue solution under fluorescent light, all samples showed improved photocatalytic properties. The best efficiency was obtained for the Ag-implanted sol–gel film with 1×1016ions/cm2 after annealing at 400°C, and it was higher by a factor of 1.6 than that of single rutile crystalline plate. The increase in the efficiency is considered to be due to existence of Ag nanoparticles in the film.

Formation of almost delta-layered nanoparticles in SiO2 thin film on Si substrate by metal negative-ion implantation

Two formation methods of delta-layered metal nanoparticles in thermally grown thin SiO2 layer on Si substrate were investigated: (1) gold negative-ion implantation at low-energy for nanoparticle formation at a shallow depth of the oxide layer and (2) thermal diffusion after silver negative-ion implantation for formation at a deep depth. Gold negative ions were implanted at 1, 15 and 35keV to SiO2/Si. At the low-energy implantation at 1keV, the almost delta-layer of Au nanoparticles (about 8nm in diameter) was obtained at a shallow depth of 5nm after annealing at 900°C. For formation at deep depth, silver negative-ions were implanted at 10keV and 1×1015ions/cm2 to 25-nm-thick SiO2/Si. After annealing at 700°C, all formed nanoparticles with 6nm in diameter aligned at the same deep depth, i.e. almost delta-layered Ag nanoparticles, at the minimum distance of 2nm from the boundary to Si. It is found from high resolution TEM observation that both almost delta-layered nanoparticles of Au and Ag were single crystals.

Silver negative-ion implantation to sol–gel TiO2 film for improving photocatalytic property under fluorescent light

We have investigated improvement of photocatalytic properties of spin-coated sol–gel anatase-type TiO2 with a bandgap of 3.2eV by negative ion-implantation. Silver-negative ions were implanted to anatase-type sol–gel TiO2 films (about 500nm in thickness) on a glass at 65 and 30keV with various doses in 1016–1017ions/cm2. Subsequent annealing was performed at up to 600°C for 1h. After annealing at above 300°C, all the samples showed strong optical absorption of surface plasmon resonance of Ag nanoparticles at around the photon energy of 2.2eV. This means that Ag nanoparticles were formed in the TiO2 films. In the photocatalytic properties under fluorescent light, the Ag-implanted sample (30keV, 1×1016ions/cm2) after annealing at 400°C showed an improved photocatalytic efficiency by 1.58 times than a rutile single crystal TiO2 with bandgap of 3.0eV less than that of the anatase. Ag nanoparticles formed at shallower depth are considered to work well for catalysis.

Delta layer formation of silver nanoparticles in thin silicon dioxide film by negative ion implantation

Two formation methods of nanoparticles distributed at a certain depth in a delta layer by negative ion implantation were investigated. Such delta layer formation of isolated metal nanoparticles is desired in a very thin gate oxide film, with thickness of less than 10 nm for the development of a single electron device using nanoparticles as very small capacitance for Coulomb blockade. We have tried two methods by metal negative ion implantation in a thermally grown SiO 2 film on Si substrate and postannealing for (1) a delta layer formation of gold nanoparticles in the surface region of SiO 2 film, and for (2) a delta layer formation of silver nanoparticles in the bottom region of SiO 2 film. In the former with gold negative ion implantation with a low energy, the depth distribution of nanoparticles formed at annealing at less than 900 °C depended on depth deviation of implanted atoms. The almost delta layer of Au nanoparticles with diameters of 4–8 nm was obtained just under the surface by Au negative ion implantation at 1 keV and annealing at 900 °C. In the latter with silver negative ion implantation at 10 keV, 1×10 15 ions/cm 2, 25-nm-thick SiO 2/Si, and annealing at 700 °C, Ag nanoparticles with a diameter of 6–8 nm were aligned at the same depth in a delta layer in the bottom region of the SiO 2 at a distance of 2 nm from the boundary.

Silver nanoparticle formation in thin oxide layer on silicon by silver-negative-ion implantation for Coulomb blockade at room temperature

Formation of silver nanoparticles formed by silver negative-ion implantation in a thin SiO2 layer and its I–V characteristics were investigated for development single electron devices. In order to obtain effective Coulomb blockade phenomenon at room temperature, the isolated metal nanoparticles should be in very small size and be formed in a thin insulator layer such as gate oxide on the silicon substrate. Therefore, conditions of a fine particles size, high particle density and narrow distribution should be controlled at their formation without any electrical breakdown of the thin insulator layer. We have used a negative-ion implantation technique with an advantage of “charge-up free” for insulators, with which no breakdown of thin oxide layer on Si was obtained. In the I–V characteristics with Au electrode, the current steps were observed with a voltage interval of about 0.12V. From the step voltage the corresponded capacitance was calculated to be 0.7aF. In one nanoparticle system, this value of capacitance could be given by a nanoparticle of about 3nm in diameter. This consideration is consistent to the measured particle size in the cross-sectional TEM observation. Therefore, the observed I–V characteristics with steps are considered to be Coulomb staircase by the Ag nanoparticles.

Formation of silver nanoparticles in thin oxide layer on Si by negative-ion implantation

Formation of nanoparticles in a 50-nm-thick SiO 2 film on Si was conducted by silver negative ion implantation at 30 keV with various doses, and the state of nanoparticles in the thin oxide film was observed by a cross-sectional scanning TEM. The sample implanted with 1 × 10 15 ions/cm 2 showed nanoparticles with diameter of about 2–3 nm distributed in the middle of the SiO 2 film on Si. The location of nanoparticles was in good agreement with the calculated profile by TRIM-DYN. Nanoparticles with various diameters of 4–8 nm formed in samples of 1 × 10 16 and 1 × 10 17 ions/cm 2. The size of silver nanoparticles was found to relate strongly to density of implanted atoms in the layer.

Improvement of photocatalytic efficiency of rutile titania by silver negative-ion implantation

Optical absorption and photocatalytic properties of metal negative-ion implanted rutile-type titania (TiO 2) were studied to show metal nanoparticle formation in the surface region and improvement of photocatalytic efficiency. Silver negative ions were implanted into titania substrate (rutile crystalline substrate) at 65 keV with a dose of 3 × 10 16–1 × 10 17 ions/cm 2. The Ag-implanted rutile also showed a strong optical absorption near 2.1 eV after annealing above 400 °C, those well agreed with theoretical absorption peaks due to surface plasmon resonance. This means that Ag nanoparticles were formed in the surface layers of the titania. Photocatalytic efficiencies for Ag-implanted titania were evaluated by means of decolorization of methylene blue solution under fluorescent light. Ag-implanted titania (Ag: 65 keV, 5 × 10 16 ions/cm 2, 500 °C annealed) showed the better photocatalytic efficiency higher 2.2 times than that of unimplanted rutile titania. In the evaluation under fluorescent light through UV-cut filter, the Ag-implanted rutile showed 6.7 times better efficiency.

Neuron cell positioning on polystyrene in culture by silver-negative ion implantation and region control of neural outgrowth

A new method to control the position of neuron cell attachment and extension region of neural outgrowth has been developed by using a pattering ion implantation with silver-negative ions into polystyrene dishes. This technique offers a promising method to form an artificially designed neural network in cell culture in vitro. Silver-negative ions were implanted into non-treated polystyrene dishes (NTPS) at conditions of 20 keV and 3×1015ions/cm2 through a pattering mask, which had as many as 67 slits of 60μm in width and 4 mm in length with a spacing of 60μm. For cell culture in vitro, nerve cells of PC-12h (rat adrenal phechromocytoma) were used because they respond to a nerve growth factor (NGF). In the first 2 days in culture without NGF, we observed a selective cell attachment only to the ion-implanted region in patterning Ag− implanted polystyrene sample (p-Ag/NTPS). In another 2 days in culture with NGF, the nerve cells expanded neurites only over the ion-implanted region. For collagen-coated p-Ag/NTPS sample of which collagen was coated after the ion implantation (Collagen/p-Ag/NTPS), most nerve cells were also attached on the ion-implanted region. However, neurites expanded in both ion-implanted and unimplanted regions. The contact angle of NTPS decreased after the ion implantation from 86° to 74°. The region selectivity of neuron attachment and neurite extension is considered to be due to contact angle lowering by the ion implantation as radiation effect on the surface.