佐藤 井一

Surface modified nanometer-sized silver particles with mercaptosuccinic acid (MSA) are crystallized in the size range of few micron-meter. The esterification of carboxylic acid of MSA on the surface of particles is performed with n-butanol, enabling phase transfer from water to organic phase. The yield of esterification was found to be more than 90%.

Optical absorption properties of Au nanoparticle crystals are studied in the near infrared-to-near ultraviolet region. Although sparsely deposited Au nanoparticles show a small absorption peak at 2.4 eV, the Au nanoparticle crystals show a significantly stronger peak at 1.9 eV. Altering the temperature from room temperature to 77 K brings about very little difference in the intensity and width of the absorption peak. A qualitative model of the electronic polarizability is introduced to explain the observed peak shift as a result of assembling the nanoparticles into the 3D lattices.

Gold nanorods were electrochemically prepared in aqueous micellar solutions consisting of a supporting electrolyte surfactant and a hydrophobic cosurfactant. With the selection of cosurfactant. the nanorods grew to be submicrometers in length with very high aspect ratios. The growth axial direction of the rod was revealed to be [110]. the direction normal to high-energy surfaces. by electron diffraction measurements.

Three-dimensional superlattices consisting of gold nanoparticles were grown at air/suspension or suspension/solid interfaces. The growth of superlattices was found to be strongly dependent on substrate materials: Micrometer-sized superlattices were grown at air/suspension interfaces and upon silver substrates, whereas no growth was observed on silicon, silicon oxide, or amorphous carbon substrates. To explain the observed substrate dependence, Lifshitz theory was used to calculate the Hamaker constants between gold nanoparticle assemblies and substrates through the suspension. Van der Waals interactions estimated from this calculation fully explain the experimental results.

Surface modified nanometer-sized gold particles with mercaptosuccinic acid (MSA) are crystallized in the size range of a few micrometer. Two dimensional superlattice was also generated in the same system. The transmission electron microscope images of these superstructures and the reflectance spectrum are reported in comparison with that of bulk sample.

Plasmon loss features of porous silicon (PS) layers consisting of Si nanoparticles with average diameters of 3.6-4.5 nm were studied using x-ray photoelectron spectroscopy. The volume plasmons of the Si nanoparticles in the as-prepared PS layers have the same energy as that of bulk Si (17.4 eV). The surfaces of the PS layers were sputter etched by argon (Ar) ion bombardment to remove any hydrogen passivation. The plasmon energy for the surface-sputtered PS layers increases from 17.4 eV to either similar to 17.9 or 18.5 eV depending on the size of the nanoparticles, while the energy for bulk Si remains the same after the surface sputtering. A qualitative model with the plasma frequency modified from the free electron model was introduced to explain the observed characteristic features in the volume plasmons. (C) 1999 American Institute of Physics. [S0021-8979(99)04111-0].

The germanium (Ge) nanocrystals were deposited on substrates whose temperature was kept at room or liquid nitrogen (LN2) temperature by the cluster-beam evaporation technique. The deposited films are found to consist of the tetragonal crystalline structure rather than the diamond structure of bulk Ge. Such a phase-transition has been theoretically predicted for sizes smaller than 4 nm, which agrees with the size measured by the transmission electron microscopy (TEM), The tetragonal Ge is expected to have a direct band gap of 1.47 eV, Furthermore, the Ge film deposited at LN, temperature exhibits unique properties, such as photo-oxidation and blue-light emission, The Ge-nanocrystal films deposited by the cluster-beam evaporation technique are attractive materials for application to light emitting devices in future.

Plasmon loss features of germanium (Ge) nanocrystals fabricated by the cluster beam evaporation technique have been measured with Xray photoelectron spectroscopy (XPS). There is a significant difference between the plasmon energies of the Ge nanostructures and that of the bulk: 17.1-17.4 eV for the Ge nanocrystals; and 16.3 eV for the bulk. The plasmon energy of the Ge nanostructure, which remains at the higher value after annealing up to 700 degrees C, shifts to the lower value ( similar to 16.5 eV) after annealing at 800 degrees C. This energy difference is attributed to the crystal structure transformation. (C) 1999 Elsevier Science S.A. All rights reserved.

Germanium (Ge) nanocrystals are synthesized by a cluster-beam deposition technique. Raman and X-ray photoelectron spectroscopy (XPS) are used to investigate the crystal structure of the clusters. The phonon spectrum and the density-of-states of the valence band of the cluster-beam-deposited Ge nanoparticles bear a close resemblance to those of ST-12, which is a high pressure polymorph of Ge with a tetragonal structure. The usual diamond-like crystal structure of bulk Ge appears to be completely absent. The optical absorption spectrum from the near infrared to the ultraviolet also indicates an ST-12-like behaviour. (C) 1998 Elsevier Science S.A. All rights reserved.

Si nanocrystals were deposited in a helium atmosphere by the gas-evaporation technique. Their average size is 3.5 nm, much smaller than those of the Si nanocrystals deposited in an argon atmosphere. The PL spectra of the as-deposited and the HF-treated Si nanocrystals were compared. A great increase in the PL intensity of the HE-treated Si nanocrystals is attributed to the hydrogen passivation of Si surface dangling bonds. A good correlation between the amount of Si-O bonds and the PL intensity suggests that the oxygen-passivation of dangling bonds is required for the red-band FL. The PL spectra of the HE-treated Si nanocrystals resemble those of porous Si and clearly indicate that the HE-treated Si nanocrystals well simulate the porous Si.

The Ge nanostructures were deposited on substrates whose temperature was kept at room or liquid nitrogen (LN2) temperature by the cluster-beam evaporation technique. The deposited film is found to consist of the tetragonal crystalline structure, not the diamond structure in bulk Ge. Such a phase transition is theoretically predicted for sizes smaller than 4 nm, which agrees with the sizes measured by the transmission electron microscopy (TEM). The Ge film deposited at LN2 temperature exhibits unique properties such as photo-oxidation and blue-light emission.

The structure and the vibrational properties of germanium films deposited by the cluster-beam technique were investigated mainly using Raman spectroscopy. Germanium films were deposited on Si substrates at room temperature (Ge-RT) and liquid N2 temperature (Ge-LNT). Raman as well as other techniques (XRD, TEM, optical absorption and PL) were used to characterize these films. The nanostructures were observed in Ge-LNT by TEM micrograph, while flat continuous images were found in Ge-RT. Raman spectra of both Ge films showed a broad peak around 270 cm-1 different from that of cubic Ge (300 cm-1). Ge-RT films showed a line at 300 cm-1 by annealing the films above 700°C, while Ge-LNT did not give such line up to the 800°C annealing. The relation between these spectra and the observed nanostructure is discussed.

Oxygen-containing silicon (Si) nanostructures have been prepared on Si and SiO2 substrates by evaporation of Si powder in an oxygen-containing argon atmosphere. The structure and composition have been investigated by transmission electron microscope (TEM), optical absorption measurements in the near infrared-to-near ultraviolet region, and electron spectroscopy for chemical analysis (ESCA). Intertwined chain-like nanostructures have been noted in the TEM micrographs of the samples. The electron diffraction pattern of the nanostructure indicates that it is an amorphous state with no detectable Si crystallites. None of the crystalline particles have been detected by ESCA, the spectrum showing the presence of SiO2 and SiOx (x < 2). The optical bandgap of the material estimated from the optical absorption measurements is 3.4 eV, which is substantially lower than that of SiO2 (similar to 8 eV). The as-deposited ultrafine particles exposed to ultraviolet light emit blue light, which is strong enough to be seen with the naked eye. The blue light emission has been attributed to a broad photoluminescence (PL) peak at 2.7 eV The decay time of the blue light emission determined from the transient PL measurements is about 3 nsec, which is seven orders of magnitude faster than the similar blue light emission from SiO2 glass.