佐藤 井一

Porous Si films were formed on electrically insulative, semiconductive, and conductive substrates by depositing aqueous and nonaqueous Si nanoparticle inks. In this study, we focused on whether the Si ink deposition resulted in the formation of uniform porous Si films on various substrates. As a result of the experiments, we found that the inks showing better substrate wettabilities did not necessarily result in more uniform film formation on the substrates. This implies that the ink-solvent wettability and the nanoparticle-substrate interactions play important roles in the uniform film formation. As one of the interactions, we discussed the influence of van der Waals interactions by calculating the Hamaker constants. The calculation results indicated that the uniform film formation was hampered when the nanoparticle surface had a repulsive van der Waals interaction with the substrate.

Hole injection barriers at the regioregular-poly-3(hexylthiophene) (RR-P3HT)/metal (Cu or Ag) interface were investigated using the accumulated charge measurement (ACM). Thermal annealing of RR-P3HT at 55 degrees C decreased the injection barrier. RR-P3HT thermally annealed in N-2 forms an ohmic contact with Ag and a Schottky contact with Cu. The obtained values of the injection barriers, phi(B) were well expressed by the Mott-Schottky rule, i. e., phi(B) = IE - W-m', where IE is the ionization energy of RR-P3HT and W-m' is the work function of the metal electrode in air. The effect of the large vacuum level shift, reported in UPS studies conducted under ultrahigh vacuum, was not observed. (C) 2017 Elsevier B.V. All rights reserved.

The effect of the offset bias voltage on the threshold voltage of the hole injection into the organic-semiconductor (OS) layer was examined in detail in the accumulated charge measurement (ACM) for the n-type Si/SiO2/OS/Ag (OS = zinc phthalocyanine [ZnPc] or metal-free phthalocyanine [H2Pc]) capacitor. The threshold highly depends on the offset bias voltages, when the OS layer is in the hole depletion regime. On the contrary, the threshold was nearly constant when the OS layer operated in the hole-accumulation regime. The hole injection barrier of the Ag/OS interface was obtained by the threshold in the accumulation regime. The obtained values were 0.41 and 0.05 eV for H2Pc/Ag and ZnPc/Ag interfaces, respectively. The study revealed that accurate estimation of the injection barrier is possible by examining the offset voltage dependence in the ACM.

The charge-injection barrier from metal electrodes to thin-film pentacene is investigated using accumulated charge measurements. When a gold electrode is deposited on a pentacene film, the interface forms a Schottky contact with a hole-injection barrier of 0.2 eV. However, interfacial carrier motion is reversible between charge injection and discharge. The result suggests that the reported electrical hysteresis in typical pentacene transistors is caused by carrier traps that are localized primarily in the SiO2/pentacene interface. The Ag/pentacene junction has a large barrier height of 0.5 eV. The barrier height is significantly reduced, and an Ohmic contact is realized by using molybdenum oxide (MoO3) as a buffer layer.

We report an experimental investigation of the magnetic field effect (MFE) in polymer bulk heterojunction devices at temperatures below 10 K using photocarrier extraction by linearly increasing voltages. The examined devices were composed of an active layer of poly(3-hexylthiophene) and [6,6]-phenyl-C-61-butyric acid methyl ester. In the experiments, the delay time (t(d)) dependence of the MFE was investigated in detail. For t(d) < 80 mu s, a positive MFE was observed in the field region B < 0.1 T and a negative MFE was observed for B > 0.2 T. For t(d) > 8 ms, only a positive MFE proportional to B-2 was observed. For the photocurrent pulse detected immediately after light irradiation, the MFE was negligibly small. In a high magnetic field of 15 T, a significant MFE exceeding 80% was observed at 1.8 K for t(d) = 800 ms. We discuss the results based on a model of triplet-singlet (or singlet-triplet) conversion in the magnetic field and estimate the exchange integral for the charge-transfer exciton in this photovoltaic cell.

Water soluble Si particles were produced by modifying the surface with mercaptosuccinic acid (MSA). Using the MSA modified Si particle ink, porous-like Si films with resistivities of 10(2) Omega cm were formed by annealing above 260 degrees C. Current-voltage (I-V) characteristics of the annealed films were ohmic in N-2. When the films were exposed to air, the current decreased and the I-V curve became nonlinear. This nonlinear characteristic could be explained by the Poole-Frenkel model, which indicated that adsorbed gas molecules produced traps for free carriers. The I-V curve returned to the original shape by changing the atmosphere gas back to N-2. The semiconductive porous-like Si films made from the Si particle inks are expected to be applied to a variety of electronic devices such as gas sensors and high capacity anodes for lithium-ion batteries. (C) 2015 Elsevier B.V. All rights reserved.

Ethoxy-terminated silicon quantum dots (Si QDs) were prepared by a simple ball milling method of bulk Si in ethanol. Size (diameter) and band-gap energy (E-g) were 2.6 +/- 0.3 nm and 2.1 eV by TEM and absorption spectroscopy, respectively. The main surface termination was found to be Si-OC2H5 by attenuated total reflection IR spectroscopy. Exciton population dynamics of Si QDs was investigated by femtosecond near-IR transient absorption spectroscopy. Intensity-dependent exciton dynamics revealed that the multiple excitons generated by high photon flux were captured to the trapping sites immediately before Auger recombination.

A conventional laser microscope can be used to derive the index of refractivity by the ratio of geometrical height of the transparent platelet to the apparent height of the normal incident light for very small crystals in the wide size range. We demonstrate that the simple method is effective for the samples from 100 mu m to 16 mu m in size using alkali halide crystals as a model system. The method is also applied for the surface fractured micro-crystals and an inclined crystal with microscopic size regime. Furthermore, we present two-dimensional refractive index mapping as well as two-dimensional height profile for the mixture of three alkali halides, KCl, KI, and NaCl, all are mu m in size. (C) 2014 AIP Publishing LLC.

We show that aqueous inks containing Au nanocrystals that were slightly modified with mercaptosuccinic acid (MSA, IUPAC name: sulfanylsuccinic acid) turn into conductive films after being cured in room-temperature air. Because of the highly hydrophilic nature of MSA, the nanoparticles maintain good dispersion in water. The conductive film exhibits an electrical resistivity in the order of 10(-5) Omega cm, and its temperature coefficient in the range from 290 to 20K was 7 x 10(-5)K(-1) without any post-treatments such as annealing, light irradiation, or solution-immersing processes.

A Si nanoparticle surface was etched in an aqueous HE solution under monochromatic light irradiation in the infrared-to-visible region. The average size of the nanoparticles was controlled by changing the photon energy of the irradiating light. After prolonged photoetching, the size distribution of the nanoparticles was narrowed significantly. The correlation between the photon energy and the final size of the nanoparticles indicated that the band gap energy change due to the quantum size effect played an important role in controlling nanoparticle size.

Micrometer-sized three-dimensional superlattices were fabricated by assembling Au nanocrystals modified with mercaptosuccinic acid in aqueous suspensions. Each superlattice formed a single crystal arrangement with a hexagonal close-packed structure where a = 5.1 nm. The superlattices were densely deposited on a glass substrate and their electrical properties were investigated. The superlattice-assembled film showed ohmic behavior in the temperature range from 20 to 285 K. The absolute value of the resistivity was in the semiconductor region, but the resistivity slightly decreased as the temperature decreased. The temperature coefficient of the film was one order of magnitude smaller than that of bulk Au. (c) 2011 The Japan Society of Applied Physics

Micrometer-sized three-dimensional superlattices with different lattice constants were fabricated by assembling Au nanocrystals modified with mercaptosuccinic acid in aqueous suspensions. The refractive index of the superlattices was varied between 2.3 and 3.3 by altering the size of the component nanocrystals between 3.11 and 4.68 nm. As a result, the metal content of the superlattices varied between 23 and 36%. The correlation between nanocrystal size and the optical permittivity estimated from the refractive index was explained using the Maxwell-Garnett theory. (C) 2010 The Japan Society of Applied Physics

Polyacrylamide gel electrophoresis (PAGE) is used to separate the component of the glutathione protected gold clusters up to Au(160) core size and compare the size with the mobility data from PAGE. Both electrospray mass spectrometry and transmission electron microscopy were systematically applied to analyze Au-glutathione clusters thus isolated. We have found a linear relationship between the mobility of the gold cluster compounds to the logarithm of the number of gold atoms of the core. Better fitting to the logarithm of the number of thiolate molecules adsorbed on the gold core was found and was expressed by a simple equation that bears conformation of ligand and charge density of Clusters. Hence the mobility of the metallic clusters was correlated to the component of the ligand of the cluster.

A germanium (Ge) nanoparticle surface was etched in an aqueous solution under monochromatic light irradiation in the infrared-to-ultraviolet region. Since the bandgap widened up to the photon energy of the irradiating light, the average size of the nanoparticles was controlled and the size distribution narrowed. The quantum size effect explained the correlation between the resulting bandgap energy and the final size. The etched nanoparticles showed blue-green or red-infrared photoluminescence (PL) after the surfaces were terminated with organic molecules or hydrogen atoms. The PL peak energy was independent of size, indicating PL was due to radiative recombination via localized states at the Ge core surface. (C) 2009 Elsevier Ltd. All rights reserved.

Crystalline SiO2 clusters of nearly square shape were fabricated from amorphous silicon deposited on a clean Si(001) surface, where the deposited silicon was oxidized in the presence of atomic hydrogen and annealed in an O-2 atmosphere. The side of these clusters was aligned along the (110) and (1 (1) over bar0) azimuths of the Si(001) substrate. The crystallinity of the clusters was verified by grazing incidence x-ray diffraction and low-energy electron diffraction. The crystal structure of SiO2 clusters was estimated to be beta-tridymite, whose (10 (1) over bar0) and (0001) axes were parallel to the (110) axes of the substrate Si(001). When the Si(001) surface without deposited amorphous silicon was oxidized by the method described above, single-crystal SiO2 clusters were also prepared. However, the shape, the size, and the number density of the clusters were different from those of the clusters made from the amorphous silicon. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3103331]

A quality superlattice made of hydrophilic An nanoparticles was grown at an air/water interface over several days or a week; during this period, it is possible for a self-correcting process to take place, which revealed both narrow angle and wide angle electron diffraction, indicating that there were both translational and orientational ordering in a superlattice. The model structure of this superlattice was constructed on the basis of a truncated octahedral shape at an atomic level. The ordering misfit angle was derived from the analysis of the crescent pattern in a wide-angle diffraction region.

We synthesized micrometer-sized three-dimensional superlattices consisting of gold (Au) nanocrystals modified with three types of water-soluble thiolates in different dimensions: mercaptosuccinic acid (MSA), N-(2-mercaptopropionyl)-glysine (MPG), and glutathione in its reduced form (GTR). The nanoparticles were assembled at an air/water interface by decreasing the repulsive interactions between the surface modifiers. This was accomplished by adjusting acid concentration. Using x ray and transmission electron diffractometry, MSA- and MPG-modified Au nanoparticle superlattices were identified as hcp single-crystal structures, while the GTR-modified nanoparticle superlattices were determined to be a fcc single-crystal structure. Optical absorption studies show that the surface plasmon peaks redshifted as the nanoparticle density of the assemblages increased and the peak widths decreased when the nanoparticles formed lattice arrangements. In contrast, optical absorption studies of randomly assembled nanoparticles demonstrated increased peak widths as nanoparticle density increased. To explain these observed characteristic features in the surface plasmons, we introduced a qualitative model for the electronic polarizability of these superlattices. (c) 2007 American Institute of Physics.

The dispersion and assembly behavior of silicon (Si) nanoclusters are controlled in organic and aqueous suspensions. Hydrogen-terminated Si nanoclusters are stably dispersed in non-polar solvents, but assemble on aqueous suspension surfaces. The nanoclusters spontaneously pack into lattice arrangements when a slow assembly rate is maintained. In addition, converting the nanocrystal surface to carboxylic acid termination stably disperses the nanocrystals in aqueous suspensions. They show size-dependent photoluminescence (PL) from yellow to green in the suspensions, while partly oxidizing the surface causes blue PL.

We have prepared silicon nanoparticles by an arc discharge method in liquid nitrogen. This system is easily adoptable in the preparation of large quantities of Si nanoparticles and has the potential for low-cost production. Etching Si nanoparticles with hydrofluoric acid reduced their size and passivated their surface such that they exhibited bright visible luminescence under UV illumination. The slow HF etching allows the formation of Si nanoparticles with a narrow size distribution. After etching, nanoparticles as small as 3-6 nm in diameter were obtained. Raman spectroscopy was used to determine the average diameter of Si nanoparticles during etching. The photoluminescence of Si nanoparticle was measured as a function of size and could be associated with a quantum confinement model.

Photoinitiated hydrosilylation was used to attach acrylic acid to the surface of photoluminescent silicon nanoparticles, thereby producing water-dispersible, propionic-acid-terminated particles. From transmission electron microscope (TEM) observations, the average diameters of the synthesized nanocrystals were 1.9-2.4 nm. It is likely that smaller particles (< 1.5 nm) were also present but could not be imaged. As the nanocrystal size decreased, both the optical absorption edge and photoluminescence (PL) emission peak blue-shifted, whereas the photoluminescence excitation (PLE) spectrum changed very little and showed a sharp onset indicative of direct interband absorption at. After prolonged ultrasonication in water, the Si nanoparticles showed strong blue PL. This can tentatively be attributed to the formation of radiative centers related to incomplete oxidation of the nanocrystals. The silicon nanocrystals could be transferred into water or methanol by dialysis without inducing this oxidation. The PA-terminated Si nanoparticles were stably dispersed in acrylic acid, water, and methanol and showed essentially the same optical properties in all three solvents. The approach used here provides a general means of producing water-dispersible silicon nanocrystals with size-dependent photoluminescence tunable over a wide range of the visible spectrum.

Small gold clusters (<1 nm), protected by monolayers of glutathione, N-(2-mercaptopropionyl)glycine, or mercaptosuccinic acid, were prepared by reducing the corresponding Au(I)-thiolate polymers and were fractionated by size using polyacrylamide gel electrophoresis (PAGE). Mass analysis of the fractionated clusters revealed that their core sizes varied with the molecular structures of the thiolates. This finding indicates that the reduction of the Au(I)-thiolate polymers yields small clusters whose growth is kinetically hindered by passivation with thiolates. Optical spectra of the clusters with identical compositions exhibited different profiles depending on the thiolate molecular structures. This observation implies that deformation of the underlying gold cores is induced by interligand interactions.

Silicon nanoparticles ranging from 2 to 16 nm were synthesized by a facile wet chemical route. in which SiO amorphous powder was annealed at 1000 degrees C, etched in hydrofluoric acid, and surface modified by alkene. After alkyl-termination of the particle surfaces, size selective precipitation technique was applied to separate the nanoparticles into uniform sized fractions. Transmission electron microscopy showed well-dispersed and highly crystalline silicon nanoparticles after the treatment by alkene. Visible room-temperature photoluminescence in the range 800-500 nm was observed from these nanoparticles. The photoluminescence intensity has significantly been enhanced by the surface functionalization. Moreover, the PL peak energy of the size-selected nanoparticles shifted to blue as the size decreased due to quantum confinement effect. The experimental result was also compared with the theoretical predictions and was found to follow the general trend of the model calculations.

In this review article, we describe a precise synthetic recipe to prepare water-soluble gold and silver nanoparticles, and present how to prepare two dimensional as well as three-dimensional superlattices utilizing a hydrophilic and hydrophobic interface. We emphasize the importance of the role of side chain groups of thiol for controlling the properties of nanoparticles at the interface. Through chemical modification of the surface of nanoparticles via ion-pair interaction or covalent-bonding, it is shown how the phase transfer extraction efficiency is tuned. The inter-particle distance in the superlattice can be varied by an appropriate selection of mediator molecules through hydrogen bonding, which is automatically inserted among particles giving ordered structure. Some examples of three dimensional superlattices produced are presented, and the formation mechanism of the superlattices at the hydrophilic/hydrophobic interface is proposed through the van der Waals force calculation. A comparison is made between the calculation and experiments. Copyright © 2006 Taylor &amp Francis Group, LLC.

In recent years, the physical characteristics of noble metal nanoparticles are of considerable interest to material research. We made an effort to get superlattices of Ag nanoparticles at the air/water interface so as to apply various types of measurement by transferring the formed superlattices to suitable substrates. Basic procedure involved is based on a process recently developed for mercaptosuccinic acid (MSA) modified Au nanoparticles. The MSA protected Ag nanoparticles powder was redispersed in distilled water. HCl was added and the solution was left for several days at room temperature. Small crystallites appeared at the air/liquid interface. The diameter of the nanoparticles constructing the 2D superlattice was found to be larger than the parent nanoparticles. © World Scientific Publishing Company.

Silicon nanopowder with a narrow size distribution was synthesized by a simple method, in which amorphous SiOx (x < 2) powder as starting material was annealed at high temperature and then etched by hydrofluoric acid (HF). Si nanoparticles thus obtained exhibited emission in the ultraviolet and visible regions under excitation at an energy corresponding to the direct band-gap transition. At the same time, they could be redispersed in various organic solvents such as octanol, toluene, etc., without surfactants or capping molecules on particle surfaces. X-ray diffraction and Fourier transform infrared spectroscopy were used to follow the change of components in the sample during annealing and HF etching processes, and the size distribution and dispersion morphology of the nanoparticles in different solvents were revealed by transmission electron microscopy analysis.

Si nanowires of diameters 5-20 nm and nanoparticles of similar to 4 nm were synthesized by a simple arc-discharge method in water. The TEM analysis reveals that the growth direction of the observed Si nanowires is parallel to the {111} crystal planes.

We have demonstrated that carboxylic acid-capped gold nanoparticles were self-assembled to form two-dimensional (2D) and/or three-dimensional (3D) superlattices at an air/water interface in the presence of a bifunctional hydrogen-bonding mediator such as 4-pyridinecarboxylic acid (PyC) or trans-3-(3-pyridyl)acrylic acid (PyA). Transmission electron microscopy revealed a hexagonal close-packed arrangement of nanoparticles in the superlattice with an extension of interparticle spacing. In the 2D superlattices, larger particles produced a higher-quality assembly having long-range translational ordering. Attenuated total reflectance IR (ATR-IR) spectroscopy revealed the presence of hydrogen bonds between the mediator used and the capping agents of carboxylic acid on nanoparticle surfaces. Since the experimentally obtained interparticle separation distance agreed approximately with that obtained by the geometrical model calculations, we conclude that the hydrogen-bonding mediation controlled the interparticle spacing or structure by monomolecular incorporation between adjacent nanoparticles in the superlattices.

Monolayers consisting of two-dimensional (2D) Si nanoparticle lattices were synthesized using a self-assembly process of Si nanoparticles at an octanol/aqueous solution interface. The component nanoparticles were prepared in an aqueous solution by dissolving Si sub-oxide powder that contained Si nanocrystals. The Si nanocrystals, which were dispersed in the aqueous solution, were transferred into an octanol phase and assembled at the octanol/aqueous solution interface. The average diameter of the component nanocrystals in the 2D lattices varied from 7 to I I urn in different lattices, which suggests that the lattices incorporate nanoparticles of the same size and expel those of different sizes during lattice growth.

We report a strong visible light photoluminescence (PL) from three-dimensional Si cluster arrays. Using a chemical self-assembly process of Si clusters in aqueous solutions, three types of array structures are prepared: (1) BCC arrays with a = 6.1 angstroms, (II) FCC arrays with a = 7.5 angstroms, and (III) BCC arrays with a = 6.9 angstroms. The distance between the centers of adjacent Si clusters is 5.3 angstroms for both sample I and II, and 6.0 angstroms for the sample III. In the PL measurements, red, orange, and blue-green are the three different colors observed at peak wavelengths of 700, 610, and 530 nm, respectively. The red, orange, and blue-green light emissions are thought to originate from the samples I, II, and III, respectively. PL is strong enough to be seen by the naked eye in all the samples. Possible mechanisms for the strong visible light luminescence are discussed. (C) 2003 Elsevier Ltd. All rights reserved.

Two-dimensional (2D) superlattices of mercaptosuccinic acid-modified gold nanoparticles were constructed at an air/water interface in the presence of 4-pyridinecarboxylic acid (PyC). Hydrogen-bonding interaction caused the incorporation of PyC into the surface-functionalized gold nanoparticles, suggesting that the interparticle spacing in the superlattices is controllable with an intercalating molecule.

The self-assembly of gold nanoparticles at gas/suspension interface was strongly affected by the presence of organic vapor in the gas phase. Nonpolar organic molecules in the gas phase induced spherical assembly of gold nanoparticles, while polar organic molecules favored the formation of irregular aggregates at the gas/suspension interfaces.

In this paper, we report on the successful preparation and characterization of large Au colloidal crystals using hydrophilic Au nanoparticles as the building units in bulk aqueous solution. Scanning electron microscopy observation shows that the Au colloidal crystals have a clear crystal appearance and well-developed facets. Elemental composition of the Au colloidal crystals is estimated using energy-dispersive X-ray spectroscopy. The crystallographic data are uniquely determined using small-angle X-ray diffraction and transmission electron diffraction. On the basis of the crystallographic data, the stacking behavior of the Au nanoparticles in the colloidal crystals is also discussed. In the three-dimensional superlattices, Au nanoparticles are hexagonal close-packed and interconnected maybe by interparticle chemical bonding thanks to the mercaptosuccinic acid molecules over the Au nanoparticle surface.

Single crystals of Si clusters were created at an air/solution interface or a hydrogen-terminated Si substrate/solution interface using a self-assembly process, which originated from the hydrophobic nature of Si clusters. Mass analysis indicated that the components of the cluster crystals were partially oxidized hydrogenated Silo clusters. At the interfaces the nearest-neighbor distances between lattice points in the Si cluster crystals were 0.53 nm and 0,60 nm, which are consistent with the diameters of the Silo clusters. The slight difference in these values seems to be due to variations in the surface passivation of the component Silo clusters.

Micrometer-sized three-dimensional Si cluster arrays have been synthesized at an air-water interface and a water-hydrogen-terminated Si (100) interface leveraging the hydrophobic nature of Si clusters. Structures of the arrays grown at these two different interfaces are compared. The array packing structures adopt either a b.c.c. structure with a = 0.69 nm or an f.c.c. structure with a = 0.75 nm at-an air-water interface. Those grown on the Si substrate form two types of b.c.c. structures with a = 0.61 and 0.69 nm. The distance between centers of adjacent Si clusters is 5.3 nm in both the b.c.c. structure with a = 0.61 nm and the f.c.c. structure with a = 0.75 nm. It is concluded that component clusters in the arrays grown at both types of interfaces are identical. (C) 2003 Elsevier Science B.V. All rights reserved.

Superlattices consisting of gold (An) nanocrystals whose surfaces are modified with hydrophilic surfactants have been synthesized at an air/suspension interface under equilibrium conditions. At the initial stage of the growth, the assembly consists of an imperfect close-packing monolayer of Au nanocrystals whose average diameter is 3.5 nm and the standard deviation of size distribution is 14%. As the growth proceeds three-dimensionally, the quality of the close-packing structure significantly improves. After the sufficient growth, high-quality micrometer-sized superlattices are obtained. (C) 2002 Elsevier Science B.V. All rights reserved.

We show a very easy method for synthesizing three-dimensional superlattices consisting of silicon (Si) clusters. The obtained superlattices were micrometer-sized single crystals of Si clusters. The packing structures of the superlattices were either face-centered cubic (FCC) or body-centered cubic (BCC). The Si superlattices showed strong visible light emission, whereby the color of luminescence was either blue-green or orange. (C) 2002 Elsevier Science B.V. All rights reserved.